A method is described for computing the induced magnetic field method, require solving a set of equations to simultaneously caused by the susceptibility difference of two materials. Arbitrary determine the field at predefined nodal points. The field at surfaces derived from 3D NMR images were modeled with trianpoints between nodes is then calculated using interpolation. gular elements of constant magnetic surface-charge density. The As the number of nodes increases, the problem can become boundary elements were generated by means of a surface triangucomputationally time-consuming. Durney et al. (11) took lation method that is typically used for displaying 3D images. The advantage of the conditions in NMR, i.e., the strong static induced field was computed from the charged triangles using a field and the small magnetic susceptibilities, to make an hierarchical multipole algorithm. Validation of the technique on accurate approximation of the magnetic surface-charge dena spherical object showed excellent agreement with the analytical sity at susceptibility boundaries. Under these conditions, the solution. The method was applied to computing histograms of the induced fields in specimens of trabecular bone. A 76 1 76 1 76 field can be explicitly calculated without solving a system 3D array of image voxels resulted in 322,169 triangles and required of equations by using the surface-charge density to approxi-ร70 minutes on a Sun Sparc 10 workstation to compute the field mate the surface integral solution of Poisson's equation for at 100,000 locations. The width of the induced field distributions magnetostatic fields. This approximation was used by Durwas found to be narrowest when the polarizing field was parallel ney et al. for models of lung tissue and by Ford et al. (12) to the principal orientation of the trabeculae, confirming previous for models of trabecular bone. experimental findings. The method is applicable to other structures Both Durney et al. and Ford et al. used manually defined in which susceptibility-induced fields are of interest, such as capilrectangular surface-charge elements to roughly approximate lary networks or blood cells.