A method is proposed for collimating an off-center sub-object of an examination subject by a collimator of an X-ray diagnostic apparatus. The apparatus has a computed tomography imaging system having a first X-ray source and a computed tomography X-ray detector disposed opposite the first X-ray source having a number of individual detectors and an angiographic imaging system having 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 with matrix shaped pixel elements. A 3D image of the subject is taken by the CT imaging system. The off-center sub-object is selected based on the 3D image. The position of the sub-object is determined for a shooting position of the angiographic imaging system according to the fixed relative disposition between the angiographic imaging system and the CT imaging system. The collimator is adjusted accordingly for collimating the off-center section.