Imaging of epithelial tissue in vivo based on excitation of multiple endogenous nonlinear optical signals

Label-free multimodal nonlinear optical microscopy reveals fundamental insights of skeletal muscle development

We developed a label-free nonlinear optical (NLO) microscope integrating the stimulated Raman scattering, multi-color two-photon excited fluorescence and second harmonic generation. The system produces multimodal images of protein content, mitochondria distribution and sarcomere structure of fresh muscle samples. With the advanced imaging technique, we studied the mal-development of skeletal muscle caused by sarcomeric gene deficiency. In addition, important development processes of normal muscle from neonatal to adult stage were also clearly revealed based on the changing sarcomere structure, mitochondria distribution and muscle fiber size. The results demonstrate that the newly developed multimodal NLO microscope is a powerful tool to assess the muscle integrity and function.

Label-free in vivo flow cytometry in zebrafish using two-photon autofluorescence imaging

We demonstrate a label-free in vivo flow cytometry in zebrafish blood vessels based on two-photon excited autofluorescence imaging. The major discovery in this work is the strong autofluorescence emission from the plasma in zebrafish blood. The plasma autofluorescence provides excellent contrast for visualizing blood vessels and counting blood cells. In addition, the cellular nicotinamide adenine dinucleotide autofluorescence enables in vivo imaging and counting of white blood cells (neutrophils).

Three-dimensional imaging of small intestine morphology using non-linear optical microscopy and endogenous signals

Two-photon microscopy (2PM) has become a gold standard for deep-tissue observations in the living animal as well as on thick samples. Using 2PM, the endofluorescence properties of biomolecules have shown an interesting potential for the imaging of tissues without any staining. In this short communication, we report a method to observe the different layers of mouse small intestine explants with subcellular resolution and without any staining or clearing. This method allows rapid observations of samples with little to no preparation thanks to the endofluorescence properties of biomolecules such as NAD(P)H or flavins and second-harmonic generation. Finally, we show different three-dimensional reconstructions of the mouse small intestine anatomy obtained with this approach to show the potential of this method in morphological studies.

Establishment of rules for interpreting ultraviolet autofluorescence microscopy images for noninvasive detection of Barrett's esophagus and dysplasia

The diagnostic potential of autofluorescence (AF) microscopy under ultraviolet (UV) excitation is explored using ex vivo human specimens. The aim is to establish optical patterns (the rules for interpretation) that correspond to normal and abnormal histologies of the esophagus, spanning from early benign modifications (Barrett's esophagus) to subsequent dysplastic change and progression toward carcinoma. This was achieved by developing an image library categorized by disease progression. We considered morphological changes of disease as they are compared with histological diagnosis of the pathological specimen, as well as control samples of normal esophagus, proximal stomach, and small intestine tissue. Our experimental results indicate that UV AF microscopy could provide real-time histological information for visualizing changes in tissue microstructure that are currently undetectable using conventional endoscopic methods.

Time-resolved detection enables standard two-photon fluorescence microscopy for in vivo label-free imaging of microvasculature in tissue

We conducted a systematic study on two-photon excited fluorescence (TPEF) of hemoglobin using the near transform-limited and Gaussian-shaped femtosecond pulse sources. We found that the two-photon action cross section of hemoglobin drops over 2 orders of magnitude in the wavelength range from 550 to 800 nm, while the spectral and temporal characteristics of hemoglobin TPEF are insensitive to the change of excitation wavelength. In particular, our new findings showed that the hemoglobin fluorescence could be excited with sufficient efficiency using a conventional Ti:sapphire laser tuned at the wavelength close to 700 nm. With the employment of a time-resolved detection method, we demonstrated that the TPEF signals of hemoglobin excited by a Ti:sapphire laser could be clearly differentiated from other nonlinear signals presented within the living biological tissues, indicating that a standard TPEF microscope can become a routine tool for in vivo label-free microangiography imaging.

