Morgan Heisler

Quantitative Noninvasive Angiography of the Fovea Centralis Using Speckle Variance Optical Coherence Tomography

PURPOSE To demonstrate the utility of speckle variance optical coherence tomography (svOCT), a noninvasive angiographic technique, for evaluating the foveal vasculature. METHODS Twelve normal human eyes were imaged with svOCT (1060-nm, 100-kHz custom-built system) and fluorescein angiography (FA; Topcon TRC-50DX with 5.0 megapixel resolution camera). Manual tracing techniques were used to quantify the foveal vasculature, including foveal avascular zone (FAZ) metrics (area, perimeter, greatest diameter, and lowest diameter). Reproducibility of these measurements was determined. The FAZ was imaged in 25 normal eyes using svOCT and 15 donor eyes using confocal scanning laser microscopy. Retinal capillary plexuses in donor eyes were perfusion-labeled with phalloidin conjugated to Alexa Fluor 546. RESULTS Speckle variance OCT is able to stratify the foveal circulation into inner and deep capillary plexuses as well as reliably quantify and assess the morphometric dimensions of the human FAZ. Capillary density measurements were significantly greater in svOCT than FA (31.2 ± 1.6% vs. 19.3 ± 1.9% of total tissue area; P < 0.001). Measurements were highly reproducible (all P > 0.366). All FAZ metrics were significantly lower in histology than svOCT (all P < 0.001). CONCLUSIONS Speckle variance OCT permits precise, reproducible, and noninvasive visualization of the human foveal vasculature. Speckle variance OCT may become an important adjunct in evaluating patients with retinal vascular diseases.

Segmentation of the foveal microvasculature using deep learning networks

Abstract. Accurate segmentation of the retinal microvasculature is a critical step in the quantitative analysis of the retinal circulation, which can be an important marker in evaluating the severity of retinal diseases. As manual segmentation remains the gold standard for segmentation of optical coherence tomography angiography (OCT-A) images, we present a method for automating the segmentation of OCT-A images using deep neural networks (DNNs). Eighty OCT-A images of the foveal region in 12 eyes from 6 healthy volunteers were acquired using a prototype OCT-A system and subsequently manually segmented. The automated segmentation of the blood vessels in the OCT-A images was then performed by classifying each pixel into vessel or nonvessel class using deep convolutional neural networks. When the automated results were compared against the manual segmentation results, a maximum mean accuracy of 0.83 was obtained. When the automated results were compared with inter and intrarater accuracies, the automated results were shown to be comparable to the human raters suggesting that segmentation using DNNs is comparable to a second manual rater. As manually segmenting the retinal microvasculature is a tedious task, having a reliable automated output such as automated segmentation by DNNs, is an important step in creating an automated output.

Lens-based wavefront sensorless adaptive optics swept source OCT.

Optical coherence tomography (OCT) has revolutionized modern ophthalmology, providing depth resolved images of the retinal layers in a system that is suited to a clinical environment. Although the axial resolution of OCT system, which is a function of the light source bandwidth, is sufficient to resolve retinal features at a micrometer scale, the lateral resolution is dependent on the delivery optics and is limited by ocular aberrations. Through the combination of wavefront sensorless adaptive optics and the use of dual deformable transmissive optical elements, we present a compact lens-based OCT system at an imaging wavelength of 1060 nm for high resolution retinal imaging. We utilized a commercially available variable focal length lens to correct for a wide range of defocus commonly found in patient’s eyes, and a novel multi-actuator adaptive lens for aberration correction to achieve near diffraction limited imaging performance at the retina. With a parallel processing computational platform, high resolution cross-sectional and en face retinal image acquisition and display was performed in real time. In order to demonstrate the system functionality and clinical utility, we present images of the photoreceptor cone mosaic and other retinal layers acquired in vivo from research subjects.

Wavefront sensorless adaptive optics OCT with the DONE algorithm for in vivo human retinal imaging [Invited]

In this report, which is an international collaboration of OCT, adaptive optics, and control research, we demonstrate the Data-based Online Nonlinear Extremum-seeker (DONE) algorithm to guide the image based optimization for wavefront sensorless adaptive optics (WFSL-AO) OCT for in vivo human retinal imaging. The ocular aberrations were corrected using a multi-actuator adaptive lens after linearization of the hysteresis in the piezoelectric actuators. The DONE algorithm succeeded in drastically improving image quality and the OCT signal intensity, up to a factor seven, while achieving a computational time of 1 ms per iteration, making it applicable for many high speed applications. We demonstrate the correction of five aberrations using 70 iterations of the DONE algorithm performed over 2.8 s of continuous volumetric OCT acquisition. Data acquired from an imaging phantom and in vivo from human research volunteers are presented.

