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.
KeywordsTransverse Magnetization Longitudinal Magnetization Total Imaging Time Speed Image Acquisition Small Flip Angle
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