In vivo photoacoustic microscopy with 7.6-µm axial resolution using a commercial 125-MHz ultrasonic transducer

Optical ultrasound sensors for photoacoustic imaging: a review

Significance

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

Aim

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

Approach

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

Results

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

Conclusions

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

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.

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

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

Spatiotemporal singular value decomposition for denoising in photoacoustic imaging with a low-energy excitation light source

Photoacoustic (PA) imaging is an emerging hybrid imaging modality that combines rich optical spectroscopic contrast and high ultrasonic resolution, and thus holds tremendous promise for a wide range of pre-clinical and clinical applications. Compact and affordable light sources such as light-emitting diodes (LEDs) and laser diodes (LDs) are promising alternatives to bulky and expensive solid-state laser systems that are commonly used as PA light sources. These could accelerate the clinical translation of PA technology. However, PA signals generated with these light sources are readily degraded by noise due to the low optical fluence, leading to decreased signal-to-noise ratio (SNR) in PA images. In this work, a spatiotemporal singular value decomposition (SVD) based PA denoising method was investigated for these light sources that usually have low fluence and high repetition rates. The proposed method leverages both spatial and temporal correlations between radiofrequency (RF) data frames. Validation was performed on simulations and in vivo PA data acquired from human fingers (2D) and forearm (3D) using a LED-based system. Spatiotemporal SVD greatly enhanced the PA signals of blood vessels corrupted by noise while preserving a high temporal resolution to slow motions, improving the SNR of in vivo PA images by 90.3%, 56.0%, and 187.4% compared to single frame-based wavelet denoising, averaging across 200 frames, and single frame without denoising, respectively. With a fast processing time of SVD (∼50 µs per frame), the proposed method is well suited to PA imaging systems with low-energy excitation light sources for real-time in vivo applications.

Two octaves spanning photoacoustic microscopy

In this study, for the first time, a Photoacoustic Microscopy instrument driven by a single optical source operating over a wide spectral range (475-2400 nm), covering slightly more than two octaves is demonstrated. Xenopus laevis tadpoles were imaged in vivo using the whole spectral range of 2000 nm of a supercontinuum optical source, and a novel technique of mapping absorbers is also demonstrated, based on the supposition that only one chromophore contributes to the photoacoustic signal of each individual voxel in the 3D photoacoustic image. By using a narrow spectral window (of 25 nm bandwidth) within the broad spectrum of the supercontinuum source at a time, in vivo hyper-spectral Photoacoustic images of tadpoles are obtained. By post-processing pairs of images obtained using different spectral windows, maps of five endogenous contrast agents (hemoglobin, melanin, collagen, glucose and lipids) are produced.

A Review of High-Frequency Ultrasonic Transducers for Photoacoustic Imaging Applications

Photoacoustic imaging (PAI) is a new and rapidly growing hybrid biomedical imaging modality that combines the virtues of both optical and ultrasonic (US) imaging. The nature of the interaction between light and ultrasound waves allows PAI to make good use of the rich contrast produced by optics while retaining the imaging depths in US imaging. High-frequency US transducers are an important part of the PAI systems, used to detect the high-frequency and broad-bandwidth photoacoustic signals excited by the target tissues irradiated by short laser pulses. Advancement in high-frequency US transducer technology has influenced the boost of PAI to broad applications. Here, we present a review on high-frequency US transducer technologies for PAI applications, including advanced piezoelectric materials and representative transducers. In addition, we discuss the new challenges and directions facing the development of high-frequency US transducers for PAI applications.

High-fidelity deconvolution for acoustic-resolution photoacoustic microscopy enabled by convolutional neural networks

Acoustic-resolution photoacoustic microscopy (AR-PAM) image resolution is determined by the point spread function (PSF) of the imaging system. Previous algorithms, including Richardson-Lucy (R-L) deconvolution and model-based (MB) deconvolution, improve spatial resolution by taking advantage of the PSF as prior knowledge. However, these methods encounter the problems of inaccurate deconvolution, meaning the deconvolved feature size and the original one are not consistent (e.g., the former can be smaller than the latter). We present a novel deep convolution neural network (CNN)-based algorithm featuring high-fidelity recovery of multiscale feature size to improve lateral resolution of AR-PAM. The CNN is trained with simulated image pairs of line patterns, which is to mimic blood vessels. To investigate the suitable CNN model structure and elaborate on the effectiveness of CNN methods compared with non-learning methods, we select five different CNN models, while R-L and directional MB methods are also applied for comparison. Besides simulated data, experimental data including tungsten wires, leaf veins, and in vivo blood vessels are also evaluated. A custom-defined metric of relative size error (RSE) is used to quantify the multiscale feature recovery ability of different methods. Compared to other methods, enhanced deep super resolution (EDSR) network and residual in residual dense block network (RRDBNet) model show better recovery in terms of RSE for tungsten wires with diameters ranging from 30 μ m to 120 μ m . Moreover, AR-PAM images of leaf veins are tested to demonstrate the effectiveness of the optimized CNN methods (by EDSR and RRDBNet) for complex patterns. Finally, in vivo images of mouse ear blood vessels and rat ear blood vessels are acquired and then deconvolved, and the results show that the proposed CNN method (notably RRDBNet) enables accurate deconvolution of multiscale feature size and thus good fidelity.

