Reflection-mode in vivo photoacoustic microscopy with subwavelength lateral resolution

Photoacoustic imaging in prostate cancer: A new paradigm for diagnosis and management

The global health issue of prostate cancer (PCa) requires better diagnosis and treatment. Photoacoustic imaging (PAI) may change PCa management. This review examines PAI's principles, diagnostic role, and therapeutic guidance. PAI uses optical light excitation and ultrasonic detection for high-resolution functional and molecular imaging. PAI uses endogenous and exogenous contrast agents to distinguish cancerous and benign prostate tissues with greater sensitivity and specificity than PSA testing and TRUS-guided biopsy. In addition to diagnosing, PAI can guide and monitor PCa therapy. Its real-time imaging allows precise biopsies and brachytherapy seed placement. Photoacoustic temperature imaging allows non-invasive monitoring of thermal therapies like cryotherapy, improving treatment precision and success. Transurethral illumination probes, innovative contrast agents, integration with other imaging modalities, and machine learning analysis are being developed to overcome depth and data complexity restrictions. PAI could become an essential tool for PCa diagnosis and therapeutic guidance as the field advances.

Photoacoustic imaging for cutaneous melanoma assessment: a comprehensive review

Significance

Cutaneous melanoma (CM) has a high morbidity and mortality rate, but it can be cured if the primary lesion is detected and treated at an early stage. Imaging techniques such as photoacoustic (PA) imaging (PAI) have been studied and implemented to aid in the detection and diagnosis of CM.

Aim

Provide an overview of different PAI systems and applications for the study of CM, including the determination of tumor depth/thickness, cancer-related angiogenesis, metastases to lymph nodes, circulating tumor cells (CTCs), virtual histology, and studies using exogenous contrast agents.

Approach

A systematic review and classification of different PAI configurations was conducted based on their specific applications for melanoma detection. This review encompasses animal and preclinical studies, offering insights into the future potential of PAI in melanoma diagnosis in the clinic.

Results

PAI holds great clinical potential as a noninvasive technique for melanoma detection and disease management. PA microscopy has predominantly been used to image and study angiogenesis surrounding tumors and provide information on tumor characteristics. Additionally, PA tomography, with its increased penetration depth, has demonstrated its ability to assess melanoma thickness. Both modalities have shown promise in detecting metastases to lymph nodes and CTCs, and an all-optical implementation has been developed to perform virtual histology analyses. Animal and human studies have successfully shown the capability of PAI to detect, visualize, classify, and stage CM.

Conclusions

PAI is a promising technique for assessing the status of the skin without a surgical procedure. The capability of the modality to image microvasculature, visualize tumor boundaries, detect metastases in lymph nodes, perform fast and label-free histology, and identify CTCs could aid in the early diagnosis and classification of CM, including determination of metastatic status. In addition, it could be useful for monitoring treatment efficacy noninvasively.

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.

State of the Art in Carbon Nanomaterials for Photoacoustic Imaging

Photoacoustic imaging using energy conversion from light to ultrasound waves has been developed as a powerful tool to investigate in vivo phenomena due to their complex characteristics. In photoacoustic imaging, endogenous chromophores such as oxygenated hemoglobin, deoxygenated hemoglobin, melanin, and lipid provide useful biomedical information at the molecular level. However, these intrinsic absorbers show strong absorbance only in visible or infrared optical windows and have limited light transmission, making them difficult to apply for clinical translation. Therefore, the development of novel exogenous contrast agents capable of increasing imaging depth while ensuring strong light absorption is required. We report here the application of carbon nanomaterials that exhibit unique physical, mechanical, and electrochemical properties as imaging probes in photoacoustic imaging. Classified into specific structures, carbon nanomaterials are synthesized with different substances according to the imaging purposes to modulate the absorption spectra and highly enhance photoacoustic signals. In addition, functional drugs can be loaded into the carbon nanomaterials composite, and effective in vivo monitoring and photothermal therapy can be performed with cell-specific targeting. Diverse applied cases suggest the high potential of carbon nanomaterial-based photoacoustic imaging in in vivo monitoring for clinical research.

