How to Measure Cortical Folding from MR Images: a Step-by-Step Tutorial to Compute Local Gyrification Index

Abstract

Cortical folding (gyrification) is determined during the first months of life, so that adverse events occurring during this period leave traces that will be identifiable at any age. As recently reviewed by Mangin and colleagues 2 , several methods exist to quantify different characteristics of gyrification. For instance, sulcal morphometry can be used to measure shape descriptors such as the depth, length or indices of inter-hemispheric asymmetry 3 . These geometrical properties have the advantage of being easy to interpret. However, sulcal morphometry tightly relies on the accurate identification of a given set of sulci and hence provides a fragmented description of gyrification. A more fine-grained quantification of gyrification can be achieved with curvature-based measurements, where smoothed absolute mean curvature is typically computed at thousands of points over the cortical surface 4 . The curvature is however not straightforward to comprehend, as it remains unclear if there is any direct relationship between the curvedness and a biologically meaningful correlate such as cortical volume or surface. To address the diverse issues raised by the measurement of cortical folding, we previously developed an algorithm to quantify local gyrification with an exquisite spatial resolution and of simple interpretation. Our method is inspired of the Gyrification Index 5 , a method originally used in comparative neuroanatomy to evaluate the cortical folding differences across species. In our implementation, which we name local Gyrification Index (lGI 1 ), we measure the amount of cortex buried within the sulcal folds as compared with the amount of visible cortex in circular regions of interest. Given that the cortex grows primarily through radial expansion 6 , our method was specifically designed to identify early defects of cortical development.

Local brain gyrification as a marker of neurological soft signs in schizophrenia

“…The LGI is defined as the amount of cortex buried within the sulcal folds as compared with the amount of visible cortex in circular regions of interest [11,12]. The development of cortical folding starts during prenatal life and undergoes only minor changes in adolescence [13,14].…”

“…surface based morphometry -SBM) based on T1-weighted structural MRI scans has made it possible to compute the local gyrification index (LGI) automatically [11,12]. The LGI is defined as the amount of cortex buried within the sulcal folds as compared with the amount of visible cortex in circular regions of interest [11,12].…”

Cortical Folding Defects as Markers of Poor Treatment Response in First-Episode Psychosis

“…11,23,46 The method by Schaer and colleagues is an automated vertexwise extension of the Zilles gyrification index, which computes a ratio of the inner folded contour to the outer perimeter of the cortex 47 using images reconstructed through the FreeSurfer pipeline. 48 It provides an…”

577. Lifespan Gyrification Trajectories of Human Brain in Healthy Individuals and Patients with Major Psychiatric Disorders

“…The reconstructed images were visually inspected by the authors to exclude the apparent reconstruction errors. The local gyrification index (GI) at each vertex of the reconstructed cortical surface mesh was calculated using the toolbox in Freesurfer with default settings 69 . Briefly, a circular region of interest was delineated on the outer surface, and its corresponding region of interest on the inner cortical surface was identified using a matching algorithm as described elsewhere 70 and the ratio between the folded inner cortical surface and its corresponding exposed outer surface was calculated as GI.…”