The robustness of T value as a trabecular structural index at multiple spatial resolutions of 7 Tesla MRI.

PURPOSE

To evaluate the robustness of MR transverse relaxation times of trabecular bone from spin-echo and gradient-echo acquisitions at multiple spatial resolutions of 7 T.

METHODS

The effects of MRI resolutions to T and T2* of trabecular bone were numerically evaluated by Monte Carlo simulations. T , T2*, and trabecular structural indices from multislice multi-echo and UTE acquisitions were measured in defatted human distal femoral condyles on a 7 T scanner. Reference structural indices were extracted from high-resolution microcomputed tomography images. For bovine knee trabecular samples with intact bone marrow, T and T2* were measured by degrading spatial resolutions on a 7 T system.

RESULTS

In the defatted trabecular experiment, both T and T2* values showed strong ( |r| > 0.80) correlations with trabecular spacing and number, at a high spatial resolution of 125 µm . The correlations for MR image-segmentation-derived structural indices were significantly degraded ( |r| < 0.50) at spatial resolutions of 250 and 500 µm . The correlations for T2* rapidly dropped ( |r| < 0.50) at a spatial resolution of 500 µm , whereas those for T remained consistently high ( |r| > 0.85). In the bovine trabecular experiments with intact marrow, low-resolution (approximately 1 mm , 2 minutes) T values did not shorten ( |r| > 0.95 with respect to approximately 0.4 mm , 11 minutes) and maintained consistent correlations ( |r| > 0.70) with respect to trabecular spacing (turbo spin echo, 22.5 minutes).

CONCLUSION

T measurements of trabeculae at 7 T are robust with degrading spatial resolution and may be preferable in assessing trabecular spacing index with reduced scan time, when high-resolution 3D micro-MRI is difficult to obtain.