1. A method for obtaining a volume data set of a mobile object of a patient by a C-arm X-ray device and an electromagnetic position detection system, comprising: arranging an electromagnetic sensor of the electromagnetic position detection system on the object; recording a plurality of X-ray projections of the object from various projection directions by the C-arm X-ray device; detecting the electromagnetic sensor in the X-ray projections; and reconstructing the volume data set from the X-ray projections in which the electromagnetic sensor corresponds to a position characterizing a determined displacement phase of the object.