Method for the determination of the luminance of two-photon vision stimuli.

Two-photon vision is a new and developing field in vision science. The phenomenon is based on visual perception of pulsed infrared lasers (800-1300 nm) due to the isomerization of visual pigments caused by two-photon absorption, with color perception corresponding to a wavelength about one-half of the stimulating wavelength in the near-infrared spectral range. Future applications of this effect, both in medical diagnostics and in virtual/augmented reality (VR/AR), require the ability to determine the luminance of the two-photon stimuli.

Chirped flicker optoretinography for in vivo characterization of human photoreceptors' frequency response to light.

Flicker electroretinography (ERG) has served as a valuable noninvasive objective tool for investigating retinal physiological function through the measurement of electrical signals originating from retinal neurons in response to temporally modulated light stimulation. Deficits in the response at certain frequencies can be used as effective biomarkers of cone-pathway dysfunction. In this Letter, we present the progress we made on its optical counterpart-photopic flicker optoretinography (f-ORG).

30 Years of Optical Coherence Tomography: introduction to the feature issue.

The guest editors introduce a feature issue commemorating the 30th anniversary of Optical Coherence Tomography.

Laser pulse train parameters determine the brightness of a two-photon stimulus.

This report presents the results of measurements of the two-photon vision threshold for various pulse trains. We employed three pulsed near-infrared lasers and pulse stretchers to obtain variations of the pulse duty cycle parameter over three orders of magnitude. We proposed and extensively described a mathematical model that combines the laser parameters with the visual threshold value.

In vivo imaging of the human retina using a two-photon excited fluorescence ophthalmoscope.

Noninvasive imaging of endogenous retinal fluorophores, including vitamin A derivatives, is vital to developing new treatments for retinal diseases. Here, we present a protocol for obtaining in vivo two-photon excited fluorescence images of the fundus in the human eye. We describe steps for laser characterization, system alignment, positioning human subjects, and data registration.

From mouse to human: Accessing the biochemistry of vision in vivo by two-photon excitation.

The eye is an ideal organ for imaging by a multi-photon excitation approach, because ocular tissues such as the sclera, cornea, lens and neurosensory retina, are highly transparent to infrared (IR) light. The interface between the retina and the retinal pigment epithelium (RPE) is especially informative, because it reflects the health of the visual (retinoid) cycle and its changes in response to external stress, genetic manipulations, and drug treatments. Vitamin A-derived retinoids, like retinyl esters, are natural fluorophores that respond to multi-photon excitation with near IR light, bypassing the filter-like properties of the cornea, lens, and macular pigments.

Spatio-temporal optical coherence tomography provides full thickness imaging of the chorioretinal complex.

Despite the rapid development of optical imaging methods, high-resolution imaging with penetration into deeper tissue layers is still a major challenge. Optical coherence tomography (OCT) has been used successfully for non-invasive human retinal volumetric imaging , advancing the detection, diagnosis, and monitoring of various retinal diseases. However, there are important limitations of volumetric OCT imaging, especially coherent noise and the limited axial range over which high resolution images can be acquired.

Red blood cell distribution width is associated with increased interactions of blood cells with vascular wall.

The mechanism underlying the association between elevated red cell distribution width (RDW) and poor prognosis in variety of diseases is unknown although many researchers consider RDW a marker of inflammation. We hypothesized that RDW directly affects intravascular hemodynamics, interactions between circulating cells and vessel wall, inducing local changes predisposing to atherothrombosis. We applied different human and animal models to verify our hypothesis.

Femtosecond Er-doped fiber laser source tunable from 872 to 1075 nm for two-photon vision studies in humans.

We report the development of a widely-tunable femtosecond fiber laser system and its application for two-photon vision studies. The source is based on an Er-doped fiber laser with spectral shift up to 2150 nm, followed by a second harmonic generation module to generate a frequency-doubled beam tunable from 872 to 1075 nm. The source delivers sub-230 fs pulses with nearly-constant duration over the entire tuning range, with output powers between 0.

