Report from Stanford MDCT: Low-dose technique makes 64-slice heart scans more palatable

Step-and-shoot prospective gating can cut the radiation exposure of 64-slice CT coronary artery assessments by nearly 70% in patients with heart rates up to 70 beats per minute, according to a study presented at the Stanford Multidetector-Row CT Symposium in San Francisco.

The prospective study compared dose and motion artifact for the new step-and-shoot software technique "SnapShot Pulse," from GE Medical Systems, with conventional cardiac spiral retrospective gating in patients presenting with chest pain for 64-slice CT performed on a GE Lightspeed VCT scanner modified for dual energy (dual kVp) imaging.

Out of 110 patients involved in the study, 40 underwent the conventional study, which entails continuous radiation exposure, and 70 had step-and-shoot, which involves intermittent exposure. Mean heart rate was similar for both groups (57 to 58 bpm).

On average, the step-and-shoot technique reduced dose by 67% with little difference in motion artifact compared to conventional spiral scans, said presenter Dr. Sachio Kuribayashi, professor of radiology at the Keio University School of Medicine in Japan. However, with step-and-shoot, there was a small increase in motion artifact in patients with higher heart rates and variation in heart rates during the scan, he said.

Potential dangers of cardiac CT have been on the minds of cardiologists in recent months, said another symposium speaker, Cynthia McCollough, Ph.D. One article published in the medical trade press indicated that one in 1000 patients undergoing cardiac CT will develop radiation-induced cancer.

"Cardiologists came to us asking if they should be referring patients, because they don't want to take this risk," said McCollough, an associate professor of radiologic physics at Mayo Clinic in Rochester, MN.

However, McCollough pointed out that it is statistically not possible to predict who will actually develop and die from cancer. It is possible to estimate increased risk, which amounts to 0.05% for a cardiac CT examination, she said.

Average risk for developing cancer is 20%, so a cardiac CT would raise this risk to 20.05%, assuming the patient does not die from other causes in the 20 to 40 years it takes to develop radiation-induced cancer.

"These figures provide perspective for justifying medical exposure. The increased risk is very small. But you still need a good reason to do the study," McCollough said.

Overall risk to the body from radiation exposure in medicine is assessed through measurements of effective dose in milisieverts (mSv). Effective dose for CTA is comparable to both sestamibi myocardial perfusion and conventional coronary angiography, McCollough said.

However, effective dose tells only part of the story. It is also important to consider how tissue of specific organs is affected. Such effects are presented as measurements of absorbed dose in miligray (mGy) units.

In nuclear medicine, the absorbed dose is higher in bone marrow and ovaries, relative to coronary angiography and CT angiography. Dose to the breast is highest for CT coronary angiography.

"Breast dose is higher for CT, so it is important to use dose-reduction techniques," McCollough said.

Estimated risks to various organs are currently being updated and it seems likely that gonad figures will drop, improving the profile of sestamibi studies. Conversely, risks to the breast are due to rise, which will unfavorably affect CT's radiation safety profile in the breast relative to other studies.