Towards 3D ultrafast Doppler imaging with a portable ultrasound scanner

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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.