BIDS multiple coordsystem in one session

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Hi,
Quick question regarding the coordsystem.json. I can’t find the source anymore, but I believe that I have read somewhere that there can only be one _coordsystem.json for test session, which is also what I see in my meg directories. However, in my study that includes multiple tasks within a single test session, I had a subject a while ago that managed to remove one HPI coil from his head during a short break between tasks. Therefore, I had to digitize the HPI locations again.

In such a scenario, I would need two coordsystems.jsons that code the respective HPI coordinates, but since only one is allowed, one would be overwritten, right?

I guess the only way to bypass this problem would be then to additionally set the acq field for each task separately. Or am I missing anything? Would I then need to specify the acq field in all recordings of all subjects for consistency within the dataset sakes, or is it fine to have only a few subjects/sessions for which the acq field was set?

Thanks!

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I guess the only way to bypass this problem would be then to additionally set the acq field for each task separately.

sounds like a valid solution to me.

is it fine to have only a few subjects/sessions for which the acq field was set?

from a BIDS side, setting this only for the cases where it’s needed should be fine.

I guess the only way to bypass this problem

for the case where the participant removed an HPI coil, you could treat the “before removal”, and “after re-application” of HPI as two sessions (irrespective of your own “terminology” that this was a single day, single visit of the participant). But I guess using acq and a good documentation of this in README is better for this case.

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