Fully integrated photoacoustic microscopy and photoplethysmography of human in vivo

Deep learning-based virtual staining, segmentation, and classification in label-free photoacoustic histology of human specimens

In pathological diagnostics, histological images highlight the oncological features of excised specimens, but they require laborious and costly staining procedures. Despite recent innovations in label-free microscopy that simplify complex staining procedures, technical limitations and inadequate histological visualization are still problems in clinical settings. Here, we demonstrate an interconnected deep learning (DL)-based framework for performing automated virtual staining, segmentation, and classification in label-free photoacoustic histology (PAH) of human specimens. The framework comprises three components: (1) an explainable contrastive unpaired translation (E-CUT) method for virtual H&E (VHE) staining, (2) an U-net architecture for feature segmentation, and (3) a DL-based stepwise feature fusion method (StepFF) for classification. The framework demonstrates promising performance at each step of its application to human liver cancers. In virtual staining, the E-CUT preserves the morphological aspects of the cell nucleus and cytoplasm, making VHE images highly similar to real H&E ones. In segmentation, various features (e.g., the cell area, number of cells, and the distance between cell nuclei) have been successfully segmented in VHE images. Finally, by using deep feature vectors from PAH, VHE, and segmented images, StepFF has achieved a 98.00% classification accuracy, compared to the 94.80% accuracy of conventional PAH classification. In particular, StepFF's classification reached a sensitivity of 100% based on the evaluation of three pathologists, demonstrating its applicability in real clinical settings. This series of DL methods for label-free PAH has great potential as a practical clinical strategy for digital pathology.

3D Multiparametric Photoacoustic Computed Tomography of Primary and Metastatic Tumors in Living Mice

Photoacoustic computed tomography (PACT), an emerging imaging modality in preclinical cancer research, can provide multiparametric 3D information about structures, physiological functions, and pharmacokinetics. Here, we demonstrate the use of high-definition 3D multiparametric PACT imaging of both primary and metastatic tumors in living mice to noninvasively monitor angiogenesis, carcinogenesis, hypoxia, and pharmacokinetics. The high-definition PACT system with a 1024-element hemispherical ultrasound transducer array provides an isotropic spatial resolution of 380 μm, an effective volumetric field-of-view of 12.8 mm × 12.8 mm × 12.8 mm without scanning, and an acquisition time of <30 s for a whole mouse body. Initially, we monitor the structural progression of the tumor microenvironment (e.g., angiogenesis and vessel tortuosity) after tumor cell inoculation. Then, we analyze the change in oxygen saturation of the tumor during carcinogenesis, verifying induced hypoxia in the tumor's core region. Finally, the whole-body pharmacokinetics are photoacoustically imaged after intravenous injection of micelle-loaded IR780 dye, and the in vivo PACT results are validated in vivo and ex vivo by fluorescence imaging. By employing the premium PACT system and applying multiparametric analyses to subcutaneous primary tumors and metastatic liver tumors, we demonstrate that this PACT system can provide multiparametric analyses for comprehensive cancer research.

Dynamic model for the strain-modulated spectral reflectance of the human skin in vivo

Hyperspectral imaging (HSI) offers a wealth of information regarding human skin. In this study, we established a dynamic skin spectral reflectance model (DSSR) relating the reflectance to skin surface strain, considering multi physiological and physical parameters of the skin. Experimentally, by HSI, we measured the reflectance variance of the forearm skin in vivo caused by the surface strain, and assessed these key parameters. For the human skin in vivo, within the strain range covered in this paper, stretching increases spectral reflectance, while compression decreases it. Our proposed model provides a possibility for non-contact strain measurement and health monitoring on the skin in vivo based on HSI.

