Collecting back-reflected photons in photoacoustic microscopy

Fast wavelet-based photoacoustic microscopy

A novel photoacoustic microscopy (PAM) structure, based on Haar wavelet patterns, is proposed in this paper. Its main goal is to mitigate the PAM imaging resolution and thus the time of its sampling process without compromising the image quality. Owing to the intrinsic nature of wavelet transform, this structure collects spatial and spectral components simultaneously, and this feature speeds up the sampling process by 33%. The selection of these patterns helps in better control of required conditions, such as multi-resolution imaging, to guarantee adequate image quality in comparison to previous microscopic structures. Simulation results prove the superior quality of the proposed approach (about 47% better peak signal-to-noise ratio) compared to the latest structures in this field, achieving a high-resolution and high-quality image.

Acoustic-resolution-based photoacoustic microscopy with non-coaxial arrangements and a multiple vertical scan for high lateral resolution in-depth

In conventional acoustic-resolution-based photoacoustic microscopy (ARPAM), a focused ultrasound transducer is placed coaxially with the laser beam to obtain the generated ultrasound signals. The information from deep regions can be greatly affected by the shallow targets. More importantly, in ARPAM the irreconcilable conflict between the lateral resolution and depth of fields has always been a major factor that lowers the imaging quality. In this work, an ARPAM system was developed, in which a non-coaxial arrangement of light illumination and acoustic detection was adopted to alleviate the influence of the tissue surface on the deep targets, and a focal zone integral algorithm was applied with a multiple scanning scheme to improve the lateral resolution. The system can achieve a consistent high lateral resolution of 0.5 mm over a large range in the axial direction. Both the phantom experiment and the chicken embryo in vivo results indicate that the proposed method can provide more in-depth information compared with the conventional ARPAM method. With the development of high repetition lasers and the advancement of image scanning technologies, the proposed method may play an important role in cerebral vascular imaging, superficial tumor imaging, and other related biomedical imaging applications.

[Focal zone integral and multiple axial scanning based acoustic resolution photoacoustic microscopy with high lateral resolution in-depth]

Acoustic resolution photoacoustic microscopy (ARPAM) combines the advantages of high optical contrast, and high ultrasonic spatial resolution and penetration. However, in photoacoustic microscopy (PAM), the information from deep regions can be greatly affected by the shallow targets, and most importantly, the irreconcilable conflict between the lateral resolution and depth of fields has always be a major factor that limits the imaging quality. In this work, an ARPAM system was developed, in which a non-coaxial arrangement of light illumination and acoustic detection was adopted to alleviate the influence of the tissue surface on the deep targets, and a novel focal zone integral algorithm was applied with multiple axial scanning to improve the lateral resolution. Phantom experiment results show that, the build system can maintain a consistent high lateral resolution of 0.6 mm over a large range in axial direction, which is close to the theoretical calculations. The following tumor imaging results on nude mice indicate that, the proposed method can provide more in-depth information compared with the conventional back detection ARPAM method. With the development of fast repetition lasers and image scanning technologies, the proposed method may play an important role in cerebral vascular imaging, cervical cancer photoacoustic endoscopic detection, and superficial tumor imaging.

The Dual-Mode Imaging of Nanogold-Labeled Cells by Photoacoustic Microscopy and Fluorescence Optical Microscopy

Photoacoustic microscopy is dominantly sensitive to the endogenous optical absorption, while a fluorescence optical microscopy can detect the fluorescence emission to obtain the image of a sample. To some extent, the physical processes of the 2 methods are opposite, one is absorption and another is emission, but both can be used to image cells. In this article, a simultaneous dual-mode imaging system of photoacoustic microscopy and fluorescence optical microscopy is set up to image tobacco cells. Furthermore, gold nanoparticles, which have a large absorption coefficient and enough fluorescence emission with wavelength of 512 nm, are used to label certain drugs and added to the tobacco cells. Then based on the simultaneous dual-mode microscopy imaging system, the photoacoustic microscopy and fluorescence optical microscopy images of gold nanoparticle-labeled tobacco cells are obtained. The final purpose of this experimental research is to detect if the labeled drugs can enter the cells by the positions of the gold nanoparticles. This will help the experts to deliver organic pesticide more accurately and effectively. The experimental results show that by gold nanoparticle labeling technology, the imaging quality of photoacoustic microscopy and fluorescence optical microscopy can be improved, which indicates that the drugs probably enter the tobacco cells successfully.

