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Genitourinary Imaging |
1 From the Division of Abdominal Imaging and Intervention, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, White 270-E, 55 Fruit St, Boston, MA 02114. Received February 19, 2004; revision requested April 29; revision received May 21; accepted July 20. Address correspondence to M.K.K. (e-mail: mannudeep_k_kalra@yahoo.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Sixteen radiopaque kidney stones (2.5–19.2 mm in diameter) were embedded in the collecting systems of two bovine kidneys immersed in a water bath. A kidney phantom was made by placing the kidneys in an elliptical Plexiglas phantom (32 x 20 x 20 cm) filled with physiologic saline. The phantom was scanned at 16–detector row CT with a fixed tube current (300 mA) and z-axis modulation at noise indexes of 14, 20, 25, 35, and 50; remaining imaging parameters were held constant. Two abdominal radiologists reviewed images from most to least noisy. Images were evaluated for presence of stones and size, site, and attenuation value of each stone. Readers also graded conspicuity and margins of each stone on a five-point scale. In addition, follow-up studies of 22 patients (mean age, 46 years; range, 26–57 years; male-female ratio, 14:8) with kidney and ureteral stones who underwent CT with z-axis modulation (noise index, 14 and 20) were evaluated in the same manner. Statistical analysis was performed with the Student t test, Wilcoxon signed rank test, and 
test of interobserver agreement. Institutional review board approval was obtained, and informed consent was not needed.
RESULTS: In the phantom study, all 16 stones were identified on images obtained with a fixed tube current and z-axis modulation at noise indexes of 14, 20, and 25 (with a reduction in radiation dose of up to 77% compared with that of fixed tube current scanning). Three stones (<5 mm) were not visualized with z-axis modulation at noise indexes of 35 and 50. No significant difference was shown for conspicuity of kidney stones in 22 patients who underwent CT with z-axis modulation (with a 43%–66% reduction in radiation dose) when compared with results of previous fixed tube current studies (P > .05).
CONCLUSION: Kidney stones (
2.5 mm) can be adequately depicted with the z-axis modulation technique, with a 56%–77% reduction in radiation dose. In patients with urinary tract stones, the technique results in a 43%–66% reduction in radiation dose at noise indexes of 14 and 20 without compromising stone depiction.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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The purpose of our study was to evaluate the CT depiction of urinary tract calculi with the z-axis modulation technique at various noise indexes so as to reduce radiation dose while preserving image quality.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Phantom Experiment
Sixteen radiopaque calcium oxalate or calcium phosphate kidney stones (with no patient identifiers) obtained from a human stone bank at the Massachusetts General Hospital were embedded in the collecting systems of two freshly harvested bovine kidneys (obtained from a slaughterhouse) following a coronal incision (M.K.K.). The incision and implantation of stones were performed in a water bath to keep air bubbles from entering into the collecting system. The location of each stone embedded in the kidneys was recorded (eight stones were embedded in each kidney). The stones ranged in size from 2.5 to 19.2 mm (mean, 8.0 mm). A kidney phantom was created by placing the bovine kidneys in an elliptical Plexiglas container (32 x 20 x 20 cm in transverse and anteroposterior dimensions) filled with physiologic saline. The phantom was scanned with a 16–detector row CT scanner (Lightspeed 4.X; GE Medical Systems, Waukesha, Wis) at six different times, once with the fixed tube current technique (300 mA) and then with z-axis modulation (AutomA; GE Medical Systems) at noise indexes of 14, 20, 25, 35, and 50. The remaining scanning parameters were kept identical for both techniques and included a 16 x 1.25-mm detector configuration (16 rows with a thickness of 1.25 mm), 0.938:1 beam pitch, 18.75-mm table speed, 140 kVp, minimum and maximum tube current of 1 mA and 380 mA, respectively, 0.5-second gantry rotation time, standard reconstruction algorithm, and 5-mm-thick reconstructed sections at full reconstruction mode. Standard CT radiation dose descriptors—the CT dose index volume, or CTDIvol, and dose length product, or DLP—were recorded from the scanner user interface for the fixed tube current and z-axis modulation techniques at each noise index.