Light-adapted flicker optoretinograms captured with a spatio-temporal optical coherence-tomography (STOC-T) system.

For many years electroretinography (ERG) has been used for obtaining information about the retinal physiological function. More recently, a new technique called optoretinography (ORG) has been developed. In one form of this technique, the physiological response of retinal photoreceptors to visible light, resulting in a nanometric photoreceptor optical path length change, is measured by phase-sensitive optical coherence tomography (OCT).

Optical coherence tomography reveals heterogeneity of the brain tissue and vasculature in the ischemic region after photothrombotic stroke in mice.

We demonstrate in vivo imaging of the ischemic area in the mouse brain after photostroke using a custom prototype Gaussian‑beam optical coherence tomography (OCT) setup in which the near infrared imaging beam and the green photoinducing light pass through the same objective lens. The goal of our research was analysis of vascularity of the ischemic area during 2‑week progress of stroke and correlating the hypo‑ and hyperreflective OCT scattering areas with the location of activated microglia and astroglia. Angiogenesis, which was assessed using angiomaps, showed that the area of vessels in the ischemic center increased until day 7.

Multimode fiber as a tool to reduce cross talk in Fourier-domain full-field optical coherence tomography.

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is an emerging tool for high-speed eye imaging. However, cross-talk formation in images limits the imaging depth. To this end, we have recently shown that reducing spatial coherence with a fast deformable membrane can suppress the noise but over a limited axial range and with substantial data processing.

In vivo imaging of the human eye using a 2-photon-excited fluorescence scanning laser ophthalmoscope.

BackgroundNoninvasive assessment of metabolic processes that sustain regeneration of human retinal visual pigments (visual cycle) is essential to improve ophthalmic diagnostics and to accelerate development of new treatments to counter retinal diseases. Fluorescent vitamin A derivatives, which are the chemical intermediates of these processes, are highly sensitive to UV light; thus, safe analyses of these processes in humans are currently beyond the reach of even the most modern ocular imaging modalities.MethodsWe present a compact, 2-photon-excited fluorescence scanning laser ophthalmoscope and spectrally resolved images of the human retina based on 2-photon excitation (TPE) with near-infrared light.

Multimode fiber enables control of spatial coherence in Fourier-domain full-field optical coherence tomography for in vivo corneal imaging.

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) has recently emerged as a fast alternative to point-scanning confocal OCT in eye imaging. However, when imaging the cornea with FD-FF-OCT, a spatially coherent laser can focus down on the retina to a spot that exceeds the maximum permissible exposure level. Here we demonstrate that a long multimode fiber with a small core can be used to reduce the spatial coherence of the laser and, thus, enable ultrafast in vivo volumetric imaging of the human cornea without causing risk to the retina.

Two-photon microperimetry with picosecond pulses.

Two-photon vision is a phenomenon associated with the perception of short pulses of near-infrared radiation (900-1200 nm) as a visible light. It is caused by the nonlinear process of two-photon absorption by visual pigments. Here we present results showing the influence of pulse duration and repetition rate of short pulsed lasers on the visual threshold.

High-Throughput Monitoring of Bacterial Cell Density in Nanoliter Droplets: Label-Free Detection of Unmodified Gram-Positive and Gram-Negative Bacteria.

Droplet microfluidics disrupted analytical biology with the introduction of digital polymerase chain reaction and single-cell sequencing. The same technology may also bring important innovation in the analysis of bacteria, including antibiotic susceptibility testing at the single-cell level. Still, despite promising demonstrations, the lack of a high-throughput label-free method of detecting bacteria in nanoliter droplets prohibits analysis of the most interesting strains and widespread use of droplet technologies in analytical microbiology.

Multi-meridian corneal imaging of air-puff induced deformation for improved detection of biomechanical abnormalities.

