Quantitative second harmonic generation imaging and modeling of the optical clearing mechanism in striated muscle and tendon

A Perspective Review: Analyzing Collagen Alterations in Ovarian Cancer by High-Resolution Optical Microscopy

High-grade serous ovarian cancer (HGSOC) is the predominant subtype of ovarian cancer (OC), occurring in more than 80% of patients diagnosed with this malignancy. Histological and genetic analysis have confirmed the secretory epithelial of the fallopian tube (FT) as a major site of origin of HGSOC. Although there have been significant strides in our understanding of this disease, early stage detection and diagnosis are still rare. Current clinical imaging modalities lack the ability to detect early stage pathogenesis in the fallopian tubes and the ovaries. However, there are several microscopic imaging techniques used to analyze the structural modifications in the extracellular matrix (ECM) protein collagen in ex vivo FT and ovarian tissues that potentially can be modified to fit the clinical setting. In this perspective, we evaluate and compare the myriad of optical tools available to visualize these alterations and the invaluable insights these data provide on HGSOC initiation. We also discuss the clinical implications of these findings and how these data may help novel tools for early diagnosis of HGSOC.

Characterization of pathological stomach tissue using polarization-sensitive second harmonic generation microscopy

Alterations in collagen ultrastructure between human gastric adenocarcinoma and normal gastric tissue were investigated using polarization-resolved second harmonic generation (PSHG) microscopy. Cylindrical and trigonal symmetries were assumed to extract quantitative PSHG parameters, ρ, κ and S, from each image pixel. Statistically significant variations in these values were observed for gastric adenocarcinoma, indicating a higher disorder of collagen. Numerical focal volume simulations of crossing fibrils indicate increased S parameter is due to more intersecting collagen fibrils of varying diameters. These parameters were also able to distinguish between different grades of gastric adenocarcinoma indicating that PSHG may be useful for automated cancer diagnosis.

Measurement of rat and human tissue optical properties for improving the optical detection and visualization of peripheral nerves

Peripheral nerve damage frequently occurs in challenging surgical cases resulting in high costs and morbidity. Various optical techniques have proven effective in detecting and visually enhancing nerves, demonstrating their translational potential for assisting in nerve-sparing medical procedures. However, there is limited data characterizing the optical properties of nerves in comparison to surrounding tissues, thus limiting the optimization of optical nerve detection systems. To address this gap, the absorption and scattering properties of rat and human nerve, muscle, fat, and tendon were determined from 352-2500 nm. The optical properties highlighted an ideal region in the shortwave infrared for detecting embedded nerves, which remains a significant challenge for optical approaches. A 1000-1700 nm hyperspectral diffuse reflectance imaging system was used to confirm these results and identify optimal wavelengths for nerve imaging contrast in an in vivo rat model. Optimal nerve visualization contrast was achieved using 1190/1100 nm ratiometric imaging and was sustained for nerves embedded under ≥600 µm of fat and muscle. Overall, the results provide valuable insights for optimizing the optical contrast of nerves, including those embedded in tissue, which could lead to improved surgical guidance and nerve-sparing outcomes.

Ex vivo confocal Raman microspectroscopy of porcine dura mater supported by optical clearing

The effect of tissue optical clearing (TOC) to increase the probing depth and observe in-depth structure of the ex vivo porcine dura mater was studied by confocal Raman microspectroscopy (CRM). Raman intensities were significantly increased at the depth of 250 μm for all collagen bands after treatment with glycerol. The influence of glycerol on collagen hydration was also investigated. The results indicate that the process of TOC can be divided into three main steps. The first one is a fast process of tissue dehydration accompanied by collagen shrinkage while the second relatively slow process is related to the glycerol penetration into the interfibrillar space of collagen combined with swelling of tissue. The third step is collagen dissociation caused by the high concentration of glycerol. To the best of our knowledge, this study is the first example to introduce the TOC technique in assisting CRM of ex vivo dura mater in-depth probing.

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.

Recent Advancements in Optical Harmonic Generation Microscopy: Applications and Perspectives

Second harmonic generation (SHG) and third harmonic generation (THG) microscopies have emerged as powerful imaging modalities to examine structural properties of a wide range of biological tissues. Although SHG and THG arise from very different contrast mechanisms, the two are complimentary and can often be collected simultaneously using a modified multiphoton microscope. In this review, we discuss the needed instrumentation for these modalities as well as the underlying theoretical principles of SHG and THG in tissue and describe how these can be leveraged to extract unique structural information. We provide an overview of recent advances showing how SHG microscopy has been used to evaluate collagen alterations in the extracellular matrix and how this has been used to advance our knowledge of cancers, fibroses, and the cornea, as well as in tissue engineering applications. Specific examples using polarization-resolved approaches and machine learning algorithms are highlighted. Similarly, we review how THG has enabled developmental biology and skin cancer studies due to its sensitivity to changes in refractive index, which are ubiquitous in all cell and tissue assemblies. Lastly, we offer perspectives and outlooks on future directions of SHG and THG microscopies and present unresolved questions, especially in terms of overall miniaturization and the development of microendoscopy instrumentation.

