[FieldTrip] correct way to read Elekta fiff-files

[michael.scholz at med.ovgu.de]

Dear community,

this is Michael Scholz from Universitätsklinik für Neurologie, Otto-von-Guericke-Universität Magdeburg.

We are working with EEG- and MEG-data from Neuromag/Elekta/Megin fif-files.
We have a Triux-system with 306 MEG-sensors installed in 2016;
Internal Active Shielding (IAS) is off,  Maxfilter is used always.

I'm working on a matlab-routine to export preprocessed EEG- and MEG-data from fieldtrip
to Compumedics Neuroscan Curry 8, which is our standard tool for source analysis.

There is some incongruity with reading fif-file by fieldtrip.
Right by reading header-info by ft_read_header(fifname) I get a lot of warnings:
% Warning: assuming that the default scaling should be amplitude/distance rather than amplitude
%  In ft_datatype_sens at line 325
%  In ft_datatype_sens at line 207
%  In ft_datatype_sens at line 180
%  In ft_read_header at line 2841
I tried to localize the source of this warnings, but I finally failed in some sub-sub-sub-routine.
It seems, reading header has kind of inconsistency, however, results of reading data seem to be fine.

I now (by chance) found the option 'coil_accuracy' for ft_read_header.
Calling it with value 0 for 'coil_accuracy' works fine, since there aren't warnings any more.

I also compared the resulting values of location and orientation of sensors with the data
I get from Curry's cdt/dpa-files, if I read and export the fif-file within Curry.

With coil_accuracy=[], I get deviation of gradiometer-positions of up to 0.3mm;
with coil_accuracy=0, I get deviation of gradiometer-positions of up to 0.00003mm,
but only after adaption of gradiometer-position-pair-means to magnetometer-position.

What is the correct way to read our fif-files into ft?

In MNE's coil_def.dat for sensor coil-type id 3024 for magnetometer and 3012 for gradiometer
there are "vector-view"-entries only, but no entry for "triux".
Is there a difference?
Is possibly the used coil_def.dat not up-to-date?
#       mne_list_coil_def version 1.12 compiled at Jul 12 2016 18:39:09

Did I miss to read a piece of ft-documentation, which answers this questions,
especially about the usage of coil-accuracy?

with best regards,
Michael Scholz