Reducing Radiation Dose in Body CT: A Practical Approach to Optimizing CT Protocols

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1 Medical Physics and Informatics Review Goldman and Maldjian Reducing Radiation Dose in ody CT Medical Physics and Informatics Review Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved FOCUS ON: lice R. Goldman 1 Pierre D. Maldjian Goldman R, Maldjian PD Keywords: body CT technique, dose reduction, protocol optimization, radiation dose, radiation safety DOI: /JR Received ugust 3, 2012; accepted after revision November 14, oth authors: Department of Radiology, UMDNJ-NJ Medical School, University Hospital, 150 ergen St, UH C-320, Newark, NJ ddress correspondence to. R. Goldman (goldrad@gmail.com). CME/SM This article is available for CME/SM credit. JR 2013; 200: X/13/ merican Roentgen Ray Society Reducing Radiation Dose in ody CT: Practical pproach to Optimizing CT Protocols OJECTIVE. The purpose of this article is to describe an approach to protocol modifications in body CT to reduce patient radiation exposure while maintaining image quality and to illustrate the effects of these modifications. CONCLUSION. One should become proficient in interpreting the CT dose report, adjusting CT technical parameters, and applying focused collimation. The goal is to intelligently use CT to answer the question of clinical concern in the most dose-efficient manner. I n this article we will discuss some general principles regarding the implementation of protocol changes to minimize radiation dose and body CT protocol modifications to reduce patient radiation exposure for specific clinical indications. The radiologist involved in this process should first become familiar with the CT dose reports, estimates of effective dose, and image quality of the CT scans resulting from different departmental CT protocols. Comparing the volume CT dose index ( ) and estimates of effective dose (effective dose = dose-length product [DLP]) with reference values available in the literature can be helpful [1]. lthough assessing image quality is subjective, the amount of tolerable noise does vary with the specific clinical indication for the CT examination. For example, we observed that there was room for radiation dose reduction for renal stone protocol CT scans (studies with high noise tolerance) if there is little discernible noise on the images. One may begin the process of dose reduction with small incremental changes, ideally altering one parameter at a time. t first, protocol modifications should be made in a provisional fashion and the studies should be assessed by the radiologist spearheading the effort. If the results are acceptable, all radiologists responsible for interpreting the relevant studies should be informed to be aware of the changes and to alert the proper individuals if the images are deemed unsatisfactory. Image quality should be assessed as part of the routine interpretation of cases. In addition, a periodic review of protocols and the resultant image quality can be made to assess whether further modifications are indicated. This process should be an ongoing process of continued quality improvement. Indication-specific protocols can be optimized by tailoring CT parameters to patient size. n additional benefit of this iterative approach is that it allows radiologists to adjust, over time, to reading images with gradually increasing noise. Radiologists should become comfortable interpreting lower-dose, highernoise CT examinations because diagnostic efficacy is not improved by more aesthetically pleasing images. The ultimate goal of this approach is to eventually determine the sweet spot where lowest dose, highest tolerated noise, and diagnostic image quality are optimally balanced. y making small, judicious, incremental changes in CT parameters, radiologists can minimize the risk of producing nondiagnostic studies. In our experience of more than 5 years, we have not had any instances of nondiagnostic CT studies as a result of this approach. There have been a handful of cases close to the limit of acceptable image quality that we have found instructive. Patient size is a critical variable for protocol optimization, so protocols are tailored by adjusting CT parameters to patient size. On the basis of the protocols used at Massachusetts General Hospital (Sahani D, written communication, 2011), we classify patients into one of the following four weight categories: less than 135 lb ( 61 kg), lb (62 90 kg), lb ( kg), and more than 300 lb (> 136 kg). For all protocols peak kilovoltage (kvp), noise index (NI), and maximum tube current (m) are modified on 748 JR:200, pril 2013

