Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging

Evoking Photothermy by Capturing Intramolecular Bond Stretching Vibration-Induced Dark-State Energy

Development of highly effective approaches to desirable photothermal conversion agents is particularly valuable. Herein, we report a concept, namely, bond stretching vibration-induced photothermy, that serves as a mechanism to construct advanced photothermal conversion agents. As a proof-of-concept, two compounds (DCP-TPA and DCP-PTPA) with donor-acceptor (D-A) structures were synthesized. The bond stretching vibration of the pyrazine-containing unit in these molecules is vigorous and insensitive to the external environmental restraint, which efficiently transforms the absorbed photons to dark-state heat energy. The nanoparticles (NPs) of DCP-TPA and DCP-PTPA show rather high photothermal conversion efficiency (52% and 59%) and stronger photoacoustic (PA) signal than commercial methylene blue and reported high-performance semiconducting polymer nanoparticles. The DCP-PTPA NPs perform better than DCP-TPA NPs in terms of photothermal conversion, PA signal production, and in vivo PA tumor imaging because of the increased bond stretching vibration in the former molecule.

Molecular imaging of inflammation - Current and emerging technologies for diagnosis and treatment

Inflammation is a key factor in multiple diseases including primary immune-mediated inflammatory diseases e.g. rheumatoid arthritis but also, less obviously, in many other common conditions, e.g. cardiovascular disease and diabetes. Together, chronic inflammatory diseases contribute to the majority of global morbidity and mortality. However, our understanding of the underlying processes by which the immune response is activated and sustained is limited by a lack of cellular and molecular information obtained in situ. Molecular imaging is the visualization, detection and quantification of molecules in the body. The ability to reveal information on inflammatory biomarkers, pathways and cells can improve disease diagnosis, guide and monitor therapeutic intervention and identify new targets for research. The optimum molecular imaging modality will possess high sensitivity and high resolution and be capable of non-invasive quantitative imaging of multiple disease biomarkers while maintaining an acceptable safety profile. The mainstays of current clinical imaging are computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (US) and nuclear imaging such as positron emission tomography (PET). However, none of these have yet progressed to routine clinical use in the molecular imaging of inflammation, therefore new approaches are required to meet this goal. This review sets out the respective merits and limitations of both established and emerging imaging modalities as clinically useful molecular imaging tools in addition to potential theranostic applications.

Spatial heterogeneity of oxygenation and haemodynamics in breast cancer resolved in vivo by conical multispectral optoacoustic mesoscopy

The characteristics of tumour development and metastasis relate not only to genomic heterogeneity but also to spatial heterogeneity, associated with variations in the intratumoural arrangement of cell populations, vascular morphology and oxygen and nutrient supply. While optical (photonic) microscopy is commonly employed to visualize the tumour microenvironment, it assesses only a few hundred cubic microns of tissue. Therefore, it is not suitable for investigating biological processes at the level of the entire tumour, which can be at least four orders of magnitude larger. In this study, we aimed to extend optical visualization and resolve spatial heterogeneity throughout the entire tumour volume. We developed an optoacoustic (photoacoustic) mesoscope adapted to solid tumour imaging and, in a pilot study, offer the first insights into cancer optical contrast heterogeneity in vivo at an unprecedented resolution of <50 μm throughout the entire tumour mass. Using spectral methods, we resolve unknown patterns of oxygenation, vasculature and perfusion in three types of breast cancer and showcase different levels of structural and functional organization. To our knowledge, these results are the most detailed insights of optical signatures reported throughout entire tumours in vivo, and they position optoacoustic mesoscopy as a unique investigational tool linking microscopic and macroscopic observations.

Transcranial photoacoustic imaging of NMDA-evoked focal circuit dynamics in the rat hippocampus

OBJECTIVE

We report the transcranial functional photoacoustic (fPA) neuroimaging of N-methyl-D-aspartate (NMDA) evoked neural activity in the rat hippocampus. Concurrent quantitative electroencephalography (qEEG) and microdialysis were used to record real-time circuit dynamics and excitatory neurotransmitter concentrations, respectively.

APPROACH

We hypothesized that location-specific fPA voltage-sensitive dye (VSD) contrast would identify neural activity changes in the hippocampus which correlate with NMDA-evoked excitatory neurotransmission.

MAIN RESULTS

Transcranial fPA VSD imaging at the contralateral side of the microdialysis probe provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration changes. qEEG validated a wide range of glutamatergic excitation, which culminated in focal seizure activity after a high NMDA dose. We conclude that transcranial fPA VSD imaging can distinguish focal glutamate loads in the rat hippocampus, based on the VSD redistribution mechanism which is sensitive to the electrophysiologic membrane potential.

SIGNIFICANCE

Our results suggest the future utility of this emerging technology in both laboratory and clinical sciences as an innovative functional neuroimaging modality.

Fluoro-Photoacoustic Polymeric Renal Reporter for Real-Time Dual Imaging of Acute Kidney Injury

Photoacoustic (PA) imaging agents detect disease tissues and biomarkers with increased penetration depth and enhanced spatial resolution relative to traditional optical imaging, and thus hold great promise for clinical applications. However, existing PA imaging agents often encounter the issues of slow body excretion and low-signal specificity, which compromise their capability for in vivo detection. Herein, a fluoro-photoacoustic polymeric renal reporter (FPRR) is synthesized for real-time imaging of drug-induced acute kidney injury (AKI). FPRR simultaneously turns on both near-infrared fluorescence (NIRF) and PA signals in response to an AKI biomarker (γ-glutamyl transferase) with high sensitivity and specificity. In association with its high renal clearance efficiency (78% at 24 h post-injection), FPRR can detect cisplatin-induced AKI at 24 h post-drug treatment through both real-time imaging and optical urinalysis, which is 48 h earlier than serum biomarker elevation and histological changes. More importantly, the deep-tissue penetration capability of PA imaging results in a signal-to-background ratio that is 2.3-fold higher than NIRF imaging. Thus, the study not only demonstrates the first activatable PA probe for real-time sensitive imaging of kidney function at molecular level, but also highlights the polymeric probe structure with high renal clearance.

