Nuclear Cardiology Works Toward Reduced Radiation Exposure



DENVER – Nuclear medicine specialists are feeling the brunt of increased public anxiety and regulatory concern regarding patient radiation exposures.

Nuclear cardiologists, in particular, find themselves in the crosshairs as a result of recent evidence of inappropriate overutilization of myocardial perfusion imaging. The profession has responded with a campaign aimed at defining appropriate use scenarios for practitioners and encouraging adoption of newer techniques that reduce radiation exposure while retaining high image quality.

"Based on these recommendations, we expect that for the population of patients referred for SPECT or PET myocardial perfusion imaging, on average a total radiation exposure of 9 mSv or less can be achieved in 50% of studies by 2014," Dr. Manuel D. Cerqueira said at the annual meeting of the American Society of Nuclear Cardiology.

Meeting that goal will require, for example, doing fewer separate-day, stress/rest technetium-99 myocardial perfusion imaging tests, which typically entail 13-16 mSv of radiation exposure. Also, the American Society of Nuclear Cardiology (ASNC) recommendations urge consideration of stress echocardiography as an alternative to nuclear imaging in younger patients because the diagnostic accuracy may be comparable and they can avoid radiation exposure altogether. In addition, the ASNC report discourages thallium-201–based imaging protocols, which involve 22-31 mSv of radiation exposure, noted Dr. Cerqueira, who was first author of the recommendations (J. Nucl. Cardiol. 2010;17:709-18), and is professor of radiology and medicine at Case Western Reserve University, Cleveland.

Something that has nuclear medicine specialists and interventional radiologists greatly concerned, according to Robert W. Atcher, Ph.D., is a proposed Nuclear Regulatory Commission (NRC) policy change that would lower the occupational radiation exposure limit from 5 to 2 rem (Roentgen equivalent man).

"The most affected groups in our field are technologists with a heavy PET [positron emission tomography] patient flow, cyclotron engineers, maintenance personnel, and radiochemists synthesizing PET tracers and therapeutic compounds. This is potentially devastating to us, because if we lower the limit then we have to double the number of people who are responsible for doing the same number of imaging studies, with no way to collect any more reimbursement to handle that task. In essence, they’re threatening to devastate our ability to do imaging," said Dr. Atcher, director of the National Isotope Development Center at the U.S. Department of Energy.

The NRC got an earful from concerned physicians and nonphysician scientists at hearings on the proposed changes held last year in Los Angeles, Houston, and Washington. The agency has not yet announced whether it plans to go ahead.

Another instance of what Dr. Atcher characterized as "regulatory overreacting" involves congressional interest in requiring hospitalization for patients who have received iodine-131. He and others have testified that there is no scientific evidence of risk to patients’ families or the general public if current guidelines for I-131 use are followed. Congressional representatives were also told that hospitalizing I-131 recipients would cost in excess of $600 million annually. In addition, critics of the idea pointed out that the risk of acquiring a serious methicillin-resistant Staphylococcus aureus infection during a hospital stay is quite real, said Dr. Atcher, a former president of the Society for Nuclear Medicine.

The new mantra at ASNC is "patient-centered imaging." The group’s recommendations for reducing radiation exposure from myocardial perfusion imaging emphasize appropriate patient selection, the use of standardized imaging protocols, radiotracers with shorter half-lives, weight-based dosing, and improved imaging systems.

Dr. E. Gordon DePuey highlighted the many new methods of optimizing image quality that have reached the market. These include resolution recovery and noise modeling software that provides superior image quality with shortened radiation exposure time. "All vendors now offer software that does this," he pointed out.

Also, hardware enhancements such as cardiofocal collimation are making a big difference. This particular technology allows half-time SPECT (single-photon emission computed tomography) with 100% myocardial radiation count density, explained Dr. DePuey of Columbia University, New York.

"All these new hardware and software methods out there are major advancements in nuclear cardiology. They need to be very seriously considered and incorporated in your practice, because they are really the keys to allowing you to decrease the radiation dose to your patients," he said.

Dr. Cerqueira drew attention to a large study that concluded myocardial perfusion imaging accounts for 22% of the total effective radiation dose accumulated from all nuclear medicine imaging procedures. Abdominal CT was the second biggest contributor, at 18% (N. Engl. J. Med. 2009;361:849-57).

Also concerning was a recent study using the very large UnitedHealthcare patient database. It showed that myocardial perfusion imaging accounted for 80% of the cumulative effective radiation dose from all cardiac imaging procedures in women age 18-34 years (J. Am. Coll. Cardiol. 2010;56:702-11).


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