Measurement of the ratio of forward-propagating to back-propagating second harmonic signal using a single objective

Quantitative Classification of 3D Collagen Fiber Organization From Volumetric Images

Collagen fibers in biological tissues have a complex 3D organization containing rich information linked to tissue mechanical properties and are affected by mutations that lead to diseases. Quantitative assessment of this 3D collagen fiber organization could help to develop reliable biomechanical models and understand tissue structure-function relationships, which impact diagnosis and treatment of diseases or injuries. While there are advanced techniques for imaging collagen fibers, published methods for quantifying 3D collagen fiber organization have been sparse and give limited structural information which cannot distinguish a wide range of 3D organizations. In this article, we demonstrate an algorithm for quantitative classification of 3D collagen fiber organization. The algorithm first simulates five groups, or classifications, of fiber organization: unidirectional, crimped, disordered, two-fiber family, and helical. These five groups are widespread in natural tissues and are known to affect the tissue's mechanical properties. We use quantitative metrics based on features such as preferred 3D fiber orientation and spherical variance to differentiate each classification in a repeatable manner. We validate our algorithm by applying it to second-harmonic generation images of collagen fibers in tendon and cervix tissue that has been sectioned in specified orientations, and we find strong agreement between classification from simulated data and the physical fiber organization. Our approach provides insight for interpreting 3D fiber organization directly from volumetric images. This algorithm could be applied to other fiber-like structures that are not necessarily made of collagen.

Tissue effects of intra-tissue refractive index shaping (IRIS): insights from two-photon autofluorescence and second harmonic generation microscopy

Intra-tissue refractive index shaping (IRIS) is a novel, non-ablative form of vision correction by which femtosecond laser pulses are tightly focused into ocular tissues to induce localized refractive index (RI) change via nonlinear absorption. Here, we examined the effects of Blue-IRIS on corneal microstructure to gain insights into underlying mechanisms. Three-layer grating patterns were inscribed with IRIS ~180 µm below the epithelial surface of ex vivo rabbit globes using a 400 nm femtosecond laser. Keeping laser power constant at 82 mW in the focal volume, multiple patterns were written at different scan speeds. The largest RI change induced in this study was + 0.011 at 20 mm/s. After measuring the phase change profile of each inscribed pattern, two-photon excited autofluorescence (TPEF) and second harmonic generation (SHG) microscopy were used to quantify changes in stromal structure. While TPEF increased significantly with induced RI change, there was a noticeable suppression of SHG signal in IRIS treated regions. We posit that enhancement of TPEF was due to the formation of new fluorophores, while decreases in SHG were most likely due to degradation of collagen triple helices. All in all, the changes observed suggest that IRIS works by inducing a localized, photochemical change in collagen structure.

Application of quantitative second-harmonic generation microscopy to posterior cruciate ligament for crimp analysis studies

We use second-harmonic generation (SHG) microscopy to quantitatively characterize collagen fiber crimping in the posterior cruciate ligament (PCL). The obtained SHG images are utilized to define three distinct categories of crimp organization in the PCL. Using our previously published spatial-frequency analysis, we develop a simple algorithm to quantitatively distinguish the various crimp patterns. In addition, SHG microscopy reveals both the three-dimensional structural variation in some PCL crimp patterns as well as an underlying helicity in these patterns that have mainly been observed using electron microscopy. Our work highlights how SHG microscopy could potentially be used to link the fibrous structural information in the PCL to its mechanical properties.

Dual CARS and SHG image acquisition scheme that combines single central fiber and multimode fiber bundle to collect and differentiate backward and forward generated photons

In coherent anti-Stokes Raman scattering (CARS) and second harmonic generation (SHG) imaging, backward and forward generated photons exhibit different image patterns and thus capture salient intrinsic information of tissues from different perspectives. However, they are often mixed in collection using traditional image acquisition methods and thus are hard to interpret. We developed a multimodal scheme using a single central fiber and multimode fiber bundle to simultaneously collect and differentiate images formed by these two types of photons and evaluated the scheme in an endomicroscopy prototype. The ratio of these photons collected was calculated for the characterization of tissue regions with strong or weak epi-photon generation while different image patterns of these photons at different tissue depths were revealed. This scheme provides a new approach to extract and integrate information captured by backward and forward generated photons in dual CARS/SHG imaging synergistically for biomedical applications.

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.

The Use of Second Harmonic Generation to Image the Extracellular Matrix During Tumor Progression

Metastasis is the leading cause of cancer mortality, resulting from changes in the tumor microenvironment which increases tumor cell migration, dispersal to distant organs, and subsequent survival. This is accompanied by changes in tumor collagen which may allow cells to travel more efficiently away from a primary tumor and invade the surrounding tissue. Second Harmonic generation (SHG) is an intrinsic optical signal that has expanded our understanding of collagen evolution throughout tumor progression. This article addresses current research into tumor progression using SHG, as well as the future prospects of using SHG to advance our understanding of the tumor microenvironment.

Characterization of collagen and cholesterol deposition in atherosclerotic arterial tissue using non-linear microscopy

Atherosclerosis is characterized by the accumulation of lipids within the arterial wall and is commonly diagnosed using standard histology. Non-linear microscopy represents a possible label-free alternative to standard diagnostic methods for imaging various tissue components. Here we employ SHG and CARS microscopy for imaging thin cross-sections of atherosclerotic arterial tissue, demonstrating that both cholesterol deposition in the lumen and collagen in the normal arterial wall can be imaged and discriminated using SHG and CARS microscopy. A simultaneous detection of both forward and backward scattered SHG signals allows distinguishing collagen fibres from cholesterol. Further analysis, based on image pattern evaluation algorithms, is used to characterize collagen organization in the healthy arterial wall against collagen found within plaques. Different values of fibre mean size, distribution and anisotropy are calculated for lumen and media prospectively allowing for automated classification of atherosclerotic lesions. The presented method represents a promising diagnostic tool for evaluating atherosclerotic tissue.

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.

Quantitative analysis of multiphoton excitation autofluorescence and second harmonic generation imaging for medical diagnosis

In recent years, two-photon excitation fluorescence and second harmonic generation microscopy has become an important tool in biomedical research. The ability of two-photon microscopy to achieve optical sectioning with minimal invasiveness is particularly advantageous for biomedical diagnosis. Advances in the miniaturization of the imaging system have increased its clinical potential, together with the development of quantitative technique for the analysis of data acquired using these imaging modalities. We present a review of the quantitative analysis techniques that have been used successfully with two-photon excitation fluorescence and SHG imaging. Specifically, quantification techniques using ratiometric, morphological, and structural differences to analyze two-photon images will be discussed, and their effectiveness at evaluating dermal and corneal pathologies and cancerous tumor growth will be described.

Two-photon and second harmonic microscopy in clinical and translational cancer research

Application of two-photon microscopy (TPM) to translational and clinical cancer research has burgeoned over the last several years, as several avenues of pre-clinical research have come to fruition. In this review, we focus on two forms of TPM-two-photon excitation fluorescence microscopy, and second harmonic generation microscopy-as they have been used for investigating cancer pathology in ex vivo and in vivo human tissue. We begin with discussion of two-photon theory and instrumentation particularly as applicable to cancer research, followed by an overview of some of the relevant cancer research literature in areas that include two-photon imaging of human tissue biopsies, human skin in vivo, and the rapidly developing technology of two-photon microendoscopy. We believe these and other evolving two-photon methodologies will continue to help translate cancer research from the bench to the bedside, and ultimately bring minimally invasive methods for cancer diagnosis and treatment to therapeutic reality.

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.