Virtual optical-resolution photoacoustic microscopy using the k-Wave method

Deep learning has been widely used in image processing, quantitative analysis, and other applications in optical-resolution photoacoustic microscopy (OR-PAM). It requires a large amount of photoacoustic data for training and testing. However, due to the complex structure, high cost, slow imaging speed, and other factors of OR-PAM, it is difficult to obtain enough data required by deep learning, which limits the research of deep learning in OR-PAM to a certain extent. To solve this problem, a virtual OR-PAM based on k-Wave is proposed. The virtual photoacoustic microscopy mainly includes the setting of excitation light source and ultrasonic probe, scanning and signal processing, which can realize the common Gaussian-beam and Bessel-beam OR-PAMs. The system performance (lateral resolution, axial resolution, and depth of field) was tested by imaging a vertically tilted fiber, and the effectiveness and feasibility of the virtual simulation platform were verified by 3D imaging of the virtual vascular network. The ability to the generation of the dataset for deep learning was also verified. The construction of the virtual OR-PAM can promote the research of OR-PAM and the application of deep learning in OR-PAM.

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.

High-resolution in vivo imaging of rhesus cerebral cortex with ultrafast portable photoacoustic microscopy

Revealing the structural and functional change of microvasculature is essential to match vascular response with neuronal activities in the investigation of neurovascular coupling. The increasing use of rhesus models in fundamental and clinical studies of neurovascular coupling presents an emerging need for a new imaging modality. Here we report a structural and functional cerebral vascular study of rhesus monkeys using an ultrafast, portable, and high resolution photoacoustic microscopic system with a long working distance and a special scanning mechanism to eliminate the relative displacement between the imaging interface and samples. We derived the structural and functional response of the cerebral vasculature to the alternating normoxic and hypoxic conditions by calculating the vascular diameter and functional connectivity. Both vasodilatation and vasoconstriction were observed in hypoxia. In addition to the change of vascular diameter, the decrease of functional connectivity is also an important phenomenon induced by the reduction of oxygen ventilatory. These results suggest that photoacoustic microscopy is a promising method to study the neurovascular coupling and cerebral vascular diseases due to the advanced features of high spatiotemporal resolution, excellent sensitivity to hemoglobin, and label-free imaging capability of observing hemodynamics.

On-Chip Multicolor Photoacoustic Imaging Flow Cytometry

On-chip imaging flow cytometry has been widely used in cancer biology, immunology, microbiology, and drug discovery. Pure optical imaging combined with flow cytometry to derive chemical, structural, and morphological features of cells provides systematic insights into biological processes. However, due to the high concentration and strong optical attenuation of red blood cells, preprocessing is necessary for optical flow cytometry while dealing with whole blood. In this study, we develop an on-chip photoacoustic imaging flow cytometry (PAIFC), which combines multicolor high-speed photoacoustic microscopy and microfluidics for cell imaging. The device employs a micro-optical scanner to achieve a miniaturized outer size of 30 × 17 × 24 mm³ and ultrafast cross-sectional imaging at a frame rate of 1758 Hz and provides lateral and axial resolutions of 2.2 and 33 μm, respectively. Using a multicolor strategy, PAIFC is able to differentiate cells labeled by external contrast agents, detect melanoma cells with an endogenous contrast in whole blood, and image melanoma cells in blood samples from tumor-bearing mice. The results suggest that PAIFC has sufficient sensitivity and specificity for future cell-on-chip applications.

Technical considerations in the Verasonics research ultrasound platform for developing a photoacoustic imaging system

Photoacoustic imaging (PAI) is an emerging functional and molecular imaging technology that has attracted much attention in the past decade. Recently, many researchers have used the vantage system from Verasonics for simultaneous ultrasound (US) and photoacoustic (PA) imaging. This was the motivation to write on the details of US/PA imaging system implementation and characterization using Verasonics platform. We have discussed the experimental considerations for linear array based PAI due to its popularity, simple setup, and high potential for clinical translatability. Specifically, we describe the strategies of US/PA imaging system setup, signal generation, amplification, data processing and study the system performance.

