PShield: an exact three-dimensional numerical solution for determining optimal shielding designs for PET/CT facilities.

ABSTRACT

PURPOSE: Calculation of radiation shielding requirements for high-workload positron emission tomography/computed tomography (PET/CT) installations using the methods proposed by Task Group 108 (TG 108) of the American Association of Physicists in Medicine can be a complex task. The principal challenge that makes PET shielding design more complex than other diagnostic imaging modalities, aside from the higher photon energy, is that it is a multisource problem for which no unique solution exists. Although many solutions may meet shielding design dose limits, each solution has a different cost for materials and construction according to the type, thickness, weight, and location of the specified shielding. Here, the authors describe PShield, an algorithm that incorporates three-dimensional (3D) numerical methods to optimize PET shielding and delivers a cost-optimized solution while making no approximations. METHODS: The PShield algorithm uses a sequential quadratic programming routine to optimize PET shielding by minimizing a cost function in three-dimensions using extrapolations of the formulas published by TG 108. PShield makes no approximations and accounts for the contribution of every radiation source to the dose rate at every location in the problem using a discrete mesh. The authors used two simple examples of shielding problems to compare PShield with the TG 108 methods. RESULTS: The benefit of applying an optimization routine to an indeterminate problem is the identification of the only solution to the problem that minimizes the desired cost function. Choosing a poorly optimized solution can result in a shielding design that requires more shielding than an optimized design to reach the same dose rate at a given control point. The increased accuracy afforded by PShield ensures that dose rates at every point in a control area never exceed the design dose, whereas approximations often used to evaluate TG 108 methods may result in hot spots where the dose rate exceeds the design dose. CONCLUSIONS: The PShield algorithm is an exact 3D numerical solution for optimal PET/CT shielding based on the methods proposed by TG 108. Selection of the optimum shielding design can minimize the total material cost and structural burden of installed shielding or achieve other goals desired by a specific site. This is especially important for modern PET/CT suites, where increases in scanner capabilities have resulted in more complex shielding problems and the potential for high occupational doses.