Multiscale sensorless adaptive optics OCT angiography system for in vivo human retinal imaging

Abstract. We present a multiscale sensorless adaptive optics (SAO) OCT system capable of imaging retinal structure and vasculature with various fields-of-view (FOV) and resolutions. Using a single deformable mirror and exploiting the polarization properties of light, the SAO-OCT-A was implemented in a compact and easy to operate system. With the ability to adjust the beam diameter at the pupil, retinal imaging was demonstrated at two different numerical apertures with the same system. The general morphological structure and retinal vasculature could be observed with a few tens of micrometer-scale lateral resolution with conventional OCT and OCT-A scanning protocols with a 1.7-mm-diameter beam incident at the pupil and a large FOV (15 deg× 15 deg). Changing the system to a higher numerical aperture with a 5.0-mm-diameter beam incident at the pupil and the SAO aberration correction, the FOV was reduced to 3 deg× 3 deg for fine detailed imaging of morphological structure and microvasculature such as the photoreceptor mosaic and capillaries. Multiscale functional SAO-OCT imaging was performed on four healthy subjects, demonstrating its functionality and potential for clinical utility.

Strip-based registration of serially acquired optical coherence tomography angiography

Abstract. The visibility of retinal microvasculature in optical coherence tomography angiography (OCT-A) images is negatively affected by the small dimension of the capillaries, pulsatile blood flow, and motion artifacts. Serial acquisition and time-averaging of multiple OCT-A images can enhance the definition of the capillaries and result in repeatable and consistent visualization. We demonstrate an automated method for registration and averaging of serially acquired OCT-A images. Ten OCT-A volumes from six normal control subjects were acquired using our prototype 1060-nm swept source OCT system. The volumes were divided into microsaccade-free en face angiogram strips, which were affine registered using scale-invariant feature transform keypoints, followed by nonrigid registration by pixel-wise local neighborhood matching. The resulting averaged images were presented of all the retinal layers combined, as well as in the superficial and deep plexus layers separately. The contrast-to-noise ratio and signal-to-noise ratio of the angiograms with all retinal layers (reported as average±standard deviation) increased from 0.52±0.22 and 19.58±4.04  dB for a single image to 0.77±0.25 and 25.05±4.73  dB, respectively, for the serially acquired images after registration and averaging. The improved visualization of the capillaries can enable robust quantification and study of minute changes in retinal microvasculature.

Quantitative comparisons between optical coherence tomography angiography and matched histology in the human eye.

ABSTRACT The aim was to quantitatively compare retinal vascular detail as seen on optical coherence tomography angiography (OCTA) and matched histology in the human eye. 13 normal human donor eyes were used. The central retinal artery was cannulated after which human packed red blood cells were perfused through the retinal vasculature. Retinal vessels were imaged using a custom‐built OCTA device during red blood cell perfusion. The eye was subsequently perfused with endothelial cell antibodies and the flat‐mounted retina studied histologically using a confocal scanning laser microscope. Qualitative and quantitative comparisons of retinal vascular information as seen on OCTA and histology from the same region of interest were performed. Gradable OCTA images were acquired from 4 of 13 eyes with mean postmortem‐to‐OCTA imaging time of 4.5 ± 1.3 h 23 pairs of OCTA‐histology matched images were evaluated. The retinal arteries and veins had similar pixel intensity on OCTA images. The diameter of retinal veins was significantly greater than its paired artery on OCTA (P < 0.001). The density of vascular structures on OCTA (40.2% ± 10.1%) was significantly less than matched histology (52.1% ± 9.3%, P < 0.001). Mean capillary diameter on OCTA (10.2 ± 2.4 &mgr;m) was significantly greater than histology (8.2 ± 2.4 &mgr;m; P < 0.001). This is the first study to directly compare OCTA against histology from the same human eye. OCTA visualizes many of the vascular structures in the human retinal circulation but does not exactly match what is seen on histologic examination. HighlightsDensity of retinal vessels was significantly greater on micro‐perfusion labeled histology compared to OCTA.Vessel density is greater on histology than OCTA for which intervening vascular beds between large vessels were sometimes not visualized.Arteries and veins could not be distinguished on OCTA using pixel intensity. Veins generally had larger diameter than their paired arteries.

