Multispectral fluorescence lifetime imaging by TCSPC

Fluorescence in depth: integration of spectroscopy and imaging with Raman, IR, and CD for advanced research

Fluorescence spectroscopy serves as a vital technique for studying the interaction between light and fluorescent molecules. It encompasses a range of methods, each presenting unique advantages and applications. This technique finds utility in various chemical studies. This review discusses Fluorescence spectroscopy, its branches such as Time-Resolved Fluorescence Spectroscopy (TRFS) and Fluorescence Lifetime Imaging Microscopy (FLIM), and their integration with other spectroscopic methods, including Raman, Infrared (IR), and Circular Dichroism (CD) spectroscopies. By delving into these methods, we aim to provide a comprehensive understanding of the capabilities and significance of fluorescence spectroscopy in scientific research, highlighting its diverse applications and the enhanced understanding it brings when combined with other spectroscopic methods. This review looks at each technique's unique features and applications. It discusses the prospects of their combined use in advancing scientific understanding and applications across various domains.

The use of fluorescence lifetime imaging (FLIM) for in situ microbial detection in complex mineral substrates

The utility of fluorescence lifetime imaging microscopy (FLIM) for identifying bacteria in complex mineral matrices was investigated. Baseline signals from unlabelled Bacillus subtilis and Euglena gracilis, and Bacillus subtilis labelled with SYTO 9 were obtained using two-photon excitation at 730, 750 and 800 nm, identifying characteristic lifetimes of photosynthetic pigments, unpigmented cellular autofluorescence, and SYTO 9. Labelled and unlabelled B. subtilis were seeded onto marble and gypsum samples containing endolithic photosynthetic cyanobacteria and the ability to distinguish cells from mineral autofluorescence and nonspecific dye staining was examined in parallel with ordinary multichannel confocal imaging. It was found that FLIM enabled discrimination of SYTO 9 labelled cells from background, but that the lifetime of SYTO 9 was shorter in cells on minerals than in pure culture under our conditions. Photosynthetic microorganisms were easily observed using both FLIM and confocal. Unlabelled, nonpigmented bacteria showed weak signals that were difficult to distinguish from background when minerals were present, though cellular autofluorescence consistent with NAD(P)H could be seen in pure cultures, and phasor analysis permitted detection on rocks. Gypsum and marble samples showed similar autofluorescence profiles, with little autofluorescence in the yellow-to-red range. Lifetime or time-gated imaging may prove a useful tool for environmental microbiology. LAY DESCRIPTION: The standard method of bacterial enumeration is to label the cells with a fluorescent dye and count them under high-power fluorescence microscopy. However, this can be difficult when the cells are embedded in soil and rock due to fluorescence from the surrounding minerals and dye binding to ambiguous features of the substrate. The use of fluorescence lifetime imaging (FLIM) can disambiguate these signals and allow for improved detection of bacteria in environmental samples.

Applications of machine learning in time-domain fluorescence lifetime imaging: a review

Many medical imaging modalities have benefited from recent advances in Machine Learning (ML), specifically in deep learning, such as neural networks. Computers can be trained to investigate and enhance medical imaging methods without using valuable human resources. In recent years, Fluorescence Lifetime Imaging (FLIm) has received increasing attention from the ML community. FLIm goes beyond conventional spectral imaging, providing additional lifetime information, and could lead to optical histopathology supporting real-time diagnostics. However, most current studies do not use the full potential of machine/deep learning models. As a developing image modality, FLIm data are not easily obtainable, which, coupled with an absence of standardisation, is pushing back the research to develop models which could advance automated diagnosis and help promote FLIm. In this paper, we describe recent developments that improve FLIm image quality, specifically time-domain systems, and we summarise sensing, signal-to-noise analysis and the advances in registration and low-level tracking. We review the two main applications of ML for FLIm: lifetime estimation and image analysis through classification and segmentation. We suggest a course of action to improve the quality of ML studies applied to FLIm. Our final goal is to promote FLIm and attract more ML practitioners to explore the potential of lifetime imaging.

64 picosecond time resolved time-correlated single photon counting imaging

High-speed imaging of dynamic scenes is a challenging and important task in many applications. However, conventional imaging methods based on charge coupled devices or complementary metal oxide semiconductors have limitations in temporal resolution and photon sensitivity. To address this problem, we propose a novel high-speed imaging scheme that combines single-pixel imaging with single photon detection and time-correlated single photon counting. Our scheme can achieve high-speed imaging with 64 ps resolution by repeating the motion scenes and using binary outputs from single photon detectors. We demonstrate our scheme by reconstructing the switching process of a digital micro-mirror device and a liquid crystal spatial light modulator. Our scheme can be further improved to 1 ps resolution by using a more accurate time-correlated single photon counting system. Moreover, our scheme can adapt to different speed scenes by adjusting the temporal resolution and reducing the sampling time. Our high temporal resolution imaging scheme further expands the application areas of single-pixel imaging and provides solutions for scenes requiring single photon detection and higher temporal resolution, such as reproducible chemical reaction processes imaging, cellular or sub-cellular bio imaging, single-molecule imaging of rotary motors, high-speed equipment inspection, and other periodic high-speed scenes imaging.

