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.