Photoacoustic Tomography Opening New Paradigms in Biomedical Imaging

After the emergence of the ultrasound, X-ray CT, PET, and MRI, photoacoustic tomography (PAT) is now in the phase of its exponential growth, with its expected full maturation being another form of mainstream clinical imaging modality. By combining the high contrast benefit of optical imaging and the high-resolution deep imaging capability of ultrasound, PAT can provide unprecedented anatomical image contrasts at clinically relevant depths as well as enable the use of a variety of functional and molecular imaging information, which is not possible with conventional imaging modalities. With these strengths, PAT has achieved numerous breakthroughs in various biomedical applications and also provided new technical platforms that may be able to resolve unmet issues in clinics. In this chapter, we provide an overview of the development of PAT technology for several major biomedical applications and provide an approximate projection of the future of PAT.

Towards non-contact photoacoustic imaging [review]

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

Photoacoustic imaging with fiber optic technology: A review

Photoacoustic imaging (PAI) has achieved remarkable growth in the past few decades since it takes advantage of both optical and ultrasound (US) imaging. In order to better promote the wide clinical applications of PAI, many miniaturized and portable PAI systems have recently been proposed. Most of these systems utilize fiber optic technologies. Here, we overview the fiber optic technologies used in PAI. This paper discusses three different fiber optic technologies: fiber optic light transmission, fiber optic US transmission, and fiber optic US detection. These fiber optic technologies are analyzed in different PAI modalities including photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and minimally invasive photoacoustic imaging (MIPAI).

Resolution-matched reflection mode photoacoustic microscopy and optical coherence tomography dual modality system

Photoacoustic microscopy (PAM) and optical coherence tomography (OCT) are sensitive to optical absorption and scattering characteristics, respectively. As such, the integration of these two modalities in order to combine important complementary information has garnered much attention. Due to the relatively low axial resolution of PAM, PAM and OCT dual modality systems generally have a large resolution gap, especially for reflection mode systems. In this study, based on a wide-band transparent pure-optical ultrasonic detector, we developed a dual modality system (PAM-OCT system) in which PAM has a similar spatial resolution (i.e. several micrometers in both the lateral and axial directions) to OCT. In addition, due to the optical transparency advantage, the integrated system works in reflection mode, which is ideal for in vivo biomedical imaging. We successfully imaged the skin of a mouse hindlimb, which cannot be done by a transmission mode dual modality system. Our work demonstrates this dual modality system has potential in biomedical studies with complementary imaging contrasts.

Transparent High-Frequency Ultrasonic Transducer for Photoacoustic Microscopy Application

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

What scans we will read: imaging instrumentation trends in clinical oncology

Oncological diseases account for a significant portion of the burden on public healthcare systems with associated costs driven primarily by complex and long-lasting therapies. Through the visualization of patient-specific morphology and functional-molecular pathways, cancerous tissue can be detected and characterized non-invasively, so as to provide referring oncologists with essential information to support therapy management decisions. Following the onset of stand-alone anatomical and functional imaging, we witness a push towards integrating molecular image information through various methods, including anato-metabolic imaging (e.g., PET/CT), advanced MRI, optical or ultrasound imaging.

This perspective paper highlights a number of key technological and methodological advances in imaging instrumentation related to anatomical, functional, molecular medicine and hybrid imaging, that is understood as the hardware-based combination of complementary anatomical and molecular imaging. These include novel detector technologies for ionizing radiation used in CT and nuclear medicine imaging, and novel system developments in MRI and optical as well as opto-acoustic imaging. We will also highlight new data processing methods for improved non-invasive tissue characterization. Following a general introduction to the role of imaging in oncology patient management we introduce imaging methods with well-defined clinical applications and potential for clinical translation. For each modality, we report first on the status quo and, then point to perceived technological and methodological advances in a subsequent status go section. Considering the breadth and dynamics of these developments, this perspective ends with a critical reflection on where the authors, with the majority of them being imaging experts with a background in physics and engineering, believe imaging methods will be in a few years from now.

Overall, methodological and technological medical imaging advances are geared towards increased image contrast, the derivation of reproducible quantitative parameters, an increase in volume sensitivity and a reduction in overall examination time. To ensure full translation to the clinic, this progress in technologies and instrumentation is complemented by advances in relevant acquisition and image-processing protocols and improved data analysis. To this end, we should accept diagnostic images as “data”, and – through the wider adoption of advanced analysis, including machine learning approaches and a “big data” concept – move to the next stage of non-invasive tumour phenotyping. The scans we will be reading in 10 years from now will likely be composed of highly diverse multi-dimensional data from multiple sources, which mandate the use of advanced and interactive visualization and analysis platforms powered by Artificial Intelligence (AI) for real-time data handling by cross-specialty clinical experts with a domain knowledge that will need to go beyond that of plain imaging.

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.

