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.

In MRI systems the radio frequency (RF) transmitter produces the RF current (oscillator) and delivers it to the transmitting coil (antenna). The RF signal produced by the transmitter is used to excite the protons in the imaging volume.

The Mx and My data (digital MR signal) sampled and stored during data acquisition as a function of time and phase from an MRI sequence are called raw data. The horizontal axis is the frequency code, the vertical axis the phase code (of the protons).
Every point in the raw data matrix contains part of the information for the complete image. A point in the raw data matrix does not correspond to a point in the image matrix. The outer rows of the raw data matrix provide information regarding the borders and contours of the image, detailed structures, and also determine the resolution. This is also called k-space data.

Reciprocals of the relaxation times, T1 and T2 (R1 = 1/T1 and R2 = 1/T2). There is often a linear relation between the concentration of MR contrast agents and the resulting change in relaxation rate. The rate of relaxation is influenced by molecules with protons that are tumbling. A slower tumble rate will result in faster relaxation rate (shorter relaxation time). Due to the molecular structure of fat with its larger size than water, fat will tumble slower than water molecules. The slower tumble rate of fat enables a faster relaxation rate.

The ability of magnetic compounds to increase the relaxation rates of the surrounding water proton spins.
Relaxivity is used to improve the contrast of the image, and to study tissue specific areas where the contrast agent better diffuses or to perform functional magnetic resonance imaging.
The relaxivity of MRI contrast agents depends on the molecular structure and kinetic of the complex. To increase the number of water molecules that are in the inner sphere of the complex, or to slow down the molecular rotational correlation time, are possibilities to improve the water relaxivity.
Relaxivity units (r1, r2) are mM-1 * sec-1 (at varying temperatures).