Hydrogen nuclei magnetic moments are randomly oriented in the absence of an external magnetic field and are considered to have a net magnetization of zero. Once hydrogen protons are placed in the presence of an external magnetic field, they align themselves in one of two directions, parallel or anti parallel to the net magnetic field, which is commonly referred to as the vector B0. The parallel and anti parallel protons cancel each other out, only the small number of low energy protons left aligned with the magnetic field create the overall net magnetization, this difference is all that counts. The magnetic moments of these protons are added together and are referred to as net magnetization vector (NMV) or the symbol 'M'.

See also Magnetization Transfer Contrast.

(Mz) The component of the net magnetization vector in the direction of the static magnetic field (z). After RF excitation, this vector returns to its equilibrium value at a rate characterized by the time constant T1.

In physics, the magnitude (length) of a vector is a scalar in the physical sense, i.e. a physical quantity independent of the coordinate system, expressed as the product of a numerical value and a physical unit, not just a number.

See also Longitudinal Magnetization and Net Magnetization Vector.

Angle between the net magnetization vector before and after a RF excitation pulse. Small tip angles allow a decrease in TR, which is used to decrease scan time in Field Echo pulse sequences. See Flip Angle.

The xy component of the net magnetization vector at right angles to the main magnetic field. The precession of the transverse magnetization at the Larmor frequency is responsible for the detectable MRI signal. In the absence of externally applied RF energy, the transverse magnetization will decay to zero with a characteristic time constant of T2, or more strictly T2*.