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.

High-speed dual-view photoacoustic imaging pen

Today, photoacoustic imaging (PAI) is widely used to study diseases in the skin, brain, cardiovascular, and other parts. However, these studies are often carried out using physiological slices or model animals, which indicate that many PAI techniques can only be used in the laboratory. In order to promote the transformation of PAI into clinical applications or, more specifically, to extend the application of photoacoustic (PA) microscopy to areas such as the oral cavity, throat, cervix, and abdominal viscera which are difficult to detect with conventional PA microscopy systems, a PAI pen was developed. The PAI pen can be handheld and can perform forward detection and lateral detection. The imaging area is a 2.4 mm diameter circular area. In addition, it can provide a high-speed imaging mode of four frames per second and a high-resolution imaging mode of 0.25 frames per second to meet the different needs of clinical users. In this Letter, the performance of the PAI pen was tested by imaging the phantom and the human oral cavity. The experimental results prove that the PAI pen can clearly image the microvessels of the oral cavity, which indicates that it has the same imaging capability for other similar areas and has a good prospect for assisting the diagnosis of related diseases.

Miniature probe for dual-modality photoacoustic microscopy and white-light microscopy for image guidance: A prototype toward an endoscope

In this study, a novel photoacoustic microscopy (PAM) probe integrating white-light microscopy (WLM) modality that provides guidance for PAM imaging and complementary information is implemented. One single core of an imaging fiber bundle is employed to deliver a pulsed laser for photoacoustic excitation for PAM mode, which provides high resolution with deep penetration. Meanwhile, for WLM mode, the imaging fiber bundle is used to transmit two-dimensional superficial images. Lateral resolution of 7.2 μm for PAM is achieved. Since miniature components are used, the probe diameter is only 1.7 mm. Imaging of phantom and animals in vivo is conducted to show the imaging capability of the probe. The probe has several advantages by introducing the WLM mode, such as being able to conveniently identify regions of interest and align the focus for PAM mode. The prototype of an endoscope shows potential to facilitate clinical photoacoustic endoscopic applications.

Evaluation of visible NIR-I and NIR-II light penetration for photoacoustic imaging in rat organs

In this study, we evaluate the penetration capability of light in visible, near-infrared-I (NIR-I) and near-infrared-II (NIR-II) optical windows for photoacoustic macroscale imaging inside 9 biological tissues with three typical penetration depths. An acoustic resolution photoacoustic microscopy is designed to guarantee the consistent experiment conditions except excitation wavelength. Experimental results show that short NIR-II (1000-1150 nm) shows the best performance inside kidney, spleen and liver tissues at all depths, while NIR-I (700-1000 nm) works better for muscle, stomach, heart and brain tissues, especially in deep imaging. This study proposes the optimal selection of illumination wavelengths for photoacoustic macroscale imaging in rat organs, which enables the best signal-to-noise ratio (SNR) of the observed target.

Iterative image reconstruction in transcranial photoacoustic tomography based on the elastic wave equation

Photoacoustic computed tomography (PACT) is an emerging computed imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the photoacoustically induced initial pressure distribution within tissue. The PACT reconstruction problem corresponds to a time-domain inverse source problem, where the initial pressure distribution is recovered from the measurements recorded on an aperture outside the support of the source. A major challenge in transcranial PACT of the brain is to compensate for aberrations and attenuation in the measured data due to the propagation of the photoacoustic wavefields through the skull. To properly account for these effects, a wave equation-based inversion method can be employed that can model the heterogeneous elastic properties of the medium. In this study, an optimization-based image reconstruction method for 3D transcranial PACT is developed based on the elastic wave equation. To accomplish this, a forward-adjoint operator pair based on a finite-difference time-domain discretization of the 3D elastic wave equation is utilized to compute penalized least squares estimates of the initial pressure distribution. Computer-simulation and experimental studies are conducted to investigate the robustness of the reconstruction method to model mismatch and its ability to effectively resolve cortical and superficial brain structures.

Quantitative comparison of frequency-domain and delay-and-sum optoacoustic image reconstruction including the effect of coherence factor weighting

Image reconstruction in optoacoustic imaging is often based on a delay-and-sum (DAS) or a frequency domain (FD) algorithm. In this study, we performed a comprehensive comparison of these two algorithms together with coherence factor (CF) weighting using phantom and in-vivo mouse data obtained with optoacoustic microscopy. For this purpose we developed an FD based definition of the CF. Our results reveal the equivalence of DAS and FD, with and without CF weighting, in terms of spatial resolution and contrast-to-noise ratio (CNR) but highlight the clear advantage of FD in terms of computational cost, making it preferable for 3D reconstruction or real-time applications. An important additional result of this research is that, contradictory to previous studies, CF weighting does not lead to any improvement in lateral resolution.

The role of noninvasive diagnostic imaging in monitoring pregnancy and detecting patients at risk for preterm birth: a review of quantitative approaches

Preterm birth (PTB) is the leading cause of neonatal morbidity and mortality worldwide. The ability to predict patients at risk for preterm birth remains a major health challenge. The currently available clinical diagnostics such as cervical length and fetal fibronectin may detect only up to 30% of patients who eventually experience a spontaneous preterm birth. This paper reviews ongoing efforts to improve the ability to conduct a risk assessment for preterm birth. In particular, this work focuses on quantitative methods of imaging using ultrasound-based techniques, magnetic resonance imaging, and optical imaging modalities. While ultrasound imaging is the major modality for preterm birth risk assessment, a summary of efforts to adopt other imaging modalities is also discussed to identify the technical and diagnostic limits associated with adopting them in clinical settings. We conclude the review by proposing a new approach using combined photoacoustic, ultrasound, and elastography as a potential means to better assess cervical tissue remodeling, and thus improve the detection of patients at-risk of PTB.

