Download - Abdominal Multislice CT for Obese Patients: Effect on Image Quality and Radiation Dose in a Phantom Study

Abdominal Multislice CT for Obese Patients: Effecton Image Quality and Radiation Dose in a

Phantom Study1

Sebastian T. Schindera, MD, Rendon C. Nelson, MD, Ellie R. Lee, MD, David M. DeLong, PhD, Giao Ngyen, MSGreta Toncheva, MS, Terry T. Yoshizumi, PhD

Rationale and Objectives. To evaluate the effect of a modified abdominal multislice computed tomography (CT) protocolfor obese patients on image quality and radiation dose.

Materials and Methods. An adult female anthropomorphic phantom was used to simulate obese patients by adding oneor two 4-cm circumferential layers of fat-equivalent material to the abdominal portion. The phantom was scanned with asubcutaneous fat thickness of 0, 4, and 8 cm using the following parameters (detector configuration/beam pitch/table feedper rotation/gantry rotation time/kV/mA): standard protocol A: 16 � 0.625 mm/1.75/17.5 mm/0.5 seconds/140/380, andmodified protocol B: 16 � 1.25 mm/1.375/27.5 mm/1.0 seconds/140/380. Radiation doses to six abdominal organs and theskin, image noise values, and contrast-to-noise ratios (CNRs) were analyzed. Statistical analysis included analysis of vari-ance, Wilcoxon rank sum, and Student’s t-test (P � .05).

Results. Applying the modified protocol B with one or two fat rings, the image noise decreased significantly (P � .05),and simultaneously, the CNR increased significantly compared with protocol A (P � .05). Organ doses significantly in-creased, up to 54.7%, comparing modified protocol B with one fat ring to the routine protocol A with no fat rings (P �.05). However, no significant change in organ dose was seen for protocol B with two fat rings compared with protocol Awithout fat rings (range �2.1% to 8.1%) (P � .05).

Conclusions. Using a modified abdominal multislice CT protocol for obese patients with 8 cm or more of subcutaneousfat, image quality can be substantially improved without a significant increase in radiation dose to the abdominal organs.

Key Words. Computed tomography; abdominal imaging; image quality; radiation dose.©
AUR, 2007
Obesity is a growing, major public health problem inmany countries around the world (1–3). According to arecent article in the Journal of the American Medical As-sociation, about one-third of adults in the United States—

Acad Radiol 2007; 14:486–494

1 From the Department of Radiology, Duke University Medical Center(S.T.S., R.C.N., E.R.L., D.M.D., T.T.Y.), and Division of Radiation Safety(G.N., G.T., T.T.Y.), Duke University Medical Center, Durham, NC 27710.Received January 3, 2007; accepted January 26, 2007. Address corre-spondence to: R.C.N. e-mail: [email protected]

©
AUR, 2007doi:10.1016/j.acra.2007.01.030
486

more than 90 million people—are obese (defined as abody mass index of 30 or higher) (3). Furthermore, a sig-nificant increase in the prevalence of obesity in male ado-lescents was observed between 1999 and 2004 (3). Obe-sity is associated with various medical conditions, includ-ing cardiovascular disease, type 2 diabetes mellitus,cholelithiasis, abdominal hernias, and different types ofcancers (eg, breast and colon cancer) (4).

The use of computed tomography (CT) as a diagnostictool plays a major role in the work-up of obese patientswith abdominal comorbidities or postoperative complica-
tion after bariatric surgery. However, insufficient image

Academic Radiology, Vol 14, No 4, April 2007 ABDOMINAL MULTISLICE CT FOR OBESE PATIENTS

quality is frequently a concern with abdominal CT exami-nation of these patients, owing to a greater absorption ofthe x-ray beam by the subcutaneous and visceral fat.Fewer incident photons contribute to image formation,which results in increased image noise. To obtain diag-nostic-quality images, the radiologist needs to modifyexisting CT protocols when examining obese patients.One effective and practical approach to improve imagequality in obese patients is to increase the x-ray tube out-put (5). In a corpulent patient, the radiation dose to theabdominal organs is expected to be smaller than in a slimpatient using the same CT protocol because the additionalsubcutaneous and visceral fat might serve as unintendedradiation protection by attenuating more incident photons.To date, no reports, to the best of our knowledge, on im-age quality and radiation dose for abdominal multisliceCT scan in obese patients have been published in the sci-entific literature.