Single-Shot Near-Field Volumetric Imaging System for Optical Ultrasound and Photoacoustics Using Capacitive Micromachined Ultrasonic Transducer Without Transmission Mode

In this article, we present a single-shot dual-mode imaging system that uses optical ultrasound (US) as an ultrasonic pulser without a transmission circuit. The ultrasonic pulse-echo system comprises an optical US pulser generated by carbon nanotubes (CNTs), which generate a high-power photoacoustic (PA) signal and a capacitive micromachined ultrasonic transducer (CMUT) receiver. By fabricating a thin CNT-polydimethylsiloxane (PDMS) composite capable of semiabsorption of the laser, a single-shot imaging system was developed. By transmitting a semipenetration light to the object, US and PA imaging were performed in a single shot. A CNT thickness of [Formula: see text] produced a maximum pressure of 154 kPa, and US was received by CMUT with a 2-MHz center frequency in PDMS. Additionally, a low-profile and near-depth imaging system was constructed with an intermediate layer of the 6-mm PDMS for the dry contact method. We performed a single-shot dual-mode imaging experiment on point and line phantoms, as well as the particle spread in the soft tissue. Thus, we examined the feasibility of the near-depth and single-shot dual-mode (US and PA) imaging system capable of a dry contact.

Photoacoustic imaging of cells in a three-dimensional microenvironment

Imaging live cells in a three-dimensional (3D) culture system yields more accurate information and spatial visualization of the interplay of cells and the surrounding matrix components compared to using a two-dimensional (2D) cell culture system. However, the thickness of 3D cultures results in a high degree of scattering that makes it difficult for the light to penetrate deeply to allow clear optical imaging. Photoacoustic (PA) imaging is a powerful imaging modality that relies on a PA effect generated when light is absorbed by exogenous contrast agents or endogenous molecules in a medium. It combines a high optical contrast with a high acoustic spatiotemporal resolution, allowing the noninvasive visualization of 3D cellular scaffolds at considerable depths with a high resolution and no image distortion. Moreover, advances in targeted contrast agents have also made PA imaging capable of molecular and cellular characterization for use in preclinical personalized diagnostics or PA imaging-guided therapeutics. Here we review the applications and challenges of PA imaging in a 3D cellular microenvironment. Potential future developments of PA imaging in preclinical applications are also discussed.

Optical-Resolution Photoacoustic Microscopy Using Transparent Ultrasound Transducer

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

In Vivo Reflection-Mode Photoacoustic Microscopy Enhanced by Plasmonic Sensing with an Acoustic Cavity

Relying on high-sensitivity refractive index sensing and a highly constrained evanescent field of surface plasmon resonance (SPR), broadband photoacoustic (PA) pressure transients were measured using an SPR sensor instead of routinely used piezoelectric ultrasonic transducers. An acoustic cavity made from stainless steel and having a designed ellipsoidal inner surface redirected laser-induced PA waves from the PA excitation spot to the SPR sensor. By incorporating the SPR sensor with the acoustic cavity, we developed optical-resolution photoacoustic microscopy (OR-PAM) with multiple advantages, including reflection-mode signal capture, improved PA detection sensitivity, increased PA spectral bandwidth as broad as ∼98 MHz, and micrometer-scale lateral resolution. This allowed label-free volumetric PA imaging of vasculature in not only the thin ear but also the thick forelimb of living mice. With these combined advantages, our OR-PAM system potentially offers more opportunities for biomedical investigation, for example, when studying microcirculations in the eye and cortex.

Optoacoustic image formation approaches-a clinical perspective

Clinical translation of optoacoustic imaging is fostered by the rapid technical advances in imaging performance as well as the growing number of clinicians recognizing the immense diagnostic potential of this technology. Clinical optoacoustic systems are available in multiple configurations, including hand-held and endoscopic probes as well as raster-scan approaches. The hardware design must be adapted to the accessible portion of the imaged region and other application-specific requirements pertaining the achievable depth, field of view or spatio-temporal resolution. Equally important is the adequate choice of the signal and image processing approach, which is largely responsible for the resulting imaging performance. Thus, new image reconstruction algorithms are constantly evolving in parallel to the newly-developed set-ups. This review focuses on recent progress on optoacoustic image formation algorithms and processing methods in the clinical setting. Major reconstruction challenges include real-time image rendering in two and three dimensions, efficient hybridization with other imaging modalitites as well as accurate interpretation and quantification of bio-markers, herein discussed in the context of ongoing progress in clinical translation.

Photoacoustic shadow-casting microscopy

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

Photoacoustic imaging with low-cost sources; A review

Benefitting from advantages of optical and ultrasound imaging, photoacoustic imaging (PAI) has demonstrated potentials in a wide range of medical applications. In order to facilitate clinical applications of PAI and encourage its application in low-resource settings, research on low-cost photoacoustic imaging with inexpensive optical sources has gained attention. Here, we review the advances made in photoacoustic imaging with low-cost sources.

