Dear SPM (VBM) users!
We have scanned a large number of patients and healthy control subjects on the same scanner over a long period of time using the same camera settings. During this time a software filter in the camera has been updated two times, the second time the camera was also moved to a different location. There are no visible differences betweeen images acquired using the different filters but I suspect that there may be slight differences in intensity distribution. The images are used for VBM analyses, segmented using DARTEL in VBM8. When checking images using "Sample Homogeneity" images from different filters does not group and are scattered "randomly" in the boxplot.
I believe I probably still have to correct for "Filter setting" in my analyses so my question is how do I do this?
Is it possible to use a categorical variable (Filter1, 2, 3) as a nuisance variable in regression analyses?
If comparing, for example, Patients vs. Controls, should I use a 3x2 ANOVA? Then my problem is that no healthy controls has been scanned using filter 1.
Any other ideas? Any other ways to check if I need to correct for filter setting in my analyses?
I would be grateful for all the help I can get!
/ Carl Johan Ekman, MD
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