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Radiography 17 (2011) 109e114

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Radiography

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Developing optimized CT scan protocols: Phantom measurements of imagequality

Francis Zarb a,*, Louise Rainford b, Mark F. McEntee b,c

aDepartment of Radiography, Faculty of Health Sciences, University of Malta, St Luke’s Raod, G’Mangia, Maltab School of Medicine & Medical Science, Health Science Centre, University College Dublin, Belfield, Dublin 4, Ireland

a r t i c l e i n f o

Article history:Received 24 May 2010Received in revised form10 October 2010Accepted 26 October 2010Available online 20 November 2010

Keywords:CTOptimizationPsychophysical testsRadiation doseImage quality

* Corresponding author. Tel.: þ356 23401833.E-mail addresses: francis.zarb@um.edu.mt (F. Z

(L. Rainford), mark.mcentee@ucd.ie (M.F. McEntee).c Tel.: þ353 17166536.

1078-8174/$ e see front matter � 2010 The College odoi:10.1016/j.radi.2010.10.004

a b s t r a c t

Purpose: The increasing frequency of computerized tomography (CT) examinations is well documented,leading to concern about potential radiation risks for patients. However, the consequences of not per-forming the CT examination and missing injuries and disease are potentially serious, impacting uponcorrect patient management. The ALARA principle of dose optimization must be employed for alljustified CT examinations.Dose indicators displayed on the CT console as either CT dose index (CTDI) and/or dose length product

(DLP), are used to indicate dose and can quantify improvements achieved through optimization. Key scanparameters contributing to dose have been identified in previous literature and in previous work by ourgroup.The aim of this study was to optimize the scan parameters of mA; kV and pitch, whilst maintaining

image quality and reducing dose. This research was conducted using psychophysical image qualitymeasurements on a CT quality assurance (QA) phantom establishing the impact of dose optimization onimage quality parameters.Method: Current CT scan parameters for head (posterior fossa and cerebrum), abdomen and chestexaminations were collected from 57% of CT suites available nationally in Malta (n ¼ 4). Current scanprotocols were used to image a Catphan�600 CT QA phantomwhereby image quality was assessed. Eachscan parameter: mA; kV and pitch were systematically reduced until the contrast resolution (CR), spatialresolution (SR) and noise were significantly lowered.The Catphan�600 images, produced by the range of protocols, were evaluated by 2 expert observers

assessing CR, SR and noise. The protocol considered as the optimization threshold was just above thesetting that resulted in a significant reduction in CR and noise but not affecting SR at the 95% confidenceinterval.Results: The limit of optimization threshold was determined for each CT suite. Employing optimizedparameters, CTDI and DLP were both significantly reduced (p � 0.001) by 0e49% and 0e51% respectively.Below the optimization threshold, CR and noise were significantly affected (p � 0.05) but not SR(p � 0.083). The threshold value is the value below which CR and noise were affected.Conclusion: Optimization was effected through the establishment of the limits at which image quality isdetrimentally reduced by dose reduction. This work established the mA, kV and pitch changes necessaryto achieve the optimization thresholds, reducing the dose and the limit at which psychophysical imagequality is significantly decreased. Further work will evaluate these settings in the clinical environment.

� 2010 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.

Introduction

Since the introduction of computerized tomography (CT) in1975, its use has largely replaced imaging modes which were

arb), louise.rainford@ucd.ie

f Radiographers. Published by Else

limited in demonstrating anatomy and pathology or more invasivediagnostic tests. The increased clinical applications of CT are mainlydue to rapid technological developments in this field of imaging.1

It was estimated in 1989 in the UK that despite constituting only2% of all requested radiological examinations, CT contributed toapproximately 20% of the collective dose.2 Over less than 10 years,the use of CT in the UK increased to 5% of all radiological exami-nations contributing to approximately 40% of the collective dose.3

In the USA, despite comprising only 11e13% of all diagnostic

vier Ltd. All rights reserved.

Figure 1. The Catphan�600 CT quality assurance phantom.

