Bone Mineral 31P and Matrix-Bound Water Densities Measured by Solid-State 1H and 31P MRI

摘要

Bone is a composite material consisting of mineral and hydrated collagen fractions. MRI of bone is challenging due to 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, ages 27-97 years, were acquired by zero-echo-time ³¹P and 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-CT, and apparent mineral density by pQCT. MRI-derived densities were compared to x-ray-based measurements by least-squares regression.Mean bone mineral ³¹P density was 6.74±1.22 mol/L (corresponding to 1129±204 mg/cc mineral), and mean bound water ¹H density was 31.3±4.2 mol/L (corresponding to 28.3±3.7 %v/v). Both ³¹P and bound water (BW) densities were correlated negatively with porosity (³¹P: R² = 0.32, p < 0.005; BW: R² = 0.63, p < 0.0005) and age (³¹P: R² = 0.39, p < 0.05; BW: R² = 0.70, p < 0.0001), and positively with pQCT density (³¹P: R² = 0.46, p < 0.05; BW: R² = 0.50, p < 0.005). In contrast, the bone mineralization ratio (expressed here as the ratio of ³¹P 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.