Analysis and Design of High-Frequency 1-D CMUT Imaging Arrays in Noncollapsed Mode

High-frequency ultrasound imaging arrays are important for a broad range of applications, from small animal imaging to photoacoustics. Capacitive micromachined ultrasonic transducer (CMUT) arrays are particularly attractive for these applications as low noise receiver electronics can be integrated for an overall improved performance. In this paper, we present a comprehensive analysis of high-frequency CMUT arrays based on an experimentally verified CMUT array simulation tool. The results obtained on an example, a 40-MHz 1-D CMUT array for intravascular imaging, are used to obtain key design insights and tradeoffs for receive only and pulse-echo imaging. For the receiver side, thermal mechanical current noise, plane wave pressure sensitivity, and pressure noise spectrum are extracted from simulations. Using these parameters, we find that the receiver performance of CMUT arrays can be close to an ideal piston, independent of gap thickness, and applied dc bias, when coupled to low noise electronics with arrays utilizing smaller membranes performing better. For pulse-echo imaging, thermal mechanical current noise limited signal-to-noise ratio is observed to be dependent on the maximum available voltage and gap thickness. In terms of bandwidth, we find that the Bragg resonance of the array, related to the fill factor, is a significant determinant of the high frequency limit and the fluid loaded single membrane resonance determines the lower limit. Based on these results, we present design guidelines requiring only fluid loaded single membrane simulations and membrane pitch to achieve a desired pulse-echo response. We also provide a design example and discuss limitations of the approach.

Isotropic-resolution photoacoustic microscopy with multi-angle illumination

We have developed photoacoustic microscopy (PAM) with three-dimensional (3D) micron-level spatial resolution. With multi-angle illumination, PAM images from different view angles can be simultaneously acquired for multi-view deconvolution, without the rotation of imaging targets. A side-by-side comparison of this multi-angle-illumination PAM (MAI-PAM) and conventional PAM, which share the same ultrasonic detector, was performed in phantoms and live mice. The phantom study showed that MAI-PAM achieved a high axial resolution of 3.7 μm, which was 10-fold higher than that of conventional PAM and approached the lateral resolution of 2.7 μm. Furthermore, the in vivo study demonstrated that MAI-PAM was able to image the 3D microvasculature with isotropic spatial resolution.

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

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

Molecular Photoacoustic Contrast Agents: Design Principles & Applications

Photoacoustic imaging (PAI) is a rapidly growing field which offers high spatial resolution and high contrast for deep-tissue imaging in vivo. PAI is nonionizing and noninvasive and combines the optical resolution of fluorescence imaging with the spatial resolution of ultrasound imaging. In particular, the development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation ranging from small molecules to macromolecular proteins, polymeric and inorganic nanoparticles. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented and a review of prior reports spanning from early-to-current literature is put forth.

Dual-view photoacoustic microscopy for quantitative cell nuclear imaging

Optical-resolution photoacoustic microscopy (OR-PAM) is an emerging imaging modality for studying biological tissues. However, in conventional single-view OR-PAM, the lateral and axial resolutions-determined optically and acoustically, respectively-are highly anisotropic. In this Letter, we introduce dual-view OR-PAM to improve axial resolution, achieving three-dimensional (3D) resolution isotropy. We first use 0.5 μm polystyrene beads and carbon fibers to validate the resolution isotropy improvement. Imaging of mouse brain slices further demonstrates the improved resolution isotropy, revealing the 3D structure of cell nuclei in detail, which facilitates quantitative cell nuclear analysis.

In vivo blind-deconvolution photoacoustic ophthalmoscopy with total variation regularization

Photoacoustic ophthalmoscopy (PAOM) is capable of noninvasively imaging anatomic and functional information of the retina in living rodents. However, the strong ocular aberration in rodent eyes and limited ultrasonic detection sensitivity affect PAOM's spatial resolution and signal-to-noise ratio (SNR) in in vivo eyes. In this work, we report a computational approach to combine blind deconvolution (BD) algorithm with a regularizing constraint based on total variation (BDTV) for PAOM imaging restoration. We tested the algorithm in retinal and choroidal microvascular images in albino rat eyes. The algorithm improved PAOM's lateral resolution by around 2-fold. Moreover, it enabled the improvement in imaging SNR for both major vessels and capillaries, and realized the well-preserved blood vessels' edges simultaneously, which surpasses conventional Richardson-Lucy BD algorithm. The reported results indicate that the BDTV algorithm potentially facilitate PAOM in extracting retinal pathophysiological information by enhancing in vivo imaging quality without physically modifying PAOM's optical configuration.

Triplex micron-resolution acoustic, photoacoustic, and optical transmission microscopy via photoacoustic radiometry

We present a new sensing technique, termed photoacoustic radiometry (PAR), for mapping the optical attenuation properties of a sample. In PAR, laser pulses attenuated via transmission through the sample impinge on the ultrasound transducer and generate a photoacoustic (PA) signal within it. Spatial variation of the optical attenuation properties of the sample influences the amplitude of the PAR signal, providing image contrast. Performed simultaneously with pulse-echo ultrasound and PA imaging, this triplex imaging technique enables rapid characterization of samples with micrometer-resolution in a single scan. In this work, we demonstrate that the PAR technique can be easily integrated into existing PA microscopy systems, with applications in imaging biological samples and non-destructive evaluation of optically opaque materials such as silicon wafers.

