Intraoperative lesion characterization after focused ultrasound thalamotomy

Outcomes after focused ultrasound ablation (FUSA) for essential tremor remain heterogeneous, despite therapeutic promise. Clinical outcomes are directly related to the volume and location of the therapeutic lesions, consistent with CNS ablative therapies. Recent data demonstrate that postoperative diffusion MRI, specifically the quantification of intracellular diffusion by restricted diffusion imaging (RDI), can accurately characterize focused ultrasound lesions. However, it is unclear whether RDI can reliably detect focused ultrasound lesions intraoperatively (i.e., within a few minutes of lesioning) and whether the intraoperative lesions predict delayed clinical outcomes.

An intraoperative imaging protocol was implemented that included RDI and T2-weighted imaging in addition to intraoperative MR thermography. Lesion characteristics were defined with each sequence and then compared. An imaging-outcomes analysis was performed to determine lesion characteristics associated with delayed clinical outcomes.

Intraoperative RDI accurately identified the volume and location of focused ultrasound lesions. Intraoperative T2-weighted imaging underestimated the lesion volume but accurately identified the location. Intraoperative RDI revealed that lesions of the ventral border of the ventral intermediate nucleus were significantly associated with postoperative tremor improvement. In contrast, the lesions extending into the inferolateral white matter were associated with postoperative ataxia.

These data support the acquisition of intraoperative RDI to characterize focused ultrasound lesions. Future research should test the histological correlates of intraoperative RDI and test whether it can be developed as feedback to optimize the current technique of FUSA.