Mueller matrix polarimetry and polar decomposition of articular cartilage imaged in reflectance.

Articular cartilage birefringence relates to zonal architecture primarily of type II collagen, which has been assessed extensively in transmission, through thin tissue sections, to evaluate cartilage repair and degeneration. Mueller matrix imaging of articular cartilage in reflection is of potential utility for non-destructive imaging in clinical and research applications. Therefore, such an imaging system was constructed to measure laser reflectance signals, calibrated, and tested with optical standards. Polar decomposition was chosen as a method to extract fundamental optical parameters from the experimental Mueller matrices, with performance confirmed by simulations. Adult bovine articular cartilage from the patellofemoral groove was found to have ∼0.93 radians retardance, low diattenuation of ∼0.2, and moderately high depolarization of 0.66. Simulations showed that variation in depolarization drives inaccuracy of depolarization and retardance maps derived by polar decomposition. These results create a basis for further investigation of the clinical utility of polarized signals from knee tissue and suggest potential approaches for improving the accuracy of polar decomposition maps.