OBJECTIVE: The objective of this study was to assess the diagnostic value of attenuation measurements of the kidney on unenhanced helical CT in patients with obstructive ureterolithiasis.
MATERIALS AND METHODS: Consecutive unenhanced helical CT scans of patients referred for acute unilateral renal colic were retrospectively reviewed. Patients with CT evidence of other urinary system diseases were excluded. Included scans (n = 145) were assessed for ureteral stone and secondary signs of obstruction such as unilateral collecting system or ure-teral dilatation, perinephric stranding, and periureteral edema. Renal attenuation in Hounsfield units was measured in the upper, middle, and lower portions of the parenchyma, and a mean value was determined for each kidney.\
RESULTS: Ureteral stones were present in 76 patients. Renal attenuation on the side with lithiasis was lower than on the opposite kidney: 27.2 � 3.9 H vs 32.6 � 3.4 H (p < 0.001). Attenuation differences between kidneys were higher for patients with ureterolithiasis: 5.4 � 3.2 H (range, -3.3 to 13.0 H) versus 1.2 � 1.0 H (range, 0-4.7 H) (p < 0.001). An attenuation difference between kidneys greater than or equal to 5.0 H had 61% sensitivity, 100% specificity, 100% positive predictive value, 69% negative predictive value, and 79% accuracy for diagnosis of ureteral lithiasis.
CONCLUSION: Attenuation difference between kidneys greater than or equal to 5.0 H was a valuable sign and had diagnostic performance similar to other secondary signs of obstructive ureterolithiasis. Furthermore, attenuation difference had the advantage of being an objective, measurement-based indicator.