Combined photoacoustic and ultrasound biomicroscopy

Highly sensitive self-focused ultrasound transducer with a bionic back-reflector for multiscale-resolution photoacoustic microscopy

In this study, we designed a self-focused ultrasonic transducer made of polyvinylidene fluoride (PVDF). This transducer involves a back-reflector, which is modeled after tapetum lucidum in the eyes of some nocturnal animals. The bionic structure reflects the ultrasound, which passes through the PVDF membrane, back to PVDF and provides a second chance for the PVDF to convert the ultrasound to electric signals. This design increases the amount of ultrasound absorbed by the PVDF, thereby improving the detection sensitivity. Both ultrasonic and photoacoustic (PA) experiments were conduct to characterize the performance of the transducer. The results show that the fabricated transducer has a center frequency of 13.07 MHz, and a bandwidth of 96% at -6 dB. With an acoustic numerical aperture (NA) of 0.64, the transducer provides a lateral resolution of 140µm. Importantly, the bionic design improves the detection sensitivity of the transducer about 30%. Finally, we apply the fabricated transducer to optical-resolution (OR) and acoustic-resolution photoacoustic microscopy (AR-PAM) to achieve multiscale-resolution PA imaging. Imaging of the bamboo leaf and the leaf skeleton demonstrates that the proposed transducer can provide high spatial resolution, better imaging intensity and contrast. Therefore, the proposed transducer design will be useful to enhance the performance of multiscale-resolution PAM.

Influence mechanism of the temporal duration of laser irradiation on photoacoustic technique: a review

Significance

In the photoacoustic (PA) technique, the laser irradiation in the time domain (i.e., laser pulse duration) governs the characteristics of PA imaging-it plays a crucial role in the optical-acoustic interaction, the generation of PA signals, and the PA imaging performance.

Aim

We aim to provide a comprehensive analysis of the impact of laser pulse duration on various aspects of PA imaging, encompassing the signal-to-noise ratio, the spatial resolution of PA imaging, the acoustic frequency spectrum of the acoustic wave, the initiation of specific physical phenomena, and the photothermal-PA (PT-PA) interaction/conversion.

Approach

By surveying and reviewing the state-of-the-art investigations, we discuss the effects of laser pulse duration on the generation of PA signals in the context of biomedical PA imaging with respect to the aforementioned aspects.

Results

First, we discuss the impact of laser pulse duration on the PA signal amplitude and its correlation with the lateral resolution of PA imaging. Subsequently, the relationship between the axial resolution of PA imaging and the laser pulse duration is analyzed with consideration of the acoustic frequency spectrum. Furthermore, we examine the manipulation of the pulse duration to trigger physical phenomena and its relevant applications. In addition, we elaborate on the tuning of the pulse duration to manipulate the conversion process and ratio from the PT to PA effect.

Conclusions

We contribute to the understanding of the physical mechanisms governing pulse-width-dependent PA techniques. By gaining insight into the mechanism behind the influence of the laser pulse, we can trigger the pulse-with-dependent physical phenomena for specific PA applications, enhance PA imaging performance in biomedical imaging scenarios, and modulate PT-PA conversion by tuning the pulse duration precisely.

Laser2: A two-domain photon-phonon laser

The laser is one of the greatest inventions in history. Because of its ubiquitous applications and profound societal impact, the concept of the laser has been extended to other physical domains including phonon lasers and atom lasers. Quite often, a laser in one physical domain is pumped by energy in another. However, all lasers demonstrated so far have only lased in one physical domain. We have experimentally demonstrated simultaneous photon and phonon lasing in a two-mode silica fiber ring cavity via forward intermodal stimulated Brillouin scattering (SBS) mediated by long-lived flexural acoustic waves. This two-domain laser may find potential applications in optical/acoustic tweezers, optomechanical sensing, microwave generation, and quantum information processing. Furthermore, we believe that this demonstration will usher in other multidomain lasers and related applications.

Rapid ultraviolet photoacoustic remote sensing microscopy using voice-coil stage scanning

There is an unmet need for fast virtual histology technologies that exhibit histological realism and can scan large sections of fresh tissue within intraoperative time-frames. Ultraviolet photoacoustic remote sensing microscopy (UV-PARS) is an emerging imaging modality capable of producing virtual histology images that show good concordance to conventional histology stains. However, a UV-PARS scanning system that can perform rapid intraoperative imaging over mm-scale fields-of-view at fine resolution (<500 nm) has yet to be demonstrated. In this work, we present a UV-PARS system which utilizes voice-coil stage scanning to demonstrate finely resolved images for 2×2 mm² areas at 500 nm sampling resolution in 1.33 minutes and coarsely resolved images for 4×4 mm² areas at 900 nm sampling resolution in 2.5 minutes. The results of this work demonstrate the speed and resolution capabilities of the UV-PARS voice-coil system and further develop the potential for UV-PARS microscopy to be employed in a clinical setting.

Achieving depth-independent lateral resolution in AR-PAM using the synthetic-aperture focusing technique

Acoustic-resolution photoacoustic microscopy (AR-PAM) is a promising imaging modality that renders images with ultrasound resolution and extends the imaging depth beyond the optical ballistic regime. To achieve a high lateral resolution, a large numerical aperture (NA) of a focused transducer is usually applied for AR-PAM. However, AR-PAM fails to hold its performance in the out-of-focus region. The lateral resolution and signal-to-noise ratio (SNR) degrade substantially, thereby leading to a significantly deteriorated image quality outside the focal area. Based on the concept of the synthetic-aperture focusing technique (SAFT), various strategies have been developed to address this challenge. These include 1D-SAFT, 2D-SAFT, adaptive-SAFT, spatial impulse response (SIR)-based schemes, and delay-multiply-and-sum (DMAS) strategies. These techniques have shown progress in achieving depth-independent lateral resolution, while several challenges remain. This review aims to introduce these developments in SAFT-based approaches, highlight their fundamental mechanisms, underline the advantages and limitations of each approach, and discuss the outlook of the remaining challenges for future advances.

