In vivo preclinical photoacoustic imaging of tumor vasculature development and therapy

A Review of Endogenous and Exogenous Contrast Agents Used in Photoacoustic Tomography with Different Sensing Configurations

Optical-based sensing approaches have long been an indispensable way to detect molecules in biological tissues for various biomedical research and applications. The advancement in optical microscopy is one of the main drivers for discoveries and innovations in both life science and biomedical imaging. However, the shallow imaging depth due to the use of ballistic photons fundamentally limits optical imaging approaches’ translational potential to a clinical setting. Photoacoustic (PA) tomography (PAT) is a rapidly growing hybrid imaging modality that is capable of acoustically detecting optical contrast. PAT uniquely enjoys high-resolution deep-tissue imaging owing to the utilization of diffused photons. The exploration of endogenous contrast agents and the development of exogenous contrast agents further improve the molecular specificity for PAT. PAT’s versatile design and non-invasive nature have proven its great potential as a biomedical imaging tool for a multitude of biomedical applications. In this review, representative endogenous and exogenous PA contrast agents will be introduced alongside common PAT system configurations, including the latest advances of all-optical acoustic sensing techniques.

Medical Imaging of Microrobots: Toward In Vivo Applications

Medical microrobots (MRs) have been demonstrated for a variety of non-invasive biomedical applications, such as tissue engineering, drug delivery, and assisted fertilization, among others. However, most of these demonstrations have been carried out in in vitro settings and under optical microscopy, being significantly different from the clinical practice. Thus, medical imaging techniques are required for localizing and tracking such tiny therapeutic machines when used in medical-relevant applications. This review aims at analyzing the state of the art of microrobots imaging by critically discussing the potentialities and limitations of the techniques employed in this field. Moreover, the physics and the working principle behind each analyzed imaging strategy, the spatiotemporal resolution, and the penetration depth are thoroughly discussed. The paper deals with the suitability of each imaging technique for tracking single or swarms of MRs and discusses the scenarios where contrast or imaging agent's inclusion is required, either to absorb, emit, or reflect a determined physical signal detected by an external system. Finally, the review highlights the existing challenges and perspective solutions which could be promising for future in vivo applications.

In Vivo Tumor Vascular Imaging with Light Emitting Diode-Based Photoacoustic Imaging System

Photoacoustic (PA) imaging has shown tremendous promise for imaging tumor vasculature and its function at deeper penetration depths without the use of exogenous contrast agents. Traditional PA imaging systems employ expensive and bulky class IV lasers with low pulse repetition rate, due to which its availability for preclinical cancer research is hampered. In this study, we evaluated the capability of a Light-Emitting Diode (LED)-based PA and ultrasound (US) imaging system for monitoring heterogeneous microvasculature in tumors (up to 10 mm in depth) and quantitatively compared the PA images with gold standard histology images. We used a combination of a 7 MHz linear array US transducer and 850 nm excitation wavelength LED arrays to image blood vessels in a subcutaneous tumor model. After imaging, the tumors were sectioned and stained for endothelial cells to correlate with PA images across similar cross-sections. Analysis of 30 regions of interest in tumors from different mice showed a statistically significant R-value of 0.84 where the areas with high blood vessel density had high PA response while low blood vessel density regions had low PA response. Our results confirm that LED-based PA and US imaging can provide 2D and 3D images of tumor vasculature and the potential it has as a valuable tool for preclinical cancer research.

Co-registration of optoacoustic tomography and magnetic resonance imaging data from murine tumour models

As optoacoustic tomography (OT) emerges as a mainstream pre-clinical imaging modality, understanding the relationship between optoacoustic and other imaging biomarkers in the context of the underlying tissue biology becomes vitally important. Complementary insight into tumour vasculature and hypoxia can be gained using OT alongside magnetic resonance imaging (MRI)-based techniques. To evaluate the relationship between these metrics and the relative performance of the two modalities in assessment of tumour physiology, co-registration of their output imaging data is required. Unfortunately, this poses a significant challenge due to differences in animal positioning during imaging. Here, we present an integrated framework for registration of OT and MR image data in mice. Our framework combines a novel MR animal holder, to improve animal positioning during imaging, and a landmark-based software co-registration algorithm. We demonstrate that our protocol significantly improves registration of both body and tumour contours between these modalities, enabling more precise multi-modal tumour characterisation.

High-speed wide-field multi-parametric photoacoustic microscopy

Capable of imaging blood perfusion, oxygenation, and flow simultaneously at the microscopic level, multi-parametric photoacoustic microscopy (PAM) has quickly emerged as a powerful tool for studying hemodynamic and metabolic changes due to physiological stimulations or pathological processes. However, the low scanning speed poised by the correlation-based blood flow measurement impedes its application in studying rapid microvascular responses. To address this challenge, we have developed a new, to the best of our knowledge, multi-parametric PAM system. By extending the optical scanning range with a cylindrically focused ultrasonic transducer (focal zone, 76µm×4.5mm) for simultaneous acquisition of 500 B-scans, the new system is 112 times faster than our previous multi-parametric system that uses a spherically focused transducer (focal diameter, 40 µm) and enables high-resolution imaging of blood perfusion, oxygenation, and flow over an area of 4.5×1mm2 at a frame rate of 1 Hz. We have demonstrated the feasibility of this system in the living mouse ear. Further development of this system into reflection mode will enable real-time cortex-wide imaging of hemodynamics and metabolism in the mouse brain.

Photothermally tunable Fabry-Pérot fiber interferometer for photoacoustic mesoscopy

An optical fiber based Fabry-Pérot interferometer whose resonant wavelength can be dynamically tuned was designed and realized for photoacoustic mesoscopy. The optical path length (OPL) of the Fabry-Pérot cavity can be modulated by a photothermal heating process, which was achieved by adjusting the power of a 650 nm heating laser. The optical heating process can effectively change the thickness and refractive index of the polymer spacer of the sensor cavity. The robustness of the sensor can be greatly improved by proper packaging. The interferometer was interrogated by a relatively cheap wavelength-fixed 1550 nm laser for broadband and sensitive ultrasound detection, eliminating the requirement for an expensive tunable interrogation laser. The sensing module was then integrated into a photoacoustic mesoscopic imaging system. Two phantom imaging experiments and an ex vivo imaging experiment demonstrated the capability of such a miniature sensor. The interferometer has an acoustic detection bandwidth of up to 30 MHz and a noise equivalent pressure of 40 mPa/Hz^1/2 (i.e., 220 Pa over the full detection bandwidth). The new tuning mechanism and the batch-production compatibility of the sensor holds promises for commercialization and parallelized detection.

