Evaluation of segmentation algorithms for generation of patient models in radiofrequency hyperthermia.

Time-efficient and easy-to-use segmentation algorithms (contour generation) are a precondition for various applications in radiation oncology, especially for planning purposes in hyperthermia. We have developed the three following algorithms for contour generation and implemented them in an editor of the HyperPlan hyperthermia planning system. Firstly, a manual contour input with numerous correction and editing options. Secondly, a volume growing algorithm with adjustable threshold range and minimal region size. Thirdly, a watershed transformation in two and three dimensions. In addition, the region input function of the Helax commercial radiation therapy planning system was available for comparison. All four approaches were applied under routine conditions to two-dimensional computed tomographic slices of the superior thoracic aperture, mid-chest, upper abdomen, mid-abdomen, pelvis and thigh; they were also applied to a 3D CT sequence of 72 slices using the three-dimensional extension of the algorithms. Time to generate the contours and their quality with respect to a reference model were determined. Manual input for a complete patient model required approximately 5 to 6 h for 72 CT slices (4.5 min/slice). If slight irregularities at object boundaries are accepted, this time can be reduced to 3.5 min/slice using the volume growing algorithm. However, generating a tetrahedron mesh from such a contour sequence for hyperthermia planning (the basis for finite-element algorithms) requires a significant amount of postediting. With the watershed algorithm extended to three dimensions, processing time can be further reduced to 3 min/slice while achieving satisfactory contour quality. Therefore, this method is currently regarded as offering some potential for efficient automated model generation in hyperthermia. In summary, the 3D volume growing algorithm and watershed transformation are both suitable for segmentation of even low-contrast objects. However, they are not always superior to user-friendly manual programs for contour generation. When the volume growing algorithm is used, the contours have to be postprocessed with suitable filters. The watershed transformation has a large potential if appropriately developed to 3D sequences and 3D interaction features. After all, the practicality and feasibility of every segmentation method critically depend on various details of the user software as pointed out in this article.

Anatomy- and physics-based facial animation for craniofacial surgery simulations.

A modelling approach for the realistic simulation of facial expressions of emotion in craniofacial surgery planning is presented. The method is different from conventional, non-physical techniques for character animation in computer graphics. A consistent physiological mechanism for facial expressions was assumed, which was the effect of contracting muscles on soft tissues. For the numerical solution of the linear elastic boundary values, the finite element method on tetrahedral grids was used. The approach was validated on a geometrical model of a human head derived from tomographic data. Using this model, individual facial expressions of emotion were estimated by the superpositioning of precomputed single muscle actions.

Improving deformable surface meshes through omni-directional displacements and MRFs.

Deformable surface models are often represented as triangular meshes in image segmentation applications. For a fast and easily regularized deformation onto the target object boundary, the vertices of the mesh are commonly moved along line segments (typically surface normals). However, in case of high mesh curvature, these lines may intersect with the target boundary at "non-corresponding" positions, or even not at all. Consequently, certain deformations cannot be achieved. We propose an approach that allows each vertex to move not only along a line segment, but within a surrounding sphere. We achieve globally regularized deformations via Markov Random Field optimization. We demonstrate the potential of our approach with experiments on synthetic data, as well as an evaluation on 2 x 106 coronoid processes of the mandible in Cone-Beam CTs, and 56 coccyxes (tailbones) in low-resolution CTs.

In vitro study of near-wall flow in a cerebral aneurysm model with and without coils.

BACKGROUND AND PURPOSE: Coil embolization procedures change the flow conditions in the cerebral aneurysm and, therefore, in the near-wall region. Knowledge of these flow changes may be helpful to optimize therapy. The goal of this study was to investigate the effect of the coil-packing attenuation on the near-wall flow and its variability due to differences in the coil structure. MATERIALS AND METHODS: An enlarged transparent model of an ACA aneurysm was fabricated on the basis of CT angiography. The near-wall flow was visualized by using a recently proposed technique called Wall-PIV. Coil-packing attenuation of 10%, 15%, and 20% were investigated and compared with an aneurysmal flow without coils. Then the flow variability due to the coil introduction was analyzed in 10 experiments by using a packing attenuation of 15%. RESULTS: A small packing attenuation of 10% already alters the near-wall flow significantly in a large part of the aneurysmal sac. These flow changes are characterized by a slow flow with short (interrupted) path lines. An increased packing attenuation expands the wall area exposed to the altered flow conditions. This area, however, depends on the coil position and/or on the 3D coil structure in the aneurysm. CONCLUSIONS: To our knowledge, this is the first time the near-wall flow changes caused by coils in an aneurysm model have been visualized. It can be concluded that future hydrodynamic studies of coil therapy should include an investigation of the coil structure in addition to the coil-packing attenuation.

