The value of prone scanning to distinguish ureterovesical junction stones from ureteral stones that have passed into the bladder: leave no stone unturned.

OBJECTIVE: When a stone is located in the region of the ureterovesical junction on CT studies, it is important to determine if the stone is impacted at the ureterovesical junction or has already passed into the bladder. The purpose of this study was to determine if CT findings can be used to distinguish stones impacted at the ureterovesical junction from stones that have already passed into the bladder. SUBJECTS AND METHODS: From February 1, 1997, to January 31, 1998, 37 patients with acute flank pain underwent unenhanced CT studies in the supine position that showed stones in the region of the ureterovesical junction. To determine stone location, limited rescans were obtained with the patients in the prone position. The supine images were reviewed retrospectively by one of the authors, who was unaware of the findings of the prone scans. Forty-one stones were classified as being impacted at the ureterovesical junction or as having passed into the bladder.
RESULTS: On CT, 23 stones were correctly classified as being impacted at the ureterovesical junction; nine stones were correctly classified as having passed into the bladder; and nine stones were incorrectly classified as being within the bladder. We calculated a sensitivity of 72%, a specificity of 100%, a positive predictive value of 100%, a negative predictive value of 50%, and an accuracy of 78% for CT in revealing stones impacted at the ureterovesical junction.
CONCLUSION: If patients are imaged in the supine position only, unenhanced helical CT cannot reliably distinguish stones impacted at the ureterovesical junction from stones that have already passed into the bladder. A prone scan can be used to make this distinction.