Hi Anderson,
I have very delayed follow-up questions.
I was a bit confused by the example to compute a difference image here (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Randomise_details-11) because it's subtracting the second image from the first image, but then I thought maybe what really matters is how you set up your contrast matrix...
That being said, I just wanted make sure I have my logic correct when finding the difference image and evaluating change in disease groups versus controls. If my hypothesis is "FA decreases faster in the disease group than the control group" and if I subtract my first time point images from my second time point images (using the command: fslmaths FA_time_2 -sub FA_time_1 FA_diff), then my contrast should be "1 -1 0 0…" where the first column is controls and the second column is the disease group. If my FA at time point 2 is indeed lower than time point 1 and the disease group has a faster decreasing FA, then my difference for the disease group will be more negative than the difference for controls and a "1 -1 0 0…" contrast would test for that. And if I'm testing to see if a DTI scalar is increasing faster in the disease group than controls, then I'd reverse my contrast. Am I thinking about this correctly?
Thank you,
Joy
|