Biomedical photoacoustics: fundamentals, instrumentation and perspectives on nanomedicine

Photoacoustic imaging (PAI) is an integrated biomedical imaging modality which combines the advantages of acoustic deep penetration and optical high sensitivity. It can provide functional and structural images with satisfactory resolution and contrast which could provide abundant pathological information for disease-oriented diagnosis. Therefore, it has found vast applications so far and become a powerful tool of precision nanomedicine. However, the investigation of PAI-based imaging nanomaterials is still in its infancy. This perspective article aims to summarize the developments in photoacoustic technologies and instrumentations in the past years, and more importantly, present a bright outlook for advanced PAI-based imaging nanomaterials as well as their emerging biomedical applications in nanomedicine. Current challenges and bottleneck issues have also been discussed and elucidated in this article to bring them to the attention of the readership.

Photoacoustic tomography imaging and estimation of oxygen saturation of hemoglobin in ocular tissue of rabbits

This study evaluated in vivo imaging capabilities and safety of qualitative monitoring of oxygen saturation of hemoglobin (sO2) of rabbit ciliary body tissues obtained with acoustic resolution (AR) photoacoustic tomography (PAT). AR PAT was used to collect trans-scleral images from ciliary body vasculature of seven New Zealand White rabbits. The PAT sO2 measurements were obtained under the following conditions: when systemic sO2 as measured by pulse oximetry was between 100% and 99% (level 1); systemic sO2 as measured by pulse oximetry was between 98% and 90% (level 2); and systemic sO2 as measured by pulse oximetry was less than 90% (level 3). Following imaging, histological analysis of ocular tissue was conducted to evaluate for possible structural damage caused by the AR PAT imaging. AR PAT was able to resolve anatomical structures of the anterior segment of the eye, viewed through the cornea or anterior sclera. Histological studies revealed no ocular damage. On average, sO2 values (%) obtained with AR PAT were lower than sO2 values obtained with pulse oximetry (all p < 0.001): 86.28 ± 4.16 versus 99.25 ± 0.28, 84.09 ± 1.81 vs. 95.3 ± 2.6, and 64.49 ± 7.27 vs. 71.15 ± 10.21 for levels 1, 2 and 3 respectively. AR PAT imaging modality is capable of qualitative monitoring for deep tissue sO2 in rabbits. Further studies are needed to validate and modify the AR PAT modality specifically for use in human eyes. Having a safe, non-invasive method of in vivo imaging of sO2 in the anterior segment is important to studies evaluating the role of oxidative damage, hypoxia and ischemia in pathogenesis of ocular diseases.

Recent Progress in Multimodal Photoacoustic Tomography

Since its first demonstration of functional imaging in small animals about a decade ago, photoacoustic tomography (PAT) has quickly become one of the fastest growing biomedical imaging modalities. Combining optical excitation with acoustic detection, PAT can provide detailed images of tissues deep in the body. While PAT technology continues to improve significantly, substantial efforts have also been made to develop multimodal PAT systems. These systems not only provide complementary information for more comprehensive characterization of tissue, they also generate data that can be used to further improve PAT reconstruction. This review will present current progress in multimodal PAT imaging, focusing on the technical aspects of integration and its applications in biomedicine.

Hybrid multiphoton and optoacoustic microscope

We present a hybrid microscope combining multiphoton microscopy incorporating second-harmonic generation contrast and optical-resolution optoacoustic (photoacoustic) microscopy. We study the relative performance of the two systems and investigate the complementarity of contrast by demonstrating the label-free imaging capabilities of the hybrid microscope on zebrafish larvae ex vivo, concurrently visualizing the fish musculature and melanocytes. This implementation can prove useful in multiparametric microscopy studies, enabling broader information to be collected from biological specimens.

A transparent broadband ultrasonic detector based on an optical micro-ring resonator for photoacoustic microscopy

Photoacoustic microscopy (PAM) does not rely on contrast agent to image the optical absorption contrast in biological tissue. It is uniquely suited for measuring several tissue physiological parameters, such as hemoglobin oxygen saturation, that would otherwise remain challenging. Researchers are designing new clinical diagnostic tools and multimodal microscopic systems around PAM to fully unleash its potential. However, the sizeable and opaque piezoelectric ultrasonic detectors commonly used in PAM impose a serious constraint. Our solution is a coverslip-style optically transparent ultrasound detector based on a polymeric optical micro-ring resonator (MRR) with a total thickness of 250 μm. It enables highly-sensitive ultrasound detection over a wide receiving angle with a bandwidth of 140 MHz, which corresponds to a photoacoustic saturation limit of 287 cm⁻¹, at an estimated noise-equivalent pressure (NEP) of 6.8 Pa. We also established a theoretical framework for designing and optimizing the MRR for PAM.