Improvement of spatial resolution in photoacoustic microscopy using transmissive adaptive optics with a low-frequency ultrasound transducer

Maintaining a high spatial resolution in photoacoustic microscopy (PAM) of deep tissues is difficult due to large aberration in an objective lens with high numerical aperture and photoacoustic wave attenuation. To address the issue, we integrate transmission-type adaptive optics (AO) in high-resolution PAM with a low-frequency ultrasound transducer (UT), which increases the photoacoustic wave detection efficiency. AO improves lateral resolution and depth discrimination in PAM, even for low-frequency ultrasound waves by focusing a beam spot in deep tissues. Using the proposed PAM, we increased the lateral resolution and depth discrimination for blood vessels in mouse ears.

A backward-mode optical-resolution photoacoustic microscope for 3D imaging using a planar Fabry-Pérot sensor

Optical-resolution photoacoustic microscopy (OR-PAM) combines high spatial resolution and strong absorption-based contrast in tissue, which has enabled structural and spectroscopic imaging of endogenous chromophores, primarily hemoglobin. Conventional piezoelectric ultrasound transducers are typically placed far away from the photoacoustic source due to their opacity, which reduces acoustic sensitivity. Optical ultrasound sensors are an alternative as their transparency allows them to be positioned close to the sample with minimal source-detector distances. In this work, a backward-mode OR-PAM system based on a planar Fabry-Pérot ultrasound sensor and coaxially aligned excitation and interrogation beams was developed. Two 3D imaging modes, using raster-scanning for enhanced image quality and continuous-scanning for fast imaging, were implemented and tested on a leaf skeleton phantom. In fast imaging mode, a scan-rate of 100,000 A-lines/s was achieved. 3D images of a zebrafish embryo were acquired in vivo in raster-scanning mode. The transparency of the FP sensor in the visible and near-infrared wavelength region makes it suitable for combined functional and molecular imaging applications using OR-PAM and multi-photon fluorescence microscopy.

Broadband surface plasmon resonance sensor for fast spectroscopic photoacoustic microscopy

High-speed optical-resolution photoacoustic microscopy (OR-PAM), integrating the merits of high spatial resolution and fast imaging acquisition, can observe dynamic processes of the optical absorption-based molecular specificities. However, it remains challenging for the evaluation to morphological and physiological parameters that are closely associated with photoacoustic spectrum due to the inadequate ultrasonic frequency response of the routinely-employed piezoelectric transducer. By utilizing the galvanometer for fast optical scanning and our previously-developed surface plasmon resonance sensor as an unfocused broadband ultrasonic detector, high-speed spectroscopic photoacoustic imaging was accessed in the OR-PAM system, achieving an acoustic bandwidth of ∼125 MHz and B-scan rate at ∼200 Hz over a scanning range of ∼0.5 mm. Our system demonstrated the dynamic imaging of the moving phantoms' structures and the simultaneous characterization of their photoacoustic spectra over time. Further, fast volumetric imaging and spectroscopic analysis of microanatomic features of a zebrafish eye ex vivo was obtained label-freely.

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.

Convolutional neural network for resolution enhancement and noise reduction in acoustic resolution photoacoustic microscopy

In acoustic resolution photoacoustic microscopy (AR-PAM), a high numerical aperture focused ultrasound transducer (UST) is used for deep tissue high resolution photoacoustic imaging. There is a significant degradation of lateral resolution in the out-of-focus region. Improvement in out-of-focus resolution without degrading the image quality remains a challenge. In this work, we propose a deep learning-based method to improve the resolution of AR-PAM images, especially at the out of focus plane. A modified fully dense U-Net based architecture was trained on simulated AR-PAM images. Applying the trained model on experimental images showed that the variation in resolution is ∼10% across the entire imaging depth (∼4 mm) in the deep learning-based method, compared to ∼180% variation in the original PAM images. Performance of the trained network on in vivo rat vasculature imaging further validated that noise-free, high resolution images can be obtained using this method.

