Applied Radiology

Traditionally, intravenous urography has been considered the definitive test for evaluating patients who have acute flank pain and are suspected of having ureteral stones. Recently, however, it has become apparent that noncontrast helical CT (HCT) has major advantages over intravenous urography in the evaluation of patients with urolithiasis.

Noncontrast HCT studies can be completed much more rapidly than intravenous urography, as there is no need for oral or IV contrast administration or other patient preparation. Patients undergoing HCT can be scanned immediately, and the scan time is less than 5 minutes, whereas intravenous urography requires a minimum of 30 minutes, and completion of the study often takes hours if ureteral obstruction is present. Noncontrast HCT avoids the discomfort, inconvenience, and risks associated with the injection of contrast material.

Virtually all types of urinary tract calculi can be visualized with noncontrast HCT (figure 1).l-3 In contrast, a substantial number of urinary tract calculi cannot be visualized with either standard radiography or with intravenous urography.4 Finally, noncontrast HCT is more useful than intravenous urography in detecting important intraabdominal abnormalities other than those associated with urolithiasis.

Acute flank pain is a common complaint in patients with any of a number of disorders. Diagnostic considerations for these patients include ureteral calculi, gynecologic disease, appendicitis, diverticulitis, and biliary tract disease. Rapid diagnosis of a ureteral stone is important to conclude the search for other etiologies and to avoid unnecessary procedures. In addition, identification and characterization of a ureteral stone assists in planning for proper management.

Recent studies have proven that noncontrast HCT is the most accurate technique for detecting urinary tract calculi, including ureteral stones.4-8 It has been demonstrated that stones are more accurately detected with noncontrast HCT than standard radiography, nephrotomography, intravenous urography, or sonography.9,10 In studies using CT as the "gold standard," it has been shown that conventional radiography has a sensitivity of only 50 to 70% for detecting ureteral calculi.9 Overall, noncontrast HCT has a sensitivity of 0.97, a specificity of 0.96, and an accuracy of 0.97 for diagnosis of ureteral stone disease.5 Additionally, research has shown that noncontrast HCT has a positive predictive value of 97% and a negative predictive value of 98% in the diagnosis of ureteral calculi,5 substantially exceeding the accuracy of intravenous urography.4

Although noncontrast HCT provides no direct functional information, it has been found that the size and location of a ureteral stone can be determined more accurately with this technique than with intravenous urography.10 The size and location of stones, coupled with patient symptomatology including duration and severity of pain, evidence of infection, and patient occupation, are the major determinants in planning treatment of ureteral stones.11

Technique

In the evaluation of patients with acute flank pain, collimation should be 5 mm, and scanning should extend from the top of the kidneys to the bottom of the bladder. A pitch of 1:1 to 1.5:1 is desirable, and scanning should be completed during a single breath-hold, if possible. However, because respiratory motion in this area is minimal, the pelvic segment of the scan can be performed during unlabored breathing, if necessary. Scanning requires approximately 30 seconds. If deemed necessary, reformatted images can be obtained after evaluation of the standard images.

Image interpretation

Diagnostic findings of ureteral stones can be categorized as primary or secondary. The primary finding indicative of a ureteral stone is visualization of the stone within the ureter. Unlike standard radiography, where 10 to 15% of urinary tract calculi are inadequately mineralized to be radiopaque, virtually all urinary tract stones are radiopaque with noncontrast HCT (figure 1).1-3 Exceptions to this rule include unmineralized stone matrices and stones related to the protease inhibitor Indinavir.12,13 However, these types constitute a minority of ureteral stones. With noncontrast HCT, the attenuation values of ureteral stones are much higher than those of surrounding soft tissues, making the stones more readily detectable. Uric acid stones, which are radiolucent with standard radiography, have attenuation values of 300 to 500 Hounsfield units (HU) (figure 1).13 Calcium stones, in comparison, have attenuation values in excess of 1000 HU.l4 Visually, all of these urinary tract calculi demonstrate homogeneously high attenuation, similar to that of bone cortex. For accurate detection of a ureteral stone, the ureter should be followed caudally on sequential HCT images.