Fibre-optic based exploration of lung cancer autofluorescence using spectral fluorescence lifetime

Fibre-optic based time-resolved fluorescence spectroscopy (TRFS) is an advanced spectroscopy technique that generates sample-specific spectral-temporal signature, characterising variations in fluorescence in real-time. As such, it can be used to interrogate tissue autofluorescence. Recent advancements in TRFS technology, including the development of devices that simultaneously measure high-resolution spectral and temporal fluorescence, paired with novel analysis methods extracting information from these multidimensional measurements effectively, provide additional insight into the underlying autofluorescence features of a sample. This study demonstrates, using both simulated data and endogenous fluorophores measured bench-side, that the shape of the spectral fluorescence lifetime, or fluorescence lifetimes estimated over high-resolution spectral channels across a broad range, is influenced by the relative abundance of underlying fluorophores in mixed systems and their respective environment. This study, furthermore, explores the properties of the spectral fluorescence lifetime in paired lung tissue deemed either abnormal or normal by pathologists. We observe that, on average, the shape of the spectral fluorescence lifetime at multiple locations sampled on 14 abnormal lung tissue, compared to multiple locations sampled on the respective paired normal lung tissue, shows more variability; and, while not statistically significant, the average spectral fluorescence lifetime in abnormal tissue is consistently lower over every wavelength than the normal tissue.

Fast data fitting scheme for compressive multispectral fluorescence lifetime imaging

A single-pixel camera combined with compressive sensing techniques is a promising fluorescence microscope scheme for acquiring a multidimensional dataset (space, spectrum, and lifetime) and for reducing the measurement time with respect to conventional microscope schemes. However, upon completing the acquisition, a computational step is necessary for image reconstruction and data analysis, which can be time-consuming, potentially canceling out the beneficial effect of compressive sensing. In this work, we propose and experimentally validate a fast-fit workflow based on global analysis and multiple linear fits, which significantly reduces the computation time from tens of minutes to less than 1 s. Moreover, as the method is interlaced with the measurement flow, it can be applied in parallel with the acquisitions.

Label-free biomedical optical imaging

Label-free optical imaging employs natural and nondestructive approaches for the visualisation of biomedical samples for both biological assays and clinical diagnosis. Currently, this field revolves around multiple broad technology-oriented communities, each with a specific focus on a particular modality despite the existence of shared challenges and applications. As a result, biologists or clinical researchers who require label-free imaging are often not aware of the most appropriate modality to use. This manuscript presents a comprehensive review of and comparison among different label-free imaging modalities and discusses common challenges and applications. We expect this review to facilitate collaborative interactions between imaging communities, push the field forward and foster technological advancements, biophysical discoveries, as well as clinical detection, diagnosis, and monitoring of disease.

High-throughput multiplexed fluorescence lifetime microscopy

Fluorescence lifetime microscopy has been widely used in quantifying cellular interaction or histopathological identification of different stained tissues. A novel, to the best of our knowledge, approach for high-throughput multiplexed fluorescence lifetime imaging is presented. To establish a high-throughput fluorescence lifetime acquisition system, a uniformed illumination optical focus array was generated by a novel computer-generated hologram algorithm based on matrix triple product. This, in conjunction with an array detector and multichannel time-correlated single-photon counting, enables the full use of the acquisition ability of each detector. By utilizing interval segmentation of photon time detection, a high-throughput multiplexed fluorescence lifetime imaging is achieved. Experimental results demonstrate that this method achieves a fivefold increase in the collection throughput of fluorescence lifetime and is capable of simultaneous dual-target fluorescence lifetime measurement.

Application of Nanosecond Pulsed Electric Field and Autofluorescence Lifetime Microscopy of FAD in Lung Cells

Exposure of nanosecond pulsed electric fields (nsPEFs) to live cells is an increasing research interest in biology and medicine. Despite extensive studies, a question still remains as to how effects of application of nsPEF on intracellular functions are different between cancerous cells and normal cells and how the difference can be detected. Herein, we have presented an approach of autofluorescence lifetime (AFL) microscopy of flavin adenine dinucleotide (FAD) to detect effects of application of nsPEF having 50 ns of a pulse width, nsPEF(50), on intracellular function in lung cancerous cells, A549 and H661, which show nsPEF(50)-induced apoptosis, and normal cells, MRC-5, in which the field effect is less or not induced. Then, the application of nsPEF(50) is shown to increase the lifetime of FAD autofluorescence in lung cancerous cells, whereas the electric field effects on the autofluorescence of FAD was not significant in normal healthy cells, which indicates that the lifetime measurements of FAD autofluorescence are applicable to detect the field-induced change in intracellular functions. Lifetime and intensity microscopic images of FAD autofluorescence in these lung cells were also acquired after exposure to the apoptosis-inducer staurosporine (STS). Then, it was found that the AFL of FAD became longer after exposure not only in the cancerous cells but also in the normal cells. These results indicate that nsPEF(50) applied to lung cells induced apoptotic cell death only in lung cancerous cells (H661 and A549) but not in lung normal cells (MRC-5), whereas STS induced apoptotic cell death both in lung cancerous cells and in lung normal cells. The lifetime microscopy of FAD autofluorescence is suggested to be very useful as a sensitive detection method of nsPEF-induced apoptotic cell death.

