Computer-aided surgery (CAS), the intraoperative application of biomedical visualization techniques, appears to be one of the most promising fields of application for augmented reality (AR), the display of additional computer-generated graphics over a real-world scene. Typically a device such as a head-mounted display (HMD) is used for AR. However, considerable technical problems connected with AR have limited the intraoperative application of HMDs up to now. One of the difficulties in using HMDs is the requirement for a common optical focal plane for both the realworld scene and the computer-generated image, and acceptance of the HMD by the user in a surgical environment. In order to increase the clinical acceptance of AR, we have adapted the Varioscope (Life Optics, Vienna), a miniature, cost-effective head-mounted operating binocular, for AR. In this paper, we present the basic design of the modified HMD, and the method and results of an extensive laboratory study for photogrammetric calibration of the Varioscope's computer displays to a real-world scene. In a series of 16 calibrations with varying zoom factors and object distances, mean calibration error was found to be 1.24 +/- 0.38 pixels or 0.12 +/- 0.05 mm for a 640 x 480 display. Maximum error accounted for 3.33 +/- 1.04 pixels or 0.33 +/- 0.12 mm. The location of a position measurement probe of an optical tracking system was transformed to the display with an error of less than 1 mm in the real world in 56% of all cases. For the remaining cases, error was below 2 mm. We conclude that the accuracy achieved in our experiments is sufficient for a wide range of CAS applications.

Development of complex software applications in image guided therapy (IGT) is often complicated by the fact that providing basic functionality for image processing and user interaction from a graphical user interfaces (GUI) requires considerable manpower for software development. We present a programming environment that combines the high-level image processing library AVW, in-house developed patient-to-image registration procedures, and an interface to position measurement hardware. A specific application can be developed by using Tcl/Tk, a simple platform-independent scripting language, for GUI development. This environment was applied to developing VISIT, a navigation system for computer-aided implant dentistry. VISIT is presented as a result of this paper. Parts of the development environment were made accessible to the public. Future work includes the implementation of video input for connecting ultrasound or fluoroscopy units. We conclude that our approach is well suited to accelerate the development of exploratory new applications of IGT.

In this article it is shown how Gauss' (1981) famous cyclotomic sum formula can be used for extracting square-roots modulo N.

In this correspondence it is shown how the elements of finite fields when mapped on the surface of a torus can be used for block coding over a three-dimensional signal space. The block codes are useful for data transmission using the recently defined Mannheim distance.

The Deutsche Telekom is currently considering the development of an en-/decryption module for widespread (internal) usage. For this purpose a block-cipher algorithm has been developed by the Deutsche Telekom. The algorithm named MAGENTA which stands for Multifunctional Algorithm for General-purpose Encryption and Network Telecommunication Applications, is based on the shuffle signal-flow structure combined with non-linear operations. The module is planned to operate in the Gigabit/sec range for fast networks. For such high data rates there is a need of block-cipher hardware.

The author shows how block codes over Gaussian integers can be used for coding over two-dimensional signal space. He introduces a two-dimensional modular distance called the Mannheim distance and proposes using codes designed for this distance. Some simple constructions of such codes are given, among them icyclic codes which belong to the class of constacyclic codes. As a special case icyclic codes include perfect one Mannheim error correcting codes. For most of the codes considered efficient decoders are given and their performance on the Gaussian channel is investigated.>

New methods are given to solve equations over finite fields. The methods exploit the structure of GF(p/sup m/) and are first developed for fields of characteristic two. Then the results are generalized for fields of characteristic p>2.>

Some properties of Zech's logarithms are stated and exploited to compute Zech's logarithms from a restricted set of key elements. Zech's logarithms are recommended for the fast solution of quadratic, cubic, and quartic equations over GF(2/sup m/).>