Moving spins have a significant effect upon the received MRI signal, which is seen, depending upon the pulse sequence utilized, as a modulation in signal intensity and/or phase relative to that of stationary spins. This can be used in MRI to distinguish between blood vessels and stationary anatomic structures. Monoplanar time-of-flight techniques [1-9] use pulse sequences which modulate the signal intensity from spins in a vessel flowing perpendicularly through a slice. This modulation is the result of the fact that the signal is composed of signal strengths from different spin populations whose relative ratio will depend upon the thickness of the slice, the velocity of fluid perpendicular to the slice, and pulse sequence parameters. If two different pulse sequences are used with identical signals from stationary anatomy, one producing increased signal in the region of flowing spins and the other producing decreased signal, then a subtraction image can be formed depicting only regions of flow . Here we develop expressions for the signal intensity of two pulse sequences which are expected to give optimum contrast for imaging flowing blood free of overlying non-vascular anatomy. This imaging technique would provide a means for imaging blood vessels without the use of ionizing radiation or contrast injection; furthermore, it provides information about the presence of flow in vessels. As such, it may be promising as a method to evaluate vessels which are not accessible to standard angiographic imaging.