Search

Bruch’s membrane (BM) is a pentalaminar structure that mediates transport between the retinal pigment epithelium (RPE) and choriocapillaris. With near-infrared Optical Coherence Tomography (OCT), it has been challenging to visualize, let alone quantify, BM non-invasively in non-pathologic eyes. First, we show that shorter wavelength visible light OCT consistently delineates BM better than longer wavelength visible light OCT in pigmented human subjects, independent of axial resolution. Second, we develop a physical model of RPE and BM reflectivity to explain this finding. Third, we employ this model to devise a morphometric algorithm to more accurately map BM thickness in the normal macula.

Neuroimaging techniques aim to image deep, with high resolution and minimal invasiveness. Here, we present a label-free optical microscopy approach that achieves a unique balance between these competing goals. Specifically, we design a high numerical aperture optical coherence microscope centered near 1700 nm, where ballistic attenuation in the mouse brain is minimized. Dynamic focusing and image fusion are employed to balance speckle reduction against multiple scattered light reduction. Imaging through the thinned skull to preserve intracranial pressure and minimize inflammation, we present volumetric imaging of cytoarchitecture and myeloarchitecture across the entire mouse neocortex and some sub-cortical regions.

Steady cerebral blood flow (CBF) is needed for normal brain function, but continuous monitoring of CBF in humans is currently challenging. Here, by leveraging a low-cost sensor technology, we introduce a class of novel near-infrared optical devices that monitor CBF continuously and non-invasively in adult humans. We achieve this by replacing expensive single photon counting detectors, currently used for optical CBF monitors, with complementary metal–oxide–semiconductor (CMOS) arrays. We maintain performance by employing an additional optical “trick” known as interferometry, which transforms each CMOS pixel into a sensitive detector for fluctuations of coherent light that probes blood flow in the brain. Our method is called interferometric Diffusing Wave Spectroscopy (iDWS). In this talk we describe technical advantages of iDWS, including recent advances in our approach, and broadly envisage how interferometry can help to advance the field of diffuse optical brain monitoring.

Measuring the point spread function (PSF) rolloff is a time-consuming part of spectral/Fourier domain Optical Coherence Tomography (SD-OCT) system characterization. Here, we introduce a method that predicts the PSF (sensitivity and axial resolution) rolloff without an interferometer. Instead, the method analyzes correlations of incoherent excess noise from the light source across the detector array to characterize the spectrometer response. We demonstrate this technique using supercontinuum and superluminescent diode sources, showing that a noise time course of just 0.5 seconds predicts axial imaging performance, as confirmed by meticulous measurements using a Michelson interferometer. This approach promises to facilitate SD-OCT system development.

Interferometric Near-Infrared Spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved method to measure diffuse optical field dynamics from the human brain. Here we demonstrate a non-contact, null source-collector separation iNIRS approach based on polarization splitting, which enables galvanometer-based scanning across large spatial fields-of-view and suppresses single and few-scattered superficial light that degrades the effective dynamic range for deep measurements. We present, for the first time, multi-dimensional TOF- and laterally-resolved data sets that describe human forehead dynamics. The resulting blood flow index images show significant spatial heterogeneity in superficial dynamics, helping to identify optimal regions for subsequent monitoring with improved brain specificity.

Central nervous system diseases start at the microscopic level; thus, in vivo deep brain imaging with cellular resolution is needed. Using the 1700 nm optical window, which has lowest ballistic attenuation for brain imaging, an optical coherence microscopy system was designed for in vivo imaging of mouse brain cellular architecture. Taking advantage of relatively low scattering at 1700 nm, neuronal cell bodies in the mouse brain were visualized through a thinned skull preparation, which minimizes inflammation and preserves intracranial pressure. Cellular architecture was co-registered with simultaneous angiographic imaging, showing the distribution of neuronal cell bodies relative to supplying capillaries.

Spectral resolution is a crucial parameter in spectral / Fourier domain Optical Coherence Tomography (OCT). The sensitivity roll-off is determined by the spectral resolution, while depth-dependent axial resolution changes are caused by spectral resolution variations with wavelength. Currently, spectral resolution assessment is performed using either a narrow linewidth light source or broadband interferometry. Although commonly used, these methods require either additional components or time-consuming procedures. Here, we present a simple method to directly measure the spectral resolution at all wavelengths based on excess noise correlations. We apply this method to a visible light OCT system and validate it against interferometry.

