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Coordinate Systems for MEG EEG iEEG
From now on MEG/EEG/iEEG is referred to as ephys.
In BIDS we can specify the following coordinate systems across ephys:
Fiducials*AnatomicalLandmarks*HeadCoil*DigitizedHeadPoints*EEG*MEG*iEEG*
for all of these, usually three fields can be defined:
*CoordinateSystem*CoordinateSystemDescription*CoordinateUnits
The "allowed" values for the CoordinateSystem fields are described in Appendix VIII of the BIDS specification. The idea is to provide a list of restricted keywords in the specification, which can then be used in the datasets. If dataset readers then want to find out more about the coordinate system, they can check the specification by searching for the relevant keyword.
If the CoordinateSystem is not part of BIDS, the value Other can be used, in which case CoordinateSystemDescription MUST be used to exhaustively describe the CoordinateSystem. However, it is generally advised that users open an issue on the BIDS specification repository to discuss adding new keywords to the Appendix VIII.
CoordinateUnits usually have to be in mm, cm, or m.
The BIDS validator checks the relevant coordinate system fields in the following three JSON schema files:
- https://github.com/bids-standard/bids-validator/blob/master/bids-validator/validators/json/schemas/coordsystem_eeg.json
- https://github.com/bids-standard/bids-validator/blob/master/bids-validator/validators/json/schemas/coordsystem_meg.json
- https://github.com/bids-standard/bids-validator/blob/master/bids-validator/validators/json/schemas/coordsystem_ieeg.json