Studies of Electromagnetic Interference of PET Detector Insert for Simultaneous PET/MRI

We are developing a brain positron emission tomography (PET) system prototype with long optical cables to minimize mutual interference between our PET components and magnetic resonance imaging (MRI) system. Our PET system consists of 16 PET detector modules which are placed in Faraday cages spaced equally in a 32 cm diameter ring. By using 20 m length optical cables rather than electrical connections, the Faraday cage ground can float relative to the MRI RF ground which permits the RF field to transmit through PET ring. This could eliminate the need for custom RF coils in whole body inserts, or the need for a custom transmit coil in brain insert PET/MRI designs. The aim of this study is to investigate the feasibility of PET detectors with a floating ground from measurements of electromagnetic interference (EMI) shielding and numerical analyses of RF field attenuation under different conditions. The shielding effectiveness equation shows that a copper plate of 30 mu M (similar to 4 x Skin Depth) thickness shields approximately 120 dB (99.9999 %) of both the 66.7 MHz analog-to-digital converter sampling frequency of the interior PET electronics and the 127.7 MHz Larmor frequency of the exterior 3-T MRI RF coil. Simulation results using ANSOFT Maxwell showed that a larger gap between PET detectors or a shorter height of PET Faraday cage results in less RF field attenuation. The two side plates of adjacent PET Faraday cage act as a capacitor. When the gap increases or height shrinks, capacitive impedance increases which then results in less RF power dissipation and thus more RF field transmission inside field of view (FOV). Simulation results showed 25 dB increase of the transmission level when the gap was increased by 2 mm and height was decreased by 20 mm. Further MR-compatibility analysis will be performed by acquiring MR images with the shielded PET detector ring inserted.