Helical CT scanning: The primary imaging modality for acute flank pain

Original Contributions

Helical CT Scanning: The Primary Imaging Modality for Acute Flank Pain MARCELLA M. NACHMANN, DO, RICHARD C. HARKAWAY, MD, SUSAN L. SUMMERTON, MD, MINDY M. HORROW, MD, CHERYL L. KIRBY, MD, RYAN G. FIELDS, BS, AND PHILLIP C. GINSBERG, DO We set out to evaluate the accuracy of nonenhanced helical computed tomography (CT) scanning at stone detection in the patient with acute flank pain, and as a means of detecting noncalculus causes of acute flank pain. Between April 1995 and April 1997, 412 consecutive patients with acute flank pain underwent noncontrast-enhanced helical CT. Two hundred eighty-one patients had confirmation of their CT diagnosis by other radiographic studies, urologic intervention, or spontaneous stone passage of calculi. We determined the presence or absence of urinary calculi, as well as the presence of other noncalculus pathology. CT scanning revealed a stone in 92/281 patients (32.7%) and no stone in 189/281 patients (67.3%). Of the 189 patients, 60/189 patients (32%) had another positive finding as a cause for flank pain. Eighty-one of 92 patients with a stone on CT (88%) had confirmation of stone disease by radiologic or surgical intervention. Eleven of 92 patients (12%) did not have confirmation of their diagnosis because of resolution of symptoms or refusal of further intervention. On helical CT scans 129/189 patients demonstrated no abnormalities. Two of 189 (1.5%) thought to be stone free by CT passed a stone. Helical CT had a sensitivity of 97%, a specificity of 92%, a positive predictive value of 88%, and a negative predictive value of 98% at stone detection. Noncontrast-enhanced helical CT is accurate and rapid in detecting calculus disease in patients with acute flank pain. Perhaps more importantly, it provides the added benefit of detecting noncalculus causes of flank pain in greater than 30% of patients. (Am J Emerg Med 2000;18:649-652. Copyright @ 2000 by W.B. Saunders Company)

Between 2% and 3% of all people will experience an episode of acute renal colic during their lifetime.1 In most institutions, excretory urography remains the primary radiologic study for the evaluation of patients with acute flank pain, especially if a urologist evaluates them. Two institutions, however, have proposed the use of noncontrastenhanced helical computed tomography (CT) as a replacement for intravenous urography (IVU) in the work-up of acute flank pain.2,3 Noncontrast-enhanced CT has several advantages. No bowel preparation is required. The risk of contrast reactions

From the Departments of Urology and Radiology, Albert Einstein Medical Center, Philadelphia, PA. Manuscript received January 6, 2000, accepted January 17, 2000. Address reprint requests to Richard C. Harkaway, MD, Division of Urology, Albert Einstein Medical Center, Philadelphia, PA 19141. Key Words: Calculi, computed tomography, renal colic, obstruction, pain, kidney, ureter, bladder. Copyright 娀 2000 by W.B. Saunders Company 0735-6757/00/1806-0001$10.00/0 doi:10.1053/ajem.2000.16292

is eliminated because no oral or intravenous (IV) administration of contrast materials is used. All calculi are visible on CT because of their higher attenuation than the surrounding tissue, regardless of stone composition.4 Both location and size of stones can be accurately assessed. Furthermore, CT is faster, with current generation slipring CT scanners (helical) imaging at 1 second per section.5 Therefore imaging of the entire genitourinary system can be accomplished within 2 minutes decreasing the likelihood of motion artifact obscuring the radiologic interpretation. Finally, noncontrast-enhanced CT can show nonurinary causes of acute flank pain, eliminating the need for further imaging studies. At busy institutions, rapid diagnosis of acute flank pain is important to obtain the correct service for consultation and treatment. The purpose of our study was to determine the value of unenhanced helical CT in the diagnosis of acute flank pain, both in the accuracy of stone detection and the ability to detect nonurinary causes of acute flank pain without further radiologic imaging. METHODS Between April 1995 and April 1997, 412 consecutive patients presenting with acute flank pain were imaged with noncontrast helical CT. The group consisted of 115 men and women, 18 to 81 years old (mean 52). All CT examinations were performed using a High Speed Advantage Helical CT Scanner (General Electric, Milwaukee, WI). Images were obtained from the mid-portion of the T1 vertebral body to the midpoint of the pubic symphysis with a collimation of 7 mm and a pitch of 1:1. Acquisition times ranged from 30 to 45 seconds with suspended respiration in two overlapping sessions. Reconstructions were created in 2 mm increments through a limited region when findings were indeterminate. If findings were still indeterminate, the patients were assigned to receive contrast-enhanced CT. Informed consent for intravenous administration of contrast agents was obtained from all 33 patients who underwent contrastenhanced CT. One hundred fifty-three patients were diagnosed with urinary calculi. Two hundred fifty-nine patients’ studies were negative for urinary calculi. A confirmed diagnosis of the presence or absence of stone disease was available in 281 patients. One hundred thirty-one patients had no confirma649

