Integrated multimodal optical microscopy for structural and functional imaging of engineered and natural skin

Correlative Multi-Modal Microscopy: A Novel Pipeline for Optimizing Fluorescence Microscopy Resolutions in Biological Applications

The modern fluorescence microscope is the convergence point of technologies with different performances in terms of statistical sampling, number of simultaneously analyzed signals, and spatial resolution. However, the best results are usually obtained by maximizing only one of these parameters and finding a compromise for the others, a limitation that can become particularly significant when applied to cell biology and that can reduce the spreading of novel optical microscopy tools among research laboratories. Super resolution microscopy and, in particular, molecular localization-based approaches provide a spatial resolution and a molecular localization precision able to explore the scale of macromolecular complexes in situ. However, its use is limited to restricted regions, and consequently few cells, and frequently no more than one or two parameters. Correlative microscopy, obtained by the fusion of different optical technologies, can consequently surpass this barrier by merging results from different spatial scales. We discuss here the use of an acquisition and analysis correlative microscopy pipeline to obtain high statistical sampling, high content, and maximum spatial resolution by combining widefield, confocal, and molecular localization microscopy.

Label-free metabolic and structural profiling of dynamic biological samples using multimodal optical microscopy with sensorless adaptive optics

Label-free optical microscopy has matured as a noninvasive tool for biological imaging; yet, it is criticized for its lack of specificity, slow acquisition and processing times, and weak and noisy optical signals that lead to inaccuracies in quantification. We introduce FOCALS (Fast Optical Coherence, Autofluorescence Lifetime imaging, and Second harmonic generation) microscopy capable of generating NAD(P)H fluorescence lifetime, second harmonic generation (SHG), and polarization-sensitive optical coherence microscopy (OCM) images simultaneously. Multimodal imaging generates quantitative metabolic and morphological profiles of biological samples in vitro, ex vivo, and in vivo. Fast analog detection of fluorescence lifetime and real-time processing on a graphical processing unit enables longitudinal imaging of biological dynamics. We detail the effect of optical aberrations on the accuracy of FLIM beyond the context of undistorting image features. To compensate for the sample-induced aberrations, we implemented a closed-loop single-shot sensorless adaptive optics solution, which uses computational adaptive optics of OCM for wavefront estimation within 2 s and improves the quality of quantitative fluorescence imaging in thick tissues. Multimodal imaging with complementary contrasts improves the specificity and enables multidimensional quantification of the optical signatures in vitro, ex vivo, and in vivo, fast acquisition and real-time processing improve imaging speed by 4-40 × while maintaining enough signal for quantitative nonlinear microscopy, and adaptive optics improves the overall versatility, which enable FOCALS microscopy to overcome the limits of traditional label-free imaging techniques.

Mechanical modeling and characterization of human skin: A review

This paper reviews the advances made in recent years on modeling approaches and experimental techniques to characterize the mechanical properties of human skin. The skin is the largest organ of the human body that has a complex multi-layered structure with different mechanical behaviors. The mechanical properties of human skin play an important role in distinguishing between healthy and unhealthy skin. Furthermore, knowing these mechanical properties enables computer simulation, skin research, clinical studies, as well as diagnosis and treatment monitoring of skin diseases. This paper reviews the recent efforts on modeling skin using linear, nonlinear, viscoelastic, and anisotropic materials. The work also focuses on aging effects, microstructure analysis, and non-invasive methods for skin testing. A detailed explanation of the skin structure and numerical models, such as finite element models, are discussed in this work. This work also compares different experimental methods that measure the mechanical properties of human skin. The work reviews the experimental results in the literature and shows how the mechanical properties of human skin vary with the skin sites, the layers, and the structure of human skin. The paper also discusses how state-of-the-art technology can advance skin research.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Real-time pixelwise phasor analysis for video-rate two-photon fluorescence lifetime imaging microscopy

