In vivo simultaneous morphological and biochemical optical imaging of oral epithelial cancer

Clinical label-free endoscopic imaging of biochemical and metabolic autofluorescence biomarkers of benign, precancerous, and cancerous oral lesions

Early detection is critical for improving the survival rate and quality of life of oral cancer patients; unfortunately, dysplastic and early-stage cancerous oral lesions are often difficult to distinguish from oral benign lesions during standard clinical oral examination. Therefore, there is a critical need for novel clinical technologies that would enable reliable oral cancer screening. The autofluorescence properties of the oral epithelial tissue provide quantitative information about morphological, biochemical, and metabolic tissue and cellular alterations accompanying carcinogenesis. This study aimed to identify novel biochemical and metabolic autofluorescence biomarkers of oral dysplasia and cancer that could be clinically imaged using novel multispectral autofluorescence lifetime imaging (maFLIM) endoscopy technologies. In vivo maFLIM clinical endoscopic images of benign, precancerous, and cancerous lesions from 67 patients were acquired using a novel maFLIM endoscope. Widefield maFLIM feature maps were generated, and statistical analyses were applied to identify maFLIM features providing contrast between dysplastic/cancerous vs. benign oral lesions. A total of 14 spectral and time-resolved maFLIM features were found to provide contrast between dysplastic/cancerous vs. benign oral lesions, representing novel biochemical and metabolic autofluorescence biomarkers of oral epithelial dysplasia and cancer. To the best of our knowledge, this is the first demonstration of clinical widefield maFLIM endoscopic imaging of novel biochemical and metabolic autofluorescence biomarkers of oral dysplasia and cancer, supporting the potential of maFLIM endoscopy for early detection of oral cancer.

Mesoscopic fluorescence lifetime imaging: Fundamental principles, clinical applications and future directions

Fluorescence lifetime imaging (FLIm) is an optical spectroscopic imaging technique capable of real-time assessments of tissue properties in clinical settings. Label-free FLIm is sensitive to changes in tissue structure and biochemistry resulting from pathological conditions, thus providing optical contrast to identify and monitor the progression of disease. Technical and methodological advances over the last two decades have enabled the development of FLIm instrumentation for real-time, in situ, mesoscopic imaging compatible with standard clinical workflows. Herein, we review the fundamental working principles of mesoscopic FLIm, discuss the technical characteristics of current clinical FLIm instrumentation, highlight the most commonly used analytical methods to interpret fluorescence lifetime data and discuss the recent applications of FLIm in surgical oncology and cardiovascular diagnostics. Finally, we conclude with an outlook on the future directions of clinical FLIm.

Piezo-type mechanosensitive ion channel component 1 (Piezo1) in human cancer

Piezo-type mechanosensitive ion channel component 1 (Piezo1) is a mechanosensitive ion channel protein that is evolutionarily conserved and multifunctional. It plays an important role as an oncogenic mediator in several malignant tumors. It mediates the proliferation, migration, and invasion of a variety of cancer cells through various mechanisms. Multiple studies have shown that the expression of Piezo1 is related to the clinical characteristics of senescence and cancer patients, making Piezo1 useful as a new biomarker for the diagnosis and prognosis of a variety of human cancers. Manipulating the expression or function of Piezo1 is a potential therapeutic strategy for different diseases. Piezo1 may be a promising tumor biomarker and therapeutic target. Here we review the biological function, mechanism of action, and potential clinical significance of Piezo1 in oncogenesis and progression.

Roles of matrix metalloproteinase-7 (MMP-7) in cancer

Matrix metalloproteinase-7 (MMP-7) is a small proteolytic enzyme that secretes zinc and calcium endopeptidases. It can degrade a variety of extracellular matrix substrates and other substrates and plays important regulatory roles in many human pathophysiological processes. Since its discovery, MMP-7 has been recognized as a regulatory protein in wound healing, bone growth, and remodeling. Later, MMP-7 was reported to regulate the occurrence and development of cancers and mediate the proliferation, differentiation, metastasis, and invasion of several types of cancer cells via various mechanisms. Thus, matrix metalloproteinase-7 may be a promising tumor biomarker and therapeutic target. The expression of MMP-7 correlates with the clinical characteristics of cancer patients, and its expression profile is a new diagnostic and prognostic biomarker for a variety of human diseases. Hence, manipulating the expression or function of MMP-7 may be a potential treatment strategy for different diseases including cancers. This review summarizes the role played by MMP-7 in carcinogenesis of several human cancers, underlying mechanisms, and its clinical significance of the occurrence and development of cancers.

