Three-dimensional non-invasive brain imaging of ischemic stroke by integrated photoacoustic, ultrasound and angiographic tomography (PAUSAT)

Non-Invasive Deep-Brain Imaging With 3D Integrated Photoacoustic Tomography and Ultrasound Localization Microscopy (3D-PAULM)

Photoacoustic computed tomography (PACT) is a proven technology for imaging hemodynamics in deep brain of small animal models. PACT is inherently compatible with ultrasound (US) imaging, providing complementary contrast mechanisms. While PACT can quantify the brain's oxygen saturation of hemoglobin (sO , US imaging can probe the blood flow based on the Doppler effect. Further, by tracking gas-filled microbubbles, ultrasound localization microscopy (ULM) can map the blood flow velocity with sub-diffraction spatial resolution. In this work, we present a 3D deep-brain imaging system that seamlessly integrates PACT and ULM into a single device, 3D-PAULM. Using a low ultrasound frequency of 4 MHz, 3D-PAULM is capable of imaging the brain hemodynamic functions with intact scalp and skull in a totally non-invasive manner. Using 3D-PAULM, we studied the mouse brain functions with ischemic stroke. Multi-spectral PACT, US B-mode imaging, microbubble-enhanced power Doppler (PD), and ULM were performed on the same mouse brain with intrinsic image co-registration. From the multi-modality measurements, we further quantified blood perfusion, sO2, vessel density, and flow velocity of the mouse brain, showing stroke-induced ischemia, hypoxia, and reduced blood flow. We expect that 3D-PAULM can find broad applications in studying deep brain functions on small animal models.

Three-dimensional diffractive acoustic tomography

Acoustically probing biological tissues with light or sound, photoacoustic and ultrasound imaging can provide anatomical, functional, and/or molecular information at depths far beyond the optical diffusion limit. However, most photoacoustic and ultrasound imaging systems rely on linear-array transducers with elevational focusing and are limited to two-dimensional imaging with anisotropic resolutions. Here, we present three-dimensional diffractive acoustic tomography (3D-DAT), which uses an off-the-shelf linear-array transducer with single-slit acoustic diffraction. Without jeopardizing its accessibility by general users, 3D-DAT has achieved simultaneous 3D photoacoustic and ultrasound imaging with optimal imaging performance in deep tissues, providing near-isotropic resolutions, high imaging speed, and a large field-of-view, as well as enhanced quantitative accuracy and detection sensitivity. Moreover, powered by the fast focal line volumetric reconstruction, 3D-DAT has achieved 50-fold faster reconstruction times than traditional photoacoustic imaging reconstruction. Using 3D-DAT on small animal models, we mapped the distribution of the biliverdin-binding serpin complex in glassfrogs, tracked gold nanoparticle accumulation in a mouse tumor model, imaged genetically-encoded photoswitchable tumors, and investigated polyfluoroalkyl substances exposure on developing embryos. With its enhanced imaging performance and high accessibility, 3D-DAT may find broad applications in fundamental life sciences and biomedical research.

Non-Invasive Photoacoustic Cerebrovascular Monitoring of Early-Stage Ischemic Strokes In Vivo

Early-stage stroke monitoring enables timely intervention that is crucial to minimizing neuronal damage and increasing the extent of recovery. By monitoring collateral circulation and neovascularization after ischemic stroke, the natural recovery process can be better understood, optimize further treatment strategies, and improve the prognosis. Photoacoustic computed tomography (PACT), a non-invasive imaging modality that captures multiparametric high-resolution images of vessel structures, is well suited for evaluating cerebrovascular structures and their function. Here 3D multiparametric transcranial PACT is implemented to monitor the early stage of a photothrombotic (PT)-stroke model in living rats. New vessels in the PT-induced region are successfully observed using PACT, and these observations are confirmed by histology. Then, using multiparametric PACT, it is found that the SO2 in the ischemic area decreases while the SO2 in newly formed vessels increases, and the SO2 in the PT region also recovers. These findings demonstrate PACT's remarkable ability to image and monitor cerebrovascular morphologic and physiological changes. They highlight the usefulness of whole-brain PACT as a potentially powerful tool for early diagnosis and therapeutic decision-making in treating ischemic stroke.

