Günther Greiner

Quantification of, visualization of, and compensation for brain shift using intraoperative magnetic resonance imaging.

OBJECTIVEModern neuronavigation systems lack spatial accuracy during ongoing surgical procedures because of increasing brain deformation, known as brain shift. Intraoperative magnetic resonance imaging was used for quantitative analysis and visualization of this phenomenon. METHODSFor a total of 64 patients, we used a 0.2-T, open-configuration, magnetic resonance imaging scanner, located in an operating theater, for pre- and intraoperative imaging. The three-dimensional imaging data were aligned using rigid registration methods. The maximal displacements of the brain surface, deep tumor margin, and midline structures were measured. Brain shift was observed in two-dimensional image planes using split-screen or overlay techniques, and three-dimensional, color-coded, deformable surface-based data were computed. In selected cases, intraoperative images were transferred to the neuronavigation system to compensate for the effects of brain shift. RESULTSThe results demonstrated that there was great variability in brain shift, ranging up to 24 mm for cortical displacement and exceeding 3 mm for the deep tumor margin in 66% of all cases. Brain shift was influenced by tissue characteristics, intraoperative patient positioning, opening of the ventricular system, craniotomy size, and resected volume. Intraoperative neuronavigation updating (n = 14) compensated for brain shift, resulting in reliable navigation with high accuracy. CONCLUSIONWithout brain shift compensation, neuronavigation systems cannot be trusted at critical steps of the surgical procedure, e.g., identification of the deep tumor margin. Intraoperative imaging allows not only evaluation of and compensation for brain shift but also assessment of the quality of mathematical models that attempt to describe and compensate for brain shift.

Interactive volume on standard PC graphics hardware using multi-textures and multi-stage rasterization

Interactive direct volume rendering has yet been restricted to high-end graphics workstations and special-purpose hardware, due to the large amount of trilinear interpolations, that are necessary to obtain high image quality. Implementations that use the 2D-texture capabilities of standard PC hardware, usually render object-aligned slices in order to substitute trilinear by bilinear interpolation. However the resulting images often contain visual artifacts caused by the lack of spatial interpolation. In this paper we propose new rendering techniques that significantly improve both performance and image quality of the 2D-texture based approach. We will show how in ulti-texturing capabilitiesof modern consumer PC graphboards are exploited to enable in teractive high quality volume visualization on low-cost hardware. Furthermore we demonstrate how multi-stage rasterization hardware can be used to efficiently render shaded isosurfaces and to compute diffuse illumination for semi-transparent volume rendering at interactive frame rates.

Variational Design and Fairing of Spline Surfaces

Variational principles have become quite popular in the design of free form surfaces. Among others they are used for fairing purposes. The choice of the ‘right’ fairness functional is a crucial step. There is always a tradeoff between high quality and computational effort. In this paper we present fairness functionals that allow fairing efficiently, i.e., produce high quality surfaces in a reasonable amount of time. These functionals can be considered as simplified thin plate energy functionals for parametric surfaces or as simplified MVC functionals.

Automatic reconstruction of personalized avatars from 3D face scans

We present a simple algorithm for computing a high‐quality personalized avatar from a single color image and the corresponding depth map which have been captured by Microsofts Kinect sensor. Due to the low market price of our hardware setup, 3D face scanning becomes feasible for home use. The proposed algorithm combines the advantages of robust non‐rigid registration and fitting of a morphable face model. We obtain a high‐quality reconstruction of the facial geometry and texture along with one‐to‐one correspondences with our generic face model. This representation allows for a wide range of further applications such as facial animation or manipulation. Our algorithm has proven to be very robust. Since it does not require any user interaction, even non‐expert users can easily create their own personalized avatars. Copyright

Fast volumetric deformation on general purpose hardware

High performance deformation of volumetric objects is a common problem in computer graphics that has not yet been handled sufficiently. As a supplement to 3D texture based volume rendering, a novel approach is presented, which adaptively subdivides the volume into piecewise linear patches. An appropriate mathematical model based on tri-linear interpolation and its approximations is proposed. New optimizations are introduced in this paper which are especially tailored to an efficient implementation using general purpose rasterization hardware, including new technologies, such as vertex programs and pixel shaders. Additionally, a high performance model for local illumination calculation is introduced, which meets the aesthetic requirements of visual arts and entertainment. The results demonstrate the significant performance benefit and allow for time-critical applications, such as computer assisted surgery.

