Application of Integrated Electronics to Ultrasonic Medical Instruments

1274 PROCEEDINGS OF THE IEEE, VOL. 67, NO. 9, SEPTEMBER 1979

Application of Integrated Electronics to Ultrasonic Medical Instruments

ROGER D. MELEN, MEMBER, IEEE, ALBERT MACOVSKI, FELLOW, IEEE, AND JAMES D. MEINDL, FELLOW, IEEE

Abstract-During the past decade, ultrasonic medical diagnostic instruments have advanced significantly as a result of the resourceful application of electronic-system and rcoustic-trPnsducer designs to the problem of noninvasive information acquisition from clinical patienta These advancements are reviewed, with emphasis on the electronic technology by which they were achieved. Current instruments that enable imaging through real-time scanning and g r a y d e display are described. It is predicted that future ultrasonic diagnostic instruments wiU be based on incmsingly sophisticated integrated electronic technology that should greatly enhance the imaging performance of these systems; it should also provide the foundation for ultrasoNc imaging instruments that will extend the state of the art in quantitative anrlysis and diagnosis though doppler processing. This growing technology most likely will include 1) highcurrent DMOS multiplexer tnnsmit/ receive switches, 2) precision preamplifiir may circuits, 3) advanced beamsteer/focus dehy lines, 4) multielement integrated transducer -s, and S ) D o p p l ~ - p ~ integrated circuib.

I. INTRODUCTION c ARDIOVASCULAR diseases, diseases of the heart and blood vessels, are by far the major cause of death in the United States. In 1975, over one-half of all deaths in the

United States were attributed to these diseases [ 1 I . Cardiovas- cular diseases affect more than twenty-nine million Americans and have been estimated to cost annually twenty billion dollars in medical expenses and eight billion dollars in lost productivity.

Cancer is the second leading source of fatality in the United States, resulting in nearly one-fourth of the deaths of people under 65 years of age. As a result, cardiovascular disease and cancer are the principal targets for diagnostic ultrasonic instruments. The major advantages of these devices are early diagnosis and new clinical insight into advanced-treatment techniques.

The ideal medical diagnostic instrument should yield defii- tive data concerning the condition of a patient, should cause no harm or discomfort, and should be convenient, reliable, and economical. Transcutaneous ultrasound has a great potential for meeting these specifications because of its noninvasive and practically harmless nature. Ultrasonic instruments are now widely used for noninvasive imaging of the heart, kidneys, spleen, pancreas, liver, abdomen, gall bladder, fetus, brain, and eye. Typical applications include detection of 1) pericardial effusion, 2) aortic- and mitral-valve defects, 3) congenital heart defects, 4) liver lesions such as cirrhosis, tumors, and cysts, 5 ) gall stones, 6 ) fetal maturity and viability and placenta localization, 7) carotid-artery lesions, and 8) differentiation of cyst versus solid breast lesions. They also include localization and extraction of intraocular foreign bodies and localization of the kidney, pancreas, and spleen.

Manuscript received December 15,1978;revised April 13, 1979. The authors are with the Center for Integrated Electronics in Medi-

cine, Stanford University, Stanford, CA 94305.

During the past decade, increasingly sophisticated ultrasonic instruments have been made available to physicians and appear in routine clinical practice. An excellent medically oriented review of these instruments was presented by Devey and Wells [ 21. Current ultrasonic devices rely heavily on the integrated- circuit electronic technology developed for the computer industry. Another generation of ultrasonic diagnostic instruments most likely will emerge over the next decade. This generation will utilize integrated-circuit technology developed specifically for medical applications and promises even greater performance. This paper reviews the 1) evolution of medical ultrasonic imaging instruments with emphasis on electronic technology, 2) desirability of ultrasound for clinical use and its complementary capabilities to those of X-ray imaging, and 3) prospects for future electronics-intensive ultrasonic imaging and diagnostic instruments.

II. HISTORICAL OVERVIEW OF ULTRASONIC IMAGING SYSTEMS

The history of medical ultrasonic imaging systems is an appropriate basis on which to establish a perspective and understanding of current electronics used for ultrasound in clinical medicine. Ultrasound in the clinical diagnostic en- vironment began with single-transducer systems. The evolu- tionary multielement transducer imaging instruments that followed incorporated advanced scanning techniques to image two physical dimensions. This section reviews the history of these instruments, with special emphasis on integrated electronic technology.

A. Single-Transducer Systems The early single-transducer ultrasonic systems [31 consisted

of a piezoelectric transducer driven by pulse-echo transmit receive electronics. An oscilloscope displayed the time amplitude response of the reflected pulseecho. This A-scan display (Fig. 1) does not generate full cross section images; instead, it presents range echoes versus time.

Other singletransducer systems utilize a continuous-wave transmitter instead of the pulses in the A-scan. Continuous- wave doppler instruments detect blood velocity through the frequency shift in the returning echoes from the red blood cells. Higher power devices generate “deep heat” for physical therapy.

An extension of the pulse-echo A-scan instrument is the time-motion (TM) ultrasonic scanner in Fig. 2(a) that generates for the physician a TM intensity-modulated display (Fig. 2(b)) of echo amplitude (intensity) versus depth (y-axis) during different times of measurement (x-axis). This information is presented on a storage oscilloscope concurrent with a traditional electrocardiographic amplitude (y-axis) versus time

0018-9219/79/0900-1274500.75 O 1979 IEEE

MELEN et al.: INTEGRATED ELECTRONICS AND ULTRASONIC MEDICAL INSTRUMENTS 1275

DEPTH [ t i m e ) - Fig. 1. A-scan display.