Three-dimensional synthetic aperture focusing photoacoustic microscopy based on the acoustic simulation generated delay time and weighted factor

Acoustic resolution photoacoustic microscopy (ARPAM) is a promising imaging tool in biomedical applications for its advantage of penetration over other optical imaging techniques. However, the lateral resolution of ARPAM deteriorates significantly in the out-of-focus region. The synthetic aperture focusing technique (SAFT) is required to restore this kind of focus-related imaging distortion. The conventional SAFT method is based on the virtual detector (VD) conception, in which the phase of the received photoacoustic (PA) signal is calculated by assuming the focus of the transducer as a VD. Nevertheless, the phase of the received PA signal is not only determined by the geometrical parameters of the transducer, but also by the transducer's electromechanic response and the original PA signal. Ignoring these two factors will reduce the quality of the imaging results. In this work, a new SAFT method, which is based on acoustic simulation, is proposed for ARPAM. The measured PA signal from a point target at the focus is employed to evaluate the convolution of the transducer's electromechanic response and the original PA signal. This measured signal is used as the excitation in an acoustic simulation. The simulation, which is based on the geometrical and acoustic parameters of the transducer, is employed to calculate the delay time and weighted coefficient for the SAFT calculation. The phantom experiments with point and line targets indicate that the proposed method obtains imaging results with better lateral resolution and improved signal-noise ratio compared with the widely used VD-based SAFT method.

Research on Local Sound Field Control Technology Based on Acoustic Metamaterial Triode Structure

Cell photoacoustic detection faces the problem where the strength of the sound wave signal is so weak that it easily gets interfered by other acoustic signals. A sonic triode model based on an artificial periodic structure is designed by COMSOL Multiphysics 5.3a software (Stockholm, Sweden), and software simulations are conducted. Experiments show that when a sound wave with a specific frequency is input by the sound wave triode, it can produce an energy amplification effect on the sound wave signals of the same frequency and a blocking effect on the sound wave signals of other frequencies. This contrast effect is more obvious after increasing the sound pressure intensity of the input sound wave signal. It can effectively filter out interference sound signals. The study of the acoustic triode model provides a new approach for the acquisition and identification of acoustic signals in cell photoacoustic detection, which can significantly improve the working efficiency and accuracy of cell photoacoustic detection.

Photoacoustic viscoelasticity imaging for the detection of acute hepatitis: a feasibility study

Biomechanical assessments are essential for the understanding of physiological states and the characterization of certain tissue pathologies such as liver cirrhosis. In this work, we showed by the photoacoustic viscoelasticity (PAVE) imaging that obvious mechanical change was also observed in the development of the acute hepatitis owing to the hepatocyte enlargement and intracellular fluid increment, indicating that the PAVE technique can be developed as a supplementary method for detecting acute hepatitis in future. The feasibility of the PAVE imaging is validated by a group of agar phantoms. Furthermore, acute hepatitis pathological animal models were established and imaged ex vivo and in situ by the PAVE technique to demonstrate its capability for the mechanical characterization of acute hepatitis, and the imaging results were consistent with pathological results. The feasibility study of detecting acute hepatitis by the PAVE technique proved that this method has potential to be developed as a clinical biomechanical imaging method to supplement current clinical strategy for liver disease detection.