Therefore, we conducted an anthropomorphic phantomstudy to evaluate the effect of a modified abdominal mul-tislice CT protocol for obese patients on image qualityand radiation dose.

MATERIALS AND METHODS

Anthropomorphic PhantomAn adult-sized, whole-body female anthropomorphic

phantom (Model 702; CIRS Inc., Norfolk, VA) was usedto determine radiation dose and image quality. Phantomsfabricated of epoxy resins simulate accurately the physicaldensity and x-ray interaction of various human tissues. Thephantom was composed of 38 sectional slabs (2.5 cm thick)with drilled holes in which to insert the dosimeters. The ab-dominal cross-sectional diameter measured 18 � 22 cm.

To model an obese patient, the anthropomorphic phantomwas modified by adding one or two custom-built, 4 cm thickcircumferential layers of fat equivalent material (CIRS Inc.)with a CT attenuation of �80 Hounsfield units (Fig 1). Tosimulate a patient with 4 or 8 cm of subcutaneous fat, oneor both fat rings, respectively, were placed around the phan-tom’s upper abdomen at the level of the thoracolumbar junc-tion (T12-L3). Before ordering the two fat rings, one authorat our institution (E.R.L.) measured the subcutaneous fatportions of 15 obese patients (10 females, 5 males, agerange: 20–71 years) with a body mass index greater than 30undergoing an abdominal CT examination. The thickness ofthe subcutaneous fat was measured on one axial CT image
at the level of the umbilicus. Patients were measured three
times at three different sides: anterior, posterior, and lateral.All measurements were performed on a separate workstation(Advantage Windows 4.2; GE Healthcare Inc., Milwaukee,WI) using an electronic ruler. The measurements were al-ways placed perpendicular to the skin surface. The anteriorand posterior measurements were performed in the lateralone third of the anterior or posterior subcutaneous fat por-tion, the lateral measurement in the middle one third of thelateral subcutaneous portion. The patients’ subcutaneous fatmeasured 3.1–7.7 cm laterally, 3.0–5.8 cm anteriorly, and3.0–7.9 cm posteriorly. Thus the one or two 4-cm fat ringsrepresent the range of fat thickness of obese patients. Thecross-sectional diameter of the phantom at the level of theupper abdomen was 18 � 22 cm without fat rings, 26 � 30cm plus one fat ring, and 34 � 38 cm plus two fat rings.Each fat ring measured 10 cm in the longitudinal directionand covered four sectional slabs (#21–24) in the upper abdo-men (Fig 2).

CT ScanningImaging of the anthropomorphic phantom was per-

formed on a 16-slice CT scanner (LightSpeed 16; GEHealthcare Inc.). The CT protocol consisted of two com-ponents: protocol A demonstrating the standard CT pa-rameters in our institution for abdominal-pelvic CT scans;protocol B demonstrating the manually adjusted CT pa-rameters for an obese patient. A detailed overview of theCT scanning parameters and the estimated dose length

Figure 1. Anthropomorphic phantom encased with two 4 cmthick fat rings covering the abdominal portion.

product of the two protocols are given in Table 1. Our

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SCHINDERA ET AL Academic Radiology, Vol 14, No 4, April 2007

goal was to make simple modifications into our routineabdominal-pelvic CT protocol with the intent of improv-ing the image quality while maintaining breathholdingcapability. Therefore, the manual adjustment of the modi-fied protocol B consisted of three parts: first, the tubecurrent-time product was increased twofold by doublingthe gantry rotation time; second, the section collimationwas doubled; and third, the table feed per gantry rotationwas increased from 17.5 mm to 27.5 mm. Because, forprotocol A, we had already applied the highest tube cur-rent allowed by the CT scanner (380 mA), we were notable to further increase the tube current for protocol B.The anthropomorphic phantom was scanned with protocolsA and B in three different setups. During the first setup, thephantom was scanned without any fat ring. In the secondsetup CT images were acquired with one 4-cm fat ring andin the third setup acquired with both 4-cm fat rings (Fig 3).

The longitudinal scan coverage was 15 cm, which cov-ered the four sectional slabs (#21–24) of the upper abdo-men and 2.5 cm cranial and caudal of them (Fig 2). Toavoid errors in the radiation dose and image quality causedby misplacement of the phantom, great effort was made toaccurately align the phantom with the gantry isocenter.

