Towards two-photon excited endogenous fluorescence lifetime imaging microendoscopy

In situ fluorescence lifetime imaging microscopy (FLIM) in an endoscopic configuration of the endogenous biomarker nicotinamide adenine dinucleotide (NADH) has a great potential for malignant tissue diagnosis. Moreover, two-photon nonlinear excitation provides intrinsic optical sectioning along with enhanced imaging depth. We demonstrate, for the first time to our knowledge, nonlinear endogenous FLIM in a fibered microscope with proximal detection, applied to NADH in cultured cells, as a first step to a nonlinear endomicroscope, using a double-clad microstructured fiber with convenient fiber length (> 3 m) and excitation pulse duration (≈50 fs). Fluorescence photons are collected by the fiber inner cladding and we show that its contribution to the impulse response function (IRF), which originates from its intermodal and chromatic dispersions, is small (< 600 ps) and stable for lengths up to 8 m and allows for short lifetime measurements. We use the phasor representation as a quick visualization tool adapted to the endoscopy speed requirements.

Volumetric fluorescence retinal imaging in vivo over a 30-degree field of view by oblique scanning laser ophthalmoscopy (oSLO)

While fluorescent contrast is widely used in ophthalmology, three-dimensional (3D) fluorescence retinal imaging over a large field of view (FOV) has been challenging. In this paper, we describe a novel oblique scanning laser ophthalmoscopy (oSLO) technique that provides 3D volumetric fluorescence retinal imaging with only one raster scan. The technique utilizes scanned oblique illumination and angled detection to obtain fluorescent cross-sectional images, analogous to optical coherence tomography (OCT) line scans (or B-scans). By breaking the coaxial optical alignment used in conventional retinal imaging modalities, depth resolution is drastically improved. To demonstrate the capability of oSLO, we have performed in vivo volumetric fluorescein angiography (FA) of the rat retina with ~25μm depth resolution and over a 30° FOV. Using depth segmentation, oSLO can obtain high contrast images of the microvasculature down to single capillaries in 3D. The multi-modal nature of oSLO also allows for seamless combination with simultaneous OCT angiography.

Two-photon microscopy imaging of oxidative stress in human living erythrocytes

Red blood cells (RBCs) are known to be the most suitable cells to study oxidative stress, which is implicated in the etiopathology of many human diseases. The goal of the current study was to develop a new effective approach for assessing oxidative stress in human living RBCs using two-photon microscopy. To mimic oxidative stress in human living RBCs, an in vitro model was generated followed by two-photon microscopy imaging. The results revealed that oxidative stress is clearly visible on the two-photon microscopy images of RBCs under oxidative stress compared to no fluorescence in controls (P<0.0001). This novel approach for oxidative stress investigation in human living RBCs could efficiently be applied in clinical research and antioxidant compounds testing.

High axial resolution imaging system for large volume tissues using combination of inclined selective plane illumination and mechanical sectioning

To resolve fine structures of biological systems like neurons, it is required to realize microscopic imaging with sufficient spatial resolution in three dimensional systems. With regular optical imaging systems, high lateral resolution is accessible while high axial resolution is hard to achieve in a large volume. We introduce an imaging system for high 3D resolution fluorescence imaging of large volume tissues. Selective plane illumination was adopted to provide high axial resolution. A scientific CMOS working in sub-array mode kept the imaging area in the sample surface, which restrained the adverse effect of aberrations caused by inclined illumination. Plastic embedding and precise mechanical sectioning extended the axial range and eliminated distortion during the whole imaging process. The combination of these techniques enabled 3D high resolution imaging of large tissues. Fluorescent bead imaging showed resolutions of 0.59 μm, 0.47μm, and 0.59 μm in the x, y, and z directions, respectively. Data acquired from the volume sample of brain tissue demonstrated the applicability of this imaging system. Imaging of different depths showed uniform performance where details could be recognized in either the near-soma area or terminal area, and fine structures of neurons could be seen in both the xy and xz sections.

Observation of live chromatin dynamics in cells via 3D localization microscopy using Tetrapod point spread functions

We report the observation of chromatin dynamics in living budding yeast (Saccharomyces cerevisiae) cells, in three-dimensions (3D). Using dual color localization microscopy and employing a Tetrapod point spread function, we analyze the spatio-temporal dynamics of two fluorescently labeled DNA loci surrounding the GAL locus. From the measured trajectories, we obtain different dynamical characteristics in terms of inter-loci distance and temporal variance; when the GAL locus is activated, the 3D inter-loci distance and temporal variance increase compared to the inactive state. These changes are visible in spite of the large thermally- and biologically-driven heterogeneity in the relative motion of the two loci. Our observations are consistent with current euchromatin vs. heterochromatin models.