Acoustic resolution photoacoustic microscopy based on microelectromechanical systems scanner

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

Review on practical photoacoustic microscopy

Photoacoustic imaging (PAI) has many interesting advantages, such as deep imaging depth, high image resolution, and high contrast to intrinsic and extrinsic chromophores, enabling morphological, functional, and molecular imaging of living subjects. Photoacoustic microscopy (PAM) is one form of the PAI inheriting its characteristics and is useful in both preclinical and clinical research. Over the years, PAM systems have been evolved in several forms and each form has its relative advantages and disadvantages. Thus, to maximize the benefits of PAM for a specific application, it is important to configure the PAM system optimally by targeting a specific application. In this review, we provide practical methods for implementing a PAM system to improve the resolution, signal-to-noise ratio (SNR), and imaging speed. In addition, we review the preclinical and the clinical applications of PAM and discuss the current challenges and the scope for future developments.

High-speed simultaneous multiscale photoacoustic microscopy

Photoacoustic microscopy (PAM) is a fast-growing biomedical imaging technique that provides high-resolution in vivo imaging beyond the optical diffusion limit. Depending on the scalable lateral resolution and achievable penetration depth, PAM can be classified into optical resolution PAM (OR-PAM) and acoustic resolution PAM (AR-PAM). The use of a microelectromechanical systems (MEMS) scanner has improved OR-PAM imaging speed significantly and is highly beneficial in the development of miniaturized handheld devices. The shallow penetration depth of OR-PAM limits the use of such devices for a wide range of clinical applications. We report the use of a high-speed MEMS scanner for both OR-PAM and AR-PAM. A high-speed, wide-area scanning integrated OR-AR-PAM system combining MEMS scanner and raster mechanical movement was developed. A lateral resolution of 5  μm and penetration depth ∼0.9-mm in vivo was achieved using OR-PAM at 586 nm, whereas a lateral resolution of 84  μm and penetration depth of ∼2-mm in vivo was achieved using AR-PAM at 532 nm.

Isometrically Resolved Photoacoustic Microscopy Based on Broadband Surface Plasmon Resonance Ultrasound Sensing

Photoacoustic microscopy (PAM) can measure optical absorption-based molecular specificities within tissues. Despite the diffraction-limited lateral resolution in optical-resolution photoacoustic microscopy (OR-PAM), the ongoing challenge is poor axial resolution because of an insufficient ultrasound detection bandwidth, which hampers PAM volumetric imaging. We propose polarization-differential surface plasmon resonance (SPR) sensing for broadband and high-sensitivity photoacoustic (PA) detection, allowing OR-PAM with comparable resolution along lateral and axial directions. This sensor possesses an estimated noise-equivalent-pressure sensitivity of ∼477 Pa over an approximately linear pressure response up to 107 kPa. Moreover, an improved PA detection bandwidth of ∼173 MHz permits an axial resolution (∼7.6 μm) that approaches the lateral resolution (∼4.5 μm) of our OR-PAM system. The capability in spatially isometric micrometer-scale resolution enables in vivo volumetric label-free imaging of the microvasculature of a mouse ear. The SPR sensing technology promises broader applications of PAM in biomedical studies such as microcirculation.

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

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

1064 nm acoustic resolution photoacoustic microscopy

Photoacoustic imaging is a noninvasive imaging technique having the advantages of high-optical contrast and good acoustic resolution at improved imaging depths. Light transport in biological tissues is mainly characterized by strong optical scattering and absorption. Photoacoustic microscopy is capable of achieving high-resolution images at greater depth compared to conventional optical microscopy methods. In this work, we have developed a high-resolution, acoustic resolution photoacoustic microscopy (AR-PAM) system in the near infra-red (NIR) window II (NIR-II, eg, 1064 nm) for deep tissue imaging. Higher imaging depth is achieved as the tissue scattering at 1064 nm is lesser compared to visible or near infrared window-I (NIR-I). Our developed system can provide a lateral resolution of 130 μm, axial resolution of 57 μm, and image up to 11 mm deep in biological tissues. This 1064-AR-PAM system was used for imaging sentinel lymph node and the lymph vessel in rat. Urinary bladder of rat filled with black ink was also imaged to validate the feasibility of the developed system to study deeply seated organs.

