Imaging and procedural features of staged bilateral magnetic resonance-guided focused ultrasound thalamotomy for essential tremor.

Background: Magnetic Resonance-guided Focused Ultrasound (MRgFUS) thalamotomy is an established incisionless treatment for medication-refractory essential tremor (ET). While staged-bilateral MRgFUS thalamotomy has recently gained clinical acceptance, detailed radiological and procedural comparisons between first- and second-side interventions remain limited. This study aimed to provide a comprehensive imaging-based and procedural characterisation of staged bilateral MRgFUS thalamotomy, with a specific focus on stereotactic targeting, sonication parameters, and lesion morphology. Materials and methods: In this retrospective-prospective, single-centre observational study, consecutive patients with ET undergoing staged bilateral MRgFUS thalamotomy were included. Procedural metrics, stereotactic targeting coordinates, and sonication parameters were compared between FUS1 and FUS2 procedures. MRI analyses were performed using a standardised protocol including morphological pre-treatment 3D T1-weighted imaging and post-treatment 3D T2-weighted imaging acquired within 24 h and at approximately 1 month to document the lesion. Treatment-related lesions were segmented using a semi-automated approach, with volumetric measurements independently obtained by two blinded raters; inter-rater agreement was assessed using intraclass correlation coefficients. Adverse events (AEs) were recorded as secondary outcomes. A systematic review of the literature on treatment strategy of staged bilateral MRgFUS thalamotomy was conducted. Results: Fifteen patients underwent staged bilateral MRgFUS thalamotomy. Most anatomical, procedural, and sonication-related parameters were comparable between FUS1 and FUS2. Final stereotactic targeting during FUS2 showed a small but consistent anterior and dorsal shift relative to FUS1. Lesion volumes measured at both 24 h and 1 month after the procedure did not differ significantly between FUS1 and FUS2, and inter-rater agreement for lesion volumetry was excellent across time points (ICC > 0.91). AEs after FUS2 were predominantly mild and transient. We found no significant differences in lesion volume or inter-side targeting displacement between patients with and without gait disturbances, the most common AE, persisting at 1 month after FUS2. Single-patient imaging analyses suggested heterogeneous spatial lesion configurations in patients with AEs persistent 6-12 months after FUS2. Conclusion: In this single-centre cohort, staged bilateral MRgFUS thalamotomy showed high procedural and radiological consistency between FUS1 and FUS2. MRI-based volumetric analyses show consistent lesion morphology across hemispheres, with small, safety-oriented refinements in second-side targeting, in line with the literature.