Photoacoustic computed microscopy

Correction of high-rate motion for photoacoustic microscopy by orthogonal cross-correlation

Photoacoustic imaging is a promising technology for in vivo imaging. However, its imaging performance can be hampered by motion artifacts, especially when dealing with high-rate motion. In this paper, we propose an orthogonal motion correction method that utilizes cross-correlation along orthogonal scan directions to extract accurate motion displacements from the photoacoustic data. The extracted displacements are then applied to remove artifacts and compensate for motion-induced distortions. Phantom experiments demonstrate that the proposed method can extract the motion information and the structural similarity index measurement after correction is increased by 26.5% and 11.2% compared to no correction and the previous correction method. Then the effectiveness of our method is evaluated in vivo imaging of a mouse brain. Our method shows a stable and effective performance under high-rate motion. The high accuracy of the motion correction method makes it valuable in improving the accuracy of photoacoustic imaging.

Testicular torsion diagnosis and injury assessment using photoacoustic oxygenation imaging

Testicular torsion (TT) is a medical emergency that requires immediate diagnostic evaluation. Photoacoustic imaging (PAI) has the potential to provide spatially resolved oxygen saturation (sO₂), which can serve as a valuable marker in TT diagnosis. We investigated the potential of PAI as an alternative method for TT diagnosis and testicular injury assessment. We measured sO₂ levels in different degrees of TT models using PAI at various time points. Based on histopathological results, we found that the averaged sO₂ per pixel (sO2®) and reduction of sO2® (rsO₂) in twisted testicles had significant correlations with hypoxic conditions. Both sO2® and rsO₂ exhibited excellent diagnostic abilities in detecting TT and identifying ischemia/hypoxia injury following TT. Furthermore, PAI-measured sO₂ demonstrated favorable diagnostic capabilities in discriminating if the testicle had suffered irreversible injury. In summary, PAI presents a potentially promising novel approach in evaluating TT and warrants further clinical investigation.

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.

Functional imaging of the exposed brain

When the brain is exposed, such as after a craniotomy in neurosurgical procedures, we are provided with the unique opportunity for real-time imaging of brain functionality. Real-time functional maps of the exposed brain are vital to ensuring safe and effective navigation during these neurosurgical procedures. However, current neurosurgical practice has yet to fully harness this potential as it pre-dominantly relies on inherently limited techniques such as electrical stimulation to provide functional feedback to guide surgical decision-making. A wealth of especially experimental imaging techniques show unique potential to improve intra-operative decision-making and neurosurgical safety, and as an added bonus, improve our fundamental neuroscientific understanding of human brain function. In this review we compare and contrast close to twenty candidate imaging techniques based on their underlying biological substrate, technical characteristics and ability to meet clinical constraints such as compatibility with surgical workflow. Our review gives insight into the interplay between technical parameters such sampling method, data rate and a technique's real-time imaging potential in the operating room. By the end of the review, the reader will understand why new, real-time volumetric imaging techniques such as functional Ultrasound (fUS) and functional Photoacoustic Computed Tomography (fPACT) hold great clinical potential for procedures in especially highly eloquent areas, despite the higher data rates involved. Finally, we will highlight the neuroscientific perspective on the exposed brain. While different neurosurgical procedures ask for different functional maps to navigate surgical territories, neuroscience potentially benefits from all these maps. In the surgical context we can uniquely combine healthy volunteer studies, lesion studies and even reversible lesion studies in in the same individual. Ultimately, individual cases will build a greater understanding of human brain function in general, which in turn will improve neurosurgeons' future navigational efforts.

