Calibration method for a gamma ray detector that comprises a pixelated scintillator array having multiple scintillator pixels for emitting scintillation photons at photo conversion positions in response to incident gamma rays , and a pixelated photodetector, PD, array having multiple PD pixels coupled to the pixelated scintillator array for determining a spatial intensity distribution of the scintillation photons, wherein the PD pixels are subdivided into multiple PD sub-pixels , the calibration method being characterized in comprising the steps of - enabling a first PD sub-pixel which is coupled to a first scintillator pixel of the pixelated scintillator array , - enabling a second PD pixel which is coupled to a second scintillator pixel of the pixelated scintillator array, wherein the enabled second PD pixel is located adjacent to a PD pixel to which the enabled first PD sub-pixel belongs, recording scintillation photons, emitted at a photo conversion position located in the first scintillator pixel , by the enabled first PD sub-pixel to obtain a first PD sub-pixel detection signal at a first time point, recording shared scintillation photons, resulting from the photo conversion in the first scintillator pixel and travelled into the second scintillator pixel , by the enabled second PD pixel to obtain a second PD pixel detection signal at a second time point, estimating a first time skew between the first time point and the second time point, and correcting the first time skew. a pixelated scintillator array having multiple scintillator pixels for emitting scintillation photons at photo conversion positions in response to incident gamma rays , and a pixelated photodetector, PD, array having multiple PD pixels coupled to the pixelated scintillator array for determining a spatial intensity distribution of the scintillation photons, wherein the PD pixels are subdivided into multiple PD sub-pixels , the calibration method being characterized in comprising the steps of - enabling a first PD sub-pixel which is coupled to a first scintillator pixel of the pixelated scintillator array , - enabling a second PD pixel which is coupled to a second scintillator pixel of the pixelated scintillator array, wherein the enabled second PD pixel is located adjacent to a PD pixel to which the enabled first PD sub-pixel belongs, recording scintillation photons, emitted at a photo conversion position located in the first scintillator pixel , by the enabled first PD sub-pixel to obtain a first PD sub-pixel detection signal at a first time point, - enabling a first PD sub-pixel which is coupled to a first scintillator pixel of the pixelated scintillator array , - enabling a second PD pixel which is coupled to a second scintillator pixel of the pixelated scintillator array, wherein the enabled second PD pixel is located adjacent to a PD pixel to which the enabled first PD sub-pixel belongs, recording scintillation photons, emitted at a photo conversion position located in the first scintillator pixel , by the enabled first PD sub-pixel to obtain a first PD sub-pixel detection signal at a first time point, recording shared scintillation photons, resulting from the photo conversion in the first scintillator pixel and travelled into the second scintillator pixel , by the enabled second PD pixel to obtain a second PD pixel detection signal at a second time point, estimating a first time skew between the first time point and the second time point, and correcting the first time skew. <EOS>