The robustness of T-2 value as a trabecular structural index at multiple spatial resolutions of 7 Tesla MRI

Abstract

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-2 and T-2* trabecular bone were numerically evaluated by Monte Carlo simulations. T-2, T-2* 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-2 and T-2* were measured by degrading spatial resolutions on a 7 T system. Results: In the defatted trabecular experiment, both T-2 and T-2* values showed strong (vertical bar r vertical bar> 0.80) correlations with trabecular spacing and number, at a high spatial resolution of 125 mu m(3). The correlations for MR image-segmentation-olutions of 250 and 500 mu m(3). The correlations for T-2* rapidly dropped (vertical bar r vertical bar< 0.50) at a spatial resolution of 500 mu m(3), whereas those for T-2* remained consistently high (vertical bar r vertical bar> 0.85). In the bovine trabecular experiments with intact marrow, low- resolution (approximately 1 mm(3), 2 minutes) T-2 values did not shorten (vertical bar r vertical bar> 0.95 with respect to approximately 0.4mm(3), 11 minutes) and maintained consistent correlations (vertical bar r vertical bar > 0.70) with respect to trabecular spacing (turbo spin echo, 22.5 minutes). Conclusion: T-2 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.