Principles of 3d Image Analysis and Synthesis

Contributing Authors. Preface. 1. Optical 3D Sensors. 2. Multiple Views and Image Sequences. 3. Recognition and Interpretation. 4. Representation and Processing of Surface Data. 5. Realistic Rendering. 6. Volume Visualization. 7. Acoustic Imaging, Rendering, and Localization. 8. Selected Applications. References. Index.

Remeshing triangulated surfaces with optimal parameterizations

The use of polygonal meshes, especially triangle meshes, is manifold, but a lot of algorithms require the mesh to be structured in a certain way and cannot be applied to an arbitrarily structured mesh. The process of replacing an arbitrarily structured mesh by a structured one is called remeshing and the most important class of structured meshes are triangle meshes with subdivision connectivity. In this paper, we present an algorithm for remeshing triangle meshes with boundary that is based on parameterizing the mesh over a planar domain. We discuss what kind of parameterizations are optimal for the purpose of remeshing and show the advantages of our approach in a series of examples.

Reconstructing animated meshes from time-varying point clouds

In this paper, we describe a novel approach for the reconstruction of animated meshes from a series of time‐deforming point clouds. Given a set of unordered point clouds that have been captured by a fast 3‐D scanner, our algorithm is able to compute coherent meshes which approximate the input data at arbitrary time instances. Our method is based on the computation of an implicit function in ℝ4 that approximates the time‐space surface of the time‐varying point cloud. We then use the four‐dimensional implicit function to reconstruct a polygonal model for the first time‐step. By sliding this template mesh along the time‐space surface in an as‐rigid‐as‐possible manner, we obtain reconstructions for further time‐steps which have the same connectivity as the previously extracted mesh while recovering rigid motion exactly.

Multi bounding volume hierarchies

Efficient tracing of single and incoherent rays is still a challenge in computer graphics. Coherent packet tracing has reached real-time performance, but ray packets bring about restrictions for the architecture of the renderer and their suitability for diverging secondary ray bundles is uncertain. The algorithm presented in this paper avoids these problems by not using ray packets at all. Instead, it uses triangle packets and bounding volume packets in a novel acceleration data structure called multi bounding volume hierarchy (MBVH). It is designed for SIMD single ray tracing. The hierarchy is built from a binary bounding volume hierarchy by collapsing subtrees of height two into SIMD nodes storing four bounding boxes. A modified cost function for construction guarantees that all but one of the leaf nodes contain exactly four triangles. The MBVH makes good use of data-level parallelism during traversal and triangle intersection, yielding speed-ups of up to 2.8times for random ray shooting. It consumes less memory than a regular bounding volume hierarchy and requires no modifications to the architecture of the rendering engine.

Modeling with triangular B-splines

Triangular B-splines are a new tool for modeling complex objects with nonrectangular topology. The scheme is based on blending functions and control points, and lets us model piecewise polynomial surfaces of degree n that are C/sup n-1/-continuous throughout. Triangular B-splines permit the construction of smooth surfaces with the lowest degree possible. Because they can represent any piecewise polynomial surface, they provide a unified data format. The new B-spline scheme for modeling complex irregular objects over arbitrary triangulations has many desirable features. Applications like filling polygonal holes or constructing smooth blends demonstrate its potential for dealing with concrete design problems. The method permits real-time editing and rendering. Currently, we are improving the editor to accept simpler user input, optimizing intersection computations and developing new applications. >