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

摘要

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 27– 97 years, were acquired by zero-echo-time 31-phosphorus (~(31)P) and 1-hydrogen (~1H) 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 (μCT), 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 ~(31)P density was 6.74 ± 1.22 mol/l (corresponding to 1129 ± 204mg/cc mineral), and mean bound water ~1H density was 31.3± 4.2mol/l (corresponding to 28.3 ± 3.7 %v/v). Both ~(31)P and bound water (BW) densities were correlated negatively with porosity (~(31)P: R~2=0.32, p<0.005; BW: R~2 = 0.63, p<0.0005) and age (~(31)P: R~2 = 0.39, p<0.05; BW: R~2=0.70, p<0.0001), and positively with pQCT density (~(31)P: 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 ~(31)P 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.