Reflection spectroscopy of atherosclerotic plaque

Multimodal microscopy for the simultaneous visualization of five different imaging modalities using a single light source

Optical microscopy has been widely used in biomedical research as it provides photophysical and photochemical information of the target in subcellular spatial resolution without requiring physical contact with the specimen. To obtain a deeper understanding of biological phenomena, several efforts have been expended to combine such optical imaging modalities into a single microscope system. However, the use of multiple light sources and detectors through separated beam paths renders previous systems extremely complicated or slow for in vivo imaging. Herein, we propose a novel high-speed multimodal optical microscope system that simultaneously visualizes five different microscopic contrasts, i.e., two-photon excitation, second-harmonic generation, backscattered light, near-infrared fluorescence, and fluorescence lifetime, using a single femtosecond pulsed laser. Our proposed system can visualize five modal images with a frame rate of 3.7 fps in real-time, thereby providing complementary optical information that enhances both structural and functional contrasts. This highly photon-efficient multimodal microscope system enables various properties of biological tissues to be assessed.

In vivo diffuse reflectance spectroscopic analysis of fatty liver with inflammation in mice

Background

Nonalcoholic steatohepatitis is a progressive liver disease that can lead to cirrhosis, hepatocellular carcinoma, and hepatic failure. Thus, the diagnosis of nonalcoholic steatohepatitis, especially discrimination from nonalcoholic fatty liver, is crucial, but reliable methods other than invasive biopsy have not been established yet. In this study, we investigated the usefulness of diffuse reflectance spectroscopy, which does not require tissue collection, to evaluate the pathological states of fatty liver with inflammation.

Materials and Methods

We performed in vivo optical fiber-based diffuse reflectance spectroscopy in both the near-infrared and visible spectral regions for livers in STAM mice, which typically show steatosis at 6 weeks, steatohepatitis at 8 weeks, and fibrosis at 12 weeks of age. After diffuse reflectance spectroscopy, all of the liver tissues were histologically analyzed and scored on the basis of the rodent nonalcoholic fatty liver disease scoring system. We examined correlations between the diffuse reflectance spectra and scores associated with steatosis and inflammation.

Results and Conclusion

The results showed that the second derivative values of reflectance at 1204 nm, the lipid absorption peak in the near-infrared region, were strongly correlated with steatosis scores (r = 0.9172, P < .0001, n = 20) and that the differences of the first derivative values of reflectance in the visible region (570 nm − 550 nm) that reflect hemoglobin deoxygenation were significantly correlated with inflammation scores (r = 0.5260, P = .0172, n = 20). These results suggest that our diffuse reflectance spectroscopy method is useful for diagnosis of the states of steatosis with inflammation in livers and hence nonalcoholic steatohepatitis.

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Optical reflectance signals are used to diagnose nonalcoholic steatohepatitis in mice.

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The near-infrared reflectance signals are strongly correlated with steatosis scores.

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The visible reflectance signals are significantly correlated with inflammation scores.

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Nonalcoholic steatohepatitis can be noninvasively detected by reflectance signals.

Hyperspectral imaging of atherosclerotic plaques in vitro

Vulnerable plaques constitute a risk for serious heart problems, and are difficult to identify using existing methods. Hyperspectral imaging combines spectral- and spatial information, providing new possibilities for precise optical characterization of atherosclerotic lesions. Hyperspectral data were collected from excised aorta samples (n = 11) using both white-light and ultraviolet illumination. Single lesions (n = 42) were chosen for further investigation, and classified according to histological findings. The corresponding hyperspectral images were characterized using statistical image analysis tools (minimum noise fraction, K-means clustering, principal component analysis) and evaluation of reflectance/fluorescence spectra. Image analysis combined with histology revealed the complexity and heterogeneity of aortic plaques. Plaque features such as lipids and calcifications could be identified from the hyperspectral images. Most of the advanced lesions had a central region surrounded by an outer rim or shoulder-region of the plaque, which is considered a weak spot in vulnerable lesions. These features could be identified in both the white-light and fluorescence data. Hyperspectral imaging was shown to be a promising tool for detection and characterization of advanced atherosclerotic plaques in vitro. Hyperspectral imaging provides more diagnostic information about the heterogeneity of the lesions than conventional single point spectroscopic measurements.

Noise and stray light characterization of a compact CCD spectrophotometer used in biomedical applications

In recent years, new generation spectrophotometers are increasingly used in biomedical applications. Handheld spectrophotometers, offering compactness, versatility, and low cost, have facilitated a broad array of applications in biomedical optics. However, despite the popularity and the diverse range of applications, a detailed characterization of many of these new spectrophotometers in terms of stray light and noise characteristics is missing from the literature. Such a popular instrument (USB2000) is characterized in detail with particular focus and emphasis on its noise and stray light characteristics. The results of the analysis may be useful to numerous users of this and other similar instruments in a diverse range of applications.

Spectroscopy to improve identification of vulnerable plaques in cardiovascular disease

Many apparent healthy persons die from cardiovascular disease, despite major advances in prevention and treatment of cardiovascular disease. Traditional cardiovascular risk factors are able to predict cardiovascular events in the long run, but fail to assess current disease activity or nearby cardiovascular events. There is a clear relation between the occurrence of cardiovascular events and the presence of so-called vulnerable plaques. These vulnerable plaques are characterized by active inflammation, a thin cap and a large lipid pool. Spectroscopy is an optical imaging technique which depicts the interaction between light and tissues, and thereby shows the biochemical composition of tissues. In recent years, impressive advances have been made in spectroscopy technology and intravascular spectroscopy is able to assess the composition of plaques of interest and thereby to identify and actually quantify plaque vulnerability. This review summarizes the current evidence for spectroscopy as a measure of plaque vulnerability and discusses the potential role of intravascular spectroscopic imaging techniques.

Label-free molecular imaging of atherosclerotic lesions using multimodal nonlinear optical microscopy

Arterial tissues collected from Ossabaw swine bearing metabolic syndrome-induced cardiovascular plaques are characterized by multimodal nonlinear optical microscopy that allows coherent anti-Stokes Raman scattering, second-harmonic generation, and two-photon excitation fluorescence imaging on the same platform. Significant components of arterial walls and atherosclerotic lesions, including endothelial cells, extracellular lipid droplets, lipid-rich cells, low-density lipoprotein aggregates, collagen, and elastin are imaged without any labeling. Emission spectra of these components are obtained by nonlinear optical microspectrometry. The nonlinear optical contrast is compared with histology of the same sample. Multimodal nonlinear optical imaging of plaque composition also allows identification of atherosclerotic regions that are vulnerable to rupture risk. The demonstrated capability of nonlinear optical microscopy for label-free molecular imaging of atherosclerotic lesions with 3-D submicrometric resolution suggests its potential application to the diagnosis of atherosclerotic plaques, determination of their rupture risk, and design of individualized drug therapy based on plaque composition.