Wide-field fast-scanning photoacoustic microscopy based on a water-immersible MEMS scanning mirror

Reconstructing Cancellous Bone From Down-Sampled Optical-Resolution Photoacoustic Microscopy Images With Deep Learning

OBJECTIVE

Bone diseases deteriorate the microstructure of bone tissue. Optical-resolution photoacoustic microscopy (OR-PAM) enables high spatial resolution of imaging bone tissues. However, the spatiotemporal trade-off limits the application of OR-PAM. The purpose of this study was to improve the quality of OR-PAM images without sacrificing temporal resolution.

METHODS

In this study, we proposed the Photoacoustic Dense Attention U-Net (PADA U-Net) model, which was used for reconstructing full-scanning images from under-sampled images. Thereby, this approach breaks the trade-off between imaging speed and spatial resolution.

RESULTS

The proposed method was validated on resolution test targets and bovine cancellous bone samples to demonstrate the capability of PADA U-Net in recovering full-scanning images from under-sampled OR-PAM images. With a down-sampling ratio of [4, 1], compared to bilinear interpolation, the Peak Signal-to-Noise Ratio and Structural Similarity Index Measure values (averaged over the test set of bovine cancellous bone) of the PADA U-Net were improved by 2.325 dB and 0.117, respectively.

CONCLUSION

The results demonstrate that the PADA U-Net model reconstructed the OR-PAM images well with different levels of sparsity. Our proposed method can further facilitate early diagnosis and treatment of bone diseases using OR-PAM.

Unsupervised deep learning enables real-time image registration of fast-scanning optical-resolution photoacoustic microscopy

A fast scanner of optical-resolution photoacoustic microscopy is inherently vulnerable to perturbation, leading to severe image distortion and significant misalignment among multiple 2D or 3D images. Restoration and registration of these images is critical for accurately quantifying dynamic information in long-term imaging. However, traditional registration algorithms face a great challenge in computational throughput. Here, we develop an unsupervised deep learning based registration network to achieve real-time image restoration and registration. This method can correct artifacts from B-scan distortion and remove misalignment among adjacent and repetitive images in real time. Compared with conventional intensity based registration algorithms, the throughput of the developed algorithm is improved by 50 times. After training, the new deep learning method performs better than conventional feature based image registration algorithms. The results show that the proposed method can accurately restore and register the images of fast-scanning photoacoustic microscopy in real time, offering a powerful tool to extract dynamic vascular structural and functional information.

Sounding out the dynamics: a concise review of high-speed photoacoustic microscopy

Significance

Photoacoustic microscopy (PAM) offers advantages in high-resolution and high-contrast imaging of biomedical chromophores. The speed of imaging is critical for leveraging these benefits in both preclinical and clinical settings. Ongoing technological innovations have substantially boosted PAM's imaging speed, enabling real-time monitoring of dynamic biological processes.

Aim

This concise review synthesizes historical context and current advancements in high-speed PAM, with an emphasis on developments enabled by ultrafast lasers, scanning mechanisms, and advanced imaging processing methods.

Approach

We examine cutting-edge innovations across multiple facets of PAM, including light sources, scanning and detection systems, and computational techniques and explore their representative applications in biomedical research.

Results

This work delineates the challenges that persist in achieving optimal high-speed PAM performance and forecasts its prospective impact on biomedical imaging.

Conclusions

Recognizing the current limitations, breaking through the drawbacks, and adopting the optimal combination of each technology will lead to the realization of ultimate high-speed PAM for both fundamental research and clinical translation.

Influence mechanism of the temporal duration of laser irradiation on photoacoustic technique: a review

Significance

In the photoacoustic (PA) technique, the laser irradiation in the time domain (i.e., laser pulse duration) governs the characteristics of PA imaging-it plays a crucial role in the optical-acoustic interaction, the generation of PA signals, and the PA imaging performance.

Aim

We aim to provide a comprehensive analysis of the impact of laser pulse duration on various aspects of PA imaging, encompassing the signal-to-noise ratio, the spatial resolution of PA imaging, the acoustic frequency spectrum of the acoustic wave, the initiation of specific physical phenomena, and the photothermal-PA (PT-PA) interaction/conversion.

Approach

By surveying and reviewing the state-of-the-art investigations, we discuss the effects of laser pulse duration on the generation of PA signals in the context of biomedical PA imaging with respect to the aforementioned aspects.

Results

First, we discuss the impact of laser pulse duration on the PA signal amplitude and its correlation with the lateral resolution of PA imaging. Subsequently, the relationship between the axial resolution of PA imaging and the laser pulse duration is analyzed with consideration of the acoustic frequency spectrum. Furthermore, we examine the manipulation of the pulse duration to trigger physical phenomena and its relevant applications. In addition, we elaborate on the tuning of the pulse duration to manipulate the conversion process and ratio from the PT to PA effect.

Conclusions

We contribute to the understanding of the physical mechanisms governing pulse-width-dependent PA techniques. By gaining insight into the mechanism behind the influence of the laser pulse, we can trigger the pulse-with-dependent physical phenomena for specific PA applications, enhance PA imaging performance in biomedical imaging scenarios, and modulate PT-PA conversion by tuning the pulse duration precisely.

Photoacoustic microscopy for real-time monitoring of near-infrared optical absorbers inside biological tissue

Significance

We developed a high-speed optical-resolution photoacoustic microscopy (OR-PAM) system using a high-repetition-rate supercontinuum (SC) light source and a two-axes Galvano scanner. The OR-PAM system enabled real-time imaging of optical absorbers inside biological tissues with excellent excitation wavelength tunability.

Aim

In the near-infrared (NIR) wavelength range, high-speed OR-PAM faces limitations due to the lack of wavelength-tunable light sources. Our study aimed to enable high-speed OR-PAM imaging of various optical absorbers, including NIR contrast agents, and validate the performance of high-speed OR-PAM in the detection of circulating tumor cells (CTCs).

Approach

A high-repetition nanosecond pulsed SC light source was used for OR-PAM. The excitation wavelength was adjusted by bandpass filtering of broadband light pulses produced by an SC light source. Phantom and in vivo experiments were performed to detect tumor cells stained with an NIR contrast agent within flowing blood samples.

Results

The newly developed high-speed OR-PAM successfully detected stained cells both in the phantom and in vivo. The phantom experiment confirmed the correlation between the tumor cell detection rate and tumor cell concentration in the blood sample.

Conclusions

The high-speed OR-PAM effectively detected stained tumor cells. Combining high-speed OR-PAM with molecular probes that stain tumor cells in vivo enables in vivo CTC detection.

