Characterizing Microstructural and Mechanical Properties of Dancer Achilles Tendon Using Ultrashort Echo Time MRI and Shear Wave Elastography Ultrasound

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, β₃, 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.