Studies of chi(2)/chi(3) tensors in submicron-scaled bio-tissues by polarization harmonics optical microscopy

Investigating cartilage-related diseases by polarization-resolved second harmonic generation (P-SHG) imaging

Establishing quantitative parameters for differentiating between healthy and diseased cartilage tissues by examining collagen fibril degradation patterns facilitates the understanding of tissue characteristics during disease progression. These findings could also complement existing clinical methods used to diagnose cartilage-related diseases. In this study, cartilage samples from normal, osteoarthritis (OA), and rheumatoid arthritis (RA) tissues were prepared and analyzed using polarization-resolved second harmonic generation (P-SHG) imaging and quantitative image texture analysis. The enhanced molecular contrast obtained from this approach is expected to aid in distinguishing between healthy and diseased cartilage tissues. P-SHG image analysis revealed distinct parameters in the cartilage samples, reflecting variations in collagen fibril arrangement and organization across different pathological states. Normal tissues exhibited distinct χ33/χ31 values compared with those of OA and RA, indicating collagen type transition and cartilage erosion with chondrocyte swelling, respectively. Compared with those of normal tissues, OA samples demonstrated a higher degree of linear polarization, suggesting increased tissue birefringence due to the deposition of type-I collagen in the extracellular matrix. The distribution of the planar orientation of collagen fibrils revealed a more directional orientation in the OA samples, associated with increased type-I collagen, while the RA samples exhibited a heterogeneous molecular orientation. This study revealed that the imaging technique, the quantitative analysis of the images, and the derived parameters presented in this study could be used as a reference for disease diagnostics, providing a clear understanding of collagen fibril degradation in cartilage.

Polarimetric second-harmonic generation microscopy of partially oriented fibers I: Digital modeling

Second-harmonic generation (SHG) in biological tissues originates predominantly from noncentrosymmetric fibrillar structures partially oriented within a focal volume (voxel) of a multiphoton excitation microscope. This study is aimed to elucidate fibrillar organization factors influencing SHG intensity, as well as achiral, R, and chiral, C, nonlinear susceptibility tensor component ratios. SHG response is calculated for various configurations of fibrils in a voxel using the digital nonlinear microscope. The R and C ratios are calculated using linear incident and outgoing polarization states that simulate polarization-in polarization-out polarimetric measurements. The investigation shows strong SHG intensity dependence on parallel/antiparallel fiber organization. The R and C ratios are strongly influenced by the fiber chirality, tilting of the fibers out of the image plane, and crossing of the fibers. The computational modeling provides the basis for the interpretation of polarimetric SHG microscopy images in terms of the ultrastructural organization of fibers in each voxel of the samples. The modeling results are employed in the accompanying paper to investigate the ultrastructures with parallel/antiparallel fibers and two-dimensional and tree-dimensional crossing fibers in biological and biomimetic structures.

Polarimetric second harmonic generation microscopy of partially oriented fibers II: Imaging study

Polarimetric second harmonic generation (SHG) microscopy imaging is employed to investigate the ultrastructural organization of biological and biomimetic partially oriented fibrillar structures. The linear polarization-in polarization-out SHG microscopy measurements are conducted with rat tail tendon, rabbit cornea, pig cartilage, and biomimetic meso-tetra(4-sulfonatophenyl)porphine (TPPS4) cylindrical aggregates, which represent different two- and three-dimensional (2D and 3D) configurations of C6 symmetry fibril structures in the focal volume (voxel) of the microscope. The polarization-in polarization-out imaging of rat tail tendon reveals that SHG intensity is affected by parallel/antiparallel arrangements of the fibers, and achiral (R) and chiral (C) susceptibility component ratio values change by tilting the tendon fibers out of image plane. The R ratio changes for the 2D crossing fibers observed in cornea tissue. The 3D crossing of fibers also affects R ratio in cartilage tissue. The distinctly different dependence of R on crossing and tilting of fibers is demonstrated in collagen and TPPS4 aggregates, due to the achiral molecular susceptibility ratio having values below and above 3, respectively. The polarimetric microscopy results correspond well with the analytical expressions of amplitude and R and C ratios dependence on the crossing angle of the fibers. The experimentally measured SHG intensity and R and C ratio maps are consistent with the computational modeling of various fiber configurations presented in the preceding article. The demonstrated SHG intensity and R and C ratio dependencies on fibril configurations provide the basis for interpreting polarimetric SHG microscopy images in terms of 3D ultrastructural organization of fibers in each voxel of the samples.

Wide-field Stokes polarimetric microscopy for second harmonic generation imaging

We employ wide-field second harmonic generation (SHG) microscopy together with nonlinear Stokes polarimetry for quick ultrastructural investigation of large sample areas (700 μm × 700 μm) in thin histology sections. The Stokes vector components for SHG are obtained from the polarimetric measurements with incident and outgoing linear and circular polarization states. The Stokes components are used to construct the images of polarimetric parameters and deduce the maps of ultrastructural parameters of achiral and chiral nonlinear susceptibility tensor components ratios and cylindrical axis orientation in fibrillar materials. The large area imaging was employed for lung tumor margin investigations. The imaging shows reduced SHG intensity, increased achiral susceptibility ratio values, and preferential orientation of collagen strands along the boarder of tumor margin. The wide-field Stokes polarimetric SHG microscopy opens a possibility of quick large area imaging of ultrastructural parameters of tissue collagen, which can be used for nonlinear histopathology investigations.

Second harmonic generation microscopy: a powerful tool for bio-imaging

Second harmonic generation (SHG) microscopy is an important optical imaging technique in a variety of applications. This article describes the history and physical principles of SHG microscopy and its more advanced variants, as well as their strengths and weaknesses in biomedical applications. It also provides an overview of SHG and advanced SHG imaging in neuroscience and microtubule imaging and how these methods can aid in understanding microtubule formation, structuration, and involvement in neuronal function. Finally, we offer a perspective on the future of these methods and how technological advancements can help make SHG microscopy a more widely adopted imaging technique.