F. Zarb et al. / Radiography 17 (2011) 109e114110

ionizing radiation exams, CT is estimated to contribute up to 67% ofthe collective dose. Worldwide, CT constitutes 5% of radiologicalexaminations yet contributes to 34% of the collective dose.4e12

Despite the benefits associated with CT, this increase in exam-inations has led to concern about potential radiation risks whichmust be considered in relation to the benefits of performing the CTexamination. The consequences of not performing the CT exami-nation and missing injuries and disease are potentially serious. Anyradiation induced cancer may have a long latency period, making itdifficult to link radiation effects to past imaging examinations andpatients may be placed at an increased risk of long term radiationeffects.13,14 Thus optimization of clinically required examinationsshould be in line with the ALARA principle.1,15e17

For the purpose of public health and safety, and because of theconsensus expert opinion, the BEIR VII report concluded that for CTa risk exists and hence the linear non threshold (LNT) model isassumed.1,17,18

Therefore, once a CT examination is considered justified andappropriate for the clinical scenario, scan parameters should beoptimized and dose reduction techniques used to perform the CTexamination at the lowest possible dose. This dose should be largerthan the intrinsic detector noise level of the CT detector. Doses in CTshould and can be lowered without compromising image quality.The three most effective ways of reducing radiation dose in CT is bylowering the tube current (mA), tube voltage (kV) and increasingthe pitch. This can lead to an increase in image noise which in turnaffects image quality in terms of both noise and contrast resolution.Noise is the most important image quality aspect as it increaseswith a reduction in dose. Contrast resolution is the ability todiscriminate between small subjects of similar density (low subjectcontrast) at a specified dose.19e23 It is important that whateverreduction in dose is achieved, radiologists must feel confident withtheir diagnosis.1,20,23

Manystrategieshavebeen introduced inCTtoadhere to theALARAprinciple based on modifications in CT scan parameters.19,24e27

The purpose of this study, which was conducted using psychophys-ical measures on a QA phantom, was to investigate the overall effectof manipulating these scan parameters: mA, kV and pitch togetherrather than individually, in order to reduce dose until the imagequality changes become significant at the 95% confidence interval. Inthis way the limit of optimization was identified.

Method

There is only one acute general public hospital in Malta equip-ped with two spiral CT units, one of which dedicated to Accident

Figure 2. (a) Low contrast module

and Emergency (A&E) cases. There are four other private hospitalsand a private clinic equipped with a spiral CT unit each makinga total of seven CT units on the island. All were invited to participatein this survey by a written invitation sent to the hospital admin-istration through the heads of their respective imaging depart-ments. Three hospitals including the public hospital participated.Thus 50% of eligible hospitals and clinics equipped with spiral CTunits participated in the study. Out of a total of seven CT units, four(57%) participated in the survey including three main CT scannermanufacturers and scanner types: Suite 1 e Siemens SomatomEmotion Duo; Suite 2 e Philips Brilliance 64-slice; Suite 3 e GEBrightSpeed 16-slice and Suite 4 e GE LightSpeed 2-slice. The mA,kV, and pitch currently used for CT scanning protocols of the mostfrequent examinations in Malta were collected by means ofa questionnaire based on NRPB W67. The scan protocols of CTexaminations of the head which comprise 43% of all CT examina-tions in Malta uses two separate protocols: one for the posteriorfossa and one for the cerebrum. The abdomen including pelviscomprises 33% and the chest, 9%. These scan protocols were used toscan a Catphan�600 CT QA phantom (Fig. 1). Scan protocols forspine (2%), extremities (2%) and special procedures (11%) were notincluded in this study.

The Catphan� 600 CT QA phantom is a well established QA tooldeveloped through scientific research and direct field experience inthe evaluation of imaging equipment used to objectively measurethe physical and diagnostic performance of CT systems.28,29 The lowcontrast resolution (CR); the high spatial resolution (SR); and theimage uniformity modules within the phantomwere used. The lowCR module consists of contrast target discs arranged in 3 groups

; (b) High resolution module.

Figure 3. Image uniformity module with ROI measurement locations.

