Figure 23. Water excitation. A 121 (1, 2, 1 at top of image) composite pulse is shown with a total flip angle of 90°. Prior to the first RF pulse (1 at bottom of image), both water (solid arrows) and fat (dashed arrows) hydrogen are unexcited. At the end of the first RF pulse (2 at bottom of image), both are excited 22.5°. Because of the difference in resonant frequencies between fat and water, the fat hydrogen becomes out of phase with the water hydrogen. The time for the second RF pulse (3) is chosen so that the fat hydrogen is exactly 180° out of phase. At the end of the second RF pulse (4), the water proton is rotated 67.5° while the fat hydrogen is rotated -22.5°. A similar delay is chosen between the second and third RF pulses (5). At the end of the third RF pulse (6), the fat hydrogen is at 0° (unexcited), while the water hydrogen is rotated 90°.