Flow compensation is based on the principle of even echo rephasing and a function of specific pulse sequences, wherein the application of strategic gradient pulses can compensate for the objectionable spin phase effects of flow motion. Gradient moment nulling of the first order of flow is another adjustment for the reduction of flow artifacts.
Gradient field changes can be configured in such a way that during an echo the magnetization signal vectors for all pixels have zero phase angle independent of velocities, accelerations etc. of the measured tissue. The simplest velocity-compensated pulse sequence is the symmetrical second echo of a spin echo pulse sequence.
Strategic gradient pulses are integrated in special sequences (e.g. CRISP, Complex Rephasing Integrated with Surface Probes) and for the most sequences flow compensation is an optional parameter.

Phenomenological (classical) equations of motion for the macroscopic magnetization vector. They include the effects of precession about the magnetic field (static and RF) and the T1 and T2 relaxation times.

RF pulse designed to rotate the macroscopic magnetization vector 180° in space as referred to the rotating frame of reference, usually about an axis at right angles to the main magnetic field. If the spins are initially aligned with the magnetic field, this pulse will produce inversion. Also called P Pulse.

RF pulse designed to rotate the macroscopic magnetization vector 90° in space as referred to the rotating frame of reference, usually about an axis at right angles to the main magnetic field. If the spins are initially aligned with the magnetic field, this pulse will produce transverse magnetization and a FID. Also called P/2 Pulse.

See Macroscopic Magnetization Vector.