MR-microscopic visualization of anisotropic internal cartilage structures using the magic angle technique

NMR microscopic studies of articular cartilage at 7.1 T are presented. Using a special experimental design, T₂-weighted spin-echo images of cartilage-bone plugs were taken under variable angles with respect to the static magnetic field B_o to visualize the angular-dependent representation of internal matrix structures mediated by the collagen network arrangement. To quantify the observed orientational effect in the MR images, exact measurements of the transverse relaxation time T₂ were taken using the CPMG sequence. The NMR experiments show the strong influence of the cartilage orientation with respect to the static magnetic field on the inhomogeneous appearance of the articular cartilage in the MR image. Additionally performed polarization light microscopic investigations demonstrate the direct relation between the oriented collagenous structures and the anisotropic regions observed in the MR images. A simple cartilage matrix model derived from the experimental findings is proposed, and consequences for the clinical assessment of the articular joint are discussed.