Fast polarization-sensitive second-harmonic generation microscopy based on off-axis interferometry

We propose polarization-sensitive second-harmonic generation microscopy based on off-axis interferometry (OI-PSHG) by recording the complex field of a wide-field second-harmonic generation (SHG) image and performing polarization measurements. With the ability to record the SHG signals associated with different positions simultaneously, the proposed method exhibits a higher imaging frame rate than raster scanning-based SHG microscopy. The molecular orientation (in terms of their symmetric axis) of tendon collagen fibrils and myosin in muscle is resolved in three dimensions from a subset of polarization-resolved SHG holograms. With the present configuration, it takes approximately 0.01 s to acquire an image with 128 × 128 pixels, which is mainly limited by the excitation power density for wide-field illumination. For the same data throughput using pixel-by-pixel scanning, 0.16-s-long acquisition is required, with the pixel dwell time of 10 µs. Offering the ability to perform wide-field imaging and polarization measurements, the present work lays the foundation for fast SHG microscopy using complex deconvolution and harmonic tomography.

Comparison between Cylindrical, Trigonal, and General Symmetry Models for the Analysis of Polarization-Dependent Second Harmonic Generation Measurements Acquired from Collagen-Rich Equine Pericardium Samples

Polarization-dependent second harmonic generation (PSHG) microscopy is used as an innovative, high-resolution, non-destructive, and label-free diagnostic imaging tool to elucidate biological issues with high significance. In the present study, information on the structure and directionality of collagen fibers in equine pericardium tissue was collected using PSHG imaging measurements. In an effort to acquire precise results, three different mathematical models (cylindrical, trigonal, and general) were applied to the analysis of the recorded PSHG datasets. A factor called the “ratio parameter” was calculated to provide quantitative information. The implementation of the trigonal symmetry model to the recorded data led to the extraction of improved results compared with the application of the widely used cylindrical symmetry model. The best outcome was achieved through the application of the general model that does not include any kind of symmetry for the data processing. Our findings suggest that the trigonal symmetry model is preferable for the analysis of the PSHG datasets acquired from the collagenous tissues compared with the cylindrical model approach although an increased computational time is required.

Monitoring aging-associated structural alterations in Caenorhabditis elegans striated muscles via polarization-dependent second-harmonic generation measurements

The in-vivo elucidation of the molecular mechanisms underlying muscles dysfunction due to aging via non-invasive label free imaging techniques is an important issue with high biological significance. In this study, polarization-dependent second-harmonic generation (PSHG) was used to evaluate structural alterations in the striated muscles during Caenorhabditis elegans lifespan. Young and old wild-type animals were irradiated. The obtained results showed that it was not feasible to detect differences in the structure of myosin that could be correlated with the aging of the worms, via the implementation of the classical, widely used, cylindrical symmetry model and the calculation of the SHG anisotropy values. A trigonal symmetry model improved the extracted results; however, the best outcome was originated from the application of a general model. Myosin structural modifications were monitored via the estimation of the difference in spectral phases derived from discrete Fourier transform analysis. Age classification of the nematodes was achieved by employing both models, proving the usefulness of the usage of PSHG microscopy as a potential diagnostic tool for the investigation of muscle diseases.

Polarization-dependent second-harmonic generation for collagen-based differentiation of breast cancer samples

Nonlinear optical imaging techniques have been widely used to reveal biological structures for accurate diagnosis at the cellular as well as the tissue level. In the present study, polarization-dependent second-harmonic generation (PSHG) was used to determine collagen orientation in breast cancer biopsy tissues (grades 0, I, II and III). The obtained data were processed using fast Fourier transform (FFT) analysis, while second-harmonic generation (SHG) anisotropy and the "ratio parameter" values were also calculated. Such measurements were shown to be able to distinguish collagen structure modifications in different cancer grades tested. The analysis presented herein suggests that PSHG imaging could provide a quantitative evaluation of the tumor state and the distinction of malignant from benign breast tissues. The obtained results also allowed the development of a biophysical model, which can explain the aforementioned differentiations and is in agreement with the simulations relating the SHG anisotropy values with the mechanical tension applied to the collagen during cancer progression. The current approach could be a step forward for the development of new, nondestructive, label free optical diagnostic tools for cancer reducing the need of recalls and unnecessary biopsies, while potentially improving cancer detection rates.

Stokes polarimetry-based second harmonic generation microscopy for collagen and skeletal muscle fiber characterization

The complete polarization state of second harmonic (SH) light was measured and characterized by collagen type I and skeletal muscle fiber using a Stokes vector-based SHG microscope. The polarization states of the SH signal are analyzed in a pixel-by-pixel manner and displayed through two dimensional (2D) Stokes vector images. Various polarization parameters are reconstructed using Stokes values to quantify the polarization properties of SH light. Also, the measurements are extended for different input polarization states to investigate the molecular structure of second harmonic generation (SHG) active molecules such as collagen type I and myosin.

Harmonic optical tomography of nonlinear structures

Second-harmonic generation microscopy is a valuable label-free modality for imaging non-centrosymmetric structures and has important biomedical applications from live-cell imaging to cancer diagnosis. Conventional second-harmonic generation microscopy measures intensity signals that originate from tightly focused laser beams, preventing researchers from solving the scattering inverse problem for second-order nonlinear materials. Here, we present harmonic optical tomography (HOT) as a novel modality for imaging microscopic, nonlinear and inhomogeneous objects. The HOT principle of operation relies on inter-ferometrically measuring the complex harmonic field and using a scattering inverse model to reconstruct the three-dimensional distribution of harmonophores. HOT enables strong axial sectioning via the momentum conservation of spatially and temporally broadband fields. We illustrate the HOT operation with experiments and reconstructions on a beta-barium borate crystal and various biological specimens. Although our results involve second-order nonlinear materials, we show that this approach applies to any coherent nonlinear process.

