【Q&A in MRI】5.2.4 Rectangular FOV 矩形 FOV

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。

Courtesy of Allen D. Elster, MRIquestions.com

网站：

https://www.mriquestions.com/

原著：Allen D. Elster, MD

译注：蒋强盛

How does one obtain a rectangular field of view? Why would you want to use it? 

矩形 FOV 是如何采集的？为什么要使用 RFOV？

In the prior Q&A we showed how the field-of- view (FOV) is inversely related to the spacing (Δk) of samples in k-space. For simplicity of explanation we considered square pixels with the same FOV in each direction. However, by altering the sampling rates along the frequency and phase axes it is possible to alter the FOV along each image dimension independently.

在前一 Q&A 中我们知道为何 FOV 与 Δk 成反比，之前为了简化说明，只考虑了频率编码方向与相位编码方向的 FOV 大小与像素大小均相同的情况。然而通过改变 k 空间频率轴与相位轴的采样率可以独立地改变图像频率编码方向与相位编码方向的 FOV，即：

FOVf = 1 / Δkf
FOVp = 1 / Δkp

For 2D-spin warp imaging, a rectangular FOV with a 1:2 ratio is obtained by sampling alternate phase-encode lines in k-space while leaving the maximum and minimum amplitudes of the phase-encoding gradient unchanged. This process halves the number of phase-encoding steps (Np), but doubles the increment between successive steps.

Rectangular (1:2) FOV obtained by sampling every other line in the k_p-direction

The sampling rate and hence the field-of-view in the frequency-encode direction (FOVf) remains unaffected.  Furthermore, because both the FOVp and Np have been reduced by one-half, pixel size in the phase-encode direction (FOVp / Np) and overall spatial resolution of the image are also unchanged.  

而读出方向的采样率保持不变，那么频率编码方向的 FOV 不变。更进一步地讲，由于 FOVp 和 Np 均变为原来的一半，那么相位编码方向的像素大小 (FOVp / Np) 保持不变，继而图像整体的空间分辨率保持不变。

All MR manufacturers offer rectangular FOV as a standard option applicable to most pulse sequences. The names differ slightly by vendor: "Rectangular FOV" (Philips, Hitachi, Canon), "Phase FOV" (Siemens), and "Partial FOV" (GE).  

所有的 MR 机器上大多数脉冲序列都可以使用 RFOV。但不同厂家的名称稍有差别：飞利浦、日立、佳能称之为 “Rectangular FOV”；西门子称作 “Phase FOV”；GE 叫 “Partial FOV”。

The rectangular FOV option is most beneficial in imaging the spine and extremities, where one anatomical dimension is much longer than the other.  Most scanners allow adjusting the phase-sampling ratio over a large range; accordingly, considerable latitude may be permitted in the size and shape of the rectangular FOV.  

在脊柱与四肢扫描时使用 RFOV 十分有好处，因为它们的解剖形态的一侧比另一侧长很多。大多数 MR 允许用户在大范围扫描时可以自定义矩形 FOV 的比例，因此在设置 RFOV 的形状比例时有很大的自由度。

Rectangular FOV techniques have two major drawbacks, however.  First, because fewer phase-encoding steps are acquired, the signal-to-noise ratio (SNR) is proportionately lower compared to full FOV imaging.  Secondly, wrap-around (aliasing) in the phase-encoding direction may occur if the object dimensions exceed this defined FOVp. Careful positioning of the patient and the use of saturation bands may be needed to eliminate this artifact.

ectangular FOV with wrap-around (aliasing) artifact from chin (red arrow)

当然，矩形 FOV 的短边可以在相位编码方向也可以在频率编码方向，在相位编码方向可以通过施加空间饱和带进行卷褶伪影的抑制，也可以进行相位编码方向的过采样。一般在脊柱矢状位扫描时，设置相位编码方向为 FH，那么同时需要进行过采样。

矩形 FOV 的短边在频率编码方向时，一般不会出现频率卷褶，因为采集时都是以最大频率的两倍进行采集的，即奈奎斯特采样频率。如下图：

每个线圈单元重建的图像也没有频率卷褶：

每个线圈单元重建的图像也都有频率卷褶：

另外，RFOV 采集的 k 空间与 SENSE 采集的 k 空间是什么关系？