Resolution Enhanced Non-Contact Thermoacoustic Imaging using Coded Pulse Excitation

Non-contact thermoacoustic and photoacoustic imaging systems which combine the high resolution of ultrasound with good dielectric/optical contrast show promise in many medical, remote sensing and non-destructive testing applications. One approach that enables meeting the challenging signal-to-noise constraints in non-contact imaging is the use of highly sensitive, resonant air-coupled capacitive micromachined ultrasound transducers (CMUT) as receivers. This sensitivity, however, is gained through a fundamental tradeoff with bandwidth and is at the cost of lower image quality due to slowly decaying residual oscillations of the underdamped, high quality factor CMUTs. In this paper, we propose a pulse-based, multi-cycle signal excitation scheme which optimally modulates the generated ultrasound pressure via the thermoacoustic effect so as to actively cancel the residual oscillations of the CMUT response. This improves achievable thermoacoustic image resolution and reduces multipath clutter. The proposed technique can also be used to improve resolution in contact-based imaging systems that use resonant sensors.