Label-free characterization of collagen fibers in cancerous esophagus tissues using ratiometric nonlinear optical microscopy

IMPACT STATEMENT

The issue of classifying esophageal cancer at various developmental stages is crucial for determining the optimized treatment protocol for the patients, as well as the prognosis. Precision improvement in staging esophageal cancer keeps seeking quantitative and analytical imaging methods that could augment histopathological techniques. In this work, we used nonlinear optical microscopy for ratiometric analysis on the intrinsic signal of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) from single collagen fibers only in submucosa of esophageal squamous cell carcinoma (ESCC). The blind tests of TPEF/SHG and forward (F)/backward (B) SHG were demonstrated to compare with the histology conclusion. The discussion of sensitivity and specificity was provided via statistical comparison between the four stages of esophageal cancer. To the best of our knowledge, this is the first study of using these two ratios in combination for staging ESCC.

Dual- and single-shot susceptibility ratio measurements with circular polarizations in second-harmonic generation microscopy

Polarization-resolved second-harmonic generation (P-SHG) microscopy is a technique capable of characterizing nonlinear optical properties of noncentrosymmetric biomaterials by extracting the nonlinear susceptibility tensor components ratio χzzz2'/χzxx2' , with z-axis parallel and x-axis perpendicular to the C₆ symmetry axis of molecular fiber, such as a myofibril or a collagen fiber. In this paper, we present two P-SHG techniques based on incoming and outgoing circular polarization states for a fast extraction of χzzz2'/χzxx2' : A dual-shot configuration where the SHG circular anisotropy generated using incident right- and left-handed circularly-polarized light is measured; and a single-shot configuration for which the SHG circular anisotropy is measured using only one incident circular polarization state. These techniques are used to extract the χzzz2'/χzxx2' of myosin fibrils in the body wall muscles of Drosophila melanogaster larva. The results are in good agreement with values obtained from the double Stokes-Mueller polarimetry. The dual- and single-shot circular anisotropy measurements can be used for fast imaging that is independent of the in-plane orientation of the sample. They can be used for imaging of contracting muscles, or for high throughput imaging of large sample areas.

Wavy nature of collagen fibrils deduced from the dispersion of their second-order nonlinear optical anisotropy parameters ρ

From P-SHG experiments, second-order nonlinear optical anisotropy parameters ρ = χ_ZZZ/χ_ZXX of collagen tissues are calculated assuming the same model of supercoiled collagen fibril characterized by a variable angle θ. Dispersion of experimental ρ values is converted into distribution of θ values based on the wavy nature of collagen fibrils deduced from EM studies. For tendon, the results show that the dispersion of experimental ρ values is mainly due to Poisson photonic shot noise assuming a slight fibrillar undulation with θ = 2.2^° ± 1.8^°. However for skin and vessels, the dispersion of experimental ρ values is mainly due to a stronger fibrillar undulation with θ = 16.2^° ± 1.3^°. The results highlight that this undulation is reduced during the development of liver fibrosis therefore, contributing to the rigidity of the tissue.

Harmonic Generation Microscopy 2.0: New Tricks Empowering Intravital Imaging for Neuroscience

Optical harmonic generation, e.g., second- (SHG) and third-harmonic generation (THG), provides intrinsic contrasts for three-dimensional intravital microscopy. Contrary to two-photon excited fluorescence (TPEF), however, they have found relatively specialized applications, such as imaging collagenous and non-specific tissues, respectively. Here we review recent advances that broaden the capacity of SHG and THG for imaging the central nervous system in particular. The fundamental contrast mechanisms are reviewed as they encode novel information including molecular origin, spectroscopy, functional probes, and image analysis, which lay foundations for promising future applications in neuroscience.

Identifying crossing collagen fibers in human corneal tissues using pSHG images

Polarization sensitive second harmonic generation (pSHG) microscopy has been used previously to characterize the structure of collagen fibers in corneal samples. Due to the typical organization of the corneal stroma, the information that pSHG provides may be misleading in points where two different collagen fiber bundles orient along different direction crossings. Here, a simulation that illustrates the problem is presented, along with a novel method that is capable of identifying these crossing points. These results can be used to improve the evaluation of corneal collagen structure, and it has been applied to analyze pSHG data acquired from healthy and keratoconic human corneal samples.

Comparison of Different Polarization Sensitive Second Harmonic Generation Imaging Techniques

Polarization sensitive second harmonic generation (pSHG) microscopy is an imaging technique able to provide, in a non-invasive manner, information related to the molecular structure of second harmonic generation (SHG) active structures, many of which are commonly found in biological tissue. The process of acquiring this information by means of pSHG microscopy requires a scan of the sample using different polarizations of the excitation beam. This process can take considerable time in comparison with the dynamics of in vivo processes. Fortunately, single scan polarization sensitive second harmonic generation (SS-pSHG) microscopy has also been reported, and is able to generate the same information at a faster speed compared to pSHG. In this paper, the orientation of second harmonic active supramolecular assemblies in starch granules is obtained on by means of pSHG and SS-pSHG. These results are compared in the forward and backward directions, showing a good agreement in both techniques. This paper shows for the first time, to the best of the authors' knowledge, data acquired using both techniques over the exact same sample and image plane, so that they can be compared pixel-to-pixel.

