GEOMETRIC MODELING OF THE HUMAN TORSO USING CUBIC HERMITE ELEMENTS

We discuss the advantages and problems associated with fitting geometr ic data of the human torso obtained from magnetic resonance imaging, w ith high-order (bicubic Hermite) surface elements. These elements pres erve derivative (C-1) continuity across element boundaries and permit smooth anatomically accurate surfaces to be obtained with relatively f ew elements. These elements are fitted to the data with a new nonlinea r fitting procedure that minimizes the error in the fit while maintain ing C-1 continuity with nonlinear constraints. Nonlinear Sobelov smoot hing is also incorporated into this fitting scheme. The structures fit ted along with their corresponding root mean-squared error, number of elements used, and number of degrees of freedom (df) per variable are: epicardium (0.91 mm, 40 elements, 142 df), left lung (1.66 mm, 80 ele ments, 309 df), right lung (1.69 mm, 80 elements, 309 df), skeletal mu scle surface (1.67 mm, 264 elements, 1,010 df), fat layer (1.79 mm, 26 4 elements, 1,010 df), and the skin layer (1.43 mm, 264 elements, 1,01 0 df). The fitted surfaces are assembled into a combined finite elemen t/boundary element model of the torso in which the exterior surfaces o f the heart and lungs are modeled with two dimensional boundary elemen ts and the layers of the skeletal muscle, fat, and skin are modeled wi th finite elements. The skeletal muscle and fat layers are modeled wit h bicubic Hermite linear elements and are obtained by joining the adja cent surface elements for each layer. Applications for the torso model include the forward and inverse problems of electrocardiography, defi brillation studies, radiation dosage studies, and heat transfer studie s.