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

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.

Reduced functional connectivity within the primary motor cortex of patients with brachial plexus injury.

This study aims at the effects of traumatic brachial plexus lesion with root avulsions (BPA) upon the organization of the primary motor cortex (M1). Nine right-handed patients with a right BPA in whom an intercostal to musculocutaneous (ICN-MC) nerve transfer was performed had post-operative resting state fMRI scanning. The analysis of empirical functional correlations between neighboring voxels revealed faster correlation decay as a function of distance in the M1 region corresponding to the arm in BPA patients as compared to the control group. No differences between the two groups were found in the face area. We also investigated whether such larger decay in patients could be attributed to a gray matter diminution in M1. Structural imaging analysis showed no difference in gray matter density between groups. Our findings suggest that the faster decay in neighboring functional correlations without significant gray matter diminution in BPA patients could be related to a reduced activity in intrinsic horizontal connections in M1 responsible for upper limb motor synergies.