Photoacoustic Imaging in Biomedicine and Life Sciences

Photo-acoustic imaging, also known as opto-acoustic imaging, has become a widely popular modality for biomedical applications. This hybrid technique possesses the advantages of high optical contrast and high ultrasonic resolution. Due to the distinct optical absorption properties of tissue compartments and main chromophores, photo-acoustics is able to non-invasively observe structural and functional variations within biological tissues including oxygenation and deoxygenation, blood vessels and spatial melanin distribution. The detection of acoustic waves produced by a pulsed laser source yields a high scaling range, from organ level photo-acoustic tomography to sub-cellular or even molecular imaging. This review discusses significant novel technical solutions utilising photo-acoustics and their applications in the fields of biomedicine and life sciences.

Self-Fluence-Compensated Functional Photoacoustic Microscopy

Optical-resolution photoacoustic microscopy (OR-PAM) can image blood oxygen saturation (sO₂) in vivo with high resolution and excellent sensitivity and offers a great tool for neurovascular study and early cancer diagnosis. OR-PAM ignores the wavelength-dependent optical attenuation in superficial tissue, which cause errors in sO₂ imaging. Monte Carlo simulation shows that variations in imaging depth, vessel diameter, and focal position can cause up to  ∼ 60 % decrease in sO₂ imaging. Here, we develop a self-fluence-compensated OR-PAM to compensate for the wavelength-dependent fluence attenuation. We propose a linearized model to estimate the fluence attenuations and use three optical wavelengths to compensate for them in sO₂ calculation. We validate the model in both numerical and physical phantoms and show that the compensation method can effectively reduce the sO₂ errors. In functional brain imaging, we demonstrate that the compensation method can effectively improve sO₂ accuracy, especially in small vessels. Compared with uncompensated ones, the sO₂ values are improved by 10~30% in the brain. We monitor ischemic-stroke-induced brain injury which demonstrates great potential for the pre-clinical study of vascular diseases.

High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror

Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an off-axis parabolic acoustic mirror with a conical bore. Quantitative simulation and experiments demonstrated that this combiner can provide better acoustic focusing performance and detection sensitivity. Moreover, OR-PAM is based on the combiner suffer low optical disorders, which guarantees the good resolution. In vivo experiments of the mouse brain and the iris were also conducted to show the practicability of the combiner in biomedicine. This proposed optical-acoustic combiner realizes a high-quality optical-acoustic confocal alignment with minimal optical disorders and acoustic insertion loss, strong acoustic focusing, and easy implementation. These characteristics might be useful for improving the performance of OR-PAM.

Noncontact evaluation of full elastic constants of perovskite MAPbBr3 via Photoacoustic eigen-spectrum analysis in one test

Elasticity is one basic property of hybrid organic-inorganic perovskites. It highly relates to many fundamental processes in solid physics. The investigation of elasticity is of interest not only to explore the intrinsic properties of a material, but also to improve their potential application performance. In this study, we predict photoacoustic eigen-spectrum (PAES) of single crystal. Then by solving the inverse problem of the generation of PAES, we propose a noncontact method to determine a complete set of elastic constants of single crystal in one test. Experiments confirm the proposed method accurately determines all elastic constants of MAPbBr₃. Since this method is totally noncontact and does not require multiple specimens cutting along different crystal axes, it could be more competent for rare, tiny and brittle specimen, or when the specimen is immersed in turbid or opaque medium. Benefitting from these superiorities, the proposed method might be found prominent values in materials science and applications.

Optical Biopsy of Melanoma and Basal Cell Carcinoma Progression by Noncontact Photoacoustic and Optical Coherence Tomography: In Vivo Multi-Parametric Characterizing Tumor Microenvironment