X-ray free-electron laser induced acoustic microscopy (XFELAM)

The X-ray free-electron laser (XFEL) has remarkably advanced X-ray imaging technology and enabled important scientific achievements. The XFEL's extremely high power, short pulse width, low emittance, and high coherence make possible such diverse imaging techniques as absorption/emission spectroscopy, diffraction imaging, and scattering imaging. Here, we demonstrate a novel XFEL-based imaging modality that uses the X-ray induced acoustic (XA) effect, which we call X-ray free-electron laser induced acoustic microscopy (XFELAM). Initially, we verified the XA effect by detecting XA signals from various materials, then we validated the experimental results with simulation outcomes. Next, in resolution experiments, we successfully imaged a patterned tungsten target with drilled various-sized circles at a spatial resolution of 7.8 ± 5.1 µm, which is the first micron-scale resolution achieved by XA imaging. Our results suggest that the novel XFELAM can expand the usability of XFEL in various areas of fundamental scientific research.

Video-rate endocavity photoacoustic/harmonic ultrasound imaging with miniaturized light delivery

Significance

Endocavity ultrasound (US) imaging is a frequently employed diagnostic technique in gynecology and urology for the assessment of male and female genital diseases that present challenges for conventional transabdominal imaging. The integration of photoacoustic (PA) imaging with clinical US imaging has displayed promising outcomes in clinical research. Nonetheless, its application has been constrained due to size limitations, restricting it to spatially confined locations such as vaginal or rectal canals.

Aim

This study presents the development of a video-rate (20 Hz) endocavity PA/harmonic US imaging (EPAUSI) system.

Approach

The approach incorporates a commercially available endocavity US probe with a miniaturized laser delivery unit, comprised of a single large-core fiber and a line beamshaping engineered diffuser. The system facilitates real-time image display and subsequent processing, including angular energy density correction and spectral unmixing, in offline mode.

Results

The spatial resolutions of the concurrently acquired PA and harmonic US images were measured at 318    μ m and 291    μ m in the radial direction, respectively, and 1.22 deg and 1.50 deg in the angular direction, respectively. Furthermore, the system demonstrated its capability in multispectral PA imaging by successfully distinguishing two clinical dyes in a tissue-mimicking phantom. Its rapid temporal resolution enabled the capture of kinetic dye perfusion into an ex vivo porcine ovary through the depth of porcine uterine tissue. EPAUSI proved its clinical viability by detecting pulsating hemodynamics in the male rat's prostate in vivo and accurately classifying human blood vessels into arteries and veins based on sO 2 measurements.

Conclusions

Our proposed EPAUSI system holds the potential to unveil previously overlooked indicators of vascular alterations in genital cancers or endometriosis, addressing pressing requirements in the fields of gynecology and urology.

Transcytosis-Inducing Multifunctional Albumin Nanomedicines with Deep Penetration Ability for Image-Guided Solid Tumor Treatment

Transcytosis is an active transcellular transportation pathway that has garnered interest for overcoming the limited deep penetration of nanomedicines in solid tumors. In this study, a charge-convertible nanomedicine that facilitates deep penetration into solid tumors via transcytosis is designed. It is an albumin-based calcium phosphate nanomedicine loaded with IR820 (mAlb-820@CaP) for high-resolution photoacoustic imaging and enhanced photothermal therapy. Biomineralization on the surface stabilizes the albumin-IR820 complex during circulation and provides calcium ions (Ca2+ ) for tissue penetration on degradation in an acidic environment. pH-triggered transcytosis of the nanomedicine enabled by caveolae-mediated endocytosis and calcium ion-induced exocytosis in 2D cellular, 3D spheroid, and in vivo tumor models is demonstrated. Notably, the extravasation and penetration ability of the nanomedicine is observed in vivo using a high-resolution photoacoustic system, and nanomedicine shows the most potent photothermal antitumor effect in vivo. Overall, the strategy provides a versatile theragnosis platform for both noninvasive photoacoustic imaging and high therapeutic efficiency resulting from deep penetration of nanomedicine.