Reflection-mode switchable subwavelength Bessel-beam and Gaussian-beam photoacoustic microscopy in vivo

We have developed a reflection-mode switchable subwavelength Bessel-beam (BB) and Gaussian-beam (GB) photoacoustic microscopy (PAM) system. To achieve both reflection-mode and high resolution, we tightly attached a very small ultrasound transducer to an optical objective lens with numerical aperture of 1.0 and working distance of 2.5 mm. We used axicon and an achromatic doublet in our system to obtain the extended depth of field (DOF) of the BB. To compare the DOF performance achieved with our BB-PAM system against GB-PAM system, we designed our system so that the GB can be easily generated by simply removing the lenses. Using a 532 nm pulse laser, we achieved the lateral resolutions of 300 and 270 nm for BB-PAM and GB-PAM, respectively. The measured DOF of BB-PAM was approximately 229 μm, which was about 7× better than that of GB-PAM. We imaged the vasculature of a mouse ear using BB-PAM and GB-PAM and confirmed that the DOF of BB-PAM is much better than the DOF of GB-PAM. Thus, we believe that the high resolution achieved at the extended DOF by our system is very practical for wide range of biomedical research including red blood cell (RBC) migration in blood vessels at various depths and observation of cell migration or cell culture.

Extended photoacoustic transport model for characterization of red blood cell morphology in microchannel flow

The dynamic response behavior of red blood cells holds the key to understanding red blood cell related diseases. In this regard, an understanding of the physiological functions of erythrocytes is significant before focusing on red blood cell aggregation in the microcirculatory system. In this work, we present a theoretical model for a photoacoustic signal that occurs when deformed red blood cells pass through a microfluidic channel. Using a Green's function approach, the photoacoustic pressure wave is obtained analytically by solving a combined Navier-Stokes and photoacoustic equation system. The photoacoustic wave expression includes determinant parameters for the cell deformability such as plasma viscosity, density, and red blood cell aggregation, as well as involving laser parameters such as beamwidth, pulse duration, and repetition rate. The effects of aggregation on blood rheology are also investigated. The results presented by this study show good agreements with the experimental ones in the literature. The comprehensive analytical solution of the extended photoacoustic transport model including a modified Morse type potential function sheds light on the dynamics of aggregate formation and demonstrates that the profile of a photoacoustic pressure wave has the potential for detecting and characterizing red blood cell aggregation.

Broadband graphene-based photoacoustic microscopy with high sensitivity

Photoacoustic microscopy (PAM) enables the measurement of properties associated with optical absorption within tissues and complements sophisticated technologies employing optical microscopy. An inadequate frequency response as determined by a piezoelectric ultrasonic transducer results, however, in poor depth resolution and inaccurate measurements of the coefficients of optical absorption. We developed a PAM system configured as an attenuated total reflectance sensor with a ten-layer graphene film sandwiched between a prism and water (the coupling medium) for photoacoustic (PA) wave detection. Transients of the PA pressure cause perturbations in the refractive index of the water thereby changing the polarization-dependent absorption of the graphene film. The signal in PA detection involves recording the difference in the temporal-varying reflectance intensity between the two orthogonally polarized probe beams. The graphene-based sensor has an estimated noise-equivalent-pressure sensitivity of ∼550 Pa over an approximately linear pressure response from 11.0 kPa to 55.0 kPa. Moreover, it enables a much broader PA bandwidth detection of up to ∼150 MHz, primarily dominated by a highly localized evanescent field. From the strong optical absorption of inherent hemoglobin, in vivo label-free PAM imaging provided a three-dimensional viewing of the microvasculature of a mouse ear. These results suggest great potential for graphene-based PAM in biomedical investigations, such as microcirculation studies.

