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

Abstract

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.

Keywords: Parkinson's disease; cumulative equivalent minutes at 43°C; functional stereotactic neurosurgery; high intensity MR-guided focused ultrasound; incisionless; minimal invasive; pallidothalamic tractotomy; subthalamotomy.

Figure 1

(left) outline (in gray) of the maximal extension of the PTT drawn from Myelin stains on an axial image. The target lesion sub-units (1 to 6) were adapted for intercommissural length <26mm, 0.9mm below the intercommissural plane. mtt (mammillothalamic tract), ic (internal capsule), mcl (mid-commissural line), tb (thalamic border). Coordinates of target lesion sub-unit 1: lateral 7.5mm (L 7.5), 2.7mm posterior to MCL (MCL-2.7) and 1mm ventral to the intercommissural plane (V1); 2: L5.5, MCL-2.7, V1; 3: L7.5, MCL-0.7, V1; 4: L6.5, MCL+1, V1; 5: L5.5, MCL-0.7, V1; and 6: L6.5, MCL-1.7, V1. (right) PTT target lesion sub-units projected on a modified map of the Morel's Stereotactic Atlas of the Human Thalamus and Basal Ganglia 0.9mm ventral to the intercommissural plane. For ICL ≥26mm see Figure 2.

Figure 2

Target lesion sub-units 1 to 7 for ICL ≥26mm depicted at the level of the intercommissural plane (DV0). Target lesion sub-units 4 and 5 are projected from V1 and sub-unit 7 from 1mm dorsal (D1). Coordinates of target lesion sub-unit 1: L 7.5, MCL-2.7, DV0; 2: L5.5, MCL-2.7, DV0; 3: L7.5, MCL-0.7, DV0; 4: L6.5, MCL+1, V1; 5: L5.5, MCL-0.7, V1; 6: L6.5, MCL-1.7, DV0; and 7: L7.0, MCL-1, D1.

Figure 3

PTT target lesion sub-units (ICL ≥26mm) (gray circles) on preoperative MR images. (A) axial scan centered on the intercommissural plane (DV0). ICL is the distance between the center of the anterior (ac) and posterior (pc) commissures. The mid-commissural line (mcl) is represented as dotted line. (B) mid-sagittal scan onto which target lesion sub-units 1 to 7 (ICL ≥26mm) are projected.

Figure 4

MR axial scans cut through the intercommissural plane after PTT taken (A) intraoperatively with the body coil and (B) 48 h after the procedure with the 32-channel head coil.