All-optical photoacoustic microscopy

Thermal-tagging photoacoustic remote sensing flowmetry

Ultrasound coupling is one of the critical challenges for traditional photoacoustic (or optoacoustic) microscopy (PAM) techniques transferred to the clinical examination of chronic wounds and open tissues. A promising alternative potential solution for breaking the limitation of ultrasound coupling in PAM is photoacoustic remote sensing (PARS), which implements all-optical non-interferometric photoacoustic measurements. Functional imaging of PARS microscopy was demonstrated from the aspects of histopathology and oxygen metabolism, while its performance in hemodynamic quantification remains unexplored. In this Letter, we present an all-optical thermal-tagging flowmetry approach for PARS microscopy and demonstrate it with comprehensive mathematical modeling and ex vivo and in vivo experimental validations. Experimental results demonstrated that the detectable range of the blood flow rate was from 0 to 12 mm/s with a high accuracy (measurement error:±1.2%) at 10-kHz laser pulse repetition rate. The proposed all-optical thermal-tagging flowmetry offers an effective alternative approach for PARS microscopy realizing non-contact dye-free hemodynamic imaging.

Optical ultrasound sensors for photoacoustic imaging: a review

Significance

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that can map optical absorption contrast in biological tissues by detecting ultrasound signal. Piezoelectric transducers are commonly used in PA imaging to detect the ultrasound signals. However, piezoelectric transducers suffer from low sensitivity when the dimensions are reduced and are easily influenced by electromagnetic interference. To avoid these limitations, various optical ultrasound sensors have been developed and shown their great potential in PA imaging.

Aim

Our study aims to summarize recent progress in optical ultrasound sensor technologies and their applications in PA imaging.

Approach

The commonly used optical ultrasound sensing techniques and their applications in PA systems are reviewed. The technical advances of different optical ultrasound sensors are summarized.

Results

Optical ultrasound sensors can provide wide bandwidth and improved sensitivity with miniatured size, which enables their applications in PA imaging.

Conclusions

The optical ultrasound sensors are promising transducers in PA imaging to provide higher-resolution images and can be used in new applications with their unique advantages.

Ultrawide-bandwidth high-resolution all-optical intravascular ultrasound using miniaturized photoacoustic transducer

Conventional intravascular ultrasound (IVUS) uses piezoelectric transducers to electrically generate and receive ultrasound. However, it remains a challenge to achieve large bandwidth for high resolution without compromising imaging depth. We report an all-optical IVUS (AO-IVUS) imaging system using picosecond laser pulse-pumped carbon composite for ultrasound excitation and π-phase-shifted fiber Bragg gratings for ultrasound detection. Using this all-optical technique, we achieved ultrawide-bandwidth (147%) and high-resolution (18.6 micrometers) IVUS imaging, which is unattainable by conventional technique. Imaging performance has been characterized in phantoms, presenting 18.6-micrometer axial resolution, 124-micrometer lateral resolution, and 7-millimeter imaging depth. Rotational pullback imaging scans are performed in rabbit iliac artery, porcine coronary artery, and rabbit arteries with drug-eluting metal stents, in parallel with commercial intravenous ultrasound scans as reference. Results demonstrated the advantages of high-resolution AO-IVUS in delineating details in vascular structures, showing great potential in clinical applications.

