Introduction: Transcranial MR-guided focused ultrasound (FUS) is a novel method for creating thalamic lesions in the treatment of essential tremor. In this procedure, MR thermography is used to verify lesion target and size, allowing precise lesion placement and predictable lesion volume. The objectives of this study are to characterize FUS thalamotomy lesion volume over time and determine if there is a correlation between lesion volume and relief of tremor, or if additional procedural factors improve outcomes.

Methods: This is a retrospective study of 15 patients who underwent stereotactic FUS thalamotomy in a pilot study for treatment of essential tremor at the University of Virginia. MRI scans were obtained at 1 day, 1 week, and 1 month postoperatively. Lesion volumes and diameters were measured on T2-weighted MR images. Procedural details including number of sonications, final power, final energy, and peak voxel temperature were collected retrospectively. Tremor severity was graded by the CRST preoperatively, at the time of MRI, and 1 year postoperatively. Statistical analysis was performed using Wilcoxon rank sum tests, Fisher’s exact tests, and univariate analysis.

Results: FUS lesions can be divided into three zones according to imaging appearance and histological characteristics. Zone 1 corresponds to the area of coagulative necrosis, and zone 3 corresponds to vasogenic edema. Average zone 1 volumes were 248, 338, and 144mm3 at 1-day, 1-week, and 1-month, respectively. Average zone 3 volumes were 1207, 1171, and 186mm3 at the same time-points. There was no statistically significant correlation between treatment conditions, lesion volume, lesion diameter, and tremor relief or transient adverse symptoms.

Conclusions: Effective lesion volumes calculated on T2-weighted imaging on postoperative day #1 ranged from 130-460 mm3. Lesion volumes peaked at 1 week and decreased thereafter. There was no correlation of lesion size with tremor relief.

Patient Care: This study provides important information regarding early imaging characteristics of thermal lesions and the lesion volumes required for effective treatment of tremor. This information is important for future trials using transcranial focused ultrasound for the treatment of movement disorders and other intracranial targets as this method is applied to other disorders.

Learning Objectives: By the conclusion of this session, participants should be able to: 1. Understand possible mechanisms for how transcranial MR-guided focused ultrasound thalamotomy is effective in tremor reduction for patients with essential tremor. 2. Describe the physical and temporal characteristics of thermal lesions produced by transcranial MR-guided focused ultrasound. 3. Identify possible reasons for why lesion volume was not correlated to tremor relief in this study.

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