mne_make_sphere_bem
Overview
mne_make_sphere_bem creates a spherical BEM model consisting of concentric spherical shells. This provides a quick analytical head model for MEG/EEG forward computation when realistic BEM surfaces from MRI segmentation are not available.
This is a C++ port of the original MNE-C tool by Matti Hämäläinen.
Usage
mne_make_sphere_bem [options]
Options
| Option | Description |
|---|---|
--out <file> | Output BEM FIFF file |
--origin <x,y,z> | Sphere origin in mm (default: 0,0,40) |
--radii <r1,r2,r3> | Shell radii in mm (default: 71,80,90 for 3-layer) |
--conductivity <s1,s2,s3> | Conductivities (default: 0.33,0.0042,0.33) |
--ico <n> | Icosahedron subdivision level (default: 4) |
--help | Print help |
--version | Print version |
Description
The spherical BEM model approximates the head as concentric spherical shells representing the brain, skull, and scalp. Default parameters create a standard three-layer model:
- Brain: radius 71 mm, conductivity 0.33 S/m
- Skull: radius 80 mm, conductivity 0.0042 S/m
- Scalp: radius 90 mm, conductivity 0.33 S/m
The origin parameter specifies the center of the spheres, typically set to the center of the head in head coordinates.
Workflow Context
The spherical BEM is an alternative to MRI-based BEM models when:
- MRI data is not available.
- A quick approximate forward model is needed for initial analysis.
- You want to compare spherical and realistic head models.
The resulting BEM file can be used directly with mne_forward_solution.
Example
# Create a default 3-layer spherical BEM model
mne_make_sphere_bem --out sphere-bem.fif
# Custom origin and radii
mne_make_sphere_bem --out sphere-bem.fif --origin 0,5,40 --radii 70,79,88