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.

In vivo photoacoustic imaging of a nonfluorescent E2 crimson genetic reporter in mammalian tissues

SIGNIFICANCE

Green-fluorescent protein (GFP)-like fluorescent proteins are used extensively as genetic reporters in fluorescence imaging due to their distinctive ability to form chromophores independent of external enzymes or cofactors. However, their use for photoacoustic (PA) imaging has not been demonstrated in mammalian tissues because they possess low PA signal generation efficiency in their native state. By engineering them to become nonfluorescent (NF), their PA generation efficiency was increased. This enabled the generation of in vivo contrast in mice, making it possible for GFP-like proteins to be used as PA genetic reporters in mammalian tissues.

AIM

The aim was to develop a darkened GFP-like protein reporter by modifying E2 crimson fluorescent protein (FP) in order to generate NF mutant proteins with high PA signal generation efficiency for in vivo imaging.

APPROACH

The absorbance, fluorescence, and PA amplitude spectra of purified protein solutions of the FP and engineered NF mutants were measured in order to identify the mutant with the highest PA signal generation efficiency. This mutant, referred to as NFA, and the native FP were then stably expressed in LS174T human colorectal tumor cells using a retroviral vector and tested for photostability under continuous pulsed illumination. To demonstrate the improvement in PA signal generation in vivo, cells expressing the FP and NFA mutant were injected subcutaneously in mice and imaged using a Fabry-Perot based PA scanner.

RESULTS

The NF mutants of E2 crimson exhibited fluorescence that was 2 orders of magnitude lower than the FP and a higher PA signal generation efficiency; the NFA-generated PA signal was approximately three times higher than the FP. Tumor cells expressing the NFA mutant provided sufficient image contrast to be visualized in vivo against a background of strong vascular contrast, whereas the FP-expressing cells did not generate visible contrast.

CONCLUSION

A GFP-like protein has been demonstrated as a genetic reporter for PA imaging in mammalian tissue for the first time. This was achieved by a mutation, which darkened the FP and increased the PA signal generation efficiency. The approach taken suggests that GFP-like proteins could be a promising addition to the current cohort of genetic reporters available for in vivo PA imaging.

Image-guided tumor surgery: The emerging role of nanotechnology

Surgical resection is a mainstay treatment for solid tumors. Yet, methods to distinguish malignant from healthy tissue are primarily limited to tactile and visual cues as well as the surgeon's experience. As a result, there is a possibility that a positive surgical margin (PSM) or the presence of residual tumor left behind after resection may occur. It is well-documented that PSMs can negatively impact treatment outcomes and survival, as well as pose an economic burden. Therefore, surgical tumor imaging techniques have emerged as a promising method to decrease PSM rates. Nanoparticles (NPs) have unique characteristics to serve as optical contrast agents during image-guided surgery (IGS). Recently, there has been tremendous growth in the volume and types of NPs used for IGS, including clinical trials. Herein, we describe the most recent contributions of nanotechnology for surgical tumor identification. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Implantable Materials and Surgical Technologies > Nanoscale Tools and Techniques in Surgery Diagnostic Tools > in vivo Nanodiagnostics and Imaging.

From tumour perfusion to drug delivery and clinical translation of in silico cancer models

In silico cancer models have demonstrated great potential as a tool to improve drug design, optimise the delivery of drugs to target sites in the host tissue and, hence, improve therapeutic efficacy and patient outcome. However, there are significant barriers to the successful translation of in silico technology from bench to bedside. More precisely, the specification of unknown model parameters, the necessity for models to adequately reflect in vivo conditions, and the limited amount of pertinent validation data to evaluate models' accuracy and assess their reliability, pose major obstacles in the path towards their clinical translation. This review aims to capture the state-of-the-art in in silico cancer modelling of vascularised solid tumour growth, and identify the important advances and barriers to success of these models in clinical oncology. Particular emphasis has been put on continuum-based models of cancer since they - amongst the class of mechanistic spatio-temporal modelling approaches - are well-established in simulating transport phenomena and the biomechanics of tissues, and have demonstrated potential for clinical translation. Three important avenues in in silico modelling are considered in this contribution: first, since systemic therapy is a major cancer treatment approach, we start with an overview of the tumour perfusion and angiogenesis in silico models. Next, we present the state-of-the-art in silico work encompassing the delivery of chemotherapeutic agents to cancer nanomedicines through the bloodstream, and then review continuum-based modelling approaches that demonstrate great promise for successful clinical translation. We conclude with a discussion of what we view to be the key challenges and opportunities for in silico modelling in personalised and precision medicine.

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).

Spatial heterogeneity of nanomedicine investigated by multiscale imaging of the drug, the nanoparticle and the tumour environment

Genetic and phenotypic tumour heterogeneity is an important cause of therapy resistance. Moreover, non-uniform spatial drug distribution in cancer treatment may cause pseudo-resistance, meaning that a treatment is ineffective because the drug does not reach its target at sufficient concentrations. Together with tumour heterogeneity, non-uniform drug distribution causes “therapy heterogeneity”: a spatially heterogeneous treatment effect. Spatial heterogeneity in drug distribution occurs on all scales ranging from interpatient differences to intratumour differences on tissue or cellular scale. Nanomedicine aims to improve the balance between efficacy and safety of drugs by targeting drug-loaded nanoparticles specifically to tumours. Spatial heterogeneity in nanoparticle and payload distribution could be an important factor that limits their efficacy in patients. Therefore, imaging spatial nanoparticle distribution and imaging the tumour environment giving rise to this distribution could help understand (lack of) clinical success of nanomedicine. Imaging the nanoparticle, drug and tumour environment can lead to improvements of new nanotherapies, increase understanding of underlying mechanisms of heterogeneous distribution, facilitate patient selection for nanotherapies and help assess the effect of treatments that aim to reduce heterogeneity in nanoparticle distribution.

In this review, we discuss three groups of imaging modalities applied in nanomedicine research: non-invasive clinical imaging methods (nuclear imaging, MRI, CT, ultrasound), optical imaging and mass spectrometry imaging. Because each imaging modality provides information at a different scale and has its own strengths and weaknesses, choosing wisely and combining modalities will lead to a wealth of information that will help bring nanomedicine forward.

