LYSO-SSPM based PET detector module for combined PET/MRI applications

We are investigating the performance of a LYSO-solid state photomultiplier (SSPM) based PET detector in a magnetic resonance imaging (MRI) scanner to exploit the potential advantage of a combined PET/MRI system. SSPMs are compact, have high gain at low bias, fast response time and they are insensitive to magnetic fields which makes them very attractive for combined PET/MRI applications. A PET detector was built by coupling a 4×4 LYSO array with each element 1.5×1.5×20 mm³ to the matching 4×4 SSPM array mounted on a non magnetic substrate and was positioned in the annulus between the RF and the gradient coils of 7T Bruker Biospec MRI system. Data was taken inside and outside the magnet, and when inside the magnet with Spin Echo and Gradient Echo pulse sequences on. The flood images were compared with the once recorded outside and inside with MRI sequences. No effects of either the static magnetic field, or the running of pulse sequences were observed. The energy resolution measured was 16% (FWHM) at 511 keV for the complete detector module. No change in the location of the photopeak was observed with and without the MRI sequences running. The measured energy resolution remained unchanged, suggesting that there was no significant loss in signal amplitude. A structured cylindrical MR phantom containing water was imaged to assess the effect of the LYSO-SSPM detector on the MR data acquisition. No obvious artifacts were observed in the resulting phantom images when compared with the ones acquired without the PET detector. Results shows that the direct coupled SSPM-LYSO detector works with no significant effects in the 7T magnet, both from the PET and MRI perspective.