Investigation of Faraday Cage Materials with Low Eddy Current and High RF Shielding Performance for PET/MRI Applications

After the successful fusion of positron emission tomography (PET) and computed tomography (CT), the attention turned to integrating PET and magnetic resonance imaging (MRI). The most significant challenges in integrating PET and MR systems arise from the potential for mutual interference between the systems. The main magnetic field B0 can be disturbed by trace amounts of magnetic materials, leading to MR susceptibility artifacts. The high-power RF field B1 may induce currents in or even damage PET electronics inside the MR system without protection. The MR system gradient fields can induce a large eddy current in conductive loops in PET detectors, causing further image distortions. The purpose of this work is to investigate shielding efficiency, gradient-induced eddy current, and MR susceptibility performance of six different shielding materials: copper plate, copper tape, carbon fiber fabric, stainless steel (304) mesh, phosphor bronze mesh, and conductive coating. The RF noise results show a lower bound shielding efficiency (SE) of 43.3dB for copper plate, copper tape, phosphor bronze mesh, and conductive coating and measured SEs 42.9dB and 42.5dB for stainless steel mesh and carbon fiber fabric. Cooper plate and copper tape generated the largest ghosting artifacts from eddy currents, increasing percent signal ghosting (PSG) 106%/12.3%/111.6%, and 20.8%/7%/24.3% in the axial/sagittal/coronal plane, respectively. For MR susceptibility evaluation, stainless steel mesh had the highest pixel value difference (7.4 ± 3.6Hz) compared with the reference. Copper tape and conductive coating also showed a slightly larger difference, 3.7 ± 2.3Hz and 3.9 ± 1.1Hz, respectively. Although conductive coating does not have the lowest susceptibility, owing to its easy manufacturing process and consistent performance, we choose it as the Faraday cage material in our MR-compatible PET insert.