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A prototype electro-optically coupled PET brain insert is being developed for 1.5 T or 3 T MRI systems. To reduce the number of electronic channels, we developed and tested a novel silicon photomultiplier (SiPM) multiplexing circuit based on a novel cross strip readout circuit using capacitors that reduces the number of channels by a factor of 4. Next, we developed and tested this multiplexing circuit with a new electro-optical coupling method that uses a small vertical cavity surface emitting laser (VCSEL) to transmit the scintillation signal out over fiber. This multiplexing circuit is specially suited for directly driving VCSEL lasers without using preamps or any other active components. Two detectors were placed inside the MR bored, and operated with and without a continuos RF field. We were able to resolve a 6 × 6 array of 2.63 mm × 2.63 mm × 20 mm LYSO crystals that were directly coupled to a 4 × 4 array of 3.2 mm × 3.2 mm SiPM pixels with a global delay corrected paired coincidence time resolution 2.50 +/- 0.015 ns FWHM and a per crystal gain corrected energy resolution of 12.6 +/- 0.6% FWHM @511 keV while operating inside an 1.5 T MRI, with or without the RF field on. The two technologies, capacitive multiplexing and electro-optical coupling, have enabled a high-performance clinical PET detector based on silicon photomultipliers, a factor 4 reduction in the number of readout channels, and, when inserted into a 1.5 T MRI system, has low levels of mutual interference between the PET detector and the MRI.