Authors: Marissa D'Souza; Kevin Chen, MD; Mor Dayan, PhD; Jarrett Rosenberg, PhD; Kimberly Butts Pauly, PhD; Casey Halpern, MD; Pejman Ghanouni, MD, PhD (Savannah, GA)

Introduction: Magnetic resonance guided focused ultrasound (MRgFUS) thalamotomy has shown to be efficacious in the treatment of Essential Tremor (ET).Skull density ratios (SDR) of less than 0.45 ± 0.05 have previously been thought to be associated with decreased treatment effectiveness and increased rates of adverse events, thereby serving as exclusion criterion for treatment eligibility in some studies. Here, we compared the efficacy and safety of MRgFUS treatment of 189 patients with ET based on an SDR threshold of 0.45. Methods: Patients treated for ET were divided into two groups: SDR < 0.45 and SDR ≥ 0.45. Efficacy was based on improvement in the Clinical Rating Scale for Tremor (CRST) scores at 1-year follow-up after MRgFUS treatment. Safety was based on rates of the most severe procedure- or thalamotomy-related adverse event reported per patient. Results: Of the 189 patients treated for ET, 28% had an SDR < 0.45. There was no difference in efficacy between groups; 68% of patients with SDR < 0.45 demonstrated at least 50% improvement in CRST at 1-year follow-up, compared to 68% of patients with SDR ≥ 0.45. The group with SDR < 0.45 had a lower rate of adverse events compared to the group with SDR ≥ 0.45 (P = 0.013). Additionally, there were no serious adverse events reported in the group with SDR < 0.45. Conclusion: SDR is an indicator of the acoustic transparency of the skull to the ultrasound beam. Usually, lower SDR correlates with higher energy required to reach ablative temperature, but this prediction is not exact. By demonstrating that MRgFUS thalamotomy of some patients with SDR less than 0.45 is beneficial and can be performed without an increase in the rate of adverse events, our analysis presents an opportunity for more patients to access this modality as a feasible, non-invasive therapeutic.