A scintillation-crystal based gamma-ray detector with photon sensors disposed on edge surface(s) of the crystal to take advantage of total internal reflection of scintillation photons within the thin-slab detector substrate to improve spatial resolution of determination of a scintillation event (including depth-of-interaction resolution) while preserving energy resolution and detection efficiency. The proposed structure benefits from the reduced—as compared with related art—total number of readout channels elimination of need in complicated and repetitive cutting and polishing operations to form pixelated crystal arrays used in conventional PET detector modules. Detectors systems utilizing stacks of such detectors, and methods of operation of same.