Volumetric Cortical Bone Porosity Assessment with MR Imaging: Validation and Clinical Feasibility.

PURPOSE: To develop a method to assess volumetric cortical bone porosity in clinically practical acquisition times by measuring the signal decay at only two echo times (TEs) as part of a single three-dimensional ultrashort TE (UTE) magnetic resonance (MR) examination. MATERIALS AND METHODS: The study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained from all subjects. A marker of cortical bone porosity called porosity index was defined as the ratio of UTE image intensities at a long and short TE, and the results were compared with biexponential analysis. Porosity index of midtibia cortical bone samples obtained from 16 donors was compared with ground-truth porosity by using micro-computed tomographic (CT) imaging and bone mineral density by peripheral quantitative CT scanner. Reproducibility of porosity index were tested in volunteers, and clinical feasibility was evaluated in postmenopausal women. Interparameter associations were assessed by using Pearson or Spearman correlation coefficient. RESULTS: Bone specimen porosity index was correlated with micro-CT imaging porosity (R(2) = 0.79) and pore size (R(2) = 0.81); age (R(2) = 0.64); peripheral quantitative CT scanner density (R(2) = 0.49, negatively); and pore water fraction (R(2) = 0.62) and T2* (R(2) = 0.64) by biexponential analysis. The reproducibility study yielded a coefficient of variation of 2.2% and intraclass correlation coefficient of 0.97. The study that involved postmenopausal women showed a wide range of porosity index (15%-38%). CONCLUSION: A two-point MR imaging method to assess cortical bone porosity in humans was conceived and validated. This approach has the potential for clinical use to assess changes in cortical bone porosity that result from disease or in response to therapy. (©) RSNA, 2015 Online supplemental material is available for this article.

Bone mineral (31)P and matrix-bound water densities measured by solid-state (31)P and (1)H 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 27-97 years, were acquired by zero-echo-time 31-phosphorus ((31)P) and 1-hydrogen ((1)H) 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 ± 204 mg/cc mineral), and mean bound water (1)H density was 31.3 ± 4.2 mol/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.

Simulation of the in vivo resorption rate of ß-tricalcium phosphate bone graft substitutes implanted in a sheep model.

A few years ago, a model was proposed to predict the effect of the pore architecture of a bone graft substitute on its cell-mediated resorption rate. The aim of the present study was to compare the predictions of the model with the in vivo resorption rate of four ß-tricalcium phosphate bone graft substitutes implanted in a sheep model. The simulation algorithm contained two main steps: (1) detection of the pores that could be accessed by blood vessels of 50 µm in diameter, and (2) removal of one solid layer at the surface of these pores. This process was repeated until full resorption occurred. Since the pore architecture was complex, µCT data and fuzzy imaging techniques were combined to reconstruct the precise bone graft substitute geometry and then image processing algorithms were developed to perform the resorption simulation steps. The proposed algorithm was verified by comparing its results with the analytical results of a simple geometry and experimental in-vivo data of ß-TCP bone substitutes with more complex geometry. An excellent correlation (r(2)>0.9 for all 4 bone graft substitutes) was found between simulation results and in-vivo data, suggesting that this resorption model could be used to (i) better understand the in vivo behavior of bone graft substitutes resorbed by cell-mediation, and (ii) optimize the pore architecture of a bone graft substitute, for example to maximize its resorption rate.