T I M E

(b)

Fig. 2. Single t r a d u c e r time-motion ultrasonic scanner. (a) Device. (b) Display.

(x-axis) display. A wide dynamic-range vacuum-tube preamplifier is then driven by nonlinear CRT intensity-modulated electronics. The transmitter contains a singlechannel pulser capable of generating severalhndred-volt pulses. Measured in terms of its number of active electronic devices, the overall electronic complexity of this TM ultrasonic Scanner is low by today’s standards. It is widely and effectively used for study- ing the heart and has reduced the need for catherization.

(b 1 Fig. 3. Mechanical-arm two-dimensional scanner. (a) Equipment.

(b) Abdominal image.

B. Mechanical Scanners The singleelement systems were followed by mechanical

scanners that extended the display to include two spatial dimensions (axial and azimuthal directions), thereby creating a full cross-sectional image. These systems are based on either manual or automatic mechanical scanning of a single transducer. Some mechanical scanners use an acoustic fixed-focus lens to reduce the dimensions of the scanning beam so as to maximize resolution at the focal length of the lens. These instruments are effective in generating highquality abdominal cross-sectional images.

The manual scanners (Fig. 3) require a single transducer connected to a mechanical ‘‘arm’’ [41 containing position- sensing potentiometers or optical encoders. These position indicators drive the deflection circuitry of a CRT storage display, and images are obtained by moving the transducer so as to “paint” the image on the CRT display. Operator skill is a significant factor in obtaining highquality images with this type of system. In automatic mechanical scanners [SI, [61, an electrical

motor rotates the transducer directly or drives a mechanical camshaft that deflects the transducer in an arc or a linear reciprocating motion. These scanners can achieve highquality images with less operator intervention than can their manual counterparts. Imaging of the heart and blood vessels is facilitated by their rapid frame rate; however, in many such sys-


Fig. 4. Eight-transducer water-bath mechanical scanner.

TRANSOUCER ARRAY

Y

t

Fig. 5. Imaging-syatem parameters.

tems, a small aperture will produce low lateral resolution, or a have a direct correspondence with a transducer element and large size will limit the organs that can be scanned. In all the acoustic scan-line CRT display. Separate elements and a fixed-focus systems, classical tradeoffs must be made between cable for each circumvented the need for high-power elec- resolution at the focus range and depth of field of the lens tronic transmitlreceive switches. This scanner requires 20 (range over which high resolution is obtained). In addition, receivers although only one is directed toward the display at erratic motion while scanning reduces the unambiguous in- any time. Gain matching of the receivers and transducers formation that may be extracted from the echoes. introduced a source of error not observed in the mechanically

is shown in Fig. 4. This system realizes high resolution A scanner with similar linear-array scanning capabilities has through eight largeaperture transducers in a water bath, been developed that incorporates a moving “window” of and microprocessors control the position of each to create a several transducer elements to generate an interpolated multi- rapid scan. line display [ 101. Scanning circuitry was located in the trans-

C. Electronic Scanners ducer probe and, unfortunately, this circuitry increased the probe size and weight.

Electronic scanners are based on arrays of transducer ele- Similar systems have been developed with refined scan se- ments that can be subdivided into collimated (near-field) and quences and digital scan conversion to obtain a more fully

An automated water-bath scanner developed by Kosoff [ 7 ] scanned predecessors.

phased (time-delay) arrays. In collimated arrays, the ultrasonic image is formed directly in the plane of the array by means.of collimated or nearly collimated beam patterns. In phased arrays, the outputs of all the transducers are delayed and combined to represent the output from any point in the plane.

The behavior of collimated arrays is illustrated in Fig. 5 . It can be observed that in the region where the depth is smaller than D*/A, the pattern is approximately a collimated replica of the transducer array and resolution is governed by transducer size. Beyond this distance, in the far-field or Fraunhofer region, the beam diverges at an angle of A/D. For a maximum depth Z,,, therefore, transducer size D required to approximate collimated imaging is d m . A frequency of 3 MHz and a maximum depth of 20 cm represents a value of D = 1 .O

interpolated image. One such linear scanning system, the SONICS scanner (Fig. 6), has been clinically demonstrated [ 11 1 . It is a high-performance ophthalmic imaging system that uses electronic transmit/receive DMOS multiplexers for scanning and a digital unit for display [ 121. This instrument is designed for near-field high-resolution eye scanning at a 7-MHz ultrasound.

The custom-made DMOS multiplexers in the SONICS scanner were developed by Plummer [ 131 and are shown in the photomicrograph in Fig. 7. They were originally designed for an earlier experimental system [ 141 in which the elements in the transducer array were positioned along two spatial dimensions, as illustrated in Fig 8; thus two-dimensional arrays could generate scans in all three major planes.

cm. This relatively poor resolution is the primary reason why Another method for creating images by means of transducer collimated arrays are limited to superficial structures at small arrays is based on timedelay electronics to equalize the time depths. At these lower depths, higher frequencies can be used delay through the propagation pathways from the imaged because the attenuation problem is greatly reduced. For object to a common summing point as is demonstrated in example, in a system with a frequency of 6 MHz and a maxi- Fig. 9. The echoes from different depths and scan angles can mum depth of 5 cm, D = 3.46 mm. be detected by varying the time delay of the delay lines. This