Dual-wavelength UV-visible metalens for multispectral photoacoustic microscopy: A simulation study

Photoacoustic microscopy is advancing with research on utilizing ultraviolet and visible light. Dual-wavelength approaches are sought for observing DNA/RNA- and vascular-related disorders. However, the availability of high numerical aperture lenses covering both ultraviolet and visible wavelengths is severely limited due to challenges such as chromatic aberration in the optics. Herein, we present a groundbreaking proposal as a pioneering simulation study for incorporating multilayer metalenses into ultraviolet-visible photoacoustic microscopy. The proposed metalens has a thickness of 1.4 µm and high numerical aperture of 0.8. By arranging cylindrical hafnium oxide nanopillars, we design an achromatic transmissive lens for 266 and 532 nm wavelengths. The metalens achieves a diffraction-limited focal spot, surpassing commercially available objective lenses. Through three-dimensional photoacoustic simulation, we demonstrate high-resolution imaging with superior endogenous contrast of targets with ultraviolet and visible optical absorption bands. This metalens will open new possibilities for downsized multispectral photoacoustic microscopy in clinical and preclinical applications.

In vivo quantitative photoacoustic monitoring of corticosteroid-induced vasoconstriction

Significance

Corticosteroids-commonly prescribed medications for skin diseases-inhibit the secretion of vasodilators, such as prostaglandin, thereby exerting anti-inflammatory action by constricting capillaries in the dermis. The effectiveness of corticosteroids is determined by the degree of vasoconstriction followed by skin whitening, namely, the blanching effect. However, the current method of observing the blanching effect indirectly evaluates the effects of corticosteroids.

Aim

In this study, we employed optical-resolution photoacoustic (PA) microscopy (OR-PAM) to directly visualize the blood vessels and quantitatively evaluate vasoconstriction.

Approach

Using OR-PAM, the vascular density in mice skin was monitored for 60 min after performing each experimental procedure for four groups, and the vasoconstriction was quantified. Volumetric PA data were segmented into the papillary dermis, reticular dermis, and hypodermis based on the vascular characteristics obtained through OR-PAM. The vasoconstrictive effect of each skin layer was quantified according to the dermatological treatment method.

Results

In the case of corticosteroid topical application, vasoconstriction was observed in the papillary ( 56.4 ± 10.9 % ) and reticular ( 45.1 ± 4.71 % ) dermis. For corticosteroid subcutaneous injection, constriction was observed solely in the reticular ( 49.5 ± 9.35 % ) dermis. In contrast, no vasoconstrictions were observed with nonsteroidal topical application.

Conclusions

Our results indicate that OR-PAM can quantitatively monitor the vasoconstriction induced by corticosteroids, thereby validating OR-PAMs potential as a practical evaluation tool for predicting the effectiveness of corticosteroids in dermatology.

Functional photoacoustic imaging: from nano- and micro- to macro-scale

Functional photoacoustic imaging is a promising biological imaging technique that offers such unique benefits as scalable resolution and imaging depth, as well as the ability to provide functional information. At nanoscale, photoacoustic imaging has provided super-resolution images of the surface light absorption characteristics of materials and of single organelles in cells. At the microscopic and macroscopic scales. photoacoustic imaging techniques have precisely measured and quantified various physiological parameters, such as oxygen saturation, vessel morphology, blood flow, and the metabolic rate of oxygen, in both human and animal subjects. This comprehensive review provides an overview of functional photoacoustic imaging across multiple scales, from nano to macro, and highlights recent advances in technology developments and applications. Finally, the review surveys the future prospects of functional photoacoustic imaging in the biomedical field.