Phantom Image Analysis
Images were reviewed on a digital picture archiving and communication system diagnostic workstation (Impax RS 3000 1K review station; AGFA Technical Imaging Systems, Richfield Park, NJ) by two abdominal radiologists (M.M.M., with 5 years of experience, R.V.D., with 2 years of experience) who were blinded to the scanning techniques. To ensure that the readers did not develop a visual impression of the abnormality (stones) from assessment of better-quality images obtained with a lower noise index or a fixed tube current, they were asked to review the image sets in a particular order—from most to least noisy. Thus, the order of review was as follows: image sets obtained at noise indexes of 50, 35, 25, 20, and 14 and image sets obtained with a fixed tube current. Each radiologist independently evaluated each image set for the presence of stones and measured the size, site, and attenuation value of each stone. The readers used a five-point scale (1, much worse; 2, slightly worse; 3, equal; 4, better; 5, excellent) to compare the conspicuity and margins of each stone on images obtained with z-axis modulation to those on images obtained with a fixed tube current. In addition, image noise and sharpness of renal outline were also graded with the same five-point scale. Both radiologists were also asked to record whether air bubbles were present in the renal parenchyma and the pelvicaliceal collecting system.
Patient Study
In addition to the phantom study, we retrospectively identified, from a review of electronic medical records, 22 consecutive, clinically indicated CT studies obtained with our stone protocol and z-axis modulation in 22 patients who had also previously undergone CT with the fixed tube current technique. All CT scans with z-axis modulation were obtained between March 1 and June 30, 2003. There were 14 men (mean age, 48 years; age range, 26–55 years) and eight women (mean age, 42 years; age range, 26–57 years) in the study cohort. The Human Research Committee of the Institutional Review Board approved the study protocol for retrospective evaluation of the CT scans; the need for informed consent was waived. These scans were obtained with a 16–detector row CT scanner (Lightspeed 4.X; GE Medical Systems) by using z-axis modulation with noise indexes of 14 (n = 7) and 20 (n = 15) and a minimum and maximum tube current of 75 and 380 mA, respectively. These minimum and maximum tube current values were chosen on the basis of the manufacturer’s recommendation (3). The remaining scanning parameters were identical to those used for evaluating the phantomwith the z-axis modulation technique and included 140 kVp, 0.5-second tube rotation time, 16 x 1.25 detector configuration, 0.938:1 beam pitch, 18.75-mm table speed per gantry rotation, 0.5-second gantry rotation time, standard reconstruction algorithm, and 5-mm-thick reconstructed sections at full reconstruction mode. For each patient, images obtained with z-axis modulation were compared with corresponding images from previous CT examinations performed at a fixed tube current (200–300 mA) with our stone protocol (in 20 patients) and with use of contrast material (in two patients). The mean interval between CT studies performed with the two techniques was 3 months (range, 1–5 months). The remaining scanning parameters for studies performed with a fixed tube current were identical to those used with z-axis modulation.
Patient Image Analysis
Patient images were independently and retrospectively assessed by the same radiologists (M.M.M. and R.V.D.) who evaluated the phantom images. The radiologists were blinded to the scanning techniques. The scans were reviewed on a digital picture-archiving system diagnostic workstation (Impax RS 3000; AFGA Technical Imaging Systems). Images were assessed for the presence of urinary tract stones (renal or ureteral) along with their number, size, and location on each image. The radiologists were asked to grade the conspicuity and margins of the smallest stone on images obtained with z-axis modulation in comparison to that on images obtained with the fixed tube current technique by using a five-point scale (1, much worse; 2, slightly worse; 3, equal; 4, better; 5, excellent). In addition, noise and diagnostic acceptability of images obtained with the two techniques were compared at two levels—at the right renal hilum and at the pelvis at the level of the acetabular roof—by using the same five-point scale. Image noise was defined with qualitative assessment of mottle or "graininess" in the images obtained with the two techniques. Diagnostic acceptability was graded on the basis of soft-tissue contrast, sharpness of tissue interfaces, stone conspicuity (if present at that level), and degree of image degradation due to noise streaking or beam-hardening artifacts.
In the assessment of radiation dose to the patients, tube current was recorded at each section position for all stone protocol CT studies performed with z-axis modulation or fixed tube current techniques. The average tube current was calculated for each study performed with z-axis modulation. Tube current–time product for each examination was calculated by multiplying tube current by tube rotation time.
Statistical Analysis
Statistical analysis of the data was performed with a statistical software program (MedCalc Software, Mariakerke, Belgium) (M.K.K.). For the phantom study, the number, size, and presence of stones on CT scans obtained with a fixed tube current and z-axis modulation with different noise indexes were compared by using a multivariate analysis of variance test, whereas stone conspicuity and margins of stones, image noise, and diagnostic acceptability were compared with the Kruskal-Wallis test.