[Musculoskeletal biomechanics of the knee joint. Principles of preoperative planning for osteotomy and joint replacement]. / Muskuloskelettale Biomechanik des Kniegelenks. Grundlagen für die präoperative Planung von Umstellung und Gelenkersatz.

The long-term clinical outcome of surgical interventions at the knee is dependent upon the quality of the restoration of normal function, together with moderate musculoskeletal loading conditions. In order to achieve this, it is essential to consider biomechanical knowledge during the planning and execution of the procedures. Until now, such knowledge has only been available in books and journal manuscripts and is merely considered during preoperative planning. Its transfer into the specific intraoperative situation is, however, primarily dependent upon the surgeon's skills and understanding. Mathematical models hold the potential to provide the surgeon with detailed, patient-specific information on the in vivo forces, as well as their spatial and temporal distribution. Their application in clinical routine, however, requires a comprehensive validation. Based on a model validated against patient data, it has been shown that - mainly as a result of the action of the muscles - both the tibiofemoral as well as the patellofemoral joints experience substantial mechanical loads even during normal activities of daily living. The calculations further indicate that malalignment at the knee in the frontal plane of more than approximately 4 degrees results in considerably increased forces across the tibiofemoral joint. The actual change in force to a given degree of malalignment might, however, vary greatly between subjects. In order to additionally determine the distribution of the forces in more detail, a sufficiently accurate model of knee joint kinematics is required. In combination with MR-based in vivo imaging techniques, new mathematical models offer the possibility to capture the individual characteristics of knee kinematics and might additionally allow the effect of muscle activity on joint kinematics to be considered. By implementing these technologies in preoperative planning and navigation systems, up-to-date biomechanical knowledge can be made available at the surgeons' fingertips. We propose that optimizing the biomechanical conditions through using these approaches will allow the long-term function of the replaced joint to be significantly enhanced.

Visualization, reconstruction, and integration of neuronal structures in digital brain atlases.

Brain atlases are used in neuroanatomy to define the spatial layout of neuronal structures. Their digital variant can serve as a database and common reference frame for integrating data from different biological experiments. This article presents an overview of methods for three-dimensional visualization of neuroanatomical image data, reconstructing neuronal structures from image data, creating digital brain atlases, and registering data in an atlas. This enables analysis of spatial relations between individual structures imaged in different experiments as well as between these structures and the atlas.

Influence of patient models and numerical methods on predicted power deposition patterns.

BACKGROUND: A hyperthermia planning system has been developed for generating patient and applicator models as well as calculating and visualizing E-field and temperature distributions. Significant dependencies on models and algorithms have been found. METHODS: Computerized tomography (CT) data sets are first transformed into so called 'labelled CT-volume'-data sets of equal resolution, which are used for segmentation. The first type of patient model obtained subsequently is based on regions with specified electrical properties representing tissues or organs (so called 'region-based model'). The second patient model renders a direct transformation of Hounsfield Units (HU) to electrical constants (so called 'HU-based model'). The FDTD-method (finite difference time domain) is then applied on a cubic lattice employing either an auxiliary 'sub-cubic lattice' (for HU-based segmentation) or a tetrahedron grid (for region-based segmentation) to assign the electrical properties, both representing the anatomy of the patient. E-field distributions are corrected by a post-processing procedure with respect to the geometry of interfaces defined by the tetrahedron grid. For comparison, the VSIE method (volume surface integral equation) is performed on the same tetrahedron grid. The applicator model assumes eight half-wavelength dipole antennas fed with constant voltages with water as background medium. RESULTS: For both numerical methods (FDTD, VSIE) the resulting antenna input impedances as well as the current distributions along the antennas were quite similar and almost insensitive to the particular geometry model (region-based, HU-based). In contrast to that, the power deposition patterns in the interior of the patient depended strongly on those models. Major differences can be related to different labels of the tissue type bone in the HU-based model in comparison to the definition via regions. Conversely, comparable results were obtained using the VSIE method and the FDTD method on the region-based patient model with a posteriori correction at the tetrahedron grid points. SAR (specific absorption rate) elevations up to a factor of 10 were predicted when employing region-based models. Those peaks might correspond to specific toxicity of electromagnetic radiation clinically known as hot spot phenomena or musculo-skeletal syndromes. Conversely, HU-based models generated quite homogeneous power deposition patterns with fluctuations of at most factor 2. CONCLUSION: The methods employing region-based geometry models such as the VSIE method and FDTD method in conjunction with a posteriori correction at tissue interfaces result in comparable E-field distributions for regional hyperthermia. Due to its shorter calculation time, the FDTD method is currently used in the clinic. Predictions derived from HU-based models without prior corrections of tissue specifications are not always supported by clinical experience.