Figure 1: Illustration of the effect of a 90° pulse that is either on resonance or off resonance. In the on-resonance condition, the magnetic field making up the 90° pulse (shown by a black arrow) rotates with the same frequency as the magnetization. In the off-resonance condition, the magnetization rotates faster than the 90° pulse. This results in a slight tilt of the magnetization, but far from the 90° flip in the on-resonance condition.