Monte Carlo simulation of polarization-sensitive second-harmonic generation and propagation in biological tissue

Polarization-sensitive second harmonic generation (p-SHG) is a nonlinear optical microscopy technique that has shown great promise in biomedicine, such as in detecting changes in the collagen ultrastructure of the tumor microenvironment. However, the complex nature of light-tissue interactions and the heterogeneity of biological samples pose challenges in creating an analytical and experimental quantification platform for tissue characterization via p-SHG. We present a Monte Carlo (MC) p-SHG simulation model based on double Stokes-Mueller polarimetry for the investigation of nonlinear light-tissue interaction. The MC model predictions are compared with experimental measurements of second-order nonlinear susceptibility component ratio and degree of polarization (DOP) in rat-tail collagen. The observed trends in the behavior of these parameters as a function of tissue thickness, as well as the overall extent of agreement between MC and experimental results, are discussed. High sensitivities of the susceptibility ratio and DOP are observed for the varying tissue thickness on the incoming fundamental light propagation pathway.

Calculations of second harmonic generation with radially polarized excitations by elliptical mirror focusing

Second harmonic generation (SHG) polarization intensity distribution illuminated with radially polarized beams by lens focusing appears two peaks, when the nonlinear optical coefficients dominate that is relevant to the transverse electric field components. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. In this paper, an elliptical mirror based system is proposed in the case of radially polarized beams illumination for SHG. The calculated predictions and numerical simulations demonstrate that for radially polarized beams, the proportion of transverse field components at the focal plane under the condition of elliptical mirror focusing is 2.6 times smaller than that with lens focusing when its numerical aperture (NA) is 1. Furthermore, the full width at half maximum (FWHM) of the total field intensity profile is approximately 81% of that in a lens focusing system. Due to the enhancement of longitudinal components of incident field, the distribution of SHG polarization presents a single-peak pattern, in which two peaks can be observed with lens focusing. The SHG polarizations in collagen fiber, KTiOPO₄ , and LiNbO₃ have been numerical simulated and discussed in detail to verify the validity of the proposed method. LAY DESCRIPTION: A novel focusing mode for second harmonic generation (SHG) microscopy is described in this paper. In the case of radially polarized excitations, the SHG polarization distributions of some specimens appear two peaks pattern with traditional lens focusing. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. By introducing elliptical mirror, the modulation of electric field at the focal plane can be realized. Thus, the distribution of SHG polarization is converted into a single-peak pattern which eliminates the ghosting and improves the spatial resolution on SHG images. This proposed method can be used for SHG in various materials and the full width at half maximum (FWHM) of SHG polarization will compress in different degrees. Besides, there is a significant improvement on signal-to-noise ratio in SHG imaging when an annular aperture illumination is applied. The focusing mode with elliptical mirror can also be utilized in other nonlinear optics areas such as polarized third harmonic generation (THG) measurements and two-photon fluorescence (TPF) microscopy.

Enabling second harmonic generation as a contrast mechanism for optical projection tomography (OPT) and scanning laser optical tomography (SLOT)

Volumetric imaging of connective tissue provides insights into the structure of biological tissue. Second harmonic generation (SHG) microscopy has become a standard method to image collagen rich tissue like skin or cornea. Due to the non-centrosymmetric architecture, no additional label is needed and tissue can be visualized noninvasively. Thus, SHG microscopy enables the investigation of collagen associated diseases, providing high resolution images and a field of view of several hundreds of μm. However, the in toto visualization of larger samples is limited to the working distance of the objective and the integration time of the microscope setup, which can sum up to several hours and days. A faster imaging technique for samples in the mesoscopic range is scanning laser optical tomography (SLOT), which provides linear fluorescence, scattering and absorption as intrinsic contrast mechanisms. Due to the advantages of SHG and the reduced measurement time of SLOT, the integration of SHG in SLOT would be a great extension. This way SHG measurements could be performed faster on large samples, providing isotropic resolution and simultaneous acquisition of all other contrast mechanisms available, such as fluorescence and absorption. SLOT is based on the principle of computed tomography, which requires the rotation of the sample. The SHG signal, however, depends strongly on the sample orientation and the polarization of the laser, which results in SHG intensity fluctuation during sample rotation and prevents successful 3D reconstruction. In this paper we investigate the angular dependence of the SHG signal by simulation and experiment and found a way to eliminate reconstruction artifacts caused by this angular dependence in SHG-SLOT data. This way, it is now possible to visualize samples in the mesoscopic range using SHG-SLOT, with isotropic resolution and in correlation to other contrast mechanisms as absorption, fluorescence and scattering.

Guiding cellular activity with polarized light

Actin, cytoskeleton protein forming microfilaments, play a crucial role in cellular motility. Here we show that exposure to very low levels of polarized light guide their orientation in-vivo within the live cell. Using a simple model to describe the role of actin-filament orientation in directional cellular motion, we demonstrate that the actin polymerization/depolymerization mechanism develops primarily along this direction and, under certain conditions, can lead to guidance of the cell movement. Our results also show a dose dependent increase in actin activity in direct correspondence to the level of laser irradiance. We found that total expression of Tau protein, which stabilize microtubules, was decreased by the irradiance, indicating that exposure to the light may change the activity of kinase, leading to increased cell activity.

μMAPPS: a novel phasor approach to second harmonic analysis for in vitro-in vivo investigation of collagen microstructure

Second Harmonic Generation (SHG) is a label-free imaging method used to monitor collagen organization in tissues. Due to its sensitivity to the incident polarization, it provides microstructural information otherwise unreachable by other intensity based imaging methods. We develop and test a Microscopic Multiparametric Analysis by Phasor projection of Polarization-dependent SHG (μMAPPS) that maps the features of the collagen architecture in tissues at the micrometer scale. μMAPPS retrieves pixel-by-pixel the collagen fibrils anisotropy and orientation by operating directly on two coupled phasor spaces, avoiding direct fitting of the polarization dependent SHG signal. We apply μMAPPS to fixed tissue sections and to the study of the collagen microscopic organization in tumors ex-vivo and in-vivo. We develop a clustering algorithm to automatically group pixels with similar microstructural features. μMAPPS can perform fast analyses of tissues and opens to future applications for in-situ diagnosis of pathologies and diseases that could assist histo-pathological evaluation.

