Gatenby J C, Gore J C
Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, Connecticut 06510.
J Magn Reson B. 1994 Jun;104(2):119-26. doi: 10.1006/jmrb.1994.1064.
The signal produced by turbulent flow in NMR pulse sequences has been analyzed by considering the effects of variations in fluid velocity on the net signal phase variance. It is shown that in a bipolar field gradient, the signal is dependent on the gradient amplitude and the precise time dependence of the velocity fluctuations. This dependence is described using an autocorrelation function, whose characteristic width is the correlation time. When the correlation time is short, the signal from fluid elements decreases as though they are diffusing in a random walk, whereas when the time is long, the signal falls more rapidly with an increase in the duration of the gradient waveform. However, many situations fall into an intermediate regime. A general expression appropriate for all correlation times has been derived and used to characterize the turbulent flow distal to a stenosis in a tube. By use of multiple images of the spatial distribution of NMR signal obtained with differing readout gradients and analyzed by application of the general expression for signal loss, the spatial variations in the correlation time and turbulent intensity have been obtained. The measured variations correlate well with computer calculations obtained by numerical simulation of the flow. This method permits turbulent and complex flows to be characterized without disturbing the flow and may have general applications.
通过考虑流体速度变化对净信号相位方差的影响，分析了核磁共振脉冲序列中湍流产生的信号。结果表明，在双极场梯度中，信号取决于梯度幅度和速度波动的精确时间依赖性。这种依赖性用自相关函数来描述，其特征宽度为相关时间。当相关时间短时，流体单元的信号下降，就好像它们在随机游动中扩散一样；而当时间长时，信号随着梯度波形持续时间的增加而下降得更快。然而，许多情况属于中间状态。已经推导出一个适用于所有相关时间的通用表达式，并用于表征管中狭窄远端的湍流。通过使用具有不同读出梯度获得的核磁共振信号空间分布的多个图像，并应用信号损失的通用表达式进行分析，得到了相关时间和湍流强度的空间变化。测量到的变化与通过流动数值模拟获得的计算机计算结果很好地相关。这种方法允许在不干扰流动的情况下表征湍流和复杂流动，并且可能具有广泛的应用。
