DEVELOPMENT OF A FLAT PANEL DETECTOR WITH AVALANCHE GAIN FOR INTERVENTIONAL RADIOLOGY

by

MATTHEW M. WRONSKI

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Graduate Department of Medical Biophysics University of Toronto

© Copyright Matthew M. Wronski 2009

Development of a Flat Panel Detector with Avalanche Gain for Interventional Radiology

Matthew Michael Wronski

Doctor of Philosophy, 2009

Department of Medical Biophysics

University of Toronto

Abstract A number of interventional procedures such as cardiac catheterization, angiography and

the deployment of endovascular devices are routinely performed using x-ray fluoroscopy.

To minimize the patient’s exposure to ionizing radiation, each fluoroscopic image is

acquired using a very low x-ray exposure (~ 1 µR at the detector). At such an exposure,

most semiconductor-based digital flat panel detectors (FPD) are not x-ray quantum noise

limited (QNL) due to the presence of electronic noise which substantially degrades their

imaging performance. The goal of this thesis was to investigate how a FPD based on

amorphous selenium (a-Se) with internal avalanche multiplication gain could be used for

QNL fluoroscopic imaging at the lowest clinical exposures while satisfying all of the

requirements of a FPD for interventional radiology.

Towards this end, it was first determined whether a-Se can reliably provide avalanche

multiplication gain in the solid-state. An experimental method was developed which

enabled the application of sufficiently large electric field strengths across the a-Se. This

method resulted in avalanche gains as high as 104 at an applied field of 105 V/µm using

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optical excitation. This was the first time such high avalanche gains have been reported in

a solid-state detector based on an amorphous material.

Secondly, it was investigated how the solid-state a-Se avalanche detector could be used to

image X-rays at diagnostic radiographic energies (~ 75 kVp). A dual-layered direct-

conversion FPD architecture was proposed. It consisted of an x-ray drift region and a

charge avalanche multiplication region and was found to eliminate depth-dependent gain

fluctuation noise. It was shown that electric field strength non-uniformities in the a-Se do

not degrade the detective quantum efficiency (DQE).

Lastly, it was determined whether the solid-state a-Se avalanche detector satisfies all of

the requirements of interventional radiology. Experimental results have shown that the

total noise produced by the detector is negligible and that QNL operation at the lowest

fluoroscopic exposures is indeed possible without any adverse effects occurring at much

larger radiographic exposures. In conclusion, no fundamental obstacles were found

preventing the use of avalanche a-Se in next-generation solid-state QNL FPDs for use in

interventional radiology.

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To my parents

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Acknowledgements First and foremost, I would like to thank my thesis supervisor, Dr. John A. Rowlands for

his infallible guidance and support and exceptional scientific training. Dr. Rowlands has

taught me the importance of patience, persistence and taking the high road. He has also

taught me to keep things simple and that research can be a rewarding experience.

I would also like to thank the members of my supervisory committee, Dr. Mike Rauth

and Dr. Don Plewes for their excellent comments, suggestions and advice. They have

helped keep me on track in my research and encouraged me to finish my thesis in a

timely way.

Thanks to Dr. Wei Zhao, Dr. Alla Reznik and Dr. Afrin Sultana for their collaboration

and helpful discussions. I would also like to thank Dr. Dylan Hunt for introducing me to

this field of research and taking the time to explain all the important details when I first

joined John’s laboratory. Dylan, your enthusiasm in this field of research has been

contagious!

A big thank you goes to Giovanni DeCrescenzo who has been instrumental in developing

and teaching me how to use the many tools used throughout this thesis. Giovanni has also

always been there to answer all my questions and reassure me of my work. Also, thanks

to Dr. Kenkichi Tanioka at NHK, for providing the samples on which a large part of this

thesis is based.

A special thanks goes out to my friends and colleagues in John’s group, particularly

Philip Komljenovic, Dr. Normand Robert, Kristina Watt, Dr. Farhad Taghibakhsh, David

Green and Sarah Cuddy. You guys have kept a permanent smile on my face throughout

my studies!

Last but not least, I would like to thank my parents for all their love and relentless

support and for always believing in my abilities. You guys are truly the best!

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Table of Contents

Chapter 1 Introduction....................................................................................................................... 1

1.1 Overview ................................................................................................................... 2 1.2 Fluoroscopy and interventional radiology ............................................................... 2 1.3 Clinical fluoroscopy requirements for interventional radiology .............................. 5 1.4 Current x-ray imaging technology for interventional radiology and its limitations 8

1.4.1 X-ray image intensifiers ..................................................................................... 8 1.4.2 Flat panel detectors ........................................................................................... 9 1.4.3 Electronic noise in flat panel detectors ........................................................... 12

1.5 Possible solutions for quantum noise limited AMFPIs........................................... 15 1.6 High-gain avalanche rushing photoconductor (HARP) technology....................... 19

1.6.1 HARP camera .................................................................................................. 19 1.6.2 The need for a solid-state HARP ..................................................................... 21

1.7 Rationale and problem formulation........................................................................ 23 1.8 Thesis outline .......................................................................................................... 24 References ..................................................................................................................... 26

Chapter 2 Development of a solid-state amorphous selenium avalanche photoreceptor........... 32

2.1 Introduction............................................................................................................. 33 2.2 Theory ..................................................................................................................... 34 2.3 Methods................................................................................................................... 40

2.3.1 Distributed resistive layer................................................................................ 40 2.3.2 Experimental setup........................................................................................... 42 2.3.3 Linearity ........................................................................................................... 43 2.3.4 Gain and dark current ..................................................................................... 44 2.3.5 Carrier transport ............................................................................................. 45

2.4 Results ..................................................................................................................... 47 2.4.1 Breakdown characteristics............................................................................... 47 2.4.2 Linearity ........................................................................................................... 50 2.4.3 Gain and dark current ..................................................................................... 51 2.4.4 Carrier transport ............................................................................................. 52

2.5 Discussion ............................................................................................................... 55 2.5.1 Breakdown characteristics............................................................................... 55 2.5.2 Linearity ........................................................................................................... 58 2.5.3 Gain and dark current ..................................................................................... 60 2.5.4 Carrier transport ............................................................................................. 61

2.6 Conclusions............................................................................................................. 62 References ..................................................................................................................... 64

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Chapter 3 Theory of x-ray imaging with avalanche amorphous selenium in the solid state ..... 67

3.1 Introduction............................................................................................................. 68 3.2 Background ............................................................................................................. 69

3.2.1 Indirect-conversion HARP imager .................................................................. 69 3.2.2 Depth-dependent gain fluctuation noise .......................................................... 72

3.3 Proposed device structure....................................................................................... 74 3.4 Calculation methods ............................................................................................... 76

3.4.1 MTF, NPS and DQE ........................................................................................ 76 3.4.2 Avalanche gain, gain nonuniformities and fill-factor...................................... 80 3.4.3 Del response..................................................................................................... 82

3.5 Results ..................................................................................................................... 83 3.5.1 MTF, NPS and DQE ........................................................................................ 83 3.5.2 Avalanche gain, gain nonuniformities and fill-factor...................................... 85 3.5.3 Del response..................................................................................................... 88

3.6 Discussion ............................................................................................................... 89 3.6.1 MTF, NPS and DQE ........................................................................................ 90 3.6.2 Avalanche gain, gain nonuniformities and fill-factor...................................... 91

3.6.2.1. Average gain and fill-factor..................................................................... 91 3.6.2.2 Avalanche multiplication noise................................................................. 92 3.6.2.3 Gain nonuniformities ............................................................................... 94

3.6.3 Del response..................................................................................................... 96 3.6.4 Response at high spatial frequencies............................................................... 96 3.6.5 Dark current..................................................................................................... 97 3.6.6 Direct x-ray interaction in the gain region...................................................... 98

3.7 Conclusions............................................................................................................. 99 References ................................................................................................................... 101

Chapter 4 Experimental characterization of DRL-HARP for interventional radiology applications .................................................................................................................... 105

4.1 Introduction........................................................................................................... 106 4.2 Methods................................................................................................................. 107

4.2.1 Noise characterization ................................................................................... 107 4.2.2 X-ray sensitivity ............................................................................................. 109 4.2.3 Dynamic range............................................................................................... 111 4.2.4 Temporal response......................................................................................... 111 4.2.5 Compatibility with TFT technology ............................................................... 115

4.2.5.1 Reverse structure .................................................................................... 115 4.2.5.2 HARP thickness....................................................................................... 117 4.2.5.3 TFT compatibility (in collaboration with A. Sultana at U. of Waterloo)............................................................................................................................. 117

4.3 Results ................................................................................................................... 119 4.3.1 Noise characterization ................................................................................... 119

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4.3.2 X-ray sensitivity ............................................................................................. 120 4.3.3 Dynamic range............................................................................................... 121 4.3.4 Temporal response......................................................................................... 122

4.3.4.1 RC and ghosting...................................................................................... 122 4.3.4.2 Lag .......................................................................................................... 124 4.3.4.3 Predicted timing response for a DRL-HARP FPD ................................. 125

4.3.5 Compatibility with active matrix technology ................................................. 126 4.3.5.1 Reverse structure .................................................................................... 126 4.3.5.2 HARP thickness....................................................................................... 127 4.3.5.3 TFT compatibility (experiments performed with Afrin Sultana U Waterloo)............................................................................................................................. 128

4.4 Discussion ............................................................................................................. 130 4.4.1 Noise characterization ................................................................................... 130 4.4.2 X-ray sensitivity ............................................................................................. 132 4.4.3 Dynamic range............................................................................................... 132 4.4.4 Temporal response......................................................................................... 135

4.4.4.1 RC and ghosting...................................................................................... 135 4.4.4.2 Lag .......................................................................................................... 136 4.4.4.3 Predicted timing response for a DRL-HARP FPD ................................. 137

4.4.5 Compatibility with active matrix technology ................................................. 138 4.4.5.1 Reverse structure .................................................................................... 138 4.4.5.2 HARP thickness....................................................................................... 140 4.4.5.3 TFT compatibility ................................................................................... 141

4.5 Conclusion ............................................................................................................ 143 References ................................................................................................................... 144

Chapter 5 Conclusions .................................................................................................................... 146

5.1 Brief summary ....................................................................................................... 147 5.2 Summary of major results ..................................................................................... 149

5.2.1 Solving the breakdown problem of electroded HARP ................................... 149 5.2.2 X-ray imaging with HARP-AMFPI ................................................................ 151 5.2.3 Addressing the requirements of interventional radiology ............................. 152

5.3 Original contributions .......................................................................................... 154 5.4 Future work........................................................................................................... 156

5.4.1 Materials characterization............................................................................. 156 5.4.2 Device optimization ....................................................................................... 157 5.4.3 Imager prototype fabrication ......................................................................... 158

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List of Figures 1.1 Three different x-ray imaging systems…………………………………………………………….11 1.2 The active matrix and detecting element…………………………………………………………..12 1.3 Simulated radiographs reflecting the significance of electronic noise…………………………….14 1.4 Principle of operation of a HARP camera tube…………………………………………………....20 1.5 Directly electroded HARP………………………………………………………………………...23 2.1 Directly electroded HARP and DRL-HARP……………………………………………………....36 2.2 Time-temperature-crystallization diagram………………………………………………………...37 2.3 Electrical discharge paths in directly electroded HARP and DRL-HARP………………………..39 2.4 Cellulose acetate casting process………………………………………………………………….42 2.5 Experimental setup used for characterization of directly electroded HARP and DRL-HARP……43 2.6 Measured dark current transient in directly electroded HARP……………………………………47 2.7 Measured dark current magnitude before and after breakdown…………………………………...48 2.8 Number of electrical discharges in DRL-HARP…………………………………………………..50 2.9 Peak measured photocurrent in DRL-HARP as a function of LED source intensity……………..50 2.10 Photocurrent transients in DRL-HARP for varying applied biases in the avalanche regime……..51 2.11 Measured DRL-HARP photocurrent and dark current as a function of high voltage bias………..52 2.12 Measured photocurrent transient in directly electroded HARP and DRL-HARP………………...53 2.13 Measured a-Se hole mobility in DRL-HARP and a Xerox a-Se plate…………………………….54 2.14 Time-of-flight traces obtained in the avalanche regime…………………………………………..55 3.1 SHARP-AMFPI imager concept………………………………………………………………….70 3.2 Calculated DQE(f) for SHARP-AMFPI for an x-ray exposure of 0.1 µR………………………..71 3.3 MICROMEGAS, GEM and dual-layered a-Se detector concepts………………………………...73 3.4 Structure of HARP-AMFPI……………………………………………………………………….76 3.5 Stages of the cascaded linear system model for HARP-AMFPI………………………………….77 3.6 Calculated MTF for a-Se and aperture function…………………………………………………..83 3.7 NPS for a direct-conversion a-Se AMFPI before and after the addition of electronic noise……...84 3.8 Calculated DQE(f) for a direct-conversion a-Se AMFPI………………………………………….84 3.9 Calculated DQE(0) for a direct-conversion a-Se AMFPI…………………………………………85 3.10 Calculated average avalanche gain for a direct-conversion a-Se AMPFI…………………………86 3.11 Calculated electric field distribution for a direct-conversion a-Se AMPFI……………………….86 3.12 Calculated effective fill factor for a direct-conversion a-Se AMPFI……………………………...87 3.13 Calculated gain nonuniformity for a direct-conversion a-Se AMPFI……………………………..87 3.14 DQE(0) calculated as a function of conversion and avalanche gain………………………………88 3.15 Avalanche gain and image charge calculated as a function of x-ray exposure……………………89 4.1 Linear cascaded noise model used to calculate the expected noise variance for DRL-HARP…..108 4.2 Experimental setup used to characterize x-ray sensitivity……………………………………….110 4.3 Circuit diagrams used to model the electrical behaviour of DRL-HARP………………………..114 4.4 The regular and reverser HARP structures………………………………………………………116 4.5 Experimental setup used to investigate combined DRL-HARP / TFT operation………………..118 4.6 Measured charge signal and noise produced by DRL-HARP……………………………………119 4.7 Measured DRL-HARP dark current and photocurrent…………………………………………..121 4.8 Measured output charge from DRL-HARP as a function of the equivalent x-ray exposure…….122 4.9 Measured photocurrent transient showing ghosting and RC effects……………………………..123 4.10 Measured photocurrent transient showing lag effect…………………………………………….124 4.11 Output signal in a FPD with a DRL-HARP avalanche layer and TFT charge readout…………..126 4.12 Measured photocurrent for normal and reverse-structured DRL-HARP………………………...127 4.13 Measured photocurrent and dark current for a DRL-HARP with a 4 µm HARP layer………….128 4.14 Measured drain-source current as a function of gate-source voltage for TFT in Fig. 4.5………..129 4.15 TFT output current measured using the experimental setup shown in Figure 4.5……………….130

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List of Tables 1.1 Several key imaging modalities used in interventional radiology……………………………….....4 1.2 Three different approaches for overcoming electronic noise at low x-ray exposures in AMFPIs...18 2.1 Measured characteristics of DRL-HARP with and without a DRL……………………………….49 2.2 Maximum electric field in a-Se layer for several different types of contacts……………………..49 3.1 Summary of factors used to characterize nonuniformities in conversion and avalanche gain…….80 3.2 Detector operating conditions and design parameters chosen for fluoroscopy and radiography….80

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List of Symbols ASe a-Se Swank factor………………………………………………………………………………….77 Aav avalanche Swank factor……………………………………………………………………………78 Asec secondary Swank factor…………………………………………………………………………...79 b electron hole pair recombination coefficient………………………………………………………45 β impact ionization coefficient………………………………………………………………………45 C capacitance………………………………………………………………………………………...12 Cd del capacitance…………………………………………………………………………………….36 d a-Se thickness……………………………………………………………………………………...38 ∆A electrical discharge area…………………………………………………………………………...36 ∆Q amount of heat dissipated from discharge region………………………………………………….38 e electronic charge…………………………………………………………………………………..14 E electric field strength………………………………………………………………………………33 EG band gap energy…………………………………………………………………………………...60 Ea-Se electric field strength in the a-Se layer…………………………………………………………….45 Ed electrical energy accumulated on del capacitance…………………………………………………36 Emax maximum electric field strength…………………………………………………………………...49 η x-ray quantum absorption efficiency………………………………………………………………77 g total gain………………………………………………………………………………………….122 gc conversion gain……………………………………………………………………………………45 gav avalanche gain……………………………………………………………………………………..45 Id dark current………………………………………………………………………………………..60 Idis peak discharge current……………………………………………………………………………..36 k Boltzmann’s constant……………………………………………………………………………...12 q0 number of incident x-ray photons per unit area…………………………………………………...77 Reff effective series resistance of DRL…………………………………………………………………38 Rlat effective lateral (sheet) resistance of DRL………………………………………………………...40 ρDRL resistivity of DRL………………………………………………………………………………….40 σav

2 avalanche multiplication gain variance……………………………………………………………77 T temperature………………………………………………………………………………………...12 Ta(f) del aperture function……………………………………………………………………………….77 Tb(f) MTF associated with electron hole pair generation in a-Se……………………………………….77 τlat time constant for lateral charge conduction……………………………………………………….40 Vd potential across del capacitance…………………………………………………………………...36 Weff required amount of absorbed x-ray energy to produce a single EHP that survives recombination.17 Z atomic number……………………………………………………………………………………..15

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List of Abbreviations ALARA as low as reasonably achievable………………………………………………………………….....4 AMFPI active matrix flat panel imager……………………………………………………………………...9 AsSe3 arsenic triselenide……………………………………………………………………………….....97 a-Se amorphous selenium……………………………………………………………………………….10 CA cellulose acetate……………………………………………………………………………………40 CCD charge coupled device………………………………………………………………………………8 CeO2 cerium oxide…………………………………………………………………………………….....97 CMOS complimentary metal oxide semiconductor……………………………………………………….22 c-Si crystalline silicon…………………………………………………………………………………..60 CsI cesium iodide……………………………………………………………………………………….9 DC direct current……………………………………………………………………………………..109 del detector element………………………………………………………………………………….....9 DQE detective quantum efficiency……………………………………………………………………...68 DSA digital subtraction angiography…………………………………………………………………….6 DRL distributive resistive layer…………………………………………………………………………35 EHP electron hole pair……………………………………………………………………………………9 EMI electro-magnetic interference……………………………………………………………………...46 FEM finite element method……………………………………………………………………………...80 FPD flat panel detector………………………………………………………………………………….13 GEM gas electron multiplier……………………………………………………………………………..72 HARP high gain avalanche rushing photoconductor……………………………………………………...19 HgI2 mercuric iodide…………………………………………………………………………………….15 HV high voltage………………………………………………………………………………………..15 ITO indium tin oxide…………………………………………………………………………………...23 LED light emitting diode………………………………………………………………………………..42 MRI magnetic resonance imaging………………………………………………………………………..3 MTF modulation transfer function………………………………………………………………………..7 NPS noise power spectrum……………………………………………………………………………...79 OCT optical coherence tomography……………………………………………………………………...3 PbI2 lead iodide…………………………………………………………………………………………15 PbO lead oxide………………………………………………………………………………………….15 PEDOT Poly(3,4-ethylenedioxythiophene)………………………………………………………………...36 PMT photomultiplier tube……………………………………………………………………………….42 QNL quantum noise limited………………………………………………………………………………7 RE readout element…………………………………………………………………………………..117 R/F radiography/fluoroscopy…………………………………………………………………………..57 SHARP scintillator-HARP………………………………………………………………………………….69 SPICE Simulation Program with Integrated Circuit Emphasis…………………………………………..113 SNR signal to noise ratio………………………………………………………………………………119 TOF time of flight……………………………………………………………………………………….45 TFT thin film transistor…………………………………………………………………………………11 TTC time-temperature-crystallization…………………………………………………………………..37 W/L width to length ratio……………………………………………………………………………...118 XRII x-ray image intensifier……………………………………………………………………………...3 RC resistive-capacitive………………………………………………………………………………...38

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Chapter 1 Introduction 1.1 Overview 1.2 Fluoroscopy and interventional radiology 1.3 Clinical fluoroscopy requirements for interventional radiology 1.4 Current x-ray imaging technology for interventional radiology and its limitations

1.4.1 X-ray image intensifiers 1.4.2 Flat panel detectors 1.4.3 Electronic noise in flat panel detectors

1.5 Possible solutions for quantum noise limited AMFPIs 1.6 High-gain avalanche rushing photoconductor (HARP) technology

1.6.1 HARP camera 1.6.2 The need for a solid-state HARP

1.7 Rationale and problem formulation 1.8 Thesis outline

1

1.1 Overview This chapter starts by reviewing the role of fluoroscopy in interventional radiology,

within the context of other imaging modalities currently used for image guidance.

Specific clinical requirements are identified for the fluoroscopic imaging system. Next,

the limitations of current x-ray imaging technologies are identified. The emphasis is

placed particularly on the solid-state flat panel detector which is a relatively new and very

promising imaging technology. Possible solutions are identified for overcoming

electronic noise, which is the single-most important problem affecting all existing flat

panel detectors in fluoroscopy. The chapter next presents a review and discusses how a

specialized technology based on avalanche multiplication of charge in an amorphous

selenium photoconductor can be used for fluoroscopic imaging in interventional

radiology. The need for a solid-state version of this promising technology is identified.

Finally, problems impeding its practical use are outlined and this establishes the main

objectives of the thesis.

1.2 Fluoroscopy and interventional radiology The field of radiography was born soon after Roentgen discovered X rays near the end of

the 19th century. Shortly after this discovery, it became possible to produce diagnostic

images or radiographs of internal human anatomy. Internal anatomical motion could also

be observed by using a device known as a fluoroscope which employed a screen with a

material such as zinc cadmium sulfide that emitted light when exposed to X rays.1

Fluoroscopic imaging, or fluoroscopy, initially required the radiologist to greatly increase

the sensitivity of their eyes to the faint blue or green light by sitting in a darkened room

2

prior to examining the image on the screen (dark adaptation). Subsequently, it was found

that wearing red goggles permitted dark adaptation to be retained in ordinary room light.

However, even with dark adapted eyes, the poor optical coupling of the screen to the

human eye resulted in a degradation of the final image on the viewer’s retina. This

situation, in which only a portion of all secondary quanta (optical photons in this case)

are used to create the final image is known as a secondary quantum sink. The problems

associated with dark adaptation and the secondary quantum sink were overcome in the

1950’s with the development of the x-ray image intensifier (XRII).

Since its inception, fluoroscopy has, and continues to be a key imaging modality used in

interventional radiology, a branch of radiology that is concerned with the use of image

guidance to conduct minimally invasive procedures for both diagnostic and therapeutic

purposes. These procedures include, for instance, angiography, angioplasty, pacemaker

insertion and embolization. Ultrasound and magnetic resonance imaging (MRI),

developed in the 1970s and 1980s, respectively, and the more recently developed optical

coherence tomography (OCT) have also been and are increasingly being used for image

guidance in interventional radiology. These imaging modalities have the important

advantage of being tomographic, meaning that they can produce image slices at a

specified depth or volumetric renderings of the anatomical regions being imaged. While

X-ray fluoroscopy is not inherently tomographic, technologies such as cone beam CT or

rotational angiography may be used in addition to fluoroscopy to provide three-

dimensional image data. Table 1.1 summarizes the key advantages and disadvantages of

each of the imaging modalities currently used in interventional radiology.

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Table 1.1. Summary of the advantages and disadvantages of several key imaging modalities used in interventional radiology. Advantages Disadvantages X-ray fluoroscopy • high spatial resolution

• high temporal resolution • excellent geometrical accuracy • low relative cost

• ionizing radiation • poor soft tissue contrast • not tomographic

Ultrasound • high temporal resolution • no ionizing radiation • low relative cost • localized imaging can be

performed at catheter tip • tomographic

• poor spatial resolution • poor imaging performance

near bones and air-filled cavities

• large operator dependency

MRI • good soft tissue contrast • no ionizing radiation • localized imaging can be

performed at catheter tip • tomographic

• requires specialized non-magnetic devices

• fundamental tradeoff between image quality and temporal resolution

• very high cost OCT • high spatial resolution

• high temporal resolution • low relative cost • tomographic

• limited to localized imaging at catheter tip

The reason X-ray fluoroscopy remains a dominant imaging modality in interventional

radiology is because no other single modality provides the same combination of high

spatial and temporal resolution which is particularly important for proper deployment of

endovascular (from within the blood vessel) devices such as stents or coils. However, the

harmful effects of ionizing radiation used in fluoroscopy, which have long been

recognized, require the patient dose to be as low as reasonably achievable during an

intervention. This is often referred to as the ALARA principle. The use of harmful x-ray

radiation is justifiable by considering that the benefit from the clinical outcome of the

intervention will outweigh the adverse biological effects of the radiation. These

biological effects include indirect and direct effects. Indirect effects of ionizing radiation

arise when electrons set in motion by x-ray photons excite and ionize water molecules,

4

creating free radicals which then cause damage to critical biological targets such as DNA.

In direct effects, electrons directly ionize DNA molecules. As a result, in certain cases

such as pediatric interventions, a particularly strict adherence to ALARA is required,

since the accrued stochastic effects due to radiation exposure are more likely to disrupt

tissue growth and development as well as lead to an increased chance of cancer over the

child’s lifetime. There are adverse effects associated with other imaging modalities as

well. The electromagnetic radiofrequency pulses used in MRI are known to cause

heating. This can be particularly problematic near metallic devices or implants such as

pacemakers or hearing aids. Ultrasound contrast agents, when exposed to ultrasound

waves, can also cause potential bio-effects (i.e. rupture of cell membranes) at the level of

the microcirculation, although the clinical relevance of such bio-effects remains unclear.2

1.3 Clinical fluoroscopy requirements for interventional radiology

A modern fluoroscope consists of a large “C” shaped mount called a C-arm with an x-ray

source on one end and an x-ray imager on the other. This assembly can be positioned

such that different projections of the patient anatomy may be acquired from different

angles. In certain cases, the C-arm assembly is rotated around the patient during injection

of a contrast dye and multiple projection images are acquired and subsequently

reconstructed into a three-dimensional rendition of the vasculature.3 The C-arm assembly

should hence be designed such that the x-ray source and x-ray imager are as small and

light as possible to facilitate its positioning or rotation and to improve patient

accessibility.