Panoramic volumetric clinical handheld photoacoustic and ultrasound imaging

Photoacoustic (PA) imaging has gained much attention, providing structural and functional information in combination with clinical ultrasound (US) imaging systems. 2D PA and US imaging is easily implemented, but its heavy dependence on operator skills makes 3D imaging preferable. In this study, we propose a panoramic volumetric clinical PA and US imaging system equipping a handheld imaging scanner weighing 600 g and measuring 70 × 62 × 110 mm³. Multiple PA/US scans were performed to cover a large field-of-view (FOV), and the acquired PA/US volumes were mosaic-stitched after manually correcting the positions and rotations in a total of 6 degrees of freedom. PA and US maximum amplitude projection images were visualized online, while spectral unmixed data was quantified offline. The performance of the system was tested via tissue-mimicking phantom experiments. The system's potential was confirmed in vivo by panoramically imaging vascular networks in human arms and necks, with FOVs of 331 × 38 and 129 × 120 mm², respectively. Further, we quantified hemoglobin oxygen saturation levels in the radial artery, brachial artery, carotid artery, and jugular vein. We hope that this system can be applied for various clinical fields such as cardiovascular imaging, dermatology, vascular surgery, internal medicine, and oncology.

In vivo photoacoustic monitoring of vasoconstriction induced by acute hyperglycemia

Postprandial hyperglycemia, blood glucose spikes, induces endothelial dysfunction, increasing cardiovascular risks. Endothelial dysfunction leads to vasoconstriction, and observation of this phenomenon is important for understanding acute hyperglycemia. However, high-resolution imaging of microvessels during acute hyperglycemia has not been fully developed. Here, we demonstrate that photoacoustic microscopy can noninvasively monitor morphological changes in blood vessels of live animals' extremities when blood glucose rises rapidly. As blood glucose level rose from 100 to 400 mg/dL following intraperitoneal glucose injection, heart/breath rate, and body temperature remained constant, but arterioles constricted by approximately -5.7 ± 1.1% within 20 min, and gradually recovered for another 40 min. In contrast, venular diameters remained within about 0.6 ± 1.5% during arteriolar constriction. Our results experimentally and statistically demonstrate that acute hyperglycemia produces transitory vasoconstriction in arterioles, with an opposite trend of change in blood glucose. These findings could help understanding vascular glucose homeostasis and the relationship between diabetes and cardiovascular diseases.

Theoretical and experimental comparison of the performance of gold, titanium, and platinum nanodiscs as contrast agents for photoacoustic imaging

Exogenous contrast agents in photoacoustic imaging help improve spatial resolution and penetration depth and enable targeted molecular imaging. To screen efficient photoacoustic signaling materials as contrast agents, we propose a light absorption-weighted figure of merit (FOM) that can be calculated using material data from the literature and numerically simulated light absorption cross-sections. The calculated light absorption-weighted FOM shows that a Ti nanodisc has a photoacoustic conversion performance similar to that of an Au nanodisc and better than that of a Pt nanodisc. The photoacoustic imaging results of Ti, Au, and Pt nanodiscs, which are physically synthesized with identical shapes and dimensions, experimentally demonstrated that the Ti nanodisc could be a highly efficient contrast agent.

Recent Advances in Photoacoustic Agents for Theranostic Applications

Photoacoustic agents are widely used in various theranostic applications. By evaluating the biodistribution obtained from photoacoustic images, the effectiveness of theranostic agents in terms of their delivery efficiency and treatment responses can be analyzed. Through this study, we evaluate and summarize the recent advances in photoacoustic-guided phototherapy, particularly in photothermal and photodynamic therapy. This overview can guide the future directions for theranostic development. Because of the recent applications of photoacoustic imaging in clinical trials, theranostic agents with photoacoustic monitoring have the potential to be translated into the clinical world.

Efficient label-free in vivo photoacoustic imaging of melanoma cells using a condensed NIR-I spectral window

In this paper, we propose an efficient label-free in vivo photoacoustic (PA) imaging of melanoma using a condensed near infrared-I (NIR-I) supercontinuum light source. Although NIR-II spectral window is advantageous such as longer penetration depth compared to the NIR-I region, supercontinuum light sources emitting both NIR-I and NIR-II region could lower the efficiency to target melanoma because of low optical power density in the melanoma's absorption spectra. To exploit efficient in vivo PA imaging of melanoma, we demonstrated the light source emitting from visible (532-600 nm) to NIR-I (600-1000 nm) by optimizing stimulated Raman scattering induced supercontinuum generation. The melanoma's structure is successfully differentiated from blood vessels at a high pulse energy of 2.5 µJ and a flexible pulse repetition rate (PRR) of 5-50 kHz. The proposed light source with the microjoules energies and tens of kHz of PRR can potentially accelerate clinical trials such as early diagnosis of melanoma.