High-speed functional photoacoustic microscopy using a water-immersible two-axis torsion-bending scanner

Optical-resolution photoacoustic microscopy (OR-PAM) can provide functional, anatomical, and molecular images at micrometer level resolution with an imaging depth of less than 1 mm in tissue. However, the imaging speed of traditional OR-PAM is often low due to the point-by-point mechanical scanning and cannot capture time-sensitive dynamic information. In this work, we demonstrate a recent effort in improving the imaging speed of OR-PAM, using a newly developed water-immersible two-axis scanner. Driven by water-compatible electromagnetic actuation force, the new scanning mirror employs a novel torsion-bending mechanism to achieve fast 2D scanning. The torsion scanning along the fast-axis works in the resonant model, and the bending scanning along the slow-axis operate at the quasi-static mode. The scanning speed and scanning range along the two axes can be independently adjusted. Steered by the two-axis torsion-bending scanning mirror immersed in water, the focused excitation light and the generated acoustic wave can be confocally aligned over the entire imaging area. Thus, a high imaging speed can be achieved without sacrificing the detection sensitivity. Equipped with the torsion-bending scanner, the high-speed OR-PAM system has achieved a cross-sectional frame rate of 400 Hz, and a volumetric imaging speed of 1 Hz over a field of view of 1.5 × 2.5 mm2. We have also demonstrated high-speed OR-PAM of the hemodynamic changes in response to pharmaceutical and physiological challenges in small animal models in vivo. We expect the torsion-bending scanner based OR-PAM will find matched biomedical studies of tissue dynamics.

Photoacoustic microscopy with sparse data by convolutional neural networks

The point-by-point scanning mechanism of photoacoustic microscopy (PAM) results in low-speed imaging, limiting the application of PAM. In this work, we propose a method to improve the quality of sparse PAM images using convolutional neural networks (CNNs), thereby speeding up image acquisition while maintaining good image quality. The CNN model utilizes attention modules, residual blocks, and perceptual losses to reconstruct the sparse PAM image, which is a mapping from a 1/4 or 1/16 low-sampling sparse PAM image to a latent fully-sampled one. The model is trained and validated mainly on PAM images of leaf veins, showing effective improvements quantitatively and qualitatively. Our model is also tested using in vivo PAM images of blood vessels of mouse ears and eyes. The results suggest that the model can enhance the quality of the sparse PAM image of blood vessels in several aspects, which facilitates fast PAM and its clinical applications.

Centimeter-scale wide-field-of-view laser-scanning photoacoustic microscopy for subcutaneous microvasculature in vivo

We developed a simple and compact laser-scanning photoacoustic microscopy (PAM) for imaging large areas of subcutaneous microvasculature in vivo. The reflection-mode PAM not only retains the advantage of high scanning speed for optical scanning, but also offers an imaging field-of-view (FOV) up to 20 × 20 mm², which is the largest FOV available in laser-scanning models so far. The lateral resolution of the PAM system was measured to be 17.5 µm. Image experiments on subcutaneous microvasculature in in vivo mouse ears and abdomen demonstrate the system's potential for fast and high-resolution imaging for injuries and diseases of large tissues and organs.

Axial accuracy and signal enhancement in acoustic-resolution photoacoustic microscopy by laser jitter effect correction and pulse energy compensation

In recent years, photoacoustic imaging has found vast applications in biomedical imaging. Photoacoustic imaging has high optical contrast and high ultrasound resolution allowing deep tissue non-invasive imaging beyond the optical diffusion limit. Q-switched lasers are extensively used in photoacoustic imaging due to the availability of high energy and short laser pulses, which are essential for high-resolution photoacoustic imaging. In most cases, this type of light source suffers from pulse peak-power energy variations and timing jitter noise, resulting in uncertainty in the output power and arrival time of the laser pulses. These problems cause intensity degradation and temporal displacement of generated photoacoustic signals which in turn deteriorate the quality of the acquired photoacoustic images. In this study, we used a high-speed data acquisition system in combination with a fast photodetector and a software-based approach to capture laser pulses precisely in order to reduce the effect of timing jitter and normalization of the photoacoustic signals based on pulse peak-powers simultaneously. In the experiments, maximum axial accuracy enhancement of 14 µm was achieved in maximum-amplitude projected images on XZ and YZ planes with ±13.5 ns laser timing jitter. Furthermore, photoacoustic signal enhancement of 77% was obtained for 75% laser pulses peak-power stability.

Simultaneous Dual-Modal Multispectral Photoacoustic and Ultrasound Macroscopy for Three-Dimensional Whole-Body Imaging of Small Animals

Photoacoustic imaging is a promising medical imaging technique that provides excellent function imaging of an underlying biological tissue or organ. However, it is limited in providing structural information compared to other imaging modalities, such as ultrasound imaging. Thus, to offer complete morphological details of biological tissues, photoacoustic imaging is typically integrated with ultrasound imaging. This dual-modal imaging technique is already implemented on commercial clinical ultrasound imaging platforms. However, commercial platforms suffer from limited elevation resolution compared to the lateral and axial resolution. We have successfully developed a dual-modal photoacoustic and ultrasound imaging to address these limitations, specifically targeting animal studies. The system can acquire whole-body images of mice in vivo and provide complementary structural and functional information of biological tissue information simultaneously. The color-coded depth information can be readily obtained in photoacoustic images using complementary information from ultrasound images. The system can be used for several biomedical applications, including drug delivery, biodistribution assessment, and agent testing.