Corneal biomechanics play a fundamental role in the genesis and progression of corneal pathologies, such as keratoconus; in corneal remodeling after corneal surgery; and in affecting the measurement accuracy of glaucoma biomarkers, such as the intraocular pressure (IOP). Air-puff induced corneal deformation imaging reveals information highlighting normal and pathological corneal response to a non-contact mechanical excitation. However, current commercial systems are limited to monitoring corneal deformation only on one corneal meridian.

Longitudinal in-vivo OCM imaging of glioblastoma development in the mouse brain.

We present in-vivo imaging of the mouse brain using custom made Gaussian beam optical coherence microscopy (OCM) with 800nm wavelength. We applied new instrumentation to longitudinal imaging of the glioblastoma (GBM) tumor microvasculature in the mouse brain. We have introduced new morphometric biomarkers that enable quantitative analysis of the development of GBM.

Frequency-doubled femtosecond Er-doped fiber laser for two-photon excited fluorescence imaging.

A femtosecond frequency-doubled erbium-doped fiber laser with an adjustable pulse repetition rate is developed and applied in two-photon excited fluorescence microscopy. The all-fiber laser system provides the fundamental pulse at 1560 nm wavelength with 22 fs duration for the second harmonic generation, resulting in 1.35 nJ, 60 fs pulses at 780 nm.

Noninvasive two-photon optical biopsy of retinal fluorophores.

High-resolution imaging techniques capable of detecting identifiable endogenous fluorophores in the eye along with genetic testing will dramatically improve diagnostic capabilities in the ophthalmology clinic and accelerate the development of new treatments for blinding diseases. Two-photon excitation (TPE)-based imaging overcomes the filtering of ultraviolet light by the lens of the human eye and thus can be utilized to discover defects in vitamin A metabolism during the regeneration of the visual pigments required for the detection of light. Combining TPE with fluorescence lifetime imaging (FLIM) and spectral analyses offers the potential of detecting diseases of the retina at earlier stages before irreversible structural damage has occurred.

Keratoconus Detection Based on a Single Scheimpflug Image.

Purpose: To introduce a new approach for keratoconus detection based on corneal microstructure observed in vivo derived from a single Scheimpflug image.

Methods: Scheimpflug single-image snapshots from 25 control subjects and 25 keratoconus eyes were analyzed; from each group, five subjects were randomly selected to provide out-of-sample data. Each corneal image was segmented, after which the stromal pixel intensities were statistically modeled with a Weibull distribution.

Corneal tissue properties following scleral lens wear using Scheimpflug imaging.

Purpose: To investigate the effect of short-term scleral lens wear on the corneal stroma at a macroscopic (thickness) and microscopic (within tissue) level, including regional variations.

Methods: Fourteen young, healthy participants wore a rotationally symmetric, 16.5 mm diameter, scleral lens for 8 h.

imaging of the human cornea with high-speed and high-resolution Fourier-domain full-field optical coherence tomography.

Corneal evaluation in ophthalmology necessitates cellular-resolution and fast imaging techniques that allow for accurate diagnoses. Currently, the fastest volumetric imaging technique is Fourier-domain full-field optical coherence tomography (FD-FF-OCT), which uses a fast camera and a rapidly tunable laser source. Here, we demonstrate high-resolution, high-speed, non-contact corneal volumetric imaging with FD-FF-OCT that can acquire a single 3D volume with a voxel rate of 7.

Computational aberration correction in spatiotemporal optical coherence (STOC) imaging.

Spatiotemporal optical coherence (STOC) imaging is a new technique for suppressing coherent cross talk noise in Fourier-domain full-field optical coherence tomography (FD-FF-OCT). In STOC imaging, the time-varying inhomogeneous phase masks modulate the incident light to alter the interferometric signal. Resulting interference images are then processed as in standard FD-FF-OCT and averaged incoherently or coherently to produce cross-talk-free volumetric optical coherence tomography (OCT) images of the sample.