FIELD-STRENGTH AND ANGLE DEPENDENCE OF TRABECULAR BONE-MARROW TRANSVERSE RELAXATION IN THE CALCANEUS

The contribution from reversible dephasing, R(2)', to the effective tr ansverse relaxation rate, R(2), in trabecular bone marrow, is governe d by the magnetic held inhomogeneity induced by the difference in diam agnetic susceptibility between bone and bone marrow and should, theref ore, scale linearly with field strength. Measurement of R(2)' in the c alcaneus at 1.5 and 4 T by means of the GESFIDE (gradient-echo samplin g of free induction decay and echo) pulse sequence showed R(2)' to inc rease by a factor of 2.73 at the higher held, which is close to the ra tio of the field strengths (2.67), at both field strengths, R(2)' domi nates R(2), contributing more than 90% to the total relaxation rate a t 4 T, The data further indicate large regional variations within the calcaneus and a notable dependence of R(2)' on the angle of the foot r elative to the direction of the static field. The findings have implic ations on the choice of the calcaneal site for assessment of trabecula r density.