Quantitative T1 mapping MRI for the assessment of extraocular muscle fibrosis in thyroid-associated ophthalmopathy

Purpose We aimed to investigate the performance of T1 mapping and its histological correlation with extraocular muscle fibrosis in thyroid-associated ophthalmopathy (TAO). Methods We prospectively recruited 12 cases of active TAO, 12 cases of inactive TAO, and 15 cases of control subjects. All participants underwent magnetic resonance imaging (MRI) scan with pre-/postcontrast T1 mapping and short-time inversion-recovery (STIR) sequence. The images were analyzed to obtain precontrast T1, extracellular-volume (ECV) fraction on T1 mapping, and signal-intensity ratio (SIR) on STIR for each rectus. Muscle biopsy was performed at lateral rectus to quantify-collagen volume fraction, glycosaminoglycan (GAG)-volume fraction, and extracellular space component. The relationship between MRI and histopathology was examined with Pearson correlation coefficient. Results The active TAO group was characterized with GAG accumulation, while the inactive TAO group presented with substantial fibrosis. The MRI parameters achieved acceptable interobserver and intraobserver agreement. The precontrast T1 and ECV remarkably increased in the TAO groups than the control group, and ECV positively correlated with collagen-volume fraction (r = 0.913) and extracellular-space component (r = 0.886) in the inactive TAO group. The SIR statistically increased in the active TAO group, and SIR positively correlated with GAG-volume fraction in all three groups. The performance of ECV (cutoff > 48.1%) to screen out extraocular muscle fibrosis in inactive TAO was 60.9% sensitivity and 93.3% specificity. Conclusions The ECV parameter on T1 mapping MRI is a reliable tool to quantify extraocular muscle fibrosis, providing insights into noninvasive evaluation of pathological changes in TAO orbit.