Objective: To determine whether restricted diffusion imaging (RDI) 30 minutes after MR-guided focused ultrasound (MRgFUS) thalamotomy in patients with essential tremor (ET) improves  the association with clinical outcomes compared to conventional T2-weighted (T2) imaging.

Background: MRgFUS thalamotomy is effective for ET, yet 10–20% of patients experience suboptimal benefit. Conventional T2 imaging, the current standard, provides high macroscopic conspicuity but limited sensitivity to the clinically relevant microstructural changes within the tremor network. RDI, a diffusion-derived metric sensitive to microstructural changes, may complement T2 imaging, and potentially enhance outcome-relevant lesion assessment after MRgFUS thalamotomy.

Method: Fifty ET patients (71.0±12.4 years, 21 women) underwent unilateral MRgFUS thalamotomy. RDI and T2 images were acquired at 30 minutes post-treatment. Lesions were segmented and quantitative metrics were derived, including lesion volume, lesion position, and overlap with key tremor-network structures such as the dentatorubrothalamic tract (DRTT) components. We also introduced novel RDI-weighted metrics that integrate lesion extent with the magnitude of restricted diffusion change to estimate cumulative microstructural burden. Associations with 6-month absolute change in the Clinical Rating Scale for Tremor (parts A + B of treated hemibody) were assessed using Spearman correlations (FDR-corrected). The top-performing metric was also explored for association with poor outcome (<50% improvement) using penalised logistic regression.

Results: RDI-weighted overlap with the decussating DRTT at 30 minutes showed the strongest correlation with 6-month tremor relief (ρ=0.58, FDR-p=0.0002), significantly outperforming its T2 analogue (ρ=0.32, p=0.06). Lower acute RDI-weighted overlap was associated with poor outcome (odds ratio =0.70, 95%CI:0.59-0.84, p=0.0001), while remaining unrelated to side effects.

Conclusion: RDI provides a mechanistically grounded framework for quantifying therapeutic microstructural lesion burden. Acute RDI-weighted overlap with the decussating DRTT captures lesion burden relevant to sustained tremor relief more accurately than T2 imaging, supporting the integration of microstructural quantification into precision MRgFUS thalamotomy workflows.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/acute-microstructural-imaging-of-focused-ultrasound-thalamotomy-lesions-is-associated-with-long-term-tremor-relief-in-essential-tremor/