Fitting a template MRI to the MEG Polhemus head shape

This example script demonstrates how to fit a sphere to the MEG Polhemus recorded head shape, how to fit a sphere to a template MRI, and subsequently use the two fitted spheres to scale the template MRI such that it fits the MEG Polhemus head shape.

clc; clear; close all;
restoredefaultpath
addpath('/nashome1/wexu/matlab/fieldtrip')
ft_defaults

load standard_mri % Colin 27 template in fieldtrip

% read MEG sensor location
MEG_sens = ft_read_sens('/nashome1/wexu/MNE_data/CN/MEG/CN19/CN19_raw_tsss_mc.fif');
MEG_sens = ft_convert_units(MEG_sens,'mm');

% read polhemus headshape
headshape = ft_read_headshape('/nashome1/wexu/MNE_data/CN/MEG/CN19/CN19_raw_tsss_mc.fif');
headshape    = ft_convert_units(headshape,'mm');

save headshape headshape
save MEG_sens MEG_sens

% realign to neuromag coordinate system
lpa    = [  7 104  26];
nas    = [ 92 210  32];
rpa    = [176 104  26];
zpoint = [ 92 106 139];

cfg                 = [];
cfg.method          = 'fiducial';
cfg.fiducial.nas    = nas;
cfg.fiducial.lpa    = lpa;
cfg.fiducial.rpa    = rpa;
cfg.fiducial.zpoint = zpoint;
cfg.coordsys        = 'neuromag';
mri_realigned_fiducial      = ft_volumerealign(cfg,mri);

cfg           = [];
cfg.output    = {'brain','skull','scalp'};
segmentedmri  = ft_volumesegment(cfg, mri);

T_neuromag = mri_realigned_fiducial.transform;
segmentedmri.transform = T_neuromag;
segmentedmri.coordsys = 'neuromag';

cfg             = [];
cfg.tissue      = {'brain'};
cfg.numvertices = 3600;
brain           = ft_prepare_mesh(cfg, segmentedmri);

cfg             = [];
cfg.tissue      = {'scalp'};
cfg.numvertices = [3600];
bnd             = ft_prepare_mesh(cfg, segmentedmri);

% remove the lower part of the head
cfg = [];
cfg.translate = [0 0 -140];
cfg.scale     = [300 300 300];
cfg.selection = 'outside';
bnd_deface = ft_defacemesh(cfg,bnd);

% remove digitized head points on the nose
cfg = [];
cfg.translate = [0 90 -50];
cfg.scale     = [400 400 100];
cfg.selection = 'outside';
headshape_denosed = ft_defacemesh(cfg, headshape);


figure
ft_plot_headshape(headshape);
hold on
ft_plot_mesh(bnd_deface, 'edgecolor', 'none', 'facecolor', 'skin', 'facealpha',0.9)
ft_plot_mesh(brain, 'edgecolor', 'none', 'facecolor', [1 0 1]/1.2, 'facealpha',  0.5)
ft_plot_axes(headshape_denosed)
camlight left
camlight right
material dull
alpha 0.8
lighting phong

% fit a sphere to the MRI template
cfg=[];
cfg.method='singlesphere';
scalp_sphere = ft_prepare_headmodel(cfg, bnd_deface);

%fit a sphere to the polhemus headshape
cfg=[];
cfg.method = 'singlesphere';
headshape_sphere = ft_prepare_headmodel(cfg, headshape_denosed);

% scale the template MRI
scale = headshape_sphere.r/scalp_sphere.r;

T2 = [1 0 0 scalp_sphere.o(1);
      0 1 0 scalp_sphere.o(2);
      0 0 1 scalp_sphere.o(3);
      0 0 0 1                ];

T1 = [1 0 0 -scalp_sphere.o(1);
      0 1 0 -scalp_sphere.o(2);
      0 0 1 -scalp_sphere.o(3);
      0 0 0 1                 ];

S = [scale 0 0 0;
     0 scale 0 0;
     0 0 scale 0;
     0 0 0 1 ];

TRANSFORM = T1*S*T2;

segmentedmri.transform = TRANSFORM*T_neuromag;

cfg             = [];
cfg.tissue      = {'scalp'};
cfg.numvertices = 3600;
scalp_scaled    = ft_prepare_mesh(cfg, segmentedmri);


cfg             = [];
cfg.tissue      = {'brain'};
cfg.numvertices = 3600;
brain_scaled    = ft_prepare_mesh(cfg, segmentedmri);

figure
%ft_plot_sens(MEG_sens, 'style', '*b');
ft_plot_headshape(headshape_denosed);
hold on
ft_plot_mesh(scalp_scaled, 'edgecolor', 'none', 'facecolor', [1 1 1]/1.2, 'facealpha',  0.5)
ft_plot_mesh(brain_scaled, 'edgecolor', 'none', 'facecolor', [1 0 1]/1.2, 'facealpha',  0.5)
ft_plot_axes(headshape_denosed)
camlight left
camlight right
material dull
alpha 0.8
lighting phong