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.

The exchange of energy at a particular frequency between two systems; a large amplitude vibration in a mechanical or electrical system caused by a relatively small periodic stimulus with a frequency at or close to a natural frequency of the system.
Resonance is referred to as the property of an atom to absorb energy only at the Larmor frequency. The energy must also be delivered at 90° to the net magnetic vector (NMV) and main magnetic field (B0). Otherwise, no energy will be absorbed, resonance will not have occurred and an image cannot be created. In MRI systems, resonance can refer to the MR itself or to the tuning of the RF circuitry.

A rotation matrix is a matrix corresponding to a rotation of a vector around a certain axis. Used for example to describe the spins of nuclei.

The T1 relaxation time (also called spin lattice or longitudinal relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types. This tissue-specific time constant for protons, is a measure of the time taken to realign with the external magnetic field. The T1 constant will indicate how quickly the spinning nuclei will emit their absorbed RF into the surrounding tissue.
As the high-energy nuclei relax and realign, they emit energy which is recorded to provide information about their environment. The realignment with the magnetic field is termed longitudinal relaxation and the time in milliseconds required for a certain percentage of the tissue nuclei to realign is termed 'Time 1' or T1. Starting from zero magnetization in the z direction, the z magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1. This is the basic of T1 weighted images.
The T1 time is a contrast determining tissue parameter. Due to the slow molecular motion of fat nuclei, longitudinal relaxation occurs rather rapidly and longitudinal magnetization is regained quickly. The net magnetic vector realigns with B0 leading to a short T1 time for fat.
Water is not as efficient as fat in T1 recovery due to the high mobility of the water molecules. Water nuclei do not give up their energy to the lattice (surrounding tissue) as quickly as fat, and therefore take longer to regain longitudinal magnetization, resulting in a long T1 time.

See also T1 Weighted Image, T1 Relaxation, T2 Weighted Image, and Magnetic Resonance Imaging MRI.