Detector Calibration MethodFor measurements of the radiation dose to the deep

Figure 2. Scout computed tomography (CT) image of the anthro-pomorphic phantom demonstrates the four sectional slabs (#21–24)in the upper abdomen covered by the fat rings. Please note that theincreased radiolucency of the upper abdomen in comparison to thechest and pelvis is caused by a rectangular hole in a wooden boardon which the phantom was placed for the CT scans.

upper abdominal organs and to the skin, we used a

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metal oxide semiconductor field effect transistor(MOSFET) system (Model TN-RD-60; Thomson-Nielsen, Ottawa, Canada) with high-sensitivity radiol-ogy TN-1002RD dosimeters. The MOSFET reader wasconnected to a laptop computer (Latitude; Dell, RoundRock, TX) and the data were read immediately aftereach CT exposure.

We calibrated the MOSFET detectors as follows: firstwe determined the thickness of copper sheets to achievethe half-value layer (7.24 mm aluminum at 120 kV) ofthe CT scanner with a conventional radiographic x-raytube (7); then we added 0.2 mm copper sheets to thex-ray tube to obtain an equivalent half-value layer of 7.37mm aluminum at 120 kV. Then individual MOSFET de-tectors were calibrated at our clinical energy level of 140kV. During calibration, our detectors were placed side byside with an ion chamber (10 � 5–6; Radcal, Monrovia,CA). Radiation exposure was read with a radiation monitor(Model 9015, Radcal), which has a built-in function for au-tomatic temperature and pressure corrections. Conversionfrom exposure to absorbed dose was computed by multiply-ing by an f-factor of 0.94 at 140 kV (7).

Radiation Dose Assessment andStatistical Analysis

To measure radiation dose to the upper abdominal or-gans, 14 MOSFET dosimeters were placed in predrilled

Table 1Multislice CT Scanning Parameters

Standardprotocol A

Modifiedprotocol B

Detector configuration 16 � 0.625 mm 16 � 1.25 mmPeak kilovoltage (kV) 140 140Tube current (mA) 380* 380*Gantry rotation time (seconds) 0.5 1.0Tube current-time product (mAs) 190 380Beam pitch 1.750 1.375Table feed per gantry rotation

(mm) 17.5 27.5Table speed (mm/second) 35 27.5Reconstructed slice thickness

(mm) 5 5Estimated dose length product

(mGy/cm)† 714.9 1853.7Acquisition time (seconds)† 14.3 18.2

*Maximum allowed by the manufacturer.†Estimated for a scan coverage of 500 mm.

holes of the anthropomorphic phantom at the following

and


six abdominal organ locations: the right lobe of the liver(n � 4 MOSFET detectors), stomach (n � 4), gallbladder(n � 2), left kidney (n � 2), spleen (n � 1), and body ofthe pancreas (n � 1). The three phantom setups with im-bedded MOSFET detectors were scanned a total of fourtimes with both protocols A and B, for a total of 24

Figure 3. Images of the three phantom sthe left were acquired with protocol A; imagImages in the first row represent the first srow, the second setup, the phantom encarow, the third setup, the phantom encasedin the second row demonstrates an examplacement in the soft tissue (white ellipse)

scans.

For skin dose measurements, 10 MOSFET detectorswere placed, according to the three setups, either on thesurface of the anthropomorphic phantom or on the surfaceof the fat rings. The detectors were contiguously placedangled approximately 45° to the z-axis. Because eachMOSFET has an epoxy-protected sensitive part and

s using protocols A and B. Images onthe right with the modified protocol B.without a fat ring; images in the secondy 1 fat ring; and the images in the third

two fat rings. The image on the left sidef the location of the region of interestin the fat ring (white circle).

etupes onetupsed b

byple o

lead wire, we found it easier to angle detectors 45°

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rather than place them side by side along the z-axis.Figure 4 illustrates the exact arrangement of the detec-tors. Because the helical beam pattern of the multisliceCT exposes the detectors in a random manner, creatinghigh and low patterns of exposure readings, we took aconservative approach to determine a representativeskin dose by accepting the four highest readings of oneCT acquisition.

