Abstract: A computer tomography apparatus has an x-ray source which is fed with a high voltage supply. Operation of the x-ray source over an extended period of time causes the focal spot on the anode to change position, thereby degrading the image which is constructed from the series of exposures, because construction of the image is based in part upon the focus being at a known position. A focus position computer is therefore provided which receives signals from the high voltage supply corresponding to the operating parameters of the x-ray source. Based on a known relationship between these parameters, the length of operating time, and the expected position of the focus, the focus position computer calculates the expected change in position of the focus, and supplies a signal corresponding to this change to the image construction computer, which takes the change in focus position into account in constructing the image for display.