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In most cases, the x-ray imager is used for fluoroscopy as well as two other imaging

modes, namely cine acquisition and radiography. We will now describe each of these

imaging modes in turn. In the fluoroscopic mode, the physician typically guides

interventional devices such as guidewires or stents through a catheter towards the lesion

and is mainly concerned with tracking the position of the device. This guidance, usually

referred to as a catheterization, is typically done at relatively high imaging frame rates

(7.5 – 30 image frames per second) so that the physician can get a good sense of the

advancement of the device towards the lesion while avoiding potential complications due

to, for example, arterial tortuosity. Catheterization is often a relatively time consuming

process (ten minutes or longer) and is done at very low x-ray exposures (mean exposure

of 1 µR per frame at the imager) to minimize patient radiation exposure. The x-ray

exposures employed are so low, in fact, that quantum noise (the stochastic variation in the

spatial distribution of x-ray photons) becomes the dominant form of noise in the

fluoroscopic images obtained using an XRII system. Because the interventional device

being imaged typically has a high degree of radio-opacity, these low exposures are often

adequate to obtain sufficient contrast to image the device. In the cine mode, sequences of

images are acquired during administration of an x-ray contrast agent (typically via a

catheter) into the vasculature. These relatively short (several seconds in duration) image

acquisitions are taken at higher exposures (~10 µR/frame) so as to provide superior

image quality (less quantum noise) and thus improve the diagnostic value of the images.

This mode is also used when accurate positioning or deployment of endovascular devices

is performed. In the radiographic mode, images are acquired at even larger exposures (~

100 µR/frame) for applications such as digital subtraction angiography (DSA) which

6

have been shown to improve the detection of certain lesions such as aneurysms or

thrombi.4

High spatial resolution is an important requirement in the cine and radiographic imaging

modes, as it can strongly affect the diagnosis or treatment outcome of an interventional

procedure. Interventional devices such as guide wires or stents typically have wire

diameters ranging from 50-200 µm. Rudin5 et al. have demonstrated that despite the

effects of x-ray scattering, imaging of individual stent wires (struts) using a high

resolution imager is possible inside a human head phantom; this should enable the

deployment of novel assymetrical stents for specialized therapeutic neurovascular

applications. Furthermore, in certain applications such as coronary angiography,

detection of small calcium deposits (tens of micrometers in size) in coronary arteries

provides an important means of assessing the degree of atherosclerosis as well as the

likelihood of a successful angioplasty 6,7. For optimal imaging of fine features in

interventional radiology, the imager should be able to resolve 5 line pairs per millimeter

(a line pair is a pair of light and dark lines) such that the relative contrast between the two

lines of each pair is greater than 0.2 (i.e. a modulation transfer function (MTF) greater

than 0.2 at a spatial frequency of 5 cycles/mm).

From the discussion above, it follows that the key requirements for a clinical x-ray

imager for interventional radiology – beyond a thin profile and providing unobstructed

access to the patient -- are: (1) quantum-noise limited (QNL) operation at the lowest

clinical fluoroscopic x-ray exposures (in conformance with the ALARA principle), (2)

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capability for modes of operation that require significantly higher x-ray exposures and (3)

capability of imaging fine features of interventional devices or lesions.

1.4 Current x-ray imaging technology for interventional radiology and its limitations 1.4.1 X-ray image intensifiers Until recently, the most widely used x-ray imaging system in interventional radiology

was the x-ray image intensifier (XRII). This is an electro-optical device that operates

inside a vacuum enclosure (Figure 1.1(a)) which contains an input phosphor used to

convert X rays into optical photons. The phosphor is coupled to a photocathode which,

upon exposure to these optical photons produces electrons. The latter are accelerated in

an electric field and hit an output phosphor screen. This process enables the production of

several thousand optical photons for each photoelectron emitted from the photocathode.

The resulting optical image is captured using an optical assembly and a video camera or

charge coupled device (CCD). XRII/video systems provide excellent x-ray sensitivity (the

degree to which small numbers of X rays - ideally a single X ray - can be detected) due to

the large internal gain of the XRII, however, the XRII also presents important limitations.

These are associated with the curvature of the input phosphor and the presence of

multiple conversion stages, leading to geometrical distortions (particularly in the

periphery of the images), spatial nonuniformities and a degradation of the imaging

resolution 8-10. Furthermore, these systems are bulky and heavy (they can weigh several

hundred pounds), compromising patient accessibility and image acquisition modes such

as rotational angiography or cone-beam computed tomography. Another important

8

limitation of the XRII is its high sensitivity to magnetic fields (including the earth’s

magnetic field) which produces a distortion in the image that is shaped like an “S” and

referred to as an S-distortion.10 Due to these substantial limitations and the availability of

new solid-state technologies, there has been a progressive trend in the past few years to

replace XRII systems with flat panel detectors.11

1.4.2 Flat panel detectors

There has been much work over the last decade on the development of solid-state flat

panel detectors, also known as active matrix flat panel imagers (AMFPI). Unlike the

XRII, these systems are thin, produce negligible spatial distortions and are insensitive to

magnetic fields.12 In principle, panels with sufficiently small pixels, or detector elements

(del) also have the potential for substantially improved spatial resolution in comparison

with the XRII because there are considerably less conversion stages involved in the

image detection process (Figure 1.1 (b) and (c)).

The majority of commercial AMFPIs are indirect conversion detectors, in which x-ray

photons strike a scintillator such as cesium iodide (CsI) and generate optical photons

which then interact with a photosensor (usually an amorphous silicon photodiode), in turn

producing electron-hole-pairs (EHP) that are capacitively stored prior to being

electronically processed (see Figure 1.1(b)). The process of detecting the photon-

generated charge from each del (referred to as readout) produces a digital image which

represents the original distribution of X rays incident at the imager’s surface. In another

class of detectors know as direct-conversion detectors, x-rays interact with a

9

photoconductor, usually amorphous selenium (a-Se), and directly generate EHPs, which

follow the parallel field lines in the presence of an electric field (see Figure 1.1(c)) prior

to being read out. Because there is no intermediary optical stage to contribute to blurring,

these systems have the important advantage of providing superior spatial resolution

compared to indirect AMFPIs, however, the photoconductor needs to be thick enough (~

1000 µm) to yield a reasonable quantum efficiency at radiographic x-ray energies (20 –

150 kV).13 The very high spatial resolution of direct-conversion a-Se detectors has

recently increased their use in digital mammographic imaging systems.14 Direct-

conversion detectors are also being considered for use in tomosynthesis, in which a series

of breast radiographs are acquired from different angles and reconstructed into a series of

slices. In contrast with mammography, the image slices produced by tomosynthesis are

largely immune to structural noise, which is the noise introduced into an image due to x-

ray attenuation in overlapping anatomical structures.15

10

c) b)a)

AMFPI AMFPI

Figure 1.1 Three different x-ray imaging systems. (a) The image intensifier consists of an input phosphor coupled to a photocathode which converts X rays into optical photons and subsequently electrons. The latter are accelerated in an electric field and hit an output phosphor screen. This produces an amplification of several thousand. The resulting optical image is captured using an optical assembly and a charge coupled device (CCD). (b) The indirect active matrix flat panel imager (AMFPI) consists of a scintillator which converts X rays into optical photons, a photoconductor which converts them in turn to electrons and a readout layer which stores and processes the resulting charge image. (c) In the direct conversion flat panel detector, X rays are directly converted into charge inside a photoconductor. In each diagram, the electric field lines are shown as two lines next to an arrow which shows the direction in which the charge travels. Adapted from Ref 12.

In both types of AMFPIs, a two-dimensional array of thin film transistors (TFT) is used

to relay the stored image charge at each del electrode to charge amplifiers, as shown in

Figure 1.2. The charge is stored (and integrated) at each del on a del storage capacitor.

Each TFT in the array (the active matrix) acts as a switch which is activated by the gate

line and is turned on in sequence for a short time interval (several microseconds) enabling

the image charge transfer to occur along data lines. Since only a single TFT is turned on

at any given time, the number of charge amplifiers needed is equal to the number of

columns in the array. This combination of TFT technology with the sequential switching

scheme – similar to that used in liquid crystal displays - reduces the number of data lines,

amplifiers and processing electronics required, thus facilitating the manufacture of

AMFPIs.

11

Figure 1.2 Diagram showing an active matrix (left) which is used for reading out photon-generated charge from a photoconductor, usually deposited on top (not shown). The active matrix consists of an array of electrodes, one for each detecting element (del). The charge is stored at each electrode on a del storage capacitor. It is periodically transferred to a charge amplifier by means of a data line when the neighboring thin film transistor (TFT) is switched on by means of activating its gate line (right). Switching control electronics deliver pulses for each gate line in such a way that the charge stored in each row of dels is transferred to the charge amplifiers simultaneously. This charge readout is performed row-by-row. Signal processing electronics are then used to digitize the charge signal and produce an image once each row has been read out.

1.4.3 Electronic noise in flat panel detectors

The principal limitation of active matrix technology for AMFPI is the generation of

electronic noise in the active matrix and the associated readout electronics. This noise

arises from various sources, the dominant ones being thermal noise, charge amplifier

noise and noise associated with electrical interference. Significant reduction of these

noise sources is unlikely, as shall now be briefly discussed. Thermal noise, also referred

to as Johnson noise, or Nyquist noise is caused by the thermal agitation of electrons

inside a conductor irrespective of an applied voltage.16 In AMFPIs, this noise arises at the

del storage capacitance and is specifically referred to as kTC noise, as it depends on the

product of Boltzmann’s constant k, the temperature T and the capacitance C. Cooling the

detector can reduce this noise, however this is not a good choice for most AMFPIs since

temperatures significantly lower than room temperature hinder the operation of the

12

TFTs.17 Significant reductions in C are also impractical because a larger potential can

develop across a smaller storage capacitance for the same amount of x-ray generated

charge. This increases the likelihood of electric breakdown of the capacitor dielectric.

Charge amplifier noise is caused by the active electronic components in the charge

amplifiers. It largely depends on the capacitance of the data lines that are sampled by

each amplifier. Since modern FPDs are quite large (i.e. 30 cm by 30 cm), the length and

hence the capacitance of the data lines is substantial. Electrical interference noise is

caused by sources outside the active matrix array such as power supplies. The noise of

these sources can capacitively couple into the array. Specialized double-sampling

circuitry is being used to reduce this noise, however it is difficult to remove the higher

frequency noise components.

The electronic noise is added to the x-ray-generated image charge and degrades the

sensitivity of the imager. This is particularly problematic at the lowest detector exposures

encountered in fluoroscopy (0.1 – 1 µR/frame). As an example, an exposure of 0.1

µR/frame in an a-Se AMFPI with 250 x 250 µm dels is equivalent to a single x-ray

photon striking each del.18,19 A 50 keV X ray generates approximately 1000 EHPs in an

a-Se photoconductor biased at 10 V/µm.20 Meanwhile, the electronic noise level in the

readout system of a state-of-the art AMFPI is in the range 1500 - 3000 electrons per del21,

1.5 – 3.0 times the x-ray-generated signal. Clearly, for an AMFPI with a smaller del size

(i.e. 100 µm by 100 µm ), the minimum x-ray exposure at which the imager is quantum

noise limited would be even larger (i.e. 0.6 µR/frame). Thus, unlike the XRII which has a

large internal gain, current-generation AMFPIs are limited at the lowest fluoroscopic

13

exposures by electronic noise and not x-ray quantum-noise. The significance of this

electronic noise in the context of interventional radiology is demonstrated in Figure 1.3

which shows a simulated radiograph of a coronary stent obtained at a large (12 µR) and a

low (0.5 µR) x-ray detector exposure. The stent is clearly visible at the large exposure. At

the low exposure, the stent can still be clearly visualized despite the added quantum noise

in the image. When the electronic noise is accounted for, however, it becomes very

difficult to see the stent. In a clinical environment, the added structural noise in the image

- due to overlapping anatomical features corresponding to a single x-ray projection -

would make the task of visualizing the stent even more difficult. Hence, the goal is to

achieve QNL images that are not affected by electronic noise.

Figure 1.3 Simulated radiographs of a coronary stent obtained for two different x-ray exposures. Also shown are radiographs obtained with and without the addition of electronic noise for a 0.5 µR fluoroscopic exposure. A detector element size of 100 µm was used and an electronic noise of 2000e was assumed. Adapted from 5.

14

1.5 Possible solutions for quantum noise limited AMFPIs

We have therefore demonstrated that significant reductions of electronic noise are

unlikely. Thus, techniques for overcoming it must involve the production of a large

amount of charge for each x-ray interaction. There have been essentially three solutions

proposed towards this end, as summarized in Table 1.2. The first involves the use of

high-gain photoconductors22,23 which generate larger numbers of EHPs for each incident

x-ray photon, thus providing a stronger signal than a-Se or amorphous silicon, the two

predominantly used materials in direct and indirect conversion AMFPIs, respectively. As

well as higher gain, the new materials being developed (eg. PbI2, PbO or HgI2) have

higher atomic numbers (Z), which in addition enables better x-ray absorption per unit

thickness than a-Se, and as such they are being developed for use as direct-conversion

imagers. Because of the relatively small thickness of these high-gain photoconductors (~

100 – 300 µm) required for a reasonable quantum efficiency, a considerably lower high

voltage (HV) bias may be used than what is being used in current direct-conversion

imagers with a relatively small Z (~10 kV).23 Many of these materials can also be directly

deposited on TFT arrays using techniques such as physical vapor deposition. A

disadvantage with high-gain photoconductors is that, although they can provide adequate

gain to overcome electronic noise at low fluoroscopic exposures, the fixed internal gain

may be too high at the larger exposures used in cine acquisitions and especially in

radiography. The large amount of image charge produced in these modes of operation

will produce a large voltage across the small del storage capacitance, leading to

breakdown of the dielectric (usually an oxide layer). High-gain photoconductors also are

difficult to deposit into large defect-free areas, suffer from limited charge range, have

15

considerably large dark currents in the polycrystalline state and raise environmental

concerns. Thus, there are still many challenges to be overcome before these materials can

be used in practical x-ray imagers.

A second method for overcoming electronic noise consists of using readout circuits

known as active pixels which incorporate low-noise amplifiers at each del.24 Unlike most

current AMFPIs in which a single TFT is used at each del as a switch that is either open

or closed, active pixel circuits combine several TFTs of which at least one operates as an

analog amplifier. These amplifiers provide the necessary gain for QNL operation in x-ray

imagers. Active pixels are compatible with existing photoconductors and photodiodes, so

they can be used in both direct and indirect conversion imagers. Specialized designs of

the active pixel circuit enable each del to be read out multiple times allowing electronic

noise to be further reduced by signal averaging.25 Similar to high-gain photoconductors,

they have also typically had a fixed internal gain, leading to the development of

undesirably large del voltages in cine and radiography (del saturation). However, a new

design has recently been proposed that enables the amplifier function to be disabled at

sufficiently large x-ray exposures. In this state, each del can be made to function as a

simple charge storage and switch, as is the case in most existing AMFPI designs (known

as the passive pixel sensor approach).25 The major disadvantage of active pixels is the

significantly increased complexity caused by the presence of multiple TFTs at each del,

rendering the manufacturing process significantly more difficult and hence costlier than

for passive pixel designs. They may also require the use of re-crystallized amorphous

silicon (a-Si) or “poly-silicon” instead of the more commonly available a-Si.26 Because

16

one or more TFTs at each del operate as an analog amplifier, these devices are also

significantly more prone to radiation damage than passive pixel devices in which each

TFT is either in an On or Off state. Furthermore, the TFTs have varying electrical

characteristics and corrections are often required so that they produce the same analog

signals for identical exposure conditions.

The third approach, and the one investigated in this thesis takes advantage of the physical

process of avalanche multiplication in a-Se.18,27 which is a very well characterized

material used in a number of current AMFPI systems, particularly in mammography, and

has a long history of use in radiographic imaging plates.28-30 It has recently been

developed for use in a low-cost laser readout imager by Fujifilm. a-Se has been the

photoconductor of choice for direct-conversion imagers, due to its high intrinsic imaging

resolution, low dark current and good charge transport of both holes and electrons.31 The

manufacturing process, which uses large-area thermal evaporation, is well established

and relatively inexpensive. Furthermore, because it is a low-temperature process, a-Se

can be directly evaporated on the TFT readout array without affecting the operation of the

array. The avalanche gain, which strongly depends on the applied HV bias, is adjustable

to a very large degree. Hence, the gain can be increased at very low fluoroscopic x-ray

exposures -- by increasing the bias -- to overcome electronic noise and reduced or

eliminated – by reducing the bias – prior to radiographic exposures to prevent del

saturation. The main disadvantages with the use of a-Se as a photoconductor have to do

with the large bias voltages that need to be applied to achieve a reasonable effective work

function Weff (the required amount of absorbed x-ray energy to produce a single EHP that

17

survives recombination and whose charge is collected). Typically, an electric field of 10

V/µm is required to achieve a Weff of about 40 eV for diagnostic x-ray energies.32 A

significantly lower Weff (about 4 times) - approaching that of high-gain photoconductors

discussed above - may be obtained by increasing the field to 75 V/µm.18 An electric field

exceeding 75 V/µm is required to initiate avalanche multiplication. Furthermore thick

films (~ 1000 µm) of a-Se are required to obtain a reasonable quantum efficiency at

radiographic x-ray energies. Despite these limitations, the low dark current, compatibility

with existing TFT array technology, adjustable avalanche gain and good charge transport

properties make a-Se a very promising photoconductor material for QNL AMFPIs.

Furthermore, a-Se can also be used as an avalanche photoreceptor in indirect-conversion

AMFPIs.33 In this case, thin films of a-Se (~ 10 µm) may be used to detect light photons

generated by a phosphor.

Table 1.2. Summary of the advantages and disadvantages of three different approaches for overcoming electronic noise at low x-ray exposures in AMFPIs. Advantages Disadvantages High-gain photoconductors (PbI2, PbO or HgI2)

• high Z materials offer better x-ray absorption than a-Se for direct-conversion

• lower bias voltage required compared to a-Se for direct-conversion

• compatible with existing TFT arrays

• fixed high-gain • limited charge range • difficult to deposit into

large defect-free areas • considerably large

dark current • environmental

concerns Active pixels • compatible with existing

photoconductor/photodiode technologies

• specialized designs enable multiple readouts of each del

• fixed high-gain • more TFTs per del • prone to radiation

damage

Avalanche a-Se multiplication gain

• good charge transport of both holes and electrons

• adjustable avalanche gain • compatible with existing TFT

arrays • largely scalable deposition • very low dark current

• large bias voltages required

• thick films required for direct-conversion

• depth gain fluctuation noise for direct-conversion

18

1.6 High-gain avalanche rushing photoconductor (HARP) technology 1.6.1 HARP camera

Soon after the discovery of avalanche multiplication in a-Se in 1980 34, Tanioka and co-

workers at the Japanese Broadcasting Corporation (NHK) Science and Technology

Laboratories developed and later commercialized the HARP broadcasting camera in

conjunction with companies such as Hamamatsu Corp.35 The key component in the

camera is a layered HARP structure which consists of a-Se and blocking layers, as

depicted in Figure 1.4. The hole and electron blocking layers enable the application of a

very large electric field E while minimizing the amount of charge injected into the a-Se at

both contacts. Optical photons create EHPs in the a-Se layer. Holes undergo avalanche

multiplication as they are swept through the layer under the influence of E. A scanning

electron beam is used to raster scan the free surface of the HARP. In regions on the

surface where photon-generated holes have accumulated, more electrons will be drawn

from the electron beam to neutralize the charge in that region, thus temporarily increasing

the beam current. By monitoring the current entering the HARP through the readout

electrode during the scanning process, the spatial distribution of photon-generated holes

can be inferred. This distribution corresponds to the distribution of light photons (i.e. the

optical image) absorbed in the a-Se near the transparent conductive electrode. Low-noise

operation of the HARP is strongly reliant on the blocking contacts. This is because

charge injection at the electrodes contributes to dark current and this produces a form of

noise known as shot noise which is caused by the statistical fluctuations of charge in the

electric current.16 The hole blocking contact is particularly important, since injected holes

can experience the same degree of avalanche multiplication as photon-generated holes,

19

producing a very large degree of shot noise. Hence, the amount of avalanche gain that

can be obtained from a HARP layer with any given thickness of a-Se is limited by the

largest electric field strength that can be applied without producing a significant amount

of dark current shot noise. Tanioka and colleagues demonstrated avalanche gains as high

as one thousand in a HARP camera with a 15 µm thick a-Se layer.36 Shown in Figure 1.4

(b) are optical images taken with a HARP camera showing a portion of an oscilloscope

panel in a nearly completely dark room. As the electric field is increased beyond 100

V/µm, the image becomes very clear. This demonstrates that avalanche gain can be used

for imaging objects in photon-starved conditions (eg. very low-light conditions).

free surface

readout electrode (a)

Figure 1.4 (a) Diagram illustrating the principle of operation of a HARP camera tube. Optical photons create electron hole pairs in the a-Se layer. Holes undergo avalanche multiplication as they are swept through the layer under the influence of an electric field E. A scanning electron beam is used to read out the resulting charge image on the free surface (i.e. no conducting electrode). (b) Optical images (showing five buttons on an oscilloscope panel) obtained from a HARP camera for different applied electric fields. These images were obtained in a nearly completely dark room.

20

Recently, Hunt has theoretically investigated the use of HARP technology for

applications in fluoroscopy.18,27 It has been identified that a HARP layer could provide

adjustable gain at the photodetector stage and that a gain of ~50 is required to completely

overcome the electronic noise at the lowest fluoroscopic exposures and thus yield a QNL

AMFPI.37 However, theoretically it is expected and experimental work on direct

interaction of diagnostic energy X rays with HARP layers has recently revealed the

presence of substantial depth-dependent gain fluctuation noise. This type of noise is

caused by the absorption of X rays at different depths in the a-Se, producing holes that

undergo largely varying degrees of avalanche gain depending on how far they have to

travel through the a-Se to reach the negative electrode (i.e. the farther the travel path, the

larger the resulting avalanche gain due to an increased number of impact ionizations). As

a result, HARP technology itself is not directly compatible with a direct-conversion x-ray

detection scheme. It has been established, however, that specialized direct-conversion

imager architectures could, in principle eliminate depth-dependent gain fluctuation

noise.27,38 A promising imager architecture consists of a thick (~ 1 mm) a-Se layer having

a low electric field strength (10 V/µm) used for x-ray charge production coupled to a

HARP layer used for avalanche multiplication of the x-ray-produced charge.

1.6.2 The need for a solid-state HARP

Although current HARP camera systems provide excellent image quality and sensitivity

and are well adapted for video and broadcasting applications, their direct applicability to

AMFPIs is greatly limited because radiological imaging applications require a large

21

photosensitive area (commensurate with the anatomical region being imaged). Clearly,

using a scanning electron beam readout approach would result in a device similar in size

to a cathode ray tube television, which is undesirable. A more compact approach, known

as field emission array39, involves the use of a number of electron emitting tips (known as

Spindt tips) at each del. The electrons emitted from these tips are focused using a mesh

electrode onto the free surface of the HARP. This approach has been demonstrated for a

small area detector.40 It remains to be seen, however, whether this technology is truly

scalable as the challenge of establishing a necessary thin vacuum gap between the

electron-emitting tips and the HARP layer over a large area is substantial.

A solid-state HARP image receptor would completely eliminate the need for vacuum and

would greatly facilitate the fabrication of scalable QNL AMFPIs with an adjustable

internal avalanche gain. There have been some efforts to develop a solid-state HARP

system, in which the HARP layer is in direct electrical contact (directly electroded) with

the readout electronics, as shown in Figure 1.5. Two Japanese groups have demonstrated

avalanche multiplication in directly electroded HARP but with limited success 41,42.

Ohshima et al. obtained an avalanche gain of up to 10 times in a-Se layers 1-6 µm thick

coated with a gold electrode 41. A similar gain was obtained by Takiguchi et al. in 500 nm

thick a-Se coupled to a complimentary metal oxide semiconductor (CMOS) readout layer

42. Unfortunately, in both cases, stable long term device operation could not be realized.

22

Figure 1.5 Diagram of a directly electroded HARP. The HARP consists of a transparent indium tin oxide (ITO) electrode deposited on a glass substrate (not shown). The hole blocking contact, a-Se and electron blocking contact are deposited on the ITO electrode. Discrete del electrodes are next deposited on top of the HARP structure.

1.7 Rationale and problem formulation

The key clinical requirements for a solid-state x-ray imager for interventional radiology

are, as recapitulated from section 1.3: (1) quantum-noise limited (QNL) operation at the

lowest clinical fluoroscopic x-ray exposures (in conformance with the ALARA

principle), (2) capability for modes of operation that require significantly higher x-ray

exposures and (3) capability of imaging fine features of interventional devices or lesions.

No existing solid-state imaging system simultaneously satisfies all three requirements.

Avalanche multiplication of charge in a-Se can provide sufficient gain to satisfy

requirement (1) and the highly adjustable avalanche gain should also satisfy requirement

(2). Furthermore, the high intrinsic imaging resolution of a-Se should also answer

requirement (3). This suggests that a-Se is a very good candidate as a photoconductor for

23

use in x-ray imaging in interventional radiology. However, the following problems need

to be addressed, and they establish the three major questions which this thesis shall

answer: (A) can a-Se reliably provide avalanche multiplication gain in the solid state? (B)

how can a solid-state avalanche a-Se photoreceptor be used for imaging X-rays? (C) can

a solid-state avalanche a-Se photoreceptor practically satisfy all the four imager

requirements for interventional radiology?

1.8 Thesis outline

First, a method is developed which enables the application of avalanche-grade electric

fields across a-Se layers in the solid state (Chapter 2). This method is aimed at addressing

the problematic occurrence of electrical discharges seen in prior attempts to directly

electrode a-Se photoconductors. The electrical breakdown properties and gain

characteristics of this solid-state avalanche device are experimentally investigated. Once

it is demonstrated that sufficient avalanche multiplication gain can be obtained for QNL

operation, the next step is to investigate suitable detector architectures for imaging X-rays

(Chapter 3). Both indirect and direct conversion architectures are examined and

compared. However, since a feasibility study of an indirect-conversion detector has

already been completed, the central focus of this chapter will be on the feasibility of a

direct-conversion x-ray detector with a built-in avalanche layer. This consists of a

theoretical investigation of several key figures of merit which evaluate the detector’s

imaging performance. At this point in the thesis, it will become clear that a solid-state

24

avalanche a-Se photoreceptor can be realized and that it can be used as a direct or indirect

x-ray detector with sufficient spatial resolution for advanced interventional radiology

applications. However, it remains to be seen whether the avalanche a-Se photoreceptor

meets all the specific requirements for use as an interventional radiology imager (Chapter

4). Hence, the last major thesis chapter will investigate such topics as the photoreceptor

noise (for proper QNL operation), x-ray exposure range of operation (for cine and

radiographic imaging modes), and compatibility with existing solid-state image readout

electronics (for realization of a compact imaging system). Lastly, the thesis results are

summarized and future research opportunities are identified. It should be noted that both

Chapters 2 and 4 are largely comprised of experimental work and Chapter 3 is entirely

theoretical.

