Investigation of artifacts in retinal and choroidal OCT angiography with a contrast agent

Birefringence-derived scleral artifacts in optical coherence tomography images of eyes with pathologic myopia

We investigated birefringence-derived scleral artifacts in optical coherence tomography (OCT) images of eyes with pathologic myopia. This study included 76 eyes of 42 patients with pathologic myopia. Five sets of OCT B-scan images of the macula were obtained using commercial swept-source OCT. A dataset of prototype swept-source polarization-diversity OCT images was used to identify polarization-dependent OCT images (i.e., complex averaging of OCT signals from two polarization channels) and polarization-independent OCT images (i.e., intensity averaging of two OCT signals). Polarization-dependent OCT images and commercial OCT images were assessed for the presence of birefringence-derived artifacts by comparison with polarization-independent OCT images. Both polarization-dependent OCT images and commercial OCT images contained scleral vessel artifacts. Scleral vessel artifacts were present in 46 of 76 eyes (60.5%) imaged by polarization-dependent OCT and 17 of 76 eyes (22.4%) imaged by commercial OCT. The proportion of images that showed scleral vessel artifacts was significantly greater among polarization-dependent OCT images than among commercial OCT images (P < 0.001). Additionally, polarization-dependent OCT images showed low-intensity band artifacts. This study demonstrated the existence of birefringence-derived scleral artifacts in commercial OCT images and indicated that polarization-diversity OCT is an effective tool to evaluate the presence of these artifacts.

Morphometric and Microstructural Changes During Murine Retinal Development Characterized Using In Vivo Optical Coherence Tomography

Purpose

The purpose of this study was to develop an in vivo optical coherence tomography (OCT) system capable of imaging the developing mouse retina and its associated morphometric and microstructural changes.

Methods

Thirty-four wild-type mice (129S1/SvlmJ) were anesthetized and imaged between postnatal (P) day 7 and P21. OCT instrumentation was developed to optimize signal intensity and image quality. Semi-automatic segmentation tools were developed to quantify the retinal thickness of the nerve fiber layer (NFL), inner plexiform layer (IPL), inner nuclear layer (INL), and the outer retinal layers (ORL), in addition to the total retina. The retinal maturation was characterized by comparing layer thicknesses between consecutive time points.

Results

From P7 to P10, the IPL increased significantly, consistent with retinal synaptogenesis. From P10 to P12, the IPL and ORL also increased, which is coherent with synaptic connectivity and photoreceptor maturation. In contrast, during these periods, the INL decreased significantly, consistent with cellular densification and selective apoptotic “pruning” of the tissue during nuclear migration. Thereafter from P12 to P21, the INL continued to thin (significantly from P17 to P21) whereas the other layers remained unchanged. No time-dependent changes were observed in the NFL. Overall, changes in the total retina were attributed to those in the IPL, INL, and ORL. Regions of the retina adjacent to the optic nerve head were thinner than distal regions during maturation.

Conclusions

Changes in retinal layer thickness are consistent with retinal developmental mechanisms. Accordingly, this report opens new horizons in using our system in the mouse to characterize longitudinally developmental digressions in models of human diseases.

Signal properties of split-spectrum amplitude decorrelation angiography for quantitative optical coherence tomography-based velocimetry

Split-spectrum amplitude-decorrelation angiography (SSADA) is a noninvasive and three-dimensional angiographic technique with a microscale spatial resolution based on optical coherence tomography. The SSADA signal is known to be correlated with the blood flow velocity and the quantitative velocimetry with SSADA has been expected; however, the signal properties of SSADA are not completely understood due to lack of comprehensive investigations of parameters related to SSADA signals. In this study, phantom experiments were performed to comprehensively investigate the relation of SSADA signals with flow velocities, time separations, particle concentrations, signal-to-noise ratios, beam spot sizes, and viscosities, and revealed that SSADA signals reflect the spatial commonality within a coherence volume between adjacent A-scans.