Two-photon fluorescence lifetime imaging microscopy (FLIM) is a widely used technique in biomedical optical imaging. Presently, many two-photon time-domain FLIM setups are limited by long acquisition and postprocessing times that decrease data throughput and inhibit the ability to image fast sub-second processes. Here, we present a versatile two-photon FLIM setup capable of video-rate (up to 25 fps) imaging with graphics processing unit (GPU)-accelerated pixelwise phasor analysis displayed and saved simultaneously with acquisition. The system uses an analog output photomultiplier tube in conjunction with 12-bit digitization at 3.2 GHz to overcome the limited maximum acceptable photon rate associated with the photon counting electronics in many FLIM systems. This allows for higher throughput FLIM acquisition and analysis, and additionally enables the user to assess sample fluorescence lifetime in real-time. We further explore the capabilities of the system to examine the kinetics of Rhodamine B uptake by human breast cancer cells and characterize the effect of pixel dwell time on the reduced nicotinamide adenine dinucleotide and reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) autofluorescence lifetime estimation accuracy.

Assessing the severity of psoriasis through multivariate analysis of optical images from non-lesional skin

Patients with psoriasis represent a heterogeneous population with individualized disease expression. Psoriasis can be monitored through gold standard histopathology of biopsy specimens that are painful and permanently scar. A common associated measure is the use of non-invasive assessment of the Psoriasis Area and Severity Index (PASI) or similarly derived clinical assessment based scores. However, heterogeneous manifestations of the disease lead to specific PASI scores being poorly reproducible and not easily associated with clinical severity, complicating the efforts to monitor the disease. To address this issue, we developed a methodology for non-invasive automated assessment of the severity of psoriasis using optical imaging. Our analysis shows that two-photon fluorescence lifetime imaging permits the identification of biomarkers present in both lesional and non-lesional skin that correlate with psoriasis severity. This ability to measure changes in lesional and healthy-appearing skin provides a new pathway for independent monitoring of both the localized and systemic effects of the disease. Non-invasive optical imaging was conducted on lesions and non-lesional (pseudo-control) skin of 33 subjects diagnosed with psoriasis, lesional skin of 7 subjects diagnosed with eczema, and healthy skin of 18 control subjects. Statistical feature extraction was combined with principal component analysis to analyze pairs of two-photon fluorescence lifetime images of stratum basale and stratum granulosum layers of skin. We found that psoriasis is associated with biochemical and structural changes in non-lesional skin that can be assessed using clinically available two-photon fluorescence lifetime microscopy systems.

Coregistered Spectral Optical Coherence Tomography and Two-Photon Microscopy for Multimodal Near-Instantaneous Deep-Tissue Imaging

Two-photon microscopy (2PM) has brought unique insight into the mechanisms underlying immune system dynamics and function since it enables monitoring of cellular motility and communication in complex systems within their genuine environment-the living organism. However, use of 2PM in clinical settings is limited. In contrast, optical coherence tomography (OCT), a noninvasive label-free diagnostic imaging method, which allows monitoring morphologic changes of large tissue regions in vivo, has found broad application in the clinic. Here we developed a combined multimodal technology to achieve near-instantaneous coregistered OCT, 2PM, and second harmonic generation (SHG) imaging over large volumes (up to 1,000 × 1,000 × 300 μm³ ) of tendons and other tissue compartments in mouse paws, as well as in mouse lymph nodes, spleens, and femurs. Using our multimodal imaging approach, we found differences in macrophage cell shape and motility behavior depending on whether they are located in tendons or in the surrounding tissue compartments of the mouse paw. The cellular shape of tissue-resident macrophages, indicative for their role in tissue, correlated with the supramolecular organization of collagen as revealed by SHG and OCT. Hence, the here-presented approach of coregistered OCT and 2PM has the potential to link specific cellular phenotypes and functions (as revealed by 2PM) to tissue morphology (as highlighted by OCT) and thus, to build a bridge between basic research knowledge and clinical observations. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Multimodal x-ray and electron microscopy of the Allende meteorite

Multimodal microscopy that combines complementary nanoscale imaging techniques is critical for extracting comprehensive chemical, structural, and functional information, particularly for heterogeneous samples. X-ray microscopy can achieve high-resolution imaging of bulk materials with chemical, magnetic, electronic, and bond orientation contrast, while electron microscopy provides atomic-scale spatial resolution with quantitative elemental composition. Here, we combine x-ray ptychography and scanning transmission x-ray spectromicroscopy with three-dimensional energy-dispersive spectroscopy and electron tomography to perform structural and chemical mapping of an Allende meteorite particle with 15-nm spatial resolution. We use textural and quantitative elemental information to infer the mineral composition and discuss potential processes that occurred before or after accretion. We anticipate that correlative x-ray and electron microscopy overcome the limitations of individual imaging modalities and open up a route to future multiscale nondestructive microscopies of complex functional materials and biological systems.

