Combining microscopy with mesoscopy using optical and optoacoustic label-free modes

Nanophotonic-enhanced photoacoustic imaging for brain tumor detection

Photoacoustic brain imaging (PABI) has emerged as a promising biomedical imaging modality, combining high contrast of optical imaging with deep tissue penetration of ultrasound imaging. This review explores the application of photoacoustic imaging in brain tumor imaging, highlighting the synergy between nanomaterials and state of the art optical techniques to achieve high-resolution imaging of deeper brain tissues. PABI leverages the photoacoustic effect, where absorbed light energy causes thermoelastic expansion, generating ultrasound waves that are detected and converted into images. This technique enables precise diagnosis, therapy monitoring, and enhanced clinical screening, specifically in the management of complex diseases such as breast cancer, lymphatic disorder, and neurological conditions. Despite integration of photoacoustic agents and ultrasound radiation, providing a comprehensive overview of current methodologies, major obstacles in brain tumor treatment, and future directions for improving diagnostic and therapeutic outcomes. The review underscores the significance of PABI as a robust research tool and medical method, with the potential to revolutionize brain disease diagnosis and treatment.

Multimodal optoacoustic imaging: methods and contrast materials

Optoacoustic (OA) imaging offers powerful capabilities for interrogating biological tissues with rich optical absorption contrast while maintaining high spatial resolution for deep tissue observations. The spectrally distinct absorption of visible and near-infrared photons by endogenous tissue chromophores facilitates extraction of diverse anatomic, functional, molecular, and metabolic information from living tissues across various scales, from organelles and cells to whole organs and organisms. The primarily blood-related contrast and limited penetration depth of OA imaging have fostered the development of multimodal approaches to fully exploit the unique advantages and complementarity of the method. We review the recent hybridization efforts, including multimodal combinations of OA with ultrasound, fluorescence, optical coherence tomography, Raman scattering microscopy and magnetic resonance imaging as well as ionizing methods, such as X-ray computed tomography, single-photon-emission computed tomography and positron emission tomography. Considering that most molecules absorb light across a broad range of the electromagnetic spectrum, the OA interrogations can be extended to a large number of exogenously administered small molecules, particulate agents, and genetically encoded labels. This unique property further makes contrast moieties used in other imaging modalities amenable for OA sensing.

Overdriven laser diode optoacoustic microscopy

Laser diodes are small and inexpensive but don't afford the pulse energy and beam profile required for optoacoustic (photoacoustic) microscopy. Using two novel modulation concepts, i.e. overdriving continuous-wave laser diodes (CWLD) and frequency-wavelength multiplexing (FWM) based on illumination pulse-trains, we demonstrate concurrent multi-wavelength optoacoustic microscopy with signal-to-noise ratios of > 17 dB, < 2 µm resolution at repetition rates of 1 MHz. This unprecedented performance based on an adaptable trigger engine allowed us to contrast FWM to wavelength alternating acquisition using identical optical components. We showcase this concept's superiority over conventional optoacoustic microscopes by visualizing vascular oxygenation dynamics and circulating tumor cells in mice. This work positions laser diodes as a technology allowing affordable, tunable, and miniaturizable optoacoustic microscopy.

A scoping review of current and emerging techniques for evaluation of peripheral nerve health, degeneration and regeneration: part 2, non-invasive imaging

Peripheral neuroregenerative research and therapeutic options are expanding exponentially. With this expansion comes an increasing need to reliably evaluate and quantify nerve health. Valid and responsive measures of the nerve status are essential for both clinical and research purposes for diagnosis, longitudinal follow-up, and monitoring the impact of any intervention. Furthermore, novel biomarkers can elucidate regenerative mechanisms and open new avenues for research. Without such measures, clinical decision-making is impaired, and research becomes more costly, time-consuming, and sometimes infeasible. Part 1 of this two-part scoping review focused on neurophysiology. In part 2, we identify and critically examine many current and emerging non-invasive imaging techniques that have the potential to evaluate peripheral nerve health, particularly from the perspective of regenerative therapies and research.