The mean radiation dose and its standard deviationwere computed for each of the six abdominal organs andthe skin. For the statistical analysis of the radiation dose,we compared the radiation dose of the phantom setupwithout a fat ring scanned with the standard protocol A(representing the baseline radiation dose) with the radia-tion dose for the phantom setup with one or two 4-cm fatrings scanned with the adjusted protocol B because webelieve that an obese patient can receive approximatelythe same abdominal organ dose as a slim patient. We de-termined the percentage difference in the abdominal organand skin doses between the first phantom setup without afat ring scanned with protocol A and the second and thirdsetups scanned with protocol B. For the abdominal organdose, values were compared by a repeated measures anal-ysis of variances, with the multiple dose measurementswithin a run comprising the repeated measure. The spleenand the pancreas were analyzed with a one-way analysisof variance, because each run included one measurement.For the skin dose, the four highest measurements of the

Figure 4. Arrangement of the metal oxide semiconductor fieldeffect transistor detectors for the skin dose measurements. Theywere placed in a contiguous manner and angled approximately45° to the z-axis.

three different setups were compared with the Wilcoxon

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rank-sum test. A P value of less than .05 was consideredto indicate a statistically significant difference. All statisti-cal analyses were performed with SAS software system,version 9.1.3 (SAS, Cary, NC).

Image Quality Assessment andStatistical Analysis

Image quality assessment was performed on recon-structed 5 mm thick images by measuring the image noiseand contrast-to-noise ratio (CNR) of the soft tissue-to-subcutaneous fat. The three phantom setups were scannedonce without the MOSFET detectors (thereby eliminatingstreak artifacts) using protocols A and B. Region-of-interest(ROI) measurements were obtained by one investigator(S.T.S.) on three different images for each of the threephantom setups scanned with protocols A and B. ROIswere measured in the soft tissue (approximately 2000mm2) and in the fat ring (approximately 800 mm2). Thelocation of ROI placement and area were identical foreach of the three measurements. An example of the exactlocation of the placed ROIs for the soft tissue and the fatring is demonstrated in Fig 3. Image noise was defined asthe standard deviation (SD) of the ROI value measured inthe soft tissue of the phantom.

Soft tissue-to-fat CNR was explicitly calculated for thesecond and third phantom setups. CNR was calculated asfollows: (ROIST – ROIFat)/SDST, where ROIST is the atten-uation value of soft tissue, ROIFat is the attenuation valueof the fat ring, and SDST is the standard deviation of thesoft tissue representing the noise. To demonstrate the im-pact of the adjusted abdominal CT protocol B on the im-age noise and soft tissue–to-fat CNR, protocol B wascompared with protocol A on the same phantom setup.The mean and standard deviation, and percentage change,with statistical difference, between protocols A and Bwere calculated for the image noise and soft tissue–to-fatCNR using the two-groups Student’s t-test. A differencewith a P value of less than .05 was considered statisti-cally significant.

RESULTS

Radiation DoseA significant increase, ranging from 37.0 to 54.7%, in

the mean abdominal organ dose was recorded, when com-paring the phantom setup with one 4-cm fat ring scannedwith the modified protocol B to the setup without fat
rings scanned with the standard protocol A (P � .05)

rings


(Table 2, Fig 5). There was, however, no statistical differ-ence in the mean abdominal organ dose between thephantom setup with two 4-cm fat rings scanned with theprotocol B and the setup without fat rings scanned withprotocol A (P � .05). The percentage change in the meanabdominal organ dose for this particular comparisonranged from –2.1% to 8.1% (Table 2).

The skin dose increased significantly for the phantomsetup with one or two fat rings scanned with modifiedprotocol B compared with the setup without a fat ringscanned with the standard protocol A (Table 2) (P �.05). Using the same CT protocol, the mean skin dosedecreased progressively with an increasing number of fatrings. For protocol A, the mean skin dose decreased from35.7 to 26.8 mGy, and for protocol B, from 86.0 to 55.2mGy (Table 2).