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.

HemoSYS: A Toolkit for Image-based Systems Biology of Tumor Hemodynamics

Abnormal tumor hemodynamics are a critical determinant of a tumor’s microenvironment (TME), and profoundly affect drug delivery, therapeutic efficacy and the emergence of drug and radio-resistance. Since multiple hemodynamic variables can simultaneously exhibit transient and spatiotemporally heterogeneous behavior, there is an exigent need for analysis tools that employ multiple variables to characterize the anomalous hemodynamics within the TME. To address this, we developed a new toolkit called HemoSYS for quantifying the hemodynamic landscape within angiogenic microenvironments. It employs multivariable time-series data such as in vivo tumor blood flow (BF), blood volume (BV) and intravascular oxygen saturation (Hb_sat) acquired concurrently using a wide-field multicontrast optical imaging system. The HemoSYS toolkit consists of propagation, clustering, coupling, perturbation and Fourier analysis modules. We demonstrate the utility of each module for characterizing the in vivo hemodynamic landscape of an orthotropic breast cancer model. With HemoSYS, we successfully described: (i) the propagation dynamics of acute hypoxia; (ii) the initiation and dissolution of distinct hemodynamic niches; (iii) tumor blood flow regulation via local vasomotion; (iv) the hemodynamic response to a systemic perturbation with carbogen gas; and (v) frequency domain analysis of hemodynamic heterogeneity in the TME. HemoSYS (freely downloadable via the internet) enables vascular phenotyping from multicontrast in vivo optical imaging data. Its modular design also enables characterization of non-tumor hemodynamics (e.g. brain), other preclinical disease models (e.g. stroke), vascular-targeted therapeutics, and hemodynamic data from other imaging modalities (e.g. MRI).

Ultrasound-Enhanced Chemiluminescence for Bioimaging

Tissue imaging has emerged as an important aspect of theragnosis. It is essential not only to evaluate the degree of the disease and thus provide appropriate treatments, but also to monitor the delivery of administered drugs and the subsequent recovery of target tissues. Several techniques including magnetic resonance imaging (MRI), computational tomography (CT), acoustic tomography (AT), biofluorescence (BF) and chemiluminescence (CL), have been developed to reconstruct three-dimensional images of tissues. While imaging has been achieved with adequate spatial resolution for shallow depths, challenges still remain for imaging deep tissues. Energy loss is usually observed when using a magnetic field or traditional ultrasound (US), which leads to a need for more powerful energy input. This may subsequently result in tissue damage. CT requires exposure to radiation and a high dose of contrast agent to be administered for imaging. The BF technique, meanwhile, is affected by strong scattering of light and autofluorescence of tissues. The CL is a more selective and sensitive method as stable luminophores are produced from physiochemical reactions, e.g. with reactive oxygen species. Development of near infrared-emitting luminophores also bring potential for application of CL in deep tissues and whole animal studies. However, traditional CL imaging requires an enhancer to increase the intensity of low-level light emissions, while reducing the scattering of emitted light through turbid tissue environment. There has been interest in the use of focused ultrasound (FUS), which can allow acoustic waves to propagate within tissues and modulate chemiluminescence signals. While light scattering is decreased, the spatial resolution is increased with the assistance of US. In this review, chemiluminescence detection in deep tissues with assistance of FUS will be highlighted to discuss its potential in deep tissue imaging.

Photoacoustic image-guided interventions

Photoacoustic imaging has demonstrated its potential for diagnosis over the last few decades. In recent years, its unique imaging capabilities, such as detecting structural, functional and molecular information in deep regions with optical contrast and ultrasound resolution, have opened up many opportunities for photoacoustic imaging to be used during image-guided interventions. Numerous studies have investigated the capability of photoacoustic imaging to guide various interventions such as drug delivery, therapies, surgeries, and biopsies. These studies have demonstrated that photoacoustic imaging can guide these interventions effectively and non-invasively in real-time. In this minireview, we will elucidate the potential of photoacoustic imaging in guiding active and passive drug deliveries, photothermal therapy, and other surgeries and therapies using endogenous and exogenous contrast agents including organic, inorganic, and hybrid nanoparticles, as well as needle-based biopsy procedures. The advantages of photoacoustic imaging in guided interventions will be discussed. It will, therefore, show that photoacoustic imaging has great potential in real-time interventions due to its advantages over current imaging modalities like computed tomography, magnetic resonance imaging, and ultrasound imaging.

Acoustic resolution photoacoustic microscopy based on microelectromechanical systems scanner

Photoacoustic microscopy (PAM) can be classified as optical resolution (OR)-PAM and acoustic resolution (AR)-PAM depending on the type of resolution achieved. Using microelectromechanical systems (MEMS) scanner, high-speed OR-PAM system was developed earlier. Depth of imaging limits the use of OR-PAM technology for many preclinical and clinical imaging applications. Here, we demonstrate the use of a high-speed MEMS scanner for AR-PAM imaging. Lateral resolution of 84 μm and an axial resolution of 27 μm with ~2.7 mm imaging depth was achieved using a 50 MHz transducer-based AR-PAM system. Use of a higher frequency transducer at 75 MHz has further improved the resolution characteristics of the system with a reduction in imaging depth and a lateral resolution of 53 μm and an axial resolution of 18 μm with ~1.8 mm imaging depth was achieved. Using the two-axis MEMS scanner a 2 × 2 .5 mm² area was imaged in 3 seconds. The capability of achieving acoustic resolution images using the MEMS scanner makes it beneficial for the development of high-speed miniaturized systems for deeper tissue imaging.