Non-uniform image reconstruction for fast photoacoustic microscopy of histology imaging

Photoacoustic microscopic imaging utilizes the characteristic optical absorption properties of pigmented materials in tissues to enable label-free observation of fine morphological and structural features. Since DNA/RNA can strongly absorb ultraviolet light, ultraviolet photoacoustic microscopy can highlight the cell nucleus without complicated sample preparations such as staining, which is comparable to the standard pathological images. Further improvements in the imaging acquisition speed are critical to advancing the clinical translation of photoacoustic histology imaging technology. However, improving the imaging speed with additional hardware is hampered by considerable costs and complex design. In this work, considering heavy redundancy in the biological photoacoustic images that overconsume the computing power, we propose an image reconstruction framework called non-uniform image reconstruction (NFSR), which exploits an object detection network to reconstruct low-sampled photoacoustic histology images into high-resolution images. The sampling speed of photoacoustic histology imaging is significantly improved, saving 90% of the time cost. Furthermore, NFSR focuses on the reconstruction of the region of interest while maintaining high PSNR and SSIM evaluation indicators of more than 99% but reducing the overall computation by 60%.

In vivo characterization and analysis of glioblastoma at different stages using multiscale photoacoustic molecular imaging

Simultaneous spatio-temporal description of tumor microvasculature, blood-brain barrier, and immune activity is pivotal to understanding the evolution mechanisms of highly aggressive glioblastoma, one of the most common primary brain tumors in adults. However, the existing intravital imaging modalities are still difficult to achieve it in one step. Here, we present a dual-scale multi-wavelength photoacoustic imaging approach cooperative with/without unique optical dyes to overcome this dilemma. Label-free photoacoustic imaging depicted the multiple heterogeneous features of neovascularization in tumor progression. In combination with classic Evans blue assay, the microelectromechanical system based photoacoustic microscopy enabled dynamic quantification of BBB dysfunction. Concurrently, using self-fabricated targeted protein probe (αCD11b-HSA@A1094) for tumor-associated myeloid cells, unparalleled imaging contrast of cells infiltration associated with tumor progression was visualized by differential photoacoustic imaging in the second near-infrared window at dual scale. Our photoacoustic imaging approach has great potential for tumor-immune microenvironment visualization to systematically reveal the tumor infiltration, heterogeneity, and metastasis in intracranial tumors.

Modeling of Rapid Pam Systems Based on Electrothermal Micromirror for High-Resolution Facial Angiography

In this paper, a portable photoacoustic microscopy (PAM) system is proposed based on a large stroke electrothermal micromirror to achieve high resolution and fast imaging. The crucial micromirror in the system realizes a precise and efficient 2-axis control. Two different designs of electrothermal actuators with "O" and "Z" shape are evenly located around the four directions of mirror plate. With a symmetrical structure, the actuator realized single direction drive only. The finite element modelling of both two proposed micromirror has realized a large displacement over 550 μm and the scan angle over ±30.43° at 0-10 V DC excitation. In addition, the steady-state and transient-state response show a high linearity and quick response respectively, which can contribute to a fast and stable imaging. Using the Linescan model, the system achieves an effective imaging area of 1 mm × 3 mm in 14 s and 1 mm × 4 mm in 12 s for the "O" and "Z" types, respectively. The proposed PAM systems have advantages in image resolution and control accuracy, indicating a significant potential in the field of facial angiography.

Three-dimensional reconstructing undersampled photoacoustic microscopy images using deep learning

Spatial sampling density and data size are important determinants of the imaging speed of photoacoustic microscopy (PAM). Therefore, undersampling methods that reduce the number of scanning points are typically adopted to enhance the imaging speed of PAM by increasing the scanning step size. Since undersampling methods sacrifice spatial sampling density, by considering the number of data points, data size, and the characteristics of PAM that provides three-dimensional (3D) volume data, in this study, we newly reported deep learning-based fully reconstructing the undersampled 3D PAM data. The results of quantitative analyses demonstrate that the proposed method exhibits robustness and outperforms interpolation-based reconstruction methods at various undersampling ratios, enhancing the PAM system performance with 80-times faster-imaging speed and 800-times lower data size. The proposed method is demonstrated to be the closest model that can be used under experimental conditions, effectively shortening the imaging time with significantly reduced data size for processing.

High-speed wide-field photoacoustic microscopy using a cylindrically focused transparent high-frequency ultrasound transducer

Combining focused optical excitation and high-frequency ultrasound detection, optical-resolution photoacoustic microscopy (OR-PAM) can provide micrometer-level spatial resolution with millimeter-level penetration depth and has been employed in a variety of biomedical applications. However, it remains a challenge for OR-PAM to achieve a high imaging speed and a large field of view at the same time. In this work, we report a new approach to implement high-speed wide-field OR-PAM, using a cylindrically-focused transparent ultrasound transducer (CFT-UT). The CFT-UT is made of transparent lithium niobate coated with indium-tin-oxide as electrodes. A transparent cylindrical lens is attached to the transducer surface to provide an acoustic focal line with a length of 9 mm. The excitation light can pass directly through the CFT-UT from the above and thus enables a reflection imaging mode. High-speed imaging is achieved by fast optical scanning of the focused excitation light along the CFT-UT focal line. With the confocal alignment of the optical excitation and acoustic detection, a relatively high detection sensitivity is maintained over the entire scanning range. The CFT-UT-based OR-PAM system has achieved a cross-sectional frame rate of 500 Hz over the scanning range of 9 mm. We have characterized the system's performance on phantoms and demonstrated its application on small animal models in vivo. We expect the new CFT-UT-based OR-PAM will find matched biomedical applications that need high imaging speed over a large field of view.

Freehand scanning photoacoustic microscopy with simultaneous localization and mapping

Optical-resolution photoacoustic microscopy offers high-resolution, label-free hemodynamic and functional imaging to many biomedical applications. However, long-standing technical barriers, such as limited field of view, bulky scanning probes, and slow imaging speed, have limited the application of optical-resolution photoacoustic microscopy. Here, we present freehand scanning photoacoustic microscopy (FS-PAM) that can flexibly image various anatomical sites. We develop a compact handheld photoacoustic probe to acquire 3D images with high speed, and great flexibility. The high scanning speed not only enables video camera mode imaging but also allows for the first implementation of simultaneous localization and mapping (SLAM) in photoacoustic microscopy. We demonstrate fast in vivo imaging of some mouse organs, and human oral mucosa. The high imaging speed greatly reduces motion artifacts and distortions from tissue moving, breathing, and unintended handshaking. We demonstrate small-lesion localization in a large region of the brain. FS-PAM offers a flexible high-speed imaging tool with an extendable field of view, enabling more biomedical imaging applications.