Hybrid confocal fluorescence and photoacoustic microscopy for the label-free investigation of melanin accumulation in fish scales

Lower vertebrates, including fish, can rapidly alter skin lightness through changes in melanin concentration and melanosomes’ mobility according to various factors, which include background color, light intensity, ambient temperature, social context, husbandry practices and acute or chronic stressful stimuli. Within this framework, the determination of skin chromaticity parameters in fish species is estimated either in specific areas using colorimeters or at the whole animal level using image processing and analysis software. Nevertheless, the accurate quantification of melanin content or melanophore coverage in fish skin is quite challenging as a result of the laborious chemical analysis and the typical application of simple optical imaging methods, requiring also to euthanize the fish in order to obtain large skin samples for relevant investigations. Here we present the application of a novel hybrid confocal fluorescence and photoacoustic microscopy prototype for the label-free imaging and quantification of melanin in fish scales samples with high spatial resolution, sensitivity and detection specificity. The hybrid images are automatically processed through optimized algorithms, aiming at the accurate and rapid extraction of various melanin accumulation indices in large datasets (i.e., total melanin content, melanophores’ area, density and coverage) corresponding to different fish species and groups. Furthermore, convolutional neural network-based algorithms have been trained using the recorded data towards the classification of different scales’ samples with high accuracy. In this context, we demonstrate that the proposed methodology may increase substantially the precision, as well as, simplify and expedite the relevant procedures for the quantification of melanin content in marine organisms.

Label-free concurrent 5-modal microscopy (Co5M) resolves unknown spatio-temporal processes in wound healing

The non-invasive investigation of multiple biological processes remains a methodological challenge as it requires capturing different contrast mechanisms, usually not available with any single modality. Intravital microscopy has played a key role in dynamically studying biological morphology and function, but it is generally limited to resolving a small number of contrasts, typically generated by the use of transgenic labels, disturbing the biological system. We introduce concurrent 5-modal microscopy (Co5M), illustrating a new concept for label-free in vivo observations by simultaneously capturing optoacoustic, two-photon excitation fluorescence, second and third harmonic generation, and brightfield contrast. We apply Co5M to non-invasively visualize multiple wound healing biomarkers and quantitatively monitor a number of processes and features, including longitudinal changes in wound shape, microvascular and collagen density, vessel size and fractality, and the plasticity of sebaceous glands. Analysis of these parameters offers unique insights into the interplay of wound closure, vasodilation, angiogenesis, skin contracture, and epithelial reformation in space and time, inaccessible by other methods. Co5M challenges the conventional concept of biological observation by yielding multiple simultaneous parameters of pathophysiological processes in a label-free mode.

Photoacoustic Neuroimaging - Perspectives on a Maturing Imaging Technique and its Applications in Neuroscience

A prominent goal of neuroscience is to improve our understanding of how brain structure and activity interact to produce perception, emotion, behavior, and cognition. The brain's network activity is inherently organized in distinct spatiotemporal patterns that span scales from nanometer-sized synapses to meter-long nerve fibers and millisecond intervals between electrical signals to decades of memory storage. There is currently no single imaging method that alone can provide all the relevant information, but intelligent combinations of complementary techniques can be effective. Here, we thus present the latest advances in biomedical and biological engineering on photoacoustic neuroimaging in the context of complementary imaging techniques. A particular focus is placed on recent advances in whole-brain photoacoustic imaging in rodent models and its influential role in bridging the gap between fluorescence microscopy and more non-invasive techniques such as magnetic resonance imaging (MRI). We consider current strategies to address persistent challenges, particularly in developing molecular contrast agents, and conclude with an overview of potential future directions for photoacoustic neuroimaging to provide deeper insights into healthy and pathological brain processes.

Recent development of near-infrared photoacoustic probes based on small-molecule organic dye

Photoacoustic imaging (PAI), which integrates the higher spatial resolution of optical imaging and the deeper penetration depth of ultrasound imaging, has attracted great attention. Various photoacoustic probes including inorganic and organic agents have been well fabricated in last decades. Among them, small-molecule based agents are most promising candidates for preclinical/clinical applications due to their favorite in vivo features and facile functionalization. In recent years, PAI, in the near-infrared region (NIR, 700-1700 nm) has developed rapidly and has made remarkable achievements in the biomedical field. Compared with the visible light region (400-700 nm), it can significantly reduce light scattering and meanwhile provide deeper tissue penetration. In this review, we discuss the recent developments of near-infrared photoacoustic probes based on small molecule dyes, which focus on their "always on" and "activatable" form in biomedicine. Further, we also suggest current challenges and perspectives.

