14.1.3 梯度回波序列的一些实际考量

一切权利归原作者所有。 仅供学习交流使用，严禁用作商业用途。 Book： 《Handbook of MRI Pulse Sequences》 原著：MATT A. BERNSTEIN, KEVIN F. KING, XIAOHONG JOE ZHOU, 译注：蒋强盛 14.1.3 P RACTICAL C ONSIDERATIONS 14.1.3 梯度回波序列的一些实际考量

    GRE Pulse Sequence Selection    Because there are so many different GRE pulse sequences, it is not always clear which one to select for a particular application. If T1 weighting is desired, then spoiled sequences are used. Spoiled GRE also has the greatest flexibility in the acquisition mode because sequential 2D, interleaved 2D, and 3D can all be used. The steady-state pulse sequences, however, require a short TR, so only the 3D or sequential 2D acquisition modes are practical. It is important to keep in mind that the signal formula for spoiled GRE (Eq. 14.8) is different from the signal formula for conventional RF spin echo (Section 14.3), so quite different contrast behavior can be observed. These differences have been particularly noted in gadolinium-enhanced studies of the brain, as described in Mugler and Brookeman (1993).

    梯度回波脉冲序列的选择    由于有许多不同的梯度回波脉冲序列，对于某个特定的应用场景选择哪一种并不总是明确的。如果需要 T1 权重，那么可以使用扰相梯度回波序列。扰相梯度回波序列的采集模式也十分灵活，因为序贯 2D 采集、交叉 2D 采集、和 3D 采集都可以应用。然而稳态脉冲序列需要短 TR，所以只有 3D 采集或序贯 2D 采集模式是可行的。需要记住的是，扰相梯度回波序列与经典自旋回波序列的信号强度公式是不同的，因此它们的对比度十分不同，这一点很重要。这些差异在颅脑钆对比剂增强研究文献 Mugler and Brookeman（1993）中也被特别指出。

%%%%%%%%%%%%%%%

【译者注1】

 As TR decreases for fast GRE acquisition, the Ernst angle (Eq. 14.9) for spoiled GRE also decreases. Attempts to obtain T1 weighting by increasing the flip angle generally result in a poor SNR when TR is short (e.g., TR < 10 ms). Instead, the preferred strategy to obtain T1 weighting with a short TR is to use magnetization preparation (e.g., inversion) followed by a series of GRE pulse sequences, as in MP-RAGE (Section 14.2). With MP-RAGE, the short TR becomes an advantage because the k-space lines are acquired closer in time, so there is less blurring introduced by the signal modulation of the inversion recovery curve.

扰相梯度回波序列的 TR 降低时，恩斯特角（方程 14.9）也减小。如果想要通过增加翻转角来获得 T1 权重，那么当 TR 很短时（比如，TR < 10ms）通常会导致信噪比很差。使用短 TR 来获得 T1 权重更好的代替方案是使用磁化准备（比如，反转脉冲），然后使用一系列梯度回波脉冲序列读出，比如 MP-RAGE（14.2 节）。对于 MP-RAGE，短 TR 反而有优势，因为 k 空间数据采集时在时间上靠得很近，受反转恢复曲线调制的信号变化而导致的模糊比较少。

【译者注2】

exp(-x) 是单调递减函数，arccos(x) 在 (0, pi/2) 内也是单调递减函数。同增异减，那么 arccos(exp(-x)) 在 exp(-x)∈(0, pi/2) 内是单调递增函数。那么当 TR 减小时，恩斯特角也减小。

Balanced SSFP with high flip angle provides the greatest SNR per unit time, so it is the method of choice if bright fluid signal is desired. Its two main drawbacks are banding artifacts in the region of susceptibility change, and a bright fat signal. To suppress the signal from lipids, the method in Scheffler et al. (2003) can be used. If banding artifacts are a problem, then multiacquisition SSFP (i.e., CISS) is a good choice unless patient motion causes mis-registration artifacts. In that case, SSFP-FID is a robust alternative, although its acquisition efficiency per unit time is lower because the SSFP-echo signal is intentionally dephased.

在平衡稳态自由进动序列中，使用大的翻转角时，单位时间内能够获得最大的信噪比。因此，如果想要液体表现为高亮信号则可以使用此序列。平衡稳态自由进动序列有两个缺点：在磁化率变化比较大的地方会出现黑带伪影、脂肪信号比较高。为了抑制脂肪信号，可以利用 Scheffler et al.（2003）提供的方法。如果有黑带伪影，可以使用多次采集 SSFP 序列（即 CISS）进行抑制，当然，不能去除病人运动造成的错配伪影。在这种情况下，SSFP-FID 可以作为一个很好的替代，尽管单位时间内 SSFP-FID 的采集效率比较低，因为需要额外施加梯度对 SSFP-echo 信号进行扰相。

【译者注3】

 With the multiple GRE sequences, DESS has found application to joint imaging because it can provide a good depiction of cartilage (Hardy et al. 1996). Dual-echo GRE is often used in a 2D mode with interleaved slice locations. The in-phase and out-of-phase images are particularly useful for body imaging because of the property described next.