Improved quantification of collagen anisotropy with polarization-resolved second harmonic generation microscopy

Imaging tissue samples by polarization-resolved second harmonic generation microscopy provides both qualitative and quantitative insights into collagen organization in a label-free manner. Polarization-resolved second harmonic generation microscopy goes beyond simple intensity-based imaging by adding the laser beam polarization component and applying different quantitative metrics such as the anisotropy factor. It thus provides valuable information on collagen arrangement not available with intensity measurements alone. Current established approaches are limited to calculating the anisotropy factor for only a particular laser beam polarization and no general guidelines on how to select the best laser beam polarization have yet been defined. Here, we introduce a novel methodology for selecting the optimal laser beam polarization for characterizing tissues using the anisotropy in the purpose of identifying cancer signatures. We show that the anisotropy factor exhibits a similar laser beam polarization dependence to the second harmonic intensity and we combine it with the collagen orientation index computed by Fast Fourier Transform analysis of the recorded images to establish a framework for choosing the laser beam polarization that is optimal for an accurate interpretation of polarization-resolved second harmonic generation microscopy images and anisotropy maps, and hence a better differentiation between healthy and dysplastic areas. SHG image of skin tissue (a) and a selected area of interest for which we compute the SHG intensity (b) and anisotropy factor (c) dependence on the laser beam polarization and also the FFT spectrum (d) to evaluate the collagen orientation index.

Third-harmonic generation Stokes-Mueller polarimetric microscopy

An experimental implementation of the nonlinear Stokes-Mueller polarimetric (NSMP) microscopy in third-harmonic generation modality is presented. The technique is able to extract all eight 2D-accessible χ⁽³⁾ components for any sample from 64 polarization measurements, and can be applied to noninvasive ultrastructural characterization. The polarization signature of an isotropic glass coverslip is presented, and carotenoid crystallites in the root of orange carrot (Daucus carota) are investigated, showing complex χ⁽³⁾ components with a significant chiral contribution.

Changes of collagen ultrastructure in breast cancer tissue determined by second-harmonic generation double Stokes-Mueller polarimetric microscopy

Second-harmonic generation (SHG) double Stokes-Mueller polarimetric microscopy is applied to study the alteration of collagen ultrastructure in a tissue microarray containing three pathological human breast cancer types with differently overexpressed estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor 2 (HER2). Kleinman symmetry is experimentally validated in breast tissue for 1028 nm laser wavelength and it has been shown that measurements with only linearly polarized incoming and outgoing states can determine molecular nonlinear susceptibility tensor component ratio, average in-plane orientation of collagen fibers and degree of linear polarization of SHG. Increase in the susceptibility ratio for ER, PgR, HER2 positive cases, reveals ultrastructural changes in the collagen fibers while the susceptibility ratio increase and decrease in degree of linear polarization for ER and PgR positive cases indicate alteration of the ultrastructure and increased disorder of the collagen fibers within each focal volume. The study demonstrates a potential use of polarimetric SHG microscopy for collagen characterization and cancer diagnostics.

(Second) Harmonic Disharmony: Nonlinear Microscopy Shines New Light on the Pathology of Atherosclerosis

There has been increasing interest in second harmonic generation (SHG) imaging approaches for the investigation of atherosclerosis due to the deep penetration and three-dimensional sectioning capabilities of the nonlinear optical microscope. Atherosclerosis involves remodeling or alteration of the collagenous framework in affected vessels. The disease is often characterized by excessive collagen deposition and altered collagen organization. SHG has the capability to accurately characterize collagen structure, which is an essential component in understanding atherosclerotic lesion development and progression. As a structure-based imaging modality, SHG is most impactful in atherosclerosis evaluation in conjunction with other, chemically specific nonlinear optics (NLO) techniques to identify additional components of the lesion. These include the use of coherent anti-Stokes Raman scattering and two-photon excitation fluorescence for studying atherosclerosis burden, and application of stimulated Raman scattering to image cholesterol crystals. However, very few NLO studies have attempted to quantitate differences in control versus atherosclerotic states or to correlate the application to clinical situations. This review highlights the potential of SHG imaging to directly and indirectly describe atherosclerosis as a pathological condition.

Connection of Jones and Mueller Tensors in Second Harmonic Generation and Multi-Photon Fluorescence Measurements

Despite the rapidly growing use of second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) microscopy, opportunities for relating polarization-dependent measurements back to local structure and molecular orientation are often confounded by losses in polarization purity. In this work, connections linking Mueller tensor and Jones tensor descriptions of polarization-dependent SHG and TPEF are shown to substantially simplify partially depolarized microscopy measurements. These connections were facilitated by the derivation of several new tensor identity relations, based on generalization of established transformations of matrices and vectors. Methods are described for integrating local-frame symmetry and azimuthal rotation angle for simplifying the Mueller tensor. Through simple expressions bridging the Mueller and Jones formalisms, mathematical models for partial depolarization can greatly simplify interpretation of SHG and TPEF measurements to reconstruct the more general Mueller tensors using the much more concise Jones descriptions for the purely polarized components. Integrating the Mueller architecture allows polarization-dependent SHG and TPEF measurements to be connected back to a relatively small set of free parameters related to local structure and orientation.

Maintaining polarization in polarimetric multiphoton microscopy

Polarimetric measurements in multiphoton microscopy can reveal information about the local molecular order of a sample. However, the presence of a dichroic through which the excitation beam propagates will generally scramble its polarization. We propose a simple scheme whereby a second properly-oriented compensation dichroic is used to negate any alteration regardless of the wavelength and the initial polarization. We demonstrate how this robust and rapid approach simplifies polarimetric measurements in second-harmonic generation, two-photon excited fluorescence and coherent anti-Stokes Raman scattering. Illustration of the polarization maintaining strategy with the compensating dichroic oriented such that its s- and p-axes are interchanged with these of the primary dichroic.