Another decade of photoacoustic imaging

Photoacoustic imaging - a hybrid biomedical imaging modality finding its way to clinical practices. Although the photoacoustic phenomenon was known more than a century back, only in the last two decades it has been widely researched and used for biomedical imaging applications. In this review we focus on the development and progress of the technology in the last decade (2010-2020). From becoming more and more user friendly, cheaper in cost, portable in size, photoacoustic imaging promises a wide range of applications, if translated to clinic. The growth of photoacoustic community is steady, and with several new directions researchers are exploring, it is inevitable that photoacoustic imaging will one day establish itself as a regular imaging system in the clinical practices.

Pushing the boundaries of optoacoustic microscopy by total impulse response characterization

Optical microscopy improves in resolution and signal-to-noise ratio by correcting for the system’s point spread function; a measure of how a point source is resolved, typically determined by imaging nanospheres. Optical-resolution optoacoustic (photoacoustic) microscopy could be similarly corrected, especially to account for the spatially-dependent signal distortions induced by the acoustic detection and the time-resolved and bi-polar nature of optoacoustic signals. Correction algorithms must therefore include the spatial dependence of signals’ origins and profiles in time, i.e. the four-dimensional total impulse response (TIR). However, such corrections have been so far impeded by a lack of efficient TIR-characterization methods. We introduce high-quality TIR determination based on spatially-distributed optoacoustic point sources (SOAPs), produced by scanning an optical focus on an axially-translatable 250 nm gold layer. Using a spatially-dependent TIR-correction improves the signal-to-noise ratio by >10 dB and the axial resolution by ~30%. This accomplishment displays a new performance paradigm for optoacoustic microscopy.

Characterizing the total impulse response (TIR) of photoacoustic microscopes has been challenging due to difficulties distributing appropriate point sources. Here, the authors present a method for 3D generation of spatially-distributed optoacoustic point sources and show that subsequent TIR correction results in improved image quality.

Photoacoustic Imaging Probes Based on Tetrapyrroles and Related Compounds

Photoacoustic imaging (PAI) is a rapidly evolving field in molecular imaging that enables imaging in the depths of ultrasound and with the sensitivity of optical modalities. PAI bases on the photoexcitation of a chromophore, which converts the absorbed light into thermal energy, causing an acoustic pressure wave that can be captured with ultrasound transducers, in generating an image. For in vivo imaging, chromophores strongly absorbing in the near-infrared range (NIR; > 680 nm) are required. As tetrapyrroles have a long history in biomedical applications, novel tetrapyrroles and inspired mimics have been pursued as potentially suitable contrast agents for PAI. The goal of this review is to summarize the current state of the art in PAI applications using tetrapyrroles and related macrocycles inspired by it, highlighting those compounds exhibiting strong NIR-absorption. Furthermore, we discuss the current developments of other absorbers for in vivo photoacoustic (PA) applications.

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.

Advances in Optoacoustic Neurotomography of Animal Models

Unlike traditional optical methods, optoacoustic imaging is less sensitive to scattering of ballistic photons, so it is capable of high-resolution interrogation at a greater depth. By integrating video-rate visualization with multiplexing and sensing a range of endogenous and exogenous chromophores, optoacoustic imaging has matured into a versatile noninvasive investigation modality with rapidly expanding use in biomedical research. We review the principal features of the technology and discuss recent advances it has enabled in structural, functional, and molecular neuroimaging in small-animal models. In extending the boundaries of noninvasive observation beyond the reach of customary photonic methods, the latest developments in optoacoustics have substantially advanced neuroimaging inquiry, with promising implications for basic and translational studies.

