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The influence of self-absorption on PET and PET/CT shielding requirements.

Med Phys. 2010 Jun;37(6):2999-3007

Authors: Elschot M, de Wit TC, de Jong HW

PURPOSE: The high energy (511 keV) annihilation photons used in positron emission tomography (PET) imaging generally require a substantial amount of lead to protect personnel and the general public from ionizing radiation. A cost-effective design of the PET facility that ensures radiation does not exceed formal dose limits requires accurate estimation of the necessary PET shielding. The American Association of Physicists in Medicine (AAPM) Task Group 108 recently published broad beam transmission factors based on Monte Carlo calculations of 511 keV photons. In this work, an extension to the AAPM model is presented, based on Monte Carlo simulations including the effects of self-absorption on the photon energy spectrum. METHODS: Monte Carlo calculations were performed using MCNPX. The photon energy spectrum after self-absorption was computed by simulating a normal 18FDG activity distribution in an anthropomorphic phantom. This spectrum was used to calculate the dose rate transmission factors for various wall thicknesses of lead, concrete, and iron. The method was validated by measurement and corresponding simulation of the transmission factors of an 18FDG source in air and in PMMA. Furthermore, a method to generate 3D area dose rate maps of PET facilities incorporating the calculated transmission tables is presented and applied to several shielding situations. RESULTS: The calculated self-absorption correction factor and the broad beam transmission factors resulting from Monte Carlo simulations of a monoenergetic point source emitting 511 keV photons were in excellent agreement with the results of the AAPM publication (0.66 vs 0.64 and R2 = 0.999, respectively). However, when all radiation physics, i.e., also the effect of self-absorption on the photon energy spectrum, is included in the Monte Carlo calculations, a substantial reduction in required shielding material was found. For example, including all radiation physics leads to 13.3 mm of lead required to obtain a typical transmission factor of 0.1, instead of 16.0 mm of lead when the AAPM data including only the self-absorption correction factor are used. These findings were confirmed by the experimental measurements. The transmission factors produced in this work can be applied in the same manner as those estimated by AAPM to allow for a cost-effective design of PET and PET/CT facilities without violating radiation safety regulations. CONCLUSIONS: Taking into account the effect of self-absorption on the photon energy spectrum results in more accurate and cost-effective shielding requirement estimations.

PMID: 20632612 [PubMed - in process]

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Krypton gas cylinders as a source of radiation.

Health Phys. 2010 Jul;99(1):58-65

Authors: Fischer HW, Bielefeld T, Hettwig B

A standard 40 foot shipping container with a cargo of pressurized krypton gas in 159 steel cylinders, which had triggered a radiation alarm, was investigated to address radiation safety and illicit nuclear trafficking concerns. The investigation included contamination and dose rate measurements as well as in situ high resolution gamma spectroscopy. The dose rate measurements gave a maximum value of 0.07 muSv h above background (0.08 to 0.11 muSv h) on the cylinder surface and no detectable increase above background at distances of 1 m and higher. Contamination monitor readings showed a similar relative increase (plus 8 cpm) above background (about 12 cpm) to the dose rate readings. Quantitative gamma spectroscopy revealed a contamination of the gas with Kr at a level of 3.5 x 10 Bq kg. This value was found to be consistent with analytical and numerical estimates based on current data for atmospheric Kr, which is captured from ambient air together with stable krypton during the production process. This incident demonstrates an apparent lack of radiation-related knowledge by those who handle krypton gas, as well as by border control personnel and emergency responders. We therefore propose to improve labeling and documentation standards for such shipments. This effort may be facilitated by introducing the new category of “technically enhanced artificial radioactive material,” or “TEARM” (similar to the existing “naturally occurring radioactive material” or “NORM” and “technically enhanced naturally occurring radioactive material” or “TENORM” categories).

PMID: 20539125 [PubMed - in process]

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