Use of a flexible optical fibre bundle to interrogate a Fabry-Perot sensor for photoacoustic imaging

Photoacoustic imaging systems based on a Fabry Perot (FP) ultrasound sensor that is read-out by scanning a free-space laser beam over its surface can provide high resolution photoacoustic images. However, this type of free-space scanning usually requires a bulky 2-axis galvanometer based scanner that is not conducive to the realization of a lightweight compact imaging head. It is also unsuitable for endoscopic applications that may require complex and flexible access. To address these limitations, the use of a flexible, coherent fibre bundle to interrogate the FP sensor has been investigated. A laboratory set-up comprising an x-y scanner, a commercially available, 1.35 mm diameter, 18,000 core flexible fibre bundle with a custom-designed telecentric optical relay at its distal end was used. Measurements of the optical and acoustic performance of the FP sensor were made and compared to that obtained using a conventional free-space FP based scanner. Spatial variations in acoustic sensitivity were greater and the SNR lower with the fibre bundle implementation but high quality photoacoustic images could still be obtained. 3D images of phantoms and ex vivo tissues with a spatial resolution and fidelity consistent with a free-space scanner were acquired. By demonstrating the feasibility of interrogating the FP sensor with a flexible fibre bundle, this study advances the realization of compact hand-held clinical scanners and flexible endoscopic devices based on the FP sensing concept.

Longitudinal Photoacoustic Imaging of the Pharmacodynamic Effect of Vascular Targeted Therapy on Tumors

PURPOSE

Photoacoustic imaging (PAI) is a novel noninvasive and nonionizing imaging technique that allows longitudinal imaging of tumor vasculature in vivo and monitoring of response to therapy, especially for vascular targeted chemotherapy agents. In this study, we used a novel high-resolution all-optical PAI scanner to observe the pharmacodynamic response to the vascular-disrupting agent OXi4503.

EXPERIMENTAL DESIGN

Two models of colorectal carcinoma (SW1222 and LS174T) that possess differing pathophysiologic vascularization were established as subcutaneous tumors in mice. Monitoring of response was performed over a 16-day "regrowth" period following treatment at 40 mg/kg, and at day 2 for a "dose response" study at 40 mg/kg, 10 mg/kg, 1 mg/kg, and sham dose.

RESULTS

Qualitative and quantitative changes in PA signal are observed, with an initial decrease followed by a plateau and subsequent return of signal indicating regrowth. Both tumor types exhibited a decrease in signal; however, the more vascularized SW1222 tumors show greater response to treatment. Decreasing the dose of OXi4503 led to a decrease in PA signal intensity of 60%, 52%, and 20% in SW1222 tumors and 30%, 26%, and 4% for LS174T tumors.

CONCLUSIONS

We have shown for the first time that PAI can observe the pharmacodynamic response of tumor vasculature to drug treatment both longitudinally and at different dose levels. Assessment of differing response to treatment based on vascular pathophysiologic differences among patients has the potential to provide personalized drug therapy; we have demonstrated that PAI, which is clinically translatable, could be a powerful tool for this purpose.

Optical-Resolution Photoacoustic Microscopy Using Transparent Ultrasound Transducer

The opacity of conventional ultrasound transducers can impede the miniaturization and workflow of current photoacoustic systems. In particular, optical-resolution photoacoustic microscopy (OR-PAM) requires the coaxial alignment of optical illumination and acoustic-detection paths through complex beam combiners and a thick coupling medium. To overcome these hurdles, we developed a novel OR-PAM method on the basis of our recently reported transparent lithium niobate (LiNbO3) ultrasound transducer (Dangi et al., Optics Letters, 2019), which was centered at 13 MHz ultrasound frequency with 60% photoacoustic bandwidth. To test the feasibility of wearable OR-PAM, optical-only raster scanning of focused light through a transducer was performed while the transducer was fixed above the imaging subject. Imaging experiments on resolution targets and carbon fibers demonstrated a lateral resolution of 8.5 µm. Further, we demonstrated vasculature mapping using chicken embryos and melanoma depth profiling using tissue phantoms. In conclusion, the proposed OR-PAM system using a low-cost transparent LiNbO3 window transducer has a promising future in wearable and high-throughput imaging applications, e.g., integration with conventional optical microscopy to enable a multimodal microscopy platform capable of ultrasound stimulation.

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.

Estimating blood oxygenation from photoacoustic images: can a simple linear spectroscopic inversion ever work?

Linear spectroscopic inversions, in which photoacoustic amplitudes are assumed to be directly proportional to absorption coefficients, are widely used in photoacoustic imaging to estimate blood oxygen saturation because of their simplicity. Unfortunately, they do not account for the spatially varying wavelength-dependence of the light fluence within the tissue, which introduces "spectral coloring," a potentially significant source of error. However, accurately correcting for spectral coloring is challenging, so we investigated whether there are conditions, e.g., sets of wavelengths, where it is possible to ignore the spectral coloring and still obtain accurate oxygenation measurements using linear inversions. Accurate estimates of oxygenation can be obtained when the wavelengths are chosen to (i) minimize spectral coloring, (ii) avoid ill-conditioning, and (iii) maintain a sufficiently high signal-to-noise ratio (SNR) for the estimates to be meaningful. It is not obvious which wavelengths will satisfy these conditions, and they are very likely to vary for different imaging scenarios, making it difficult to find general rules. Through the use of numerical simulations, we isolated the effect of spectral coloring from sources of experimental error. It was shown that using wavelengths between 500 nm and 1000 nm yields inaccurate estimates of oxygenation and that careful selection of wavelengths in the 620- to 920-nm range can yield more accurate oxygenation values. However, this is only achievable with a good prior estimate of the true oxygenation. Even in this idealized case, it was shown that considerable care must be exercised over the choice of wavelengths when using linear spectroscopic inversions to obtain accurate estimates of blood oxygenation. This suggests that for a particular imaging scenario, obtaining accurate and reliable oxygenation estimates using linear spectroscopic inversions requires careful modeling or experimental studies of that scenario, taking account of the instrumentation, tissue anatomy, likely sO₂ range, and image formation process.

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.

