Duerk J L, Simonetti O P
Department of Radiology, MetroHealth Medical Center, Cleveland, OH 44109.
J Magn Reson Imaging. 1991 Nov-Dec;1(6):643-50. doi: 10.1002/jmri.1880010605.
Magnetic resonance (MR) imaging can be performed on or below the time scale of most anatomic motion via echo-planar imaging (EPI) techniques and their derivatives. The goal is to image rapidly and reduce artifacts that typically result from view-to-view changes in the spatial distribution of spins due to motion. However, the required time-dependent magnetic field gradient waveforms remain sensitive to the dephasing effects of motion. Sources of motion artifact are simulated for spins moving along the imaging axes and are shown to be an important source of reduced image quality in EPI. A novel method of EPI is proposed that (a) refocuses single or multiple derivatives of motion at all echoes and (b) prevents accumulation of velocity (or higher derivative)--induced dephasing along the phase-encoding axis by moment nulling all phase-encoding-step waveforms about a single instant of time. Theoretical EPI sequences with considerable reductions in ghosts, blurring, and signal loss due to motion sensitivity are produced and compared with other EPI methods. Their time efficiency is presented as a function of available (relative) gradient strength for a variety of sequence waveforms.
通过回波平面成像（EPI）技术及其衍生技术，可以在大多数解剖运动的时间尺度上或其以下进行磁共振（MR）成像。目的是快速成像并减少由于运动导致自旋空间分布在视图间变化而通常产生的伪影。然而，所需的随时间变化的磁场梯度波形对运动的去相位效应仍然敏感。对沿成像轴移动的自旋的运动伪影源进行了模拟，结果表明它是EPI中图像质量下降的一个重要来源。提出了一种新颖的EPI方法，该方法（a）在所有回波处对运动的单个或多个导数进行重新聚焦，并且（b）通过在单个时刻对所有相位编码步长波形进行矩归零，防止速度（或更高阶导数）引起的去相位沿相位编码轴累积。生成了由于运动敏感性导致重影、模糊和信号损失显著减少的理论EPI序列，并与其他EPI方法进行了比较。给出了它们的时间效率与各种序列波形的可用（相对）梯度强度的函数关系。