Simultaneous Photoacoustic Imaging and Cavitation Mapping in Shockwave Lithotripsy

Kidney stone disease is a major health problem worldwide. Shockwave lithotripsy (SWL), which uses high-energy shockwave pulses to break up kidney stones, is extensively used in clinic. However, despite its noninvasiveness, SWL can produce cavitation in vivo. The rapid expansion and violent collapse of cavitation bubbles in small blood vessels may result in renal vascular injury. To better understand the mechanism of tissue injury and improve treatment safety and efficiency, it is highly desirable to concurrently detect cavitation and vascular injury during SWL. Current imaging modalities used in SWL ( e.g. , C-arm fluoroscopy and B-mode ultrasound) are not sensitive to vascular injuries. By contrast, photoacoustic imaging is a non-invasive and non-radiative imaging modality that is sensitive to blood, by using hemoglobin as the endogenous contrast. Moreover, photoacoustic imaging is also compatible with passive cavitation detection by sharing the ultrasound detection system. Here, we have integrated shockwave treatment, photoacoustic imaging, and passive cavitation detection into a single system. Our experimental results on phantoms and in vivo small animals have collectively demonstrated that the integrated system is capable of capturing shockwave-induced cavitation and the resultant vascular injury simultaneously. We expect that the integrated system, when combined with our recently developed internal-light-illumination photoacoustic imaging, will find important applications for monitoring shockwave-induced vascular injury in deep tissues during SWL.

Assembly strategies of organic-based imaging agents for fluorescence and photoacoustic bioimaging applications

The results of numerous studies have led to the development of supramolecular (assembled) organic substances for use in biomedical imaging as part of comprehensive approaches to the diagnosis of diseases. This review summarizes recent advances that have been made in the design and fabrication of assembled organic dyes for fluorescence and photoacoustic bioimaging.

Sensitivity Enhanced Photoacoustic Imaging Using a High-Frequency PZT Transducer with an Integrated Front-End Amplifier

Photoacoustic (PA) imaging is a hybrid imaging technique that can provide both structural and functional information of biological tissues. Due to limited permissible laser energy deposited on tissues, highly sensitive PA imaging is required. Here, we developed a 20 MHz lead zirconium titanate (PZT) transducer (1.5 mm × 3 mm) with front-end amplifier circuits for local signal processing to achieve sensitivity enhanced PA imaging. The electrical and acoustic performance was characterized. Experiments on phantoms and chicken breast tissue were conducted to validate the imaging performance. The fabricated prototype shows a bandwidth of 63% and achieves a noise equivalent pressure (NEP) of 0.24 mPa/√Hz and a receiving sensitivity of 62.1 μV/Pa at 20 MHz without degradation of the bandwidth. PA imaging of wire phantoms demonstrates that the prototype is capable of improving the detection sensitivity by 10 dB compared with the traditional transducer without integrated amplifier. In addition, in vitro experiments on chicken breast tissue show that structures could be imaged with enhanced contrast using the prototype and the imaging depth range was improved by 1 mm. These results demonstrate that the transducer with an integrated front-end amplifier enables highly sensitive PA imaging with improved penetration depth. The proposed method holds the potential for visualization of deep tissue structures and enhanced detection of weak physiological changes.

Deep tissue photoacoustic imaging of nickel(II) dithiolene-containing polymeric nanoparticles in the second near-infrared window

Photoacoustic imaging is gaining great attention in the medical world due to its significant potential for clinical translation. Light excitation in the second near-infrared (NIR-II) window (1000-1350 nm) has resolution and penetration depth suitable for several clinical applications. However, the significant challenge exists for clinical translation because of the absence of notable intrinsic chromophores in this clinically significant optical range to generate diagnostic images. Methods: We present newly developed a biocompatible nickel dithiolene-based polymeric nanoparticle (NiPNP), which have a strong and sharp absorption peak at 1064 nm, as a photoacoustic contrast agent to boost specific absorbance in the NIR-II window for in vivo deep tissue imaging. Results: We confirm the enhanced PA signal by NiPNP's strong light absorption in the NIR-II window (287% higher than that of NIR-I) and deep tissue imaging capability (~5.1 cm) through in vitro experiment. We have successfully acquired diagnostic-quality in vivo photoacoustic images in deep tissue (~3.4 cm) of sentinel lymph nodes, gastrointestinal tracts, and bladders of live rats by using clinically viable imaging system. Conclusions: Our results prove that with strong absorption in the NIR-II window and with deeper imaging depth, the clinical translation of photoacoustic imaging with NiPNP is feasible for preclinical studies and thus would facilitate further clinical investigations.

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.

Optical and Optoacoustic Imaging

The spatiotemporal determination of molecular events and cells is important for understanding disease processes, especially in oncology, and thus for the development of novel treatments. Equally important is the knowledge of the biodistribution, localization, and targeted accumulation of novel therapies as well as monitoring of tumor growth and therapeutic response. Optical imaging provides an ideal versatile platform for imaging of all these problems and questions.

Gold nanoclusters for biomedical applications: toward in vivo studies

In parallel with the rapidly growing and widespread use of nanomedicine in the clinic, we are also witnessing the development of so-called theranostic agents that combine diagnostic and therapeutic properties.