Integrating photoacoustic microscopy with other imaging technologies for multimodal imaging

As a hybrid optical microscopic imaging technology, photoacoustic microscopy images the optical absorption contrasts and takes advantage of low acoustic scattering of biological tissues to achieve high-resolution anatomical and functional imaging. When combined with other imaging modalities, photoacoustic microscopy-based multimodal technologies can provide complementary contrast mechanisms to reveal complementary information of biological tissues. To achieve intrinsically and precisely registered images in a multimodal photoacoustic microscopy imaging system, either the ultrasonic transducer or the light source can be shared among the different imaging modalities. These technologies are the major focus of this minireview. It also covered the progress of the recently developed penta-modal photoacoustic microscopy imaging system featuring a novel dynamic focusing technique enabled by OCT contour scan.

Photoacoustic and ultrasound (PAUS) dermoscope with high sensitivity and penetration depth by using a bimorph transducer

A bimorph transducer was proposed to improve the detection sensitivity and imaging depth of photoacoustic and ultrasound (PAUS) dermoscope. By applying the bimorph transducer, the imaging depth and sensitivity of PAUS dermoscope were enhanced by simultaneously improving excitation efficiency and reception bandwidth. The integrated design of the imaging head of the dermoscope makes it highly convenient for detecting human skin. The PAUS imaging performance was demonstrated via visualizing subcutaneous tumor and depicting full structures of different skin layers from epidermis to subcutaneous tissue. The results confirm that the dermoscope with the bimorph transducer is well suited for PA and US dual-modality imaging, which can provide multi-information for skin disease.

From Light to Sound: Photoacoustic and Ultrasound Imaging in Fundamental Research of Alzheimer's Disease

Alzheimer's disease (AD) causes severe cognitive dysfunction and has long been studied for the underlining physiological and pathological mechanisms. Several biomedical imaging modalities have been applied, including MRI, PET, and high-resolution optical microscopy, for research purposes. However, there is still a strong need for imaging tools that can provide high spatiotemporal resolutions with relatively deep penetration to enhance our understanding of AD pathology and monitor treatment progress in fundamental research. Photoacoustic (PA) imaging and ultrasound (US) imaging can potentially address these unmet needs in AD research. PA imaging provides functional information with endogenous and/or exogenous contrast, while US imaging provides structural information. Recent studies have demonstrated the ability to monitor physiological parameters in small-animal brains with PA and US imaging as well as the feasibility of using US imaging as a therapeutic tool for AD. This concise review aims to introduce recent advances in AD research using PA and US imaging, provide the fundamentals, and discuss the potentials and challenges for future advances.

Super Wide-Field Photoacoustic Microscopy of Animals and Humans In Vivo

Acoustic-resolution photoacoustic micro-scopy (AR-PAM) is an emerging biomedical imaging modality that combines superior optical sensitivity and fine ultrasonic resolution in an optical quasi-diffusive regime (~1-3 mm in tissues). AR-PAM has been explored for anatomical, functional, and molecular information in biological tissues. Heretofore, AR-PAM systems have suffered from a limited field-of-view (FOV) and/or slow imaging speed, which have precluded them from routine preclinical and clinical applications. Here, we demonstrate an advanced AR-PAM system that overcomes both limitations of previous AR-PAM systems. The new AR-PAM system demonstrates a super wide-field scanning that utilized a 1-axis water-proofing microelectromechanical systems (MEMS) scanner integrated with two linear stepper motor stages. We achieved an extended FOV of 36 ×80 mm² by mosaicking multiple volumetric images of 36 ×2.5 mm² with a total acquisition time of 224 seconds. For one volumetric data (i.e., 36 ×2.5 mm²), the B-scan imaging speed over the short axis (i.e., 2.5 mm) was 83 Hz in humans. The 3D volumetric image was also provided by using MEMS mirror scanning along the X-axis and stepper-motor scanning along the Y-axis. The super-wide FOV mosaic image was realized by registering and merging all individual volumetric images. Finally, we obtained multi-plane whole-body in-vivo PA images of small animals, illustrating distinct multi-layered structures including microvascular networks and internal organs. Importantly, we also visualized microvascular networks in human fingers, palm, and forearm successfully. This advanced MEMS-AR-PAM system could potentially enable hitherto not possible wide preclinical and clinical applications.

Fully waterproof two-axis galvanometer scanner for enhanced wide-field optical-resolution photoacoustic microscopy

A large field-of-view and fast scanning of photoacoustic microscopy (PAM) relatively have been difficult to obtain due to the water-drowned structure of the system for the transmission of ultrasonic signals. Researchers have widely studied the achievement of a waterproof scanner for dynamic biological applications with a high-resolution and high signal-to-noise ratio. This Letter reports a novel, to the best of our knowledge, waterproof galvanometer scanner-based PAM system with a successfully attainable ${9.0}\;{\rm mm} \times {14.5}\;{\rm mm}$9.0mm×14.5mm scan region, amplitude scan rate of 40 kHz, and spatial resolution of 4.9 µm. The in vivo characterization of a mouse brain in intact-skull microvascular visualization demonstrated its capability in biomedical imaging and is anticipated to be an effective technique for various preclinical and clinical studies.