A Mathematical Model for Simulating Photoacoustic Signal Generation Process in Biological Tissues.. [DOI: 10.21203/rs.3.rs-2928563/v2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Background: Biomedical photoacoustic imaging (PAI) is a hybrid imaging modality based on the laser-generated ultrasound waves due to the photoacoustic (PA) effect physical phenomenon that has been reported firstly by A. G. Bell in 1880. Numerical modeling-based simulation for the PA signal generation process in biological tissues helps researchers for decreasing error trials in-vitro and hence decreasing error rates for in-vivo experiments. Numerical modeling methods help in obtaining a rapid modeling procedure comparable to pure mathematics. However, if a proper simplified mathematical model can be founded before applying numerical modeling techniques, it will be a great advantage for the overall numerical model. Most scientific theories, equations, and assumptions, been proposed to mathematically model the complete PA signal generation and propagation process in biological tissues, are so complicated. Hence, the researchers, especially the beginners, will find a hard difficulty to explore and obtain a proper simplified mathematical model describing the process. That’s why this paper is introduced. Methods: In this paper we have tried to simplify understanding for the biomedical PA wave’s generation and propagation process, deducing a simplified mathematical model for the whole process. The proposed deduced model is based on three steps: a- pulsed laser irradiance, b- diffusion of light through biological tissue, and c- acoustic pressure wave generation and propagation from the target tissue to the ultrasound transducer surface. COMSOL Multiphysics, which is founded due to the finite element method (FEM) numerical modeling principle, has been utilized to validate the proposed deduced mathematical model on a simulated biological tissue including a tumor inside. Results and Conclusion: The time-dependent study been applied by COMSOL has assured that the proposed deduced mathematical model may be considered as a simplified, easy, and fast startup base for scientific researchers to numerically model and simulate biomedical PA signals’ generation and propagation process utilizing any proper software like COMSOL.

A Mathematical Model for Simulating Photoacoustic Signal Generation Process in Biological Tissues.. [DOI: 10.21203/rs.3.rs-2928563/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Background Biomedical photoacoustic imaging (PAI) is a hybrid imaging modality based on the laser-generated ultrasound waves due to the photoacoustic (PA) effect physical phenomenon that has been reported firstly by A. G. Bell in 1880. Numerical modeling based simulation for PA signal generation process in biological tissues helps researchers for decreasing error trials in-vitro and hence decreasing error rates for in-vivo experiments. Numerical modeling methods help in obtaining a rapid modeling procedure comparable to pure mathematics. However, if a proper simplified mathematical model can be founded before applying numerical modeling techniques, it will be a great advantage for the overall numerical model. More scientific theories, equations, and assumptions through the biomedical PA imaging research literature have been proposed trying to mathematically model the complete PA signal generation and propagation process in biological tissues. However, most of them have so complicated details. Hence, the researchers, especially the beginners, will find a hard difficulty to explore and obtain a proper simplified mathematical model describing the process. That’s why this paper is introduced. Methods In this paper we have tried to simplify understanding for the biomedical PA wave’s generation and propagation process, deducing a simplified mathematical model for the whole process. The proposed deduced model is based on three steps: a- pulsed laser irradiance, b- diffusion of light through biological tissue, and c- acoustic pressure wave generation and propagation from the target tissue to the ultrasound transducer surface.

Optical ultrasound sensors for photoacoustic imaging: a narrative review

Optical ultrasound sensors have been increasingly employed in biomedical diagnosis and photoacoustic imaging (PAI) due to high sensitivity and resolution. PAI could visualize the distribution of ultrasound excited by laser pulses in biological tissues. The information of tissues is detected by ultrasound sensors in order to reconstruct structural images. However, traditional ultrasound transducers are made of piezoelectric films that lose sensitivity quadratically with the size reduction. In addition, the influence of electromagnetic interference limits further applications of traditional ultrasound transducers. Therefore, optical ultrasound sensors are developed to overcome these shortcomings. In this review, optical ultrasound sensors are classified into resonant and non-resonant ones in view of physical principles. The principles and basic parameters of sensors are introduced in detail. Moreover, the state of the art of optical ultrasound sensors and applications in PAI are also presented. Furthermore, the merits and drawbacks of sensors based on resonance and non-resonance are discussed in perspectives. We believe this review could provide researchers with a better understanding of the current status of optical ultrasound sensors and biomedical applications.

Recent advances in high-speed photoacoustic microscopy

Photoacoustic (PA) microscopy (PAM) has achieved remarkable progress in biomedicine in the past decade. It is a fast-rising imaging modality with diverse applications, such as hemodynamics, oncology, metabolism, and neuroimaging. Combining optical excitation and acoustic detection, the hybrid nature of PAM provides advantages of rich contrast and deep penetration. In recent years, high-speed PAM has flourished and enabled high-speed wide-field imaging of functional activity. In this review, we summarize the most recent advances in high-speed PAM technologies, including high-repetition-rate multi-wavelength laser development, fast scanning techniques, and novel PA signal acquisition strategies.

