Report from SCCT: Coronary CT angiography dose reduction techniques prove consistency

July 17, 2007

Cardiac imagers can obtain coronary CT angiograms of diagnostic quality with significant radiation dose reductions using either step-and-shoot gating on 64-slice scanners or ECG-pulsing on dual-source systems, according to several studies released at the 2007 Society of Cardiovascular Computed Tomography meeting. Patients’ weight and heart rates must be normal or low, however.

Cardiac imagers can obtain coronary CT angiograms of diagnostic quality with significant radiation dose reductions using either step-and-shoot gating on 64-slice scanners or ECG-pulsing on dual-source systems, according to several studies released at the 2007 Society of Cardiovascular Computed Tomography meeting. Patients' weight and heart rates must be normal or low, however.

Narrowing the scan range, adapting the table's pitch, and lowering tube current and voltage are some of the tricks imagers can use to reduce radiation exposure in dual-source coronary CTA. Physicians should add ECG-gated dose modulation, also known as ECG-pulsing, to bring doses to the lowest level achievable without affecting image interpretation, said Dr. Tobias Pflederer from the department of internal medicine at the University of Erlangen in Germany.

Pflederer and colleagues prospectively enrolled 100 consecutive patients who had been referred for coronary CTA. They excluded pregnant patients and those with renal disease, known allergies to contrast agents, or weight of more than 85 kg. Patients were distributed randomly and equally into two groups that underwent scanning protocols of 120 kv/330 mAs (group 1) and 100 kV/330 mAs (group 2). Both protocols included ECG-pulsing to pace x-ray exposure according to a patient's heart rate (lower or higher than 65 bpm). The researchers assessed image quality using a four-point rating scale.

They found that by using reduced tube current and ECG-pulsing they could reduce the full effective dose from about 12 mSv for group 1 to less than 8 mSv for group 2. The difference was statistically significant (p<

The study had several limitations, including the arbitrary weight threshold and the formula used to calculate radiation dose, Pflederer said. It assessed only image quality and not each protocol's accuracy in terms of stenosis detection. It did confirm, however, that the scan protocol with lower tube voltage leads to substantial reduction of radiation exposure in patients with a relatively low body weight and a heart rate under 65 bpm while preserving image quality.

In a different study, investigators from the University of South Alabama College of Medicine tried a prospectively gated protocol (SnapShot Pulse, GE Healthcare) during 64-slice coronary CTA. They assessed 51 patients with heart rates below 65 bpm, according to Dr. Jason Cole, who presented the study.

In addition to using a step-and-shoot technique, Cole and colleagues narrowed the field of interest from the top of the pulmonary arch to the diaphragm for further radiation exposure reductions. They were able to achieve images of good diagnostic quality and a mean effective radiation dose of 9.13 mSv.

"Low-dose prospectively gated coronary CTA with 64-slice scanners is feasible," Cole said. "More recently, we have been able to achieve standard effective dose reductions below 5 mSv."

The protocol's benefits may be restricted to patients of normal body size and weight and those with a heart rate below 70 bpm, however. This leaves larger patients and small children out of the picture.

In a third study, researchers at the Fairfax Radiological Consultants in Virginia evaluated the same step-and-shoot technique in 1026 patients referred for coronary CTA between October 2006 and May 2007. Led by Dr. James K. Min, who presented the study, they assessed the technique's reliability as well as the effective radiation dose reductions it could achieve on 64-slice scanning.

The investigators obtained studies of good diagnostic quality in 995 (96.9%) patients who successfully completed the prospectively gated coronary CTA protocols. They also achieved effective dose reductions of up to 80%, Min said.

As in the previous studies, patient selection bias represented the most significant limitation. Although the average body mass index recorded 28, most patients' BMI was below average. On the brighter side, prospectively gated coronary CTA software is commercially available and could become clinically widespread soon, facilitating further studies.

"Prospectively gated coronary CTA is feasible, robust, and makes our lives easier. Images don't have to be beautiful, only of diagnostic quality," Min said.

For more information from the Diagnostic Imaging archives:

Low-dose 64-slice cardiac CTA vies with catheter angiography

Low-dose technique makes 64-slice heart scans more palatable

Dual-source CT boosts patient flow, diagnostic confidence

Variety, ingenuity spice up latest CT scanner designs