Measuring the structural and functional status of tumor microenvironment for malignant melanoma (MM) and basal cell carcinoma (BCC) is of profound significance in understanding dermatological condition for biopsy. However, conventional optical imaging techniques are limited to visualize superficial skin features and parameter information is deficient to depict pathophysiology correlations of skin diseases. Here, we demonstrate a preclinical device, all-optically integrated photoacoustic and optical coherence tomography (AOPA/OCT), that, for the first time, can simultaneously provide label-free biomarkers of vascular patterns, temporal and spatial heterogeneity of blood flow, and tissue micro-structure changes during tumor growth with pathophysiological correlations in mice models. We found that tumor microenvironment of MM and BCC led to the alternation in spatial-temporal heterogeneity that affected morphological and functional parameters, performing the AOPA/OCT quantitative metrics. A robust correlation between imaging biomarkers derived from this in vivo technique and histopathology validation ex vivo in distinguishing benign from malignant is also presented. In receiver operating characteristics (ROC) analysis, multi-parametric AOPA/OCT yields improved diagnostic accuracy of 98.4% and 95.8% for MM and BCC respectively, which indicate that AOPA/OCT represents a high-performance and clinically translatable technique for accurate diagnosis and therapy monitoring in dermatology.

Photoacoustic assessment of hemodynamic changes in foot vessels

Monitoring the blood supply in the lower extremities is critical for individuals who are vulnerable to vascular dysfunction. Current clinical approaches are ineffective in observing hemodynamic changes in peripheral vessels. In this paper, we investigate the potential of photoacoustic tomography (PAT) as an alternative way to in vivo monitor hemodynamic changes in foot vessels. High spatial and temporal resolution maps of hemoglobin in major arteries and veins are shown. Results from twelve human subjects are presented here to visualize vascular perfusion of healthy volunteers in two age groups (young vs aged). Significant differences between the two groups are observed and verify the declining in vascular function with aging, highlighting the potential of PAT as a new tool to evaluate vascular function in the lower extremities.

Isolated Murine Brain Model for Large-Scale Optoacoustic Calcium Imaging

Real-time visualization of large-scale neural dynamics in whole mammalian brains is hindered with existing neuroimaging methods having limited capacity when it comes to imaging large tissue volumes at high speeds. Optoacoustic imaging has been shown to be capable of real-time three-dimensional imaging of multiple cerebral hemodynamic parameters in rodents. However, optoacoustic imaging of calcium activity deep within the mammalian brain is hampered by strong blood absorption in the visible light spectrum as well as a lack of activity labels excitable in the near-infrared window. We have developed and validated an isolated whole mouse brain preparation labeled with genetically encoded calcium indicator GCaMP6f, which can closely resemble in vivo conditions. An optoacoustic imaging system coupled to a superfusion system was further designed and used for rapid volumetric monitoring of stimulus-evoked calcium dynamics in the brain. These new imaging setup and isolated preparation's protocols and characteristics are described here in detail. Our new technique captures calcium fluxes as true three-dimensional information across the entire brain with temporal resolution of 10 ms and spatial resolution of 150 μm, thus enabling large-scale neural recording at penetration depths and spatio-temporal resolution scales not covered with any existing neuroimaging techniques.

Imaging acoustic sources through scattering media by using a correlation full-matrix filter

In the inhomogeneous medium, acoustic scattering is always a fundamental challenge for photoacoustic imaging. We implement a correlation full-matrix filter (CFMF) combing with a time reversal operator to improve the imaging quality of acoustic sources in complex media. The correlation full-matrix filtering process extracts the direct wave component from the detected signal and preserve all the useful information at the same time. A location factor is considered in the time reversal operator to compensate for the image distortion and false contrast caused by the limited-view detection. The numerical simulations demonstrate that the proposed approach can perform good imaging quality with the higher image signal-noise ratio and better resolution in an acoustic scattering environment. This scheme might be applied to improve the photoacoustic imaging for inhomogeneous biological tissues.

Photoacoustic imaging in scattering media by combining a correlation matrix filter with a time reversal operator

Acoustic scattering medium is a fundamental challenge for photoacoustic imaging. In this study, we reveal the different coherent properties of the scattering photoacoustic waves and the direct photoacoustic waves in a matrix form. Direct waves show a particular coherence on the antidiagonals of the matrix, whereas scattering waves do not. Based on this property, a correlation matrix filter combining with a time reversal operator is proposed to preserve the direct waves and recover the image behind a scattering layer. Both numerical simulations and photoacoustic imaging experiments demonstrate that the proposed approach effectively increases the image contrast and decreases the background speckles in a scattering medium. This study might improve the quality of photoacoustic imaging in an acoustic scattering environment and extend its applications.

