Price R R
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN 37232-2675.
Radiographics. 1995 Jan;15(1):165-78; quiz 149-50. doi: 10.1148/radiographics.15.1.7899595.
The strengths of spin-echo sequences in clinical magnetic resonance imaging are its relative insensitivity to magnetic field inhomogeneities and its versatility; however, they require relatively long imaging times. Use of gradient-echo sequences largely circumvents the problem. A gradient echo is created by means of a gradient reversal following a single radio-frequency (RF) pulse, typically with a flip angle that is less than 90 degrees, compared with a spin-echo sequence, which requires at least two RF pulses, typically a 90 degree excitation pulse followed by a 180 degree refocusing pulse. The shortened repetition times (TRs) in gradient-echo sequences together with small flip angles enable maintaining adequate image signal-to-noise ratios while still achieving a short imaging time. Numerous gradient-echo sequences have been developed, with the most common being (a) fast low angle shot (FLASH), which uses small flip angles and short TRs; (b) gradient-recalled acquisition in the steady state and fast imaging with steady-state precession (GRASS/FISP); (c) magnetization-prepared rapid gradient echo (MP-RAGE); and (d) echo planar. The shorter imaging times made possible through gradient-echo sequences are used clinically to minimize patient motion artifacts and to allow isotropic three-dimensional imaging with acceptable imaging times.
自旋回波序列在临床磁共振成像中的优势在于其对磁场不均匀性相对不敏感且具有通用性；然而，它们需要相对较长的成像时间。梯度回波序列的使用在很大程度上规避了这个问题。与自旋回波序列（至少需要两个射频脉冲，通常是一个90度激发脉冲后跟一个180度重聚焦脉冲）相比，梯度回波是通过在单个射频（RF）脉冲后进行梯度反转产生的，通常翻转角小于90度。梯度回波序列中缩短的重复时间（TR）以及小翻转角能够在保持足够图像信噪比的同时实现短成像时间。已经开发了许多梯度回波序列，最常见的有：（a）快速低角度激发（FLASH），它使用小翻转角和短TR；（b）稳态梯度回波采集和稳态进动快速成像（GRASS/FISP）；（c）磁化准备快速梯度回波（MP-RAGE）；以及（d）回波平面成像。通过梯度回波序列实现的较短成像时间在临床上用于最小化患者运动伪影，并允许在可接受的成像时间内进行各向同性三维成像。
