7T MRI quantitative susceptibility mapping for magnetic resonance-guided focused ultrasound thalamotomy: methodology and anatomo-clinical correlations in a retrospective pilot study

Ventral intermediate nucleus (VIM) thalamotomy is an effective treatment for essential tremor (ET). It can be performed incisionlessly by transcranial magnetic resonance-guided focused ultrasound (MRgFUS), aiming for accurate targeting and thermal dose control. Conventional magnetic resonance imaging (MRI) sequences performed at 1.5T or 3T provide limited case-specific information on the topology of intra-thalamic sub-nuclei. Quantitative susceptibility mapping (QSM) MRI enhances the visualization of subcortical structures by leveraging the magnetic properties of tissues, typically the iron content and the myelin microstructure. QSM may help depict the high-intensity focused ultrasound (HIFU) lesion's position and extent relative to the thalamic sub-nuclei, and correlate this information with clinical outcomes. To the best of our knowledge, this study addresses for the first time the integration of 7T MRI QSM into the clinical workflow of MRgFUS VIM thalamotomy. We retrospectively analyzed the radiological and clinical data of five patients undergoing routine MRgFUS VIM thalamotomy. A pre-operative multi-echo gradient-echo sequence was acquired at 7T MRI at 0.6 mm isotropic resolution. These data were processed to generate QSM images, which were combined with a paired T1-weighted (T1w) magnetization prepared rapid gradient echo (MPRAGE) dark-cerebrospinal fluid (CSF) sequence acquired both pre- and 6-week post-operatively. The MPRAGE data ensured the preservation of key anatomical references for indirect MRgFUS targeting, such as the commissures and ventricular wall. It also enabled sharp delineation of the ablative lesions, which were retro-projected "back-in-time" onto the paired pre-operative QSM. In this preliminary report, the enhanced intra-thalamic contrast provided by 7T MRI QSM supported a comprehensive correlation between the neuroradiological findings and the clinical outcomes, advocating for the interest in integrating 7T QSM MRI into the clinical workflow of MRgFUS VIM thalamotomy.