2 Reducing Radiation Dose in ody CT Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved the basis of the clinical indication and patient weight category, with these parameters typically being increased as weight increases. lthough CT protocol changes have resulted in a marked reduction in patient radiation exposure at our institution, we still continue to refine each indication-specific protocol. simple but robust technique that can be applied to reduce radiation exposure with CT is to strategically limit the anatomic coverage to the specific organs of clinical concern, thereby minimizing the scan length and radiation of other tissues. We have termed this approach of tailoring CT protocols to the anatomy of interest focused collimation. This strategy can be especially useful for lowering dose in multiphase examinations as illustrated later in this article. ltering protocols to minimize the anatomy radiated should be done in consultation with the other radiologists at one s institution because they may have differing opinions regarding the critical anatomy. Depending on the particular indication, discussion with the referring clinicians may also be necessary to ensure that the relevant clinical issues and limitations of the modified protocol are addressed. Estimates of the effective radiation dose to any particular patient from the DLP data are crude; in the following examples, the and DLP data accurately reflect relative dose changes because the same patient is used in each illustrative case, which controls for any patient-specific factors. is helpful in comparing dose when changes are made to CT technical parameters. Comparing scan length directly is helpful when assessing the effects of focused collimation. Changes in DLP reflect the combined effects of modifications to both and scan length. The effective dose is useful for comparing an examination with dose values referenced in the literature. The following examples of protocol modifications are arranged by indication, with an emphasis on unenhanced CT for the detection of urinary tract calculi and multiphase liver CT as paradigms for radiation dose reduction strategies. In addition, in some instances we have found that making individualized protocol adjustments specific to the area of clinical concern is advantageous, especially in patients who require reevaluation by CT over a short period of time. ll the CT examinations that we discuss were performed on a 16-MDCT scanner (Light- Speed 16, GE Healthcare). These cases were selected from an institutional review board approved retrospective review of our experience over a 5-year period of CT protocol revisions to reduce radiation exposure of our patients. Fig year-old man with flank pain. and, xial 5-mm-thick images at level of kidneys from two successive renal stone protocol CT examinations. oth images show hydronephrosis of right kidney (arrows) that occurred because of obstructing calculus in distal right ureter (not shown). was obtained with maximum tube current of 350 m, volume CT dose index ( ) of 11.2 mgy, and dose-length product (DLP) of 460 mgy cm. was obtained with maximum tube current of 275 m, of 8.9 mgy, and DLP of 370 mgy cm. For both studies, peak tube voltage setting was 100 kvp, noise index was 23, and exposure time was 0.8 second. oth studies are of diagnostic quality. Decreasing maximum m resulted in dose reduction of approximately 20%. Single-Phase ody CT Protocols: Renal Stone and bdominopelvic Examinations Modification of unenhanced CT protocols for urinary tract calculi detection is a straightforward starting point to develop an eye for the effects of dose reducing modifications to CT parameters. ecause of the inherent high contrast of urinary tract calculi, radiologists have a higher tolerance for image noise in interpreting these studies [2]. CT parameters that can be modified include the NI, range of tube currents (minimum and maximum tube currents), and peak tube voltage (kvp). Simply reducing the maximum m can result in a significant reduction in dose without impeding image interpretation (Fig. 1). Increasing the NI while lowering the maximum m can also result in significant dose reduction (Fig. 2). In Figure 2, for the initial CT scan, the of 32.5 mgy is too high, exceeding current merican College of Radiology guidelines, which clearly indicates the need for protocol modification. This scan was obtained before CT dose management was implemented at our institution. We now know that the NI was too low and the maximum m was too high for that study. In our experience with unenhanced CT for urinary calculi, a in the range of 3 10 mgy for patients weighing up to 300 lb ( 136 kg) provides diagnostic image quality. s illustrated in