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Dear FSL users:
 
I am a question about the range of calculated FA value using dtifit: should it be less than 1 determinately? By the definition of FA, I think the answer is yes. and when I use dtifit on example data (fdt_subj1), I got very reasonable results, with FA ranging from 0 to 0.770387. But when I calculated FA maps using dtifit on our data (256*256*45, 13 directions, eddy current corrected), FA maps were ranging from 0 to 1.224745. The answer maps looked like reasonable, however, with high FA values where they should be (e.g., corpus callosum and CST). I tested L1, L2 and L3 value (using fslstats) at the same time. Their range were as follow:
L1: -0.002313 0.004803
L2: -0.002548 0.004386
L3: -0.002707 0.004128
I have a question why negative value came out? when I compute FA maps using another program (DTIstudio) on the same data, the FA is from 0 to 1 with positive L1, L2 and L3.
In the mannul of TBSS, it said that: " If you created your FA data using dtifit in FDT then the data will indeed be roughly in the range 0:1. " I am wondering why FA were bigger than 1 if it is correct. Were there something wrong in my progress?
 
Thank you very much!