Integration of Decay Time Analysis and Radiation Measurement for Quantum-Dot-Based Scintillator’s Characterization

In this study, we demonstrated the process of an integrated apparatus for decay time analysis and gamma radiation measurement with a liquid-scintillator-based cadmium-doped zinc oxide (CZO) nanomaterial. Generally, time-resolved photon counting is an essential analysis method in the field of precision measurement in the quantum domain. Such photon counting equipment requires a pulse laser that can be repeated quickly while having a sharp pulse width of picoseconds or femtoseconds as a light source. Time-correlated single photon counting (TCSPC) equipment, which is currently a commercial product, is inconvenient for recent development research because the scintillator size and shape are limited. Here, neodymium-doped yttrium aluminum garnet (Nd/YAG) laser TCSPC equipment was constructed to analyze the fluorescence characteristics of scintillators having various sizes and shapes. Then, a liquid scintillator added with CZO nanomaterial was prepared and the Nd/YAG laser TCSPC equipment test was performed. As a result of measuring the scintillator using the manufactured Nd/YAG laser TCSPC equipment, the non-CZO liquid scintillator was analyzed at 2.30 ns and the liquid scintillator equipped with CZO-loaded nanomaterial was analyzed at 11.95 ns. It showed an error within 5% when compared with the result of commercial TCSPC equipment. In addition, it was verified that the Nd/YAG laser TCSPC system can sufficiently measure the decay time in nanoseconds (ns). Moreover, it was presented that the Compton edge energy of Cs−137 is 477.3 keV, which hardly generates a photoelectric effect, and Compton scattering mainly occurs.

Halide perovskites and perovskite related materials for particle radiation detection

Radiation detectors are widely used in physics, materials science, chemistry, and biology. Halide perovskites are known for their superior properties including tunable bandgaps and chemical compositions, high defect tolerance, solution-processable synthesis of films and crystals, and high carrier diffusion length. Recently, halide perovskites have attracted enormous interest as particle radiation detectors for both charged (α and β) and uncharged (neutrons) particles. Solid-state detectors based on single crystal perovskites can detect α particles and thermal neutrons with energy-resolved spectra. Halide perovskite scintillators are also able to detect β particles and fast neutrons. In this review, we briefly introduce the fundamentals of radiation detection and summarize the recent progress on halide perovskite detectors for particle radiation.

Deep Learning in Biomedical Optics

This article reviews deep learning applications in biomedical optics with a particular emphasis on image formation. The review is organized by imaging domains within biomedical optics and includes microscopy, fluorescence lifetime imaging, in vivo microscopy, widefield endoscopy, optical coherence tomography, photoacoustic imaging, diffuse tomography, and functional optical brain imaging. For each of these domains, we summarize how deep learning has been applied and highlight methods by which deep learning can enable new capabilities for optics in medicine. Challenges and opportunities to improve translation and adoption of deep learning in biomedical optics are also summarized. Lasers Surg. Med. © 2021 Wiley Periodicals LLC.

Simultaneous multi-spectral, single-photon fluorescence imaging using a plasmonic colour filter array

We present the first realisation of simultaneous multi-spectral fluorescence imaging using a single-photon avalanche diode (SPAD) array, where the spectral unmixing is facilitated by a plasmonic metasurface mosaic colour filter array (CFA). A 64 × 64 pixel format silicon SPAD array is used to record widefield fluorescence and brightfield data from four biological samples. A plasmonic metasurface composed of an arrangement of circular and elliptical nanoholes etched into an aluminium thin film deposited on a glass substrate provides the high transmission efficiency CFA, enabling a bespoke spectral unmixing algorithm to reconstruct high fidelity, full colour images from as few as ∼3 photons per pixel. This approach points the way toward real-time, single-photon sensitive multi-spectral fluorescence imaging. Furthermore, this is possible without additional bulky components such as a filter wheel, prism or diffraction grating, nor the need for multiple sample exposures or multiple detectors.

Multispectral fluorescence lifetime imaging device with a silicon avalanche photodetector

We report the design, development, and characterization of a novel multi-spectral fluorescence lifetime measurement device incorporating solid-state detectors and automated gain control. For every excitation pulse (∼1 µJ, 600 ps), this device records complete fluorescence decay from multiple spectral channels simultaneously within microseconds, using a dedicated UV enhanced avalanche photodetector and analog to digital convert (2.5 GS/s) in each channel. Fast (<2 ms) channel-wise dynamic range adjustment maximizes the signal-to-noise ratio. Fluorophores with known lifetime ranging from 0.5-6.0 ns were used to demonstrate the device accuracy. Current results show the clear benefits of this device compared to existing devices employing microchannel-plate photomultiplier tubes. This is demonstrated by 5-fold reduction of lifetime measurement variability in identical conditions, independent gain adjustment in each spectral band, and 4-times faster imaging speed. The use of solid-state detectors will also facilitate future improved performance and miniaturization of the instrument.

Achieving a high photon count rate in digital time-correlated single photon counting using a hybrid photodetector

We report an enhanced photon count rate in a digitally implemented time-correlated single-photon counting (TCSPC) system by utilizing a hybrid photodetector (HPD). In our digital TCSPC scheme, the photoelectronic responses from a single photon-sensitive photodetector are digitally analyzed through a high-speed analog-to-digital convertor (ADC). By virtue of the HPD which provides nearly a constant signal gain, the single-photon pulses can be effectively distinguished from pulses of simultaneously detected multiple photons by the pulse heights. Consequently, our digital TCSPC system can selectively collect single-photon signals even in the presence of intense multi-photon detections with its temporal accuracy not to be compromised. In our experiment of fluorescence lifetime measurement, the maximum count rate of single photons nearly reached the theoretical limit given by the Poisson statistics. This demonstrated that the digital TCSPC combined with the HPD provides an ultimate solution for the TCSPC implementation for high photon count rates.

