Characterization of anisotropy in articular cartilage samples using magnetic resonance imaging (MRI) and multiphoton microscopy (MPM)
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- Institutt for fysikk 
Articular cartilage is a thin flexible tissue found at the ends of the bones in a joint that provides a lubricated surface for articulation and distribute loads with low friction. However, due to degenerative diseases, such as osteoarthritis (OA) its principal function is lost. Such diseases required noninvasive, specific and sensitive imaging modality to assess the structure and biochemical properties of the articular cartilage for identifying the cartilage disorders. MRI can be a possible noninvasive modality for characterizing the structure of macromolecule and pathology of articular cartilage. The collagen fibers are the most plentiful macromolecule present in the extracellular matrix (ECM) that give the cartilage tensile strength and stiffness. The purpose of this study is to investigate the structural anisotropy and orientational dependence of T2 relaxation time in the articular cartilage. For each of three samples; two samples of chicken knee joints (samples CFKC1 and CFKC2) and one sample of pig (juvenile pig) elbow joint (sample PFEC), independent T2-weighted images were acquired using 7-T small animal scanner with in-plane resolution of 78 µm × 78 µm for 2, 5 and 3 different specimen orientations for samples CFKC1, CFKC2 and PFEC respectively. The specimen was rotated using a sample-rotating device that was designed for this purpose. T2 maps were analyzed using in-house-software based on three fitting parameters. Cartilage T2 profiles (T2 versus depth, from surface toward bone) were evaluated as a function of sample (tissue) orientation with respect the static magnetic field (Bo). Similarly, for each three samples, diffusion-weighted images were acquired with in-plane resolution of 156 µm × 156 µm, and the images were analyzed using FSL software. FA profiles (FA versus depth) were evaluated. Moreover, for each samples, multiphoton microscopy (second harmonic generation and two-photon fluorescence) images were acquired for further characterization of collagen fibers and chondrocyte cells and correlated with the T2 maps. In T2-weighted imaging of sample PFEC, approximately five-laminae were visualized when the normal to the articular surface is oriented at 0 with respect to the static magnetic field, and appeared approximately homogenous when the normal to the articular surface is oriented at 55 and 35 with respect to the static magnetic field. The greater angular variation was observed in radial zone and nearly follow the orientational dependence of nuclear dipolar interaction. In contrast, less angular variation was observed at the transitional zone. The results obtained using multiphoton microscopy and T2 maps from sample PFEC were approximately correlated. The result indicates that sensitivity of T2 relaxation anisotropy measurement could be used as an indicator for orientations of collagen fibers in the articular cartilage. The cartilage part (excluding cell layer) of samples CFKC1 and CFKC2 were almost visualized as tri-laminar, in T2-weighted imaging, when the normal to the articular surface is oriented at 0 and 90 for sample CFKC1 and 0, 25 and 35 for sample CFKC2 with respect to the static magnetic field. And sample CFKC2 were nearly visualized as bi-laminar when the normal to the articular surface is oriented at 55 with respect to the static magnetic field. Correlating the results obtained using MPM and T2 mapping from samples CFKC1 and CFKC2 was challenging due to the presence of blood vessels. Generally, FA mapping results did not show specific trend due to insufficient SNR and it was difficult to probe the collagen fiber anisotropy and to correlate with the T2 maps.