Bone mineral 31P and matrix-bound water densities measured by solid-state 31P and 1H MRI

Bone is a composite material consisting of mineral and hydrated collagen fractions. MRI of bone is challenging because of extremely short transverse relaxation times, but solid-state imaging sequences exist that can acquire the short-lived signal from bone tissue. Previous work to quantify bone density via MRI used powerful experimental scanners. This work seeks to establish the feasibility of MRI-based measurement on clinical scanners of bone mineral and collagen-bound water densities, the latter as a surrogate of matrix density, and to examine the associations of these parameters with porosity and donors' age. Mineral and matrix-bound water images of reference phantoms and cortical bone from 16 human donors, aged 2797 years, were acquired by zero-echo-time 31-phosphorus (P-31) and 1-hydrogen (H-1) MRI on whole body 7T and 3T scanners, respectively. Images were corrected for relaxation and RF inhomogeneity to obtain density maps. Cortical porosity was measured by micro-computed tomography (mu CT), and apparent mineral density by peripheral quantitative CT (pQCT). MRI-derived densities were compared to X-ray-based measurements by least-squares regression. Mean bone mineral P-31 density was 6.74 +/- 1.22 mol/l (corresponding to 1129 +/- 204mg/cc mineral), and mean bound water H-1 density was 31.3 +/- 4.2mol/l (corresponding to 28.3 +/- 3.7 % v/v). Both P-31 and bound water (BW) densities were correlated negatively with porosity (P-31: R-2= 0.32, p< 0.005; BW: R-2 = 0.63, p< 0.0005) and age (P-31: R-2 = 0.39, p< 0.05; BW: R-2= 0.70, p< 0.0001), and positively with pQCT density (P-31: R-2= 0.46, p< 0.05; BW: R-2= 0.50, p< 0.005). In contrast, the bone mineralization ratio (expressed here as the ratio of P-31 density to bound water density), which is proportional to true bone mineralization, was found to be uncorrelated with porosity, age or pQCT density. This work establishes the feasibility of image-based quantification of bone mineral and bound water densities using clinical hardware. Copyright (C) 2014 John Wiley & Sons, Ltd.