Coherent Raman Scattering Unravelling Mechanisms Underlying Skull Optical Clearing for Through-Skull Brain Imaging

Amphiphilic hemicyanine molecular probes crossing the blood-brain barrier for intracranial optical imaging of glioblastoma

Intracranial optical imaging of glioblastoma (GBM) is challenging due to the scarcity of effective probes with blood-brain barrier (BBB) permeability and sufficient imaging depth. Herein, we describe a rational strategy for designing optical probes crossing the BBB based on an electron donor-π-acceptor system to adjust the lipid/water partition coefficient and molecular weight of probes. The amphiphilic hemicyanine dye (namely, IVTPO), which exhibits remarkable optical properties and effective BBB permeability, is chosen as an efficient fluorescence/photoacoustic probe for in vivo real-time imaging of orthotopic GBM with high resolution through the intact skull. Abnormal leakage of IVTPO adjacent to the developing tumor is unambiguously observed at an early stage of tumor development prior to impairment of BBB integrity, as assessed by commercial Evans blue (EB). Compared with EB, IVTPO demonstrates enhanced optical imaging capability and improved tumor-targeting efficacy. These results offer encouraging insights into medical diagnosis of intracranial GBM.

Strongly absorbing molecules make tissue transparent: a new insight for understanding tissue optical clearing

Optical imaging plays a central role in the field of biomedicine, but it suffers from the light scattering of tissues. The research group from Stanford University has reported a counterintuitive observation that strongly absorbing molecules could achieve optical transparency in live animals, providing a new insight for understanding tissue optical clearing. It empowers scientists to leverage optical imaging techniques for in vivo observation of a wide range of deep-seated structures and activities.

Delivery and kinetics of immersion optical clearing agents in tissues: Optical imaging from ex vivo to in vivo

Advanced optical imaging provides a powerful tool for the structural and functional analysis of tissues with high resolution and contrast, but the imaging performance decreases as light propagates deeper into the tissue. Tissue optical clearing technique demonstrates an innovative way to realize deep-tissue imaging and have emerged substantially in the last two decades. Here, we briefly reviewed the basic principles of tissue optical clearing techniques in the view of delivery strategies via either free diffusion or external forces-driven advection, and the commonly-used optical techniques for monitoring kinetics of clearing agents in tissue, as well as their ex vivo to in vivo applications in multiple biomedical research fields. With future efforts on the even distribution of both clearing agents and probes, excavation of more effective clearing agents, and automation of tissue clearing processes, tissue optical clearing should provide more insights into the fundamental questions in biological events clinical diagnostics.

Transcranial photobiomodulation improves insulin therapy in diabetic microglial reactivity and the brain drainage system

The dysfunction of microglia in the development of diabetes is associated with various diabetic complications, while traditional insulin therapy is insufficient to rapidly restore the function of microglia. Therefore, the search for new alternative methods of treating diabetes-related dysfunction of microglia is urgently needed. Here, we evaluate the effects of transcranial photobiomodulation (tPBM) on microglial function in diabetic mice and investigate its mechanism. We find tPBM treatment effectively improves insulin therapy on microglial morphology and reactivity. We also show that tPBM stimulates brain drainage system through activation of meningeal lymphatics, which contributes to the removal of inflammatory factor, and increase of microglial purinergic receptor P2RY12. Besides, the energy expenditure and locomotor activity of diabetic mice are also improved by tPBM. Our results demonstrate that tPBM can be an efficient, non-invasive method for the treatment of microglial dysfunction caused by diabetes, and also has the potential to prevent diabetic physiological disorders.

Tissue Clearing and Its Application in the Musculoskeletal System

The musculoskeletal system is an integral part of the human body. Currently, most skeletal muscle research is conducted through conventional histological sections due to technological limitations and the structure of skeletal muscles. For studying and observing bones and muscles, there is an urgent need for three-dimensional, objective imaging technologies. Optical tissue-clearing technologies seem to offer a novel and accessible approach to research of the musculoskeletal system. Using this approach, the components which cause refraction or prevent light from penetrating into the tissue are physically and chemically eliminated; then the liquid in the tissue is replaced with high-refractive-index chemicals. This innovative method, which allows three-dimensional reconstruction at the cellular and subcellular scale, significantly improves imaging depth and resolution. Nonetheless, this technology was not originally developed to image bones or muscles. When compared with brain and nerve organs which have attracted considerable attention in this field, the musculoskeletal system contains fewer lipids and has high levels of hemoglobin, collagen fibers, and inorganic hydroxyapatite crystals. Currently, three-dimensional imaging methods are widely used in the diagnosis and treatment of skeletal and muscular illnesses. In this regard, it is vitally important to review and evaluate the optical tissue-clearing technologies currently employed in the musculoskeletal system, so that researchers may make an informed decision. In the meantime, this study offers guidelines and recommendations for expanding the use of this technology in the musculoskeletal system.

