Background: T2 anisotropy in cartilage is usually attributed to residual dipolar couplings in water molecules bound to oriented collagen fibres1,2. It is known that dipolar interactions are weaker when protons are exchanged with deuterium nuclei3. In this study, we present the effects of deuteration on transverse relaxation anisotropy in articular cartilage using MR micro-imaging experiments.
Methods: Four cartilage plugs were excised from visually normal bovine patellae. T1- and T2-weighted imaging was performed at two sample orientations θAS = 0° and θAS = 55°, where θAS is the angle between the static magnetic field B0 and the normal to the articular surface of the sample. Each sample was soaked for at least 12 hours in four solutions with molar concentration ratios of %D2O:%H2O equal to 0:100, 25:75, 50:50, 75:25. The imaging protocol was repeated after each soaking session. Non-linear curve fitting yielded R1 and R2 relaxation rates. Isotropic and anisotropic components of transverse relaxation were calculated for each voxel as R2I = R2 (θ=55°) and R2A = R2 (θ=0°) - R2 (θ=55°) respectively.
Results: R1 and R2I values varied significantly with both deuteration as well as depth from the articular surface. Predictably, they exhibited an almost linear decrease with increasing deuteration levels indicative of the contributions of dipolar interactions to relaxation. However, while R2A values also varied with depth from articular surface, they were surprisingly unaffected by deuteration.
Conclusion: The absence of any change in R2A values with increasing deuteration levels in articular cartilage indicates that intramolecular dipolar interactions in preferentially aligned water molecules bound to oriented collagen fibres may not be the predominant source of T2 anisotropy in tissues such as cartilage and tendon.