Quantitative in situ correlation between microscopic MRI and polarized light microscopy studies of articular cartilage.

OBJECTIVE To establish the correlation between the non-invasive imaging by magnetic resonance microscopy (microMRI) and the histological imaging by polarized light microscopy (PLM) accurately, quantitatively, at the highest possible MRI resolution (13.7 microm), and based on the same piece of tissue (articular cartilage from canine shoulder joint). DESIGN In microMRI experiments, the laminar appearance (the magic angle effect) of the proton intensity images and the anisotropic characteristics of the T(2)relaxation images were analysed. In PLM experiments, the images of the optical retardation and collagen-fibre orientation in cartilage were constructed in two dimensions. RESULTS The T(2)profile has a distinctly asymmetric bell-shaped curve and three featured zones. The retardation profile has a non-zero minimum at the middle of the transitional zone of the tissue. The angle profile has a smooth variation across the transitional zone. These facts suggest that the collagen fibres in the transitional zone are not entirely random but have a residual order. In addition, the peak of the T(2)profile coincides with the minimum of the retardation profile, both represent the most isotropic region of the tissue. A hyperbolic tangent function was found to best describe the transition of the collagen fibres in cartilage. A set of criteria was developed for each technique to define the features in the quantitative measurements. CONCLUSIONS The criteria offer, for the first time, a set of quantitative and objective means to subdivide the tissue thickness into the zones in histology and in MRI. It is shown that the microMRI zones based on the T(2)characteristics are statistically equivalent to the histological zones based on the collagen fibre orientation (t-probabilities of 0.730, 0.973, 0.647, 0.850 for the superficial, transitional, radial zones and the total thickness).