Second harmonic generation microscopy investigation of the crystalline ultrastructure of three barley starch lines affected by hydration

Second harmonic generation (SHG) microscopy is employed to study changes in crystalline organization due to altered gene expression and hydration in barley starch granules. SHG intensity and susceptibility ratio values (R'SHG ) are obtained using reduced Stokes-Mueller polarimetric microscopy. The maximum R'SHG values occur at moderate moisture indicating the narrowest orientation distribution of nonlinear dipoles from the cylindrical axis of glucan helices. The maximum SHG intensity occurs at the highest moisture and amylopectin content. These results support the hypothesis that SHG is caused by ordered hydrogen and hydroxyl bond networks which increase with hydration of starch granules.

The use of optical trap and microbeam to investigate the mechanical and transport characteristics of tunneling nanotubes in tumor spheroids

The use of optical trap and microbeam for investigating mechanical and transport properties of inter cellular tunneling nanotubes (TnTs) in tumor spheroids has been demonstrated. TnTs in tumor spheroids have been visualized by manipulating TnT connected cells using optical tweezers. Functionality of the TnTs for transferring cytoplasmic vesicles and injected dye molecules by optoporation method has been studied. Further, the TnTs could be longitudinally stretched by manipulating the connected cells and their elastic response was studied. Manipulation of cells at the surface of tumor spheroid using optical tweezers and injection of fluorescent dye into a trapped cell using optoporation technique.

Translational label-free nonlinear imaging biomarkers to classify the human corneal microstructure

Diseases that affect the cornea can lead to severe vision loss and have tremendous social impact. These diseases are associated to deviations from normal structural order and orientation of collagen fibril bundles. Unfortunately, resolving non-invasively the corneal collagen structure is not possible to date. In this work, polarization sensitive second harmonic generation (pSHG) microscopy is used to obtain information with molecular specificity on microstructure of human corneas. This information is used to develop a set of label-free imaging biomarkers that were generated by means of a novel methodology based on mathematical tensorial calculus. The method is proven to be highly sensitive and robust. The use of these biomarkers permits accurate characterization of the anisotropic, depth-dependent, structural organization of corneal collagen fibril bundles without any a priori information. The method can be valuable to improve understanding of microstructural pathophysiological changes of the human cornea close to in vivo conditions.

Enabling the detection of UV signal in multimodal nonlinear microscopy with catalogue lens components

Using an optical system made from fused silica catalogue optical components, third-order nonlinear microscopy has been enabled on conventional Ti:sapphire laser-based multiphoton microscopy setups. The optical system is designed using two lens groups with straightforward adaptation to other microscope stands when one of the lens groups is exchanged. Within the theoretical design, the optical system collects and transmits light with wavelengths between the near ultraviolet and the near infrared from an object field of at least 1 mm in diameter within a resulting numerical aperture of up to 0.56. The numerical aperture can be controlled with a variable aperture stop between the two lens groups of the condenser. We demonstrate this new detection capability in third harmonic generation imaging experiments at the harmonic wavelength of ∼300 nm and in multimodal nonlinear optical imaging experiments using third-order sum frequency generation and coherent anti-Stokes Raman scattering microscopy so that the wavelengths of the detected signals range from ∼300 nm to ∼660 nm.

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.

Crystal lattice determination of ZnSe nanowires with polarization-dependent second harmonic generation microscopy

We demonstrate a noninvasive optical microscopy technique based on polarization-dependent second harmonic generation for determining the crystal lattice structure and microscopic heterogeneities within individual nanostructures. Differentiation between periodically twinned and wurtzite ZnSe nanowires (NWs) was demonstrated, and measurement of the cubic lattice rotation orientation around the NW axis was determined within 1° accuracy. Zinc blende NWs were differentiated from wurtzite. The technique can be used for quality inspection and optimization of growth conditions for nanostructures.

Fast monitoring of in-vivo conformational changes in myosin using single scan polarization-SHG microscopy

Fast imaging of molecular changes under high-resolution and label-free conditions are essential for understanding in-vivo processes, however, current techniques are not able to monitor such changes in real time. Polarization sensitive second harmonic generation (PSHG) imaging is a minimally invasive optical microscopy technique capable of quantifying molecular conformational changes occurring below the diffraction limit. Up to now, such information is generally retrieved by exciting the sample with different linear polarizations. This procedure requires the sample to remain static during measurements (from a few second to minutes), preventing the use of PSHG microscopy from studying moving samples or molecular dynamics in living organisms. Here we demonstrate an imaging method that is one order of magnitude faster than conventional PSHG. Based on circular polarization excitation and instantaneous polarimetry analysis of the second harmonic signal generated in the tissue, the method is able to instantaneously obtain molecular information within a pixel dwell time. As a consequence, a single scan is only required to retrieve all the information. This allowed us to perform PSHG imaging in moving C. elegans, monitoring myosin's dynamics during the muscular contraction and relaxation. Since the method provides images of the molecular state, an unprecedented global understanding of the muscles dynamics is possible by correlating changes in different regions of the sample.

Third harmonic generation from mid-IR to near-IR regions in a phase-matched silicon-silicon-nanocrystal hybrid plasmonic waveguide

The conversion efficiency of third harmonic generation (THG) from mid-IR (3600 nm) to near-IR (1200 nm) regions in a silicon-silicon-nanocrystal hybrid plasmonic waveguide (SSHPW) was calculated. The required modal phase-matching condition (PMC) between the 0-th mode at fundamental wave (FW) and the 2-nd mode at third harmonic (TH) is achieved by carefully designing the waveguide geometry. Benefiting from the hybridized surface plasmon polariton (SPP) nature of the two guided modes, the SSHPW is capable of achieving both high THG nonlinear coefficient |I₆| and reasonable linear propagation loss, thereby resulting in large figure-of-merits (FOMs) for both FW and TH. According to our simulation, THG conversion efficiency up to 0.823% is achieved at 62.9 ����m SSHPW with pump power of 1 W.