The Core of Medical Imaging: State of the Art and Perspectives on the Detectors

In this review, the roles of detectors in various medical imaging techniques were described. Ultrasound, optical (near-infrared spectroscopy and optical coherence tomography) and thermal imaging, magnetic resonance imaging, computed tomography, single-photon emission tomography, positron emission tomography were the imaging modalities considered. For each methodology, the state of the art of detectors mainly used in the systems was described, emphasizing new technologies applied.

Optoacoustic imaging of the skin

Optoacoustic (OA, photoacoustic) imaging capitalizes on the synergistic combination of light excitation and ultrasound detection to empower biological and clinical investigations with rich optical contrast while effectively bridging the gap between micro and macroscopic imaging realms. State-of-the-art OA embodiments consistently provide images at micron-scale resolution through superficial tissue layers by means of focused illumination that can be smoothly exchanged for acoustic-resolution images at diffuse light depths of several millimetres to centimetres via ultrasound beamforming or tomographic reconstruction. Taken together, this unique multi-scale imaging capacity opens unprecedented capabilities for high-resolution in vivo interrogations of the skin at scalable depths. Moreover, diverse anatomical and functional information is retrieved via dynamic mapping of endogenous chromophores such as haemoglobin, melanin, lipids, collagen, water and others. This, along with the use of non-ionizing radiation, facilitates a clinical translation of the OA modalities. We review recent progress in OA imaging of the skin in preclinical and clinical studies exploiting the rich contrast provided by endogenous substances in tissues. The imaging capabilities of existing approaches are discussed in the context of initial translational studies on skin cancer, inflammatory skin diseases, wounds and other conditions.

Review recent developments in photoacoustic imaging and sensing for nondestructive testing and evaluation

Photoacoustic (PA) imaging has been widely used in biomedical research and preclinical studies during the past two decades. It has also been explored for nondestructive testing and evaluation (NDT/E) and for industrial applications. This paper describes the basic principles of PA technology for NDT/E and its applications in recent years. PA technology for NDT/E includes the use of a modulated continuous-wave laser and a pulsed laser for PA wave excitation, PA-generated ultrasonic waves, and all-optical PA wave excitation and detection. PA technology for NDT/E has demonstrated broad applications, including the imaging of railway cracks and defects, the imaging of Li metal batteries, the measurements of the porosity and Young’s modulus, the detection of defects and damage in silicon wafers, and a visualization of underdrawings in paintings.

Improvement in resolution of fiber-laser photoacoustic tomography based on a virtual-point concept

In this study, a virtual-point concept was introduced into fiber-laser photoacoustic tomography to improve the elevational image resolution. The flexible fiber laser was bent into an arc shape to conform to the ultrasound wavefront, which formed an ultrasound focus at the center of the arc. The synthetic aperture focusing technique was utilized to reconstruct the images; as a result, the elevational resolution particularly within the out-of-focus region was considerably improved compared to the resolution of an image retrieved by multiplexing the PA time-resolved signals with sound velocity. The all-optical fiber-laser photoacoustic tomography system with a high spatial resolution has potential for various applications, including biomedical research and preclinical/clinical diagnosis.

Another decade of photoacoustic imaging

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

Towards non-contact photoacoustic imaging [review]

Photoacoustic imaging (PAI) takes advantage of both optical and ultrasound imaging properties to visualize optical absorption with high resolution and contrast. Photoacoustic microscopy (PAM) is usually categorized with all-optical microscopy techniques such as optical coherence tomography or confocal microscopes. Despite offering high sensitivity, novel imaging contrast, and high resolution, PAM is not generally an all-optical imaging method unlike the other microscopy techniques. One of the significant limitations of photoacoustic microscopes arises from their need to be in physical contact with the sample through a coupling media. This physical contact, coupling, or immersion of the sample is undesirable or impractical for many clinical and pre-clinical applications. This also limits the flexibility of photoacoustic techniques to be integrated with other all-optical imaging microscopes for providing complementary imaging contrast. To overcome these limitations, several non-contact photoacoustic signal detection approaches have been proposed. This paper presents a brief overview of current non-contact photoacoustic detection techniques with an emphasis on all-optical detection methods and their associated physical mechanisms.

