Hi Salil,
The trace image is not used much quantitatively because it confounds T2 effects with diffusion effects ("T2 shine-through"). That being said, it can still be very useful for picking out early ischemia with restricted diffusion.
As far as I know, it is not a direct output from dtifit or other FSL tools. However, you can compute it yourself pretty easily. The trace image is just the geometric mean (not arithmetic mean) of the individual DWI-weighted direction images (without the b0 image included).
trace = (bimg_1 * bimg_2 * bimg_3 * ... * bimg_n) ^ (1/n)
Because there are a lot of images, you will likely run into floating point numerical issues if you try to calculate it like that, though. However, you can also calculate it as the exponential of the average of the logarithms of the individual images. You can do this using fslmaths as follows:
First, put together all your non-b0 images. Assuming the b0s are at the beginning, you can do:
> fslroi dwi_series dwi_nonb0 5 -1
If they aren't at the beginning, you'll have to take them out more manually and splice things back together.
Then take the logarithm:
> fslmaths dwi_nonb0 -log dwi_nonb0_log
Then average:
> fslmaths dwi_non_b0_log -Tmean dwi_non_b0_log_mean
Then exponentiate:
> fslmaths dwi_non_b0_log_mean -exp trace
You can do the last three steps in one line if you like:
fslmaths dwi_nonb0 -log -Tmean -exp trace
Hope this helps!
Best,
Mike