F. Zarb et al. / Radiography 17 (2011) 109e114 111

with nominal contrast of 0.3%, 0.5% and 1.0% and decreasing indiameters of 15mm, 9mm, 8mm, 7mm, 6mm, 5mm, 4mm, 3mmand 2 mm. For the purpose of this study the total number ofcontrast disc targets was recorded as a measure of CR (Fig. 2a). Thehigh SR module has a 1 through 21 line pair per centimetre highresolution test gauge. The SR was gauged by the number of linepairs discernible out of the 21 demonstrated on the scanned image(Fig. 2b). The image uniformity module is made up from a materialof uniform density. The CT number of the material is within 2% ofthe density of water at standard scan protocols.29 This module wasused for the measurement of noise. A 3 mm2 circular region ofinterest (ROI) was used to take measurements of the mean andcorresponding standard deviations which were recorded from thecentre and 4 peripheral locations on the module (Fig. 3). Theaverage of the standard deviations’ readings was taken as anaverage of the noise within the image.

The phantom was scanned on each of the 4 CT units with thecurrent scan protocol for the 4 different anatomical regions (Tables1e4). The scanned length was kept constant at 20 cm. CTDI and DLPreadings were recorded as an indication of the dose per slice andthe total examination dose respectively.

The Catphan� 600 was re-scanned using systematically lowerscan parameters. As mA is the most significant contributor to CRand noise, in the first instance the mA setting was reduced in stepsand CR, noise and SR readings were recorded. When the lowest mAreading which produced a change in all three criteria: CR, noise andSR was observed by the research team, the limit of mA reduction

Table 1Current and new scanning protocols for Posterior Fossa.

Post Fossa mA kV Pitch CTDI (mGy)

Suite 1 Old 89 130 0.3 19.6%change �24 0 0 �23New 68 130 0.3 15

Suite 2 Old 471 120 0.67 39.6%change �33 0 0 �33New 314 120 0.67 26.6

Suite 3 Old 160 120 0.88 39.2%change 0 �17 0 �37New 160 100 0.88 25.3

Suite 4 Old 130 120 1.5 33.2%change �23 0 0 �23New 100 120 1.5 25.5

was considered to be achieved and the mA step previous to thelower limit was selected. This identified mA setting was appliedwith kV and the kV systematically reduced until again changes inall of: CR, noise and SR were seen. Once again the kV level justabove this limit was chosen. This was followed by changes to pitchin conjunction with the identified mA and kV settings until onceagain changes in all three of: CR, noise and SR were seen. Thecombination of lowest mA, lowest kV and lowest pitch resulted inthe identification of the limit of optimization below which signifi-cant changes would be seen to CR, noise and SR.

The Catphan� 600 images produced by the different protocolswere evaluated by 2 observers who were not aware of the changesin the protocols. The two observers were considered as experts inCT, one being a medical physicist and the other a senior CT radi-ographer. The medical physicist, originally a radiographer, is theonly medical physicist with a specialization in diagnostic imagingin Malta and co-ordinates the QA programs and equipment tests,and therefore is experienced in viewing and evaluating QA imagesproduced from QA phantoms. The senior radiographer is the headof one of the CT suites from the participating hospitals and hadbeen involved in the set up of a new CT suite, thus having experi-ence in the commissioning of 2 CT units. The radiographer hasmorethan 5 years experience in the field. The choice of observers wasbased on their expertise and the type of images being evaluated.The images are not clinical images but images produced from QAphantom tests. The evaluation of the images was undertaken on anAdvantage Workstation (AW) v. 4.3_07. Advantage Workstation isone of the most widely used 3D visualization and analysis solu-tions.30 Display conditions were conducting as recommended inthe American Association of Physicists in Medicine (AAPM) taskgroup 18.31

Descriptive and inferential statistics were used in the analysisand presentation of the data. These are tabulated in Tables 1e5.Since the sample data followed a normal distribution, as demon-strated by the K-S test, the paired sample t-test was used tocompare the means of dose and image quality of the current andnew protocols32 (Table 5). The Statistical Package for Social Sciences(SPSS) version 17 was used to compute statistics.

Results and discussion

Seventy different scan parameter changes for the 3 CT exami-nations including 4 anatomical regions: posterior fossa, cerebrum,abdomen and chest, for the 4 CT suites were assessed to identify thelimit of optimization. New scan protocols for each specific CT suitewere determined at the limit of image quality change. Below thislimit CR and noise are seen to be significantly reduced (p � 0.05)

DLP (mGycm) SR(L/pairs) CR (spheres) Noise (SD)

406 6 17 7.3�23 0 �12 þ3311 6 15 7.5

1037 7 25 3.8�33 �14 �12 þ27698 6 22 4.82

834 7 18 10.9�36 0 0 þ28537 7 18 14

717 7 23 3.4�23 �14 0 þ24552 6 23 4.2

Table 2Current and new scanning protocols for Cerebrum.

