Production of spin echo by repeated RF pulses. First observed using equal (90°) RF pulses, now commonly used to describe refocusing of transverse magnetization by a 180° RF pulse. By choosing long echo delay times, the spins in a Hahn echo first dephase for a long time, then rephase, which makes the Hahn pulse sequence more susceptible to diffusion effects.

Use of a magnetic field gradient to effectively eliminate residual transverse magnetization by producing a strong position dependence of phase within a resolution element. See Spoiler Gradient Pulse.

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.

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.

An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Proton density contrast is a quantitative summary of the number of protons per unit tissue. The higher the number of protons in a given unit of tissue, the greater the transverse component of magnetization, and the brighter the signal on the proton density contrast image. Conversely the lower the number of protons in a given unit of tissue, the less the transverse magnetization and the darker the signal on the proton density image. Also called (Rho) ρ-weighted.

See also Density Weighted Imaging and Image Contrast Characteristics.