A three-dimensional detector module for use in detecting annihilation photons generated by positrons emitted from radio-labeled sites within a body is formed from multiple solid state photo-detectors attached to one or more scintillators. Each photo-detector can be attached to a scintillator to form a photo-detector/scintillator combination and multiple photo-detector/scintillator combinations can be arranged in an array. Alternatively, multiple photo-detectors can be attached to the surface of a single scintillator to form an array. Multiple arrays are then stacked to form a photo-detector module. The modules can then be assembled to form a sheet of photo-detector modules. Multiple sheets or multiple modules can then be arranged around a body to detect emissions from radio-labeled sites in the body. Multiple position sensors attached to the photo-detectors, arrays or modules provide the ability to locate the source of the positron emissions from the labeled sites in the body and generate an image of the emission site. A series of novel PET configurations can be constructed from these detector modules, making PET scanners portable, more sensitive and flexible to be used in numerous different operational configurations, such as operating room, emergency rooms, critical care units, or battlefield.