PC-Based Control Unit for a Head Mounted Operating Microscope for Augmented Reality Visualization in Surgical Navigation Michael Figl 1,4 , Wolfgang Birkfellner 1,2 , Franz Watzinger 3 , Felix Wanschitz 3 , Johann Hummel 1 , Rudolf Hanel 1 , Rolf Ewers 3 , and Helmar Bergmann 1,4 1 Department of Biomedical Engineering and Physics, AKH, Vienna, Austria 2 CARCAS Group, University Hospital of Basle, Switzerland 3 Department of Oral and Maxillofacial Surgery, AKH, Vienna, Austria 4 Ludwig-Boltzmann Institute of Nuclear Medicine, Vienna, Austria Abstract. We have adapted a miniature head mounted operating mi- croscope for AR by integrating two very small computer displays. To calibrate the projection parameters of this so called Varioscope AR we have used Tsai’s Algorithm for camera calibration. Connection to a sur- gical navigation system was performed by defining an open interface to the control unit of the Varioscope AR. The control unit consists of a standard PC with an dual head graphics adapter. We connected this control unit to an computer aided surgery (CAS) system by the TCP/IP interface. In this paper we present the control unit for the HMD and its software design. We tested two different optical tracking systems, the Flashpoint (Image Guided Technologies, Boulder, CO), which provided about 10 frames per second, and the Polaris (Northern Digital, Ontario, Can) which provided at least 30 frames per second, both with a time delay of one frame. 1 Introduction Due to the fact that computer aided surgery (CAS) was first introduced to the field of neurosurgery, many efforts were focused on introducing Augmented Reality (AR),the overlay of computer-generated graphics for providing ”target information” into operating microscopes [3,4,5,6]. and the computer graphics are to be displayed in the optical system of the microscope, therefore no focusing problems when merging the computer graphics and the optical image are encoun- tered. On the other hand, the operating microscope is bulky, expensive, and has a rather limited field of clinical applications. In parallel, head-mounted displays (HMD) were also used for AR in medicine [7,8]. HMDs are lightweight, cheaper, and can be attached directly to the surgeon’s head. A major problem, lies in the fact that a common focal plane is not easily achieved in commercial HMDs. Furthermore, rapid head motions cause perceptible delay in the display of the computer graphics, thus causing simulator sickness and reduced acceptance. Two methods have been proposed in the past to overcome these problems. In [7] and [8], the authors describe video see-through systems. The real world T. Dohi and R. Kikinis (Eds.): MICCAI 2002, LNCS 2489, pp. 44–51, 2002. c  Springer-Verlag Berlin Heidelberg 2002