Figure 2, when multiple parameters are involved, the effects of changes can lead to confusing results regarding radiation exposure unless the data are analyzed carefully. It is important, when possible, to recognize which parameter is having the greater impact on dose. lthough one would expect dose to decrease when the NI is increased, note that in Figure 2 the lowest exposure is associated with the lowest maximum m and not the highest NI. Therefore, in this example maximum m was the limiting parameter affecting dose for the third CT scan, not the NI. The reason is that when automatic exposure control (EC) is active, the tube current is modulated for each slice to maintain the preselected NI throughout the examination. If one sets the maximum m at a value lower than necessary to maintain the selected noise level, the maximum m overrides the NI setting and results in images with noise levels that exceed the prescribed NI. This information can be ascertained by checking the numeric value for the actual m used for each slice, which can be viewed as an annotation overlay on the corresponding image in the PCS. If the actual m values used for scanning the patient are consistently close to the maximum m, then the maximum m setting is the dose-limiting parameter rather than the NI. For the second CT scan in Figure 2, because the maximum m was never reached, the NI was the limiting parameter. Given the relatively high of 12.8 mgy, adjustments to either the NI or maximum m are warranted. Note that the value of the maximum m is usually not recorded in the PCS, so one needs to be familiar with the designated protocol settings of the maximum m. If this information is not available, one can examine the range of the actual m values used to scan the patient displayed in the PCS, and if the numeric values consistently peak at an upper limit, that upper limit likely represents the maximum m. JR:200, pril

3 Goldman and Maldjian Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved Scan NI m Max (m) m ctual (m) (mgy) DLP (mgy cm) Dose Reduction % % Fig year-old man with nephrolithiasis who underwent three unenhanced CT studies over period of 2 years., Chart depicts changes in noise index (NI) and maximum tube current (m Max) setting with dose data for three studies. ll three studies were performed at 120 kvp; m ctual shows range of tube current values used in each study. Dose Reduction column indicates percentage dose reduction compared with dose from initial scan. Note that in this case lowest dose is associated with lowest maximum m, not highest NI. s explained in text, because actual tube current for third study is consistently at maximum m setting, maximum tube current for third study was dose-limiting parameter, not NI. = volume CT dose index, DLP = dose-length product., Corresponding representative 5-mm-thick images through urinary bladder for each of three CT scans discussed in. SD of attenuation values within region-of-interest caliper (arrows) approximates selected NI for each of first two studies. Discrepancy for third CT scan confirms that selected NI was not maintained. When imaging thin patients with low-dose techniques (low kvp and low maximum m) with EC active, setting the minimum m too low can result in excessive image noise (Fig. 3). ecause of a lack of intraabdominal fat to provide contrast between tissues and organs, the noise tolerance for interpreting CT studies in thin patients is lower than that for larger patients [3]. Therefore, for imaging small patients, when EC is active, either the minimum m should be raised or the NI should be lowered. Once again, evaluating the actual m used for scanning the patient displayed on the image overlay in the PCS is informative. If the actual m is consistently near the minimum m, then the minimum m was the critical factor regarding the lower limit of radiation output rather than the NI. s with maximum m, the value of minimum m is also usually not recorded in the PCS, so one needs to be familiar with the designated protocol settings of the minimum m. If this information is not available, one can examine the range of the actual m values used to scan the patient displayed in the PCS; if the numeric values consistently bottom out at a lower limit, the lower limit likely represents the minimum m. One should note that the default values of minimum m for the CT scanner may be very low (sometimes set to 10 m). This setting may not be consequential until other parameters (such as the NI) are modified. When an appropriate minimum m is chosen, the minimum m setting can override the NI setting in thin patients to avoid unacceptable noise levels. Therefore, for almost all of our adult body CT