The inner plexiform layer (IPL) of the retina comprises extremely thin sublaminae with connections between bipolar cells, amacrine cells, and ganglion cells. So far, observations of IPL lamination in near-infrared Optical Coherence Tomography (OCT) images have been anecdotal. Visible light OCT theoretically provides higher axial resolution than near-infrared OCT for a given wavelength bandwidth. Imaging of the human retina with ultrahigh resolution visible light OCT and longitudinal chromatic aberration correction was recently shown, with a focus on the outer retina. Here, we demonstrate in vivo imaging of lamination in the inner plexiform layer using achromatized visible light Optical Coherence Tomography (OCT). To further improve the achievable axial resolution and contrast, we incorporate a grating light valve spatial light modulator (GLV-SLM) spectral shaping stage into our setup. The GLV-SLM rapidly and dynamically shapes the source spectrum to either reduce sidelobes in the axial point spread function, improve axial resolution by reducing the width of the axial point spread function, or switch between red light alignment mode and white light acquisition mode. In vivo retinal OCT images acquired from human subjects show that the IPL consists of 3 hyper-reflective bands and 2 hypo-reflective bands, corresponding well with the standard anatomical division of the IPL into 5 layers. Strategies to improve contrast of the subtle bands representing the IPL sublaminae are investigated. Possible explanations for the ability of visible light OCT to visualize IPL sublaminae, based only on backscattering or backreflection contrast, and implications for glaucoma progression monitoring, are discussed.

Over the past 5 years, visible light Optical Coherence Tomography (OCT) has emerged as a promising technique for ultrahigh resolution microstructural imaging and depth-resolved imaging of chromophores. In the retina, visible light OCT can simultaneously induce and observe retinal changes during the phototransduction cascade, including bleaching-related absorption changes, as well as intrinsic scattering, cell swelling, and possible longer-term changes in retinal chromophores. Here we investigate outer retinal reflectance changes during visible light OCT in mice to better understand the contributions of these various signals. All experiments were performed on pigmented (C57BL/6J) and albino (BALB/c) mice in an initially dark-adapted state. There were no consistent reflectance changes in any layers including and proximal to the External Limiting Membrane (ELM). However, reflectance increased in the inner segment / outer segment (IS / OS) junction and outer segments tips (OST) of both strains. Layers distal to the photoreceptors such as the Retinal Pigment Epithelium (RPE), Bruch’s membrane (BM), and choroid showed a consistent increase in pigmented mice and showed no significant change in albino mice. Though our results are qualitatively well-explained by results from photopigment bleaching and intrinsic optical signal experiments in the literature, the time scale of some of the changes observed in our study is too long, which could indicate either signals with a different physiological origin or the need for a more precise model to describe imaging and stimulation using the same beam profile.

Interferometric near-infrared spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved sensing method for direct and simultaneous quantification of tissue optical properties (absorption and reduced scattering) and dynamics (blood flow index) in vivo with a single modality. The technique has previously been validated in Intralipid phantoms, and applied to continuously and non-invasively monitor optical properties and blood flow index in the brains of head-fixed, anesthetized mice. A demonstration of robust iNIRS measurements in human tissues with motion would support the viability of iNIRS for clinical applications. Here, we perform non-contact iNIRS in human tissues. We show that phase drift caused by involuntary motion during acquisition significantly distorts the optical field autocorrelation, particularly at early TOFs. To solve this issue, we present a novel numerical phase drift correction method to isolate field dynamics due to just red blood cell motion within the sample. Upon correction, TOF-resolved autocorrelations exhibit exponential decay behavior, whether acquired from Intralipid, the human forearm, or the human forehead. We confirm the link between bulk motion artifacts and phase drift by simultaneous, co-registered iNIRS and Optical Coherence Tomography measurements. By applying conventional, time-resolved diffusion theory and diffusing wave spectroscopy theory, we quantify optical properties and time-of-flight-resolved dynamics in Intralipid, the human forearm, and the human brain. Finally, we explore strategies for increased photon collection through parallelization of iNIRS, to probe greater depths in the human brain. This work conclusively shows that diffuse optical measurements of field dynamics are possible, even in the presence of motion artifacts.

