Simultaneous ultra-high frequency photoacoustic microscopy and photoacoustic radiometry of zebrafish larvae in vivo

30-100 kHz, 2 ns passively Q-switched laser for fast and efficient photoacoustic microscopy

Actively Q-switched (AQS) fiber laser and solid-state laser (SSL) are widely used for photoacoustic microscopy (PAM). In contrast, passively Q-switched (PQS) SSL not only maintains most of the merits of AQS lasers, but also exhibits unique advantages, including the pulse width (PW), pulse repetition rate (PRR) tunability, wavelength, compactness, and cost. These advantages all benefit the PAM. However, there are few reports demonstrating the performance of PQS-SSL on PA imaging. Here, we demonstrate a compact PQS-SSL for fast and efficient PA imaging. The laser uniquely maintains a constant PW (~2 ns) and pulse energy (~3 μJ) during the PRR variation (30-100 kHz), which is valuable for preserving a stabilized imaging performance at different scanning rates. The PA imaging performance is compared by a resolution target and showcased by whole-body scanning of an embryonic zebrafish in vivo. The performance indicates that PQS-SSL is a promising candidate for PAM.

Multicolor Photoacoustic Volumetric Imaging of Subcellular Structures

Photoacoustic imaging (PAI) has been widely used in multiscale and multicontrast imaging of biological structures and functions. Optical resolution photoacoustic microscopy (OR-PAM), an emerging submodality of PAI, features high lateral resolution and rich optical contrast, indicating great potential in visualizing cellular and subcellular structures. However, three-dimensional (3D) imaging of subcellular structures using OR-PAM has remained a challenge due to the limited axial resolution. In this study, we propose a multicolor 3D photoacoustic microscopy with high lateral/axial resolutions of 0.42/2 and 0.5/2.5 μm at 532 and 780 nm excitation, respectively. Owing to the significantly increased axial resolution, we could visualize the volumetric subcellular structures of melanoma cells using intrinsic contrast. In addition, we carried out multicolor imaging of labeled microtubules/clathrin-coated pits (CCP) and microtubules/mitochondria, respectively, with one scanning by using two different excitation wavelengths. The internal connections between different subcellular structures are revealed by quantitatively comparing the spatial distributions of microtubules/CCP and microtubules/mitochondria in a single cell. Current results suggest that the proposed OR-PAM may serve as an efficient tool for subcellular and cytophysiological studies.

Label-free photoacoustic microscopy: a potential tool for the live imaging of blood disorders in zebrafish

The zebrafish has emerged as a useful model for human hematological disorders. Transgenic zebrafish that express green fluorescence protein (GFP) in red blood cells (RBCs) visualized by fluorescence microscopy (FLM) is a fundamental approach in such studies to understand the cellular processes and biological functions. However, additional and cumbersome efforts are required to breed a transgenic zebrafish line with reliable GFP expression. Further, the yolk autofluorescence and finite GFP fluorescence lifetimes also have an adverse impact on the observation of target signals. Here, we investigate the identification of intracerebral hemorrhage (ICH) and hemolytic anemia (HA) in zebrafish embryos using label-free photoacoustic microscopy (PAM) for imaging. First, ICH and HA in transgenic LCR-EGFP zebrafish are mainly studied by PAM and FLM. The results show that PAM is comparable to FLM in good identification of ICH and HA. Besides, PAM is more advantageous in circumventing the issue of autofluorescence. Secondly, ICH and HA in the transparent casper zebrafish without fluorescent labeling are imaged by PAM and bright-field microscopy (BFM). Because of the high contrast to reveal RBCs, PAM obviously outperforms BFM in the identification of both ICH and HA. Note that FLM cannot observe casper zebrafish due to its lack of fluorescent labeling. Our work proves that PAM can be a useful tool to study blood disorders in zebrafish, which has advantages: (i) Reliable results enabled by intrinsic absorption of RBCs; (ii) wide applicability to zebrafish strains (no requirement of a transgene); (iii) high sensitivity in identification of ICH and HA compared with BFM.

Anti-HER2 PLGA-PEG polymer nanoparticle containing gold nanorods and paclitaxel for laser-activated breast cancer detection and therapy

Phase-transition nanoparticles have been identified as effective theragnostic, anti-cancer agents. However, non-selective delivery of these agents results in inaccurate diagnosis and insufficient treatment. In this study, we report on the development of targeted phase-transition polymeric nanoparticles (NPs) for the imaging and treatment of breast cancer cell lines over-expressing human epidermal growth factor receptor 2 (HER2). These NPs contain a perfluorohexane liquid interior and gold nanorods (GNRs) stabilized by biodegradable and biocompatible copolymer PLGA-PEG. Water-insoluble therapeutic drug Paclitaxel (PAC) and fluorescent dye were encapsulated into the PLGA shell. The NP surfaces were conjugated to HER2-binding agent, Herceptin, to actively target HER2-positive cancer cells. We evaluated the potential of using these NPs as a photoacoustic contrast agent. The efficacy of cancer cell treatment by laser-induced vaporization and stimulated drug release were also investigated. The results showed that our synthesized PLGA-PEG-GNRs (mean diameter 285 ± 29 nm) actively targeted HER2 positive cells with high efficacy. The laser-induced vaporization caused more damage to the targeted cells versus PAC-only and negative controls. This agent may provide better diagnostic imaging and therapeutic potential than current methods for treating HER2-positive breast cancer.

