Hi Elvira,
Any differences in FD or FDC will be difficult to interpret: unlike with high b-value data, it is not only the intra-axonal space that will have contributed to your DWI signal, and hence the FOD amplitudes will be confounded by e.g. diffusivity / tortuosity of the extra-cellular space. You will also struggle to resolve some crossing fibre regions, both in individual subjects and in the FOD template. However this does not totally preclude you from performing FBA using your data, and indeed you may still be able to achieve some fibre specificity & sensitivity; it’s primarily the capability to interpret any changes observed that will be lacking.
Apart from the capability to resolve crossing fibres in the definition of fixels, the FC metric should not be influenced by the lower b-value: It is a morphological metric based on registration to the template, and hence the specific sensitivity to intra-axonal space is less relevant.
Rob