Assessment of Age-related Oxygenation Changes in Calf Skeletal Muscle by Photoacoustic Imaging: A Potential Tool for Peripheral Arterial Disease

Peripheral artery disease is often asymptomatic, and various imaging and nonimaging techniques have been used for assessment and monitoring treatments. This study is designed to demonstrate the ability of photoacoustic imaging to noninvasively determine changes in tissue oxygenation that occur in mice's hind limb skeletal muscle as they age. Mice from two age cohorts were scanned bilaterally with a pulsed laser. The photoacoustic signal was unmixed to generate a parametric map of estimated oxygen saturation and then overlaid on grayscale ultrasound images. Tissue oxygenation measured in young and old mice was compared. Photoacoustic imaging visually and quantitatively showed the decrease in skeletal muscle oxygenation that occurs with age. Percent tissue oxygenation decreased from 30.2% to 3.5% (p < 0.05). This reduction corresponded to reduced fractional area of oxygenation, which decreased from 60.6% to 6.0% (p < 0.05). The change in oxygenation capacity of the still active vascular regions was insignificant (p > 0.05). Intrasubject, intra-, and interobserver comparisons showed low variability in measurements, exhibited by high regression and intraclass correlations exceeding 0.81 for all ages. The decrease in oxygenation detected by photoacoustic imaging paralleled the known oxygenation decrease observed in aging tissues, demonstrating that photoacoustic imaging can assess age-related changes in a mouse calf muscle. These intramuscular changes could potentially act as a strong diagnostic marker for peripheral artery disease. This study thus opens the doors for a novel, affordable, noninvasive method of evaluation free of radiation or exogenous material.

Photoacoustic imaging as a tool to probe the tumour microenvironment

The tumour microenvironment (TME) is a complex cellular ecosystem subjected to chemical and physical signals that play a role in shaping tumour heterogeneity, invasion and metastasis. Studying the roles of the TME in cancer progression would strongly benefit from non-invasive visualisation of the tumour as a whole organ in vivo, both preclinically in mouse models of the disease, as well as in patient tumours. Although imaging techniques exist that can probe different facets of the TME, they face several limitations, including limited spatial resolution, extended scan times and poor specificity from confounding signals. Photoacoustic imaging (PAI) is an emerging modality, currently in clinical trials, that has the potential to overcome these limitations. Here, we review the biological properties of the TME and potential of existing imaging methods that have been developed to analyse these properties non-invasively. We then introduce PAI and explore the preclinical and clinical evidence that support its use in probing multiple features of the TME simultaneously, including blood vessel architecture, blood oxygenation, acidity, extracellular matrix deposition, lipid concentration and immune cell infiltration. Finally, we highlight the future prospects and outstanding challenges in the application of PAI as a tool in cancer research and as part of a clinical oncologist's arsenal.

Summary: This Review details the potential of photoacoustic imaging to visualise features of the tumour microenvironment such as blood vessels, hypoxia, fibrosis and immune infiltrate to provide unprecedented insight into tumour biology.

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.

High-Resolution 3D NIR-II Photoacoustic Imaging of Cerebral and Tumor Vasculatures Using Conjugated Polymer Nanoparticles as Contrast Agent

Exogenous contrast-agent-assisted NIR-II optical-resolution photoacoustic microscopy imaging (ORPAMI) holds promise to decipher wide-field 3D biological structures with deep penetration, large signal-to-background ratio (SBR), and high maximum imaging depth to depth resolution ratio. Herein, NIR-II conjugated polymer nanoparticle (CP NP) assisted ORPAMI is reported for pinpointing cerebral and tumor vasculatures. The CP NPs exhibit a large extinction coefficient of 48.1 L g^-1 at the absorption maximum of 1161 nm, with an ultrahigh PA sensitivity up to 2 µg mL^-1 . 3D ORPAMI of wide-field mice ear allows clear visualization of regular vasculatures with a resolution of 19.2 µm and an SBR of 29.3 dB at the maximal imaging depth of 539 µm. The margin of ear tumor composed of torsional dense vessels among surrounding normal regular vessels can be clearly delineated via 3D angiography. In addition, 3D whole-cortex cerebral vasculatures with large imaging area (48 mm² ), good resolution (25.4 µm), and high SBR (22.3 dB) at a depth up to 1001 µm are clearly resolved through the intact skull. These results are superior to the recently reported 3D NIR-II fluorescence confocal vascular imaging, which opens up new opportunities for NIR-II CP-NP-assisted ORPAMI in various biomedical applications.

Three-dimensional quantitative photoacoustic tomography using an adjoint radiance Monte Carlo model and gradient descent

Quantitative photoacoustic tomography aims to recover maps of the local concentrations of tissue chromophores from multispectral images. While model-based inversion schemes are promising approaches, major challenges to their practical implementation include the unknown fluence distribution and the scale of the inverse problem. We describe an inversion scheme based on a radiance Monte Carlo model and an adjoint-assisted gradient optimization that incorporates fluence-dependent step sizes and adaptive moment estimation. The inversion is shown to recover absolute chromophore concentrations, blood oxygen saturation, and the Grüneisen parameter from in silico three-dimensional phantom images for different radiance approximations. The scattering coefficient is assumed to be homogeneous and known a priori.

Volumetric Optoacoustic Imaging Unveils High-Resolution Patterns of Acute and Cyclic Hypoxia in a Murine Model of Breast Cancer

Mapping tumor heterogeneity and hypoxia within a living intact organism is essential for understanding the processes involved in cancer progression and assessing long-term responses to therapies. Efficient investigations into tumor hypoxia mechanisms have been hindered by the lack of intravital imaging tools capable of multiparametric probing of entire solid tumors with high spatial and temporal resolution. Here, we exploit volumetric multispectral optoacoustic tomography (vMSOT) for accurate, label-free delineation of tumor heterogeneity and dynamic oxygenation behavior. Mice bearing orthotopic MDA-MB-231 breast cancer xenografts were imaged noninvasively during rest and oxygen stress challenge, attaining time-lapse three-dimensional oxygenation maps across entire tumors with 100 μm spatial resolution. Volumetric quantification of the hypoxic fraction rendered values of 3.9% to 21.2%, whereas the oxygen saturation (sO₂) rate declined at 1.7% to 2.3% per mm in all tumors when approaching their core. Three distinct functional areas (the rim, hypoxic, and normoxic cores) were clearly discernible based on spatial sO₂ profiles and responses to oxygen challenge. Notably, although sO₂ readings were responsive to the challenge, deoxyhemoglobin (HbR) trends exhibited little to no variations in all mice. Dynamic analysis further revealed the presence of cyclic hypoxia patterns with a 21% average discrepancy between cyclic fractions assessed via sO₂ (42.2% ± 17.3%) and HbR fluctuations (63% ± 14.1%) within the hypoxic core. These findings corroborate the strong potential of vMSOT for advancing preclinical imaging of cancer and informing clinical decisions on therapeutic interventions. SIGNIFICANCE: vMSOT provides quantitative measures of volumetric hypoxic fraction and cyclic hypoxia in a label-free and noninvasive manner, providing new readouts to aid tumor staging and treatment decision making. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/79/18/4767/F1.large.jpg.See related commentary by Klibanov and Hu, p. 4577.