Optical and Optoacoustic Imaging

The present chapter summarizes progress with optical methods that go beyond human vision. The focus is on two particular technologies: fluorescence molecular imaging and optoacoustic (photoacoustic) imaging. The rationale for the selection of these two methods is that in contrast to optical microscopy techniques, both fluorescence and optoacoustic imaging can achieve large fields of view, i.e., spanning several centimeters in two or three dimensions. Such fields of views relate better to human vision and can visualize large parts of tissue, a necessary premise for clinical detection. Conversely, optical microscopy methods only scan millimeter-sized dimensions or smaller. With such operational capacity, optical microscopy methods need to be guided by another visualization technique in order to scan a very specific area in tissue and typically only provide superficial measurements, i.e., information from depths that are of the order of 0.05-1 mm. This practice has generally limited their clinical applicability to some niche applications, such as optical coherence tomography of the retina. On the other hand, fluorescence molecular imaging and optoacoustic imaging emerge as more global optical imaging methods with wide applications in surgery, endoscopy, and non-invasive clinical imaging, as summarized in the following. The current progress in this field is based on a volume of recent review and other literature that highlights key advances achieved in technology and biomedical applications. Context and figures from references from the authors of this chapter have been used here, as it reflects our general view of the current status of the field.

Complete-noncontact photoacoustic microscopy by detection of initial pressures using a 3×3 coupler-based fiber-optic interferometer

We demonstrate a 3×3 coupler-based fiber-optic interferometric system to detect the local initial photoacoustic pressure. In contrast with the existing interferometric photoacoustic microscopy (PAM) relying on the measurement of the phase change of the probe light caused by the sample surface vibration, the present method measures the intensity change of the probe light caused by the initial photoacoustic pressure. Compared with the conventional interferometric PAMs, this method has the advantages: (1) it is free from the influence of the rough tissue surface, achieving complete noncontact in vivo imaging; (2) the probe light and the excitation light are focused at a same point below the sample surface, and the confocal configuration makes it more convenient for in vivo imaging; and (3) there is no need for phase stabilization, allowing a high imaging speed. These advantages show that the method will be a promising technique for in vivo imaging. This method is verified by imaging of a resolution test target and in vivo imaging of the blood vessels in a mouse ear.

Indocyanine Green J Aggregates in Polymersomes for Near-Infrared Photoacoustic Imaging

Clinical translation of photoacoustic imaging (PAI) has been limited by the lack of near-infrared (NIR) contrast agents with low toxicity required for regulatory approval. Herein, J aggregates of indocyanine green (ICG) with strong NIR absorbance were encapsulated at high loadings within small 77 nm polymersomes (nanocapsules) composed of poly(lactide-co-glycolide-b-poly(ethylene glycol)) (PLGA-b-PEG) bilayers, thus enabling PAI of of breast and ovarian cancer cells with high specificity and a sensitivity at the level of ∼100 total cells. All of the major components of the polymersomes are FDA approved and used in the clinic. During formation of polymersomes with a water-in-oil-in-water double emulsion process, loss of ICG from the ICG J aggregates was minimized by coating them with a layer of branched polyethylenimine and by providing excess "sacrificial" ICG to adsorb at the oil-water interfaces. The encapsulated J aggregates were protected against dissociation by the polymersome shell for 24 h in 100% fetal bovine serum, after which the polymersomes biodegraded and the J aggregates dissociated to ICG monomers.

Near-Infrared Light-Harvesting Fullerene-Based Nanoparticles for Promoted Synergetic Tumor Phototheranostics

A synergetic phototheranostic system, combining diagnostic photo-imaging and phototherapies [such as photothermal therapy and photodynamic therapy (PDT)], shows great potential in today's tumor precise therapy. Herein, we fabricate near-infrared (NIR) light-harvesting fullerene-based nanoparticles (DAF NPs) for photoacoustic (PA) imaging-guided synergetic tumor photothermal and PDT. The fullerene derivatives (DAF) absorbing in the NIR region have been synthesized by conjugating NIR-absorbing antenna with fullerene. In addition, DAF NPs with good biocompatibility have been fabricated via a nanoprecipitation approach. The as-prepared DAF NPs can accumulate and generate PA signals around the tumor site 6 h post injection via enhanced permeability and retention effect in vivo. More importantly, the DAF NPs exhibit better reactive oxygen species and heat generation efficacy compared with fullerene and antenna nanoparticles (DA NPs), respectively. Further in vitro and in vivo studies demonstrate that DAF NPs can effectively inhibit tumor growth through synergetic photodynamic and photothermal therapies, which provides a new sight of photosensitizer design for enhanced cancer phototheranostics.

Fast and High-Resolution Three-Dimensional Hybrid-Domain Photoacoustic Imaging Incorporating Analytical-Focused Transducer Beam Amplitude

Recently, many reconstruction methods have been developed to improve the lateral resolution of acoustic-resolution photoacoustic microscopy (ARPAM) in out-of-focus regions. Though these methods enhance image resolution to some extent, they require advanced computational hardware and large computational time, especially for three-dimensional (3-D) cases. However, some methods do not consider the finite size of a transducer, while others employ numerical discretization to build a focused transducer model that is less efficient and accurate. To overcome these problems, we propose a 3-D ARPAM imaging reconstruction method with high precision, high efficiency, and low memory cost. It inherits the framework of model-based reconstructions and incorporates the forward acoustic model in the hybrid domain. This hybrid-domain acoustic model promotes an analytical solution to establish a focused transducer model. Furthermore, the non-uniform fast Fourier transform (NUFFT) and deconvolution methods are introduced to reduce the required computational time and memory volume for 3-D reconstructions. According to the experimental results reconstructed by the proposed method, the lateral resolution of an ARPAM image recorded by a 20-MHz focused transducer (NA 0.393) can reach 88.39 [Formula: see text]. This resolution exceeds the diffraction limitation of the focused transducer ( [Formula: see text]). When reconstructing a 3-D image with 200×200×150 pixels, the proposed method takes only 8.15 s using a laptop loaded with Intel Core i7-8550U CPU at 1.8 GHz and 1.06-GB memory.