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

Visible CCD Camera-Guided Photoacoustic Imaging System for Precise Navigation during Functional Rat Brain Imaging

In photoacoustic (PA) imaging, tissue absorbs specific wavelengths of light. The absorbed energy results in thermal expansion that generates ultrasound waves that are reconstructed into images. Existing commercial PA imaging systems for preclinical brain imaging are limited by imprecise positioning capabilities and inflexible user interfaces. We introduce a new visible charge-coupled device (CCD) camera-guided photoacoustic imaging (ViCPAI) system that integrates an ultrasound (US) transducer and a data acquisition platform with a CCD camera for positioning. The CCD camera accurately positions the US probe at the measurement location. The programmable MATLAB-based platform has an intuitive user interface. In vitro carbon fiber and in vivo animal experiments were performed to investigate the precise positioning and imaging capabilities of the ViCPAI system. We demonstrated real-time capturing of bilateral cerebral hemodynamic changes during (1) forelimb electrical stimulation under normal conditions, (2) forelimb stimulation after right brain focal photothrombotic ischemia (PTI) stroke, and (3) progression of KCl-induced cortical spreading depression (CSD). The ViCPAI system accurately located target areas and achieved reproducible positioning, which is crucial in animal and clinical experiments. In animal experiments, the ViCPAI system was used to investigate bilateral cerebral cortex responses to left forelimb electrical stimulation before and after stroke, showing that the CBV and SO₂ in the right primary somatosensory cortex of the forelimb (S1FL) region were significantly changed by left forelimb electrical stimulation before stroke. No CBV or SO₂ changes were observed in the bilateral cortex in the S1FL area in response to left forelimb electrical stimulation after stroke. While monitoring CSD progression, the ViCPAI system accurately locates the S1FL area and returns to the same position after the probe moves, demonstrating reproducible positioning and reducing positioning errors. The ViCPAI system utilizes the real-time precise positioning capability of CCD cameras to overcome various challenges in preclinical and clinical studies.

Optical Light Sources and Wavelengths within the Visible and Near-Infrared Range Using Photoacoustic Effects for Biomedical Applications

The photoacoustic (PA) effect occurs when sound waves are generated by light according to the thermodynamic and optical properties of the materials; they are absorption spectroscopic techniques that can be applied to characterize materials that absorb pulse or continuous wave (CW)-modulated electromagnetic radiation. In addition, the wavelengths and properties of the incident light significantly impact the signal-to-ratio and contrast with photoacoustic signals. In this paper, we reviewed how absorption spectroscopic research results have been used in applying actual photoacoustic effects, focusing on light sources of each wavelength. In addition, the characteristics and compositions of the light sources used for the applications were investigated and organized based on the absorption spectrum of the target materials. Therefore, we expect that this study will help researchers (who desire to study photoacoustic effects) to more efficiently approach the appropriate conditions or environments for selecting the target materials and light sources.