在多回波梯度回波序列中，DESS 可以应用于关节成像，因为它对软骨有很好的显示效果（Hardy et al. 1996）。双回波梯度回波序列经常使用交叉多层 2D 采集模式。它的同相位与反相位图像特别适用于体部成像，因为它具有下面将会描述的特性。

 Chemical Shift Artifact of the Second Kind and Dual-Echo GRE    The standard chemical shift artifact manifests itself as a spatial shift between fat and water in the frequency-encoded direction. It occurs both in RF spin-echo and in GRE images. If, however, fat and water are present in the same voxel, then at certain TE values GRE images display a second kind of chemical shift artifact. (That artifact is not seen on RF spin-echo images, although a similar artifact can occur when STIR is used; see Section 14.2.) This has been called the phase cancellation artifact, the phase elimination artifact, the black boundary artifact, or the chemical shift artifact of the second kind.

第二类化学位移伪影与双回波梯度回波序列    标准的化学位移伪影表现为在频率编码方向上脂肪与水在空间位置上有位移。在射频自旋回波序列与梯度回波序列中都存在。然而，如果在同一体素中既有脂质子又有水质子，那么在梯度回波序列的某些 TE 值会表现出第二类化学位移伪影。（这一伪影在射频自旋回波序列中看不到，然而在 IR 序列图像中可能会出现类似的伪影，见 14.2 节）这一伪影被称为相位抵消伪影，相位消除伪影，黑边伪影，或第二类化学位移伪影。

【译者注4】

这里提到在 RF spin-echo 中也有与 GRE 序列中类似的第二类化学伪影，应该不是在 STIR 序列中（STIR 是设置脂肪过零点采集），而应该是在普通的 IR 序列中，多数为做 T1W 权重时，在模图 M 图中会现，而在 PSIR 图中不会出现。

关于这个之前已经讨论过多次： 【磁共振成像序列研究】T1W_IR    [The Basics of MRI 8.7] 磁共振成像基本原理 8.7 它是由于在 M 图重建时，不考虑磁化矢量的极性，导致恢复到正向的脑干组织与仍在负向的脑积水/眼球房水组织的磁化矢量大小相当。那么在 M 图重建时，两者信号表现为相近的灰阶。而此处箭头所指的黑线勾边伪影，是由于这些地方的体素内，同时有水质子与脂质子，它们磁化矢量极性相反，相互抵消，导致信号强度下降。

关于这个在 Q&A 中也有阐述：

↑↑ from Questions and Answers in MRI

在 IR 序列中的这个 IR Bounce Point Artifact 其实也是 phase cancellation artifact，这里的 phase 表示磁化矢量的极性，+180°，-180°。

Because the Larmor frequency of protons in water is approximately 3.3-3.5 ppm higher than protons in fat, the transverse magnetization of protons in water will continually accumulate phase relative to those in fat. In RF spin-echo pulse sequences, the 180° refocusing pulse negates the phase accumulated from 0 to TE/2, so that at the echo time TE fat and water are back in phase. In spoiled GRE and SSFP-FID, however, the phase accumulation from chemical shift is never negated. If a TE is selected so that the phase of the transverse magnetization of fat and water is opposed, signal cancellation will result. This process is explained in detail in Section 17.3 on Dixon's method and illustrated on Figure 14.10. Table 14.3 lists TE values at which fat and water are in- and out-of-phase for several field strengths, assuming a chemical shift of 3.4 ppm.

