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Decreasing the Radiation Dose for Renal Stone CT

A Feasibility Study of Single- and Multidetector CT

¹ Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710.
² Radiation Safety Division, Duke University Health System, Durham, NC 27710.

Received May 30, 2001; accepted after revision October 23, 2001.

 
Presented at the annual meeting of the American Roentgen Ray Society, Seattle, April—May 2001.

Address correspondence to J.P. Heneghan.

Introduction

Top Introduction Subjects and Methods Results Discussion References

 
The evaluation of renal colic using unenhanced helical CT was originally described by Smith et al. [1] in 1995. This technique has gained widespread acceptance among radiologists, emergency department physicians, and urologists. In the acute setting, most patients currently undergo unenhanced helical CT for the initial evaluation of nephrolithiasis [2]. The frequency of this request in our institution has increased substantially during the past 2 years because of an expanding indication for unenhanced helical CT in the emergency setting [3] and because the clinicians are increasingly familiar with the technique. From July 1999 through June 2000, 863 unenhanced CT examinations were ordered to evaluate for renal stones, compared with 650 such studies in the same time period in 1998-1999. Of these 863 examinations, 420 were ordered in the acute setting from the emergency department. In the period studied, 595 (69%) of all examinations were performed on patients younger than 50 years old, 196 (33%) of whom were women. Clearly, a great number of these examinations are being requested, frequently in a young population. As with all pelvic CT, there is direct gonadal radiation exposure, particularly in female patients. Thermoluminescent dosimeter measurements, using our institution's standard CT protocols for renal stone examinations, revealed doses to the uterus of 18 mGy (1.8 rad) on our single-detector helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI) and 23 mGy (2.3 rad) on our multidetector scanner (QX/i LightSpeed; General Electric Medical Systems). Most renal calculi are of high attenuation relative to soft tissue and are readily visualized because of high inherent contrast, which raises the question: Is it possible to perform a diagnostic renal stone CT examination at a lower radiation dose? The purpose of our study was to assess detectability of human renal stones in a porcine kidney phantom at various radiation exposures on both single-detector and multidetector helical CT scanners.

Subjects and Methods

Top Introduction Subjects and Methods Results Discussion References

 
Forty human calcium oxalate stones measuring 2-8 mm were implanted in the parenchyma of two porcine kidneys (20 in each kidney) via two longitudinal incisions made in each kidney. All stones were implanted while the kidneys were submersed in water to avoid placement of gas bubbles inside the kidneys, which could affect detectability. The incisions were then sutured closed, also during submersion, to prevent movement of the calculi. The location and size of each stone as measured before implantation were recorded. The kidneys were fully submersed in a water-filled elliptic phantom (Data Spectrum, Hillsborough, NC). We scanned the kidneys in the transverse plane on a QX/i LightSpeed multidetector scanner and a CT/i single-detector helical CT scanner. All scans were obtained at 140 kVp, with 5-mm collimation and a 0.8-sec gantry rotation speed. The tube current was decreased serially as follows: 170, 120, 80, 60, 40, 30 and 20 mA on the QX/i at a pitch of 3:1 (15 mm per gantry rotation) and 220, 170, 110, 80, 60, 40, 30, and 20 mA on the CT/i at a pitch of 1 (5 mm per gantry rotation). The CT dose index was measured at the center of a 32-cm diameter acrylic CT body phantom (model 20 CT14; Radcal, Monrovia, CA) with a pencil ion chamber (model 6000-200; Victoreen, Solon, OH) and monitor (Nero 8000; Victoreen). We used the CT dose index initially because it provided a convenient method of first-order dose comparison between two models of CT scanners. CT dose index data were analyzed by a linear regression method to fit the experimentally obtained dose data from the single- and multidetector scanners. We found a goodness of fit of 0.9915 for the QX/i scanner and 0.9975 for the CT/i scanner.

All scans were obtained during the same session. Individual organ doses and the effective dose equivalent were estimated at a different session using thermoluminescent dosimeters (Harshaw TLD-100; Saint-Gobain Cystals & Detectors, Solon, OH) and an anthropomorphic phantom (RANDO phantom; The Phantom Laboratory, Salem, NY). Thermoluminescent dosimeter calibration was performed using a simulated CT beam with a half-value layer of 8 mm of aluminum at 120 kVp. Two thermoluminescent dosimeter chips were placed in various organs of the phantom. CT scans were obtained using the clinical renal-stone protocols at our institution for the CT/i scanner (peak kilovoltage, 140 kVp; varied amperage; pitch, 1; slice thickness, 5 mm) and the QX/i scanner (peak kilovoltage, 140 kVp; varied amperage; pitch, 3; slice thickness, 5 mm).

