Report from NCRP: Hybrid imaging poses radiation exposure challenges

Nuclear medicine applications have boomed since PET studies secured reimbursement, and the use of hybrid modalities combining nuclear imaging with multislice CT scanning is growing. The potential for increasing radiation exposure for patients and medical workers requires beefing up radiation safety measures, according to two leading nuclear medicine experts who spoke at the 2007 National Council on Radiation Protection and Measurements meeting.

Studies show that the fusion of PET with CT yields higher diagnostic accuracy. The scientific evidence reflects the increasing clinical utilization of PET/CT as well as SPECT/CT scanners, said Dr. Alan H. Maurer, director of nuclear medicine at Temple University Hospital in Philadelphia.

Fusion imaging, along with known and new radionuclides, will extend the frontiers of the developing field of in vivo molecular imaging. Gains from hybrid imaging, however, come at the expense of increasing radiation exposure to patients, Maurer said.

Several factors should be considered in the design of PET or PET/CT facilities, according to physicist Pat Zanzonico, Ph.D. Considerations prompting calls for special shielding requirements include the amount of energy produced by positron-electron annihilation (two 511-keV gamma-ray photons per particle collision), the half-life of radionuclides, and patient throughput. Zanzonico, who manages the Memorial Sloan-Kettering Cancer Center's nuclear medicine research laboratory, outlined several strategies to reduce radiation exposure risks:

• design and build an imaging room with concrete, lead, or iron shielding for floors, ceilings, and adjacent walls based on broadbeam transmission factors for each of these materials

• provide a radionuclide injection/uptake room for patients that follows similar shielding requirements

• take into account the time required for the exam, the time the patient is the radioactive source, and the half-life of the radiopharmaceutical used

• evaluate the patient's body mass for proper attenuation correction, as x-ray tube current and effective doses from multislice CT scanners for adult and pediatric exams can vary considerably

• keep an eye on radiation regulatory limits for areas restricted to nuclear medicine personnel (50 mSv/year) and uncontrolled areas near PET facilities (1 mSv/year or 20 mSv in any one hour) open to the public

• manage increasing patient throughput in a way consistent with shielding and regulatory requirements

A report published by the American Association of Physicists in Medicine provides a comprehensive description of PET and PET/CT radiation safety requirements (Med Phys 2006; 33[1]).

Using short-half-life radionuclides provides an alternative to those with a longer half-life, such as F-18 FDG. The alternative, however, requires an onsite cyclotron to be cost-effective. Cyclotrons come with radiation safety issues of their own, including the potential for internal and external radiation contamination, Zanzonico said.

Nuclear imaging departments considering installation will have to decide between unshielded and self-shielded cyclotrons. The former is popular among experienced personnel because it allows easier servicing and repair, but it needs to be placed and operated inside a concrete vault. The latter is considerably smaller and easier to install, but it does not eliminate the need for careful health physics consultation in designing the facility that will house it, he said.

Special section: Exclusive coverage of the 2007 National Council on Radiation Protection and Measurements meeting

This year's historic NCRP meeting reveals the disturbing growth of patient exposure to ionizing radiation from medical imaging and proposes practical solutions to regulate its growth. Extended coverage from Diagnostic Imaging lays out the facts and recommendations to better protect patients, physicians, and medical staff.

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