Novel endoscopic optical diagnostic technologies in medical trial research: recent advancements and future prospects

Novel endoscopic biophotonic diagnostic technologies have the potential to non-invasively detect the interior of a hollow organ or cavity of the human body with subcellular resolution or to obtain biochemical information about tissue in real time. With the capability to visualize or analyze the diagnostic target in vivo, these techniques gradually developed as potential candidates to challenge histopathology which remains the gold standard for diagnosis. Consequently, many innovative endoscopic diagnostic techniques have succeeded in detection, characterization, and confirmation: the three critical steps for routine endoscopic diagnosis. In this review, we mainly summarize researches on emerging endoscopic optical diagnostic techniques, with emphasis on recent advances. We also introduce the fundamental principles and the development of those techniques and compare their characteristics. Especially, we shed light on the merit of novel endoscopic imaging technologies in medical research. For example, hyperspectral imaging and Raman spectroscopy provide direct molecular information, while optical coherence tomography and multi-photo endomicroscopy offer a more extensive detection range and excellent spatial-temporal resolution. Furthermore, we summarize the unexplored application fields of these endoscopic optical techniques in major hospital departments for biomedical researchers. Finally, we provide a brief overview of the future perspectives, as well as bottlenecks of those endoscopic optical diagnostic technologies. We believe all these efforts will enrich the diagnostic toolbox for endoscopists, enhance diagnostic efficiency, and reduce the rate of missed diagnosis and misdiagnosis.

Ski: Double roles in cancers

The Ski (Sloan-Kettering Institute) is an evolutionarily conserved protein that plays a dual role as an oncoprotein and tumor suppressor gene in the development of human cancer. The Ski oncogene was first identified as a transforming protein of the avian Sloan-Kettering retrovirus in 1986. Since its discovery, Ski has been identified as a carcinogenic regulator in a variety of malignant tumors. Later, it was reported that Ski regulates the occurrence and development of some cancers by acting as an oncogene. Ski mediates the proliferation, differentiation, metastasis, and invasion of numerous cancer cells through various mechanisms. Several studies have shown that Ski expression is correlated with the clinical characteristics of cancer patients and is a promising biomarker and therapeutic target for cancer. In this review, we summarize the mechanisms and potential clinical implications of Ski in dimorphism, cancer occurrence, and progression in various types of cancer.

Clinical label-free biochemical and metabolic fluorescence lifetime endoscopic imaging of precancerous and cancerous oral lesions

INTRODUCTION

Incomplete head and neck cancer resection occurs in up to 85% of cases, leading to increased odds of local recurrence and regional metastases; thus, image-guided surgical tools for accurate, in situ and fast detection of positive margins during head and neck cancer resection surgery are urgently needed. Oral epithelial dysplasia and cancer development is accompanied by morphological, biochemical, and metabolic tissue and cellular alterations that can modulate the autofluorescence properties of the oral epithelial tissue.

OBJECTIVE

This study aimed to test the hypothesis that autofluorescence biomarkers of oral precancer and cancer can be clinically imaged and quantified by means of multispectral fluorescence lifetime imaging (FLIM) endoscopy.

METHODS

Multispectral autofluorescence lifetime images of precancerous and cancerous lesions from 39 patients were imaged in vivo using a novel multispectral FLIM endoscope and processed to generate widefield maps of biochemical and metabolic autofluorescence biomarkers of oral precancer and cancer.

RESULTS

Statistical analyses applied to the quantified multispectral FLIM endoscopy based autofluorescence biomarkers indicated their potential to provide contrast between precancerous/cancerous vs. healthy oral epithelial tissue.

