Optical-resolution photoacoustic microscopy (OR-PAM) is widely utilized in biomedical applications. However, performance of OR-PAM has long been affected by the tradeoff between the field of view (FOV) and sensitivity of the ultrasound transducer (UT). Commercially available unfocused UTs possess a millimeter-scale FOV, enabling a large field imaging via rapid optical scanning. However, their sensitivity is limited and hence large laser dosage is required, making them unsuitable for imaging tissues like brain and eyes. Focused UTs or focused acoustic detection methods (e.g. applying acoustic lens with unfocused UTs) offer excellent sensitivity but have an FOV of only a few tens of micrometers. Thus a mechanical scanning is needed to improve the FOV, resulting in very slow imaging speed. To address this challenge, we propose a solution based on miniature transducer (mini-UT) and developed a series of high-performance OR-PAM using one or multiple mini-UTs suitable for various biomedical imaging scenarios. This home-made unfocused mini-UT has a size of 0.4 mm × 0.5 mm × 0.25 mm, a center frequency of 50 MHz, a bandwidth exceeding 60%, and an effective FOV over 400 μm ×400 μm. It exhibits a sensitivity tens of times higher than traditional unfocused UT and a substantially increased FOV than focused UT. Additionally, the miniature size enables versatile usage of mini-UT in multiple imaging scenarios. Initially, we developed an OR-PAM with an imaging rate of 8 frames per second (FPS) using dual-axis optical scanning, enabling real-time tracking of red blood cells in live tissue. Benefited by the miniature size of the mini-UT, we further developed a dual-mode photoacoustic/fluorescence microscope with sub-micrometer resolution, enabling concurrent imaging of single-neuron firing and blood oxygen metabolism in awake mouse brain. Further, we developed a head-mounted microscope based on the above dual-mode system and realized neurovascular imaging in freely moving mice. We expanded the FOV of the imaging system by a factor of four based on a ring of 4 mini-UTs around the optical objective. We also developed a fast wide-filed OR-PAM (1 FPSFOV: 6 mm*6 mm) based on a linear array of 8 mini-UTs combined with a polygon scanning mirror, enabling real-time cortex-wide imaging of a mouse brain. The proposed method based on mini-UT provides a new insight to break through the constraint between FOV and sensitivity within OR-PAM and will consequently promote the biomedical applications of the technology.

Photoacoustic imaging is a new medical imaging modality that has shown great promise in various clinical applications. Recent advances in deep learning algorithms further enhance the photoacoustic imaging capability in terms of imaging speed, spatial resolution, and imaging depth. This talk highlights the deep-learning enhanced translational photoacoustic imaging studies at the University at Buffalo. More specifically, we will present results in biometric identification, breast cancer imaging, and foot ulcer imaging.

The direct and non-invasive visualization of neurons, glia, and microvasculature in vivo is crucial for neuroscience research. When it comes to imaging the brain in vivo, the opaque skull and brain tissue pose challenges as they significantly reduce the excitation and emission photons, leading to optical aberration and scattering. Consequently, even in the superficial brain, the image quality is poor. To address this, we have developed a microscope that combines three-photon excitation with two specific adaptive optics (AO) techniques. This innovative approach enables fast measurements and the correction of both low-order and high-order aberrations in tissue at great depth. As a result, we can achieve high-resolution in vivo imaging of fine neuronal structures in the mouse cortex through the intact skull. Similarly, though the retina is the only part of the central nervous system that can be directly visualized, optical aberrations in the eye degrade imaging resolution and hinder the visualization of subcellular retinal structures. To overcome this limitation, we have developed an adaptive optics two-photon excitation fluorescence microscopy (AO-TPEFM) system capable of correcting ocular aberrations. This system has achieved subcellular resolution for in vivo fluorescence imaging of the mouse retina. By leveraging accurate wavefront sensing and rapid aberration correction, AO-TPEFM allows for both structural and functional imaging of the mouse retina with submicron resolution. We look forward to sharing more about our latest applications and results at the Photonics North conference.