Quantitative Tractography-Based Evaluations in Essential Tremor Patients after MRgFUS Thalamotomy

Abstract

Background: Magnetic resonance-guided focused ultrasound (MRgFUS) targeting the thalamic ventral intermediate nucleus (VIM) is an innovative treatment for drug-refractory essential tremor (ET). The relationship between lesion characteristics, dentate-rubro-thalamic-tract (DRTT) involvement and clinical benefit remains unclear.

Objectives: To investigate whether clinical outcome is related to lesion volume and/or its overlap with the DRTT. To compare the reliability of probabilistic versus deterministic tractography in reconstructing the DRTT and improving VIM targeting.

Methods: Forty ET patients who underwent MRgFUS thalamotomy between 2019 and 2022 were retrospectively analyzed. Clinical outcomes and adverse effects were recorded at 1/6/12 months after the procedure. The DRTT was generated using deterministic and probabilistic tractography on preoperative diffusion-tensor 3 T-images and location and volume of the lesion were calculated.

Results: Probabilistic tractography identified both decussating (d-DRTT) and non-decussating (nd-DRTT) components of the DRTT, whereas the deterministic approach only identified one component overlapping with the nd-DRTT. Despite the lesions predominantly intersecting the medial portion of the d-DRTT, with a significantly greater overlap in responder patients, we observed only a non-significant correlation between tremor improvement and increased d-DRTT-lesion overlap (r = 0.22, P = 0.20). The lesion volume demonstrated a significant positive correlation with clinical improvement at 1-day MRI (r = 0.42, P < 0.01).

Conclusion: Variability in the reconstructed DRTT position relative to the lesion center of mass, even among good responders, suggests that this fiber bundle is unlikely to be considered the sole target for a successful MRgFUS thalamotomy in ET. Indirect individualized targeting allows for more precise and reproducible identification of actual treatment coordinates than the direct method.

Keywords: MR‐guided focused ultrasound; dentato‐rubro‐thalamic tract; essential tremor; thalamotomy; tractography.

Figure 1

MRI appearance of thalamotomy lesion. MRI appearance of the thalamotomy lesion at 1‐ and 50‐days post‐treatment. Representative axial images include T1‐weighted sequences with and without contrast, fluid attenuated inversion recovery (FLAIR), T2‐weighted sequences, diffusion‐weighted imaging with apparent diffusion coefficient, and SWI‐phase contrast enhancement imaging (post‐gadolinium).

Figure 2

3D rendering of the deterministic and probabilistic dentate‐rubro‐thalamic‐tract (DRTT). Example of 3D rendering of the DRTT tract obtained with deterministic tractography (blue) and the nd‐DRTT component (yellow) obtained from maps of probabilistic tractography: panel a. Panel b also includes the probabilistic decussating (d‐DRTT) component (orange). The software used for rendering was TrackVis (http://trackvis.org/).

Figure 3

Relationship between average centroids of the deterministic and probabilistic tracts. Relationship between the average centroids of deterministic and probabilistic tracts and the lesion center overlaid on a T1 (1 mm isotropic) template. The standard deviations of the distances are represented as ellipsoids with major and minor axes corresponding to x and y directions. Lesion (white); probabilistic d‐dentate‐rubro‐thalamic‐tract (DRTT) (red), non‐decussating (nd‐DRTT) (orange), corticospinal tract (CST) (green); deterministic DRTT (blue), CST (violet).

Figure 4

Example of hyper‐responder patients who underwent magnetic resonance‐guided focused ultrasound (MRgFUS) left‐thalamotomy. Relationship from coronal and axial views between the lesion (yellow) and the decussating (d‐DRTT) (red = d‐DRTT M1, cyan = d‐DRTT supplementary motor area [SMA]) traced by probabilistic tractography after co‐registration between pre‐operative DTI and 1‐day T1 sequences. (A) Example featuring an HR patient who underwent MRgFUS left‐thalamotomy showing a good clinical response but a poor d‐DRTT M1‐lesion overlap (3D intersection index: 0,138). (B) Example of an HR patient exhibiting both a good clinical response and a good d‐DRTT M1‐lesion overlap (3D intersection index: 0,429).