Augmented line-scan focal modulation microscopy for multi-dimensional imaging of zebrafish heart in vivo

Multi-dimensional fluorescence imaging of live animal models demands strong optical sectioning, high spatial resolution, fast image acquisition, and minimal photobleaching. While conventional laser scanning microscopes are capable of deep penetration and sub-cellular resolution, they are generally too slow and causing excessive photobleaching for volumetric or time-lapse imaging. We demonstrate the performance of an augmented line-scan focal modulation microscope (aLSFMM), a high-speed imaging platform that affords above video-rate imaging speed by the use of line scanning. Exceptional background rejection is accomplished by combining a confocal slit with focal modulation. The image quality is further improved by merging the information from simultaneously acquired focal modulation and confocal images. Such a hybrid imaging scheme makes it possible to use very low power excitation light in high-speed imaging, and therefore leads to reduced photobleaching that is desirable for three-dimensional (3D) and four-dimensional (4D) in vivo image acquisition.

Three-dimensional side-view endomicroscope for tracking individual cells in vivo

We demonstrate a side-view endomicroscope using a monolithic 3-axis scanner placed in the post-objective position that performs either tilt or piston motion to achieve either optical scan angles >10° or large vertical displacements, respectively. This configuration allows for scaling down of instrument dimensions for high maneuverability and accurate positioning in vivo. Images exceeded either 700 × 600 μm² in the horizontal plane or vertical depths of 200 μm. Resolution of 1.19 and 3.46 μm was obtained in the horizontal and oblique planes, respectively. Optical sections were collected from dysplastic colonic epithelium in vivo in mice that express tdTomato at 10 Hz to visualize individual cells.

Improved contrast in inverted selective plane illumination microscopy of thick tissues using confocal detection and structured illumination

Inverted selective plane illumination microscopy (iSPIM) enables fast, large field-of-view, long term imaging with compatibility with conventional sample mounting. However, the imaging quality can be deteriorated in thick tissues due to sample scattering. Three strategies have been adopted in this paper to optimize the imaging performance of iSPIM on thick tissue imaging: electronic confocal slit detection (eCSD), structured illumination (SI) and the two combined. We compared the image contrast when using SPIM, confocal SPIM (using eCSD alone), SI SPIM (using SI alone) or confocal-SI SPIM (combining both methods) on images of gelatin phantom and highly-scattering fluorescently-stained human tissue. We demonstrate that all the three methods showed remarkable contrast enhancement on both samples compared to iSPIM alone, and SI SPIM and the combined confocal-SI mode outperformed confocal SPIM in contrast enhancement. Moreover, the use of SI at high pattern frequencies outperformed confocal SPIM in terms of optical sectioning capability. However, image signal-to-noise ratio (SNR) was decreased at high pattern frequencies when imaging scattering samples with SI SPIM. By combining eCSD with SI to reduce background signal and noise, the superior optical sectioning performance of SI could be achieved while also maintaining high image SNR.

Optical projection tomography for rapid whole mouse brain imaging

In recent years, three-dimensional mesoscopic imaging has gained significant importance in life sciences for fundamental studies at the whole-organ level. In this manuscript, we present an optical projection tomography (OPT) method designed for imaging of the intact mouse brain. The system features an isotropic resolution of ~50 µm and an acquisition time of four to eight minutes, using a 3-day optimized clearing protocol. Imaging of the brain autofluorescence in 3D reveals details of the neuroanatomy, while the use of fluorescent labels displays the vascular network and amyloid deposition in 5xFAD mice, an important model of Alzheimer's disease (AD). Finally, the OPT images are compared with histological slices.

