Disappearing iliac vessels: An MR phase cancellation phenomenon

Magnetic resonance images often fail to distinguish the iliac vessels on one side. Phantom studies and theoretical analysis demonstrated that this phenomenon arises from a loss of the flow void effect. At a certain angle, the additional phase accumulation that results from motion in the section-selection direction exactly cancels the additional phase accumulation from motion in the frequency-encoding direction. Thus, vessels that cut through the imaging plane near this critical angle have substantial signal intensity. This artifact can be eliminated by alternating the direction of the section-selection gradient on sequential phase encodings.