Enabling fast and high quality LED photoacoustic imaging: a recurrent neural networks based approach

Photoacoustic (PA) techniques have shown promise in the imaging of tissue chromophores and exogenous contrast agents in various clinical applications. However, the key drawback of current PA technology is its dependence on a complex and hazardous laser system for the excitation of a tissue sample. Although light-emitting diodes (LED) have the potential to replace the laser, the image quality of an LED-based system is severely corrupted due to the low output power of LED elements. The current standard way to improve the quality is to increase the scanning time, which leads to a reduction in the imaging speed and makes the images prone to motion artifacts. To address the challenges of longer scanning time and poor image quality, in this work we present a deep neural networks based approach that exploits the temporal information in PA images using a recurrent neural network. We train our network using 32 phantom experiments; on the test set of 30 phantom experiments, we achieve a gain in the frame rate of 8 times with a mean peak-signal-to-noise-ratio of 35.4 dB compared to the standard technique.

Laser scanning laser diode photoacoustic microscopy system

The development of low-cost and fast photoacoustic microscopy systems enhances the clinical applicability of photoacoustic imaging systems. To this end, we present a laser scanning laser diode-based photoacoustic microscopy system. In this system, a 905 nm, 325 W maximum output peak power pulsed laser diode with 50 ns pulsewidth is utilized as the light source. A combination of aspheric and cylindrical lenses is used for collimation of the laser diode beam. Two galvanometer scanning mirrors steer the beam across a focusing aspheric lens. The lateral resolution of the system was measured to be ∼21 μm using edge spread function estimation. No averaging was performed during data acquisition. The imaging speed is ∼370 A-lines per second. Photoacoustic microscopy images of human hairs, ex vivo mouse ear, and ex vivo porcine ovary are presented to demonstrate the feasibility and potentials of the proposed system.

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.

Label-free cell nuclear imaging by Grüneisen relaxation photoacoustic microscopy

Photoacoustic microscopy (PAM) with ultraviolet (UV) laser illumination has recently been demonstrated as a promising tool that provides fast, label-free, and multilayered histologic imaging of human breast tissue. Thus far, the axial resolution has been determined ultrasonically. To enable optically defined axial resolution, we exploit the Grüneisen relaxation (GR) effect. By imaging mouse brain slices, we show that GRUV-PAM reveals detailed information about three-dimensional cell nuclear distributions and internal structures, which are important diagnostic features for cancers. Due to the nonlinear effect, GRUV-PAM also provides better contrast in images of cell nuclei.

Correcting the limited view in optical-resolution photoacoustic microscopy

Optical-resolution photoacoustic microscopy (OR-PAM) has proven useful for anatomical and functional imaging with high spatial resolutions. However, the coherent signal generation and the desired reflection-mode detection in OR-PAM can result in a limited detectability of features aligned with the acoustic axis (ie, vertical structures). Here, we investigated the limited-view phenomenon in OR-PAM by simulating the generation and propagation of the acoustic pressure waves and determined the key optical parameters affecting the visibility of vertical structures. Proof-of-concept numerical experiments were performed with different illumination angles, optical foci and numerical apertures (NA) of the objective lens. The results collectively show that an NA of 0.3 can readily improve the visibility of vertical structures in a typical reflection-mode OR-PAM system. This conclusion was confirmed by numerical simulations on the cortical blood vessels in a mouse brain and by experiments in a suture-cross phantom and in a mouse brain in vivo.

Label-free automated three-dimensional imaging of whole organs by microtomy-assisted photoacoustic microscopy

Three-dimensional (3D) optical imaging of whole biological organs with microscopic resolution has remained a challenge. Most versions of such imaging techniques require special preparation of the tissue specimen. Here we demonstrate microtomy-assisted photoacoustic microscopy (mPAM) of mouse brains and other organs, which automatically acquires serial distortion-free and registration-free images with endogenous absorption contrasts. Without tissue staining or clearing, mPAM generates micrometer-resolution 3D images of paraffin- or agarose-embedded whole organs with high fidelity, achieved by label-free simultaneous sensing of DNA/RNA, hemoglobins, and lipids. mPAM provides histology-like imaging of cell nuclei, blood vessels, axons, and other anatomical structures, enabling the application of histopathological interpretation at the organelle level to analyze a whole organ. Its deep tissue imaging capability leads to less sectioning, resulting in negligible sectioning artifact. mPAM offers a new way to better understand complex biological organs.