Photoacoustic diagnosis of pharmacokinetics and vascular shutdown effects in photodynamic treatment with indocyanine green-lactosome for a subcutaneous tumor in mice

Indocyanine green lactosome (ICG-lactosome) is an attractive new-generation agent for photodynamic therapy (PDT) that is characterized by a near-infrared excitation wavelength and high stability in the bloodstream. Fluorescence imaging has been used to examine its pharmacokinetics in vivo, but no depth-resolved information can be obtained with this method. In this study, we applied photoacoustic (PA) imaging to visualize the depth distribution of ICG-lactosome in a mouse subcutaneous tumor model. With this method, the depth distribution of blood vessels can also be visualized, enabling detection of vascular shutdown effects due to PDT. We performed PA imaging of both the distributions of ICG-lactosome and blood vessels in a tumor before and after PDT, and we found that PA signals originating from ICG-lactosome were greatly increased at 18 h after drug injection but rapidly decreased after PDT. These results indicate efficient accumulation of ICG-lactosome and rapid photobleaching due to the PDT reaction in the tumor, respectively. After PDT, PA amplitudes of hemoglobin were significantly decreased, being attributable to vascular shutdown effects. These results show the usefulness of PA imaging for monitoring not only photosensitizer accumulation and bleaching but also vascular responses in PDT with ICG-lactosome. This method can be applied to the diagnosis of many types of PDT processes.

3-D Longitudinal Imaging of Tumor Angiogenesis in Mice in Vivo Using Ultrafast Doppler Tomography

Angiogenesis, the formation of new vessels, is one of the key mechanisms in tumor development and an appealing target for therapy. Non-invasive, high-resolution, high-sensitivity, quantitative 3-D imaging techniques are required to correctly depict tumor heterogeneous vasculature over time. Ultrafast Doppler was recently introduced and provides an unprecedented combination of resolution, penetration depth and sensitivity without requiring any contrast agents. The technique was further extended to three dimensions with ultrafast Doppler tomography (UFD-T). In this work, UFD-T was applied to the monitoring of tumor angiogenesis in vivo, providing structural and functional information at different stages of development. UFD-T volume renderings revealed that our murine model's vasculature stems from pre-existing vessels and sprouts to perfuse the whole volume as the tumor grows until a critical size is reached. Then, as the network becomes insufficient, the tumor core is no longer irrigated because the vasculature is concentrated mainly in the periphery. In addition to spatial distribution and growth patterns, UFD-T allowed a quantitative analysis of vessel size and length, revealing that the diameter distribution of vessels remained relatively constant throughout tumor growth. The network is dominated by small vessels at all stages of tumor development, with more than 74% of the vessels less than 200 µm in diameter. This study also found that cumulative vessel length is more closely related to tumor radius than volume, indicating that the vascularization becomes insufficient when a critical mass is reached. UFD-T was also compared with dynamic contrast-enhanced ultrasound and found to provide complementary information regarding the link between structure and perfusion. In conclusion, UFD-T is capable of in vivo quantitative assessment of the development of tumor vasculature (vessels with blood speed >1 mm/s [sensitivity limit] assessed with a resolution limit of 80 µm) in 3 dimensions. The technique has very interesting potential as a tool for treatment monitoring, response assessment and treatment planning for optimal drug efficiency.

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.

Raster-scan optoacoustic angiography of blood vessel development in colon cancer models

Raster-scan optoacoustic angiography at 532 nm wavelength with 50 μm lateral resolution at 2 mm diagnostic depth was used for quantitative characterization of neoangiogenesis in colon cancer models. Two tumor models of human colon adenocarcinoma (HT-29) and murine colon carcinoma (CT26) different in their histology and vascularization were compared. Tumors of both origins showed an inhomogeneous distribution of areas with high and low vascularization. Rapidly growing CT26 tumor demonstrated a higher rate of vessel growth from the periphery to the center. Peculiarities of the vascularity of tumor models revealed by optoacoustic imaging were confirmed by fluorescent microscopy with FITC-dextran and morphological analysis. The obtained results may be important for the investigation of tumor development and for improvement of colon cancer treatment strategies.

Monitoring neovascularization and integration of decellularized human scaffolds using photoacoustic imaging

Tissue engineering is a branch of regenerative medicine that aims to manipulate cells and scaffolds to create bioartificial tissues and organs for patients. A major challenge lies in monitoring the blood supply to the new tissue following transplantation: the integration and neovascularization of scaffolds in vivo is critical to their functionality. Photoacoustic imaging (PAI) is a laser-generated ultrasound-based technique that is particularly well suited to visualising microvasculature due to the high optical absorption of haemoglobin. Here, we describe an early proof-of-concept study in which PAI in widefield tomography mode is used to image biological, decellularized human tracheal scaffolds. We found that PAI allowed the longitudinal tracking of scaffold integration into subcutaneous murine tissue with high spatial resolution at depth over an extended period of time. The results of the study were consistent with post-imaging histological analyses, demonstrating that PAI can be used to non-invasively monitor the extent of vascularization in biological tissue-engineered scaffolds. We propose that this technique may be a valuable tool for studies designed to test interventions aimed at improving the speed and extent of scaffold neovascularization in tissue engineering. With technological refinement, it could also permit in vivo monitoring of revascularization in patients, for example to determine timing of heterotopic graft transfer.