The first imaging systems to employ multiple transducers to approach is similar to the techniques commonly used in radar scan electronically were reported by Bom [ 81 and Somer [91. systems. The delay correction may have two components-a The Somer system is discussed later. The Bom system con- spherical (or focus) range correction and a beamsteering (or tained 20 elements in an 8-cm array operating at 2.25 MHz. angle-of-approach) correction. The focus component is Images were generated by transmitting and receiving through a typically approximated by a parabolic correction and is rectilinear array of transducers (a near-field array) that pro- electronically more difficult to generate. The resolution or duced collimated beam patterns.’ Near-field imaging systems field pattern is determined by the resultant amplitude at

MELEN et d.: INTEGRATED ELECTRONICS AND ULTRASONIC MEDICAL INSTRUMENTS 1277

CONTROL MICRO

PROCESSOR

FRAME CALIBRATION VIOEO FREEZE GENERATOR ADC

4

A TRACE PROGRAM MEMORY MEMORY

RETICLE GENERATOR

I DISPLAY \ COMPOSITE TV SIGNAL GENERATOR

( 4

Fig. 6. The SONICS scanner. (a) Near-fiild 7-MHz ultrasound scanner using DMOS multiplexers. (b) Typical acoustic image of the eye. (c) Block diagram (lineax array system).

the delaycompensated point where the array has been steered and focused; at other points, the delays are incorrect and result in varying degrees of field-pattern cancellation. For pulse lengths of a few cycles, it is reasonable to estimate lateral resolution based on steady-state diffraction. For example, an array “focused” or delay-compensated at a depth of z will experience a lateral response which is the Fourier transform of the array aperture; here, the effective width of the central lobe, which determines resolution, is approximately (A/D)z. As a result, in an electronically focused system, resolution im- proves with large apertures, and size is often limited by the available “windows” in the body such as the cardiac notch in heart imaging.

The complete field pattern of a focused array without deflection is plotted in Fig. 10. Again, the width of the centtal

lobe is controlled by array size. The pattern of the individual transducers (denoted by the dashed line) governs the field of view or maximum deflection angle and has an effective angular width of A/W where W is the width of each transducer. The grating lobes are the undesired responses resulting from the periodic structure of the array. They have the same shape as the main lobe and are separated from it by A/d where d is the transducer spacing. These grating lobes can be reduced by the overall pattern of the individual transducers; however, as can be observed in Fig. 1 1, this is of little value when the beam is deflected to an angle 6. The main lobe slides down, and the grating lobe rises to an amplitude comparable to that of the main lobe.

There are several approaches to resolving these gratinplobe problems. The most straightforward is to fii the aperture with

PROCEEDINGS OF THE IEEE, VOL. 67, NO. 9, SEPTEMBER 1979

Fig. 7. Photomicrograph of DMOS multiplexer.

GLASS INWLATOR-- SILICON DICE

UTIPLEXING MOHOLlTHlC

PIEZOELECTRIC ARRAY WAFER

ELEMENT ISOLATION GROOVES

-EXTERNAL LEADS

CONNECTIONS NECTIW'BUYPS*

-Y. KTIVE ELECTROOES

ECHO WAVE FRONT

ECHO WAVEFRONTS

PREAMPLIFIERS "BEAM STEER'

-. DELAY LINES

OUTPUT

Fig. 9. Focusing and beamsteering of ultrasonic waves with delay lines.

Fig. 10. Element and array beam patterns.

AMPLITUDE RELATIVE

t

AMPLITUM RELATIVE

1

(b)

Fig. 8. Two-dimensional electronically scanned acoustic-transducer array. (a) Cross section. (b) Photomicrograph. Fig. 11. Effect of beamsteering on grating lobes.

MELEN et al.: INTEGRATED ELECTRONICS AND ULTRASONIC MEDICAL INSTRUMENTS 1279

a large number of small transducer elements, which will place the grating lobes increasingly farther away from the main lobe with diminishing amplitude until they disappear at transducer spacings of X/2. This, however, results in additional electronic complexity. Another method relates to the insonification structure. If it is identical to the receiver structure, based on the same linear array, the grating lobes will be in both the insonification and receiver beam patterns. The insonification system should have a different pattern and should not insonify the grating-lobe region significantly. It must be emphasized that the patterns in Figs. l0.and 11 represent steady-state con- ditions. With relatively short pulses, the main lobe remains undisturbed and the grating lobes disappear with lower peak amplitudes. Norton 1151 proposes that the peak amplitudes are reduced by the ratio of the number of transducer elements to the number of cycles in the pulse waveform.

Electronically scanned arrays, as compared to mechanical scanning systems, have the advantages of high speed, reliability, flexibility, and a compact probe. The round-trip time per line is 200 to 300 ps, depending on the depth required for analysis. A typical frame rate of 30/s, therefore, will produce a real-time image of 100 to 150 lines/frame. With electronic scanning, angular deflection, scan rate, and scan-line density can be controlled electronically. The array can also interre gate any scan line repeatedly for an A- or TM-mode presentation. Here, the reflectivity of a single line is displayed as either deflection or intensity versus time, and each line can be related directly to the cross sectional displayed image. In this way, the clinician can acquire detailed knowledge concerning an area of disease observed in the two-dimensional display.

Somer [ 91 constructed a beamsteering clinical scanner based on a receiver network comprised of fmed coaxialcable delay lines. This system established a fixed offset to the steering, and a parallel sample/bold delay circuit obtained a variable delay for the electronically controlled deflection of the received-beam angle of approach.

The time-delay scanned imaging system generates a sector scan or pie-shaped display format. Sector scanning has the following advantages.

-A small transducer can create a large field of view through a limited acoustic aperture such as the rib cage.

