1. A method for extending the effective FOV of a CT scanner, in which the first step is a), b) or c): a) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV; b) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV between the maximum and minimum extent of the patient from top to bottom and from right to left. c) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV and inside the outer boundary of the patient as defined by a PET or SPECT image.