An advanced optical clearing protocol allows label-free detection of tissue necrosis via multiphoton microscopy in injured whole muscle

Rationale: Structural remodeling or damage as a result of disease or injury is often not evenly distributed throughout a tissue but strongly depends on localization and extent of damaging stimuli. Skeletal muscle as a mechanically active organ can express signs of local or even systemic myopathic damage, necrosis, or repair. Conventionally, muscle biopsies (patients) or whole muscles (animal models) are mechanically sliced and stained to assess structural alterations histologically. Three-dimensional tissue information can be obtained by applying deep imaging modalities, e.g. multiphoton or light-sheet microscopy. Chemical clearing approaches reduce scattering, e.g. through matching refractive tissue indices, to overcome optical penetration depth limits in thick tissues.

Methods: Here, we optimized a range of different clearing protocols. We find aqueous solution-based protocols employing (20-80%) 2,2'-thiodiethanol (TDE) to be advantageous over organic solvents (dibenzyl ether, cinnamate) regarding the preservation of muscle morphology, ease-of-use, hazard level, and costs.

Results: Applying TDE clearing to a mouse model of local cardiotoxin (CTX)-induced muscle necrosis, a complete loss of myosin-II signals was observed in necrotic areas with little change in fibrous collagen or autofluorescence (AF) signals. The 3D aspect of myofiber integrity could be assessed, and muscle necrosis in whole muscle was quantified locally via the ratios of detected AF, forward- and backward-scattered Second Harmonic Generation (fSHG, bSHG) signals.

Conclusion: TDE optical clearing is a versatile tool to study muscle architecture in conjunction with label-free multiphoton imaging in 3D in injury/myopathy models and might also be useful in studying larger biofabricated constructs in regenerative medicine.

Optical clearing for photoacoustic lympho- and angiography beyond conventional depth limit in vivo

Photoacoustic (PA) imaging (PAI) is an emerging powerful tool for noninvasive real-time mapping of blood and lymphatic vessels and lymph nodes in vivo to diagnose cancer, lymphedema and other diseases. Among different PAI instruments, commercially available raster-scanning optoacoustic mesoscopy (RSOM) (iThera Medical GmbH., Germany) is useful for high-resolution imaging of different tissues with high potential of clinical translation. However, skin light scattering prevents mapping vessels and nodes deeper than 1-2 mm, that limits diagnostic values of PAI including RSOM. Here we demonstrate that glycerol-based tissue optical clearing (TOC) overcomes this challenge by reducing light scattering that improves RSOM depth penetration. In preclinical model of mouse limb in vivo, the replacement of conventional acoustic coupling agents such as water on the mixture of 70 % glycerol and 30 % ultrasound (US) gel resulted in the increase of tissue imaging depth in 1.5-2 times with 3D visualization of vessels with diameter down to 20 μm. To distinguish blood and lymphatic networks, we integrated label-free PA angiography (i.e., imaging of blood vessels), which uses hemoglobin as endogenous contrast agent, with PA lymphography based on labeling of lymphatic vessels with exogenous PA contrast agents. Similar to well-established clinical lymphography, contrast agents were injected in tissue and taken up by lymphatic vessels within a few minutes that provided quick RSOM lymphography. Furthermore, co-injection of PA contrast dye and multilayer nanocomposites as potential low-toxic drug-cargo showed selective prolonged accumulation of nanocomposites in sentinel lymph nodes. Overall, our findings open perspectives for deep and high resolution 3D PA angio- and lymphography, and for PA-guided lymphatic drug delivery using new RSOM & TOC approach.

Magnetic resonance contrast agents in optical clearing: Prospects for multimodal tissue imaging

Skin optical clearing effect ex vivo and in vivo was achieved by topical application of low molecular weight paramagnetic magnetic resonance contrast agents. This novel feature has not been explored before. By using collimated transmittance the diffusion coefficients of three clinically used magnetic resonance contrast agents, that is Gadovist, Magnevist and Dotarem as well as X-ray contrast agent Visipaque in mouse skin were determined ex vivo as (4.29 ± 0.39) × 10^-7 cm² /s, (5.00 ± 0.72) × 10^-7 cm² /s, (3.72 ± 0.67) × 10^-7 cm² /s and (1.64 ± 0.18) × 10^-7 cm² /s, respectively. The application of gadobutrol (Gadovist) resulted in efficient optical clearing that in general, was superior to other contrast agents tested and allowed to achieve: (a) more than 12-fold increase of transmittance over 10 minutes after application ex vivo; (b) markedly improved images of skin architecture obtained with optical coherence tomography; (c) an increase of the fluorescence intensity/background ratio in TagRFP-red fluorescent marker protein expressing tumor by five times after 15 minutes application into the skin in vivo. The obtained results have immediate implications for multimodality imaging because many contrast agents are capable of simultaneously enhancing the contrast of multiple imaging modalities.

