MR-compatible shields for 511 keV gamma photons: a feasibility study

An MR-compatible PET scanner capable of acquiring PET and MR images simultaneously will require 511 keV gamma shielding to suppress the influence of activity outside the PET field of view. Suitable materials must have a magnetic permeability close to that of water and air, high density, high Z and ideally a low conductivity. Eleven materials were selected on the basis of the criteria above, including various MRI magnetically-compatible metals and metal oxides, 2 scintillating crystals (bismuth germanate and lead tungstate), and 3 metal/epoxy compounds. Samples of these materials, immersed in a water-filed phantom, were first imaged on a human MRI-scanner to assess their MR-compatibility. A similar protocol was then applied to 3 annular shields, respectively made of red lead oxide, lead powder and bulk lead. Lastly, a water-filled phantom was imaged on a small-animal MRI scanner with the coil within annular shields made of bismuth germanate, sintered lead monoxide or bulk lead. These experiments showed that lead cannot be generally used for MR-compatible gamma shielding, since it yields conductivity-related artefacts or signal loss. The most promising candidates are high-density insulating compounds such as lead oxides, and crystalline scintillator materials such as bismuth germanate