face and approximately centered thereon angular orientation monitors 22 1 22 2 and 22 3 track the angular orientation of the respective collimator detector units a rotating gantry 24 rotates all three detector heads about the subject 18 an angular orientation monitor 26 determines the angular orientation of each of the heads at each angular data collection position each of the detector heads records radiation events in terms of which detector element received the radiation angle of the collimator detector unit within the head and angle of the head around the subject a data acquisition system 30 receives the projection data along with the corresponding positional and angular parameters this data is stored in a projection memory 32 the projection data is processed by reconstruction processor 34 which reconstructs an image from the projection data the reconstruction processor includes at least a preconditioning processor 36 denoted by p and a projection processor 38 denoted by s which are used in conjunction with an iterative loop processor 40 to perform the reconstruction the reconstructed image is preferably stored in an image memory 42 from which it is retrieved by a video processor 44 and displayed on a video monitor 46 of course other output devices such as a color.