Second-harmonic generation microscopy analysis reveals proteoglycan decorin is necessary for proper collagen organization in prostate

Collagen remodeling occurs in many prostate pathologies; however, the underlying structural architecture in both normal and diseased prostatic tissues is largely unexplored. Here, we use second-harmonic generation (SHG) microscopy to specifically probe the role of the proteoglycan decorin (Dcn) on collagen assembly in a wild type (wt) and Dcn null mouse (Dcn  -    /    -  ). Dcn is required for proper organization of collagen fibrils as it regulates size by forming an arch-like structure at the end of the fibril. We have utilized SHG metrics based on emission directionality (forward-backward ratio) and relative conversion efficiency, which are both related to the SHG coherence length, and found more disordered fibril organization in the Dcn  -    /    -  . We have also used image analysis readouts based on entropy, multifractal dimension, and wavelet transforms to compare the collagen fibril/fiber architecture in the two models, where all these showed that the Dcn  -    /    -   prostate comprised smaller and more disorganized collagen structures. All these SHG metrics are consistent with decreased SHG phase matching in the Dcn  -    /    -   and are further consistent with ultrastructural analysis of collagen in this model in other tissues, which show a more random distribution of fibril sizes and their packing into fibers. As Dcn is a known tumor suppressor, this work forms the basis for future studies of collagen remodeling in both malignant and benign prostate disease.

Intravital Imaging of Blood Flow and HSPC Homing in Bone Marrow Microvessels

Two-photon intravital microscopy (2P-IVM) is an advanced imaging technique that allows the visualization of dynamic cellular behavior deeply inside tissues and organs of living animals. Due to the deep tissue penetration, imaging of highly light-scattering tissue as the bone becomes feasible at subcellular resolution.To better understand the influence of blood flow on hematopoietic stem and progenitor cell (HSPC) homing to the bone marrow (BM) microvasculature of the calvarial bone, we analyzed blood flow dynamics and the influence of flow on the early homing behavior of HSPCs during their passage through BM microvessels. Here, we describe a 2P-IVM approach for direct measurements of red blood cell (RBC) velocities in the BM microvasculature using repetitive centerline scans at the level of individual arterial vessels and sinusoidal capillaries to obtain a detailed flow profile map. Furthermore, we explain the isolation and enrichment of HSPCs from long bones and the transplantation of these cells to study the early homing behavior of HSPCs in BM sinusoids at cellular resolution. This is achieved by high-resolution spatiotemporal imaging through a chronic cranial window using transgenic reporter mice.

Recent progress in tissue optical clearing for spectroscopic application

This paper aims to review recent progress in optical clearing of the skin and over naturally turbid biological tissues and blood using this technique in vivo and in vitro with multiphoton microscopy, confocal Raman microscopy, confocal microscopy, NIR spectroscopy, optical coherence tomography, and laser speckle contrast imaging. Basic principles of the technique, its safety, advantages and limitations are discussed. The application of optical clearing agent on a tissue allows for controlling the optical properties of tissue. Optical clearing-induced reduction of tissue scattering significantly facilitates the observation of deep-located tissue regions, at the same time improving the resolution and image contrast for a variety of optical imaging methods suitable for clinical applications, such as diagnostics and laser treatment of skin diseases, mucosal tumor imaging, laser disruption of pathological abnormalities, etc.

3D second harmonic generation imaging tomography by multi-view excitation

Biological tissues have complex 3D collagen fiber architecture that cannot be fully visualized by conventional second harmonic generation (SHG) microscopy due to electric dipole considerations. We have developed a multi-view SHG imaging platform that successfully visualizes all orientations of collagen fibers. This is achieved by rotating tissues relative to the excitation laser plane of incidence, where the complete fibrillar structure is then visualized following registration and reconstruction. We evaluated high frequency and Gaussian weighted fusion reconstruction algorithms, and found the former approach performs better in terms of the resulting resolution. The new approach is a first step toward SHG tomography.