High-speed dual-layer scanning photoacoustic microscopy using focus tunable lens modulation at resonant frequency

The range of imaging depth in optical resolution photoacoustic microscopy (PAM) is limited by the short depth of focus of high-numerical aperture objective lenses. In this paper, focus tunable lens modulation has been employed at the resonant frequency of focus tunable lenses in order to enhance both the range of imaging depth and the scanning speed. By electrically controlling the focal length in the axial direction of the sample, the range of imaging depth was extended approximately 1.22 times and the scanning speed was enhanced by approximately 7.40 times, in comparison to corresponding values of conventional PAM systems.

Multiview Hilbert transformation in full-ring transducer array-based photoacoustic computed tomography

Based on the photoacoustic (PA) effect, PA tomography directly measures specific optical absorption, i.e., absorbed optical energy per unit volume. We recently developed a full-ring ultrasonic transducer array-based photoacoustic computed tomography (PACT) system for small-animal whole-body imaging. The system has a full-view detection angle and high in-plane resolution (∼100  μm). However, due to the bandpass frequency response of the piezoelectric transducer elements and the limited elevational detection coverage of the full-ring transducer array, the reconstructed images present bipolar (i.e., both positive and negative) pixel values, which cause ambiguities in image interpretation for physicians and biologists. We propose a multiview Hilbert transformation method to recover the unipolar initial pressure for full-ring PACT. The effectiveness of the proposed algorithm was first validated by numerical simulations and then demonstrated with ex vivo mouse brain structural imaging and in vivo mouse whole-body imaging.

Optical Detection of Ultrasound in Photoacoustic Imaging

OBJECTIVE

Photoacoustic (PA) imaging emerges as a unique tool to study biological samples based on optical absorption contrast. In PA imaging, piezoelectric transducers are commonly used to detect laser-induced ultrasonic waves. However, they typically lack adequate broadband sensitivity at ultrasonic frequency higher than 100 MHz, whereas their bulky size and optically opaque nature cause technical difficulties in integrating PA imaging with conventional optical imaging modalities. To overcome these limitations, optical methods of ultrasound detection were developed and shown their unique applications in PA imaging.

METHODS

We provide an overview of recent technological advances in optical methods of ultrasound detection and their applications in PA imaging. A general theoretical framework describing sensitivity, bandwidth, and angular responses of optical ultrasound detection is also introduced.

RESULTS

Optical methods of ultrasound detection can provide improved detection angle and sensitivity over significantly extended bandwidth. In addition, its versatile variants also offer additional advantages, such as device miniaturization, optical transparency, mechanical flexibility, minimal electrical/mechanical crosstalk, and potential noncontact PA imaging.

CONCLUSION

The optical ultrasound detection methods discussed in this review and their future evolution may play an important role in PA imaging for biomedical study and clinical diagnosis.

Development of a Fiber Laser with Independently Adjustable Properties for Optical Resolution Photoacoustic Microscopy

Photoacoustic imaging is based on the detection of generated acoustic waves through thermal expansion of tissue illuminated by short laser pulses. Fiber lasers as an excitation source for photoacoustic imaging have recently been preferred for their high repetition frequencies. Here, we report a unique fiber laser developed specifically for multiwavelength photoacoustic microscopy system. The laser is custom-made for maximum flexibility in adjustment of its parameters; pulse duration (5–10 ns), pulse energy (up to 10 μJ) and repetition frequency (up to 1 MHz) independently from each other and covers a broad spectral region from 450 to 1100 nm and also can emit wavelengths of 532, 355, and 266 nm. The laser system consists of a master oscillator power amplifier, seeding two stages; supercontinuum and harmonic generation units. The laser is outstanding since the oscillator, amplifier and supercontinuum generation parts are all-fiber integrated with custom-developed electronics and software. To demonstrate the feasibility of the system, the images of several elements of standardized resolution test chart are acquired at multiple wavelengths. The lateral resolution of optical resolution photoacoustic microscopy system is determined as 2.68 μm. The developed system may pave the way for spectroscopic photoacoustic microscopy applications via widely tunable fiber laser technologies.

Multiparametric photoacoustic microscopy of the mouse brain with 300-kHz A-line rate

Enabling simultaneous high-resolution imaging of the total concentration of hemoglobin ([Formula: see text]), oxygen saturation of hemoglobin ([Formula: see text]), and cerebral blood flow (CBF), multiparametric photoacoustic microscopy (PAM) holds the potential to quantify the cerebral metabolic rate of oxygen at the microscopic level. However, its imaging speed has been severely limited by the pulse repetition rate of the dual-wavelength photoacoustic excitation and the scanning mechanism. To address these limitations, we have developed a new generation of multiparametric PAM. Capitalizing on a self-developed high-repetition dual-wavelength pulsed laser and an optical-mechanical hybrid-scan configuration, this innovative technique has achieved an unprecedented A-line rate of 300 kHz, leading to a 20-fold increase in the imaging speed over our previously reported multiparametric PAM that is based on pure mechanical scanning. The performance of the high-speed multiparametric PAM has been examined both in vitro and in vivo. Simultaneous PAM of microvascular [Formula: see text], [Formula: see text], and CBF in absolute values over a [Formula: see text]-mm-diameter brain region of interest can be accomplished within 10 min.