Direct Photon-by-Photon Analysis of Time-Resolved Pulsed Excitation Data using Bayesian Nonparametrics

Lifetimes of chemical species are typically estimated by either fitting time-correlated single-photon counting (TCSPC) histograms or phasor analysis from time-resolved photon arrivals. While both methods yield lifetimes in a computationally efficient manner, their performance is limited by choices made on the number of distinct chemical species contributing photons. However, the number of species is encoded in the photon arrival times collected for each illuminated spot and need not be set by hand a priori. Here, we propose a direct photon-by-photon analysis of data drawn from pulsed excitation experiments to infer, simultaneously and self-consistently, the number of species and their associated lifetimes from a few thousand photons. We do so by leveraging new mathematical tools within the Bayesian nonparametric. We benchmark our method for both simulated and experimental data for 1-4 species.

A fraction of NADH in solution is "dark": Implications for metabolic sensing via fluorescence lifetime

The metabolic cofactor and energy carrier NADH (nicotinamide adenine dinucleotide, reduced) has fluorescence yield and lifetime that depends strongly on conformation, a fact that has enabled metabolic monitoring of cells via FLIM (Fluorescence Lifetime Microscopy). Using femtosecond fluorescence upconversion, we show that this molecule in solution participates in ultrafast self-quenching along with both bulk solvent relaxation and spectral relaxation on 1.4 and 26 ps timescales. This, in effect, means up to a third of NADH is effectively "dark" for FLIM in the 400-500 nm observation window commonly employed. Methods to compensate for, avoid or measure dark species corrections are outlined.

Multiparametric Time-Correlated Single Photon Counting Luminescence Microscopy

Classic time-correlated single photon counting (TCSPC) technique involves detection of single photons of a periodic optical signal, registration of the photon arrival time in respect to the reference pulse, and construction of photon distribution with regard to the detection times. This technique achieves extremely high time resolution and near-ideal detection efficiency. Modern TCSPC is multi-dimensional, i.e., in addition to the photon arrival time relative to the excitation pulse, spatial coordinates within the image area, wavelength, time from the start of the experiment, and many other parameters are determined for each photon. Hence, the multi-dimensional TCSPC allows generation of photon distributions over these parameters. This review describes both classic and multi-dimensional types of TCSPC microscopy and their application for fluorescence lifetime imaging in different areas of biological studies.

Macromolecular crowding effects on energy transfer efficiency and donor-acceptor distance of hetero-FRET sensors using time-resolved fluorescence

Living cells are crowded with macromolecules and organelles, which affect a myriad of biochemical processes. As a result, there is a need for sensitive molecular sensors for quantitative, site-specific assessment of macromolecular crowding. Here, we investigated the excited-state dynamics of recently developed hetero-FRET sensors (mCerulean3-linker-mCitrine) in homogeneous and heterogeneous environments using time-resolved fluorescence measurements, which are compatible with fluorescence lifetime imaging microscopy (FLIM). The linker in these FRET constructs, which tether the mCerulean3 (the donor) and mCitrine (the acceptor), vary in both length and flexibility. Glycerol and Ficoll-70 solutions were used for homogeneous and heterogeneous environments, respectively, at variable concentrations. The wavelength-dependent studies suggest that the 425-nm excitation and the 475-nm emission of the donor are best suited for quantitative assessment of the energy transfer efficiency and the donor-acceptor distance of these FRET probes. Under the same experimental conditions, the enzymatically cleaved counterpart of these probes was used as a control as well as a means to account for the changes in the environmental refractive indices. Our results indicate that the energy transfer efficiency of these FRET probes increases as the linker becomes shorter and more flexible in pure buffer at room temperature. In addition, the FRET probes favor a compact structure with enhanced energy transfer efficiency and a shorter donor-acceptor distance in the heterogeneous, polymer-crowded environment due to steric hindrance. In contrast, the stretched conformation of these FRET probes is more favorable in the viscous, homogeneous environment with a reduced energy transfer efficiency and relatively larger donor-acceptor distance as compared with those in pure buffer, which was attributed to a reduced structural fluctuation of the mCerulean3-mCitrine FRET pair in the viscous, more restrictive glycerol-enriched buffer. Our findings will help to advance the potential of these hetero-FRET probes using FLIM for spatio-temporal assessment of the compartmentalized crowding in living cells.

Crowding Effects on Energy-Transfer Efficiencies of Hetero-FRET Probes As Measured Using Time-Resolved Fluorescence Anisotropy

Macromolecular crowding is prevalent in all living cells due to the presence of large biomolecules and organelles. Cellular crowding is heterogeneous and is known to influence biomolecular transport, biochemical reactions, and protein folding. Emerging evidence suggests that some cell pathologies may be correlated with compartmentalized crowding. As a result, there is a need for robust biosensors that are sensitive to crowding as well as quantitative, noninvasive fluorescence methods that are compatible with living cells studies. Here, we have developed a model that describes the rotational dynamics of hetero-Förster resonance energy transfer (FRET) biosensors as a means to determine the energy-transfer efficiency and donor-acceptor distance. The model was tested on wavelength-dependent time-resolved fluorescence anisotropy of hetero-FRET probes (mCerulean3-linker-mCitrine) with variable linkers in both crowded (Ficoll-70) and viscous (glycerol) solutions at room temperature. Our results indicate that the energy-transfer efficiencies of these FRET probes increase as the linker becomes shorter and more flexible in pure buffer at room temperature. In addition, the FRET probes favor compact structures with enhanced energy-transfer efficiencies and a shorter donor-acceptor distance in the heterogeneous, polymer-crowded environment due to steric hindrance. In contrast, the extended conformation of these FRET probes is more favorable in viscous, homogeneous environments with a reduced energy-transfer efficiency compared to those in pure buffer, which we attribute to reduced structural fluctuations of the mCerulean3-mCitrine FRET pair in the glycerol-enriched buffer. Our results represent an important step toward the application of quantitative and noninvasive time-resolved fluorescence anisotropy of hetero-FRET probes to investigate compartmentalized macromolecular crowding and protein-protein interactions in living cells as well as in controlled environments.