25

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architectures for digital medical imaging applications," ECS Transactions 8, 289-

293 (2007).

26 L. Antonuk, M. Koniczek, Y. El-Mohri, and Q. Zhao, "Active pixel and photon

counting imagers based on poly-Si TFTs: rewriting the rule book on large area

flat panel x-ray devices," Proc. SPIE 7258, 725810-725814 (2009).

27 D. C. Hunt, "Investigation of Avalanche Multiplication in Amorphous Selenium

for Use in Digital Fluoroscopy", PhD Thesis, University of Toronto (2005).

28 H. Guilleminot, "Use of selenium in the radiometry of Rontgen rays," Archives

d'Electricite Medicale 23, 168-173 (1915).

29 C. Luraschi, "New apparatus for measuring the intensity and quantity of Rontgen

rays," Archives d'Electricite Medicale 16, 14-26 (1908).

30 A. Nemet, A. W. Balls, and W. F. Cox, "Xeroradiography applied to the

inspection of electrical equipment," Proc. Inst. Electr. Eng. 109A, 184-188

(1962).

31 J. A. Rowlands and G. DeCrescenzo, "X-ray imaging using amorphous selenium:

Determination of x-ray sensitivity by pulse height spectroscopy," Med. Phys. 19,

1065-1069 (1992).

29

32 J.A. Rowlands and J. Yorkston, "Flat Panel Detectors for Digital Radiography," in

Handbook of Medical Imaging, edited by J. Beutel, H. Kundel, and R. Van Metter

(SPIE, Bellingham, Washington, 2000), Vol. I.

33 W. Zhao, D. Li, A. Reznik, B. J. M. Lui, D. C. Hunt, J. A. Rowlands, Y. Ohkawa,

and K. Tanioka, "Indirect flat-panel detector with avalanche gain: Fundamental

feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat

panel imager)," Med. Phys. 32, 2954-2966 (2005).

34 G. Juska and K. Arlauskas, "Impact Ionization and Mobilities of Charge Carriers

at High Electric Fields in Amorphous Selenium," Phys. Stat. Sol. 59, 389-395

(1980).

35 K. Miyakawa, Y. Ohkawa, T. Matsubara, T. Takahata, S. Suzuki, and M. Kubota,

"Ultrahigh-sensitivity HDTV New Super-HARP Camera," Proc. SPIE 5677, 26

(2005).

36 K. Tanioka, T. Matsubara, Y. Ohkawa, K. Miyakawa, S. Suzuki, T. Takahata, N.

Egami, K. Ogusu, A. Kobayashi, T. Hirai, T. Kawai, M. Hombo, and T. Yoshida,

"Ultra-high-sensitivity new super-HARP pickup tube and its camera," IEICE

Trans. on Elec. E86C, 1790-1795 (2003).

37 W. Zhao, D. Li, A. Reznik, B. Lui, D. C. Hunt, Kenkichi Tanioka, and J. A.

Rowlands, "Indirect flat-panel detector with avalanche gain: Design and operation

of the avalanche photoconductor," Proc. SPIE 5745, 352-364 (2005).

38 D. C. Hunt, K. Tanioka, and J. A. Rowlands, "X-ray imaging using avalanche

multiplication in amorphous selenium: Investigation of depth dependent

avalanche noise," Med. Phys. 34, 976-986 (2007).

30

39 D. Li and W. Zhao, "Avalanche detector with field emitter readout," in IEEE

Bioengineering Conference (2007), pp. 67-68.

40 D. Li and W. Zhao, "SAPHIRE (scintillator avalanche photoconductor with high

resolution emitter readout) for low dose x-ray imaging: Spatial resolution," Med.

Phys. 35, 3151-3161 (2008).

41 T. Ohshima, K. Tsuji, K. Sameshima, T. Hirai, K. Shidara, and K. Taketoshi,

"Excess Noise in Amorphous Selenium Avalanche " Japanese Journal of Applied

Physics 30 (6B), L1071-L1074 (1991).

42 Y. Takiguchi, H. Maruyama, M. Kosugi, F. Andoh, T. Kato, and K. Tanioka, "A

CMOS Imager Hybridized to an Avalanche Multiplied Film," IEEE Trans.

Electronic Devices 44 (10), 1783-1788 (1997).

31

Chapter 2 Development of a solid-state amorphous selenium avalanche photoreceptor 2.1 Introduction 2.2 Theory 2.3 Methods 2.3.1 Distributed resistive layer 2.3.2 Experimental setup 2.3.3 Linearity 2.3.4 Gain and dark current 2.3.5 Carrier transport 2.4 Results

2.4.1 Breakdown characteristics 2.4.2 Linearity 2.4.3 Gain and dark current 2.4.4 Carrier transport

2.5 Discussion 2.5.1 Breakdown characteristics 2.5.2 Linearity 2.5.3 Gain and dark current 2.5.4 Carrier transport

2.6 Conclusions

This chapter combines selected material from the following: • U.S.A. Provisional Patent No. 61/129389 Matthew M. Wronski et al.

Photodetector/Imaging Device with Avalanche Gain, filing date: June 23, 2008 • M. Wronski et al., “A solid-state amorphous selenium avalanche technology for large

area photon counting and photon starved imaging applications” to be submitted as a letter to Med. Phys.

• M. Wronski et al., “A solid-state avalanche photoreceptor for low-exposure x-ray imaging applications” to be submitted to Med. Phys.

32

2.1 Introduction The X-ray image intensifier is able to overcome electronic noise because it provides an

internal gain. One of the objectives of this thesis, and the purpose of this chapter, is to

develop a solid-state device, based on amorphous selenium (a-Se), with a similar

capability.

The principal challenge with realizing avalanche multiplication in a-Se is that it requires

the application of a large electric field (E ~ 100 V/µm). HARP technology has been

shown to provide avalanche gains gav ~ 10 with metal contacts deposited on the HARP

(electroded HARP).1,2 This, however, was the maximum gain that could be achieved

prior to failure of the device. The amount of avalanche gain required for quantum noise

limited (QNL) operation throughout the entire clinical range of fluoroscopic x-ray

exposures is approximately 50.3 Since avalanche multiplication gain is exponentially

dependent on E, only a slight improvement in the HARP structure is required to provide

the required gain for QNL operation.

To better understand the failure process in electroded HARP, we first develop a model

that estimates the maximum amount of energy in a spontaneous electric discharge. The

model provides insight into how this energy may be limited, leading to the development

of an experimental method which enables the practical application of significantly larger

electric field strengths across the HARP. The application of larger electric field strengths

in turn enables much larger avalanche gains. Prior to measuring the gain, however, the

linear range of operation is identified for the avalanche detector such that its response

33

remains unaffected by the significant amounts of charge produced by avalanche

multiplication. This enables an accurate gain and dark current characterization, providing

an indication on whether the avalanche detector can achieve a sufficient degree of

sensitivity for QNL operation. The final goal of this chapter is to confirm the presence of

avalanche multiplication in the newly-developed solid-state detector. This is done by

examining the transport of electric charge carriers inside the detector at both sub-

avalanche and avalanche electric field strengths. This analysis also enables the

measurement of carrier mobility (holes in particular), providing a useful point of

comparison between HARP a-Se and standard a-Se used in conventional radiographic

imaging plates.

2.2 Theory

Thus far, electroded HARP has been limited1 to avalanche gains gav < 10. Much larger

gains have been achieved in the HARP camera tube (gav ~ 103). This suggests that there

is no limitation in the HARP structure itself to achieving high gain. Rather, the means by

which the HARP is electrically contacted needs improvement to accomplish a completely

solid state solution.

There are two important mechanisms that potentially underlie the breakdown process in

electroded HARP. The first has to do with the electric field enhancement near the sharp

edges of conductive electrodes. This large field can cause excessive charge injection at

34

the electrode, despite the presence of blocking contacts, causing high localized current

densities. The second mechanism is a positive feedback process in which a large current

density resulting from either excessive charge injection or simply from an electric

discharge results in a localized deposition of heat (Joule heating), causing the a-Se to

crystallize. Since crystalline selenium has a larger electrical conductivity than a-Se, the

discharge region begins to draw a larger current which in turn produces yet more heat.

Due to sustained heating, this process causes the crystallization to spread to a larger area

over time.

Hence, in principle, failure in electroded HARP can be mitigated by (1) eliminating the

electric field enhancement near the sharp edges of conductive electrodes and (2)

quenching electrical discharges or excessive charge injection which ultimately lead to

crystallization of the a-Se. In principle, problems (1) and (2) stated above may be

addressed by introducing a distributed resistance layer (DRL), as shown in Figure 2.1.

This layer, which can be deposited on top of the HARP structure should (1) reduce the

electric field in the vicinity of electrode edges and (2) provide a mechanism to limit the

current flow in regions of incipient failure.

35

Figure 2.1. (b) Diagram of the directly electroded HARP where the photon-generated charge is collected by means of conductive del electrodes. The dashed horizontal lines represent electric field lines. The bending of field lines near the del electrode edges produces regions of electric field enhancement (stress points) (c) In DRL-HARP, the addition of a distributed resistive layer (DRL) eliminates stress points near del electrodes - due to the negligible electric field in the DRL - and enables the significant reduction of discharge currents (Idis) - due to the series resistance of the DRL - while maintaining the same voltage bias VHARP across the HARP.

A model is now developed to better understand the failure process and quantify the

current-limiting effect of the DRL. One could use Joule’s first law which relates the

amount of heat produced by a current flowing through a resistance, however the exact

current and resistance in the discharge region are unknown and change rapidly during a

discharge event. Rather, an energy model can be used to estimate the upper bound on the

energy available for instantaneous heating during an electric discharge. The energy Ed

accumulated on the del capacitance Cd with an applied voltage Vd is 2

21

ddVC .

When an electrical discharge occurs over some small area ∆A of the HARP, this region

provides a conductive path for Ed (Figure 2.3). Assuming a reasonable ∆A =10 µm x 10

µm, Cd = 1 pF (for a 1 mm2 PEDOT (Poly(3,4-ethylenedioxythiophene)) electrode) an a-

Se heat capacity and density of 25 J/mol·K and 5 g/cm3, respectively, and adiabatic

conditions, then if Ed was entirely dissipated as heat, it would raise the temperature in the

discharge region by over 600 degrees Kelvin. Since heat would not be removed fast

36

enough from the a-Se, this could lead to a-Se crystallization. From a materials science

perspective this can be conveniently represented in the form of a time-temperature-

crystallization (TTC) diagram as shown in Figure 2.2. This diagram shows how the

duration of an electrical discharge in a-Se affects the occurrence of phase transformation.

In the case of a rapid discharge, the a-Se temperature rises quickly. During the slow

cooling process in which the a-Se temperature returns to room temperature, there is

sufficient time and thermal energy for the a-Se to undergo a phase transformation and

assume the lower energy crystalline state. However, if the electrical discharge is slow,

heat generation is distributed in time and because cooling occurs simultaneously, the

maximum a-Se temperature reached is much lower. If the discharge is sufficiently slow,

the a-Se will not crystallize.

Figure 2.2. Time-temperature-crystallization (TTC) diagram showing how the duration of an electrical discharge in a-Se affects the occurrence of phase transformation. In the case of a rapid discharge, the a-Se temperature (T) rises quickly. During the following slow cooling process in which T returns to room termperature, there is sufficient time (t) and thermal energy for the a-Se to undergo a phase transformation and assume the lower energy crystalline state. If the electrical discharge is slow, however, heat generation is distributed in time and because cooling occurs simultaneously, the maximum a-Se temperature reached is much lower. If the discharge is sufficiently slow, the a-Se will not undergo a phase transformation.

37

Using the right-hand circuit diagram in Figure 2.3, we can develop a model that takes

into account the slower rate of discharge due to the DRL. In this case, the discharge

current traverses through a resistance-capacitance (RC) network whose time constant can

be approximated as follows:

effd

DRL

N

x xSe

N

xxSe

RC

RR

C≈

∆+

∆

∆≈

−= −

=−

∑

∑

12

2

11τ , Eq. 2.1

where N is the number of distributed RC elements, Reff is the effective series resistance in

the discharge region and the ∆RSe-x term can be ignored because the a-Se resistance is

very high (> 100 TΩ/mm2). Within τ, the amount of heat ∆Q dissipated from the

discharge region (to the underlying glass substrate) can be calculated as follows:

dTAkQ ∆

∆⋅⋅=∆ τ , Eq. 2.2

where k is the thermal conductivity of a-Se, ∆T is the difference between the temperature

in the discharge region and room temperature and d is the a-Se thickness (i.e. the longest

distance the heat must travel to exit the discharge region into the glass substrate). Then,

Ed/∆Q can be used to express the ratio between the heat generating and heat sinking

processes. Using Eqs. 2.1 and 2.2 above, this gives:

TAkRdV

TAkRC

dVC

QE

eff

d

effd

ddd

∆⋅∆⋅∆⋅∆⋅=

∆

22

~21

, Eq. 2.3

38

Figure 2.3. Diagrams illustrating the electrical discharge paths in electroded HARP (left) and electroded DRL-HARP (right). A distributed resistance-capacitance network is assumed with an arbitrarily large series current limiting resistance RS. The HARP and DRL are modeled as cells (numbered 1 through x) each consisting of a capacitance ∆C and resistance ∆R in parallel. When a discharge occurs (i.e. ∆R 0 in one or more of the HARP cells), the total energy E stored in the HARP flows through one or more cells. When a DRL layer is present, the discharge current is forced through a narrow region of the DRL having a very large resistance.

It can be appreciated from Eq. 2.3 that for a discharge region size ∆A ~ 10 µm x 10 µm,

using RDRL ~ 10 GΩ/mm2 and thus Reff >> 1 TΩ and Vd = 1500 V, then even for an

arbitrarily small temperature gradient ∆T = 1 deg K, Ed/∆Q << 1. Furthermore, it can be

seen that for this to remain true, RDRL > 0.1 GΩ/mm2. According to this conservative

model, this would be a sufficient (although not necessary) resistance for the DRL to

prevent a-Se crystallization, because the heat produced by the discharge is removed from

the a-Se at a faster rate than it is generated and so it does not accumulate.

39

2.3 Methods

2.3.1 Distributed resistive layer

In addition to preventing a-Se crystallization as discussed in section 2.2, the material for

the DRL needs to be chosen such that (1) during normal operation, essentially all of the

HV bias is applied across the a-Se layer and (2) there is no significant lateral conduction

(leakage) of charge between neighboring dels. If we assume a del size and capacitance of

100 µm x 100 µm and 0.5 pF, respectively, and a dark current of 10 pA/mm2, then an

insignificant potential drop across the DRL = 0.1 V per micrometer of thickness

would require a material with a volume resistivity of the order

DRLV

≈RLρ 1012 Ω·cm, thus

satisfying the first condition. Assuming a DRL thickness of the order of 1 µm, this would

be equivalent to a lateral (sheet) resistance ≈latR 1016 Ω/square. Assuming a 10 µm

spacing between dels, this translates into a time constant for lateral charge conduction

≈latτ 600 s. Since the time during which the latent image charge is stored at each del of

the imager prior to being converted into a digital signal is on the order of 10 ms,4 lateral

charge leakage between neighboring dels should be negligible, thus satisfying the second

condition.

Cellulose acetate (CA) satisfies the resistivity requirements outlined above

( Ω·cm). CA films having a thickness of 1 µm or more would have an

estimated series resistance R

105 12⋅=CAρ

DRL > 50 GΩ/mm2, which, according to the model in section

2.2 should be more than sufficient to quench electrical discharges leading to a-Se

crystallization. Furthermore, CA bonds well to a-Se and has been used as a protective

40

overcoating in a-Se xeroradiographic plates and in this application was shown5 to extend

the plate life by about a factor of 10. For these reasons, it is a good choice for the DRL.

In order to optimize the thickness and electrical performance of the CA layer

experimentally, we used different concentrations of CA in an acetone solvent and then

measured the resulting film thickness and tested its capability of eliminating adverse

electrical discharges. The CA polymer was obtained in powder form from Eastman

Chemical. It was dissolved in CMOS grade acetone (J. T. Baker) in the following

concentrations: 2%, 4% and 8% by weight. The CA solution was then cast onto the

HARP samples. This process consisted of applying a controlled amount (2 mL) of CA

solution onto the surface of the HARP sample and the rate of evaporation of the solvent

was controlled by using an enclosure with a controlled ventilation (Figure 2.4). This was

found to prevent the denaturation (specifically, an increase in opacity) of the polymer and

enabled the fabrication of high-quality transparent films whose thickness scales with the

concentration of the CA solution. A profilometer (Dektak) was used to measure the

thickness of the CA layer. Conductive electrodes were formed on the CA layer by

applying a conductive polymer (PEDOT). A spring-loaded gold-plated pin was used to

form a pressure contact with the conductive electrode. When not in use, the coated

samples were stored in the dark in a dessicator at 23 0C.

41

Figure 2.4. Diagram illustrating the cellulose acetate (CA) casting process. A controlled amount of CA solution is first dispensed on the HARP target which is mounted on a copper plate. This is done such that the entire plate is coated with the solution. Next, the solvent in the CA solution is given time to evaporate. This is performed in an enclosure with a small opening to control the rate of evaporation. The HARP target is then removed from the plate by cutting the CA around its edge.

2.3.2 Experimental setup

For the purposes of gain measurements (described in section 2.3.4), optical excitation

using a light emitting diode (LED) was used. Since a-Se has a quantum efficiency > 95%

at blue wavelengths, a blue LED was used.3 The experimental setup is shown in Figure

2.5. There are several advantages of using this setup: (1) an optical fiber bundle is used to

couple the excitation light to the HARP. This eliminates any electrical interference that

could potentially be caused by the pulse generator; (2) blue light photons are absorbed in

a very shallow region at the HARP surface (first several-hundred nanometers). This

results in negligible gain-depth fluctuation noise;6 (3) the relative pulse intensity of the

LED can easily be calibrated using an auxiliary photon detector such as a photomultiplier

tube (PMT); (4) the LED pulse intensity can be varied over a very wide range (i.e. 6

orders of magnitude).

42

Figure 2.5. Diagram illustrating the experimental setup used for basic characterization of directly electroded HARP and DRL-HARP. Dark current measurements are performed by replacing the charge amplifier with an electrometer.

The HARP photocurrent was measured as a function of the applied voltage using a low-

noise current amplifier (Stanford Research) and the voltage was supplied from a high

voltage (HV) generator (Stanford Research). A fiber optic bundle (Thor Labs) was used

to channel the LED excitation to the HARP, and a photomultiplier tube (PMT)

(Hamamatsu) biased in the linear region of operation was used to quantify the intensity of

the LED source and ensure that the output signal from the device is linear with the LED

intensity.

2.3.3 Linearity

Since avalanche multiplication can produce a large amount of charge even for relatively

low exposures, this can result in an effect known as ghosting. This is usually manifested

as a change (usually a reduction) in sensitivity of a photoreceptor due to prior exposures.4

It is generally caused by the trapping of charge either in the bulk of the photoreceptor or

at interfaces between the photoreceptor and other materials and it typically produces

artifacts in images that are manifested as latent images from previous exposures (ghosts).

43

Any effect such as ghosting that changes the a-Se sensitivity as a result of prior exposures

affects the linearity of the photoreceptor response, that is the relationship between the

signal produced by the photoreceptor and the number of photons interacting with it.

Clearly, this can also affect the measurement of avalanche gain.

To determine the conditions in which gain measurements can be made accurately and

unaffected by any non-linearities, two experiments were performed using the setup

shown in Figure 2.5. In the first, the peak DRL-HARP photon-generated current was

measured as a function of LED pulse intensity at several different biases in the avalanche

regime. This would provide the range of LED intensities for which the detector response

remains linear, at avalanche electric field strengths. In the second experiment, the LED

pulse intensity remained constant and the transient response was recorded at different HV

biases, once again in the avalanche regime. In both experiments, 1 ms LED excitation

pulses were used.

2.3.4 Gain and dark current

The setup shown in Figure 2.5 was used to measure the gain of HARP-DRL as a function

of the applied electric field strength E. The amount of charge produced by a single optical

pulse in the HARP-DRL was controlled by reducing the source LED pulse intensity in

steps (thus maintaining it in the linear response range) as the HV bias – and thus the

avalanche gain – was increased. By recording the reduction factor in source intensity for

each step, the entire photocurrent gain characteristic was reconstructed. The DRL-HARP

44

gain data were compared with gain data obtained from HARP camera tubes with HARP

targets having the same a-Se layer thickness (15 µm).

Two models were used to fit the DRL-HARP gain characteristics. A three parameter

model was used to fit the conversion gain gc (for ESe < 75 V/µm) and McIntyre theory

was used to model the avalanche gain (ga) characteristics (for ESe > 75 V/µm). We

assumed that only holes avalanche in the a-Se which is a reasonable assumption for a-Se

biased near 100 V/µm.7,8 The models for the conversion and avalanche gains gc and ga

are given by the following two equations, respectively:

baEEgSe

Sec += , Eq. 2.4

SeESe ed

a eg/2

1ββ −

= Eq. 2.5

where 1/a is the value of gc at infinite field, b is the electron-hole pair recombination

term, dSe is the a-Se thickness and β1 and β2 are the impact ionization factors of a-Se.

Dark current was measured using the setup shown in Figure 2.5, with an electrometer

(Keithley) in place of the charge amplifier. Each measurement was taken several minutes

after a change in the HV bias to allow the dark current to stabilize.

2.3.5 Carrier transport The transport of electric charge carriers in DRL-HARP is examined to confirm the

presence of avalanche multiplication in the detector. Towards this end, we employed a

time of flight (TOF) characterization method which measures the transient (function of

45

time) photon-generated current in DRL-HARP immediately following a very short

excitation pulse (impulse). This technique provides the mobility of carriers inside the

device as well as information regarding trapping effects associated with non-uniformities

in the electric field distribution.

The experimental setup was the same as that shown in Figure 2.5, except that a nitrogen

laser was used in place of the LED. The laser produced 0.8 ns pulses at 337 nm. These

pulses were coupled into a 40 meter long ultraviolet-grade optical fiber which produced

sufficient delay (~ 200 ns) so that the electro-magnetic interference (EMI) spike produced

by the laser on the oscilloscope could be clearly differentiated from the photon-generated

signal. Also, the charge amplifier was not used since the laser produced sufficient signal

for a low impedance (50 Ω) oscilloscope termination, which also permitted a higher

frequency response (~ 1 GHz).

First, TOF measurements were performed on electroded HARP and DRL-HARP at non-

avalanche electric field strengths (E < 75 V/µm). This enabled: (1) investigation of the

effect of the DRL on hole transport in DRL-HARP and (2) the measurement of the a-Se

mobility of holes. Lastly, TOF traces were obtained for DRL-HARP at avalanche electric

field strengths (E > 75 V/µm).

46

2.4 Results 2.4.1 Breakdown characteristics

Figure 2.6 shows the measured dark current in an electroded 15 µm HARP with a 1 mm2

PEDOT contact (left). Shown on the right is an optical microscope image (in

transmission mode) of the contact and surrounding region on the HARP surface. The

measured dark current characteristic before and after breakdown is shown in Figure 2.7.

The contact damage shown in Figure 2.6 could be prevented by including a large (100

GΩ) series resistance in the current path, however even with this resistance in place, a

significant shift in the dark current characteristic occurs, similar to that shown in Figure

2.7.

Figure 2.6. Left: measured dark current in electroded HARP (15 µm thick) for increasing voltage steps. Failure or breakdown (BD) of the device typically occurs as the applied voltage is increased from 1100 V to 1300 V. Each transient in dark current is due to the increasing of the voltage by 100 V. Right: Optical transmission image of the HARP surface after breakdown.

0 200 400 600 800 1000 1200 1400 160010

-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

time (s)

dark

cur

rent

(A/m

m2 )

FI 15 µm no CA

200 V500 V

800 V 1100 V

BD

1300 V

47

1

10

100

1000

10000

100000

1000000

0 20 40 60 80 100

E a-Se (V/um)

I dc

( pA

/mm

2 ) Before breakdown

After breakdown

Figure 2.7. Measured dark current (idc) as a function of the a-Se electric field strength (Ea-Se) before and after breakdown of the HARP.

The measured series resistance for a DRL layer fabricated as described in section 2.3.1

and having a measured thickness of 0.9±0.2 µm using a (1±0.5 mm)2 electrical contact

was RDRL = 10±4 GΩ. This resistance is lower than the expected series resistance of 50

GΩ/mm2 per micrometer of film thickness calculated using the manufacturer’s specified

resistivity. The discrepancy could be due to impurities in the cellulose acetate or the

acetone, which could lower the effective resistivity of the material by introducing mobile

ions. The measured electrical properties of a 15 µm HARP layer with DRLs of varying

thicknesses are summarized in Table 2.1.

48

Table 2.1. Measured characteristics of DRL-HARP with and without a DRL. The DRL is a cellulose acetate (CA) polymer film having a thickness of 1, 2 or 4 µm and the HARP structure consists of a 15 µm thick a-Se layer. Shown are values of the DRL resistance (RDRL), the DRL-HARP dark current (idc), and the maximum attainable bias (Vmax), electric field (Emax) and avalanche gain (ga_max). No DRL DRL 1 µm 2 µm 4 µm Vmax (V) 1260 1435 1575 > 1800 RDRL (Ω/mm2) 0 1x1010 2x1010 4x1010

idc (pA) 8 8.5 15 > 5x103

Emax (V/µm) 84 95 105 105 ga_max 2 40 ~10,000 ~10,000 Shown in Table 2.2 is a summary of the maximum electric field strength Ea-Se-max and the

device operating time (without breakdown) at Ea-Se-max for electroded HARP with

sputtered platinum (Pt) and gold (Au) contacts, a PEDOT contact and the CA/PEDOT

combination (DRL-HARP).

Table 2.2. Maximum electric field in a-Se layer and operating time at this field for several different types of contacts. The HARP sample used had a 15 µm thick a-Se layer. The Pt and Au contacts were deposited using a sputtering process. Contact Type Ea-Se-max Operating time at Ea-Se-max Pt 53 V/µm ~ 10 s Au 59 V/µm ~ 10 s PEDOT 84 V/µm ~ 10 s CA/PEDOT 105 V/µm 10 000 s +

Shown in Figure 2.8 is the number of electrical discharges occurring in DRL-HARP with

a 2 µm CA layer within a 15 minute time interval (chosen arbitrarily).

