Iterative reconstruction slashes radiation levels

CT vendors have responded to public concerns about radiation dose with innovations that promise to cut patient exposure substantially without affecting imaging service operations.

Backed by results from a large, multicenter cohort study, the vendors are saying that iterative reconstruction can be relied on to substantially reduce worrisome ionizing radiation.

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 determined that an iterative reconstruction technique developed by GE Healthcare decreases the median dose of 64-slice coronary CTA by up to 90%, when combined with other dose-reducing strategies.

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 dose reported from the international PROTECTION I trial, which involved nearly 2000 patients from 50 sites.

Adaptive Statistical Iterative Re-construction (ASIR) uses statistical remodeling to diminish noise while reducing reliance on filtered back projection reconstruction techniques that were adopted soon after the invention of computed tomography, Earls said during a scientific session at the 2009 RSNA meeting. This change allows for lower tube current, resulting in reduced dose for all CT applications to which it is applied.

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.

The protocol recommended 100-kV imaging, reducing current based on the patient's body mass index, and a minimal use of padding.

In the trial, procedures employing filtered back projection as part of the hospitals' 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.

With ASIR introduced as a commercial product, GE is moving forward with its next generation of radiation-reducing software. Model-based iterative reconstruction (MBIR) features substantially more computing power than ASIR, giving it possible advantages during image reconstruction, according to the company. Clinical tests are planned this summer.

Siemens plans to launch iterative reconstruction in image space (IRIS) this year. This approach promises to cut radiation dose as much as 60% for numerous CT applications.

Philips unveiled in its RSNA exhibit booth iDose for the company's Brilliance iCT and 64-channel scanners. It was designed to deliver images that have the same look as conventional-process images whose data are acquired at a much higher dose. The company is angling to release the software in the second half of 2010.

Toshiba is also developing iterative reconstruction capabilities for its 320-slice Aquilion One and other multislice scanners.

Dr. Boris Nikolic of Beth Israel Deaconess Medical Center demonstrated that the maximum absorbed dose can be cut significantly in the breast, lung, and thyroid gland during cardiac imaging with the 320-slice Toshiba Aquilion One scanner by slowing heart rate or reducing tube voltage.