The invention relates to an image processing method and a medical observation device (1) such as a microscope (2) or endoscope. The device and method are used for displaying output image data (54) of soft biological tissue (12). In image-guided surgery, pre-operative three-dimensional image data (6) of the soft biological tissue (12) are elastically matched to interoperative image data (14) which are acquired during surgery. By displaying the elastically matched pre-operative three-dimensional image data (6) together with the interoperative image data (14), the surgeon may be made aware of the consistence of the soft biological tissue (12) below the visible surface layer. Existing systems for image-guided surgery need to be manually readjusted if surgery is done on soft biological tissue (12), which may deform and shift. To avoid this, the device and method according to the invention perform an elastic matching of the pre-operative three-dimensional image data (6) based on the interoperative image data (14) of the soft biological tissue (12). At least one vascular plexus structure (48) is first identified in the interoperative image data (14) and then the same vascular plexus structure (48) is identified in the pre-operative three-dimensional image data (6). The vascular plexus structure (48) in the pre-operative three-dimensional image data (6) is then elastically matched to the vascular plexus structure (48) in the interoperative image data (14). Output image data (54) are generated combining the elastically matched pre-operative three-dimensional image data (6) to the interoperative image data (14). Preferably, the at least one vascular plexus structure (48) is identified using fluorescent light from a fluorophore (22) which has been injected into the soft biological tissue (12).
