Ultra-Low CT Dose Accurately Detects Renal Stones
Ultra-low-dose CT compares favorably with standard CT for renal stone imaging.
BANFF, Alberta—Ultra-low-dose computed tomography (CT) can accurately discriminate between true cases of renal colic and pathologies with symptoms due to other causes, according to a study presented at the Canadian Urological Association (CUA) annual meeting.
In a prospective study of 56 patients with suspected renal colic, ultra-low-dose CT had a sensitivity of 92% and a specificity of 100% for detecting renal stones. The sensitivity increased to more than 97% when the stones were larger than 2 mm in diameter.
“Once we publish a manuscript on our results, our plan is to come up with guidelines between urology, radiology and emergency medicine for the use of the ultra-low-dose protocol,” said Iain Kirkpatrick, MD, Associate Professor of Diagnostic Radiology at the University of Manitoba in Winnipeg and Adjunct Clinical Instructor in diagnostic radiology at Stanford University in Palo Alto, Calif. “We would want to target it at patients who have a high pretest probability of calculi and obstructive uropathy, since the ability to detect other pathology is hindered. For now, we are still performing them under the study protocol.”
The American Urological Association and the European Association of Urology have both adopted recommendations for the use of low-dose CT for renal colic. The new study takes this a step further by focusing on ultra-low-dose CT, which gives a radiation dose of just 20 mA. This translates into a greater than 95% reduction in radiation dose compared with standard CT when the length of the scans is taken into consideration.
Jason Archambault, MD, a senior urology resident at the University of Manitoba, led the study under Dr. Kirkpatrick's supervision. The study began in November 2010 and is ongoing. The analysis presented at the CUA meeting included 56 emergency-department patients with suspected renal colic who underwent a standard CT of the kidneys, ureters, and bladder. An ultra-low-dose CT scan was also performed on each patient, taking less than one additional minute. The two sets of CTs from each patient were each interpreted by four blinded radiologists.
The subjects had a mean age of 58 years and a mean body mass index (BMI) of 29 kg/m2.
Thirty-six of the patients were found on standard CT to have an obstructing stone and 33 of these were also found to have a stone on ultra-low-dose CT. There were no false positives with the ultra-low-dose approach. There was a high inter-observer agreement between the four radiologists who read the CT scans. This included a high level of agreement about the presence of hydronephrosis, which was a secondary outcome measure.
The average size of the stones in patients with false negatives—that is, in whom ultra-low-dose CT did not show a stone—was 2.4 mm. In comparison, the true positives' average stone size was 3.8 mm. There was not a significant difference in BMI between the true-positive and false-negative groups.
The investigators found that image quality was worse in high-BMI patients, but the study was not large enough to show a statistically significant difference.
“We did not find that there was a significant difference in diagnostic utility of the scan by BMI, but again this could be because of the low patient numbers,” Dr. Kirkpatrick noted. “Once all the data are in we may end up recommending a modified protocol for high-BMI patients, but this is still to be worked out.”
The radiologists had an average 75% rating of being ‘very confident' in diagnosis with standard CT and an average 48% rating of this level of confidence with ultra-low-dose CT.
“One of the things we intend to look at is whether tests interpreted at the end of the study tend to be read with more confidence than those at the beginning,” Dr. Kirkpatrick said. “The relatively low observer confidence with ultra-low-dose CT that we found overall does not reflect low enthusiasm at all for the protocol. The idea for the ultra-low dose exams was a joint one between urology and radiology, who have an excellent working relationship in Winnipeg.”