Hi Zhong,
> Question 1: If the bvals are not the same across subjects, would it be okay to compare their DTI data (probabilistic tractography results)?
Not in general. As you can see below, the choice of b value can influence in many ways orientation estimation and this differences in accuracy and precision will be reflected in probabilistic tractography results.
By the way, a b value of 300 is too low for diffusion-weighting.
> Question 2: In the past I've only dealt with b=1000 s/mm2 for each direction. What are the advantages of having multiple b values?
There is a trade-off on choosing b values. Low b values have a little diffusion contrast (i.e. what we are interested in when doing diffusion MRI), but relatively good SNR. High b values have lots of diffusion contrast (for instance you can more easily differentiate complex fibre geometries), but low SNR. A scheme with multiple b values deals better with this trade-off.
Hope this helps
Stam
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