Real-time functional optical-resolution photoacoustic microscopy using high-speed alternating illumination at 532 and 1064 nm

Nanophotonic-enhanced photoacoustic imaging for brain tumor detection

Photoacoustic brain imaging (PABI) has emerged as a promising biomedical imaging modality, combining high contrast of optical imaging with deep tissue penetration of ultrasound imaging. This review explores the application of photoacoustic imaging in brain tumor imaging, highlighting the synergy between nanomaterials and state of the art optical techniques to achieve high-resolution imaging of deeper brain tissues. PABI leverages the photoacoustic effect, where absorbed light energy causes thermoelastic expansion, generating ultrasound waves that are detected and converted into images. This technique enables precise diagnosis, therapy monitoring, and enhanced clinical screening, specifically in the management of complex diseases such as breast cancer, lymphatic disorder, and neurological conditions. Despite integration of photoacoustic agents and ultrasound radiation, providing a comprehensive overview of current methodologies, major obstacles in brain tumor treatment, and future directions for improving diagnostic and therapeutic outcomes. The review underscores the significance of PABI as a robust research tool and medical method, with the potential to revolutionize brain disease diagnosis and treatment.

Recent advances in nanomaterials for neural applications: opportunities and challenges

Nanomedicines are promising for delivering drugs to the central nervous system, though their precision is still being improved. Fortifying nanoparticles with vital molecules can interact with the blood-brain barrier, enabling access to brain tissue. This study summarizes recent advances in nanomedicine to treat neurological complications. The integration of nanotechnology into cell biology aids in the study of brain cells' interactions. Magnetic microhydrogels have exhibited superior neuron activation compared with superparamagnetic iron oxide nanoparticles and hold promise for neuropsychiatric disorders. Nanomaterials have shown notable results, such as tackling neurodegenerative diseases by hindering harmful protein buildup and regulating cellular processes. However, further studies of the safety and effectiveness of nanoparticles in managing neurological diseases and disorders are still required.

Photoacoustic (532 and 1064 nm) and ultrasonic coscanning microscopy for in vivo imaging on small animals: A productized strategy

Photoacoustic microscopy provides a new dimension of observation in microscopic life science. However, due to the high complexity of building a photoacoustic microscopy system, for many life science practitioners, it usually takes several years to build a stable photoacoustic microscopy system. For the above situation, in this article, a productized strategy of photoacoustic (532 and 1064 nm) and ultrasonic coscanning microscopy for in vivo imaging on small animals is presented. A 532 nm laser is applied to image blood vessels and pigments in label-free manner, whereas 1064 nm laser is applied to image pigments and some novel probes developed for NIR-II windows. Ultrasound is applied to assist photoacoustic imaging to accurately locate its imaging site in tissues. All 3D results are obtained with one single scan. The strategy presented here will help life science practitioners to build a stable photoacoustic microscopy platform.

Recent Trend of Ultrasound-Mediated Nanoparticle Delivery for Brain Imaging and Treatment

In view of the fact that the blood-brain barrier (BBB) prevents the transport of imaging probes and therapeutic agents to the brain and thus hinders the diagnosis and treatment of brain-related disorders, methods of circumventing this problem (e.g., ultrasound-mediated nanoparticle delivery) have drawn much attention. Among the related techniques, focused ultrasound (FUS) is a favorite means of enhancing drug delivery via transient BBB opening. Photoacoustic brain imaging relies on the conversion of light into heat and the detection of ultrasound signals from contrast agents, offering the benefits of high resolution and large penetration depth. The extensive versatility and adjustable physicochemical properties of nanoparticles make them promising therapeutic agents and imaging probes, allowing for successful brain imaging and treatment through the combined action of ultrasound and nanoparticulate agents. FUS-induced BBB opening enables nanoparticle-based drug delivery systems to efficiently access the brain. Moreover, photoacoustic brain imaging using nanoparticle-based contrast agents effectively visualizes brain morphologies or diseases. Herein, we review the progress in the simultaneous use of nanoparticles and ultrasound in brain research, revealing the potential of ultrasound-mediated nanoparticle delivery for the effective diagnosis and treatment of brain disorders.

Looking deep inside tissue with photoacoustic molecular probes: a review

Significance

Deep tissue noninvasive high-resolution imaging with light is challenging due to the high degree of light absorption and scattering in biological tissue. Photoacoustic imaging (PAI) can overcome some of the challenges of pure optical or ultrasound imaging to provide high-resolution deep tissue imaging. However, label-free PAI signals from light absorbing chromophores within the tissue are nonspecific. The use of exogeneous contrast agents (probes) not only enhances the imaging contrast (and imaging depth) but also increases the specificity of PAI by binding only to targeted molecules and often providing signals distinct from the background.

