A system for improving the resolution of nuclear medicine gamma cameras. A pixellated crystal scintillator is optically connected to position sensitive photomultiplier tubes. Photons incident on the crystal scintillator travel a distance through the crystal that is directly proportional to the energy level of the photon before converting to photoelectrons. The system uses this depth of interaction information to classify the received photons according to their energy levels. Images may then be displayed using information from only those photons that are within the selected energy class.