van Hc Hans Assen

Quantification of collagen orientation in 3D engineered tissue

Tissue engineered heart valves are a promising alternative for current heart valve replacements. However, the mechanical properties of these valves are insufficient for implantation at the aortic position [1]. Collagen orientation is important to improve the mechanical properties of tissue engineered valves. Two-photon laser-scanning microscopy allows us to study the influence of strain on collagen orientation in 3D. A method based on the 2nd order derivative of the 3D image structure was used to determine the general orientation of the collagen fibers with automatic scale selection of the operator. We studied the effect of strain on collagen orientation. Alignment in the direction of the applied strain is seen. Histograms show that the distribution of orientations becomes smaller for increased strain. This indicates that the collagen fibers align more.

Dense Multiscale Motion Extraction from Cardiac Cine MR Tagging using HARP Technology

We propose an operational method to extract the left ventricle (LV) systole dynamics using harmonic phase (HARP) images extracted from tagged cardiac MR sequences. Established techniques to generate HARP sequences provide independent evidence for motion extraction, in the sense that the combined linear system for scalar brightness conservation, applied to the HARP images, can be uniquely solved for a dense field of motion parameters without the need for regularization. In contrast to some of the previously proposed popular methods, no segmentation or tracking of tags over time, nor interpolation of a sparse motion field explicitly coupled to the tag pattern is required, and the problem of tag fading is bypassed. An important novelty is the incorporation of automatic local scale selection so as to obtain a robust solution, which not only yields a stable, but also a smoothly varying motion field of the (healthy) LV myocardial wall. The scheme relies on an integer parameter representing order of approximation, and allows one to simultaneously obtain a dense field of differential tensors capturing the low order differential structure of the motion field, which is useful for the computation of relevant local quantities such as strain rates and material acceleration fields. The methodology is generic and straightforward to implement, and can be generalized to 3D and, in principle, to account for higher order differential structure.

Cardiac left atrium CT image segmentation for ablation guidance

Catheter ablation is an increasingly important curative procedure for atrial fibrillation. Knowledge of the local wall thickness is essential to determine the proper ablation energy. This paper presents the first semi-automatic atrial wall thickness measurement method for ablation guidance. It includes both endocardial and epicardial atrial wall segmentation on CT image data. Segmentation is based on active contours, Otsus multiple threshold method and hysteresis thresholding. Segmentation results were compared to contours manually drawn by two experts, using repeated measures analysis of variance. The root mean square differences between the semi-automatic and the manually drawn contours were comparable to intra-observer variation (endocardium: p = 0.23, epicardium: p = 0.18). Mean wall thickness difference is significant between one of the experts on one side, and the presented method and the other expert on the other side (p ≪ 0.001). Wall thicknesses found were in the range of 0.5–5.5mm, corresponding to values presented in literature.

Cardiac Strain and Rotation Analysis Using Multi-scale Optical Flow

Tagging MRI enables analysis of the local contractility of the cardiac left ventricle. It permits reliable assessment of local contractile dysfunction related to various cardiomyopathies. We present a multi-scale optical flow method, with Gabor filtering, for the extraction of dense motion fields from cardiac MR tagging. It is based on a multi-scale first-order extension of the classical optical flow constraint equation enabling the extraction first-order parameters such as rotation and strain. A quantitative validation study based on the phantom proposed by Young et al. showed excellent performance. Furthermore, strain patterns are presented for one ischemic patient case with known wall motion abnormalities and two volunteers. Patient circumferential strain abnormalities co-localize with enhanced areas in late-enhancement MRI. Rotation patterns are presented for the same patient and four volunteers. The rotation pattern described in the patient is strikingly different from that describing the volunteers.

3D saddle point detection and applications in cardiac imaging

We investigate saddle points in 3D cardiac images. We do so by improving a critical point detection algorithm, the 3D winding number, or Poincaré index. We consider two different applications. We estimate cardiac motion from 3D tagged MRI data, based on tracking of saddle points. We also employ our method for saddle point extraction in blood flow data, acquired by phase contrast MRI.

Automatic blood pool identification in contrast ultrasound using principal component analysis

Several cardiovascular parameters of clinical interest can be assessed by indicator dilution techniques. Ultrasound contrast agents have been proposed as non-invasive indicator, showing promising results for blood volume estimation. However, the definition of an optimal region of interest for quantification of the indicator remains a critical step in the procedure, usually performed manually. In this work we present an automatic method to extract indicator dilution curves. Dimensionality reduction is achieved by principal component analysis followed by clustering to identify the different regions of interest. The method is evaluated on in vitro and in vivo datasets, compared to manually defined regions. The average difference was -3.47% ± 3.58% for in vitro volume estimates and the error was 1.29% ± 2.54% for trans-pulmonary mean transit time estimation. The new method allows kinetic parameter estimates in close agreement with those obtained manually; therefore it is a promising alternative for indicator dilution curve extraction.