Endomicroscopy imaging of epithelial structures using tissue autofluorescence

We explore autofluorescence endomicroscopy as a potential tool for real-time visualization of epithelial tissue microstructure and organization in a clinical setting. The design parameters are explored using two experimental systems--an Olympus Medical Systems Corp. stand-alone clinical prototype probe, and a custom built bench-top rigid fiber conduit prototype. Both systems entail ultraviolet excitation at 266 nm and/or 325 nm using compact laser sources. Preliminary results using ex vivo animal and human tissue specimens suggest that this technology can be translated toward in vivo application to address the need for real-time histology.

Two-photon excited hemoglobin fluorescence provides contrast mechanism for label-free imaging of microvasculature in vivo

Direct visualization of microvasculature provides significant insights in microcirculation and critically impacts the diagnosis and treatment of microcirculatory diseases. Recently, we discovered that the high-energy Soret fluorescence of hemoglobin peaked at 438 nm with an extremely short lifetime becomes strongly visible under two-photon excitation. Based on the distinct spectral and temporal characteristics of hemoglobin fluorescence, we demonstrated that two-photon fluorescence microscopy could become a powerful tool for label-free in vivo imaging of microvasculature in tissue.

Maximum imaging depth of two-photon autofluorescence microscopy in epithelial tissues

Endogenous fluorescence provides morphological, spectral, and lifetime contrast that can indicate disease states in tissues. Previous studies have demonstrated that two-photon autofluorescence microscopy (2PAM) can be used for noninvasive, three-dimensional imaging of epithelial tissues down to approximately 150 μm beneath the skin surface. We report ex-vivo 2PAM images of epithelial tissue from a human tongue biopsy down to 370 μm below the surface. At greater than 320 μm deep, the fluorescence generated outside the focal volume degrades the image contrast to below one. We demonstrate that these imaging depths can be reached with 160 mW of laser power (2-nJ per pulse) from a conventional 80-MHz repetition rate ultrafast laser oscillator. To better understand the maximum imaging depths that we can achieve in epithelial tissues, we studied image contrast as a function of depth in tissue phantoms with a range of relevant optical properties. The phantom data agree well with the estimated contrast decays from time-resolved Monte Carlo simulations and show maximum imaging depths similar to that found in human biopsy results. This work demonstrates that the low staining inhomogeneity (∼ 20) and large scattering coefficient (∼ 10 mm(-1)) associated with conventional 2PAM limit the maximum imaging depth to 3 to 5 mean free scattering lengths deep in epithelial tissue.

Two-photon excited hemoglobin fluorescence

We discovered that hemoglobin emits high energy Soret fluorescence when two-photon excited by the visible femtosecond light sources. The unique spectral and temporal characteristics of hemoglobin fluorescence were measured by using a time-resolved spectroscopic detection system. The high energy Soret fluorescence of hemoglobin shows the spectral peak at 438 nm with extremely short lifetime. This discovery enables two-photon excitation fluorescence microscopy to become a potentially powerful tool for in vivo label-free imaging of blood cells and vessels.

Characterizing the origin of autofluorescence in human esophageal epithelium under ultraviolet excitation

The autofluorescence under ultraviolet excitation arising from normal squamous and columnar esophageal mucosa is investigated using multispectral microscopy. The results suggest that the autofluorescence signal arises from the superficial tissue layer due to the short penetration depth of the ultraviolet excitation. As a result, visualization of esophageal epithelial cells and their organization can be attained using wide-field autofluorescence microscopy. Our results show tryptophan to be the dominant source of emission under 266 nm excitation, while emission from NADH and collagen are dominant under 355 nm excitation. The analysis of multispectral microscopy images reveals that tryptophan offers the highest image contrast due to its non-uniform distribution in the sub-cellular matrix. This technique can simultaneously provide functional and structural imaging of the microstructure using only the intrinsic tissue fluorophores.