gyrification index from graph

[guillaume]

Hello,

I play with the gyrification index tool and have now few questions.

1) How is computed the brain_hull_area (rawhull = ingraph['brain_hull_area'])?
I suppose that it is done in a similar way as in the paper of A.Cachia (ref below) (morpho closing, meshing and computing the area of the resulting mesh) but
-which volume is closed?
-how much closing?

2) Since your great work making morphologist working on various dataset, would it be possible for you to give an average (and variance) value of the global gyrification index for "healthy subjects not too old and not too young"?
Such a value could serve e.g. as a sort of "graph validation index" in the sense that if I work with "healthy subjects not too old and not too young" and get an abnormal global gyrification index for a given subject I would check for the graph and morphologist pipeline for any problem...
Of course, this could be done for any graph property but the global gyrification index seems to be a good first candidate to me as e.g. it does not rely on the sulci identification process (as pointed in the paper).
(Of course also you may answer that the inter subjects variability is so important that such statistics are useless?)

3)From the paper of A.Cachia computing the local grification index consists in normalizing the sulcus-part area with the "brain_hull_area", is there a brainvisa process or command to do so (even it is not hard to code)?

Thanks for your help,
cheers,

gui

@article{cachia2008cortical,
title={Cortical folding abnormalities in schizophrenia patients with resistant auditory hallucinations},
author={Cachia, A. and Paill{\`e}re-Martinot, M.L. and Galinowski, A. and Januel, D. and de Beaurepaire, R. and Bellivier, F. and Artiges, E. and Andoh, J. and Bartr{\'e}s-Faz, D. and Duchesnay, E.},
journal={Neuroimage},
volume={39},
number={3},
pages={927--935},
year={2008},
publisher={Elsevier}
}

[riviere]

Hi Guillaume,

1) Yes, it's the area of the brain hull mesh. I don't remember if Arnaud did something different to compute the hull (I don't think so) but in any way the idea is the same. If I remember we take if from the external part of the skeleton image from which we extract the folds.

2) It would be possible, it just has to be done...

3) No there is no process ready for use in brainvisa for the local gyrification index. I thing Arnaud did it either manually, either using a script of his own. But doing so is not difficult: the morphometry process can output the surfaces of each sulcus, and you just have to divide them by the hull area.

Denis