Transparent High-Frequency Ultrasonic Transducer for Photoacoustic Microscopy Application

We report the development of an optically transparent high-frequency ultrasonic transducer using lithium niobate single-crystal and indium-tin-oxide electrodes with up to 90% optical transmission in the visible-to-near-infrared spectrum. The center frequency of the transducer was at 36.9 MHz with 33.9%, at -6 dB fractional bandwidth. The photoacoustic imaging capability of the fabricated transducer was also demonstrated by successfully imaging a resolution target and mouse-ear vasculatures in vivo, which were irradiated by a 532 nm pulse laser transmitted through the transducer.

Microbubble Resonators for All-Optical Photoacoustics of Flowing Contrast Agents

In this paper, we implement a Whispering Gallery mode microbubble resonator (MBR) as an optical transducer to detect the photoacoustic (PA) signal generated by plasmonic nanoparticles. We simulate a flow cytometry experiment by letting the nanoparticles run through the MBR during measurements and we estimate PA intensity by a Fourier analysis of the read-out signal. This method exploits the peaks associated with the MBR mechanical eigenmodes, allowing the PA response of the nanoparticles to be decoupled from the noise associated with the particle flow whilst also increasing the signal-to-noise ratio. The photostability curve of a known contrast agent is correctly reconstructed, validating the proposed analysis and proving quantitative PA detection. The experiment was run to demonstrate the feasible implementation of the MBR system in a flow cytometry application (e.g., the detection of venous thrombi or circulating tumor cells), particularly regarding wearable appliances. Indeed, these devices could also benefit from other MBR features, such as the extreme compactness, the direct implementation in a microfluidic circuit, and the absence of impedance-matching material.

Infrared spectra of micro-structured samples with microPhotoacoustic spectroscopy and synchrotron radiation

Photoacoustic spectroscopy (PAS) measures the photon absorption spectrum of a sample through detection of the acoustic wave generated by the photothermal effect as one modulates the intensity of the incident radiation at each wavelength. We have recently demonstrated the implementation of PAS in a microscopy configuration with mid-infrared radiation (microPAS). In the present work, we describe the performance of microPAS using synchrotron radiation (SR) in diffraction-limited spectromicroscopy and imaging experiments. Spectra were obtained for polystyrene beads, polypropylene fibres, and single fibres of human hair. SR produced microPAS spectra of much higher intensity as compared with those obtained using conventional mid- and near-infrared sources. For hair samples, the penetration depth of mid-infrared light, even with bright SR, is significantly shorter than the probed sample thickness at very low modulation frequencies resulting in saturated PAS spectra. In contrast, microPAS spectra of polymer beads were in general of much better quality than those obtained with conventional sources. We also demonstrated the capability to collect line profiles and line spectra at diffraction limited spatial resolution. The microPAS spectra of beads appear free from appreciable bandshape distortions arising from the real part of the refractive index of the sample. This observation confirms microPAS as an absorption-only technique and establishes it as a valuable new tool in the microspectroscopic analysis of particulates and of samples with a complex topography.

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.