Artifacts in Optical Coherence Tomography Angiography

We performed a comprehensive search of the published literature in PubMed and Google Scholar to identify types, prevalence, etiology, clinical impact, and current methods for correction of various artifacts in optical coherence tomography angiography (OCTA) images. We found that the prevalence of OCTA image artifacts is fairly high. Artifacts associated with eye motion, misidentification of retinal layers, projections, and low optical coherence tomography signal are the most prevalent types. Artifacts in OCTA images are the major limitations of this diagnostic modality in clinical practice and identification of these artifacts and measures to mitigate them are essential for correct diagnosis and follow-up of patients.

High-resolution, depth-resolved vascular leakage measurements using contrast-enhanced, correlation-gated optical coherence tomography in mice

Vascular leakage plays a key role in vision-threatening retinal diseases such as diabetic retinopathy and age-related macular degeneration. Fluorescence angiography is the current gold standard for identification of leaky vasculature in vivo, however it lacks depth resolution, providing only 2D images that complicate precise identification and localization of pathological vessels. Optical coherence tomography (OCT) has been widely adopted for clinical ophthalmology due to its high, micron-scale resolution and rapid volumetric scanning capabilities. Nevertheless, OCT cannot currently identify leaky blood vessels. To address this need, we have developed a new method called exogenous contrast-enhanced leakage OCT (ExCEL-OCT) which identifies the diffusion of tracer particles around leaky vasculature following injection of a contrast agent. We apply this method to a mouse model of retinal neovascularization and demonstrate high-resolution 3D vascular leakage measurements in vivo for the first time.

Advances in multimodal imaging in ophthalmology

Multimodality ophthalmic imaging systems aim to enhance the contrast, resolution, and functionality of existing technologies to improve disease diagnostics and therapeutic guidance. These systems include advanced acquisition and post-processing methods using optical coherence tomography (OCT), combined scanning laser ophthalmoscopy and OCT systems, adaptive optics, surgical guidance, and photoacoustic technologies. Here, we provide an overview of these ophthalmic imaging systems and their clinical and basic science applications.

Choroidal neovascularization fusion visualization for spectral-domain optical coherence tomography and optical coherence tomography angiography images

PURPOSE

The spectral-domain optical coherence tomography (SD-OCT) images and OCT angiography (OCTA) images can provide complementary information for choroidal neovascularization (CNV) visualization. We expected to simultaneously display multifaceted characteristics of CNV in a single projection image.

METHODS

We proposed a fusion method for CNV visualization by combining structural and angiographic images, which mainly involves four steps: (a) Generate SD-OCT and OCTA projection images from original volumetric data with retinal layer restriction. (b) For SD-OCT projection images, enhance retinal vessels and CNV. (c) For OCTA images, detect CNV region based on multimodal data and display the neovascularization in false color. (d) A maximum fusion strategy was adopted to generate the fused images.

RESULTS

Experimental results with 30 cases from 30 patients demonstrate that the fused images are more effective in displaying CNV than single-modality projection images. The average information entropy and the mean gradient in the CNV regions for SD-OCT projection images, OCTA images, and the fusion images are 4.66/0.21, 5.45/0.45, and 6.8/0.58, respectively.

CONCLUSIONS

The proposed method is more effective for CNV visualization than the conventional single-modality image-based method. The proposed method can combine complementary information from multimodal images and provide a satisfying visual effect.

Use of the Retinal Vascular Histology to Validate an Optical Coherence Tomography Angiography Technique

Purpose

To determine the fidelity of optical coherence tomography angiography (OCTA) techniques by direct comparison of the retinal capillary network images obtained from the same region as imaged by OCTA and high-resolution confocal microscope.

Method

Ten porcine eyes were perfused with red blood cells for OCTA image acquisition from the area centralis and then perfusion-fixed, and the vessels were labeled for confocal imaging. Two approaches involving post-processing of two-dimensional projection images and vessel tracking on three dimensional image stacks were used to obtain quantitative measurements. Data collected include vessel density, length of visible vessel track, count of visible branch points, vessel track depth, vessel diameter, angle of vessel descent, and angle of dive for comparison and analysis.

