Triple rule-out study showing a pulmonary embolus (arrow). Source: Halpern, Ethan J. Clinical Cardiac CT: Anatomy and Function. Thieme Medical Publishers, 2008. Reprinted with permission by the author.

Physicians can reduce the radiation dose delivered to patients undergoing coronary CT angiography in a "triple rule-out" protocol by more than half just by using tube current modulation, according to a study published in the April issue of the American Journal of Roentgenology.

The study, performed at Thomas Jefferson University Hospital in Philadelphia, included 172 patients who were evaluated using coronary CT angiography (CCTA) without tube current modulation and 95 patients who were evaluated with tube current modulation. The objective was to evaluate the reduction in effective radiation dose through the use of tube current modulation in patients who underwent a triple rule-out CCTA evaluation, which also includes evaluation for pumonary embolism and aortic dissection, and to document how effective radiation dose was impacted by patient age, sex and body mass index (BMI).

The effective radiation dose ranged between 9.9 and 31.3 mSv in patients without modulation; with modulation, the dose ranged between 5.4 and 16.6 mSv.

Researchers retrospectively analyzed 267 consecutive triple rule-out CCTA exams performed on a 64-slice CT scanner in low- to moderate-risk patients suspected of having acute coronary syndrome. Patients presented to the emergency department of a single university hospital between October 2006 and March 2008. Tube current modulation was generally used in patients with heart rates below 65 beats per minute during the second half of the study period as a way to reduce radiation exposure.

According to the results, among the 172 patients evaluated without tube current modulation, effective dose averaged (± SD) 18.0 ± 5.6 mSv (range, 9.9-31.3 mSv). Of the 95 patients who underwent CTA examination with tube current modulation, effective dose was significantly lower at 8.75 ± 2.64 mSv (range, 5.4-16.6 mSv) and image quality was better as compared with exams without tube current modulation. There were no significant radiation differences by patient age, but tube current modulation decreased radiation exposure by at least half.

Among the studies in which tube current modulation was not used, women received less radiation than men (17 vs. 19.5 mSv, respectively). For the studies with tube current modulation, there were no radiation differences by sex. Obese patients received significantly more radiation than overweight and normal-weight patients in the non-tube current modulation groups (20.9 mSv vs. 15 and 14.9 mSv, respectively) and in the tube current modulation groups (10.3 mSv vs. 7.6 and 7.1 mSv).

The overall effective radiation dose for triple rule-out CCTA was reduced by more than 50 percent with ECG-based tube current modulation without loss of image quality, according to Kevin M. Takakuwa, MD, lead author of the study.

"One of the major criticisms of the triple rule-out coronary CT angiography study is the concern about the high amount of radiation given, which has been estimated by some to be as high as 30-40 mSv. Our study demonstrates that the radiation is a lot less, averaging less than 9 mSv when using tube current modulation. In addition, it uses less radiation than a nuclear stress test, a common alternative study to the triple rule-out cardiac CT," he said. "Our goal is to be able to perform cardiac CT using tube current modulation 24/7 and as a means to be able to admit or discharge patients more rapidly."

The authors concluded, "We can ill afford to dismiss the 64-MDCT angiography triple rule-out evaluation because of the theoretic future cancer risk when there is potential to miss immediate life-threatening entities in an emergency department setting--especially when alternative tests often require a similar or higher level of radiation exposure. Tube current modulation should be used for triple rule-out CTA examinations whenever possible."