In Vivo Imaging of Microvasculature during Anesthesia with High-Resolution Photoacoustic Microscopy

Anesthesia monitoring is extremely important in improving the quality of anesthesia and ensuring the safety of patients in operation. Photoacoustic microscopy (PAM) is proposed to in vivo image the skin microvasculature of 10 nude mice undergoing general anesthesia by using the isoflurane gas with a concentration of 3%. Benefiting from strong optical absorption of hemoglobin, PAM has good contrast and high resolution in mapping of microvasculature. A series of high quality images can clearly reveal the subtle changes of capillaries in morphology over time. Two indices, vessel intensity and vessel density, are extracted from these images to measure the microvasculature quantitatively. The imaging results show that the vessel intensity and density are increased over time. After 65 min, the vessel intensity increased 42.7 ± 8.6% and the density increased 28.6 ± 12.2%. These indices extracted from photoacoustic images accurately reflect the greater blood perfusion undergoing general anesthesia. Additionally, abnormal reductions of vessel intensity and density are also observed as overtime anesthesia. This preclinical study suggests that PAM holds potential to monitor anesthesia by imaging the skin microvasculature.

Portable optical-resolution photoacoustic microscopy for volumetric imaging of multiscale organisms

Photoacoustic microscopy (PAM) provides a fundamentally new tool for a broad range of studies of biological structures and functions. However, the use of PAM has been largely limited to small vertebrates due to the large size/weight and the inconvenience of the equipment. Here, we describe a portable optical-resolution photoacoustic microscopy (pORPAM) system for 3-dimensional (3D) imaging of small-to-large rodents and humans with a high spatiotemporal resolution and a large field of view. We show extensive applications of pORPAM to multiscale animals including mice and rabbits. In addition, we image the 3D vascular networks of human lips, and demonstrate the feasibility of pORPAM to observe the recovery process of oral ulcer and cancer-associated capillary loops in human oral cavities. This technology is promising for broad biomedical studies from fundamental biology to clinical diseases.

Ultracompact high-resolution photoacoustic microscopy

Optical resolution photoacoustic microscopy (ORPAM), benefiting from rich optical contrast, scalable acoustic resolution, and deep penetration depth, is of great importance for the fields of biology and medicine. However, limited by the size and performance of reported optical/acoustic scanners, existing portable/handheld ORPAMs are bulky and heavy, and suffer from low imaging quality/speed. Here, we present an ultracompact ORPAM probe, which is miniature and light, and has high imaging quality. The probe only weighs 20 grams and has an outer size of 22  mm×30  mm×13  mm, a high lateral resolution of 3.8 μm, and an effective imaging domain of 2  mm×2  mm. To show its advantages over existing ORPAMs, we apply this probe to image vasculatures of internal organs in a rat abdominal cavity and inspect the entire human oral cavity.

Optical resolution photoacoustic microscopy based on multimode fibers

Photoacoustic microscopy (PAM) is a multiscale imaging technique. In optical-resolution photoacoustic microscopy (OR-PAM), a single mode (SM) fiber is normally used as the source of optical excitation to be focused into a diffraction-limited spot. Recent advances in OR-PAM have improved its imaging speed using microelectromechanical systems (MEMS). Here we report for the first time the use of a multimode (MM) fiber as the optical excitation source for high resolution OR-PAM in vivo imaging. A high-speed MEMS scanner based OR-PAM system combined with the mechanical movement to provide wide area imaging was used. The use of multimode fiber for achieving tight optical focus would make the optical alignment easier and high repetition rate light delivery possible for high-speed OR-PAM imaging. A lateral resolution of 3.5 µm and axial resolution of 27 µm with ~1.5 mm imaging depth was successfully demonstrated using the system. The efficacy of multimode fibers for achieving tight focus is beneficial for developing high-resolution photoacoustic endoscopy systems and can be combined with other optical endoscopic imaging modalities as well.

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.

