Ultrasound imaging (USI) and optical imaging (OI) modalities, such as photoacoustic imaging (PAI), offer complementary benefits for medical diagnosis and monitoring. However, their seamless integration poses a challenge. Transparent ultrasound transducers (TUTs) offer a potential solution, but existing TUTs suffer from inferior acoustic performance compared to conventional opaque ultrasound transducers (OUTs). This study addresses this limitation by developing a TUT with acoustic performance comparable to OUTs, facilitating simultaneous high-quality USI and PAI.

Previous attempts to create high-performance TUTs have fallen short due to difficulties in creating transparent materials with the required acoustic impedance. While some studies have demonstrated fusion imaging modalities, the acoustic performance of the TUTs used was inferior to OUTs. Others have focused on transparent optical sensors for PAI but lacked US imaging capabilities. The key challenges are creating transparent matching layers with an acoustic impedance of 7-9 MRayl to maximize transmission efficiency, a transparent backing layer with an impedance greater than 5 MRayl to minimize ringdowns, and ensuring a strong connection between all layers to maintain transducer quality.

This research aimed to create a transparent adhesive with suitable acoustic, rheological, and optical properties. The researchers designed a silicon dioxide-epoxy composite material for both the matching and backing layers. They used simulations and experiments to optimize the material's composition and properties, focusing on achieving the target acoustic impedances (7.5 MRayl for the front matching layer and 4–6 MRayl for the backing layer) while maintaining high optical transparency (>80%). The fabrication process involved mixing ceramic particles into epoxy, degassing under vacuum, curing in two stages, and lapping/polishing to the desired thickness. The TUTs were constructed using lithium niobate (LNO) crystals, ITO electrodes, and the developed composite materials. The researchers used simulations based on the Krimholtz, Leedom, and Matthae (KLM) model to analyze the transducer's performance. They measured the electrical input impedance and pulse-echo response to validate the simulation results and compared the performance to conventional OUTs and TUTs. For imaging, an integrated US/PA microscopy system was built, incorporating the fabricated TUT. The system's performance was evaluated in phantoms and in vivo using chicken breast tissue, live mice, and human palm tissue. The in-vivo experiments included monitoring dye perfusion to assess the system’s capabilities for functional imaging. The experimental setup comprised an imaging head, a raster scanning mechanism, two angle-adjustable mirrors, and a fixed right-angle prism mirror. A parabolic mirror focused both the laser beam and the ultrasound beam. Axial and lateral resolutions were measured, along with signal-to-noise ratio (SNR) at various depths. In-vivo imaging was performed on mice and a human volunteer's palm, acquiring both US and PA data. Data analysis involved creating average intensity projections (AIPs), average amplitude projections (AAPs), and maximum amplitude projections (MAPs) of the image data. Depth-sliced images were also generated and analyzed to extract detailed anatomical information. Ex-vivo experiments validated the dye accumulation in the liver through PA imaging.

The study successfully fabricated a transparent ultrasound transducer (TUT) with significantly improved acoustic performance compared to previous designs. The developed silicon dioxide-epoxy composite material achieved the target acoustic impedances while maintaining high optical transparency. The fabricated TUT exhibited a 63% bandwidth at a single resonance frequency (30 MHz) and pulse-echo sensitivity comparable to a conventional opaque ultrasound transducer (OUT). In-vivo dual-modal ultrasound and photoacoustic imaging experiments in mice and a human palm demonstrated high-definition images with an imaging depth of >15 mm in both modalities. The axial resolution reached 32.6 µm for USI and 40.4 µm for PAI, leading to depth-to-resolution ratios exceeding 500 and 370 respectively. The signal-to-noise ratio (SNR) for USI and PAI in chicken breast tissue reached 55 dB and 49 dB, respectively. The perfusion kinematics of an infrared dye in a live mouse were successfully monitored using the TUT-based imaging system, demonstrating the capability of the system for functional imaging. Ex-vivo experiments validated the in-vivo findings regarding dye accumulation in the liver. The performance of the proposed TUT was compared to various commercial and custom-made transducers, demonstrating comparable noise-equivalent pressure (NEP) and superior coaxial alignment.

The results demonstrate a significant advancement in TUT technology, overcoming the long-standing limitations of acoustic impedance mismatch. The high-performance TUT enables simultaneous, high-resolution ultrasound and photoacoustic imaging at unprecedented depths, surpassing the capabilities of previous systems. The achievement of comparable acoustic performance to OUTs opens new avenues for integrating optical and ultrasonic imaging for various applications. The ability to monitor perfusion kinematics highlights the potential of this technology for functional imaging studies. This study provides a new standard for TUT design and fabrication, paving the way for enhanced sensor fusion in biomedical imaging and other fields.

This study successfully designed and fabricated a highly sensitive and broadband transparent ultrasound transducer (TUT) using a novel silicon dioxide-epoxy composite material. The resulting TUT exhibits performance comparable to conventional opaque ultrasound transducers while maintaining high optical transparency. The integrated US/PA imaging system demonstrated high-resolution, high-contrast imaging in live animals and humans, achieving significantly improved depth-to-resolution ratios compared to existing systems. Future research could explore optimizing the material properties for further performance enhancement, investigating applications in various clinical settings, and exploring miniaturization for minimally invasive procedures.

While the study achieved exceptional results, potential limitations include the use of a parabolic mirror, which might constrain the acoustic numerical aperture (NAA) compared to a concave lens. The imaging depth was influenced by the sample's acoustic wave attenuation, leading to shallower penetration in the human palm compared to the mouse model. The sample size for in-vivo animal and human studies was relatively small. Further studies with larger sample sizes are needed to confirm the generalizability of these results.