Multimodal Imaging to Support Vim Targeting in Magnetic Resonance-Guided Focused Ultrasound Thalamotomy: Two Technically and Anatomically Challenging Cases

Essential tremor is the most common adult movement disorder, typically presenting with kinetic and postural tremor that interferes with daily activities. For patients unresponsive to medications, surgical options such as radiofrequency thalamotomy, deep brain stimulation, and magnetic resonance-guided focused ultrasound can provide therapeutic benefit. Magnetic resonance-guided focused ultrasound, an incisionless lesioning modality, has gained clinical use; however, accurate targeting of the ventral intermediate nucleus remains challenging because the nucleus cannot be directly visualized on routine magnetic resonance imaging and exhibits substantial individual variability. This report describes 2 cases of medication-refractory essential tremor treated with magnetic resonance-guided focused ultrasound ventral intermediate nucleus thalamotomy under structurally complex conditions. One patient had a thick skull with a low skull density ratio, and the other exhibited marked thalamic distortion. In both cases, individualized ventral intermediate nucleus targeting was performed using multimodal imaging that combined a stereotactic planning platform, dentatorubrothalamic tract tractography, and Fast Gray Matter Acquisition T1 Inversion Recovery. The Fast Gray Matter Acquisition T1 Inversion Recovery sequence, which enhances gray-white matter contrast, provided relative contrast of intrathalamic structures, including signal patterns corresponding to the internal medullary lamina. When integrated with connectivity-based dentatorubrothalamic tract tractography, this approach provided complementary information to support anatomically guided targeting. Stepwise sonication with intraoperative thermal monitoring resulted in substantial tremor reduction without new neurological deficits. These observations suggest that combining Fast Gray Matter Acquisition T1 Inversion Recovery with tractography offers practical, complementary guidance for ventral intermediate nucleus targeting, including in settings without access to advanced commercial integration software. Although limited to 2 patients, this work suggests the value of accessible multimodal imaging for improving confidence in anatomy-informed targeting in magnetic resonance-guided focused ultrasound and potentially other lesioning procedures.