Bayesian probabilistic density mapping of the decussating dentato-rubro-thalamic tract to predict clinical tremor...

Magnetic resonance-guided focused ultrasound (MRgFUS) is increasingly recognized as an effective treatment option for patients with medication-refractory essential tremor (ET). Indirect coordinates of the ventral intermediate nucleus of the thalamus, as well as the dentato-rubro-thalamic tract (DRTT) originating from the ipsilateral dentate nucleus, known as the "nondecussating DRTT" (nd-DRTT), are commonly used as targets for sonication. Anatomically, the DRTT originating from the contralateral dentate nucleus, referred to as the "decussating DRTT" (d-DRTT), constitutes the predominant component of the two fiber populations. However, the d-DRTT is rarely visualized using conventional diffusion tensor imaging (DTI) because of the technical challenges associated with resolving crossing fiber orientations. Probabilistic tractography enables the differentiation of crossing fibers, thus allowing for visualization of both the d-DRTT and nd-DRTT. Authors of this study aimed to evaluate whether the d-DRTT delineated by probabilistic tractography represents an anatomical target more important than indirect coordinates or the nd-DRTT.

Consecutive patients with medically refractory ET who underwent unilateral MRgFUS thalamotomy at a single institution between May 2022 and August 2024 were analyzed. Tremor severity was assessed using the Clinical Rating Scale for Tremor Part B, and the percentage improvement at 3 months after treatment was calculated as an indicator of functional recovery. Probabilistic tractography of the DRTT was performed post hoc using preoperative diffusion MRI and Bayesian modeling (BedpostX) and probabilistic tracking (ProbtrackX). The distances between the sonicated lesion as detected on postoperative MRI and each of the following were compared: indirect coordinates, nd-DRTT, and d-DRTT. Subgroup analysis was performed on patients with a peak lesion temperature ≥ 55°C. Pearson correlation was used to assess the relationships between distance metrics and clinical outcomes.

Probabilistic tractography successfully visualized the d-DRTT in all 28 patients included in the study. The d-DRTT was more lateral than both the indirect coordinate and the nd-DRTT (p < 0.01 for both), with a nonsignificant tendency for a more anterior position relative to the nd-DRTT (p = 0.054). Among the patients with a peak lesion temperature ≥ 55°C, the distance between the sonicated lesion and the d-DRTT showed a strong correlation with clinical outcomes, whereas that between the lesion and nd-DRTT showed a moderate correlation; the indirect coordinates showed no significant correlation.

Probabilistic tractography successfully visualized the d-DRTT, and its location appears to capture the "tremor-relevant" neural pathway more accurately than either the indirect coordinate or the nd-DRTT.