How are MRAs and MRVs created?

Several methods may be used, including time of flight (TOF), phase contrast, and gadolinium techniques.
In the most commonly used technique, TOF, blood flows into the selected slice between the time that the radio wave pulse is applied (i.e., when the slice is excited) and when the echo is returned.
The blood flowing in has all of its magnetization and gives a high signal (the entry slice phenomenon), producing a clear image of the vessel.