Figure 7

This diagram shows how a SPAMM pulse can produce a magnetisation pattern consisting of a series of parallel lines that are alternately fully magnetised and fully saturated, appearing as bright and dark lines (shown on the right). In this example, the composite rf pulse is a 1-2-1 binomial pulse, consisting of three rf pulses with relative amplitudes in the ratio of 1:2:1. As the effective flip angle of the composite pulse is set to be 90°, the flip angles of the three individual rf pulses is therefore 22.5°, 45° and 22.5° respectively. Starting at equilibrium (a), the first rf pulse causes all spins to flip through 22.5° (b). The first modulating gradient is then applied causing the spins to move out of phase along the gradient until there is a 180° phase shift between points that correspond to the desired spacing between adjacent bright and dark lines. The second 45° rf pulse is then applied, causing the magnetisation that is 180° out of phase to be flipped from −22.5° to +22.5°, while magnetisation that is in phase is flipped from +22.5° to 67.5° (d). The second modulating gradient introduces a further 180° phase shift between adjacent bright and dark tag lines (e). The third 22.5° rf pulse is then applied. This causes the magnetisation that is 180° out of phase to be flipped from −22.5° to 0 (aligned along the z-axis), while magnetisation that is in phase is flipped from 67.5° to 90° to become saturated (f).