Photoacoustic imaging of cells in a three-dimensional microenvironment

Imaging live cells in a three-dimensional (3D) culture system yields more accurate information and spatial visualization of the interplay of cells and the surrounding matrix components compared to using a two-dimensional (2D) cell culture system. However, the thickness of 3D cultures results in a high degree of scattering that makes it difficult for the light to penetrate deeply to allow clear optical imaging. Photoacoustic (PA) imaging is a powerful imaging modality that relies on a PA effect generated when light is absorbed by exogenous contrast agents or endogenous molecules in a medium. It combines a high optical contrast with a high acoustic spatiotemporal resolution, allowing the noninvasive visualization of 3D cellular scaffolds at considerable depths with a high resolution and no image distortion. Moreover, advances in targeted contrast agents have also made PA imaging capable of molecular and cellular characterization for use in preclinical personalized diagnostics or PA imaging-guided therapeutics. Here we review the applications and challenges of PA imaging in a 3D cellular microenvironment. Potential future developments of PA imaging in preclinical applications are also discussed.

A versatile dark-field acoustic-resolution photoacoustic microscopy system aided by 3D printing

Since the dark-field acoustic-resolution photoacoustic microscopy (AR-PAM) was invented over one decade ago, its powerful imaging capability made this system successful. In this work, we designed and tested an AR-PAM system whose key parts are relied on 3D printing and fiber bundles. This new design not only makes it much simpler to develop a robust PAM system, but also the illumination angle is adjustable to aid for different applications. Our simulation study and phantom experiments demonstrated that this design has the comparable performance with traditional dark-field AR-PAM.

Towards microvascular pressure estimation using ultrasound and photoacoustic imaging

Microvascular pressure drives perfusion in tissues but is difficult to measure. A method is proposed here to estimate relative pressures in microvessels using photoacoustic and ultrasound tracking of small vessels during calibrated tissue compression. A photoacoustic-ultrasound dual imaging transducer is used to directly compress on tissue in vivo. Photoacoustic signals from blood vessels diminish as an external load is applied and eventually reaches a minimum or vanishes when external pressure is sufficiently greater than the internal pressure. Two methods were proposed to estimate relative pressures. In the first approach, vessels were tracked during compression and when the vessel photoacoustic signals vanished below a set threshold, the internal pressures were assigned as the external loading pressure at the respective collapse point. In this approach pressures required to collapse vessel signatures completely were found to be much greater than physiological blood pressures. An alternative approach was to track the cross-sectional area of small vessels with changing external load and fitting the data to a known Shapiro model for thin-walled vessel compression. This approach produced estimates of internal pressures which were much more realistic. Both approaches produced the same rank-ordering of relative pressures of various vessels in vivo. Approaches thus far require future work to become fully quantitative but the present contributions represent steps towards this goal.

Optoacoustic mesoscopy for biomedicine

Fuelled by innovation, optical microscopy plays a critical role in the life sciences and medicine, from basic discovery to clinical diagnostics. However, optical microscopy is limited by typical penetration depths of a few hundred micrometres for in vivo interrogations in the visible spectrum. Optoacoustic microscopy complements optical microscopy by imaging the absorption of light, but it is similarly limited by penetration depth. In this Review, we summarize progress in the development and applicability of optoacoustic mesoscopy (OPAM); that is, optoacoustic imaging with acoustic resolution and wide-bandwidth ultrasound detection. OPAM extends the capabilities of optical imaging beyond the depths accessible to optical and optoacoustic microscopy, and thus enables new applications. We explain the operational principles of OPAM, its placement as a bridge between optoacoustic microscopy and optoacoustic macroscopy, and its performance in the label-free visualization of tissue pathophysiology, such as inflammation, oxygenation, vascularization and angiogenesis. We also review emerging applications of OPAM in clinical and biological imaging.

Concurrent photoacoustic and ultrasound microscopy with a coaxial dual-element ultrasonic transducer

Simultaneous photoacoustic and ultrasound (PAUS) imaging has attracted increasing attention in biomedical research to probe the optical and mechanical properties of tissue. However, the resolution for majority of the existing PAUS systems is on the order of 1 mm as the majority are designed for clinical use with low-frequency US detection. Here we developed a concurrent PAUS microscopy that consists of optical-resolution photoacoustic microscopy (OR-PAM) and high-frequency US pulse-echo imaging. This dual-modality system utilizes a novel coaxial dual-element ultrasonic transducer (DE-UST) and provides anatomical and functional information with complementary contrast mechanisms, achieving a spatial resolution of 7 μm for PA imaging and 106 μm for US imaging. We performed phantom studies to validate the system’s performance. The vasculature of a mouse’s hind paw was imaged to demonstrate the potential of this hybrid system for biomedical applications.

Estimation of cerebral metabolic rate of oxygen consumption using combined multiwavelength photoacoustic microscopy and Doppler microultrasound

The metabolic rate of oxygen consumption is an important metric of tissue oxygen metabolism and is especially critical in the brain, yet few methods are available for measuring it. We use a custom combined photoacoustic-microultrasound system and demonstrate cerebral oxygen consumption estimation in vivo. In particular, the cerebral metabolic rate of oxygen consumption was estimated in a murine model during variation of inhaled oxygen from hypoxia to hyperoxia. The hypothesis of brain autoregulation was confirmed with our method even though oxygen saturation and flow in vessels changed.

