Objective: To determine the “functional” target within  VIM Nucleus of the Thalamus during HIFU with high field MRI.

Background: Targeting of neuronal structures has been historically achieved by indirect methods where the position  is obtained by extrapolation, measuring distances from visible reference points and intraoperative neurophysiological mapping. HIFU is an emerging non invasive technology that enables thermal thalamotomy for the treatment of Essential Tremors (ET). The drawback of HIFU is the lack of electrophysiological recording, and the decreased ability to transfer ultrasonic energy with multiple sonication attempts (thus decreasing the effectiveness of the procedure). The Vim is identified within the ventrolateral thalamic mass . It appears as a hypointense band crossing the thalamus in the coronal plane, with slight bending from posterolateral to anteromedial.
In this paper we demonstrated the use of direct targeting of the VIM with  FSE MRI  during MRgHIFU.

Method: 8 patients had  HIFU for the treatment of ET. The VIM coordinates as obtained by FSE MRI were used in targeting the ultrasonic waves in order to achieve thermal ablation of the VIM during MRgHIFU. All patients underwent a post procedure MRI studies. These imaging studies were fused with the preprocedural MRI  images and the stereotactic coordinates were compared. In addition, all patients underwent comprehensive clinical assessment of the clinical effect of MRgHIFU with regards to the exact lesioned area within the identified VIM.

Results: Within our cohort  we were able to identify the VIM nucleus by using FSE sequence in all patients. The stereotactic coordinates that were obtained  proved to be of positive clinical importance in 7 of the patients. The use of these coordinates also enabled us to reduce the average number of sonication from 9 to 6.5 (as compared to data from other treated patients). As with regards to negative side effects 3 patients reported minimal ataxia which resolved within a maximum of 3 weeks. There where no other serious permanent side effects.

Conclusion: With direct identification of the VIM thermal ablation with minimal number of sonications and good clinical outcome is feasible. Post HIFU MRI studies demonstrated that the anatomical VIM as seen on FSE MRI can be used as a therapeutic target during MRgHIFU for the treatment of ET. This study has the potential to allow treatment with MRgHIFU even.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/direct-targeting-of-the-vim-nucleus-during-high-intensity-focused-ultrasound-hifu-by-direct-visualization/