In vivo label-free functional photoacoustic monitoring of ischemic reperfusion

Pressure ulcer formation is a common problem among patients confined to bed or restricted to wheelchairs. The ulcer forms when the affected skin and underlying tissues go through repeated cycles of ischemia and reperfusion, leading to inflammation. This theory is evident by intravital imaging studies performed in immune cell-specific, fluorescent reporter mouse skin with induced ischemia-reperfusion (I-R) injuries. However, traditional confocal or multiphoton microscopy cannot accurately monitor the progression of vascular reperfusion by contrast agents, which leaks into the interstitium under inflammatory conditions. Here, we develop a dual-wavelength micro electro mechanical system (MEMS) scanning-based optical resolution photoacoustic microscopy (OR-PAM) system for continuous label-free functional imaging of vascular reperfusion in an IR mouse model. This MEMS-OR-PAM system provides fast scanning speed for concurrent dual-wavelength imaging, which enables continuous monitoring of the reperfusion process. During reperfusion, the revascularization of blood vessels and the oxygen saturation (sO₂ ) changes in both arteries and veins are recorded, from which the local oxygen extraction ratios of the ischemic tissue and the unaffected tissue can be quantified. Our MEMS-OR-PAM system provides novel perspectives to understand the I-R injuries. It solves the problem of dynamic label-free functional monitoring of the vascular reperfusion at high spatial resolution.

The integrated high-resolution reflection-mode photoacoustic and fluorescence confocal microscopy

A dual modality microscopy with the highest imaging resolution reported so far based on reflection-mode photoacoustic and confocal fluorescence is presented in this study. The unique design of the imaging head of the microscope makes it highly convenient for scalable high-resolution imaging by simply switching the optical objectives. The submicron resolution performance of the system is demonstrated via in vivo imaging of zebrafish, normal mouse ear, and a xenograft tumor model inoculated in the mouse ear. The imaging results confirm that the presented dual-modality microscopy imaging system could play a vital role in observing model organism, studying tumor angiogenesis and assessment of antineoplastic drugs.

Cardiovascular optoacoustics: From mice to men - A review

Imaging has become an indispensable tool in the research and clinical management of cardiovascular disease (CVD). An array of imaging technologies is considered for CVD diagnostics and therapeutic assessment, ranging from ultrasonography, X-ray computed tomography and magnetic resonance imaging to nuclear and optical imaging methods. Each method has different operational characteristics and assesses different aspects of CVD pathophysiology; nevertheless, more information is desirable for achieving a comprehensive view of the disease. Optoacoustic (photoacoustic) imaging is an emerging modality promising to offer novel information on CVD parameters by allowing high-resolution imaging of optical contrast several centimeters deep inside tissue. Implemented with illumination at several wavelengths, multi-spectral optoacoustic tomography (MSOT) in particular, is sensitive to oxygenated and deoxygenated hemoglobin, water and lipids allowing imaging of the vasculature, tissue oxygen saturation and metabolic or inflammatory parameters. Progress with fast-tuning lasers, parallel detection and advanced image reconstruction and data-processing algorithms have recently transformed optoacoustics from a laboratory tool to a promising modality for small animal and clinical imaging. We review progress with optoacoustic CVD imaging, highlight the research and diagnostic potential and current applications and discuss the advantages, limitations and possibilities for integration into clinical routine.

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.

Integrated multimodal photoacoustic microscopy with OCT- guided dynamic focusing

Combining different contrast mechanisms to achieve simultaneous multimodal imaging is always desirable but is challenging due to the various optical and hardware requirements for different imaging systems. We developed a multimodal microscopic optical imaging system with the capability of providing comprehensive structural, functional and molecular information of living tissues. This imaging system integrated photoacoustic microscopy (PAM), optical coherence tomography (OCT), optical Doppler tomography (ODT) and confocal fluorescence microscopy in one platform. By taking advantage of the depth resolving capability of OCT, we developed a novel OCT-guided surface contour scanning methodology for dynamic focusing adjustment. We have conducted phantom, in vivo, and ex vivo tests to demonstrate the capability of the multimodal imaging system for providing comprehensive microscopic information of biological tissues. Integrating all the aforementioned imaging modalities with OCT-guided dynamic focusing for simultaneous multimodal imaging has promising potential for preclinical research and clinical practice in the future.

