Hi Ryan,
FreeSurfers Longitudinal pipeline assumes head size is relatively fixed. The problem with growing heads is that the surfaces from the subject-template will not fit well to later (or earlier) time points and that the algorithm can potentially not recover. If surfaces look OK it means it worked. I have no idea how large the time delta is for it to fail but I guess if you scan a 4 year old and then again when they are 12 that could be problematic.
You should use eTIV from the cross stream. In the long stream eTiv is fixed across time.
Best, Martin
On 9. Jul 2018, at 20:51, Nillo, Ryan Michael R RyanMichael.Nillo@ucsf.edu wrote:
External Email - Use CautionHello FreeSurfer experts,
I have a few questions on the longitudinal processing stream. I am interested in studying longitudinal morphometry changes in a pediatric population. The age range of this population is between 4-20 years with age deltas between a few months to < 2 years. I want to know what are appropriate/acceptable age deltas used for longitudinal processing. I looked through the archives and one researcher asked about age deltas. One of the answers was that “if the time delta is relatively small, this has good chances to work”. What does “relatively small” mean? If the time points exit the longitudinal pipeline without errors and the surfaces look correct, does this mean the data is relevant?? When I correct for head size, should I use the eTIV values from the cross-sectional directories or the longitudinal directories?
Thanks in advance,
Ryan Michael Nillo Staff Research Associate I University of California San Francisco Department of Radiology and Biomedical Imaging
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