[FieldTrip] question on coil orientations for elekta systems

Schoffelen, J.M. (Jan Mathijs) jan.schoffelen at donders.ru.nl
Tue Aug 21 11:11:51 CEST 2018

Dear Thomas,

I would guess that the 3x3 ‘rotation’ part of the affine transformation matrix here describes the direction plane of the sensor in the first two columns (x/y axis), and the normal to this plane in the third column, which is the vector perpendicular to the coil. Try and verify whether the cross-product between the first two columns gives the the third.

Best wishes,


On 21 Aug 2018, at 10:24, Thomas Hartmann <thomas.hartmann at th-ht.de<mailto:thomas.hartmann at th-ht.de>> wrote:


while diving into the import code for elekta systems, i noticed that coil orientations are imported like this:

t = orig.chs(n).coil_trans;

grad.coilpos(kCoil,:) = t(1:3,4)+0.0003*t(1:3,3);
grad.coilori(kCoil,:) = t(1:3,3);

(full code is here: https://github.com/fieldtrip/fieldtrip/blob/master/fileio/private/mne2grad.m#L321)

t (or coil_trans) is a 4x4 matrix, that if i understood it correctly, is a 3x3 rotation matrix combined with xyz coordinates in the last column.

if i understand everything correctly, coilori is supposed to store a unit vector pointing in the direction of the coil's orientation.

so, why is this the third column?

thanks a lot in advance!


Dr. Thomas Hartmann

Centre for Cognitive Neuroscience
FB Psychologie
Universität Salzburg
Hellbrunnerstraße 34/II
5020 Salzburg

Tel: +43 662 8044 5109
Email: thomas.hartmann at th-ht.de<mailto:thomas.hartmann at th-ht.de>

"I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)

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