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

With their optically transparent appearance, zebrafish larvae are readily imaged with optical-resolution photoacoustic (PA) microscopy (OR-PAM). Previous OR-PAM studies have mapped endogenous chromophores (e.g. melanin and hemoglobin) within larvae; however, anatomical features cannot be imaged with OR-PAM alone due to insufficient optical absorption. We have previously reported on the photoacoustic radiometry (PAR) technique, which can be used simultaneously with OR-PAM to generate images dependent upon the optical attenuation properties of a sample. Here we demonstrate application of the duplex PAR/PA technique for label-free imaging of the anatomy and vasculature of zebrafish larvae in vivo at 200 and 400 MHz ultrasound detection frequencies. We then use the technique to assess the effects of anti-angiogenic drugs on the development of the larval vasculature. Our results demonstrate the effectiveness of simultaneous PAR/PA for acquiring anatomical images of optically transparent samples in vivo, and its potential applications in assessing drug efficacy and embryonic development.

Blood vessel imaging using radiofrequency-induced second harmonic acoustic response

We introduce a contrast mechanism for visualizing blood vessels based on radiofrequency-induced second harmonic acoustic (RISHA) signals sensing blood conductivity. We develop a novel imaging system using commonly available inexpensive components, and demonstrate in vivo RISHA visualization of blood vessels based on low-power quasi-continuous radiofrequency excitation of tissue at frequencies of a few MHz. We show how the novel approach also implicitly enables radiofrequency-induced passive ultrasound imaging and can be readily applied to non-invasive imaging of blood vessels ex vivo and in vivo. We discuss the implications of non-invasive conductivity measurements in the context of biomedical applications.

Triplex micron-resolution acoustic, photoacoustic, and optical transmission microscopy via photoacoustic radiometry

We present a new sensing technique, termed photoacoustic radiometry (PAR), for mapping the optical attenuation properties of a sample. In PAR, laser pulses attenuated via transmission through the sample impinge on the ultrasound transducer and generate a photoacoustic (PA) signal within it. Spatial variation of the optical attenuation properties of the sample influences the amplitude of the PAR signal, providing image contrast. Performed simultaneously with pulse-echo ultrasound and PA imaging, this triplex imaging technique enables rapid characterization of samples with micrometer-resolution in a single scan. In this work, we demonstrate that the PAR technique can be easily integrated into existing PA microscopy systems, with applications in imaging biological samples and non-destructive evaluation of optically opaque materials such as silicon wafers.

Quad-mode functional and molecular photoacoustic microscopy

A conventional photoacoustic microscopy (PAM) system typically has to make tradeoffs between its spatial resolution and penetration depth, by choosing a fixed configuration of optical excitation and acoustic detection. The single-scale imaging capability of PAM may limit its applications in biomedical studies. Here, we report a quad-mode photoacoustic microscopy (QM-PAM) system with four complementary spatial resolutions and maximum penetration depths. For this we first developed a ring-shaped focused ultrasound transducer that has two independent elements with respective central frequencies at 20 MHz and 40 MHz, providing complementary acoustically-determined spatial resolutions and penetration depths. To accommodate the dual-element ultrasound transducer, we implemented two optical excitation modes to provide tightly- and weakly-focused light illumination. The dual-element acoustic detection combined with the two optical focusing modes can thus provide four imaging scales in a single imaging device, with consistent contrast mechanisms and co-registered field of views. We have demonstrated the multiscale morphological, functional, and molecular imaging capability of QM-PAM in the mouse head, leg and ear in vivo. We expect the high scale flexibility of QM-PAM will enable broad applications in preclinical studies.

Non-linear optical microscopy and histological analysis of collagen, elastin and lysyl oxidase expression in breast capsular contracture

Background

Capsular contracture is one of the most common complications in surgical interventions for aesthetic breast augmentation or post-mastectomy breast reconstruction involving the use of silicone prostheses. Although the precise cause of capsular contracture is yet unknown, the leading hypothesis is that it is caused by long-term unresolved foreign body reaction towards the silicone breast implant. To authors’ best knowledge, this is the first study that elucidates the presence of lysyl oxidase (LOX)—an enzyme that is involved in collagen and elastin crosslinking within fibrous capsules harvested from patients with severe capsular contracture. It was hypothesized that over-expression of LOX plays a role in the irreversible crosslinking of collagen and elastin which, in turn, stabilizes the fibrous proteins and contributes to the progression of capsular contracture.

