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A single-slice magnetic resonance (MR) compatible positron emission tomography (PET) system with a transaxial field of view ∼6 cm has been designed and is currently under construction. It will consist of four concentric rings of lutetium oxyorthosilicate (LSO) scintillation crystals, each coupled to one of eight multi-channel photomultiplier tubes (PMTs) via 3.5 m optical fibers. The annulus containing the crystals has an outside diameter of 12 cm and is placed within the MR field of view whilst the PMTs are located outside the main magnetic field. A highly reproducible method that optimizes the amount of scintillation light that reaches the PMTs has been established. Two small sections of the scanner, each containing 4 by 4 crystal arrays, demonstrated good flood position histograms with all sixteen channels clearly identifiable. The light loss through a fiber of length of 3.25 m was approximately 70%. The spatial resolution of the two arrays in coincidence was measured at 1.6 mm full-width half-maximum (FWHM), and the temporal resolution of one array in coincidence with a single LSO crystal was measured to be 10.9 ns. The design incorporates a novel technique for improving sampling at the center of the field of view within the scanner. The concentric rings are offset with respect to one another by one quarter of the crystal width between layers resulting in significantly improved sampling. The results indicate that the PET scanner will have a performance comparable with that of current small animal systems and it will be used to investigate the utility and potential applications of combining PET and MR in small animals.