Abstract
Previous observations of multi-echo ultrashort echo time (UTE) magnetic resonance imaging (MRI) decay data from the Achilles tendon (AT) report an off-resonance non-water signal associated with non-collagenous extracellular matrix (ECM) constituents. This cross-sectional study investigates the relationship between this MRI-derived tissue matrix signal and mechanical stiffness of the AT in professional ballet dancers and non-dancer adults. Multiexponential analysis of multi-echo UTE MRI was used to quantify water components and an off-resonance AT matrix component. To compare AT structure with its functionality, shear wave elastography (SWE) ultrasound US was used to measure tendon stiffness along both longitudinal (V(L)) and transverse (V(S)) axes. 34 participants, including 15 ballet dancers and 19 non-dancers, were studied. Dancers exhibited significantly larger V(S) (p = 0.013) compared to non-dancers, consistent with prior observations of a training effect in tendon from repeated loading with exercise. UTE-derived off-resonance relaxation component amplitude, β(3), was positively associated with V(L) in dancers (p = 0.029) and V(S) in non-dancers (p = 0.024), suggesting a microstructural role of this matrix component. While additional work is needed to unambiguously assign this off-resonance signal, these findings suggest its association with non-collagenous ECM and show potential for combined use of UTE and SWE imaging to assess tendon structure-function relationships and adaptations to mechanical loading in vivo.