Documentation/4.8/Modules/GeodesicSlicer

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GeodesicSlicer logo

The users enters 1) the T1-weighted whole-brain anatomical image 2) Place four points: the nasion, the inion, the left tragus and the right tragus. The program make a 3D mesh morphed to the structural MRI data of a participant and calculates the 10-20 system EEG with T3P3, and outputs the distance between the anatomical target and the T3 electrode.

Introduction and Acknowledgements

Author(s)/Contributor(s): Frederic Briend (ISTS EA 7466, UNICAEN), Antoine Nourry (UMS 3408) Acknowledgements: This work was supported by a Perceneige-Fondamental prize, CHU Caen, Region Normandie and UNICAEN. Contact: Frederic Briend, <email>briend@cyceron.fr</email>

The module has been developed based on ideas and feedbacks from the community. We would like to especially thank:

• Dr. Olivier Etard, M.D., Ph.D., CHU de Caen.
• Dr. Clément Nathou, M.D., Ph.D., CHU de Caen.
• Dr. Sonia Dollfus, M.D., Ph.D., CHU de Caen, header of ISTS.
• Dr. Csaba Pinter, MSc, Queen's University.
• Dr. Andras Lasso, Ph.D., Queen's University.

If you use this module, please cite the following article: .

Module Description

This module calculates geodesic path in 3D structure. Thanks to this geodesic path, this module can draw an EEG 10-20 system, determine the projected scalp stimulation site (MRI guided brain stimulation without the use of a neuronavigation System) and correct the rTMS resting motor threshold by correction factor.

Terminology

• Mesh A mesh or polygon mesh is a collection of vertices, edges and faces that defines the shape of a polyhedral object in 3D computer graphics and solid modeling.
• Shortest path In graph theory, the shortest path problem is the problem of finding a path between two vertices (or nodes) in a graph such that the sum of the weights of its constituent edges is minimized.
• 10-20 EEG system The International 10-20 system is commonly used for electroencephalogram (EEG) electrode placement and to correlate external skull locations with underlying cortical areas.

Use Cases

The overall goal is to allow users to find the shortest paths between nodes in a graph and via the Dijkstra's algorithm to make 10-20 system. This module can be used for:

• Stimulation in psychiatry: MRI guided brain stimulation without the use of a neuronavigation system.
• Surgery measurement.
• 3D printing.

Panels and their use

Create a mesh

A typical straightforward Geodesic Slicer workflow for consists of the following steps:

1. Load a volume.nii.
2. Enter in the Geodesic Slicer module using either the toolbar or the Modules menu button.
3. Press the button "Create a mesh".
   • Wait a moment.
4. If the segmentation is fine, press the button "Use this mesh".
   • If your image was named 'T1.nii' the output will be called 'T1.stl', in the same directory of your initial image.
5. Go to Parameters to find the shortest path or Make 10-20 EEG system electrode section.

Parameters to find the shortest path

1. Source points: The list of fiducial points on the curve, since the "Create-and-place Fiducial" button (in green in the figure above).
2. Input STL model: The model you use (after "use this mesh", the T1.stl created).
   • Find the shortest path: Calculate in centimeter the geodesic (shortest) path via the Dijkstra's algorithm.
   • Draw the shortest path: Draw the Dijkstra's algorithm shortest path.
     • Length (cm): The length of the current curve is shown in centimeter.

10-20 system electrode

• Run the Dijkstra's algorithm to make the 10-20 system electrode.

Four anatomical landmarks are used for the essential positioning of the electrodes: the nasion, the inion, the pre auricular to the left ear and the pre auricular to the right ear.

1. 4 anatomical landmarks: (Sources Points) The list of fiducial points on the curve, since the "Create-and-place Fiducial" button (in green in the figure above). Four anatomical landmarks are used for the essential positioning of the electrodes (in this order!):
   • 1/ The nasion
   • 2/ The inion
   • 3/ The pre auricular to the left ear
   • 4/ The pre auricular to the right ear
2. Input STL model: The model you use (after "use this mesh", the T1.stl created).
3. Press the button "Make 10-20 EEG system electrode" to draw the 10-20 EEG system via the Dijkstra's algorithm.
   • The traditional T3P3 site according to the International 10–20 system of electroencephalogram was identified.

• Project the stimulation site on the 10-20 system electrode distances and characterize it.

1. Stimulation Site placed: Place on the T1-weighted anatomical image the stimulation point that you want since the "Create-and-place Fiducial" button. Once this point given, click on 'Yes'.
2. Press the button "Project the stimulation site" to project the stimulation point on the scalp and find the 3nearest electrodes around it.
   • Nearest electrode 1: The distance in centimeter between the first nearest electrode and the projected stimulation site.
   • Nearest electrode 2: The distance in centimeter between the second nearest electrode and the projected stimulation site.
   • Nearest electrode 3: The distance in centimeter between the third nearest electrode and the projected stimulation site.

rTMS resting motor threshold- Correction factor

Calculate correction factors to adjust the rTMS dose for the treatment (according to the depth of the stimulation site).

Localization of the motor hand area via a knob on the precentral gyrus

1. M1 Point Placed: Place on the T1-weighted anatomical image a point targeting the human motor cortex since the "Create-and-place Fiducial" button. Once this point given, click on 'Yes'. Help via the Yousry's method.
2. Set the stimulation intensity of the resting motor threshold.
3. Press the button "Correct the motor threshold" to correct the unadjusted motor threshold (rMT) in % stimulator output.
   • Two adjusted motor threshold (AdjMT%) in % stimulator output are given where SCDx is the scalp-to-cortex distance between the scalp and and the Stimulation Site, SCDm is the scalp-to-cortex distance between the scalp and M1.
   • 1/ The first according to Stokes et al. Clin Neurophysiol 2007 , where [AdjMT% = 2,7 x (SCDx - SCDm) + rMT]
   • 2/ The second according to Hoffman et al. Biol Psychiatry 2013 , where [AdjMT% = 0.90*rMT*e0.036*(SCDt-SCDm)]

Information for Developers

The code is available at Github.