Minimally invasive photoacoustic imaging: Current status and future perspectives

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality that is based on optical absorption contrast, capable of revealing distinct spectroscopic signatures of tissue at high spatial resolution and large imaging depths. However, clinical applications of conventional non-invasive PAI systems have been restricted to examinations of tissues at depths less than a few cm due to strong light attenuation. Minimally invasive photoacoustic imaging (miPAI) has greatly extended the landscape of PAI by delivering excitation light within tissue through miniature fibre-optic probes. In the past decade, various miPAI systems have been developed with demonstrated applicability in several clinical fields. In this article, we present an overview of the current status of miPAI and our thoughts on future perspectives.

Advanced Nanotechnology Leading the Way to Multimodal Imaging-Guided Precision Surgical Therapy

Surgical resection is the primary and most effective treatment for most patients with solid tumors. However, patients suffer from postoperative recurrence and metastasis. In the past years, emerging nanotechnology has led the way to minimally invasive, precision and intelligent oncological surgery after the rapid development of minimally invasive surgical technology. Advanced nanotechnology in the construction of nanomaterials (NMs) for precision imaging-guided surgery (IGS) as well as surgery-assisted synergistic therapy is summarized, thereby unlocking the advantages of nanotechnology in multimodal IGS-assisted precision synergistic cancer therapy. First, mechanisms and principles of NMs to surgical targets are briefly introduced. Multimodal imaging based on molecular imaging technologies provides a practical method to achieve intraoperative visualization with high resolution and deep tissue penetration. Moreover, multifunctional NMs synergize surgery with adjuvant therapy (e.g., chemotherapy, immunotherapy, phototherapy) to eliminate residual lesions. Finally, key issues in the development of ideal theranostic NMs associated with surgical applications and challenges of clinical transformation are discussed to push forward further development of NMs for multimodal IGS-assisted precision synergistic cancer therapy.

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

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

Ultrasound-Responsive Conversion of Microbubbles to Nanoparticles to Enable Background-Free in Vivo Photoacoustic Imaging

Photoacoustic (PA) imaging based on the photon-to-ultrasound conversion allows the imaging of optical absorbers in deep tissues with high spatial resolution. However, the inherent optical absorbance of biomolecules (e.g., hemoglobin, melanin, etc.) would show up as tissue background signals to interfere with signals from the contrast agent during in vivo PA imaging, limiting the imaging sensitivity. Herein, an ultrasound (US)-responsive PA imaging probe based on microbubbles (MBs) containing gold nanoparticles (Au NPs) is designed for in vivo "background-free" PA imaging. The obtained Au@lip MBs with separated Au NPs decorated within the lipid shell of MBs show low PA signals under near-infrared (NIR) excitation. Interestingly, under exposure to US pulses, those Au@lip MBs would burst to form nanoscale aggregates of Au@lip NPs, which exhibit significantly enhanced NIR PA signals due to their red-shifted surface plasmon resonance. Therefore, by subtracting the PA image captured pre-US burst from that captured post-US burst, the tissue background PA signals could be deducted to enable background-free PA imaging with high sensitivities as demonstrated by multiple ex vivo and in vivo experiments. This work presents a simple yet effective strategy to deduct background signals during PA imaging, which is promising for accurate PA detection of targets in tissues with a strong background.

Enhanced contrast acoustic-resolution photoacoustic microscopy using double-stage delay-multiply-and-sum beamformer for vasculature imaging

In acoustic-resolution photoacoustic microscopy (AR-PAM) systems, the lateral resolution in the focal zone of the ultrasound (US) transducer is determined by the numerical aperture (NA) of the transducer. To have a high lateral resolution, a large NA is used. However, the larger the NA, the smaller the depth of focus [DOF]. As a result, the lateral resolution is deteriorated at depths out of the focal region. The synthetic aperture focusing technique (SAFT) along with a beamformer can be used to improve the resolution outside the focal region. In this work, for image formation in AR-PAM, we propose the double-stage delay-multiply-and-sum (DS_DMAS) algorithm to be combined with SAFT. The proposed method is evaluated experimentally using hair targets and in vivo vasculature imaging. It is shown that DS_DMAS provides a higher resolution and contrast compared to other methods. For the B-mode images obtained using the hair phantom, the proposed method reduces the average noise level for all the depths by about 134%, 57% and 23%, compared to the original low- resolution, SAFT+DAS and SAFT+DMAS methods, respectively. All the results indicate that the proposed method can be an appropriate algorithm for image formation in AR-PAM systems.

Photoacoustic imaging of living mice enhanced with a low-cost contrast agent

One of the advantages of photoacoustic imaging (PAI) is that its image contrast may come from exogenous agents. Such advantage leads to the development of a great number of exogenous probes. However, the biosafety of most of these contrast agents has not yet been confirmed, thus hindering their clinical translation. In this work, we report on the utilization of a clinically commonly used nutritional medicine, the Intralipid, as a new contrast agent for photoacoustic imaging. Intralipid consists of soybean oil, lecithin and glycerin and has long been adapted in clinical practices, mainly as a parenteral nutrition. In our study, we found that with Intralipid, the imaging sensitivity of PAI can be effectively enhanced, as demonstrated in in vivo imaging of different organs of nude mice. Further imaging studies on cancerous mice showed not only a twofold PA signal enhancement, but also a strong and long-lasting signal aggregation in the tumor region. Our result revealed the potential of Intralipid to be used in clinical PAI applications, since it is clinically safe, and can be easily prepared at very low cost.

