Figure 19

Use of low flip angles with gradient echo pulse sequences. Gradient echo sequences can use a variable (low) flip angle for the excitation pulse which allows much shorter TR values to be used without losing too much signal. When a 90° rf pulse is used (top row), the short TR allows very little recovery between rf pulses. The z-magnetisation quickly reduces, resulting in a low signal amplitude when it is transferred into the xy plane. The use of a low flip angle (in this case, 30°, bottom row), allows the z-magnetisation to remain much closer to its equilibrium value. This, when transferred into the xy plane, results in a much larger signal in comparison.