We present here a study of blood oxygen saturation (sO2) in the parafoveal region from healthy subjects using visible light optical coherence tomography. Sixteen eyes of the normal subjects were analyzed. The sO2 of arterioles was significantly higher than the venules’ (92.1 ± 7.1 (vol %) for arterioles, 48.4 ± 5.0 (vol %) for venules, p<0.001), indicating that VIS-OCT can be a powerful tool to investigate the retinal sO2 in parafoveal micro-vessels in pathological conditions.
Current state-of-the-art human retinal vis-OCT systems have a limited field of view due to the large curvature of the retina and the system sensitivity roll-off. Here we presented the first linear-in-k visible range OCT spectrometer and achieved a sub-5dB roll-off over the entire imaging depth. The first full-range vis-OCT imaging was demonstrated to extend the system imaging depth. Thanks to 140 nm bandwidth in visible light range, the system axis resolution can achieve 1-2 um in air and a 65˚ by 65˚ wide-field human retinal full range vis-OCT imaging was demonstrated.
To evaluate the clinical utility of Visible and near infrared optical coherence tomography (vnOCT) for glaucoma early detection, A total of 55 eyes from three groups of subjects (normal subjects, glaucoma suspects, glaucoma patients) were scanned by a custom-designed vnOCT device and Zeiss Spectralis OCT. The peripapillary retinal nerve fiber layer reflectivity in visible light OCT and the ratio between visible and NIR channel is more sensitive in separating suspect eyes from normal ones than clinical OCT thickness measurements. It could be a useful metric in early detection of glaucoma upon further longitudinally validation.
LED array microscopy is an emerging platform for computational imaging . Existing LED array systems often exploit transmission imaging geometries of standard brightfield microscopes that leave the rich backscattered field undetected. Here, we develop an LED array reflectance microscope capturing the sample’s backscattered signal. In particular, we demonstrate multimodal brightfield, darkfield, and differential phase contrast imaging on fixed and living biological specimens including Caenorhabditis elegans (C. elegans), zebrafish embryos, and live cell cultures. Video-rate multimodal imaging records real-time features of freely-moving C. elegans and the fast beating heart of zebrafish embryo. Our new reflectance mode is a valuable addition to the LED array microscopic toolbox.
Although vis-OCT oximetry has been successfully demonstrated in vivo in human retina, it is so far limited on large vessels. To precisely locate the small vessels in depth dimension, OCTA is the natural method since it can effectively enhance the vessel contrast. In this paper, we demonstrate the first visible OCTA image in human retina, the small blood vessel can be located with high fidelity by using both the structural and angiographic contrasts. This technical advance lays a foundation for the absolute sO2 calculation in small vessels in human retina in order to assess local oxygen extraction and metabolism.
The retina, as part of the central nervous system, has distinct anatomical and structural properties for its visual function. Light scattering spectroscopy, while widely used for tissue structural characterization and disease diagnosis, has been relatively unexplored in the living retina. Recently, we have developed a fiber-based visible and near-infrared optical coherence tomography system (vnOCT) for in vivo retinal imaging, to uniquely measure a spectroscopic marker (VN ratio) sensitive to nanoscale pathological changes. In the present study, we applied vnOCT in an animal model of glaucoma (dexamethasone-induced ocular hypertension mouse) and tested the capabilities of four optical markers, VN ratio, peripapillary retinal nerve fiber layer (RNFL) thickness, total retinal blood flow, and hemoglobin oxygen saturation (sO2), for the detection of retinal ganglion cell (RGC) damage in association with ocular hypertension. We found that RNFL-RGC VN ratio and arteriovenous (A-V) sO2 are capable of detecting early retinal alteration in ocular hypertensive eyes, preceding measurable change of RNFL thickness. This study suggests a potential clinical application of vnOCT in early detection of glaucoma.
Despite the recent development of advanced ophthalmic imaging techniques, volumetric fluorescence angiography (vFA) over a large field of view is still lacking. Fundus photography techniques have significant limitations due to the lack of 3D imaging capability. Scanning laser ophthalmoscopy (SLO) and confocal SLO (cSLO) use confocal gating to remove diffused light, resulting in crisper image quality. However, the volumetric imaging of SLO requires to compile z stacks, which can be challenging and time-consuming. Adaptive optics SLO (AOSLO) allows diffraction-limited resolution in both axial and lateral resolution. This technique is limited however, by its small field of view (FOV) and also the necessity of z stacks for volumetric imaging. To fill the technical void of vFA over a large field of view (FOV), we developed a novel retinal imaging modality called oblique scanning laser ophthalmoscopy (oSLO) for in vivo volumetric fluorescence retinal imaging. By using oblique illumination and detection, oSLO essentially allows “OCT-like” cross-sectional images contributed solely by the fluorescent contrast, without the need for z stacking. We will demonstrate 3D vFA over a 30˚x30˚ FOV in vivo in mouse retina. We will further report a high-speed oSLO in imaging capillary hemodynamics. The new capability allows the calculation of capillary hematocrit and blood speed in 3D, which can be potentially valuable in diabetic retinopathy and macular degeneration.
Biological functions rely on local microvasculature for delivering oxygen and nutrients and carrying away metabolic waste. Alterations to local oxygenation level are manifested in diseases including cancer, diabetes mellitus, etc. The ability to in vivo quantify oxygen saturation (sO2) of single vessels down to capillary level to assess local tissue oxygenation and metabolic function is highly sought after. Visible light optical coherence tomography (vis-OCT) has shown promise in reaching this goal. However, to achieve reliable measurement in small vessels are challenging due to the reduced signal and requires data averaging to improve the spectral data quality. Therefore, a method to quality control the vis-OCT data from small vessels becomes essential to reject unreliable readings. In this work, we present a generalized method with several quantitative metrics to evaluate the spectral data for reliable sO2 measurements. Parameters of the scanning protocol and the statistical data cleaning can be flexibly adjusted according to different applications and system performances. We used this method to measure sO2 of C57BL/6J mice lower extremity microvasculature and validated it via introducing hyperoxia for expected sO2 changes. After validation, we applied this method on C57BL/J mouse ear microvasculature to conduct in vivo single capillary OCT oximetry. This work intends to standardize the data processing method for in vivo oximetry in small vessels by vis-OCT.
Elastic light scattering spectroscopy (ELSS) has been proven as a powerful tool in characterizing tissue native structures with superb sensitivity. As a widely used technique, optical coherence tomography (OCT) would have been well suited for ELSS measurement by using a broadband light source. However, OCT-based ELSS is largely hampered by the limited k-space spectral bandwidth from all existing OCT systems. To overcome this barrier, we report a simple all fiber-based setup to implement dual-channel visible and near infrared (NIR) optical coherence tomography (vnOCT) for human retinal imaging, bridging over 300nm spectral gap. Remarkably, we discovered a newly available fiber that supports single-mode propagation and maintains high interference efficiency for both visible and NIR light with fringe visibility of 97% and 90%, respectively, which was previously considered impossible to use the same fiber components for such a broad range of wavelengths. Longitudinal chromatic aberration from the eye is corrected by a custom-designed achromatizing lens. As retinal imaging being an important OCT application, we demonstrated vnOCT on human retina and further developed robust ELSS analysis method to quantify spectroscopic contrast in several import layers of human retina. This vnOCT platform and method of ELSS analysis open new opportunities in understanding structure-function relationship in the human retina and in exploring new biomarkers for retinal diseases.
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