The spins (protons) go out of phase in the transverse plane when the Rf pulse is switched of. This dephasing is due to static magnetic field inhomogeneities intrinsic (i.e. susceptibility changes at tissue interfaces) and extrinsic (i.e. field inhomogeneities of the main magnetic field) to the examined object.

See also T2*.

Magnetic field gradient pulse used to create spatial variation of phase of transverse magnetization. For example, it may be applied prior to signal detection in the presence of a magnetic field gradient with opposite polarity (or of the same polarity if separated by a refocusing RF pulse) so that the resulting gradient echo signal will represent a more complete sampling of the Fourier transformation of the desired image.

See also Spoiler Gradient Pulse.

The gradient recalled echo MRI sequence generates gradient echoes as a consequence of echo refocusing. The initial slice selective RF pulse applied to the tissue is less than 90° (typically rotation angles are between 10° and 90°). Immediately after this RF pulse, the spins begin to dephase.
Instead of a refocusing 180° RF pulse, reversing the gradient polarity produces a gradient echo. A negative phase encoding gradient and a dephasing frequency encoding gradient are used simultaneous. The switch on of the frequency encoding gradient produces an echo caused by refocusing of the dephasing, which is caused by the dephasing gradient.
TR and flip angle together control the T1, and TE control T2* weighting.

Diffusion weighted imaging can be performed similar to the phase contrast angiography sequence. The gradients must be increased in amplitude to depict the much slower motions of molecular diffusion in the body.
While a T1 weighted MRI pulse sequence is diffusion sensitive, a quantitative diffusion pulse sequence was introduced by Steijskal and Tanner. Its characteristic features are two strong symmetrical gradient lobes placed on either side of the 180° refocusing pulse in a spin echo sequence. These symmetrical gradient lobes have the sole purpose of enhancing dephasing of spins, thereby accelerating intravoxel incoherent motion (IVIM) signal loss.
Dephasing is proportional to the square of the time (diffusion time) during which the gradients are switched on and the strength of the applied gradient field. Therefore, the use of high field gradient systems with faster and more sensitive sequences, make diffusion weighting more feasible.
Areas in which the protons diffuse rapidly (swollen cells in early stroke, less restriction to diffusion) will show an increased signal when the echo is measured relative to areas in which diffusion is restricted. For increased accuracy of diffusion measurement and image enhancement, useful motion correction techniques such as navigator echo and other methods should be used. In addition to this, applying the b-value calculated by the strength and duration of motion probing gradients with a high rate of accuracy is very important.

See also Apparent Diffusion Coefficient, ADC Map, Lattice Index Map.

Quick Overview
Please note that there are different common names for this artifact.

Flow effects in MRI produce a range of artifacts, e.g. intravascular signal void by time of flight effects; turbulent dephasing and first echo dephasing, caused by flowing blood.
Through movement of the hydrogen nuclei (e.g. blood flow), there is a location change between the time these nuclei experience a radio frequency pulse and the time the emitted signal is received (because the repetition time is asynchronous with the pulsatile flow).
The blood flow occasionally produces intravascular high signal intensities due to flow related enhancement, even echo rephasing and diastolic pseudogating. The pulsatile laminar flow within vessels often produces a complex multilayered band that usually propagates outside the head in the phase encoded direction. Blood flow artifacts should be considered as a special subgroup of motion artifacts.

Artifacts can be reduced by reduction of phase shifts with flow compensation (gradient moment nulling), suppression of the blood signal with saturation pulses parallel to the slices, synchronization of the imaging sequence with the heart cycle (cardiac triggering) or can be flipped 90° by swapping the phase//frequency encoding directions.

See also Flow Related Enhancement and Flow Effects.