Photoacoustic Imaging with Capacitive Micromachined Ultrasound Transducers: Principles and Developments

Photoacoustic imaging (PAI) is an emerging imaging technique that bridges the gap between pure optical and acoustic techniques to provide images with optical contrast at the acoustic penetration depth. The two key components that have allowed PAI to attain high-resolution images at deeper penetration depths are the photoacoustic signal generator, which is typically implemented as a pulsed laser and the detector to receive the generated acoustic signals. Many types of acoustic sensors have been explored as a detector for the PAI including Fabry-Perot interferometers (FPIs), micro ring resonators (MRRs), piezoelectric transducers, and capacitive micromachined ultrasound transducers (CMUTs). The fabrication technique of CMUTs has given it an edge over the other detectors. First, CMUTs can be easily fabricated into given shapes and sizes to fit the design specifications. Moreover, they can be made into an array to increase the imaging speed and reduce motion artifacts. With a fabrication technique that is similar to complementary metal-oxide-semiconductor (CMOS), CMUTs can be integrated with electronics to reduce the parasitic capacitance and improve the signal to noise ratio. The numerous benefits of CMUTs have enticed researchers to develop it for various PAI purposes such as photoacoustic computed tomography (PACT) and photoacoustic endoscopy applications. For PACT applications, the main areas of research are in designing two-dimensional array, transparent, and multi-frequency CMUTs. Moving from the table top approach to endoscopes, some of the different configurations that are being investigated are phased and ring arrays. In this paper, an overview of the development of CMUTs for PAI is presented.

Photoacoustic shadow-casting microscopy

We present photoacoustic shadow-casting microscopy (PASM), a technique that allows high-resolution imaging of weakly absorbing biological samples with unprecedented sensitivity. In PASM, a uniform optical absorbing layer is placed in contact with the samples and is excited by the light transmitted through the sample, producing photoacoustic (PA) waves with an increased signal-to-noise ratio compared with that generated by the sample itself. Therefore, given a desired image quality, the required excitation fluence is much reduced, alleviating the photothermal damage to the specimen. The system provides a lateral resolution of 5 μm when using a 0.30 NA microscope objective lens. To demonstrate PASM, we present images of bovine red blood cells and microbeads.

Transparent capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic applications

Integration of acoustic and optical techniques prompted the need for transparent ultrasonic transducers to guide the light through the transducer and improve the signal to noise ratio. In the presented paper, capacitive micromachined ultrasound transducers (CMUTs) using glass substrate and indium-tin-oxide electrodes were fabricated by adhesive wafer bonding technique presenting a transparency of up to 82% in the visible range. A receive sensitivity of 65.5 μV/Pa was measured with noise equivalent sensitivity of 95 Pa. Capacity of the produced CMUTs for photoacoustic imaging was also demonstrated by successfully detecting the photoacoustic signal from an aluminum foil target, which was irradiated by a 532-nm pulse laser transmitted through the CMUT. The centre frequency of the detected photoacoustic signal was at 2 MHz with 52.3% -6-dB fractional bandwidth.

Miniature probe for all-optical double gradient-index lenses photoacoustic microscopy

A novel all-optical double gradient-index (GRIN) lens optical-resolution photoacoustic microscopy (OR-PAM), termed as DGL-PAM, is demonstrated. The miniature probe consists of a single-mode fiber and double GRIN lenses for optical focusing and a miniature fiber Fabry-Perot sensor for ultrasound detection. The new design is simple and realizes high resolution with long working distance (WD) by virtue of the double GRIN lenses. The overall size of the probe is 2.7 mm in diameter. High lateral resolution of 3.7 μm (at 532 nm laser wavelength) and long WD of 5.5 mm are achieved. In vivo OR-PAM of mouse ear demonstrates the imaging ability of DGL-PAM. Since precise alignment of optical and acoustic foci is not needed, the proposed DGL-PAM is relatively easy to implement. It has potential to be developed as a low-cost, disposable OR-PAM probe and for endoscopic applications. The proposed double GRIN lenses for making miniature endoscopic probes can also be applied to other modalities, such as optical coherence tomography and confocal fluorescence microscopy, to enable high resolution and long WD.

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.

The Progress in Photoacoustic and Laser Ultrasonic Tomographic Imaging for Biomedicine and Industry: A Review

The current paper reviews a set of principles and applications of photoacoustic and laser ultrasonic imaging, developed in the Laser Optoacoustic Laboratories of ILIT RAS, NUST MISiS, and ILC MSU. These applications include combined photoacoustic and laser ultrasonic imaging for biological objects, and tomographic laser ultrasonic imaging of solids. Principles, algorithms, resolution of the developed methods, and related problems are discussed. The review is written in context of the current state-of-art of photoacoustic and laser ultrasonic imaging.

