[FieldTrip] MEG-anatomical MRI image coregistration

Stephen Whitmarsh stephen.whitmarsh at gmail.com
Mon Jul 30 16:48:33 CEST 2012

Dear Nenad,

My 2 cents:

1) As long as you are convinced that both give appropriately accurate
segmentations - something easily checked along the way by
superimposing stripped brain,stull etc on the original scan - I don't
see any reason against using them together in one study. Also, as you
will probably not work on the high resolution of your T3 (i.e. 1mm
isomorphic), but on downsampled grids for beamformer (i.e. 5 mm
isomorphic), below the resolution of your 1.5T scans, differences in
resolution should not confound your results on source level.
Ofcourse in some specific cases in between- or within-group
comparisons you could do a control analysis, e.g. using scan device as
a covariate in your analysis, just to be sure, but I can think of only
some very specific situations where that could be the case.

2) The localisation error would be related to how close you think you
can co-register [sic]. I would consider that addive uncertainty above
the localisation errors you'll unavoidably already have with your
recording (movement) and source reconstruction (i.e. point-spread of
the beamformer and such). I don't know what you mean exactly with
'software-based marking'. If you know where your MEG coils where in
respect to anatomical landmarks, you can intelligently 'guess' where
they are on the anatomical and just put them there by hand (nasion
would be pretty close, ears probably put them e.g. 1.5 cm anterior to
ear canals). The added uncertainty will be the degree of uncertainty
you have being able to do that, right?
If with 'software-based marking' you mean the use of a polemus device
than that would probably be better.
In any case, being conservative i would say the by-eye-method would
increase your uncertainly by another cm or 2, instead of about a cm
when appropriately using vitamin markers. I have no data to back that
claim up though.

Good luck!

On 30 July 2012 16:10, Nenad Polomac <polomacnenad at gmail.com> wrote:
> Hello,
> I am analyzing MEG CTF data. I want to do source localisation and I have two
> particular questions regarding the anatomical MRI images.
> 1. Is it allowed to use anatomical MRI images from a two different MRI
> devices in a one source localisation study? We already have routinely obtain
> anatomical images for some patients and those are obtained on the 1.5 Tesla
> MRI. And for other patients we will record anatomical MRI on 3 Tesla. So,
> what do you think can we use this data in the same source analysis study.
>  2. The anatomical MRI images from a few subject were not marked with the
> vitamin E during imaging for relevant skull points (nasion and ear canals).
> So, I think to mark them now in some imaging software. The MEG data were
> properly marked during acquisition for this skull points.
> What is your opinion about accuracy of software based marking. How big
> localisation error might be?
> Thank you in advance!
> Kind regards!
> Nenad
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