Development of the Varioscope AR. A see-through HMD for computer-aided surgery

In computer-aided surgery (CAS), an undesired side-effect of the necessity of handling sophisticated equipment in the operating room is the fact that the surgeon´s attention is drawn from the operating field, since surgical progress is partially monitored on the computer´s screen. Augmented reality (AR), the overlay of computer-generated graphics over a real-world scene, provides a possibility to solve this problem. The technical problems associated with this approach, such as viewing of the scenery within a common focal range on the head-mounted display (HMD) or latency in the display on the HMD, have, however, kept AR from widespread usage in CAS. The concept of the Varioscope AR, a lightweight head-mounted operating microscope used as a HMD, is introduced. The registration of the patient to the pre-operative image data, as well as pre-operative planning, take place on VISIT, a surgical navigation system developed at our hospital. Tracking of the HMD and stereoscopic visualisation take place on a separate POSIX.4-compliant real-time operating system running on PC hardware. We were able to overcome the technical problems described above; our work resulted in an AR visualisation system with an update rate of 6 Hz and a latency below 130 ms. It integrates seamlessly into a surgical navigation system and provides a common focus for both virtual and real-world objects. First evaluations of the photogrammetric 2D/3D registration have resulted in a match of 1.7 pixels on the HMD display. The Varioscope AR with its real-time visualisation unit is a major step towards the introduction of AR into clinical routine