An apparatus, method and computer program for operating an apparatus. The apparatus comprises: a scintillator and an array of photodetectors; the scintillator configured to be rotatable around the periphery of a computed tomography scanner, the scintillator configured to receive X-rays incident on the scintillator, convert the received X-rays to visible light and transmit the visible light towards a corresponding photodetector of the array of photodetectors; and the array of photodetectors fixed around the periphery of the computed tomography scanner, each of the photodetectors in the array of photodetectors configured to output an electrical signal in response to detecting the visible light received from the scintillator.