PET MR compatibility references
From 2014 PET/MR conference, a nice review of potential difficulties in PET/MR interference
More precisely, the PET detectors ideally have to be unaffected by the strong static magnetic B0 and gradient fields as well as the electromagnetic (EM) radio-frequency (RF) field (B1) of the MRI scanner (MR compatibility) while preserving the MRI system's B0 homogeneity, gradient field linearity and pulse shape in addition to a high RF Signal-to-Noise Ratio (SNR) of the MRI (RF silence).
Besides B0 field distortion related to susceptibility-shifts which may be introduced by materials residing within the MRI B0 -field, conducting materials act as a source of eddy-currents when being exposed to RF fields and alternating magnetic fields generated by the MRI gradient coils, leading to MRI image disturbances (Wehner2015, Hennel1997).
[Hennel1997] F. Hennel "Multiple-shot echo-planar imaging" Concepts Magn. Reson., vol. 9,no. 1, pp. 43-58, 1997
In addition to PET-materials-related MRI magnetic field distortions and MRI RF disturbances, operating PET electronics emit RF fields which might couple into the MRI RF receive chain via the RF coil(s), leading to a possible deterioration of the MRI SNR (Wehner2014, Gebhardt2013, Maramraju2012, Maramraju2011).
To reduce this unwanted RF field coupling which we hereinafter define as noise, EM interference (EMI) shielding of PET modules and power supplies is commonly applied (Catana2008, Judenhofer2007, Hong2013, Peng2010, Maramraju2012, Weissler2014, Olcott2009, Yamamoto2012), but may lead to MR image artifacts due to shielding-related eddy currents. EMI shielding (in context of the MRI RF excitation: RF shielding) at the same time may serve the purpose of shielding operating PET electronics from MRI-related RF pulses and gradient switching which might negatively affect the PET operation (Maramraju2012, Yoon2012, Weirich2012, Yamamoto2011). To completely avoid shielding-related eddy currents and potentially related MRI image degradation, a PET detector with no shielding would be the ultimate goal, but would require RF emissions from the PET electronics to be sufficiently low.