Eduard Gröller

CPR - curved planar reformation

Visualization of tubular structures such as blood vessels is an important topic in medical imaging. One way to display tubular structures for diagnostic purposes is to generate longitudinal cross-sections in order to show their lumen, wall, and surrounding tissue in a curved plane. This process is called curved planar reformation (CPR). We present three different methods to generate CPR images. A tube-phantom was scanned with computed tomography (CT) to illustrate the properties of the different CPR methods. Furthermore we introduce enhancements to these methods: thick-CPR, rotating-CPR and multi-path-CPR.

GEOMETRY OF MIXED-MODE OSCILLATIONS IN THE 3-D AUTOCATALATOR

We present a geometric explanation of a basic mechanism generating mixed-mode oscillations in a prototypical simple model of a chemical oscillator. Our approach is based on geometric singular perturbation theory and canard solutions. We explain how the small oscillations are generated near a special point, which is classified as a folded saddle-node for the reduced problem. The canard solution passing through this point separates small oscillations from large relaxation type oscillations. This allows to define a one-dimensional return map in a natural way. This bimodal map is capable of explaining the observed bifurcation sequence convincingly.

The VesselGlyph: Focus & Context Visualization in CT-Angiography

Accurate and reliable visualization of blood vessels is still a challenging problem, notably in the presence of morphologic changes resulting from atherosclerotic diseases. We take advantage of partially segmented data with approximately identified vessel centerlines to comprehensively visualize the diseased peripheral arterial tree. We introduce the VesselGlyph as an abstract notation for novel focus & context visualization techniques of tubular structures such as contrast-medium enhanced arteries in CT-angiography (CTA). The proposed techniques combine direct volume rendering (DVR) and curved planar reformation (CPR) within a single image. The VesselGlyph consists of several regions where different rendering methods are used. The region type, the used visualization method and the region parameters depend on the distance from the vessel centerline and on viewing parameters as well. By selecting proper rendering techniques for different regions, vessels are depicted in a naturally looking and undistorted anatomic context. This may facilitate the diagnosis and treatment planning of patients with peripheral arterial occlusive disease. In this paper we furthermore present a way of how to implement the proposed techniques in software and by means of modern 3D graphics accelerators.

Computed tomography angiography: a case study of peripheral vessel investigation

This paper deals with vessel exploration based on computed tomography angiography. Large image sequences of the lower extremities are investigated in a clinical environment. Two different approaches for peripheral vessel diagnosis dealing with stenosis and calcification detection are introduced. The paper presents an automated vessel-tracking tool for curved planar reformation. An interactive segmentation tool for bone removal is proposed.

Memory efficient acceleration structures and techniques for CPU-based volume raycasting of large data

Most CPU-based volume raycasting approaches achieve high performance by advanced memory layouts, space subdivision, and excessive pre-computing. Such approaches typically need an enormous amount of memory. They are limited to sizes which do not satisfy the medical data used in daily clinical routine. We present a new volume raycasting approach based on image-ordered raycasting with object-ordered processing, which is able to perform high-quality rendering of very large medical data in real-time on commodity computers. For large medical data such as computed tomographic (CT) angiography run-offs (512 /spl times/ 512 /spl times/ 1202) we achieve rendering times up to 2.5 fps on a commodity notebook. We achieve this by introducing a memory efficient acceleration technique for on-the-fly gradient estimation and a memory efficient hybrid removal and skipping technique of transparent regions. We employ quantized binary histograms, granular resolution octrees, and a cell invisibility cache. These acceleration structures require just a small extra storage of approximately 10%.

Mastering windows: improving reconstruction

Ideal reconstruction filters, for function or arbitrary derivative reconstruction, have to be bounded in order to be practicable since they are infinite in their spatial extent. This can be accomplished by multiplying them with windowing functions. In this paper we discuss and assess the quality of commonly used windows and show that most of them are unsatisfactory in terms of numerical accuracy. The best performing windows are Blackman, Kaiser and Gaussian windows. The latter two are particularly useful since both have a parameter to control their shape, which, on the other hand, requires to find appropriate values for these parameters. We show how to derive optimal parameter values for Kaiser and Gaussian windows using a Taylor series expansion of the convolution sum. Optimal values for function and first derivative reconstruction for window widths of two, three, four and five are presented explicitly.

Illustrative visualization: new technology or useless tautology?

The computer graphics group at TU Vienna has created some of most beautiful and effective illustrative visualizations. In this article, they share with us their unique perspective on illustrative visualization. --- Kwan-Liu Ma

Virtual colon flattening

We present a new method to visualize virtual endoscopic views. We propose to flatten the organ by the direct projection of the surface onto a set of cylinders. Two sampling strategies are presented and the introduced distortions are studied. A non-photorealistic technique is presented to enhance the perception of the images. Finally, an approximate but real-time endoscopic fly-through is possible by using the data obtained by the projection technique.

Visualizing Poincaré Maps together with the Underlying Flow

We present a set of advanced techniques for the visualization of 2D Poincare maps. Since 2D Poincare maps are a mathematical abstraction of periodic or quasi-periodic 3D flows, we propose to embed the 2D visualization with standard 3D techniques to improve the understanding of the Poincare maps. Methods to enhance the representation of the relation x ↔ P(x), e.g., the use of spot noise, are presented as well as techniques to visualize the repeated application of P, e.g., the approximation of P as a warp function. It is shown that animation can be very useful to further improve the visualization. For example, the animation of the construction of Poincare map P is a very intuitive visualization. During the paper we present a set of examples which demonstrate the usefulness of our techniques.

Mastering interactive virtual Bronchioscopy on a Low—end PC

Virtual endoscopy presents the cross-sectional acquired 3D-data of a computer tomograph as an endoluminal view. The common approach for the visualization of a virtual endoscopy is surface rendering, yielding images close to a real endoscopy. If external structures are of interest, volume rendering techniques have to be used. These methods do not display the exact shape of the inner lumen very well. For certain applications, e.g. operation planning of a transbronchial biopsy, both the shape of the inner lumen as well as outer structures like blood vessels and the tumor have to be delineated. A method is described, that allows a quick and easy hybrid visualization using overlays of different visualization methods like different surfaces or volume renderings with different transfer functions in real time on a low-end PC. To achieve real time frame rates, image based rendering techniques have been used.