I am doing several measurements in a row on one subject preceded by the
head digitization at the beginning. The coil-position is determined at the
beginning of each measurement (Neuromag-122). If I understood correctly,
there are two transformations:
The first is determined by the coil-measurement at the beginn of the
measurement(or in between with other equipment). The second by the
coregistration in mne. Right?
Would it be incorrect to reuse the trans-file created in mne coreg from one
measurement for all other measurements done with the same head-digitization
on the same subject?
Or is the transformation saved in mne coreg already a combination of both
transformations?
The first is determined by the coil-measurement at the beginn of the
measurement(or in between with other equipment). The second by the
coregistration in mne. Right?
Correct.
Would it be incorrect to reuse the trans-file created in mne coreg from
one measurement for all other measurements done with the same
head-digitization on the same subject?
In principle it should not change, in practice it will depend on your
digitzation accuracy. While the MRI coordinate frame is determined by your
one Freesurfer recon so it (of course) will not change between
measurements, the head coordinate frame on the other hand is determined by
the locations of LPA/Nasion/RPA that you digitized for any given
measurement. So if you are confident that your digitization procedure
recorded the locations of these physical landmarks identically in each
measurement, then the trans should not change. Regardless, I would check by
loading the subsequent recordings in mne coreg, and load the trans from the
first recording to see how good it is. At the very least it should give you
a very good starting point for making a new, slightly adjusted one if
necessary.
allright, thank you for your response.
For my experiments, I do the digitization only once at the beginning of the
session with several measurements with a subject.
Is it advisable to digitize more often in between the measurements?