A comparative study of ex vivo skin optical clearing using two-photon microscopy

Multiphoton tomography (MPT) is a prospective tool for imaging the skin structure. Aiming to increase the probing depth, a comparative ex vivo study of optical clearing of porcine ear skin was performed by using two optical clearing agents (OCAs), i.e., glycerol and iohexol (Omnipaque^TM ) at different concentrations, which exhibit different osmotic properties. The results show that a topical application of glycerol or Omnipaque^TM solutions onto the skin for 60 min significantly improved the depth and contrast of the MPT signals. By utilizing 40%, 60% and 100% glycerol, and 60% and 100% Omnipaque^TM it was demonstrated that both agents improve autofluorescence and SHG (second harmonic generation) signals from the skin. At the applied concentrations and agent time exposure, glycerol is more effective than Omnipaque^TM . However, tissue shrinkage and cell morphology changes were found for highly concentrated glycerol solutions. Omnipaque^TM , on the contrary, increases the safety and has no or minimal tissue shrinkage during the optical clearing process. Moreover Omnipaque^TM allows for robust multimodal optical/X-ray imaging with automatically matched optically cleared and X-ray contrasted tissue volumes. These findings make Omnipaque^TM more prospective than glycerol for some particular application.

Wavelength-Dependent Second Harmonic Generation Circular Dichroism for Differentiation of Col I and Col III Isoforms in Stromal Models of Ovarian Cancer Based on Intrinsic Chirality Differences

Extensive remodeling of the extracellular matrix (ECM) occurs in many epithelial cancers. For example, in ovarian cancer, upregulation of collagen isoform type III has been linked to invasive forms of the disease, and this change may be a potential biomarker. To examine this possibility, we implemented wavelength-dependent second harmonic generation circular dichroism (SHG-CD) imaging microscopy to quantitatively determine changes in chirality in ECM models comprised of different Col I/Col III composition. In these models, Col III was varied between 0 and 40%, and we found increasing Col III results in reduced net chirality, consistent with structural biology studies of Col I and III in tissues where the isoforms comingle in the same fibrils. We further examined the wavelength dependence of the SHG-CD to both optimize the response and gain insight into the underlying mechanism. We found using shorter SHG excitation wavelengths resulted in increased SHG-CD sensitivity, where this is consistent with the electric-dipole-coupled oscillator model suggested previously for the nonlinear chirality response from thin films. Moreover, the sensitivity is further consistent with the wavelength dependency of SHG intensity fit to a two-state model of the two-photon absorption in collagen. We also provide experimental calibration protocols to implement the SHG-CD modality on a laser scanning microscope. We last suggest that the technique has broad applicability in probing a wide range of diseased states with changes in collagen molecular structure.

Collaborative effects of wavefront shaping and optical clearing agent in optical coherence tomography

We demonstrate that simultaneous application of optical clearing agents (OCAs) and complex wavefront shaping in optical coherence tomography (OCT) can provide significant enhancement of penetration depth and imaging quality. OCA reduces optical inhomogeneity of a highly scattering sample, and the wavefront shaping of illumination light controls multiple scattering, resulting in an enhancement of the penetration depth and signal-to-noise ratio. A tissue phantom study shows that concurrent applications of OCA and wavefront shaping successfully operate in OCT imaging. The penetration depth enhancement is further demonstrated for <italic<ex vivo</italic< mouse ears, revealing hidden structures inaccessible with conventional OCT imaging.

Multi-view second-harmonic generation imaging of mouse tail tendon via reflective micro-prisms

Here we experimentally show that second-harmonic generation (SHG) imaging is not sensitive to collagen fibers oriented parallel to the direction of laser propagation and, as a consequence, can potentially miss important structural information. As an alternative approach, we demonstrate the use of reflective micro-prisms to enable multi-view SHG imaging of mouse tail tendon by redirecting the focused excitation and collection of subsequent emission. Our approach data corroborates the theoretical treatment on vanishing and nonvanishing orientations, where fibers along the laser direction are largely transparent by SHG. In strong contrast, the two-photon excited fluorescence of dye-labeled collagen fibers is isotropic and is not subject to this constraint. We utilized Pearson correlation to quantify differences in fluorescent and backward detected SHG images of the tendon fiber structure, where the SHG and TPEF were highly statistically correlated (0.6-0.8) for perpendicular excitation but were uncorrelated for excitation parallel to the fiber axis. The results suggest that improved imaging of 3D collagen structure is possible with multi-view SHG microscopy.

