Question about fsaverage_sym - Freesurfer

26 Feb 2019


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Hello,
I have subject's surfaces where I first register the rh to the lh using
xhemireg, then register both hemispheres to fsaverage_sym using surfreg,
and then resample the registered surface to the isocahedron using
mris_surf2surf (see code below if that's helpful, part of the RSA toolbox,
written by Joern Diedrichsen). This means that for the two hemispheres of
these surfaces, each vertex number corresponds to an exact anatomical
location (vertex i in rh will be in the exact mirroring location of vertex
i).
(I then also move the coordinates of the surface to correspond to world,
rather than RAS_tkreg coords, but I think that is irrelevant to my
question. This is used to create searchlights ).
I then compute curvature maps on these surfaces, and would like to view
them in Freeview overlaid on a common surface. As the remapped surfaces
were registered to fsaverage_sym, I thought it would make sense to load
them as overlays on the fsaverage_sym surfaces. However I'm not sure on
which hemisphere of fsaverage_sym should the curv of the rh be overlaid on.
I originally thought it shouldn't matter -  that the rh of fsaverage_sym
was an exact mirror of the lh, so it shouldn't matter if I load the curv as
overlay on the lh or rh. But for a reason I don't understand, that is not
true, and it really changes how the curvature looks on the surface.
Moreover, it looks exactly the same if I load the curvature as overlay on
the rh of fsaverage or fsaverage_sym - I thought these two should be
different.
What am I getting wrong? which is the correct way of viewing the curvature?
Thanks very much for the help.
------------------------
function S=freesurfer_mapicosahedron_xhem(subj,subject_dir,varargin);
% function S=freesurfer_mapicosahedron(subj,subject_dir,varargin);
% Resampels a registered subject surface to a regular isocahedron
% This allows things to happen exactly in atlas space - each vertex number
% corresponds exactly to a anatomical location
% Makes a new folder, called ['x' subj] that contains the remapped subject
% Uses function mri_surf2surf
% INPUT:
%   subj: subject name
%   subjects_dir: freesurfer's SUBJECT_DIR
% VARARGIN:
%   'hemisphere',[1 2]  : left / right or both hemispheres
%   'surf_files',{'',''}: Surface files to be resampled
%
{'.white','.pial','.inflated','.sphere.reg','.sphere'};
%   'curv_files',{'',''}: Curvature files to be resampled
%                       {'.curv','.sulc','.area'};
%   'smoothing',1:        Smoothing iterations applied (otherwise nearest
neighbour)
% ---------------------------
% v1.0 Joern Diedrichsen (j.diedrichsen@ucl.ac.uk
%
current_dir=pwd;
direct=[getenv('FREESURFER_HOME') filesep 'average' filesep 'surf' ];
old_dir=getenv('SUBJECTS_DIR');
% if (isempty(subject_dir))
%    subject_dir=getenv('SUBJECTS_DIR');
% else
%    setenv('SUBJECTS_DIR',subject_dir);
% end;
subject_dir=getenv('SUBJECTS_DIR');
structname={'left','right'};
dirname={'LeftHem','RightHem'};
hem={'lh','rh'};
surf_files={'.white','.pial','.inflated'};
curv_files={'.curv','.sulc','.area'};
smoothing=1;
hemisphere=[1:2]; % Do both hemispheres
vararginoptions(varargin,{'smoothing','surf_files','curv_files','hemisphere'
});
% read freesurfer version
ver_file = fullfile(getenv('FREESURFER_HOME'),'build-stamp.txt');
if exist(ver_file)
fid     = fopen(ver_file);
verStr  = fgetl(fid);
fclose(fid);
verStr  = strrep(verStr,'-v',' ');
verStr  = textscan(verStr,'%s%f');
fsl_ver = verStr{2};
end
new_dir = [subject_dir filesep 'x' subj filesep 'surf'];
if (~exist(new_dir))
mkdir(new_dir);
end;
num_surf=length(surf_files);
files={surf_files{:},curv_files{:}};
for h=hemisphere
% Original subjects dir
if (h==1)
orig_dir =fullfile(subject_dir,subj,'surf');
else
orig_dir =fullfile(subject_dir,subj,'xhemi','surf');
end;
cd(orig_dir);
% [vertex_coords, faces]
[Vico,F]=read_surf([direct filesep 'lh.sphere.reg']); %
/opt/fmrib/FreeSurfer_releases/6.0/average/surf/lh.sphere.reg
% Topology file
for i=1:length(files)
if i<=num_surf
% e.g
/home/fs0/abaram/scratch/freesurfer-subjects/s99/surf/lh.white
[V]=read_surf([orig_dir filesep 'lh' files{i}]);
else
[V]=read_curv([orig_dir filesep 'lh' files{i}]);
end;
B=[];
for j=1:size(V,2) % Alon: number of coords - usually 3
% Alon's comment: write the coords of the vertices (one
dimension at a
% time) as curv files
write_curv([orig_dir filesep 'lh.temp.curv'],V(:,j),10000); %
e.g [$SUBJECTS_DIR/s99/surf/lh.temp.curv, only x coords of V, number of
faces (why only 10000?! I think it's just not used)]
if (h==1)
system(['DYLD_LIBRARY_PATH=/opt/fmrib/FreeSurfer_releases/6.0/lib/gcc/lib
'...
'mri_surf2surf --srcsubject ' subj ' --hemi lh' ...
' --trgsubject ico  --trgicoorder 7'...
' --surfreg fsaverage_sym.sphere.reg'...
' --srcsurfval temp.curv --src_type curv' ...
' --trgsurfval tempN.curv --trg_type curv --mapmethod
nnf --nsmooth-out ' num2str(smoothing)]);
else
% Reslice the left hemisphere of the xhemi subject - this
% is originally the right hemisphere
system(['DYLD_LIBRARY_PATH=/opt/fmrib/FreeSurfer_releases/6.0/lib/gcc/lib
'...
'mri_surf2surf --srcsubject ' subj '/xhemi --hemi lh'
...
' --trgsubject ico  --trgicoorder 7'...
' --surfreg fsaverage_sym.sphere.reg'...
' --srcsurfval temp.curv --src_type curv' ...
' --trgsurfval tempN.curv --trg_type curv --mapmethod
nnf --nsmooth-out ' num2str(smoothing)]);
end;
%              B(:,i)=read_curv([orig_dir filesep hem{h} '.tempN.curv']);
%  For different alignment, I would have to use -surf_reg option
%  here
% Alon's comment: read in the resampled curv file of a single
dim of the
% coordinates per vertex. At the end of the j loop, B will have
% the all 3 resamped dims of the coordinates.
if fsl_ver <= 5.1
B(:,j)=read_curv(fullfile(orig_dir,'tempN.curv'));
else
B(:,j)=read_curv(fullfile([orig_dir filesep 'lh.tempN.curv'
]));
end;
end;
% write the surfaces (3 col) and curvs (1 col).
if i<=num_surf
freesurfer_write_surf([new_dir filesep hem{h}
files{i}],B(:,1:3),F);
else
write_curv([new_dir filesep hem{h} files{i}],B(:,1),size(F,1));
end;
end;
% deleting temporary files
try
delete(fullfile(orig_dir,['lh.temp.curv']));
if fsl_ver <= 5.1
delete(fullfile(orig_dir,['tempN.curv']));
else
delete(fullfile(orig_dir,['lh.tempN.curv']));
end
catch err
disp(getReport(err));
end;
end;
setenv('SUBJECTS_DIR',old_dir);