Cardiac CT triple rule-out debunks radiation fears

For years, high radiation dose exposure has been the bogeyman that kept the cardiac CT triple rule-out exam for chest pain from widespread application. That concern has been addressed with the use of radiation dose reduction techniques, according to a Thomas Jefferson University study.

"The amount of radiation that people get from a triple rule-out study is not as high as everybody thinks it is," said principal investigator Dr. Kevin Takakuwa, an assistant professor of emergency medicine at TJU.

That finding will be welcomed by supporters of the triple rule-out approach. It proposes using a wide field-of-view to image the entire chest during multislice CT to rule out myocardial infarction for chest pain patients considered at low risk of infarction from an initial evaluation in a hospital emergency room. Triple rule-out also examines these patients for possible aortic dissection and pulmonary emboli, the other leading causes of acute angina.

Takakuwa and colleagues retrospectively assessed 267 consecutive patients who underwent triple rule-out cardiac CT angiography after presenting to the ER with symptoms of acute coronary syndrome. Ninety-five patients underwent the triple rule-out CTA protocol with tube current modulation, while the other 172 did so without it.

While patients getting triple rule-out without tube current modulation received average effective radiation doses of 18 mSv, the investigators found those getting the exam with tube current modulation had their effective radiation dose reduced by more than 50% without image quality loss. Findings were published in the American Journal of Roentgenology (AJR 2009;192:866-872).

Average effective dose for patients undergoing triple rule-out CTA without tube current modulation was 18 mSv (range, 9.9 to 31.3 mSv). Average effective dose for patients undergoing the exam with tube current modulation was 8.75 mSv (range, 5.4 to 16.6 mSv). The difference was statistically significant (p<0.0001). There were no significant radiation differences by patient age. Women received less radiation than men (17 versus 19.5 mSv, respectively) among patients in which tube current modulation was not used (p<0.001).

Some studies have estimated effective doses to be as high as 40 mSv, causing radiologists in the audience for the presentation of such results to shift nervously in their seats from the prospect of exposing their patients to so much radiation. The Thomas Jefferson study demonstrates, however, that the radiation can be far less when using tube current modulation. It can be even less radiation than a nuclear stress test, which is the alternative most people use, Takakuwa said.

The average effective dose for nuclear stress tests ranges from 10 to 17 mSv, according to the clinical literature.

Misconceptions about radiation dose are not the only obstacle to mainstream acceptance of the triple rule-out, Takakuwa said. Only a few studies on the technique have been published. Not many physicians really know how to perform the test properly, and thus they cannot get adequate imaging.

Previous studies by the Thomas Jefferson research team have shown that the test can produce consistent results. One in four patients presenting to the ER at TJU hospital already gets a definitive diagnosis with it, he said.

"The triple rule-out study is going to become the diagnostic imaging study of choice, particularly for undifferentiated chest pain," Takakuwa said.

Takakuwa's group has just submitted another paper for publication that compares the triple rule-out observation protocols with those of the nuclear stress test strategy. Findings of that study suggest results from the triple rule-out exam can reduce hospital stays.

"We hope that this helps people realize that the triple rule-out is actually a good study," Takakuwa said.