QUANTITATION OF T-2' ANISOTROPIC EFFECTS ON MAGNETIC-RESONANCE BONE-MINERAL DENSITY-MEASUREMENT

In this paper, the authors quantitate the anisotropy of susceptibility effects in an uniaxial trabecular bone model and show its relevance t o clinical MR bone mineral density measurements. A physical model is d escribed that quantitates the anisotropic MR behavior of uniaxial trab ecular bone. To test the model, a phantom of parallel polyethylene fil aments was scanned every 15 degrees between 0 degrees and 90 degrees w ith respect to the system's main magnetic field ((B) over right arrow( 0)). The distal radial metaphysis of a healthy female volunteer was sc anned in orthogonal projections. The signal from each phantom image an d each radial image was separated in a pixel-wise fashion into R(2) an d R(2)' maps. As predicted, R(2)' relaxation showed anisotropic behavi or and changed according to sin(2) (theta), confirming that columnar s tructures parallel with <(B)over right arrow(0)> will cause no MR susc eptibility effects. Scans of the distal radius showed that R(2)' relax ation was twice as great with the forearm perpendicular to <(B)over ri ght arrow(0)> as when it was parallel to it, demonstrating different c ontributions from struts and columns. For both phantom and radial bone scans, R(2) relaxation was isotropic and did not change with object o rientation.