Distance between Head and sensors

[afo]

Hi again,

I know with BST it is possible to fit the electrode exactly to the head,
With meg sensors, there is a small distance with respect to head,
can we change the default distance of the meg channel in BST
relative to the head?

Thanks

[SBeumer]

Hi Afo,

Usually the information is in the dataset , because the location of the head is quite important in that kind of systems.

You might find your answer in the following thread about MEG sensors:

Senors organisation

The file coil_def.dat contains the geometry of various types of MEG sensors, independently from any acquisition device. The functions mne_add_coil_defs.m or ctf_add_coil_defs.m apply this geometry to the description of an acquisition device (position and orientation of each sensor in the MEG system), as it can be found in a .fif file (Elekta) or .res4 file (CTF). https://github.com/mne-tools/mne-matlab/blob/master/matlab/mne_add_coil_defs.m https://github.com/brainstorm-tools/brainstorm3/blob…

Kind regards,
Steven

[afo]

Hi Steven,

A) It is not mentioned here info regarding the distance between sensors and head,
@Francois said in the discussion you shared that BST do not allow to construct new senors coordinates, I understand this, and it is not difficult to do, but my question is about the distance of the sensors and head of the already existing in BST datasets

B) but as you share this discusson; I have question regarding the coil_def.data if we take for example these lines

1 5002 1 4 1.600e-02 0.000e+00 "CTF reference magnetometer size = 16.00 mm"
0.2500 4.000e-03 4.000e-03 0.000e+00 0.000 0.000 1.000
0.2500 -4.000e-03 4.000e-03 0.000e+00 0.000 0.000 1.000
0.2500 -4.000e-03 -4.000e-03 0.000e+00 0.000 0.000 1.000
0.2500 4.000e-03 -4.000e-03 0.000e+00 0.000 0.000 1.000

I understand everything here, but I just need to know how do you chose the coordinate here
for example: 4.000e-03 4.000e-03 0.000e+00, it is a random coordinates?

Thanks

[SBeumer]

Hi Afo,

I wouldn't know for A, though in B)The points that you mention are called integration points and it is to better capture what a real coil would capture of the magnetic field, in the following thread that is quite extensively explained: Computing the vector field B on the MEG sensors (forward problem) - #7 by John_Mosher

Kind regards,
Steven

[afo]

What do you mean by better capture?
and how do you chose those coordinates

[Francois]

With meg sensors, there is a small distance with respect to head, can we change the default distance of the meg channel in BST relative to the head?

There is no "default distance". The information is provided by the MEG acquisition system itself, it is saved in the recordings.
This has no sense to change it after, this is the geometry of the MEG system that recorded the data...

I understand everything here, but I just need to know how do you chose the coordinate here
for example: 4.000e-03 4.000e-03 0.000e+00, it is a random coordinates?

This represent the geometry of the CTF reference magnetometer coils...

[SBeumer]

This is explained in the thread that I send you,

Basically using those points you use a weighted average of the magnetic field that is flowing through the coil. that's why it has 0.25 per point in front of it. Doing full numerical integration would be way too much work for fields that are relatively constant with respect to your coil.

The choice of those coordinates is indeed governed by the geometry of the magnetometer coils and is explained by John Mosher in the thread