-The entire array is used for transmitting and receiving, thereby distributing the power density and maximizing the reception of weak echoes.

-The array does not create an absolute shadow; blind spots caused by intervening objects are reduced.

-Full-aperture electronic focusing will ensure maximum lateral resolution.

The disadvantages are that the targets in proximity to the transducer are difficult to image because of a limited field of view and receiver overload, the display format is difficult to interface to standard raster-scan television equipment, and the abeyant shadow of intervening obstacles grows wider with depth.

The first clinical application of an electronically focused system was introduced by Thurstone [ 161. This scanner utilized discrete inductor-capacitor delay lines and a linear array of transducers operating at 2 MHz. The numerous delay lines are bulky, however, and greatly increased the size of the system. A variant of the Somer and Thurstone approaches was developed by Anderson [ 171, based on compact inductor- capacitor delay lines and a cylindrical 7cm focal-length fixed-

Fig. 12. Phased-array ultrasonic sectorscan system. (a) Device. (b) Cardiac image.

focus acoustic lens attached to the front of the transducer array. This electronic beamsteering acoustic-focusing device (Fig. 12) achieves focused resolution optimized to image the center of the adult heart.

During the past few years, a number of system components have been specifically designed to extend the performance of the transducer array and the preprocessor electronics of imaging systems by increasing volumetric resolution, sidelobe suppression, and dynamic range. These custom components include thetaconfiguration transducer arrays, cascadecon- trolled attenuation preamplifier circuits, cascade charge- coupled electronic lenses, and polyvinylidene PVFl integrated transducers.

The transducer array establishes many of the resolution limits in an imaging system. Macovski and Norton [ 181 designed the theta array (Fig. 13) to obtain high-resolution images at all ranges. They circumvented the fixed-range focus transmit limitations by using the symmetry properties of the annular array for transmit and the rectilinear array and electronic delay lines on receive. The combined transmit and receive beam patterns of apodized array elements yield good resolution in all three spatial dimensions.

The sidelobe level of a time-delay scanned imaging system is determined primarily by the mismatch in channel gain and


L O U T P U T

Fig. 13. Electronically focused theta array.

I P-TYPE SI

THICK OXIDE THIN OXIDE ' POLYSILICON

(a)

'I '2

(C)

Fig. 14. Charge-coupled device delay line. (a) Cross section. @) Stor. age potential diagrams. (c) Clock waveforms.

phase of the preprocessor electronics before the summing node (see Fig. 9). These include mismatches in the summing node, delay lines, preamplifiers, and transducers.

Batch-fabricated monolithic integrated circuits result in a good match among components fabricated in the same batch, especially those fabricated on the same integratedcircuit die. Chargecoupleddevice (CCD) delay lines are integrated-circuit capacitor arrays that can be operated as a frequencycontrolled delay-line device. Fig. 14 describes a CCD delay line comprised of a closely spaced array of metal-oxide-semiconductor capacitors. In operation, samples of charge representing the electrical signal are stored beneath the capacitor electrode at the surface of the silicon substrate. This signal charge is then transferred between plates in response to level changes in

Fig. 15. Twenty-channel cascade charge-coupled device.

clocks 81 and 82. During each high-blow transition of the clocking signals, the charge advances one electrode in the array, thereby setting the charge velocity in proportion to clock frequency; as a result, the delay time of the delay line is inversely proportional to clock frequency. These CCD delay lines have the potential of performing a number of sonar and ultrasonic signal-processing functions [ 191. A cascade charge- coupled device (C3D) has been developed [201 that performs the summing, beamsteering, and focusingdelay functions of a sector-scan ultrasonic imaging system on a single monolithic integrated-circuit chip less than cubic inches in volume. Fig. 15 is a photomicrograph of the silicon chip that may be combined with the theta array. Focusing and beamsteering are changed electronically by varying the clock frequencies of the parabolically shaped and triangular sections, respectively.

Other investigators have required a six-foot electronic rack of delay lines to perform these sum-delay functions. The delay mismatches of C3D lenses are sufficiently low for Eaton [21 I to observe that transducer mismatch is the most significant source of mismatch in his C3Dbased system. Eaton used lead-zirconate-titanate transducer elements and a preamplifier array controlled in gain by a microprocessor to reduce preamplifier mismatch. He further observed that the dynamic range of the delay-sum preprocessor was limited by the dynamic range of the variable-gain preamplifier.

The preamplifier in medical ultrasonic systems requires a 6GdB dynamic range and 604B attenuation control. A common choice for the preamplifier is an integrated circuit designed for television-receiver intermediate-frequency automatic gain-control applications; such circuits have a 304B dynamic range at minimum attenuation, which does not increase as the input attenuation is increased. Because gain- control accuracy was not considered important for the original use of this IC, the desired O.ldB matching in gain between channels in the array is difficult to obtain over the entire 6GdB attenuationcontrol range. Mussman 1221 developed a technique for extending the dynamic range through cascade controlled attenuation as illustrated in Fig. 16.

Matching of transducer elements in an electronically scanned system is a limiting factor in determining sidelobe response. Swartz [231 has developed a wide-band polyvinylidene fluo- ride (PVF2) transducer array directly attached to the face of the silicon integratedcircuit preamplifier array (Fig. 17).

MELEN et aL: INTEGRATED ELECTRONICS AND ULTRASONIC MEDICAL INSTRUMENTS 1281

I + l i l - l - J 1 1 - - ---+ - +

.65V .65V c

VGAlN

Fig. 16. Cascade-controlled attenuation preamptifiir.