Imaging plant tissues: advances and promising clearing practices

The study of the organ structure of plants and understanding their physiological complexity requires 3D imaging with subcellular resolution. Most plant organs are highly opaque to light, and their study under optical sectioning microscopes is therefore difficult. In animals, many protocols have been developed to make organs transparent to light using clearing protocols (CPs). By contrast, clearing plant tissues is challenging because of the presence of fibers and pigments. We describe progress in the development of plant CPs over the past 20 years through a modified taxonomy of CPs based on their physical and optical parameters that affect tissue properties. We also discuss successful approaches that combine CPs with new microscopy methods and their future applications in plant science research.

In vivo tissue optical clearing assisted through-skull targeted photothrombotic ischemic stroke model in mice

SIGNIFICANCE

Photothrombotic stroke is an important and widely used model for ischemic stroke research. However, the significant scattering of the skull during the procedure limits the light's ability to penetrate and focus on its target. Targeted photothrombosis uses surgery-based skull windows to obtain optical access to the brain, but it renders the brain's environment unnatural even before a stroke is established.

AIM

To establish a targeted, controllable ischemic stroke model in mice through an intact skull.

APPROACH

The in vivo skull optical clearing technique provides a craniotomy-free "optical window" that allows light to penetrate. Alongside the local photodynamic effect, we have established targeted photothrombosis without skull removal, effectively controlling the degree of thrombotic occlusion by changing the light dose.

RESULTS

Ex vivo and in vivo results demonstrated that skull optical clearing treatment significantly enhanced light's ability to penetrate the skull and focus on its target, contributing to thrombotic occlusion. The skull optical clearing window was also used for continuous blood flow mapping, and the relationship between light dose and injury degree was evaluated over 14 days of monitoring. Per our findings, increasing the light dose was accompanied by more severe infarction, indicating that the model was easily controllable.

CONCLUSIONS

Herein, a targeted, controllable ischemic stroke model was established by combinedly running an in vivo skull optical clearing technique and a photothrombotic procedure, avoiding unnecessary damage or environmental changes to the brain caused by surgery on the skull. Our established model should offer significant value to research on ischemic stroke.

Optical clearing imaging assisted evaluation of urokinase thrombolytic therapy on cerebral vessels with different sizes

Ischemic stroke is caused by occlusion of the blood vessels in the brain, where intravenous thrombolytic therapy is the most effective treatment. Urokinase is a commonly used drug for intravenous thrombolytic therapy, while the effect of vessel size has not been thoroughly studied on urokinase. In this work, using the thrombin-combined photothrombosis model and craniotomy-free skull optical clearing window, we studied the recanalization of different cortical vessels after urokinase treatment. The results demonstrated that, compared to small vessels in distal middle cerebral artery (MCA) and large MCA, urokinase has the best therapeutic effect on secondary branches of MCA. This study holds potential to provide references for the clinical applications of urokinase.

In vivo simultaneous nonlinear absorption Raman and fluorescence (SNARF) imaging of mouse brain cortical structures

Label-free multiphoton microscopy is a powerful platform for biomedical imaging. Recent advancements have demonstrated the capabilities of transient absorption microscopy (TAM) for label-free quantification of hemoglobin and stimulated Raman scattering (SRS) microscopy for pathological assessment of label-free virtual histochemical staining. We propose the combination of TAM and SRS with two-photon excited fluorescence (TPEF) to characterize, quantify, and compare hemodynamics, vessel structure, cell density, and cell identity in vivo between age groups. In this study, we construct a simultaneous nonlinear absorption, Raman, and fluorescence (SNARF) microscope with the highest reported in vivo imaging depth for SRS and TAM at 250–280 μm to enable these multimodal measurements. Using machine learning, we predict capillary-lining cell identities with 90% accuracy based on nuclear morphology and capillary relationship. The microscope and methodology outlined herein provides an exciting route to study several research topics, including neurovascular coupling, blood-brain barrier, and neurodegenerative diseases.