Applications of second-harmonic generation imaging microscopy in ovarian and breast cancer

In this perspective, we discuss how the nonlinear optical technique of second-harmonic generation (SHG) microscopy has been used to greatly enhance our understanding of the tumor microenvironment (TME) of breast and ovarian cancer. Striking changes in collagen architecture are associated with these epithelial cancers, and SHG can image these changes with great sensitivity and specificity with submicrometer resolution. This information has not historically been exploited by pathologists but has the potential to enhance diagnostic and prognostic capabilities. We summarize the utility of image processing tools that analyze fiber morphology in SHG images of breast and ovarian cancer in human tissues and animal models. We also describe methods that exploit the SHG physical underpinnings that are effective in delineating normal and malignant tissues. First we describe the use of polarization-resolved SHG that yields metrics related to macromolecular and supramolecular structures. The coherence and corresponding phase-matching process of SHG results in emission directionality (forward to backward), which is related to sub-resolution fibrillar assembly. These analyses are more general and more broadly applicable than purely morphology-based analyses; however, they are more computationally intensive. Intravital imaging techniques are also emerging that incorporate all of these quantitative analyses. Now, all these techniques can be coupled with rapidly advancing miniaturization of imaging systems to afford their use in clinical situations including enhancing pathology analysis and also in assisting in real-time surgical determination of tumor margins.

Theoretical, numerical and experimental study of geometrical parameters that affect anisotropy measurements in polarization-resolved SHG microscopy

Polarization-resolved second harmonic generation (P-SHG) microscopy is an efficient imaging modality for in situ observation of biopolymers structure in tissues, providing information about their mean in-plane orientation and their molecular structure and 3D distribution. Nevertheless, P-SHG signal build-up in a strongly focused regime is not throroughly understood yet, preventing reliable and reproducible measurements. In this study, theoretical analysis, vectorial numerical simulations and experiments are performed to understand how geometrical parameters, such as excitation and collection numerical apertures and detection direction, affect P-SHG imaging in homogeneous collagen tissues. A good agreement is obtained in tendon and cornea, showing that detection geometry significantly affects the SHG anisotropy measurements, but not the measurements of collagen in-plane orientation.

Articular cartilage zonal differentiation via 3D Second-Harmonic Generation imaging microscopy

PURPOSE

The collagen structure throughout the patella has not been thoroughly investigated by 3D imaging, where the majority of the existing data come from histological cross sections. It is important to have a better understanding of the architecture in normal tissues, where this could then be applied to imaging of diseased states.

METHODS

To address this shortcoming, we investigated the combined use of collagen-specific Second-Harmonic Generation (SHG) imaging and measurement of bulk optical properties to characterize collagen fiber orientations of the histologically defined zones of bovine articular cartilage. Forward and backward SHG intensities of sections from superficial, middle and deep zones were collected as a function of depth and analyzed by Monte Carlo simulations to extract the SHG creation direction, which is related to the fibrillar assembly.

RESULTS

Our results revealed differences in SHG forward-backward response between the three zones, where these are consistent with a previously developed model of SHG emission. Some of the findings are consistent with that from other modalities; however, SHG analysis showed the middle zone had the most organized fibril assembly. While not distinct, we also report bulk optical property values for these different zones within the patella.

CONCLUSIONS

Collectively, these results provide quantitative measurements of structural changes at both the fiber and fibril assembly of the different cartilage zones and reveals structural information not possible by other microscope modalities. This can provide quantitative insight to the collagen fiber network in normal cartilage, which may ultimately be developed as a biomarker for osteoarthritis.

Simultaneous determination of the second-harmonic generation emission directionality and reduced scattering coefficient from three-dimensional imaging of thick tissues

Second-harmonic generation (SHG) microscopy has intrinsic contrast for imaging fibrillar collagen and has shown great promise for disease characterization and diagnostics. In addition to morphology, additional information is achievable as the initially emitted SHG radiation directionality is related to subresolution fibril size and distribution. We show that by two parameter fittings, both the emission pattern (FSHG/BSHG)creation and the reduced scattering coefficient μs', can be obtained from the best fits between three-dimensional experimental data and Monte Carlo simulations. The improved simulation framework accounts for collection apertures for the detected forward and backward components. We apply the new simulation framework to mouse tail tendon for validation and show that the spectral slope of μs' obtained is similar to that from bulk optical measurements and that the (FSHG/BSHG)creation values are also similar to previous results. Additionally, we find that the SHG emission becomes increasingly forward directed at longer wavelengths, which is consistent with decreased dispersion in refractive index between the laser and SHG wavelengths. As both the spectral slope of μs' and (FSHG/BSHG)creation have been linked to the underlying tissue structure, simultaneously obtaining these parameters on a microscope platform from the same tissue provides a powerful method for tissue characterization.

Recent progress in tissue optical clearing

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical-clearing agents into tissue, such as physical methods, chemical-penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced. [Formula: see text].