Methods

Eight fibrous capsules were collected from patients undergoing capsulectomy procedure, biomechanical testing was performed for compressive Young’s moduli and evaluated for Type I and II collagen, elastin and LOX by means of non-linear optical microscopy and immunohistology techniques.

Results

Observations revealed the heterogeneity of tissue structure within and among the collected fibrous capsules. Regardless of the tissue structure, it has been shown that LOX expression was intensified at the implant-to-tissue interface.

Conclusion

Our results indicate the involvement of LOX in the initiation of fibrous capsule formation which ultimately contributes towards the progression of capsular contracture.

Electronic supplementary material

The online version of this article (10.1186/s40001-018-0322-0) contains supplementary material, which is available to authorized users.

Optoacoustic microscopy at multiple discrete frequencies

Optoacoustic (photoacoustic) sensing employs illumination of transient energy and is typically implemented in the time domain using nanosecond photon pulses. However, the generation of high-energy short photon pulses requires complex laser technology that imposes a low pulse repetition frequency (PRF) and limits the number of wavelengths that are concurrently available for spectral imaging. To avoid the limitations of working in the time domain, we have developed frequency-domain optoacoustic microscopy (FDOM), in which light intensity is modulated at multiple discrete frequencies. We integrated FDOM into a hybrid system with multiphoton microscopy, and we examine the relationship between image formation and modulation frequency, showcase high-fidelity images with increasing numbers of modulation frequencies from phantoms and in vivo, and identify a redundancy in optoacoustic measurements performed at multiple frequencies. We demonstrate that due to high repetition rates, FDOM achieves signal-to-noise ratios similar to those obtained by time-domain methods, using commonly available laser diodes. Moreover, we experimentally confirm various advantages of the frequency-domain implementation at discrete modulation frequencies, including concurrent illumination at two wavelengths that are carried out at different modulation frequencies as well as flow measurements in microfluidic chips and in vivo based on the optoacoustic Doppler effect. Furthermore, we discuss how FDOM redefines possibilities for optoacoustic imaging by capitalizing on the advantages of working in the frequency domain.

Delineating the anatomy of the ciliary body using hybrid optical and photoacoustic imaging

We demonstrate the application of an extended field of view hybrid microscope, integrating distinct optical and photoacoustic (PA) contrast modes, for the precise three-dimensional anatomy delineation of the ciliary body/iris structures in healthy rabbit eyes ex vivo. The glutaraldehyde-induced autofluorescence and the intrinsic PA signals provided by each of the employed imaging modalities were characterized by a high spatial complementarity, offering thus rich morphological information regarding the pars plana and pars plicata ciliary body portions, the iris, as well as, the attached zonule fiber strands. The bimodal microscopy approach presented could find application on studies involving the ocular accommodation mechanism or pathological ciliary body conditions, as a powerful diagnostic technique contributing to the understanding of ocular physiology and function.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

Contrast-enhanced dual mode imaging: photoacoustic imaging plus more

Conventional biomedical imaging modalities in wide clinical use, such as ultrasound imaging, X-ray computed tomography, magnetic resonance imaging, and positron emission tomography, can provide morphological, anatomical, and functional information about biological tissues. However, single mode imaging in conventional medicine provides only limited information for definitive diagnoses. Thus, combinational diagnosis using multiple imaging modalities has become increasingly important. Recently, photoacoustic imaging (PAI) has gained significant attention, and several PAI prototypes have been used in clinical trials. At the same time, PAI has been tested in combination with conventional imaging modalities. For all these imaging modalities, various contrast-enhancing agents have been developed for various purposes. In this review article, we will focus on recent progress in developing dual mode contrast agents for PAI in combination with other conventional imaging modalities.

