Sparse crystal setting and large axial FOV for integrated whole-body PET/MR

Integrated PET/MR imaging will open a broad field of new clinical applications and diagnostic opportunities by combining high resolution and excellent tissue contrast of MR with high sensitive functional diagnostics of PET. Technical challenges occurred to realize compatibility between the MR and PET, i.e., the magnetic field compliance of the PET device and its neutrality towards gradient and RF fields. While novel detector technologies such as APDs and SiPMs can be used to replace photomultipliers (PMTs) which are highly affected by magnetic fields, shielding of the PET modules against RF signals is normally realized by a copper housing for the photon detector. Unfortunately, this reduces the performance of the MR system or the remaining field-of-view (FOV) because of the additional volume that is required to integrate the PET detector into the MR. Here we propose a novel distributed detector concept which allows an integration of the PET detector partly into the RF-coil of the MR system. This concept increases the axial FOV by distributing detector elements and crystals according to a sparse setting which then enables the crystals to be located mainly inside the RF coil, while preserving the RF shielding between the crystals. After integration of additional copper stripes, influences between the PET detector electronics and the MR are reduced without diminishing the sensitivity and spatial resolution of the detector. Moreover, additional space can be accessed for the bonding connections of the individual SiPM dies and the detector electronics. Resolution and sensitivity of three different scanner geometries were investigated in detail, and numerical simulations of the magnetic shielding behavior were done. It turned out that the sparse crystal concept introduces significant fortunes generally for whole body PET in terms of additional place for connectors, and more specific for a partly integration of the PET detector into the RF coil of the MR system.