Enhanced dual-mode imaging: Superior photoacoustic and ultrasound endoscopy in live pigs using a transparent ultrasound transducer

Dual-mode photoacoustic/ultrasound endoscopy (ePAUS) is a promising tool for preclinical and clinical interventions. To be clinically useful, ePAUS must deliver high-performance ultrasound imaging comparable to commercial systems and maintain high photoacoustic imaging performance at long working distances. This requires a transducer with an intact physical aperture and coaxial alignment of acoustic and optical beams within the probe, a challenging integration task. We present a high-performance ePAUS probe with a miniaturized, optically transparent ultrasonic transducer (TUT) called ePAUS-TUT. The 1.8-mm-diameter probe, fitting into standard endoscopic channels, aligns acoustic and optical beams efficiently, achieving commercial-level ultrasound and high-resolution photoacoustic imaging over long distances. These imaging capabilities were validated through in vivo imaging of a rat's rectum and a pig's esophagus. The ePAUS-TUT system substantially enhances feasibility and potential for clinical applications.

Deep tissue photoacoustic imaging with light and sound

Photoacoustic computed tomography (PACT) can harvest diffusive photons to image the optical absorption contrast of molecules in a scattering medium, with ultrasonically-defined spatial resolution. PACT has been extensively used in preclinical research for imaging functional and molecular information in various animal models, with recent clinical translations. In this review, we aim to highlight the recent technical breakthroughs in PACT and the emerging preclinical and clinical applications in deep tissue imaging.

Enhancing Ischemic Stroke Evaluation by a Model-Based Photoacoustic Tomography Algorithm

Ischemic stroke (IS) is characterized by the sudden interruption of blood supply to the brain, resulting in neurological impairments and even mortality. Photoacoustic computed tomography (PACT) integrates the high contrast of optical imaging and the penetration of ultrasound imaging, enabling non-invasive IS evaluation. However, the image reconstruction quality significantly affects the oxyhemoglobin saturation (sO2) estimation. This study investigates a model-based with total variation minimized by augmented Lagrangian and alternating direction (MB-TVAL3) approach and compared it with the widely used back-projection (BP) and delay-and-sum (DAS) algorithms. Both simulations and in vivo experiments are conducted to validate the performance of the MB-TVAL3 algorithm, showing a higher sO2 estimation accuracy and sensitivity in detecting infarct area compared to BP and DAS. The findings of this study emphasize the impact of acoustic inverse problem on the accuracy of sO2 estimation and the proposed approach offers valuable support for IS evaluation and cerebrovascular diagnosis.

Oxygenation heterogeneity facilitates spatiotemporal flow pattern visualization inside human blood vessels using photoacoustic computed tomography

Hemodynamics can be explored through various biomedical imaging techniques. However, observing transient spatiotemporal variations in the saturation of oxygen (sO2) within human blood vessels proves challenging with conventional methods. In this study, we employed photoacoustic computed tomography (PACT) to reconstruct the evolving spatiotemporal patterns in a human vein. Through analysis of the multi-wavelength photoacoustic (PA) spectrum, we illustrated the dynamic distribution within blood vessels. Additionally, we computationally rendered the dynamic process of venous blood flowing into the major vein and entering a branching vessel. Notably, we successfully recovered, in real time, the parabolic wavefront profile of laminar flow inside a deep vein in vivo-a first-time achievement. While the study is preliminary, the demonstrated capability of dynamic sO2 imaging holds promise for new applications in biology and medicine.

Simultaneous photoacoustic and ultrasound imaging: A review

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that combines the advantages of optical and ultrasound imaging, enabling the generation of images with both optical resolution and acoustic penetration depth. By leveraging similar signal acquisition and processing methods, the integration of photoacoustic and ultrasound imaging has introduced a novel hybrid imaging modality suitable for clinical applications. Photoacoustic-ultrasound imaging allows for non-invasive, high-resolution, and deep-penetrating imaging, providing a wealth of image information. In recent years, with the deepening research and the expanding biomedical application scenarios of photoacoustic-ultrasound bimodal systems, the immense potential of photoacoustic-ultrasound bimodal imaging in basic research and clinical applications has been demonstrated, with some research achievements already commercialized. In this review, we introduce the principles, technical advantages, and biomedical applications of photoacoustic-ultrasound bimodal imaging techniques, specifically focusing on tomographic, microscopic, and endoscopic imaging modalities. Furthermore, we discuss the future directions of photoacoustic-ultrasound bimodal imaging technology.