3D PHOVIS: 3D photoacoustic visualization studio

Photoacoustic (PA) imaging (or optoacoustic imaging) is a novel biomedical imaging method in biological and medical research. This modality performs morphological, functional, and molecular imaging with and without labels in both microscopic and deep tissue imaging domains. A variety of innovations have enhanced 3D PA imaging performance and thus has opened new opportunities in preclinical and clinical imaging. However, the 3D visualization tools for PA images remains a challenge. There are several commercially available software packages to visualize the generated 3D PA images. They are generally expensive, and their features are not optimized for 3D visualization of PA images. Here, we demonstrate a specialized 3D visualization software package, namely 3D Photoacoustic Visualization Studio (3D PHOVIS), specifically targeting photoacoustic data, image, and visualization processes. To support the research environment for visualization and fast processing, we incorporated 3D PHOVIS onto the MATLAB with graphical user interface and developed multi-core graphics processing unit modules for fast processing. The 3D PHOVIS includes following modules: (1) a mosaic volume generator, (2) a scan converter for optical scanning photoacoustic microscopy, (3) a skin profile estimator and depth encoder, (4) a multiplanar viewer with a navigation map, and (5) a volume renderer with a movie maker. This paper discusses the algorithms present in the software package and demonstrates their functions. In addition, the applicability of this software to ultrasound imaging and optical coherence tomography is also investigated. User manuals and application files for 3D PHOVIS are available for free on the website (www.boa-lab.com). Core functions of 3D PHOVIS are developed as a result of a summer class at POSTECH, "High-Performance Algorithm in CPU/GPU/DSP, and Computer Architecture." We believe our 3D PHOVIS provides a unique tool to PA imaging researchers, expedites its growth, and attracts broad interests in a wide range of studies.

Label-free optical imaging in developmental biology [Invited]

Application of optical imaging in developmental biology marks an exciting frontier in biomedical optics. Optical resolution and imaging depth allow for investigation of growing embryos at subcellular, cellular, and whole organism levels, while the complexity and variety of embryonic processes set multiple challenges stimulating the development of various live dynamic embryonic imaging approaches. Among other optical methods, label-free optical techniques attract an increasing interest as they allow investigation of developmental mechanisms without application of exogenous markers or fluorescent reporters. There has been a boost in development of label-free optical imaging techniques for studying embryonic development in animal models over the last decade, which revealed new information about early development and created new areas for investigation. Here, we review the recent progress in label-free optical embryonic imaging, discuss specific applications, and comment on future developments at the interface of photonics, engineering, and developmental biology.

Functional optical coherence tomography and photoacoustic microscopy imaging for zebrafish larvae

We present a dual modality functional optical coherence tomography and photoacoustic microscopy (OCT-PAM) system. The photoacoustic modality employs an akinetic optical sensor with a large imaging window. This imaging window enables direct reflection mode operation, and a seamless integration of optical coherence tomography (OCT) as a second imaging modality. Functional extensions to the OCT-PAM system include Doppler OCT (DOCT) and spectroscopic PAM (sPAM). This functional and non-invasive imaging system is applied to image zebrafish larvae, demonstrating its capability to extract both morphological and hemodynamic parameters in vivo in small animals, which are essential and critical in preclinical imaging for physiological, pathophysiological and drug response studies.

Single-shot photoacoustic microscopy of hemoglobin concentration, oxygen saturation, and blood flow in sub-microseconds

We present fast functional optical-resolution photoacoustic microscopy (OR-PAM) that can simultaneously image hemoglobin concentration, blood flow speed, and oxygen saturation with three-pulse excitation. To instantaneously determine the blood flow speed, dual-pulse photoacoustic flowmetry is developed to determine the blood flow speed from photoacoustic signal decay in sub-microseconds. Grueneisen relaxation effect is compensated for in the oxygen saturation calculation. The blood flow imaging is validated in phantom and in vivo experiments. The results show that the flow speed can be measured accurately in sub-microseconds by comparing the dual-pulse flowmetric method with photoacoustic Doppler flowmetry. Wide-field OR-PAM of hemoglobin concentration, blood flow speed, and oxygen saturation are demonstrated in the mouse ear. This technical advance enables more biomedical applications for fast functional OR-PAM.

Insights into photoacoustic speckle and applications in tumor characterization

In ultrasound imaging, fully-developed speckle arises from the spatiotemporal superposition of pressure waves backscattered by randomly distributed scatterers. Speckle appearance is affected by the imaging system characteristics (lateral and axial resolution) and the random-like nature of the underlying tissue structure. In this work, we examine speckle formation in acoustic-resolution photoacoustic (PA) imaging using simulations and experiments. Numerical and physical phantoms were constructed to demonstrate that PA speckle carries information related to unresolved absorber structure in a manner similar to ultrasound speckle and unresolved scattering structures. A fractal-based model of the tumor vasculature was used to study PA speckle from unresolved cylindrical vessels. We show that speckle characteristics and the frequency content of PA signals can be used to monitor changes in average vessel size, linked to tumor growth. Experimental validation on murine tumors demonstrates that PA speckle can be utilized to characterize the unresolved vasculature in acoustic-resolution photoacoustic imaging.