Orientation of trabecular bone in human vertebrae assessed by MRI

The aim of the study was to assess the relative number of bone trabeculae in different orientations by using magnetic inhomogeneity measurements by MR imaging. Twelve defatted human vertebrae (L2-L4) were studied by MR imaging and CT. In the MR measurements the reversible transaxial decay rate, R2', was determined using the GESFIDE sequence. The relative contribution to R2' of bone trabeculae oriented along the x (R2'(x)), y (R2'(y)) and z (R2'(y)) axes was assessed, by rotation of the specimen in the magnetic field. The results were validated by CT measurements of trabecular structure at a resolution of 0.2 x 0.2 x 1 mm, using custom-made software R2' ranged from 4.9 to 32 s(-1). After separation, theoretical R2', ranged from 2.3 to 10.7 s(-1), R2'(y), ranged from 2.6 to 14.0 s(-1) and R2'(y) ranged from 4.7 to 17.9 s(-1). The number of bone trabeculae per millimeter identified in axial CT images ranged from 0.15 to 0.38 and from 0.039 to 0.22 per millimeter in sagittal images. The difference between axial and sagittal CT images was statistically significant and due to anisotropic voxels. The correlation between R2' and the number of bone trabeculae per millimeter was statistically significant (r = 0.83, p < 0.001 for x orientation; r = 0.63 for y orientation, and r = 0.59 for z orientation, p < 0.05 for both). The relative amount of bone trabeculae in different orientations can be assessed in vitro using R2' measurements.