Abstract: A positron emission tomography scanner having a plurality of detector rings positioned side-by-side. Each detector plane contains a plurality of photomultiplier tubes with at least two columns of scintillation crystals positioned on each photomultiplier tube defining a module in ring. Each crystal is uniform and has a length in the axial direction less than the size of the photomultiplier tube. The fundamental crystal identification module is limited to two photomultiplier tubes. Each alternate module in the ring is offset by one-half or less of the center-to-center intra-module crystal spacing in the axial direction.