THE CALCULATION OF THE SUSCEPTIBILITY-INDUCED MAGNETIC-FIELD FROM 3D NMR IMAGES WITH APPLICATIONS TO TRABECULAR BONE

A method is described for computing the induced magnetic held caused b y the susceptibility difference of two materials, Arbitrary surfaces d erived from 3D NMR images were modeled with triangular elements of con stant magnetic surface-charge density. The boundary elements were gene rated by means of a surface triangulation method that is typically use d for displaying 3D images. The induced field was computed from the ch arged triangles using a hierarchical multipole algorithm. Validation o f the technique on a spherical object showed excellent agreement with the analytical solution. The method was applied to computing histogram s of the induced fields in specimens of trabecular bone. A 76 x 76 x 7 6 3D array of image voxels resulted in 322,169 triangles and required similar to 70 minutes on a Sun Spare 10 workstation to compute the fie ld at 100,000 locations. The width of the induced field distributions was found to be narrowest when the polarizing held was parallel to the principal orientation of the trabeculae, confirming previous experime ntal findings. The method is applicable to other structures in which s usceptibility-induced fields are of interest, such as capillary networ ks or blood cells. (C) 1995 Academic Press, Inc.