Helmet and head points

Discussions
1

Dear Francois,
Qustion 1: I get a raw data by using Neuromag. I find that the head points don’t fit the helmet, as shown in the attachment picture. Is there any problem with my data acquisition process?
Quistion 2: When i get the raw data in Neuromag, i pre-process the dada with Maxfilter/tsss immediately. And i do other pre-process in brainstorm such as apply SSP & CTF compensation, notch filter and detect other artifacts after Maxfilter/tsss algorithm. Is my processing proper?
Looking forward to your reply.

Cheers,
chenying

2

Hi Chenying,

  1. It looks like there is something wrong with the digitized head points. Maybe the receivers moved during the subject preparation… There only thing you can do is probably to ignore them, and hope the positions of the HPI coils are correct.
  2. If you are happy with tsss, yes, there is nothing wrong with what you are doing (the “Apply SSP” step is optional, in the notch filter you could also select the option “Process all file at once”).

Francois

3

Hi Francois.

Do you mean that i should align the four HPI coils with the MRI volumn not using head points to align with the MRI volumn?

4

If the positions of the head points are incorrect, the positions of the HPI coils is probably incorrect as well. This happens sometimes when there is some problem occurring during the acquisition of the points with the Polhemus.
If these points are incorrect, there is no way to fix it, so there is no way to know where the head was in the MEG. The registration MEG/MRI is probably wrong, but you have no way to know how off it is.
If you have a lot of subjects for your study, maybe you could ignore this one…
I’m sorry, I cannot help you much with this. It’s an acquisition issue, not a processing issue.
Francois

5

It’s very kind of you for answering my questions in detail. Thank you very much!

chenying