Fig. 17. Integrated4rcuit PVF, buffer amplifier prior to transducer attachment.

These arrays promise excellent phase matching and highly accu- rate geometric transducer placement. Computer simulations of the frequency and phase response of the PVFz/oxide/silicon/fieldeffect transistor (POSFET) are compared in Fig. 18 to the conventional lead-zirconate-titanate PZT transducer.

These technological advances have the potential for generating time-delay scanned imaging systems with greatly en- hanced performance. One such system is the volumetric pyramidal scanner (Fig. 19) described by Beaver [24]. The ability to scan nonplanar cross sections may be of value in determining cardiac stroke volume and relative timing of the heart valves and may provide the necessary means for quantitative measurements in advanced systems designed for vessel tracking to obtain dynamic-flow measurements.

D. Acoustic Holographic Scanners

Acoustic holography [25], [26] is an alternative to collimated and time-delay scanned ultrasonic imaging and is a direct analog of optical holography. In echo ultrasound, the time-of-flight information delineates depth, and this is made possible by the slow propagation velocity of ultrasound compared to EM waves. In acoustic holography, this valuable information is not used, and depth is determined by focusing

PZT-5A X12 RESONANT AT ~ M H Z MATCHING, HEAVY BACKING

a o . m v . \ s 0 0 5 10 15 20 25

FREQUENCY (MHz)

Fig. 18. Comparison of computer simulations of PVF, and PZT frequency responses.

Fig. 19. Pyramidal sector scanning in three dimensions.

onto specific depth regions. This approach is not satisfactory in a coherent system in which out-of-focus artifacts are numerous; another problem is the severe speckle patterns resulting from coherence. Although all ultrasound systems exhibit these patterns, they are most critical in holographic systems having large regions of coherence. These systems, however, can acquire data from an entire volume where any part of the volume can be displayed. Unfortunately, holographic reconstruction of the total volume can cause serious distortion because of the large ratio of sonic to optical wavelengths.

111. COMPARISON OF ULTRASOUND TO X-RAY Ultrasonic and X-ray systems have many complementary

capabilities. Diagnostic X-rays having wavelengths on the order of lo-" m exhibit no diffraction in tissue or in the X-ray apparatus; however, ultrasonic systems having wavelengths of zl mm are generally limited by diffraction because the wavelength is on the same order as that of the structures of interest. Similarly, X-rays display no refractive effects because velocity is essentially identical for all media. Ultra-


sound waves are subject to a wide variety of refractive indices which enable the construction of lenses and prisms. For- tunately, variations within the body are small so that refrac- tion does not severely distort the images. Because compres- sion waves have the highly desirable characteristic of traveling APRO J

at only 1500 m/s in water, range gating can be accomplished through simple circuitry and, as a result, reflectivity informa- &T-Q tion can be directly accessed in three dimensions. This c a p ability is unavailable in X-ray systems where measurements produce the line integral of the X-ray density rather than the (includes Vol. 2)

density at each point.

TRANSDUCER

VOLUME 2

VOLUME 1

Clinically, because ultrasound has the significant advantage Fig. 20. Attenuation-compented flow-measurement system.

of being f i e i of ionizing radiation, the radiaition-sensitive fetus, eyes, thyroid, and reproductive organs can be examined without hazard. Ultrasound generally performs well where a soft tissue path leads from the skin to the entire abdominal region; this is also true in the cardiac region of the thorax because of the cardiac notch in the left lung. Regions having intervening air or bone, however, cannot be visualized (as the in- terior of the lungs) or are visualized poorly (as in the brain). Ultrasonic systems are well-suited to dynamic three-dimensional studies because of the rapid acquisition of data and the lack of ionizing radiation, and these advantages should prove extremely valuable in cardiac applications; they are nonexis- tent in radiographic systems.

lV. PROSPECTS FOR ULTWUND IN MEDICINE The history of ultrasonic instruments in Section I1 describes

the evolution of electronic complexity, from the relatively simple electronics of the A-scan devices to the timadelay scanned-array instruments that contain thousands of electronic components. This was followed by a comparison of the X-ray and ultrasonic modalities, indicating the complementary nature of their capabilities and discussing the distinct advantage of ultrasound in being free from ionizing radiation. This advantage has promoted the use of ultrasound in nonimaging applications that will be facilitated by the technological developments discussed in this section.

A. Potential Copabilities

Research into and development of the nonimaging properties of ultrasound for medical diagnosis have been extensive, with particular emphasis on data collection for diagnostic applications. Quantitative data are being obtained through signal processing of the ultrasonic echoes. Current research is focused on quantitative and noninvasive measurements of 1) blood-flow rate, velocity distribution, and turbulence, 2)heart- wall velocity and roughness, 3) heart-valve abnormality, 4) temperature, 5) blood pressure, and 6 ) respiration [ 271.

Most of the clinical information is being obtained from the display-reflectivity data. Many studies are under way to acquire tissue-signature information which would be useful in distinguishing between diseased and normal tissue, using such parameters as velocity and attenuation [281 as derived from transmission measurements. Results obtained from reflectivity studies of tissue signatures, based on in vitro material, appear promising for such diseases as cirrhosis of the liver. Recent in vivo analyses of normal and diseased myocardium [ 291 are also encouraging. Amplitude histograms of the signals from 1ocaI volume elements reveal distinct differences between infarcted and normal myocardium, and these have been correlated with the biological states of the tissue.

