I came across several papers e.g. http://www.sciencedirect.com/science/article/pii/S2213158215000856 where gyrification index (GI) is indicated in 'mm'.

By definition GI is the ratio of two quantities: inner contour/outer contour (http://en.citizendium.org/wiki/Gyrification), therefore, shouldn't GI be 'unitless' then?

I am not sure which way is this correct?

Thanks.