Multistage adaptive noise reduction technique for optical resolution photoacoustic microscopy

Photoacoustic microscopy has received great attention due to the benefits of the optical resolution contrast as well as its superior spatial resolution and relatively deep depth. Like other imaging modalities, photoacoustic images suffer from noise, and filtering techniques are required to remove them. To overcome the noise, we proposed a combination of filters, including an adaptive median filter, an effective filter for impulsive noise, and a nonlocal means filter, an effective filter for background noise, for noise removal and image quality enhancement. Our proposed method enhanced the signal-to-noise ratio by 16 dB in an in vivo study compared to the traditional image reconstruction approach and preserved the image detail with minimal blurring, which usually occurs when filtering. These experimental results verified that the proposed adaptive multistage denoising techniques could effectively improve image quality under noisy data acquisition conditions, providing a strong foundation for photoacoustic microscopy with limited laser power.

Metasurface Micro/Nano-Optical Sensors: Principles and Applications

Metasurfaces are 2D artificial materials consisting of arrays of metamolecules, which are exquisitely designed to manipulate light in terms of amplitude, phase, and polarization state with spatial resolutions at the subwavelength scale. Traditional micro/nano-optical sensors (MNOSs) pursue high sensitivity through strongly localized optical fields based on diffractive and refractive optics, microcavities, and interferometers. Although detections of ultra-low concentrations of analytes have already been demonstrated, the label-free sensing and recognition of complex and unknown samples remain challenging, requiring multiple readouts from sensors, e.g., refractive index, absorption/emission spectrum, chirality, etc. Additionally, the reliability of detecting large, inhomogeneous biosamples may be compromised by the limited near-field sensing area from the localization of light. Here, we review recent advances in metasurface-based MNOSs and compare them with counterparts using micro-optics from aspects of physics, working principles, and applications. By virtue of underlying the physics and design flexibilities of metasurfaces, MNOSs have now been endowed with superb performances and advanced functionalities, leading toward highly integrated smart sensing platforms.

Ultrathin, high-speed, all-optical photoacoustic endomicroscopy probe for guiding minimally invasive surgery

Photoacoustic (PA) endoscopy has shown significant potential for clinical diagnosis and surgical guidance. Multimode fibres (MMFs) are becoming increasingly attractive for the development of miniature endoscopy probes owing to their ultrathin size, low cost and diffraction-limited spatial resolution enabled by wavefront shaping. However, current MMF-based PA endomicroscopy probes are either limited by a bulky ultrasound detector or a low imaging speed that hindered their usability. In this work, we report the development of a highly miniaturised and high-speed PA endomicroscopy probe that is integrated within the cannula of a 20 gauge medical needle. This probe comprises a MMF for delivering the PA excitation light and a single-mode optical fibre with a plano-concave microresonator for ultrasound detection. Wavefront shaping with a digital micromirror device enabled rapid raster-scanning of a focused light spot at the distal end of the MMF for tissue interrogation. High-resolution PA imaging of mouse red blood cells covering an area 100 µm in diameter was achieved with the needle probe at ∼3 frames per second. Mosaicing imaging was performed after fibre characterisation by translating the needle probe to enlarge the field-of-view in real-time. The developed ultrathin PA endomicroscopy probe is promising for guiding minimally invasive surgery by providing functional, molecular and microstructural information of tissue in real-time.

Low-cost high-resolution photoacoustic microscopy of blood oxygenation with two laser diodes

Optical-resolution photoacoustic microscopy (OR-PAM) has been widely used for imaging blood vessel and oxygen saturation of hemoglobin (sO₂), providing high-resolution functional images of living animals in vivo. However, most of them require one or multiple bulky and costly pulsed lasers, hindering their applicability in preclinical and clinical settings. In this paper, we demonstrate a reflection-mode low-cost high-resolution OR-PAM system by using two cost-effective and compact laser diodes (LDs), achieving microvasculature and sO₂ imaging with a high lateral resolution of ∼6 µm. The cost of the excitation sources has dramatically reduced by ∼20-40 times compared to that of the pulsed lasers used in state-of-the-art OR-PAM systems. A blood phantom study was performed to show a determination coefficient R ² of 0.96 in linear regression analysis. Experimental results of in vivo mouse ear imaging show that the proposed dual-wavelength LD-based PAM system can provide high-resolution functional images at a low cost.

Visualizing tumor angiogenesis and boundary with polygon-scanning multiscale photoacoustic microscopy

Recently, we developed an integrated optical-resolution (OR) and acoustic-resolution (AR) PAM, which has multiscale imaging capability using different resolutions. However, limited by the scanning method, a tradeoff exists between the imaging speed and field of view, which impedes its wider applications. Here, we present an improved multiscale PAM which achieves high-speed wide-field imaging based on a homemade polygon scanner. Encoder trigger mode was proposed to avoid jittering of the polygon scanner during imaging. Distortions caused by polygon scanning were analyzed theoretically and compared with traditional types of distortions in optical-scanning PAM. Then a depth correction method was proposed and verified to compensate for the distortions. System characterization of OR-PAM and AR-PAM was performed prior to in vivo imaging. Blood reperfusion of an in vivo mouse ear was imaged continuously to demonstrate the feasibility of the multiscale PAM for high-speed imaging. Results showed that the maximum B-scan rate could be 14.65 Hz in a fixed range of 10 mm. Compared with our previous multiscale system, the imaging speed of the improved system was increased by a factor of 12.35. In vivo imaging of a subcutaneously inoculated B-16 melanoma of a mouse was performed. Results showed that the blood vasculature around the melanoma could be resolved and the melanoma could be visualized at a depth up to 1.6 mm using the multiscale PAM.

