a case he already knows how the path of the structure beyond his current open incision continues particularly in vascular surgery in which vascular injuries are absolutely to be avoided such a technical operating aid is greatly advantageous to this end however the volume data set 10 first has to be aligned which is possible if it is registered the problem here is that the volume data set 10 does not comprise any external patient landmarks or artificial landmarks and a registration method in accordance with the invention is explained in the following the 3d volume data set 10 from the rotational angiography shows only a small tissue structure which is not accessible from outside for the purpose of registration in accordance with the present embodiment of the invention a predefined angular coordinate system is therefore used for example on the rotational angiography scanner 1 in order to ascertain the spatial position of the structure 10 in the three dimensional volume data set to this end the current position of the patient in the rotational angiography scanner is first ascertained by means of the localization device 6 with respect to the coordinate system of the scanner as shown in fig7 the scanner 1 is aligned such that the localization device is distinguishable in an exactly fixed vertical and horizontal alignment in each of a lateral sample recording an x ray image from right to left fig9 and an anterior posterior recording fig8 this is the case if the markers 5 are situated on a line which in fig8 for example is provided with the reference numeral 7 the angles set for this purpose.