Figure 2.

Fat suppression techniques. Dashed arrow = water, solid arrow = fat. (a) Diagram shows the mechanism of CHESS fat suppression, in which fat is selectively excited by an RF pulse, and a gradient pulse is applied to disperse the magnetization in the transverse plane. (b) Diagram shows the mechanism of coronal STIR fat suppression, in which the fat signal is inverted with a non-selective 180° pulse, and acquisition is begun after the inversion time (TI) that nulls the fat signal, resulting in a water signal with reduced magnitude. (c) Diagram shows the mechanism of spectral adiabatic inversion-recovery (SPAIR) fat suppression, in which the fat signal is inverted with an adiabatic spectrally selective pulse, and acquisition is begun after the inversion time that nulls the fat signal. (d) Diagram shows the mechanism of water excitation, in which an excitation pulse with an FA that equals one-half the desired FA is applied, and a delay is introduced to allow the water and fat magnetization to fall out of phase. At this point, a second excitation pulse is applied, which brings the fat magnetization back to the longitudinal axis and finishes flipping the water magnetization onto the transverse plane. (e) Diagram shows the mechanism of the Dixon technique, in which both water and fat are excited, and the first echo (TE1) is acquired when the two magnetization vectors in the transverse plane are out of phase, creating destructive interference. The second echo (TE2) is acquired when the two vectors are in phase, creating constructive addition.