Fig. 20 is a system developed for the measurement of total flow at the aortic arch [ 30 J wherein echoes from a large and small acoustic sample volume are electronically processed simultaneously to obtain an attenuationcompensated flow measurement. This measurement is insensitive to acoustic tissue attenuation because it is based on the ratio of two derived signals that are proportional to the attenuation and, as a result, the acoustic-attenuation dependence is factored out. The doppler angle (angle between the flow and transducer beam) is not a direct source of error because volume flow rather than velocity is being measured.

A novel silicon integrated-circuit f i ter (Figs. 21 and 22) has been developed by Genberg [31 I for first- and second- moment doppler signal processing to realize quantitative flow and turbulence measurements. These monolithic fiiters are fabricated on integrated-circuit substrates by forming polycrystalline-silicon electrodes that are doped to become a re& tive conductor. The silicon-dioxide layer is used as the di- electric of a capacitor formed between the silicon substrate and polycrystalline electrode. The resistor/capacitor properties of the structure are controlled by the resistance and shape of the electrode and by substrate capacitance. The frequency- attenuation slope is governed primarily by the shape of the electrode. In spectral-moment processing, the slope of the filter rolloff determines the moment of the filter. Previously, spectral-moment fdters required many precision components to approximate the desired rolloff characteristics.

One of the first clinical applications of very-high-power ultrasound was to warm muscle tissue to facilitate the per- fusion of blood for physical therapy. Other therapeutic applications have been proposed, including 1) neurosurgery, 2) pitu- itary inhibition, 3) termination of pregnancy, 4) cleaning and decalcifying blood vessels and cardiac valves, 5) elimination of oxygen bubbles from the blood, and 6) cell disintegration [ 321. A combination imaging/therapy device may prove use ful for experiments in these areas by assisting in localizing the application of ultrasound energy.

B. Economic Impact In the next generation of noninvasive ultrasonic systems for

imaging and diagnosis, electronic complexity may be one of the technological factors most likely to play a major role. This is evident in Fig. 23 wherein the complexity of ultrasonic imaging instruments can be seen to increase over a 20 year span from the simple singletransducer scanner to the proposed volumetric pyramidal scanner with its millions of active electronic devices. To date, this rise in electronic complexity has developed

with a very moderate or no increase in cost, as is supported by the semiconductor-industry data obtained by Moore [331 and

DISTRIBUTED RC LINE

W R C

(b) Fig. 2 1. Silicon monolithic integrated-circuit Doppler-moment fit-.

(a) Filter. (b) Frequency response.

Fig. 22. Photomicrograph of IC doppler fiier.

Mohan Rao [34] in Fig. 24. The price per function of semiconductor components has dropped by two orders of magni-

E

0 VI

P 1,000,000

. 4 VI W

> W

i!

n

/ VOLUMETRIC ELECTRONIC

SCANNER

/ /

/ PHASED ARRAY ELECTRONIC

SCANNER /

W 10,000

a t NEARFIELD ELECTRONIC SCANNERS

/ - i- u

/ > / !z X W t / MECHANICAL SCANNERS A a / 0 I u

1965 1975 1985 YEAR OF INTRODUCTION

Fig. 23. Electronic complexity of ultrasonic scanners.

S COST/POUND(" $ 5 0 0 / l b )

S COST / WATT ( - $50 I WATT)

- WATTS/VOLUME(- 2 0 0 WATT I c u . f t . )

\

YEAR OF INTRODUCTION

Fig. 24. Cost, power, size, and speed factors in electronic systems.

fabricating more active components per unit area on a single semiconductor substrate.

Device density has been made possible through circuit inn* vations and photolithographic refinement. The more compact semiconductor devices have less parasitic capacitance and are capable of operating at higher frequencies than do their larger predecessors. Higher frequency operation means less parasitic circuit delays. The overall power consumption of an integrated circuit is limited by the thermal properties of the package in which it is contained. It is not surprising, then, that power consumption per function parallels the cost per function because additional functions are incorporated in the package only if there is no increase in the total package power dissipation.

Electronic system factors of cost per pound and per watt and watts per dollar have been constant over the past ten years and are likely to continue through the next decade. The ratio between cost and power per function is also a constant, and the overall expense of electronic systems is roughly proportional to the price of the electronic components. It is interest-

tude each decade, and this dramatic reduction is the result of ing to note that circuit delays are not being reduced as rapidly

1284 PROCEEDINGS OF THE IEEE. VOL. 67, NO. 9, SEPTEMBER 1979

as the other variables. As a result, future electronic systems should be designed to take advantage of complexity increases by requiring more parallel processing rather than additional speed. These variables and constants support the premise that even more complex instruments can be developed with no rise in cost, and no use in size, weight, and power consumption as well.

The semiconductor industry has found a large and compatible market in the computer industry that has fostered this rapidly growing electronics technology. Medicalelectronics has also benefited from these low cost per function components.

Although the cost of the detection and control electronics in the phased-array scanners is only moderately higher than that of the single-transducer systems, the overall difference in system price is much greater than this modest amount. This phenomenon is similar to one found in the computer industry where the price of the central processing unit is small in comparison to the price of the peripheral inputloutput devices required for the successful operation of the computer. In phased-array ultrasonic scanners, the input device (or system front end) contains the transducer, interface preamplifiers, and beamsteer/focus delay lines; the output device is the scan converter/cathode ray tube. The actual investment in microprocessor-control and image-detection electronics is small, and this portion of the system contains most of the electronic complexity. Current research projects using custom silicon integrated-circuit devices for ultrasonic phased-array frontend signal processing undoubtedly will enhance performance of the front end electronics with little increase in overall system manufacturing cost.

