UNENHANCED HELICAL CT OF URETEROLITHIASIS - VALUE OF THE TISSUE RIM SIGN

OBJECTIVE. The tissue rim sign-a rim or halo of soft-tissue attenuatio n seen around the circumference of an intraureteral calculus on unenha nced axial CT-has been described as in differentiating ureteral calcul i from extraurinary abdominal or pelvic calcifications. The purpose of this study was to determine the prevalence of the tissue rim sign in patients with ureterolithiasis and extraurinary calcifications and to determine the relationship between the tissue rim sign, the size of a calculus, and the degree of urinary obstruction. MATERIALS AND METHODS . Unenhanced helical CT studies followed by excretory urography were o btained in 59 patients with suspected acute ureterolithiasis. Each cal cification along the expected course of the ureter seen on axial CT sc ans was categorized as a ureteral calculus or as an extraurinary calci fication, Each categorization was based on CT, urographic, and clinica l findings and the presence or absence of a tissue rim sign, When the outer wall of the ureter could not be seen because there was no clear fat plane at the level of the calcification on CT, the sign was catego rized as ''indeterminate.'' The size of the calculus was measured on C T, and the degree of urinary obstruction was estimated on the basis of the urograms. RESULTS. Thirty-two patients each had a single ureteral calculus. Of these patients, CT revealed a positive tissue rim sign i n 16 patients (50%), was negative in five patients (16%), and was inde terminate in 11 patients (34%), In addition, we saw 57 extraurinary ca lcifications in 18 patients (11 patients with ureteral calculi and sev en patients without ureteral calculi). None of the 57 extraurinary cal cifications was associated with a positive tissue rim sign, The tissue rim sign was negative in 39 (68%) of the 57 extraurinary calcificatio ns and indeterminate in the remaining 18 (32%). Ureteral calculi with a negative tissue rim sign were larger than ureteral calculi with a po sitive tissue rim sign (p <.01). A high degree of obstruction was pres ent in four of five patients with ureteral calculi for which CT showed a negative tissue rim sign. Conversely, six of 16 patients in whom CT revealed a positive tissue rim sign also had a high degree of obstruc tion. Therefore, no clear relationship was found between the degree of obstruction and the presence of a positive tissue rim sign. CONCLUSIO N. A positive tissue rim sign is specific for the diagnosis of uretero lithiasis. However, a negative tissue rim sign does not preclude such a diagnosis. The presence or absence of this tissue rim sign correlate s with the size of a calculus but not with the degree of urinary obstr uction. When CT reveals an indeterminate tissue rim sign, careful insp ection for other CT findings, such as ipsilateral ureteral dilatation, perinephric edema, dilatation of the intrarenal collecting system, an d renal swelling, is necessary.