aezponda@unav.es

MRI-guided high-intensity focused ultrasound (HIFU) is an effective therapeutic approach for the ablation of the ventral intermediate nucleus (VIM) of the thalamus in drug refractory tremor. Cortical connections of VIM might differ from person to person. For treatment planning best seeds points at cortex are not defined. The aim of this study was to assess the cortical connections of VIM nucleus using diffusion tensor Imaging (DTI)-based tractography that overlap with the lesion location.

Twenty-two consecutive patients (20 right-handed) with medication-refractory ET (n=17) or PD (n=5) were recruited. All of them underwent VIM ablations contralateral to the patient's hand dominance using HIFU equipment compatible with the 3-T MR scanner. Pre-treatment and immediately after treatment structural and DTI MRI data were acquired. Pre-treatment DTI was co-registered with the post-treatment 3D T2WI sequences. The treatment-induced VIM lesion was used as seed for the DTI-based tractography. Topography of the VIM lesions and cortical connections were registered. Distance to the mid-sagittal plane was quantified at the juxtacortical white matter on axial T2WI.

Overall, HIFU was effective for immediate tremor control, awaiting for a longer follow-up. Mean size of the focused-sonography lesions was 6.3±2.7mm on axial 3D-T2WI. Mean distances to the midline and lateral wall of the third ventricle were 14.7±1.1 and 10.5±0.6 mm, respectively. Lesions were 1.6±1.4 mm above the intercommissural plane and 6.8±1.2 anterior to the posterior commissure. According to the cortical connections of the VIM nucleus, patients were allocated in 4 groups: medial aspect of the primary motor cortex (mM, n=7); intermediate region of the primary motor cortex (between m-M and hand-knob, iM, n=10); hand-knob region of the primary motor cortex (hM, n=2) and medial premotor area (preM, n=3). Mean distance from mid-sagittal plane at these sites were 10.7±1 (mM), 17.9±3.4 (iM), 21.9±4.5 (hM) and 8.6±3.1 (preM) mm.

Seeding of the VIM lesions on pre-treatment DTI shows connections predominantly to the primary motor cortex, usually medial to the hand-knob region.

DTI tractography defines the topography of juxtacortical white matter projections of the VIM. For treatment planning, cortical seeds should more frequently be placed at the primary motor cortex, medial to the hand-knob region.