Molecular fingerprinting of nanoparticles in complex media with non-contact photoacoustics: beyond the light scattering limit

Optical instruments can probe physical systems even to the level of individual molecules. In particular, every molecule, solution, and structure such as a living cell has a unique absorption spectrum representing a molecular fingerprint. This spectrum can help identify a particular molecule from others or quantify its concentration; however, scattering limits molecular fingerprinting within a complex compound and must be overcome. Here, we present a new, non-contact photoacoustic (PA)-based method that can almost completely remove the influence of background light scattering on absorption measurements in heterogeneous highly scattering solutions and, furthermore, separate the intrinsic absorption of nanoscale objects from their scattering. In particular, we measure pure absorption spectra for solutions of gold nanorods (GNRs) as an example of a plasmonic agent and show that these spectra differ from the extinction measured with conventional UV-VIS spectrophotometry. Finally, we show how the original GNR absorption changes when nanoparticles are internalized by cells.

Fast photoacoustic imaging systems using pulsed laser diodes: a review

Photoacoustic imaging (PAI) is a newly emerging imaging modality for preclinical and clinical applications. The conventional PAI systems use Q-switched Nd:YAG/OPO (Optical Parametric Oscillator) nanosecond lasers as excitation sources. Such lasers are expensive, bulky, and imaging speed is limited because of low pulse repetition rate. In recent years, the semiconductor laser technology has advanced to generate high-repetitions rate near-infrared pulsed lasers diodes (PLDs) which are reliable, less-expensive, hand-held, and light-weight, about 200 g. In this article, we review the development and demonstration of PLD based PAI systems for preclinical and clinical applications reported in recent years.

[Focal zone integral and multiple axial scanning based acoustic resolution photoacoustic microscopy with high lateral resolution in-depth]

Acoustic resolution photoacoustic microscopy (ARPAM) combines the advantages of high optical contrast, and high ultrasonic spatial resolution and penetration. However, in photoacoustic microscopy (PAM), the information from deep regions can be greatly affected by the shallow targets, and most importantly, the irreconcilable conflict between the lateral resolution and depth of fields has always be a major factor that limits the imaging quality. In this work, an ARPAM system was developed, in which a non-coaxial arrangement of light illumination and acoustic detection was adopted to alleviate the influence of the tissue surface on the deep targets, and a novel focal zone integral algorithm was applied with multiple axial scanning to improve the lateral resolution. Phantom experiment results show that, the build system can maintain a consistent high lateral resolution of 0.6 mm over a large range in axial direction, which is close to the theoretical calculations. The following tumor imaging results on nude mice indicate that, the proposed method can provide more in-depth information compared with the conventional back detection ARPAM method. With the development of fast repetition lasers and image scanning technologies, the proposed method may play an important role in cerebral vascular imaging, cervical cancer photoacoustic endoscopic detection, and superficial tumor imaging.

Label-free photoacoustic imaging guided sclerotherapy for vascular malformations: a feasibility study

We used high-resolution photoacoustic imaging (PAI) to guide sclerotherapy of vascular malformations in an in vivo animal model. A focus-adjustable PAI system was developed. It can adapt to the imaging needs of different depths by adjusting the focus. Blood samples drawn before and after sclerosis were examined with PAI, which could distinguish whether or not the blood had been exposed to a sclerosing agent. Superficial and deep vessels in the animal model were examined in vivo to prove the feasibility of guiding sclerotherapy. We found that PAI can distinguish sclerotic vessels from normal vessels within a certain depth range. Our findings suggest the potential of PAI to find accurate injection points and to localize thrombi, making it possible to reduce the dosage of sclerosing agents.

