A state of a set of spins in which the ensemble of spins in a voxel are uniformly distributed with phases between 0 and 2p reducing the transverse magnetization in a voxel to essentially zero.

(GRE - sequence) A gradient echo is generated by using a pair of bipolar gradient pulses. In the pulse sequence timing diagram, the basic gradient echo sequence is illustrated. There is no refocusing 180° pulse and the data are sampled during a gradient echo, which is achieved by dephasing the spins with a negatively pulsed gradient before they are rephased by an opposite gradient with opposite polarity to generate the echo.
See also the Pulse Sequence Timing Diagram. There you will find a description of the components.
The excitation pulse is termed the alpha pulse α. It tilts the magnetization by a flip angle α, which is typically between 0° and 90°. With a small flip angle there is a reduction in the value of transverse magnetization that will affect subsequent RF pulses. The flip angle can also be slowly increased during data acquisition (variable flip angle: tilt optimized nonsaturation excitation). The data are not acquired in a steady state, where z-magnetization recovery and destruction by ad-pulses are balanced. However, the z-magnetization is used up by tilting a little more of the remaining z-magnetization into the xy-plane for each acquired imaging line.
Gradient echo imaging is typically accomplished by examining the FID, whereas the read gradient is turned on for localization of the signal in the readout direction. T2* is the characteristic decay time constant associated with the FID. The contrast and signal generated by a gradient echo depend on the size of the longitudinal magnetization and the flip angle. When α = 90° the sequence is identical to the so-called partial saturation or saturation recovery pulse sequence. In standard GRE imaging, this basic pulse sequence is repeated as many times as image lines have to be acquired. Additional gradients or radio frequency pulses are introduced with the aim to spoil to refocus the xy-magnetization at the moment when the spin system is subject to the next α pulse.
As a result of the short repetition time, the z-magnetization cannot fully recover and after a few initial α pulses there is an equilibrium established between z-magnetization recovery and z-magnetization reduction due to the α pulses.
Gradient echoes have a lower SAR, are more sensitive to field inhomogeneities and have a reduced crosstalk, so that a small or no slice gap can be used. In or out of phase imaging depending on the selected TE (and field strength of the magnet) is possible. As the flip angle is decreased, T1 weighting can be maintained by reducing the TR. T2* weighting can be minimized by keeping the TE as short as possible, but pure T2 weighting is not possible. By using a reduced flip angle, some of the magnetization value remains longitudinal (less time needed to achieve full recovery) and for a certain T1 and TR, there exist one flip angle that will give the most signal, known as the "Ernst angle".
Contrast values:
PD weighted: Small flip angle (no T1), long TR (no T1) and short TE (no T2*)
T1 weighted: Large flip angle (70°), short TR (less than 50ms) and short TE
T2* weighted: Small flip angle, some longer TR (100 ms) and long TE (20 ms)

Classification of GRE sequences can be made into four categories:

• T1 weighted or incoherent/(RF or gradient) spoiled GRE sequences
• T1/T2* weighted or coherent/refocused GRE sequences
• T2 weighted contrast enhanced GRE sequences
• ultrafast GRE sequences

See also Gradient Recalled Echo Sequence, Spoiled Gradient Echo Sequence, Refocused Gradient Echo Sequence, Ultrafast Gradient Echo Sequence.

A gradient echo is generated by using a pair of bipolar gradient pulses. The gradient field is negatively pulsed, causing the spins of the xy-magnetization to dephase. A second gradient pulse is applied with the opposite polarity. During the pulsing, the spins that dephased begin to rephase and generate a gradient echo.
Spoiling can be accomplished by RF or a gradient. The incoherent RF spoiled type of a gradient echo sequence use a continuous shifting of the RF pulse to spoil the residual transverse magnetization. The phase of the RF excitation and receiver channel are varied pseudo randomly with each excitation cycle to prevent the xy magnetization from achieving steady state. T2* does not dominate image contrast, so T1 and PD weighting is practical. This method is effective and can be used to achieve a shorter TR, due to a lack of additional gradients. Spoiling eliminates the effect of the remaining xy-magnetization and leads to steady state longitudinal magnetization. These sequences can be used for breath hold, dynamic imaging and in cine and volume acquisitions.

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.

Pulse sequences, designed to be insensitive to flow, e.g. at every even echo, a spin echo sequence is not flow sensitive. Velocity compensation is achieved by using gradients, which are either symmetrical around a 180° pulse and switched on twice as is the case for motion compensated spin echo pulse sequences, or two antisymmetrical gradient lobes without 180° pulse, which is the way to produce a velocity compensated gradient echo pulse sequence.
The signal of the second echo (and all other even echoes) is independent of the velocity of the object. Thus, velocity-based motion effects stemming from the entire voxel or from spins within a voxel (intravoxel incoherent motion) are suppressed with such pulse sequences.
If higher order motion is relevant, as it may be in turbulent jets across valves, acceleration and jerk effects can also be compensated for by the use of appropriate combinations of gradient- and radio frequency pulses.
With the increasingly stronger gradients, echo times in MR systems can be shortened to the point at which effects other than velocity effects hardly ever become relevant.