Abstract: An apparatus and method are disclosed for detecting and locating the origin of a gamma ray in a medical diagnostic imaging system. At least one primary fiber, which is a scintillating optical fiber, is positioned to receive radiation from a gamma ray source. At least one secondary fiber intersects the primary fiber at a non-zero angle and is in optical contact with the primary fiber. Both the primary and secondary fibers are provided with means for detecting light propagated in the fibers. The interaction of a gamma ray with the primary fiber will result in the propagation of light in both the primary and secondary fibers, thereby permitting the determination of the site of impact of the gamma ray in the detector, and possibly also enabling the determination of the path of incidence of the gamma ray.