Real-time whole-brain imaging of hemodynamics and oxygenation at micro-vessel resolution with ultrafast wide-field photoacoustic microscopy

High-speed high-resolution imaging of the whole-brain hemodynamics is critically important to facilitating neurovascular research. High imaging speed and image quality are crucial to visualizing real-time hemodynamics in complex brain vascular networks, and tracking fast pathophysiological activities at the microvessel level, which will enable advances in current queries in neurovascular and brain metabolism research, including stroke, dementia, and acute brain injury. Further, real-time imaging of oxygen saturation of hemoglobin (sO₂) can capture fast-paced oxygen delivery dynamics, which is needed to solve pertinent questions in these fields and beyond. Here, we present a novel ultrafast functional photoacoustic microscopy (UFF-PAM) to image the whole-brain hemodynamics and oxygenation. UFF-PAM takes advantage of several key engineering innovations, including stimulated Raman scattering (SRS) based dual-wavelength laser excitation, water-immersible 12-facet-polygon scanner, high-sensitivity ultrasound transducer, and deep-learning-based image upsampling. A volumetric imaging rate of 2 Hz has been achieved over a field of view (FOV) of 11 × 7.5 × 1.5 mm³ with a high spatial resolution of ~10 μm. Using the UFF-PAM system, we have demonstrated proof-of-concept studies on the mouse brains in response to systemic hypoxia, sodium nitroprusside, and stroke. We observed the mouse brain's fast morphological and functional changes over the entire cortex, including vasoconstriction, vasodilation, and deoxygenation. More interestingly, for the first time, with the whole-brain FOV and micro-vessel resolution, we captured the vasoconstriction and hypoxia simultaneously in the spreading depolarization (SD) wave. We expect the new imaging technology will provide a great potential for fundamental brain research under various pathological and physiological conditions.

Performance comparison of high-speed photoacoustic microscopy: opto-ultrasound combiner versus ring-shaped ultrasound transducer

Photoacoustic microscopy (PAM) embedded with a 532 nm pulse laser is widely used to visualize the microvascular structures in both small animals and humans in vivo. An opto-ultrasound combiner (OUC) is often utilized in high-speed PAM to confocally align the optical and acoustic beams to improve the system's sensitivity. However, acoustic impedance mismatch in the OUC results in little improvement in the sensitivity. Alternatively, a ring-shaped ultrasound transducer (RUT) can also accomplish the confocal configuration. Here, we compare the performance of OUC and RUT modules through ultrasound pulse-echo tests and PA imaging experiments. The signal-to-noise ratios (SNRs) of the RUT-based system were 15 dB, 12 dB, and 7 dB higher when compared to the OUC-based system for ultrasound pulse-echo test, PA phantom imaging test, and PA in-vivo imaging test, respectively. In addition, the RUT-based system could image the microvascular structures of small parts of a mouse body in a few seconds with minimal loss in SNR. Thus, with increased sensitivity, improved image details, and fast image acquisition, we believe the RUT-based systems could play a significant role in the design of future fast-PAM systems.

Tissue-mimicking phantoms for performance evaluation of photoacoustic microscopy systems

Phantom-based performance test methods are critically needed to support development and clinical translation of emerging photoacoustic microscopy (PAM) devices. While phantoms have been recently developed for macroscopic photoacoustic imaging systems, there is an unmet need for well-characterized tissue-mimicking materials (TMMs) and phantoms suitable for evaluating PAM systems. Our objective was to develop and characterize a suitable dermis-mimicking TMM based on polyacrylamide hydrogels and demonstrate its utility for constructing image quality phantoms. TMM formulations were optically characterized over 400-1100 nm using integrating sphere spectrophotometry and acoustically characterized using a pulse through-transmission method over 8-24 MHz with highly confident extrapolation throughout the usable band of the PAM system. This TMM was used to construct a spatial resolution phantom containing gold nanoparticle point targets and a penetration depth phantom containing slanted tungsten filaments and blood-filled tubes. These phantoms were used to characterize performance of a custom-built PAM system. The TMM was found to be broadly tunable and specific formulations were identified to mimic human dermis at an optical wavelength of 570 nm and acoustic frequencies of 10-50 MHz. Imaging results showed that tungsten filaments yielded 1.1-4.2 times greater apparent maximum imaging depth than blood-filled tubes, which may overestimate real-world performance for vascular imaging applications. Nanoparticles were detectable only to depths of 120-200 µm, which may be due to the relatively weaker absorption of single nanoparticles vs. larger targets containing high concentration of hemoglobin. The developed TMMs and phantoms are useful tools to support PAM device characterization and optimization, streamline regulatory decision-making, and accelerate clinical translation.

Photoacoustic imaging of 3D-printed vascular networks

Thrombosis in the circulation system can lead to major myocardial infarction and cardiovascular deaths. Understanding thrombosis formation is necessary for developing safe and effective treatments. In this work, using digital light processing (DLP)-based 3D printing, we fabricated sophisticatedin vitromodels of blood vessels with internal microchannels that can be used for thrombosis studies. In this regard, photoacoustic microscopy (PAM) offers a unique advantage for label-free visualization of the 3D-printed vessel models, with large penetration depth and functional sensitivity. We compared the imaging performances of two PAM implementations: optical-resolution PAM and acoustic-resolution PAM, and investigated 3D-printed vessel structures with different patterns of microchannels. Our results show that PAM can provide clear microchannel structures at depths up to 3.6 mm. We further quantified the blood oxygenation in the 3D-printed vascular models, showing that thrombi had lower oxygenation than the normal blood. We expect that PAM can find broad applications in 3D printing and bioprinting forin vitrostudies of various vascular and other diseases.

Acoustic-resolution photoacoustic microscope based on compact and low-cost delta configuration actuator

The state-of-the-art configurations for acoustic-resolution photoacoustic (PA) microscope (AR-PAM) are large in size and expensive, hindering their democratization. While previous research on AR-PAMs introduced a low-cost light source to reduce the cost, few studies have investigated the possibility of optimizing the sensor actuation, particularly for the AR-PAM. Additionally, there is an unmet need to evaluate the image quality deterioration associated with the actuation inaccuracy. A low-cost actuation device is introduced to reduce the system size and cost of the AR-PAM while maintaining the image quality by implementing the advanced beamformers. This work proposes an AR-RAM incorporating the delta configuration actuator adaptable from a low-cost off-the-shelf 3D printer as the sensor actuation device. The image degradation due to the data acquisition positioning inaccuracy is evaluated in the simulation. We further assess the mitigation of potential actuation precision uncertainty through advanced 3D synthetic aperture focusing algorithms represented by the Delay-and-Sum (DAS) with Coherence Factor (DAS+CF) and Delay-Multiply-and-Sum (DMAS) algorithms. The simulation study demonstrated the tolerance of image quality on actuation inaccuracy and the effect of compensating the actuator motion precision error through advanced reconstruction algorithms. With those algorithms, the image quality degradation was suppressed to within 25% with the presence of 0.2 mm motion inaccuracy. The experimental evaluation using phantoms and an ex-vivo sample presented the applicability of low-cost delta configuration actuators for AR-PAMs. The measured full width at half maximum of the 0.2 mm diameter pencil-lead phantom were 0.45 ± 0.06 mm, 0.31 ± 0.04 mm, and 0.35 ± 0.07 mm, by applying the DAS, DAS+CF, and DMAS algorithms, respectively. AR-PAMs with a compact and low-cost delta configuration provide high-quality PA imaging with better accessibility for biomedical applications. The research evaluated the image degradation contributed by the actuation inaccuracy and suggested that the advanced beamformers are capable of suppressing the actuation inaccuracy.