Image QualityUsing the modified protocol B for the phantom setup

with one or two 4-cm fat rings resulted in a significantimage noise decrease of 39.8% and 45.8%, respectively,compared with protocol A (Table 3, Fig 6) (P � .05).Thus soft tissue–to-fat CNR increased significantly by65.7% and 80.5% on the images of the phantom setupwith one or two fat rings, respectively, comparing proto-col B with A (P � .05). However, image noise was stillhigher for the phantom setup with one or two fat rings,scanned with the modified protocol B compared withthe setup without fat rings scanned with the standard

Table 2Mean Organ Dose, Mean Skin Dose, and Percent Differences C1 or 2 Fat Rings (4 or 8 cm Fat)

Liver Stomach Gallbla

Prot A0 cm 30.0 � 3.2 31.3 � 5.2 28.5 �

Prot B0 cm 68.2 � 1.5 63.1 � 2.0 67.9 �

Prot A4 cm 19.0 � 1.2 22.1 � 0.8 20.2 �

Prot B4 cm 46.4 � 3.1 48.3 � 3.4 43.6 �

Prot A8 cm 13.7 � 1.5 15.0 � 1.5 13.6 �

Prot B8 cm 30.7 � 2.7 33.8 � 1.6 29.8 �

Prot B4 cm vs. Prot A0 cm

(P value) 54.7% (�.05) 54.6% (�.05) 53.0% (Prot B8 cm vs. Prot A0 cm

(P value) 2.3% (�.05) 8.1% (�.05) 4.7% (

Mean organ and skin dose � standard deviation in mGy.Prot A0 cm: protocol A without a fat ring; Prot B0 cm: protocol B w

Prot B4 cm: protocol B with one 4 cm thick fat ring; Prot A8 cm: prot4 cm thick fat rings; Prot B8 cm: protocol B with two 4 cm thick fat

protocol A.

DISCUSSION

Image noise on abdominal CT examination is ex-tremely important, because high noise levels might ob-scure low-contrast lesions in parenchymal organs such asthe liver, spleen, or pancreas (5,8–10). The limited visi-bility of low-contrast lesions can reduce the diagnosticaccuracy of abdominal CT studies. Therefore, to achievediagnostic-quality images, adjustments of abdominal CT

Figure 5. Radiation dose to different abdominal organs inthree different phantom setups. Gray bar: phantom setup with-out any fat rings scanned with protocol A; spotted bar: phan-tom setup with one 4 cm thick fat ring scanned with protocol B;bar with diagonal lines: phantom setup with two 4 cm thickfat rings scanned with protocol B. Error bars represent �standard deviation. Abdominal organ doses did not increasesignificantly for the phantom setup with two 4 cm thick fatrings compared with the phantom setup without any fat rings(P � 0.05).

aring Protocol A without Fat Ring with Protocol B Including

Left kidney Spleen Pancreas Skin dose

28.0 � 2.7 31.0 � 3.1 28.0 � 1.1 35.7 � 4.753.4 � 1.3 59.7 � 2.1 55.9 � 1.5 86.0 � 8.118.3 � 0.9 18.7 � 0.9 17.8 � 1.0 29.6 � 1.240.4 � 2.7 42.5 � 2.4 41.3 � 1.0 72.0 � 2.112.3 � 1.0 13.9 � 0.9 12.4 � 0.5 26.8 � 3.028.7 � 1.6 30.3 � 1.1 28.5 � 1.4 55.2 � 4.0

44.3% (�.05) 37.0% (�.05) 47.5% (�.05) 101.7% (�.05)

2.4% (�.05) �2.1% (�.05) 1.8% (�.05) 54.6% (�.05)

t a fat ring; Prot A4 cm: protocol A with one 4 cm thick fat ring;A with two.

omp

dder

1.22.40.72.30.72.3

�.05)

�.05)

ithouocol

protocols to accommodate obese patients are needed. Ei-

491


ther manual or automatic adjustments by tube currentmodulation are feasible. Our manual modification of theabdominal CT protocol not only included higher photonfluence (increased tube current-time product), but also anincrease in the section collimation to further decrease theimage noise. Hence the table feed was also adjusted,namely accelerated, to keep the scanning time within thescope of a single, comfortable breathhold. Of the threeadjustments, the most fundamental for improving imagequality is the increased tube current-time product, whichcan be achieved by increasing the tube current or slowingdown the gantry rotation time, or both. Although all theseadjustments increase the radiation exposure, the exact impacton the abdominal organ doses of corpulent individuals hasnot been reported. Are current concerns about an increased

Table 3Mean Image Noise and Contrast-to-Noise Ratio

Image noise(Hounsfield Unites) CNR

Prot A0 cm 5.2 � 0.2Prot B0 cm 3.0 � 0.1% change (P value) �42.3% (�.05)Prot A4 cm 11.3 � 0.4 13.4 � 0.6Prot B4 cm 6.8 � 0.5 25.5 � 1.2% change (P value) �39.8% (�.05) 65.7% (�.05)Prot A8 cm 23.6 � 0.6 5.1 � 0.1Prot B8 cm 12.8 � 0.4 12.3 � 0.7% change (P value) �45.8% (�.05) 80.5% (�.05)

Mean image noise and contrast-to-noise ratio (CNR) � stan-dard deviation.

