Characterization of trabecular bone density with ultra-short echo-time MRI at 1.5, 3.0 and 7.0 T - comparison with micro-computed tomography

The goal of this study was to test the potential of ultra-short echo-time (UTE) MRI at 1.5, 3.0 and 7.0T for depiction of trabecular bone structure (of the wrist bones), to evaluate whether T-2* relaxation times of bone water and parametric maps of T-2* of trabecular bone could be obtained at all three field strengths, and to compare the T-2* relaxation times with structural parameters obtained from micro-computed tomography (micro-CT) as a reference standard. Ex vivo carpal bones of six wrists were excised en bloc and underwent MRI at 1.5, 3.0 and 7.0T in a whole-body MR imager using the head coil. A three-dimensional radial fat-suppressed UTE sequence was applied with subsequent acquisitions, with six different echo times T-E of 150, 300, 600, 1200, 3500 and 7000 mu s. The T-2* relaxation time and pixel-wise computed T-2* parametric maps were compared with a micro-computed-tomography reference standard providing trabecular bone structural parameters including porosity (defined as the bone-free fraction within a region of interest), trabecular thickness, trabecular separation, trabecular number and fractal dimension (D-k). T-2* relaxation curves and parametric maps could be computed from datasets acquired at all field strengths. Mean T-2* relaxation times of trabecular bone were 4580 +/- 1040 mu s at 1.5T, 2420 +/- 560 mu s at 3.0T and 1220 +/- 300 mu s at 7.0T, when averaged over all carpal bones. A positive correlation of T-2* with trabecular bone porosity and trabecular separation, and a negative correlation of T-2* relaxation time with trabecular thickness, trabecular number and fractal dimension, was detected (p<0.01 for all field strengths and micro-CT parameters). We conclude that UTE MRI may be useful to characterize the structure of trabecular bone, comparable to micro-CT. Copyright (c) 2014 John Wiley & Sons, Ltd.