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.

In MR, saturation is a nonequilibrium state with no net magnetization. The same amount of nuclear spins is aligned against and with the magnetic field. Saturation methods like FatSat, SPIR etc., work with a frequency selective saturation pulse for a specific chemical shift applied before the actual sequence starts. This saturation pulse adjusts the magnetization from tissue components to zero. The hydrogen nuclei of fat and water resonate at different frequencies, which makes it possible to excite just the fat with repeatedly applying RF pulses at the Larmor frequency with interpulse times compared to T1. The resulting signal is then destroyed with a gradient pulse (Spoiler Gradient Pulse). Fat is the chemical compound to be saturated at a fat saturation sequence. When the actual sequence follows, (e.g., a spin echo sequence) the unwanted suppressed component will not resonate.

See also Saturation Recovery.