Non-linear optical microscopy sheds light on cardiovascular disease

Many cardiac diseases have been associated with increased fibrosis and changes in the organization of fibrillar collagen. The degree of fibrosis is routinely analyzed with invasive histological and immunohistochemical methods, giving a limited and qualitative understanding of the tissue's morphological adaptation to disease. Our aim is to quantitatively evaluate the increase in fibrosis by three-dimensional imaging of the collagen network in the myocardium using the non-linear optical microscopy techniques Two-Photon Excitation microscopy (TPE) and Second Harmonic signal Generation (SHG). No sample staining is needed because numerous endogenous fluorophores are excited by a two-photon mechanism and highly non-centrosymmetric structures such as collagen generate strong second harmonic signals. We propose for the first time a 3D quantitative analysis to carefully evaluate the increased fibrosis in tissue from a rat model of heart failure post myocardial infarction. We show how to measure changes in fibrosis from the backward SHG (B_SHG) alone, as only backward-propagating SHG is accessible for true in vivo applications. A 5-fold increase in collagen I fibrosis is detected in the remote surviving myocardium measured 20 weeks after infarction. The spatial distribution is also shown to change markedly, providing insight into the morphology of disease progression.

Second-harmonic generation circular dichroism studies of osteogenesis imperfecta

We report the use of second-harmonic generation (SHG) microscopy in conjunction with circular dichroism (CD) to differentiate normal skin from that in the connective tissue disorder osteogenesis imperfecta (OI). Osteogenesis imperfecta results from mutations in the collagen triple helix, where the individual chains are defective, leading to abnormal folding, and ultimately, abnormal fibril/fiber organization. Second-harmonic-generation circular dichroism successfully differentiated normal human and OI skin tissues, whereas other SHG polarization schemes did not provide discrimination, suggesting this approach has high sensitivity for studying the difference in chirality in the mutated collagen. We further suggest that the method has clinical diagnostic value, as it could be performed with minimal invasion.

Enhanced optical clearing of skin in vivo and optical coherence tomography in-depth imaging

The strong optical scattering of skin tissue makes it very difficult for optical coherence tomography (OCT) to achieve deep imaging in skin. Significant optical clearing of in vivo rat skin sites was achieved within 15 min by topical application of an optical clearing agent PEG-400, a chemical enhancer (thiazone or propanediol), and physical massage. Only when all three components were applied together could a 15 min treatment achieve a three fold increase in the OCT reflectance from a 300 μm depth and 31% enhancement in image depth Z(threshold).

Second harmonic generation microscopy for quantitative analysis of collagen fibrillar structure

Second-harmonic generation (SHG) microscopy has emerged as a powerful modality for imaging fibrillar collagen in a diverse range of tissues. Because of its underlying physical origin, it is highly sensitive to the collagen fibril/fiber structure, and, importantly, to changes that occur in diseases such as cancer, fibrosis and connective tissue disorders. We discuss how SHG can be used to obtain more structural information on the assembly of collagen in tissues than is possible by other microscopy techniques. We first provide an overview of the state of the art and the physical background of SHG microscopy, and then describe the optical modifications that need to be made to a laser-scanning microscope to enable the measurements. Crucial aspects for biomedical applications are the capabilities and limitations of the different experimental configurations. We estimate that the setup and calibration of the SHG instrument from its component parts will require 2-4 weeks, depending on the level of the user's experience.

Optical clearing of porcine skin tissue in vitro studied by Raman microspectroscopy

In present work, we studied the effect of optical clearing on porcine skin in vitro with glycerol by Raman microspectroscopy, denoted as RM, at various time intervals of 0, 15, 30, 45, 60, and 75 min respectively. The results showed that the addition of glycerol significantly improved the depth of RM measurement, and enhanced the recovery of skin tissue Raman spectra that were not overlapped with the glycerol Raman spectra over time. Moreover, it was found that the Raman signals resembled the native spectrum of the molecules in porcine skin with a negligible frequency shift. Furthermore, we evaluated the extent of optical clearing in porcine skin by utilizing various concentrations of 40%, 60%, and 80% glycerol solution. The results demonstrated that with the increase of concentration of glycerol, the optical clearing of porcine skin was much improved.

Structural changes in mixed Col I/Col V collagen gels probed by SHG microscopy: implications for probing stromal alterations in human breast cancer

Second Harmonic Generation (SHG) microscopy has been previously used to describe the morphology of collagen in the extracellular matrix (ECM) in different stages of invasion in breast cancer. Here this concept is extended by using SHG to provide quantitative discrimination of self-assembled collagen gels, consisting of mixtures of type I (Col I) and type V (Col V) isoforms which serve as models of changes in the ECM during invasion in vivo. To investigate if SHG is sensitive to changes due to Col V incorporation into Col I fibrils, gels were prepared with 0-20% Col V with the balance consisting of Col I. Using the metrics of SHG intensity, fiber length, emission directionality, and depth-dependent intensities, we found similar responses for gels comprised of 100% Col I, and 95% Col I/5% Col V, where these metrics were all significantly different from those of the 80% Col I/20% Col V gels. Specifically, the gels of lower Col V content produce brighter SHG, are characterized by longer fibers, and have a higher forward/backward emission ratio. These attributes are all consistent with more highly organized collagen fibrils/fibers and are in agreement with previous TEM characterization as well as predictions based on phase matching considerations. These results suggest that SHG can be developed to discriminate Col I/Col V composition in tissues to characterize and follow breast cancer invasion.

