To systematically review magnetic resonance guided focused ultrasound (MRgFUS) thalamotomy targeting techniques for tremor, beyond traditional atlas/coordinate-based systems.

MRgFUS thalamotomy is used effectively to reduce tremor [1-5]. Currently, most targeting techniques rely on an indirect atlas/coordinate-based system which approximates the ventral intermediate nucleus (ViM) using anatomic structures. With advancing imaging modalities, we aimed to systematically review other targeting techniques in treating tremor using MRgFUS thalamotomy.

A systematic review was conducted in Embase, Cochran, and PubMed adhering to PRISMA guidelines [6]. Studies including MRgFUS for tremor, description of targeting methods, tremor outcomes, and side effects were included. Studies using only coordinate-based targeting were excluded. After review of 292 studies, 17 met predefined inclusion criteria.

Of the 17 studies, 16 used tractography to further guide the MRgFUS target area. Tractography was used to visualize white matter tracts such as the medial lemniscus , cortical spinal tract, and dentato-rubro-thalamic tracts. All studies reported at least 50% reduction in tremor. The most common side effects were paresthesia and gait disturbances, but these were mostly mild and transient. Further implications of tractography include fewer sonication trials required and decreased lesion sizes. 

Refined ViM targeting using adjustments from direct visualization of these various fiber locations could expand the patient population that stand to benefit from MRgFUS thalamotomy. Reduction in trial sonications required could improve patient tolerability particularly in those with low skull density ratios. As smaller lesion size has been correlated to fewer side effects [7], tractography could provide better control of side effect profile. However, larger studies, standardization of techniques, and quantitative analysis are needed to confirm the benefit of tractography in MRgFUS thalamotomy for tremor.