Cereb mA kV Pitch CTDI (mGy) DLP (mGycm) SR (L/pairs) CR (spheres) Noise (SD)

Suite 1 Old 113 130 0.5 26 564 6 26 2.62%change �24 �15 0 �49 �49 0 0 þ50New 86 110 0.5 13.3 289 6 26 3.92

Suite 2 Old 471 120 0.67 39.6 1037 7 25 3.8%change �33 0 0 �33 �33 �14 �12 þ27New 314 120 0.67 26.6 698 6 22 4.82

Suite 3 Old 140 120 0.88 34.3 743.4 7 22 4.89%change 0 0 þ53 �35 �35 0 �18 þ25New 140 120 1.35 22.3 486 7 18 6.1

Suite 4 Old 130 120 1.5 30.22 698 7 30 2.59%change �23 0 0 �23 �23 0 0 þ4New 100 120 1.5 23.2 537 7 30 2.7

F. Zarb et al. / Radiography 17 (2011) 109e114112

without significant impact on SR. Above this threshold CR and noisewere not significantly affected (p � 0.05).

Scan parameters, CTDI, DLP, CR, noise and SR for all 4 anatomicalregions and CT suits are presented in Tables 1e4, together withtheir respective percentage (%) changes at the threshold level.

A decrease in mA (20%e33%) produced the same percentagedecrease in CTDI and DLP (20%e33%). However, the same decreasein mA also increased the noise (3%e27%). These findings are inbroad agreement with McNitt-Gray33 who concluded that a 50%reduction in mA produces a 50% reduction in dose together witha 40% increase in noise.33 Several studies investigating the use oflow mA values in specific anatomical regions such as the head,chest and abdomen, have been performed showing that it ispossible to reduce the mA values and still obtain images adequatefor diagnosis with a considerable reduction in patient dose.25,34e45

The subsequent reduction in kV (14%e17%) reduced the dose by32%e38% with a considerable increase in noise (16%e29%). Thisconcurred with the findings by Sigal-Cinqualbre et al.,46 andNakayama et al.,47 who concluded that for abdominal and chestcontrast enhanced scans, decreasing the kV allowed the amount ofcontrast material administered to be reduced without compro-mising image quality.46,47 The alterations in kV on the differenttypes of scanners produce different levels of noise increase attrib-uted to differences in hardware, software and scanner designs.35

The final changes (where achievable) involved an increase of thepitch by 0%e69%. This produced a decrease in dose (35%) and anincrease in noise (25%). As pitch increases, the time at any one pointin space spent in the x-ray beam also decreases, hence a lesser doseis given to that point. Dose is inversely proportional to pitch.33,48

Crawley et al.,49 suggest that a 4-fold decrease in dose producesa 2-fold increase in noise.49 This current study concurred with their

Table 3Current and new scanning protocols for Abdomen.

Abdo mA kV Pitch CTDI (mGy)

Suite 1 Old 105 130 1.2 7.56%change 0 �15 0 �38New 105 110 1.2 4.69

Suite 2 Old 375 120 0.89 16.5%change �20 0 þ69 �20New 300 120 1.5 13.2

Suite 3 Old 300 120 0.94 14.8%change �33 0 �47 �49New 200 120 1.38 7.6

Suite 4 Old 100 120 1.5 5.09%change �20 0 0 �20New 80 120 1.5 4.07

findings, where the greatest dose reduction (49%e51%) was ach-ieved by a combination of reduction inmA and increase in pitch butwith a 70% increase in noise. A 49% decrease in dose with a lesserincrease in noise (50%) was obtained by combining a reduction ofboth mA and kV. SR was not affected by the changes in scanparameters until the optimization threshold was achieved andwhen it was affected the SR decreased by 14% during scanning ofthe posterior fossa and cerebrum in CT suites 2 and 4. As expected,the quality parameter mostly affected due to the increase in noisewas the CR which had a decrease of 7%e23%.