49

0

1

2

3

4

7 13 20 27 33 40 47 53 60 67 73 80 83 87 90 93 97

Electric Field (V/um)

Num

ber o

f dis

char

ges

in 1

5 m

in

inte

rval

l

Figure 2.8. Graph showing the number of electrical discharges occurring in DRL-HARP with a 2 µm CA layer and a 15 µm a-Se layer within a 15 minute time interval.

2.4.2 Linearity

Shown in Figure 2.9 is the peak DRL-HARP photon-generated current (photocurrent) as

a function of LED intensity and three HV biases. Figure 2.10 shows the photocurrent

transient as a function of the HV bias. The results in Figures 2.9 and 2.10 were obtained

by measuring the photocurrent from the same PEDOT contact at different times.

Figure 2.9. Graph showing the peak measured photocurrent from DRL-HARP as a function of LED source intensity at three different biases in the avalanche regime.

50

Figure 2.10. Graph of photocurrent transients obtained from DRL-HARP for a 1 ms long source excitation pulse and for varying applied biases in the avalanche regime.

2.4.3 Gain and dark current

The measured DRL-HARP gain and room temperature dark current are shown in Figure

2.11. The fitting parameters a and b used in the conversion gain model (see section 2.3.4)

were 6.6 and 440 respectively, which are the same as the published values,9 and the

impact ionization coefficients β1 and β2 were 1000 and 800 which are very close to the

published values.8,10 The measured gain agrees very well with both the conversion (for

HV < 1100 V) and avalanche (HV > 1100 V) gain models.

51

0 500 1000 1500 200010

0

101

102

103

104

105

106

107

108

HV bias (V)

pA/m

m2

DRL-HARP photocurrentgc model for 15 µm HARP

ga model for 15 µm HARP

DRL-HARP dark current

Figure 2.11. Graph showing measured peak photocurrent (circles) and dark current (triangles) obtained from DRL-HARP as a function of the applied high voltage (HV) bias. Also shown are the fitted conversion gain (solid line) and avalanche gain (dashed line) models.

2.4.4 Carrier transport

Figure 2.12 compares TOF traces obtained for directly electroded HARP with a 15 µm a-

Se layer and a 1 mm2 PEDOT contact. For comparison are shown TOF traces for DRL-

HARP with 1 µm and 8 µm CA layers.

52

-2 0 2 4 6 8 10

x 10-7

-5

0

5

10x 10-3

-2 0 2 4 6 8 10

x 10-7

-2

0

2

4x 10-3

-3 -2 -1 0 1 2 3 4 5 6 7

x 10-7

-2

0

2

4x 10-3

Reference (no CA)PEDOT contact220 V bias

UHSe1 (15% CA, 2mL)PEDOT contact220 V bias

UHSe2 (1% CA, 2mL)PEDOT contact220 V bias

phot

ocur

rent

(mA

)

time (s)

phot

ocur

rent

(mA

)

time (s)

phot

ocur

rent

(mA

)

time (s)

Figure 2.12. Graph showing measured photocurrent transient in (a) directly electroded HARP, (b) DRL-HARP with 8 µm CA layer and (c) DRL-HARP with 1 µm CA layer. A PEDOT electrode was used in all three cases and the applied bias was 220 V.

TOF traces were also obtained for DRL-HARP with a 2 µm CA layer and a 1 mm2

PEDOT contact for electric field strengths in the range of 2-20 V/µm. This enabled the

calculation of hole mobility in a-Se for DRL-HARP as a function of electric field. The

mobility was calculated from the TOF traces by measuring the transit time of holes. The

effective hole velocity in the a-Se could be deduced by dividing the a-Se thickness (15

µm) by the hole transit time. The hole mobility is then obtained by dividing the effective

hole velocity by the magnitude of the electric field in the a-Se. The room-temperature

53

hole mobility is ploted in Figure 2.13, along with the mobility of holes in a standard 150

µm thick Xerox a-Se imaging plate.

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

0 5 10 15 20 25

E a-Se (V/um)

hole

mob

ility

(m2 /V

s)

Xerox a-Se plate

DRL-HARP

DRL-HARP

Xerox a-Se

Figure 2.13. Graph showing measured a-Se hole mobility as a function of electric field strength in DRL-HARP and a Xerox a-Se plate.

TOF traces obtained from DRL-HARP with a 15 µm a-Se layer, a 2 µm CA layer and a 1

mm2 PEDOT contact at avalanche electric field strengths are shown in Figure 2.14. The

figure shows three traces obtained for HV biases of 1410 V, 1500 V and 1530 V. The

measured dark current was below 20 pA. The calculated a-Se electric field strengths

corresponding to each of the traces are 94, 100 and 102 V/µm.

54

0 50 100 150 200

0

5

10

15

20

25

time (ns)

sign

al (m

V)

E = 94 V/µm

E = 100 V/µm

E = 102 V/µm

Figure 2.14. Measured time-of-flight traces obtained for three different electric field strengths in the avalanche regime.

2.5 Discussion

2.5.1 Breakdown characteristics

We begin by discussing the breakdown characteristics of a directly electroded HARP

layer. As shown in Figure 2.6 (left panel), after each HV step, the dark current idc

stabilizes. This reduction in idc has previously been observed in a-Se structures with metal

contacts and is thought to be caused by interfacial charge trapping which can

significantly influence the amount of injected charge near the contact.11 However, once

the HV is increased beyond 1200 V, idc rises instantly by over four orders of magnitude,

suggesting the presence of an electrical discharge. As seen in Figure 2.6, an optical

microscope image reveals clear damage along the periphery of the circular electrode

55

(right panel in Figure 2.6). This suggests there is an electric field strength enhancement

near the edges of the PEDOT electrode leading to electrical discharges which occur at the

larger electric fields in this region.

The macroscopic damage shown in Figure 2.6 can be prevented by including a 100 GΩ

series resistance in the current path. This resistance limits the current traversing the

HARP. However, using the large series resistance does not prevent the occurrence of

breakdown which is manifested as an irreversible change in the dark current

characteristic after the electroded HARP has been biased with HV > 1200 V (Figure 2.7).

Hence, breakdown in electroded HARP must be caused by microscopic changes in the

state of the a-Se as a result of an electric discharge. The increased current density within

the region of an electric discharge (or a defective spot which causes an increase in

current) likely produces localized Joule heating which causes crystallization of the a-Se

and an associated increase in its electrical conductivity. These observations confirm that

two key limitations with electroded HARP, as identified earlier in section 2.2, are (1)

electric field enhancement near electrode edges and (2) deposition of heat due to high

localized current densities.

As seen in Table 2.2, metallized contacts consisting of Pt or Au cannot sustain the high

electric field strengths necessary for avalanche multiplication in electroded HARP. It can

be appreciated that the conductive polymer PEDOT contact enables the application of a

significantly larger E (84 V/µm). This is likely due to differences in the microstructure at

the interface between the HARP and the conductive contact. Unlike sputtered or

56

thermally evaporated metallization layers which are prone to diffusion through the

amorphous photoconductive material, the conductive polymer material (PEDOT) is

comprised of large cross-linked molecular chains that cannot diffuse into the

photoconductive material and alter its electrical properties over time. Kasap has also

suggested that the interface at a metal-Se junction is complicated by the formation of

metal selenides.11 It can thus be appreciated that a polymer overcoating on the HARP

provides a chemically stable contact that protects the a-Se surface from environmental

insults. It is likely one of the reasons that Xerox used protective polymer overcoatings in

a-Se xeroradiographic plates.5

We shall now discuss the effect of incorporating a CA polymer layer into the HARP

structure. The CA layer acts as the DRL. As seen in Table 2.1, the DRL thickness is

crucial in determining the maximum electric field strength that can be applied across the

HARP. With no DRL, a maximum avalanche gain 2max_ =ag could be obtained. It has

been previously determined that an avalanche gain 50≈ag is sufficient to overcome the

presence of electronic noise over the entire R/F range of exposures.3,7 A DRL thickness

of 2 µm thus enables much more gain (ga < 104) than is required. Although a thicker

DRL (4 µm) enables the application of even larger biases without causing breakdown

(i.e. V > 1800 V), there was no further apparent increase in ga_max. This is likely due to

the large dark current (several nA) which generates a voltage drop across the DRL and

limits the potential difference across the HARP structure. This dramatic increase in dark

current is probably caused by an inefficiency of the hole blocking contact at very high

electric field strengths, resulting in holes being injected into the a-Se layer from the

57

electrode. These holes avalanche to the same extent as photon-generated holes and

strongly contribute to the dark current, in turn causing a reduction in the avalanche

electric field strength. This negative feedback process results in the gain characteristic

flattening out at very large HV biases. These observations suggest that an improvement in

the hole blocking contact could dramatically increase ga_max for large biases.

The high electric field strengths required for avalanche multiplication in DRL-HARP

increase the probability of electrical discharges. The chart shown in Figure 2.8 provides a

good indication of the rate of occurrence of discharges in DRL-HARP. It can be seen that

below Ea-Se = 50 V/µm, the occurrence of discharges is relatively rare. For 50 < Ea-Se < 80

V/µm, one discharge occurs on average within the 15 minute period. For Ea-Se > 80

V/µm, the discharge rate is two to three times greater. Electric discharges can potentially

introduce noise into the integrated charge signal at each del. The pulse repetition rate in

fluoroscopy, however, is on the order of 10 Hz. Hence the likelihood of a discharge

occurring within the duration of a single fluoroscopic pulse (corresponding to a single

image frame) is negligible.

2.5.2 Linearity

It can be seen that for a bias of 1500 V (corresponding to Ea-Se = 100 V/µm), the DRL-

HARP response is linear over a relatively wide range. At larger biases however (1600

and 1700 V), the linear regime becomes limited and the photocurrent tends to saturate at

larger LED intensities. This is likely due to the increased dependence of avalanche gain

on a built-in bias caused by trapped charge, which is a form of ghosting. It can be argued

58

that two counter-opposing effects influence ghosting in the avalanche regime: (1) the

larger field strength reduces the probability of charge trapping and hence the amount of

trapped charge, leading to a reduction of ghosting, and (2) a built-in bias resulting from

trapped charge can strongly affect the avalanche multiplication gain, since the gain is

strongly dependent on the net electric field in the a-Se, leading to an enhancement of

ghosting. The results in Figure 2.9 suggest that the second effect dominates over the first.

Figure 2.10 provides more insight on how this ghosting is manifested: at lower biases

(HV < 1560 V), the top of the photocurrent pulse remains flat, indicating that Ea-Se does

not change during the pulse duration. At larger biases (HV > 1560 V), the signal is

increased due to avalanche multiplication. The distinctive shape of the pulses suggests a

change in Ea-Se throughout the duration of the pulse: a larger initial signal due to the

presence of a large Ea-Se and a large associated avalanche gain gav, followed by a

decaying signal resulting from the reduction of gav due to the progressive accumulation of

trapped charge. This effect is more prominent at larger biases since more charge is

initially produced and gav is more sensitive to trapped charge.

These results indicate that gain measurements should be performed carefully such that the

amount of charge produced inside the DRL-HARP is controlled. These results also

suggest that the linear dynamic range of DRL-HARP may be limited. The temporal

response and dynamic range, as they apply specifically to fluoroscopy, will be examined

in detail in Chapter 4.

59

2.5.3 Gain and dark current

The measured photocurrent as a function of applied HV bias demonstrates that DRL-

HARP enables avalanche multiplication gains as high as 104. This is the first time such

high avalanche gains are reported in an electroded solid-state amorphous

photoreceptor in the linear (proportional) regime.12 The good agreement between the

theoretical and experimentally-measured conversion and avalanche gains confirms that

only avalanche multiplication of holes is occurring at the electric fields involved in the

experiment (Ea-Se 105 V/µm). ≤

The measured dark current in DRL-HARP (Fig. 2.11) is Id ≤ 20 pA/mm2 at room

temperature for Ea-Se 105 V/µm. This is significantly lower than the dark current in

crystalline silicon (c-Si) detectors (

≤

≈dI 1 nA/mm2) because of the relatively large

bandgap energy of a-Se ( =−SeaEG 2.3 eV compared to =−SicEG 1.1 eV for c-Si) and the

associated low thermal generation of carriers in the a-Se bulk. Dark current produces a

type of noise known as shot noise which is caused by the stochastic fluctuations of

electrons traversing a conductor.13 The shot noise in conventional a-Se flat panel

detectors biased at 10 V/µm (and having dark currents Id ≤ 1 pA/mm2) is negligible.

Since shot noise is proportional to the square root of the dark current, we anticipate in the

worst case an increase in shot noise by a factor of only approximately 4 in the case of the

DRL-HARP, which should also be negligible.

60

2.5.4 Carrier transport

The TOF traces shown in Figure 2.12 reveal no significant difference at sub-avalanche

electric field strengths between hole transport for electroded HARP and DRL-HARP with

a 1 µm CA layer. However, when the CA layer thickness is increased to 8 µm - which

corresponds to approximately half the thickness of the HARP - there is an apparent

effect. In this case, the TOF trace consists of two distinct signals: a short pulse, which

corresponds to the movement of holes across the a-Se and a long exponential tail which

corresponds to the much slower movement of holes in the CA. When the thickness of the

CA layer approaches the thickness of the HARP, a significant amount of the total signal

remains to be slowly discharged through the CA layer. The upward inclination in the

pulses of the TOF traces in Figure 2.12 is likely due to trapped holes in the a-Se bulk due

to previous exposures. Such trapped charge can result in electric field non-uniformities

across the a-Se. For the purposes of the present investigation, it is sufficient to observe

that for CA layers which are substantially thinner than the 15 µm HARP – which is the

case for the 2 µm layers employed throughout this thesis – hole transport is not

significantly affected.

It can be seen in Figure 2.13 that the measured mobility of holes in a-Se for DRL-HARP

increases with increasing field strength. This is due to the lower probability of holes

being trapped at larger fields. More importantly, the measured mobility of holes in a-Se is

essentially the same in both DRL-HARP and standard Xerox a-Se. This is an expected,

but nonetheless important observation since it suggests that the DRL deposition process

does not significantly alter the hole transport properties in the a-Se layer in the HARP. It

61

is known that organic solvents - such as acetone which is used in the CA deposition

process described in section 2.3.1 – may act as catalysts for phase transformations in a-

Se, leading to partial crystallization14 and thus altering its electrical properties. Although

significant a-Se phase transformations are unlikely, the formation of microscopic regions

of crystallization, particularly at the a-Se surface is possible. A materials characterization

technique such as differential scanning calorimetry or X-ray diffraction could be used to

obtain the degree of crystallization in a-Se before and after the CA deposition, however

this is not within the scope of this thesis.

Direct evidence of avalanche mulitiplication is provided by the TOF traces obtained at

avalanche electric field strengths shown in Figure 2.14. The traces reveal a sharp initial

peak, which corresponds to the avalanche multiplication of photon-generated holes. The

remaining signal, which is manifested as a plateau followed by a decaying exponential

tail is due to electrons, which have a lower mobility in a-Se.15 The rapidly-increasing

magnitude of the hole peak as a function of E as well as the production of electrons

throughout the bulk of the a-Se confirm that avalanche multiplication of holes is indeed

taking place in DRL-HARP.

2.6 Conclusions

An experimental method was developed which enabled the practical application of

avalanche-grade electric field strengths in a-Se. The method consists of casting a resistive

polymer layer on an existing HARP structure. Avalanche gains as high as 104 were

62

obtained in the DRL-HARP device. This is the first time such high avalanche gains have

been reported in a solid-state amorphous photoreceptor in the linear (proportional)

regime. The measured dark current in DRL-HARP was found to be less than 20 pA/mm2

at room temperature for electric fields less than 105 V/µm. This is significantly lower

than the dark current in crystalline silicon detectors and the corresponding dark current

shot noise is negligible. A TOF analysis revealed an expected a-Se hole mobility of ~

1.5x10-5 m2/Vs (at 10 V/µm) in DRL-HARP and the DRL was found to have a negligible

effect on hole transport. The TOF analysis was also used to confirm the presence of

avalanche multiplication gain in DRL-HARP.

63

References

1 T. Ohshima, K. Tsuji, K. Sameshima, T. Hirai, K. Shidara, and K. Taketoshi,

"Excess noise in amorphous selenium avalanche photodiodes," Jap. J. of App.

Phys. 30, L1071 - L1074 (1991).

2 M. Yamauchi, T. Hayashida, M. Kosugi, K. Moroboshi, T. Watabe, Y. Ishiguro,

K. Yamano, H. Ohtake, T. Tajima, T. Watanabe, H. Kokubun, M. Abe, and K.

Tanioka, " CMOS image sensor overlaid with a HARP photoconversion film," in

Optoelectronic and Microelectronic Materials and Devices (COMMAD, 2000),

pp. 89-92.

3 W. Zhao, D. Li, A. Reznik, B. J. M. Lui, D. C. Hunt, J. A. Rowlands, Y. Ohkawa,

and K. Tanioka, "Indirect flat-panel detector with avalanche gain: Fundamental

feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat

panel imager)," Med. Phys. 32, 2954-2966 (2005).

4 J.A. Rowlands and J. Yorkston, "Flat Panel Detectors for Digital Radiography," in

Handbook of Medical Imaging, edited by J. Beutel, H. Kundel, and R. Van Metter

(SPIE, Bellingham, Washington, 2000), Vol. I.

5 T. S. Curry, J. E. Dowdey, and R. C. Murry, Christensen's Physics of Diagnostic

Radiology. (Lipnicott Williams & Wilkins, 1990), fourth ed.

6 D. C. Hunt, K. Tanioka, and J. A. Rowlands, "X-ray imaging using avalanche

multiplication in amorphous selenium: Investigation of depth dependent

avalanche noise," Med. Phys. 34, 976-986 (2007).

64

7 Dylan C. Hunt, "Investigation of Avalanche Multiplication in Amorphous

Selenium for Use in Digital Fluoroscopy", PhD Thesis, University of Toronto

(2005).

8 D. C. Hunt, S. S. Kirby, and J. A. Rowlands, "X-ray imaging with amorphous

selenium: X-ray to charge conversion gain and avalanche multiplication gain,"

Med. Phys. 29, 2464-2471 (2002).

9 S.O. Kasap, Optoelectronics and Photonics: Principles and Practices with

CDROM Optoelectronics and Photonics. (Prentice Hall, Upper Saddle River,

2000).

10 K. Tsuji, Y. Takasaki, T. Hirai, J. Yamazaki, and K. Tanioka, "Avalanche

phenomenon in amorphous selenium," Optoelectron., Devices Technol. 9, 367-

378 (1994).

11 R. Johanson, S. Kasap, J. Rowlands, and B. Polischuk, "Metallic electrical

contacts to stabilized amorphous selenium for use in X-ray image detectors,"

Journal of Non-Crystalline Solids 227-230, 1359-1362 (1998).

12 M.M. Wronski, W. Zhao, J.A. Rowlands, A. Reznik, G. Decrescenzo, and J.

Segui, USA patent Patent No. 61/129389 (2008).

13 S. M. Sze, Physics of Semiconductor Devices. (John Wiley & Sons, 1981).

14 T. Ohtani, N. Takayama, K. Ikeda, and M. Araki, "Unusual crystallization

behavior of selenium in the presence of organic molecules at room temperature,"

Chemistry Letters 33, 100-106 (2004).

65

15 C. Juhasz, S.M. Vaezinejad, and S.O. Kasap, "Electron and hole drift mobility in

amorphous selenium-based photoreceptors," Journal of Imaging Science 29, 144-

147 (1985).

66

Chapter 3 Theory of x-ray imaging with avalanche amorphous selenium in the solid state 3.1 Introduction 3.2 Background 3.2.1 Indirect-conversion HARP imager 3.2.2 Depth-dependent gain fluctuation noise 3.3 Proposed direct-conversion HARP imager 3.4 Calculation methods 3.4.1 MTF, NPS and DQE 3.4.2 Avalanche gain, gain nonuniformities and fill factor 3.4.3 Del response 3.5 Results

3.5.1 MTF, NPS and DQE 3.5.2 Avalanche gain, gain nonuniformities and fill factor 3.5.3 Del response

3.6 Discussion 3.6.1 MTF, NPS and DQE 3.6.2 Avalanche gain, gain nonuniformities and fill factor 3.6.2.1 Average gain and fill-factor 3.6.2.2 Avalanche multiplication noise 3.6.2.3 Gain nonuniformities

3.6.3 Del response 3.6.4 Response at high spatial frequencies 3.6.5 Dark current 3.6.6 Direct x-ray interaction in the gain region

3.7 Conclusions

A paper based on this chapter has been published as: M. Wronski et al., "Direct-conversion flat-panel imager with avalanche gain: Feasibility investigation for HARP-AMFPI (HARP active matrix flat panel imager)”, Med. Phys. (2008) 35: 5207-5218

67

3.1 Introduction In the previous chapter, a method was developed which enables the application of large

electric fields across a-Se layers in the solid state and without breakdown. The gain and

dark current characteristics of the resulting avalanche device, known as DRL-HARP were

measured and it was determined that it could provide sufficient gain for QNL operation.

The goal of this chapter is to investigate suitable detector architectures based on DRL-

HARP for x-ray imaging. Both indirect and direct conversion architectures are examined,

however, since a feasibility study of an indirect-conversion imager has already been

completed1, the central focus of this chapter will be on the feasibility of a direct-

conversion x-ray imager with a built-in HARP avalanche layer. The present study is

theoretical and a significant part will focus on the spatial frequency-dependent detective

quantum efficiency (DQE) of the imager. Although a physical implementation of the

imager is beyond the scope of this thesis, the present investigation is focused on a solid-

state avalanche x-ray imager suited for deploying advanced endovascular instrumentation

in interventional radiology.

In what follows, we will briefly review the expected imaging performance of an indirect-

conversion HARP x-ray imager as well as the general problem of depth-dependent gain

fluctuation noise that affects most direct-conversion imagers. An imager architecture will

next be proposed for a direct-conversion HARP x-ray imager which overcomes depth-

dependent noise. We will next perform a theoretical investigation of the detective

quantum efficiency (DQE) for the direct-conversion imager and examine other important

68

characteristics such as the mean avalanche gain, avalanche gain nonuniformities, fill

factor and linearity. We will finish the investigation by discussing the significance of

dark current and the effect of direct x-ray interaction in the HARP layer.

3.2 Background 3.2.1 Indirect-conversion HARP imager

A feasibility investigation of an indirect-conversion HARP x-ray imager has recently

been conducted by Zhao et al. It is called SHARP-AMFPI (scintillator HARP active

matrix flat panel imager) and consists of a structured CsI phosphor optically coupled to a

HARP layer (Figure 3.1). The phosphor is used to absorb diagnostic X-rays and convert

their energy into optical photons. The optical photons are next absorbed at the HARP

surface where they produce electron-hole pairs (EHPs). The holes undergo avalanche

multiplication in the HARP and an active matrix array is used to collect and store the

charge on del electrodes. Thin film transistors (TFT) are used at each pixel to read out the

stored image charge.

69

Figure 3.1. Diagram showing the concept of SHARP-AMFPI in which an avalanche a-Se photoconductor is used to detect light photons produced in a CsI phosphor. A mushroom TFT structure is used to maximize the geometrical fill factor. Reproduced from Reference 1.

The investigation showed that in SHARP-AMFPI, the optical quantum efficiency of the

HARP at a wavelength of 540 nm -- which corresponds to the peak emission wavelength

of a CsI:Tl phosphor – is in the range 20-30% at avalanche electric field strengths. This is

significantly lower than the quantum efficiency of 80% for an amorphous silicon

photodiode used in most existing indirect-conversion flat panel detectors. Despite this

low quantum efficiency, however, it was found that SHARP-AMFPI could provide x-ray

quantum noise limited (QNL) imaging performance by employing an avalanche

multiplication gain of 46 during fluoroscopy. This would require an electric field strength

of 110 V/µm in the 8 µm thick HARP layer. It was also shown that direct x-ray

interaction in the HARP did not hinder the imaging performance and that the HARP

thickness uniformity should be kept within 4% to maintain the signal within the dynamic

range of the readout electronics and within the capability of gain correction algorithms.

70

Figure 3.2. Calculated DQE(f) for SHARP-AMFPI for an x-ray exposure of 0.1 µR and assuming a 200 µm pixel pitch, 1500 noise electrons (rms) and a 600 µm thick CsI:Tl phosphor with a reflective layer. Reproduced from Reference 1.

This previous investigation did not take into account the practical means to establish an

electrode connection to the HARP layer. Thus a distributed resistive layer (DRL) – as

discussed in Chapter 2 -- should be incorporated in between the HARP and the active

matrix array. This layer is not expected to affect the overall modulation transfer function

(MTF) of the imager because (1) the DRL thickness is ~ 1 µm while the del pitch is ~

100 µm so all charge produced within a single del will arrive at the del electrode and not

spread to adjacent del electrodes and (2) according to the analysis in section 2.3.1, the

time constant associated with charge leakage between neighboring dels is on the order of

1000 s, while the charge integration time at each del is on the order of only 10 ms.

Experimental results from Chapter 2 have shown that a 2 µm cellulose acetate (CA) DRL

is sufficient to prevent breakdown in HARP, however it remains to be seen whether

proper operation of the low-voltage (~10 V) thin film transistors (TFT) in the active

matrix array can be maintained while the array is in direct electrical contact with the high

71

voltage (~1000 V) HARP. The TFT and DRL-HARP compatibility will be examined

experimentally in Chapter 4.

3.2.2 Depth-dependent gain fluctuation noise

Avalanche multiplication has been used in radiographic gas imagers as early as 1965,

when Reiss developed an image-forming chamber which relied on avalanche

multiplication of electrons in a gas.2 The associated gain helped overcome the low

sensitivity of previous imagers and it provided better sensitivity than a-Se imaging plates.

However, it was later recognized by Boag3 that the avalanche process produces a

‘random assortment of large charge deposits’, which significantly degrade the image

quality.3 This was attributed to secondary electrons initiating avalanche multiplication at

various depths in the gas, thus contributing to a depth-dependent gain fluctuation noise.

The depth-dependent gain problem may be overcome, in principle, by dividing the

imaging chamber into a conversion or drift region in which impinging radiation ionizes

the gas and generates free electrons and an amplification or gain region in which the

electrons avalanche. Two CERN developments, the micro-mesh gaseous structure

(MICROMEGAS)4 and the gas electron multiplier (GEM)5,6 use this approach. A

conceptual diagram of both devices is shown in Figure 3.3. The depth-dependent gain

problem has been overcome so successfully in these devices that they can provide

excellent energy resolutions.7 However, high gas pressures (several atmospheres) and

thick layers of gas (several centimeters) are required to provide reasonable quantum

efficiencies at radiographic energies. The window of the imaging chamber needs to be

72

made sufficiently thick to support the high gas pressures. The window attenuates the

incident x-rays before they enter the gas, thus limiting the quantum efficiency of the

imaging chamber. The large gas thickness also leads to a degradation of imaging

resolution for obliquely-incident x-rays. This may, in principle, be corrected by using a

spherical detector geometry 3, however this is often impractical.