Results

Comparing vascular images acquired from OCTA and confocal microscopy, we found (1) a good representation of the larger caliber retinal vessels, (2) an underrepresentation of retinal microvessels smaller than 10 µm and branch points in all four retinal vascular plexuses, particularly the intermediate capillary plexus, (3) reduced visibility associated with an increase in the angle of descent, (4) a tendency to loss visibility of vessel track at a branch point or during a sharp dive, and (5) a reduction in visibility with increase in retinal depth on OCTA images.

Conclusions

Current OCTA techniques can visualize the retinal capillary network, but some types of capillaries cannot be detected by OCTA, particularly in the middle to deeper layers.

Translational Relevance

The information indicates the limitation in clinical use and scopes for improvement in the current OCTA technologies.

Vascular morphology and blood flow signatures for differential artery-vein analysis in optical coherence tomography of the retina

Differential artery-vein (AV) analysis is essential for retinal study, disease detection, and treatment assessment. This study is to characterize vascular reflectance profiles and blood flow patterns of retinal artery and vein systems in optical coherence tomography (OCT) and OCT angiography (OCTA), and establish them as robust signatures for objective AV classification. A custom designed OCT was employed for three-dimensional (3D) imaging of mouse retina, and corresponding OCTA was reconstructed. Radially resliced OCT B-scans revealed two, i.e. top and bottom, hyperreflective wall boundaries in retinal arteries, while these wall boundaries were absent in OCT of retinal veins. Additional OCTA analysis consistently displayed a layered speckle distribution in the vein, which may indicate the venous laminar flow. These OCT and OCTA differences offer unique signatures for objective AV classification in OCT and OCTA.

Visibility of microvessels in Optical Coherence Tomography angiography depends on angular orientation

Optical Coherence Tomography angiography (OCTA) is a widespread tool for depth-resolved imaging of chorioretinal vasculature with single microvessel resolution. To improve the clinical interpretation of OCTA, the conditions affecting visualization of microvessels must be defined. Here we inject a scattering plasma tracer (Intralipid) during OCTA imaging of the anesthetized rat eye. In the retina, we find that interlaminar (vertical) vessels that connect laminae have one-fourth to one-third the OCTA red blood cell to tracer (RBC-to-tracer) signal ratio of intralaminar (horizontal) vessels. This finding suggests that the OCTA signal from microvessels depends on angular orientation, making vertically-oriented vessels more difficult to visualize using intrinsic contrast alone. Clinicians should be aware of this potential artifact when interpreting OCTA.

Approaches to quantify optical coherence tomography angiography metrics

Optical coherence tomography (OCT) has revolutionized the field of ophthalmology in the last three decades. As an OCT extension, OCT angiography (OCTA) utilizes a fast OCT system to detect motion contrast in ocular tissue and provides a three-dimensional representation of the ocular vasculature in a non-invasive, dye-free manner. The first OCT machine equipped with OCTA function was approved by U.S. Food and Drug Administration in 2016 and now it is widely applied in clinics. To date, numerous methods have been developed to aid OCTA interpretation and quantification. In this review, we focused on the workflow of OCTA-based interpretation, beginning from the generation of the OCTA images using signal decorrelation, which we divided into intensity-based, phase-based and phasor-based methods. We further discussed methods used to address image artifacts that are commonly observed in clinical settings, to the algorithms for image enhancement, binarization, and OCTA metrics extraction. We believe a better grasp of these technical aspects of OCTA will enhance the understanding of the technology and its potential application in disease diagnosis and management. Moreover, future studies will also explore the use of ocular OCTA as a window to link ocular vasculature to the function of other organs such as the kidney and brain.

Analysis of longitudinal sections of retinal vessels using Doppler OCT

We present a new method for imaging retinal vessels that provides both structural and hemodynamic information. Our technique is based on a single beam OCT system with an integrated retinal tracker that enables recording of arbitrary scan patterns. We record longitudinal sections along the traces of retinal vessels. The tracker function enables the acquisition of multiple longitudinal sections along the same trace to provide high-quality averaged OCT scans as well as temporal changes of flow dynamics. The vessel walls are clearly identified as narrow, bright lines from which the vessel diameter can be retrieved as a function of position along the vessel. Furthermore, the Doppler angle can be obtained at each position along the vessel trace, enabling measurement of absolute blood flow by Doppler OCT analysis. The method is demonstrated in flow phantoms and in-vivo on retinal vessel bifurcations in healthy volunteers. In 7 of 9 imaged bifurcations, measured in- and outflow deviate by less than 11%, demonstrating the consistency of the method.

