Hinks R S, Xiang Q S, Henkelman R M
GE Medical Systems, Waukesha, WI 53188.
J Magn Reson Imaging. 1993 Sep-Oct;3(5):777-85. doi: 10.1002/jmri.1880030514.
Motion artifacts are a dominant cause of magnetic resonance image quality degradation. Periodic or nearly periodic motion results in image replicates of the moving structures in spin-warp Fourier imaging. The replicates, or ghosts, propagate in the image in the phase-encoding, or y, direction. These ghosted images can be considered to consist of the time-averaged spin density I0 and a ghost mask g. A set of j ghosted images Ij may be acquired in which the ghost mask is intentionally phase shifted by varying amounts relative to I0 with interleaved acquisitions that have shifted phase-encoding orders or by acquiring multiple images during a single readout period in the presence of an oscillating phase-encoding gradient. The resulting complex images Ij have the same time-averaged spin density I0 but have ghost contributions gj that, on a pixel-by-pixel basis, trace part of a circle around I0. The source images Ij can then be used to estimate I0. Simulations and experiments with the phase-encoding gradient modulation method show good general ghost suppression for a variety of quasi-periodic motion sources including both respiratory-type artifacts and flow artifacts. The primary limitation of the method is the need for rapid gradient switching.
运动伪像是磁共振图像质量下降的主要原因。周期性或近乎周期性的运动会在自旋扭曲傅里叶成像中导致运动结构的图像复制。这些复制图像，即重影，在图像中沿相位编码方向（或y方向）传播。这些带有重影的图像可被视为由时间平均自旋密度I0和一个重影掩码g组成。可以采集一组j个带有重影的图像Ij，其中通过具有移位相位编码顺序的交错采集，或者在存在振荡相位编码梯度的情况下在单个读出周期内采集多个图像，使重影掩码相对于I0有意地进行不同量的相位偏移。所得的复数图像Ij具有相同的时间平均自旋密度I0，但具有重影贡献gj，在逐个像素的基础上，gj围绕I0描绘出部分圆周。然后可以使用源图像Ij来估计I0。使用相位编码梯度调制方法进行的模拟和实验表明，对于包括呼吸型伪像和流动伪像在内的各种准周期性运动源，该方法具有良好的总体重影抑制效果。该方法的主要局限性在于需要快速的梯度切换。