CONCLUSION

To the best of our knowledge, this study represents the first demonstration of label-free biochemical and metabolic clinical imaging of precancerous and cancerous oral lesions by means of widefield multispectral autofluorescence lifetime endoscopy. Future studies will focus on demonstrating the capabilities of endogenous multispectral FLIM endoscopy as an image-guided surgical tool for positive margin detection during head and neck cancer resection surgery.

In vivo multiphoton microscopy detects longitudinal metabolic changes associated with delayed skin wound healing

Chronic wounds are difficult to diagnose and characterize due to a lack of quantitative biomarkers. Label-free multiphoton microscopy has emerged as a useful imaging modality capable of quantifying changes in cellular metabolism using an optical redox ratio of FAD/(NADH+FAD) autofluorescence. However, the utility of an optical redox ratio for long-term in vivo monitoring of tissue metabolism has not been robustly evaluated. In this study, we demonstrate how multiphoton microscopy can be used to monitor changes in the metabolism of individual full-thickness skin wounds in vivo. 3D optical redox ratio maps and NADH fluorescence lifetime images identify differences between diabetic and control mice during the re-epithelialization of wounds. These metabolic changes are associated with a transient increase in keratinocyte proliferation at the wound edge. Our study demonstrates that high-resolution, non-invasive autofluorescence imaging can be performed in vivo and that optical redox ratios can serve as quantitative optical biomarkers of impaired wound healing.

Dispersion mapping as a simple postprocessing step for Fourier domain Optical Coherence Tomography data

Optical Coherence Tomography (OCT) was originally conceived as a volumetric imaging method. Quickly, OCT images went beyond structural data and started to provide functional information about an object enabling for example visualization of blood flow or tissue elasticity. Minimal or no need for system alterations make functional OCT techniques useful in performing multimodal imaging, where differently contrasted images are produced in a single examination. We propose a method that further extends the current capabilities of OCT and requires no modifications to the system. Our algorithm provides information about the sample's Group Velocity Dispersion (GVD) and can be easily applied to any OCT dataset acquired with a Fourier domain system. GVD is calculated from the difference in material's optical thickness measured from two images obtained for different spectral ranges. Instead of using two separate light sources, we propose to apply a filter-based, numerical procedure that synthesizes two spectra from one broadband spectrum. We discuss the limitations of the method and present GVD values for BK7 and sapphire and ocular media: cornea and aqueous humour of a rat eye. Results corroborate previous measurements using two different light sources.

Quantitative Live Cell FLIM Imaging in Three Dimensions

In this chapter, the concept of fluorescence lifetime and its utility in quantitative live cell imaging will be introduced, along with methods to record and analyze FLIM data. Relevant applications in 3D tissue and live cell imaging, including multiplexed FLIM detection, will also be detailed.

Early detection and personalized treatment in oral cancer: the impact of omics approaches

BACKGROUND

Oral cancer is one of the most common malignant lesions of the head and neck. This cancer is an aggressive and lethal disease with no significant improvements in the overall survival in the last decades. Moreover, the incidence of oral HPV-positive tumors is rising, especially in young people. This oral neoplasm develops through numerous molecular imbalances that affect key genes and signaling pathways; however, the molecular mechanisms involved in the pathogenesis and progression of oral tumors are still to be fully determined. In order to improve the quality of life and long-term survival rate of these patients, it is vital to establish accurate biomarkers that help in the early diagnosis, prognosis and development of target treatments. Such biomarkers may possibly allow for selection of patients that will benefit from each therapy modality, helping in the optimization of intensity and sequence of the treatments in order to decrease side effects and improve survival.

CONCLUSION

In this review we discuss the current knowledge of oral cancer and the potential role of omics approaches to identify molecular biomarkers in the improvement of early diagnosis, treatment and prognosis. The pursuit to improve the quality of life and decrease mortality rates of the oral patients needs to be centralized on the identification of critical genes in oral carcinogenesis. Understanding the molecular biology of oral cancer is vital for search new therapies, being the molecular-targeted therapies the most promising treatment for these patients.