Second harmonic generation imaging microscopy: applications to diseases diagnostics

Second Harmonic Generation microscopy has emerged as a powerful new optical imaging modality. This Feature describes its chemical and physical principles and highlights current applications in disease diagnostics.

Tissue optical immersion clearing

In this article, we discuss the optical immersion method based on refractive index matching of scatterers (e.g., collagen, elastin fibers, cells and cell compartments) and the ground material (interstitial fluid and/or cytoplasm) of tissue and blood under the action of exogenous optical clearing agents. We analyze the optical clearing of fibrous and cell-structured tissues and blood from the point of view of receiving more valuable, normally hidden, information from spectroscopic and polarization measurements, confocal microscopy, optical coherence and optical projection tomography, as well as from nonlinear spectroscopies, such as two-photon fluorescence and second-harmonic generation techniques. Some important applications of the immersion technique to glucose sensing, drug delivery monitoring, improvements of image contrast and imaging depth, nondistortive delivery of laser radiation and precision tissue laser photodisruption, among others, are also described.

Rat muscle opacity decrease due to the osmosis of a simple mixture

It is known that the fibrous structure of muscle causes light scattering. This phenomenon occurs due to the refractive index discontinuities located between muscle fibers and interstitial fluid. To study the possibility of reducing light scattering inside muscle, we consider its spectral transmittance evolution during an immersion treatment with an optical clearing solution containing ethanol, glycerol, and distilled water. Our methodology consists of registering spectral transmittance of muscle samples while immersed in that solution. With the spectral data collected, we represent the transmittance evolution for some wavelengths during the treatment applied. Additionally, we study the variations that the treatment has caused on the samples regarding tissue refractive index and mass. By analyzing microscopic photographs of tissue cross section, we can also verify changes in the internal arrangement of muscle fibers caused by the immersion treatment. Due to a mathematical model that we develop, we can explain the variations observed in the studied parameters and estimate the amount of optical clearing agent that has diffused into the tissue samples during the immersion treatment. At the end of the study, we observe and explain the improvement in tissue spectral transmittance, which is approximately 65% after 20 min.

Polarization-resolved Second Harmonic microscopy in anisotropic thick tissues

We thoroughly analyze the linear propagation effects that affect polarization-resolved Second Harmonic Generation imaging of thick anisotropic tissues such as collagenous tissues. We develop a theoretical model that fully accounts for birefringence and diattenuation along the excitation propagation, and polarization scrambling upon scattering of the harmonic signal. We obtain an excellent agreement with polarizationresolved SHG images at increasing depth within a rat-tail tendon for both polarizations of the forward SHG signal. Most notably, we observe interference fringes due to birefringence in the SHG depth profile when excited at π/4 angle from the tendon axis. We also measure artifactual decrease of ρ = Χxxx/Χxyy with depth due to diattenuation of the excitation. We therefore derive a method that proves reliable to determine both ρ and the tendon birefringence and diattenuation.

Alterations of the extracellular matrix in ovarian cancer studied by Second Harmonic Generation imaging microscopy

Background

Remodeling of the extracellular matrix (ECM) has been implicated in ovarian cancer, and we hypothesize that these alterations may provide a better optical marker of early disease than currently available imaging/screening methods and that understanding their physical manifestations will provide insight into invasion.

Methods

For this investigation we use Second Harmonic Generation (SHG) imaging microcopy to study changes in the structure of the ovarian ECM in human normal and malignant ex vivo biopsies. This method directly visualizes the type I collagen in the ECM and provides quantitative metrics of the fibrillar assembly. To quantify these changes in collagen morphology we utilized an integrated approach combining 3D SHG imaging measurements and bulk optical parameter measurements in conjunction with Monte Carlo simulations of the experimental data to extract tissue structural properties.

Results

We find the SHG emission attributes (directionality and relative intensity) and bulk optical parameters, both of which are related to the tissue structure, are significantly different in the tumors in a manner that is consistent with the change in collagen assembly. The normal and malignant tissues have highly different collagen fiber assemblies, where collectively, our findings show that the malignant ovaries are characterized by lower cell density, denser collagen, as well as higher regularity at both the fibril and fiber levels. This further suggests that the assembly in cancer may be comprised of newly synthesized collagen as opposed to modification of existing collagen.