Metabolic Profiling of Live Cancer Tissues Using NAD(P)H Fluorescence Lifetime Imaging

Altered metabolism is a hallmark of cancer, both resulting from and driving oncogenesis. The NAD and NADP redox couples play a key role in a large number of the metabolic pathways involved. In their reduced forms, NADH and NADPH, these molecules are intrinsically fluorescent. As the average time for fluorescence to be emitted following excitation by a laser pulse, the fluorescence lifetime, is exquisitely sensitive to changes in the local environment of the fluorophore, imaging the fluorescence lifetime of NADH and NADPH offers the potential for label-free monitoring of metabolic changes inside living tumors. Here, we describe the biological, photophysical, and methodological considerations required to establish fluorescence lifetime imaging (FLIM) of NAD(P)H as a routine method for profiling the metabolism of living cancer cells and tissues.

Two-Photon Techniques in Tissue Engineering

Purpose

NIR radiation in the range of about 800 nm is less absorbed by biological tissues and is suited for triggering photonic effects using femtosecond pulsed Ti: Sa lasers. Especially in the life sciences, two-photon techniques are gaining greater importance. We introduce two laser applications for tissue engineering: the autofluorescent visualization of cells seeded on 3D scaffolds after two-photon excitation; and the manufacturing of 3D-structured hydrogel-like scaffolds by triggering free-radical polymerization processes within polymerizable precursors.

Methods

Primary bovine chondrocytes were cultivated on collagen I/III scaffolds using a flow chamber system coupled with a two-photon laser scanning microscope (2PLSM). During the incubation the cell population was hydrostatically stimulated. The selective visualization of unlabeled cells and scaffolds was achieved by spectral autofluorescence imaging. To gain some insight into scaffold-mediated effects on cell growth and cell differentiation, hydrogel-like scaffolds with well defined 3D structures were generated by two-photon polymerization (2PP) using methacrylated urethane and polyethyleneglycol diacrylate.

Results

We were able to show that spectral autofluorescence imaging provides spatially resolved data for the non-invasive online control of the tissue engineering process as well as the quantification of cell distribution within the scaffold. The fabrication of 3D 2PP scaffolds made from hydrogel-forming monomers and their effect on cell attachment and cell growth were also shown.

Conclusions

Two-photon techniques provide powerful tools for both the non-invasive online visualization of 3D cell-scaffold constructs and the structuring of 3D cultivation environments. The application of these techniques is also suitable for integration into micro-systems technology (e.g. BioMEMS, Cells-on-Chip, Lab-on-a Chip).

Time- and frequency-resolved fluorescence with a single TCSPC detector via a Fourier-transform approach

We introduce a broadband single-pixel spectro-temporal fluorescence detector, combining time-correlated single photon counting (TCSPC) with Fourier transform (FT) spectroscopy. A birefringent common-path interferometer (CPI) generates two time-delayed replicas of the sample's fluorescence. Via FT of their interference signal at the detector, we obtain a two-dimensional map of the fluorescence as a function of detection wavelength and emission time, with high temporal and spectral resolution. Our instrument is remarkably simple, as it only requires the addition of a CPI to a standard single-pixel TCSPC system, and it shows a readily adjustable spectral resolution with inherently broad bandwidth coverage.

Two-Photon Intravital Fluorescence Lifetime Imaging of the Kidney Reveals Cell-Type Specific Metabolic Signatures

In the live animal, tissue autofluorescence arises from a number of biologically important metabolites, such as the reduced form of nicotinamide adenine dinucleotide. Because autofluorescence changes with metabolic state, it can be harnessed as a label-free imaging tool with which to study metabolism in vivo Here, we used the combination of intravital two-photon microscopy and frequency-domain fluorescence lifetime imaging microscopy (FLIM) to map cell-specific metabolic signatures in the kidneys of live animals. The FLIM images are analyzed using the phasor approach, which requires no prior knowledge of metabolite species and can provide unbiased metabolic fingerprints for each pixel of the lifetime image. Intravital FLIM revealed the metabolic signatures of S1 and S2 proximal tubules to be distinct and resolvable at the subcellular level. Notably, S1 and distal tubules exhibited similar metabolic profiles despite apparent differences in morphology and autofluorescence emission with traditional two-photon microscopy. Time-lapse imaging revealed dynamic changes in the metabolic profiles of the interstitium, urinary lumen, and glomerulus-areas that are not resolved by traditional intensity-based two-photon microscopy. Finally, using a model of endotoxemia, we present examples of the way in which intravital FLIM can be applied to study kidney diseases and metabolism. In conclusion, intravital FLIM of intrinsic metabolites is a bias-free approach with which to characterize and monitor metabolism in vivo, and offers the unique opportunity to uncover dynamic metabolic changes in living animals with subcellular resolution.

