The frequency at which the resonance phenomenon occurs. The resonancefrequency 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 resonancefrequency 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 resonancefrequency at 1.5 T for 31P is 25.85 MHz, for 1H, 63.86 MHz.