protocols, we have set the minimum tube current to 75 m. In our experience with unenhanced CT for the evaluation of renal colic, noise tolerance of this examination is increased; therefore, we currently use an NI of 23 for examinations of all our adult patients regardless of size. Lowering the kvp in these studies has also been very effective in minimizing radiation dose. We use 100 kvp for all but the largest patients with steadily increasing maximum m according to patient weight. In thin patients this technique can produce diagnostic studies at estimated effective doses of less than 2 msv without the use of iterative reconstruction (Fig. 3). Focused collimation of the CT examination to cover just the urinary tract is a straightforward method for limiting radiation dose. s illustrated in Figure 4, identification of the location of the kidneys on the CT topogram and appropriately adjusting the scan length can lead to dose reductions of 20% or more with a reduction in scan length, on average, of approximately 10 cm. The CT technologist can attempt to begin scanning as close as possible to the superior margins of the kidneys; even if the upper Fig year-old man with left ureteral calculus. and, Unenhanced 5-mm-thick CT images of abdomen show left hydronephrosis (arrow, ) due to obstructing stone in left proximal ureter (arrow, ). (Punctate high-attenuation focus in porta hepatis in is cholecystectomy clip.) Technical parameters for study were as follows: peak tube voltage of 100 kvp, minimum tube current (m) of 75 m, maximum m of 240 m, noise index (NI) of 23, exposure time of 0.8 second, volume CT dose index of 2.8 mgy, dose-length product of 115 mgy cm, and estimated effective dose of 1.7 msv. lthough study is diagnostic, in this thin patient with little intraabdominal fat, images are grainy reaching limits of tolerable noise. ctual m values used for study ranged from 75 to 96 m indicating that minimum m setting was critical factor regarding lower limit of radiation output rather than NI. If minimum m had been set lower than 75 m, image noise likely would have been unacceptably high. 750 JR:200, pril 2013

4 Reducing Radiation Dose in ody CT Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved Scan NI kvp (kv) m Max (m) Scan Length (cm) (mgy) DLP (mgy cm) Estimated Effective Dose (msv) pole of one kidney is initially missed, additional imaging to include it can easily be performed because there are no timing issues related to the use of IV contrast material. dditional advantages of focused collimation include reduced radiation exposure of the breasts in women and the decreased likelihood of detecting incidentalomas. The combined effects of modifying technical parameters and focused collimation result in dramatic dose reductions. Reducing dose is especially important for controlling cumulative radiation exposure in young patients with benign diseases who require multiple CT examinations (Fig. 4). For routine contrast-enhanced CT of the abdomen and pelvis (such as CT for abdominal pain), dose reductions can be achieved with an incremental reduction in maximum m and an increase in NI; in addition, the peak tube voltage (kvp) setting can be lowered in small patients. ecause of the lower noise tolerance of routine abdominopelvic CT compared with the renal stone CT protocol, we use a lower NI and higher maximum m for routine abdominopelvic CT; a lower kvp setting must be used more judiciously for routine abdominopelvic CT. lso, the NI progressively increases with increasing body weight because greater noise is tolerated in larger patients (for the reasons discussed previously). Each CT protocol is subdivided into the four previously described Fig year-old woman with left ureteral stent who developed encrustation of pigtail portions of stent in renal pelvis and bladder., CT topogram is used to illustrate principle of focused collimation. Scan range for standard CT examination of abdomen and pelvis would extend from just above dome of diaphragm to just below ischial tuberosities (). For evaluation of urinary tract with unenhanced CT, one can limit coverage from top of kidneys to just below bladder (mid pubic symphysis) (). In this case, 20% reduction in scan range results in 20% reduction in doselength product (DLP)., Patient underwent four unenhanced CT examinations of abdomen and pelvis over 4 years for evaluation of urinary tract calculi. Chart depicts changes in noise index (NI), peak tube voltage (kvp), maximum tube current (m), and scan length with dose data for four studies. ll studies were of diagnostic quality. These examinations illustrate cumulative changes resulting from iterative approach we have used over past 5 years in