Optical coherence tomography velocimetry based on decorrelation estimation of phasor pair ratios (DEPPAIR)

Quantitative velocity estimations in optical coherence tomography requires the estimation of the axial and lateral flow components. Optical coherence tomography measures the depth resolved complex field reflected from a sample. While the axial velocity component can be determined from the Doppler shift or phase shift between a pair of consecutive measurements at the same location, the estimation of the lateral component for in vivo applications is still challenging. One approach to determine lateral velocity is multiple simultaneous measurements at different angles. In another approach the lateral component can be retrieved through repeated measurements at (nearly) the same location by an analysis of the decorrelation over time. In this paper we follow the latter approach. We describe a model for the complex field changes between consecutive measurements and use it to predict the uncertainties for amplitude-based, phase-based and complex algorithms. The uncertainty of the flow estimations follows from a statistical analysis and is determined by the number of available measurements and the applied analysis method. The model is verified in phantom measurements and the dynamic range of velocity estimations is investigated. We demonstrate that phase-based and complex (phasor) based lateral flow estimation methods are superior to amplitude-based algorithms.

Temporal speckle-averaging of optical coherence tomography volumes for in-vivo cellular resolution neuronal and vascular retinal imaging

It has been recently demonstrated that structures corresponding to the cell bodies of highly transparent cells in the retinal ganglion cell layer could be visualized noninvasively in the living human eye by optical coherence tomography (OCT) via temporal averaging. Inspired by this development, we explored the application of volumetric temporal averaging in mice, which are important models for studying human retinal diseases and therapeutic interventions. A general framework of temporal speckle-averaging (TSA) of OCT and optical coherence tomography angiography (OCTA) is presented and applied to mouse retinal volumetric data. Based on the image analysis, the eyes of mice under anesthesia exhibit only minor motions, corresponding to lateral displacements of a few micrometers and rotations of a fraction of 1 deg. Moreover, due to reduced eye movements under anesthesia, there is a negligible amount of motion artifacts within the volumes that need to be corrected to achieve volume coregistration. In addition, the relatively good optical quality of the mouse ocular media allows for cellular-resolution imaging without adaptive optics (AO), greatly simplifying the experimental system, making the proposed framework feasible for large studies. The TSA OCT and TSA OCTA results provide rich information about new structures previously not visualized in living mice with non-AO-OCT. The mechanism of TSA relies on improving signal-to-noise ratio as well as efficient suppression of speckle contrast due to temporal decorrelation of the speckle patterns, enabling full utilization of the high volumetric resolution offered by OCT and OCTA.

Noninvasive, in vivo rodent brain optical coherence tomography at 2.1 microns

In biological tissue, longer near-infrared wavelengths generally experience less scattering and more water absorption. Here we demonstrate an optical coherence tomography (OCT) system centered at 2.1 microns, whose bandwidth falls in the 2.2 micron water absorption optical window, for in vivo imaging of the rodent brain. We show in vivo that at 2.1 microns, the OCT signal is actually attenuated less in cranial bone than at 1.3 microns, and is also less susceptible to multiple scattering tails. We also show that the 2.2 micron window enables direct spectroscopic OCT assessment of tissue water content. We conclude that with further optimization, 2.2 micron OCT will have advantages in low-water-content tissue such as bone, as well as applications where extensive averaging is possible to compensate absorption losses.

