Tension-Dependent Variability and Repeatability of Achilles Tendon UTE-T₂* Mapping Using Mono- and Bi-Exponential...

Quantitative MRI using ultrashort echo time (UTE)-T₂* mapping is sensitive to collagen-bound water and tendon microstructure, enabling noninvasive assessment of tendon integrity and laxity. However, the extent to which UTE-T₂* measures reflect changes in tendon tension, and their repeatability, remain incompletely understood. We evaluated the sensitivity of mono- and bi-exponential UTE-T₂* measures of the Achilles tendon to changes in tendon tension induced by ankle positioning and assessed the test-retest repeatability of these metrics across repeated scan sessions. In this study, healthy adult volunteers underwent UTE-MRI of the Achilles tendon at two scan sessions, spaced 1 week apart, with the ankle positioned in dorsiflexion (higher tendon tension) and plantarflexion (lower tendon tension). Mono-exponential T₂* (T₂*_mono) and bi-exponential parameters (T₂*_short, T₂*_long, and short-component fraction ρ_short) were quantified. We performed a two-way repeated-measures ANOVA to assess the main effects of ankle position and scan session, and their interaction. Repeatability was evaluated using root mean square error (RMSE), coefficient of variation (CV%), and Bland-Altman analysis. We observed that short-component T₂* metrics demonstrated significant sensitivity to tendon tension. A main effect of ankle position was observed for T₂*_mono (p < 0.001) and T₂*_short (p = 0.02), with lower values in dorsiflexion compared with plantarflexion. No significant effect of scan session or ankle position × scan session interactions were observed. T₂*_long and ρ_short showed no significant dependence on position or scan session, suggesting that bulk hydration and relative water-compartment contributions remained stable across loading conditions. Dorsiflexion demonstrated lower RMSE and CV% across metrics than plantar flexion, indicating improved repeatability under passive tendon tension. UTE-T₂* relaxometry of the Achilles tendon is repeatable and sensitive to changes in tendon tension. Short T₂* measures may provide quantitative imaging markers related to tendon mechanical integrity and laxity, while highlighting the importance of standardized tendon tension for longitudinal quantitative tendon MRI.