Visualization of medical data requires the extraction of surfaces that represent the boundaries of objects of interest. This paper describes a method that combines finding these boundaries accurately and ensuring that this surface consists of high quality triangles. The latter is important for subsequent visualization and simulation. We show that the surfaces created using this method are both accurate and have good quality triangles.@en

Finding a correct a priori back-to-front (BTF) visibility ordering for the perspective projection of the voxels of a rectangular volume poses interesting problems. The BTF ordering presented by Frieder et al. [6] and the permuted BTF presented by Westover [14] are correct for parallel projection but not for perspective projection [12]. Swan presented a constructive proof for the correctness of the perspective BTF (PBTF) ordering [12]. This was a significant improvement on the existing orderings. However, his proof assumes that voxel projections are not larger than a pixel, i.e. voxel projections do not overlap in screen space. Very often the voxel projections do overlap, e.g. with splatting algorithms. In these cases, the PBTF ordering results in highly visible and characteristic rendering artefacts. In this paper we analyse the PBTF and show why it yields these rendering artefacts. We then present an improved visibility ordering that remedies the artefacts. Our new ordering is as good as the PBTF, but it is also valid for cases where voxel projections are larger than a single pixel, i.e. when voxel projections overlap in screen space. We demonstrate why and how our ordering works at fundamental and implementation levels. Keywords Volume rendering - Visibility ordering - Splatting@en

The field of volume visualization has undergone rapid development during the past years, both due to advances in suitable computing hardware and due to the increasing availability of large volume datasets. Recent work has focused on increasing the visual realism in Direct Volume Rendering (DVR) by integrating a number of visually plausible but often effect-specific rendering techniques, for instance modeling of light occlusion and depth of field. Besides yielding more attractive renderings, especially the more realistic lighting has a positive effect on perceptual tasks. Although these new rendering techniques yield impressive results, they exhibit limitations in terms of their exibility and their performance. Monte Carlo ray tracing (MCRT), coupled with physically based light transport, is the de-facto standard for synthesizing highly realistic images in the graphics domain, although usually not from volumetric data. Due to the stochastic sampling of MCRT algorithms, numerous effects can be achieved in a relatively straight-forward fashion. For this reason, we have developed a practical framework that applies MCRT techniques also to direct volume rendering (DVR). With this work, we demonstrate that a host of realistic effects, including physically based lighting, can be simulated in a generic and flexible fashion, leading to interactive DVR with improved realism. In the hope that this improved approach to DVR will see more use in practice, we have made available our framework under a permissive open source license.@en

We present a combination of techniques for interactive out-of-core visualisation of isosurfaces from large time-dependent data sets. We make use of an index tree, computed in a pre-processing stage, which effectively captures temporal coherence in the data set. This tree data structure enables fast extraction of all isovalue-spanning cells from any time step and for any isovalue. For very large time-dependent data sets, such as those resulting from CFD simulations, this data structure can easily become too large to fit in main memory. Therefore, we have adapted the generation of the data structure, as well as the data structure itself for out-of-core application. During generation, the data set is spatially divided into several regions, each resulting in a separate tree. For visualisation, the application uses all these trees simultaneously, but will use only part of each of the trees. Only a user-specified time window will be kept in main memory and other parts of the tree will be read and released on demand. Finally, to avoid time-consuming triangulation and surface reconstruction, we have used a hardware-assisted direct point rendering algorithm for displaying the isosurfaces. These combined techniques allow interactive exploration and visualisation of very large time-varying data sets on a normal PC. Keywords: Scientific visualization; Large data handling; Isosurfaces; Out-of-core techniques; Time-dependent data@en

Piecewise removal of myomata (fibroids, benign tumors) from the uterine wall is performed using a hysteroresectoscope inserted through the vagina. Only radical removal of the myoma prevents regrowth, whereas penetration of the uterine wall should be avoided. Preoperative MR (Magnetic Resonance) images show the full outline and the location of the myoma in the uterine wall. Preoperative 3D US (ultrasound) reveals the part of the myoma that protrudes into the uterine cavity, but the part located in the uterine wall is almost invisible. The images and extracted preoperative information must be registered to the preoperative situation. A pragmatic solution in the form of three point incremental matching is applied. Three patients were scanned and the matching was performed on the data. The procedure provided a good match on two patients.@en

A research programme that aims to improve the state of the art in shoulder joint replacement surgery has been initiated at the Delft University of Technology. Development of improved endoprostheses for the upper extremities (DIPEX), as this effort is called, is a clinically driven multidisciplinary programme consisting of many contributory aspects. A part of this research programme focuses on the pre-operative planning and per-operative guidance issues. The ultimate goal of this part of the DIPEX project is to create a surgical support infrastructure that can be used to predict the optimal surgical protocol and can assist with the selection of the most suitable endoprosthesis for a particular patient. In the pre-operative planning phase, advanced biomechanical models of the endoprosthesis fixation and the musculo-skeletal system of the shoulder will be incorporated, which are adjusted to the individual's morphology. Subsequently, the support infrastructure must assist the surgeon during the operation in executing his surgical plan. In the per-operative phase, the chosen optimal position of the endoprosthesis can be realised using camera-assisted tools or mechanical guidance tools. In this article, the pathway towards the desired surgical support infrastructure is described. Furthermore, we discuss the pre-operative planning phase and the per-operative guidance phase, the initial work performed, and finally, possible approaches for improving prosthesis placement. Author Keywords: pre-operative planning; per-operative guidance; navigation; computer-assisted orthopaedic surgery; shoulder arthroplasty; medical applications; data registration; tracking; visualisation; MRI; CT; X-ray@en

In projection-based Virtual Reality (VR) systems, typically only one headtracked user views stereo images rendered from the correct view position. For other users, who are presented a distorted image, moving with the first user's head motion, it is difficult to correctly view and interact with 3D objects in the virtual environment. In close-range VR systems, such as the Virtual Workbench, distortion effects are especially large because objects are within close range and users are relatively far apart. On these systems, multi-user collaboration proves to be difficult. In this paper, we analyze the problem and describe a novel, easy to implement method to prevent and reduce image distortion and its negative effects on close-range interaction task performance. First, our method combines a shared camera model and view distortion compensation. It minimizes the overall distortion for each user, while important user-personal objects such as interaction cursors, rays and controls remain distortion-free. Second, our method retains co-location for interaction techniques to make interaction more consistent. We performed a user experiment on our Virtual Workbench to analyze user performance under distorted view conditions with and without the use of our method. Our findings demonstrate the negative impact of view distortion on task performance and the positive effect our method introduces. This indicates that our method can enhance the multi-user collaboration experience on close-range, projection-based VR systems.@en