Field strength and angle dependence of trabecular bone marrow transverse relaxation in the calcaneus.

The contribution from reversible dephasing, R2', to the effective transverse relaxation rate, R2*, in trabecular bone marrow, is governed by the magnetic field inhomogeneity induced by the difference in diamagnetic susceptibility between bone and bone marrow and should, therefore, scale linearly with field strength. Measurement of R2' in the calcaneus at 1.5 and 4 T by means of the GESFIDE (gradient-echo sampling of free induction decay and echo) pulse sequence showed R2' to increase by a factor of 2.73 at the higher field, which is close to the ratio of the field strengths (2.67). At both field strengths, R2' dominates R2*, contributing more than 90% to the total relaxation rate at 4 T. The data further indicate large regional variations within the calcaneus and a notable dependence of R2' on the angle of the foot relative to the direction of the static field. The findings have implications on the choice of the calcaneal site for assessment of trabecular density.