[FieldTrip] Extracting virtual EEG channel from a atlas based ROI

[gab dippel]

Dear Fieldtrippers, 

my current question(s) has previously been answered here by Julian Keil: https://mailman.science.ru.nl/pipermail/fieldtrip/2015-December/009882.html yet I appear to face a conceptual problems in my understanding. 

So the goal is to compute the virtual channels of a certain atlas region let say the SMA. 
As we do not have individual MRI, I thought it should be possible to make use of the MNI template MRI the 'standard_mri.mat' as included in fieldtrip. 

Now according to Julian's answer from the above link the steps to go about the virtual channels reconstruction is: 
1. Compute the virtual channels for all grid points in the cortex (regular grid /sourcemodel)
--> So here I construct an MRI based grid


load standard_mri.mat
cfg = [];
cfg.mri = mri;
template_grid  = ft_prepare_sourcemodel(cfg);

this yields a grid with dimensions of 19x22x18 and 4093 gridpoints within the brain.

2. If you have computed the virtual channels on an  individual MRI, make sure  that the position corresponds  to the standard MRI (I think that's described in the source analysis -tutorial)
--> this is obsolete in my case i guess

3. Build a high resolution 3D-Grid on the standard MRI (It's important to use one grid point per voxel)
--> I am not sure how. the following gives me an Error 'incorrect cfg specification for constructing a dipole grid':
cfg = [];
cfg.grid.xgrid  = -90:1:90;
cfg.grid.ygrid  = -108:1:108;
cfg.grid.zgrid  = -90:1:90;
cfg.grid.unit   = 'mm';
cfg.grid.tight  = 'yes';
cfg.mri = mri;
high_res_grid_mm  = ft_prepare_sourcemodel(cfg)


4. Select all grid points from the grid of step 3 which belongs to an atlas (ft_volumelookup)
--> Now atlas has a dimension of 91x109x91 but the standard mri of 181x217x181. Here, I am wondering how to align the two to use logical indexing for finding the ROI voxels.

5. Use pythagoras to find the virtual channels from step 2 closest to the grid points from step 4.
--> This is clear if the grid from step 1 and 3 are of the same dimension which is however not the case here.

I would appreciate any feedback and ideas on this issue.

Kind regards, 

Gabriel
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