Compressive sensing-based sparse transducers for ultrasound imaging

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Ultrasound imaging is used routinely for material testing and for the diagnosis of many diseases. The resolution of ultrasound imaging is determined by various factors, including transducer design and frequency of operation. To maximize resolution, high operating frequencies (> 1 MHz) and multi-element transducers are usually employed, leading to two major limitations: 1) the penetration depth is limited by the large attenuation of the ultrasonic waves in the propagating medium and by the aberration and reverberation associated with the interfaces at high frequencies, 2) the complexity of the signal processing associated with a large number of transducer elements still limits the achievable frame rate for a given image quality. Until now, most of the work conducted has focused on either imaging algorithms or transducers design. A unique combination of enabling technologies will allow a major breakthrough in the penetration depth and frame rate in ultrasound imaging: 1) the imaging algorithm Excitelet, which exploits lower frequency ultrasound and inherently considers a detailed model of the full wave propagation path, 2) compressive sensing algorithms, which exploit diversity in encoded propagation paths, and 3) additive manufacturing technologies, which enable encoding wave propagation paths within metamaterials. The objective of this program is to create a new generation of sparse ultrasound transducer arrays with a low number of elements, for lower signal processing complexity and higher frame rate, and working at low frequency for improved penetration depth, therefore breaking with existing approaches. The arrays will be used for ultrasound imaging of media which can be either engineering materials or biological tissues. Lateral resolution down to 1/30 of the wavelength is sought at large penetration depth and at a frame rate in the order of 1-10 kHz. This research program articulates along three research thrusts, parallel but related: 1) compressive sensing with ultrasound transducers, where the type of waves and frequencies will be used to enrich information encoding, 2) optimization of sparse transducer arrays, where Excitelet will be considered within the optimization process, and 3) metamaterials in ultrasound probe fabrication, where additive manufacturing techniques will be used to fabricate the metamaterials for encoding the information and other components of the probe. In material testing, this work will allow better characterization of damage precursor in advanced materials, such as composite structures (micro-cracks, voids, micro-buckling, local fiber breakage, or local fiber-matrix debonding). In medical diagnosis, this work will contribute to answer an urgent need for fast imaging frame rates and better resolution in both near- and far-field to avoid the risks in the surrounding organs in guided procedures, in addition to extended penetration depth in order to adapt to the increasing proportion of the obese population.