Photoacoustic Microscopy

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (~1 mm in soft tissue). With its excellent scalability, PAM can provide high-resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering-based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence-based methods, such as wide-field, confocal, and multi-photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state-of-the-art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies.

Integrating photoacoustic ophthalmoscopy with scanning laser ophthalmoscopy, optical coherence tomography, and fluorescein angiography for a multimodal retinal imaging platform

Photoacoustic ophthalmoscopy (PAOM) is a newly developed retinal imaging technology that holds promise for both fundamental investigation and clinical diagnosis of several blinding diseases. Hence, integrating PAOM with other existing ophthalmic imaging modalities is important to identify and verify the strengths of PAOM compared with the established technologies and to provide the foundation for more comprehensive multimodal imaging. To this end, we developed a retinal imaging platform integrating PAOM with scanning laser ophthalmoscopy (SLO), spectral-domain optical coherence tomography (SD-OCT), and fluorescein angiography (FA). In the system, all the imaging modalities shared the same optical scanning and delivery mechanisms, which enabled registered retinal imaging from all the modalities. High-resolution PAOM, SD-OCT, SLO, and FA images were acquired in both albino and pigmented rat eyes. The reported in vivo results demonstrate the capability of the integrated system to provide comprehensive anatomic imaging based on multiple optical contrasts.

Photoacoustic ophthalmoscopy for in vivo retinal imaging: current status and prospects

Photoacoustic ophthalmoscopy (PAOM) is a new retinal imaging technology that offers the unique capability to measure optical absorption in the retina. Because PAOM is compatible with optical coherence tomography, scanning laser ophthalmoscopy, and autofluorescence imaging, registered multimodal images can be acquired from a single device at comparable resolution for comprehensive anatomic and functional retinal characterizations. Therefore, PAOM is anticipated to have applications in both research and clinical diagnosis of many blinding diseases. The authors explain the basic principles of the photoacoustic effect and imaging. Then, different types of photoacoustic microscopy are introduced and compared. Finally, the current status of photoacoustic imaging in animal eyes is presented and the prospects of future development of PAOM are suggested.

Optical coherence photoacoustic microscopy: accomplishing optical coherence tomography and photoacoustic microscopy with a single light source

We developed optical coherence photoacoustic microscopy (OC-PAM) to demonstrate that the functions of optical coherence tomography (OCT) and photoacoustic microscopy (PAM) can be achieved simultaneously by using a single illuminating light source. We used a pulsed broadband laser centered at 580 nm and detected the absorbed photons through photoacoustic detection and the back-scattered photons with an interferometer. In OC-PAM, each laser pulse generates both one OCT A-line and one PAM A-line simultaneously; as a result, the two imaging modalities are intrinsically co-registered in the lateral directions. In vivo images of the mouse ear were acquired to demonstrate the capabilities of OC-PAM.

Photoacoustic tomography: fundamentals, advances and prospects

Optical microscopy has been contributing to the development of life science for more than three centuries. However, due to strong optical scattering in tissue, its in vivo imaging ability has been restricted to studies at superficial depths. Advances in photoacoustic tomography (PAT) now allow multiscale imaging at depths from sub-millimeter to several centimeters, with spatial resolutions from sub-micrometer to sub-millimeter. Because of this high scalability and its unique optical absorption contrast, PAT is capable of performing anatomical, functional, molecular and fluid-dynamic imaging at various system levels, and is playing an increasingly important role in fundamental biological research and clinical practice. This review discusses recent technical progress in PAT and presents corresponding applications. It ends with a discussion of several prospects and their technical challenges.

Combined photoacoustic microscopy and optical coherence tomography can measure metabolic rate of oxygen

We proposed to measure the metabolic rate of oxygen (MRO(2)) in small animals in vivo using a multimodal imaging system that combines laser-scanning optical-resolution photoacoustic microscopy (LSOR-PAM) and spectral-domain optical coherence tomography (SD-OCT). We first tested the capability of the multimodal system to measure flow rate in a phantom made of two capillary tubes of different diameters. We then demonstrated the capability of measuring MRO(2) by imaging two parallel vessels selected from the ear of a Swiss Webster mouse. The hemoglobin oxygen saturation (sO(2)) and the vessel diameter were measured by the LSOR-PAM and the blood flow velocity was measured by the SD-OCT, from which blood flow rate and MRO(2) were further calculated. The measured blood flow rates in the two vessels agreed with each other.