Motion Compensation for 3D Multispectral Handheld Photoacoustic Imaging

Three-dimensional (3D) handheld photoacoustic (PA) and ultrasound (US) imaging performed using mechanical scanning are more useful than conventional 2D PA/US imaging for obtaining local volumetric information and reducing operator dependence. In particular, 3D multispectral PA imaging can capture vital functional information, such as hemoglobin concentrations and hemoglobin oxygen saturation (sO₂), of epidermal, hemorrhagic, ischemic, and cancerous diseases. However, the accuracy of PA morphology and physiological parameters is hampered by motion artifacts during image acquisition. The aim of this paper is to apply appropriate correction to remove the effect of such motion artifacts. We propose a new motion compensation method that corrects PA images in both axial and lateral directions based on structural US information. 3D PA/US imaging experiments are performed on a tissue-mimicking phantom and a human wrist to verify the effects of the proposed motion compensation mechanism and the consequent spectral unmixing results. The structural motions and sO₂ values are confirmed to be successfully corrected by comparing the motion-compensated images with the original images. The proposed method is expected to be useful in various clinical PA imaging applications (e.g., breast cancer, thyroid cancer, and carotid artery disease) that are susceptible to motion contamination during multispectral PA image analysis.

Deep Learning Enhances Multiparametric Dynamic Volumetric Photoacoustic Computed Tomography In Vivo (DL-PACT)

Photoacoustic computed tomography (PACT) has become a premier preclinical and clinical imaging modality. Although PACT's image quality can be dramatically improved with a large number of ultrasound (US) transducer elements and associated multiplexed data acquisition systems, the associated high system cost and/or slow temporal resolution are significant problems. Here, a deep learning-based approach is demonstrated that qualitatively and quantitively diminishes the limited-view artifacts that reduce image quality and improves the slow temporal resolution. This deep learning-enhanced multiparametric dynamic volumetric PACT approach, called DL-PACT, requires only a clustered subset of many US transducer elements on the conventional multiparametric PACT. Using DL-PACT, high-quality static structural and dynamic contrast-enhanced whole-body images as well as dynamic functional brain images of live animals and humans are successfully acquired, all in a relatively fast and cost-effective manner. It is believed that the strategy can significantly advance the use of PACT technology for preclinical and clinical applications such as neurology, cardiology, pharmacology, endocrinology, and oncology.

Opto-ultrasound biosensor for wearable and mobile devices: realization with a transparent ultrasound transducer

Mobile and wearable healthcare electronics are widely used for measuring bio-signals using various fusion sensors that employ photoplethysmograms, cameras, microphones, ultrasound (US) sensors, and accelerometers. However, the consumer demand for small form factors has significantly increased as the integration of multiple sensors is difficult in small mobile or wearable devices. This study proposes two novel opto-US sensors, namely (1) a wearable photoplethysmography (PPG)-US device and (2) a PPG sensor built-in mobile smartphone with a US sensor, seamlessly integrated using a transparent ultrasound transducer (TUT). The TUT exhibits a center frequency of 6 MHz with a 50% bandwidth and 82% optical transparency in visible and near-infrared regions. We developed an integrated wearable PPG-US device to demonstrate its feasibility and coupled the TUT sensor with a smartphone. We measured the heart rates optically and acoustically in human subjects and quantified the oxygen saturation optically by passing light through the TUT. The proposed proof-of-concept is a novel sensor fusion for mobile and wearable devices that require a small form factor and aim to improve digital healthcare. The results of this study can form the basis for innovative developments in sensor-based high-tech industrial applications, such as automobiles, robots, and drones, in addition to healthcare applications.

Whole-Body Photoacoustic Imaging Techniques for Preclinical Small Animal Studies

Photoacoustic imaging is a hybrid imaging technique that has received considerable attention in biomedical studies. In contrast to pure optical imaging techniques, photoacoustic imaging enables the visualization of optical absorption properties at deeper imaging depths. In preclinical small animal studies, photoacoustic imaging is widely used to visualize biodistribution at the molecular level. Monitoring the whole-body distribution of chromophores in small animals is a key method used in preclinical research, including drug-delivery monitoring, treatment assessment, contrast-enhanced tumor imaging, and gastrointestinal tracking. In this review, photoacoustic systems for the whole-body imaging of small animals are explored and summarized. The configurations of the systems vary with the scanning methods and geometries of the ultrasound transducers. The future direction of research is also discussed with regard to achieving a deeper imaging depth and faster imaging speed, which are the main factors that an imaging system should realize to broaden its application in biomedical studies.