RF interference reduction for simultaneous digital PET/MR using an FPGA-based, optimized spatial and temporal clocking distribution

Positron Emission Tomography (PET) combined with Magnetic Resonance Imaging (MRI) as a hybrid imaging modality is about to become the next-generation imaging technique in the field of molecular imaging. The integration of PET detectors into an MR-gantry enabling simultaneous acquisitions with unaffected performance of PET and MRI is challenging, as PET detectors need to be unaffected by the MR operation, RF-silent (low RF emission) and compact. Especially the RF silence of a fully digital PET detector is demanding, as the digital detection and data acquisition architecture may produce electromagnetic (EM) field emission which can result in noise artifacts in MR images. The RF fields emitted by PET detectors, which may be coupled into the MRI RF coil, are therefore unwanted and are from an MRI point of view disturbances considered as noise. A conventional way to overcome RF emission is to use thick RF shielding which however leads to MR image artifacts due to eddy currents distorting the MR image. In this paper, we present investigations of novel interference reduction techniques which were implemented by solely modifying firmware designs used in FPGAs of our MR-compatible PET modules used in the PET Insert Hyperion-II^D while keeping the entire hardware untouched. The principles apply on a more fundamental level namely the EM field coupling mechanism to the RF receive coil. We propose to reduce the coupling by shifting the clocking frequencies and by applying clock phase patterns of the PET sensors, leading to an optimization of the EM field emission with the aim for as little as possible RF shielding. The initial results presented in this paper demonstrate how our flexible PET architecture can be used to reduce the noise coupled into the MRI receive chain. Measurements performed with our near-field scanner in the lab and with the MRI confirm, that the frequency shifting approach can be applied to successfully reduce the noise coupled into the MRI receive- chain. Least noise was measured at the Larmor frequency with the PET sensors clocked at 160 MHz and 100 MHz.