Photoacoustics and speed-of-sound dual mode imaging with a long depth-of-field by using annular ultrasound array

Speed-of-sound and optical absorption reflect the structure and function of tissues from different aspects. A dual-mode microscopy system based on a concentric annular ultrasound array is proposed to simultaneously acquire the long depth-of-field images of speed-of-sound and optical absorption of inhomogeneous samples. First, speed-of-sound is decoded from the signal delay between each element of the annular array. The measured speed-of-sound could not only be used as an image contrast, but also improve the resolution and accuracy of spatial location of photoacoustic image in inhomogeneous acoustic media. Secondly, benefitting from dynamic focusing of annular array and the measured speed-of-sound, it is achieved an advanced acoustic-resolution photoacoustic microscopy with a precise position and a long depth-of-field. The performance of the dual-mode imaging system has been experimentally examined by using a custom-made annular array. The proposed dual-mode microscopy might have the significances in monitoring the biological physiological and pathological processes.

Functional optoacoustic neuro-tomography for scalable whole-brain monitoring of calcium indicators

Non-invasive observation of spatiotemporal activity of large neural populations distributed over entire brains is a longstanding goal of neuroscience. We developed a volumetric multispectral optoacoustic tomography platform for imaging neural activation deep in scattering brains. It can record 100 volumetric frames per second across scalable fields of view ranging between 50 and 1000 mm³ with respective spatial resolution of 35-200 μm. Experiments performed in immobilized and freely swimming larvae and in adult zebrafish brains expressing the genetically encoded calcium indicator GCaMP5G demonstrate, for the first time, the fundamental ability to directly track neural dynamics using optoacoustics while overcoming the longstanding penetration barrier of optical imaging in scattering brains. The newly developed platform thus offers unprecedented capabilities for functional whole-brain observations of fast calcium dynamics; in combination with optoacoustics' well-established capacity for resolving vascular hemodynamics, it could open new vistas in the study of neural activity and neurovascular coupling in health and disease.

Wearable 3-D Photoacoustic Tomography for Functional Brain Imaging in Behaving Rats

Understanding the relationship between brain function and behavior remains a major challenge in neuroscience. Photoacoustic tomography (PAT) is an emerging technique that allows for noninvasive in vivo brain imaging at micrometer-millisecond spatiotemporal resolution. In this article, a novel, miniaturized 3D wearable PAT (3D-wPAT) technique is described for brain imaging in behaving rats. 3D-wPAT has three layers of fully functional acoustic transducer arrays. Phantom imaging experiments revealed that the in-plane X-Y spatial resolutions were ~200 μm for each acoustic detection layer. The functional imaging capacity of 3D-wPAT was demonstrated by mapping the cerebral oxygen saturation via multi-wavelength irradiation in behaving hyperoxic rats. In addition, we demonstrated that 3D-wPAT could be used for monitoring sensory stimulus-evoked responses in behaving rats by measuring hemodynamic responses in the primary visual cortex during visual stimulation. Together, these results show the potential of 3D-wPAT for brain study in behaving rodents.

Noninvasive Assessment of Early Dental Lesion Using a Dual-Contrast Photoacoustic Tomography

Dental hard tissue lesions, including caries, cracked-tooth, etc., are the most prevalent diseases of people worldwide. Dental lesions and correlative diseases greatly decrease the life quality of patients throughout their lifetime. It is still hard to noninvasively detect these dental lesions in their early stages. Photoacoustic imaging is an emerging hybrid technology combining the high spatial resolution of ultrasound in deep tissue with the rich optical contrasts. In this study, a dual-contrast photoacoustic tomography is applied to detect the early dental lesions. One contrast, named B-mode, is related to the optical absorption. It is good at providing the sharp image about the morphological and macro-structural features of the teeth. Another contrast, named S-mode, is associated with the micro-structural and mechanical properties of the hard tissue. It is sensitive to the change of tissue properties induced by the early dental lesions. Experiments show that the comprehensive analysis of dual-contrast information can provide reliable information of the early dental lesions. Moreover, the imaging parameter of S-mode is device-independent and it could measure tissue properties quantitatively. We expect that the proposed scheme could be beneficial for improving safety, accuracy and sensitivity of the clinical diagnosis of the dental lesion.