In this study a microscope is constructed that carries out simultaneous nonlinear absorption, Raman, and fluorescence (SNARF). Machine learning is then used to predict capillary-lining cell identities with 90% accuracy based on nuclear morphology and capillary relationship, which in combination with the developed microscope, can provide a means to study several fields such as neurovascular coupling.

Improving SERS bioimaging of subcutaneous phantom in vivo with optical clearing

Surface-enhanced Raman scattering (SERS) has proven to be a promising technique for different types of imaging including preoperative and intraoperative in vivo tumor visualization. However, the strong scattering of the turbid tissue limits its use in subcutaneous areas. In this article, we used an optical clearing technique to improve the SERS signal from a subcutaneous tumor phantom. The phantom is a 2 mm sphere of calcium alginate with incorporated petal-like gap-enhanced Raman tags. The use of optical clearing increases the SERS signal target-to-background ratio for 5 times and allow to decrease the total imaging time for at least 10 times. In addition, SERS imaging assisted with optical clearing made it possible to more precisely determine the shape and boundaries of the implanted phantom. The combination of optical clearing and SERS is a promising strategy for the clinical imaging of subcutaneous objects that are usually shielded by dermal tissue.

Advances in nonlinear optical microscopy techniques for in vivo and in vitro neuroimaging

Understanding the mechanism of the brain via optical microscopy is one of the challenges in neuroimaging, considering the complex structures. Advanced neuroimaging techniques provide a more comprehensive insight into patho-mechanisms of brain disorders, which is useful in the early diagnosis of the pathological and physiological changes associated with various neurodegenerative diseases. Recent advances in optical microscopy techniques have evolved powerful tools to overcome scattering of light and provide improved in vivo neuroimaging with sub-cellular resolution, endogenous contrast specificity, pinhole less optical sectioning capability, high penetration depth, and so on. The following article reviews the developments in various optical imaging techniques including two-photon and three-photon fluorescence, second-harmonic generation, third-harmonic generation, coherent anti-Stokes Raman scattering, and stimulated Raman scattering in neuroimaging. We have outlined the potentials and drawbacks of these techniques and their possible applications in the investigation of neurodegenerative diseases.

Assessment of tissue-specific changes in structure and function induced by in vivo skin/skull optical clearing techniques

BACKGROUND OBJECTIVES

Newly developed in vivo skin and skull optical clearing techniques can greatly improve the optical imaging performance, showing great advantages and clinical prospects. However, there is a poor understanding of in vivo optical clearing-induced changes in the skin and skull.

MATERIALS AND METHODS

Here, we employed in vivo skin/skull optical clearing techniques to improve the optical coherence tomography (OCT) imaging quality. And we also used polarization-sensitive OCT to monitor the dynamic changes in the polarization characteristics of the skin and skull during in vivo optical clearing processes. Two-photon imaging was used to evaluate changes in tissue barrier function and structure. Additionally, Raman spectra were employed for assessing the changes of each component in the skin and skull before and after optical clearing treatment.

RESULTS

The results indicated that the polarization states of the skin and skull were altered with the usages of optical clearing agents. And the barrier permeability and collagen fiber distribution of them became disordered. Furthermore, the Raman spectra of tissue demonstrated that the applications of in vivo tissue optical clearing methods could lead to the reduction of proteins, lipids, and inorganic salts in these two organs. Interestingly, after recovery treatment, the structure and function of the skin and skull could almost recover to the initial states.

CONCLUSION

In vivo tissue optical clearing can lead to changes in the structure and function of tissue, which was reversible to some extent. This study plays an important role in revealing the underlying mechanisms of tissue optical clearing techniques; moreover, it is conducive to the development and optimization of a novel in vivo tissue optical clearing approaches in future.

Tissue Optical Clearing for Biomedical Imaging: From In Vitro to In Vivo

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This chapter firstly gives a brief introduction to mechanisms of tissue optical clearing techniques, from the physical mechanism to chemical mechanism, which is the most important foundation to develop tissue optical clearing methods. During the past years, in vitro and in vivo tissue optical clearing methods were developed. In vitro tissue optical clearing techniques, including the solvent-based clearing methods and the hydrophilic reagents-based clearing methods, combined with labeling technique and advanced microscopy, can be applied to image 3D microstructure of tissue blocks or whole organs such as brain and spinal cord with high resolution. In vivo skin or skull optical clearing, promise various optical imaging techniques to detect cutaneous or cortical cell and vascular structure and function without surgical window.