There is a significant difference in both CTDI and DLP (p� 0.001)when using the new scan protocol. Below the threshold of the newprotocol there is a significant reduction in CR and noise (p � 0.05),while there is no difference in SR between the current and newprotocol (p � 0.083). The threshold value is the value below whichCR and noise were affected.

For the 2 scanners having the option of automatic tube currentmodulation (suites 2 and 3), this optionwas switched off during thescanning of the Catphan� 600 and the mA was manually reduced,as otherwise there would not have been any changes to mA sincethere is no variation in the scanned phantom. No change in mAbetween the old and new protocol is an indication that the lowestlimit of mA is already being used.

Image noise is generally considered as the most importantimage quality parameter in CT for the detection of low contrastobjects. Quantum noise is the primary contributor to noise in theimage however other noise components include: detector anddigitization noise which are usually of negligible importance. Thetotal number of detected x-ray photons that determine the amountof noise on the image is affected by a number of factors which arewithin the control of the radiographer such as mAs, kV and pitch.50

DLP (mGycm) SR (L/pairs) CR (spheres) Noise (SD)

171 7 26 5.26�38 0 �23 þ16106 7 20 6.08

445 7 23 5.5�18 0 0 �13367 7 23 4.76

333 6 14 9.02�51 0 �7 þ70163 6 13 15.3

118 7 22 4.32�20 0 0 þ1194 7 22 4.8

Table 4Current and new scanning protocols for Chest.

Chest mA kV Pitch CTDI (mGy) DLP (mGycm) SR (L/pairs) CR (spheres) Noise (SD)

Suite 1 Old 75 130 1.2 4.32 95 7 15 7.4%change 0 �15 0 �38 �38 0 �27 þ29New 75 110 1.2 2.68 59 7 11 9.56

Suite 2 Old 300 120 0.89 13.2 356 7 23 5.44%change 0 0 �28 0 �3 0 0 �18New 300 120 0.64 13.2 345 7 23 4.44

Suite 3 Old 300 120 0.94 14.8 333 6 14 9.02%change �33 0 þ47 �49 �51 0 �7 þ70New 200 120 1.38 7.6 163 6 13 15.4

Suite 4 Old 150 140 1 2.15 45.1 7 19 8.76%change 0 �14 0 �32 �32 0 0 þ24New 150 120 1 1.47 30.9 7 19 10.9

Table 5Results of paired sample t-test for current and new protocol.

Mean Std. Dev 95% Lower confidenceintervals Upper

t d.f. p-value

CTDI 6.75 4.95 4.1 9.39 5.45 15 0.000DLP 156.05 116.52 93.97 218.14 5.36 15 0.000SR 0.19 0.4 �0.03 0.4 1.86 15 0.083CR 1.5 1.9 0.47 2.53 3.1 15 0.007Noise �1.59 2.04 �2.68 �0.5 �3.11 15 0.007

F. Zarb et al. / Radiography 17 (2011) 109e114 113

The changes to these scan parameters were investigated as to howthese affect the patient dose in terms of CTDI and DLP and imagequality in terms of CR, SR and noise.

The effect on image quality and dose by the changes in the scanparameters is not the same on each CT unit but depends on the typeand design of scanner; hence protocols need to be specificallydesigned for the particular CT scanner, whether single slice, dualslice or multi-slice and whether automatic mA is an option or not.

Conclusions

In this study we have demonstrated that changes in scanparameters can have a great impact on CTDI and DLP in CT. The mA,kV and pitch can be changed to reduce the dosewith no effect on SRbut with a reduction in CR and an increase in noise. The threshold ofmA, kv and pitch changes have been identified below which imagequality is significantly affected.

The effects of the decrease in CR and increase in noise have beeninvestigated using a Catphan� 600 QA phantom. The changesreflected in the phantom images have been evaluated however willthe same effects be manifested in clinical images? The effect of thescan parameter changes on image diagnosis will be investigated ina future phase of this study where the new protocols will be usedon an animal model and the images evaluated by visual gradinganalysis. The results of the second phase of the studywill be used torefine the new protocols before clinical use.

Acknowledgements

The authors would like to thank all participants who contrib-uted to the collection and analysis of the data.

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