EVALUATION OF BONE-MINERAL DENSITY USING 3-DIMENSIONAL SOLID-STATE P-31 NMR PROJECTION IMAGING

A solid state magnetic resonance imaging technique is used to measure true three-dimensional mineral density of synthetic hydroxyapatite pha ntoms and specimens of bone ex vivo. The phosphorus-31 free induction decay at 2.0 T magnetic field strength is sampled following applicatio n of a short, hard radiofrequency excitation pulse in the presence of a fixed amplitude magnetic field gradient. Multiple gradient direction s covering the unit sphere are used in an efficient spherical polar to Cartesian interpolation and Fourier transform projection reconstructi on scheme to image the three-dimensional distribution of phosphorus wi thin the specimen. Using 3-6 Gauss/cm magnetic field gradients, a spat ial resolution of 0.2 cm over a field of view of 10 cm is achieved in an imaging time of 20-35 minutes. Comparison of solid state magnetic r esonance imaging with dual energy X-ray absorptiometry (DXA), gravimet ric analysis, and chemical analysis of calcium and phosphorus demonstr ates good quantitative accuracy. Direct measurement of bone mineral by solid state magnetic resonance opens up the possibility of imaging va riations in mineral composition as well as density. Advantages of the solid state magnetic resonance technique include avoidance of ionizing radiation; direct measurement of a constituent of the mineral without reliance on assumptions about, or models of, tissue composition; the absence of shielding, beam hardening, or multiple scattering artifacts ; and its three-dimensional character. Disadvantages include longer me asurement times and lower spatial resolution than DXA and computed tom ography, and the inability to scan large areas of the body in a single measurement, although spatial resolution is sufficient to resolve cor tical from trabecular bone for the purpose of measuring bone mineral d ensity.