In vivo quantitative molecular absorption of glycerol in human skin using coherent anti-Stokes Raman scattering (CARS) and two-photon auto-fluorescence

The penetration of small molecules through the human skin is a major issue for both safety and efficacy issues in cosmetics and pharmaceutic domains. To date, the quantification of active molecular compounds in human skin following a topical application uses ex vivo skin samples mounted on Franz cell diffusion set-up together with appropriate analytical methods. Coherent anti-Stokes Raman scattering (CARS) has also been used to perform active molecule quantification on ex vivo skin samples, but no quantification has been described in human skin in vivo. Here we introduce and validate a framework for imaging and quantifying the active molecule penetration into human skin in vivo. Our approach combines nonlinear imaging microscopy modalities, such as two-photon excited auto-fluorescence (TPEF) and coherent anti-Stokes Raman scattering (CARS), together with the use of deuterated active molecules. The imaging framework was exemplified on topically applied glycerol diluted in various vehicles such as water and xanthan gel. In vivo glycerol quantitative percutaneous penetration over time was demonstrated, showing that, contrary to water, the xanthan gel vehicle acts as a film reservoir that releases glycerol continuously over time. More generally, the proposed imaging framework provides an enabling platform for establishing functional activity of topically applied products in vivo.

Depth resolved label-free multimodal optical imaging platform to study morpho-molecular composition of tissue

Multimodal imaging platforms offer a vast array of tissue information in a single image acquisition by combining complementary imaging techniques. By merging different systems, better tissue characterization can be achieved than is possible by the constituent imaging modalities alone. The combination of optical coherence tomography (OCT) with non-linear optical imaging (NLOI) techniques such as two-photon excited fluorescence (TPEF), second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) provides access to detailed information of tissue structure and molecular composition in a fast, label-free and non-invasive manner. We introduce a multimodal label-free approach for morpho-molecular imaging and spectroscopy and validate the system in mouse skin demonstrating the potential of the system for colocalized acquisition of OCT and NLOI signals.

Digital staining through the application of deep neural networks to multi-modal multi-photon microscopy

Deep neural networks have been used to map multi-modal, multi-photon microscopy measurements of a label-free tissue sample to its corresponding histologically stained brightfield microscope colour image. It is shown that the extra structural and functional contrasts provided by using two source modes, namely two-photon excitation microscopy and fluorescence lifetime imaging, result in a more faithful reconstruction of the target haematoxylin and eosin stained mode. This modal mapping procedure can aid histopathologists, since it provides access to unobserved imaging modalities, and translates the high-dimensional numerical data generated by multi-modal, multi-photon microscopy into traditionally accepted visual forms. Furthermore, by combining the strengths of traditional chemical staining and modern multi-photon microscopy techniques, modal mapping enables label-free, non-invasive studies of in vivo tissue samples or intravital microscopic imaging inside living animals. The results show that modal co-registration and the inclusion of spatial variations increase the visual accuracy of the mapped results.

In Vivo Assessment of Engineered Skin Cell Delivery with Multimodal Optical Microscopy

The healing process is often significantly impaired under conditions of chronic or large area wounds, which are often treated clinically using autologous split-thickness skin grafts. However, in many cases, harvesting of donor tissue presents a serious problem such as in the case of very large area burns. In response to this, engineered biomaterials have emerged that attempt to mimic the natural skin environment or deliver a suitable therapy to assist in the healing process. In this study, a custom-built multimodal optical microscope capable of noninvasive structural and functional imaging is used to investigate both the engineered tissue microenvironment and the in vivo wound healing process. Investigation of various engineered scaffolds show the strong relationship among the microenvironment of the scaffold, the organization of the cells within the scaffold, and the delivery pattern of these cells onto the healing wound. Through noninvasive tracking of these processes and parameters, multimodal optical microscopy provides an important tool in the assessment of engineered scaffolds both in vitro and in vivo.