Large-field-of-view optical resolution photoacoustic microscopy

The use of existing optical resolution photoacoustic microscopy (ORPAM) has been limited to small organs or part of large organs due to the millimeter-scale field of view (FOV) in both lateral and axial directions. Here, we report a large-field-of-view ORPAM (L-ORPAM) using a combination of a new scanning mechanism and an ultrafast pulsed laser. Phantom and in vivo experiments show that L-ORPAM has a spatial FOV of 40 mm in lateral and 12 mm in axial, which expends the effective imaging domain to one order that of existing ORPAMs. To show the advantages of L-ORPAM, we apply it to imaging vasculatures of both brain and ears simultaneously in mice, and to visualizing intestinal vasculatures in rats. The result suggests that L-ORPAM has sufficient contrast, resolution and spatial FOV to carry out studies of large rodents.

Inverted multiscale optical resolution photoacoustic microscopy

Optical resolution photoacoustic microscopy (ORPAM) is one of the fastest evolving photoacoustic imaging modalities. It not only has a comparable spatial resolution to pure optical microscopies such as epifluorescence microscopy, confocal microscopy, and two-photon microscopy, but also owns a higher penetration depth. In this study, we report an inverted multiscale (IM)-ORPAM that utilizes a two-dimensional galvanometric scanner integrated with two microscopic imaging lenses to achieve rotary-scanning-based multiscale imaging. A 15 MHz cylindrically focused ultrasonic transducer is mounted on a motorized rotator to synchronously follow the optical scanning paths. To minimize the loss of signal-to-noise ratio, the acoustic focal line is precisely adjusted confocal with the optical focal plane. Black tapes, carbon fibers, and sharp blades are imaged to evaluate the performance of the system, and in vivo multiscale imaging of vasculatures inside the ears and brains of mice is demonstrated using this system.

Development of low-cost photoacoustic imaging systems using very low-energy pulsed laser diodes

With the growing application of photoacoustic imaging (PAI) in medical fields, there is a need to make them more compact, portable, and affordable. Therefore, we designed very low-cost PAI systems by replacing the expensive and sophisticated laser with a very low-energy laser diode. We implemented photoacoustic (PA) microscopy, both reflection and transmission modes, as well as PA computed tomography systems. The images obtained from tissue-mimicking phantoms and biological samples determine the feasibility of using a very low-energy laser diode in these configurations.

Photoacoustic microscopy in vivo using synthetic-aperture focusing technique combined with three-dimensional deconvolution

Acoustic-resolution photoacoustic microscopy (ARPAM) plays an important role in studying the microcirculation system of biological tissues with deep penetration. High lateral resolution of ARPAM is achieved by using a high numerical aperture acoustic transducer. The deteriorated lateral resolution in the out-of-focus region can be alleviated by synthetic aperture focusing technique (SAFT). Previously, we reported a three-dimensional (3D) deconvolution ARPAM to improve both lateral and axial resolutions in the focus region. In this study, we present our extension of resolution enhancement to the out-of-focus region based on two-dimensional SAFT combined with the 3D deconvolution (SAFT+Deconv). In both the focus and out-of-focus regions, depth-independent lateral resolution provided by SAFT, together with inherently depth-independent axial resolution, ensures a depth-independent point spread function for 3D deconvolution algorithm. Imaging of 10 μm polymer beads shows that SAFT+Deconv ARPAM improves the -6 dB lateral resolutions from 65-700 μm to 20-29 μm, and the -6 dB axial resolutions from 35-42 μm to 12-19 μm in an extended depth of focus (DOF) of ∼2 mm. The signal-to-noise ratio is also increased by 6-30 dB. The resolution enhancement in three dimensions is validated by in vivo imaging of a mouse's dorsal subcutaneous microvasculature. Our results suggest that SAFT+Deconv ARPAM may allow fine spatial resolution with deep penetration and extended DOF for biomedical photoacoustic applications.

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.

