Design and fabrication of transparent ultrasound transducer for photoacoustic imaging application

Abstract

Photoacoustic imaging (PAI) plays a pivotal role within the domain of biomedical engineering, a field that integrates the high contrast and optical imaging capabilities of ultrasound imaging with its profound penetration depth. However, conventional ultrasound transducers are opaque, thereby obstructing the light pathway, so the necessity of complex light components in PAI systems to deliver light results in bulky system designs. Transparent ultrasound transducers (TUTs) are regarded as a viable solution to this challenge, enabling the transmission of light through the transducers and direct illumination of targets. This study aims to enhance the performance of single-element TUTs by investigating both transparent active and passive materials. Additionally, the TUT array is fabricated and characterized for its potential to offer a novel co-axial design for photoacoustic computed tomography (PACT). Adoption of transparent polymethyl methacrylate (PMMA) for acoustic matching layers in high-frequency TUTs results in a significant improvement of -6 dB bandwidth (BW) from 20% to 50%, leading to enhanced axial resolution of PAI. The efficacy of the proposed TUT has been demonstrated through successful phantom and in vivo PAI with different frequencies of TUTs, indicating that the novel transparent acoustic matching method is universally applicable. To increase sensitivity of TUTs in vivo ultrasound imaging, transparent piezoelectric materials are studied to explore ultrasound/photoacoustic dual-modality imaging. Cutting-edge alternating current (AC) poling technology is studied for high-frequency (>20 MHz) TUT applications, with a focus on voltage and cycle number conditions. The [001]-oriented Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) single crystal is used for TUT due to its high piezoelectric constant and great piezoelectric voltage constant, and the transducer shows insertion loss (IL) of 23.8 dB, achieving successful in vivo ultrasound imaging. Additionally, Eu doped PMN-PT ceramic is utilized to develop a TUT with dimensions of 2x2 mm² for photoacoustic microscopy. The dual-modality imaging capability of this ceramic has been demonstrated with a high signal-to-noise ratio, and its imaging depth has also been successfully demonstrated. Eu doped PMN-PT ceramic exhibits very large clamped dielectric constant, making it a promising candidate for photoacoustic endoscopy applications. The TUT array is fabricated using a novel fabrication method. The fabrication process includes initial bonding of the flexible circuit, followed by the separate dicing of the elements based on the alignment of the flexible circuit. The 64-element array exhibits a center frequency of 17 MHz and a -6 dB bandwidth of 35%, demonstrating excellent uniformity. Additionally, the photoacoustic A-line of each element is measured, and the TUT array demonstrates considerable potential for further in vivo PACT applications, as evidenced by the measured high performance of each element. The findings of this study indicate a promising future for TUTs and related imaging applications.