Characterization of the temperature rise in a single cell during photoacoustic tomography at the nanoscale

We are developing a label-free nanoscale photoacoustic tomography (nPAT) for imaging a single living cell. nPAT uses a laser-induced acoustic pulse to generate a nanometer-scale image. The primary motivation behind this imaging technique is the imaging of biological cells in the context of diagnosis without fluorescent tagging. During this procedure, thermal damage due to the laser pulse is a potential risk that may damage the cells. A physical model is built to estimate the temperature rise and thermal relaxation during the imaging procedure. Through simulations using finite element methods, two lasers (532 nm at 5 ps pulse duration and 830 nm at 0.2 ps pulse duration) were simulated for imaging red blood cells (RBCs). We demonstrate that a single 5-ps pulse laser with a 400-Hz repetition rate will generate a steady state temperature rise of less than a Kelvin on the surface of the RBCs. All the simulation results show that there is no significant temperature rise in an RBC in either single pulse or multiple pulse illumination with a 532-nm laser with 219 W fluence. Therefore, our simulation results demonstrate the thermal safety of an nPAT system. The photoacoustic signal generated by this laser is on the order of 2.5 kPa, so it should still be large enough to generate high-resolution images with nPAT. Frequency analysis of this signal shows a peak at 1.47 GHz, with frequencies as high as 3.5 GHz still being present in the spectrum. We believe that nPAT will open an avenue for disease diagnosis and cell biology studies at the nanometer-level.

Frequency domain photoacoustic and fluorescence microscopy

We report on simultaneous frequency domain optical-resolution photoacoustic and fluorescence microscopy with sub-µm lateral resolution. With the help of a blood smear, we show that photoacoustic and fluorescence images provide complementary information. Furthermore, we compare theoretically predicted signal-to-noise ratios of sinusoidal modulation in frequency domain with pulsed excitation in time domain.

Grueneisen relaxation photoacoustic microscopy in vivo

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

In vivo deconvolution acoustic-resolution photoacoustic microscopy in three dimensions

Acoustic-resolution photoacoustic microscopy (ARPAM) provides a spatial resolution on the order of tens of micrometers, and is becoming an essential tool for imaging fine structures, such as the subcutaneous microvasculature. High lateral resolution of ARPAM is achieved using high numerical aperture (NA) of acoustic transducer; however, the depth of focus and working distance will be deteriorated correspondingly, thus sacrificing the imaging range and accessible depth. The axial resolution of ARPAM is limited by the transducer's bandwidth. In this work, we develop deconvolution ARPAM (D-ARPAM) in three dimensions that can improve the lateral resolution by 1.8 and 3.7 times and the axial resolution by 1.7 and 2.7 times, depending on the adopted criteria, using a 20-MHz focused transducer without physically increasing its NA and bandwidth. The resolution enhancement in three dimensions by D-ARPAM is also demonstrated by in vivo imaging of the microvasculature of a chick embryo. The proposed D-ARPAM has potential for biomedical imaging that simultaneously requires high spatial resolution, extended imaging range, and long accessible depth.

All-optical photoacoustic microscopy

Three-dimensional photoacoustic microscopy (PAM) has gained considerable attention within the biomedical imaging community during the past decade. Detecting laser-induced photoacoustic waves by optical sensing techniques facilitates the idea of all-optical PAM (AOPAM), which is of particular interest as it provides unique advantages for achieving high spatial resolution using miniaturized embodiments of the imaging system. The review presents the technology aspects of optical-sensing techniques for ultrasound detection, such as those based on optical resonators, as well as system developments of all-optical photoacoustic systems including PAM, photoacoustic endoscopy, and multi-modality microscopy. The progress of different AOPAM systems and their representative applications are summarized.

In vivo sub-femtoliter resolution photoacoustic microscopy with higher frame rates

Microscopy based on non-fluorescent absorption dye staining is widely used in various fields of biomedicine for 400 years. Unlike its fluorescent counterpart, non-fluorescent absorption microscopy lacks proper methodologies to realize its in vivo applications with a sub-femtoliter 3D resolution. Regardless of the most advanced high-resolution photoacoustic microscopy, sub-femtoliter spatial resolution is still unattainable, and the imaging speed is relatively slow. In this paper, based on the two-photon photoacoustic mechanism, we demonstrated a in vivo label free laser-scanning photoacoustic imaging modality featuring high frame rates and sub-femtoliter 3D resolution simultaneously, which stands as a perfect solution to 3D high resolution non-fluorescent absorption microscopy. Furthermore, we first demonstrated in vivo label-free two-photon acoustic microscopy on the observation of non-fluorescent melanin distribution within mouse skin.