Portable optical resolution photoacoustic microscopy (pORPAM) for human oral imaging

Optical resolution photoacoustic microscopy (ORPAM) represents one of the fastest evolving optical microscopic techniques. However, due to the bulky size and complicated system configuration of conventional ORPAM, it is largely limited to small animal experiments. In this Letter, we present the design and evaluation of a portable ORPAM with a high spatiotemporal resolution and a large field of view. In this system, we utilize a rotatory scanning mechanism instead of the conventional raster scanning to achieve translationless imaging of the probe/samples, making it accessible to the human oral lip and tongue. After phantom evaluation, we applied this system to monitor longitudinal neo-angiogenesis of tumor growth and, for the first time, to the best of our knowledge, image the oral vascular network of humans to show its potential in clinical detection of early-stage oral cancer.

Development of a Fiber Laser with Independently Adjustable Properties for Optical Resolution Photoacoustic Microscopy

Photoacoustic imaging is based on the detection of generated acoustic waves through thermal expansion of tissue illuminated by short laser pulses. Fiber lasers as an excitation source for photoacoustic imaging have recently been preferred for their high repetition frequencies. Here, we report a unique fiber laser developed specifically for multiwavelength photoacoustic microscopy system. The laser is custom-made for maximum flexibility in adjustment of its parameters; pulse duration (5–10 ns), pulse energy (up to 10 μJ) and repetition frequency (up to 1 MHz) independently from each other and covers a broad spectral region from 450 to 1100 nm and also can emit wavelengths of 532, 355, and 266 nm. The laser system consists of a master oscillator power amplifier, seeding two stages; supercontinuum and harmonic generation units. The laser is outstanding since the oscillator, amplifier and supercontinuum generation parts are all-fiber integrated with custom-developed electronics and software. To demonstrate the feasibility of the system, the images of several elements of standardized resolution test chart are acquired at multiple wavelengths. The lateral resolution of optical resolution photoacoustic microscopy system is determined as 2.68 μm. The developed system may pave the way for spectroscopic photoacoustic microscopy applications via widely tunable fiber laser technologies.

Engineering Dark Chromoprotein Reporters for Photoacoustic Microscopy and FRET Imaging

A subset of the family of fluorescent proteins are the non-fluorescent chromoproteins which are promising probe molecules for use in photoacoustic imaging and as acceptor chromophores in Förster resonance energy transfer (FRET)-based biosensors. Typical approaches for fluorescent protein optimization by screening of large libraries of variants cannot be effectively applied to chromoproteins due to their characteristic lack of fluorescence. To address this challenge, we have developed a directed evolution method to iteratively screen large libraries of protein variants on the basis of their photoacoustic signal levels. By applying this procedure to the promising Ultramarine and cjBlue chromoprotein templates, we were able to identify improved variants with a 02–04 fold increase in photoacoustic signal-to-noise ratio after only a few evolutionary steps. These improved variants enable more accurate spectral de-mixing and localization of protein-producing bacteria in vivo and serve as effective FRET acceptors for both fluorescence- and photoacoustic-based detection of protease activity.

Glucose-functionalized Au nanoprisms for optoacoustic imaging and near-infrared photothermal therapy

Targeted imaging and tumor therapy using nanomaterials has stimulated research interest recently, but the high cytotoxicity and low cellular uptake of nanomaterials limit their bioapplication. In this paper, glucose (Glc) was chosen to functionalize Au nanoprisms (NPrs) for improving the cytotoxicity and cellular uptake of Au@PEG-Glc NPrs into cancer cells. Glucose is a primary source of energy at the cellular level and at cellular membranes for cell recognition. A coating of glucose facilitates the accumulation of Au@PEG-Glc NPrs in a tumor region much more than Au@PEG NPrs. Due to the high accumulation and excellent photoabsorbing property of Au@PEG-Glc NPrs, enhanced optoacoustic imaging of a tumor in vivo was achieved, and visualization of the tumor further guided cancer treatment. Based on the optical-thermal conversion performance of Au@PEG-Glc NPrs, the tumor in vivo was effectively cured through photothermal therapy. The current work demonstrates the great potential of Au@PEG-Glc NPrs in optoacoustic imaging and photothermal cancer therapy in future.

Multiscale Functional and Molecular Photoacoustic Tomography

Photoacoustic tomography (PAT) combines rich optical absorption contrast with the high spatial resolution of ultrasound at depths in tissue. The high scalability of PAT has enabled anatomical imaging of biological structures ranging from organelles to organs. The inherent functional and molecular imaging capabilities of PAT have further allowed it to measure important physiological parameters and track critical cellular activities. Integration of PAT with other imaging technologies provides complementary capabilities and can potentially accelerate the clinical translation of PAT.

Aqueous synthesis of PEGylated copper sulfide nanoparticles for photoacoustic imaging of tumors

By integrating high imaging sensitivity and high resolution in a single modality, photoacoustic (PA) imaging emerges as a promising diagnostic tool for clinical applications. Benefiting from the absorption in the near-infrared region (NIR), copper sulfide nanoparticles (NPs) as a contrast agent are potentially useful for increasing the sensitivity of PA imaging. However, the aqueous synthesis of size-tunable, biocompatible and colloidally stable copper sulfide NPs remains challenging due to the intrinsic dipole-dipole interactions among particles. In this work, aqueous synthesis of PEGylated copper sulfide NPs with controllable size between 3 and 7 nm was developed. The particle size-dependent contrast enhancement effect of the copper sulfide NPs for PA imaging was carefully studied both in vitro and in vivo. Although the contrast enhancement effect of the copper sulfide NPs is proportional to particle size, the in vivo studies revealed that copper sulfide NPs smaller than 5 nm presented higher tumor imaging performance, especially at the tumor boundary site, which was further discussed in combination with the pharmacokinetic behaviors of differently sized particles.

