User preference affects dose for dual-source CT

A head-to-head comparison of CT scanner performance has found that user behavior is as important as performance in explaining why the dual-source Siemens Definition often exposes patients to more ionizing radiation than a single-source 64-slice CT system.

Sandra S. Halliburton, Ph.D, a medical physicist at The Cleveland Clinic, examined CT scans of 536 patients to reconcile differences in the claims and counterclaims of Siemens Medical Solutions and its competitors regarding the effective patient radiation exposure generated by the unique CT platform. Siemens officials have argued since introducing the dual-source Definition at the 2005 RSNA meeting that it requires half the radiation dose of single-source multislice scanners for some applications. Competitors say the configuration doubles radiation dose.

Halliburton's study uncovered problems with both arguments. Numerous features of the Definition limit radiation dosage, but patients scanned on the dual-source system for cardiac, cardiovascular, and thoracic applications were exposed to more radiation than the equivalent protocols performed on a 64-slice Siemens single-source platform in five of six setups. All the differences were statistically significant, she said.

Coronary imaging

For coronary artery imaging, 83 patients examined on the dual-source imager were exposed to an average of 13.2 mSv of radiation compared with 10.2 mSv for the 124 patients scanned on a 64-slice system. The mean exposure for pulmonary vein stenosis was 11.1 mSv with dual-source and 9.4 mSv on the single-source system. For thoracic/aortic disease, the mean effective dose of dual-source imaging was 11.8 mSv for 30 patients compared with 9.9 mSv for 42 patients examined on a 64-slice scanner (see table).

The dual-source system exposed patients to less radiation than the single-source scanner, however, for the diagnosis of arrhythmogenic right ventricular dysplasia, pericardial disease, and congenital heart abnormalities. The mean effective dose for this set of indications was 11.8 mSv compared with 12.8 mSv for single-source MSCT.

Context matters

Halliburton warned not to take the results out of context. The higher doses recorded on the dual-source Definition stemmed mainly from the users' preference for high mAs settings, possible because of the dual-source system's higher energy reserve.

"Presumably, a higher mA was selected for imaging the coronary arteries of our heavier patients," Halliburton said. "We are choosing higher values because the Definition gives us that option. It is not simply the dual-source design. It reflects technologist preference."

Halliburton also pointed to engineering innovations that limit patient radiation exposure on the Siemens Definition despite a core dual-detector design that would have doubled the exposure, if such radiation reduction strategies had not been applied.

"It is not just the addition of another [x-ray] source that influences the difference in radiation dose," Halliburton said. "It is these software issues and the other things that come together to affect what happens in the end."

The results challenge the manufacturer to properly educate physicians about their options when they establish protocols for the Definition, said Scott Goodwin, vice president of CT business at Siemens Medical Solutions. They can either use the extra power reserve for optimal coronary artery imaging or imaging of obese patients, or they can rely more heavily on built-in radiation reduction techniques for routine applications.