Simultaneous, label-free, multispectral fluorescence lifetime imaging and optical coherence tomography using a double-clad fiber

We present a novel fiber-based imaging platform that allows simultaneous fluorescence lifetime imaging (FLIm) and optical coherence tomography (OCT) using a double-clad fiber. This platform acquires co-registered images showing structural and compositional contrast in unlabeled biological samples by scanning the fiber tip across the sample surface. In this Letter, we report a characterization of each modality and show examples of co-registered FLIm and OCT images acquired from a lemon segment and a section of human coronary artery. The close comparison between the combined FLIm and OCT images and a co-registered histology section provides a qualitative validation of the technique and highlights its potential for minimally invasive, multimodal imaging of tissue structure and composition.

A quantitative framework for the analysis of multimodal optical microscopy images

BACKGROUND

Multimodal optical microscopy, a set of imaging techniques based on unique, yet complementary contrast mechanisms and spatially and temporally co-registered data acquisition, has emerged as a powerful biomedical tool. However, the analysis of the dense, high-dimensional datasets acquired by these instruments remains mostly qualitative and restricted to analysis of each modality individually.

METHODS

Using a custom-built multimodal nonlinear optical microscope, high dimensional datasets were acquired for automated classification of functional cell states as well as identification of histopathological features in tissues slices. Supervised classification of cell death modes was performed through support vector machines (SVM) and semi-supervised classification of tissue slices was performed through the use of the expectation maximization (EM) algorithm.

RESULTS

Applications of these techniques to the automated classification of cell death modes as well as to the identification of tissue components in fixed ex vivo tissue slices are presented. The analysis techniques developed provide a direct link between multimodal image contrast and biological structure and function, resulting in highly accurate classification in both settings.

CONCLUSIONS

Quantification of multimodal optical microscopy images through statistical modeling of the high dimensional data acquired gives a strong correlation between biological structure and function and image contrast. These methods are sensitive to the identification of diagnostic, cellular-level features important in a variety of clinical settings.

Label-free in vivo cellular-level detection and imaging of apoptosis

Cell death plays a critical role in health and homeostasis as well as in the pathogenesis and treatment of a broad spectrum of diseases and can be broadly divided into two main categories: apoptosis, or programmed cell death, and necrosis, or acute cell death. While these processes have been characterized extensively in vitro, label-free detection of apoptosis and necrosis at the cellular level in vivo has yet to be shown. In this study, for the first time, fluorescence lifetime imaging microscopy (FLIM) of intracellular reduced nicotinamide adenine dinucleotide (NADH) was utilized to assess the metabolic response of in vivo mouse epidermal keratinocytes following induction of apoptosis and necrosis. Results show significantly elevated levels of both the mean lifetime of NADH and the intracellular ratio of protein bound-to-free NADH in the apoptotic compared to the necrotic tissue. In addition, the longitudinal profiles of these two cell death processes show remarkable differences. By identifying and extracting these temporal metabolic signatures, apoptosis in single cells can be studied in native tissue environments within the living organism.

Reflectance confocal microscopy of skin in vivo: From bench to bedside

Following more than two decades of effort, reflectance confocal microscopy (RCM) imaging of skin was granted codes for reimbursement by the US Centers for Medicare and Medicaid Services. Dermatologists in the USA have started billing and receiving reimbursement for the imaging procedure and for the reading and interpretation of images. RCM imaging combined with dermoscopic examination is guiding the triage of lesions into those that appear benign, which are being spared from biopsy, against those that appear suspicious, which are then biopsied. Thus far, a few thousand patients have been spared from biopsy of benign lesions. The journey of RCM imaging from bench to bedside is certainly a success story, but still much more work lies ahead toward wider dissemination, acceptance, and adoption. We present a brief review of RCM imaging and highlight key challenges and opportunities. The success of RCM imaging paves the way for other emerging optical technologies, as well-and our bet for the future is on multimodal approaches. Lasers Surg. Med. 49:7-19, 2017. © 2016 Wiley Periodicals, Inc.

