1. A medical system for examination or treatment comprising: an image recording device; an image processing device for processing sensor signals recorded in an examination area, the examination area being a blood vessel; a catheter comprising a first sensor of an imaging system for optical coherence tomography; and a second sensor of an intravascular ultrasound image recording system and a catheter tip, the catheter connected to the image recording system, the first and/or second sensor connected to the image processing device, the catheter further comprising a position sensor or position detector; a display device for displaying images generated by the image processing device; an external locating device comprising a position detector operatively connected to the position sensor in the catheter, or a position sensor operatively connected to the position detector in the catheter, for detecting a position or an orientation of the catheter tip, wherein the locating device includes a computing unit for calculating at least one geometric property of the examination area based on the detected position or orientation respectively of the catheter tip, the at least one geometric property comprising the centerline and/or an envelope curve of the examination area calculated based on a plurality of boundary points where the catheter strikes a boundary layer of the vessel; and a 3D-image generator for generating a three dimensional image using a plurality of two-dimensional images recorded by the image recording device, the 3D-image generator further processing the detected position or orientation respectively of the catheter tip in combination with said at least one geometric property of the examination area for generating the three-dimensional image.