Quantitative photoacoustic imaging study of tumours in vivo: Baseline variations in quantitative measurements

Photoacoustic imaging (PAI) provides information on haemoglobin levels and blood oxygenation (sO2). To facilitate assessment of the variability in sO2 and haemoglobin in tumours, for example in response to therapies, the baseline variability of these parameters was evaluated in subcutaneous head and neck tumours in mice, using a PAI system (MSOTinVision-256TF). Tumours of anaesthetized animals (midazolam-fentanyl-medetomidine) were imaged for 75 min, in varying positions, and repeatedly over 6 days. An increasing linear trend for average tumoural haemoglobin and blood sO2 was observed, when imaging over 75 min. There were no significant differences in these temporal trends, when repositioning tumours. A negative correlation was found between the percent decrease in blood sO2 over 6 days and tumour growth rate. This paper shows the potential of PAI to provide baseline data for assessing the significance of intra- and inter-tumoural variations that may eventually have value for predicting and/or monitoring cancer treatment response.

Strategies for Image-Guided Therapy, Surgery, and Drug Delivery Using Photoacoustic Imaging

Photoacoustic imaging is a rapidly maturing imaging modality in biological research and medicine. This modality uses the photoacoustic effect ("light in, sound out") to combine the contrast and specificity of optical imaging with the high temporal resolution of ultrasound. The primary goal of image-guided therapy, and theranostics in general, is to transition from conventional medicine to precision strategies that combine diagnosis with therapy. Photoacoustic imaging is well-suited for noninvasive guidance of many therapies and applications currently being pursued in three broad areas. These include the image-guided resection of diseased tissue, monitoring of disease states, and drug delivery. In this review, we examine the progress and strategies for development of photoacoustics in these three key areas with an emphasis on the value photoacoustics has for image-guided therapy.

Photoacoustic oximetry imaging performance evaluation using dynamic blood flow phantoms with tunable oxygen saturation

Multispectral photoacoustic oximetry imaging (MPOI) is an emerging hybrid modality that enables the spatial mapping of blood oxygen saturation (SO2) to depths of several centimeters. To facilitate MPOI device development and clinical translation, well-validated performance test methods and improved quantitative understanding of physical processes and best practices are needed. We developed a breast-mimicking blood flow phantom with tunable SO2 and used this phantom to evaluate a custom MPOI system. Results provide quantitative evaluation of the impact of phantom medium properties (Intralipid versus polyvinyl chloride plastisol) and device design parameters (different transducers) on SO2 measurement accuracy, especially depth-dependent performance degradation due to fluence artifacts. This approach may guide development of standardized test methods for evaluating MPOI devices.

Evaluation of renal oxygen saturation using photoacoustic imaging for the early prediction of chronic renal function in a model of ischemia-induced acute kidney injury

PURPOSE

To evaluate the utility of photoacoustic imaging in measuring changes in renal oxygen saturation after ischemia-induced acute kidney injury, and to compare these measurements with histological findings and serum levels of kidney function.

MATERIAL AND METHODS

Acute kidney injury was induced by clamping the left renal pedicle in C57Bl/6 mice, with a 35-min ischemic period used to induce mild renal injury (14 mice) and a 50-min period for severe injury (13 mice). The oxygen saturation was measured before induction, and at 5 time-points over the first 48 h after induction, starting at 4 h after induction. Oxygen saturation, histological score, kidney volume, and the 24 h creatinine clearance rate and serum blood urea nitrogen were also measured on day 28. Between-group differences were evaluated using a Mann-Whitney U-test and Dunn's multiple comparisons. The association between oxygen saturation and measured variables was evaluated using Spearman's correlation. A receiver operator characteristic curve was constructed from oxygen saturation values at 24 h after heminephrectomy to predict chronic renal function.

RESULTS

The oxygen saturation was higher in the mild than severe renal injury group at 24 h after induction (73.7% and 66.9%, respectively, P<0.05). Between-group comparison on day 28 revealed a higher kidney volume (P = 0.007), lower tubular injury (P<0.001), lower serum level of blood urea nitrogen level (P = 0.016), and lower 24 h creatinine clearance rate (P = 0.042) in the mild compared with the severe injury group. The oxygen saturation at 24 h correlated with the 24 h creatinine clearance rate (P = 0.036) and serum blood urea nitrogen (P<0.001) on day 28, with an area under the receiver operating curve of 0.825.

CONCLUSION

Oxygen saturation, measured by photoacoustic imaging at 24 h after acute kidney injury can predict the extent of subsequent histological alterations in the kidney early after injury.

Receive Beam-Steering and Clutter Reduction for Imaging the Speed-of-Sound Inside the Carotid Artery

Handheld imaging of the tissue’s speed-of-sound (SoS) is a promising multimodal addition to diagnostic ultrasonography for the examination of tissue composition. Computed ultrasound tomography in echo mode (CUTE) probes the spatial distribution of SoS, conventionally via scanning the tissue under a varying angle of ultrasound transmission, and quantifying—in a spatially resolved way—phase variations of the beamformed echoes. So far, this technique is not applicable to imaging the lumen of vessels, where blood flow and tissue clutter inhibit phase tracking of the blood echoes. With the goal to enable the assessment of atherosclerotic plaque composition inside the carotid artery, we propose two modifications to CUTE: (a) use receive (Rx) beam-steering as opposed to transmit (Tx) beam-steering to increase acquisition speed and to reduce flow-related phase decorrelation, and (b) conduct pairwise subtraction of data obtained from repetitions of the scan sequence, to highlight blood echoes relative to static echo clutter and thus enable the phase tracking of blood echoes. These modifications were tested in a phantom study, where the echogenicity of the vessel lumen was chosen to be similar to the one of the background medium, which allows a direct comparison of SoS images obtained with the different techniques. Our results demonstrate that the combination of Rx-steering with the subtraction technique results in an SoS image of the same quality as obtained with conventional Tx-steering. Together with the improved acquisition speed, this makes the proposed technique a key step towards successful imaging of the SoS inside the carotid artery.

Nanoscale photoacoustic tomography for label-free super-resolution imaging: simulation study

Resolutions higher than the optical diffraction limit are often desired in the context of cellular imaging and the study of disease progression at the cellular level. However, three-dimensional super-resolution imaging without reliance on exogenous contrast agents has so far not been achieved. We present nanoscale photoacoustic tomography (nPAT), an imaging modality based on the photoacoustic effect. nPAT can achieve a dramatic improvement in the axial resolution of the photoacoustic imaging. We derive the theoretical resolution and sensitivity of nPAT and demonstrate that nPAT can achieve a maximum axial resolution of 9.2 nm. We also demonstrate that nPAT can theoretically detect smaller numbers of molecules (∼273) than conventional photoacoustic microscopy due to its ability to detect acoustic signals very close to the photoacoustic source. We simulate nPAT imaging of malaria-infected red blood cells (RBCs) using digital phantoms generated from real biological samples, showing nPAT imaging of the RBC at different stages of infection. These simulations show the potential of nPAT to nondestructively image RBCs at the nanometer resolutions for in vivo samples without the use of exogenous contrast agents. Simulations of nPAT-enabled functional imaging show that nPAT can yield insight into malarial metabolism and biocrystallization processes. We believe that the experimental realization of nPAT has important applications in biomedicine.

