1. A method for collimating an off-center sub-object of an examination subject by a collimator of an imaging system of an X-ray diagnostic apparatus with a rotating gantry, comprising: disposing a computed tomography imaging system in the rotating gantry comprising a first X-ray source and a computed tomography X-ray detector disposed opposite the first X-ray source and comprising an array of individual detectors; disposing an angiographic imaging system comprising a second X-ray source offset to the first X-ray source, and a flat panel X-ray detector disposed opposite the second X-ray source and comprising a matrix-shaped pixel elements; taking a 3D X-ray image of the examination subject by the computed tomography imaging system; selecting the off-center examination sub-object based on the 3D X-ray image; determining a collimator setting for a shooting position of the angiographic imaging system based on a fixed relative disposition between the angiographic imaging system and the computed tomography imaging system; and collimating the off-center examination sub-object by adjusting the collimator according to the collimator setting.