Noninvasive Assessment of Early Dental Lesion Using a Dual-Contrast Photoacoustic Tomography

Dental hard tissue lesions, including caries, cracked-tooth, etc., are the most prevalent diseases of people worldwide. Dental lesions and correlative diseases greatly decrease the life quality of patients throughout their lifetime. It is still hard to noninvasively detect these dental lesions in their early stages. Photoacoustic imaging is an emerging hybrid technology combining the high spatial resolution of ultrasound in deep tissue with the rich optical contrasts. In this study, a dual-contrast photoacoustic tomography is applied to detect the early dental lesions. One contrast, named B-mode, is related to the optical absorption. It is good at providing the sharp image about the morphological and macro-structural features of the teeth. Another contrast, named S-mode, is associated with the micro-structural and mechanical properties of the hard tissue. It is sensitive to the change of tissue properties induced by the early dental lesions. Experiments show that the comprehensive analysis of dual-contrast information can provide reliable information of the early dental lesions. Moreover, the imaging parameter of S-mode is device-independent and it could measure tissue properties quantitatively. We expect that the proposed scheme could be beneficial for improving safety, accuracy and sensitivity of the clinical diagnosis of the dental lesion.

Photoacoustic imaging of cerebral hypoperfusion during acupuncture

Using acupuncture to treat cerebral hypoperfusion is a hot topic. However, there is a lack of effective tools to clarify the therapeutic effect of acupuncture on cerebral hypoperfusion. Here, we show in a mouse model of cerebral hypoperfusion that photoacoustic tomography (PAT) can noninvasively image cerebral vasculature and track total hemoglobin (HbT) concentration changes in cerebral hypoperfusion with acupuncture stimulation on the YangLingQuan (GB34) point. We measured the changes of HbT concentration and found that the HbT concentration in hypoperfusion regions was clearly lower than that in the control regions when the acupuncture was absent; however, it was significantly increased when the acupuncture was implemented on the GB34 point. We also observed the increase of vessel size and the generation of new vessels in cerebral hypoperfusion during acupuncture. Laser speckle imaging (LSI) was employed to validate some of the PAT findings.

Miniature probe integrating optical-resolution photoacoustic microscopy, optical coherence tomography, and ultrasound imaging: proof-of-concept

In this Letter, we present a novel tri-modal miniature side-view probe, through which optical-resolution photoacoustic microscopy (OR-PAM), optical coherence tomography (OCT), and pulse-echo ultrasound (US) images can be coaxially acquired and displayed simultaneously. The probe consists of a common optical path for OR-PAM (light delivery) and OCT (light delivery/detection), and a 40-MHz unfocused ultrasound transducer for OR-PAM (photoacoustic detection) and US (ultrasound transmission/receiving) with an overall diameter of 2 mm. Combining OR-PAM, OCT, and US would provide complementary information including optical absorption (OR-PAM), optical back-scattering (OCT), and deep tissue structures (US) about biological tissue. Based on an integrated imaging system consisting of OR-PAM, time-domain OCT, and US, phantom images and in vivo images of rat ear were acquired to demonstrate the capabilities of the integrated tri-modality imaging probe. The probe yields a lateral resolution of 13.6 μm for OR-PAM and OCT, and an axial resolution of 43 μm for OR-PAM and US. Currently, for a scanning area of 1 ×1  mm, it took ∼25  min to acquire data for tri-modal volumetric imaging.