Differentiation of Col I and Col III isoforms in stromal models of ovarian cancer by analysis of second harmonic generation polarization and emission directionality

A profound remodeling of the extracellular matrix occurs in many epithelial cancers. In ovarian cancer, the minor collagen isoform of Col III becomes upregulated in invasive disease. Here we use second harmonic generation (SHG) imaging microscopy to probe structural differences in fibrillar models of the ovarian stroma comprised of mixtures of Col I and III. The SHG intensity and forward-backward ratios decrease with increasing Col III content, consistent with decreased phasematching due to more randomized structures. We further probe the net collagen α-helix pitch angle within the gel mixtures using what is believed to be a new pixel-based polarization-resolved approach that combines and extends previous analyses. The extracted pitch angles are consistent with those of peptide models and the method has sufficient sensitivity to differentiate Col I from the Col I/Col III mixtures. We further developed the pixel-based approach to extract the SHG signal polarization anisotropy from the same polarization-resolved image matrix. Using this approach, we found that increased Col III results in decreased alignment of the dipole moments within the focal volume. Collectively, the SHG measurements and analysis all indicate that incorporation of Col III results in decreased organization across several levels of collagen organization. Furthermore, the findings suggest that the collagen isoforms comingle within the same fibrils, in good agreement with ultrastructural data. The pixel-based polarization analyses (both excitation and emission) afford determination of structural properties without the previous requirement of having well-aligned fibers, and the approaches should be generally applicable in tissue.

Organization of astaxanthin within oil bodies of Haematococcus pluvialis studied with polarization-dependent harmonic generation microscopy

Nonlinear optical microscopy was used to image the localization of astaxanthin accumulation in the green alga, Haematococcus pluvialis. Polarization-in, polarization-out (PIPO) second harmonic generation (SHG) and third harmonic generation (THG) microscopy was applied to study the crystalline organization of astaxanthin molecules in light-stressed H. pluvialis in vivo. Since astaxanthin readily forms H- and J-aggregates in aqueous solutions, PIPO THG studies of astaxanthin aggregates contained in red aplanospores were compared to PIPO THG of in vitro self-assembled H- and J-aggregates of astaxanthin. The PIPO THG data clearly showed an isotropic organization of astaxanthin in red aplanospores of H. pluvialis. This is in contrast to the highly anisotropic organization of astaxanthin in synthetic H- and J-aggregates, which showed to be uniaxial. Since carotenoids in vitro preferentially form H- and J-aggregates, but in vivo form a randomly organized structure, this implies that astaxanthin undergoes a different way of packing in biological organisms, which is either due to the unique physical environment of the alga or is controlled enzymatically.

Nonlinear optical microscopy signal processing strategies in cancer

This work reviews the most relevant present-day processing methods used to improve the accuracy of multimodal nonlinear images in the detection of epithelial cancer and the supporting stroma. Special emphasis has been placed on methods of non linear optical (NLO) microscopy image processing such as: second harmonic to autofluorescence ageing index of dermis (SAAID), tumor-associated collagen signatures (TACS), fast Fourier transform (FFT) analysis, and gray level co-occurrence matrix (GLCM)-based methods. These strategies are presented as a set of potential valuable diagnostic tools for early cancer detection. It may be proposed that the combination of NLO microscopy and informatics based image analysis approaches described in this review (all carried out on free software) may represent a powerful tool to investigate collagen organization and remodeling of extracellular matrix in carcinogenesis processes.

Differential polarization nonlinear optical microscopy with adaptive optics controlled multiplexed beams

Differential polarization nonlinear optical microscopy has the potential to become an indispensable tool for structural investigations of ordered biological assemblies and microcrystalline aggregates. Their microscopic organization can be probed through fast and sensitive measurements of nonlinear optical signal anisotropy, which can be achieved with microscopic spatial resolution by using time-multiplexed pulsed laser beams with perpendicular polarization orientations and photon-counting detection electronics for signal demultiplexing. In addition, deformable membrane mirrors can be used to correct for optical aberrations in the microscope and simultaneously optimize beam overlap using a genetic algorithm. The beam overlap can be achieved with better accuracy than diffraction limited point-spread function, which allows to perform polarization-resolved measurements on the pixel-by-pixel basis. We describe a newly developed differential polarization microscope and present applications of the differential microscopy technique for structural studies of collagen and cellulose. Both, second harmonic generation, and fluorescence-detected nonlinear absorption anisotropy are used in these investigations. It is shown that the orientation and structural properties of the fibers in biological tissue can be deduced and that the orientation of fluorescent molecules (Congo Red), which label the fibers, can be determined. Differential polarization microscopy sidesteps common issues such as photobleaching and sample movement. Due to tens of megahertz alternating polarization of excitation pulses fast data acquisition can be conveniently applied to measure changes in the nonlinear signal anisotropy in dynamically changing in vivo structures.

Thermal transitions of fibrillar collagen unveiled by second-harmonic generation microscopy of corneal stroma

The thermal transitions of fibrillar collagen are investigated with second-harmonic generation polarization anisotropy microscopy. Second-harmonic generation images and polarization anisotropy profiles of corneal stroma heated in the 35-80°C range are analyzed by means of a theoretical model that is suitable to probe principal intramolecular and interfibrillar parameters of immediate physiological interest. Our results depict the tissue modification with temperature as the interplay of three destructuration stages at different hierarchical levels of collagen assembly including its tertiary structure and interfibrillar alignment, thus supporting and extending previous findings. This method holds the promise of a quantitative inspection of fundamental biophysical and biochemical processes and may find future applications in real-time and postsurgical functional imaging of collagen-rich tissues subjected to thermal treatments.