Photoacoustic imaging platforms for multimodal imaging

Photoacoustic (PA) imaging is a hybrid biomedical imaging method that exploits both acoustical Epub ahead of print and optical properties and can provide both functional and structural information. Therefore, PA imaging can complement other imaging methods, such as ultrasound imaging, fluorescence imaging, optical coherence tomography, and multi-photon microscopy. This article reviews techniques that integrate PA with the above imaging methods and describes their applications.

Gold nanoparticles for photoacoustic imaging

Photoacoustic (PA) imaging is a biomedical imaging modality that provides functional information regarding the cellular and molecular signatures of tissue by using endogenous and exogenous contrast agents. There has been tremendous effort devoted to the development of PA imaging agents, and gold nanoparticles as exogenous contrast agents have great potential for PA imaging due to their inherent and geometrically induced optical properties. The gold-based nanoparticles that are most commonly employed for PA imaging include spheres, rods, shells, prisms, cages, stars and vesicles. This article provides an overview of the current state of research in utilizing these gold nanomaterials for PA imaging of cancer, atherosclerotic plaques, brain function and image-guided therapy.

In vivo study of endometriosis in mice by photoacoustic microscopy

Endometriosis (EM) impacts the healthcare and the quality of life for women of reproductive age. However, there is no reliable noninvasive diagnosis method for either animal study or clinical use. In this work, a novel imaging method, photoacoustic microscopy (PAM) was employed to study the EM on the mouse model. Our results demonstrated the PAM noninvasively provided the high contrast and 3D imaging of subcutaneously implanted EM tissue in the nude mouse in vivo. The statistical study also indicated PAM had high sensitivity and specificity in the diagnosis of EM in this animal study. In addition, we also discussed the potential clinical application for PAM in the diagnosis of EM.

Integrated photoacoustic, ultrasound and fluorescence platform for diagnostic medical imaging-proof of concept study with a tissue mimicking phantom

The structural and molecular heterogeneities of biological tissues demand the interrogation of the samples with multiple energy sources and provide visualization capabilities at varying spatial resolution and depth scales for obtaining complementary diagnostic information. A novel multi-modal imaging approach that uses optical and acoustic energies to perform photoacoustic, ultrasound and fluorescence imaging at multiple resolution scales from the tissue surface and depth is proposed in this paper. The system comprises of two distinct forms of hardware level integration so as to have an integrated imaging system under a single instrumentation set-up. The experimental studies show that the system is capable of mapping high resolution fluorescence signatures from the surface, optical absorption and acoustic heterogeneities along the depth (>2cm) of the tissue at multi-scale resolution (<1µm to <0.5mm).

Simultaneous viewing of individual cells and ambient microvasculature using optical absorption and fluorescence contrasts

Viewing individual cells and ambient microvasculature simultaneously is crucial for understanding tumor angiogenesis and microenvironments. We developed a confocal fluorescence microscopy (CFM) and photoacoustic microscopy (PAM) dual-modality imaging system that can assess fluorescent contrast and optical absorption contrast in biologic samples simultaneously. After staining tissues with fluorescent dye at an appropriate concentration, each laser pulse can generate not only sufficient fluorescent signals from cells for CFM but also sufficient photoacoustic signals from microvessels for PAM. To explore the potential of this system for diagnosis of bladder cancer, experiments were conducted on a rat bladder model. The CFM image depicts the morphology of individual cells, showing not only large polygonal umbrella cells but also intracellular components. The PAM image acquired at the same time provides complementary information on the microvascular distribution in the bladder wall, ranging from large vessels to capillaries. This device provides an opportunity to realize both histologic assay and microvascular characterization simultaneously. The combination of the information of individual cells and local microvasculature in the bladder offers the capability of envisioning the viability and activeness of these cells and holds promise for more comprehensive study of bladder cancer in vivo.

Interlaced photoacoustic and ultrasound imaging system with real-time coregistration for ovarian tissue characterization

Coregistered ultrasound (US) and photoacoustic imaging are emerging techniques for mapping the echogenic anatomical structure of tissue and its corresponding optical absorption. We report a 128-channel imaging system with real-time coregistration of the two modalities, which provides up to 15 coregistered frames per second limited by the laser pulse repetition rate. In addition, the system integrates a compact transvaginal imaging probe with a custom-designed fiber optic assembly for in vivo detection and characterization of human ovarian tissue. We present the coregistered US and photoacoustic imaging system structure, the optimal design of the PC interfacing software, and the reconfigurable field programmable gate array operation and optimization. Phantom experiments of system lateral resolution and axial sensitivity evaluation, examples of the real-time scanning of a tumor-bearing mouse, and ex vivo human ovaries studies are demonstrated.

High frequency label-free photoacoustic microscopy of single cells

Photoacoustic measurements of melanoma cells and red blood cells (RBCs) using ultra-high frequency (UHF) wide-bandwidth transducers are reported. In this detection system, the resolution typically depends on the parameters of the receiving transducer, and not the focus of the laser. A single melanoma cell was imaged with 200, 375 and 1200 MHz transducers. As the frequency increased, the resolution increased, resulting in greater detail observed. A single RBC was imaged at 1200 MHz, showing the contours of the cell. While lateral and axial resolutions approaching 1 μm are possible with this microscope, the key advantage is the ability to perform a wide-bandwidth quantitative signal analysis of the photoacoustic signals. The power spectrum of the signals measured from RBCs showed distinct spectral minima around 800 and 1500 MHz which are directly related to the RBC geometry. This study reports on the high-resolution imaging capabilities and quantitative analyses using UHF photoacoustic microscopy.

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.

Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement

A DBR fiber grating laser acoustic sensor based on polarization beat signal modulation analysis has been demonstrated for directional acoustic signal measurement. The acoustic sensor was fabricated in birefringent erbium-doped fiber, and the influences of external-acoustic pressure on fiber grating laser sensor were analyzed, considering the effect of relative orientation of the acoustic wave on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasonic pressure. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different directions (0-360 degrees in 15 degrees intervals) corresponding to a nonlinearly change in beat frequency modulation rates. The results demonstrate that the DBR fiber grating laser acoustic sensor has an orientation recognizable ability, offering a potential for acoustic vector signal detection.

Simultaneous three-dimensional photoacoustic and laser-ultrasound tomography

A tomographic setup that provides the co-registration of photoacoustic (PA) and ultrasound (US) images is presented. For pulse-echo US-tomography laser-induced broadband plane ultrasonic waves are produced by illuminating an optically absorbing target with a short near-infrared laser pulse. Part of the same pulse is frequency doubled and used for the generation of PA waves within the object of interest. The laser-generated plane waves are scattered at the imaging object and measured with the same interferometric detector that also acquires the photoacoustic signals. After collection and separation of the data image reconstruction is done using back-projection resulting in three-dimensional, co-registered PA and US images. The setup is characterized and the resolution in PA and US mode is estimated to be about 85 µm and 40 µm, respectively. Besides measurements on phantoms the performance is also tested on a biological sample.

High frequency photoacoustic imaging for in vivo visualizing blood flow of zebrafish heart

A technique on high frame rate(28fps), high frequency co-registered ultrasound and photoacoustic imaging for visualizing zebrafish heart blood flow was demonstrated. This approach was achieved with a 40MHz light weight(0.38g) ring-type transducer, serving as the ultrasound transmitter and receiver, to allow an optic fiber, coupled with a 532nm laser, to be inserted into the hole. From the wire target study, axial resolutions of 38µm and 42µm were obtained for ultrasound and photoacoustic imaging, respectively. Carbon nanotubes were utilized as contrast agents to increase the flow signal level by 20dB in phantom studies, and zebrafish heart blood flow was successfully observed.

Photoacoustic Imaging for Cancer Detection and Staging

Cancer is one of the leading causes of death in the world. Diagnosing a cancer at its early stages of development can decrease the mortality rate significantly and reduce healthcare costs. Over the past two decades, photoacoustic imaging has seen steady growth and has demonstrated notable capabilities to detect cancerous cells and stage cancer. Furthermore, photoacoustic imaging combined with ultrasound imaging and augmented with molecular targeted contrast agents is capable of imaging cancer at the cellular and molecular level, thus opening diverse opportunities to improve diagnosis of tumors, detect circulating tumor cells and identify metastatic lymph nodes. In this paper we introduce the principles of photoacoustic imaging, and review recent developments in photoacoustic imagingas an emerging imaging modality for cancer diagnosis and staging.

Video-rate functional photoacoustic microscopy at depths

We report the development of functional photoacoustic microscopy capable of video-rate high-resolution in vivo imaging in deep tissue. A lightweight photoacoustic probe is made of a single-element broadband ultrasound transducer, a compact photoacoustic beam combiner, and a bright-field light delivery system. Focused broadband ultrasound detection provides a 44-μm lateral resolution and a 28-μm axial resolution based on the envelope (a 15-μm axial resolution based on the raw RF signal). Due to the efficient bright-field light delivery, the system can image as deep as 4.8 mm in vivo using low excitation pulse energy (28 μJ per pulse, 0.35  mJ/cm² on the skin surface). The photoacoustic probe is mounted on a fast-scanning voice-coil scanner to acquire 40 two-dimensional (2-D) B-scan images per second over a 9-mm range. High-resolution anatomical imaging is demonstrated in the mouse ear and brain. Via fast dual-wavelength switching, oxygen dynamics of mouse cardio-vasculature is imaged in realtime as well.

FPGA-based reconfigurable processor for ultrafast interlaced ultrasound and photoacoustic imaging

In this paper, we report, to the best of our knowledge, a unique field-programmable gate array (FPGA)-based reconfigurable processor for real-time interlaced co-registered ultrasound and photoacoustic imaging and its application in imaging tumor dynamic response. The FPGA is used to control, acquire, store, delay-and-sum, and transfer the data for real-time co-registered imaging. The FPGA controls the ultrasound transmission and ultrasound and photoacoustic data acquisition process of a customized 16-channel module that contains all of the necessary analog and digital circuits. The 16-channel module is one of multiple modules plugged into a motherboard; their beamformed outputs are made available for a digital signal processor (DSP) to access using an external memory interface (EMIF). The FPGA performs a key role through ultrafast reconfiguration and adaptation of its structure to allow real-time switching between the two imaging modes, including transmission control, laser synchronization, internal memory structure, beamforming, and EMIF structure and memory size. It performs another role by parallel accessing of internal memories and multi-thread processing to reduce the transfer of data and the processing load on the DSP. Furthermore, because the laser will be pulsing even during ultrasound pulse-echo acquisition, the FPGA ensures that the laser pulses are far enough from the pulse-echo acquisitions by appropriate time-division multiplexing (TDM). A co-registered ultrasound and photoacoustic imaging system consisting of four FPGA modules (64-channels) is constructed, and its performance is demonstrated using phantom targets and in vivo mouse tumor models.