Surgical Guidance via Multiplexed Molecular Imaging of Fresh Tissues Labeled with SERS-Coded Nanoparticles

The imaging of dysregulated cell-surface receptors (or biomarkers) is a potential means of identifying the presence of cancer with high sensitivity and specificity. However, due to heterogeneities in the expression of protein biomarkers in tumors, molecular imaging technologies should ideally be capable of visualizing a multiplexed panel of cancer biomarkers. Recently, surface-enhanced Raman-scattering (SERS) nanoparticles (NPs) have attracted wide interest due to their potential for sensitive and multiplexed biomarker detection. In this review, we focus on the most recent advances in tumor imaging using SERS-coded NPs. A brief introduction of the structure and optical properties of SERS NPs is provided, followed by a detailed discussion of key imaging issues such as the administration of NPs in tissue (topical versus systemic), the optical configuration and imaging approach of Raman imaging systems, spectral demultiplexing methods for quantifying NP concentrations, and the disambiguation of specific vs. nonspecific sources of contrast through ratiometric imaging of targeted and untargeted (control) NP pairs. Finally, future challenges and directions are briefly outlined.

Miniature, minimally invasive, tunable endoscope for investigation of the middle ear

We demonstrate a miniature, tunable, minimally invasive endoscope for diagnosis of the auditory system. The probe is designed to sharply image anatomical details of the middle ear without the need for physically adjusting the position of the distal end of the endoscope. This is achieved through the addition of an electrowetted, tunable, electronically-controlled lens to the optical train. Morphological imaging is enabled by scanning light emanating from an optical coherence tomography system. System performance was demonstrated by imaging part of the ossicular chain and wall of the middle ear cavity of a normal mouse. During the experiment, we electronically moved the plane of best focus from the incudo-stapedial joint to the stapedial artery. Repositioning the object plane allowed us to image anatomical details of the middle ear beyond the depth of field of a static optical system. We also demonstrated for the first time to our best knowledge, that an optical system with an electrowetted, tunable lens may be successfully employed to measure sound-induced vibrations within the auditory system by measuring the vibratory amplitude of the tympanic membrane in a normal mouse in response to pure tone stimuli.

Extended output phasor representation of multi-spectral fluorescence lifetime imaging microscopy

In this paper, we investigate novel low-dimensional and model-free representations for multi-spectral fluorescence lifetime imaging microscopy (m-FLIM) data. We depart from the classical definition of the phasor in the complex plane to propose the extended output phasor (EOP) and extended phasor (EP) for multi-spectral information. The frequency domain properties of the EOP and EP are analytically studied based on a multiexponential model for the impulse response of the imaged tissue. For practical implementations, the EOP is more appealing since there is no need to perform deconvolution of the instrument response from the measured m-FLIM data, as in the case of EP. Our synthetic and experimental evaluations with m-FLIM datasets of human coronary atherosclerotic plaques show that low frequency indexes have to be employed for a distinctive representation of the EOP and EP, and to reduce noise distortion. The tissue classification of the m-FLIM datasets by EOP and EP also improves with low frequency indexes, and does not present significant differences by using either phasor.

Functional optical coherence tomography: principles and progress

In the past decade, several functional extensions of optical coherence tomography (OCT) have emerged, and this review highlights key advances in instrumentation, theoretical analysis, signal processing and clinical application of these extensions. We review five principal extensions: Doppler OCT (DOCT), polarization-sensitive OCT (PS-OCT), optical coherence elastography (OCE), spectroscopic OCT (SOCT), and molecular imaging OCT. The former three have been further developed with studies in both ex vivo and in vivo human tissues. This review emphasizes the newer techniques of SOCT and molecular imaging OCT, which show excellent potential for clinical application but have yet to be well reviewed in the literature. SOCT elucidates tissue characteristics, such as oxygenation and carcinogenesis, by detecting wavelength-dependent absorption and scattering of light in tissues. While SOCT measures endogenous biochemical distributions, molecular imaging OCT detects exogenous molecular contrast agents. These newer advances in functional OCT broaden the potential clinical application of OCT by providing novel ways to understand tissue activity that cannot be accomplished by other current imaging methodologies.