Multimodal subcellular imaging with microcavity photoacoustic transducer

Photoacoustic microscopy (PAM) is dominantly sensitive to the endogenous optical absorption compared with the confocal microscopy which images with scattering photons. PAM has similar structure such as optical transportation system, the optical scanning, and light source with the laser scanning confocal microscopy (LSCM). In order to match the PAM with LSCM, a special design microcavity photoacoustic (PA) transducer with high sensitivity is developed to detect the photoacoustic signals induced by modulated continuous wave (CW) laser. By employing a microcavity PA transducer, a PAM can be integrated with LSCM. Thus a simultaneous multimodal imaging can be obtained with the same laser source and optical system. The lateral resolutions of the PAM and the LSCM are both tested to be better than 1.25 μm. Then subcellular multimodal imaging can be achieved. Images from the two modes are corresponding with each other but functionally complementary. Combining PAM and LSCM provides more comprehensive information for the cytological test. This technique is demonstrated for imaging red-blood cells and meristematic cells.

Simultaneous dual molecular contrasts provided by the absorbed photons in photoacoustic microscopy

We investigated the feasibility of simultaneously imaging two distinctive molecular contrasts provided by the absorbed photons in biological tissues with a single light source. The molecular contrasts are based on two physical effects induced by the absorbed photons: photoacoustics (PA) and autofluorescence (AF). In an integrated multimodal imaging system, the PA and AF signals were detected by a high-sensitivity ultrasonic transducer and an avalanche photodetector, respectively. The system was tested by imaging ocular tissue samples, including the retinal pigment epithelium and the ciliary body. The acquired images provided information on the spatial distributions of melanin and lipofuscin in these samples.

Adaptive optics photoacoustic microscopy

We have developed an adaptive optics photoacoustic microscope (AO-PAM) for high-resolution imaging of biological tissues, especially the retina. To demonstrate the feasibility of AO-PAM we first designed the AO system to correct the wavefront errors of the illuminating light of PAM. The aberrations of the optical system delivering the illuminating light to the sample in PAM was corrected with a close-loop AO system consisting of a 141-element MEMS-based deformable mirror (DM) and a Shack-Hartmann (SH) wavefront sensor operating at 15 Hz. The photoacoustic signal induced by the illuminating laser beam was detected by a custom-built needle ultrasonic transducer. When the wavefront errors were corrected by the AO system, the lateral resolution of PAM was measured to be better than 2.5 µm using a low NA objective lens. We tested the system on imaging ex vivo ocular samples, e.g., the ciliary body and retinal pigment epithelium (RPE) of a pig eye. The AO-PAM images showed significant quality improvement. For the first time we were able to resolve single RPE cells with PAM.

Integrated photoacoustic and fluorescence confocal microscopy

We have developed a dual-modality imaging system by integrating optical-resolution photoacoustic microscopy and fluorescence confocal microscopy to provide optical absorption and fluorescence contrasts simultaneously. By sharing the same laser source and objective lens, intrinsically registered photoacoustic and fluorescence images are acquired in a single scan. The micrometer resolution allows imaging of both blood and lymphatic vessels down to the capillary level. Simultaneous photoacoustic angiography and fluorescence lymphangiography were demonstrated, presenting more information to study tumor angiogenesis, vasculature, and microenvironments in vivo.

Photoacoustic ophthalmoscopy for in vivo retinal imaging

We have developed a non-invasive photoacoustic ophthalmoscopy (PAOM) for in vivo retinal imaging. PAOM detects the photoacoustic signal induced by pulsed laser light shined onto the retina. By using a stationary ultrasonic transducer in contact with the eyelids and scanning only the laser light across the retina, PAOM provides volumetric imaging of the retinal micro-vasculature and retinal pigment epithelium at a high speed. For B-scan frames containing 256 A-lines, the current PAOM has a frame rate of 93 Hz, which is comparable with state-of-the-art commercial spectral-domain optical coherence tomography (SD-OCT). By integrating PAOM with SD-OCT, we further achieved OCT-guided PAOM, which can provide multi-modal retinal imaging simultaneously. The capabilities of this novel technology were demonstrated by imaging both the microanatomy and microvasculature of the rat retina in vivo.