Figure 3.3. Cross sectional diagrams showing the concept of (a) MICROMEGAS and (b) GEM, where X rays ionize a gas and the resulting electrons undergo avalanche in a gain region. (c) Cross sectional diagram showing a solid state a-Se structure proposed by Lee (Ref. 13) in which the a-Se layer is partitioned into drift and gain regions. Shaded areas denote region of a single del.

The solid state HARP-DRL avalanche device introduced in Chapter 2 overcomes many

of the problems of gas imaging chambers. However, it shares the problem of gas

detectors in that higher energy photons on average penetrate deeper into the active

medium (gas or a-Se) prior to absorption and generation of EHPs. At diagnostic x-ray

energies, an a-Se layer thickness of 200-1000 µm is required in order to achieve a

reasonable quantum efficiency.8 Establishing avalanche multiplication throughout such

thick layers of a-Se is problematic because very high potentials (20-100 kV) need to be

applied to reach avalanche fields (~100 V/µm), and absorption of x-rays at varying

73

depths in the a-Se produces depth-dependent gain fluctuation noise.9-11 To reduce the

potentials required for avalanche multiplication and suppress the depth-dependent gain

fluctuation noise, the a-Se layer could be partitioned into a low-field drift region and a

high-field gain region, analogously to the MICROMEGAS gaseous detector4 shown in

Figure 3.3(a). The general concept of such a dual-layered a-Se structure has been

presented earlier12,13, and here we will expand on this concept to inquire in more detail

what is required to make this concept practical.

Most of the remainder of this chapter will focus on the feasibility of a direct-conversion

HARP X-ray imager. In the discussion section, we will compare the key aspects of the

direct-conversion and indirect-conversion imagers.

3.3 Proposed device structure

In light of what has been discussed in section 3.2.2, we propose the HARP-AMFPI

structure shown in Figure 3.4. Figure 3.4 (a) shows a cross section of the device. It

consists of a thick (~1000 µm) a-Se drift region, in which x-rays are absorbed and

generate EHPs. The electric field in this region is comparable to what is currently used in

direct-conversion a-Se imagers (10 V/µm).14,15 A mesh electrode sets up a higher electric

field (70-110 V/µm) in the thin (~10-50 µm) gain region. As shown, the electric field

lines are shaped in such a way that most holes that drift towards the mesh electrode enter

the gain region and undergo avalanche multiplication. The top view of the device is

shown in Figure 3.4 (b).

74

The top and mesh electrodes have blocking contacts to prevent holes from being injected.

The x-ray generated holes entering the gain region are tightly focused and are collected

by the del electrodes, in which the charge is stored on del capacitors and is periodically

read out by a TFT array (i.e. once every 30 ms). Guard electrodes are used to establish a

uniform potential in the readout plane of the AMFPI. Blocking contacts on the guard and

del electrodes prevent electrons from being injected into the high-field region.

Lee et al. have recently proposed a solid-state avalanche imager using a-Se13, shown in

Figure 3.3 (c), consisting of distinct drift and gain regions. Our structure in Figure 3.4,

however, improves on this in two important ways. First, the mesh electrode apertures are

larger (on the same order as the del size) and are aligned with the del electrodes. Second,

guard electrodes are used in the readout plane of the imager. Both these differences in

structure enable a significant reduction in dark current and the amount of noise resulting

from direct x-ray interaction in the gain region, as will be discussed in sections 3.6.5 and

3.6.6.

For the same reasons as those stated in section 3.2.1, the DRL is not expected to affect

the spatial frequency response of the imager. Temporal artifacts and exposure limitations

caused by the DRL will be characterized experimentally in Chapter 4 on a single del

basis.

75

(a) (b)

Figure 3.4. (a) Side view showing the structure of HARP-AMFPI. The a-Se photoconductor is used to detect X rays and convert them to charge in the drift region. Holes undergo avalanche multiplication in the gain region and are collected at the del electrodes. Shaded area denotes region of a single del. Electric field lines are shown as continuous lines. (b) Top view of the HARP-AMFPI structure. The square area (dotted line) at the top denotes the region of a single del.

3.4 Calculation methods

In what follows is a description of the calculation methods used for a theoretical

investigation of the direct-conversion AMFPI structure described in section 2.3. This

includes an analysis of the imagers’s detective quantum efficiency (DQE) over a range of

spatial frequencies, f,as well as a determination of the gain nonuniformities arising from

electric field strength non-uniformities. The linearity of the del response to x-ray

exposure will also be investigated.

3.4.1 MTF, NPS and DQE

The MTF associated with EHP generation in a-Se is dependent on physical effects such

as Compton scattering of x-ray photons in the material, diffusion and space charge

76

effects. The model developed by Que et al.16 was used to obtain the a-Se MTF used in

this work.

The DQE model was obtained from cascaded linear systems theory.17 Our

implementation ignores second-order effects such as K-fluorescence reabsorption. Shown

in Figure 3.5 is a flow diagram of the signal and noise propagation through the various

stages of the complete imaging system in Figure 3.4. The noise at the output of the

system prior to aliasing and the addition of electronic noise is given by:

)()(1)()( 22206 fTfT

AggfTqgfS aavb

Secavbc ⎥

⎦

⎤⎢⎣

⎡+= σβηβ , Eq. 1

where Tb(f) and Ta(f) are the MTFs associated with EHP generation in a-Se and the del

aperture function, respectively. The number of incident x-ray photons per unit area and

the charge coupling efficiency between the drift and gain regions are denoted by q0 and β.

ASe, η and gc are the Swank factor, x-ray quantum absorption efficiency and x-ray to

charge conversion gain of amorphous selenium, respectively. The avalanche gain and its

variance are denoted by gav and σav2, respectively.

Figure 3.5. Flow diagram showing the stages of the cascaded linear system model for HARP-AMFPI.

77

Both charge conversion and avalanche multiplication of charge are non-deterministic

processes and hence have a certain amount of noise associated with them. Each x-ray

generates a variable number of EHPs in the a-Se layer, and this variation in conversion

gain is characterized by ASe. Each hole that dissociates from an electron, in turn,

undergoes a variable amount of avalanche multiplication. The variance of this process is

represented by σ2av. In the case where only holes avalanche, which is a valid assumption

for a-Se biased near 100 V/µm,9,18 Tager19 derived the relationship for the avalanche

variance,

avavav gg −= 22σ , Eq. 2

used in our model. The holes generated by a single x-ray photon interaction each undergo

avalanche multiplication, and the avalanche Swank factor Aav, which can be expressed as

)(

22

2

fTgAg

gA

bc

Seavav

avav

βσ

+= Eq. 3

denotes the overall variation in avalanche gain associated with this single x-ray

interaction. This, however, is under the assumption that each hole that undergoes

avalanche multiplication travels along the same path through the a-Se. In practice, holes

travel along different paths in the gain region of the AMFPI and are subject to varying

electric field strengths along these paths. This effect is characterized by the secondary

78

Swank factor Asec which accounts for avalanche gain variations due to differences in

travel paths for holes generated by a single x-ray interaction. Table 3.1 summarizes these

various factors used to quantify the conversion and avalanche gain variation. Using Eq. 1

and Eq. 3 and including the effect of path length variation, the noise may simply be

expressed as

sec

222220

6)()()(

AAAfTfTggqfS

avSe

abavc βη= Eq. 4

The aliased noise variance S7(f) is given by aliasing the noise power spectrum (NPS)

given by Eq. 4 with respect to the Nyquist frequency 1/2ap, where ap is the imager del

size. The output NPS, Sout(f) is given by the addition of the electronic noise variance Sn to

the aliased noise variance. The normalized output NPS is then given by

( )20

)()(qgg

fSfNScav

outout ηβ

= Eq. 5

and the DQE is taken as

)()()()(

0

22

fNSqfTfTfDQE

out

ab= . Eq. 6

Table 3.2 summarizes the values of all AMFPI design parameters and operating

conditions chosen for R/F applications.

79

Table 3.1. Summary of factors used to characterize nonuniformities in conversion and avalanche gain. N/A denotes that no information is available. afrom Ref. 9. bfrom Tager’s derivation, given by Eq. (2), assuming that only holes avalanche (from Ref. 19). cAssuming gc = 1000. dresults for proposed detector structure in Fig. 3.4. efrom Ref. 10 for a 25 µm a-Se layer due to variable depths of x-ray interaction in the selenium bulk, for 73.8 keV photons.

Table 3.2. Detector operating conditions and design parameters chosen for fluoroscopy and radiography. afrom Ref. 9. bfrom Ref. 15. cfrom Refs. 10 and 22. d75 kVp, 21 mm Al filtration. efrom Ref. 12.

3.4.2 Avalanche gain, gain nonuniformities and fill-factor

Numerical calculations based on the finite element method (FEM) were done in Matlab

(Mathworks, Natic, MA) and used to obtain the distribution of the electric field in the

proposed AMFPI structure. The imager parameters of interest were the mesh electrode

aperture size, the separation between the mesh electrode and the image readout plane and

the electric field in the drift and gain regions. Key AMFPI metrics were calculated and

plotted. These include the average avalanche gain, the nonuniformities in avalanche gain

80

arising from electric field strength nonuniformities and the fill factor, which is the

fraction of x-ray generated holes that is collected by the del electrodes. Our model

enabled the investigation of a large parameter space and DQE optimization.

The currently accepted model for charge transport in a-Se states that charge in a-Se

acquires energy when subjected to an external electric field and, in doing so, undergoes

both elastic and inelastic collisions.20 The bend radius of the electric field lines in our

proposed AMFPI structure is on the order of micrometers, which is much larger than the

mean free paths of elastic and inelastic collisions, 0.6 and 7.2 nm, respectively.20,21

Hence, the field lines obtained from our FEM analysis will coincide with the charge

carrier travel paths. The avalanche gain gav is obtained by integrating the incremental

gains experienced by a single charge traveling along a given field line according to

( ) ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −= ∫

d

av dssE

g0

21 expexp ββ , Eq. 7

where d is the total path length along the field line, E(s) is the electric field strength at a

point s along the path and β1 and β2 are the impact ionization coefficients for a-Se.18,22

The nonuniformity of the avalanche gain due to field strength nonuniformities was

assessed by comparing the avalanche gain experienced by charge traveling along the

central and lateral field lines arriving at the same del electrode.

81

3.4.3 Del response

For R/F, the imager signal should have a piecewise linear response as a function of

exposure over a range of up to five orders of magnitude. At the largest exposures, or with

high avalanche gain, the amount of charge generated at each del can lead to an increase in

del potential to the point of dielectric breakdown of the TFT gate oxide.

This problem can be adressed by protecting the TFTs from high voltage damage using a

dual-gate TFT structure.23 While this will lead to del saturation, meaning that no useful

signal may be obtained, it will prevent permanent damage if an inappropriate

combination of gain and exposure parameters were to be selected. Also, close to

saturation, linearity can be affected because the increase in del potential during the

exposure can decrease the electric field strength in the del gain region. This, in turn, can

potentially reduce the avalanche gain, which is very sensitive to the field strength and

result in a nonlinear del response.

To determine potential nonlinearities in the del response, each del of the AMFPI may be

modeled as two series capacitors, namely the intrinsic a-Se capacitance occurring

between the top and del electrodes (see Figure 3.4 (a)), and the del storage capacitance

used to integrate the signal charge. The a-Se capacitance in our proposed AMFPI

structure is very small (~ 0.5 fF) due to the large thickness of the a-Se layer (1000 µm).

Hence the much larger del storage capacitance (≥ 200 fF) largely determines the del

response. In modeling the latter, we assume that an x-ray pulse produces holes which

avalanche and produce more holes. These holes accumulate on the del storage

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capacitance and decrease the avalanche field, which decreases the gain, thus reducing the

amount of charge generated for an identical successive x-ray pulse.

3.5 Results

3.5.1 MTF, NPS and DQE

Shown in Figure 3.6 is the MTF associated with direct x-ray interaction in a-Se obtained

from our numerical model.16 The noise power spectrum was obtained using Eq. 4 and is

presented in Figure 3.7, for an average fluoroscopic exposure of 10-6 R/frame. The DQE,

calculated using Eq. 6, is shown for different avalanche gain factors in Figure 3.8 for

several exposures ranging from the lowest exposure to the detector occurring in

fluoroscopy (10-7 R/frame) to a radiographic exposure of 3x10-6 R/frame. Figure 3.9, also

obtained using Eq. 6, demonstrates the relationship between DQE(0) and the per-frame

exposure at different avalanche gains throughout the fluoroscopic exposure range.

Figure 3.6. Calculated MTF for a-Se and aperture function for detector with 100 x 100 µm del size.

83

Figure 3.7. Comparison of the NPS for a direct-conversion a-Se AMFPI before and after the addition of electronic noise at an average fluoroscopic exposure of 10−6 R/frame and for operating conditions shown in Table 3.2.

Figure 3.8. DQE(f) for a direct-conversion a-Se AMFPI calculated using the detector parameters and operating conditions shown in Table 3.2 for an x-ray exposure of (a) 1x10−7, (b) 1x10−6, and (c) 3x10−5 R/frame and varying levels of gain. For large enough gains, DQE(f) approaches the theoretical limit where there is no electronic noise (top-most curve in each graph). This represents the quantum noise limited DQE(f).

84

Figure 3.9. DQE(0) for a direct-conversion a-Se AMFPI calculated as a function of x-ray exposure using the detector parameters and operating conditions shown in Table 3.2 and varying levels of gain. For large gains, DQE(0) approaches the theoretical limit where there is no electronic noise (top-most curve). This represents the quantum noise limited DQE(0).

3.5.2 Avalanche gain, gain nonuniformities and fill-factor

The average avalanche gain of the direct-conversion a-Se AMFPI device, obtained using

Eq. 7, varies as a function of aperture size and mesh distance and is shown in Figure 3.10.

The effect of changing the electric field strength in the drift region is also shown. Figure

3.11 depicts the distribution of the electric field strength in the direction normal to the

readout plane and calculated at different distances from the mesh electrode. Shown are

the field distributions at the del center and 15 µm away from the center. Calculated fill

factors are presented in Figure 3.12 and Figure 3.13 shows how the gain nonuniformity,

associated with this nonuniform field-strength distribution, changes as a function of

aperture size, mesh distance and the electric field strength in the drift region. The relative

significance of the conversion and avalanche gains gc and gav is shown in Figure 3.14 for

the case where the imager is quantum noise limited and at an exposure of 10-5 R/frame.

85

Figure 3.10. Average avalanche gain for a direct-conversion a-Se AMPFI calculated as a function of aperture size, mesh distance, and the electric field strength in the drift region. The detector parameters and operating conditions used are shown in Table 3.2.

Figure 3.11. Comparison between the electric field strength experienced by a charge that travels from the drift region (region A) into the gain region (region B) along an axis that crosses the del center (denoted by the thick solid line) and along an axis 15 µm from the del center (denoted by the thin solid line), as shown in the inset at the top left. The x-axis provides the position of the charge along these axes. Negative values indicate the charge is in region A and positive values indicate it is in region B. The numerical value refers to how far the charge is situated from the horizontal mesh electrode plane (denoted by the dashed line).

86

Figure 3.12. Effective fill factor for a direct-conversion a-Se AMFPI calculated as a function of aperture size, mesh distance, and the electric field strength in the drift region. The detector parameters and operating conditions used are shown in Table 3.2.

Figure 3.13. Gain nonuniformity for a direct-conversion a-Se AMFPI calculated as a function of aperture size, mesh distance, and the electric field strength in the drift region. The detector parameters and operating conditions used are shown in Table 3.2.

87

Figure 3.14. DQE(0) calculated as a function of conversion gain gc and avalanche gain gav using the detector parameters and operating conditions shown in Table 3.2 for (a) an infinitely large x-ray exposure (quantum noise limited case) and (b) an average fluoroscopic x-ray exposure of 10−6 R/frame.

3.5.3 Del response

The effect associated with the decrease in gain due to the accumulation of charge on the

del storage capacitance is shown in Figure 3.15 (a). This effect was investigated for a

range of x-ray exposures and several different avalanche gains. The reduction in

avalanche gain was calculated using the numerical model described in section 3.4.2 and

based on Eq. 7. The amount of charge stored at each del as a function of x-ray exposure is

shown in Figure 3.15 (b), for a 200 fF del storage capacitance.

88

Figure. 3.15. (a) Avalanche gain calculated as a function of x-ray exposure for varying levels of nominal avalanche gain gav. A del storage capacitor Cp of 200 fF was assumed. (b) Calculated image charge on each del electrode as a function of x-ray exposure for varying levels of nominal avalanche gain gav (Cp=200 fF was assumed).

3.6 Discussion

We shall discuss the theoretical effects of electronic noise in HARP-AMFPI with 100 µm

dels, and how the presence of the avalanche gain stage changes DQE(f) at fluoroscopic

and radiographic exposures. Next, we will examine the significance of various noise

sources introduced in HARP-AMFPI. We will also examine the fill factor, del response,

dark current as well as the effect of direct x-ray interaction in the gain layer of the imager

and compare the imaging performance of HARP-AMFPI with the indirect-conversion

SHARP-AMPFI.

89

3.6.1 MTF, NPS and DQE

Amorphous selenium has been used in direct-conversion AMFPIs because it is a well-

characterized material and has a high intrinsic imaging resolution.8,12,16,24 As seen in the

calculated MTF curve (Figure 3.6), despite Compton scattering, charge diffusion and

space charge effects, the degradation of the a-Se MTF is less than 15% at a spatial

frequency of 10 mm-1. In comparison, the aperture function drops more rapidly, thus, for

the HARP-AMFPI, the spatial resolution is limited not by the photoconductor, but rather

by the del size (100 µm) and the associated del aperture function. This is not the case for

SHARP-AMFPI where the MTF of a CsI:Tl phosphor is worse than a 100 µm del

aperture function and so the phosphor plays a key role in reducing the imager DQE at

high spatial frequencies.

Applying the cascaded linear system model in Figure 3.5 we obtain the NPS in Figure 3.7

for a conventional direct-conversion a-Se AMFPI with 100 x 100 µm dels at an average

fluoroscopic exposure of 10-6 R/frame. The presence of an output electronic noise of

1500 electrons per del (rms) increases the NPS at all spatial frequencies by over 40%.

Electronic noise is thus a significant component of the NPS at 10-6 R/frame.

Using the calculated MTF(f) and NPS(f), we obtain the spatial frequency dependent DQE

for HARP-AMFPI in Figure 3.8 for a range of x-ray exposures. As expected, we see that

the presence of an avalanche gain stage provides a significant improvement in DQE: an

avalanche gain of 20 at a fluoroscopic exposure of 10-7 R/frame increases the DQE by

nearly an order of magnitude in the 0-5 mm-1 spatial frequency range (Figure 3.9).

90

Furthermore, optimal DQE(0) is sustained throughout the entire fluoroscopic exposure

range and a reduction of only 18% occurs at an exposure of 10-8 R/frame, as observed in

Figure 3.9. Thus, for HARP-AMFPI with 100 x 100 µm dels, an avalanche gain of 20 is

sufficient for quantum noise limited operation at all clinically relevant R/F exposures.

Zhao and co-workers1 found that an avalanche gain of 46 is required for quantum noise

limited operation in SHARP-AMFPI. Compared to HARP-AMFPI, the larger (46 > 20)

required avalanche gain is likely due to optical coupling losses at the interface between

the CsI:Tl phosphor and the HARP layer.

3.6.2 Avalanche gain, gain nonuniformities and fill-factor

3.6.2.1. Average gain and fill-factor

Numerical calculations of the avalanche gain and fill factor in HARP-AMFPI, shown in

Figs. 3.10 and 3.11, indicate that these imager characteristics are largely dependent on the

aperture size and the mesh spacing (spacing between the mesh electrode and readout

plane). The average gain drops as the aperture size is increased from 10 to 70 µm, since

the transition in the electric field strength between the drift and gain regions becomes

more gradual. As expected, increasing the mesh spacing has the opposite effect, since

avalanche multiplication gain is exponentially dependent on the thickness of the gain

layer.

The results of the numerical calculations indicate that a number of configurations exist

that have desirable operating characteristics. For example, an aperture size and mesh

spacing of 45 µm and field strengths of 104 and 8 V/µm in the gain and drift regions

91

respectively, enable avalanche gains of up to 50 with a 100% fill factor. As discussed in

section 3.6.1, this is enough gain to produce a DQE which is independent of exposure in

both the radiographic and fluoroscopic modes of operation.

In comparison with SHARP-AMFPI in which the del electrode size should be maximized

relative to the total del size to improve the fill factor, HARP-AMFPI does not suffer from

this problem. Because of the focusing effect of the electric field lines in HARP-AMFPI, a

fill factor of 100% may be achieved using small del electrodes (as shown in Figure 3.12).

However, as guard electrodes are required, the mushroom TFT structure shown in Figure

3.1 should be used for HARP-AMFPI.

It should be noted that, in comparison with SHARP-AMFPI, a disadvantage of HARP-

AMFPI is the significantly thicker HARP layer required to achieve the necessary

avalanche gain for quantum noise limited operation. This is caused by a reduced electric

field strength near the apertures of the mesh electrode. The thicker HARP layer will

require larger voltage biases to be applied.

3.6.2.2 Avalanche multiplication noise

The noise associated with avalanche multiplication can adversely affect the DQE under

unfavorable conditions. Figure 3.14 (a) shows the dependence of DQE(0) on the

avalanche and conversion gains gav and gc, respectively, for the case where the imager is

quantum noise limited. For conversion gains gc greater than 10, the DQE(0) is largely

insensitive to the mean avalanche gain gav and is only limited by the quantum efficiency

92

of the AMFPI. However, as gc is reduced, DQE(0) increasingly depends on gav: in this

regime, higher avalanche gains result in a lower DQE(0). At an average fluoroscopic

exposure of 1 µR/frame, the imager is no longer quantum noise limited and the presence

of avalanche gain unconditionally improves the DQE(0) (Figure 3.14 (b)), however the

maximum attainable DQE(0) remains limited by gc. These results are consistent with a

previous characterization of avalanche multiplication noise in a-Se.9

Hence, avalanche multiplication noise reduces the DQE for small conversion gains.

However, at beam energies which are clinically relevant to most R/F applications (50

keV and higher), gc in a-Se is at least several hundred, thus the effect of avalanche

multiplication noise is negligible. Furthermore, the imager would be unaffected by

avalanche multiplication noise in low energy x-ray applications such as mammography,

tomosynthesis or protein crystallography, all of which operate at energies greater than 1

keV (corresponding to a gc on the order of 10). In these applications, avalanche

multiplication would still be an effective means of overcoming the electronic noise - at

low frame exposures in tomosynthesis or in low exposure regions in protein

crystallography – without any associated degradation of DQE. These results apply

equally to both SHARP-AMFPI and HARP-AMFPI as they have no bearing on the front

end of the detector (i.e. the phosphor or the drift regions) and the conversion efficiency of

a CsI phosphor and a-Se (at 10 V/µm) are comparable.

93

3.6.2.3 Gain nonuniformities

HARP-AMFPI consists of a mesh electrode that partitions the imager into two distinct

regions and establishes a different field strength in each region, as seen in Figure 3.3.

Electric charge traverses from the drift region into the gain region through apertures in

the mesh electrode. The presence of apertures produces electric field strength

nonuniformities at the interface between the two regions. Here, we discuss the effect of

these nonuniformities on the DQE of the imager. These nonuniformities only concern

HARP-AMFPI and not SHARP-AMFPI.

The electric field strength distribution along an axis that traverses the centre of a single

AMFPI del is shown in Figure 3.11. The field strength rises over a distance of 80 µm,

whereas the rise along an axis 15 µm away from the del center occurs over only half that

distance. This distortion of the field is due to the proximity of the mesh electrode. The

associated lateral field strength nonuniformities in HARP-AMFPI reach up to 30%.

Shown in Figure 3.13, are the simulated gain nonuniformities resulting from electric field

strength nonuniformities for different imager configurations. We have identified in

section 3.6.2.1 that certain HARP-AMFPI configurations can produce a fill factor of

100% and avalanche gains of up to 50 enabling quantum noise limited operation over the

entire range of clinically-relevant R/F exposures. The results in Figure 3.13 indicate that,

for this configuration, charges generated at different lateral positions in HARP-AMFPI

experience at most an 18% gain nonuniformity in the avalanche region. This corresponds

to a secondary avalanche variance of approximately 3 which is much less than the

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selenium avalanche variance (90 for an avalanche gain of 10, refer to Table 3.1)

associated with avalanche multiplication noise. Thus, the field strength nonuniformities

in HARP-AMFPI have essentially no effect on the DQE.

The rationale behind the HARP-AMFPI structure is to enable direct conversion of x-rays

and avalanche multiplication of charge while overcoming the problem of depth-

dependent gain fluctuation noise. In a single layer of a-Se operating in the avalanche

multiplication regime, depth-dependent gain fluctuations can significantly degrade the

DQE: in previous work, Lui measured a secondary avalanche Swank factor of

approximately 0.5 for a 25 µm thick layer of a-Se biased at 100 V/µm and subjected to

monoenergetic x-rays in the 30.9 – 73.8 keV energy range.10 By contrast, HARP-AMFPI

has a predicted secondary avalanche Swank factor of unity. This demonstrates that, in

principle, decoupling the charge conversion and avalanche gain regions is an effective

means of overcoming the depth-dependent gain fluctuation noise, while maintaining a

high quantum efficiency. It should be noted that the HARP-AMFPI and SHARP-AMFPI

detector structures are conceptually very similar in the sense that most X-rays are

absorbed at the front end of the detector (i.e. the drift region or the CsI:Tl phosphor) and

direct x-ray interactions in the relatively thin avalanche region are extremely rare (this

will be examined in more detail in section 3.6.6).

95

3.6.3 Del response

The del response for HARP-AMFPI is shown in Figure 3.15. For a del storage

capacitance Cp of 200 fF, the response ceases to be linear for gains greater than 20 and

exposures larger than 300 µR/frame and this is associated with a steep drop in avalanche

gain. For a gain of 50, the del response is linear within the regular fluoroscopic region of

10-7 – 10-5 R/frame and saturates at an exposure of 10-3 R/frame, with a maximum

accumulated electric charge of 7x107 electrons, corresponding to a maximum del

potential Pp of 56 V.

