Relaxation time measurements of bone marrow protons in the calcaneus using a compact MRI system at 0.2 Tesla field strength.

Relaxation times (T(1) and T(2)) of the bone marrow protons and trabecular bone volume fraction (TBVF) in the calcaneus were measured for 100 female volunteers using a compact MRI system at 0.2 T field strength. The speed of sound (SOS) through the calcaneus was measured also for the same subjects using a quantitative ultrasound system. Both relaxation times were found to have positive correlations with age (R = 0.40; P < 0.0001 and R = 0.31; P < 0.002, respectively) and negative correlations with SOS (R = -0.38; P < 0.0001 and R = -0.38; P < 0.0001, respectively). Although TBVF had a fairly high positive correlation with the SOS (R = 0.67), neither T(1) nor T(2) were correlated with TBVF (R = -0.062 and -0.024, respectively). These results suggest that the age dependence of both T(1) and T(2) is caused by the microdynamic properties of the lipid molecules in bone marrow observed using acoustic or elastic modalities.