5.3.4.2\ Thalamotomy

Essential tremor can be treated by lesioning of the ventralis intermedius nucleus of the thalamus. This can be accomplished via transcranial MR imaging-guided focused ultrasound lesion inducement, which is a noninvasive technique that generates heat at the site where numerous ultrasound beams intersect, utilizing MRI thermography guidance. Lesions are visible on T2-weighted images already apparent immediately after sonication and the lesions enhancement due to blood-brain barrier disruption. Over the course of a week after sonication, edema becomes more prominent, enhance, and become more distinct on T1-weighted images. Beyond 1 month after sonication, the lesions decrease in size (Fig. 5.36). Thalamotomy has effects on many other structures, which may be apparent on DTI. For example, there is decreased fractional anisotropy in the ipsilateral preand postcentral subcortical white matter in the hand knob area; the region of the corticospinal tract in the centrum semiovale, in the posterior limb of the internal capsule, and in the cerebral peduncle; the thalamus; the region of the red nucleus; the location of the central tegmental tract; and the region of the inferior olive. The magnitude of the DTI

changes after thalamotomy correlates with the degree of clinical improvement in essential tremor. Skull characteristics, however, can affect the size of the lesions and should be evaluated prior to focused ultrasound.

Fig. 5.36  Thalamotomy by focused ultrasound for essential tremor. Postoperative axial T1-weighted MRI shows a focal defect in the ViM nucleus of the left thalamus created by ultrasound (Courtesy of Daniel Cavalcante and Ryder Gwinn)