[FieldTrip] beamformer results smaller than brain

[Akiko Ikkai]

Hi Fieldtrip users,

I'm hoping that someone could give me advice on segmentation and beamformer
on EEG data. I have EEG data set based on 128 channel cap. Thanks to the
help I got in mid-Feb, I'm now able to create a decent
segmentation (seg_results image attached) and volume conduction model.
However, when I run beamformer based on these models, resulting beamforming
image is often smaller than the brain (beamformer_result attached).
Particularly posterior parietal and occipital map is NaN.

I have tried going back to segmentation and expanded brain by
using imdilate after ft_volumesegment such as:

newbrain = imdilate(seg2.brain,strel_bol(1)); % seg2.brain is original
brain tissue from segmentation

seg2.brain = newbrain;
seg2.seg = seg2.scalp + seg2.skull*3 + seg2.brain*6;

and run ft_prepare_mesh_new on seg2. Of course, I have to make sure there
is no intersect between different tissue types, so using imdilate has
limitation.

Could someone explain why this shrinkage might be happening, and how I
could fix it?

Thanks in advance! Akiko

-- 
Akiko Ikkai, Ph.D.
Postdoctoral Fellow
Department of Psychological and Brain Sciences
Johns Hopkins University
Ames Hall, 3400 N. Charles St.
Baltimore, MD 21218
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