Time-resolved fluorescence spectroscopy for clinical diagnosis of actinic cheilitis

Actinic cheilitis is a potentially malignant disorder of the lips. Its first cause is believed to be UV sun radiation. The lesion is highly heterogeneous, making the choice of area to be biopsied difficult. This study exploits the capabilities of time-resolved fluorescence spectroscopy for the identification of the most representative area to be biopsied. A preliminary study was performed on fourteen patients. A classification algorithm was used on data acquired on nine different biopsies. The algorithm discriminated between absent, mild, and moderate dysplasia with a sensitivity of 92.9%, 90.0%, and 80.0%, respectively. The false positive rate for healthy tissue (specificity) was 88.8%.

A wide-field TCSPC FLIM system based on an MCP PMT with a delay-line anode

We report on the implementation of a wide-field time-correlated single photon counting (TCSPC) method for fluorescence lifetime imaging (FLIM). It is based on a 40 mm diameter crossed delay line anode detector, where the readout is performed by three standard TCSPC boards. Excitation is performed by a picosecond diode laser with 50 MHz repetition rate. The photon arrival timing is obtained directly from the microchannel plates, with an instrumental response of ∼190 to 230 ps full width at half maximum depending on the position on the photocathode. The position of the photon event is obtained from the pulse propagation time along the two delay lines, one in x and one in y. One end of a delay line is fed into the "start" input of the corresponding TCSPC board, and the other end is delayed by 40 ns and fed into the "stop" input. The time between start and stop is directly converted into position, with a resolution of 200-250 μm. The data acquisition software builds up the distribution of the photons over their spatial coordinates, x and y, and their times after the excitation pulses, typically into 512 × 512 pixels and 1024 time channels per pixel. We apply the system to fluorescence lifetime imaging of cells labelled with Alexa 488 phalloidin in an epi-fluorescence microscope and discuss the application of our approach to other fluorescence microscopy methods.

Correlative NAD(P)H-FLIM and oxygen sensing-PLIM for metabolic mapping

Cellular responses to oxygen tension have been studied extensively. Oxygen tension can be determined by considering the phosphorescence lifetime of a phosphorescence sensor. The simultaneous usage of FLIM of coenzymes as NAD(P)H and FAD(+) and PLIM of oxygen sensors could provide information about correlation of metabolic pathways and oxygen tension. We investigated correlative NAD(P)H-FLIM and oxygen sensing-PLIM for simultaneously analyzing cell metabolism and oxygen tension. Cell metabolism and pO2 were observed under different hypoxic conditions in squamous carcinoma cell cultures and in complex ex vivo systems. Increased hypoxia induced an increase of the phosphorescence lifetime of Ru(BPY)3 and in most cases a decrease in the lifetime of NAD(P)H which is in agreement to the expected decrease of the protein-bound NAD(P)H during hypoxia. Oxygen was modulated directly in the mitochondrial membrane. Blocking of complex III and accumulation of oxygen could be observed by both the decrease of the phosphorescence lifetime of Ru(BPY)3 and a reduction of the lifetime of NAD(P)H which was a clear indication of acute changes in the redox state of the cells. For the first time simultaneous FLIM/PLIM has been shown to be able to visualize intracellular oxygen tension together with a change from oxidative to glycolytic phenotype.

FRAP, FLIM, and FRET: Detection and analysis of cellular dynamics on a molecular scale using fluorescence microscopy

The combination of fluorescent-probe technology plus modern optical microscopes allows investigators to monitor dynamic events in living cells with exquisite temporal and spatial resolution. Fluorescence recovery after photobleaching (FRAP), for example, has long been used to monitor molecular dynamics both within cells and on cellular surfaces. Although bound by the diffraction limit imposed on all optical microscopes, the combination of digital cameras and the application of fluorescence intensity information on large-pixel arrays have allowed such dynamic information to be monitored and quantified. Fluorescence lifetime imaging microscopy (FLIM), on the other hand, utilizes the information from an ensemble of fluorophores to probe changes in the local environment. Using either fluorescence-intensity or lifetime approaches, fluorescence resonance energy transfer (FRET) microscopy provides information about molecular interactions, with Ångstrom resolution. In this review, we summarize the theoretical framework underlying these methods and illustrate their utility in addressing important problems in reproductive and developmental systems.

256 × 2 SPAD line sensor for time resolved fluorescence spectroscopy

We present a CMOS chip 256 × 2 single photon avalanche diode (SPAD) line sensor, 23.78 µm pitch, 43.7% fill factor, custom designed for time resolved emission spectroscopy (TRES). Integrating time-to-digital converters (TDCs) implement on-chip mono-exponential fluorescence lifetime pre-calculation allowing timing of 65k photons/pixel at 200 Hz line rate at 40 ps resolution using centre-of-mass method (CMM). Per pixel time-correlated single-photon counting (TCSPC) histograms can also be generated with 320 ps bin resolution. We characterize performance in terms of dark count rate, instrument response function and lifetime uniformity for a set of fluorophores with lifetimes ranging from 4 ns to 6 ns. Lastly, we present fluorescence lifetime spectra of multicolor microspheres and skin autofluorescence acquired using a custom built spectrometer. In TCSPC mode, time-resolved spectra are acquired within 5 minutes whilst in CMM mode spectral lifetime signatures are acquired within 2 ms for fluorophore in cuvette and 200 ms for skin autofluorescence. We demonstrate CMOS line sensors to be a versatile tool for time-resolved fluorescence spectroscopy by providing parallelized and flexible spectral detection of fluorescence decay.