High-speed functional photoacoustic microscopy using a water-immersible two-axis torsion-bending scanner

Optical-resolution photoacoustic microscopy (OR-PAM) can provide functional, anatomical, and molecular images at micrometer level resolution with an imaging depth of less than 1 mm in tissue. However, the imaging speed of traditional OR-PAM is often low due to the point-by-point mechanical scanning and cannot capture time-sensitive dynamic information. In this work, we demonstrate a recent effort in improving the imaging speed of OR-PAM, using a newly developed water-immersible two-axis scanner. Driven by water-compatible electromagnetic actuation force, the new scanning mirror employs a novel torsion-bending mechanism to achieve fast 2D scanning. The torsion scanning along the fast-axis works in the resonant model, and the bending scanning along the slow-axis operate at the quasi-static mode. The scanning speed and scanning range along the two axes can be independently adjusted. Steered by the two-axis torsion-bending scanning mirror immersed in water, the focused excitation light and the generated acoustic wave can be confocally aligned over the entire imaging area. Thus, a high imaging speed can be achieved without sacrificing the detection sensitivity. Equipped with the torsion-bending scanner, the high-speed OR-PAM system has achieved a cross-sectional frame rate of 400 Hz, and a volumetric imaging speed of 1 Hz over a field of view of 1.5 × 2.5 mm2. We have also demonstrated high-speed OR-PAM of the hemodynamic changes in response to pharmaceutical and physiological challenges in small animal models in vivo. We expect the torsion-bending scanner based OR-PAM will find matched biomedical studies of tissue dynamics.

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.

High-speed photoacoustic microscopy: A review dedicated on light sources

In recent years, many methods have been investigated to improve imaging speed in photoacoustic microscopy (PAM). These methods mainly focused upon three critical factors contributing to fast PAM: laser pulse repetition rate, scanning speed, and computing power of the microprocessors. A high laser repetition rate is fundamentally the most crucial factor to increase the PAM speed. In this paper, we review methods adopted for fast PAM systems in detail, specifically with respect to light sources. To the best of our knowledge, ours is the first review article analyzing the fundamental requirements for developing high-speed PAM and their limitations from the perspective of light sources.

Recent Technical Progression in Photoacoustic Imaging—Towards Using Contrast Agents and Multimodal Techniques

For combining optical and ultrasonic imaging methodologies, photoacoustic imaging (PAI) is the most important and successful hybrid technique, which has greatly contributed to biomedical research and applications. Its theoretical background is based on the photoacoustic effect, whereby a modulated or pulsed light is emitted into tissue, which selectively absorbs the optical energy of the light at optical wavelengths. This energy produces a fast thermal expansion in the illuminated tissue, generating pressure waves (or photoacoustic waves) that can be detected by ultrasonic transducers. Research has shown that optical absorption spectroscopy offers high optical sensitivity and contrast for ingredient determination, for example, while ultrasound has demonstrated good spatial resolution in biomedical imaging. Photoacoustic imaging combines these advantages, i.e., high contrast through optical absorption and high spatial resolution due to the low scattering of ultrasound in tissue. In this review, we focus on advances made in PAI in the last five years and present categories and key devices used in PAI techniques. In particular, we highlight the continuously increasing imaging depth achieved by PAI, particularly when using exogenous reagents. Finally, we discuss the potential of combining PAI with other imaging techniques.

Recent Progress on Molecular Photoacoustic Imaging with Carbon-Based Nanocomposites

For biomedical imaging, the interest in noninvasive imaging methods is ever increasing. Among many modalities, photoacoustic imaging (PAI), which is a combination of optical and ultrasound imaging techniques, has received attention because of its unique advantages such as high spatial resolution, deep penetration, and safety. Incorporation of exogenous imaging agents further amplifies the effective value of PAI, since they can deliver other specified functions in addition to imaging. For these agents, carbon-based materials can show a large specific surface area and interesting optoelectronic properties, which increase their effectiveness and have proved their potential in providing a theragnostic platform (diagnosis + therapy) that is essential for clinical use. In this review, we introduce the current state of the PAI modality, address recent progress on PAI imaging that takes advantage of carbon-based agents, and offer a future perspective on advanced PAI systems using carbon-based agents.

Dual-foci fast-scanning photoacoustic microscopy with 3.2-MHz A-line rate

We report fiber-based dual-foci fast-scanning OR-PAM that can double the scanning rate without compromising the imaging resolution, the field of view, and the detection sensitivity. To achieve fast scanning speed, the OR-PAM system uses a single-axis water-immersible resonant scanning mirror that can confocally scan the optical and acoustic beams at 1018 Hz with a 3-mm range. Pulse energies of 45∼100-nJ are sufficient for acquiring vascular and oxygen-saturation images. The dual-foci method can double the B-scan rate to 2036 Hz. Using two lasers and stimulated Raman scattering, we achieve dual-wavelength excitation on both foci, and the total A-line rate is 3.2-MHz. In in vivo experiments, we inject epinephrine and monitor the hemodynamic and oxygen saturation response in the peripheral vessels at 1.7 Hz over 2.5 × 6.7 mm². Dual-foci OR-PAM offers a new imaging tool for the study of fast physiological and pathological changes.

An Adjustable Dark-Field Acoustic-Resolution Photoacoustic Imaging System with Fiber Bundle-Based Illumination

Photoacoustic (PA) imaging has become one of the major imaging methods because of its ability to record structural information and its high spatial resolution in biological tissues. Current commercialized PA imaging instruments are limited to varying degrees by their bulky size (i.e., the laser or scanning stage) or their use of complex optical components for light delivery. Here, we present a robust acoustic-resolution PA imaging system that consists of four adjustable optical fibers placed 90° apart around a 50 MHz high-frequency ultrasound (US) transducer. In the compact design concept of the PA probe, the relative illumination parameters (i.e., angles and fiber size) can be adjusted to fit different imaging applications in a single setting. Moreover, this design concept involves a user interface built in MATLAB. We first assessed the performance of our imaging system using in vitro phantom experiments. We further demonstrated the in vivo performance of the developed system in imaging (1) rat ear vasculature, (2) real-time cortical hemodynamic changes in the superior sagittal sinus (SSS) during left-forepaw electrical stimulation, and (3) real-time cerebral indocyanine green (ICG) dynamics in rats. Collectively, this alignment-free design concept of a compact PA probe without bulky optical lens systems is intended to satisfy the diverse needs in preclinical PA imaging studies.

