My pleasure :)
>This is phase contrast tomography, so you can get the refractive index of the materials inside, and their speed.
This is way outside of my domain of competence, but if I understand correctly, this would not constitute a BOLD signal?
I'm skeptical of the idea that blood diffusion alone correlates with cerebral activity. With fMRI, you're looking at temporary gradients of de-oxygenated hemoglobin rather than a net direction of blood movement. This is sometimes confused with "blood flow", and I've been guilty of interchanging the two terms myself :/ I suspect the fluid dynamics of blood in the head is outrageously noisy and difficult to contrast out or filter though statistical means (because it's roughly random).
Concerning the relationship between neural activity and BOLD signal this paper [https://www.nature.com/articles/35084005] is a good starting point. I'll see if I can dig up the PDF for you.