Hence, for a del storage capacitance of 200 fF, the linearity of the imager will remain

uncompromised over the entire range of clinically-relevant fluoroscopic and radiographic

exposures. In typical TFT designs, however, del potentials greater than 10 V could

produce excessive current leakage.25 To eliminate this leakage, the del capacitance should

be increased (eg. Cp = 2 pF, Pp < 10 V).

3.6.4 Response at high spatial frequencies

As seen in Figure 3.8, the predicted response of HARP-AMFPI at high spatial

frequencies is excellent (DQE ~ 0.4 at 5 cycles/mm) and should provide good detection

of very narrow (~100 µm diameter) high-contrast objects used in clinical interventions

such as stent struts (individual stent wires). Zhao1 estimated a DQE of 0.3 at 2.5

cycles/mm for the indirect-conversion SHARP-AMFPI with a 200 µm del pitch and a

600 µm thick CsI phosphor with a reflective layer (Figure 3.2). This is less than optimal

for advanced clinical interventions – such as imaging guidewires or stent struts having a

96

100 µm diameter --, however the DQE at high spatial frequencies could be significantly

improved by using a 100 µm del pitch. Furthermore, a thinner phosphor could be used

with no reflective layer, yielding an improved spatial resolution at the expense of a lower

quantum efficiency.

3.6.5 Dark current

Dark current is reduced in two ways in HARP-AMFPI. First, specialized blocking layers

are used in the gain region to limit hole and electron injection. Blocking layers between

the a-Se and electrodes consisting of polycrystalline CeO2 and AsSe3 have been shown to

efficiently control the injection of holes and electrons at the anode and cathode.26,27

Unlike the original HARP structure which operates in the optical regime and thus

requires an optically transparent anode blocking layer,27 the anode blocking layer could

be made slightly thicker in HARP-AMFPI – since x-rays are more penetrating than

optical photons – in order to improve the blocking efficiency.

Second, the combination of guard electrodes and small del electrodes on the AMFPI can

significantly reduce the amount of dark current entering the imager signal path. Without

guard electrodes, injection of holes at the mesh electrode would be an important source of

dark current, because of avalanche multiplication of holes. In HARP-AMFPI, however,

holes injected from the mesh electrode into the high-field gain region are absorbed by the

guard electrodes. Thus, since they are not collected by the del electrodes, they do not

contribute to the signal.

97

Electron injection is less of a concern, because electrons do not avalanche at the electric

field strengths employed in this work. However, the presence of guard electrodes in

HARP-AMFPI also reduces the dark current associated with electron injection: electrons

are, to a large extent, injected from the guard electrodes. The expected reduction in

electron dark current is in fact directly related to the area of the del electrodes relative to

the total del area. Hence, for a del size of 100 µm and a del electrode size of 10 µm, we

would expect a reduction of electron dark current by two orders of magnitude.

Localized high electric field regions near the edges of the mesh electrode apertures can be

a strong source of dark current injection. Lee et al. have proposed to insulate the mesh

electrode, such that it is not in direct electrical contact with the a-Se photoconductor.13

Although this approach entirely reduces dark current injection from the mesh electrode,

charge trapping and accumulation at the interface between the photoconductor and

dielectric can produce unpredictable space charge effects leading to unexpected

avalanche conditions. Perhaps a better way is to control the electrode fabrication process

in such a way as to produce a mesh electrode with smooth rounded edges which do not

induce excessively high localized electric fields.

3.6.6 Direct x-ray interaction in the gain region

Direct interaction of x-rays in the gain region of HARP-AMFPI is a potential source of

added noise because of interaction depth dependent gain fluctuations, however we will

show that it is in fact negligible in practice. The significance of this noise is expected to

be similar at all spatial frequencies, because the spatial frequency response of the drift

98

and gain regions is the same, to a first approximation: the amount of electron trapping in

the a-Se varies with the applied electric field strength, and this can influence the

frequency response, but generally not more than by 20%.28 Hence, we can estimate the

significance of direct x-ray interaction simply by comparing the relative number of x-rays

that interact in each region. For a drift region thickness of 1000 µm, a 45 µm gain region

and an RQA5 x-ray spectrum, 26.3% of x-rays are transmitted through the drift region,

out of which 7% interact in the gain region. Hence, only 1.8% of incident x-rays are

absorbed in the gain region. On average, the charge generated by these x-rays

experiences less avalanche gain than the signal charge generated in the drift region. Also,

a large proportion (i.e. 100 to 1) of charge directly generated in the gain region is

absorbed by the guard and mesh electrodes and does not enter the signal path. Thus, the

estimated proportion of image signal that is subjected to depth dependent gain fluctuation

noise through direct x-ray interaction in the gain region is at most 0.02% of the total

image signal, which is negligible.

3.7 Conclusions

In this chapter, we have reviewed the feasibility of SHARP-AMFPI, an indirect-

conversion HARP x-ray imager and investigated the feasibility of HARP-AMFPI, a novel

solid-state direct-conversion imager with avalanche gain. The direct-conversion imager

consists of an a-Se photoconductor which is partitioned into a thick drift region for x-ray-

to-charge conversion and a much thinner gain region in which the charge undergoes

avalanche multiplication. This approach eliminates depth-dependent gain fluctuation

99

noise. Design considerations were made towards optimizing the imaging performance of

HARP-AMFPI for R/F applications. We examined and modeled the effects on the imager

DQE due to electronic noise, avalanche noise, electric field strength nonuniformities and

direct interaction of x-rays in the gain region. Our results showed that avalanche gains of

20 enable x-ray quantum noise limited performance for fluoroscopy. It was shown that

HARP-AMFPI can provide the required gain while maintaining a 100% fill factor and a

piecewise dynamic range of up to five orders of magnitude, while only requiring four

orders of magnitude for fluoroscopy and radiography. We have also shown that imaging

performance for both indirect and direct conversion detectors is not affected by avalanche

noise for x-ray energies above 1 keV. For HARP-AMFPI, it was shown that the effects of

electric field strength nonuniformities and direct x-ray interaction in the gain region are

negligible. As expected, the spatial frequency response of HARP-AMFPI was found to be

significantly superior to that of SHARP-AMFPI (f > 5 cycles/mm). However, with proper

optimization, both imagers should provide adequate imaging resolution for advanced

interventional radiology applications.

100

References

1 W. Zhao, D. Li, A. Reznik, B. J. M. Lui, D. C. Hunt, J. A. Rowlands, Y. Ohkawa,

and K. Tanioka, "Indirect flat-panel detector with avalanche gain: Fundamental

feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat

panel imager)," Med. Phys. 32, 2954-2966 (2005).

2 K. H. Reiss and G. Lange, "Electroradiography: some remarks on its technique

and future," Phys. Med. Biol. 18, 695-703 (1973).

3 J. W. Boag, "New ways with x-rays: Xeroradiography and ionography," Phys.

Technol. 6, 209-216 (1975).

4 Y. Giomataris, Ph. Rebourgeard, J. P. Robert, and G. Charpak, "MICROMEGAS:

a high-granularity position-sensitive gaseous detector for high particle-flux

environments," Nucl. Instrum. Methods A. 376, 29-35 (1996).

5 F. Sauli, "GEM: A new concept for electron amplification in gas detectors," Nucl.

Instrum. Methods A. 386, 531-534 (1997).

6 F. Sauli, "Imaging with the gas electron multiplier," Nucl. Instrum. Methods A.

580, 971-973 (2007).

7 M. Li, M. S. Dixit, and P. C. Johns, "Photon-counting digital radiography using

high-pressure xenon filled detectors," Nucl. Instrum. Methods A. 471, 215-221

(2001).

8 M. Z. Kabir, S. O. Kasap, W. Zhao, and J. A. Rowlands, "Direct conversion X-ray

sensors: Sensitivity, DQE and MTF," IEE Proc.-Circuits Devices Syst. 150, 258-

266 (2003).

101

9 Dylan C. Hunt, "Investigation of Avalanche Multiplication in Amorphous

Selenium for Use in Digital Fluoroscopy", PhD Thesis, University of Toronto

(2005).

10 B. J. M. Lui, D. C. Hunt, A. Reznik, K. Tanioka, and J. A. Rowlands, "X-ray

imaging with amorphous selenium: Pulse height measurements of avalanche gain

fluctuations," Med. Phys. 33, 3183-3192 (2006).

11 D. C. Hunt, K. Tanioka, and J. A. Rowlands, "X-ray imaging using avalanche

multiplication in amorphous selenium: Investigation of depth dependent

avalanche noise," Med. Phys. 34, 976-986 (2007).

12 J.A. Rowlands and J. Yorkston, "Flat Panel Detectors for Digital Radiography," in

Handbook of Medical Imaging, edited by J. Beutel, H. Kundel, and R. Van Metter

(SPIE, Bellingham, Washington, 2000), Vol. I.

13 D. L. Y. Lee, "Selenium detector with a grid for selenium charge gain," Proc.

SPIE 5745, 216-222 (2005).

14 O. Tousignant, M. Choquette, Y. Demers, L. Laperriere, J. Leboeuf, M. Honda,

M. Nishiki, A. Takahashi, and A. Tsukamoto, "Progress report on the

performance of real-time selenium flat-panel detectors for direct X-ray imaging "

Proc. SPIE 4682, 503-510 (2002).

15 J. A. Rowlands and G. DeCrescenzo, "X-ray imaging using amorphous selenium:

Determination of x-ray sensitivity by pulse height spectroscopy," Med. Phys. 19,

1065-1069 (1992).

16 W. Que and J. A. Rowlands, "X-ray imaging using amorphous selenium: Inherent

spatial resolution," Med. Phys. 22, 365-374 (1995).

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17 I. A. Cunningham and R. Shaw, "Signal-to-noise optimization of medical imaging

systems," J. Opt. Soc. Am. A 16, 621-632 (1999).

18 D. C. Hunt, S. S. Kirby, and J. A. Rowlands, "X-ray imaging with amorphous

selenium: X-ray to charge conversion gain and avalanche multiplication gain,"

Med. Phys. 29, 2464-2471 (2002).

19 A. S. Tager, "Current fluctuations in a semiconductor (dielectric) under the

conditions of impact ionization and avalanche breakdown," Soviet Physics-Solid

State 6, 1919-1925 (1965).

20 A. Reznik, S.D. Baranovskii, O.Rubel, G. Juska, S.O. Kasap, Y.Ohkawa, K.

Tanioka, and J. A. Rowlands, "Avalanche multiplication phenomenon in

amorphous semiconductors: a-Se vs a-Si:H," Journal of Applied Physics 102,

53711-53715 (2007).

21 A. Reznik, S.D. Baranovskii, O. Rubel, K. Jandieri, S.O. Kasap, Y. Ohkawa, M.

Kubota, K. Tanioka, and J. A. Rowlands, "Avalanche multiplication in

amorphous selenium and its utilization in imaging," Journal of Non-Crystalline

Solids 354, 2691-2696 (2008).

22 K. Tsuji, Y. Takasaki, T. Hirai, J. Yamazaki, and K. Tanioka, "Avalanche

phenomenon in amorphous selenium," Optoelectron., Devices Technol. 9, 367-

378 (1994).

23 W. Zhao, J. Law, D. Waechter, Z. Huang, and J. A. Rowlands, "Digital radiology

using active matrix readout of amorphous selenium: Detectors with high voltage

protection," Med. Phys. 25, 539-549 (1998).

103

24 H. Guilleminot, "Use of selenium in the radiometry of Rontgen rays," Archives

d'Electricite Medicale 23, 168-173 (1915).

25 L. E. Antonuk, K.-W. Jee, Y. El-Mohri, M. Maolinbay, S. Nassif, X. Rong, Q.

Zhao, R. A. Street, and K. S. Shah, "Strategies to improve the signal and noise

performance of active matrix, flat-panel imagers for diagnostic x-ray

applications," Med. Phys. 27, 289-306 (1999).

26 K. Miyakawa, Y. Ohkawa, T. Matsubara, T. Takahata, S. Suzuki, and M. Kubota,

"Ultrahigh-sensitivity HDTV new Super-HARP camera," Proc. SPIE 5677, 26-34

(2005).

27 K. Tanioka, J. Yamazaki, K. Shidara, K. Taketoshi, T. Kawamura, S. Ishioka,

and Y. Takasaki, "An avalanche-mode amorphous selenium photoconductive

layer for use as a camera tube target," IEEE Electron. Device Letters 8, 392-394

(1987).

28 L. Chowdhury, O. Tousignant, G. DeCrescenzo, P. Gauthierb, J. Leboeufb, and J.

A. Rowlands, "Effect of ghosting on the modulation transfer function of

amorphous selenium based flat panel detectors," Proc. SPIE 6142, 1S1 - 1S7

(2006).

104

Chapter 4 Experimental characterization of DRL-HARP for interventional radiology applications 4.1 Introduction 4.2 Methods

4.2.1 Noise characterization 4.2.2 X-ray sensitivity 4.2.3 Dynamic range 4.2.4 Temporal response

4.2.5 Compatibility with active matrix technology 4.3 Results

4.3.1 Noise characterization 4.3.2 X-ray sensitivity 4.3.3 Dynamic range 4.3.4 Temporal response 4.3.4.1 RC and ghosting 4.3.4.2 Lag 4.3.4.3 Predicted response for a DRL-HARP FPD

4.3.5 Compatibility with active matrix technology 4.3.5.1 Reverse structure 4.3.5.2 HARP thickness 4.3.5.3 TFT compatibility

4.4 Discussion 4.4.1 Noise characterization

4.4.2 X-ray sensitivity 4.4.3 Dynamic range 4.4.4 Temporal response 4.4.4.1 RC and ghosting 4.4.4.2 Lag 4.4.4.3 Predicted response for a DRL-HARP FPD

4.4.5 Compatibility with active matrix technology 4.4.5.1 Reverse structure 4.4.5.2 HARP thickness 4.4.5.3 TFT compatibility

4.5 Conclusions

This chapter combines selected material from the following: • M. Wronski et al., “A solid-state avalanche photoreceptor for low-exposure x-ray

imaging applications” to be submitted to Med. Phys. • A. Sultana, M. Wronski et al., “Digital X-ray Imaging Using an Avalanche a-Se

Photoconductor” submitted to IEEE Sensors.

105

4.1 Introduction

The general requirements for a solid-state fluoroscopic imaging system in interventional

radiology are (1) quantum-noise limited operation at the lowest clinical x-ray exposures,

(2) compatibility with modes of operation that require significantly larger exposures

(radiography) and (3) capability of imaging fine features of complex interventional

devices. The imager should also be able to operate at up to 30 frames per second.

In Chapter 2, a solid-state amorphous photoreceptor known as DRL-HARP has been

developed and has shown to produce very large avalanche multiplication gains (gav ~ 104)

and it has been shown theoretically that its added noise during avalanche multiplication

should be negligible. It has also been shown that, by virtue of the strong electric field

strength dependence of the avalanche multiplication gain, the DRL-HARP should be able

to accommodate a very wide range of x-ray exposures. Although constructing an x-ray

imager based on the DRL-HARP is beyond the scope of this thesis, we shall investigate

whether, in conformity with requirement (1) the measured noise in DRL-HARP is indeed

as low as expected and demonstrate its sensitivity to diagnostic X rays at the lowest mean

exposures encountered in fluoroscopy. We shall next characterize the dynamic range

within the entire range of x-ray exposures employed in interventional radiology and the

temporal charge response of DRL-HARP. Lastly, we will discuss the compatibility of

DRL-HARP with low-voltage thin film transistor (TFT) image readout arrays which are

currently being used in FPD systems and investigate several alternative device structures.

106

4.2 Methods In all of the experiments described in this section, a light emitting diode (LED) was used

as the radiation source to probe the DRL-HARP. The only exception was an experiment

designed to measure the x-ray sensitivity of the device, in which an R/F x-ray source was

used (section 4.2.2). The rationale behind this is that important properties such as noise,

linearity or temporal characteristics largely depend on the effects of the electric charge -

directly or indirectly resulting from x-ray interaction – which experiences the full extent

of avalanche multiplication gain throughout the a-Se thickness. An important assumption

is that direct x-ray interaction in DRL-HARP does not substantially affect these

properties. This is a reasonable assumption based on previous investigations showing that

the NPS and DQE are not substantially affected by direct x-ray interaction.1,2

Furthermore only 1-2% of all x-rays transmitted through the patient will directly interact

in DRL-HARP.2 Only a small fraction of them will contribute to generation of traps that

could potentially affect device linearity or temporal response and the charge generated by

directly-interacting x-rays will on average experience less avalanche gain than the x-ray-

generated charge produced at or entering the top surface of the HARP.

4.2.1 Noise characterization The experimental setup used to measure the noise characteristics of a DRL-HARP with a

15 µm thick HARP layer and a 2 µm thick cellulose acetate (CA) layer was the same as

that shown in Figure 2.5 (Chapter 2). The measurement device was a multi-channel

analyzer and a spectroscopy amplifier (with pulse shaping) was used in place of the

107

charge amplifier. A generalized linear cascaded noise model can be obtained using the

stages shown in Figure 4.1. This is a simplified model that applies to a single pixel and

hence does not take into account any spatial frequency dependencies.

σ2

N = (ga2-ga)gc

2ηq0/Ac + ga2gcηq0

N = gcηq0conversion

G = ga

avalanche multiplication

N = q0

σ2N = q0

absorption N = ηq0

σ2N = ηq0

selection stochastic gain, Ac

electronic noise

stochastic gain, ga

2-ga

σ2N = gc

2ηq0/Ac

N = gagcηq0

σ2N = (ga

2-ga)gc2ηq0/Ac + ga

2gcηq0 + σe2

N = gagcηq0

photon source

G = gcG = η

noise addition, σe2

Figure 4.1. Flow chart showing the linear cascaded noise model used to calculate the expected noise variance σ2

N and signal N for a DRL-HARP device with conversion and avalanche gains gc and ga, a quantum efficiency η, a Swank factor Ac and an electronic noise variance σe

2. The initial number of input photons is q0.

In the case where the photon source is an optical source (i.e. LED), the conversion gain gc

and the Swank factor Ac associated with the conversion gain are unity. The total noise

variance is then given by:

σ2N = (ga

2-ga)ηq0 + ga2ηq0 + σe

2 . Eq. 4.1

For large ga, the avalanche multiplication variance ga2 - ga can be approximated as ga

2.

The total noise variance then simplifies to:

σ2N = 2ga

2ηq0 + σe2 . Eq. 4.2

The noise measured using the setup in Figure 2.5 can now be compared with the noise

predicted by equation 4.2.

108

4.2.2 X-ray sensitivity

The x-ray sensitivity of DRL-HARP was measured by coupling it to a CsI:Tl phosphor

(Hammamatsu) and using the experimental setup shown in Figure 4.2. The DRL-HARP

consisted of a 15 µm a-Se layer and a 2 µm CA layer. A 75 kVp x-ray beam was used in

continuous fluoroscopy mode, with 2 mm of aluminum filtration and a patient phantom

consisting of a 30 cm thick block of acrylic. A low-pass filter (3 Hz) was used to reduce

noise in the direct current (DC) signal. The distance between the x-ray source and the

DRL-HARP was 1.5 meters. An ionization chamber (Keithley) and dosimeter (Keithley)

were used to measure the detector exposure at the DRL-HARP.

The x-ray sensitivity is expected to be the same for both the direct and indirect

conversion implementations of DRL-HARP discussed in Chapter 3. The rationale behind

this reasoning is as follows: in the direct-conversion case, a-Se has a Wa-Se (absorbed x-

ray energy in the photoconductor necessary to produce a single electron-hole pair that

does not immediately recombine) of approximately 40 eV at 10 V/µm.3 In the indirect-

conversion case, the CsI:Tl phosphor has a WCsI:Tl (absorbed x-ray energy in the phosphor

necessary to produce a single light photon) of 18 eV. Taking into account optical losses

within the phosphor, the coupling losses between the phosphor and a-Se and the non-

ideal quantum efficiency of a-Se at a 560 nm wavelength (peak emission wavelength of

CsI:Tl), the effective WCsI:Tl can be considerably larger (i.e. 2-4 times). Thus, in both

cases, the amount of charge quanta produced in a-Se biased at 10 V/µm for each

interacting diagnostic X-ray (i.e. 70 kVp) is on the order of 103.

109

As the x-ray sensitivity is expected to be the same for both direct and indirect conversion

implementations of DRL-HARP, it will suffice to experimentally demonstrate the

required x-ray sensitivity for QNL operation at fluoroscopic x-ray exposures using the

indirect conversion implementation, as shown in Figure 4.2. This implementation only

requires a phosphor to be optically coupled to the DRL-HARP, and is simpler to realize

in practice than the direct-conversion implementation which requires a mesh electrode

and an a-Se x-ray conversion or drift region.

Figure 4.2. Diagram showing the experimental setup used to characterize x-ray sensitivity. The a-Se detector (top) consisted of a DRL-HARP device with a a 15 µm HARP layer and a 2 µm CA layer which was coupled to a structured CsI:Tl phosphor and exposed to diagnostic X-rays. The x-ray setup (bottom) was similar to that used in interventional radiology. An x-ray tube with 75 kVp, 1.5 mA current and 2 mm of Al filtration was used as the source. The x-ray beam was attenuated using a 30 cm acrylic (lucite) phantom (radiological equivalent to a patient). An ion chamber was used to measure the detector exposure.

110

4.2.3 Dynamic range

The experimental setup shown in Figure 2.5 was used to determine the linear range of

operation of a DRL-HARP with a 15 µm a-Se layer and a 2 µm CA layer. A

photomultiplier tube (PMT) was used to relate the intensity of the light emitting diode

(LED) to the amount of charge produced in a-Se biased at 10 V/µm. Assuming a constant

value of Wa-Se or WCsI:Tl (see section 4.2.2), the amount of charge produced in DRL-

HARP with an a-Se layer biased at 10 V/µm may then be related to an equivalent x-ray

exposure at a typical radiographic energy (i.e. 70 kVp). In this way, the PMT reading can

provide the equivalent x-ray exposure at the detector for any given LED intensity.

4.2.4 Temporal response

HARP technology is currently in use in high-definition cameras employed in

broadcasting applications. As such, it can support real-time imaging rates of 30 frames/s

or more. The DRL could potentially degrade the temporal response and reduce the rate at

which images are acquired using the DRL-HARP. This is possible through three

mechanisms: (1) Resistance/Capacitance (RC) effects, (2) ghosting and (3) lag. The

specific aim here is to characterize the relative importance of these effects.

The photocurrent from DRL-HARP was measured as a function of time for an applied

electric field strength of 10 V/µm. This was compared with the photocurrent in an

electroded HARP biased at the same field strength. Next, the field strength was increased

to 70 V/µm – which corresponds to field strength just prior to the onset of avalanche

111

multiplication - and the photocurrent trace was obtained again from the DRL-HARP. The

rationale behind this experiment is that any similarities between the traces obtained at

both fields are indicative of RC effects. Any dissimilarities, on the other hand could be

indicative of mechanisms associated with charge trapping such as ghosting. Lag, which is

manifested as an increase in the signal rise or fall times is also caused by trapped charge

and can be identified by comparing the measured photocurrent with the theoretical RC

response.

The circuit model used to evaluate the RC effects of DRL-HARP is shown in Figure 4.3

(a) and assumes a del size of 1 x 1 mm. The model includes the HV power supply and

filter -- which removes the noise from the power supply – as well as a current limiting

resistance and the RC loading of the oscilloscope. The a-Se layer is modeled as a current

source in parallel with the selenium capacitance CSe and resistance RSe. The DRL is

modeled as a parallel combination of DRL resistance RDRL and capacitance RDRL. The two

key time constants of this circuit are associated with the RC interactions between (1) CSe

and RDRL and (2) CDRL and RDRL. The respective time constants (the time required to

charge or discharge CSe or CDRL to 63% of the final steady-state value) are:

τ1 = RDRL CSe Eq. 4.1

τ2 = RDRL CDRL Eq. 4.2

Of these two, τ2 is dominant for the HARP and DRL layers used in this thesis, because

the dielectric constants ε of a-Se and the cellulose acetate DRL are approximately the

112

same (ε ≈ 6) and the thickness of the DRL (2 µm) is significantly smaller than that of the

a-Se (15 µm) leading to CDRL > CSe.

The theoretical output voltage Vout measured by the oscilloscope may then be calculated

using the capacitor charge and discharge equations,

2/)( τtpout eVtV −= Eq. 4.3

)1()( 2/τtpout eVtV −−= Eq. 4.4

, where t is time and Vp is a constant and corresponds to the peak voltage across the

capacitance.

The RC behavior of an integrated DRL-HARP in a flat panel detector with 100 x 100 µm

dels is expected to be the same as that for the DRL-HARP in our laboratory model

(Figure 4.3 (a)) . The reason for this is that CDRL and CSe scale linearly with del area and

RDRL scales inversely with del area, so that the RC product remains constant regardless of

del size. There could however be second-order effects caused by RC interactions between

the a-Se or DRL and other circuit elements such as those loading the DRL. For this

reason, a numerical circuit simulation tool known as SPICE (Simulation Program with

Integrated Circuit Emphasis) is used to verify that τ2 is indeed the dominant time constant

and there are no significant second-order effects. The program uses numerical integration

methods in the time domain to approximate the state of each circuit element as a function

of time.4 For the purposes of simulation, a second circuit model is used (Figure 4.3 (b)) to

113

include the effects of the flat panel detector readout electronics. These include a charge

storage capacitance, a TFT and a charge readout amplifier.

(a)

t

(b)

t

Figure 4.3. Circuit diagrams used to model the electrical behaviour of (a) the laboratory Dexperimental prototype with a 1 x 1 mm electrode and oscilloscope signal readout and (b) anDRL-HARP in a flat panel detector with 100 x 100 µm dels and signal readout elements consiststorage capacitance, a TFT and a charge readout amplifier. The voltage at the oscilloscope ostorage capacitance is denoted by Vout. In both cases a 15 µm HARP layer and a 2 µm CAassumed.

114

Vou

Vou

RL-HARP integrated ing of a del r at the del

layer are

4.2.5 Compatibility with TFT technology

4.2.5.1 Reverse structure

Most experiments in this thesis were performed using HARP targets obtained from NHK-

STRL very similar in structure to the ones used in HARP camera tubes Figure 4.4 (a).

The key difference was in the electron blocking contact. In conventional HARP camera

targets, a porous layer of Sb2S3 is used. For our purposes, however this was replaced by

NHK-STRL with a solid layer which is better suited for DRL deposition since it provides

better protection of the underlying a-Se layer which may be sensitive to organic solvents

(section 2.5.4).

