Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and the North Carolina State University, Chapel Hill, North Carolina, United States of America. Instituto de Ciencias de la Ingeniería, Universidad de O'Higgins, Avenida Libertador Bernardo O'Higgins 611, Rancagua, Chile.
Biomed Phys Eng Express. 2020 Apr 21;6(3):035019. doi: 10.1088/2057-1976/ab7f26.
Super-resolution ultrasound imaging relies on the sub-wavelength localization of microbubble contrast agents. By tracking individual microbubbles, the velocity and flow within microvessels can be estimated. It has been shown that the average flow velocity, within a microvessel ranging from tens to hundreds of microns in diameter, can be measured. However, a 2D super-resolution image can only localize bubbles with sub-wavelength resolution in the imaging plane whereas the resolution in the elevation plane is limited by conventional beamwidth physics. Since ultrasound imaging integrates echoes over the elevation dimension, velocity estimates at a single location in the imaging plane include information throughout the imaging slice thickness. This slice thickness is typically a few orders or magnitude larger than the super-resolution limit. It is shown here that in order to estimate the velocity, a spatial integration over the elevation direction must be considered. This operation yields a multiplicative correction factor that compensates for the elevation integration. A correlation-based velocity estimation technique is then presented. Calibrated microtube phantom experiments are used to validate the proposed velocity estimation method and the proposed elevation integration correction factor. It is shown that velocity measurements are in excellent agreement with theoretical predictions within the considered range of flow rates (10 to 90 μl/min) in a microtube with a diameter of 200 μm. Then, the proposed technique is applied to two in-vivo mouse tail experiments imaged with a low frequency human clinical transducer (ATL L7-4) with human clinical concentrations of microbubbles. In the first experiment, a vein was visible with a diameter of 140 μm and a peak flow velocity of 0.8 mm s. In the second experiment, a vein was observed in the super-resolved image with a diameter of 120 μm and with maximum local velocity of ≈4.4 mm s. It is shown that the parabolic flow profiles within these micro-vessels are resolvable.
超分辨率超声成像是依赖于微泡造影剂的亚波长定位。通过跟踪单个微泡，可以估计微血管内的速度和流动。已经表明，可以测量直径从数十微米到数百微米的微血管内的平均流速。然而，二维超分辨率图像只能在成像平面内以亚波长分辨率定位微泡，而在高程平面内的分辨率受到传统波束宽度物理学的限制。由于超声成像是在高程维度上对回波进行积分，因此在成像平面上的单个位置的速度估计包括整个成像切片厚度内的信息。该切片厚度通常比超分辨率极限大几个数量级。这里表明，为了估计速度，必须考虑在高程方向上的空间积分。该操作产生一个乘性校正因子，以补偿高程积分。然后提出了一种基于相关的速度估计技术。使用校准的微管体模实验来验证所提出的速度估计方法和所提出的高程积分校正因子。结果表明，在所考虑的流速范围内（10 至 90μl/min），在直径为 200μm 的微管中，速度测量与理论预测非常吻合。然后，将所提出的技术应用于两个用低频人临床换能器（ATL L7-4）成像的体内小鼠尾巴实验，并用人类临床浓度的微泡进行实验。在第一个实验中，可见直径为 140μm 的静脉，峰值流速为 0.8mm/s。在第二个实验中，在超分辨率图像中观察到直径为 120μm 的静脉，最大局部速度约为 4.4mm/s。结果表明，这些微血管内的抛物线流动剖面是可分辨的。
