Inherent to the field of computer-aided surgery (CAS) is the necessity to handle sophisticated equipment in the operating room; an undesired side-effect of this development 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. The overlay of computer-generated graphics over a real-world scene, also referred to as augmented reality (AR), provides a possibility to solve this problem. The considerable 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 display on the HMD have, however, kept AR from widespread usage in CAS. In this paper, 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 preoperative image data as well as preoperative 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. While being in a very early stage of laboratory testing, 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. First tests with 2D/3D registration have shown a match between 3D world coordinates and 2D HMD display coordinates in the range of 1.7 pixels. It integrates seamlessly into a surgical navigation system and provides a common focus for both virtual and real world objects. On the basis of our current results, we conclude that the Varioscope AR with the real-time visualisation unit is a major step towards the introduction of AR into clinical routine.
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