Unfortunately, the structure in Figure 4.4 (a) cannot be easily integrated into a flat panel

imager device which involves an electronic readout layer such as an active matrix of thin

film transistors. Since the HARP is normally deposited on a glass substrate with a

transparent conductive (ITO) electrode, it cannot be easily coupled to an image readout

array. Rather, it would be desirable to deposit the HARP directly on top of the readout

array. This is facilitated by the fact that a-Se deposition is a relatively low temperature

process and hence should not adversely affect the readout array. Since the charge

resulting from avalanche multiplication should accumulate at each del of the readout

array, the avalanche multiplication process should occur in the other direction (i.e.

starting at the free surface). This requires the positions of the hole and electron blocking

contacts to be reversed. The reverse HARP structure is shown in Figure 4.4 (b).

115

A 15 µm HARP target having a reverse structure was obtained from NHK, Japan and it

was coated with a 2 µm thick CA layer using the casting process described in section

2.3.1. The gain and dark current were measured using methods described in section 2.3.4.

(a)

(b)

Figure 4.4. Diagrams of (a) the regular HARP structure and (b) the reverse HARP structure. In the regular structure, the indium tin oxide (ITO) electrode near the glass faceplate is biased positively and radiation traverses the faceplate and interacts inside the a-Se. In the reverse structure, the radiation arrives from the opposite side and may directly interact with the a-Se without traversing the faceplate. Hence, if the faceplate is replaced with an active matrix TFT array and the Sb2S3, a-Se, LiF doped a-Se and CeO2 layers as deposited as shown in (b), this yields a practical FPD device structure where the radiation sensitive side is located on the end of the device opposite of the active matrix.

116

4.2.5.2 HARP thickness

The same avalanche gain may be obtained in HARP layers of different thicknesses by

adjusting the applied electric field strength. The key advantage of using a thinner HARP

is that it enables lower biases to be used. A 4 µm thick HARP target was obtained from

NHK-STRL, Japan and it was coated with a 1 µm thick CA layer using the casting

process described in section 2.3.1. Its gain and dark current characteristics were obtained

using the experimental methods described in section 2.3.5.

4.2.5.3 TFT compatibility (in collaboration with A. Sultana at U. of Waterloo)

Thin film transistors used in active matrix image readout arrays are low-voltage (~ 15 V)

devices. Even for a 1 µm thick HARP layer, the voltage bias required for avalanche

multiplication (~ 100 V) is enough to damage the TFTs, in the event of an electrical

discharge in the HARP. We have demonstrated in Chapter 2 that the DRL is sufficient to

prevent breakdown of the HARP. The goal in this section is to demonstrate

experimentally that the DRL can also prevent TFT breakdown. Towards this end, a single

a-Si:H readout element (RE) integrated on a small piece (5mm x 5mm) of silicon

(referred to as a die) containing an array of REs was electrically connected to a DRL-

HARP consisting of a 15 µm HARP and a 2 µm CA layer. The die was obtained from the

Giga to Nano Electronics Laboratory, at the University of Waterloo. The experimental

setup is shown in Figure 4.5.

117

The RE consists of a readout electrode, a TFT, a storage capacitor and gate and data

lines. It has a mushroom architecture, meaning that the readout electrode is positioned

overtop of the actual TFT, storage capacitor and the gate and data lines. This enables a fill

factor (relative fraction of the active matrix array area that is used for charge collection)

of 95%.5 The RE size is 175 x 175 µm, the TFT has an inverted staggered structure with

an aspect ratio (W/L) of 54 µm/18 µm and the storage capacitance has a value of 5 pF.

The TFT is fabricated using standard lithography with five masks.5 The die including the

REs was diced and wire bonded in a ceramic package. A micrograph of a single RE is

shown in Figure 4.5.

50 µm

Figure 4.5. Left: diagram showing the experimental setup used to investigate the electrical compatibility of DRL-HARP with a readout element consisting of an integrated thin film transistor (TFT) and storage capacitor Cst. An LED was used as the excitation source and was coupled through a fiber optic cable to the DRL-HARP. A signal generator was used to switch the TFT on and off and the output signal was amplified using a charge amplifier and displayed on an oscilloscope. Right: Micrograph of the readout element. Please refer to text for fabrication details.

118

4.3 Results

4.3.1 Noise characterization

Shown in Figure 4.6 (a) is the magnitude of the photocurrent of DRL-HARP exposed to

LED light pulses as well as the mean total input-referred noise plotted as a function of the

avalanche multiplication gain ga. The DRL-HARP consisted of a 15 µm thick HARP

layer, a 2 µm thick CA layer and a 2 mm2 PEDOT contact. Figure 4.6 shows the signal to

noise ratio (SNR) calculated using the same data. The source intensity has been adjusted

such that the signal and noise are represented by the same number of electrons in the

absence of avalanche gain (ga = 1). The measured dark current at an avalanche gain of

100 was approximately 1 nA.

100

101

102

102

103

104

105

avalanche multiplication gain ga

rms

elec

trons

measured signalsignal linear fitmeasured noisetheory (σ2 = M2 - M)

electronic noise

(a)

100

101

102

100

101

102

avalanche multiplication gain ga

SN

R

measured SNRtheory

(b)

Figure 4.6. (a) graph showing the measured charge signal and noise produced by DRL-HARP for avalanche multiplication gains in the range 1-100. The experimental setup used is shown in Figure 2.5. Measured data are shown as squares and circles and solid lines represent theoretical models. The noise model is shown in Figure 4.1. (b) graph showing the corresponding signal-to-noise ration (SNR) as a function of avalanche multiplication. The DRL-HARP consisted of a 15 µm thick HARP layer, a 2 µm thick CA layer and a 2 mm2 PEDOT contact. It should be noted that SNR is dimensionless.

119

4.3.2 X-ray sensitivity

Shown in Figure 4.7 is the measured photocurrent resulting from x-ray exposures using

the setup in Figure 4.2. The photocurrent corresponding to the optical measurement is

also shown. The avalanche characteristic associated with the x-ray dataset was limited in

avalanche multiplication gain because of an increase in dark current, likely due to

premature degradation (i.e. crystallization) of the HARP which was made of high purity

a-Se and not stabilized a-Se. The x-ray dataset was obtained in two parts. For high

voltage (HV) biases below 1100V, an x-ray tube current of 20 mA was used,

corresponding to a measured exposure at the DRL-HARP of 20 µR per x-ray pulse. In the

avalanche region, the tube current was reduced to 1 mA (corresponding to a 1 µR x-ray

exposure) to avoid saturating the HARP with charge resulting from the increased

avalanche gain.

120

0 500 1000 1500 200010

0

101

102

103

104

105

106

107

108

HV bias (V)

phot

ocur

rent

(A.U

.)

CsI+EHARP data for 75 kVp x-raysEHARP data for fiber-coupled LEDconversion gain model

avalanche gain modelEHARP data for leakage current

signal level required to overcome electronic noise

signal level of current FPD systems

Figure 4.7. Graph showing the measured dark current (triangles) and DRL-HARP photocurrent using an optical (circles) and x-ray (squares) source excitation. The solid and dashed lines represent the conversion and avalanche gain models, respectively given by Equations 2.4 and 2.5.

4.3.3 Dynamic range

Shown in Figure 4.8 is the measured charge produced by DRL-HARP as a function of the

equivalent x-ray exposure. The DRL-HARP consisted of a 15 µm a-Se layer, a 2 µm CA

layer and a 1 mm2 PEDOT electrode. The equivalent x-ray exposures used in the

fluoroscopy and radiography modes of operation are shown, as well as the electronic

121

noise level. The LED light pulses were applied at a repetition rate of 30 pulses per second

with a pulse duration of 2 ms. This closely mimics the timing of the x-ray pulse sequence

delivered by most modern fluoroscopic x-ray delivery systems.

10-8

10-6

10-4

10-2

100

104

105

106

107

108

109

1010

equivalent X-ray exposure (R/frame)

outp

ut c

harg

e (e

- /mm

2 )

10 V/µm (g = 1)73 V/µm (g = 6)87 V/µm (g = 20)93 V/µm (g = 80)10 V/µm (g = 1) DC

electronic noise level

fluoroscopy radiography

Figure 4.8. Graph showing the measured output charge from DRL-HARP as a function of the equivalent x-ray exposure at different electric field strenghts. The corresponding total gain g is also shown (relative to HARP biased at 10 V/µm). All data were obtained using an excitation source repetition rate of 30 Hz except for the 10 V/µm series with the DC descriptor, which was obtained using an interval of 1 minute between sucessive pulses.

4.3.4 Temporal response

4.3.4.1 RC and ghosting

Shown in Figure 4.9 (a) is the photocurrent in directly electroded HARP (red trace) and

DRL-HARP (green trace) as a function of time, for a continuous 150 ms LED exposure,

equivalent to a 12 mR X-ray exposure (assuming a 60 kVp source). The DRL-HARP

consisted of a 15 µm a-Se layer, a 2 µm CA layer and a 1 mm2 PEDOT electrode. The

122

thin blue line shows the calculated photocurrent using the circuit model shown in Figure

4.3 (a) with a DRL capacitance of 25 pF and resistance of 1.5 GΩ and a HARP

capacitance of 4 pF and resistance of 10 TΩ. The thick black line shows the calculated

photocurrent using the same circuit model and including the effect of hole trapping at the

a-Se-CA interface. Figure 4.9 (b) shows the measured photocurrent at 10 V/µm and at 67

V/µm.

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1

0

1

2

3

4

5x 10-3

150 V bias1000 V bias

(b)

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1

0

1

2

3

4

5x 10-3

time (s)

phot

ocur

rent

(nA

with

out t

he e

xpon

ent)

No CA4% CAPSPICE mo

phot

ocur

rent

(nA

)

12 mR equivalent X-rayexposure150 ms LED pulseA = 1 mm2

CCA = 25 pF/mm2

RCA = 1.5 GΩ /mm2

CSe = 4 pF/mm2

1 MΩ scope no LR150 V bias

phot

ocur

rent

(nA

)

(a) del

time (s)

Figure 4.9. (a): Graph showing the measured photocurrent as a function of time obtained for directly electroded HARP (red) and DRL-HARP (green) for a 150 ms excitation. In both cases a 15 µm HARP layer was used and was biased at 10 V/µm. The DRL-HARP consisted of the HARP layer and a 2 µm layer of CA. The thin blue and thick black solid traces show the modelled photocurrent without and with accounting for ghosting. (b): graph showing the measured photocurrent as a function of time obtained for DRL-HARP at 10 V/µm and at 67 V/µm. All experimental data were obtained using a 12 mR equivalent x-ray exposure.

123

4.3.4.2 Lag

Shown in Figure 4.10 is the photocurrent data obtained from HARP-DRL for a 2 ms LED

exposure. The same DRL-HARP sample and experimental setup were used as for the

measurements in section 4.2.4.1. The dashed blue trace is the signal calculated using the

circuit model shown in Figure 4.3 (a). The dominant RC time constant on the timescale

shown in this plot is associated with the oscilloscope loading. The red solid trace is

obtained from the same circuit model with the lag mechanism additionally taken into

account, with a mean trapping lifetime of 400 µs.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 10-3

-2

0

2

4

6

8

10

12x 10

-3

0.5 mR equivalent X-ray exposure2 ms LED pulseA = 1 mm2

CCA = 25 pF/mm2

RCA = 1.5 GΩ /mm2

CSe = 4 pF/mm2

1 MΩ scope no LR150 V bias

lag.m with:tf = 20a = 1.4lb = 15

time (ms)

phot

ocur

rent

(nA

)

Figure 4.10. Graph showing the measured photocurrent as a function of time obtained for the same DRL-HARP device as in Figure 4.8. The circles represent measured data. The blue and red traces denote the response given by the model without and with accounting for lag.

124

4.3.4.3 Predicted timing response for a DRL-HARP FPD Shown in Figure 4.11 is the timing response (output voltage as a function of time) of the

integrated DRL-HARP circuit model in Figure 4.3 (b) to 5 ms pulses of exposure at 33

Hz (corresponding to a nominal 33 frame-per-second fluoroscopic imaging rate). Two

traces are shown, one for the DRL-HARP and one for the HARP only showing the

voltage at the del storage capacitance. This model takes into account the smaller del size

as well as the periodic charge transfer and readout. The RC circuit parameters were taken

from section 4.2.4.1., however scaled appropriately to reflect the smaller del size: a DRL

capacitance of 0.25 pF and resistance of 1.5 TΩ and a HARP capacitance of 0.04 pF were

used. For each pulse, the leading edge corresponds to charging of the del storage

capacitance by the HARP, which acts as a current source. The top portion of the pulse

corresponds to charge storage on the del capacitance and slight leakage through the Off

resistance of the TFT (the equivalent drain-to-source resistance of the TFT in the Off

state). The trailing edge of each pulse corresponds to a rapid redistribution of charge

between the small (~ 1 pF) del storage capacitance and the much larger (~ 500 pF) input

capacitance of the charge amplifier.

125

L

Out

put v

olta

ge

Figure 4.11. Graph showing in a FPD with a DRL-HARP with and without the DRL. AThe duration of a single x-ray

4.3.5 Compatibility with

4.3.5.1 Reverse structure

Figure 4.12 (left) compa

obtained for the normal

15 µm a-Se layer, a 2 µm

HARP camera tube with

Figure 4.12 (right) show

HARP. Also shown is

without the CeO2 layer i

with DR

without DRL

the expected voltage produced on an integrated 1 pF del storage capacitance avalanche layer and TFT charge readout. Two traces are shown for the device fluoroscopic x-ray pulse repetition rate of 33 pulses per second was assumed. pulse is 5 ms.

active matrix technology

res the measured photocurrent as a function of applied HV bias

and reverse structured DRL-HARP. Both devices consisted of a

CA layer and a 1 mm2 PEDOT electrode. Data obtained from a

a HARP layer of the same thickness (15 µm) are also shown.

s the measured dark current for the normal and reverse DRL-

the measured dark current for a normal DRL-HARP (15 µm)

n place, a 2 µm CA layer and a 1 mm2 PEDOT electrode.

126

0 500 1000 1500 200010

2

103

104

105

106

107

108

HV bias (V)

phot

ocur

rent

(A.U

.)

15 µm DRL-HARP15 µm HARP + camera15 µm DRL-HARP(reverse structure)

0 500 1000 1500 200010

0

101

102

103

104

HV bias (V)

dark

cur

rent

(pA

/mm

2 )

15 µm DRL-HARP

15 µm DRL-HARP(reverse structure)

15 µm DRL-HARP w/o CeO2

(a) (b)

Figure 4.12. (a): Graph showing the measured photocurrent as a function of high voltage (HV) bias for DRL-HARP (circles) and the reverse-structured DRL-HARP (diamonds). In both cases, 15 µm and 2 µm HARP and CA layers were used. Data is also shown for the measured photocurrent for a HARP camera with a 15 µm HARP target (solid squares). (b): graph showing the measured dark current as a function of HV for DRL-HARP (circles), the reverse-structured DRL-HARP (diamonds) and DRL-HARP without the CeO2 film.

4.3.5.2 HARP thickness

Figure 4.13 shows the measured photocurrent and dark current for a reverse structure

DRL-HARP with a 4 µm thick a-Se layer, a 1 µm CA layer and a 1 mm2 PEDOT

electrode. Photocurrent data obtained from a HARP camera tube with a 4 µm thick a-Se

layer are also shown, as well as a fit obtained using the conversion and avalanche gain

127

models described in section 2.34. The impact ionization factors used to fit the avalanche

characteristic were β1 = 1004 and β2 = 802. The best agreement between the DRL-HARP

data and the model could be achieved assuming a 4.5 µm thick a-Se thickness rather than

a 4 µm thickness.

0 200 400 600 80010

0

101

102

103

104

105

HV bias (V)

curre

nt (p

A/m

m2 )

4 µm DRL-HARPgc model for 4.5 µm HARP

ga model for 4.5 µm HARP

4 µm HARP + camera4 µm DRL-HARP (dark current)

Figure 4.13. Graph showing the measured photocurrent (circles) and dark current (triangles) as a function of high voltage (HV) bias for a DRL-HARP device with a 4 µm HARP layer and a 1 µm CA layer. The conversion and avalanche gain models are showed as solid and dashed traces. Also shown is the photocurrent for a HARP camera with a 4 µm HARP target.

4.3.5.3 TFT compatibility (experiments performed with Afrin Sultana U Waterloo)

Shown in Figure 4.14 is the characteristic of the TFTs manufactured at Waterloo.

Figure 4.15 shows the recorded oscilloscope traces using the experimental setup shown in

Figure 4.5. The signal used to drive the excitation LED is shown in blue. It is a periodic

30 Hz pulse sequence with a pulse width of several milliseconds, representative of a

128

fluoroscopic pulse sequence. The signal used to drive the gate of the TFT (i.e. VGS) is

shown in green. It is also a periodic 30 Hz pulse sequence with a swing from -10 to +5 V,

effectively switching the TFT on and off. The TFT output signal is shown in magenta.

It was also observed (but not shown here) that the excitation pulse produced a very small

output signal in the Off state.

Shown in Figure 4.15 is magnitude of the TFT output signal as a function of the HV bias

across the DRL-HARP and the excitation source intensity. The measured TFT

characteristic after this experiment was essentially unchanged (i.e. almost identical to that

shown in Figure 4.14).

Figure 4.14. Graph showing the measured drain-source current as a function of gate-source voltage for the thin film transistor shown in Figure 4.5. Characteristics are shown for drain-source voltages of 1 V and 10 V.

129

Figure 4.15. Graph showing the applied LED and TFT gate pulses and the TFT output current using the experimental setup shown in Figure 4.5 consisting of a DRL-HARP electrically coupled to a TFT.

4.4 Discussion

4.4.1 Noise characterization

From Figure 4.6, which shows the DRL-HARP photocurrent magnitude and variance as a

function of avalanche multiplication gain ga, it can be seen that as the avalanche gain ga

is increased, the signal rises linearly and the noise remains largely unaffected for ga < 10.

However for ga >10, the noise starts to increase linearly with gain. As ga approaches 100,

there is an apparent drop in SNR relative to the model (Figure 4.6). Based on the dark

current measurement and discussion in section 2.5.3, we would not expect this to be

caused by dark current shot noise for ga < 104. The measured dark current of 1 nA,

however, suggests that shot noise could in fact be causing this drop in SNR. The

particular HARP target used in this experiment did not contain stabilized a-Se. It is

130

possible that it had started to crystallize, resulting in much larger dark currents and

thereby limiting the maximum achievable ga.

Nonetheless, for the purposes of noise characterization, we can see that within the range

of avalanche gains investigated ( 1< ga <100) – which includes the ga = 50 required for

quantum noise limited (QNL) operation at the lowest fluoroscopic exposures - the noise

measurements agree well with our avalanche noise model described in section 4.2.1. This

is an important finding since it experimentally confirms that the solid-state electroded

DRL-HARP does not produce significant additional noise, while providing sufficient

avalanche multiplication gain for QNL operation, as speculated in section 2.5.3.

It should be noted that, using the linear cascade model shown in Figure 4.1, the maximum

theoretical DQE can be calculated as follows:

ηηηηη

ηηη

ccc

c

c

c

cac

ca

ca

in

out

gAgA

gg

qqggA

qggqgg

SNRSNRDQE 11

112

22

00

2022

20

222

2

2

max

+=

+=

+== Eq. 4.5

In the optical case, gc = 1, leading to a maximum DQE of 0.5. Thus, in this case, the

penalty due to avalanche multiplication gain corresponds to a reduction in DQE by a

factor of 2. Fortunately, in the x-ray case, in which each X-ray produces many secondary

quanta (charge in the direct-conversion case and optical photons in the indirect-

conversion case), gc >> 1 and the maximum DQE now approaches unity.

131

4.4.2 X-ray sensitivity

From the x-ray sensitivity measurement shown in Figure 4.7, it can be seen that there is

very good correlation between the gain measured using x-ray and optical methods. This is

despite the difference in peak emission wavelengths of the LED (blue) and CsI:Tl

phosphor (green). This is as expected, since the quantum efficiency of a-Se is very high

(> 95 %) at both wavelengths for a-Se biased at high electric field strengths (E > 70

V/µm).1 Furthermore, the x-ray-generated signal exceeded the minimum level required to

overcome electronic noise by over an order of magnitude. It is very encouraging that such

a distinct signal could be obtained at an x-ray exposure of 1 µR, which corresponds to the

lowest mean fluoroscopic exposure at the detector used in interventional radiology. As

suggested earlier in section 4.2.2, these results are sufficient to demonstrate the required

sensitivity of DRL-HARP to overcome the effects of electronic noise throughout the

entire clinical fluoroscopic exposure range for both indirect and direct conversion flat

panel detector implementations.

4.4.3 Dynamic range

From Figure 4.8 which shows the measured DRL-HARP output charge as a function of

equivalent x-ray exposure at different electric field strengths Ea-Se, it can be seen that for

Ea-Se = 10 V/µm and at 30 frames/second, the DRL-HARP response is linear up to an

exposure of approximately 1 mR/frame. At larger exposures, distinct signal saturation

occurs. For radiography however, a rapid imaging rate is not necessary. When the

experiment was repeated at a much lower imaging rate of 1 frame/second, the response

132

remained linear beyond the radiographic exposure range. It is interesting to note that at a

larger Ea-Se = 73 V/µm which is the highest electric field strength at which avalanche

multiplication does not occur, the linear range of operation at 30 frames/s was

significantly extended and encompassed most of the radiographic region. This suggests

that an important mechanism behind the signal saturation involves charge trapping, since

the probability of charge trapping increases with decreasing electric field. However, the

total gain g (relative to a-Se biased at 10 V/µm) at Ea-Se = 73 V/µm is only 6 and is due

only to an increase in conversion gain. This is not enough to overcome electronic noise,

since at an exposure of 10-7 R/frame, the extrapolated output charge produced by DRL-

HARP is the same as the electronic noise charge (Figure 4.8). Hence, a larger Ea-Se is

required to overcome the electronic noise.

The datasets obtained for Ea-Se = 87 V/µm and Ea-Se = 93 V/µm reveal the presence of

signal saturation and the onset of saturation occurs at lower exposures with increasing Ea-

Se. In other words, an increase in avalanche multiplication gain is causing larger signal

saturation at the same level of exposure. This is expected, since the same amount of

uncollected charge inside the device will reduce the avalanche multiplication gain more

prominently at larger fields since the avalanche gain characteristic is steeper at these

fields.

This saturation effect can be thought of as a self-limiting mechanism that restricts the

total amount of charge produced in the device for each radiation pulse. It is, in fact, an

133

important advantage for a flat panel detector used in interventional radiology applications

since the entire sensitive region of the panel may not necessarily be behind the patient. In

other words, certain portions of the panel may be exposed to the direct diagnostic x-ray

beam. The attenuation factor Ap of the x-ray beam through the patient is given by Beer’s

law:

tp eA ⋅−= µ Eq. 4.6

where µ is the linear attenuation coefficient of the patient (we can approximate using a µ

for water of 0.22/cm) and t is the patient thickness. Thus, for a 30 cm patient, the direct x-

ray beam will have a 103 times larger exposure than the mean exposure of the beam

exiting the patient. It can now be appreciated from Figure 4.8, that if a direct exposure

were to occur in the fluoroscopic mode with the detector having some arbitrary amount of

avalanche gain, the sudden increase in photon-generated charge would reduce the

avalanche gain to unity, meaning that the response would be similar to the response at 73

V/µm. Thus, a 103 time increase in radiation exposure would result in a maximum output

charge of 1010 e/mm2. Assuming a del size and capacitance of 100 µm x 100 µm and 2

pF, respectively, this would result in a maximum voltage across the del storage

capacitance VC of 8 V. In typical TFT designs, it is desirable to maintain a VC less than 10

V to prevent excessive current leakage through the TFT.6 VC could be restricted further

by increasing the size of the del storage capacitance. Clearly, without the self-limiting

avalanche gain mechanism however, either a much larger VC or a specialized TFT such as

a dual-gate TFT7 would be required, which might significantly complicate the active

matrix manufacturing process.

134

4.4.4 Temporal response

4.4.4.1 RC and ghosting

From the photocurrent transients shown in Figure 4.9, it can be seen that the DRL-HARP

signal (green trace) differs from the HARP signal (red trace) in two ways: (1) during

exposure, the DRL-HARP signal decreases in time and (2) after exposure, an exponential

tail is apparent. The first effect cannot be explained by using the capacitor charge and

discharge equations 4.3 and 4.4. It is likely due to ghosting, which is a reduction in

sensitivity of the a-Se due to trapped charge. Since this effect in apparent only in the

DRL-HARP, the charge trapping does not predominantly occur in the a-Se bulk. Rather,

holes are likely being trapped at the a-Se-CA interface and this generates an electric field

inside the a-Se layer which opposes the main applied electric field. The circuit model in

Figure 4.3 (a) does not take charge trapping into account and this explains the

discrepancy between the green trace and blue trace throughout the duration of the pulse in

Figure 4.9. By including the effect of hole trapping at the a-Se-CA interface in addition to

the charge/discharge effects, excellent agreement is obtained between the model and

experimental data (see black trace in Figure 4.9 (a)). The sensitivity reduction due to

ghosting during the 150 ms exposure can be estimated by subtracting the relative area

between the blue and green traces shown in Figure 4.9 (b), yielding a reduction factor of

8%.

135

With regards to the second effect (exponential tail), the circuit model approximated by

Equations 4.3 and 4.4 agrees well with the measured DRL-HARP photocurrent. This

confirms that the exponential tail is due to resistance-capacitance (RC) effects caused by

the DRL. The time constant corresponding to the exponential tail in Figure 4.9 is on the

order of 10 ms, which is in good agreement with the time constant (τ2 in Eq. 4.2) of the

CA layer used for the DRL.

As seen in Figure 4.9 (b), when a significantly larger electric field strength is applied in

the DRL-HARP (E = 67 V/µm), the ghosting effect becomes insignificant (blue trace).

The inherent symmetry of the photocurrent response in this case (along the rising and

falling edges) re-confirms that the rounded edges of the photocurrent pulse are due to the

RC mechanism.

4.4.4.2 Lag

There is good correlation between the model and experimental data for the rising and

falling edges of the pulse, however a discrepancy exists during and after the pulse: during

the pulse, there is less photocurrent than expected and the opposite is true immediately

following the pulse. This suggests that there is a lag mechanism at play, in which some

charge is being temporarily trapped in the bulk of the a-Se, reducing the current and is

subsequenty released, leading to an increase in current. Taking the lag mechanism into

account in the model and using a mean charge trapping lifetime of 1000 µs yields the red

trace in Figure 4.10, which is in good agreement with the experimental data. The mean

136

bulk trapping lifeftime of holes in a-Se is 400 µs which is on the same order as the value

used in the model.

