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

Objective To evaluate how data acquisition and post-processing choices influence ultrashort echo time (UTE)–based characterization of the short-T 2 * compositional structure of the Achilles tendon. Methods Ten healthy volunteers (33.3 ± 6.9 years) underwent Achilles tendon MRI using a multi-echo, time-interleaved UTE sequence. T 2 * 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). Results Estimated short vs. long T 2 * 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). T 2 * time constants were comparatively stable, particularly in the MID part ( p > 0.05). The fitting method affected short T 2 * component in the MTJ ( p MTJ = 0.032) and long T 2 * component in the INS and MTJ ( p INS = 0.025, p MTJ < 0.017). Discussion Non-fat-suppressed vs. fat-suppressed (FS) strategy and fitting approach substantially affect bi-component T 2 * 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.