Feinberg D A, Johnson G, Kiefer B
Department of Radiology, New York University Medical Center, New York 10016, USA.
Magn Reson Med. 1995 Aug;34(2):149-55. doi: 10.1002/mrm.1910340204.
GRASE (GRadient and spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is with T2 and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity in k space but instead creates periodicity in the T2 modulation function that can also cause artifacts. In addition, with this order, the effective TE must be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due to T2 decay, permits greater control of T2 contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency in k space (hence "k banding"). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this method.
梯度与自旋回波（GRASE）是一种将梯度和射频重聚焦相结合的回波链成像技术。尽管总体信号衰减遵循T2规律且场不均匀性相位误差不会累积，但微小的残余相位误差随回波数呈周期性变化。前文所述的回波顺序消除了k空间中的相位误差周期性，但却在T2调制函数中产生了周期性，这也可能导致伪影。此外，采用这种顺序时，有效回波时间（TE）必须是回波链时间的一半，而且非对称傅里叶采样难以实现。本文描述了一种新方法，该方法可大幅减少因T2衰减产生的伪影，能更好地控制T2对比度，并且适用于非对称傅里叶采样。回波链的不同时间段在k空间中用不同的空间频率带进行编码（因此称为“k带化”）。计算机模拟和实验结果均表明，采用该方法采集的GRASE图像有所改善。
