T1/T2-weighted Ratio to Improve MRI-guided Focused Ultrasound–induced Chronic Lesion Contrast in Patients with Tremor Syndrome

Background T2-weighted MRI is the standard for visualizing MRI-guided focused ultrasound (MRgFUS) thalamotomy lesions in essential tremor (ET) and tremor-dominant Parkinson disease (TDPD); however, chronic lesion visibility is often variable, including low-contrast or undetectable lesions, complicating long-term assessment, especially in patients with tremor recurrence. Purpose To determine whether T1/T2-weighted ratio imaging improves the lesion normalized contrast (NC) of chronic MRgFUS thalamotomy-induced lesions compared with T2-weighted imaging. Materials and Methods This retrospective study included patients with ET or TDPD who underwent MRgFUS thalamotomy (January 2019 to November 2023). Lesion NC was quantified on T1/T2-weighted ratio and T2-weighted images at the 6-month follow-up. Between-modality NC comparisons used paired <i>t</i> tests. Similar analyses in patients with tremor recurrence used the Wilcoxon signed rank test. Between-evaluator agreement for lesion segmentation was assessed by comparing the average intraclass correlation coefficients (ICCs) via bootstrapping (95% CI for the difference). Spearman-skipped correlation analysis was performed to assess whether lesion overlap with the ventrolateral posterior ventral thalamic nucleus (VLpv) correlated with tremor relief from baseline to the 6-month follow-up. Results A total of 54 patients were included, 27 with ET and 27 with TDPD (mean age, 71.3 years ± 9.2 [SD]; 32 male patients). Lesion NC was higher for T1/T2-weighted ratio images (mean, 36.3% ± 18.2) than for T2-weighted images (mean, 19.2% ± 8.9) (<i>P</i> < .001). Similar results were found in patients with tremor recurrence (median NC at T1/T2-weighted ratio imaging, 45.2% [IQR, 14%-64%]; median NC at T2-weighted imaging, 25.3% [IQR, 9%-31%]). T1/T2-weighted ratio imaging had higher between-evaluator agreement (ICC = 0.94) than T2-weighted imaging (ICC = 0.81; 95% CI for the difference: 0.01, 0.32). Lesion VLpv overlap was positively correlated with tremor relief in patients with ET (T2-weighted imaging: ρ = 0.47 [95% CI: 0.10, 0.74]; T1/T2-weighted ratio: ρ = 0.53 [95% CI: 0.13, 0.76]) and TDPD (T2-weighted imaging: ρ = 0.42 [95% CI: 0.08, 0.67]; T1/T2-weighted imaging ratio: ρ = 0.48 [95% CI: 0.17, 0.66]). Conclusion T1/T2-weighted ratio imaging substantially improved chronic lesion NC compared with T2-weighted imaging, reducing interobserver variability and measurement error. © RSNA, 2025 <i>Supplemental material is available for this article.</i>