NEAR-RESONANCE SATURATION PULSE IMAGING OF THE EXTRAOCULAR-MUSCLES INTHYROID-RELATED OPHTHALMOPATHY

PURPOSE: We examined the utility of near-resonance saturation pulse im aging (magnetization transfer [MT] and spin lock) in characterizing mi crostructural changes occurring in the extraocular muscles of patients with thyroid-related ophthalmopathy (TRO). METHODS: Eight healthy vol unteers and 10 patients with TRO were imaged using an off-resonance sa turation pulse in conjunction with conventional spin-echo T1-weighted imaging at frequency offsets of 500, 1000, 1500, and 2000 Hz from wate r resonance. The relative contributions of MT and spin-lock excitation to image contrast at each frequency offset were estimated using a com puter simulation model. Suppression ratios were calculated for the con trol and TRO groups from measurements obtained on two successive coron al sections in the widest portion of the inferior and medial rectus mu scles bilaterally, A repeated measures analysis of variance and a para metric correlation analysis were performed to evaluate maximum cross-s ectional area, MR-generated signal, and suppression ratios for the ext raocular muscles examined. RESULTS: Our computer model suggested that saturation of extraocular muscles was due to pure MT effects with our off-resonance pulse at 2000 and 1500 Hz, to a combination of MT and sp in lock at 1000 Hz frequency offset, and, primarily, to spin-lock exci tation at 500 Hz frequency offset. Suppression ratios for the extraocu lar muscles of the TRO patients were significantly lower than that obs erved for the control subjects at 1500, 1000, and 500 Hz frequency off set. This differential saturation effect aas maximal at 500 Hz frequen cy offset, with mean suppression ratios for the inferior and medial re ctus muscles of 27% for the healthy subjects and 20% for the TRO group . CONCLUSION: Both MT and spin-lock contrast of the extraocular muscle s in patients with TRO differ significantly from that observed in cont rol subjects. Near-resonance saturation pulse imaging may enhance our understanding of the microstructural changes occurring in the extraocu lar muscles of these patients.