4.4.4.3 Predicted timing response for a DRL-HARP FPD

The relative significance of ghosting and lag is expected to remain the same in a DRL-

HARP FPD with 100 times smaller dels (100 x 100 µm) than those characterized in this

work. This is because both the amount of trapped charge per del Qt as well as the del

capacitance Cd will be reduced by the same factor (100) and so the built-in bias resulting

from the trapped charge, which is simply given by the ratio Qt/ Cd should remain

constant. Hence, we would not expect charge trapping to degrade the linearity of DRL-

HARP beyond what is shown in Figure 4.8.

The RC effects are slightly more complex, in the sense that the del resistance and

capacitance can interact with other resistances and capacitances which are external to the

del such as those present in the HV filter. We can use the results of the circuit model in

Figure 4.3 (b) to understand the particular significance of RC effects introduced by the

DRL for an integrated DRL-HARP with 100 x 100 µm dels. Shown in Figure 4.11 is the

timing response of the integrated DRL-HARP circuit model to 5 ms pulses of exposure at

30 Hz (corresponding to a nominal 30 frame-per-second imaging rate). This model not

only takes into account the smaller del size but also the perdiodic charge transfer and

readout. It provides a reasonably good approximation of the RC response in a HARP-

DRL FPD. As seen, the presence of the DRL results in a reduction of the peak signal by

15%. More importantly, however, the difference between the magnitude of the first peak

137

and subsequent peaks in the pulse sequence is negligible, indicating that the exponential

tail associated with the RC discharge observed in Figure 4.9 has no adverse effects on the

timing response. Stated another way, there is no apparent shift in the signal baseline,

indicating that nearly all the charge produced by each light pulse can be removed from

the device prior to the next pulse. This is a reasonable result, since for pulses much

shorter in duration than the 150 ms pulse shown in Figure 4.9, we would expect most of

the charge signal to be coupled through the capacitance of the DRL, leaving only a

minute fraction to discharge through the resistance of the DRL.

4.4.5 Compatibility with active matrix technology

4.4.5.1 Reverse structure

As shown in the gain measurement of the reverse-structured DRL-HARP in Figure 4.12

(a), it can be seen that there is generally good agreement between the gain characteristics

of the normal and reverse DRL-HARP devices up to HV = 1200 V. For larger biases (i.e.

the avalanche multiplication region), the reverse DRL-HARP avalanche gain

characteristic rises slower than for the normal DRL-HARP. A maximum avalanche gain

of 30 could be obtained at 1575 V which is much lower than the gain of 104 obtained for

the normal DRL-HARP at the same bias. The photocurrent data obtained from the HARP

camera followed the normal DRL-HARP data more closely, however a significant

discrepancy is also observed, in both the conversion and avalanche gain regions.

It can be seen that the dark current for both the normal and reverse DRL-HARP

structures (with CeO2) is very similar (within a factor of 1.5), however there is a marked

138

increase in dark current for the DRL-HARP without CeO2 at HV = 1470V (Figure 4.12

(b)). This result confirms that CeO2 effectively acts as a blocking contact. Further studies

should be performed to understand how the CeO2 thickness affects its hole blocking

efficiency at high electric field strengths.

The reason for the discrepancies between the avalanche gain characteristics in Figure

4.12 is unclear. In the case of the normal and reverse DRL-HARP devices, the significant

difference in gain cannot be due to voltage drops across the DRL, since the dark current

for both devices is approximately the same (Figure 4.12 (b)). The discrepancies are likely

due to differences in the a-Se impact ionization factors (IIF) for the different devices. The

IIF is very dependent on the material properties of the a-Se. These, in turn can vary

considerably depending on the material deposition process. Since a-Se is organized in the

form of chains that grow in an orthogonal direction outward from the substrate plane8, the

structure of these chains can also depend on the nature of the substrate on which the a-Se

is deposited. The resulting change in the IIFs can lead to a significant difference in

avalanche gain, since the gain is exponentially dependent on these factors.

The sharp rise in dark current for the DRL-HARP without the CeO2 layer confirms the

importance of this as a hole-blocking layer. It is reasonable to expect a thicker CeO2 layer

to block holes at larger field strengths, however the CeO2 layer in the HARP samples

used in this work was limited to 400 nm. The reason for this was to allow sufficient

transparency so as to maintain a high optical quantum efficiency. For the direct-

conversion avalanche detector structure proposed in Chapter 3, however, optical

139

transparency is not required, since electric charge is coupled directly into the HARP layer

from the drift region. In this case, significantly thicker CeO2 layers could be used,

potentially enabling better blocking efficiency.

4.4.5.2 HARP thickness

As seen in Figure 4.13 which shows the gain and dark current measurements for the

DRL-HARP with a 4 µm thick HARP layer, there is also (as previously seen in section

4.2.5.1), a very significant discrepancy between the HARP camera and the DRL-HARP

data, and the reverse DRL-HARP once again produces significantly less avalanche gain

than expected.

It can be seen that the dark current, when considered as a function of ESe is very similar

for the measurements involving both the 4 and 15 µm HARP layers. However, a much

larger electric field strength could be applied across the 4 µm DRL-HARP than across the

15 µm DRL-HARP (E = 170 V/µm). Interestingly, the dark current at this field

(corresponding to HV = 700 V), is only 45 pA/mm2 and the avalanche gain saturates at

gav = 10. This suggests that there is a drop in E somewhere within the DRL-HARP

structure such that ESe is significantly lower than the nominal field (E ) applied across the

entire structure. Assuming a resistance of 20 GΩ/mm2 for the 2 µm CA layer (as

measured in section 2.4.1) used as the DRL, the total potential drop across the DRL is not

expected to exceed 1 V. This is not sufficient to warrant a sharp decrease in avalanche

gain. Hence there must be a mechanism within the reverse HARP structure which leads

to a decrease in ESe. Since the materials used in the reverse HARP are the same as those

140

used in the normal HARP (except for the Au contact, however this is a conductor so it

should not produce any potential drop), it is most likely that the cause of this ESe

reduction is a space charge mechanism caused by significant interfacial trapping. This

could include, for instance, hole trapping within the a-Se/a-Se:LiF or a-Se:LiF/CeO2

interfaces or electron trapping within the a-Se/Sb2S3 or Sb2S3/Au interfaces. Although

most of these interfaces also exist in the normal HARP, their material properties may be

different in the reverse HARP because of the different material deposition sequence. For

example, CeO2 deposited on a-Se:LiF could produce an interface with a significantly

larger number of electron traps than a-Se:LiF deposited on CeO2.

4.4.5.3 TFT compatibility

It can be seen from the oscilloscope traces in Figure 4.14 that the potential applied at the

gate of the transistor (VGS) determines the state of the transistor: for negative potentials,

the TFT is in the Off state and has a very high resistance (ROff ~ 10 TΩ) and for VGS > 5

V, the TFT is in the On state and has a much lower resistance (ROn ~ 10 MΩ). This

switch-like behaviour enables photon-generated charge to be integrated in the Off state

and be transferred to the charge amplifier in the On state. It can also be seen in Figure

4.15 that, as expected, the TFT output signal is strongly modulated by the gate pulse.

Futhermore, even though the gate pulse that switches the TFT into the On state occurs

many milliseconds after the excitation pulse, the signal is still present. These observations

indicate the proper functioning of the TFT and storage capacitance.

141

It was also observed (but not shown here) that the excitation pulse produced a very small

output signal in the Off state (less than 5% of the signal produced during the On state).

This is likely due to signal coupling through the drain-source capacitance of the TFT.

This effect is present in any TFT and can be thought of as a source of noise in the readout

electronics. The results in Figure 4.16, which show the response of the TFT/DRL-HARP

system are very similar to the results of output charge as a function of input exposure

shown in Figure 4.8 and demonstrate a piece-wise linear mode of operation and a clear

increase in signal due to avalanche multiplication gain. Most importantly, however,

proper operation of the DRL-HARP and TFT could be maintained without any adverse

effects to either of the devices, even when HV biases as large as 1485 V were applied

across the DRL-HARP.

The measured TFT characteristic after this experiment was almost identical to that shown

in Figure 4.14. This indicates that the high-voltage DRL-HARP is compatible with low-

voltage TFT readout technology. In particular, any electrical discharges produced in the

DRL-HARP are not sufficiently large to cause breakdown of the TFT gate oxide.

These results are very encouraging and are an important step towards the realization of a

practical avalanche FPD. Considering that, as discussed in section 4.2.5.2, thinner HARP

layers could be used to achieve the same amount of gain for QNL operation, the

likelihood of breakdown of the TFT gate oxide is expected to be even further reduced.

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4.5 Conclusion

We have demonstrated in this chapter that there are no fundamental obstacles preventing

the use of a DRL-HARP avalanche photoreceptor for FPD applications in interventional

radiology. Experimental results have confirmed that, for the avalanche multiplication

gains required for quantum noise limited operation in fluoroscopy, DRL-HARP does not

introduce any additional noise beyond that theoretically predicted in Chapter 3. The high

x-ray sensitivity of DRL-HARP in an indirect conversion implementation has been

demonstrated at the lowest clinical exposure levels and it has been discussed that a direct-

conversion implementation would provide an equally high sensitivity. DRL-HARP was

found to have a very wide piecewise linear range of operation extending over five orders

of magnitude and encompassing both the clinically-relevant fluoroscopic and

radiographic x-ray exposures. It has been identified that the key mechanisms leading to

nonlinear operation in the avalanche regime are due to resistive-capacitive effects of the

DRL and ghosting due to charge trapping at the DRL-a-Se interface. These effects,

however, are negligible at the low exposures encountered in fluoroscopy and at imaging

rates of 33 frames per second or less. Finally, it has been demonstrated that DRL-HARP

is compatible with existing active matrix technology used in FPDs; a reverse HARP

structure -- facilitating the deposition of HARP on active matrix arrays -- has been

proposed and tested and electrical compatibility of DRL-HARP with integrated TFT

readout elements has been demonstrated.

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References

1 W. Zhao, D. Li, A. Reznik, B. J. M. Lui, D. C. Hunt, J. A. Rowlands, Y. Ohkawa,

and K. Tanioka, "Indirect flat-panel detector with avalanche gain: Fundamental

feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat

panel imager)," Med. Phys. 32, 2954-2966 (2005).

2 M.M. Wronski and J.A. Rowlands, "Direct-conversion flat panel imager with

avalanche gain: Feasibility investigation for HARP-AMFPI," Med. Phys. 35 (12),

5207-5218 (2008).

3 J.A. Rowlands and J. Yorkston, "Flat Panel Detectors for Digital Radiography," in

Handbook of Medical Imaging, edited by J. Beutel, H. Kundel, and R. Van Metter

(SPIE, Bellingham, Washington, 2000), Vol. I.

4 P. W. Tuinenga, SPICE: a Guide to Circuit Simulation and Analysis Using

PSpice. (Prentice-Hall, 1988).

5 K.S. Karim, M.H. Izadi, F. Taghibakhsh, and G. Sanaie, "Intelligent pixel

architectures for digital medical imaging applications," ECS Transactions 8, 289-

293 (2007).

6 L. E. Antonuk, K.-W. Jee, Y. El-Mohri, M. Maolinbay, J. H. Siewerdsen, S.

Nassif, X. Rong, Q. Zhao, R. A. Street, and K. S. Shah, "Strategies to improve the

signal and noise performance of active matrix, flat-panel imagers for diagnostic x-

ray applications," Med. Phys. 27, 289-306 (2000).

144

7 W. Zhao, J. Law, D. Waechter, Z. Huang, and J. A. Rowlands, "Digital radiology

using active matrix readout of amorphous selenium: Detectors with high voltage

protection," Med. Phys. 25, 539-549 (1998).

8 J. Hegedüs and S. Kugler, "Growth of amorphous selenium thin films: classical

versus quantum mechanical molecular dynamics simulation," J. Phys.: Condens.

Matter 17, 6459-6468 (2005).

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Chapter 5 Conclusions 5.1 Brief summary 5.2 Summary of major results 5.3 Original contributions 5.4 Future work

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5.1 Brief summary

Current AMFPIs are not x-ray quantum noise limited at the lowest fluoroscopic

exposures used in interventional radiology. This is due to the presence of substantial

electronic noise in the active matrix which is used to read out the x-ray-generated charge

image. In this work, the feasibility of an AMFPI with internal gain has been

demonstrated. With sufficient gain, the electronic noise can be overcome throughout the

entire clinical x-ray exposure range.

The work has focused on avalanche multiplication in an a-Se photoconductor. The

motivation behind this approach was twofold: first, a-Se is a well characterized

photoconductor currently in use in a number of AMFPI systems. Secondly, high

sensitivity a-Se layers with internal avalanche multiplication gain (HARP) are currently

being used in specialized broadcasting cameras for very low light imaging applications.

The overarching goal of this thesis was to bridge the gap between the AMFPI and HARP

technologies. There are currently two key limitations with HARP: first, it operates inside

a vaccum tube with a scanning electron beam for readout of the charge image, and second

it is used for imaging of visible light photons. Towards this end, it was necessary to: (1)

demonstrate a fully solid-state implementation of HARP; (2) investigate how this solid-

state avalanche detector could be used to image X-rays with sufficient spatial resolution

for the imaging of specialized endovascular devices or important anatomical features

such as coronary microcalcifications; (3) demonstrate that a HARP-AMFPI imager would

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satisfy not only the high x-ray sensitivity and spatial resolution requirements of

interventional radiology, but would also satisfy other important requirements such as

being able to accommodate a very wide range of x-ray exposures and having a linear

response at real-time imaging frame rates.

These three specific aims were addressed in each of the three major chapters of the thesis.

In Chapter 2, it was shown that HARP could provide sufficient avalanche gain in the

solid state to overcome electronic noise in AMFPIs by incorporating an additional layer

into its structure. Next, an investigation in Chapter 3 demonstrated the feasibility of using

HARP in the solid state for the imaging of X-rays using both indirect and direct

conversion approaches. The main focus of the chapter was on a high spatial resolution

direct conversion implementation of HARP-AMFPI. Finally, the last major chapter of the

thesis (Chapter 4) demonstrated that HARP-AMFPI could accommodate the very wide

clinical range of exposures used in interventional radiology and could provide a linear

response to incident X-rays at the highest imaging frame rates used in fluoroscopy.

Once implemented in a clinical system, AMFPIs with internal avalanche gain will

provide the highest physically possible image quality at the lowest possible patient doses

of ionizing radiation. These systems, particularly direct-conversion implementations, will

also provide very high spatial resolutions facilitating the deployment of intricate

endovascular devices with extremely small wire diameters.

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5.2 Summary of major results

5.2.1 Solving the breakdown problem of electroded HARP

It was experimentally confirmed that the two key limitations of electroded HARP are: (1)

electric field enhancement near electrode edges and (2) deposition of heat due to high

localized current densities during electrical discharges. These effects caused irreversible

breakdown of HARP despite the presence of a large series current-limiting resistance. It

was found that conductive polymer electrodes (PEDOT) directly deposited on HARP

were able to sustain significantly higher electric field strengths across the HARP than

metallized contacts (Au or Pt) of the same size, possibly due to the relatively large size of

polymer molecules which cannot diffuse into the a-Se and the absence of free electrons in

PEDOT.

The incorporation of a polymer distributed resistance layer (DRL) into the HARP

structure was shown to essentially eliminate the breakdown problem and enabled the

HARP to sustain electric field strengths as high as 105 V/µm. A casting process was

developed for depositing layers of a cellulose acetate polymer on existing HARP layers

in a reproducible fashion. The cellulose acetate polymer was chosen based on its

resistivity (ρ = 5x1012 Ωcm), excellent transparency and adhesion to a-Se. It served as a

DRL and its primary functions were to (1) reduce the electric field in the vicinity of

electrode edges and (2) provide a current limiting mechanism to limit localized discharge

currents.

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It was found that a cellulose acetate (DRL) thickness of 2 µm enabled avalanche

multiplication gains as high as 104 in a 15 µm thick HARP layer for an applied high

voltage bias of 1575 V (ESe = 105 V/µm). This gain in DRL-HARP was attributed solely

to impact ionization of holes. Furthermore, it was found that increasing the thickness of

the DRL and applying biases larger than 1575 V did not yield consistently larger

avalanche gains. This was attributed to failure of the hole blocking contact for E > 105

V/µm. To completely overcome the electronic noise in either direct or indirect AMFPI, a

maximum avalanche gain of only 50 is required (corresponding to a bias of 1450 V for a

15 µm HARP layer). For these conditions, an average electrical discharge rate of 1

discharge per 7 minutes was observed. As such, electrical discharges are not expected to

add any noise during fluoroscopy in which images are acquired only once every 100 ms

or less.

The measured dark current in DRL-HARP was less than 20 pA/mm2 at room temperature

for ESe < 105 V/µm. The measured a-Se hole mobility in DRL-HARP was found to be

same as in standard Xerox a-Se. Direct evidence of avalanche multiplication in DRL-

HARP was obtained using a time of flight analysis.

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5.2.2 X-ray imaging with HARP-AMFPI

A direct-conversion HARP-AMFPI flat panel imager structure was proposed. It consists

of a thick a-Se drift region in which X-rays are absorbed and generate charge and a thin

avalanche multiplication region (HARP). This structure consists of guard electrodes and

this enables a significant reduction of dark current compared to similar dual-layered

structures proposed earlier.1

A model based on the finite element method was implemented and used to demonstrate

that the direct-conversion HARP-AMFPI imager can provide the required amount of

avalanche gain for quantum noise limited operation, while overcoming the problem of

depth-dependent gain fluctuation noise. The concept of secondary Swank factor was

introduced and was used to quantify the degradation of DQE due to differences in charge

travel path variation throughout a non-uniform electric field distribution.

It was found that the predicted imaging response of HARP-AMFPI at high spatial

frequencies is excellent (DQE ~ 0.4 at 5 cycles/mm) and should provide good detection

of very narrow (~100 µm diameter) high-contrast objects used in clinical interventions

such as stent struts (individual stent wires). Furthermore, it was found that the presence of

electric field nonuniformities does not affect the DQE of the detector, that 100% fill

factors could be obtained and that avalanche multiplication noise is negligible for x-ray

energies greater than 1 keV, which is the case at the diagnostic energies used in

interventional radiology.

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5.2.3 Addressing the requirements of interventional radiology

It was first experimentally confirmed that DRL-HARP does not produce significant

additional noise while providing sufficient avalanche multiplication gain for x-ray

quantum noise limited operation. The high sensitivity to diagnostic energy X-rays at low

fluoroscopic exposures was experimentally demonstrated. Next, it was shown that DRL-

HARP has a very wide piecewise linear exposure range which encompasses both the

clinically relevant fluoroscopic and radiographic x-ray exposures. Both ghosting and

resistive-capacitive effects have been identified as dominant effects leading to signal

saturation at large x-ray exposures, however this should not limit the detector linearity for

fluoroscopic (30 frames/second or less) or radiographic modes of operation. In fact, it

was suggested that signal saturation during avalanche multiplication at unexpectedly

large detector exposures (such as an exposure to the direct x-ray beam un-attenuated by

the patient) is a beneficial effect which can alleviate the overproduction of charge leading

to potential damage of the thin film transistors or storage capacitors in the active matrix.

It was investigated how DRL-HARP could be made more amenable to the AMFPI

manufacturing process. Towards this end, a reverse HARP structure was proposed,

fabricated and experimentally characterized. It was found to produce sufficient avalanche

gain for quantum noise limited operation at low fluoroscopic x-ray exposures, however

the measured gain was much lower than for a regular HARP structure of the same

thickness. Two potential underlying causes have been identified: (1) possible differences

in the a-Se impact ionization factors which are dependent on the material properties of

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the a-Se and can vary considerably depending on the material deposition process; (2)

possible space charge mechanism caused by enhanced interfacial trapping in the reverse

structure, also resulting from changes in the material deposition process.

It was experimentally determined that CeO2 plays a key role in limiting hole injection in

15 µm HARP targets for field strengths exceeding 98 V/µm. It was discussed that

although existing CeO2 blocking structures are sufficient for the gain requirements of

interventional radiology using 15 µm thick HARP layers, improved blocking layers could

potentially enable similar avalanche gains (i.e. 50) using significantly thinner HARP

layers (4 µm or less). This would translate into substantially lower high voltage

requirements which would facilitate integration with low-voltage active matrix

technology.

As a proof of concept, it was experimentally demonstrated that DRL-HARP consisting of

a 15 µm HARP and a 2 µm cellulose acetate DRL could be electrically contacted to a

low-voltage integrated thin film transistor (TFT) and that proper operation of the TFT

could be maintained while the DRL-HARP was biased at avalanche electric field

strengths (~100 V/µm). These results have shown that there are no fundamental physical

problems standing in the way of an integrated DRL-HARP-AMFPI device for

interventional radiology applications.

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5.3 Original contributions

1. The work in this thesis has led to the development of the first solid-state detector

capable of producing linear avalanche gains as high as 104. The detector is based

on an amorphous selenium photoconductor which can be fabricated inexpensively

over large areas. As a result, this detector holds great promise for very high

sensitivity medical imagers. This contribution has provided a basis for U.S.A.

Patent No. 61/129389, Photodetector/Imaging Device with Avalanche Gain, filing

date: June 23, 2008. It has also provided the basis for the letter submitted to

Medical Physics under the name “A solid-state amorphous selenium avalanche

technology for large area photon counting and photon starved imaging

applications”.

2. A potentially largely scalable and low-cost cellulose acetate casting process was

developed. The process can provide uniform, optically transparent resistive

polymer layers of controllable thickness which provide a chemically stable

electrical contact to amorphous chalcogenide photodonductors. This enables the

elimination of electric field strength enhancement near conductive electrode edges

at the surface of such photoconductors and provides an electrical quenching

mechanism. Both functions prevent undesirable crystallization of the amorphous

chalcogenide photoconductor. This contribution has provided a basis for U.S.A.

Patent No. 61/129389, Photodetector/Imaging Device with Avalanche Gain, filing

date: June 23, 2008.

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3. This work has demonstrated that, in principle, decoupling the charge conversion

and avalanche gain regions is an effective means of overcoming depth-dependent

gain fluctuation noise, while maintaining a high detective quantum efficiency.

This opens the door to a new generation of direct-conversion flat panel imagers

with internal gain which simultaneously provide both quantum noise limited x-ray

sensitivity and very high spatial resolution imaging. This contribution has

provided a basis for the paper: "Direct-conversion flat-panel imager with

avalanche gain: Feasibility investigation for HARP-AMFPI (HARP active matrix

flat panel imager)”, Med. Phys. (2008) 35: 5207-5218

4. This work has demonstrated that there are no fundamental physical problems

precluding the integration of high-voltage HARP and low-voltage AMFPI

technologies. This contribution has provided the basis for the paper submitted to

IEEE Sensors under the name: “Digital x-ray imaging using an avalanche a-Se

photoconductor”.

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5.4 Future work

The work in this thesis has been largely exploratory and a number of material and device

engineering challenges have yet to be overcome before a-Se avalanche layers can be

successfully used in robust clinical flat panel imagers. Several key considerations will

now be outlined for future work.

5.4.1 Materials characterization

The polymer deposition process described in this work makes use of an acetone solvent

which could potentially crystallize a-Se. A materials characterization technique such as

differential scanning calorimetry or X-ray diffraction should be used to obtain the degree

of crystallization in a-Se before and after the deposition of cellulose acetate. If

crystallization is observed, then an alternative process should be developed, possibly

using a different resistive material.

Studies should be performed to understand how the thickness of the CeO2 layer affects its

hole blocking efficiency at high electric field strengths. Thicker (> 400 nm) CeO2 layers

could be used for the direct-conversion detector structure proposed in Chapter 3,

however, for indirect-conversion detectors, the hole blocking efficiency of this layer

should be balanced against its optical transparency to ensure that a thicker CeO2 layer

does not substantially degrade the optical quantum efficiency of the HARP layer.

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It is crucial to better understand how the effects of HARP defects can be mitigated. The

main goal is to contain defects and prevent them from spreading to adjacent dels. It will

be necessary to investigate the growth dynamics of defects as a function of the magnitude

of current density traversing the del and determine if the DRL can be used to keep the

current density sufficiently low.

The avalanche gain discrepancies between the normal- and reverse-structured HARP

should be understood. A time-of-flight analysis could be carried out on both types of

samples and this could help determine whether the discrepancies are due to space charge

effects. If no substantial differences are noted, then it is possible that the a-Se ionization

factors are different. An improved deposition process may be required for the reverse-

structured HARP.

5.4.2 Device optimization

The HARP and DRL thicknesses should be optimized for interventional radiology

applications. Thinner HARP layers will require lower biases and this should enable

thinner DRLs as well which could potentially improve the temporal response. However,

the avalanche gain will also be more sensitive to any HARP thickness non-uniformities.

Thinner HARP layers may also require improved blocking contacts because the electric

field inside the a-Se will need to be made larger to sustain a sufficiently large avalanche

multiplication gain.

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The design of the active matrix should be optimized such that it can support dels with and

without a-Se defects. For instance, the size and maximum tolerable potential of the del

storage capacitors should be carefully considered such that any defective del drawing an

unusually large current would be able to quench this current by reducing the electric field

strength and associated avalanche gain in the HARP.

5.4.3 Imager prototype fabrication

A DRL-HARP-AMFPI prototype should be fabricated, initially for optical imaging.

Stabilized a-Se should be used. A sandwich approach could initially be used in which a

regular-structured HARP could be coupled to an active matrix array with a DRL layer in

between them. Spacers may be necessary to control the thickness of the DRL. A better

approach would be to deposit the reverse-structured HARP on an active matrix array

coated with a DRL. The imager could then be used at both non-avalanche and avalanche

electric field strengths. It should be confirmed that the DRL does not affect the spatial

resolution of the imager and the defect kinetics should also be studied.

Next, the indirect-conversion SHARP-AMFPI should be realized. The focus should be on

improving the spatial resolution. This could be done by using a structured CsI:Tl

phosphor with no reflective layer and a sufficiently small del pitch such that the del

aperture function does not degrade the overall MTF of the imager.

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Significant technical challenges remain to be overcome in order to realize the direct-

conversion HARP-AMFPI. These include, for instance, developing an a-Se deposition

process which allows for the deposition of a mesh electrode and controlling the electrode

fabrication process in such as way as to produce smooth rounded electrode edges which

do not induce excessively high localized electric fields. However, the fundamental results

presented here, along with preliminary experiments indicate that the method is robust and

practical and that the necessary developments are therefore worthwhile.

1 D. L. Y. Lee, "Selenium detector with a grid for selenium charge gain," Proc.

SPIE 5745, 216-222 (2005).

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