Lowering voltage for CT viable in stone analysis

June 1, 2012

In patients with stone disease, lowering the tube voltage set in dual-energy computed tomography by as much as 38% allowed a dose reduction without impairing stone analysis, a recent investigation showed.

This dose-reduced, dual-energy protocol enabled a significant lowering in patient radiation exposure without a reduction in the quality of results, reported first author Arkadiusz Miernik, MD, senior resident and fellow in minimally invasive urologic surgery at Freiburg University, Freiburg im Breisgau, Germany.

Ultra low-dose CT is considered the current gold standard for the detection of urinary stones. A tomographic differentiation of urinary stone composition between uric and non-uric acid stones using the dual-energy mode makes this modality of particular interest to clinicians.

The study, which was carried out in conjunction with the Freiburg University department of radiology, involved a retrospective evaluation for dose-reduced dual-energy CT in 46 patients with urinary stones who underwent dual-energy CT (tube A: 140 kV/55 mAs; tube B: 80 kV/303 mAs). The investigators lowered the tube voltage by 38.8% in both tubes. They performed conventional stone analysis as a reference standard to objectify results after treatment was finished.

"We had some problems with smaller, mixed stones. There were three or four mixed urinary stones under 4 mm that were classified as non-uric acid stones, but this bore no clinical relevance," Dr. Miernik observed.

In spite of these encouraging results, urologists attending Dr. Miernik's presentation expressed concern about the small sample size, as only eight out of 35 stones turned out to be uric acid stones.

"The numbers are small in this study. To validate the findings, a larger patient group is needed," said session co-chair Noor N.P. Buchholz, MD, director of endourology and stone service at Barts and the London NHS Trust, London.

"We are, in fact, currently working on a larger comparative study that will be published in which we compare the same groups statistically," said Dr. Miernik. "The point is really that we are trying to find out how much we can lower the tube voltage and thereby the radiation exposure to the patient, without any loss in quality. That is the reason for the new voltage set. We found the actual patient radiation exposure to be by two conventional KUBs. This voltage set does not allow us to differentiate, for instance, between calcium oxalate stones, monohydrate, or dihydrate stones, etc.

"If I wanted to differentiate between different forms of calcium oxalate, I would need to go up to four to six KUBs per patient. This setting really only allows a differentiation between uric acid and non-uric acid stones, however, with very low radiation exposure to the patient," Dr. Miernik said.