Inhomogeneity of rat vertebrae trabecular architecture by high-field 3D μ-magnetic resonance imaging and variable threshold image segmentation

Purpose: To analyze the 3D microarchitecture of rat lumbar vertebrae by micro-magnetic resonance imaging (μ-MRI). Materials and Methods: μ-MR images (20 x 20 x 20 μm³ apparent voxel size) were acquired with a three-dimensional spin-echo pulse sequence on four lumbar vertebrae of two rats. Apparent microarchitectural parameters like trabecular bone fraction (BV/TV), specific bone surface (BS/TV), mean intercept length (MIL), and Euler number per unit volume (Euler density, E_V) were calculated using a novel semiquantitative variable threshold segmentation technique. The threshold value T* was obtained as a point of minimum or maximum of the function E_V = E_V(T). Results: Quantitative 3D analysis of μ-MRI images revealed a higher connectivity in the peripheral regions (E_V = -570 ± 70 mm^-3) than in the central regions (E_V = -130 ± 50 mm^-3) of the analyzed rat lumbar vertebrae. Smaller intertrabecular cavities and larger bone volume fractions were observed in peripheral regions as compared to central ones (MIL = 0.18 ± 0.01 mm and 0.26 ± 0.01 mm; BV/TV = 34 ± 3% and 29 ± 3%, respectively). The quantitative 3D study of MIL showed a structural anisotropy of the trabeculae along the longitudinal axis seen on the images. The inhomogeneity of the bone architecture was validated by micro-computed tomography (μ-CT) images at the same spatial resolution. Conclusion: 3D high-field μ-MRI is a suitable technique for the assessment of bone quality in experimental animal models.