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

Laser beam array spot steering customized trajectories using the acousto-optic effect

We demonstrate a proof of principle of a technique for array laser beam steering according to a generalized elliptical path, using the acousto-optic effect. We explore the dynamic trajectories of the diffraction orders of a Gaussian beam, diffracted by a dynamic 2D sinusoidal phase grating. The latter is generated by two crossed acoustic waves propagating in a transparent medium, giving rise to 2D dynamic sinusoidal refractive index variation. We particularly emphasize the case where the two crossed acoustic waves are modulated in frequency and have a phase difference. In such case, the resulting diffraction orders are dynamic and follow some particular trajectory's shapes, from linear and circular to generalized elliptical shapes. Inspired by Poincaré and Bloch spheres used to represent polarization and quantum states, we suggest the sphere volume trajectory shape to represent all trajectories' shapes of all diffraction orders.

Barium sulfate and pigment admixture for photoacoustic and x-ray contrast imaging of the gut

Significance

X-ray imaging is frequently used for gastrointestinal imaging. Photoacoustic imaging (PAI) of the gastrointestinal tract is an emerging approach that has been demonstrated for preclinical imaging of small animals. A contrast agent active in both modalities could be useful for imaging applications.

Aim

We aimed to develop a dual-modality contrast agent comprising an admixture of barium sulfate with pigments that absorb light in the second near-infrared region (NIR-II), for preclinical imaging with both x-ray and PAI modalities.

Approach

Eleven different NIR-II dyes were evaluated after admixture with a 40% w/v barium sulfate mixture. The resulting NIR-II absorption in the soluble fraction and in the total mixture was characterized. Proof-of-principle imaging studies in mice were carried out.

Results

Pigments that produced more uniform suspensions were assessed further for photoacoustic contrast signal at a wavelength of 1064 nm that corresponds to the output of the Nd:YAG laser used. Phantom imaging studies demonstrated that the pigment-barium sulfate mixture generated imaging contrast in both x-ray and PAI modalities. The optimal pigment selected for further study was a cyanine tetrafluoroborate salt. Ex-vivo and whole-body mouse imaging demonstrated that photoacoustic and x-ray contrast signals co-localized in the intestines for both imaging modalities.

Conclusion

These data demonstrate that commercially-available NIR-II pigments can simply be admixed with barium sulfate to generate a dual-modality contrast agent appropriate for small animal gastrointestinal imaging.

Ultra-widefield and high-speed spiral laser scanning OR-PAM: System development and characterization

Photoacoustic microscopy (PAM) is a high-resolution imaging modality that has been mainly implemented with small field of view applications. Here, we developed a fast PAM system that utilizes a unique spiral laser scanning mechanism and a wide acoustic detection unit. The developed system can image an area of 12.5 cm² in 6.4 s. The system has been characterized using highly detailed phantoms. Finally, the imaging capabilities of the system were further demonstrated by imaging a sheep brain ex vivo and a rat brain in vivo.

Three-dimensional reconstructing undersampled photoacoustic microscopy images using deep learning

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

Recent progress of second near-infrared (NIR-II) fluorescence microscopy in bioimaging

Visualizing biological tissues in vivo at a cellular or subcellular resolution to explore molecular signaling and cell behaviors is a crucial direction for research into biological processes. In vivo imaging can provide quantitative and dynamic visualization/mapping in biology and immunology. New microscopy techniques combined with near-infrared region fluorophores provide additional avenues for further progress in vivo bioimaging. Based on the development of chemical materials and physical optoelectronics, new NIR-II microscopy techniques are emerging, such as confocal and multiphoton microscopy, light-sheet fluorescence microscopy (LSFM), and wide-field microscopy. In this review, we introduce the characteristics of in vivo imaging using NIR-II fluorescence microscopy. We also cover the recent advances in NIR-II fluorescence microscopy techniques in bioimaging and the potential for overcoming current challenges.

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

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

Target ischemic stroke model creation method using photoacoustic microscopy with simultaneous vessel monitoring and dynamic photothrombosis induction

The ischemic stroke animal model evaluates the efficacy of reperfusion and neuroprotective strategies for ischemic injuries. Various conventional methods have been reported to induce the ischemic models; however, controlling specific neurological deficits, mortality rates, and the extent of the infarction is difficult as the size of the affected region is not precisely controlled. In this paper, we report a single laser-based localized target ischemic stroke model development method by simultaneous vessel monitoring and photothrombosis induction using photoacoustic microscopy (PAM), which has minimized the infarct size at precise location with high reproducibility. The proposed method has significantly reduced the infarcted region by illuminating the precise localization. The reproducibility and validity of suggested method have been demonstrated through repeated experiments and histological analyses. These results demonstrate that our method can provide the ischemic stroke model closest to the clinical pathology for brain ischemia research from inducement, occurrence mechanisms to the recovery process.

