====== VIM targeting ======

{{ ::magnetic_resonance_guided_focused_ultrasound_thalamotomy_for_essential_tremor.jpg?400|}}

The [[ventral intermediate nucleus]] of the [[thalamus]] is not readily visible on structural magnetic resonance imaging. Therefore, a method for its visualization for [[stereotactic]] [[target]]ing is desirable.

The objective of a study of Sammartino et al., from the [[Toronto Western Hospital]] was to define a [[tractography]]-based methodology for the stereotactic targeting of the ventral intermediate nucleus.

The lateral and posterior borders of the ventral intermediate nucleus were defined by tracking the [[pyramidal tract]] and [[medial lemniscus]], respectively. A thalamic seed was then created 3 mm medial and anterior to these borders, and its structural connections were analyzed. The application of this method was assessed in an imaging cohort of 14 [[tremor]] patients and 15 healthy controls, in which they compared the tractography-based targeting to conventional targeting. In a separate surgical cohort (3 tremor and 3 tremor-dominant Parkinson's disease patients), they analyzed the [[accuracy]] of this method by correlating it with [[intraoperative]] [[neurophysiology]].

Tractography of the thalamic seed revealed the [[tract]]s corresponding to [[cerebellar]] input and motor cortical output [[fiber]]s. The tractography-based target was more lateral (12.5 [1.2] mm vs 11.5 mm for conventional targeting) and anterior (8.5 [1.1] mm vs 6.7 [0.3] mm, anterior to the [[posterior commissure]]). In the surgical cohort, the [[Euclidean distance]] between the ventral intermediate nucleus identified by tractography and the surgical target was 1.6 [1.1] mm. The locations of the sensory thalamus, lemniscus, and pyramidal tracts were concordant within <1 mm between tractography and neurophysiology.

The tractography-based methodology for identification of the ventral intermediate nucleus is accurate and useful. This method may be used to improve stereotactic targeting in functional neurosurgery procedures
((Sammartino F, Krishna V, King NK, Lozano AM, Schwartz ML, Huang Y, Hodaie M.
Tractography-Based Ventral Intermediate Nucleus Targeting: Novel Methodology and 
Intraoperative Validation. Mov Disord. 2016 Aug;31(8):1217-25. doi:
10.1002/mds.26633. Epub 2016 May 23. PubMed PMID: 27214406; PubMed Central PMCID:
PMC5089633.
)).
----
Krishna et al., from the Center for Neuromodulation, [[The Ohio State University Wexner Medical Center]],[[prospective]]ly assessed the [[outcome]]s of [[Focused ultrasound thalamotomy]] (FUS-T) in 10  [[essential tremor]] (ET) patients using [[tractography]]-based [[target]]ing of the [[ventral intermediate nucleus]] ([[VIM]]).

VIM was identified at the [[intercommissural]] plane based on its neighboring [[tract]]s: the [[pyramidal tract]] and [[medial lemniscus]]. FUS-T was performed at the center of tractography-defined VIM. [[Tremor]] outcomes, at baseline and 3 months, were assessed independently by the [[Tremor Research Group]]. They analyzed targeting [[coordinate]]s, clinical [[outcome]]s, and [[adverse event]]s. The FUS-T lesion location was analyzed in relation to unbiased thalamic parcellation using [[probabilistic tractography]]. Quantitative [[diffusion weighted imaging]] changes were also studied in [[fiber tract]]s of interest.

The [[tractography]] [[coordinate]]s were more anterior than the standard. Intraoperatively, therapeutic [[sonication]]s at the tractography target improved tremor (>50% improvement) without motor or sensory side effects. Sustained [[improvement]] in tremor was observed at 3 mo (tremor score: 18.3 ± 6.9 vs 8.1 ± 4.4, P = .001). No motor weakness and sensory deficits after FUS-T were observed during 6-mo follow-up. [[Ataxia]] was observed in 3 patients. FUS-T lesions overlapped with the VIM parcellated with probablisitic tractography. Significant microstructural changes were observed in the [[white matter]] connecting VIM with [[cerebellum]] and [[motor cortex]].

This is the first report of prospective [[VIM targeting]] with tractography for FUS-T. These results suggest that tractography-guided targeting is safe and has satisfactory short-term clinical outcomes
((Krishna V, Sammartino F, Agrawal P, Changizi BK, Bourekas E, Knopp MV, Rezai
A. Prospective Tractography-Based Targeting for Improved Safety of Focused
Ultrasound Thalamotomy. Neurosurgery. 2019 Jan 1;84(1):160-168. doi:
10.1093/neuros/nyy020. PubMed PMID: 29579287.
)).
===== Clinical trials =====
Optimization of VIM Targeting in Essential Tremor Surgery (Opti-VIM) https://clinicaltrials.gov/ct2/show/NCT03760406

===== References =====