adjusting parameters gradually to decrease patient dose. Reductions in scan length are from aggressive use of focused collimation to urinary tract. Combination of adjusting technical factors and focused collimation results in dramatic dose reduction of 75% since initial scan. For patients who require multiple CT examinations, cumulative doses will be excessively high unless proper steps are taken to manage radiation exposure. In this case, cumulative dose would have been four times higher than necessary if all examinations were performed exactly like first examination. = volume CT dose index. weight-based categories with parameters optimized to patient weight. Multiphase Contrast-Enhanced bdominal CT Protocols for Imaging the Liver and Pancreas ecause of the potential for large radiation doses from multiphase abdominal CT protocols, particular attention to detail is required Fig year-old man with chronic hepatitis C and cirrhosis. and, 2.5-mm-thick arterial phase images from multiphase CT examinations of liver performed 6 months apart for detection of hepatocellular carcinoma. oth images show small peripheral enhancing lesion (arrows). This finding is consistent with small arterioportal shunt because this region was isodense to liver parenchyma on subsequent phases and stable compared with multiple prior studies. For both studies, noise index was 15 and exposure time was 0.8 second. was performed at 120 kvp with maximum tube current of 450 m and volume CT dose index ( ) of 23.5 mgy. was performed at 100 kvp with maximum m of 550 m and of 17.8 mgy. Lowering kvp and increasing m resulted in 24% dose reduction for arterial phase of examination. Lowering kvp may also increase conspicuity of small enhancing lesions. to control radiation exposure, especially given that some patients will undergo repeated examinations for surveillance frequently for benign disease. t our institution, a liver transplant center, we perform a large number of multiphase CT examinations of the liver for the detection of hepatocellular carcinoma in cirrhotic patients. This study serves as a model to show approaches to radiation JR:200, pril

5 Goldman and Maldjian Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved dose management with multiphase CT. Our CT protocol for imaging the cirrhotic liver includes four phases: unenhanced CT of the liver, arterial phase CT of the liver, portal venous phase CT of the abdomen, and delayed phase CT of the liver 3 minutes after the administration of IV contrast material. Image thickness for the unenhanced phase is reformatted to 5 mm, and a 2.5-mm thickness is used for the contrast-enhanced phases. n easy first step to radiation dose management that can result in significant dose reduction is to lower the kvp and increase the maximum m. s an added benefit, because of the increased sensitivity for the detection of contrast enhancement, small enhancing lesions may appear more conspicuous [4] (Fig. 5). When decreasing kvp from 120 to 100 kvp (intervals for kvp changes are preset and smaller increments are not available), one has three choices regarding the radiation dose: maintain the radiation output used at 120 kvp (by raising maximum m at 100 kvp to match the at 120 kvp if the x-ray tube limits allow); accept the full dose reduction of the lowered kvp, which can be considerable, without changing the ms; or offset some of the kvp dose reduction by raising the maximum m to improve image quality. To assess these choices, one can use the, which is displayed on the scanner console, before imaging the patient after the CT topograms have been obtained. The study can be planned first at 120 kvp to check the. The can then be assessed after the kvp is changed to 100 kvp. If the is acceptable, one can proceed without changing the maximum m. Otherwise, the maximum m may need to be raised until the is acceptable. For the multiphase liver studies, we have observed to date that in most patients weighing less than 200 lb (90 kg) a in the range of mgy provides diagnostic-quality images. To achieve this, we offset some of the kvp dose reduction by raising the maximum Fig year-old man who presented for evaluation of cirrhosis and hepatocellular carcinoma. Images are from multiphase CT examination of liver performed for surveillance after liver transplant. High-attenuation punctate foci around inferior vena cava are surgical clips from liver transplant., xial 5-mm-thick image from low-dose unenhanced phase performed at 100 kvp with constant m fixed at 90 m and automatic exposure control (EC) was turned off. Volume CT dose index ( ) was 4.0 mgy and dose-length product (DLP), 79 mgy cm., xial 2.5-mm-thick image from delayed phase performed