The role of backscattering in SHG tissue imaging

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.

Signal improvement in multiphoton microscopy by reflection with simple mirrors near the sample

In conventional fluorescence or confocal microscopy, emitted light is generated not only in the focal plane but also above and below. The situation is different in multiphoton-induced fluorescence and multiphoton-induced higher harmonic generation. Here, restriction of signal generation to a single focal point permits that all emitted photons can contribute to image formation if collected, regardless of their path through the specimen. Often, the intensity of the emitted light is rather low in biological specimens. We present a method to significantly increase the fraction of photons collected by an epi (backward) detector by placing a simple mirror, an aluminum-coated coverslip, directly under the sample. Samples investigated include fluorescent test slides, collagen gels, and thin-layered, intact mouse skeletal muscles. Quantitative analysis revealed an intensity increase of second- and third-harmonic generated signal in skeletal muscle of nine- and sevenfold respectively, and of fluorescent signal in test slides of up to twofold. Our approach thus allows significant signal improvement also for situations were a forward detection is impossible, e.g., due to the anatomy of animals in intravital microscopy.

Second-harmonic generation imaging of collagen fibers in myocardium for atrial fibrillation diagnosis

Atrial fibrillation (AF) is the most common irregular heart rhythm and the mortality rate for patients with AF is approximately twice the mortality rate for patients with normal sinus rhythm (NSR). Some research has indicated that myocardial fibrosis plays an important role in predisposing patients to AF. Therefore, realizing the relationship between myocardial collagen fibrosis and AF is significant. Second-harmonic generation (SHG) is an optically nonlinear coherent process to image the collagen network. We perform SHG microscopic imaging of the collagen fibers in the human atrial myocardium. Utilizing the SHG images, we can identify the differences in morphology and the arrangement of collagen fibers between NSR and AF tissues. We also quantify the arrangement of the collagen fibers using Fourier transform images and calculating the values of angle entropy. We indicate that SHG imaging, a nondestructive and reproducible method to analyze the arrangement of collagen fibers, can provide explicit information about the relationship between myocardial fibrosis and AF.

Second harmonic generation imaging as a potential tool for staging pregnancy and predicting preterm birth

We use second harmonic generation (SHG) microscopy to assess changes in collagen structure of murine cervix during cervical remodeling of normal pregnancy and in a preterm birth model. Visual inspection of SHG images revealed substantial changes in collagen morphology throughout normal gestation. SHG images collected in both the forward and backward directions were analyzed quantitatively for changes in overall mean intensity, forward to backward intensity ratio, collagen fiber size, and porosity. Changes in mean SHG intensity and intensity ratio take place in early pregnancy, suggesting that submicroscopic changes in collagen fibril size and arrangement occur before macroscopic changes become evident. Fiber size progressively increased from early to late pregnancy, while pores between collagen fibers became larger and farther apart. Analysis of collagen features in premature cervical remodeling show that changes in collagen structure are dissimilar from normal remodeling. The ability to quantify multiple morphological features of collagen that characterize normal cervical remodeling and distinguish abnormal remodeling in preterm birth models supports future studies aimed at development of SHG endoscopic devices for clinical assessment of collagen changes during pregnancy in women and for predicting risk of preterm labor which occurs in 12.5% of all pregnancies.

Quantitative analysis of forward and backward second-harmonic images of collagen fibers using Fourier transform second-harmonic-generation microscopy

Fourier transform second-harmonic generation (SHG) microscopy has been applied to quantitatively compare the information content between SHG images obtained from the forward and backward direction for three tissue types: porcine tendon, sclera, and ear cartilage. Both signal types yield consistent information on the preferred orientation of collagen fibers. For all specimens, the Fourier transform of the forward and backward SHG images produces several overlapping peaks in the magnitude spectrum at various depths into the tissues, indicating that some information present in the forward SHG images can be extracted from the backward SHG images. This study highlights the potential of backward SHG microscopy for medical diagnostics.

Multimodal, multiphoton microscopy and image correlation analysis for characterizing corneal thermal damage

We used the combination of multiphoton autofluorescence (MAF), forward second-harmonic generation (FWSHG), and backward second-harmonic generation (BWSHG) imaging for the qualitative and quantitative characterization of thermal damage of ex vivo bovine cornea. We attempt to characterize the structural alterations by qualitative MAF, FWSHG, and BWSHG imaging in the temperature range of 37 to 90 degrees C. In addition to measuring the absolute changes in the three types of signals at the stromal surface, we also performed image correlation analysis between FWSHG and BWSHG and demonstrate that with increasing thermal damage, image correlation between FWSHG and BWSHG significantly increases. Our results show that while MAF and BWSHG intensities may be used as preliminary indicators of the extent of corneal thermal damage, the most sensitive measures are provided by the decay in FWSHG intensity and the convergence of FWSHG and BWSHG images.