Quantifying Variability in Longitudinal Peripapillary RNFL and Choroidal Layer Thickness Using Surface Based Registration of OCT Images

Purpose To assess within-subject variability of retinal nerve fiber layer (RNFL) and choroidal layer thickness in longitudinal repeat optical coherence tomography (OCT) images with point-to-point measurement comparison made using nonrigid surface registration. Methods Nine repeat peripapillary OCT images were acquired over 3 weeks from 12 eyes of 6 young, healthy subjects using a 1060-nm prototype swept-source device. The RNFL, choroid and the Bruchs membrane opening (BMO) were segmented, and point-wise layer thicknesses and BMO dimensions were measured. For each eye, the layer surfaces of eight follow-up images were registered to those of the baseline image, first by rigid alignment using blood vessel projections and axial height and tilt correction, followed by nonrigid registration of currents-based diffeomorphisms algorithms. This mapped all follow-up measurements point-wise to the common baseline coordinate system, allowing for point-wise statistical analysis. Measurement variability was evaluated point-wise for layer thicknesses and BMO dimensions by time-standard deviation (tSD). Results The intraclass correlation coefficients (ICCs) of BMO area and eccentricity were 0.993 and 0.972, respectively. Time-mean and tSD were computed point-wise for RNFL and choroidal thickness and color-mapped on the baseline surfaces. tSD was less than two coherence lengths of the system 2ℓ = 12 μm at most vertices. High RNFL thickness variability corresponded to the locations of retinal vessels, and choroidal thickness varied more than RNFL thickness. Conclusions Our registration-based end-to-end pipeline produced point-wise correspondence among time-series retinal and choroidal surfaces with high measurement repeatability (low variability). Blood vessels were found to be the main sources contributing to the normal variability of the RNFL thickness measure. The computational pipeline with a measurement of normal variability can be used in future longitudinal studies to identify changes that are above the threshold of normal point-wise variability and track localized changes in retinal layers in high spatial resolution. Translational Relevance Using the registration-based approach presented in this study, longitudinal changes in retinal and choroidal layers can be detected with higher sensitivity and spatial precision.

Anterior Segment Optical Coherence Tomography for Targeted Transconjunctival Suture Placement in Overfiltering Trabeculectomy Blebs.

Purpose: To demonstrate the utility of swept-source anterior segment optical coherence tomography (SS-AS-OCT) in guiding placement of transconjunctival sutures in hypotonous patients after a trabeculectomy. Patients and Methods: This is a longitudinal case series of 10 eyes from 10 patients who required transconjunctival sutures after a trabeculectomy. SS-AS-OCT was used to aid in the placement of the sutures to improve the function of the overfiltering bleb. Results: SS-AS-OCT reliably identified localized areas of overfiltering, allowing for targeted suture placement in 8 eyes. The 2 eyes in which localized areas of overfiltering were not found required further surgical intervention. Conclusions: SS-AS-OCT enhances transconjunctival suturing for overfiltering blebs when focal fluid accumulation is visualized.

Label-free volumetric imaging of conjunctival collecting lymphatics ex vivo by optical coherence tomography lymphangiography

We employ optical coherence tomography (OCT) and optical coherence microscopy (OCM) to study conjunctival lymphatics in porcine eyes ex vivo. This study is a precursor to the development of in vivo imaging of the collecting lymphatics for potentially guiding and monitoring glaucoma filtration surgery. OCT scans at 1300 nm and higher-resolution OCM scans at 785 nm reveal the lymphatic vessels via their optical transparency. Equivalent signal characteristics are also observed from blood vessels largely free of blood (and devoid of flow) in the ex vivo conjunctiva. In our lymphangiography, vessel networks were segmented by compensating the depth attenuation in the volumetric OCT/OCM signal, projecting the minimum intensity in two dimensions and thresholding to generate a three-dimensional vessel volume. Vessel segmentation from multiple locations of a range of porcine eyes (n = 21) enables visualization of the vessel networks and indicates the varying spatial distribution of patent lymphatics. Such visualization provides a new tool to investigate conjunctival vessels in tissue ex vivo without need for histological tissue processing and a valuable reference on vessel morphology for the in vivo label-free imaging studies of lymphatics to follow.