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.

(RE MRI) There are several approaches to speeding up the MRI data acquisition process by repeating the excitation by RF pulses in times short compared to T1, typically using small flip angles and gradient echo refocusing. When TR is also on the order of or shorter than T2, the repeated RF pulses will tend to refocus transverse magnetization remaining from prior excitations, setting up a condition of steady state free precession, and a dependence of signal strength (and image contrast) on both T1 and T2.
This can be modified in various ways, particularly:
1) to spoil the tendency to build up a steady state by reducing coherence between excitations, e.g. by variation of the phase or timing of consecutive RF pulses or of the strength of spoiler gradient pulses, thus increasing the relative dependence of signal strength on T1 or
2) acquire the signal when it is refocusing immediately prior to the next RF pulse, thus increasing the relative dependence of signal strength on T2.

See also Ultrafast Gradient Echo Sequence.

A rephasing or refocusing occurs when spins return to the same starting phase they had directly after the initial exciting RF pulse. E.g. spin echo sequences use 180° pulses to refocus the spins to generate signal echoes and gradient echo sequences use a refocusing gradient to maximize remaining transverse magnetization.

Magnetic field gradient pulse applied to reverse the spatial variation of phase of transverse magnetization caused by a dephasing gradient. For example, in selective excitation, it is a magnetic field gradient applied for a brief period after a selective excitation pulse, in the opposite direction to the gradient used for the selective excitation. The result of the gradient reversal is a rephasing of the spins (which will have gotten out of phase with each other along the direction of the selection gradient), forming a gradient echo and improving the sensitivity of imaging after the selective excitation process.

(T2) The return of the transverse magnetization to its equilibrium value, zero.