Laboratory of Biorheology and Medical Ultrasonics, University of Montreal Hospital Research Center (CRCHUM), Montreal, Quebec, Canada.
Physiol Meas. 2012 Oct;33(10):1585-602. doi: 10.1088/0967-3334/33/10/1585. Epub 2012 Sep 4.
Micro particle image velocimetry (µPIV) is a common method to assess flow behavior in blood microvessels in vitro as well as in vivo. The use of red blood cells (RBCs) as tracer particles, as generally considered in vivo, creates a large depth of correlation (DOC), even as large as the vessel itself, which decreases the accuracy of the method. The limitations of µPIV for blood flow measurements based on RBC tracking still have to be evaluated. In this study, in vitro and in silico models were used to understand the effect of the DOC on blood flow measurements using µPIV RBC tracer particles. We therefore employed a µPIV technique to assess blood flow in a 15 µm radius glass tube with a high-speed CMOS camera. The tube was perfused with a sample of 40% hematocrit blood. The flow measured by a cross-correlating speckle tracking technique was compared to the flow rate of the pump. In addition, a three-dimensional mechanical RBC-flow model was used to simulate optical moving speckle at 20% and 40% hematocrits, in 15 and 20 µm radius circular tubes, at different focus planes, flow rates and for various velocity profile shapes. The velocity profiles extracted from the simulated pictures were compared with good agreement with the corresponding velocity profiles implemented in the mechanical model. The flow rates from both the in vitro flow phantom and the mathematical model were accurately measured with less than 10% errors. Simulation results demonstrated that the hematocrit (paired t tests, p = 0.5) and the tube radius (p = 0.1) do not influence the precision of the measured flow rate, whereas the shape of the velocity profile (p < 0.001) and the location of the focus plane (p < 0.001) do, as indicated by measured errors ranging from 3% to 97%. In conclusion, the use of RBCs as tracer particles makes a large DOC and affects the image processing required to estimate the flow velocities. We found that the current µPIV method is acceptable to estimate the flow rate on the condition that the measurement takes place at the equatorial plane of the vessel. Otherwise, it is not an appropriate method to estimate the shape of the velocity profile.
微粒子图像测速（µPIV）是一种常用的方法，用于评估体外和体内血液微血管中的流动行为。通常在体内使用红细胞（RBC）作为示踪粒子，会产生较大的相关深度（DOC），甚至与血管本身一样大，从而降低了该方法的准确性。基于 RBC 跟踪的µPIV 用于血流测量的局限性仍有待评估。在这项研究中，我们使用体外和计算机模拟模型来了解 DOC 对使用 µPIV RBC 示踪粒子进行血流测量的影响。因此，我们使用 µPIV 技术来评估高速 CMOS 相机中 15 µm 半径玻璃管中的血液流动。该管中灌注有 40%血细胞比容的样本。通过互相关散斑跟踪技术测量的流量与泵的流速进行比较。此外，还使用三维机械 RBC 流动模型模拟了在 20%和 40%血细胞比容下，在 15 µm 和 20 µm 半径的圆形管中，在不同的聚焦平面、流速和各种速度分布形状下的光学运动散斑。从模拟图像中提取的速度分布与机械模型中实施的相应速度分布吻合良好。体外流动仿体和数学模型的流量均以小于 10%的误差准确测量。模拟结果表明，血细胞比容（配对 t 检验，p = 0.5）和管半径（p = 0.1）不会影响测量流速的精度，而速度分布的形状（p < 0.001）和聚焦平面的位置（p < 0.001）会影响测量精度，误差范围从 3%到 97%不等。总之，使用 RBC 作为示踪粒子会产生较大的 DOC，并影响估计流速所需的图像处理。我们发现，当前的 µPIV 方法在测量发生在血管赤道平面的情况下，可用于估计流量。否则，它不是一种合适的方法来估计速度分布的形状。