The penetration depth of optical microscopy in biological tissue is limited by attenuation due to absorption and scattering. Scattering decreases with wavelength, while water absorption is locally minimized at so-called infrared “optical windows.” Of the four infrared optical windows, light in the longest wavelength window at 2200 nm experiences the least scattering and the most water absorption. Therefore, the 2200 nm window is rarely used for biological microscopy. However, fractional water content differs greatly between tissues. Therefore, the best optical window for deep imaging may depend on tissue type. Here, we demonstrate the benefits of the 2200 nm optical window for imaging through the skull, which is highly turbid with a relatively low water content. A spectral domain optical coherence tomography (OCT) system at ~2200 nm was built. A maximum sensitivity of 86 dB and a tissue axial resolution of 13.9 µm were achieved. To assess relative contributions of scattering and water absorption, 2200 nm was compared with 1300 nm. In vivo cortical vasculature was imaged angiographically through the intact skull in mice. Overall 2200 nm experienced less attenuation through the intact skull. In cortical layer I, 1300 nm, which experiences more scattering but less water absorption, and 2200 nm, which experiences less scattering but more water absorption, exhibit similar attenuation. In deeper cortical layer II/III, higher attenuation was observed at 2200 nm due to higher water absorption. Thus the infrared window at 2200 nm may provide advantages for imaging layers at or near the cortical surface through thick skull.

Optical Coherence Tomography Angiography (OCTA) has recently emerged for imaging vasculature in clinical ophthalmology. Yet, apparent OCTA image artifacts remain challenging to interpret. Here, contrast-enhanced OCTA is employed in rats to help explain these apparent artifacts. By quantifying enhancement due to an intravascular contrast agent with rheological and scattering properties that are different from red blood cells (RBCs), OCTA image features are ascribed to specific rheological and scattering properties of RBCs. By imaging pigmented and unpigmented rats at a wavelength where scattering dominates image contrast (1300 nm), the impact of melanosome scattering on OCT and OCTA signals is determined.

Most flying-spot Optical Coherence Tomography (OCT) and Optical Coherence Microscopy (OCM) systems use a symmetric confocal geometry, where the detection path retraces the illumination path starting from and ending with the spatial mode of a single mode optical fiber. Here, we describe a visible light OCM instrument that breaks this symmetry to improve transverse resolution without sacrificing collection efficiency in scattering tissue. This was achieved by overfilling a 0.3 numerical aperture (NA) water immersion objective on the illumination path, while maintaining a conventional Gaussian mode detection path (1/e2 intensity diameter ~0.82 Airy disks), enabling ~1.1 μm full-width at half-maximum (FWHM) transverse resolution. At the same time, a ~0.9 μm FWHM axial resolution in tissue, achieved by a broadband visible light source, enabled femtoliter volume resolution. We characterized this instrument according to paraxial coherent microscopy theory, and then used it to image the meningeal layers, intravascular red blood cell-free layer, and myelinated axons in the mouse neocortex in vivo through the thinned skull. Finally, by introducing a 0.8 NA water immersion objective, we improved the lateral resolution to 0.44 μm FWHM, which provided a volumetric resolution of ~0.2 fL, revealing cell bodies in cortical layer I of the mouse brain with OCM for the first time.

Interferometric near-infrared spectroscopy (iNIRS) is a recently introduced time-of-flight- (TOF-) resolved sensing method for quantifying optical and dynamical properties of turbid media non-invasively. iNIRS measures the interference spectrum of light traversing a turbid medium using a rapidly tunable, or frequency swept, light source. While the modality was successfully demonstrated in vivo in the nude mouse brain for monitoring absorption, reduced scattering, and blood flow index, translation towards human measurements requires improving light collection efficiency. Particularly, interrogating cortical tissue beneath the adult human scalp and skull remains challenging due to the limited core size and throughput of the single mode fiber currently used for detection. To tackle this problem, we implement a short to null source-detector separation geometry setup to significantly improve the number of detected diffuse photons. We discuss both hardware and post-processing improvements to isolate the desired diffuse signal from the large, non-diffuse and specular signals. Furthermore, key improvements in the iNIRS optical system, including higher TOF resolution (22-60 ps), optimized dynamic range (36-47 dB), faster sweep rate (50-500 kHz), and a technique for combining the forward and backward sweeps to double the effective optical field autocorrelation sampling rate, are presented. These allow for more precise and quantitative extraction of in vivo optical properties and TOF-resolved dynamics at long path lengths. Collectively, these key advances in the technology pave the way for translating iNIRS towards non-invasive, real-time, and quantitative measurements of oxygen metabolism and blood perfusion in deep human tissues.

