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Dear Reza, > We are processing DTI data in our LAB in University of california , > San Diego with fsl software version 4.1.3 .Currently we are using > fslstats utility of your program. I have noticed a paradox in the > output of the program and hereby want to notify you about it : > > I have used the following command on the FA map of the DTI of a > healthy subject processed by fsl : > > fslstats 1_FAfsl.nii.gz -m -M -s -S -r -R > > The FA values are between 0 and 1 . the results of the program are : > > > 0.050226 0.212998 0.120878 0.165211 0.000000 0.487615 > 0.000000 1.195134 > > so in the output the program calculated the range for non-zero > voxels between zero and 1.195 which is not correct at all (0<FA > value <1) FA values > 1 is a consequence of a negative eigenvalue of the tensor. It is true that this is something that is "physically" impossible, but in practice something that is frequently observed with noisy data. Consider a voxel in an area with little/no signal. The true eigenvalues should be [0 0 0] (i.e. we have no signal and no diffusion). However, the estimated eigenvalues will be associated with some uncertainty e, so you'll really observe [0ħe 0ħe 0ħe], and hence some of those values will be negative leading to FA>1. > -in the second stage we used the internal capsule , and purely > internal capsule as the ROI and the results are as follows : > > -fslstats IC_FA.nii.gz -m -M -s -S -r -R > > 0.000010 0.607764 0.002601 0.251947 0.565753 0.791101 > 0.000000 0.793482 > > So according to the program results the range of data f is 0.56 - > 0.79 , the mean for non-zero is 0.607 , however the SD for non-zero > is 0.2519 .It looks completely unacceptable . 0.56 -- 0.79 is the "robust" range, i.e. ignoring the tails of the observed distribution of values. The full range is 0 -- 0.79, the next two values. And then an s.d. of 0.25 looks more plausible. Good Luck Jesper