A combined detector comprising a gamma radiation detector and an X-ray radiation detector; the gamma radiation detector comprising: a gamma scintillator array (101 x,y) comprising a plurality of first scintillator elements (101 1,1, 101 2,1) for generating first scintillation light in response to received gamma quanta; an optical modulator; and a first photodetector array (103 a,b) for detecting the first scintillation light generated by the gamma scintillator array (101 x,y); wherein the gamma scintillator array (101 x,y), the optical modulator and the first photodetector array (103 a,b) are arranged in a stacked configuration along a gamma radiation receiving direction; and wherein the optical modulator is disposed between the gamma scintillator array (101 x,y) and the first photodetector array (103 a,b) for modulating a transmission of the first scintillation light between the gamma scintillator array (101 x,y) and the first photodetector array (103 a,b); wherein the optical modulator comprises at least one optical modulator pixel, the optical modulator pixel having a cross sectional area in a plane that is perpendicular to the gamma radiation receiving direction; wherein the first photodetector array (103 a,b) comprises a plurality of photodetector pixels (103 1,1, 103 2,1), each photodetector pixel having a cross sectional area (103' a,b) in the plane that is perpendicular to the gamma radiation receiving direction; wherein the cross sectional area of each optical modulator pixel is greater than or equal to the cross sectional area of each photodetector pixel (103' a,b); and wherein the X-ray radiation detector is disposed along the gamma radiation receiving direction (104, 604) and on the opposite side of the gamma scintillator array (101 x,y) to the first photodetector array (103 a,b). a gamma scintillator array (101 x,y) comprising a plurality of first scintillator elements (101 1,1, 101 2,1) for generating first scintillation light in response to received gamma quanta; an optical modulator; and a first photodetector array (103 a,b) for detecting the first scintillation light generated by the gamma scintillator array (101 x,y); wherein the gamma scintillator array (101 x,y), the optical modulator and the first photodetector array (103 a,b) are arranged in a stacked configuration along a gamma radiation receiving direction; and wherein the optical modulator is disposed between the gamma scintillator array (101 x,y) and the first photodetector array (103 a,b) for modulating a transmission of the first scintillation light between the gamma scintillator array (101 x,y) and the first photodetector array (103 a,b); wherein the optical modulator comprises at least one optical modulator pixel, the optical modulator pixel having a cross sectional area in a plane that is perpendicular to the gamma radiation receiving direction; wherein the first photodetector array (103 a,b) comprises a plurality of photodetector pixels (103 1,1, 103 2,1), each photodetector pixel having a cross sectional area (103' a,b) in the plane that is perpendicular to the gamma radiation receiving direction; wherein the cross sectional area of each optical modulator pixel is greater than or equal to the cross sectional area of each photodetector pixel (103' a,b); and wherein the X-ray radiation detector is disposed along the gamma radiation receiving direction (104, 604) and on the opposite side of the gamma scintillator array (101 x,y) to the first photodetector array (103 a,b).