Abstract
The orientation dependence of the free induction decay (FID) of ^1^H NMR water signal in ex vivo bovine digital flexor tendon at the native level of hydration is reported. Residual dipolar coupling due to the overall tissue anisotropy produces a 6:1 change in the signal intensity as an angle between the long axis of a specimen and the external magnetic field is changed from the βmagic angleβ of 54.7Β° to 0Β°. The strength of residual dipolar interactions between water protons was estimated by orientational analysis of the signal intensity to be equal to 780 Hz. Apparent signal maxima are observed at orientations 8β13Β° away from 54.7Β° due to an inhomogeneous contribution to the decay. A small fraction of total water in tendon is detectable at all orientations and exhibits a shift in the precession frequency. It is hypothesized that this water fraction resides in the interconnecting gaps at the ends of collagen molecules. The gaps have a disordered environment that allows for a zero time average of dipolar interactions. Measured frequency and phase shifts are interpreted as signatures of the bulk magnetic susceptibility effect due to geometry of the cavity formed by adjacent gaps at the ends of the collagen molecules. The multiexponentiality of the FID decay is hypothesized to be due to the exchange between orientationally restricted water structured along the length of the collagen molecule and disordered water in the cavity. Magn Reson Med 54:280β288, 2005. Β© 2005 WileyβLiss, Inc.