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tory imaging, surgery, or were unable to be contacted for clinical follow-up and were therefore excluded from the study. RESULTS Unenhanced helical CT was positive for stone disease in 92 patients and negative for stone disease in 189 patients. In the latter group, one hundred twenty-nine patients had no other abnormalities detected on CT and 60 had positive CT findings unrelated to stone disease. Stone Disease Ninety-two patients showed urinary calculi on unenhanced helical CT scans. Fifty-one patients underwent ureteroscopy with visualization of stone, laser lithotripsy, or stone extraction. Nine patients underwent successful extracorporeal shock-wave lithotripsy. Thirty-two patients who did not undergo surgical treatment of their calculi were available for follow-up by chart review. Twenty-one patients recovered a stone in their urine and 11 did not. These 11 patients had resolution of their flank pain and did not wish further imaging or surgical intervention. No Abnormalities One hundred twenty-nine available patients showed no abnormalities on unenhanced helical CT scans. Two patients reported stone recovery. All other patients followed denied any recovery of calculi. Abnormalities Unrelated to Stone Disease Nonurinary causes of flank pain were discovered in 60 patients (Table 1). Data Analysis Noncontrast-enhanced helical CT at our institution had a sensitivity of 97%, a specificity of 92%, a positive predictive value of 88%, and a negative predictive value of 98% (Table 2). DISCUSSION In many institutions, IVU remains the primary imaging modality for the evaluation of patients with acute flank pain. TABLE 1. Findings on 60 Patients With Acute Flank Pain but no Stone on Helical CT Noncalculus Findings on Helical CT

No. Patients

Renal mass Adnexal/uterine mass Ovarian cyst Cholelithiasis Appendicitis Ureteral stricture disease Colon carcinoma Diverticulitis Abdominal aortic aneurysm Splenic infarct

13 11 8 7 6 5 4 4 1 1

TABLE 2. Comparison of Actual Diagnosis to CT Diagnosis in Presence or Absence of Ureteral Calculi Unenhanced CT Diagnosis Actual Diagnosis

Stone⫹

Stone⫺

Total

Stone⫹ Stone⫺ Total

81 11 92

2 127 129

83 138

Advantages of IVU include the acquisition of both anatomic and functional information. The site and degree of obstruction can be easily derived from an excretory urogram. However, despite its widespread use, IVU does have its disadvantages. Nearly 2 hours may pass by the time an IVU is performed and completed, particularly if delayed films are required to fully opacify the collecting system. Excretory urography requires the administration of IV contrast material and in the general population, the incidence of contrast reactions is 5% to 10%.6,7 These range from mild reactions, such as vomiting and urticaria, to severe reactions, such as bronchospasm and anaphylaxis. Their incidence can be decreased by the use of low osmolar contrast agents, but these are expensive to use and do not completely eliminate the risk of contrast reactions. Finally, in patients with preexisting renal insufficiency and diabetes mellitus, the risk of contrast-induced nephrotoxicity is 25%.8 All of these factors have led several institutions to seek alternate radiographic imaging studies for the evaluation of patients with acute flank pain. Plain abdominal radiography has been used to determine the presence of a calculus in patients with acute flank pain. Traditional beliefs are that 90% of urinary calculi contain either calcium phosphate or calcium oxalate and therefore plain radiography should be sufficient as a primary diagnostic modality.9 A recent study by Mutgi in 1991, however, revealed only 58% sensitivity for the detection of urinary calculi by nonblinded retrospective reviews of plain radiographs.10 Other institutions use a combination of plain radiography and ultrasonography to elucidate the presence of urinary calculi or ureteral obstruction. Positive findings include the presence of hydronephrosis, hydroureter, perinephric fluid, a ureteral calculus, or significant asymmetry of ureteral jets within the bladder. Still only 81% of proven calculi were correctly identified on kidney ultrasound biopsy (KUB) and ultrasonography.2 Nonurinary causes of acute flank pain were difficult to assess as well. The use of noncontrast-enhanced helical CT has several advantages. No bowel preparation is necessary. There is no oral or IV administration of contrast agents, obviating the potential for contrast reactions. Since no oral or IV contrast agents are used, the examination does not preclude the patient from undergoing further diagnostic imaging if necessary. Acquisition of information was accomplished during two suspended respirations by the patients. Successful imaging of the entire genitourinary system ranged from 2 to 3 minutes. Quality images appear at the CT computer workstation immediately, without having to process hardcopies of the scans, which allows a rapid initial review of the films by the physician.

