Zur Y, Bendel P
Med Phys. 1987 Mar-Apr;14(2):172-7. doi: 10.1118/1.596109.
The definition and mapping of oblique planes by magnetic resonance imaging (MRI) requires the simultaneous application of two or three orthogonal gradients to define the desired intermediate direction of the frequency encoding or "readout" gradient. Each of the three main gradient coils produces different patterns of eddy currents. Consequently, the application of dephasing and rephasing lobes of these gradients will produce echoes at slightly different times for each gradient. If two or three gradients are applied simultaneously to create an arbitrary view direction, the resulting echo will therefore be shifted in time and considerably reduced in intensity. In this article, we present an analysis of the behavior of the magnetization in a typical two-dimensional Fourier transform pulse sequence for the imaging of oblique slices. The theoretical displacements in time and reduction in intensity of the echo amplitudes are calculated and compared to the experimental behavior. We show that, in spite of this phenomenon, the final image suffers only marginally in signal-to-noise ratio, provided the slice width is small compared to the field of view. This is due to the fact that there always exists a cycle in the sequence in which the phase-encoding gradient almost completely compensates for the above described effect.
通过磁共振成像（MRI）定义和映射斜平面需要同时施加两个或三个正交梯度，以定义频率编码或“读出”梯度所需的中间方向。三个主要梯度线圈中的每一个都会产生不同模式的涡流。因此，这些梯度的去相位和重相位波瓣的应用将为每个梯度在略微不同的时间产生回波。如果同时施加两个或三个梯度以创建任意视图方向，则产生的回波将因此在时间上发生偏移并且强度会大幅降低。在本文中，我们对用于斜切片成像的典型二维傅里叶变换脉冲序列中的磁化行为进行了分析。计算了回波幅度在时间上的理论位移和强度降低，并与实验行为进行了比较。我们表明，尽管存在这种现象，但只要切片宽度与视野相比很小，最终图像的信噪比仅会受到轻微影响。这是因为在序列中总是存在一个周期，其中相位编码梯度几乎完全补偿了上述效应。