Prot A0 cm: protocol A without a fat ring; Prot B0 cm: protocol Bwithout a fat ring; % change: percentage change; Prot A4 cm: pro-tocol A with one 4 cm thick fat ring; Prot B4 cm: protocol B withone 4-cm-thick fat ring; Prot A8 cm: protocol A with two 4 cm thickfat rings; Prot B8 cm: protocol B with two 4 cm thick fat rings.

Figure 6. Image noise using protocols A and B in the three dif-ferent phantom setups, 0 cm fat, 4 cm fat, and 8 cm fat. Therewas a significant decrease in image noise comparing the CT pro-tocols B with protocol A for all three phantom setups (P � .05).

radiation dose in obese patients truly justified?

492

Based on the results of the present anthropomorphicphantom study, an obese patient with a subcutaneous fatlayer of 8 cm or more undergoing an abdominal CT ex-amination with the manually adjusted protocol B does notreceive significantly more radiation to the organs than aslim patient undergoing an abdominal CT examinationwith the standard protocol A. This is because the in-creased incident photons of protocol B are attenuated bythe additional subcutaneous and visceral fat in obese pa-tients and so do not reach the abdominal organs. Obesepatients with 4 cm or less subcutaneous fat scanned withthe modified protocol would receive a significant increasein abdominal organ dose compared with slim patientsscanned with the standard protocol. The degree that theextra subcutaneous fat in obese patients serves as an unin-tended form of radiation protection greatly affects theorgan dose in abdominal CT scans. Because the fat depo-sition in obese patients occurs not only in the subcutane-ous region, but also in the visceral and retroperitonealcompartments, resulting in additional absorption of thephoton fluence, our results on the anticipated abdominalorgan doses are very conservative. The study’s resultsalso demonstrated that the exposed radiation dose to theskin of obese patients undergoing an abdominal CT scanwith the modified protocol B would increase significantlycompared with the dose of a slim patient scanned withthe standard protocol A. The highest entrance skin doses,recorded for the phantom setups scanned with protocol B,ranged from 55.2 to 86.0 mGy. This is well below thethreshold dose of approximately 2,000 mGy required tocause radiation-induced deterministic effects such as ery-thema and epilation (11). Using the same protocol, theentrance skin dose decreased steadily with increasing di-ameters of the phantom. We conclude that obese patientsreceive lower radiation doses to the skin than do slimpatients. A plausible explanation for this finding might bethe greater attenuation of the traversing photon fluencethrough the wider girth of the subject, resulting in a lowerradiation detection of the skin detectors located on thesites distal to the x-ray tube.

Adapting the tube current-time product to individualpatient size for abdominal CT scans to maintain imagequality was first described in 1981 by Haaga et al (12).Theoretically, a twofold increase in the tube current-timeproduct reduces image noise by approximately 41%, be-cause the image noise is proportional to the square root ofthe number of incident photons (5). In our phantom study,we found a 40%–46% decrease in image noise comparing
protocol A with protocol B. Extreme increases of the tube


current-time product are required to maintain the samelevel of image noise in obese patients. For example, ac-cording to McCollough et al, to maintain a constant noiselevel in a water phantom with a changing diameter from30 to 40 cm, the tube-current time product has to be in-creased from 130 to 869 mA (13). Even though a con-stant noise level is desirable for obese patients, it is notpracticable due to technical limitations of the CT scannersand the excessive radiation exposure required (13). Be-sides, it has been shown that radiologists tend to accept aslightly greater noise level for diagnostic-quality imagesin corpulent individuals (10,13).

