Selective excitation

By applying an adapted combination of selective RF waves, rather than a single RF pulse, it is possible to flip proton magnetization of the water only (or conversely of the fat). The combination of RF wave amplitudes (flip angles) is binomial (1:1, 1:2:1, 1:3:3:1) (figure 7.6 and 7.7), with delays between each excitation corresponding to water and fat being out of phase. More complex pulse combinations give a better separation between water and fat, but to the detriment of global excitation time.

This technique is sensitive to B0 magnetic field heterogeneities, and shimming improves its performances.

It has the advantage of being faster than fat saturation or inversion-recovery.

 

 

Selective excitation of fat

The selective excitation technique can also be used to suppress the water signal, with for example, a combination of pulses 1:-1 (+45° then -45°) that only flips fat magnetization in the transverse plane.