 由于水质子的拉莫尔频率比脂质子的拉莫尔频率快大约 3.3-3.5 ppm，那么水质子的横向磁化相对于脂质子的横向磁化矢量持续有相位的累积。在射频自旋回波脉冲序列中，180° 重聚焦脉冲消除了 0 到 TE/2 时间内的相位累积，因此在 TE 时刻水和脂肪达到同相位。然而在扰相梯度回波序列和 SSFP-FID 序列中，由化学位移所产生的相位累积始终不能被消除。如果选择的 TE 时刻，水质子与脂质子的横向磁化矢量反相，那么将导致信号相互抵消。这一过程如图 14.10 所示，并且在 17.3 节 Dixon's method 中详细解释了这一过程。表 14.3 列出了一些场强下水脂同反相位的 TE 时间，假设化学位移为 3.4ppm。

%%%%%%%%%%%%%%%

【译者注5】

[The Basics of MRI 8.1~8.2] 磁共振成像基本原理 8.1~8.2 Gradient Echo

[The Basics of MRI 8.6] 磁共振成像基本原理 8.6 Spin Echo

关于水脂同反相位的计算如下： 【MR技术】你能通过只看参数来判断手头的机器是 1.5T 还是 3.0T 的吗？ 【视频】新说磁共振脂肪抑制

The effect of the chemical shift artifact of the second kind is to outline fatbanded anatomical structures (such as the kidneys). Although this is considered an artifact, it can be useful. Figure 14.15 shows an example of a dual-echo GRE image with TE1 = 2.2 ms, and TE2 = 4.5 ms, corresponding to fat and water out-of- and in-phase at 1.5T, respectively. Because the information from the two echoes is complementary, dual-echo GRE pulse sequences have gained popularity, particularly for body imaging applications.

第二类化学位移伪影的表现效果就是勾勒出被脂肪包裹的解剖结构（比如肾脏）。尽管它被认为是一种伪影，但它仍有有用的一面。图 14.15 显示的是一个双回波 GRE 序列，TE1 = 2.2 ms，TE2 = 4.5 ms，分别对应于 1.5T 中水脂反相位与同相位。由于这两个回波的图像提供的信息互相补充，使得双回波 GRE 序列十分流行，尤其在腹部成像应用中。

【译者注6】

当然，双回波扰相梯度回波序列，不仅可以对组织脏器进行勾边显示，还可以用于判断组织内是否含有脂肪，对于肝脏的脂肪浸润的检出十分敏感。同时对于，顺磁化性物质的沉积也有检出效果，比如肝脏、心肌的铁沉积。当然，现在结合 DIXON 有定量成像序列，mDIXON-Quant，IDEAL-IQ，Liver-Lab。

Flow Effects    For SSFP-echo and SSFP-FID, flow and motion can cause the precession of the transverse magnetization to experience inconsistent values of Φ among the TR intervals, which can spoil the transverse steady state (Patz 1988). For this reason, spoiled GRE is generally preferred over SSFP-FID for time-of-flight angiography (Section 15.3). The signal obtained from flowing blood is usually the same as SSFP-FID, whereas the distracting signal from other fluids such as cerebrospinal fluid is attenuated. These properties are illustrated in the row of images obtained with 70° flip angle in Figure 14.4.

流动效应    对于 SSFP-echo 与 SSFP-FID，流动和运动能够导致横向磁化矢量在每个 TR 间期内的进动产生不一致的相位 Φ，这将会扰乱横向磁化矢量的稳态（Patz 1988）。为此，对于 TOF 血管成像，扰相梯度回波序列比 SSFP-FID 序列更受青睐（15.3 节）。扰相梯度回波得到的流动血流的信号通常与 SSFP-FID 相同，而其他液体（比如脑脊液）的信号将被减弱。这些特点从图 14.4 中的 θ = 70° 那一行所采集的图像中可以看出。

 In balanced SSFP, flowing blood can set up a steady state because the net gradient area in any TR interval is zero (Haacke et al. 1990; Zur et al. 1990). Balanced SSFP generally works well as a bright-blood technique and already has found an important application in cardiac imaging. The contrast is excellent because the myocardium reaches steady state and has a low signal (due to its low T2/T1 ratio), whereas in-flowing blood shows a strong transient signal. One problem with balanced SSFP is that sometimes an anomalous enhancement of in-flowing blood occurs. These bright flashes occur when magnetization that is outside the imaging slice flows in and is rephased, effectively increasing the slice thickness (Markl et al. 2003).

在平衡稳态自由进动序列中，由于每个 TR 间期内净梯度面积为零，那么流动的血流能够达到一稳态（Haacke et al. 1990; Zur et al. 1990）。平衡稳态自由进动序列作为一亮血技术效果通常很好，并且在心脏成像方面得到了重要的应用。Balanced SSFP 的对比十分优越，因为心肌达到稳态时的信号强度较低（由于心肌的 T2/T1 值小），而流入的血流表现为较强的暂态信号。Balanced SSFP 的一个问题是，有时流入的血流会出现异常的信号增强。当流入的磁化矢量流出成像层面并且被重新聚相时会出现这些增强的信号，从而实际上“增加”了扫描层厚（Markl et al. 2003）。

【译者注7】