Aim

We aim to review the current development and future progression of photoacoustic molecular probes/contrast agents.

Approach

First, PAI and the need for using contrast agents are briefly introduced. Then, the recent development of contrast agents in terms of materials used to construct them is discussed. Then, various probes are discussed based on targeting mechanisms, in vivo molecular imaging applications, multimodal uses, and use in theranostic applications.

Results

Material combinations are being used to develop highly specific contrast agents. In addition to passive accumulation, probes utilizing activation mechanisms show promise for greater controllability. Several probes also enable concurrent multimodal use with fluorescence, ultrasound, Raman, magnetic resonance imaging, and computed tomography. Finally, targeted probes are also shown to aid localized and molecularly specific photo-induced therapy.

Conclusions

The development of contrast agents provides a promising prospect for increased contrast, higher imaging depth, and molecularly specific information. Of note are agents that allow for controlled activation, explore other optical windows, and enable multimodal use to overcome some of the shortcomings of label-free PAI.

Spectroscopic photoacoustic microscopic imaging during single spatial scan using broadband excitation light pulses with wavelength-dependent time delay

In most multispectral optical-resolution photoacoustic microscopy (OR-PAM), spatial scanning is repeated for each excitation wavelength, which decreases throughput and causes motion artifacts during spectral processing. This study proposes a new spectroscopic OR-PAM technique to acquire information on the photoacoustic signal intensity and excitation wavelength from single spatial scans. The technique involves irradiating an imaging target with two broadband optical pulses with and without wavelength-dependent time delays. The excitation wavelength of the sample is then calculated by measuring the time delay between the photoacoustic signals generated by the two optical pulses. This technique is validated by measuring the excitation wavelengths of dyes in tubes. Furthermore, we demonstrate the three-dimensional spectroscopic OR-PAM of cells stained with suitable dyes. Although the tradeoff between excitation efficiency and excitation bandwidth must be adjusted based on the application, combining the proposed technique with fast spatial scanning methods can significantly contribute to recent OR-PAM applications, such as monitoring quick biological events and microscale tracking of moving materials.

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.

Bifocal 532/1064 nm alternately illuminated photoacoustic microscopy for capturing deep vascular morphology in human skin

BACKGROUND

As a promising technology, photoacoustic microscopy (PAM) plays a critical role in diagnosis and assessment of dermatological conditions by providing subtle vascular networks non-invasively. However, the established PAMs are insufficient for clinical dermatology when faced with complex structures of human skin instead of animal models owing to high melanin content and superimposed vasculature for Asians, which cannot balance the spatial resolution and the imaging depth.

OBJECTIVES

To evaluate the ability of bifocal 532/1064-nm alternately illuminated photoacoustic microscopy (BF-PAM) to non-invasively reveal the morphological structure of human skin for improving the diagnosis and therapeutic efficacy of skin diseases.

METHODS

A BF-PAM was developed to capture biopsy-like information of human skin from epidermis to hypodermis. The optical foci of the two excitation beams are staggered in the axial direction to form an extended depth-of-field, which can maintain the lateral resolution and the contrast of PA image.

RESULTS

The imaging capability of the BF-PAM was demonstrated by depicting the vascular morphology of multilayered skin with imaging depth of ˜3 mm. Furtherly, vascular malformations in port-wine stains skin were quantitatively assessed without the need for any contrast agent, and the distribution, depth and diameter of the ectatic vessels can determine an optimal treatment protocol for port-wine stains lesions.

CONCLUSIONS

The quantitative vascular morphology in the dermis can be used to accurately assess vascular characteristics, in which case it enables clinicians to determine optimum treatment parameters in individual patients. As a non-invasive imaging technique, BF-PAM holds great potential to provide objective assessment to enhance the therapeutic efficacy.

ETHICAL STATEMENT

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Chinese Ethics Committee of Registering Clinical Trials (ChiECRCT20200184) and registered with Chinese Clinical Trial Registry (ChiCTR2000034400). Before skin imaging, written informed consent was taken from all individual participants.

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.

Another decade of photoacoustic imaging

Photoacoustic imaging - a hybrid biomedical imaging modality finding its way to clinical practices. Although the photoacoustic phenomenon was known more than a century back, only in the last two decades it has been widely researched and used for biomedical imaging applications. In this review we focus on the development and progress of the technology in the last decade (2010-2020). From becoming more and more user friendly, cheaper in cost, portable in size, photoacoustic imaging promises a wide range of applications, if translated to clinic. The growth of photoacoustic community is steady, and with several new directions researchers are exploring, it is inevitable that photoacoustic imaging will one day establish itself as a regular imaging system in the clinical practices.