Nonlinear optoacoustic readings from diffusive media at near-infrared wavelengths

Optoacoustic (photoacoustic) imaging assumes that the detected signal varies linearly with laser energy. However, nonlinear intensity responses as a function of light fluence have been suggested in optoacoustic microscopy, that is, within the first millimeter of tissue. In this study, we explore the presence of nonlinearity deeper in tissue (~4 mm), as it relates to optoacoustic mesoscopy, and investigate the fluence required to delineate a switch from linear to nonlinear behavior. Optoacoustic signal nonlinearity is studied for different materials, different wavelengths and as a function of changes in the scattering and absorption coefficient of the medium imaged. We observe fluence thresholds in the mJ/cm² range and preliminary find that different materials may exhibit different nonlinearity patterns. We discuss the implications of nonlinearity in relation to image accuracy and quantification in optoacoustic tomography.

Miniature all-optical probe for large synthetic aperture photoacoustic-ultrasound imaging

A miniature all-optical probe for high-resolution photoacoustic (PA)-ultrasound (US) imaging using a large synthetic aperture is developed. The probe consists of three optical fibers for PA excitation, US generation, and detection of acoustic waves, respectively. The fiber for PA excitation has a large numerical aperture (NA) for wide-angle laser illumination. On the other hand, the fiber with a carbon black-polydimethylsiloxane composite coated on the end face of the optical fiber is used for wide-angle US transmission through laser-US conversion. Both the excited PA and backscattered US signals are detected by a fiber-tip Fabry-Perot cavity for wide-angle acoustic detection. The probe outer diameter is only ~2 mm. The synergy of the three optical fibers makes a large-NA synthetic aperture focusing technique for high-resolution PA and US imaging possible. High PA lateral resolutions of 104-154 μm and high US lateral resolutions of 64-112 μm over a depth range of > 4 mm are obtained. Compared with other existing miniature PA-US probes, to our knowledge, our probe achieves by far the best performance in terms of lateral resolutions and imaging depth range. The constructed probe has potential for endoscopic and intravascular imaging applications that require PA and US contrasts with high resolutions over a large depth range.

Sensitivity characteristics of broadband fiber-laser-based ultrasound sensors for photoacoustic microscopy

High-frequency fiber laser sensor is a new acoustic detector for photoacoustic imaging. However, its performance has not been thoroughly studied. Here, we present a comprehensive characterization of a fiber laser sensor for photoacoustic imaging. Ultrasound waves deform the fiber laser cavity and induce frequency changes in the heterodyning output signal. The sensitivity peaks at 22 MHz, which is associated with an azimuthal mode number l = 2 and a radial mode number n = 1. The broadband acoustic sensitivity in terms of frequency shift is 2.25 MHz/kPa and the noise-equivalent pressure reaches 45 Pa with a sampling rate of 100 MHz. The 3-dB bandwidth is 18 MHz for spherical-wave detection. We characterized the spatial distribution of acoustic sensitivity. The sensitivity along the fiber longitudinal direction varies with the laser spatial mode and is determined by the grating and cavity parameters. The sensitivity at the azimuthal direction presents a |cos(2θ)| dependence as a result of fiber core asymmetry. In the radial direction, the sensitivity is inversely proportional to the square root of the distance between the source and the detector. The acoustic sensitivity can be enhanced by reducing the cavity length. We experimentally show that a short sensor can enhance the contrast and penetration depth of PAM than a long one.

Microsphere enabled subdiffraction-limited optical-resolution photoacoustic microscopy: a simulation study

Optical resolution photoacoustic microscopy (ORPAM) is a high-resolution hybrid imaging modality having potential for microscale in vivo imaging. Optical diffraction limits the lateral resolution of ORPAM. A photonic nanojet (PNJ) was used to break this diffraction limit. A single round microsphere can generate a PNJ with subwavelength waist, but its short axial length limits its applications to surface imaging only. We investigate different sphere designs to achieve ultralong nanojets that will make the nanojet more viable in far-field applications, such as photoacoustic imaging. The PNJ properties, including effective length, waist size, working distance, and peak intensity, can be tuned and controlled by changing the sphere design and its refractive index. A truncated multilayer microsphere design could generate an ultraelongated PNJ with length larger than ∼172λ (∼138  μm) while retaining a large working distance ∼32λ (∼26  μm). Through simulation study, we observed ∼11-fold enhancement in lateral resolution with 5  μm round sphere (refractive index 2.2) when used in a conventional ORPAM setup with NA=0.1 and λ=800  nm.

