Transparent ultrasound transducers (TUTs) have gained significant traction in the fields of photoacoustic (PA) and ultrasound (US) applications. TUTs possess the unique capability to transmit and receive ultrasound waves while maintaining optical transparency. As a result, TUTs simplify the PA imaging process and facilitate seamless integration with other optical imaging modalities. However, the limited sensitivity of TUTs has been a primary challenge hampering their widespread adoption in PA setups. One often overlooked factor contributing to this limitation is the electrical impedance mismatch between the transducer and the data acquisition system. Here, we designed and studied the utilization of a filter-based electrical impedance matching (EIM) circuits to enhance the sensitivity of lithium niobate-based TUTs. In our approach, the fabricated TUTs incorporate a quarter-wavelength Parylene-C matching layer and epoxy as a backing layer. Our results demonstrate that the integration of the EIM circuit yields substantial improvements in the sensitivity, bandwidth and axial resolution of both pulse-echo US signals and PA signals. PA imaging of leaf phantoms were compared with and without EIMs to further showcase the performance enhancements that can be achieved by integrating EIM with TUTs. Overall, these results demonstrate that EIM circuits can be employed to improve the performance of TUTs.
Recently, piezoelectric transparent ultrasound transducers (TUTs) have gained the attention of the photoacoustic imaging community. The ability to illuminate living subjects through the TUT and minimal acoustic coupling requirements allows for miniaturization, reduces cost, and increases the ease of multimodal integrations. TUTs, therefore, have great potential for photoacoustic imaging; however, challenges remain on matching their acoustic sensitivity to conventional opaque piezoelectric transducers. Various noise sources arising from TUT fabrication and electronics limit their sensitivity. This paper discusses various TUT-specific noise considerations that are detrimental to signal to noise ratio. We also assess the effect of different design considerations by comparing different TUTs. The results can help optimize TUTs design for improving performance, thus paving the way for powerful TUT based photoacoustic applications.
Photoacoustic imaging (PAI) is a hybrid imaging modality that uses ultrasound waves generated from light absorbing tissue chromophores to provide high spatial resolution and depth-resolved molecular information. However, conventional PAI setups involve complicated arrangement of optical components surrounding opaque ultrasound transducers to achieve a co-aligned optical illumination and ultrasound receiving field. This opacity of traditional ultrasound transducers impedes the miniaturization of the imaging head, besides precluding integration with other imaging modalities. To overcome these limitations, we recently fabricated a single element transparent ultrasound transducer (TUT) window using indium tin oxide (ITO) coated lithium niobate (LiNbO3) piezoelectric material and demonstrated its application for endoscopy and microscopy PAI applications. Extending on this work, we report new developments of TUTs to improve their detection bandwidth, sensitivity, and signal to noise ratio (SNR) while maintaining sufficient transparency. This includes investigating LiNbO3 and PMN-PT as transparent piezoelectric materials with different matching layer designs. Fabricated TUTs were characterized using pulse echo and electrical impedance analysis. The PAI performance of the fabricated TUTs were characterized using photoacoustic A-line signals from light absorbing targets. The proposed TUTs are low cost, easy to fabricate, and can be scaled and easily integrated into different PAI geometries such as: endoscopy, microscopy, and computed tomography systems for high-throughput imaging applications.
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