Animation 8.4 (Gradient Moment Nulling)

This animation supports Figure 8.43. It shows how the gradient configuration used in gradient moment nulling or flow compensation corrects for phase dispersal of flowing spins. The first lobe of the gradient causes the magnetic moment of the flowing spin to phase shift clockwise by 90°. The second lobe of the gradient is switched on to twice the amplitude of the first lobe, but with opposite polarity. As the second lobe has twice the amplitude of the first, it causes the magnetic moment of the spin to alter its phase by 180°. However, as the second lobe of the gradient has an opposite polarity to the first, the magnetic moment of the spin is phase shifted 180° anticlockwise rather than clockwise. The third lobe of the gradient is applied to the same polarity and amplitude as the first lobe. This, therefore, causes the magnetic moment of the spin to phase shift 90° in a clockwise direction, so that it ends up with the same phase position as before it started its travel through the gradients. In this way flowing spins, that would normally produce a signal void due to phase dispersal of their magnetic moments, maintain their phase coherency. This strategy, therefore, reduces flow artefact.