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.

Selective imaging in second-harmonic-generation microscopy with anisotropic radiation

As a novel modality of optical microscopy, second-harmonic generation (SHG) provides attractive features including intrinsic optical sectioning, noninvasiveness, high specificity, and high penetrability. For a biomedical application, the epicollection of backward propagating SHG is necessary. But due to phase-matching constraint, SHG from thick tissues is preferentially forward propagation. Myosin and collagen are two of the most abundant fibrous proteins in vertebrates, and both exhibit a strong second-harmonic response. We find that the radiation patterns of myosin-based muscle fibers and collagen fibrils are distinct due to coherence effects. Based on these asymmetric radiation patterns, we demonstrate selective imaging between intertwining muscle fibers and type I collagen fibrils with forward and backward SHG modalities, respectively. Thick muscle fibers dominate the forward signal, while collagen fibril distribution is preferentially resolved in the backward channel without strong interference from muscle. Moreover, we find that well-formed collagen fibrils are highlighted by forward SHG, while loosely arranged collagen matrix is outlined by backward signal.

Multimodal nonlinear optical imaging of collagen arrays

We report multimodal nonlinear optical imaging of fascia, a rich collagen type I sheath around internal organs and muscle. We show that second harmonic generation (SHG), third harmonic generation (THG) and Coherent anti-Stokes Raman scattering (CARS) microscopy techniques provide complementary information about the sub-micron architecture of collagen arrays. Forward direction SHG microscopy reveals the fibrillar arrangement of collagen type I structures as the main matrix component of fascia. SHG images detected in the backward direction as well as images of forward direction CARS microscopy show that the longitudinal collagen fiber bundles are further arranged in sheet-like bands. Forward-THG microscopy reveals the optically homogeneous content of the collagen sheet on a spatial scale of the optical wavelength. This is supported by the fact that the third harmonic signal is observed only at the boundaries between the sheets as well as by the CARS data obtained in both directions. The observations made with THG and CARS microscopy are explained using atomic force microscopy images.

Detection and imaging of non-contractile inclusions and sarcomeric anomalies in skeletal muscle by second harmonic generation combined with two-photon excited fluorescence

The large size of the multinucleated muscle fibers of skeletal muscle makes their examination for structural and pathological defects a challenge. Sections and single fibers are accessible to antibodies and other markers but imaging of such samples does not provide a three-dimensional view of the muscle. Regrettably, bundles of fibers cannot be stained or imaged easily. Two-photon microscopy techniques overcome these obstacles. Second harmonic generation (SHG) by myosin filaments and two-photon excited fluorescence (2PEF) of mitochondrial and lysosomal components provides detailed structural information on unstained tissue. Furthermore, the infrared exciting light can penetrate several layers of muscle fibers and the minimal processing is particularly valuable for fragile biopsies. Here we demonstrate the usefulness of SHG, combined with 2PEF, to reveal enlarged lysosomes and accumulations of non-contractile material in muscles from the mouse model for the lysosomal storage disorder Pompe disease (PD), and in biopsies from adult and infant PD patients. SHG and 2PEF also detect sarcomeric defects that may presage the loss of myofibrils in atrophying muscle and signify loss of elasticity. The combination of SHG and 2PEF should be useful in the analysis and diagnosis of a wide range of skeletal muscle pathologies.

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

We have investigated the mechanisms and capabilities of optical clearing in conjunction with second harmonic generation (SHG) imaging in tendon and striated muscle. Our approach combines three-dimensional (3-D) SHG imaging of the axial attenuation and directional response with Monte Carlo simulation (based on measured bulk optical properties) of the creation intensity and propagation through the tissues. Through these experiments and simulations, we show that reduction of the primary filter following glycerol treatment dominates the axial attenuation response in both muscle and tendon. However, these disparate tissue types are shown to clear through different mechanisms of the glycerol-tissue interaction. In the acellular tendon, glycerol application reduces scattering by both index matching as well as increasing the interfibril separation. This results in an overall enhancement of the 3-D SHG intensity, where good agreement is found between experiment and simulation. Through analysis of the axial response as a function of glycerol concentration in striated muscle, we conclude that the mechanism in this tissue arises from matching of the refractive index of the cytoplasm of the muscle cells with that of the surrounding higher-index collagenous perimysium. We further show that the proportional decrease in the scattering coefficient mu(s) with increasing glycerol fraction can be well-approximated by Mie theory.