Connectivity between MS lesion
density and cortical thickness
Keith Worsley
Arnaud Charil
Jason Lerch
Department of Mathematics and Statistics,
McConnell Brain Imaging Centre,
Montreal Neurological Institute,
McGill 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(voxel1, voxel2)
> cor(voxel1, neighbours of voxel2),
> cor(neighbours of voxel1, voxel2),
MS lesions and cortical thickness
(Arnaud et al., 2004)
•
•
•
•
N = 425 mild MS patients
Lesion density, smoothed 10mm
Cortical thickness, smoothed 20mm
Hypothesis: increased lesion density in white
matter -> thinner cortex in connected grey matter
• Find connectivity i.e. find voxels in 3D, nodes in
2D with high
cor(lesion density, cortical thickness)
n=425 subjects, correlation = -0.56826
Average cortical thickness
5.5
5
4.5
4
3.5
3
2.5
2
1.5
0
10
20
30
40
50
60
Average lesion volume
70
80
Same hemisphere
0.1
1
-0.3
threshold
-0.4
-0.5
0
50
100
150
distance (mm)
Correlation = 0.091943
0.1
correlation
0
correlation
1.5
-0.2
-0.1
-0.2
1.5
-0.3
1
0
-0.5
1
0.1
-0.2
0.4
threshold
2
-0.4
0.6
-0.3
-0.1
0.5
threshold
0
50
100
150
distance (mm)
Correlation = -0.1257
0.5
0
1
0
0.8
5
x 10
2.5
0
2
-0.1
Different hemisphere
0.1
correlation
correlation
0
5
x 10
2.5
0.8
-0.1
0.6
-0.2
0.4
-0.3
threshold
-0.4
0.2
-0.4
0.2
-0.5
0
-0.5
0
0
50
100
150
distance (mm)
0
50
100
150
distance (mm)
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.