Lithium niobate-based transparent ultrasound transducers for photoacoustic imaging

This Letter demonstrates lithium niobate (LiNbO₃)-based transparent ultrasound transducers (TUTs) for photoacoustic imaging applications. The TUTs were fabricated by coating the top and bottom surfaces of a 0.25 mm thick LiNbO₃wafer with transparent indium-tin-oxide (ITO) electrodes. The resulting transducers showed ∼80% optical transparency in the wavelength range of 690-970 nm. The TUTs had a resonant frequency of 14.5 MHz and ∼70% photoacoustic bandwidth. The versatility of the TUT approach is demonstrated by introducing two different transparent photoacoustic imaging (PAI) geometries. In one method, which suits endoscopy applications, an optical fiber of a laser diode is directly fixed on the backside of a 2.5 mm diameter TUT, and the fiber-TUT device is raster scanned to form 3D photoacoustic images. In the second method, which suits high-throughput applications, a free-space optical-only raster scanning of the laser fiber across a 1  cm×1  cm planar TUT yielded 3D photoacoustic images. The proposed TUT approach is low in cost, easy to manufacture, compatible with conventional clinical ultrasound electronics, and scalable for different configurations, including 2D TUT arrays to achieve real-time 3D high-throughput PAI.

Evaluation of Retinal Pigment Epithelium and Choroidal Neovascularization in Rats Using Laser-Scanning Optical-Resolution Photoacoustic Microscopy

PURPOSE

To demonstrate the value of the laser-scanning optical-resolution (LSOR)-photoacoustic (PA) microscopy (PAM) system and the conventional multimodal imaging techniques in the evaluation of laser-induced retinal injury and choroidal neovascularization (CNV) in rats.

METHODS

Different degrees of retinal injury were induced using laser photocoagulation. We compared the LSOR-PAM system with conventional imaging techniques in evaluating retinal injury with or without CNV. Six additional rats, treated with an anti-VEGF antibody or immunoglobulin G immediately after photocoagulation, were imaged 7 and 14 days after injection, and CNV lesion areas were compared.

RESULTS

In the retinal injury model, fundus autofluorescence showed well-defined hyperreflection, while the lesion displayed abundant PA signals demonstrating nonuniform melanin distribution in retinal pigment epithelium (RPE). RPE was detected with higher contrast in the PAM B-scan image than optical coherence tomography (OCT). Additionally, the CNV lesion was present with multiple PA signal intensities which distinctly characterized the location and area of CNV as found in fundus fluorescein angiography. Furthermore, the decreased PA signals extending from the CNV lesion were similar to those of the vascular bud in ex vivo imaging, which was invisible in other in vivo images. When treated with anti-VEGF agents, statistically significant differences can be demonstrated by PAM similar to other modalities.

CONCLUSIONS

LSOR-PAM can detect the melanin distribution of RPE in laser-induced retinal injury and CNV in rats. PAM imaging provides a potential new tool to evaluate the vitality and functionality of RPE in vivo as well as to monitor the development and treatment of CNV.

Deep-learning-based motion-correction algorithm in optical resolution photoacoustic microscopy

In this study, we propose a deep-learning-based method to correct motion artifacts in optical resolution photoacoustic microscopy (OR-PAM). The method is a convolutional neural network that establishes an end-to-end map from input raw data with motion artifacts to output corrected images. First, we performed simulation studies to evaluate the feasibility and effectiveness of the proposed method. Second, we employed this method to process images of rat brain vessels with multiple motion artifacts to evaluate its performance for in vivo applications. The results demonstrate that this method works well for both large blood vessels and capillary networks. In comparison with traditional methods, the proposed method in this study can be easily modified to satisfy different scenarios of motion corrections in OR-PAM by revising the training sets.

Recent advances in the development of nanoparticles for multimodality imaging and therapy of cancer

This review explores recent work directed toward the development of nanoparticles (NPs) for multimodality cancer imaging and targeted cancer therapy. In the growing era of precision medicine, theranostics, or the combined use of targeted molecular probes in diagnosing and treating diseases is playing a particularly powerful role. There is a growing interest, particularly over the past few decades, in the use of NPs as theranostic tools due to their excellent performance in receptor target specificity and reduction in off-target effects when used as therapeutic agents. This review discusses recent advances, as well as the advantages and challenges of the application of NPs in cancer imaging and therapy.

Renal Clearable Ru-based Coordination Polymer Nanodots for Photoacoustic Imaging Guided Cancer Therapy

Rationale: Despite the promises of applying theranostic nanoagents for imaging-guided cancer therapy, the chronic retention of these nanoagents may cause safety concerns that hinder their future clinical applications. The metabolizable nanoagents with rapid renal excretion to avoid long-term toxicity is a possible solution for this issue. Method: Herein, we synthesize ultra-small metal-organic coordination polymer nanodots based on ruthenium ion (Ru3+) / phenanthroline (Phen) (Ru-Phen CPNs) with superior near-infrared (NIR) absorption. The size, photothermal conversion, cytotoxicity, photoacoustic imaging, in vivo & in vitro cancer treatment efficiency and biosafety are tested. Results: The size of the ultra-small Ru-Phen CPNs is 6.5 nm. The photothermal conversion efficiency is measured to be ~ 60.69 %, much higher than that of previously reported photothermal agents. The Ru-Phen CPNs could be employed for photoacoustic (PA, 808 nm) imaging-guided photothermal therapy (PTT, 808 nm, 0.5 W/cm2) with great performance. Notably, the intrinsic PA signals (808 nm) of Ru-Phen CPNs are observed in kidneys of treated mice, illustrating efficient renal clearance of those ultra-small CPNs. Moreover, the clearance of CPNs is further confirmed by detecting Ru levels in urine and feces. Conclusion: Our work presents a new type of ultra-small Ru-based CPNs with a record high photothermal conversion efficiency, efficient tumor retention after systemic administration, and rapid renal excretion to avoid long-term toxicity, promising for imaging-guided photothermal therapy.