Conclusions

Due to the large structural changes in tissue assembly and the SHG sensitivity to these collagen alterations, quantitative discrimination is achieved using small patient data sets. Ultimately these measurements may be developed as intrinsic biomarkers for use in clinical applications.

Controling the scattering of intralipid by using optical clearing agents

Optical clearing agents (OCAs) with high refractive indices and hyperosmolarity can enhance the penetration of light in tissues by reducing scattering in tissues. However, the mechanism of tissue optical clearing is not much clear for the complex interaction between tissues and OCAs. In this work, Intralipid was mixed with different concentrations of OCAs, i.e. dimethyl sulfoxide (DMSO), glycerol, 1,4-butanediol, 1,2-propanediol, poly-ethylene glycol 200 (PEG200) and poly-ethylene glycol 400 (PEG400). Except for PEG200 and PEG400 that make aggregation of particles, the others kept the mixture uniform. The reduced scattering coefficients of uniform mixtures were predicted with Mie theory and measured by a commercially available spectrophotometer with an integrating sphere. The results show that all of the OCAs used enhance the optical clearing effect of Intralipid. If OCAs do not change the structure of Intralipid, Mie theory prediction matches well with the measurements. And the higher the refractive index of OCA, the smaller the reduced scattering coefficient. A simple formula deduced can quantitatively predict the optical clearing effect caused by OCAs. This work is helpful for clarifying the mechanism of tissue optical clearing, which will make the effect of optical clearing of tissue predictable and controllable.

Integration of optical clearing and optical sectioning microscopy for three-dimensional imaging of natural biomaterial scaffolds in thin sections

The intrinsic turbidity of scaffolds formed by natural biomaterials such as collagen fibers prevents high-resolution light microscopy in depth. In this research, we have developed a new method of using light microscopy for penetrative three-dimensional (3-D) visualization of scaffolds formed by collagen, chitosan, or cellulose. First, we applied an optical-clearing solution, FocusClear, to permeate and reduce the turbidity of the scaffolds. The improved photon penetration allowed fluorophores for efficient excitation and emission in the FocusClear solution. Confocal microscopy was applied to achieve cellular-level resolution up to 350 microm for both the fibroblast/collagen and the osteoblast/chitosan constructs and micrometer-level resolution up to 40 microm for the cellulose membrane. The depth of imaging of the cellulose membrane was further improved to 80 microm using two-photon microscopy. Significantly, these voxel-based confocal/two-photon micrographs allowed postrecording image processing via Amira projection algorithms for 3-D visualization and analysis of the scanned region. Although this optical method remains limited in viewing block scaffolds in thin sections, our approach provides a noninvasive way to microscopically examine the scaffold structure, which would be a valuable tool to studying biomaterials and their interactions with the molecule/cell of interest within the scaffold in an integrated fashion.

Following dimethyl sulfoxide skin optical clearing dynamics with quantitative nonlinear multimodal microscopy

Second-harmonic generation (SHG) imaging is combined with coherent anti-Stokes Raman scattering (CARS) microscopy to follow the process of optical clearing in human skin ex vivo using dimethyl sulfoxide (DMSO) as the optical clearing agent. SHG imaging revealed that DMSO introduces morphological changes to the collagen I matrix. By carefully measuring the dynamic tissue attenuation of the coherent nonlinear signal, using CARS reference signals during the clearing process, it is found that DMSO reduces the overall SHG response from dermal collagen. Evidence is provided for a role of DMSO in compromising the structure of collagen fibers, associated with a reduction of the tissue's scattering properties.

Retention of polarization signatures in SHG microscopy of scattering tissues through optical clearing

Polarization responses in Second Harmonic Generation (SHG) imaging microscopy are a valuable method to quantify aspects of tissue structure, and may be a means to differentiate normal and diseased tissues. Due to multiple scattering, the polarization data is lost in turbid tissues. Here we investigate if this information can be retained through the use of optical clearing which greatly reduces the scattering coefficient and increases the corresponding mean free path. To this end, we have measured the SHG intensity as a function of laser polarization and the SHG signal anisotropy in murine tendon and striated muscle over a depth range of 200 microns. We find that the laser polarization is highly randomized in the uncleared tissues at depths corresponding to only 2-3 scattering collisions (50- 10 microns). This depolarization of the laser is also reflected in the randomized anisotropy of the SHG signal as it is created over a range of polarization states. In strong contrast, both polarization signatures are significantly retained through ~200 microns of tissue thickness following treatment with 50% glycerol. Moreover, the measured polarization responses for both tendon and striated muscle are consistent with the extent of reduction of the respective scattering coefficients upon clearing. We suggest the method will be applicable to SHG imaging of connective disorders as well as cancer through several hundred microns of extracellular matrix.