Photoacoustic imaging of in vivo hemodynamic responses to sodium nitroprusside

The in vivo hemodynamic impact of sodium nitroprusside (SNP), a widely used antihypertensive agent, has not been well studied. Here, we applied functional optical-resolution photoacoustic microscopy (OR-PAM) to study the hemodynamic responses to SNP in mice in vivo. As expected, after the application of SNP, the systemic blood pressure (BP) was reduced by 53%. The OR-PAM results show that SNP induced an arterial vasodilation of 24% and 23% in the brain and skin, respectively. A weaker venous vasodilation of 9% and 5% was also observed in the brain and skin, respectively. The results show two different types of blood oxygenation response. In mice with decreased blood oxygenation, the arterial and venous oxygenation was respectively reduced by 6% and 13% in the brain, as well as by 7% and 18% in the skin. In mice with increased blood oxygenation, arterial and venous oxygenation was raised by 4% and 22% in the brain, as well as by 1% and 9% in the skin. We observed venous change clearly lagged the arterial change in the skin, but not in the brain. Our results collectively show a correlation among SNP induced changes in systemic BP, vessel size and blood oxygenation.

Deep image prior for undersampling high-speed photoacoustic microscopy

Photoacoustic microscopy (PAM) is an emerging imaging method combining light and sound. However, limited by the laser's repetition rate, state-of-the-art high-speed PAM technology often sacrifices spatial sampling density (i.e., undersampling) for increased imaging speed over a large field-of-view. Deep learning (DL) methods have recently been used to improve sparsely sampled PAM images; however, these methods often require time-consuming pre-training and large training dataset with ground truth. Here, we propose the use of deep image prior (DIP) to improve the image quality of undersampled PAM images. Unlike other DL approaches, DIP requires neither pre-training nor fully-sampled ground truth, enabling its flexible and fast implementation on various imaging targets. Our results have demonstrated substantial improvement in PAM images with as few as 1.4 % of the fully sampled pixels on high-speed PAM. Our approach outperforms interpolation, is competitive with pre-trained supervised DL method, and is readily translated to other high-speed, undersampling imaging modalities.

Progress of clinical translation of handheld and semi-handheld photoacoustic imaging

Photoacoustic imaging (PAI), featuring rich contrast, high spatial/temporal resolution and deep penetration, is one of the fastest-growing biomedical imaging technology over the last decade. To date, numbers of handheld and semi-handheld photoacoustic imaging devices have been reported with corresponding potential clinical applications. Here, we summarize emerged handheld and semi-handheld systems in terms of photoacoustic computed tomography (PACT), optoacoustic mesoscopy (OAMes), and photoacoustic microscopy (PAM). We will discuss each modality in three aspects: laser delivery, scanning protocol, and acoustic detection. Besides new technical developments, we also review the associated clinical studies, and the advantages/disadvantages of these new techniques. In the end, we propose the challenges and perspectives of miniaturized PAI in the future.

Centimeter-scale wide-field-of-view laser-scanning photoacoustic microscopy for subcutaneous microvasculature in vivo

We developed a simple and compact laser-scanning photoacoustic microscopy (PAM) for imaging large areas of subcutaneous microvasculature in vivo. The reflection-mode PAM not only retains the advantage of high scanning speed for optical scanning, but also offers an imaging field-of-view (FOV) up to 20 × 20 mm², which is the largest FOV available in laser-scanning models so far. The lateral resolution of the PAM system was measured to be 17.5 µm. Image experiments on subcutaneous microvasculature in in vivo mouse ears and abdomen demonstrate the system's potential for fast and high-resolution imaging for injuries and diseases of large tissues and organs.

Fast Acoustic Steering Via Tilting Electromechanical Reflectors (FASTER): A Novel Method for High Volume Rate 3-D Ultrasound Imaging

The 3-D ultrasound imaging is essential for a wide range of clinical applications in diagnostic and interventional cardiology, radiology, and obstetrics for prenatal imaging. 3-D ultrasound imaging is also pivotal for advancing technical developments of emerging imaging technologies, such as elastography, blood flow imaging, functional ultrasound (fUS), and super-resolution microvessel imaging. At present, however, existing 3-D ultrasound imaging methods suffer from low imaging volume rate, suboptimal imaging quality, and high costs associated with 2-D ultrasound transducers. Here, we report a novel 3-D ultrasound imaging technique, fast acoustic steering via tilting electromechanical reflectors (FASTER), which provides both high imaging quality and fast imaging speed while at low cost. Capitalizing upon unique water immersible and fast-tilting microfabricated mirror to scan ultrafast plane waves in the elevational direction, FASTER is capable of high volume rate, large field-of-view (FOV) 3-D imaging with conventional 1-D transducers. In this article, we introduce the fundamental concepts of FASTER and present a series of calibration and validation studies for FASTER 3-D imaging. In a wire phantom and tissue-mimicking phantom study, we demonstrated that FASTER was capable of providing spatially accurate 3-D images with a 500-Hz imaging volume rate and an imaging FOV with a range of 48° (20 mm at 25-mm depth) in the elevational direction. We also showed that FASTER had comparable imaging quality with conventional mechanical translation-based 3-D imaging. The principles and results presented in this study establish the technical foundation for the new paradigm of high volume rate 3-D ultrasound imaging based on ultrafast plane waves and fast-tilting, water-immersible microfabricated mirrors.

Wide-field polygon-scanning photoacoustic microscopy of oxygen saturation at 1-MHz A-line rate

We report wide-field polygon-scanning functional OR-PAM that for the first time achieves 1-MHz A-line rate of oxygen saturation in vivo. We address two technical challenges. The first is a 1-MHz dual-wavelength pulsed laser that has sufficient pulse energy and ultrafast wavelength switching. The second is a polygon-scanning imaging probe that has a fast scanning speed, a large field of view, and great sensitivity. The OR-PAM system offers a B-scan rate of 477.5 Hz in a 12-mm range and a volumetric imaging rate of ∼1 Hz over a 12 × 5 mm2 scanning area. We image microvasculature and blood oxygen saturation in a 12 × 12 mm2 scanning area in 5 s. Dynamic imaging of oxygen saturation in the mouse ear is demonstrated to monitor fast response to epinephrine injection. The new wide-field fast functional imaging ability broadens the biomedical application of OR-PAM.

