Porter D A, Calamante F, Gadian D G, Connelly A
Radiology and Physics Unit, Institute of Child Health, University College London Medical School, London, United Kingdom.
Magn Reson Med. 1999 Aug;42(2):385-92. doi: 10.1002/(sici)1522-2594(199908)42:2<385::aid-mrm21>3.0.co;2-j.
Single-shot diffusion-weighted echo-planar imaging (EPI) is typically used for most clinical diffusion studies due to its low sensitivity to patient motion. Although the Nyquist ghost artifact in EPI can be substantially reduced, there is frequently a residual ghost with low signal intensity. As reported in this study, this residual ghost can produce severe artifacts when maps of the apparent diffusion coefficient (ADC) are calculated from single-shot echo-planar images. The artifacts presented in this paper appear as regions of apparently low ADC which simulate regions of reduced diffusion, but are in fact generated by b-value dependent Nyquist ghosts of the orbits. Data acquired in vivo were used to demonstrate that these artifacts can be avoided by including standard methods of spatial presaturation or fluid-suppression in the diffusion-weighted EPI protocol. In addition, phantom studies were used to illustrate how phase and amplitude variations in the ghost generate the artifacts and theoretical expressions, derived elsewhere, were used to provide a detailed understanding of the artifacts observed in vivo. The level of Nyquist ghost reported for the current generation of commercial scanners suggests that this is a general phenomenon which should be a consideration in all EPI-based diffusion studies. Magn Reson Med 42:385-392, 1999.
单次激发扩散加权回波平面成像（EPI）由于对患者运动的低敏感性，通常用于大多数临床扩散研究。尽管EPI中的奈奎斯特鬼影伪影可以大幅减少，但通常仍会存在低信号强度的残余鬼影。如本研究报道，当从单次激发回波平面图像计算表观扩散系数（ADC）图时，这种残余鬼影会产生严重的伪影。本文中呈现的伪影表现为表观ADC明显较低的区域，这些区域模拟了扩散降低的区域，但实际上是由眼眶的b值依赖性奈奎斯特鬼影产生的。利用体内采集的数据证明，通过在扩散加权EPI协议中纳入空间预饱和或液体抑制的标准方法，可以避免这些伪影。此外，通过体模研究来说明鬼影中的相位和幅度变化如何产生伪影，并使用在其他地方推导的理论表达式来详细理解体内观察到的伪影。当前一代商业扫描仪报告的奈奎斯特鬼影水平表明，这是一种普遍现象，在所有基于EPI的扩散研究中都应予以考虑。《磁共振医学》42:385 - 392, 1999年。
