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Magnetic resonance angiography (MRA) and Flow MRI

Flow compensation

Автор Denis Hoa

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.