Yuan C, Gullberg G T, Parker D L
Department of Radiology, University of Utah, Salt Lake City 84132.
Med Phys. 1987 Nov-Dec;14(6):914-21. doi: 10.1118/1.596129.
The movement of spins during periods of selective pulses result in a modulation of the signal intensity and phase of the received magnetic resonance imaging (MRI) signal, and is a major cause of signal loss from vessels imaged with slice-selective pulses. Methods are well developed for compensation of phase perturbations for spins flowing at constant velocity during the time of applied gradients. However, for spins flowing during selective pulses, the magnitude of the amplitude and phase perturbations has not been understood nor to this time has any method of flow compensation been proposed. This is due in part to the difficulty in using the Bloch equations to quantify the amplitude and phase modulation during radiofrequency (rf) excitation since solutions cannot be obtained analytically. In this paper a finite difference method is used to solve Bloch equations for flowing spins during a 90 degrees selective pulse. Compared with stationary spins, the magnetization distribution for flowing spins exhibits a shift of the slice profile in the direction of the flow, an expansion of the profile, phase shifts, and changes in profile shape. The profiles show residual phase errors which become more severe with higher flow velocities, with flow compensation schemes which apply in the case of spins flowing during applied gradients, and in the absence of an rf pulse. The measurement and understanding of the magnetization distribution is important to designing pulse sequences that compensate for flow. Flow compensated pulse sequences are necessary to reduce image flow artifacts and to increase signal of vessels in MR angiographic images.
在选择性脉冲期间，自旋的运动会导致所接收的磁共振成像（MRI）信号的强度和相位发生调制，并且是使用切片选择性脉冲成像的血管信号丢失的主要原因。对于在施加梯度期间以恒定速度流动的自旋，已经开发出了很好的方法来补偿相位扰动。然而，对于在选择性脉冲期间流动的自旋，幅度和相位扰动的大小尚未被理解，并且到目前为止也没有提出任何流动补偿方法。这部分是由于在射频（rf）激发期间使用布洛赫方程来量化幅度和相位调制存在困难，因为无法通过解析方法获得解。在本文中，使用有限差分法来求解90度选择性脉冲期间流动自旋的布洛赫方程。与静止自旋相比，流动自旋的磁化分布在流动方向上呈现切片轮廓的偏移、轮廓的扩展、相移以及轮廓形状的变化。这些轮廓显示出残余相位误差，随着流速的增加，以及在应用于施加梯度期间流动的自旋且不存在rf脉冲的情况下应用的流动补偿方案，残余相位误差会变得更加严重。磁化分布的测量和理解对于设计补偿流动的脉冲序列很重要。流动补偿脉冲序列对于减少图像流动伪影和增加磁共振血管造影图像中血管的信号是必要的。
