1. An apparatus suitable for screening a receptacle, said apparatus comprising: a) an input for receiving an image signal conveying an input image related to contents of the receptacle, the image signal having been produced by subjecting the receptacle to penetrating radiation using a device having a radiation source and radiation detectors, the device being characterized by introducing distortion into the input image so that the input image signal conveys a distorted representation of the receptacle and its contents, wherein part of the distortion introduced in the input image is dependent on a location of the receptacle relative to the radiation source and radiation detectors while the receptacle was being subjected to penetrating radiation; b) a processing unit in communication with said input, said processing unit being programmed for: applying a distortion correction process to the image signal to remove at least in part from the input image the part of the distortion dependent on the location of the receptacle relative to the radiation source and radiation detectors while the receptacle was being subjected to penetrating radiation, thereby to generate a corrected image signal related to contents of the receptacle, the corrected image signal conveying a modified representation of the receptacle and its contents, wherein applying the distortion correction process to the image signal includes: selecting a candidate height from a range of possible heights, the possible heights in said range being related to respective locations relative to the radiation source and detectors of the device; and using the selected candidate height to condition the distortion correction process applied to the image signal; processing the corrected image signal in an attempt to detect a presence in the receptacle of a target object; generating a detection signal in response to detection of the presence of the target object in the receptacle; c) an output for releasing the detection signal.