Reappraisal of the anatomical landmarks of motor and premotor cortical regions for image-guided brain navigation in TMS practice

Abstract

Image-guided navigation systems dedicated to transcranial magnetic stimulation (TMS) have been recently developed and offer the possibility to visualize directly the anatomical structure to be stimulated. Performing navigated TMS requires a perfect knowledge of cortical anatomy, which is very variable between subjects. This study aimed at providing a detailed description of sulcal and gyral anatomy of motor cortical regions with special interest to the inter-individual variability of sulci. We attempted to identify the most stable structures, which can serve as anatomical landmarks for motor cortex mapping in navigated TMS practice. We analyzed the 3D reconstruction of 50 consecutive healthy adult brains (100 hemispheres). Different variants were identified regarding sulcal morphology, but several anatomical structures were found to be remarkably stable (four on dorsoventral axis and five on rostrocaudal axis). These landmarks were used to define a grid of 12 squares, which covered motor cortical regions. This grid was used to perform motor cortical mapping with navigated TMS in 12 healthy subjects from our cohort. The stereotactic coordinates (x-y-z) of the center of each of the 12 squares of the mapping grid were expressed into the standard Talairach space to determine the corresponding functional areas. We found that the regions whose stimulation produced almost constantly motor evoked potentials mainly correspond to the primary motor cortex, with rostral extension to premotor cortex and caudal extension to posterior parietal cortex. Our anatomy-based approach should facilitate the expression and the comparison of the results obtained in motor mapping studies using navigated TMS.

Keywords: motor cortex; premotor cortex; sulcal anatomy; transcranial magnetic stimulation.

Figure 1

(A,B) Examples of clearly separated sulci at a certain depth of the brain that merge at the surface of the cortex. (C,D) Examples of sulci observed at the cortical surface and not observed deeper. diaS: diagonal sulcus; iPCS: inferior precentral sulcus; sPCS: superior precentral sulcus; CS: central sulcus; *: a superficial sulcus linking the sPCS to the CS.

Figure 2

Morphology of the CS at the level of hand representation, usually characterized by an inverted “omega” aspect (A,B). In some cases, an interruption of the CS can be observed at a depth of 30 mm (A,D). In other cases, the “omega” aspect is absent (C,D) or replaced by an “epsilon” aspect (E).

Figure 3

Break of the PCS between its superior (sPCS) and inferior (iPCS) segments (*). paraS: paramedian sulcus; mePCS: median precentral sulcus; maPCS: marginal precentral sulcus; F1: superior frontal gyrus; F2: middle frontal gyrus; sFS: superior frontal sulcus; iFS: inferior frontal sulcus; CS: central sulcus.

Figure 4

The morphological variants of the superior precentral sulcus (sPCS), especially regarding its dorsal and ventral branches (A–C) and its relationship with the superior frontal sulcus (sFS) (D–F). mePCS: median precentral sulcus.

Figure 5

Relationship between the posterior segment of the inferior frontal sulcus (iFS) and the horizontal branch of the inferior precentral sulcus (hiPCS).

Figure 6

Organization of the posterior part of the inferior frontal gyrus, including the pars orbitalis (Por), pars triangularis (PTr), and pars opercularis (Pop) separated by the horizontal (hSF) and ascending (aSF) branches of the SF. iFS: inferior frontal sulcus; iPCS: inferior precentral sulcus; CS: central sulcus; PoCS: postcentral sulcus.

Figure 7

Segmentation of the motor cortex based on stable anatomical landmarks observed at the cortical surface. paraS: paramedian sulcus; I1: intersection between the superior frontal sulcus (sFS) and the superior precentral sulcus (sPCS); hkCS: hand knob of the CS; I2: intersection between the inferior frontal sulcus (iFS) and the inferior precentral sulcus (iPCS); aSF: ascending branch of the sylvian fissure; PoCS: postcentral sulcus. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com]

Figure 8

Segmentation of the motor cortex in 12 squares, based on stable anatomical landmarks. (A) Numbered squares of the grid (from S1 to S12). (B) Percentages of motor evoked potentials obtained in contralateral hand muscle for the stimulation of each square of the grid in a series of 12 healthy subjects.

Figure 9

Influence of coil orientation (postero‐anterior, 45°, or latero‐medial) and stimulation intensity (100% or 120% of rest motor threshold, RMT) on motor maps on the left hemisphere of two subjects. The squares whose stimulation produced motor evoked potentials are shown in red, and those whose stimulation produced no responses are shown in green. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com]

Figure 10

Definition of the anatomical limits of the dPMC (red shade) according to literature data. Numbers refer to references listed in Table 2. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com]

Figure 11

Representative areas corresponding to the PMC, including its dorsal (dPMC) and ventral (vPMC) parts. AC: anterior commissure; PbCS: paracentral branch of the cingulate sulcus; SMA: supplementary motor area; sFS: superior frontal sulcus; iFS: inferior frontal sulcus; sPCS: superior precentral sulcus; PCS: precentral sulcus; iPCS: inferior precentral sulcus; CS: central sulcus. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com]