Detector shielding design of a PET inserts in a 3T MRI system

For a PET insert to be placed inside an MR system, shielding for the detector modules is crucial as otherwise there can be mutual interference between the two modalities. In this work, we have assessed various shielding designs for both RF shielding effectiveness and gradient-induced eddy current performances. We designed six shielding configurations with various thicknesses, patterns and materials: 36, 18 and 9 urn thick solid copper; 36 urn thick segmented copper; 36 urn thick phosphor bronze mesh; and 0.8 mm thick carbon fiber composite. To evaluate the shielding performance, we performed transmission measurements (S21) and acquired the induced voltage from a MRI spin echo sequence using an RF probe. The eddy current performance was evaluated by the reflection measurement (S11) and observing the ghosting artifact and temperature rise from a MRI echo planar imaging (EPI) sequence. For the shielding effectiveness in both in/outside the MR system, the S21 magnitude for the solid copper (-43.1±1.7, -42.5±3.4, and -42.3±2.6 dB) and phosphor bronze mesh (-37.6±3.6 dB) configurations performed significantly better compared to other configurations (carbon fiber -29.2±1.5 dB and segmented copper -11.0±0.7 dB). For the eddy current performance, the phase delay acquired from the S11 measurement was less than 45° for segmented copper, phosphor bronze mesh and carbon fiber composites and significantly higher at ~120° for the solid copper configurations. The EPI ghosting artifact and temperature measurements showed similar trends inside the MR system as seen in the outside. In summary, we have evaluated the shielding effectiveness and the eddy current performances of several shielding designs, with the phosphor bronze mesh showing the best overall outcomes.