Anatomical and Technical Reappraisal of the Pallidothalamic Tractotomy With the Incisionless Transcranial MR-Guided Focused Ultrasound. A Technical Note.

Background: MR-guided focused ultrasound (MRgFUS) offers new perspectives for safe and efficient lesioning inside the brain. The issue of target coverage remains primordial and sub-optimally addressed or solved in the field of functional neurosurgery. Objective: To provide an optimized planning and operative strategy to perform a pallidothalamic tractotomy (PTT) in chronic therapy-resistant Parkinson's disease (PD) with the technology of MRgFUS. Methods and results: Histological sections and maps from 6 human brain hemispheres were analyzed and outlines of the pallidothalamic tract on Myelin-stained sections were drawn and superimposed. We determined a standardized PTT target coverage characterized by 5 to 7 preplanned target lesion sub-units of 1.5 × 1.5 × 3.0 mm, which were placed using focal point displacements and shortest possible times, under thermal dose control. Conclusion: We hereby present our current approach to the MRgFUS PTT on the basis of a histological reappraisal and optimized heat application to the pallidothalamic tract in the H1 field of Forel.

Introduction

Purpose Thermal ablation

Study Objective To develop an optimized MR-guided focused ultrasound planning and operative strategy for performing pallidothalamic tractotomy in patients with chronic therapy-resistant Parkinson's disease.

Animal model / Human subject Human (Homo sapiens); no strain; patients mean age 64.1 ± 12.9 years (histological brain cases ages not specified); sex not specified

Disease model Parkinson's disease

MRI or image guidance method MR-guided (MRgFUS) using ExAblate Neuro in a 3T MRI with stereotactic targeting based on AC–PC (intercommissural) line/MCL and Morel atlas coordinates; intraoperative MR thermometry and T2 imaging for spot adjustment and monitoring

Targeted brain region(s) Pallidothalamic Tract (H1 Field Of Forel)

Target coordinates ICL <26 mm (V1 = 1 mm ventral to AC‑PC): subunit1: ML L7.5; AP MCL-2.7 (posterior); DV V1. subunit2: L5.5; MCL-2.7; V1. subunit3: L7.5; MCL-0.7; V1. subunit4: L6.5; MCL+1; V1. subunit5: L5.5; MCL-0.7; V1. subunit6: L6.5; MCL-1.7; V1. ICL ≥26 mm (DV0 = intercommissural plane, V1 = 0.9–1 mm ventral, D1 = 1 mm dorsal): subunit1: L7.5; MCL-2.7; DV0. subunit2: L5.5; MCL-2.7; DV0. subunit3: L7.5; MCL-0.7; DV0. subunit4: L6.5; MCL+1; V1. subunit5: L5.5; MCL-0.7; V1. subunit6: L6.5; MCL-1.7; DV0. subunit7: L7.0; MCL-1; D1. PTT central target (natural transducer focal point): ML L6.5 (6.5 mm from medial thalamic border); AP MCL-1 (1 mm posterior to mid-commissural line); DV DV0 (on intercommissural plane) — corresponding to V1 when ICL <26 mm.

Outcomes and Safety

Summary of Outcomes MRgFUS pallidothalamic tractotomy for therapy‑resistant Parkinson's disease produced improved target coverage and more consistent symptom control when using multiple small, spatially distributed thermolesions rather than repeated high‑temperature or prolonged stationary sonications. Successful parameters were 5–7 preplanned sub‑target lesions ~1.5×1.5×3.0 mm applied with focal‑point displacements using the shortest possible sonication times under thermal‑dose control (avoiding repeated 54–60°C sonications or prolonged sonication without moving the focal point).

Safety-related matter Temperature monitoring was used to protect the mammailothalamic tract, keeping it below 43 degrees and below 2 CEM cumulative dose. The author noted that edema and increased local blood flow after thermolesioning can affect neighboring sonications.

Brain Region

Ultrasound Parameters

Ultrasound instrument ExAblate Neuro device (InSightec, Haifa, Israel); transducer aperture/diameter: None

Focal Characteristics Focal depth: None; Focal length: 1.5 × 1.5 × 3.0 mm; Aperture size: None

Treatment frequency Single session

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