Head Position

Detecting head movements can be done by attaching a small set of coils, called cHPI coils, to specific locations of the participant's head before MEG recording begins (usually three to five coils).

MEGIN Systems

This process relies on three key commands from the MNE library. The first, compute_chpi_amplitudes, extracts the amplitude of waves generated by the cHPI coils at each sensor. Next, compute_chpi_locs calculates the position of each coil during recording, using their initial positions obtained with a Digitizer before the session. Finally, compute_head_pos integrates data from all coils to compute the head’s position over time. The resulting head position data is saved and used in the temporal Signal Space Separation (tSSS) stage to correct for head movements.

Parameters for this step can be adjusted in the pipeline_config.cfg:

"head_position": {
    "amplitudes": {
        "t_step_min": 0.25,
        "t_window": "auto",
        "ext_order": 1,
        "tmin": 0,
        "tmax": null
    },
    "locs": {
        "t_step_max": 1,
        "too_close": "raise",
        "adjust_dig": false
    },
    "head_pos": {
        "dist_limit": 0.005,
        "gof_limit": 0.98,
        "adjust_dig": false
    }

The step size balances accuracy and speed in amplitude estimation. Elekta recommends a 10-millisecond step, but it slows down calculations, especially for long sessions like resting-state data, with minimal impact on accuracy. This is because participants typically remain relaxed, leading to minimal head movement during MEG recordings.

CTF Systems

The extract_chpi_locs_ctf function is used to retrieve head position information from CTF data. In CTF systems, the x, y, and z coordinates (in meters) of each cHPI coil are stored as individual channels within the dataset.

MEGAP automatically checks whether cHPI data is present and identifies the type of MEG system being used. Based on this information, it then selects the appropriate method to extract head position data, ensuring compatibility with both MEGIN and CTF systems.