Longitudinal magnetism and transverse magnetism are components of the net magnetism vector.
Longitudinal magnetism
Longitudinal magnetisation is the component of the net magnetisation vector parallel to the magnetic field (z-axis). This is due to a difference in the number of spins in parallel (low energy) and anti-parallel (high energy) state – i.e. before a radiofrequency pulse.
Transverse magnetisation
Transverse magnetisation is the component of the net magnetisation vector perpendicular to the magnetic field (x-y plane). This is due to the spins of individual protons getting more or less into phase (coherence) – i.e. after a radiofrequency pulse.
Transverse vs longitudinal
During excitation, the longitudinal magnetisation decreases. After a 90-degree flip angle, the populations of the two spins states are identical. Therefore the longitudinal magnetisation is null. Transverse magnetisation appears due to phase coherence, except for a 180-degree flip angle.
It is worth noting that it is only the net magnetisation vector that lies in the transverse plane, not the magnetic moments or the nuclei themselves. The magnetic moments of the nuclei only move in alignment with or against magnetic field strength (B0) – i.e. low or high energy state.
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