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Low-Dose Nonenhanced Helical CT of Renal Colic: Assessment of Ureteric Stone Detection and Measurement of Effective Dose Equivalent¹

¹ From the Department of Radiology, McMaster University, 1200 Main St West, Hamilton, Ontario, Canada L8N 3Z5 (W.L., S.J.E., B.J.K., R.H.G., A.J.R., G.W.S.); and the Department of Diagnostic Imaging, Hamilton Health Sciences Corporation, McMaster Site, Hamilton (W.L., S.J.E., B.J.K., R.H.G., G.W.S.). Received February 15, 1999; revision requested April 5; revision received July 29; accepted August 25. Address reprint requests to W.L. (e-mail: weldon@torfree.net).

	Abstract

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

 
PURPOSE: To evaluate a low-dose, nonenhanced helical computed tomographic (CT) protocol in the detection of ureteric stones and measure the associated effective dose equivalent (H_E) of radiation.

MATERIALS AND METHODS: Sixty patients suspected of having renal colic and referred by emergency department physicians underwent nonenhanced helical CT with 7-mm collimation and a 2:1 pitch and then conventional intravenous urography (IVU). The two studies were prospectively and independently interpreted. The diagnostic accuracy of CT for ureteric stone detection was determined by comparing the scans with the IVU images and with a combination of clinical, surgical, and other imaging findings. The radiation risk from typical CT and IVU examinations (five images) was measured in terms of H_E and compared with the estimated risk from two previously reported CT protocols.

RESULTS: CT correctly depicted 36 of 37 ureteric stones, and one false-positive case was recorded, for a sensitivity of 97%, specificity of 96%, and accuracy of 97%. The H_E for our CT protocol was determined to be 2.8 mSv, which is about double that for IVU and about 75% and 50% of that for two previously reported CT protocols.

CONCLUSION: Our low-dose CT protocol is superior to IVU and clinically adequate for diagnosis of renal colic.

Index terms: Computed tomography (CT), helical, 828.12111, 828.12115 • Radiations, exposure to patients and personnel • Radiations, measurement • Ureter, calculi, 828.811 • Ureter, CT, 828.12111, 828.12115 • Urography, 828.1221

	Introduction

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When nonenhanced helical computed tomography (CT) first gained popularity in the examination of patients with acute flank pain, intravenous urography (IVU) was the standard screening tool at our institution. There are distinct advantages to using nonenhanced helical CT rather than IVU, as described in earlier reports (1–3). These advantages include shorter examination time, avoidance of the cost and the complications of intravenous contrast material administration, greater sensitivity for stone detection, and higher potential for the detection of abnormalities that are unrelated to stone disease. On the other hand, patients are usually exposed to a higher dose of radiation with CT. We decided to use a nonenhanced helical CT protocol that involves less radiation exposure than do previously reported CT protocols.

The purpose of our study was twofold: first, to assess the sensitivity and specificity of this low-dose CT protocol in the detection of ureteric stones in patients suspected of having renal colic, and second, to determine the relative radiation risk from this protocol, in terms of effective dose equivalent (H_E), compared with that from the typical IVU examination at our institution and with that from two previously reported CT protocols at other institutions.

	MATERIALS AND METHODS

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Patient Selection
After discussions with urologists and emergency department physicians, it was agreed that a consecutive series of patients who presented with symptoms of renal colic to our emergency department would be imaged with a specific CT protocol followed immediately by a tailored IVU examination. Informed consent was verbally obtained after the nature of the procedures had been fully explained, including the use of additional radiation. The study was approved by the department research review committee and also sanctioned by the McMaster University Research Ethics Board.

Patients younger than 18 years were excluded from the study. The other exclusion criteria were pregnancy and inability to give informed consent. From December 31, 1996, to July 30, 1997, our patient population consisted of 60 individuals (38 men, 22 women; mean age, 42 years; age range, 19–87 years).

Ureteric Stone Detection
The patients first underwent a nonenhanced helical CT examination with a HiSpeed CT/i scanner (GE Medical Systems, Milwaukee, Wis). Single breath-hold, continuous, transverse helical acquisition was performed from the top of the kidneys to the base of the bladder with 7-mm collimation, a 2:1 pitch, 120 kVp, 280 mAs, and reconstruction at 3.5-mm intervals. No oral or intravenous contrast material was administered. A typical examination lasted less than 28 seconds. The images were reviewed at a workstation that was capable of reconstruction processing, and film hard-copy images were created from alternate images.

Immediately after CT, the patients underwent a conventional IVU examination. On average, one abdominal scout image was obtained before and four images were obtained after the administration of intravenous contrast material. Fifty milliliters of iohexol (Omnipaque 350; Nycomed Amersham, Princeton, NJ) solution was used.