Physical and chemical mechanisms of tissue optical clearing

Advanced optical methods combined with various probes pave the way toward molecular imaging within living cells. However, major challenges are associated with the need to enhance the imaging resolution even further to the subcellular level for the imaging of larger tissues, as well as for in vivo studies. High scattering and absorption of opaque tissues limit the penetration of light into deep tissues and thus the optical imaging depth. Tissue optical clearing technique provides an innovative way to perform deep-tissue imaging. Recently, various optical clearing methods have been developed, which provide tissue clearing based on similar physical principles via different chemical approaches. Here, we introduce the mechanisms of the current clearing methods from fundamental physical and chemical perspectives, including the main physical principle, refractive index matching via various chemical approaches, such as dissociation of collagen, delipidation, decalcification, dehydration, and hyperhydration, to reduce scattering, as well as decolorization to reduce absorption.

Tissue clearing technique: Recent progress and biomedical applications

Organisms are inherently three dimensional, thus comprehensive understanding of the complicated biological system requires analysis of organs or even whole bodies in the context of three dimensions. However, this is a tremendous task since the biological specimens are naturally opaque, a major obstacle in whole-body and whole-organ imaging. Tissue clearing technique provides a prospective solution and has become a powerful tool for three-dimensional imaging and quantification of organisms. Tissue clearing technique aims to make tissue transparent by minimizing light scattering and light absorption, thus allowing deep imaging of large volume samples. When combined with diverse molecular labeling methods and high-throughput optical sectioning microscopes, tissue clearing technique enables whole-body and whole-organ imaging at cellular or subcellular resolution, providing detailed and comprehensive information about the intact biological systems. Here, we give an overview of recent progress and biomedical applications of tissue clearing technique. We introduce the mechanisms and basic principles of tissue clearing, and summarize the current tissue clearing methods. Moreover, the available imaging techniques and software packages for data processing are also presented. Finally, we introduce the recent advances in applications of tissue clearing in biomedical fields. Tissue clearing contributes to the investigation of structure-function relationships in intact mammalian organs, and opens new avenues for cellular and molecular mapping of intact human organs. We hope this review contributes to a better understanding of tissue clearing technique and can help researchers to select the best-suited clearing protocol for their experiments.

3-photon microscopy of myelin in mouse digital skin excited at the 1700-nm window

Myelin is a key component of the peripheral nervous system, whose structure anomaly in the digital skin is implicated in neuropathy. Here we demonstrate an in vivo labeling and imaging technique, capable of visualizing myelin sheaths deep in the mouse digital skin. Through material characterization, we verify that 3-photon fluorescence (3PF) can be generated from a commonly used dye- FluoroMyelin Red for labeling myelin, excited at the 1700-nm window. Topical injection of FluoroMyelin Red in the mouse digit leads to bright labeling of myelin sheaths. Harnessing the deep-penetration capability of 3-photon microscopy excited at the 1700-nm window, we demonstrate that 3PF imaging of FluoroMyelin Red-labeled myelin sheaths in the mouse digit in vivo can be achieved to a depth 340 μm below the skin surface, revealing both branching bundle of and individual myelin sheaths.

Mammalian cell and tissue imaging using Raman and coherent Raman microscopy

Direct imaging of metabolism in cells or multicellular organisms is important for understanding many biological processes. Raman scattering (RS) microscopy, particularly, coherent Raman scattering (CRS) such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), has emerged as a powerful platform for cellular imaging due to its high chemical selectivity, sensitivity, and imaging speed. RS microscopy has been extensively used for the identification of subcellular structures, metabolic observation, and phenotypic characterization. Conjugating RS modalities with other techniques such as fluorescence or infrared (IR) spectroscopy, flow cytometry, and RNA-sequencing can further extend the applications of RS imaging in microbiology, system biology, neurology, tumor biology and more. Here we overview RS modalities and techniques for mammalian cell and tissue imaging, with a focus on the advances and applications of CARS and SRS microscopy, for a better understanding of the metabolism and dynamics of lipids, protein, glucose, and nucleic acids in mammalian cells and tissues. This article is categorized under: Laboratory Methods and Technologies > Imaging Biological Mechanisms > Metabolism Analytical and Computational Methods > Analytical Methods.

