See Pulse 90°.

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.

A pulse is a rapid change in the amplitude of a RF signal or in some characteristic a RF signal, e.g., phase or frequency, from a baseline value to a higher or lower value, followed by a rapid return to the baseline value. For radio frequencies near the Larmor frequency, it will result in rotation of the macroscopic magnetization vector. The amount of rotation will depend on the strength and duration of the RF pulse; commonly used examples are 90° (p/2) and 180° (p) pulses.
RF pulses are used in the spin preparation phase of a pulse sequence, which prepare the spin system for the ensuing measurements. In many sequences, RF pulses are also applied to the volumes outside the one to be measured. This is the case when spatial presaturation techniques are used to suppress artifacts. Many preparation pulses are required in MR spectroscopy to suppress signal from unwanted spins. The simplest preparation pulse making use of spectroscopic properties is a fat saturation pulse, which specifically irradiates the patient at the fat resonant frequency, so that the magnetization coming from fat protons is tilted into the xy-plane where it is subsequently destroyed by a strong dephasing gradient.
The frequency spectrum of RF pulses is critical as it determines the spatial extension and homogeneity over which the spin magnetization is influenced while a gradient field is applied.