In the triglyceride molecules of fat, the electronic environment (shield) of the protons is not the same as that of water molecules.
The 2 kinds of molecules have different proton resonance frequencies, measured by chemical shift (3.5 ppm).
This difference in proton resonance frequency in lipid molecules can be used to suppress their signal. To do so, a selective RF excitation wave is applied with a narrow bandwidth.
The selective 90° RF wave will flip the magnetization of the fat protons without affecting the proton magnetization of other tissues. Gradients then destroy the magnetization in the transverse plane and the imaging sequence begins just after that, to avoid leaving the fat magnetization time to regrow. The fat signal will thus be weaker in relation to the other tissues.
This method requires good magnetic field homogeneity (figure 7.4) namely in the case of a large field of view (so the lipid protons resonate at the same frequency throughout the explored volume) and a selective pulse frequency bandwidth that is well adapted (to saturate the signal of all the lipid protons without affecting the soft tissue signal).
It can be used before virtually all types of sequence.
This technique can be combined with inversion-recovery: SPIR (Philips) / SPECIAL (GE) combine a fat-selective 100-120° RF inversion pulse, followed by dephasing gradients and an adapted TI.