Flow
Flowing blood (or CSF) can have either lower or higher signal than stationary blood (or CSF) due to two basic phenomena:
high velocity signal loss
Artifactually lower signal loss is called “high velocity signal loss”. In order for tissue to give signal, it must be exposed to the 90 degree pulse followed by the 180 degree pulse. If blood is flowing rapidly and only experiences the 90 or the 180 degree pulse, no signal will be generated. This is the basis of “black blood” MRA (not commonly used.)
flow related enhancement
Artifactually higher signal is typically from “flow related enhancement”. The imaging volume is partially saturated after multiple 90-180 cycles. When unsaturated blood flows into the imaging volume, it has more signal than the tissues in the imaging volume. This is the basis of “white blood” MRA (our typical protocols.) This explains some of the artifacts you see when unsaturated blood flows into the imaging volume.
There is additional explanation with diagrams at:
http://www.revisemri.com/questions/pulse_sequences/black_blood_bright_blood
Q: What is the differential of abnormal signal in a vessel?
A: Occlusion or slow flow.
fellows and interested residents should read:
Whittemore AR, Bradley WG, Jinkins JR. Comparison of cocurrent and countercurrent flow-related enhancement in MR imaging. Radiology 1989:170:265-271.
http://radiology.rsna.org/content/170/1/265.abstract
for the interested student, a classic Scientific American article on countercurrent systems
https://www.scientificamerican.com/article/countercurrent-systems-in-animals/