The upper ureter courses from the renal pelvis adjacent to the psoas muscle. The ureter is positioned in the upper abdomen, behind the ipsilateral gonadal vein (figure 2A). In the lower abdomen, it extends medially; the gonadal vein extends laterally. At the level of the iliac crest, the ureter is medial to the gonadal vein. In the pelvis, the ureter is anterior to the internal iliac artery (figure 2B). At the level of the seminal vesicle, or the vaginal fornix in female patients, the course of the ureter is almost directly horizontal as it enters the bladder (figure 2C).

In addition to primary visualization of a ureteral stone, secondary signs

are important for diagnosis. These signs include unilateral ureteral dilatation (sensitivity, 0.87; specificity, 0.90) (figure 3A), unilateral perinephric soft-tissue stranding (sensitivity, 0.76; specificity, 0.90) (figures 3B,4), asymmetric intrarenal collecting system dilatation (sensitivity, 0.80; specificity, 0.91), and unilateral renal enlargement (sensitivity 0.64; specificity 0.89).15 A combination of both unilateral renal dilatation and ipsilateral perinephric stranding has a positive predictive value of 0.96 for ureteral stones.l5 The absence of both of these signs has a negative predictive value of 0.93 for the presence of a ureteral stone.l5

In practice, ureteral dilatation and perinephric stranding are almost always

present in conjunction with a ureteral stone. The absence of these two signs almost completely excludes the possibility of a ureteral stone. However, because secondary signs are absent in a small percentage of patients even when a ureteral stone is present, a diligent search of the images for a ureteral stone is necessary in every case. Furthermore, in symptomatic patients with secondary signs, such as perinephric stranding and ureteral dilatation, but no ureteral stone or evidence to support an alternative diagnosis, the likely diagnosis is recent passage of a ureteral stone, with persistent edema at the ureterovesicular junction.

Interpretation pitfalls

The major difficulty in interpreting noncontrast HCT images of patients with acute flank pain is in distinguishing between phleboliths and ureteral stones, particularly in the pelvis. In addition to secondary signs that, when present, strongly suggest a ureteral stone and when absent suggest that pelvic calcifications are more likely to be outside the urinary tract, other signs have proven helpful in making this distinction. The soft-tissue "rim" sign (figures 5,6) describes a circumferential soft-tissue ring around a calcification.16-18 This sign is thought to be indicative of ureteral wall edema at the site of an impacted stone. Presence of this sign strongly indicates that the calcification represents a ureteral stone rather than a phlebolith (figure 7). The rim sign has a reported specificity of 92% for distinguishing a stone from a phlebolith.l6 The absence of a rim sign is less useful, as it is not seen in approximately one-third of stones larger than 5 mm in diameter. Smaller stones almost always are associated with a rim sign; 90% of stones 4 mm or smaller have associated tissue rim signs.l6 Hence, a 4 mm or smaller calcification without a tissue rim sign is almost certainly a calcified phlebolith. In ambiguous cases, image reformatting may improve detection of this rim sign. Scrutiny of standard 5-mm HCT images reformatted at 3-mm intervals often improves detection of the tissue rim sign, enhancing confidence in the diagnosis of a ureteral stone.

Another secondary sign that is useful for differentiating stones from phleboliths is the "comet-tail" sign.17 This term describes a curvilinear soft-tissue band extending from the suspect calcification on serial images. This soft-tissue band is thought to represent a vein in which the phlebolith has developed, therefore indicating that the associated calcification is a phlebolith rather than a ureteral stone.

