A medical imaging system includes an x-ray source (112) having a focal SPOT that emits radiation that traverses an examination region (108). The position of the focal SPOT along a longitudinal direction is a function of a temperature of one or more x-ray source components. The system further includes a detector (120) that detects the radiation and a collimator (116), disposed between the x-ray source (112) and the examination region (108), that collimates that radiation along the longitudinal direction. A focal SPOT position estimator (132) dynamically computes an estimated position of the focal SPOT along the longitudinal direction based ON the temperature of one or more x-ray source components. A collimator positioner (128) positions the collimator (116) along the longitudinal direction based ON the estimated focal SPOT position prior to performing a scan.