High-speed label-free ultraviolet photoacoustic microscopy for histology-like imaging of unprocessed biological tissues

Ultraviolet photoacoustic microscopy (UV-PAM) has recently been demonstrated as a potential imaging tool for surgical margin analysis (SMA). UV-PAM does not require staining or micrometer-thick slicing, which is inevitable in conventional histological imaging. To promote UV-PAM as a practical intraoperative diagnostic tool, the imaging speed should be improved while preserving the high-resolution imaging capability and simplistic system design. In this Letter, we developed a galvanometer mirror-based UV-PAM (GM-UV-PAM) system for high-speed histology-like imaging. By using a UV laser with a high repetition rate (55 kHz) and a one-dimensional galvanometer mirror, our GM-UV-PAM system can generate subcellular images in less than 15 min for a typical brain biopsy (5mm×5mm), with a lateral resolution of ∼1.0µm. The images of mouse brain slices obtained by our GM-UV-PAM system show that it can provide histological information for SMA.

Pushing the boundaries of optoacoustic microscopy by total impulse response characterization

Optical microscopy improves in resolution and signal-to-noise ratio by correcting for the system’s point spread function; a measure of how a point source is resolved, typically determined by imaging nanospheres. Optical-resolution optoacoustic (photoacoustic) microscopy could be similarly corrected, especially to account for the spatially-dependent signal distortions induced by the acoustic detection and the time-resolved and bi-polar nature of optoacoustic signals. Correction algorithms must therefore include the spatial dependence of signals’ origins and profiles in time, i.e. the four-dimensional total impulse response (TIR). However, such corrections have been so far impeded by a lack of efficient TIR-characterization methods. We introduce high-quality TIR determination based on spatially-distributed optoacoustic point sources (SOAPs), produced by scanning an optical focus on an axially-translatable 250 nm gold layer. Using a spatially-dependent TIR-correction improves the signal-to-noise ratio by >10 dB and the axial resolution by ~30%. This accomplishment displays a new performance paradigm for optoacoustic microscopy.

Characterizing the total impulse response (TIR) of photoacoustic microscopes has been challenging due to difficulties distributing appropriate point sources. Here, the authors present a method for 3D generation of spatially-distributed optoacoustic point sources and show that subsequent TIR correction results in improved image quality.

Wave of single-impulse-stimulated fast initial dip in single vessels of mouse brains imaged by high-speed functional photoacoustic microscopy

SIGNIFICANCE

The initial dip in hemoglobin-oxygenation response to stimulations is a spatially confined endogenous indicator that is faster than the blood flow response, making it a desired label-free contrast to map the neural activity. A fundamental question is whether a single-impulse stimulus, much shorter than the response delay, could produce an observable initial dip without repeated stimulation.

AIM

To answer this question, we report high-speed functional photoacoustic (PA) microscopy to investigate the initial dip in mouse brains.

APPROACH

We developed a Raman-laser-based dual-wavelength functional PA microscope that can image capillary-level blood oxygenation at a 1-MHz one-dimensional imaging rate. This technology was applied to monitor the hemodynamics of mouse cerebral vasculature after applying an impulse stimulus to the forepaw.

RESULTS

We observed a transient initial dip in cerebral microvessels starting as early as 0.13 s after the onset of the stimulus. The initial dip and the subsequent overshoot manifested a wave pattern propagating across different microvascular compartments.

CONCLUSIONS

We quantified both spatially and temporally the single-impulse-stimulated microvascular hemodynamics in mouse brains at single-vessel resolution. Fast label-free imaging of single-impulse response holds promise for real-time brain-computer interfaces.

Electrothermal-MEMS-induced nonlinear distortion correction in photoacoustic laparoscopy

Micro-electro-mechanical systems (MEMS) scanner has significant advantages of miniature size, fast response and high stability, which is particularly applicable to photoacoustic laparoscopy (PAL). However, tilt angle-voltage curve of electrothermal MEMS shows a nonlinear character, which leads to inevitable nonlinear distortion in photoacoustic imaging. To overcome this problem, a nonlinear distortion correction was developed for the high-resolution forward-scanning electrothermal-MEMS-based PAL. The adaptive resampling method (ARM) was introduced to adaptively calibrate the projection of non-uniform scanning region to match the uniform scanning region. The correction performed low time complexity and high portability owing to the adaptive capacity of distortion decomposition in the reconstruction of physical models. Compared with the sample structure, phantom experiments demonstrated that the distortion was calibrated in all directions and the corrected image provided up to 96.82% high structural similarity in local subset. Furthermore, ARM was applied to imaging the abdominal cavity of rat and the vascular morphology was corrected in real-time display within a delay less than 2 seconds. All these results demonstrated that the nonlinear distortion correction possessed timely and effective correction in PAL, which suggested that it had the potential to employ to any other electrothermal-MEMS-based photoacoustic imaging systems for accurate and quantitative functional imaging.

Super Wide-Field Photoacoustic Microscopy of Animals and Humans In Vivo

Acoustic-resolution photoacoustic micro-scopy (AR-PAM) is an emerging biomedical imaging modality that combines superior optical sensitivity and fine ultrasonic resolution in an optical quasi-diffusive regime (~1-3 mm in tissues). AR-PAM has been explored for anatomical, functional, and molecular information in biological tissues. Heretofore, AR-PAM systems have suffered from a limited field-of-view (FOV) and/or slow imaging speed, which have precluded them from routine preclinical and clinical applications. Here, we demonstrate an advanced AR-PAM system that overcomes both limitations of previous AR-PAM systems. The new AR-PAM system demonstrates a super wide-field scanning that utilized a 1-axis water-proofing microelectromechanical systems (MEMS) scanner integrated with two linear stepper motor stages. We achieved an extended FOV of 36 ×80 mm² by mosaicking multiple volumetric images of 36 ×2.5 mm² with a total acquisition time of 224 seconds. For one volumetric data (i.e., 36 ×2.5 mm²), the B-scan imaging speed over the short axis (i.e., 2.5 mm) was 83 Hz in humans. The 3D volumetric image was also provided by using MEMS mirror scanning along the X-axis and stepper-motor scanning along the Y-axis. The super-wide FOV mosaic image was realized by registering and merging all individual volumetric images. Finally, we obtained multi-plane whole-body in-vivo PA images of small animals, illustrating distinct multi-layered structures including microvascular networks and internal organs. Importantly, we also visualized microvascular networks in human fingers, palm, and forearm successfully. This advanced MEMS-AR-PAM system could potentially enable hitherto not possible wide preclinical and clinical applications.

