Registration of preoperatively acquired MRI images of soft parts to intraoperatively acquired X-ray images of soft parts is not possible. The invention shows a way of nevertheless using such preoperatively acquired images for superimposition with 2D projections of the soft parts, taking an indirect route via 3D/3D registration of images of the spinal column. For this purpose, 3D image data sets of the spinal column must be obtained separately on the one hand using MRI and on the other using the X-ray imaging system so that the 3D/3D registration produces a mapping rule which then also applies to the preoperatively acquired images of the soft part if the soft part images and the spinal images are acquired without intervening change in the patient position in the MRI scanner.