The decision as to whether to use custom or standard integrated circuits depends on such factors as performance, size, power consumption, cost, and reliability. There is great economic advantage to high-volume manufacturing of semiconductor components; one million components per year is considered a nominal number for economical production. Because medical instruments are generally more expensive than electronic wristwatches and calculators, however, custom integrated circuits can be economical when they provide significant performance advantages over standard components. The preprocessor electronics of an ultrasonic imaging system is a likely candidate, therefore, because of the performance demand, size, power consumption, and reliability of this application. Digital scap-converter circuits have benefited from computer and television research, and standard integrated circuits are available for these applications.

Digital electronics is now appearing in the imaging-system output devices, particularly in the CRT scan converter. Large amounts of semiconductor memory are used to store the image temporarily and then to read it out compatibly with television displays and video-tape equipment. These output devices have benefited directly from research in the computer and television industries and will most likely continue to do so in the future. Due to the economics of volume manufacturing and the existence of suitable components, standard integrated circuits will likely prevail for display electronics.

Advanced custom and standard integrated-circuit technology will lead to imaging systems designed for greater performance and diagnostic capabilities as well as enabling the development of portable devices, such as the hand-held electronic near-field scanner in Fig. 25. The benefits are many for systems that utilize well-chosen combinations of custom and standard integrated circuits.

Fig. 25. Hand-held portable battery-powered electronic scanner.

V. CONCLUSIONS

Trends in the development of advanced instruments for ultrasound in medicine have been described and analyzed. These trends indicate that performance can be greatly en- hanced by the implementation of sophisticated multitrane ducer arrays and electronic circuitry. Although most of the transducer developments in medical instruments have resulted directly from medical needs, the majority of electronic components in ultrasonic systems were originally designed for computer and consumer applications. Custom integrated circuits developed for medical ultrasonic applications most likely will have a major impact on the design of future generations of instruments, particularly in such areas as 1) beamsteer/focus delay-line arrays, 2) acoustic-transducer-compatible digitally controlled amplifier arrays, 3) Doppler-signal spectral-processing arrays, 4) high-power low-noise transmitlreceive multiplexer arrays, 5 ) digital scan conversion and imageprocessing circuits, and 6) multielement transducer arrays.

The future of ultrasound in medicine is very promising because critical diagnostic information can be obtained non- invasively. New imaging instruments will have higher resolution and wider dynamic range than the present instruments through the use of custom integrated-circuit electronics developed for medical applications. Advanced quantitative data analysis and diagnostic functions should be incorporated into imaging systems to assist in localizing a measurement. Ultrasonic imaging in the medical field is now widespread. The actual success of future instruments will be determined by the high quality of their performance made possible, most likely, through custom integrated circuits.

ACKNOWLEDGMENT

The authors gratefully acknowledge the valuable contribu- tions of Drs. J. Shott, J. Plummer, T. Walker, and A. Susal of Stanford University, Stanford, CA. They are also grateful to Dr. R. Popp of Stanford Medical Center.

PROCEEDINGS OF THE IEEE, VOL. 67, NO. 9, SEPTEMBER 1979 1285

REFERENCES

[ 11 J. L. M a u and G. B. Kolata, “Combating the #1 Killer, The

vancement of Science, Washington, DC, 1978. Science Report on Heart Research,” Amer. h c . for the Ad-

[2 ] G. Devey and P. N. T. Web, “Ultrasound in m e d i d diagnosis,” Scientifi Amer., vol. 238, no. 5, pp. 98-113, May 1978.

[ 31 P. N. T. Web, Physical Principles of Ultmsonic Diagnarlp. New York: Academic Press, 1969.

[ 4 ] P. N. T. Wells, ibid. ( 5 1 J. M. G f l i and W. L. Henry, “A sector scanner for real-time

two-dimensional echoaudiography,” Cirdut ion, vol. 49, pp.

[ 61 R. C. EggIeton e t al., “Visualization of cardiac dynamicswith real- 1147-1152, June 1974.

time Emode ultrasonic scanner,” Circulation, vot. 50, pp. 21 52- 2159, Dec. 1974.

(71 G. Kosoff, Ultrasonics,vol. 10,p. 221, 1972. (81 N. Born et f., ‘‘Multiscan echocardiography. Part I: Technical

[9] I. C. Somer, “Electronic sector scanning for ultrasonic diagno- Desaiption, Circulation, vol. 48, pp. 1066-1074, Nov. 1973.

[ l o ] K. Iinuma, T. Kidokoro, and Y. Takemura, “High resolution ultrasonic imaging systems for medical use,” Toshiba Rev., vol.

[ 1 1 ] A. L. Susal, “Gray scale echography with simultaneous A mode 114, pp. 13-17, Mar./Apr. 1978.

presentation: A new apparatus for high resolution ultrasono-

[ 121 J. T. Walker, A. L. Susal, and J. D. Meindl, “High resolution grams,” J. Clin Ul tmsound, vol. 5, no. 6, pp. 393-397, 1977.

Photo-Optical Instmmentation Engineers Int Tech. Symp., San dynamic ultrasonic imaging,” presented at 22nd Society of

sis,” Ul t7aS0&S, VOI. 6, pp. 153-159, 1968.

[ 131 J. D. Plummer and J. D. Meindl, “A monolithic 200 volt CMOS Diego, Aug. 1978.

analoz switch.” IEEE J. SdidSta te Circuits, vol. SC-11, pp. 806-

[ 141 M. Maginness et al., “Stateaf-the-art in two dimensional ultra- 817, LC. 1976.

sonic transducer array technology,” Med. Phys., vol. 3, no. 5 , pp. 312-318, Sept. 1976.

