【Handbook of MRI Pulse Sequences】14.1.2 梯度波形与梯度回波脉冲序列名称

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Book： 《Handbook of MRI Pulse Sequences》 原著：MATT A. BERNSTEIN, KEVIN F. KING, XIAOHONG JOE ZHOU, 译注：蒋强盛 14.1.2 G RADIENT W AVEFORMS AND GRE P ULSE S EQUENCE N AMES 14.1.2 梯度波形与梯度回波脉冲序列名称 As discussed, there are a variety of SSFP signals that can result from the application of a string of excitation pulses: SSFP-FID, SSFP-echo, and balanced SSFP. The desired signal is selected by applying the proper gradient waveform. This is somewhat analogous to how crusher gradients are used to select the desired signal pathway, as discussed in Section 10.2. Also, the gradient waveform is necessary to rephase the FID into the desired GRE. 

在前面已经讨论过，经受一系列激励脉冲后，会产生各种各样的 SSFP 信号：SSFP-FID，SSFP-echo，和 bSSFP。通过施加合适的梯度波形来采集想要采集的信号。这跟 10.2 节所讲的使用损毁梯度来选择想要的信号有点类似。此外，梯度波形在重聚 FID 得到想要的梯度回波信号中也是必须的。 【译者注1】

Gradient waveforms for GRE pulse sequences can be divided into three main groups: forward, reversed, and balanced. Table 14.2 lists which gradient waveforms are used with various GRE pulse sequences. Within each group, further divisions can be made, for example, whether or not gradient moment nulling or partial echo is used. 

梯度回波脉冲序列中所使用的梯度波形可以主要分为三组：位于前面的、位于后面的和均衡的。表 14.2 列出了各类梯度回波脉冲序列中所使用的梯度波形。在每一个分组内，还可以进一步分类，例如，是否使用梯度矩归零或部分回波。

【译者注2】

Figure 14.1 shows a forward gradient waveform with partial-echo sampling. This waveform is used to rephase a GRE from an SSFP-FID or a spoiled signal. The end of sequence spoiler pulse dephases any residual SSFP-echo signal. The spoiler pulse is shown on the slice axis, although it could be applied on any one, two, or all three of the logical axes. The pulse sequence can be extended to rephase two gradient echoes, as shown in Figure 14.10. Alternatively, the pulse sequence can be time-reversed as shown in Figure 14.11, in order to form an image from the SSFP-echo signal. The spoiler pulse at the beginning of the pulse sequence eliminates the SSFP-FID signal.

图 14.1 显示的是梯度波形位于前面的梯度回波序列，并联合部分回波采样。这一梯度波形用于重聚 SSFP-FID 或 Spoiled GRE 得到一梯度回波信号。在 TR 间期未尾施加扰相梯度将任何残留的 SSFP-echo 信号散相掉。图中所示的扰相梯度脉冲施加在选层方向，然而它可以施加在任一个、任两个轴方向，或三个轴方向都施加 。这一脉冲序列可以做一个延伸，重聚两次梯度回波，如图 14.10 所示。或者，这一脉冲序列还可以作时间反转，如图 14.11 所示，为的是采集 SSFP-echo 信号。而序列每个 TR 间期之初的扰相梯度脉冲是为了消除 SSFP-FID 信号。

【译者注3】

Figure 14.12 shows a typical gradient waveform used to acquire a fullecho, balanced SSFP signal. This same gradient waveform can be used regardless of whether the RF pulses are sign alternated or not. The key feature is the gradient area on any of the logical axes is zero when integrated over one TR interval. Any slight deviation from this requirement causes the SSFP-FID and SSFP-echo signals to separate from one another in the readout window and interfere, leading to banding artifacts in the image. Figure 14.13 shows how the gradient waveform is modified to acquire a partial-echo balanced SSFP signal. The readout prephasing lobe is reduced in area by the same amount as the readout lobe while the rephasing lobe is left unchanged, so that the total area on the frequency-encoding gradient waveform remains zero. 

图 14.2 所示的是一典型的采集完整的 bSSFP 回波的梯度波形。不管射频脉冲的极性是否正负交替，都可以使用这一相同的梯度波形。关键一点就是，任一逻辑轴在 TR 间期内梯度面积的积分都为零。任何轻微的偏差都将导致 SSFP-FID 与 SSPF-echo 信号在信号读出时分离开来，并且相干扰，最终在图像上产生黑带伪影。图 14.13 所示的是将梯度波形修改下，采集半回波的 bSSFP 信号。 减少读出预散相梯度叶的面积，同时读出梯度叶也减少相同的面积，另外保持信号读完后的聚相梯度叶保持不变，因此频率编码梯度波形的总面积仍保持为零。

Sometimes the readout gradient is extended so that the SSFP-echo and SSFP-FID are intentionally separated enough not to interfere with one another and are collected as two distinct GREs (Bruder et al. 1988). This is called dual-echo steady state (DESS) free precession and representative gradient waveforms are shown in Figure 14.14. The readout gradient between the two echoes serves as a spoiler so that they do not interfere with one another. The two GREs have different contrast weightings, which can be seen from Eq. (14.15), and more easily from Eq. (14.21). They can be viewed as separate images or else combined with postprocessing, such as summing the two magnitude images. Note the differences between DESS and dual echo GRE (Figure 14.10), which rephases the same FID twice. 

有时延长读出梯度，有意地让 SSFP-echo 与 SSFP-FID 信号充分地分离，这样就不会相互干扰，并且采集到两个独立的梯度回波（Bruder et al. 1988）。这一脉冲序列称作双回波稳态（DESS）自由进动，典型的梯度波形如图 14.14 所示。两回波之间的读出梯度充当一扰相梯度，因此两回波之间不会相互干扰。这两个梯度回波拥有不同的对比度权重，从方程（14.15）就能看出，更简单地，从方程（14.21）也能看出。它们可以看成是两个独立的图像，或者通过后处理把两者联合起来，比如将两者的模图进行相加。注意 DESS 与 Dual-echo GRE（图 14.10）的区别，Dual-echo GRE 是将 FID 信号重聚两次。

Various equipment manufacturers traditionally have given different names to equivalent or nearly equivalent pulse sequences (Elster 1993), which has complicated the study of GRE. Table 14.1 cross-references some of the commonly used names for GRE pulse sequences against the genetic names that we have used.

各个磁共振设备厂商按惯例给同等或近似同等的脉冲序列起了不同的名字（Elster 1993），这就增加了学习梯度回波的复杂性。表 14.1 按照我们所使用的通用名称给出了一些梯度回波序列常用的一些名称之间的相互参考。

【译者注4】

Holden H. Wu, Ph.D.   Department of Radiological Sciences  David Geffen School of Medicine at UCLA