1. A method for registering an intra-operative image data set with a pre-operative 3D image data set for a medical examination, comprising: providing a pre-operative 3D image data set for a medical examination, said data set including information descriptive of a surface along a feature of a patient such that an image of the surface is obtainable from the data set; obtaining a pre-operative surface mask for the surface from the pre-operative 3D image data set; positioning an optical 3D sensor system to provide a side view of the surface of the feature; calibrating the optical 3D sensor system with respect to an intra-operative imaging modality; illuminating the surface with a light pattern such that the side view of the illuminated surface indicates displacement of lines in the pattern along the feature; detecting an area of interest including the illuminated surface of the feature with the optical 3D sensor system and obtaining an intra-operative surface mask of the feature based on data acquired with the optical 3D sensor system; recording the area of interest with the intra-operative imaging modality at least partly containing the intra-operative surface mask to obtain the intra-operative image data set; registering the intra-operative surface mask with the pre-operative surface mask; determining a mapping specification between the intra-operative image data set and the pre-operative 3D image data set based on the calibration of the optical 3D sensor system with the intra-operative imaging modality and the registration of the intra-operative surface mask with the pre-operative surface mask; and overlaying the intra-operative image data set with the pre-operative 3D data set based on the mapping specification.