Towards 3D ultrafast Doppler imaging with a portable ultrasound scanner

超声成像是临床上常见的一种成像方式。利用超声换能器，结合生物组织中的声波发射，然后记录后向散射回波，可以重建解剖图像。换能器是由压电元件组成的线性阵列，通常最多有256个。每个元件都连接到自己的电气通道，该通道将其信号传输到系统。然而，传统的超声扫描仪不能同时对所有这些通道进行采样，这意味着一次只能记录和显示一部分反向散射回波。因此，逐行重建完整的2D图像，每行具有一次发送/接收。这种扫描方法本质上限制了方法的帧速率(50张图像/S)。这在研究瞬时现象时是有问题的，例如血液流动。最近，具有32x32矩阵元素阵列的换能器实现了体积成像。然而，对有限通道数量的同样限制大大阻碍了4D超声的发展。我的项目研究了克服传统超声成像局限性的方法：它的低帧速率限制了血流检测，以及它提供体积成像的有限能力。使用尖端的可编程超声波扫描仪，我使用所谓的超快超声技术来实现非常高的帧速率(高达10000图像/S)。这些超快扫描仪可以对256个通道进行完全采样，并通过一次非聚焦的超声波传输/接收重建完整的图像。这种高数量的通道仍然不足以驱动32x32矩阵探头。将四个超高速扫描仪组合在一起驱动一个矩阵探头的复杂原型存在，但有两个主要缺点：巨大的硬件成本和系统便携性的损失，这是超声成像的一个关键特征。我建议用更少的资源做更多的事情：只用便携式256通道扫描仪进行4D超快成像，并将其应用于体积血流成像。我将利用多路复用器的最新发展，它使得用一个通道驱动4个传感器的元件成为可能。使用稀疏数量的元素，可以在保持高帧速率和高图像质量的同时重建体积。我正在开发模拟工具，以评估这种使用稀疏光圈的超快体积方法的物理参数。我还在开发两个体外实验装置。一种是带有高精度水听器的校准水箱，可以直接评估声压场。另一种是多普勒体模，模拟脉动动脉中的血流，我将在其上设计时空过滤器来检测血液后向散射信号。最后，我还将致力于活体概念验证在新生儿上的应用。我的方法将使床边新生儿脑血管形成的4D成像成为可能。

Ultrasound imaging is a common imaging modality in clinical practice. Using ultrasound transducers, the combination of acoustic wave emission in biological tissues, followed by the recording of backscattered echoes allows the reconstruction of anatomical images. The transducers are linear arrays of piezoelectric elements, and typically have up to 256 of them. Each element is connected to its own electrical channel that transmits its signal to the system. However, conventional ultrasound scanners cannot sample all these channels simultaneously, which means that only a part of the backscattered echoes can be recorded and displayed at once. The full 2D image is thus reconstructed line-per-line, with one transmission/reception for each line. This scanning approach intrinsically limits the framerate of the method (50 images/s). This is problematic when transient phenomenon are studied, such as blood flows. Recently, transducers with 32x32 matrix arrays of elements have enabled volumetric imaging. However, the same constraint on limited channel number considerably hinders the development of 4D ultrasound. My program investigates methods to overcome the limitations of conventional ultrasound imaging: its low framerate constraining blood flow detection, and its limited capacity to provide volumetric imaging. Using cutting-edge programmable ultrasound scanners, I use the so-called Ultrafast Ultrasound technology to achieve very-high framerates (up to 10000 images/s). These ultrafast scanners can fully sample 256 channels and reconstruct a full image with a single unfocused ultrasound transmission/reception. This high number of channels is still insufficient to drive a 32x32 matrix probe. Sophisticated prototypes combining four ultrafast scanners to drive one matrix probe exist but have two main drawbacks: their huge hardware cost and the loss of the system's portability, a key feature of ultrasound imaging. I propose to do "more with less": perform 4D ultrafast imaging with only portable 256-channels scanner, and apply it to volumetric blood-flow imaging. I will leverage on the recent developments of multiplexers, which makes it possible to drive 4 transducer's elements with a single channel. Using a sparse number of elements, it is possible to reconstruct volumes while keeping a high framerate and high image quality. I am developing simulation tools to evaluate the physical parameters for this ultrafast volumetric approach using sparse apertures. I am also developing two in vitro setups. One is a calibration water tank with a high-precision hydrophone to directly assess the acoustic pressure fields. The other is a Doppler phantom, mimicking a blood stream in a pulsatile artery, on which I will design spatiotemporal filters to detect blood backscattering signals. Lastly, I will also work on the in vivo proof-of-concept application on newborns. My approach will enable transfontanellar 4D imaging of neonate brain vascularization at the bedside.