Optical imaging of post-embryonic zebrafish using multi orientation raster scan optoacoustic mesoscopy

Whole-body optical imaging of post-embryonic stage model organisms is a challenging and long sought-after goal. It requires a combination of high-resolution performance and high-penetration depth. Optoacoustic (photoacoustic) mesoscopy holds great promise, as it penetrates deeper than optical and optoacoustic microscopy while providing high-spatial resolution. However, optoacoustic mesoscopic techniques only offer partial visibility of oriented structures, such as blood vessels, due to a limited angular detection aperture or the use of ultrasound frequencies that yield insufficient resolution. We introduce 360° multi orientation (multi-projection) raster scan optoacoustic mesoscopy (MORSOM) based on detecting an ultra-wide frequency bandwidth (up to 160 MHz) and weighted deconvolution to synthetically enlarge the angular aperture. We report unprecedented isotropic in-plane resolution at the 9-17 μm range and improved signal to noise ratio in phantoms and opaque 21-day-old Zebrafish. We find that MORSOM performance defines a new operational specification for optoacoustic mesoscopy of adult organisms, with possible applications in the developmental biology of adulthood and aging.

Multimodal optoacoustic and multiphoton microscopy of human carotid atheroma

Carotid artery atherosclerosis is a main cause of stroke. Understanding atherosclerosis biology is critical in the development of targeted prevention and treatment strategies. Consequently, there is demand for advanced tools investigating atheroma pathology. We consider hybrid optoacoustic and multiphoton microscopy for the integrated and complementary interrogation of plaque tissue constituents and their mutual interactions. Herein, we visualize human carotid plaque using a hybrid multimodal imaging system that combines optical resolution optoacoustic (photoacoustic) microscopy, second and third harmonic generation microscopy, and two-photon excitation fluorescence microscopy. Our data suggest more comprehensive insights in the pathophysiology of atheroma formation and destabilization, by enabling congruent visualization of structural and biological features critical for the atherosclerotic process and its acute complications, such as red blood cells and collagen.

Hybrid optical and acoustic resolution optoacoustic endoscopy

We propose the implementation of hybrid optical and acoustic resolution optoacoustic endoscopy. Laser light is transmitted to tissue by two types of illumination for achieving optical and acoustic resolution imaging. A 20 MHz ultrasound detector is used for recording optoacoustic signals. The endoscopy probe attains a 3.6 mm diameter and is fully encapsulated into a catheter system. We validate the imaging performance of the hybrid endoscope on phantoms and ex vivo, and discuss the necessity for the extended resolution and depth range of endoscopy achieved.

All-optical optoacoustic microscope based on wideband pulse interferometry

Optical and optoacoustic (photoacoustic) microscopy have been recently joined in hybrid implementations that resolve extended tissue contrast compared to each modality alone. Nevertheless, the application of the hybrid technique is limited by the requirement to combine an optical objective with ultrasound detection collecting signal from the same micro-volume. We present an all-optical optoacoustic microscope based on a pi-phase-shifted fiber Bragg grating (π-FBG) with coherence-restored pulsed interferometry (CRPI) used as the interrogation method. The sensor offers an ultra-small footprint and achieved higher sensitivity over piezoelectric transducers of similar size. We characterize the spectral bandwidth of the ultrasound detector and interrogate the imaging performance on phantoms and tissues. We show the first optoacoustic images of biological specimen recorded with π-FBG sensors. We discuss the potential uses of π-FBG sensors based on CRPI.

Hybrid photoacoustic and optical imaging of pigments in vegetative tissues

Pigments in vegetative tissues have been a subject of intense research during the previous decades, since they play an active role in several molecular mechanisms regarding plants' physiology and function. Towards this direction, the imaging modality that has been extensively employed and represents the state of the art for mapping pigments' distribution is confocal microscopy. Despite the advantage of a high spatial resolution however, confocal microscopy provides a rather limited imaging depth and requires necessarily strong fluorescence properties from the specimen under observation. To overcome such limitations, we propose a hybrid, photoacoustic and optical imaging methodology for the delineation of various vegetative pigments, such as chlorophylls, anthocyanins and betalains in different plant species. The superior sensitivity and the high contrast complementarity of the hybrid technique, render it a powerful alternative to the conventional fluorescence imaging modalities, significantly expanding the current state of the art.