Multimodality Molecular Imaging of Cardiac Cell Transplantation: Part I. Reporter Gene Design, Characterization, and Optical in Vivo Imaging of Bone Marrow Stromal Cells after Myocardial Infarction

Purpose To use multimodality reporter-gene imaging to assess the serial survival of marrow stromal cells (MSC) after therapy for myocardial infarction (MI) and to determine if the requisite preclinical imaging end point was met prior to a follow-up large-animal MSC imaging study. Materials and Methods Animal studies were approved by the Institutional Administrative Panel on Laboratory Animal Care. Mice (n = 19) that had experienced MI were injected with bone marrow-derived MSC that expressed a multimodality triple fusion (TF) reporter gene. The TF reporter gene (fluc2-egfp-sr39ttk) consisted of a human promoter, ubiquitin, driving firefly luciferase 2 (fluc2), enhanced green fluorescent protein (egfp), and the sr39tk positron emission tomography reporter gene. Serial bioluminescence imaging of MSC-TF and ex vivo luciferase assays were performed. Correlations were analyzed with the Pearson product-moment correlation, and serial imaging results were analyzed with a mixed-effects regression model. Results Analysis of the MSC-TF after cardiac cell therapy showed significantly lower signal on days 8 and 14 than on day 2 (P = .011 and P = .001, respectively). MSC-TF with MI demonstrated significantly higher signal than MSC-TF without MI at days 4, 8, and 14 (P = .016). Ex vivo luciferase activity assay confirmed the presence of MSC-TF on days 8 and 14 after MI. Conclusion Multimodality reporter-gene imaging was successfully used to assess serial MSC survival after therapy for MI, and it was determined that the requisite preclinical imaging end point, 14 days of MSC survival, was met prior to a follow-up large-animal MSC study. (©) RSNA, 2016 Online supplemental material is available for this article.

Longitudinal in vivo tracking of adverse effects following topical steroid treatment

Topical steroids are known for their anti-inflammatory properties and are commonly prescribed to treat many adverse skin conditions such as eczema and psoriasis. While these treatments are known to be effective, adverse effects including skin atrophy are common. In this study, the progression of these effects is investigated in an in vivo mouse model using multimodal optical microscopy. Utilizing a system capable of performing two-photon excitation fluorescence microscopy (TPEF) of reduced nicotinamide adenine dinucleotide (NADH) to visualize the epidermal cell layers and second harmonic generation (SHG) microscopy to identify collagen in the dermis, these processes can be studied at the cellular level. Fluorescence lifetime imaging microscopy (FLIM) is also utilized to image intracellular NADH levels to obtain molecular information regarding metabolic activity following steroid treatment. In this study, fluticasone propionate (FP)-treated, mometasone furoate (MF)-treated and untreated animals were imaged longitudinally using a custom-built multimodal optical microscope. Prolonged steroid treatment over the course of 21 days is shown to result in a significant increase in mean fluorescence lifetime of NADH, suggesting a faster rate of maturation of epidermal keratinocytes. Alterations to collagen organization and the structural microenvironment are also observed. These results give insight into the structural and biochemical processes of skin atrophy associated with prolonged steroid treatment.

Longitudinal label-free tracking of cell death dynamics in living engineered human skin tissue with a multimodal microscope

We demonstrate real-time, longitudinal, label-free tracking of apoptotic and necrotic cells in living tissue using a multimodal microscope. The integrated imaging platform combines multi-photon microscopy (MPM, based on two-photon excitation fluorescence), optical coherence microscopy (OCM), and fluorescence lifetime imaging microscopy (FLIM). Three-dimensional (3-D) co-registered images are captured that carry comprehensive information of the sample, including structural, molecular, and metabolic properties, based on light scattering, autofluorescence intensity, and autofluorescence lifetime, respectively. Different cell death processes, namely, apoptosis and necrosis, of keratinocytes from different epidermal layers are longitudinally monitored and investigated. Differentiation of the two cell death processes in a complex living tissue environment is enabled by quantitative image analysis and high-confidence classification processing based on the multidimensional, cross-validating imaging data. These results suggest that despite the limitations of each individual label-free modality, this multimodal imaging approach holds the promise for studies of different cell death processes in living tissue and in vivo organs.

