Lesions of the cerebello-thalamic tract rather than the ventral intermediate nucleus determine the outcome of focused ultrasound therapy in essential tremor: A 3T and 7T MRI-study.

The ventral intermediate nucleus of the thalamus (VIM) is an important relay station receiving cerebellar and pallidal fiber tracts. Data on structural visualization of the VIM however is limited and uncertainty prevails to what extent lesional approaches to treat tremor affect the VIM itself or passing tracts. The aim of the study was to analyze the localization of individual lesions with respect to the VIM and the cerebello-thalamic tract (CTT). We employed ultrahigh resolution (7 Tesla) MRI to delineate the VIM and performed 3 T-DTI-imaging pre- and post-interventional in seven ET patients undergoing transcranial magnetic resonance guided focused ultrasound (tcMRgFUS). Tremor improvement was measured using a modified subscore of the Clinical Rating Scale for Tremor. All subjects showed substantial tremor improvement (88.5%, range 80.7%-94,8%) after tcMRgFUS. We found only a minor overlap of the lesions with the VIM (4%, range 1%-7%) but a larger overlap with the CTT (43%, range 23%-60%) in all subjects. Lesions within the CTT rather than the VIM seem to drive the tremorlytic response and clinical improvement in tcMRgFUS.