Figure 13: Incorporation of fiber orientations employing a rule based approach. Fibers are rotating from endocardium towards epicardium. In the endocardial trabeculations the fibers are following the macro-anatomy. At the root of the papillary muscle the fibers a running into the papillary muscle in a ways similar to what is found in histological slices.
It is well established that excitation spread in cardiac tissue is faster along the long axis (longitudinal direction) of myocarcial cells than along the short axis (transverse direction). This is mainly attributed to the geometry of the cell (roughly cylindrical) and to the intercell connections (gap junctions) which are distributed non-uniformely around a cell. In the tissue context, the direction of fastest propagation is referred to as fiber orientation, transverse to it there are sheet and sheet normal directions. Fiber orientations and the ratios between the preferred axes in both intracellular and interstitial spaces is of major importance. Ideally, these main axes are know for each finite element in a grid based on physical measurements (Diffusion Tensor MRI), but, unfortunately, such data are not always available. Quite often, from serial histological section one can estimate the preferred fiber orientation with reasonable accuracy, however, incorporating these information into FE grids is non-trivial. CARP has built-in routines which aid in defining fiber orientations in a rule-based fashion. An example is given in Fig. 13 where a wedge preparation has been equipped with fiber orientations following a rule based approach.
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