Blood flow patterns and kinetics in the choriocapillaris are poorly understood owing to a lack of quantitative ophthalmic imaging techniques for studying microvascular flow in the eye. Compared with the proximal retinal vasculature, the more distal choroidal vasculature is relatively more challenging to probe. Magnetic Resonance Imaging and Doppler Ultrasound can assess the retina and choroid, but do not resolve the finer layers or microvasculature. While Optical Coherence Tomography (OCT) angiography produces high-quality choroidal images, attempts at quantification through Doppler-based methods have had mixed success. Here, we use a new technique called Dynamic Contrast OCT (DyC-OCT), which tracks the passage of an intravascular scattering contrast agent, to reveal laminar blood flow patterns in the retina and choroid in vivo. While conceptually similar to fluorescence angiography, DyC-OCT has the substantial benefit of depth resolution, which enables separation of retinal and choroidal microvasculature. The scattering contrast agent enables improved angiography of both macro- and microvasculature in the retina and choroid. Blood plasma transit times are measured in individual vessels, while flow and volume are quantified for each of the microvascular layers. As expected, the choriocapillaris had the highest volume and flow. Blood flow rates were estimated with an average retinal blood flow of 9.1 ± 4.3 μL/min and an average choroidal blood flow of 40 ± 18.3 μL/min in the rat eye. These rates are consistent with previous literature. DyC-OCT affords a new perspective on the poorly understood choriocapillaris blood flow and kinetics and may be useful for studying outer retinal diseases.

The Western diet, causative in the development of atherosclerotic cardiovascular disease, has recently been associated with the development of diffuse white matter disease (WMD) and other subcortical changes. Yet, little is known about the pathophysiological mechanisms by which a high-fat diet can cause WMD. Mechanistic studies of deep brain regions in mice have been challenging due to a lack of non-invasive, high-resolution, and deep imaging technologies. Here we used Optical Coherence Tomography to study mouse cortical/subcortical structures noninvasively and in vivo. To better understand the role of Western Diet in the development of WMD, intensity and Doppler flow OCT images, obtained using a 1300 nm spectral / Fourier domain OCT system, were used to observe the structural and functional alterations in the cortex and corpus callosum of Western Diet and control diet mouse models. Specifically, we applied segmentation to the OCT images to identify the boundaries of the cortex/corpus callosum, and further quantify the layer thicknesses across animals between the two diet groups. Furthermore, microvasculature alterations such as changes in spatiotemporal flow profiles within diving arterioles, arteriole diameter, and collateral tortuosity were analyzed. In the current study, while the arteriole vessel diameters between the two diet groups was comparable, we show that collateral tortuosity was significantly higher in the Western diet group, compared to control diet group, possibly indicating remodeling of brain vasculature due to dietary changes. Moreover, there is evidence showing that the corpus callosum is thinner in Western diet mice, indicative of tissue atrophy.

Visible light is absorbed by intrinsic chromophores such as photopigment, melanin, and hemoglobin, and scattered by subcellular structures, all of which are potential retinal disease biomarkers. Recently, high-resolution quantitative measurement and mapping of hemoglobin concentrations was demonstrated using visible light Optical Coherence Tomography (OCT). Yet, most high-resolution visible light OCT systems adopt free-space, or bulk, optical setups, which could limit clinical applications. Here, the construction of a multi-functional fiber-optic OCT system for human retinal imaging with

We introduce and implement interferometric near-infrared spectroscopy (iNIRS), which simultaneously extracts the optical and dynamic properties of turbid media from the analysis of the spectral interference fringe pattern. The spectral interference fringe pattern is measured using a Mach-Zehnder interferometer with a frequency swept narrow bandwidth light source such that the temporal intensity autocorrelations can be determined for all photon path lengths. This approach enables time-of-flight (TOF) resolved measurement of scatterer motion, which is a feature inaccessible in well-established diffuse correlation spectroscopy techniques. We prove this by analyzing intensity correlations of the light transmitted through diffusive fluid phantoms with photon random walks of up to 55 (approximately 110 scattering events) using laser sweep rates on the order of 100kHz. Thus, the results we present here advance diffuse optical methods by enabling simultaneous determination of depth-resolved optical properties and dynamics in highly scattering samples.

