Anisotropy of spin relaxation of water protons in cartilage and tendon

Abstract

Transverse spin relaxation rates of water protons in articular cartilage and tendon depend on the orientation of the tissue relative to the applied static magnetic field. This complicates the interpretation of magnetic resonance images of these tissues. At the same time, relaxation data can provide information about their organisation and microstructure. We present a theoretical analysis of the anisotropy of spin relaxation of water protons observed in fully hydrated cartilage. We demonstrate that the anisotropy of transverse relaxation is due almost entirely to intramolecular dipolar coupling modulated by a specific mode of slow molecular motion: the diffusion of water molecules in the hydration shell of a collagen fibre around the fibre, such that the molecular director remains perpendicular to the fibre. The theoretical anisotropy arising from this mechanism follows the 'magic-angle' dependence observed in magnetic-resonance measurements of cartilage and tendon and is in good agreement with the available experimental results. We discuss the implications of the theoretical findings for MRI of ordered collagenous tissues.

How best to preserve and reveal the structural intricacies of cartilaginous tissue

“…The anisotropic organization of the coarser collagenous fibrils can be revealed also by implementing specific stains, such as Sirius Red (Junqueira et al, 1980), as well as by polarized light microscopy (Mittelstaedt et al, 2011) and by magnetic resonance imaging (Momot et al, 2010;Koller et al, 2014), or by an analysis of birefringence (Panula et al, 1998). Scanning electron microscopy is the least useful methodology.…”

Translational and Rotational Dynamics of Water Molecules in Aligned Collagenous Tissues: Implications for Assessing Collagen Organisation with Magnetic Resonance Imaging

“…This motion is such that the time averaged director was in the direction of the collagen fibres. None of the six models developed by Momot et al [52] included reorientational motions which would be expected from water occupying water bridges.…”

“…While it is commonly accepted that the orientation dependence of T 2 is the result of anisotropic rotational motion of water molecules associated with the collagen fibres in the tissues [32,[51][52][53], the exact molecular motions responsible are still a subject of research. Unlike, T 2 , the molecular mechanisms which result in diffusion anisotropy are well understood.…”

“…An upper limit of the exchange rate would be provided by the movement of the entire water molecule between different pools. Alternatively, an analysis similar to that of the analytic study by Momot et al [52] could be used;…”

“…Momot et al [52] would allow spin relaxation rates for a network of fibres defined by some analytic fibre orientation distribution to be calculated.…”