Localizing protein-protein interactions in living cells using fluorescence lifetime imaging microscopy

In the past decade, advances in fluorescence lifetime imaging have extensively applied in the life sciences, from fundamental biological investigations to advanced clinical diagnosis. Fluorescence lifetime imaging microscopy (FLIM) is now routinely used in the biological sciences to monitor dynamic signaling events inside living cells, e.g., Protein-Protein interactions. In this chapter, we describe the calibration of both time-correlated single-photon counting (TCSPC) and frequency domain (FD) FLIM systems and the acquisition and analysis of FLIM-FRET data for investigating Protein-Protein interactions in living cells.

High-throughput gated photon counter with two detection windows programmable down to 70 ps width

We present the design and characterization of a high-throughput gated photon counter able to count electrical pulses occurring within two well-defined and programmable detection windows. We extensively characterized and validated this instrument up to 100 Mcounts/s and with detection window width down to 70 ps. This instrument is suitable for many applications and proves to be a cost-effective and compact alternative to time-correlated single-photon counting equipment, thanks to its easy configurability, user-friendly interface, and fully adjustable settings via a Universal Serial Bus (USB) link to a remote computer.

Multiphoton laser tomography and fluorescence lifetime imaging of melanoma: morphologic features and quantitative data for sensitive and specific non-invasive diagnostics

Multiphoton laser tomography (MPT) combined with fluorescence lifetime imaging (FLIM) is a non-invasive imaging technique, based on the study of fluorescence decay times of naturally occurring fluorescent molecules, enabling a non-invasive investigation of the skin with subcellular resolution. The aim of this retrospective observational ex vivo study, was to characterize melanoma both from a morphologic and a quantitative point of view, attaining an improvement in the diagnostic accuracy with respect to dermoscopy. In the training phase, thirty parameters, comprising both cytological descriptors and architectural aspects, were identified. The training set included 6 melanomas with a mean Breslow thickness±S.D. of 0.89±0.48 mm. In the test phase, these parameters were blindly evaluated on a test data set consisting of 25 melanomas, 50 nevi and 50 basal cell carcinomas. Melanomas in the test phase comprised 8 in situ lesions and had a mean thickness±S.D. of 0.77±1.2 mm. Moreover, quantitative FLIM data were calculated for special areas of interest. Melanoma was characterized by the presence of atypical short lifetime cells and architectural disorder, in contrast to nevi presenting typical cells and a regular histoarchitecture. Sensitivity and specificity values for melanoma diagnosis were 100% and 98%, respectively, whereas dermoscopy achieved the same sensitivity, but a lower specificity (82%). Mean fluorescence lifetime values of melanocytic cells did not vary between melanomas and nevi, but significantly differed from those referring to basal cell carcinoma enabling a differential diagnosis based on quantitative data. Data from prospective preoperative trials are needed to confirm if MPT/FLIM could increase diagnostic specificity and thus reduce unnecessary surgical excisions.

Förster resonance energy transfer microscopy and spectroscopy for localizing protein-protein interactions in living cells

The fundamental theory of Förster resonance energy transfer (FRET) was established in the 1940s. Its great power was only realized in the past 20 years after different techniques were developed and applied to biological experiments. This success was made possible by the availability of suitable fluorescent probes, advanced optics, detectors, microscopy instrumentation, and analytical tools. Combined with state-of-the-art microscopy and spectroscopy, FRET imaging allows scientists to study a variety of phenomena that produce changes in molecular proximity, thereby leading to many significant findings in the life sciences. In this review, we outline various FRET imaging techniques and their strengths and limitations; we also provide a biological model to demonstrate how to investigate protein-protein interactions in living cells using both intensity- and fluorescence lifetime-based FRET microscopy methods.

pH dependence of the fluorescence lifetime of FAD in solution and in cells

We have studied physiological parameters in a living cell using fluorescence lifetime imaging of endogenous chromophores. In this study, pH dependence of the fluorescence lifetime of flavin adenine dinucleotide (FAD), that is a significant cofactor exhibiting autofluorescence, has been investigated in buffer solution and in cells. The fluorescence lifetime of FAD remained unchanged with pH 5 to 9 in solution. However, the fluorescence lifetime in HeLa cells was found to decrease with increasing intracellular pH, suggesting that pH in a single cell can be estimated from the fluorescence lifetime imaging of FAD without adding exogenous fluorescent probes.

Techniques for fluorescence detection of protoporphyrin IX in skin cancers associated with photodynamic therapy

Photodynamic therapy (PDT) is a treatment modality that uses a specific photosensitizing agent, molecular oxygen, and light of a particular wavelength to kill cells targeted by the therapy. Topically administered aminolevulinic acid (ALA) is widely used to effectively treat cancerous and precancerous skin lesions, resulting in targeted tissue damage and little to no scarring. The targeting aspect of the treatment arises from the fact that ALA is preferentially converted into protoporphyrin IX (PpIX) in neoplastic cells. To monitor the amount of PpIX in tissues, techniques have been developed to measure PpIX-specific fluorescence, which provides information useful for monitoring the abundance and location of the photosensitizer before and during the illumination phase of PDT. This review summarizes the current state of these fluorescence detection techniques. Non-invasive devices are available for point measurements, or for wide-field optical imaging, to enable monitoring of PpIX in superficial tissues. To gain access to information at greater tissue depths, multi-modal techniques are being developed which combine fluorescent measurements with ultrasound or optical coherence tomography, or with microscopic techniques such as confocal or multiphoton approaches. The tools available at present, and newer devices under development, offer the promise of better enabling clinicians to inform and guide PDT treatment planning, thereby optimizing therapeutic outcomes for patients.