In vivo wide-field reflectance/fluorescence imaging and polarization-sensitive optical coherence tomography of human oral cavity with a forward-viewing probe

We report multimodal imaging of human oral cavity in vivo based on simultaneous wide-field reflectance/fluorescence imaging and polarization-sensitive optical coherence tomography (PS-OCT) with a forward-viewing imaging probe. Wide-field reflectance/fluorescence imaging and PS-OCT were to provide both morphological and fluorescence information on the surface, and structural and birefringent information below the surface respectively. The forward-viewing probe was designed to access the oral cavity through the mouth with dimensions of approximately 10 mm in diameter and 180 mm in length. The probe had field of view (FOV) of approximately 5.5 mm in diameter, and adjustable depth of field (DOF) from 2 mm to 10 mm by controlling numerical aperture (NA) in the detection path. This adjustable DOF was to accommodate both requirements for image-based guiding with high DOF and high-resolution, high-sensitivity imaging with low DOF. This multimodal imaging system was characterized by using a tissue phantom and a mouse model in vivo, and was applied to human oral cavity. Information of surface morphology and vasculature, and under-surface layered structure and birefringence of the oral cavity tissues was obtained. These results showed feasibility of this multimodal imaging system as a tool for studying oral cavity lesions in clinical applications.

Molecular Imaging in Optical Coherence Tomography

Optical coherence tomography (OCT) is a medical imaging technique that provides tomographic images at micron scales in three dimensions and high speeds. The addition of molecular contrast to the available morphological image holds great promise for extending OCT's impact in clinical practice and beyond. Fundamental limitations prevent OCT from directly taking advantage of powerful molecular processes such as fluorescence emission and incoherent Raman scattering. A wide range of approaches is being researched to provide molecular contrast to OCT. Here we review those approaches with particular attention to those that derive their molecular contrast directly from modulation of the OCT signal. We also provide a brief overview of the multimodal approaches to gaining molecular contrast coincident with OCT.

Rapid ratiometric biomarker detection with topically applied SERS nanoparticles

Multiplexed surface-enhanced Raman scattering (SERS) nanoparticles (NPs) offer the potential for rapid molecular phenotyping of tissues, thereby enabling accurate disease detection as well as patient stratification to guide personalized therapies or to monitor treatment outcomes. The clinical success of molecular diagnostics based on SERS NPs would be facilitated by the ability to accurately identify tissue biomarkers under time-constrained staining and detection conditions with a portable device. In vitro, ex vivo and in vivo experiments were performed to optimize the technology and protocols for the rapid detection (0.1-s integration time) of multiple cell-surface biomarkers with a miniature fiber-optic spectral-detection probe following a brief (5 min) topical application of SERS NPs on tissues. Furthermore, we demonstrate that the simultaneous detection and ratiometric quantification of targeted and nontargeted NPs allows for an unambiguous assessment of molecular expression that is insensitive to nonspecific variations in NP concentrations.

Estimation of the number of fluorescent end-members for quantitative analysis of multispectral FLIM data

Multispectral fluorescence lifetime imaging (m-FLIM) can potentially allow identifying the endogenous fluorophores present in biological tissue. Quantitative description of such data requires estimating the number of components in the sample, their characteristic fluorescent decays, and their relative contributions or abundances. Unfortunately, this inverse problem usually requires prior knowledge about the data, which is seldom available in biomedical applications. This work presents a new methodology to estimate the number of potential endogenous fluorophores present in biological tissue samples from time-domain m-FLIM data. Furthermore, a completely blind linear unmixing algorithm is proposed. The method was validated using both synthetic and experimental m-FLIM data. The experimental m-FLIM data include in-vivo measurements from healthy and cancerous hamster cheek-pouch epithelial tissue, and ex-vivo measurements from human coronary atherosclerotic plaques. The analysis of m-FLIM data from in-vivo hamster oral mucosa identified healthy from precancerous lesions, based on the relative concentration of their characteristic fluorophores. The algorithm also provided a better description of atherosclerotic plaques in term of their endogenous fluorophores. These results demonstrate the potential of this methodology to provide quantitative description of tissue biochemical composition.

