Gradient-echo imaging was introduced in 1985 by Frahm and Haase as a way to speed image acquisition.1–5 The gradient echo pulse sequence is simpler than the spin-echo sequence and can be performed more rapidly, enabling a variety of tasks, including real-time MRI, flow imaging3,4, and 3D or volume imaging.5 There are two primary differences between spin-echo and gradient echo imaging. In spinecho imaging, a 90° pulse is used on each repetition of the pulse sequence to flip all longitudinal magnetization into the transverse plane. In gradient-echo imaging, a smaller flip angle is used to leave some longitudinal magnetization undisturbed.6,7 For example, if a 30° flip angle is used (Figure 5.1), then half of the longitudinal magnetization (M0sin30°) is flipped into the transverse plane where it can contribute to measurable signal. On the other hand, 87% of the longitudinal magnetization (M0cos30°) remains along the direction of B0, so little time is needed to let the longitudinal magnetization recover along B0 before beginning the next repetition of the pulse sequence in gradient-echo imaging. This means that TR, the time between successive repetitions of the pulse sequence, can be made much shorter, which reduces total imaging time.
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