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.