(Hz) The standard SI unit of frequency.
Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz.
Larger units are
kilohertz (kHz) = 1 000 Hz
megahertz (MHz) = 1 000 kHz
gigahertz (GHz) = 1 000 MHz

(RF) Radio frequency refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.
The RF pulses used in MRI are commonly in the 1-100 megahertz range, and their effect upon a body is potential heating of tissues and foreign bodies, such as metallic implants, mainly at the surface.
This is a principal area of concern for MRI safety limits caused by absorption (see specific absorption rate) of the applied pulses of RF energy.

The higher the frequency, the larger will be the amount of heat developed.
The more ionic the biochemical environment in the tissue, the more energy that will be deposited as heat.
This effect is well known for homogeneous model systems, but the complex structure of various human tissues makes detailed theoretical calculations very difficult, if not impossible. By scanning problems, it is important to verify the transmission frequency. If the RF transmitted into the patient was, e.g. 5000 Hz lower than the resonance frequency of the protons, no protons was excited, and no signal returns.