A Microwave-Induced Thermoacoustic Imaging System With Non-Contact Ultrasound Detection

Abstract

Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with noncontact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as 278 µPa rms and we explore two different designs-one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air-sample interface presents a tradeoff with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than 10× smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance. Index Terms-Capacitive micromachined ultrasonic transducer (CMUT), non-contact microwave-induced thermoacoustics, piecewise synthetic aperture, ultrasound (US) imaging. I. INTRODUCTION P ORTABLE imaging systems are in demand for a variety of applications, from nondestructive testing and sensing to security screening and point-of-care diagnostic imaging [1]-[3]. Such systems must also be low-cost, safe, Manuscript

An Airborne Sonar System for Underwater Remote Sensing and Imaging

“…The multi-cycle, narrowband source requirement is in contrast to other sensing modalities which aim to use either short, delta-like pulses or wideband frequency-domain signals for enhanced imaging resolution [62]. Despite the narrowband CMUTs, with a sufficiently large aperture our system achieves reasonable imaging resolution on the order of the acoustic wavelength [19], which is approximately 2 cm in water for a 71 kHz acoustic wave.…”

“…This one-way transmission loss is still substantial and poses signal-to-noise (SNR) challenges on the received signals. To maintain high-sensitivity ultrasonic detection despite the large acoustic impedance mismatch, the CMUTs are designed to be narrowband, resonant sensors with high quality factors [19]. In contrast to the wideband LDV-based detection, the acoustic signals are now required to have multiple cycles at the CMUTs' resonance frequency as depicted in Fig.…”

“…The CMUT used herein operates at a center frequency of 71 kHz and has been previously reported in prior works [19], [59], [60]. The CMUT was designed to operate at a low acoustic frequency to limit the attenuation in air: approximately 165 dB/m at 1 M Hz, while only 3 dB/m at 100 kHz [61].…”

“…The CMUT was designed to operate at a low acoustic frequency to limit the attenuation in air: approximately 165 dB/m at 1 M Hz, while only 3 dB/m at 100 kHz [61]. It is clear that operating at acoustic frequencies in the M Hz-range would limit receiver standoff to a few centimeters; however, at lower acoustic frequencies, meter standoffs are possible with reasonable attenuation while still achieving cm-scale resolution [19].…”

“…Since the CMUTs are micro-electromechanical (MEMS) devices, they can easily interface with circuits fabricated on silicon and can be integrated into large arrays [18]. While a large physical array of CMUTs is not in the scope of this work, mechanical scanning of a single CMUT device with coherent detection allows sufficient spatial sampling of the acoustic echoes needed for proof-of-concept image reconstruction [19]. Our proposed CMUT detection technique requires a considerably different system design approach but offers higher sensitivity than the LDV-based methods while also offering potential to scale to large sensor arrays.…”