We are attempting to develop a laboratory PET scanner for mouse imaging that utilizes far fewer detectors and channels of electronics, thus reducing cost, whilst retaining state-of-the-art performance. The detectors are based on LSO arrays read out by wavelength shifting (WLS) fibres placed on the top and the bottom of the arrays. Depth of interaction information will be obtained from the ratio of the signals at either end of the array. For acceptable performance, it is critical to maximize collection of light photons from the ends of the fibres and to minimize the optical crosstalk between adjacent fibres. Factors which can affect the light collection and crosstalk were studied, including coupling materials between fibres and crystals, reflectors wrapped around the fibre sides and ends, fibre size and shape, and number of layers of fibre cladding. Properties of WLS fibres such as the transmission attenuation and transverse absorption were also studied. The light yield from 2 × 2 × 10 mm³ LSO crystals collected from the end of a 2 × 2 × 30 mm³ WLS fibre was up to 24% (typical values 16–20%) of that obtained by direct coupling of the LSO crystal. This light collection efficiency appears to be sufficient for decoding interaction position in these detectors.