Recent Progress on Near-Infrared Photoacoustic Imaging: Imaging Modality and Organic Semiconducting Agents

Over the past few decades, the photoacoustic (PA) effect has been widely investigated, opening up diverse applications, such as photoacoustic spectroscopy, estimation of chemical energies, or point-of-care detection. Notably, photoacoustic imaging (PAI) has also been developed and has recently received considerable attention in bio-related or clinical imaging fields, as it now facilitates an imaging platform in the near-infrared (NIR) region by taking advantage of the significant advancement of exogenous imaging agents. The NIR PAI platform now paves the way for high-resolution, deep-tissue imaging, which is imperative for contemporary theragnosis, a combination of precise diagnosis and well-timed therapy. This review reports the recent progress on NIR PAI modality, as well as semiconducting contrast agents, and outlines the trend in current NIR imaging and provides further direction for the prospective development of PAI systems.

Experimental design and numerical investigation of a photoacoustic sensor for a low-power, continuous-wave, laser-based frequency-domain photoacoustic microscopy

We have developed a photoacoustic (PA) sensor using a low-power, continuous- wave laser and a kHz-range microphone. The sensor is simple, flexible, cost-effective, and compatible with commercial optical microscopes. The sensor enables noncontact PA measurements through air, whereas most current existing PA techniques require an acoustic coupling liquid for detection. The PA sensor has three main components: one is the chamber that holds the sample, the second is a resonator column used to amplify the weak PA signals generated within the sample chamber, and the third is a microphone at the end of the resonator column to detect the amplified signals. The chamber size was designed to be 8  mm  ×  3  mm as the thermal diffusion length and viscous-thermal damping of air at room pressure and temperature are 2 and 1 mm, respectively. We numerically and experimentally examined the effect of the resonator column size on the frequency response of the PA sensor. The quality factor decreased significantly when the sample chamber size was reduced from 4  mm  ×  3  mm to 2  mm  ×  3  mm due to thermos-viscous damping of the air. The quality factor decreased by 27%, demonstrating the need for optimal design for the sample chamber and resonator column size. The system exhibited noise equivalent molecular sensitivity (NEM) per unit bandwidth (NEM  /  √  Δf) of ∼19,966  Hz ^−1/2 or 33  ×  10^−21  mol or 33 zeptomol, which is an improvement of 2.2 times compared to the previous system design. This PA sensor has the potential for noncontact high-resolution PA imaging of materials without the need for coupling fluids.

Listening to tissues with new light: recent technological advances in photoacoustic imaging

Photoacoustic tomography (PAT), or optoacoustic tomography, has achieved remarkable progress in the past decade, benefiting from the joint developments in optics, acoustics, chemistry, computing and mathematics. Unlike pure optical or ultrasound imaging, PAT can provide unique optical absorption contrast as well as widely scalable spatial resolution, penetration depth and imaging speed. Moreover, PAT has inherent sensitivity to tissue's functional, molecular, and metabolic state. With these merits, PAT has been applied in a wide range of life science disciplines, and has enabled biomedical research unattainable by other imaging methods. This Review article aims at introducing state-of-the-art PAT technologies and their representative applications. The focus is on recent technological breakthroughs in structural, functional, molecular PAT, including super-resolution imaging, real-time small-animal whole-body imaging, and high-sensitivity functional/molecular imaging. We also discuss the remaining challenges in PAT and envisioned opportunities.

Optoacoustic image formation approaches-a clinical perspective

Clinical translation of optoacoustic imaging is fostered by the rapid technical advances in imaging performance as well as the growing number of clinicians recognizing the immense diagnostic potential of this technology. Clinical optoacoustic systems are available in multiple configurations, including hand-held and endoscopic probes as well as raster-scan approaches. The hardware design must be adapted to the accessible portion of the imaged region and other application-specific requirements pertaining the achievable depth, field of view or spatio-temporal resolution. Equally important is the adequate choice of the signal and image processing approach, which is largely responsible for the resulting imaging performance. Thus, new image reconstruction algorithms are constantly evolving in parallel to the newly-developed set-ups. This review focuses on recent progress on optoacoustic image formation algorithms and processing methods in the clinical setting. Major reconstruction challenges include real-time image rendering in two and three dimensions, efficient hybridization with other imaging modalitites as well as accurate interpretation and quantification of bio-markers, herein discussed in the context of ongoing progress in clinical translation.

Spatiotemporally Coupled Photoactivity of Phthalocyanine-Peptide Conjugate Self-Assemblies for Adaptive Tumor Theranostics

Spatiotemporally coupled tumor phototheranostic platforms offer a flexible and precise system that takes the biological interaction between tumors and photoactive agents into consideration for optimizing treatment, which is highly consistent with precision medicine. However, the fabrication of monocomponent-based photoactive agents applicable to multifold imaging techniques and multiple therapies in a facile way remains challenging. In this study, we developed simple phthalocyanine-peptide (PF) conjugate-based monocomponent nanoparticles with spatiotemporally coupled photoactivity for adaptive tumor theranostics. The self-assembled PF nanoparticles possess well-defined spherical nanostructures and excellent colloidal stability along with supramolecular photothermal effects. Importantly, the PF nanoparticles showed switchable photoactivity triggered by their interactions with the cell membrane, which enables an adaptive transformation from photothermal therapy (PTT) and photoacoustic imaging (PAI) to photodynamic therapy (PDT) and corresponding fluorescence imaging (FI). Theranostic modalities are integrated in a spatiotemporally coupled manner, providing a facile, biocompatible and effective route for localized tumor phototherapy. This study offers a flexible and versatile strategy to integrate multiple theranostic modalities into a single component so that it can realize its full potential and thereby amplify its therapeutic efficacy, creating promising opportunities for the design of theranostics and further highlighting their clinical prospects to the diagnosis and treatment of cancers.