Advances in Imaging Techniques and Genetically Encoded Probes for Photoacoustic Imaging

Photoacoustic (PA) imaging is a rapidly emerging biomedical imaging modality that is capable of visualizing cellular and molecular functions with high detection sensitivity and spatial resolution in deep tissue. Great efforts and progress have been made on the development of various PA imaging technologies with improved resolution and sensitivity over the past two decades. Various PA probes with high contrast have also been extensively developed, with many important biomedical applications. In comparison with chemical dyes and nanoparticles, genetically encoded probes offer easier labeling of defined cells within tissues or proteins of interest within a cell, have higher stability in vivo, and eliminate the need for delivery of exogenous substances. Genetically encoded probes have thus attracted increasing attention from researchers in engineering and biomedicine. In this review, we aim to provide an overview of the existing PA imaging technologies and genetically encoded PA probes, and describe further improvements in PA imaging techniques and the near-infrared photochromic protein BphP1, the most sensitive genetically encoded probe thus far, as well as the potential biomedical applications of BphP1-based PA imaging in vivo.

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.

Fully integrated reflection-mode photoacoustic, two-photon, and second harmonic generation microscopy in vivo

The ability to obtain comprehensive structural and functional information from intact biological tissue in vivo is highly desirable for many important biomedical applications, including cancer and brain studies. Here, we developed a fully integrated multimodal microscopy that can provide photoacoustic (optical absorption), two-photon (fluorescence), and second harmonic generation (SHG) information from tissue in vivo, with intrinsically co-registered images. Moreover, using a delicately designed optical-acoustic coupling configuration, a high-frequency miniature ultrasonic transducer was integrated into a water-immersion optical objective, thus allowing all three imaging modalities to provide a high lateral resolution of ~290 nm with reflection-mode imaging capability, which is essential for studying intricate anatomy, such as that of the brain. Taking advantage of the complementary and comprehensive contrasts of the system, we demonstrated high-resolution imaging of various tissues in living mice, including microvasculature (by photoacoustics), epidermis cells, cortical neurons (by two-photon fluorescence), and extracellular collagen fibers (by SHG). The intrinsic image co-registration of the three modalities conveniently provided improved visualization and understanding of the tissue microarchitecture. The reported results suggest that, by revealing complementary tissue microstructures in vivo, this multimodal microscopy can potentially facilitate a broad range of biomedical studies, such as imaging of the tumor microenvironment and neurovascular coupling.

MHz-ultrasound generation by chirped femtosecond laser pulses from gold nano-colloidal suspensions

Strong absorption of femtosecond laser pulses in Au nano-colloidal suspensions was used to generate coherent ultrasound signals at 1-20 MHz frequency range. The most efficient ultrasound generation was observed at negative chirp values and was proportional to the pulse duration. Maximization of a dimensionless factor A ≡ αc₀tp defined as the ratio of pulse duration tp and the time required for sound at speed c₀ to cross the optical energy deposition length (an inverse of the absorption coefficient α) given by 1/(αc₀). Chirp controlled pulse duration allows effective enhancement of ultrasound generation at higher frequencies (shorter wavelengths) and is promising for a high spatial resolution acoustic imaging.

Toward in vivo biopsy of melanoma based on photoacoustic and ultrasound dual imaging with an integrated detector

Melanoma is the most dangerous type of skin cancer with high lethal rate. Tumor thickness and tumor-associated vasculature are two key parameters for staging melanoma. Previous techniques for diagnosing melanoma have insurmountable restrictions, such as invasive, low specificity, or inaccurate depth measurement. Here we develop an integrated photoacoustic (PA) and ultrasound (US) imaging system dedicated to overcome these limitations. An integrated detector with sound-light coaxial/confocal design and flexible coupling mode is employed for the combined PA/US imaging strategy. PA imaging results enable a clear characterization of tumor angiogenesis with high resolution and high contrast. Furthermore, accurate thickness measurements of melanoma in different stages can be resolved with the simultaneously obtained PA/US image. Phantom experiments and in vivo animal experimental results demonstrate the integrated PA/US system could provide potential for noninvasive biopsy of melanoma.