See Larmor Frequency.

Time duration of a pulse. For an RF pulse near the Larmor frequency, the longer the pulse length, the greater the angle of rotation of the macroscopic magnetization vector will be (greater than 180° can bring it back toward its original orientation). For an RF pulse of a given shape as a function of time, the longer the pulse length, the narrower the equivalent range of frequencies in the pulse will be.

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.

The frequency at which the resonance phenomenon occurs. The resonance frequency is given by the Larmor equation for MRI and is determined by the inductance and capacitance for RF circuits. An atom will only absorb external energy if that energy is delivered at precisely it's resonant frequency.
The Larmor equation states that the resonance frequency of a magnetic nucleus (the radio frequency needed to excite a nucleus to the higher spin rate) is directly proportional to the magnetic environment it experiences. Atoms such as hydrogen-1 (1H) and phosporous-31 (31P) resonate at different Larmor radio frequencies because of differences in the magnetic properties of their nuclei. The resonance frequency at 1.5 T for 31P is 25.85 MHz, for 1H, 63.86 MHz.

See also Larmor Frequency.