NACHMAN ET AL 䊏 HELICAL CT SCANNING FOR ACUTE FLANK PAIN

Stones have higher attenuation values (200 to 400 HU) than any surrounding soft tissue, regardless of their composition.4 Therefore CT scans can provide precise localization and sizing of a calculus within the collecting system even if it were radiolucent on KUB. Unenhanced CT can exclude stone disease with a high negative predictive value (97%).3 However, false-negative studies are possible because of to volume averaging, small, stone size, low stone attenuation, respiratory variation between acquisitions, and paucity of retroperitoneal fat to provide contrast.11 Smith believes it is reasonable to use 5 mm cuts from kidney top to bladder base. When helical acquisition is used, volume-averaging effects can be additionally reduced by retrospectively reconstructing images at arbitrary section locations with overlap of adjacent sections. Because of this, images can be reconstructed at 1 to 2 mm increments through any suspicious high attenuation foci. We have used 7 mm cuts with 1 to 2 mm reconstructions if necessary without the loss of sensitivity and specificity. Smith et al were also first to note the presence of the ‘‘tissue-rim sign,’’ a thin circumferential rim of soft tissue attenuation when a stone is lodged in the ureter.11 The most common calcific area of attenuation that can be confused with a ureteral stone on CT is a pelvic phlebolith.12 Heneghan et al states that if an indeterminate calcification shows a positive rim sign, it is most likely a ureteral stone. A negative rim sign is less helpful. They studied 136 ureteral calculi and 259 phleboliths and found a sensitivity of 77% and a specificity of 92% in distinguishing a calculus from phlebolith by a positive rim sign.12 Smaller stones (⬍4 mm) have a rim sign more often than larger stones (⬎5 mm), likely because of stretching of the ureteral wall by the larger stones, thereby decreasing the thickness of the rim sign. Smaller stones are less conspicuous because of volume averaging and are less likely to be associated with secondary signs of obstruction, such as ureteral dilatation and perinephric stranding, so the rim sign becomes even more significant when it is present. When an indeterminate calcification is present on CT along the course of the ureter, the presence of these secondary signs is helpful in determining the calcific area of attenuation to be an obstructing calculus versus a phlebolith. Katz relates a 96% rate of at least one associated finding (hydronephrosis, hydroureter, and perinephric/periureteral edema) being present on unenhanced helical CT when a calculus is noted.13 Smith repeated Katz’ study and observed that with the combination of two signs, positive and negative predictive values were high.11 When ureteral dilation and perinephric stranding were present or absent, a positive predictive value of 99% and a negative predictive value of 95% were obtained. The presence of unilateral intrarenal collecting system dilation and unilateral or asymmetric perinephric stranding had a positive predictive value of 98% and the absence of these signs yielded a negative predictive value of 91%. The cost of a CT scan at our institution is comparable with that of an IVU, because of a special rate assigned to ‘‘stone search’’ studies. Radiation dosing is also comparable. Multiple studies have documented radiation dosimetry of CT scan and IVU.14 The skin entry radiation dose of unenhanced CT examination is approximately 3 to 5 rads when using