The two studies were prospectively interpreted by different radiologists (including R.H.G. and G.W.S.) or radiology fellows and residents (including W.L., S.J.E., and B.J.K.). Separate, standardized evaluation forms were completed in a blinded manner when specific findings regarding the detected ureteric stones (including location and size) and signs of obstruction were recorded. Incidental findings also were noted. Ultimately, one radiologist or radiology fellow was responsible for issuing a combined diagnostic imaging report for the patient's medical record.

Ureteric stone detection was chosen to be the primary diagnostic end point of the imaging studies. Analysis of the data recorded on the evaluation forms was performed. Follow-up data regarding spontaneous stone passage, surgical stone retrieval, and other imaging and clinical findings were used with the IVU findings to determine the diagnostic accuracy of CT in the detection of ureteric stone disease.

H_E Assessment
The radiation risk from diagnostic radiologic procedures can be assessed by using the concept of an H_E (4) or a somatic dose index (5). Both the H_E and the somatic dose index may be considered as the uniform whole-body radiation dose that is associated with the same radiation risk as that from the set of nonuniform doses absorbed by individual organs during the diagnostic procedure. The results of a study by Huda and Bissessur (4) showed that the values of H_E per unit of energy imparted (H_E per millijoule) are relatively constant for a given type of radiologic examination. In this way, calculated values of energy imparted to the patient in any radiologic examination can be combined with the appropriate values of H_E per millijoule to obtain an estimate of the H_E for the procedure. Use of the H_E parameter allows intercomparisons of the radiation risks from different radiologic procedures as well as comparisons of radiologic procedure risks with occupational exposure and natural background exposure risks (4).

In the current study, the total energy imparted to a typical patient was determined for each CT and IVU examination. This value was combined with an appropriate value of H_E per millijoule to obtain the H_E for the procedure. The calculations of total energy imparted were based on the equipment and protocols currently used at our institution or the protocols previously reported by others (1,2). A similar approach was used previously to compare the radiation risk between CT and conventional radiography in the examination of sacroiliac joints (6).

The typical IVU examination at our institution consists of one posteroanterior and four anteroposterior views, each of which involves a 14 x 17-inch screen-film combination acquired at 70 kVp and 64 mAs. The skin entrance exposure values for these views were calculated for an average-sized patient by using the radiation output data of the x-ray installation. The exposure in air without backscatter was measured at 90 cm from the focal spot by using a recently calibrated ionization chamber and dosimeter (Model 35055; Keithley Instruments, Cleveland, Ohio). The x-ray beam quality used during the procedure was assessed by measuring the beam half-value layer at 80 kVp. The entrance exposure in air without backscatter was calculated by using the output data and knowledge of the technique used, with the inverse square law and an appropriate skin-to-focus distance taken into account (7). An appropriate area of the x-ray beam at the skin entrance was used to calculate the exposure-area product for each view, and the energy imparted to the patient was determined by extrapolating data from that of Shrimpton et al (8) for an appropriate x-ray beam quality.

For the CT examinations, the skin entrance exposure was determined by using a film dosimetry method similar to that described by others (9,10). Two 10 x 12-inch verification films (Eastman Kodak, Rochester, NY) were taped in direct contact to an average-sized patient. One film was taped anteriorly, and the other was taped posteriorly, with the long axis of the film parallel to the long axis of the patient. The developed films showed a series of exposed sections that corresponded to the helical movement of the CT scanner and were used to determine both the beam width at skin entrance and the mean skin exposure. The optical density of the films was measured at 5-cm intervals for every fifth beam section, and the mean optical density of the anterior and posterior films was used to determine the mean skin entrance exposure. The mean skin exposure for three average-sized patients was determined for our CT protocol. The skin exposure for the two previously reported helical CT protocols was estimated by using the skin exposure used at our institution and making appropriate adjustments based on differences in beam width and pitch between our protocol and the two previously reported protocols.

The mean energy imparted to the patient was calculated by using two approaches. In the first approach, the energy imparted was calculated by using the mean skin exposure, with the assumption of a uniformly absorbed dose distribution throughout a volume of the patient created by compressing the helix of entrance exposure to a 20 x 35-cm (or 700-cm²) cylindrical cross-section. The energy imparted to the volume defined by using the skin areas that were not exposed to the entrance x-ray beam was calculated by collapsing the helix defined by using the area of the patient surface in a similar manner.

In the second approach, the skin entrance exposure, together with knowledge of the area of skin exposed to the entrance x-ray beam (determined from the exposed film), was used to calculate the exposure-area product for the procedure. The energy imparted to the patient was then determined by using the exposure-area product and data extrapolated from that of Shrimpton et al (8) for an appropriate beam quality.

