Iterative reconstruction cuts radiation up to 90% during coronary CTA

CT vendors have been talking for months about the potential of iterative reconstruction as the next big thing to substantially reduce the worrisome radiation dose patients are exposed to during multislice CT imaging. Now they have results of a large multicenter cohort study to add substance to their enthusiasm.

Based on experience with 1150 consecutive patients at three centers, the Estimated Radiation Dose of Coronary CT Angiography Using Adaptive Statistical Iterative Reconstruction (ERASIR I) study has determined that an iterative reconstruction technique developed by GE Healthcare decreases the median dose of 64-slice coronary CTA by 90% when considering three different imaging scenarios applied to subgroups in the study.

Presenter Dr. James P. Earls, medical director of Fairfax Radiological Consultants in Fairfax, VA, based this finding on the median dose of 1.3 mSv the ERASIR I subjects received compared with the median 14 mSv reported from the international PROTECTION I trial, which involved nearly 2000 patients from 50 sites.

Adaptive Statistical Iterative Reconstruction (ASIR) is a new technique that uses statistical remodeling to reduce noise, Earls explained during a scientific session on Tuesday. This allows for reduced tube current, resulting in reduced dose for all CT applications to which it is applied.

Previous studies determined that increasing the percentage of ASIR reconstruction in relation to accompanying filtered background steadily reduces data set noise. The noise can be held steady by decreasing mA while ASIR percentage is increased. The end result is an image that has comparable noise but lower mA and radiation dose. This is the basis for dose savings, Earls said.

High ASIR percentage can impart a washed out appearance to images. Users typically employ a 30% to 40% mixture of ASIR with filtered back projection for reconstruction purposes.

ERASIR I examined the impact of ASIR on radiation dose and image quality when used along with other common dose-reducing strategies for patients referred for coronary CTA at three high-volume cardiac imaging services facilities. Beta blockers were administered if a patient's heart rate was above 65 bpm. All studies were enhanced with 60 to 90 mL of iodinated contrast material.

The protocol recommended 100-kV imaging, reducing current based on the patient's body mass index, and a minimal use of padding. Prospective gating was used in 90% of the scans involving filtered back projection, 88% of the scans in which ASIR was applied during a preprotocol, and 92% of scans where ASIR was used in the postprotocol.

Procedures employing standard filtered back projection as part of the hospital's standard battery of radiation-reduction strategies exposed patients to a median 3.8 mSv. Procedures using ASIR alone involved 2.6 mSv, and the combination of ASIR and other radiation-reduction strategies produced a median 1.3 mSv dose, Earls said.

Ninety-nine percent of coronary segments among all the patients were interpretable. Other measures of image quality were not described during the session or in the abstract.

ASIR has implications for reducing dose for clinical applications of multislice CT in addition to coronary CTA. Though clinicians who adopt it will face what Earls characterized as a "slight learning curve" initially, ASIR promises to operate unobtrusively as part of the clinical imaging routine.