A Versatile Pipeline for High-fidelity Imaging and Analysis of Vascular Networks Across the Body

Structural and functional changes in vascular networks play a vital role during development, causing or contributing to the pathophysiology of injury and disease. Current methods to trace and image the vasculature in laboratory settings have proven inconsistent, inaccurate, and labor intensive, lacking the inherent three-dimensional structure of vasculature. Here, we provide a robust and highly reproducible method to image and quantify changes in vascular networks down to the capillary level. The method combines vasculature tracing, tissue clearing, and three-dimensional imaging techniques with vessel segmentation using AI-based convolutional reconstruction to rapidly process large, unsectioned tissue specimens throughout the body with high fidelity. The practicality and scalability of our protocol offer application across various fields of biomedical sciences. Obviating the need for sectioning of samples, this method will expedite qualitative and quantitative analyses of vascular networks. Preparation of the fluorescent gel perfusate takes < 30 min per study. Transcardiac perfusion and vasculature tracing takes approximately 20 min, while dissection of tissue samples ranges from 5 to 15 min depending on the tissue of interest. The tissue clearing protocol takes approximately 24-48 h per whole-tissue sample. Lastly, three-dimensional imaging and analysis can be completed in one day. The entire procedure can be carried out by a competent graduate student or experienced technician. Key features • This robust and highly reproducible method allows users to image and quantify changes in vascular networks down to the capillary level. • Three-dimensional imaging techniques with vessel segmentation enable rapid processing of large, unsectioned tissue specimens throughout the body. • It takes approximately 2-3 days for sample preparation, three-dimensional imaging, and analysis. • The user-friendly pipeline can be completed by experienced and non-experienced users.

Photoacoustic imaging plus X: a review

Significance

Photoacoustic (PA) imaging (PAI) represents an emerging modality within the realm of biomedical imaging technology. It seamlessly blends the wealth of optical contrast with the remarkable depth of penetration offered by ultrasound. These distinctive features of PAI hold tremendous potential for various applications, including early cancer detection, functional imaging, hybrid imaging, monitoring ablation therapy, and providing guidance during surgical procedures. The synergy between PAI and other cutting-edge technologies not only enhances its capabilities but also propels it toward broader clinical applicability.

Aim

The integration of PAI with advanced technology for PA signal detection, signal processing, image reconstruction, hybrid imaging, and clinical applications has significantly bolstered the capabilities of PAI. This review endeavor contributes to a deeper comprehension of how the synergy between PAI and other advanced technologies can lead to improved applications.

Approach

An examination of the evolving research frontiers in PAI, integrated with other advanced technologies, reveals six key categories named "PAI plus X." These categories encompass a range of topics, including but not limited to PAI plus treatment, PAI plus circuits design, PAI plus accurate positioning system, PAI plus fast scanning systems, PAI plus ultrasound sensors, PAI plus advanced laser sources, PAI plus deep learning, and PAI plus other imaging modalities.

Results

After conducting a comprehensive review of the existing literature and research on PAI integrated with other technologies, various proposals have emerged to advance the development of PAI plus X. These proposals aim to enhance system hardware, improve imaging quality, and address clinical challenges effectively.

Conclusions

The progression of innovative and sophisticated approaches within each category of PAI plus X is positioned to drive significant advancements in both the development of PAI technology and its clinical applications. Furthermore, PAI not only has the potential to integrate with the above-mentioned technologies but also to broaden its applications even further.

In vivo monitoring of hemodynamic changes in ischemic stroke using photoacoustic tomography

Ischemic stroke occurs when a blood vessel supplying the brain is blocked, leading to decreased blood flow. Early diagnosis and treatment are crucial. However, existing clinical imaging methods have limitations, such as safety issues and low time resolution. To address these challenges, we propose using photoacoustic tomography (PAT) with a contrast agent, known for its high resolution and contrast capabilities. Our study involved imaging brain vasculature in three groups: normal, unilateral common carotid artery ligation (UCAL), and middle cerebral artery occlusion (MCAO). On the ischemic stroke side, we observed reduced blood vessel density and hemodynamic changes were evident after injecting indocyanine green for PAT. The photoacoustic intensity was notably lower in the ligated sides of the UCAL and MCAO groups, with statistically significant differences between the three groups. This work highlights PAT's potential as a powerful tool for early diagnosis and guidance in ischemic stroke cases.

Noise insensitive volumetric fusion method for enhanced photoacoustic microscopy

Significance

Photoacoustic imaging is an emerging imaging modality that combines the high contrast of optical imaging and the high penetration of acoustic imaging. However, the strong focusing of the laser beam in optical-resolution photoacoustic microscopy (OR-PAM) leads to a limited depth-of-field (DoF).

