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Half-dose SPECT Cardiac Perfusion Yields Excellent Image Quality

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Cardia imaging Performing SPECT myocardial perfusion to image coronary arteries at half the usual radiation exposure yielded good or excellent images 95 percent of the time according to researchers in Israel.

The study led by Dr. Nili Zafrir, of the Rabin Medical Center in Petah Tikva, Israel, used special imaging software to produce the enhanced images that were comparable to full-dose protocols. She presented her study results at the ICNC10 - Nuclear Cardiology and Cardiac CT meeting being held in Amsterdam, Netherlands this week.

"It's our view that myocardial perfusion imaging is feasible with significant radiation dose reduction," Zafrir said in a prepared statement. "We found that image quality using the new processing software was similar to that in conventional protocols. Indeed, the clinical results identified with the half-dose protocol were equivalent to those determined by full dose imaging. But significantly, the half dose protocol reduced radiation exposure to a minimum of 1.9 mSv in one-third of our patients, far below the dose range we see in conventional perfusion scanning.”

SPECT or single-photon emission computed tomography is a nuclear medicine imaging technique using gamma rays to produce an image in sections that are assembled by computer into a 3D image. SPECT scans are significantly less expensive than PET scans, in part because they are able to use longer-lived more easily-obtained radioisotopes than PET. The down side is that SPECT using a radioisotope such as technetium-99m is limited to an imaging scan of 14 to 30 minutes because the radiation dose in that time can be as high as 25 millisieverts (mSv).

In the study, Zafrir’s team randomly assigned 218 patients referred for myocardial perfusion imaging with 109 assigned to the half-dose protocol and 109 to the full-dose protocol. The groups were well matched with an average age of 66, an average weight of 174 pounds, an average BMI of 28kg/m2. A total of 56 percent were male. All patients had a history of myocardial infarction, percutaneous coronary intervention or coronary artery bypass graft.

Patients in the full-dose protocol received standard injections of technetium doses ranging between 12 and 32 milicurries (mCi) depending on patient weight. Patients in the half-dose group received doses ranging between 5 and 17 mCi. Images from the half-dose group were processed using imaging software originally designed to reduce the time needed to acquire a full diagnostic image. The goal of this study was to use the software to test the feasibility of reducing the tracer dose instead of reducing the acquisition time.

The image analysis showed that 94 percent of the images from the half-dose protocol were judged as “excellent to good,” with little or no loss of image quality or diagnostic accuracy. Overall, the total effective dose for the stress-rest studies was 7.19 mSv in the half-dose group compared to 14.4 mSv in the standard dose protocol. In addition, 38 of the patients in the half-dose group had only single stress-only testing which resulted in an average radiation exposure of only 1.9 mSv, which the researchers noted as being far below the dose range used in conventional perfusion scanning.

Zafrir cautioned that this is preliminary study primarily aimed at looking at the feasibility of using half the radiation dose, and that more research will be needed to confirm diagnostic accuracy. "Clearly, we cannot be certain what the long-term benefit of reducing radiation exposure might be, but theoretically it would seem important," she said.

By Michael O’Leary, contributing writer, Health Imaging Hub


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