Net magnetization vector

The net magnetization vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei.

In the absence of an external magnetic field, the individual magnetic moments are randomly oriented and since they are in opposition, the net magnetization vector is considered to be zero.

If hydrogen nuclei are placed within a strong external magnetic field, they become aligned within the field in one of two directions parallel to the direction of the field (see: energy difference between spin up and spin down states). 

In MRI, the main magnetic field is termed B₀.

• aligned in the direction of B₀ (parallel)
• aligned in the opposite direction of B₀ (antiparallel)

A parallel and antiparallel hydrogen nuclei have equal but opposite magnetic moments and cancel each other out. However, there are always slightly more hydrogen nuclei parallel to B₀ and this slight difference is termed the NMV (net magnetization vector) and given the symbol M.