at 100 kvp and maximum tube current of 550 m. EC was on. was 14.0 mgy and DLP was 279 mgy cm. Exposure time was 0.8 second for both series. Radiation dose for unenhanced phase is 72% less than that for delayed phase and makes up only 5% of total scan dose. Increased reconstructed slice thickness of 5 mm for unenhanced phase () offsets increased noise levels of low-dose technique. Focused Collimation to Liver on Three Phases m from 450 m at 120 kvp to 550 m at 100 kvp. One should be aware that CT images produced with lower kvp settings will likely require manual adjustment of the window and level settings to optimize viewing. lthough some investigators advise omitting the unenhanced phase from the multiphase liver CT protocol to reduce radiation exposure because it rarely contributes to the detection of hepatocellular carcinoma [5], we have found that low-dose unenhanced CT of the liver provides valuable information. For example, the presence of radiopaque material such as iodized oil (Lipiodol, Guerbet) from prior chemoembolization on contrastenhanced images can be problematic without unenhanced images for comparison. In addition, identification of a hypodense lesion on unenhanced images can, on occasion, improve diagnostic confidence if the findings on the contrast-enhanced phases are equivocal. Most importantly, the CT technologist can use the unenhanced series to precisely localize the liver and use the principle of focused collimation to the liver on subsequent phases to further minimize radiation exposure while ensuring complete coverage of the liver on the critical arterial phase. The unenhanced phase can be tightly collimated to the liver to avoid excess radiation because the CT technologist can easily add images if a portion of the liver is initially missed given that there are no contrast timing issues in this phase. For these reasons, we have chosen to retain the unenhanced phase in our protocol but have modified it to significantly reduce the radiation dose. The unenhanced phase is scanned at either 100 or 120 kvp (depending on the patient weight, with the same kvp used for all the subsequent phases) with the tube current fixed at 90 m. The m is manually set at 90 m and the EC is off. This latter step results in constant m output throughout scanning. lthough these images would be noisy at a 2.5-mm slice thickness (because of the Dose for Four-Phase Low-Dose Unenhanced Phase Liver CT (msv) No No 29.2 No Yes % Yes No % Yes Yes % Percentage Dose Reduction Fig. 7 Chart depicts effects of various combinations of focused collimation and low-dose unenhanced phase on estimated dose. For example, estimated effective doses for various phases of multiphase liver CT calculated from CT dose report on one of our typical patients are as follows: low-dose unenhanced phase, 0.8 msv; arterial phase (liver only), 4.3 msv; portal venous phase (diaphragm to iliac crest), 7.3 msv; and delayed phase (liver only), 4.3 msv. With neither focused collimation nor low-dose unenhanced phase, effective dose is estimated at 29.2 msv, which is equivalent of scanning abdomen from diaphragm to iliac crest four times (4 7.3 msv). With both focused collimation and low-dose unenhanced phase, dose is markedly reduced. 752 JR:200, pril 2013

6 Reducing Radiation Dose in ody CT Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved Slice Thickness natomy Scan Length m Max DLP Effective Phase (mm) Scanned (cm) kvp (kv) (m) (mgy) (mgy cm) Dose (msv) Noncontrast 5.0 Pancreas rterial 2.5 Pancreas Venous 2.5 Entire abdomen Total low m), reformatting to a 5-mm slice thickness provides acceptable image quality from reduced noise in the thicker sections. On average, the unenhanced phase accounts for less than 6% of the total dose for a four-phase examination (Figs. 6 and 7). The arterial and delayed phases of the multiphase sequence are predominantly used to evaluate the liver, whereas the portal venous phase includes the entire abdomen (from diaphragm to iliac crest) for complete evaluation. To obtain diagnostic image quality on the thinner 2.5-mm-thick images of the contrast-enhanced phases of multiphase liver CT, one must use lower NIs (for the same reconstructed slice thickness) than those used for routine abdominopelvic CT. Using a lower NI increases the amount of radiation. Therefore, once the liver has been adequately localized on the low-dose unenhanced phase, focused collimation can be applied to include only the liver on the arterial and delayed phases. This limits the scan length in those acquisitions, which can provide significant dose reduction (Fig. 7). The dose reduction for each patient will vary depending on the size of the liver. Patients with small