Fluorescence lifetime imaging and reflectance confocal microscopy for multiscale imaging of oral precancer

Optical imaging techniques using a variety of contrast mechanisms are under evaluation for early detection of epithelial precancer; however, tradeoffs in field of view (FOV) and resolution may limit their application. Therefore, we present a multiscale multimodal optical imaging system combining macroscopic biochemical imaging of fluorescence lifetime imaging (FLIM) with subcellular morphologic imaging of reflectance confocal microscopy (RCM). The FLIM module images a 16×16 mm² tissue area with 62.5 μm lateral and 320 ps temporal resolution to guide cellular imaging of suspicious regions. Subsequently, coregistered RCM images are acquired at 7 Hz with 400 μm diameter FOV, <1  μm lateral and 3.5 μm axial resolution. FLIM-RCM imaging was performed on a tissue phantom, normal porcine buccal mucosa, and a hamster cheek pouch model of oral carcinogenesis. While FLIM is sensitive to biochemical and macroscopic architectural changes in tissue, RCM provides images of cell nuclear morphology, all key indicators of precancer progression.

High-resolution optical molecular imaging of changes in choline metabolism in oral neoplasia

This study was aimed at developing an optical molecular imaging approach to measure differences in uptake and intracellular retention of choline in clinically isolated tissue biopsies from head and neck cancer patients. An optically detectable analogue of choline (propargyl choline) was synthesized and evaluated in 2D and 3D models and clinically isolated paired biopsies (n = 22 biopsies). Fluorescence contrast between clinically abnormal and normal tissues based on uptake and intracellular retention of propargyl choline was measured and correlated with pathologic diagnosis. Results in 2D and 3D models demonstrated a rapid uptake of propargyl choline in cancer cells, uniform permeation in tissue models, and specific detection of intracellular entrapped propargyl choline using the click chemistry reaction with an azide-modified Alexa 488 dye. Fluorescence imaging measurements following topical delivery of propargyl choline in clinically isolated biopsies showed that the mean fluorescence intensity (MFI) of neoplastic tissues was four-fold to five-fold higher than the MFI of clinically and pathologically normal samples. This difference in fluorescence contrast was measured on the basis of comparison of paired biopsy sets isolated from individual patients as well as comparison of clinically abnormal and normal biopsies independent of anatomic locations in the head and neck cavity and across diverse patients. In conclusion, a novel imaging approach based on monoalkyne-modified choline was developed and validated using cell and tissue models. Results in clinically isolated tissue biopsies demonstrate a significant fluorescent contrast between neoplastic and normal tissues and illustrate high specificity of the optical imaging approach.

A prospective study to assess in vivo optical coherence tomography imaging for early detection of chemotherapy-induced oral mucositis

BACKGROUND AND OBJECTIVE

Oral mucositis (OM) is a common and severe complication of many cancer therapies. Currently, prediction and early detection are not possible and objective monitoring remains problematic. Goal of this prospective study is to assess non-invasive imaging using optical coherence tomography (OCT) for early detection and evaluation of chemotherapy-induced OM in 48 patients, 12 of whom developed clinical mucositis.

STUDY DESIGN/MATERIALS AND METHODS

In 48 patients receiving neoadjuvant chemotherapy for primary breast cancer, oral mucosal health was assessed clinically, and imaged using non-invasive OCT. Images were evaluated for mucositis using an imaging-based scoring system ranging from 0 to 6. Conventional clinical assessment using the OM assessment scale (OMAS) was used as the gold standard. Patients were evaluated on Days 0-11 after commencement of chemotherapy. OCT images were visually scored by three blinded investigators.

