Connectivity between MS lesion density and cortical thickness

Connectivity between MS lesion density and cortical thickness Keith Worsley Arnaud Charil Jason Lerch Department of Mathematics and Statistics, Mc. Connell Brain Imaging Centre, Montreal Neurological Institute, Mc. Gill University

Connectivity • Measured by the correlation between data at pairs of voxels: Activation only Voxel 2 ++ + +++ Correlation only Voxel 2 Voxel 1 + + ++ Voxel 1 + +

Method 1: ‘Seed’ Friston et al. (19? ? ): Pick one voxel, then find all others that are correlated with it: Problem: how to pick the ‘seed’ voxel?

Method 2: Iterated ‘seed’ • Problem: how to find the rest of the connectivity network? • Hampson et al. , (2002): Find significant correlations, use them as new seeds, iterate.

Method 3: All correlations • Problem: how to find isolated parts of the connectivity network? • Cao & Worsley (1998): find all correlations (!)

Practical details • Find threshold first, using random field theory • Keep only correlations > threshold • Then keep only local maxima i. e. cor(voxel 1, voxel 2) > cor(voxel 1, neighbours of voxel 2), > cor(neighbours of voxel 1, voxel 2),

MS lesions and cortical thickness (Arnaud et al. , 2004) • • N = 425 mild MS patients Lesion density, smoothed 10 mm Cortical thickness, smoothed 20 mm Hypothesis: increased lesion density in white matter -> thinner cortex in connected grey matter • Find connectivity i. e. find voxels in 3 D, nodes in 2 D with high cor(lesion density, cortical thickness)

n=425 subjects, correlation = -0. 56826 Average cortical thickness 5. 5 5 4 3. 5 3 2. 5 2 1. 5 0 10 20 30 40 50 60 Average lesion volume 70 80

threshold

What would be really nice: • Use DTI fibre tracking data to get a measure of probability of connection between each point in WM and each point on the cortical surface • Huge fibre tracking problem: – Seeds at every voxel! – Repeat over subjects? – How to combine subjects? • Plot correlation vs. probability of connection should be a positive relationship …

Dinner: meet in Hilton Coffee Shop, 6 p. m.