Two-photon laser scanning microscopy with electrowetting-based prism scanning

Laser scanners are an integral part of high resolution biomedical imaging systems such as confocal or 2-photon excitation (2PE) microscopes. In this work, we demonstrate the utility of electrowetting on dielectric (EWOD) prisms as a lateral laser-scanning element integrated in a conventional 2PE microscope. To the best of our knowledge, this is the first such demonstration for EWOD prisms. EWOD devices provide a transmissive, low power consuming, and compact alternative to conventional adaptive optics, and hence this technology has tremendous potential. We demonstrate 2PE microscope imaging of cultured mouse hippocampal neurons with a FOV of 130 × 130 μm² using EWOD prism scanning. In addition, we show simulations of the optical system with the EWOD prism, to evaluate the effect of propagating a Gaussian beam through the EWOD prism on the imaging quality. Based on the simulation results a beam size of 0.91 mm full width half max was chosen to conduct the imaging experiments, resulting in a numerical aperture of 0.17 of the imaging system.

Comparison and combination of rotational imaging optical coherence tomography and selective plane illumination microscopy for embryonic study

Several optical imaging techniques have been applied for high-resolution embryonic imaging using different contrast mechanisms, each with their own benefits and limitations. In this study, we imaged the same E9.5 mouse embryo with rotational imaging optical coherence tomography (RI-OCT) and selective plane illumination microscopy (SPIM). RI-OCT overcomes optical penetration limits of traditional OCT imaging that prohibit full-body imaging of mouse embryos at later stages by imaging the samples from multiple angles. SPIM enables high-resolution, 3D imaging with less phototoxicity and photobleaching than laser scanning confocal microscopy (LSCM) by illuminating the sample with a focused sheet of light. Side by side comparisons are supplemented with co-registered images. The results demonstrate that SPIM and RI-OCT are highly complementary and could provide more comprehensive tissue characterization for mouse embryonic research.

In vivo cellular-level real-time pharmacokinetic imaging of free-form and liposomal indocyanine green in liver

Indocyanine green (ICG) is a near-infrared fluorophore approved for human use which has been widely used for various clinical applications. Despite the well-established clinical usage, our understanding about the microscopic in vivo pharmacokinetics of systemically administered ICG has been relatively limited. In this work, we successfully visualized real-time in vivo pharmacokinetic dynamics of the intravenously injected free-form and liposomal ICG in cellular resolution by utilizing a custom-built video-rate near infrared laser-scanning confocal microscopy system. Initial perfusion and clearance from blood stream, diffusion into perisinusoidal space, and subsequent absorption into hepatocyte were directly visualized in vivo. The quantification analysis utilizing the real-time image sequences revealed distinct dynamic in vivo pharmacokinetic behavior of free-form and liposomal ICG.

sideSPIM - selective plane illumination based on a conventional inverted microscope

Previously described selective plane illumination microscopy techniques typically offset ease of use and sample handling for maximum imaging performance or vice versa. Also, to reduce cost and complexity while maximizing flexibility, it is highly desirable to implement light sheet microscopy such that it can be added to a standard research microscope instead of setting up a dedicated system. We devised a new approach termed sideSPIM that provides uncompromised imaging performance and easy sample handling while, at the same time, offering new applications of plane illumination towards fluidics and high throughput 3D imaging of multiple specimen. Based on an inverted epifluorescence microscope, all of the previous functionality is maintained and modifications to the existing system are kept to a minimum. At the same time, our implementation is able to take full advantage of the speed of the employed sCMOS camera and piezo stage to record data at rates of up to 5 stacks/s. Additionally, sample handling is compatible with established methods and switching magnification to change the field of view from single cells to whole organisms does not require labor intensive adjustments of the system.

Multifocus structured illumination microscopy for fast volumetric super-resolution imaging

We here report for the first time the synergistic implementation of structured illumination microscopy (SIM) and multifocus microscopy (MFM). This imaging modality is designed to alleviate the problem of insufficient volumetric acquisition speed in super-resolution biological imaging. SIM is a wide-field super-resolution technique that allows imaging with visible light beyond the classical diffraction limit. Employing multifocus diffractive optics we obtain simultaneous wide-field 3D imaging capability in the SIM acquisition sequence, improving volumetric acquisition speed by an order of magnitude. Imaging performance is demonstrated on biological specimens.

Improving mesoscopic fluorescence molecular tomography through data reduction

Mesoscopic fluorescence molecular tomography (MFMT) is a novel imaging technique that aims at obtaining the 3-D distribution of molecular probes inside biological tissues at depths of a few millimeters. To achieve high resolution, around 100-150μm scale in turbid samples, dense spatial sampling strategies are required. However, a large number of optodes leads to sizable forward and inverse problems that can be challenging to compute efficiently. In this work, we propose a two-step data reduction strategy to accelerate the inverse problem and improve robustness. First, data selection is performed via signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) criteria. Then principal component analysis (PCA) is applied to further reduce the size of the sensitivity matrix. We perform numerical simulations and phantom experiments to validate the effectiveness of the proposed strategy. In both in silico and in vitro cases, we are able to significantly improve the quality of MFMT reconstructions while reducing the computation times by close to a factor of two.