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kVp of 120 and 200 to 400 mAs. The skin dose of IVU is dependent on the number of delayed films taken. Fielding et al quoted averages of 4.6 rads for unenhanced helical CT and 4.4 rads for complete intravenous urography.15 We believe there is a short learning curve to unenhanced helical CT interpretation. Helical CT has replaced all radiologic modalities for acute flank pain presenting to the ED. Of secondary benefit has been CT scan’s ability to provide information about nonurinary causes of acute flank pain. In a study by Dalrymple of 229 patients without evidence of stone disease on CT, 65 patients were noted to have noncalculus causes for their acute flank pain, which correlates closely with our observations.16 Rapid intervention was accomplished in many cases when nonurologic causes of acute flank pain were discovered on the unenhanced helical CT, and in fact, 32% of CT scans performed for acute flank pain were noted to have positive noncalculous findings. However, we believe that Dalrymple’s inclusion of KUB in the algorithm for diagnosis and management of acute flank pain may be unnecessary. Studies by Mutgi in 1991 revealed only a 58% sensitivity for the detection of urinary calculi by nonblinded retrospective review of plain radiographs. Additionally, in both studies, 27% to 37% of helical CT scans with no visible stone disease were found to have noncalculus causes for acute flank pain. Most of these findings would not have been diagnosed by an initial KUB. In their comparison of noncontrast CT with excretory urography, Niall et al were able to show a high sensitivity and specificity for CT as a primary imaging modality in diagnosis of renal colic.17 Our study confirms these results, with a vastly greater sample size. Sheley et al also looked at the use of helical CT as a diagnostic tool for ED presentation of acute flank pain.18 However, their follow-up protocol was devoid of image comparison. Both previous studies also failed to quantify the incidental detection of noncalculus findings with nonenhanced helical CT. In conclusion, at our institution, the use of nonenhanced helical CT scan for the evaluation of acute flank pain has provided rapid and accurate results. Information can be obtained within minutes. Reconstructions can be accomplished immediately at nearby workstations, through any indeterminate sections, without having to rescan the patient. Positive and negative predictive values of 88% and 98% were obtained and information is immediately available regarding other intraabdominal pathology when a calculus is absent on CT. REFERENCES 1. Brenner BM (ed): The Kidney. Philadelphia, PA, Saunders, 1996 2. Sommer FG, Jeffrey RB, Rubin GD, et al: Detection of ureteral calculi in patients with suspected renal colic: Value or reformatted noncontrast helical CT. AJR Am J Roentgenol 1995;165:509-513 3. Smith RC, Verga M, McCarthy S, et al: Diagnosis of acute flank pain: Value of unenhanced helical CT. AJR Am J Roentgenol 1996;166:97-101 4. Federle MP, McAnnich JW, Kaiser JA, et al: Computed tomography of urinary calculi. AJR Am J Roentgenol 1981;136:255-258 5. Smith RC, Rosenfield AT, Choe KA, et al: Acute flank pain:

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Comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995;194:789-794 6. Hartman GW, Hattery RR, Witten DM, et al: Mortality during excretory urography: Mayo clinic experience. AJR Am J Roentgenol 1982;139:919-923 7. Shehadi WM, Toniolo G: Adverse reactions to contrast media: A report from the Committee on Safety of Contrast Media of the International Society of Radiology. Radiology 1980;137:299-308 8. Barrett BJ, Carlisle EJ: Metanalysis of the relative nephrotoxicity of high and low osmolarity iodinated contrast media. Radiology 1993;188:171-179 9. Herring LC: Observations on the analysis of ten thousand urinary calculi. J Urol 1962;88:545-562 10. Mutgi A, Williams JW, Nettleman M: Renal colic: Utility of plain abdominal roentgenogram. Arch Intern Med 1991;151:1589-1592 11. Smith RC, Verga M, Dalrymple NC, et al: Acute ureteral obstruction: Value of secondary signs on helical unenhanced CT. AJR Am J Roentgenol 1996;167:1109-1113 12. Heneghan JP, Dalrymple NC, Verga M, et al: Soft-tissue ‘‘rim’’

sign in the diagnosis of ureteral calculi with use of unenhanced helical CT. Radiology 1997;202:709-711 13. Katz DS, Lane MJ, Sommer FG: Unenhanced helical CT of ureteral stones: incidence of associated urinary tract findings. AJR Am J Roentgenol 1996;166:1319-1322 14. Schultz RJ, Gignac C: Application of tissue-air ratios for patient dosage in diagnostic radiology. Radiology 1976;120:687-690 15. Fielding JR, Steele G, Fox LA, et al: Spiral CT in the evaluation of acute flank pain: A replacement for excretory urography. J Urol 1997;157:2071-2073 16. Dalrymple NC, Verga M, Anderson KR, et al: The value of unenhanced helical computerized tomography in the management of acute flank pain. J Urol 1998;159:735-740 17. Niall O, Russell J, MacGregor R, et al: A comparison of noncontrast computerized tomography with excretory urography in the assessment of acute flank pain. J Urol 1999;161:534-537 18. Sheley RC, Semonsen KG, Quinn SF: Helical CT in the evaluation of renal colic. Am J Emerg Med 1999;17:279-82