T 2 orientation anisotropy mapping of articular cartilage using qMRI.

To provide orientation-independent MR parameters potentially sensitive to articular cartilage degeneration by measuring isotropic and anisotropic components of T2 relaxation, as well as 3-D fiber orientation angle and anisotropy via multi-orientation MR scans.
Approach: Seven bovine osteochondral plugs were scanned with a high angular resolution of thirty-seven orientations spanning 180° at 9.4 T. The obtained data was fitted to the magic angle model of anisotropic T2 relaxation to produce pixel-wise maps of the parameters of interest. Quantitative Polarized Light Microscopy (qPLM) was used as a reference method for the anisotropy and fiber orientation.
Main results: The number of scanned orientations was found to be sufficient for estimating both fiber orientation and anisotropy maps. The relaxation anisotropy maps demonstrated a high correspondence with qPLM reference measurements of the collagen anisotropy of the samples. The scans also enabled calculating orientation-independent T2 maps. Little spatial variation was observed in the isotropic component of T2 while the anisotropic component was much faster in the deep radial zone of cartilage. The estimated fiber orientation spanned the expected 0° to 90° in samples that had a sufficiently thick superficial layer. The orientation-independent MRI measures can potentially reflect the true properties of articular cartilage more precisely and robustly.
Significance: The methods presented in this study will likely improve the specificity of cartilage qMRI by allowing the assessment of the physical properties such as orientation and anisotropy of collagen fibers in articular cartilage.