【Handbook of MRI Pulse Sequences】14.1.1 多次采集稳态自由进动

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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

Multiple-Acquisition SSFP (CISS)    In some applications such as imaging the structures of the inner ear, very small fields of view (10 cm or less) are desired. This necessarily increases the minimum TR, so that banding artifacts are unavoidable in balanced SSFP. Using a technique known as constructive interference in the steady state (CISS), or multiacquisition SSFP, can suppress the bands (Casselman et al. 1993). Two acquisitions are obtained sequentially, for example, one with sign alternation of the excitation pulses and the other without. The two images are separately reconstructed. Because of the phase cycling, the locations of the bands are shifted relative to one another by one-half the spatial period on the two corresponding images. Any given pixel can lie within a band on one of the images, but unless there is extreme susceptibility variation, the pixel will not lie within a band on both images. Therefore, the bands can be suppressed on the final image by calculating a maximum intensity of the two magnitude images on a pixel-by-pixel basis, as shown in Figure 14.9.

多次采集稳态自由进动（CISS）    在一些应用中，比如内耳结构成像，需要非常小的视野（10cm 或更小）。这必然会延长最短 TR，因此在平衡稳态自由运动序列中不可避免地会产生黑带伪影。使用一种称作建设性相干平衡稳态（CISS）或多次采集稳态自由进动的技术能够抑制这种伪影（Casselman et al. 1993）。两次采集是顺序完成，例如，一次射频脉冲使用相位交替，另一次不使用相位交替，两次图像分别重建。由于相位周期的存在，在两次相对应的图像中，黑带伪影在一次中的位置相对于另一次偏移了一半的空间周期。任何给定的像素可能位于其中某一次图像的黑带内，但不会位于位于另一次图像的黑带内，除非磁敏感差异极大。因此，通过两次采集到的幅度图像，基于像素对像素地计算最大密度可以抑制黑带伪影，如图 14.9 所示。

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做小范围成像时，体素势必要给的相对比较小。当读出方向 FOV 一定时，读出方向体素越小，所需要采集的数据点就越多，那么信号采集也是需要时间的，那么就会延长 Min. TR，TR 延长了，那么黑带伪影也会随之变多/重。

spatial period 即空间周期，与之对应的是 temporal period 时间周期；spatial frequency 即空间频率，与之对应的是 temporal frequency 时间频率。空间频率的单位为 /cm，空间周期的单位为 cm；时间频率的单位为 /s，时间周期的单位为 s，它们互为倒数。 the locations of the bands are shifted relative to one another by one-half the spatial period on the two corresponding images. θ x-θx-θx-... 型 RF 相位不变化时所产生的黑带伪影在一个地方，比如：

那么 θ x-θ(-x)-θx-θ(-x)... 型 RF 相位交替时所产生的黑带伪影就会出现在另一个地方，比如：

具体是怎么偏移的呢？是二分之一的空间周期，单位是 cm.

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Multiacquisition SSFP has several variations. For example, instead of two acquisitions, four can be used, each with a different phase cycle: and The bands are then shifted by one-quarter of the spatial period on each image. Four-acquisition can provide a smoother image appearance than two-acquisition, not only because of improved SNR but also because there should be three or four images on which any pixel does not lie in a band. A further variation is to replace the maximum intensity calculation with the square root of the sum of the squares, which can further improve the SNR (Bangerter and Nishimura 2003).

多次采集稳态自由进动有几种变体，例如，不是采集两次，而是采集四次，那么每一次拥有不同的相位周期：和 那么黑带伪影在每次图像上移位四分之一空间周期。四次采集比两次采集能够获得更加平滑的图像表现，不仅是因为信噪比提高了，而且因为任一体素会在三组或四组图像上任意体素都没有伪影。最大密度计算方法还可以换成均方根算法，进一步提升了图像的信噪比（Bangerter and Nishimura 2003）。

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这一技术在飞利浦平台叫 BFFE_XD，西门子平台叫 CISS，GE 平台叫 FIESTA-C。

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