To visually support a catheter ablation in the heart, three-dimensional image data have been used prior to the intervention. During ablation, the position of the catheter is pinpointed by an orientation system. The orientation system acquires electroanatomical 3D mapping data. The two-dimensional image data is assigned to the 3D mapping data in the correct position and dimensions which is a time-consuming step. The invention makes provision for the orientation system being in a fixed location relative to the X-ray system so that a positionally and dimensionally correct alignment of the X-ray image data set with the 3D mapping data is no longer required. An image or surface based 3D-3D alignment of the three-dimensional data acquired prior to the intervention with the three-dimensional X-ray image data is considerably less time-consuming than alignment thereof with the 3D mapping data and is more reliable because more structures is recognized in the three-dimensional X-ray image data.