To analyze data from the patient study, the paired t test was used to compare the number and size of stones seen on images obtained with a fixed tube current and the z-axis modulation technique. Qualitative scores for stone conspicuity and margins were compared with the Wilcoxon signed rank test. Image noise and diagnostic acceptability at the level of the right renal hilum and the pelvis at the level of the upper margin of the acetabulum were compared with the Kruskal-Wallis test. The radiation dose used with the z-axis modulation and fixed tube current techniques were compared with the Student t test. The latter test was also used to determine whether there was a statistically significant difference between the ages of male and female patients in the study cohort. Interobserver concordance was determined with the test. A P value of less than .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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= 0.8, P < .01). The mean tube current–time product with fixed tube current scanning in 22patients was 182.4 mAs (range, 160–240 mAs). For studies obtained with z-axis modulation, the mean tube current-time product was 104 mAs (range, 50–160 mAs) with a noise index of 14 and 62.6 mAs (range, 37.5–186.9 mAs) with a noise index of 20. Thus, compared with fixed tube current scanning, z-axis modulation resulted in a 43% reduction in tube current–time product at a noise index of 14 (P = .009) and a 66% reduction in mean tube current–time product at a noise index of 20 (P < .001) (Fig 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Published reports of previous phantom and patient studies (7–14) have reported that unenhanced CT can provide excellent diagnostic information in patients suspected of having urinary tract calculi at a substantially lower radiation dose than that at excretory urography. Indeed, unenhanced CT has been reported to have a sensitivity of 95%–100% and a specificity of 92%–100% in the detection of urinary tract calculi in patients with acute flank pain (7). Unenhanced helical CT for suspected renal colic is now liberally performed in the United States and other countries in adults of all ages (15). Because many of these patients are relatively young and are at substantial lifetime risk for recurrent episodes of renal colic, radiologists must pay attention to the radiation dose delivered to these patients because they may need to undergo repeated CT examinations over time (15).
With an anthropomorphic torso phantom, Spielmann et al (8) reported unimpaired visualization of renal calculi with a significant reduction in tube current on single– and multi–detector row CT scans, with more than 75% reduction in radiation dose. In a study comprising 209 patients with ureteral calculi (9), a half-dose helical CT protocol was reported to have a sensitivity and specificity of 97.7% and 96.8%, respectively, in the detection of ureteral calculi. Similarly, some investigators (10,11) have reported a reduced radiation dose and satisfactory results with unenhanced helical CT performed for suspected renal colic with a pitch of at least 2. Heneghan et al (7), in a recent study of 50 patients with acute flank pain assessed with standard and low-dose CT, documented radiation dose reductions of 25% at multi–detector row CT and 42% at single–detector row CT. Because their study was performed in patients weighing less than 90 kg, however, the fixed tube current technique (100 mA) used by those authors may not be applicable to patients weighing more than 90 kg.
Indeed, prior reports (16,17) suggest that there is a linear correlation between patient size and image quality and that an equivalent reduction in radiation dose for "lighter" and "heavier" patients may not provide acceptable image quality in the latter group of patients. Thus, separate fixed tube current scanning protocols may be needed for "heavier" or "larger" patients, and a threshold quantitative measure (of weight, body mass index, or cross-sectional dimension) would be needed to divide the patient population into "lighter/smaller" and "heavier/larger" groups.
Another consideration with fixed tube current scanning is highlighted in the low-dose unenhanced multi–detector row CT study by Tack et al (14), which comprised 106 patients suspected of having renal colic. Although abdominal CT scans obtained at 30 mAs were acceptable in the diagnosis of urinary tract calculi, in the pelvis, the authors reported that additional images obtained with 60 mAs were required to supplement images obtained at 30 mAs. This may be explained by presence of greater image noise in the pelvis than in the abdomen at the same tube current owing to the presence of pelvic bones.
In these circumstances, the z-axis modulation technique, unlike the fixed tube current technique, offers an opportunity to select desired image quality (noise index) to automatically reduce or increase tube current depending on patient size ("lighter" vs "heavier" patients) and attenuation (abdomen vs pelvis). In contrast to "arbitrary" selection of tube current for low-dose fixed tube current scanning for specific clinical indications that can tolerate greater image noise, z-axis modulation requires selection of an appropriate noise index to obtain images with the lowest radiation dose without compromising diagnostic acceptability for that particular indication. Although automatic tube current modulation techniques have been available for multi–detector row CT scanners for more than 2 years, to the best of our knowledge no study with published results has been performed to evaluate the application of this technique in the reduction of radiation dose for CT examinations performed in patients with urinary tract calculi.