A recent trend in optical coherence tomography (OCT) hardware has been the move towards higher A-scan rates. However, the estimation of axial blood flow velocities is affected by the presence and type of noise, as well as the estimation method. Higher acquisition rates alone do not enable the accurate quantification of axial blood velocity. Moreover, decorrelation is an unavoidable feature of OCT signals when there is motion relative to the OCT beam. For in-vivo OCT measurements of blood flow, decorrelation noise affects Doppler frequency estimation by broadening the signal spectrum. Here we derive a maximum likelihood estimator (MLE) for Doppler frequency estimation that takes into account spectral broadening due to decorrelation. We compare this estimator with existing techniques. Both theory and experiment show that this estimator is effective, and outperforms the Kasai and additive white Gaussian noise (AWGN) ML estimators. We find that maximum likelihood estimation can be useful for estimating Doppler shifts for slow axial flow and near transverse flow. Due to the inherent linear relationship between decorrelation and Doppler shift of scatterers moving relative to an OCT beam, decorrelation itself may be a measure of flow speed.

In optical coherence tomography (OCT), unbiased and low variance Doppler frequency estimators are desirable for blood velocity estimation. Hardware improvements in OCT mean that ever higher acquisition rates are possible. However, it is known that the Kasai autocorrelation estimator, unexpectedly, performs worse as acquisition rates increase. Here we suggest that maximum likelihood estimators (MLEs) that utilize prior knowledge of noise statistics can perform better. We show that the additive white Gaussian noise maximum likelihood estimator (AWGN MLE) has a superior performance to the Kasai autocorrelation estimate under additive shot noise conditions. It can achieve the Cramer-Rao Lower Bound (CRLB) for moderate data lengths and signal-to-noise ratios (SNRs). However, being a parametric estimator, it has the disadvantages of sensitivity to outliers, signal contamination and deviations from noise model assumptions. We show that under multiplicative decorrelation noise conditions, the AWGN MLE performance deteriorates, while the Kasai estimator still gives reasonable estimates. Hence, we further develop a multiplicative noise MLE for use under multiplicative noise dominant conditions. According to simulations, this estimator is superior to both the AWGN MLE and the Kasai estimator under these conditions, but requires knowledge of the decorrelation statistics. It also requires more computation. For actual data, the decorrelation MLE appears to perform adequately without parameter optimization. Hence we conclude that it is preferable to use a maximum likelihood approach in OCT Doppler frequency estimation when noise statistics are known or can be accurately estimated.

Optical Coherence Tomography (OCT) has recently been used to produce 3D angiography of microvasculature in the rodent brain in-vivo and blood flow maps of large vessels. Key enabling developments were novel algorithms for detecting Doppler shifts produced by moving scatterers and new scanning protocols tailored to increase sensitivity to small flow speeds. These progresses were pushed by the need for a non invasive imaging modality to monitor quantitative blood flow at a higher resolution and a greater depth than could be achieved by other means. The rationale for this work originates from new hypotheses regarding the role of blood regulation in neurodegenerative diseases and from current investigations of animal models of vascular degeneration. In this work we demonstrate the synthetic reconstruction of dynamic blood flow in mice over the course of a single cardiac cycle in an 800μm wide by ~ 3mm deep B-Frame slice with a lateral resolution of 10μm and a depth resolution of 7μm. Images were taken using a cranial window over the exposed parietal bone of mice skull. Electrocardiography (ECG) recordings were co registered with the OCT A lines at high temporal resolution. QRS peak detection was then used to locate the time value of each A-line in the cardiac cycle and to reconstruct a synthetic temporal frame over one cardiac cycle. Doppler speed in this cardiac cycle was used to measure temporal variations of flow inside arteries and of their area. Three dimensional volume scans yielded measurements of quantitative blood flow on the same arteries. Using these informations a measure of compliance could be established. Comparing measures between atherosclerotic (ATX) and wild type (WT) mice revealed higher blood flow in WT mice, suggested lower systemic compliance in the ATX group but higher compliance of cerebral vasculature on these mice. These results are consistent with expectations showing that OCT is a potential tool for in-vivo arterial compliance evaluation.