livers will benefit the most. Patients with large livers may not have any benefit, but if the liver extends below the iliac crest on the unenhanced phase, the CT technologist will know to include the entire liver on subsequent contrast-enhanced phases, especially the critical arterial phase. The cumulative effects of dose reduction measures are Fig year-old thin woman weighing 124 lb (56 kg) who presented with epigastric pain. Patient underwent CT of pancreas because of concern for pancreatic malignancy., Chart displays some technical parameters and dosimetric data for different phases of examination. ecause of small size of this patient, peak tube voltage was set at 80 kvp. Exposure time was 0.8 second for all phases. Maximum tube current (m) of 300 m is relatively low based on our experience with multiphase liver studies but it was sufficient in this patient. Combination of low kvp and low maximum m results in low estimated effective dose of only 3.3 msv. Low volume CT dose index ( ) shows effect of low-dose localizing phase and dose-length product (DLP) shows effect of focused collimation to pancreas when comparing unenhanced and arterial phases to venous phase. ggressive radiation dose management is particularly important in small patients. For same amount of incident radiation, internal organs of small patient receive higher radiation doses than those of large patient because of less attenuation of x-rays by overlying soft tissues., xial 2.5-mm image at level of pancreatic head from portal venous phase of study shows good image quality. Pancreas was atrophic and no malignancy was found. magnified in these relatively high-dose studies because patients with liver disease may have multiple frequent CT examinations. We have extended these dose reduction techniques to multiphase CT examinations of the pancreas. Our multiphase pancreatic CT protocol is composed of unenhanced, arterial, and portal venous phase imaging. very-lowdose, fixed-m unenhanced examination with images reformatted at a 5.0-mm slice thickness and limited to the pancreatic region is used solely for the purpose of localizing the pancreas. This examination is performed with the same technique ( kvp and 90 m) as the previously described unenhanced liver CT. The unenhanced phase is followed by arterial phase imaging with focused collimation to the pancreas only and with portal venous phase imaging of the entire abdomen; all images are reformatted at 2.5-mm slice thickness. Fig year-old woman with right lower quadrant abscess secondary to appendicitis diagnosed at outside hospital. bscess was percutaneously drained and patient presented for follow-up imaging at our institution 2 weeks later., xial 5-mm-thick CT image of abdomen and pelvis after administration of oral and IV contrast media obtained at outside hospital shows abscess (arrow) in right lower quadrant contains small amount of air. Free fluid (arrowhead) is also seen in left lower quadrant. This study was performed at 120 kvp. Volume CT dose index ( ) was 8.0 mgy and dose-length product (DLP) was 534 mgy cm. Estimated effective dose was 8.0 msv. Scan length was 43 cm., xial 5-mm-thick CT image from examination limited to region of previously seen abscess in pelvis after administration of oral and IV contrast media performed at our institution shows resolution of abnormalities. This study was performed at 80 kvp. was 5.0 mgy and DLP was 95 mgy cm. Estimated effective dose was 1.4 msv. Scan length for examination was 16 cm, whereas scan length of entire pelvis in this patient is approximately 20 cm. ecause patient weighed 135 lb (61 kg), low peak tube voltage (kvp) setting was used for follow-up study. Limiting anatomic coverage and lowering exposure factors for follow-up study resulted in 82% dose reduction compared with dose of initial scan. JR:200, pril

7 Goldman and Maldjian perience, which we have applied to the modification of our other departmental protocols. Downloaded from by lice Goldman on 03/26/13 from IP address Copyright RRS. For personal use only; all rights reserved Fig year-old man weighing 220 lb (100 kg) with known diverticulosis presented with left lower quadrant pain that required CT examination for suspicion of acute diverticulitis. Patient was concerned about radiation exposure and requested that CT study be performed with as little radiation as possible. He agreed that if customized low-dose examination was not informative, he would undergo required conventional study. fter site of maximal tenderness was localized, limited CT of pelvis was performed without oral or IV contrast material at 120 kvp, manual tube current (m) setting of 100 m (fixed m with automatic exposure control [EC] off), and 0.8-second exposure time. Volume CT dose index ( ) was 5.4 mgy, dose-length product (DLP) was 69 mgy cm, and estimated effective dose was 1 msv., xial 5-mm-thick initial CT test slice obtained to ensure that images would be of acceptable quality shows inflammatory stranding of fat in left lower quadrant with thickening of overlying peritoneum (arrow). Focus of high attenuation (arrowhead) represents retained barium within diverticulum from remote prior upper gastrointestinal series., xial 5-mm-thick image slightly inferior to shows inflamed diverticulum containing retained barium (arrow) with infiltration of adjacent mesenteric fat. In this case, limiting exposure factors and limiting scan length allowed diagnosis of diverticulitis to be made with estimated effective dose of 1 msv. Estimated effective dose for conventional CT of abdomen and pelvis for patient of comparable size is approximately 15 msv. The radiation dose reduction feasible with this strategy is depicted in Figure 8. The kvp can be lowered in thin patients, which may have the added benefit of increased conspicuity of small hypervascular pancreatic tumors [4]. Individualized Protocols The principles for modifying CT parameters and anatomic coverage described earlier can be applied to tailor a CT protocol to an individual patient for a specific indication. patient may require short-term repeat CT examinations to reevaluate an abnormality detected on a prior study or to assess a complication related to the patient s medical condition. fter reviewing recent prior CT studies to assess the image quality and technical factors, a dose-efficient CT protocol focused to the area of clinical concern can be constructed. Repeat imaging to assess a pelvic abscess is a typical scenario (Fig. 9). We have also customized a study to the lower limits of radiation dose and anatomic coverage (Fig. 10) in response to a patient s concerns about radiation exposure. If CT exposure factors are going to be lowered to an unusually low setting for a special case, we recommend that a single test slice be obtained first to ensure that the modified parameters provide diagnostic image quality. We advise radiologists to monitor individualized focused CT studies while the studies are being performed in case the focused study does not adequately answer the question of clinical concern so that the patient is still on the table if additional anatomic coverage or a more conventional CT examination is needed. We have found that customizing a CT protocol lowers the radiation dose for the individual patient and frequently provides an informative ex- Conclusion In this article, we have described a common sense approach to more efficient radiation dose management for body CT. The general principles can be applied regardless of the type of CT scanner or the status of upgrades in CT equipment. It is not possible to provide recipes or one size fits all protocols for body CT because of differences in patient body habitus, clinical indications, and CT scanner capabilities. Understanding the interrelationships between CT technical factors, image quality, and radiation dose is essential. One should develop proficiency in interpreting the CT dose report, adjusting CT technical parameters, and applying focused collimation. y using an iterative approach, making small incremental changes when applying the described strategies, one can move in the direction of the right dose without compromising patient care. The goal is to intelligently use CT to answer the question of clinical concern in the most dose-efficient manner. References 1. Tamm EP, Rong XP, Cody DC, Ernst RD, Fitzgerald NE, Kundra V. Quality initiatives: CT radiation dose reduction how to implement change without sacrificing diagnostic quality. RadioGraphics 2011; 31: Coakley FV, Gould R, Yeh M, renson RL. CT radiation dose: what can you do right now in your practice? JR 2011; 196: McCollough CH, ruesewitz MR, Kofler JM. CT dose reduction and dose management tools: overview of available options. RadioGraphics 2006; 26: Marin D, Nelson R, Rubin G, Schindera S. ody CT: technical advances for improving safety. JR 2011; 197: Iannaccone R, Laghi, Catalano C. Hepatocellular carcinoma: role of unenhanced and delayed phase multi-detector row helical CT in patients with cirrhosis. Radiology 2005; 234: FOR YOUR INFORMTION This article is available for CME/SM credit. To access the exam for this article, follow the prompts associated with the online version of the article. The reader s attention is directed to a related article, titled Reducing Radiation Dose in ody CT: Practical pproach to Optimizing CT Protocols, which begins on page JR:200, pril 2013