Fluorescence depth estimation from wide-field optical imaging data for guiding brain tumor resection: a multi-inclusion phantom study

Studies have shown that fluorescent agents demarcate tumor from surrounding brain tissue and offer intraoperative guidance during resection. However, visualization of fluorescence signal from tumor below the surgical surface or through the appearance of blood in the surgical field is challenging. We have previously described red light imaging techniques for estimating fluorescent depths in turbid media. In this study, we evaluate these methods over a broader range of fluorophore concentrations, and investigate the ability to resolve multiple fluorescent emissions in the same plane or at different depths along the axis of imaging. A tungsten halogen lamp is used as a broadband white light source for reflectance imaging. Fluorescence from Alexa Fluor 647 is excited with a 635 nm diode laser. Reflectance and fluorescence spectral data are gathered between 670 and 720 nm with the use of a liquid crystal tunable filter and recorded on a sCMOS camera. Results show that two fluorescent emissions can be resolved within 2 mm if they are in the same plane or within 3 mm if they are at different depths along the axis of imaging up to 6 mm below the surface.

Optical coherence tomography angiography of retinal vascular occlusions produced by imaging-guided laser photocoagulation

Retinal vascular occlusive diseases represent a major form of vision loss worldwide. Rodent models of these diseases have traditionally relied upon a slit-lamp biomicroscope to help visualize the fundus and subsequently aid delivery of high-power laser shots to a target vessel. Here we describe a multimodal imaging system that can produce, image, and monitor retinal vascular occlusions in rodents. The system combines a spectral-domain optical coherence tomography system for cross-sectional structural imaging and three-dimensional angiography, and a fluorescence scanning laser ophthalmoscope for Rose Bengal monitoring and high-power laser delivery to a target vessel. This multimodal system facilitates the precise production of occlusions in the branched retinal veins, central retinal vein, and branched retinal arteries. Additionally, changes in the retinal morphology and retinal vasculature can be longitudinally documented. With our device, retinal vascular occlusions can be easily and consistently created, which paves the way for futures studies on their pathophysiology and therapeutic targets.

Erratum: Measuring the effect of Western diet on liver tissue architecture by FLIM autofluorescence and harmonic generation microscopy: erratum

[This corrects the article on p. 3143 in vol. 8.].

Measuring the effect of a Western diet on liver tissue architecture by FLIM autofluorescence and harmonic generation microscopy

The phasor approach to auto-fluorescence lifetime imaging was used to identify and characterize a long lifetime species (LLS) (~7.8 ns) in livers of mice fed with a Western diet. The size of the areas containing this LLS species depends on the type of diet and the size distribution shows Western diet has much larger LLS sizes. Combination of third harmonic generation images with FLIM identified the LLS species with fat droplets and the droplet size distribution was estimated. Second harmonic generation microscopy combined with phasor FLIM shows that there is an increase in fibrosis with a Western diet. A new decomposition in three components of the phasor plot shows that a Western diet is correlated with a higher fraction of free NADH, signifying more reducing condition and more glycolytic condition. Multiparametric analysis of phasor distribution shows that from the distribution of phasor points, a Western diet fed versus a low fat diet fed samples of mice livers can be separated. The phasor approach for the analysis of FLIM images of autofluorescence in liver specimens can result in discovery of new fluorescent species and then these new fluorescent species can help assess tissue architecture. Finally integrating FLIM and second and third harmonic analysis provides a measure of the advancement of fibrosis as an effect of diet.

Development of bimolecular fluorescence complementation using rsEGFP2 for detection and super-resolution imaging of protein-protein interactions in live cells

Direct visualization of protein-protein interactions (PPIs) at high spatial and temporal resolution in live cells is crucial for understanding the intricate and dynamic behaviors of signaling protein complexes. Recently, bimolecular fluorescence complementation (BiFC) assays have been combined with super-resolution imaging techniques including PALM and SOFI to visualize PPIs at the nanometer spatial resolution. RESOLFT nanoscopy has been proven as a powerful live-cell super-resolution imaging technique. With regard to the detection and visualization of PPIs in live cells with high temporal and spatial resolution, here we developed a BiFC assay using split rsEGFP2, a highly photostable and reversibly photoswitchable fluorescent protein previously developed for RESOLFT nanoscopy. Combined with parallelized RESOLFT microscopy, we demonstrated the high spatiotemporal resolving capability of a rsEGFP2-based BiFC assay by detecting and visualizing specifically the heterodimerization interactions between Bcl-x_L and Bak as well as the dynamics of the complex on mitochondria membrane in live cells.