The performance and imaging characteristics of ultrahigh speed ophthalmic optical coherence tomography (OCT)are investigated. In vivo imaging results are obtained at 850nm and 1050nm using dierent con“gurations ofspectral and swept source / Fourier domain OCT. A spectral / Fourier domain instrument using a high speedCMOS linescan camera with SLD light source centered at 850nm achieves speeds of 91,000 axial scans per secondwith 3um axial resolution in tissue. A spectral / Fourier domain instrument using an InGaAs linescan camerawith SLD light source centered at 1050nm achieves 47,000 axial scans per second with 7um resolution in tissue.A swept source instrument using a novel wavelength swept laser light source centered at 1050nm achieves 100,000axial scans per second. Retinal diseases seen in the clinical setting are imaged using the 91kHz 850nm CMOScamera and 47kHz 1050nm InGaAs camera based instruments to investigate the combined eects of varyingspeed, axial resolution, center wavelength, and ins trument sensitivity on image quality. The novel 1050nm sweptsource / Fourier domain instrument using a recently developed commercially available short cavity laser sourceimages at 100,000 axial scans per second and is demonstrated in the normal retina. The dense 3D volumetricdata sets obtained with ultrahigh speed OCT promise to improve reproducibility of quantitative measurements,enabling early diagnosis as well as more sensitive assessment of disease progression and response to therapy.Keywords: Ultrahigh Speed Optical Coherence Tomography; Spectral and Swept Source / Fourier DomainOCT; Ophthalmology; Retina, Fovea and Optic Nerve Head; Medical and biological imaging.

Advanced Imaging Group, Thorlabs, Inc., Newton, NJ ABSTRACT We performed OCT imaging of the rat retin a at 70,000 axial scans per second with ~3 m axial resolution. Three-dimensional OCT (3D-OCT) data sets of the rat retina were acquired. The high speed and high density data sets enable improved en face visualization by reducing eye moti on artifacts and improve Doppler OCT measurements. Minimal motion artifacts were visible and the OCT fundus images offer more precise registration of individual OCT images to retinal fundus features. Projection OCT fundus images show features such as the nerve fiber layer, retinal capillary networks and choroidal vasculature. Doppler OCT images and quantitative measurements show pulsatility in retinal blood vessels. Doppler OCT provides non-invasive in vivo quantitative measurements of retinal blood flow properties and may benefit studies of diseases such as glaucoma and diabetic retinopathy. Ultrahigh speed imaging using ultrahigh resolution spectral / Fourier domain OCT promises to enable novel protocols for measuring small animal retinal structure and retinal blood flow. This non-invasive imaging technology is a promising tool for monitoring disease progression in rat and mouse models to assess ocul ar disease pathogenesis and response to treatment. Keywords: Ultrahigh resolution OCT, ultrahigh speed OCT, spectral/Fourier domain OCT, Doppler OCT, small animal imaging * liujj@mit.edu; phone 1 617 253-8140; fa x 1 617 253-9611; www.rle.mit.edu

Measuring oxygen delivery in brain tissue is important for identifying the pathophysiological changes associated with brain injury and various diseases such as cancer, stroke, and Alzheimer's disease. We have developed a multi-modal imaging system for minimally invasive measurement of cerebral oxygenation and blood flow in small animals with high spatial resolution. The system allows for simultaneous measurement of blood flow using Fourier-domain optical coherence tomography, and oxygen partial pressure (pO2) using either confocal or multiphoton phosphorescence lifetime imaging with exogenous porphyrin-based dyes sensitive to dissolved oxygen. Here we present the changes in pO2 and blood flow in superficial cortical vessels of Sprague Dawley rats in response to conditions such as hypoxia, hyperoxia, and functional stimulation. pO2 measurements display considerable heterogeneity over distances that cannot be resolved with more widely used oxygen-monitoring techniques such as BOLD-fMRI. Large increases in blood flow are observed in response to functional stimulation and hypoxia. Our system allows for quantification of cerebral metabolic rate of oxygen (CMRO2) with high spatial resolution, providing a better understanding of metabolic dynamics during functional stimulation and under various neuropathologies. Ultimately, better insight into the underlying mechanisms of neuropathologies will facilitate the development of improved therapeutic strategies to minimize damage to brain tissue.

Ultrahigh speed Spectral/Fourier domain ophthalmic OCT imaging at 70,000–312,500 axial scans per second is demonstrated using a high speed CMOS camera at 800 nm. Comparative imaging results of the fovea illustrate the performance tradeoffs between different imaging speeds and spectrometer configurations. Dense 3D volumetric acquisitions show minimal motion artifacts when acquired at 250,000 axial scans per second. The porous structure of the lamina cribrosa is shown in en face images extracted from a dense volumetric acquisition of the optical nerve head acquired at 106, 382 axial scans per second. Rapid repeated volume imaging (4D-OCT) shows blood flow in retinal capillaries. Boundaries of the capillary network are enhanced by motion contrast. 3D volumetric data acquired at 49,000 axial scans per second using an InGaAs camera at 1050 nm is compared to volumetric data acquired at 101, 010 axial scans per second using a CMOS camera at 800nm. Averaging of adjacent cross sectional scans in the volume is shown to increase contrast in the images and reduce speckle. The enhanced penetration of the 1050 nm compared to the 800 nm OCT imaging system is shown. Dense 2D/3D data sets and 4D-OCT repeated volume imaging promise alternative methods for diagnosis and monitoring of disease.