Two investigators, working by consensus, recorded the total number of calculi identified in each kidney at each dose. The reviewers were aware of the technical parameters of the scan, and one of the reviewers had participated in the stone implantation in the porcine kidneys. Three calculi in the left kidney shifted position during suturing of the kidney, and they became inseparable from other calculi on the CT scans. As a result, only 17 discrete stones were identified in the left kidney on the initial scans obtained using the standard diagnostic technique. This finding then served as a reference for subsequent scans obtained at lower exposures. The stones did not shift their position during transfer of the phantom from the QX/i to the CT/i scanner.

Six indicator calculi were identified on the initial scan. The size of each indicator calculi was evaluated at each varied exposure to assess for size distortion with differing techniques. A single investigator measured the size of the six indicator calculi at a workstation using electronic calipers. Kendall tau b correlation coefficients were computed for pairs of variables.

Results

Top Introduction Subjects and Methods Results Discussion References

 
On the single-detector CT, all calculi were visible in the right kidney on scans with exposures ranging from 220 to 60 mA and in the left kidney on scans with exposures ranging from 220 to 80 mA (Figs. 1A,1B,1C and 2). On the multidetector CT scanner, all calculi were visualized in the right kidney with exposures ranging from 170 to 60 mA and in the left kidney with exposures ranging from 170 to 30 mA (Figs. 3A,3B,3C and 4). At all levels of amperage, no statistically significant change was found in the measured size of the six indicator stones (p = 0.38 for scans obtained on the CT/i and p = 0.07 for scans obtained on the QX/i scanner).

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on single-detector helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through the phantom at 140 kVp and 220 mA shows nine renal calculi.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on single-detector helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through phantom at 140 kVp and 80 mA shows visualization of all nine renal calculi seen on higher-dose scan A, although small calculi are not as well seen.

View larger version (142K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on single-detector helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through phantom at 140 kVp and 20 mA shows poor visualization of only six of nine renal calculi seen on A.

View larger version (25K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —Bar graph shows stone detection in right kidney (white bars) and left kidney (gray bars) as function of decreasing amperage in scans obtained on single-detector CT.

View larger version (107K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on multidetector scanner (QX/i LightSpeed; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through phantom at 140 kVp and 170 mA shows nine renal calculi.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on multidetector scanner (QX/i LightSpeed; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through phantom at 140 kVp and 60 mA shows visualization of all nine renal calculi seen on higher-dose scan A, although calculi are not as well seen.

View larger version (141K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —CT images of simulated torso phantom with water-submersed porcine kidneys, obtained on multidetector scanner (QX/i LightSpeed; General Electric Medical Systems, Milwaukee, WI). Transverse scan obtained through phantom at 140 kVp and 20 mA shows poor visualization of only seven of nine renal calculi seen on A.

View larger version (23K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —Bar graph shows stone detection in right kidney (white bars) and left kidney (gray bars) as function of decreasing amperage in scans obtained on multidetector-row CT.

The calculated dose to the phantom decreased in a linear fashion with the decreasing amperage on both the single-detector and multidetector scanners (Fig. 5). Specifically, on the CT/i, a decrease from 220 to 80 mA reduced the CT dose index from 11.1 to 3.8 mGy. The corresponding thermoluminescent dosimeter measurement for kidneys decreased from 28.5 to 10.3 mGy and the estimated effective dose equivalent decreased by 64% from 16.3 to 5.9 mSv (Fig. 6). On the QX/i, a decrease from 170 to 60 mA reduced the CT dose index from 14.9 to 5.2 mGy. The corresponding thermoluminescent dosimeter measurement for kidneys decreased from 34.6 to 12.2 mGy, and the estimated effective dose equivalent decreased by 65%—from 22 to 7.8 mSv (Fig. 7).

View larger version (13K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5. —Line graph shows measured CT dose index for varied exposures on single-detector (circles) helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI) and multidetector (squares) scanner (QX/i LightSpeed; General Electric Medical Systems). Linear reduction in radiation dose is function of decreasing amperage.

View larger version (12K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6. —Line graph shows thermoluminescent detector measurements obtained at varied exposures on single-detector helical scanner (CT/i; General Electric Medical Systems, Milwaukee, WI), with circles representing doses to kidneys in milligrays and squares representing effective dose equivalents in millisieverts.

View larger version (12K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —Line graph shows thermoluminescent detector measurements obtained at varied exposures on multidetector scanner (QX/i LightSpeed; General Electric Medical Systems, Milwaukee, WI), with circles representing doses to kidneys in milligrays and squares representing effective dose equivalents in millisieverts.

Discussion

Top Introduction Subjects and Methods Results Discussion References

 
Unenhanced helical CT has been shown to have a sensitivity of 97%, a specificity of 96%, and an accuracy of 97% for the diagnosis of urolithiasis [4]. This examination has become the primary technique in many centers for the evaluation of renal calculi in the acute setting, and it has been well received in the clinical community, particularly in emergency departments [5, 6].

The main disadvantage of CT is that it exposes patients to a relatively high radiation dose. This high dose is of particular concern in those young patients who are repeated stone formers and thus may require multiple CT examinations in their lifetime.

