Analysis of Multi-Component Echo Decay in Achilles Tendon by NMR Spectroscopy

Fibrous tissues are characterized by angular anisotropy of relaxation times at the nuclear magnetic resonance (NMR) testing. The anisotropy is associated with the water distribution inside such tissues, and its determination requires multiple experiments with the sample rotation relative to the magnetic field. In this paper a quantitative link between a single NMR echo decay and the structure and fluctuations of the water-filled nanocavity in tendon is established, without orientation-dependent relaxation measurements. Echo decays were acquired from Achilles tendon using the Carr-Purcell-Meiboom-Gill and spin-locking sequences. To determine the relationship between the NMR relaxation and the structure and fluctuations of water-filled nanocavities, a physical model was used based on the representation of the tendon as a set of nanocavities and the averaged Hamiltonian of the dipole–dipole spin interaction. Two distinct nanocavity types were identified, which differ in the parameters of the nanocavity structure and their fluctuations. Non-invasive quantitative assessment of the nanocavity structural characteristics opens a way to develop biomarkers for detecting pathological changes in a tendon.