local Gyrification Index (lGI)

Overview

Gyrification index is a metric that quantifies the amount of cortex buried within the sulcal folds as compared with the amount of cortex on the outer visible cortex. A cortex with extensive folding has a large gyrification index, whereas a cortex with limited folding has a small gyrification index. The method to be incorporated into Freesurfer is based on that of Marie Schaer, [http://ltswww.epfl.ch/~schaer/Schaer_TMI_accepted.pdf "A Surface-based Approach to Quantify Local Cortical Gyrification"], which computes local measurements of gyrification at thousands of points over the whole cortical surface.

Processing Stream

Stages

  1. From a freesurfer surface file (primarily ?h.pial, but ?.white or other surface files are possible), an 'outer surface' surface file is created, called ?h.outer-pial, which is basically an 'enveloped', version of the (pial) input. It is created by a morphological closing operation which operates on a volume created using mris_fill (taking as input the pial surface file), using a sphere as the structural element (se).
  2. Once the outer-surface file is available, then the local gyrification measurements are calculated for each vertex of the outer-surface. To do this, circular regions of interest are defined for each vertex on the outer-surface, and the corresponding areas of each ROI are found on the pial surface. Finally, the local gyrification index (lGI) values are calculated at each pial vertices, and a scalar file is output (used in a similar manner as a 'thickness' scalar file).

I/O Overview

Input

Process

Output

Notes

Computation time

?h.pial

mris_fill -c -r 1

?h.pial.filled.mgz

<1min

?h.pial.filled.mgz

make_outer_surface -se 15

?h.pial-outer

matlab commands, morphological closing*

<2min

?h.pial-outer

mris_smooth -nw -n 3

?h.pial-outer-smoothed

<1min

?h.pial-outer-smoothed

mris_convert -n (output normals as ascii)

?h.pial-outer-smoothed.asc

<1 min

?h.pial, ?h.pial-outer-smoothed

find_corresponding_center_FSformat

?h.pial.*.center

matlab commands

<1 min

?h.pial, ?h.pial-outer-smoothed, ?h.pial-outer-smoothed.asc

make_roi_paths

?h.pial.*.label

matlab command

~2h

?h.pial.*.path, ?h.pial.*.center

make_roi_labels

?h.pial.*.label

wrapper for mri_path2label

per vertex: 17.5 s in average, total: ~4h (but probably 5s per vertex are devoted to read the surface only, which is done up to 1'000 times)

?h.pial.*.labels, ?h.pial-outer-smoothed-mesh

compute_lgi

?h.pial.lgi.mgh

matlab commands

average per region: ~3s, total: ~45min

* decreasing the diameter of the se sphere below 10mm will produce an outer surface entering in the sulci, increasing it do not substantially affect the morphology of the outer surfaces (but increase computation time!)

Detailed Stages' Description

1. Computation of the regions of interest on the outer surface in matlab (=make_roi_paths)

For each one on 100 vertex of the outer surface (that we denote iV), a circular region of interest is defined (referred as ROIo in the validation paper). The area of this region will become the above term of the lGI ratio computation at that vertex.

Input

Process

Output

Notes

?h.outer-smoothed-mesh

definition of the circular region of interest

based on intersection with a sphere with a geodesical constraint

measurement of its area

?h.outerROIareas.mat

transfer of the points on the pial surface

?h.*.path

including reorganization of the vertices' list in the right order

find_corresponding_center

?h.c_*

seed points are defined as the vertex of the pial surface which is closest to the iV on the outer surface (< 2min)

2. Computation of the regions of interest on the outer surface using FreeSurfer (=make_roi_labels)

For each region of interest on the outer surface (ROIo), a corresponding region of interest is obtained on the pial surface (referred as ROIp in the validation paper). The correspondance is based on both the proximity with the ROIo's perimeter and geodesical constraints.

Input

Process

Output

Notes

?h.*.path

mri_path2label --confill SUBJID ?h ?h.pial.*.center --i ?h.*.path --o ./?h.pial.*.label

?h.pial.*.label

to compute iteratively for each iV

3. Computation of the lGI measurements and creation of cortical gyrification maps in matlab (=compute_lgi)

Input

Process

Output

Notes

?h.ROIp_*.label

area measurement, lGI ratio computation

?h.lGI_outer.mat

lGI values sampled on the outer surface

?h.lGI_outer.mat

propagation to the pial surface

?h.lGI_pial.mgh

lGI values are weighted according to their prior involvement in the whole computational process with weighting inversely proportional to the distance to the outer surface's normal