In all classic and angiographic imaging sequences, aside from phase contrast MRA, phase is designed to relate to spatial information only (determined by phase encoding).
A bipolar gradient, whose positive and negative lobes are of equal importance, will have no effect on the phase of stationary spins. On the other hand, if a spin is moving on the gradient axis, its gradient effect will be subject to its position. Consequently, moving spins will be poorly located and a motion artifact will be seen in vascular structures and moving fluids.
To correct this artifact, a third lobe is added to the slice selection and readout gradients. The gradient lobe surface ratio is 1: -2: 1, which will not induce dephasing in the stationary spins and avoids dephasing in flows at constant velocity .
To compensate the dephasing of flows at variable velocity (non-null acceleration), gradient lobes again have to be added on, resulting in longer TE and a reduced signal.
Flow compensation gradients can be used in any sequence requiring the suppression of these artifacts, except for phase contrast MRA.
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