Our study results show that the z-axis modulation technique of automatic tube current modulation can be used for CT of the abdomen and pelvis in patients suspected of having urinary tract calculi. Because the attenuation of urinary tract calculi is higher than that of the surrounding soft tissues, greater image noise (and, therefore, a greater noise index) can be tolerated with stone protocol CT. In addition, the results of our study show the need for distinct scanning protocols (noise index) for this indication if the full potential for radiation dose reduction is to be achieved without compromising the image quality necessary to make appropriate diagnosis. In comparison to the noise indexes of 10.5–15.0 evaluated in previous studies for routine abdominal-pelvic scanning with z-axis modulation (4), our phantom study shows that stones measuring 2.5–19.0 mm can be detected at noise indexes of up to 25 with a 76.33% reduction in radiation dose, although noise indexes of 25 and more are associated with inferior stone conspicuity and poorer definition of margins, as well as with increased image noise and decreased diagnostic acceptability, compared with noise indexes of 14 and 20. We found that stones smaller than 5 mm can be missed at a higher noise index (noise index, 35 and 50) because of a greater increase in image noise. Results of the patient study revealed that CT for suspected urinary tract calculi with z-axis modulation can be performed at a noise index of 20 without compromising the diagnostic acceptability of images and without affecting the detection of tiny stones measuring up to 1 mm.
On the basis of the manufacturer’s suggestion, we used a minimum tube current of 75 mA in our patient study (at a noise index of 20), which is similar to that used for routine abdominal-pelvic CT (at a noise index of 12.5–15.0). This could have prevented a more than two-thirds reduction in radiation dose at a higher noise index (noise index, 20), as the minimum tube current threshold of 75 mA used in our study did not allow the use of a tube current of less than 75 mA. Although a minimum tube current of 75 mA (37.5 mAs) limits a further reduction in radiation dose, it diminishes the possibility of unacceptable image quality if technologists inappropriately select a greater noise index for the study. In addition, it maintains image quality at higher or acceptable levels for routine abdominal-pelvic CT and, thus, decreases the possibility of missing subtle lesions such as appendicitis, diverticulitis, and epiploic appendagitis and stones with lower CT attenuation values with lower tube currents. Further studies should focus on defining clinical indications for which a higher noise index combined with a lower minimum tube current (less than 75 mA) can be used, for example at follow-up CT for urinary tract calculi to obtain an additional reduction in radiation dose.
Our study has several limitations. The kidney phantom used in our study was a simplified model and did not contain other abdominal visceral and skeletal structures. In addition, we did not study the effect of stone composition or type on detection and conspicuity at different noise indexes. The manufacturer suggests the use of a noise index of 11–12 for routine CT of the abdomen and pelvis. Because reported results of previous studies have shown that diagnostic-quality CT scans of urinary tract calculi can be obtained with a substantial reduction in radiation dose, we chose an initial noise index of 14 for scanning with z-axis modulation (to attain a reduction in radiation dose of approximately 40% compared to that with a noise index of 12) (7–11). The noise indexes of 20, 25, 35, and 50 in the phantom study were chosen to assess the maximum noise index value or lowest radiation dose level at which urinary tract stones can be visualized. Another limitation of our study is that we did not perform power analysis to determine the appropriate sample size of patients suspected of having urinary tract stones, as we did not have any prior baseline data and were not aware of any prior studies assessing the use of z-axis modulation in patients with urinary tract stones. In addition, in the patient study, we did not compare standard CT radiation dose descriptors (eg, CT dose index volume and dose length product) with fixed tube current scanning and z-axis modulation. Previous studies, however, have compared radiation dose with automatic tube current modulation and fixed tube current techniques by using a mean milliampere-second value for each technique (4).
In conclusion, appropriate selection of noise index with the z-axis modulation technique for CT in patients with urinary tract stones results in a substantial reduction in radiation dose. The results of our phantom experiment show that kidney stones (up to 2.5 mm) can be adequately evaluated by using z-axis modulation, with a 56%–77% reduction in radiation dose. In patients with urinary tract stones, z-axis modulation resulted in a 43%–66% reduction in radiation dose at noise indexes of 14 and 20 without compromising stone conspicuity.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, S.S., M.K.K.; study concepts and design, M.K.K.; literature research, M.K.K.; clinical studies, M.K.K., M.M.M., R.D., S.R., M.A.B., S.S.; experimental studies, M.M.M., M.K.K., R.D.; data acquisition, M.K.K., M.M.M., R.D., S.R.; data analysis/interpretation, M.K.K., E.F.H.; statistical analysis, E.F.H., M.K.K.; manuscript preparation and definition of intellectual content, M.K.K.; manuscript editing, M.K.K., M.M.M.; manuscript revision/review and final version approval, M.K.K., S.S., M.M.M., S.R., R.D., M.A.B.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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