Zurich, Switzerland - In what researchers say is the first published study to evaluate the feasibility of prospective ECG gating for CT angiography, a Swiss group has demonstrated that radiation dose can be reduced to as low as 1 to 3 mSV, without sacrificing image quality [1]. The study is published online December 18, 2007 in the European Heart Journal.

The study is welcome news in light of recent papers in the New England Medical Journal and the Journal of the American Medical Association that raised alarm bells over the potential radiation risks associated with the increasing popularity of CT scans, including CT angiography [2,3]. In the UK, a government-based radiation watchdog, the Committee on the Medical Aspects of Radiation in the Environment (COMARE), recently issued an advisory stating that private companies should stop offering full-body scans on the basis of growing fears about excess radiation exposure from CT.

According to the authors of this new study, however, Drs Lars Husmann (University Hospital Zurich, Switzerland) and colleagues, prospective ECG gating, while still "in its infancy," is an important step forward for CT coronary angiography and has the potential to be combined with further refinements in CT technology, including higher rotation speeds and higher detector numbers.

Prospective ECG gating, using the GE 64-slice scanner studied in this paper, employs "snapshot" software that converts the scanner mode from conventional helical acquisition to a "step-and-shoot" mode with prospective gating, so that radiation is administered only in predefined snapshots during the cardiac cycle, rather than during the whole cycle, as it is in helical imaging. All 64-slice CT machines can now be used with dose-modulation algorithms that reduce tube current during certain cycles, but the nonhelical step-and-shoot approach takes this notion even further. As previously reported by heartwire, many of the leading figures in the CT field have viewed this development as highly promising, but no studies had previously confirmed that scans using these protocols still produce adequate images of the heart.

In Husmann et al's feasibility study, 40 patients with known or suspected coronary artery disease underwent 64-slice CT according to the step-and-shoot protocol. In all, the scan yielded 519 evaluable coronary artery segments >1.5 mm (out of a theoretical maximum of 640 segments). The authors report that the mean effective radiation dose was 2.1 mSv (ranging from 1.1 to 3.0 mSv), yet the image quality was reasonably high: only 26 out of 519 segments, or 5%, were of "nondiagnostic" image quality. Of these, artifacts resulting from lack of overlap in the step-and-shoot protocol were the cause of just under half of the nondiagnostic images.

Commenting on the study for heartwire, Dr Paul Schoenhagen (Cleveland Clinic, OH) said that the study is an important first validation of this technological advance, particularly given recent comments, reported by heartwire, calling for a "moratorium" on widespread use of CT. A radiation dose of 2 to 3 mSv, he points out, is "in the range of diagnostic cardiac catheterizations"—the current gold standard.

What the Husmann et al study shows is the feasibility of radically reducing radiation—more work is needed to figure out whether a slightly higher radiation exposure would reduce the number of "step" artifacts, Schoenhagen said.

"There are obviously techniques where you can overlap the information a little bit more, so you can reconstruct more at the step artifact, and that can be chosen in the protocol you use," he explained. "But the more overlap you have, the more radiation exposure, so you really need to balance the two goals. This was a feasibility study, where the goal was to show the maximum possible reduction in radiation dose, and they didn't overlap the segments at all. The study mainly shows that it is feasible to reduce radiation exposure significantly and to some extent still get diagnostic images."

Of note, the COMARE report in the UK singles out certain medical applications for CT technology, including diseases of the heart and colon, as having medical merit. But it rejected a role for whole-body scans, scans of the lungs, or scans for spinal disorders, osteoporosis, or body-fat composition.