In our phantom study, we found excellent detectability of all renal calculi, which ranged in size from 2-8 mm, when we used a much lower amperage and measured radiation dose than those of the standard protocols. At an exposure of 80 mA for single-detector CT and 60 mA for multidetector CT, all renal calculi were visualized in the porcine kidneys. These findings correspond to a 2.9-fold decrease in estimated radiation dose (11.1-3.8 mGy) on the single-detector CT scanner and a 2.9-fold decrease (14.9-5.2 mGy) on the multidetector CT scanner. In addition, the findings correspond to a 2.8-fold decrease in the estimated effective dose equivalent (16.3-5.9 mSv) on the single-detector CT scanner and a 2.8-fold decrease (22-7.8 mSv) on the multidetector CT. We measured a significantly greater dose on the QX/i (multidetector) scanner as compared with the CT/i (single-detector) scanner. We believe that this finding is primarily associated with the inherent design of multidetector CT scanners; that is, the QX/i produces a broadened beam profile beyond the umbra (the main beam over the detectors) and generates a very large penumbra radiation contribution.

Denton et al. [7] have raised the concern of an increased radiation dose with helical CT compared with conventional excretory urography. Their study found that the average effective dose of unenhanced helical CT was more than three times that of three-film excretory urography. They commented that the radiation risk is somewhat offset by the risks of contrast media reaction and contrast media—induced nephrotoxicity. They concluded that the increased radiation risk may be justifiable if information that alters subsequent patient treatment is gained from CT that would not have been obtained from excretory urography. Liu et al. [8] described a low-dose CT protocol for evaluation of renal colic compared with exposure on excretory urography. The effective dose equivalent of their protocol was 2.8 mSv, which is double that of excretory urography. However, they used parameters of 120 kVp and 280 mAs on a single-detector CT, producing a greater dose than the standard renal-stone protocol at our institution. A study by Diel et al. [9] explored increasing the pitch as a means of reducing radiation dose in CT of patients with suspected renal colic. The average entrance exposures were estimated as 461 mR (1.19 x 10^-4 C/kg), 553 mR (1.43 x 10^-4 C/kg), and 913 mR (2.36 x 10^-4 C/kg) at pitches of 3.0, 2.5, and 1.5, respectively. Although accuracy did not significantly change when they used the higher pitch of 3.0 versus 2.5, the image quality did decrease.

A limitation of our study is that the investigators were aware of the technical parameters of the scan during the evaluation of stone size and conspicuity; as the noise progressively increased with decreasing amperage, it was obvious to the reviewers which were the reduced-dose scans. A further limitation of our study is that we were unable to evaluate for ureteric stones in this phantom model; we were only able to evaluate stone size and detectability in the kidney. Visualization of a calculus in the lumen of a dilated ureter is direct evidence of acute urinary obstruction resulting from renal stone disease. Occasionally, the patient is scanned after passage of a stone, or the ureteric stone remains undetected because of volume averaging, lack of retroperitoneal fat, respiratory motion, or an abundance of phleboliths [10]. It is not known whether the ureter and the secondary signs of ureteral obstruction would be adequately evaluated with a reduced-dose technique. Furthermore, it is unclear how well this technique would visualize the remainder of the abdomen and pelvis to evaluate for other causes of acute flank or abdominal pain. A low-dose renal-stone protocol may prove most valuable in patients requiring follow-up scanning in the setting of known nephrolithiasis when other diagnoses are less likely. Clearly, a low-dose technique would not be possible for obese patients, who routinely require a higher exposure for diagnostic images.

In conclusion, this phantom study shows that renal stone detectability and size remain constant on both the single- and multidetector helical CT scanners at much lower radiation doses than those called for in the standard renal-stone protocol. Evaluation of the clinical performance of a low-dose renal-stone technique is warranted in this frequently ordered examination.

Acknowledgments
 
We thank G. Allan Johnson for his contribution in the formulation of the study design and David DeLong for his help with statistical analysis.

References

Top Introduction Subjects and Methods Results Discussion References

 

1. Smith RC, Rosenfield AT, Choe KA, et al. Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995;194:789 -794[Abstract/Free Full Text]
2. Chen MY, Zagoria RJ. Can noncontrast helical computed tomography replace intravenous urography for evaluation of patients with acute urinary tract colic? J Emerg Med 1999;17:299 -303[Medline]
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4. Smith RC, Verga M, McCarthy S, Rosenfield AT. Diagnosis of acute flank pain: value of unenhanced helical CT. AJR 1996;166:97 -101[Abstract/Free Full Text]
5. Preminger GM, Vieweg J, Leder RA, Nelson RC. Urolithiasis: detection and management with unenhanced spiral CT: a urologic perspective. Radiology 1998;207:308 -309[Free Full Text]
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This Article Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Spielmann, A. L. Articles by Nelson, R. C. Search for Related Content PubMed PubMed Citation Articles by Spielmann, A. L. Articles by Nelson, R. C. Social Bookmarking                      What's this? Hotlight (NEW!) What's Hotlight?