To improve the spatial resolution of power Doppler (PD) imaging, we explored null subtraction imaging (NSI) as an alternative beamforming technique to delay-and-sum (DAS). NSI is a nonlinear beamforming approach that uses three different apodizations on receive and incoherently sums the beamformed envelopes. NSI uses a null in the beam pattern to improve the lateral resolution, which we apply here for improving PD spatial resolution both with and without contrast microbubbles. In this study, we used NSI with three types of singular value decomposition (SVD)-based clutter filters and noise equalization to generate high-resolution PD images. An element sensitivity correction scheme was also proposed as a crucial component of NSI-based PD imaging. First, a microbubble trace experiment was performed to evaluate the resolution improvement of NSI-based PD over traditional DAS-based PD. Then, both contrast-enhanced and contrast free ultrasound PD images were generated from the scan of a rat brain. The cross-sectional profile of the microbubble traces and microvessels were plotted. FWHM was also estimated to provide a quantitative metric. Furthermore, iso-frequency curves were calculated to provide a resolution evaluation metric over the global field of view. Up to six-fold resolution improvement was demonstrated by the FWHM estimate and four-fold resolution improvement was demonstrated by the iso-frequency curve from the NSI-based PD microvessel images compared to microvessel images generated by traditional DAS-based beamforming. A resolvability of $39~\mu \text{m}$ was measured from the NSI-based PD microvessel image. The computational cost of NSI-based PD was only increased by 40 percent over the DAS-based PD.

中文翻译：

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使用零减影成像的高分辨率功率多普勒


为了提高功率多普勒 （PD） 成像的空间分辨率，我们探索了零减法成像 （NSI） 作为延迟和和 （DAS） 的替代波束形成技术。NSI 是一种非线性波束成形方法，它在接收时使用三种不同的切趾，并将波束成形包络相加。NSI 在光束模式中使用零来提高横向分辨率，我们在这里将其应用于提高有和没有对比微泡的 PD 空间分辨率。在这项研究中，我们使用 NSI 与三种类型的基于奇异值分解 （SVD） 的杂波滤波器和噪声均衡来生成高分辨率的 PD 图像。还提出了一种元素灵敏度校正方案作为基于 NSI 的 PD 成像的关键组成部分。首先，进行了微泡痕量实验，以评估基于 NSI 的 PD 相对于传统基于 DAS 的 PD 的分辨率提升。然后，从大鼠脑部扫描生成对比增强和无对比超声 PD 图像。绘制微气泡痕迹和微血管的横截面剖面图。FWHM 也被估计为提供了一个定量指标。此外，计算等频曲线以提供全局视场的分辨率评估指标。FWHM 估计证明了高达 6 倍的分辨率提高了，与基于 DAS 的传统波束成形生成的微血管图像相比，基于 NSI 的 PD 微血管图像的等频曲线证明了分辨率提高了 4 倍。从基于 NSI 的 PD 微血管图像中测得的可分辨性为 $39~\mu \text{m}$。基于 NSI 的 PD 的计算成本仅比基于 DAS 的 PD 增加了 40%。