Another important pitfall when interpreting noncontrast HCT scans in patients with suspected urolithiasis is the tendency to overlook the presence of other abnormalities. Abnormalities causing symptoms are detected in about one-third of patients who have flank pain but do not have ureteral stones.5 These include extraurinary abnormalities and urinary tract abnormalities other than stone disease. Standard noncontrast HCT scanning is usually adequate for diagnosis of commonly encountered abnormalities that mimic stone disease, such as appendicitis (figure 8), leaking abdominal aortic aneurysm, diverticulitis, and adnexal diseases. However, additional imaging may be useful to better delineate abnormalities suspected after a noncontrast HCT scan. Contrast-enhanced CT may be a helpful alternate modality in this scenario, due to its ability to improve detection of renal infections (figure 9), renal neoplasms, and numerous extraurinary tract abnormalities (figure 10). Oral or rectal contrast agents can be administered to aid in detection of bowel-related diseases.

Techniques other than CT also can be utilized for further evaluation of patients with suspected urolithiasis. On occasion, limited intravenous urography can be performed after a nondiagnostic noncontrast HCT study. This technique is particularly useful when the distinction between phlebolith and ureteral stone cannot be made on the basis of noncontrast HCT results. An intravenous urogram usually demonstrates clearly the ureteral course and its relationship to pelvic calcifications.

Other applications of noncontrast HCT for urolithiasis

Several studies have shown that noncontrast HCT is superior to radiography, nephrotomography, and sonography for detection and measurement of urinary tract stones.9,l0,l9 Therefore, it may be surmised that noncontrast HCT can be useful in the detection of nonobstructing urolithiasis and in accurate assessment of urinary tract stone burden and adequacy of stone removal therapies. There also is evidence that noncontrast HCT findings can be used to predict which patients will spontaneously pass their stones with conservative management alone.20 Small stones (less than 3 mm in diameter) associated with high-grade perinephric stranding and perinephric fluid collections are significantly more likely than other stones to pass spontaneously.20 Noncontrast HCT also can be useful in three-dimensional reconstructions of the upper urinary tract for planning treatment of branched calculi.2l Finally, there is evidence that the chemical composition of urinary tract calculi can accurately be determined by using the CT attenuation characteristics of detected urinary tract stones.l4 This may be useful in treatment planning, as some stones may be resistant to extracorporeal lithotripsy, thereby indicating the need for an alternative treatment.22

Conclusion

Noncontrast HCT is more accurate than any other imaging technique for the detection of urinary tract stones. It is a noninvasive, rapid technique with numerous advantages over more traditional imaging studies. Noncontrast HCT accurately supplies information crucial to clinical decision making in these cases. In addition, the ability of noncontrast HCT to detect abnormalities other than urinary tract stone disease is a major advantage. This modality should replace intravenous urography for imaging of patients with acute flank pain. AR

References

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13. Gentle DL, Stoller ML, Jarrett TW, et al: Protease inhibitor-induced urolithiasis. Urology 50:508, 1997.

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16. Heneghan JP, Dalrymple NC, Verga M: Soft-tissue "rim" sign in the diagnosis of ureteral calculi with use of unenhanced helical CT. Radiology 202:709-711, 1997.

17. Bell TV, Fenlon HM, Davison BD, et al: Unenhanced helical CT criteria to differentiate distal ureteral calculi from pelvic phleboliths. Radiology 207:363-367, 1998.

18. Kawashima A, Sandler CM, Boridy IC, et al: Unenhanced helical CT of ureterolithiasis: Value of the tissue rim sign. AJR 168:997-1000, 1997.

19. Remer EK, Herts BR, Streem SB, et al: Spiral noncontrast CT versus combined plain radiography and renal US after extracorporeal shock wave lithotripsy: cost-identification analysis. Radiology 204:33-37, 1997.

20. Takahashi N, Kawashima A, Ernst RD, et al: Ureterolithiasis: Can clinical outcome be predicted with unenhanced helical CT? Radiology 208:97-102, 1998.

21. Liberman SN, Halpern EJ, Sullivan K, et al: Spiral computed tomography for staghorn calculi. Urology 50:519-524, 1997.

22. Dyer RB, Zagoria RJ: Radiological patterns of mineralization as predictor of urinary stone etiology, associated pathology, and therapeutic outcome. J Stone Dis 4:272-282, 1992.

Dr. Zagoria is Professor of Radiology at Wake Forest University School of Medicine in Winston-Salem, NC.