Towards 3D ultrafast Doppler imaging with a portable ultrasound scanner

超声成像是临床常用的一种成像方式。利用超声波换能器，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。换能器是压电元件的线性阵列，通常有多达256个。每个元件都连接到自己的电气通道，将其信号传输到系统。然而，传统的超声波扫描仪不能同时对所有这些通道进行采样，这意味着只能记录和显示一部分后向散射回波。因此，完整的二维图像是逐行重建的，每条线有一个传输/接收。这种扫描方法本质上限制了该方法的帧率（50张图像/秒）。这在研究瞬时现象（如血液流动）时是有问题的。最近，具有32x32矩阵阵列元件的换能器已经实现了体积成像。然而，同样的受限通道数的限制也极大地阻碍了4D超声的发展。我的计划研究克服传统超声成像局限性的方法：低帧率限制血流检测，以及提供体积成像的有限能力。使用尖端的可编程超声扫描仪，我使用所谓的超快超声技术来实现非常高的帧率（高达10000张图像/秒）。这些超快扫描仪可以完全采样256通道，并重建一个完整的图像与单一的无聚焦超声传输/接收。如此高的通道数量仍然不足以驱动32x32矩阵探头。目前已经有了将四个超快扫描仪驱动一个矩阵探针的复杂原型，但有两个主要缺点：硬件成本高昂，以及系统的便携性不足，而便携性是超声成像的一个关键特征。我建议做“少花钱多”：仅用便携式256通道扫描仪进行4D超快成像，并应用于体积血流成像。我将利用多路复用器的最新发展，这使得有可能用单个通道驱动4个换能器的元件。使用稀疏数量的元素，可以在保持高帧率和高图像质量的同时重建体积。我正在开发模拟工具来评估这种使用稀疏孔径的超快速体积方法的物理参数。我还在开发两个体外装置。一种是带有高精度水听器的校准水箱，可直接评估声压场。另一个是多普勒幻像，模拟搏动动脉中的血流，我将在其上设计时空滤波器来检测血液反向散射信号。最后，我还将在新生儿体内进行概念验证应用。我的方法将使经囟门四维成像新生儿脑血管在床边。

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.