The Effect of Cranial Sutures Should Be Considered in Transcranial Electrical Stimulation

Abstract

Background: Computational modeling is used to optimize transcranial electrical stimulation (tES) approaches, and the precision of these models is dependent on their anatomical accuracy. We are unaware of any computational modeling of tES that has included cranial sutures.

Objectives: The aims of the study were to review the literature on the timing of closure of the coronal and squamous sutures, which are situated under electrode placements used in tES; to review the literature regarding differences in skull and suture conductivity and to determine a more accurate conductivity for sutures; and to identify magnetic resonance image (MRI) techniques that could be used to detect cranial sutures.

Methods: A scoping review of medical literature was conducted. We conducted computational modeling of a cranial bone plug using COMSOL Multiphysics finite element software, utilizing methodology and results from a previous study. We assessed use of the "3D Slicer" software to identify sutures in routine T1-weighted MRI scans.

Results: Reports from forensic examinations and computed tomography (CT) scans showed suture closure does not correlate with age. Our computational modeling determined a cranial suture conductivity of 0.32 S/m, which is much higher than for skull (compact skull 0.004 S/m, standard trilayer 0.013 S/m). 3D slicer enabled rapid and precise identification of the anatomy and location of cranial sutures.

Conclusions: Cranial sutures persist throughout the lifespan and have a far higher conductivity than skull bone. Cranial sutures can be localized quickly and precisely using a combination of MRI and readily available modeling software. Sutures should be included in tES computational modeling and electroencephalography source imaging to improve the accuracy of results.

Keywords: EEG; computational modeling; cranial suture conductance; transcranial electrical stimulation.

FIGURE 1

Major cranial sutures underlying ECT electrode placement. Human skull with the cranial sutures that lie below 2 ECT electrode placements; the BF—5 cm superior to the lateral canthus, and BT—3 cm superior to the midpoint of the lateral canthus and tragus. The red circles depict circular electrodes that are 5 cm in diameter. Image is licensed under Creative Commons Attribution 3.0 generic (Author OpenStax College from Wikimedia).

FIGURE 2

Closure of the right coronal suture with age from an autopsy study inmales and females. Ascardi and Nemeskeri technique (4 item scoring from 0 to 3) however partially closed sutures (1 and 2) were not included given low numbers.

FIGURE 3

Closure of the right coronal suture with age assessed by head CT. A, Scatter plots showing linear regression models of age estimation using the right coronal suture segments (RC1—medial part, RC2—middle part, and RC3—lateral part (for score see “B”). B, Three-dimensional CT images of the skull representing the ectocranial scoring system: 1) incipient closure indicated by the evidence of bony bridging up to 50% closure; 2) significant closure indicated by the evidence of bony bridging greater than 50%; 3)* (not represented in image from referenced paper) obliteration with no trace remaining of the suture margins. Copyright 2023 (Creative Commons Attribution License).

FIGURE 4

Closure of right squamous suture with age as assessed by Head CT in living subjects. The rating scale: completely open—clear joint space in whole length, partially closed—thinner and interrupted by partially closed areas, and closed—suture location is not recognizable.

FIGURE 5

Distribution of seizure threshold for RUL-UB ECT with age. The number of male and female subjects is in parentheses.

FIGURE 6

Conductivity of bone plugs taken from the skull from Tang et al. A, Graph representing the average conductivity (converted from resistance) of 6 bone plugs taken from approximate locations illustrated on the skull. B, Cross-sectional views of the 1.4 cm bone plugs with average thickness (Copyright 2008, IEEE). The vertical shaded column includes the coronal suture whereas the cross-hatched shaded column includes the squamous suture.

FIGURE 7

Error distribution for the EEG forward solution when omitting the cranial sutures. Relative error (RE) is presented, through a color scale, (RE is a ratio of the error compared to the measurement being taken) (Copyright IOP Publishing Limited 2019).

FIGURE 8

Computational simulation of electrical flow through bone plugs—using COMSOL Multiphysics finite element software and the experimental methods from Tang et al. Cylindrical saline layers (1 S/m) were placed each side of bone plugs and electrical current applied across 0.15 mm compact bone layers and variable spongiform bone geometries reproduced.

FIGURE 9

Volumetric rendering of T1 and FRACTURE MRI scan images—DICOM images of 45-year-old man (AC) uploaded to “3D Slicer v5.0.2” and volume rendering shift adjusted using the preset “CT-Lung” so the skull and sutures were visible. The sutures are clearly demonstrated in relation to soft tissue landmarks. HREC number HC190222 (research MRI scanning for protocol development).