Ultrahigh speed spectral / Fourier domain optical coherence tomography (OCT) using a CMOS line scan camera with image acquisition rates of 70,000 - 312,500 axial scans per second is investigated. Several design configurations are presented to illustrate design trade-offs between acquisition speed, sensitivity, resolution and sensitivity roll-off performance. We demonstrate: extended imaging range and improved sensitivity roll-off at 70,000 axial scans per second with 4096 camera pixels, high speed and ultrahigh resolution imaging at 106,382 axial scans per second, and ultrahigh speed imaging at 250,000-312,500 axial scans per second. Each configuration is characterized through optical testing and the trade-offs demonstrated with in vivo imaging of the fovea and optic disk in the human retina. We show dense homogeneous 3D-OCT volumetric data sets that were acquired by raster scanning at 250,000 axial scans per second, which is an order of magnitude faster than most current generation spectral / Fourier OCT instruments. OCT fundus images constructed from the 3D-OCT data have no noticeable discontinuity of retinal features and show that there are minimal motion artifacts. Using an improved sensitivity roll-off configuration at 70,000 axial scans per second, long cross sectional scans are acquired at high resolution for imaging large areas of the retina, including the fovea and optic disk. Using an ultrahigh speed configuration at 250,000 axial scans per second, the fine porous structures of the lamina cribrosa can be seen from slices extracted from a dense 3D data set. Rapid repeated imaging of a small volume (4D-OCT) enables time resolved visualization of the capillary network surrounding the INL and may show individual red blood cells. This capability could create the possibility for alternative techniques for quantifying capillary blood flow, which cannot be measured with Doppler OCT methods because of the capillary’s perpendicular orientation to the optical beam. The results of this study suggest that high speed CMOS cameras can achieve a significant improvement in performance for ophthalmic imaging. This promises to have a powerful impact in clinical applications by improving early diagnosis, reproducibility of quantitative measurements and enabling more sensitive assessment of disease progression or response to therapy.

Ophthalmic OCT was performed using a novel, compact external cavity semiconductor laser at ~850 nm. Retinal imaging was demonstrated with a resolution of 10 mm, enabling an axial measurement range of ~2.5 mm. Real-time display and data streaming capabilities enable video-rate imaging of the retina at >30 frames per second. High-definition and three-dimensional imaging were demonstrated on normal retinas. The resolution of

Retinal imaging ranks amongst the most important clinical applications for optical coherence tomography (OCT) [1, 2]. The recent demonstration of increased sensitivity [3-6] in Fourier Domain detection [7, 8] has opened the way for dramatically higher imaging speeds, up to axial scan rates of several tens of kilohertz. However, these imaging speeds are still not sufficient for high density 3D datasets and a further increase to several hundreds of kilohertz is necessary. In this paper we demonstrate a swept laser source at 1050 nm with a sweep rate of 202 kHz. The laser source provides ~10 mW average output power, up to 60 nm total sweep range and a sensitivity roll off of less than 10 dB over 4 mm. In vivo 2D and 3D imaging of the human retina at a record axial scan rate of 101 kHz is demonstrated. These results suggest that swept source OCT has the potential to significantly outperform spectral/Fourier domain OCT for ophthalmic imaging applications in the future.

Non-invasive in vivo functional optical imaging is emonstrated using high-speed, ultrahigh resolution optical coherence tomography (UHR-OCT). A high-speed, UHR-OCT system using spectral/Fourier domain detection was developed for functional imaging experiments in the rodent retina. Using a spectrally multiplexed superluminescent diode light source, imaging was performed with 2.8 μm resolution at a rate of 24,000 axial scans per second. OCT measurement protocols were designed to minimize noise sources that cause undesired fluctuations in the measured OCT signal. A white light stimulus was applied to the retina and the average reflectivity from each intraretinal layer was monitored over time using OCT. A white light stimulus induces a response consisting of an increase in the reflectance of the photoreceptor outer segments. To our knowledge, this is the first in vivo demonstration of functional imaging using OCT in the retina. Further systematic investigation will be required to fully characterize the observed optical changes. Eventually, this may prove to be an objective method for measuring photoreceptor function in the human retina.

