An image sensor comprising: a two-dimensional array of pixel sensors; a layer of scintillation material adjacent to said two-dimensional array, said scintillation material arranged to emit light in response to x-rays impinging thereon, said pixel sensors being arranged to detect said light; and a controller that is arranged to read out data stored in said two-dimensional array of pixel sensors and to form an image comprising a two-dimensional array of pixels therefrom, said controller being arranged to correct that data for errors resulting from x-rays that generate electrons that are stored in said pixel sensors in the process of forming said image; wherein each of said pixel sensors comprises a photodetector that is arranged to store a charge in response to said light being received by that photodetector during a preceding time period; and wherein said controller is arranged to form said image by causing said array of pixel sensors to form a plurality of frames, each frame comprising an array of measurements determined by said charges stored by said pixel sensors during a corresponding preceding time period; characterised in that said controller is arranged to identify pixel sensors in which said stored charge is greater than a threshold value, and said controller is arranged to cumulatively combine data from a plurality of frames for each pixel sensor to determine a corresponding pixel in said image and wherein said controller is arranged such that, for each pixel sensor, it does not combine data from a frame in which said pixel sensor has been identified as storing a charge greater than said threshold value. a two-dimensional array of pixel sensors; a layer of scintillation material adjacent to said two-dimensional array, said scintillation material arranged to emit light in response to x-rays impinging thereon, said pixel sensors being arranged to detect said light; and a controller that is arranged to read out data stored in said two-dimensional array of pixel sensors and to form an image comprising a two-dimensional array of pixels therefrom, said controller being arranged to correct that data for errors resulting from x-rays that generate electrons that are stored in said pixel sensors in the process of forming said image; wherein each of said pixel sensors comprises a photodetector that is arranged to store a charge in response to said light being received by that photodetector during a preceding time period; and wherein said controller is arranged to form said image by causing said array of pixel sensors to form a plurality of frames, each frame comprising an array of measurements determined by said charges stored by said pixel sensors during a corresponding preceding time period; characterised in that said controller is arranged to identify pixel sensors in which said stored charge is greater than a threshold value, and said controller is arranged to cumulatively combine data from a plurality of frames for each pixel sensor to determine a corresponding pixel in said image and wherein said controller is arranged such that, for each pixel sensor, it does not combine data from a frame in which said pixel sensor has been identified as storing a charge greater than said threshold value.