Fiber-Laser-Based Ultrasound Sensor for Photoacoustic Imaging

Photoacoustic imaging, especially for intravascular and endoscopic applications, requires ultrasound probes with miniature size and high sensitivity. In this paper, we present a new photoacoustic sensor based on a small-sized fiber laser. Incident ultrasound waves exert pressures on the optical fiber laser and induce harmonic vibrations of the fiber, which is detected by the frequency shift of the beating signal between the two orthogonal polarization modes in the fiber laser. This ultrasound sensor presents a noise-equivalent pressure of 40 Pa over a 50-MHz bandwidth. We demonstrate this new ultrasound sensor on an optical-resolution photoacoustic microscope. The axial and lateral resolutions are 48 μm and 3.3 μm. The field of view is up to 1.57 mm2. The sensor exhibits strong resistance to environmental perturbations, such as temperature changes, due to common-mode cancellation between the two orthogonal modes. The present fiber laser ultrasound sensor offers a new tool for all-optical photoacoustic imaging.

Non-Contact Photoacoustic Imaging Using a Commercial Heterodyne Interferometer

Most current photoacoustic imaging (PAI) systems employ piezoelectric transducers to receive photoacoustic signals, which requires coupling medium to facilitate photoacoustic wave propagation and are not favored in many applications. Here, we report an all-optical non-contact PAI system based on a commercial heterodyne interferometer working at 1550 nm. The interferometer remotely detects ultrasound-induced surface vibration and does not involve any physical contact with the sample. The theoretically predicated and experimentally measured noise equivalent detection limits of the optical sensor are about 4.5 and 810 Pa over 1.2 MHz bandwidth. Using a raster-scan PAI system equipped with the non-contact design, stereotactic boundaries of an artificial tumor in a pig brain were accurately delineated. The non-contact design also enables the tomographic PAI of biological tissue samples in a non-invasive manner. The preliminary results and analyses reveal that the heterodyne interferometer-based non-contact PAI system holds good potential in biomedical imaging.

All-optical optoacoustic microscopy based on probe beam deflection technique

Optoacoustic (OA) microscopy using an all-optical system based on the probe beam deflection technique (PBDT) for detection of laser-induced acoustic signals was investigated as an alternative to conventional piezoelectric transducers. PBDT provides a number of advantages for OA microscopy including (i) efficient coupling of laser excitation energy to the samples being imaged through the probing laser beam, (ii) undistorted coupling of acoustic waves to the detector without the need for separation of the optical and acoustic paths, (iii) high sensitivity and (iv) ultrawide bandwidth. Because of the unimpeded optical path in PBDT, diffraction-limited lateral resolution can be readily achieved. The sensitivity of the current PBDT sensor of 22 μV/Pa and its noise equivalent pressure (NEP) of 11.4 Pa are comparable with these parameters of the optical micro-ring resonator and commercial piezoelectric ultrasonic transducers. Benefits of the present prototype OA microscope were demonstrated by successfully resolving micron-size details in histological sections of cardiac muscle.

Grueneisen relaxation photoacoustic microscopy in vivo

Grueneisen relaxation photoacoustic microscopy (GR-PAM) can achieve optically defined axial resolution, but it has been limited to ex vivo demonstrations so far. Here, we present the first in vivo image of a mouse brain acquired with GR-PAM. To induce the GR effect, an intensity-modulated continuous-wave laser was employed to heat absorbing objects. In phantom experiments, an axial resolution of 12.5  μm was achieved, which is sixfold better than the value achieved by conventional optical-resolution PAM. This axial-resolution improvement was further demonstrated by imaging a mouse brain in vivo, where significantly narrower axial profiles of blood vessels were observed. The in vivo demonstration of GR-PAM shows the potential of this modality for label-free and high-resolution anatomical and functional imaging of biological tissues.