Computed optical interferometric tomography for high-speed volumetric cellular imaging

Three-dimensional high-resolution imaging methods are important for cellular-level research. Optical coherence microscopy (OCM) is a low-coherence-based interferometry technology for cellular imaging with both high axial and lateral resolution. Using a high-numerical-aperture objective, OCM normally has a shallow depth of field and requires scanning the focus through the entire region of interest to perform volumetric imaging. With a higher-numerical-aperture objective, the image quality of OCM is affected by and more sensitive to aberrations. Interferometric synthetic aperture microscopy (ISAM) and computational adaptive optics (CAO) are computed imaging techniques that overcome the depth-of-field limitation and the effect of optical aberrations in optical coherence tomography (OCT), respectively. In this work we combine OCM with ISAM and CAO to achieve high-speed volumetric cellular imaging. Experimental imaging results of ex vivo human breast tissue, ex vivo mouse brain tissue, in vitro fibroblast cells in 3D scaffolds, and in vivo human skin demonstrate the significant potential of this technique for high-speed volumetric cellular imaging.

Accumulating advantages, reducing limitations: multimodal nonlinear imaging in biomedical sciences - the synergy of multiple contrast mechanisms

Multimodal nonlinear microscopy has matured during the past decades to one of the key imaging modalities in life science and biomedicine due to its unique capabilities of label-free visualization of tissue structure and chemical composition, high depth penetration, intrinsic 3D sectioning, diffraction limited resolution and low phototoxicity. This review briefly summarizes first recent advances in the field regarding the methodology, e.g., contrast mechanisms and signal characteristics used for contrast generation as well as novel image processing approaches. The second part deals with technologic developments emphasizing improvements in penetration depth, imaging speed, spatial resolution and nonlinear labeling strategies. The third part focuses on recent applications in life science fundamental research and biomedical diagnostics as well as future clinical applications.

Long-term time-lapse multimodal intravital imaging of regeneration and bone-marrow-derived cell dynamics in skin

A major challenge for translating cell-based therapies is understanding the dynamics of cells and cell populations in complex in vivo environments. Intravital microscopy has shown great promise for directly visualizing cell behavior in vivo. However, current methods are limited to relatively short imaging times (hours), by ways to track cell and cell population dynamics over extended time-lapse periods (days to weeks to months), and by relatively few imaging contrast mechanisms that persist over extended investigations. We present technology to visualize and quantify complex, multifaceted dynamic changes in natural deformable skin over long time periods using novel multimodal imaging and a non-rigid image registration method. These are demonstrated in green fluorescent protein (GFP) bone marrow (BM) transplanted mice to study dynamic skin regeneration. This technology provides a novel perspective for studying dynamic biological processes and will enable future studies of stem, immune, and tumor cell biology in vivo.

Structural and dynamical aspects of skin studied by multiphoton excitation fluorescence microscopy-based methods

This mini-review reports on applications of particular multiphoton excitation microscopy-based methodologies employed in our laboratory to study skin. These approaches allow in-depth optical sectioning of the tissue, providing spatially resolved information on specific fluorescence probes' parameters. Specifically, by applying these methods, spatially resolved maps of water dipolar relaxation (generalized polarization function using the 6-lauroyl-2-(N,N-dimethylamino)naphthale probe), activity of protons (fluorescence lifetime imaging using a proton sensitive fluorescence probe--2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) and diffusion coefficients of distinct fluorescence probes (raster imaging correlation spectroscopy) can be obtained from different regions of the tissue. Comparative studies of different tissue strata, but also between equivalent regions of normal and abnormal excised skin, including applications of fluctuation correlation spectroscopy on transdermal penetration of liposomes are presented and discussed. The data from the different studies reported reveal the intrinsic heterogeneity of skin and also prove these strategies to be powerful noninvasive tools to explore structural and dynamical aspects of the tissue.