Local-flexible coupling optical-resolution photoacoustic microscopy with enhanced sensitivity

An acoustic coupling scheme largely determines the performance of optical-resolution photoacoustic microscopy (OR-PAM), including practicability, sensitivity, and stability. In this study, we propose OR-PAM based on a local-flexible acoustic coupling scheme, which includes a well-designed combiner connecting a set of circulating systems. The combiner integrates an objective lens and an ultrasonic transducer, controls the water level, restricts the flow rate, and drains bubbles. The circulating system provides sustained and steady flowing water. The flowing water constrained in the combiner and the circulating system forms a flexible and stable local contact between the sample and the transducer. Phantom experiments demonstrate that the proposed method can maintain high optical resolution but improve the detection sensitivity by approximately 1.9 times in comparison to dry coupling. In vivo imaging experiments of the mouse eyeground are conducted to examine the practicability of the proposed system in biomedicine. Moreover, in vivo experiments show that OR-PAM based on local-flexible coupling can reveal more details of eyeground microvasculatures, benefiting from its enhanced sensitivity. These merits promise that OR-PAM based on local-flexible coupling may have broad applications in biomedical fields.

Video-rate high-resolution single-pixel nonscanning photoacoustic microscopy

Optical-resolution photoacoustic microscopy (OR-PAM) is widely utilized in biomedical applications because of its ability to noninvasively image biological tissues in vivo while providing high-resolution morphological and functional information. However, one drawback of conventional OR-PAM is its imaging speed, which is restricted by the scanning technique employed. To achieve a higher imaging frame rate, we present video-rate high-resolution single-pixel nonscanning photoacoustic microscopy (SPN-PAM), which utilizes Fourier orthogonal basis structured planar illumination to overcome the above-mentioned limitations. A 473 × 473 µm² imaging field of view (FOV) with 3.73 µm lateral resolution and video-rate imaging of 30 Hz were achieved. In addition, in both in vitro cell and in vivo mouse vascular hemodynamic imaging experiments, high-quality images were obtained at ultralow sampling rates. Thus, the proposed high-resolution SPN-PAM with video-rate imaging speed provides new insights into high-speed PA imaging and could be a powerful tool for rapid biological imaging.

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

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

Recent advances in high-speed photoacoustic microscopy

Photoacoustic (PA) microscopy (PAM) has achieved remarkable progress in biomedicine in the past decade. It is a fast-rising imaging modality with diverse applications, such as hemodynamics, oncology, metabolism, and neuroimaging. Combining optical excitation and acoustic detection, the hybrid nature of PAM provides advantages of rich contrast and deep penetration. In recent years, high-speed PAM has flourished and enabled high-speed wide-field imaging of functional activity. In this review, we summarize the most recent advances in high-speed PAM technologies, including high-repetition-rate multi-wavelength laser development, fast scanning techniques, and novel PA signal acquisition strategies.

Waterproof Galvanometer Scanner-Based Handheld Photoacoustic Microscopy Probe for Wide-Field Vasculature Imaging In Vivo

Photoacoustic imaging (PAI) is a hybrid non-invasive imaging technique used to merge high optical contrast and high acoustic resolution in deep tissue. PAI has been extensively developed by utilizing its advantages that include deep imaging depth, high resolution, and label-free imaging. As a representative implementation of PAI, photoacoustic microscopy (PAM) has been used in preclinical and clinical studies for its micron-scale spatial resolution capability with high optical absorption contrast. Several handheld and portable PAM systems have been developed that improve its applicability to several fields, making it versatile. In this study, we developed a laboratory-customized, two-axis, waterproof, galvanometer scanner-based handheld PAM (WP-GVS-HH-PAM), which provides an extended field of view (14.5 × 9 mm2) for wide-range imaging. The fully waterproof handheld probe enables free movement for imaging regardless of sample shape, and volume rate and scanning region are adjustable per experimental conditions. Results of WP-GVS-HH-PAM-based phantom and in vivo imaging of mouse tissues (ear, iris, and brain) confirm the feasibility and applicability of our system as an imaging modality for various biomedical applications.

Photoacoustic imaging of in vivo hemodynamic responses to sodium nitroprusside

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

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

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