ACUTE URETERAL OBSTRUCTION - VALUE OF SECONDARY SIGNS ON HELICAL UNENHANCED CT

OBJECTIVE. The purpose of our study was to determine the value of seco ndary signs of ureteral obstruction on helical unenhanced CT. MATERIAL S AND METHODS. Over a 19-month interval, 312 patients with acute flank pain were imaged with helical unenhanced CT. Ureteral stone disease w as confirmed to be present in 109 patients and confirmed to be absent in 111 patients. Ninety-two remaining patients had no confirmatory ima ging studies or surgery and were unable to be contacted for follow-up. For each of the 220 patients with a confirmed diagnosis, we determine d the presence or absence of ureteral or collecting system dilatation, perinephric stranding, symmetry of renal size, and renal stones. In a ll patients with a ureteral stone, we noted the presence or absence of a circumferential rim of soft-tissue attenuation (''tissue-rim'' sign ) surrounding each stone and each phlebolith. RESULTS. The sensitivity of each secondary sign was ureteral dilatation, 90%; perinephric stra nding, 82%; collecting system dilatation, 83%; and renal enlargement, 71%. The specificity of each secondary sign was ureteral dilatation, 9 3%; perinephric stranding, 93%; collecting system dilatation, 94%; and renal enlargement, 89%. Ureteral dilatation and perinephric stranding were both present or both absent in 181 of the 220 patients with a co nfirmed diagnosis. In this subgroup, this combination of signs had a p ositive predictive value of 99% and a negative predictive value of 95% . The odds ratio for the frequency of the tissue-rim sign with stones versus tissue-rim with phleboliths was 31:1. CONCLUSION. When using un enhanced CT to help diagnose acute flank pain, if one does not see a u reteral stone or does see an indeterminate but suspicious calcificatio n, then secondary signs of obstruction are important for diagnosis. Th e results of this study form the basis of an imaging algorithm that ca n be used when interpreting unenhanced CT images of patients with acut e flank pain.