Feasibility of co-registered ultrasound and acoustic-resolution photoacoustic imaging of human colorectal cancer

Colorectal cancer is the second leading cause of cancer death in the United States. Significant limitations in screening and surveillance modalities continue to hamper early detection of primary cancers or recurrences after therapy. In this study, we describe a new registered ultrasound (US) and acoustic-resolution photoacoustic microscopy (AR-PAM) system and report its initial testing in ex vivo human colorectal tissue. A total of 8 colorectal specimens were imaged, which included 2 polyps, 4 malignant colon cancers, and 2 treated colorectal cancers. In each specimen, normal tissue was also imaged for internal control. Initial data have demonstrated the feasibility of identifying colorectal cancer imaging features and the invasion depth using co-registered US and an AR-PAM system. In normal tissue, we found that our system consistently demonstrates the multi-layer structure of normal colonic tissue while differentiating layers with elevated vascularity; these findings highly correlated with histologic findings of each specimen. For malignant colorectal samples, the tissue structure is highly disorganized as seen in US, and photoacoustic imaging revealed distorted vascular distribution inside the tumor. Notably, AR-PAM of tumor beds after complete tumor destruction by radiation and chemotherapy yielded a pattern identical to benign tissue. Quantitative analysis of photoacoustic spectral slope has demonstrated more high-frequency components in malignant tissue as compared to the normal colon tissue, which may be caused by significantly increased microvessel networks. In summary, we demonstrate the successful differentiation of benign and malignant colorectal tissue with our co-registered ultrasound and photoacoustic system.

Noninvasive Anatomical and Functional Imaging of Orthotopic Glioblastoma Development and Therapy using Multispectral Optoacoustic Tomography

PURPOSE

Here we demonstrate the potential of multispectral optoacoustic tomography (MSOT), a new non-invasive structural and functional imaging modality, to track the growth and changes in blood oxygen saturation (sO2) in orthotopic glioblastoma (GBMs) and the surrounding brain tissues upon administration of a vascular disruptive agent (VDA).

METHODS

Nude mice injected with U87MG tumor cells were longitudinally monitored for the development of orthotopic GBMs up to 15 days and observed for changes in sO2 upon administration of combretastatin A4 phosphate (CA4P, 30 mg/kg), an FDA approved VDA for treating solid tumors. We employed a newly-developed non-negative constrained approach for combined MSOT image reconstruction and unmixing in order to quantitatively map sO2 in whole mouse brains.

RESULTS

Upon longitudinal monitoring, tumors could be detected in mouse brains using single-wavelength data as early as 6 days post tumor cell inoculation. Fifteen days post-inoculation, tumors had higher sO2 of 63 ± 11% (n = 5, P < .05) against 48 ± 7% in the corresponding contralateral brain, indicating their hyperoxic status. In a different set of animals, 42 days post-inoculation, tumors had lower sO2 of 42 ± 5% against 49 ± 4% (n = 3, P < .05) in the contralateral side, indicating their hypoxic status. Upon CA4P administration, sO2 in 15 days post-inoculation tumors dropped from 61 ± 9% to 36 ± 1% (n = 4, P < .01) within one hour, then reverted to pre CA4P treatment values (63 ± 6%) and remained constant until the last observation time point of 6 hours.

CONCLUSION

With the help of advanced post processing algorithms, MSOT was capable of monitoring the tumor growth and assessing hemodynamic changes upon administration of VDAs in orthotopic GBMs.

Real-Time in Vivo Photoacoustic Imaging in the Assessment of Myocardial Dynamics in Murine Model of Myocardial Ischemia

Photoacoustic imaging (PAI) is an evolving real-time imaging modality that combines the higher contrast of optical imaging with the higher spatial resolution of ultrasound imaging. We utilized dual-wavelength PAI for the diagnosis and monitoring of myocardial ischemia by assessing variations in blood oxygen saturation estimated in a murine model. The use of high-frequency ultrasound in conjunction with PAI enabled imaging of anatomic and functional changes associated with ischemia. Myocardial ischemia was established in eight mice by ligating the left anterior descending artery (LAD). Longitudinal results reveal that PAI is sensitive to acute myocardial ischemia, with a rapid decline in blood oxygen saturation (p ˂ 0.001) observed after LAD ligation (30 min: 33.05 ± 6.80%, 80 min: 36.59 ± 5.22%, 120 min: 36.70 ± 9.46%, 24 h: 40.55 ± 13.04%) compared with baseline (87.83 ± 5.73%). Variation in blood oxygen saturation was found to be linearly correlated with ejection fraction (%), fractional shortening (%) and stroke volume (µL), with Pearson's correlation coefficient values of 0.66, 0.67 and 0.77, respectively (p ˂ 0.001). Our results indicate that PAI has the potential for real-time diagnosis and monitoring of acute myocardial ischemia.

Hand-held optoacoustic imaging: A review

Optoacoustic imaging is a medical imaging modality that uses optical excitation and acoustic detection to generate images of tissue structures based up optical absorption within a tissue sample. This imaging modality has been widely explored as a tool for a number of clinical applications, including cancer diagnosis and wound healing tracking. Recently, the optoacoustic imaging community has published a number of reports of hand-held optoacoustic imaging devices and platforms; these hand-held configurations improve the modality's potential for commercial clinical implementation. Here, we review recent advancements in hand-held optoacoustic imaging platforms and methods, including recent pre-clinical applications, and we present an overview of the remaining limitations in optoacoustic imaging that must be addressed to increase the translation of the modality into commercial and clinical use.

Recent progress in photoacoustic molecular imaging

By acoustically detecting the optical absorption contrast, photoacoustic (PA) tomography (PAT) has broken the penetration limits of traditional high-resolution optical imaging. Through spectroscopic analysis of the target's optical absorption, PAT can identify a wealth of endogenous and exogenous molecules and thus is inherently capable of molecular imaging with high sensitivity. PAT's molecular sensitivity is uniquely accompanied by non-ionizing radiation, high spatial resolution, and deep penetration in biological tissues, which other optical imaging modalities cannot achieve yet. In this concise review, we summarize the most recent technological advancements in PA molecular imaging and highlight the novel molecular probes specifically made for PAT in deep tissues. We conclude with a brief discussion of the opportunities for future advancements.

