Comparison of bi- and tri-component approaches for the analysis of short-T₂^* tissues in the presence of fat: application to the Achilles tendon

To evaluate how data acquisition and post-processing choices influence ultrashort echo time (UTE)–based characterization of the short-T₂^* compositional structure of the Achilles tendon.

Ten healthy volunteers (33.3 ± 6.9 years) underwent Achilles tendon MRI using a multi-echo, time-interleaved UTE sequence. T₂^* data were analyzed with and without fat suppression using two models: (i) a bi-component decay with two water pools and (ii) a tri-component decay including two water components and one fat component. The effect of magnitude versus complex fitting on compositional estimates was also assessed. Three Achilles tendon regions were analyzed: insertion (INS), mid-portion (MID), and muscle–tendon junction (MTJ).

Estimated short vs. long T₂^* components differed significantly depending on the use of fat suppression (p < 0.001 for all tendon regions) and fitting method (magnitude vs. complex) (p_INS = 0.002, p_MID < 0.001, p_MTJ < 0.001). T₂^* time constants were comparatively stable, particularly in the MID part (p > 0.05). The fitting method affected short T₂^* component in the MTJ (p_MTJ = 0.032) and long T₂* component in the INS and MTJ (p_INS = 0.025, p_MTJ < 0.017).

Non-fat-suppressed vs. fat-suppressed (FS) strategy and fitting approach substantially affect bi-component T₂^* quantification of the Achilles tendon, limiting comparability of UTE-based metrics. Tri-component modeling provides a more realistic description of tendon signal and may improve sensitivity to pathological changes. Further studies in patients are needed.