Fluorescence lifetime imaging microscopy for quantitative biological imaging

Fluorescence lifetime imaging microscopy (FLIM) is a method for measuring fluorophore lifetimes with microscopic spatial resolution, providing a useful tool for cell biologists to detect, visualize, and investigate structure and function of biological systems. In this chapter, we begin by introducing the basic theory of fluorescence lifetime, including the characteristics of fluorophore decay, followed by a discussion of factors affecting fluorescence lifetimes and the potential advantages of fluorescence lifetime as a source of image contrast. Experimental methods for creating lifetime maps, including both time- and frequency-domain experimental approaches, are then introduced. Then, FLIM data analysis methods are discussed, including rapid lifetime determination, multiexponential fitting, Laguerre polynomial fitting, and phasor plot analysis. After, data analysis methods are introduced that improve lifetime precision of FLIM maps based upon optimal virtual gating and total variation denoising. The chapter concludes by highlighting several recent FLIM applications for quantitative biological imaging, including Förster resonance energy transfer-FLIM, fluorescence correlation spectroscopy-FLIM, multispectral-FLIM, and multiphoton-FLIM.

Elimination of autofluorescence background from fluorescence tissue images by use of time-gated detection and the AzaDiOxaTriAngulenium (ADOTA) fluorophore

Sample autofluorescence (fluorescence of inherent components of tissue and fixative-induced fluorescence) is a significant problem in direct imaging of molecular processes in biological samples. A large variety of naturally occurring fluorescent components in tissue results in broad emission that overlaps the emission of typical fluorescent dyes used for tissue labeling. In addition, autofluorescence is characterized by complex fluorescence intensity decay composed of multiple components whose lifetimes range from sub-nanoseconds to a few nanoseconds. For these reasons, the real fluorescence signal of the probe is difficult to separate from the unwanted autofluorescence. Here we present a method for reducing the autofluorescence problem by utilizing an azadioxatriangulenium (ADOTA) dye with a fluorescence lifetime of approximately 15 ns, much longer than those of most of the components of autofluorescence. A probe with such a long lifetime enables us to use time-gated intensity imaging to separate the signal of the targeting dye from the autofluorescence. We have shown experimentally that by discarding photons detected within the first 20 ns of the excitation pulse, the signal-to-background ratio is improved fivefold. This time-gating eliminates over 96 % of autofluorescence. Analysis using a variable time-gate may enable quantitative determination of the bound probe without the contributions from the background.

High-resolution imaging of basal cell carcinoma: a comparison between multiphoton microscopy with fluorescence lifetime imaging and reflectance confocal microscopy

AIMS

The aim of this study was to compare morphological aspects of basal cell carcinoma (BCC) as assessed by two different imaging methods: in vivo reflectance confocal microscopy (RCM) and multiphoton tomography with fluorescence lifetime imaging implementation (MPT-FLIM).

METHODS

The study comprised 16 BCCs for which a complete set of RCM and MPT-FLIM images were available. The presence of seven MPT-FLIM descriptors was evaluated. The presence of seven RCM equivalent parameters was scored in accordance to their extension. Chi-squared test with Fisher's exact test and Spearman's rank correlation coefficient were determined between MPT-FLIM scores and adjusted-RCM scores.

RESULTS

MPT-FLIM and RCM descriptors of BCC were coupled to match the descriptors that define the same pathological structures. The comparison included: Streaming and Aligned elongated cells, Streaming with multiple directions and Double alignment, Palisading (RCM) and Palisading (MPT-FLIM), Typical tumor islands, and Cell islands surrounded by fibers, Dark silhouettes and Phantom islands, Plump bright cells and Melanophages, Vessels (RCM), and Vessels (MPT-FLIM). The parameters that were significantly correlated were Melanophages/Plump Bright Cells, Aligned elongated cells/Streaming, Double alignment/Streaming with multiple directions, and Palisading (MPT-FLIM)/Palisading (RCM).

CONCLUSION

According to our data, both methods are suitable to image BCC's features. The concordance between MPT-FLIM and RCM is high, with some limitations due to the technical differences between the two devices. The hardest difficulty when comparing the images generated by the two imaging modalities is represented by their different field of view.

Spectrally resolved fluorescence lifetime imaging microscope using tunable bandpass filters

A simple structure of spectral fluorescence lifetime imaging microscope (SLIM) is designed with the use of tunable bandpass filter, a kind of Fabry-perot filter that transmission wavelength is varying according to incident angle of light. Feasibility tests of this angle-tuned bandpass filter (ATBF) are performed and it shows high transmission and constant spectral bandwidth (20 nm) with respect to angle of incidence. Furthermore, using two ATBFs in series, spectral bandwidth can be adjustable down to 4 nm. In this paper, dual ATBFs are implemented to the detection part of fluorescence lifetime imaging microscope (FLIM) system so that we obtained spectrally resolved FLIM images. We compare these SLIM images with an original FLIM image and confirm that the former case provides high accuracy to analyze lifetime distribution as well as high contrast of images. The proposed SLIM microscope with good wavelength selectivity has many opportunities to utilize to other applications such as FLIM-Föster resonant energy transfer and autofluorescence imaging.