Cationic poly-l-lysine-encapsulated melanin nanoparticles as efficient photoacoustic agents targeting to glycosaminoglycans for the early diagnosis of articular cartilage degeneration in osteoarthritis

Cartilage degeneration is the hallmark of osteoarthritis (OA) and its early diagnosis is essential for effective cartilage repair. However, until now, there was still a lack of imaging modalities that can accurately detect and evaluate cartilage degeneration in its early stage. Herein, we introduce endogenous melanin nanoparticles (MNPs) encapsulated by poly-l-lysine (PLL) as positively charged contrast agents for the accurate photoacoustic (PA) imaging of cartilage degeneration through its strong electrostatic interaction with anionic glycosaminoglycans (GAGs) in the cartilage. PLL-MNPs presented high PA intensity, photostability and biocompatibility. In vitro PAI studies showed that PLL-MNPs with a zeta potential of +32.5 ± 9.3 mV had more cartilage uptake and longer retention time than anionic MNPs, and generated a positive relationship with the GAG content in the cartilage. After administration via intra-articular injection in living mouse models, PLL-MNPs exhibited about a two-fold stronger PA signal in a normal joint (with high GAG content) than an OA joint (with low GAG content). Furthermore, the obtained PAI results provided accurate information of the GAG content distribution in the OA knee joint. Consequently, by detecting and analyzing the changes of the GAG content in OA cartilage using PAI, we can clearly distinguish early OA from late OA and monitor the therapeutic efficacy in OA after drug treatment. All PAI results were examined histologically.

Sonophore-enhanced nanoemulsions for optoacoustic imaging of cancer

Optoacoustic imaging offers the promise of high spatial resolution and, at the same time, penetration depths well beyond the conventional optical imaging technologies, advantages that would be favorable for a variety of clinical applications. However, similar to optical fluorescence imaging, exogenous contrast agents, known as sonophores, need to be developed for molecularly targeted optoacoustic imaging. Despite numerous optoacoustic contrast agents that have been reported, there is a need for more rational design of sonophores. Here, using a library screening approach, we systematically identified and evaluated twelve commercially available near-infrared (690-900 nm) and highly absorbing dyes for multi-spectral optoacoustic tomography (MSOT). In order to achieve more accurate spectral deconvolution and precise data quantification, we sought five practical mathematical methods, namely direct classical least squares based on UV-Vis (UV/Vis-DCLS) or optoacoustic (OA-DCLS) spectra, non-negative LS (NN-LS), independent component analysis (ICA) and principal component analysis (PCA). We found that OA-DCLS is the most suitable method, allowing easy implementation and sufficient accuracy for routine analysis. Here, we demonstrate for the first time that our biocompatible nanoemulsions (NEs), in combination with near-infrared and highly absorbing dyes, enable non-invasive in vivo MSOT detection of tumors. Specifically, we found that NE-IRDye QC1 offers excellent optoacoustic performance and detection compared to related near-infrared NEs. We demonstrate that when loaded with low fluorescent or dark quencher dyes, NEs represent a flexible and new class of exogenous sonophores suitable for non-invasive pre-clinical optoacoustic imaging.

Estimating relative chromophore concentrations from multiwavelength photoacoustic images using independent component analysis

Independent component analysis (ICA) is an unmixing method based on a linear model. It has previously been applied in in vivo multiwavelength photoacoustic imaging studies to unmix the components representing individual chromophores by assuming that they are statistically independent. Numerically simulated and experimentally acquired two-dimensional images of tissue-mimicking phantoms are used to investigate the conditions required for ICA to give accurate estimates of the relative chromophore concentrations. A simple approximate fluence correction was applied to reduce but not completely remove the nonlinear fluence distortion, as might be possible in practice. The results show that ICA is robust against the residual effect of the partially corrected fluence distortion. ICA is shown to provide accurate unmixing of the chromophores when the absorption coefficient is within a certain range of values, where the upper absorption threshold is comparable to the absorption of blood. When the absorption is increased beyond these thresholds, ICA abruptly fails to unmix the chromophores accurately. The ICA approach was compared to a linear spectroscopic inversion (SI) with known absorption spectra. In cases where the mixing matrix with the specific absorption spectra is ill-conditioned, ICA is able to provide accurate unmixing when SI results in large errors.

In vivo three-dimensional photoacoustic imaging of the renal vasculature in preclinical rodent models

Noninvasive imaging of the kidney vasculature in preclinical murine models is important for the assessment of renal development, studying diseases and evaluating new therapies but is challenging to achieve using existing imaging modalities. Photoacoustic imaging is a promising new technique that is particularly well suited to visualizing the vasculature and could provide an alternative to existing preclinical imaging methods for studying renal vascular anatomy and function. To investigate this, an all-optical Fabry-Perot-based photoacoustic scanner was used to image the abdominal region of mice. High-resolution three-dimensional, noninvasive, label-free photoacoustic images of the mouse kidney and renal vasculature were acquired in vivo. The scanner was also used to visualize and quantify differences in the vascular architecture of the kidney in vivo due to polycystic kidney disease. This study suggests that photoacoustic imaging could be utilized as a novel preclinical imaging tool for studying the biology of renal disease.

Rapid scanning wide-field clutter elimination in epi-optoacoustic imaging using comb LOVIT

Epi-style optoacoustic (OA) imaging provides flexibility by integrating the irradiation optics and ultrasound receiver, yet clutter generated by optical absorption near the probe obscures deep OA sources. Localised vibration tagging (LOVIT) retrieves OA signal from images that are acquired with and without a preceding ultrasonic pushing beam: Radiation force leads to a phase shift of signals coming from the focal area resulting in their visibility in a difference image, whereas clutter from outside the pushing beam is eliminated. Disadvantages of a single-focus approach are residual clutter from inside the pushing beam above the focus, and time-intensive scanning of the focus to retrieve a large field-of-view. To speed up acquisition, we propose to create multiple foci in parallel, forming comb-shaped ARF patterns. By subtracting OA images obtained with interleaved combs, this technique moreover results in greatly improved clutter reduction in phantoms mimicking optical, acoustic and elastic properties of breast tissue.