[ 151 S. Norton, “Theory of acoustic imaging,” Ph.D. dissertation, TR

sity, Stanford, CA, Dec. 1976. No. 4956-2, Stanford Electronics Laboratories, Stanford Univer-

[ 161 F. L. Thurstone and 0. T. Von Ramm, “A new ultrasound imaging technique employing two-dimensional electronic beamsteering,” Acoust. Holography, vol. 5, pp. 249-259, July 1973.

[17] W. A. Anderson et al., “A new real-time phased array sector

Med., vol. 3B, pp. 1547-1558, 1977. scanner for imaging the entire adult human heart,” Ultrasound

[ 181 A. Macovski and S. Norton, “High-resolution Bscan systems using a circular may,” in Acoustical Holography, vol. 6. New York: Plenum Press, 1975, pp. 121-143.

[19] R. Melen, J. D. Shott, B. T. Lee, and C. L. Granger, “Charge coupled devices for holographic and beamsteering sonar system,” in Acoustical Hologmphy, vol. 27. New York: Plenum Res, 1975.

[20] J. D. Shott, R. D. Melen, and I. D. Meindl, “The cascade charge coupled device: A singleahip lens for ultrasonic imaging systems,” 1976 IEEE Solid State Circuits Conf., Philadelphia, PA, pp. 200-201, Feb. 17, 1976.

[‘rl ] M. Eaton, “High performance preprocessor electronics for ultrasound imaging,” Ph.D. dissertation, Stanford University, Stan-

[22] H. E. Mussman, R. W. Dutton, and J. D. Meindl, “A monolithic ford, CA, expected June 1979.

analog signal processor for ultrasonic imaging systems,” Roc. 1975IEEEInt. SolidSmteCircuitsConf., pp. 180-181,229.

[ 231 R. G. Swartz and J. D. Plummer, “Monolithic silicon-PVF, piezoelectric arrays for ultrasonic imaging,” Acoustical Imaging, Key

I241 W. L. Beaver e t al., “Ultrasonic imaging using two-dimensional Biscayne, June 1978.

transducer arrays,” CanliovaJcuhr Imaging and Image Procemhg: Theory and Practice-1 975, SPIE (palos Verdes Estates, CA), pp.

[25] B. B. Brendon, “Ultrasonic holography-A practical system,” in Ultrasonic Imaging and Holography. New York: Plenum Press,

[26] A. F. Metherell, “Holography with sound,” Science, vols. 7, 4 , no. 11, pp. 57-62, 1968.

[27] P. N. T. Wells, Biomedical Ultrasonics. London, England: Aca- demic Press, 1977.

[28] J. F. Greenleaf, S. A. Johnson, S. L. Lee, G. T. Herman, and

acoustic adsorption within tissue from their two-dimensional E. H. Wood, “Algebraic reconstruction of spatial distributions of

acoustic projections,” in Ultrawnic Imaging and Holography, vol.

[29 ] L. Joynt and A. Macovski, “Techniques of in vivo tissue char- 5. New York: Plenum Press, pp. 591-604, 1973.

acterization,” Acoustical Holography, vol. 8, Key Biscayne, FL, May 29, 1978.

[ 301 C. F. Hottinger, “Volume-flow measurement using doppler ultrasound: A unified approach,” Ph.D. dissertation, TR No. 4959-2, Stanford Electronics Laboratories, Stanford University, Stanford, Ca., Aug 1978.

[ 311 L. Gerzberg, “Monolithic power-spectrum centroid detector,”

June 1979. Ph.D. dissertation, Stanford University, Stanford, CA, expected

17-23.

pp. 87-103, 1974.

[ 321 P. N. T. Wells, op cit., Ch. 10. [33 ] G. E. Moore, “Progress in digital integrated electronics,” IEEE

IEDN 1975, Washington, D.C., Dec. 1975, pp. 11-13. [34] G. R. Mohan Rao and J. Hewkin, “64-K dynamic RAM needs

only one 5-Volt supply to outstrip 16-K parts,” Electronics, pp. 109-116, Sept. 28,1978.

Decision Making in Medical Technology Innovation: The Case of Ultrasound

RICHARD A. RETTIG

Abrmrct-Generrl chnrcteristics of i m o m t i o n . i n medical techwlogy, Center for Health Care Technology. Reasons for hmasing f e d d in- which are revealed in ultnswad, are indicated. Institutiod changes in vdmen t rre indicated, the concedlls of cost, risk, and efficacy are deciaion makiug about m e d i d -ogy am dkussed. The basic identified, and the implicrtions for decision making are wrmmnized. dunge is fiom prior reliance upon the nongova~lencll sector to in- +invokememtofthefededl0overnmeqt. Thisdungeisrerrerled m t h e r e g r t l . t i o n o f m e d i c r l ~ . n d t h e ~ t i o n o f t h e N ~ ~ HE development of new medical technology, when

viewed historicallv. reveals several characteristics that _ I

Manuscript received February 27, 1979; revised April 9,1979. The author is with the Rand Corporation, 2100 M Street, N.W., advances in

1 are also seen in the case of ultrasound technology in

Washington, DC 20037. ongmate from sources outside of medical research, drawing

0018-9219/79/0900-1285$00.75 0 1979 IEEE

Application of Integrated Electronics to Ultrasonic Medical Instruments

Documents

Transcript of Application of Integrated Electronics to Ultrasonic Medical Instruments