Bristol, UK - Multislice computed tomography (MSCT) using a 16-slice scanner delivers more than twice the radiation than conventional angiography, a new study shows. In a paper published online April 19, 2006 in the Journal of the American College of Cardiology, Duncan R Coles and colleagues (Bristol Heart Institute, UK) say operators need to pay closer attention to radiation dose when using MSCT.

"Coronary CT delivers a relatively high radiation dose for a purely diagnostic procedure," the authors write. "It also delivers a significantly higher effective dose than conventional angiography to patients being investigated for coronary artery disease."

Excitement over the possibility of detecting coronary artery disease using MSCT has mounted in recent years, particularly with the advent of increasingly sophisticated machines, including 64- and 128-slice scanners. As Coles et al point out, however, actual radiation doses from cardiac MSCT in the clinical setting are unknown.

For their study, Coles and colleagues estimated effective radiation dose in 180 patients with suspected coronary artery disease undergoing MSCT or conventional angiography. A subset of 91 patients underwent both types of imaging tests. During MSCT, patients scanned with 16 detectors instead of 12 received significantly higher effective radiation doses (15.3 mSv vs 14.2 mSv). When compared with conventional angiography, MSCT was associated with statistically significant higher estimated radiation doses.

The authors emphasize that radiation dose results relate to the protocols in use at the time the study was conducted. "Ideally, operators should optimize protocols to achieve adequate image quality for different-sized patients and should be aware of the impact this will have on patient dose," they say.

"With the increasing availability of MSCT cardiac scanners, operators must be aware of the radiation dose and the factors that affect it for both clinical and research protocols," Coles et al conclude.

In an accompanying Viewpoint, Dr Pat Zanzonico (Memorial Sloan Kettering Cancer Center, New York, NY) and colleagues argue that the radiation risk associated with MSCT must be put in perspective.

"When weighing the relative risk of any medical procedure, it is imperative that one consider the overall risk of alternative procedures," they write. The lifetime cancer mortality risk associated with the radiation doses reported by Coles et al for the general population would be 0.02% for conventional angiography and 0.07% for MSCT. However, they add, nonradiogenic risk of mortality associated with conventional angiography—because of its invasive nature—is 0.11% by some estimates; accordingly, the overall mortality risk of conventional angiography is 0.13%—nearly twice that of the radiogenic risks of MSCT, they estimate.

"For an equivalent diagnostic efficacy of MSCT angiography and conventional angiography, MSCT angiography emerges as the safer of these two alternatives, despite its higher radiation dose," Zanzonico et al write. If the entire US population between ages 50 and 55 were to be screened for CAD using MSCT until age 70, the anticipated aggregate increased risk of fatal cancer would be 42 900; however, screening would also identify many patients with significant stenosis, they write. "If this procedure prevented even 10% of the estimated 355 000 sudden deaths each year, the trade-off would be well worthwhile," Zanzonico et al conclude.

Also commenting on the study for heartwire, Dr Jörg Hausleiter (Deutsches Herzzentrum, Munich, Germany) called the research "very interesting . . . supporting the need for physicians to be aware of the dose that a patient could obtain from cardiac MSCT."

The magnitude of dose described in this study for MSCT angiography is certainly not helpful to promote this very exciting new technology.

Hausleiter pointed out that while the methodology used by the authors to calculate effective dose for both modalities appeared to be valid, the values for MSCT are higher than other reports. In research conducted by Hausleiter et al, in a larger population, the MSCT average dose was roughly half that of what Coles et al reported.

Reasons for this difference include the fact that Coles et al scanned longer ranges: 155 mm instead of the more standard 125 mm; a 25% increase in length corresponds to a 25% increase in dose, Hausleiter pointed out. While CABG patients, who require longer scan lengths, were included in Coles et al's study, this does not account for all of the increase in scan length, he told heartwire. In addition, dose-saving algorithms—a topic recently explored by Hausleiter and colleagues, as reported by heartwire [3]—were mentioned by the authors, but were not applied.

"If they would have taken care of these issues, the MSCT dose would have been significantly lower, probably about 50% to 60%," Hausleiter stated. As such, the study stands as a good example of the type of risk a patient is exposed to if such algorithms are not used, he added.

"The magnitude of dose described in this study for MSCT angiography is certainly not helpful to promote this very exciting new technology."