Aim

Here, a volumetric photoacoustic information fusion method was proposed to achieve large volumetric photoacoustic imaging at low cost.

Approach

First, the initial decision map was built through the focus detection based on the proposed three-dimensional Laplacian operator. Majority filter-based consistency verification and Gaussian filter-based map smoothing were then utilized to generate the final decision map for the construction of photoacoustic imaging with extended DoF.

Results

The performance of the proposed method was tested to show that our method can expand the limited DoF by a factor of 1.7 without the sacrifice of lateral resolution. Four sets of multi-focus vessel data at different noise levels were fused to verify the effectiveness and robustness of the proposed method.

Conclusions

The proposed method can efficiently extend the DoF of OR-PAM under different noise levels.

Review of Three-Dimensional Handheld Photoacoustic and Ultrasound Imaging Systems and Their Applications

Photoacoustic (PA) imaging is a non-invasive biomedical imaging technique that combines the benefits of optics and acoustics to provide high-resolution structural and functional information. This review highlights the emergence of three-dimensional handheld PA imaging systems as a promising approach for various biomedical applications. These systems are classified into four techniques: direct imaging with 2D ultrasound (US) arrays, mechanical-scanning-based imaging with 1D US arrays, mirror-scanning-based imaging, and freehand-scanning-based imaging. A comprehensive overview of recent research in each imaging technique is provided, and potential solutions for system limitations are discussed. This review will serve as a valuable resource for researchers and practitioners interested in advancements and opportunities in three-dimensional handheld PA imaging technology.

Non-invasive Deep-Brain Imaging with 3D Integrated Photoacoustic Tomography and Ultrasound Localization Microscopy (3D-PAULM)

Photoacoustic computed tomography (PACT) is a proven technology for imaging hemodynamics in deep brain of small animal models. PACT is inherently compatible with ultrasound (US) imaging, providing complementary contrast mechanisms. While PACT can quantify the brain's oxygen saturation of hemoglobin (sO2), US imaging can probe the blood flow based on the Doppler effect. Further, by tracking gas-filled microbubbles, ultrasound localization microscopy (ULM) can map the blood flow velocity with sub-diffraction spatial resolution. In this work, we present a 3D deep-brain imaging system that seamlessly integrates PACT and ULM into a single device, 3D-PAULM. Using a low ultrasound frequency of 4 MHz, 3D-PAULM is capable of imaging the whole-brain hemodynamic functions with intact scalp and skull in a totally non-invasive manner. Using 3D-PAULM, we studied the mouse brain functions with ischemic stroke. Multi-spectral PACT, US B-mode imaging, microbubble-enhanced power Doppler (PD), and ULM were performed on the same mouse brain with intrinsic image co-registration. From the multi-modality measurements, we future quantified blood perfusion, sO2, vessel density, and flow velocity of the mouse brain, showing stroke-induced ischemia, hypoxia, and reduced blood flow. We expect that 3D-PAULM can find broad applications in studying deep brain functions on small animal models.

Integrated Photoacoustic, Ultrasound, and Angiographic Tomography (PAUSAT) for NonInvasive Whole-Brain Imaging of Ischemic Stroke

Presented here is an experimental ischemic stroke study using our newly developed noninvasive imaging system that integrates three acoustic-based imaging technologies: photoacoustic, ultrasound, and angiographic tomography (PAUSAT). Combining these three modalities helps acquire multi-spectral photoacoustic tomography (PAT) of the brain blood oxygenation, high-frequency ultrasound imaging of the brain tissue, and acoustic angiography of the cerebral blood perfusion. The multi-modal imaging platform allows the study of cerebral perfusion and oxygenation changes in the whole mouse brain after stroke. Two commonly used ischemic stroke models were evaluated: the permanent middle cerebral artery occlusion (pMCAO) model and the photothrombotic (PT) model. PAUSAT was used to image the same mouse brains before and after a stroke and quantitatively analyze both stroke models. This imaging system was able to clearly show the brain vascular changes after ischemic stroke, including significantly reduced blood perfusion and oxygenation in the stroke infarct region (ipsilateral) compared to the uninjured tissue (contralateral). The results were confirmed by both laser speckle contrast imaging and triphenyltetrazolium chloride (TTC) staining. Furthermore, stroke infarct volume in both stroke models was measured and validated by TTC staining as the ground truth. Through this study, we have demonstrated that PAUSAT can be a powerful tool in noninvasive and longitudinal preclinical studies of ischemic stroke.