Blood oxygen flux estimation with a combined photoacoustic and high-frequency ultrasound microscopy system: a phantom study

The metabolic rate of oxygen consumption, an important indicator of tissue metabolism, can be expressed as the change of net blood oxygen flux into and out of a tissue region per 100 g of tissue. In this work, we propose a photoacoustic and Doppler ultrasound method for imaging local blood oxygen flux of a single vessel. An imaging system for combined photoacoustic and high-frequency ultrasound microscopy is presented. This system uses a swept-scan 25-MHz ultrasound transducer with confocal dark-field laser illumination optics. A pulse-sequencer enables ultrasonic and laser pulses to be interlaced so that photoacoustic and Doppler ultrasound images are co-registered. Since the mean flow speed can be measured by color Doppler ultrasound, the vessel cross-sectional area can be measured by power Doppler or structural photoacoustic imaging, and multi-wavelength photoacoustic methods can be used to estimate oxygen saturation (sO(2)) and total concentration of haemoglobin (C(Hb)), all of the parameters necessary for oxygen flux estimation can be provided. The accuracy of the flow speed and sO(2) estimation has been investigated. In vitro sheep blood phantom experiments have been performed at different sO(2) levels and mean flow speeds. Blood oxygen flux has been estimated, and the uncertainty of the measurement has been quantified.

3D-visual laser-diode-based photoacoustic imaging

We present a 3D-visual laser-diode-based photoacoustic imaging (LD-PAI) system with a pulsed semiconductor laser source, which has the properties of being inexpensive, portable, and durable. The laser source was operated at a wavelength of 905 nm with a repetition rate of 0.8 KHz. The energy density on the sample surface is about 2.35 mJ/cm(2) with a pulse energy as low as 5.6 μJ. By raster-scanning, preliminary 3D volumetric renderings of the knotted and helical blood vessel phantoms have been visualized integrally with an axial resolution of 1.1 mm and a lateral resolution of 0.5 mm, and typical 2D photoacoustic image slices with different thickness and orientation were produced with clarity for detailed comparison and analysis in 3D diagnostic visualization. In addition, the pulsed laser source was integrated with the optical lens group and the 3D adjustable rotational stage, with the result that the compact volume of the total radiation source is only 10 × 3 × 3 cm(3). Our goal is to significantly reduce the costs and sizes of the deep 3D-visual PAI system for future producibility.

Tyrosinase as a dual reporter gene for both photoacoustic and magnetic resonance imaging

Reporter genes are useful scientific tools for analyzing promoter activity, transfection efficiency, and cell migration. The current study has validated the use of tyrosinase (involved in melanin production) as a dual reporter gene for magnetic resonance and photoacoustic imaging. MCF-7 cells expressing tyrosinase appear brown due to melanin. Magnetic resonance imaging of tyrosinase-expressing MCF-7 cells in 300 μL plastic tubes displayed a 34 to 40% reduction in T1 compared to normal MCF-7 cells when cells were incubated with 250 μM ferric citrate. Photoacoustic imaging of tyrosinase-expressing MCF-7 cells in 700 μm plastic tubes displayed a 20 to 57-fold increase in photoacoustic signal compared to normal MCF-7 cells. The photoacoustic signal from tyrosinase-expressing MCF-7 cells was significantly greater than blood at 650 nm, suggesting that tyrosinase-expressing cells can be differentiated from the vasculature with in vivo photoacoustic imaging. The imaging results suggest that tyrosinase is a useful reporter gene for both magnetic resonance and photoacoustic imaging.

Fast voice-coil scanning optical-resolution photoacoustic microscopy

We developed a photoacoustic imaging system that has real-time imaging capability with optical resolution. The imaging system is capable of scanning at 20 Hz over a 9 mm range and up to 40 Hz over a 1 mm scanning range. A focused laser beam provides a lateral resolution of 3.4 μm as measured in an optically nonscattering medium. Flows of micrometer-sized carbon particles or whole blood in a silicone tube and individual red blood cells (RBCs) in mouse ear capillaries were also imaged in real time, demonstrating the capability to image highly dynamic processes in vivo at a micrometer-scale resolution.

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.

Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source

Photoacoustic microscopy (PAM) provides high resolution images with excellent image contrast based on optical absorption. The compact size and high repetition rate of pulsed microchip lasers make them attractive sources for PAM. However, their fixed wavelength output precludes their use in spectroscopic PAM. We are developing a tunable optical source based on a microchip laser that is suitable for spectroscopic PAM. Pulses from a 6.6 kHz repetition rate Q-switched Nd:YAG microchip laser are sent through a photonic crystal fiber with a zero dispersion wavelength at 1040 nm. The highly nonlinear optical propagation produces a supercontinuum spectrum spanning 500-1300 nm. A tunable band pass filter selects the desired wavelength band from the supercontinuum. Our PAM system employs optical focusing and a 25 MHz spherically focused detection transducer. En-face imaging experiments were performed at seven different wavelengths from 575 to 875 nm. A simple discriminant analysis of the multiwavelength photoacoustic data produces images that clearly distinguish the different absorbing regions of ink phantoms. These results suggest the potential of this compact tunable source for spectroscopic photoacoustic microscopy.

Scanning acoustic-photoacoustic microscopy using axicon transducers

A dual mode scanning acoustic microscope is investigated, yielding simultaneously images with optical and acoustical contrast. Short laser pulses are used to excite acoustic waves in a sample for the photoacoustic imaging mode. At the same time the pulses irradiate a conical target generating limited diffraction acoustic waves (X-waves) for large depth of field ultrasound imaging. For photoacoustic as well as for ultrasound imaging a focusing, ring shaped detector is applied. First phantom experiments demonstrate the possibility to acquire data for both imaging modes in a single scan, by separating images due to their different time of flight.