Three-dimensional multiphoton/optical coherence tomography for diagnostic applications in dermatology

A preliminary clinical trial using state-of-the-art multiphoton tomography (MPT) and optical coherence tomography (OCT) for three-dimensional (3D) multimodal in vivo imaging of normal skin, nevi, scars and pathologic skin lesions has been conducted. MPT enabled visualization of sub-cellular details with axial and transverse resolutions of <2 μm and <0.5 μm, respectively, from a volume of 0.35 × 0.35 × 0.2 mm(3) at a frame rate of 0.14 Hz (512 × 512 pixels). State-of-the-art OCT, operating at a center wavelength of 1300 nm, was capable of acquiring 3D images depicting the layered architecture of skin with axial and transverse resolutions ~8 μm and ~20 μm, respectively, from a volume of 7 × 3.5 × 1.5 mm(3) at a frame rate of 46 Hz (1024 × 1024 pixels). This study demonstrates the clinical diagnostic potential of MPT/OCT for pre-screening relatively large areas of skin using 3D OCT to identify suspicious regions at microscopic level and subsequently using high resolution MPT to obtain zoomed in, sub-cellular level information of the respective regions.

Imaging and tracking of bone marrow-derived immune and stem cells

Bone marrow (BM)-derived stem and immune cells play critical roles in maintaining the health, regeneration, and repair of many tissues. Given their important functions in tissue regeneration and therapy, tracking the dynamic behaviors of BM-derived cells has been a long-standing research goal of both biologists and engineers. Because of the complex cellular-level processes involved, real-time imaging technologies that have sufficient spatial and temporal resolution to visualize them are needed. In addition, in order to track cellular dynamics, special attention is needed to account for changes in the microenvironment where the cells reside, for example, tissue contraction, stretching, development, etc. In this chapter, we introduce methods for real-time imaging and longitudinal tracking of BM-derived immune and stem cells in in vivo three-dimensional (3-D) tissue environments with an integrated optical microscope. The integrated microscope combines multiple imaging functions derived from optical coherence tomography (OCT) and multiphoton microscopy (MPM), including optical coherence microscopy (OCM), microvasculature imaging, two-photon excited fluorescence (TPEF), and second harmonic generation (SHG) microscopy. Short- and long-term tracking of the dynamic behavior of BM-derived cells involved in cutaneous wound healing and skin grafting in green fluorescent protein (GFP) BM-transplanted mice is demonstrated. Methods and algorithms for nonrigid registration of time-lapse images are introduced, which allows for long-term tracking of cell dynamics over several months.

Tri-modal microscopy with multiphoton and optical coherence microscopy/tomography for multi-scale and multi-contrast imaging

Multi-scale multimodal microscopy is a very useful technique by providing multiple imaging contrasts with adjustable field of views and spatial resolutions. Here, we present a tri-modal microscope combining multiphoton microscopy (MPM), optical coherence microscopy (OCM) and optical coherence tomography (OCT) for subsurface visualization of biological tissues. The advantages of the tri-modal system are demonstrated on various biological samples. It enables the visualization of multiple intrinsic contrasts including scattering, two-photon excitation fluorescence (TPEF), and second harmonic generation (SHG). It also enables a rapid scanning over a large tissue area and a high resolution zoom-in for cellular-level structures on regions of interest. The tri-modal microscope can be important for label-free imaging to obtain a sufficient set of parameters for reliable sample analysis.

An all-fiber-optic endoscopy platform for simultaneous OCT and fluorescence imaging

We present an all-fiber-optically based endoscope platform for simultaneous optical coherence tomography (OCT) and fluorescence imaging. This design entails the use of double-clad fiber (DCF) in the endoscope for delivery of OCT source and fluorescence excitation light while collecting the backscattered OCT signal through the single-mode core and fluorescence emission through the large inner cladding of the DCF. Circumferential beam scanning was performed by rotating a 45° reflector using a miniature DC motor at the distal end of the endoscope. Additionally, a custom DCF coupler and a wavelength division multiplexer (WDM) were utilized to seamlessly integrate both imaging modalities to achieve an entirely fiber-optically based dual-modality imaging system. We demonstrated simultaneous intraluminal 3D OCT and 2D (surface) fluorescence imaging in ex vivo rabbit esophagus using the dual-modal endomicroscopy system. Structural morphologies (provided by OCT) and fluorophore distribution (provided by the fluorescence module) could be clearly visualized, suggesting the potential of the dual-modality system for future in vivo and clinical applications.