RESULTS

The following events were identified from OCT images (1) change in epithelial thickness and subepithelial tissue integrity (beginning on Day 2), (2) loss of surface keratinized layer continuity (beginning on Day 4), (3) loss of epithelial integrity (beginning on Day 4). Imaging data gave higher scores compared to clinical scores early in treatment, suggesting that the imaging-based diagnostic scoring was more sensitive to early mucositic change than the clinical scoring system. Once mucositis was established, imaging and clinical scores converged.

CONCLUSION

Using OCT imaging and a novel scoring system, earlier, more sensitive detection of mucositis was possible than using OMAS. Specific imaging-based changes were a consistent predictor of clinical mucositis.

Multimodal in vivo imaging of oral cancer using fluorescence lifetime, photoacoustic and ultrasound techniques

This work reports a multimodal system for label-free tissue diagnosis combining fluorescence lifetime imaging (FLIm), ultrasound backscatter microscopy (UBM), and photoacoustic imaging (PAI). This system provides complementary biochemical, structural and functional features allowing for enhanced in vivo detection of oral carcinoma. Results from a hamster oral carcinoma model (normal, precancer and carcinoma) are presented demonstrating the ability of FLIm to delineate biochemical composition at the tissue surface, UBM and related radiofrequency parameters to identify disruptions in the tissue microarchitecture and PAI to map optical absorption associated with specific tissue morphology and physiology.

Ex vivo optical metabolic measurements from cultured tissue reflect in vivo tissue status

Optical measurements of metabolism are ideally acquired in vivo; however, intravital measurements are often impractical. Accurate ex vivo assessments would greatly broaden the applicability of optical measurements of metabolism. We investigate the use of live tissue culture experiments to serve as a surrogate for in vivo metabolic measurements. To validate this approach, NADH and FAD fluorescence intensity and lifetime images were acquired with a two-photon microscope from hamster cheek pouch epithelia in vivo, from biopsies maintained in live tissue culture up to 48 h, and from flash-frozen and thawed biopsies. We found that the optical redox ratio (fluorescence intensity of NADH/FAD) of the cultured biopsy was statistically identical to the in vivo measurement until 24 h, while the redox ratio of the frozen-thawed samples decreased by 15% (p<0.01). The NADH mean fluorescence lifetime (τm) remained unchanged (p>0.05) during the first 8 h of tissue culture, while the NADH τm of frozen-thawed samples increased by 13% (p<0.001). Cellular morphology did not significantly change between in vivo, cultured, and frozen-thawed tissues (p>0.05). All results were consistent across multiple depth layers in this stratified squamous epithelial tissue. Histological markers for proliferation and apoptosis also confirm the viability of tissues maintained in culture. This study suggests that short-term ex vivo tissue culture may be more appropriate than frozen-thawed tissue for optical metabolic and morphologic measurements that approximate in vivo status.

Application of non-negative matrix factorization to multispectral FLIM data analysis

Existing methods of interpreting fluorescence lifetime imaging microscopy (FLIM) images are based on comparing the intensity and lifetime values at each pixel with those of known fluorophores. This method becomes unwieldy and subjective in many practical applications where there are several fluorescing species contributing to the bulk fluorescence signal, and even more so in the case of multispectral FLIM. Non-negative matrix factorization (NMF) is a multivariate data analysis technique aimed at extracting non-negative signatures of pure components and their non-negative abundances from an additive mixture of those components. In this paper, we present the application of NMF to multispectral time-domain FLIM data to obtain a new set of FLIM features (relative abundance of constituent fluorophores). These features are more intuitive and easier to interpret than the standard fluorescence intensity and lifetime values. The proposed approach, unlike several FLIM data analysis methods, is not limited by the number of constituent fluorescing species or their possibly complex decay dynamics. Moreover, the new set of FLIM features can be obtained by processing raw multispectral FLIM intensity data, thereby rendering time deconvolution unnecessary and resulting in lesser computational time and relaxed SNR requirements. The performance of the NMF method was validated on simulated and experimental multispectral time-domain FLIM data. The NMF features were also compared against the standard intensity and lifetime features, in terms of their ability to discriminate between different types of atherosclerotic plaques.