The tube current-time product for abdominal CT scansof obese patients can be selected manually either on thebasis of the patient’s weight or cross-sectional abdominaldimensions. Various reports have shown that both tech-niques are feasible when attempting to optimize the radia-tion dose and acquire diagnostic-quality images (12,14–16).Haaga et al postulated that the use of the patient’s cross-sectional diameter is a more accurate predictor than bodyweight for the individual adjustment of tube current-timeproduct (12,17). Using the body weight alone would ig-nore the patient’s height; that is, a wide variety of ab-dominal diameters exists for patients with the same bodyweight but different heights. For obese patients, the use ofthe subcutaneous fat thickness in addition to the patient’scross-sectional diameter might improve the manual proto-col adjustment.

Besides the manual adjustment of CT parameters, vari-ous types of automatic tube current modulation, angularand z-axis modulation, and combined modulation areavailable in modern multislice CT scanners. Based on thepatient’s anatomy and attenuation characteristics, thesetechniques modulate tube current to maintain constantimage quality at a lower radiation exposure. The z-axismodulation technique, in particular, regulates automati-cally the tube current to obtain images with a preselected,constant noise level independent of the patient’s bodyhabitus (18). Though this technique represents a verypromising tool for semiautomatic optimization of imagequality in obese patients undergoing abdominal CT scans,only very limited data exist on the extent of the increasein the effective tube current and radiation exposure toobese patients scanned with z-axis modulation (8). Futureinvestigation of these novel techniques, with both phan-tom and clinical studies, must concentrate on the potentialimprovement in image quality and the risks of increasedradiation exposure on abdominal CT scans for obese
patients.
There were some limitations to our study. First, ourmodified protocol for obese patients comprised three si-multaneous adjustments instead of adjusting one CT pa-rameter while holding all other parameters constant. Thelatter approach would simplify the comparison of themodified protocol with the standard protocol and the re-sults would be more translatable to scenarios using auto-mated tube current modulation. However, the authors an-ticipated to design a protocol for manual adjustments thatis practicable and yields a substantial increase in imagequality at a required spatial resolution. Second, the twofat rings covered only 10 cm of the upper abdomen in-stead of the entire abdomen. Total coverage of the phan-tom’s abdomen was not technically feasible. A completeabdominal CT examination of the three phantom setupsusing the two protocols should increase the absolute num-bers of the organ and skin doses but not the percentagechange between the setups. Third, the study consisted of afemale phantom and did not include a male phantom.Obese men tend to deposit fat mainly in the visceral re-gion, whereas obese women tend to deposit fat mainly inthe subcutaneous region (19,20). Our female phantom,with a smaller cross-sectional diameter than a male phan-tom’s and without extra visceral fat deposition, simulatesthe worst-case scenario concerning radiation exposure.Furthermore, because our simulation included the increaseof subcutaneous fat layers only and not the increase ofvisceral fat components, our abdominal organ radiationdose results have to be considered higher than those foran actual obese patient of corresponding size. Fourth,changes in the x-ray spectrum from attenuation in the fatring was not taken into account in this study based on thefollowing initial observations: 1) An organ dose compari-son between MOSFET and thermoluminescent detectorsas gold standard using an anthropomorphic adult phantomshowed good agreement, despite the changes in the x-rayspectra from different projection angles (21). This may, inpart, have been due to CT’s unique scanning geometry,360° tube rotation, which minimized the effect ofMOSFET sensitivity changes. 2) A preliminary MonteCarlo study at our institution on effective energy changeas a function of tissue depth showed that MOSFET sensi-tivity change was estimated to be 4%–5% for a 140-kVpbeam at 9-cm tissue depth. Given that the uncertainty ofMOSFET detectors is approximately 10%–15% for typi-cal CT dose measurements, we concluded that the effectof energy dependence on detectors may be ignored. It isalso not practical or feasible to apply sensitivity variation
corrections to individual detectors where detector depth
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may vary at each projection angle. We recognize that fur-ther study is needed in this area. A new investigation iscurrently underway at our institution.

In conclusion, in obese patients with a fat layer of 8cm or more, our anthropomorphic phantom study demon-strates that image quality can be improved on abdominalmultislice CT scans using a manually adjusted CT proto-col without increasing the radiation dose to the abdominalorgans. However, in individuals with a subcutaneous fatlayer of less than 4 cm, the manually adjusted abdominalCT scan for obese patients has to be applied judiciously.Furthermore, it is essential to demonstrate in a clinicalstudy that the manual adjusted abdominal CT protocol hasthe potential to improve not only the image quality inobese patients, but also the lesion detectability, particu-larly those with inherently low-contrast.

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