MEG Sensor Coordinate Files

MEG sensor coordinate files can be used to define coordinates and positions of gradiometer and magnetometer sensors. Our convention is to save magnetometer information in *.pmg, and gradiometer information in *.pos files. In practice, the distinction between gradiometers and magnetometers is based on the number of values on each line in the file.

File Header

MEG sensor coordinate files do not require a file header.

Format

Every line in the files represent the coordinates for one sensor. Labels in these files are ignored.

• Magnetometers: consist of six coordinates per line (3x location, 3x orientation).
• Gradiometers: consit of nine coordinates per line (3x location of primary sensor, 3x location of secondary sensor, 3x orientation). The program decides whether gradiometers are planar or axial based on the distance between the primary and secondary sensor locations and the center of the head.

Examples

1.) Magnetometer A1 of a BTI recording system:
-0.0019193 0.0304846 0.1081738 0.1188222 0.2394208 0.9636177

2.) Gradiometer of a Neuromag recording system:
0.108510 -0.000143 -0.044954 0.108510 0.000463 -0.028766 0.999999 0.001450 0.000000