Hi! Could anyone tell me that it matters if the multiple b-values are in the same or different directions, in diffusion imaging with multiple b-values (such as NODDI，DKI)?
In general, to fit tensors (or other ODFs) to diffusion data you need to sample b-weighted images that have different directions. The default Siemens diffusion directions are all half-sphere, but tools like Eddy can leverage full-sphere data to provide a better spatial undistortion estimate. There are online tools that help for generating b-vectors. You typically want to acquire multiple b=0 images if you are interested in MD/ADC as this forms the denominator for these estimates. As b-value increases the sensitivity to spatial orientation increases, but the signal to noise decreases.
I strongly suggest you consider the advice from the FSL team and Frank Yeh, the creator of dsi-studio.
Thanks for your guidance and advice, could you tell me if the same direction has effiect under different b value conditions? For example 1) b=1000 and b=2000 have the 30 same directions (but different b-value ); 2) b=1000 and b=2000 have the 30 different directions.
Solution (2) is a much better approach.
To understand why, I recommend this article and references therein (in particular 21, 24, 25):
Caruyer, E.; Lenglet, C.; Sapiro, G.; Deriche, R. Design of Multishell Sampling Schemes with Uniform Coverage in Diffusion MRI: Design of Multishell Sampling Schemes. Magnetic Resonance in Medicine 2013, 69 (6), 1534–1540. https://doi.org/10.1002/mrm.24736.
And a link to a tool helping you creating your own gradient directions (from the author of the above paper):
Great! Thanks for your guidance and advice.