Phosphorus-31 magnetic resonance imaging of hydroxyapatite: a model for bone imaging

One-dimensional 31P nuclear magnetic resonance images (projections) of synthetic calcium hydroxyapatite, Ca10(OH)2(PO4)6, have been obtained for samples on the order of 0.5 to 1.0 cm in linear extent at 7.4 T magnetic field strength. Because of the solid state nature of these samples, short 31P spin-spin relaxation times under 1 ms occur, necessitating echo times of 1 ms and phase-encoding magnetic field gradient pulses shorter than 500 microseconds. Optimal projection quality and shortest acquisition times result from pulsed gradient phase-encoding of the spatial dimension, using a compensating gradient pulse to cancel the distorting effects of gradient waveform transients. The exceedingly long 31P spin-lattice relaxation times could lead to potentially intolerable image acquisition times; these have been reduced with a flipback pulse technique. In addition to holding great potential as a novel research tool in the study of biomineralization of those organisms containing calcium phosphate solid phases, these methods should be of general utility in the multinuclear imaging of a wide variety of solids of interest in biophysics and materials science.