A constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field.
Definition: The ratio of the magnetic moment (field strength = T) to the angular momentum (frequency = ν) of a particle.
The gyromagnetic effect happens if a magnetic substance is subjected to a magnetic field. Upon a change in direction of the magnetic field, the magnetization of the substance must change. In order for this to happen, the atoms must change their angular momentum. Since there are no external torques acting on the system, the total angular momentum must remain constant. This mass rotation may be measured. The gyromagnetic ratio is different for each nucleus of different atoms. The value of the gyromagnetic ratio for hydrogen (1H) is 4,258 (Hz/G) (42.58 MHz/T).

(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

The uniformity of the main magnetic field measured in ppm. In a defined volume it is the difference between maximum and minimum field strength and this multiplied by 1 million.
In MR, the homogeneity of the static magnetic field is an important criterion of the quality of the magnet. Homogeneity requirements for MR imaging are generally lower than the homogeneity requirements for NMR spectroscopy, but for most imaging techniques must be maintained over a large region.

(J) The SI unit of work or energy.
Definition: The work done by a force of 1 newton acting to move an object through a distance of 1 meter in the direction in which the force is applied.
Since kinetic energy is one half the mass times the square of the velocity, 1 joule is the kinetic energy of a mass of two kilograms moving at a velocity of 1 m/sec.
The joule is named for the British physicist James P. Joule.

(K) The SI unit of temperature.
Definition: One Kelvin is 1/273.16 of the difference between the triple point of water (at exactly 273.16 K) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states. Absolute zero is the temperature at which all molecular motion discontinues.
0 K is according to -273,15° Celsius and -459° Fahrenheit.

(kg) The base SI unit of mass of the metric system.
Definition: 1 kilogram is defined as the mass of the standard kilogram, a platinum-iridium bar in the custody of the International Bureau of Weights and Measures (BIPM) near Paris, France.
A traditional unit of mass or weight is also the pound (in general use, e.g. in the United States and Great Britain), with the symbol lb (derived from the Latin word libra).
1 kg is 2.204627 pound (lb. av.)
1 pound (lb. av.) is 0.453 kg.
Smaller units are, e.g.
1 000 gram (g) are 1 kg
1 000 milligram (mg) are 1 g
1 000 microgram (μg) are 1 mg

Magnetic field gradients are used to change the strength of the magnetic field Bo in a certain direction. Gradients are used in MR imaging with selective excitation to select a region for imaging and also to be able to encode the location of MR signals received from the object being imaged. The field strength is measured in Tesla per meter (T/m).