We present clinical data obtained with the aid of a novel instrumentation using, high speed, ultrahigh resolution Spectral/Fourier domain Optical Coherence Tomography (SOCT). This method allows performing video rate, ultrahigh resolution cross-sectional images with 98 dB of sensitivity, 100 times faster that previously reported UHR OCT system with comparable axial resolution of 3 um. Ultrahigh resolution imaging enables improved visualization of retinal architectural morphology compared to standard resolution OCT. High speed, ultrahigh resolution OCT using Spectral/Fourier domain detection promises to significantly enhance the utility of OCT for clinical applications. High speed imaging enables high density data sets to be acquired which can increase the quality of reconstructed cross-sectional images and can help to visualize small focal pathologic changes. This technique helps reconstructing true retinal topography without motion artifacts. Submitted manuscript describes the technology, its clinical performance and present preliminary data obtained for various retinal pathologies.

Ultrahigh resolution OCT using broadband light sources achieves improved axial image resolutions of ~2-3 um compared to standard 10 um resolution OCT used in current commercial instruments. High-speed OCT using Fourier/spectral domain detection enables dramatic increases in imaging speeds. 3D OCT retinal imaging is performed in human subjects using high-speed, ultrahigh resolution OCT, and the concept of an OCT fundus image is introduced. Three-dimensional data and high quality cross-sectional images of retinal pathologies are presented. These results show that 3D OCT may be used to improve coverage of the retina, precision of cross-sectional image registration, quality of cross-sectional images, and visualization of subtle changes in retinal topography. 3D OCT imaging and mapping promise to help elucidate the structural changes associated with retinal disease as well as to improve early diagnosis and monitoring of disease progression and response to treatment.

An ultrahigh resolution spectral domain optical coherence tomography (OCT) system capable of performing high speed imaging in the ophthalmology clinic has been developed. An OCT system using spectral/Fourier domain enables high speed imaging rates of up to 25,000 axial scans (A-scan) per second. Using a low threshold femtosecond Ti:sapphire laser light source, which can generate bandwidths of ~125 nm at 800 nm, cross-sectional imaging of the retina with ~3 μm axial resolution is possible. High speed imaging has been performed in the ophthalmology clinic on patients with various retinal pathologies using the ultrahigh resolution spectral domain OCT system. High pixel density OCT images containing 1024 pixels and 2048 transverse lines (A-scans) can be acquired in 0.08 seconds, which represents a ~100 fold improvement in imaging speed over previously reported time-domain ultrahigh resolution OCT systems. High speed imaging also enables three dimensional scanning and mapping of intraretinal architectural morphology with unprecedented resolution. High speed ultrahigh resolution OCT is a powerful tool for visualizing retinal pathologies, especially those involving the details of the photoreceptor segments; it will enable three-dimensional retinal imaging and the rendering of image information from volumetric data, and it has the potential to improve the early diagnosis of retinal diseases.

Leaky vasculature is a key feature in a number of retinal diseases such as diabetic retinopathy and age related macular degeneration and is commonly associated with neovascularization. Currently, the only way to identify leaky vasculature is through fluorescence angiography, which lacks depth resolution and the ability to precisely localize leaky vessels. Here we present the first 4D tracking of leaky vasculature in a mouse model of sub-retinal neovascularization using contrast-enhanced OCT. A very-low-density-lipoprotein receptor kockout mouse model was imaged with OCT angiography at multiple time points following intravenous injection of Intralipid 20%, an OCT contrast agent. Compared to healthy vessels, leaky vessels appeared to become broader over time. By fitting a model to mean intensity projection profiles, the apparent width of the vessels was quantified as an indicator of leakage. A clear trend of increased leakage following the injection of contrast was observed in vessels that derive from retinal lesions. This finding was likely caused by the infiltration of Intralipid particles into the surrounding retinal tissue. Intralipid is an ideal OCT contrast agent as it is FDA approved for human use as an intravenous nutritional supplement and is highly scattering, which makes it a strong candidate for future clinical translation. To summarize, we have demonstrated 4D tracking of individual leaky vessels for the first time using contrast-enhanced OCT in a mouse model of neovascularization. This technique improves upon the capabilities of fluorescence angiography and may help pave the way for clinical translation of contrast-enhanced OCT methods for enhanced diagnostic specificity.