(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of for example ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
Usual frequencies in medical imaging are e.g., for ultrasound in the range of 1-20 MHz, for x-rays above 3x10¹⁶ Hz and for gamma rays less than 10²⁰ Hz.

(gf) The grid frequency is the number of lead strips per inch or centimeter. An increased grid frequency involves thinner stripes. The most used grid frequency is 85 -103 lines per inch.

(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.

(EMR) Electromagnetic radiation consists of an electric and a magnetic field component. All EMR travels in a vacuum at the speed of light. EMR is classified related to the frequency//length of the wave.
An EM wave consists of discrete packets of energy, named photons (quantization). The energy of the photons depends on the frequency of the wave. Planck-Einstein equation:
E = h * f
E (energy); h (Planck's constant); f (frequency)
EMR types include in order of increasing frequency//decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. EMR contains energy and momentum, which may be imparted when it interacts with matter.

See Gamma Radiation.

A photon is a discrete packet of electromagnetic energy. The amount of energy depends on the frequency (wavelength) of the photon. Highest frequency, most energetic photon radiations are gamma rays, up to 300 EHz - 1.24 MeV. In addition to energy, photons are also carrying momentum.
Photons have no electrical charge or rest mass and exhibit both particle and wave behavior.
Photons are traveling in vacuum (without interactions with matter) with the constant velocity of 2.9979 x 10⁸ m/s (c, speed of light).
Photons get absorbed or scattered away from their original direction of travel when interacting with matter.
High energy photons as for example x-rays cause damages to exposed tissue and cells. Radiation exposure is measured in roentgen, radiation absorption in Roentgen//min.
Photon radiation in the frequency ranges of x-rays and gamma rays are used for medical diagnostic and treatment.

See also Photon Energy and Gamma Ray.