A Novel Device for Quantifying Improvement in Tremor During Focused Ultrasound Thalamotomy

Introduction: Thermoablative procedures targeting thalamic ventral intermediate nucleus (Vim) using focused ultrasound is rapidly gaining popularity as treatment of choice for essential tremor (ET). However, one persistent limitation lies in identifying the correct area to target, based on subjective assessment of patient spiral drawings after low-dose treatment. We hypothesize that developing a quantitative method to assess patient tremors will provide a more objective assessment of tremor improvement than visual inspection of the spirals and could improve patient care during FUS treatments.

Methods: 41 patients undergoing FUS treatment for ET were enrolled. Tremor improvement throughout the procedure was evaluated as follows: 1) A motion detection-based (accelerometer) device, 2) an image processing algorithm of the drawn spiral, and 3) neurologist TETRAS ratings of the drawn spiral. The accelerometer and image processing methods quantified tremor using power in the Fourier (frequency) domain.

Results: The average improvement measured by digital accelerometer was 67.7 ± 3.7%, 78.3 ± 8.5% from the analysis of the drawn spiral, and 50 ± 4.9% from the TETRAS rating (mean ± SEM). Temperatures between 45-49°C resulted in the most rapid improvement, with a steady change in tremor reduction from 10% to 60%, respectively. High ablation temperatures (50-56°C) revealed a steady change from 60-80% tremor reduction. When comparing the TETRAS rating with the accelerometer-measured improvement, a positive correlation was found with an R2 value of 0.33.

Conclusion : Quantitative image processing of the spiral and accelerometer recordings provide a more precise measure of tremor improvement. Moreover, we find that these analysis methods can improve the FUS procedure, providing an objective measure for subtle improvements during the procedure. We also report on the expected reduction in tremor for each temperature used for ablation, which can serve as a guide for determining if targeting is correctly chosen during procedures.