Non-invasive cellular-resolution retinal imaging with two-photon excited fluorescence

Two-photon excited fluorescence (TPEF) imaging of the retina is a developing technique that provides non-invasive compound-specific measurements from the retina. In this report, we demonstrate high-resolution TPEF imaging of the mouse retina using sensorless adaptive optics (SAO) and optical coherence tomography (OCT). A single near-infrared light source was used for simultaneous multi-modal imaging with OCT and TPEF. The image-based SAO could be performed using the en face OCT or the TPEF for aberration correction. Our results demonstrate OCT and TPEF for angiography. Also, we demonstrate non-invasive cellular-resolution imaging of fluorescently labelled cells and the Retinal Pigment Epithelium (RPE) mosaic.

Investigating spontaneous retinal venous pulsation using Doppler optical coherence tomography

We demonstrate the advantages of optical coherence tomography (OCT) imaging for investigation of spontaneous retinal venous pulsation (SRVP). The pulsatile changes in venous vessel caliber are analyzed qualitatively and quantitatively using conventional intensity-based OCT as well as the functional extension Doppler OCT (DOCT). Single-channel and double-channel line scanning protocols of our multi-channel OCT prototype are employed to investigate venous pulsatile caliber oscillations as well as venous flow pulsatility in the eyes of healthy volunteers. A comparison to recordings of scanning laser ophthalmoscopy (SLO) – a standard en-face imaging modality for evaluation of SRVP – is provided, emphasizing the advantages of tomographic image acquisition. To the best of our knowledge, this is the first quantitative time-resolved investigation of SRVP and associated retinal perfusion characteristics using OCT.

Measurement of flow-mediated dilation of mouse femoral artery in vivo by optical coherence tomography

Flow-mediated vasodilation (FMD) is used for assessment of vascular endothelial function in humans as a predictor of cardiovascular events. It has been challenging to carry it on preclinical murine models due to the diminutive size of the femoral artery. Here, we present a new approach to accurately measure the blood velocity and femoral artery diameters of mice by acquiring Doppler optical coherence tomography and optical coherence tomography angiography continuously within 1 single experimental scanning protocol. Using the 3-dimensional imaging and new velocity algorithm, the measurement precision of diameter, blood flow, velocity and wall shear stress are improved to 0.91%, 11.0%, 10.7% and 14.0%, respectively. FMD of healthy mouse femoral artery measured by this method was 11.96% ± 0.98%, which was blunted to 5.69% ± 0.4% by intravenous administration of endothelial nitric oxide synthase inhibitor (L-NG -Nitroarginine methyl ester), in agreement with that reported in the literature.

Principles of OCTA and Applications in Clinical Neurology

PURPOSE OF REVIEW

The article reviews the recent findings on the use of optical coherence tomography angiography (OCTA) in neurology.

RECENT FINDINGS

OCTA is a new addition to the powerful and complementary technology of the OCT. Due to its noninvasiveness, and reproducibility, it is possible to obtain high-resolution 3D images of the vessels of the human eye. As the vessels of the retina with the presence of endothelial cell's tight junctions resemble the brain vessels, it was hypothesized that the imaging of the retinal vessels might bring insight into brain vessels. OCTA has been effectively used to predict retinal vessel abnormalities in dementia, demyelization, optic disc neuropathies, and inherited degenerative diseases. Most common findings were decrease of vascular density and flow and an increase of avascular zones. Although OCTA is a relative new technology, recent studies show that it can be successfully applied in neurology.

Enhancement of morphological and vascular features in OCT images using a modified Bayesian residual transform

A novel image processing algorithm based on a modified Bayesian residual transform (MBRT) was developed for the enhancement of morphological and vascular features in optical coherence tomography (OCT) and OCT angiography (OCTA) images. The MBRT algorithm decomposes the original OCT image into multiple residual images, where each image presents information at a unique scale. Scale selective residual adaptation is used subsequently to enhance morphological features of interest, such as blood vessels and tissue layers, and to suppress irrelevant image features such as noise and motion artefacts. The performance of the proposed MBRT algorithm was tested on a series of cross-sectional and enface OCT and OCTA images of retina and brain tissue that were acquired in-vivo. Results show that the MBRT reduces speckle noise and motion-related imaging artefacts locally, thus improving significantly the contrast and visibility of morphological features in the OCT and OCTA images.