Phase effects concern the transverse component of the magnetization. They occur whenever spins are moving in the presence of magnetic field gradients, as are applied for spatial encoding of the MR signal.
Magnetic field gradients provoke a change in the Larmor frequency depending on gradient strength and spin position. A gradient pulse of certain length and amplitude therefore induces a phase shift of the transverse magnetization, which can be compensated by a second gradient pulse with identical strength and duration but opposite sign. Thus, for stationary spins the net phase shift is zero. In contrast, the same gradients applied on a flowing spin generate a non-zero phase shift. Since the spins change their position during the bipolar gradient application, the second gradient pulse is no longer able to completely compensate for the phase shifts induced by the first gradient. The remaining phase shift ® is proportional to the velocity component v of the spins along the gradient direction (Fig. 7).
On standard MR imaging, this flow-induced phase shift causes a spatial misencoding of the sig-
nal leading to ghost artifacts that are typically found in the phase-encoding direction.
Spins in a blood vessel are moving with different velocities. Often, a parabolic flow profile is found (Fig. 8). Spins moving faster experience a larger phase shift than those moving more slowly. If there is a velocity distribution inside a voxel, phase dispersion (intra-voxel dephasing) occurs resulting in decreased signal in the blood vessel (Fig. 9). The extent of spin-dephasing depends on the strength and time interval of the gradient pulses, as well as the distribution of spin velocities. When complex flow patterns are encountered, for example in vessels with turbulent flow, there may be a very broad spectrum of velocities within a voxel, leading to total signal loss in the vessel (Fig. 9 c).
Using additional gradient pulses of appropriate amplitude and duration, flow-induced phase shifts can be compensated, thus eliminating any signal loss. This technique is called "gradient motion rephasing (GMR)" or just "flow compensation" . However, GMR is normally restricted to first-order movements, i.e. spins that move at a constant velocity. Turbulent flow and effects of acceleration cannot be completely compensated by GMR.
Optimal reduction of flow-induced phase effects can be achieved by combining GMR with as short a TE as possible, in order to reduce the time available for spin dephasing. Short echo times also diminish the impact of pulsatile blood flow and turbulence.
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