Glasgow Experimental MRI Centre, Institute of Neuroscience and Psychology, University of Glasgow, UK; Department of Physics, College of Science, Misan University, Iraq.
Glasgow Experimental MRI Centre, Institute of Neuroscience and Psychology, University of Glasgow, UK; Centre for Clinical Brain Sciences, University of Edinburgh, UK.
J Magn Reson. 2021 Apr;325:106929. doi: 10.1016/j.jmr.2021.106929. Epub 2021 Feb 4.
Phase-contrast magnetic resonance velocimetry (PC-MRI) has been widely used to investigate flow properties in numerous systems. In a horizontal cylindrical pipe (3 mm diameter), we investigated the accuracy of PC-MRI as the flow transitioned from laminar to turbulent flow (Reynolds number 352-2708). We focus primarily on velocimetry errors introduced by skewed intra-voxel displacement distributions, a consequence of PC-MRI theory assuming symmetric distributions. We demonstrated how rapid fluctuations in the velocity field, can produce broad asymmetric intravoxel displacement distributions near the wall. Depending on the shape of the distribution, this resulted in PC-MRI measurements under-estimating (positive skewness) or over-estimating (negative skewness) the true mean intravoxel velocity, which could have particular importance to clinical wall shear stress measurements. The magnitude of these velocity errors was shown to increase with the variance and decrease with the kurtosis of the intravoxel displacement distribution. These experimental results confirm our previous theoretical analysis, which gives a relationship for PC-MRI velocimetry errors, as a function of the higher moments of the intravoxel displacement distribution (skewness, variance, and kurtosis) and the experimental parameters q and Δ. This suggests that PC-MRI errors in such unsteady/turbulent flow conditions can potentially be reduced by employing lower q values or shorter observation times Δ.
相位对比磁共振流速测量法（PC-MRI）已广泛用于研究众多系统中的流动特性。在水平圆柱管（3mm 直径）中，我们研究了 PC-MRI 在从层流向湍流过渡时的准确性（雷诺数 352-2708）。我们主要关注 PC-MRI 理论假设对称分布所导致的偏置体素内位移分布引起的速度测量误差。我们展示了速度场的快速波动如何在靠近壁面处产生宽不对称的体素内位移分布。根据分布的形状，这导致 PC-MRI 测量值低估（正偏度）或高估（负偏度）真实的平均体素速度，这对于临床壁面切应力测量可能具有特殊意义。这些速度误差的大小被证明随体素内位移分布的方差增加而增加，随峰度减小而减小。这些实验结果证实了我们之前的理论分析，该分析给出了 PC-MRI 速度测量误差作为体素内位移分布的更高阶矩（偏度、方差和峰度）和实验参数 q 和Δ的函数的关系。这表明，在这种非稳态/湍流流动条件下，通过采用较低的 q 值或较短的观测时间Δ，可能会降低 PC-MRI 误差。
