Figure 4.

Finding boundary nodes and creating the boundary surface. A convenient origin within the heart serves as the basis for fitting a spherical harmonic expansion to the outer heart surface, which generates a surface characterized by the radius R_h(θ,Φ). Voxels that have nodes both within and without of this surface are boundary voxels. Within these voxels, the nodes outside of the heart are boundary nodes (red). A surface ("boundary surface") is fitted to these nodes by performing another spherical harmonic expansion about the heart origin. This surface is characterized by the radius R_b(θ,Φ).