From molecular structure to tissue architecture: collagen organization probed by SHG microscopy

Second-harmonic generation (SHG) microscopy is a fantastic tool for imaging collagen and probing its hierarchical organization from molecular scale up to tissue architectural level. In fact, SHG combines the advantages of a non-linear microscopy approach with a coherent modality able to probe molecular organization. In this manuscript we review the physical concepts describing SHG from collagen, highlighting how this optical process allows to probe structures ranging from molecular sizes to tissue architecture, through image pattern analysis and scoring methods. Starting from the description of the most relevant approaches employing SHG polarization anisotropy and forward - backward SHG detection, we then focus on the most relevant methods for imaging and characterizing collagen organization in tissues through image pattern analysis methods, highlighting advantages and limitations of the methods applied to tissue imaging and to potential clinical applications.

Precise, motion-free polarization control in Second Harmonic Generation microscopy using a liquid crystal modulator in the infinity space

Second Harmonic Generation (SHG) microscopy coupled with polarization analysis has great potential for use in tissue characterization, as molecular and supramolecular structural details can be extracted. Such measurements are difficult to perform quickly and accurately. Here we present a new method that uses a liquid crystal modulator (LCM) located in the infinity space of a SHG laser scanning microscope that allows the generation of any desired linear or circular polarization state. As the device contains no moving parts, polarization can be rotated accurately and faster than by manual or motorized control. The performance in terms of polarization purity was validated using Stokes vector polarimetry, and found to have minimal residual polarization ellipticity. SHG polarization imaging characteristics were validated against well-characterized specimens having cylindrical and/or linear symmetries. The LCM has a small footprint and can be implemented easily in any standard microscope and is cost effective relative to other technologies.

Imaging the bipolarity of myosin filaments with Interferometric Second Harmonic Generation microscopy

We report that combining interferometry with Second Harmonic Generation (SHG) microscopy provides valuable information about the relative orientation of noncentrosymmetric structures composing tissues. This is confirmed through the imaging of rat medial gastrocnemius muscle. The inteferometric Second Harmonic Generation (ISHG) images reveal that each side of the myosin filaments composing the A band of the sarcomere generates π phase shifted SHG signal which implies that the myosin proteins at each end of the filaments are oriented in opposite directions. This highlights the bipolar structural organization of the myosin filaments and shows that muscles can be considered as a periodically poled biological structure.

Coupled and uncoupled dipole models of nonlinear scattering

Dipole models are one of the simplest numerical models to understand nonlinear scattering. Existing dipole model for second harmonic generation, third harmonic generation and coherent anti-Stokes Raman scattering assume that the dipoles which make up a scatterer do not interact with one another. Thus, this dipole model can be called the uncoupled dipole model. This dipole model is not sufficient to describe the effects of refractive index of a scatterer or to describe scattering at the edges of a scatterer. Taking into account the interaction between dipoles overcomes these short comings of the uncoupled dipole model. Coupled dipole model has been primarily used for linear scattering studies but it can be extended to predict nonlinear scattering. The coupled and uncoupled dipole models have been compared to highlight their differences. Results of nonlinear scattering predicted by coupled dipole model agree well with previously reported experimental results.

Measurement of orientation and susceptibility ratios using a polarization-resolved second-harmonic generation holographic microscope

Three-dimensional second-harmonic fields, sample orientation, and susceptibility ratios of biological samples are measured using polarization-resolved second-harmonic generation (SHG) microscopy. The three-dimensional (3D) polarization is gathered by measurement of a series of holograms for which excitation and analyzer polarizations are systematically varied, and the 3D SHG field is recovered through numerical back propagation. Harmonophore orientation is resolved in 3D from a sub-set of polarization-resolved SHG holograms. We further expand on previous approaches for the determination of susceptibility ratios, adding the calculation of multiple ratio values to allow intrinsic verification.

Polarization-resolved second harmonic generation microscopy with a four-channel Stokes-polarimeter

We developed a four-channel photon counting based Stokes-polarimeter for spatial characterization of polarization effects in second harmonic generation (SHG). We have implemented a calibration technique allowing quantitative measurement of polarization parameters, such as the degree of polarization (DOP), degree of linear polarization (DOLP), degree of circular polarization (DOCP), as well as anisotropy from the acquired Stokes parameters. The technique is used as contrast mechanism to characterize the polarization properties from two potassium dihydrogen phosphate (KDP) micro-crystals and collagen type-I in SHG microscopy.

Characterization of oral squamous cell carcinoma based on higher-harmonic generation microscopy

In vivo higher-harmonic generation microscopy (HGM) performed on healthy human oral mucosa not only provides images with a <500 nm lateral resolution at a 280 μm penetration depth, but also leaves no photodamages in the tissues. These advantages suggest that HGM could serve as an ideal virtual biopsy tool for in vivo, in situ, and immediate histopathological diagnosis of oral cancer. However, translation of such mechanism for clinical cancer diagnosis requires evidence based algorithm capable to differentiate cancerous tissues from normal. It is thus critical to investigate if the endogenous contrast provided by the HGM would be high enough to differentiate cancerous versus normal tissues in human oral mucosa. In this report, ex vivo HGM study was performed on the cancerous mucosa from 10 patients with oral squamous cell carcinoma. Compared with histology, HGM revealed histopathological features including the cytological abnormalities, loss of differentiation, interruption of basement membrane, and irregular epithelial stratification in all 10 specimens. In addition, distinct patterns of collagen fibers and increased distribution area of actin filaments in tumor cells were noted. These results indicate HGM holds great potential for the optical biopsy screening of oral cancer lesions.