1. A method for extracting photon depth of interaction information in a scintillator crystal of a positron emission tomography system, wherein the scintillator crystal has a rod-like shape with a length with a single photodetector at a first end of the length and a light absorbing coating at a second end of the length and a uniform reflective coating on sides of the scintillator crystal extending from the first end of the length to the second end of the length to optically isolate the scintillator crystal, comprising: detecting a single pulse in the photodetector; measuring a height of the said single pulse resulted from scintillation photons within the scintillator crystal; determining if said single pulse height is within a set range; extracting photon depth of interaction along the said length of the said rod-like crystal only from said single pulse height, wherein the scintillator crystal is optically isolated to prevent scintillation photons of other scintillator crystals from entering the scintillator crystal; calculating an energy of interaction within the scintillator crystal only from said pulse height and calibration data; and using the extracted photon depth and calculated energy spectrum in image reconstruction by the positron emission tomography system.