CT urography gets low-dose makeover

The need to deliver as little radiation as possible to patients was evident during a Monday scientific session on CT imaging of the urinary tract. Researchers presented studies that ranged in dose from 20 mAs to 200 mAs. Those on the high end were sure to be grilled.

Dr. Ulrike G. Mueller-Lisse and colleagues from the University of Munich posited that CT urography, as an add-on to contrast-enhanced CT of the abdomen and pelvis, may replace excretory urography in patients with pelvic tumors. But radiation exposure is a concern. So the researchers retrospectively compared upper urinary tract delineation in low-dose and routine CT urography.

Fourteen patients (116 upper urinary tract segments) received routine CT urography (120 kVp, pitch 0.875, 175 mAs), while 26 patients (344 upper urinary tract segments) received the low-dose protocol (29 mAs, other parameters unchanged). The scanner was a four-slice machine.

Delineation of the upper urinary tract was equally good for all segments in routine and low-dose CT urography, said coauthor Dr. Eva M. Coppenrath. The intrarenal collecting system and upper and middle ureter were clearly delineated as often on both protocols. Image quality of the lower ureter suffered with low-dose CT, however.

Coppenrath concluded that low-dose CT urography was sufficient to determine the course of the upper urinary tract and perhaps locate obstruction and dilation, but it would have been unlikely to locate intraluminal lower ureter lesions.

Dr. Eike Hein and colleagues from Charite Hospital in Berlin found that an ultralow-dose CT protocol - with a radiation dose comparable to a conventional abdominal survey (KUB) - can reliably detect urolithiasis.

Researchers prospectively examined 104 patients suspected of having renal colic. Sixteen-slice CT parameters included 120 kVp, 20 mAs, and pitch 23. All patients also received an ultrasound.

The mean effective whole-body dose of the CT scan was 0.5 mSv in men and 0.7 mSv in women. These values are fairly equivalent to the dose of KUB (0.5 to 0.9 mSv), Hein said.

The incidence of renal calculi detected by CT was 69% in all cases, 78% in the presence of hematuria, and 54% without hematuria. All renal calculi identified by ultrasound were detected on CT.

"Significantly more ureteral calculi were identified on CT than on ultrasound, whereas there was no significant difference in the diagnosis of renal calculi or urinary tract dilatation," Hein said.

The overall sensitivity for low-dose CT was 97% compared with 63% for ultrasound. Coronal views are suitable for quickly determining the size and location of calculi, she said

Radiologists' subjective diagnostic confidence in interpreting the ultralow-dose CT scans was 80% for detecting calculi and 91% for diagnosing of urinary tract dilatation.