Omnidirectional displacements for deformable surfaces

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 not intersect with the target boundary at all. Consequently, certain deformations cannot be achieved. We propose omnidirectional displacements for deformable surfaces (ODDS) to overcome this limitation. ODDS allow each vertex to move not only along a line segment but within the volumetric inside of a surrounding sphere, and achieve globally optimal deformations subject to local regularization constraints. However, allowing a ball-shaped instead of a linear range of motion per vertex significantly increases runtime and memory. To alleviate this drawback, we propose a hybrid approach, fastODDS, with improved runtime and reduced memory requirements. Furthermore, fastODDS can also cope with simultaneous segmentation of multiple objects. We show the theoretical benefits of ODDS with experiments on synthetic data, and evaluate ODDS and fastODDS quantitatively on clinical image data of the mandible and the hip bones. There, we assess both the global segmentation accuracy as well as local accuracy in high curvature regions, such as the tip-shaped mandibular coronoid processes and the ridge-shaped acetabular rims of the hip bones.     

Reference region‐based pharmacokinetic modeling in quantitative dynamic contract‐enhanced MRI allows robust treatment monitoring in a rat liver tumor model despite cardiovascular changes

In this work, two pharmacokinetic modeling techniques, population arterial input function model, and reference region model, were applied to dynamic contract‐enhanced MRI data, to test the influence of a change in heart rate on modeling parameters. A rat population arterial input function was generated by dynamic contrast‐enhanced computed tomography measurements using the MR contrast agent gadolinium diethylenetriamine penta‐acetic acid. Then, dynamic contract‐enhanced MRI was used for treatment monitoring in two groups of hepatocellular carcinoma bearing rats. Whereas group 1 had the same heart rate as animals analyzed for the population arterial input function (263 ± 20 bpm), group 2 had a higher heart rate (369 ± 11 bpm) due to a different anesthesia protocol. The pharmacokinetic modeling parameters volume transfer constant K^trans and relative extravascular extracellular space v_e were calculated with both models and statistically compared. For group 1, good correlation and agreement was found between the models showing no difference in K^trans and v_e (ΔK^trans: 4 ± 19% and Δv_e: 4 ± 12%, P = 0.2). In contrast, for group 2, a bias in parameter values for the population arterial input function model was detected (ΔK^trans: ?45 ± 7% and Δv_e: ?31 ± 7%, P ≤ 0.001). The presented work underlines the value of the reference region model in longitudinal treatment monitoring and provides a straightforward approach for the generation of a rat population arterial input function. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.