	RESULTS

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Ureteric Stone Detection
By using CT, we detected 25 of the 26 ureteric stones that were demonstrated at IVU. The one false-negative case involved a 2-mm calculus near the vesicoureteric junction that was clearly visible at CT retrospectively but dismissed as vascular calcification initially. In addition, CT depicted 12 ureteric stones that were not demonstrated at IVU. Six of these stones were nonobstructing. Eleven of these cases were associated with a combination of spontaneous stone passage (n = 2), surgical stone retrieval (n = 2), follow-up imaging findings (n = 2), and other clinical information (n = 5), that supported the diagnosis of renal colic. One case was classified as false-positive because a subsequent cystoscopic procedure performed 18 days after the imaging studies failed to depict a ureteric stone.

In 22 patients, no ureteric stones were detected at either CT or IVU. None of these patients had follow-up clinical or imaging data that suggested true ureteric stone disease, and nine patients had alternative diagnoses, which included pyelonephritis (n = 2), Crohn disease (n = 2), epididymitis (n = 1), congenital ureteropelvic obstruction (n = 1), inguinal hernia (n = 1), degenerative disk disease (n = 1) and psychosomatism from depression (n = 1).

The described data indicated a sensitivity of 97% (36 of 37 stones), specificity of 96% (22 of 23 patients with no stones), and overall accuracy of 97% (58 of 60 patients) for the detection of ureteric stones with our CT protocol. The location and size of the ureteric stones detected at CT are listed in Table 1.

	DISCUSSION

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

The 2-mm stone that was demonstrated at IVU but overlooked at initial CT interpretation was clearly visible at retrospective examination. This was an observer error that occurred early in the study when interpretation was performed mainly with the film hard-copy images. We soon discovered that it was much easier to interpret the studies by paging through the data on a workstation that was capable of reconstruction processing. In this way, the course of both a dilated and a normal ureter could be followed more easily from the kidney down into the pelvis and medially to the base of the bladder. This method of viewing the data allows ureters to be more readily distinguished from vessels and calcified phleboliths outside the ureter to be identified with greater certainty (2). Others (13–18) have described the associated signs of ureteric stones and the criteria used to analyze pelvic calcifications, and these also will help to minimize interpretative errors.

The case of the 5-mm ureteric stone that was detected at CT but not confirmed at cystoscopy with attempted ureteric extraction 18 days later was recorded as false-positive. However, review of the images indicated that this was probably a correct diagnosis in which the patient was not aware of having passed the stone in the interval.

The results of this study showed also that our CT protocol is associated with a lower radiation risk compared with the risk from the protocols reported by Smith et al (1) and Sommer et al (2), given reasonable assumptions. The reduction in H_E in our study was attributed to differences in the selected x-ray beam width and pitch of the examinations. The increased x-ray beam width used in our protocol results in thicker transverse sections, and theoretically, it predisposes smaller stones to be missed during the examination. In addition, the higher pitch also theoretically reduces the stone detection capability of the examination.

We attempted to use a protocol that would reduce the radiation dose without decreasing the sensitivity for stone detection. The data suggest that no substantial number of ureteric stones escaped detection by using our CT protocol. In fact, 17 stones smaller than 5 mm were depicted, five of which were not detected at IVU (Table 1).

In summary, our low-dose, nonenhanced helical CT protocol is superior to conventional IVU and clinically adequate for the detection of ureteric stones. Since August 1997, this examination has been the standard screening tool for examining patients who present with symptoms of renal colic at our institution.

	Acknowledgments

 
We acknowledge the technical assistance of the CT and IVU technologists at the McMaster Site of Hamilton Health Sciences Corporation, particularly Patty Ellis and Sheila Hagel. We also thank Monika Ferrier for her assistance in literature search.

	Footnotes

 
Abbreviations: H_E = effective dose equivalent IVU = intravenous urography

Author contributions: Guarantors of integrity of entire study, W.L., R.H.G., G.W.S.; study concepts and design, all authors; definition of intellectual content, W.L., R.H.G., A.J.R., G.W.S.; literature research, W.L., S.J.E., B.J.K., A.J.R.; clinical studies, W.L., S.J.E., B.J.K., R.H.G., G.W.S.; experimental studies, A.J.R.; data acquisition, W.L., S.J.E., B.J.K., A.J.R.; data analysis, W.L., S.J.E., R.H.G., A.J.R.; manuscript preparation, W.L.; manuscript editing, W.L., R.H.G., A.J.R., G.W.S.; manuscript review, all authors.

	References

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

 

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