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

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

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

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

Live feedback and 3D photoacoustic remote sensing

Background

As photoacoustic (PA) techniques progress towards clinical adoption, providing a high-speed live feedback becomes a high priority. To keep up with the instantaneous optical feedback of conventional light microscopes, PA imaging would need to provide a high-resolution video-rate live feed to the user. However, conventional PA microscopy typically trades resolution, sensitivity and imaging speed when optically scanning due to the difficult opto-acoustic confocal geometry. Here, we employ photoacoustic remote sensing (PARS), an all-optical technique that relies on optical confocal geometry, to provide a high-resolution live display in a reflection-mode PA architecture.

Methods

Employing a conventional x-y galvanometer scanner and a 600 KHz pulse repetition rate laser we implement a system capable of acquiring 2.5 frames per second in 2D. To complement this fast scanning optical system, we implement a computationally inexpensive image reconstruction method that is able to render the frames with minimal overhead, providing a live display.

Results

Employing the proposed method, we demonstrate a live feedback with frame rates as high as 2.5 Hz in 2D and also report the first results of 3D imaging with a non-contact label-free reflection-mode technique. The method is validated with phantom studies and in-vivo imaging. Employing a repetition rate of 600 KHz, a live feed of carbon fibers is realized with a C-scan rate of 2.5 Hz. The imaging resolution was measured to be 1.2 µm, the highest reported for a real-time reflection-mode architecture. The mean and peak SNR were measured to be 44 and 62 dB respectively in-vivo. 3D visualizations of carbon fiber phantoms and mouse ear microvasculature structure are also demonstrated.

Conclusions

In summary, we present a method that has a small computational overhead for image rendering, resulting in a live display capable of real-time frame rates. We also report the first 3D imaging with a non-contact label-free reflection-mode PA technique. The all-optical confocal geometry required by PARS is significantly easier to implement and maintain than the opto-acoustic geometry of conventional PA microscopy techniques. This results in a system capable of high resolution and sensitivity, imaging at real-time rates. The authors believe this work represents a vital step towards a clinical high-resolution reflection-mode video-rate PA imaging system.

Fully waterproof two-axis galvanometer scanner for enhanced wide-field optical-resolution photoacoustic microscopy

A large field-of-view and fast scanning of photoacoustic microscopy (PAM) relatively have been difficult to obtain due to the water-drowned structure of the system for the transmission of ultrasonic signals. Researchers have widely studied the achievement of a waterproof scanner for dynamic biological applications with a high-resolution and high signal-to-noise ratio. This Letter reports a novel, to the best of our knowledge, waterproof galvanometer scanner-based PAM system with a successfully attainable ${9.0}\;{\rm mm} \times {14.5}\;{\rm mm}$9.0mm×14.5mm scan region, amplitude scan rate of 40 kHz, and spatial resolution of 4.9 µm. The in vivo characterization of a mouse brain in intact-skull microvascular visualization demonstrated its capability in biomedical imaging and is anticipated to be an effective technique for various preclinical and clinical studies.

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.

Toward Operations in a Surgical Scenario: Characterization of a Microgripper via Light Microscopy Approach

Micro Electro Mechanical Systems (MEMS)-Technology based micro mechanisms usually operate within a protected or encapsulated space and, before that, they are fabricated and analyzed within one Scanning Electron Microscope (SEM) vacuum specimen chamber. However, a surgical scenario is much more aggressive and requires several higher abilities in the microsystem, such as the capability of operating within a liquid or wet environment, accuracy, reliability and sophisticated packaging. Unfortunately, testing and characterizing MEMS experimentally without fundamental support of a SEM is rather challenging. This paper shows that in spite of large difficulties due to well-known physical limits, the optical microscope is still able to play an important role in MEMS characterization at room conditions. This outcome is supported by the statistical analysis of two series of measurements, obtained by a light trinocular microscope and a profilometer, respectively.

Super-resolution localization photoacoustic microscopy using intrinsic red blood cells as contrast absorbers

Photoacoustic microscopy (PAM) has become a premier microscopy tool that can provide the anatomical, functional, and molecular information of animals and humans in vivo. However, conventional PAM systems suffer from limited temporal and/or spatial resolution. Here, we present a fast PAM system and an agent-free localization method based on a stable and commercial galvanometer scanner with a custom-made scanning mirror (L-PAM-GS). This novel hardware implementation enhances the temporal resolution significantly while maintaining a high signal-to-noise ratio (SNR). These improvements allow us to photoacoustically and noninvasively observe the microvasculatures of small animals and humans in vivo. Furthermore, the functional hemodynamics, namely, the blood flow rate in the microvasculature, is successfully monitored and quantified in vivo. More importantly, thanks to the high SNR and fast B-mode rate (500 Hz), by localizing photoacoustic signals from captured red blood cells without any contrast agent, unresolved microvessels are clearly distinguished, and the spatial resolution is improved by a factor of 2.5 in vivo. L-PAM-GS has great potential in various fields, such as neurology, oncology, and pathology.

Transvaginal fast-scanning optical-resolution photoacoustic endoscopy

Photoacoustic endoscopy offers in vivo examination of the visceral tissue using endogenous contrast, but its typical B-scan rate is ∼10  Hz, restricted by the speed of the scanning unit and the laser pulse repetition rate. Here, we present a transvaginal fast-scanning optical-resolution photoacoustic endoscope with a 250-Hz B-scan rate over a 3-mm scanning range. Using this modality, we not only illustrated the morphological differences of vasculatures among the human ectocervix, uterine body, and sublingual mucosa but also showed the longitudinal and cross-sectional differences of cervical vasculatures in pregnant women. This technology is promising for screening the visceral pathological changes associated with angiogenesis.

Recent Progress on Photoacoustic Imaging Enhanced with Microelectromechanical Systems (MEMS) Technologies

Photoacoustic imaging (PAI) is a new biomedical imaging technology currently in the spotlight providing a hybrid contrast mechanism and excellent spatial resolution in the biological tissues. It has been extensively studied for preclinical and clinical applications taking advantage of its ability to provide anatomical and functional information of live bodies noninvasively. Recently, microelectromechanical systems (MEMS) technologies, particularly actuators and sensors, have contributed to improving the PAI system performance, further expanding the research fields. This review introduces cutting-edge MEMS technologies for PAI and summarizes the recent advances of scanning mirrors and detectors in MEMS.