Cardiac imaging dose strategies attract close evaluation

A Monday morning scientific session covering cardiac CT dose and noise broke down into newsworthy matched pairs: two studies examining the implication of CT radiation on public health, two investigating the impact of radiation reduction strategies on patient dose and image quality, and a third pair considering how noise reduction from iterative reconstruction affects the quality of images of highly calcified vessels and morbidly obese patients. 

The appearance at this meeting of two randomized clinical trials on the effects of CT radiation reflected the growing sophistication of research sparked by public concern about high doses of ionizing radiation associated with multislice CT, according to session moderator Dr. Konstantin Nikolaou, an associate professor of radiology at the University of Munich in Germany.

Nikolaou credited multislice CT manufacturers for a series of innovations, including prospective triggering, retrospective gating, and FLASH mode for dual-source imaging, that have engineered substantial reductions in patient radiation exposure during cardiac CT.

“Iterative reconstruction is now on the market, and we will see another dose reduction of 30% to 40% in normal-weight patients,” he said in an interview. “If you are talking about a prospectively triggered scan, no matter which 64-slice scanner you are using, a normal step-and-shoot prospective triggered scan will expose the patient to about 4 mSv. This is very good compared with spiral modes that involve 15 to 20 mSv.”

In terms of actual radiation exposure, Konstantinos Perisinakis, Ph.D., and colleagues at the University Hospital of Heraklion, on the island of Crete in Greece, presented specific data on the absolute and relative risks of lung, breast, and total cancer for 136 patients who underwent ECG-gated cardiac CT angiography on a 256-slice scanner. The average ages of men and women in the trial were 41 and 45 years respectively. Lifetime attributable risks of site-specific solid cancers were estimated using sex- and age-specific risk factors and criteria from the National Academy of Sciences’ BEIR (Biological Effects of Ionizing Radiation) VII report.

Perisinakis estimated that the lifetime risk for a solid cancer associated with axial coronary CT angiography is 0.025% and 0.007% for women and men respectively. The relative risk of lung cancer due to CCTA is 1.003 and 1.008 for women and men respectively. He concluded that a CCTA scan may add 1.3 and 0.3 new cases to every 1000 natural breast cancer cases for women who are younger than 50 years and older than 65 respectively.

Dr. Dominik Geisel described the sophisticated evaluation of the genetic effects of radiation from 320-slice CT and CCTA conducted by his group, from the Charité Hospital in Berlin. Their randomized study of 31 patients found that radiation from both modalities resulted in DNA double-strand breaks within hours of imaging, though the genetic damage was repaired spontaneously within 24 hours.

Dr. Jonathan Leipsic, an assistant professor of radiology and cardiology at the University of British Columbia in Canada, tested the impact of tube voltage reduction on the image quality of 64-slice CCTA compared with x-ray coronary angiography.

Fifty consecutive patients with suspected coronary artery disease were randomly assigned to receive CCTA either with tube current reduced to 80 or 100 kVp, depending on body mass, or a standard tube current of 100 or 120 kVp, again determined by body mass. No significant differences between the groups in age, sex, heart rate or body mass index were identified between the two groups.

Reduced tube voltage was associated with significantly less radiation dose. The median interquartile radiation dose was 2.6 mSv for standard kVp compared with 1.5 mSv for reduced kVp. Three blinded physicians read the scans. No significant differences in persegment sensitivity or specificity were identified for the standard and reduced tube voltage groups.

Matthias Renker, a colleague of Dr. U. Joseph Schoepf in the cardiac imaging lab at the Medical University of South Carolina, demonstrated in a study involving 25 patients that reducing noise with iterative reconstruction reduces artifacts from coronary artery calcifications to reveal stenoses that are otherwise obscured.

Noise reduction from iterative reconstruction is also proving useful at the Oregon Health and Science University in Portland, where Dr. Dianna M.E. Bardo and colleagues are applying it to morbidly obese patients before bariatric surgery. In a study of 30 patients with an average body mass index of 38.6 kg/m², Bardo and colleagues used the lower noise characteristics possible with iterative reconstruction to significantly improve the coronary artery image quality for these hard-to-image patients.