Quickly Alternating Green and Red Laser Source for Real-time Multispectral Photoacoustic Microscopy

Multispectral photoacoustic microscopy uses a wavelength-dependent absorption difference as a contrast mechanism to image the target molecule. In this paper, we present a novel multispectral pulsed fiber laser source, which selectively alternates the excitation wavelengths between green and red colors based on the stimulated Raman scattering (SRS) effect for imaging. This laser has a high pulse repetition rate (PRR) of 300 kHz and high pulse energy of more than 200 nJ meeting the real-time requirements of optical-resolution photoacoustic microscopy imaging. By switching the polarization state of the pump light and optical paths of the pump light, the operating wavelengths of the light source can be selectively alternated at the same fast PRR for any two SRS peak wavelengths between 545 and 655 nm. At 545 nm excitation wavelength, molecular photoacoustic signals from both blood vessels and gold nanorods were obtained simultaneously. However, at 655 nm, the photoacoustic signals of gold nanorods were dominant because the absorption of light by the blood vessels decreased drastically in the spectral region over 600 nm. Thus the multispectral photoacoustic system designed using the novel laser source implemented here could simultaneously monitor the time-dependent fast movement of two molecules independently, having different wavelength-dependent absorption properties at a high repetition rate of 0.49 frames per second (fps).

Parallel Computing for Quantitative Blood Flow Imaging in Photoacoustic Microscopy

Photoacoustic microscopy (PAM) is an emerging biomedical imaging technology capable of quantitative measurement of the microvascular blood flow by correlation analysis. However, the computational cost is high, limiting its applications. Here, we report a parallel computation design based on graphics processing unit (GPU) for high-speed quantification of blood flow in PAM. Two strategies were utilized to improve the computational efficiency. First, the correlation method in the algorithm was optimized to avoid redundant computation and a parallel computing structure was designed. Second, the parallel design was realized on GPU and optimized by maximizing the utilization of computing resource in GPU. The detailed timings and speedup for each calculation step were given and the MATLAB and C/C++ code versions based on CPU were presented as a comparison. Full performance test shows that a stable speedup of ~80-fold could be achieved with the same calculation accuracy and the computation time could be reduced from minutes to just several seconds with the imaging size ranging from 1 × 1 mm² to 2 × 2 mm². Our design accelerates PAM-based blood flow measurement and paves the way for real-time PAM imaging and processing by significantly improving the computational efficiency.

Pulsed laser damage of gold nanorods in turbid media and its impact on multi-spectral photoacoustic imaging

Innovative biophotonic modalities such as photoacoustic imaging (PAI) have the potential to provide enhanced sensitivity and molecule-specific detection when used with nanoparticles. However, high peak irradiance levels generated by pulsed lasers can lead to modification of plasmonic nanoparticles. Thus, there is an outstanding need to develop practical methods to effectively predict the onset nanoparticle photomodification as well as a need to better understand the process during PAI. To address this need, we studied pulsed laser damage of gold nanorods (GNRs) using turbid phantoms and a multi-spectral near-infrared PAI system, comparing results with spectrophotometric measurements of non-scattering samples. Transmission electron microscopy and Monte Carlo modeling were also performed to elucidate damage processes. In the phantoms, shifts in PAI-detected spectra indicative of GNR damage were initiated at exposure levels one-third of that seen in non-scattering samples, due to turbidity-induced enhancement of subsurface fluence. For exposures approaching established safety limits, damage was detected at depths of up to 12.5 mm. Typically, GNR damage occurred rapidly, over the course of a few laser pulses. This work advances the development of test methods and numerical models as tools for assessment of nanoparticle damage and its implications, and highlights the importance of considering GNR damage in development of PAI products, even for exposures well below laser safety limits.

Photoacoustic imaging in the second near-infrared window: a review

Photoacoustic (PA) imaging is an emerging medical imaging modality that combines optical excitation and ultrasound detection. Because ultrasound scatters much less than light in biological tissues, PA generates high-resolution images at centimeters depth. In recent years, wavelengths in the second near-infrared (NIR-II) window (1000 to 1700 nm) have been increasingly explored due to its potential for preclinical and clinical applications. In contrast to the conventional PA imaging in the visible (400 to 700 nm) and the first NIR-I (700 to 1000 nm) window, PA imaging in the NIR-II window offers numerous advantages, including high spatial resolution, deeper penetration depth, reduced optical absorption, and tissue scattering. Moreover, the second window allows a fivefold higher light excitation energy density compared to the visible window for enhancing the imaging depth significantly. We highlight the importance of the second window for PA imaging and discuss the various NIR-II PA imaging systems and contrast agents with strong absorption in the NIR-II spectral region. Numerous applications of NIR-II PA imaging, including whole-body animal imaging and human imaging, are also discussed.