Improvement of light penetration in biological tissue using an ultrasound-induced heating tunnel

The major obstacles of optical imaging and photothermal therapy in biomedical applications is the strong scattering of light within biological tissues resulting in light defocusing and limited penetration. In this study, we propose high intensity focused ultrasound (HIFU)-induced heating tunnel to reduce the photon scattering. To verify our idea, Monte Carlo simulation and intralipid-phantom experiments were conducted. The results show that the thermal effect created by HIFU could improve the light fluence at the targeted region by 3% in both simulation and phantom experiments. Owing to the fluence increase, similar results can also be found in the photoacoustic experiments. In conclusion, our proposed method shows a noninvasive way to increase the light delivery efficiency in turbid medium. It is expected that our finding has a potential for improving the focal light delivery in photoacoustic imaging and photothermal therapy.

Rapid skin optical clearing enhancement with salicylic acid for imaging blood vessels in vivo

BACKGROUND

Light penetration in deeper tissue is impeded by the skin scattering properties, which significantly limits the clinical applications of light in medical diagnosis and therapy. To overcome this problem, skin optical clearing methods using different optical clearing agents (OCAs) have been extensively developed to clear the dermis tissue. It is critically important to remove the outmost stratum corneum (SC) before the OCAs were applied for optical clearing, since the SC works as a natural barrier to the OCAs. For this, a controllable approach for the SC disruption through physical or chemical methods is highly required for enhanced skin optical clearing.

METHODS

Salicylic acid (SA) was combined with OCAs as a rapid skin optical clearing method to create a transparent window within 5 min. The clearing efficacy of this method was demonstrated by using dorsal skin model of mice. In addition, the intensity variations of vessel gray images and diffuse reflectance (DR) spectra were used to quantify the optical clearing efficacy, which were acquired by a low-cost self-built white light imaging system and optical fiber spectrometer, respectively.

RESULTS

Within a specific action time of the OCAs to the skin tissue, the enhanced images of the deeper blood vessels were obtained through the removal of the SC. It takes 5 min for the skin to turn transparent and 15 min to visualize the microvascular morphology for naked eyes. Furthermore, the intensity of blood vessel gray images was identified to be an evaluation parameter for quantifying the optical clearing efficacy.

CONCLUSIONS

An efficient and easy-to-handle method for enhanced skin optical clearing was established by combining SA with OCAs, which could boost the clinical applications of light in medical diagnosis and therapy.

Facile Synthesis of Efficient Luminogens with AIE Features for Three-Photon Fluorescence Imaging of the Brain through the Intact Skull

Visualization of the brain in its native environment is important for understanding common brain diseases. Herein, bright luminogens with remarkable aggregation-induced emission (AIE) characteristics and high quantum yields of up to 42.6% in the solid state are synthesized through facile reaction routes. The synthesized molecule, namely BTF, shows ultrabright far-red/near-infrared emission and can be fabricated into AIE dots by a simple nanoprecipitation procedure. Due to their high brightness, large Stokes shift, good biocompatibility, satisfactory photostability, and large three-photon absorption cross section, the AIE dots can be utilized as efficient fluorescent nanoprobes for in vivo brain vascular imaging through the intact skull by a three-photon fluorescence microscopy imaging technique. This is the first example of using AIE dots for the visualization of the cerebral stroke process through the intact skull of a mouse with high penetration depth and good image contrast. Such good results are anticipated to open up a new venue in the development of efficient emitters with strong nonlinear optical effects for noninvasive bioimaging of living brain.

Tissue imaging depth limit of stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) microscopy is a promising technique for studying tissue structure, physiology, and function. Similar to other nonlinear optical imaging techniques, SRS is severely limited in imaging depth due to the turbidity and heterogeneity of tissue, regardless of whether imaging in the transmissive or epi mode. While this challenge is well known, important imaging parameters (namely maximum imaging depth and imaging signal to noise ratio) have rarely been reported in the literature. It is also important to compare epi mode and transmissive mode imaging to determine the best geometry for many tissue imaging applications. In this manuscript we report the achievable signal sizes and imaging depths using a simultaneous epi/transmissive imaging approach in four different murine tissues; brain, lung, kidney, and liver. For all four cases we report maximum signal sizes, scattering lengths, and achievable imaging depths as a function of tissue type and sample thickness. We report that for murine brain samples thinner than 2 mm transmissive imaging provides better results, while samples 2 mm and thicker are best imaged with epi imaging. We also demonstrate the use of a CNN-based denoising algorithm to yield a 40 µm (24%) increase in achievable imaging depth.