Fiber bundle-based integrated platform for wide-field fluorescence imaging and patterned optical stimulation for modulation of vasoconstriction in the deep brain of a living animal

We report a fiber optics-based intravital fluorescence imaging platform that includes epi-fluorescence microscopy and laser patterned-light stimulation system. The platform can perform real-time fluorescence imaging with a lateral resolution of ~4.9 μm while directly inserted into the intact mouse brain, optically stimulate vasoconstriction during real-time imaging, and avoid vessel damage in the penetration path of imaging probe. Using 473-nm patterned-light stimulation, we successfully modulated the vasoconstriction of a single targeted 37-μm-diameter blood vessel located more than 4.7 mm below the brain surface of a live SM22-ChR2 mouse. This platform may permit the hemodynamic studies associated with deeper brain neurovascular disorders.

Line-scanning hyperspectral imaging based on structured illumination optical sectioning

Line-scanning hyperspectral imaging (LHSI) is known to have a higher acquisition rate but lower sectioning capability than point-scanning hyperspectral imaging. To further increase the axial imaging contrast of LHSI, structured illumination was integrated into line excitation to remove the off-focus and scattered on-focus fluorescence signals. In an unsectioned leaf, the imaging contrast can be enhanced by 8 times, while in sectioned mouse skin tissues, a 4.5-fold enhancement can be achieved. With a spectral resolution of 1.15 nm, the fluorophores with seriously-overlapped spectra was proved to be separated without cross-talk by applying linear unmixing to the recorded spectral information.

Reconstruction of super-resolution STORM images using compressed sensing based on low-resolution raw images and interpolation

Single-molecule-localization-based super-resolution microscopic technologies, such as stochastic optical reconstruction microscopy (STORM), require lengthy runtimes. Compressed sensing (CS) can partially overcome this inherent disadvantage, but its effect on super-resolution reconstruction has not been thoroughly examined. In CS, measurement matrices play more important roles than reconstruction algorithms. Larger measurement matrices have better restricted isometry properties (RIPs). This paper proposes, analyzes, and compares uses of higher resolution cameras and interpolation to achieve better outcomes. Statistical results demonstrate that super-resolution reconstructions is significantly improved by interpolating low-resolution STORM raw images and using point-spread-function-based measurement matrices with better RIPs. The analysis of publically accessible experimental data confirms this conclusion.

High-dynamic-range fluorescence laminar optical tomography (HDR-FLOT)

Three-dimensional fluorescence laminar optical tomography (FLOT) can achieve resolutions of 100-200 µm and penetration depths of 2-3 mm. FLOT has been used in tissue engineering, neuroscience, as well as oncology. The limited dynamic range of the charge-coupled device-based system makes it difficult to image fluorescent samples with a large concentration difference, limits its penetration depth, and diminishes the quantitative accuracy of 3D reconstruction data. Here, incorporating the high-dynamic-range (HDR) method widely used in digital cameras, we present HDR-FLOT, increasing penetration depth and improving the ability to image fluorescent samples with a large concentration difference. The method was tested using an agar phantom and a B6 mouse for brain imaging in vivo.

Lenslet array tunable snapshot imaging spectrometer (LATIS) for hyperspectral fluorescence microscopy

Snapshot hyperspectral imaging augments pixel dwell time and acquisition speeds over existing scanning systems, making it a powerful tool for fluorescence microscopy. While most snapshot systems contain fixed datacube parameters (x,y,λ), our novel snapshot system, called the lenslet array tunable snapshot imaging spectrometer (LATIS), demonstrates tuning its average spectral resolution from 22.66 nm (80x80x22) to 13.94 nm (88x88x46) over 485 to 660 nm. We also describe a fixed LATIS with a datacube of 200x200x27 for larger field-of-view (FOV) imaging. We report <1 sec exposure times and high resolution fluorescence imaging with minimal artifacts.