Three-dimensional visualization of choroidal vessels by using standard and ultra-high resolution scattering optical coherence angiography

In Vivo Capillary Structure and Blood Cell Flux in the Normal and Diabetic Mouse Eye

Purpose

To characterize the early structural and functional changes in the retinal microvasculature in response to hyperglycemia in the Ins2^Akita mouse.

Methods

A custom phase-contrast adaptive optics scanning light ophthalmoscope was used to image retinal capillaries of 9 Ins2^Akita positive (hyperglycemic) and 9 Ins2^Akita negative (euglycemic) mice from postnatal weeks 5 to 18. A 15 kHz point scan was used to image capillaries and measure red blood cell flux at biweekly intervals; measurements were performed manually. Retinal thickness and fundus photos were captured monthly using a commercial scanning laser ophthalmoscope/optical coherence tomography. Retinal thickness was calculated using a custom algorithm. Blood glucose and weight were tracked throughout the duration of the study.

Results

Elevated blood glucose (>250 mg/dL) was observed at 4 to 5 weeks of age in Ins2^Akita mice and remained elevated throughout the study, whereas euglycemic littermates maintained normal glucose levels. There was no significant difference in red blood cell flux, capillary anatomy, lumen diameter, or occurrence of stalled capillaries between hyperglycemic and euglycemic mice between postnatal weeks 5 and 18. Hyperglycemic mice had a thinner retina than euglycemic littermates (p < 0.001), but retinal thickness did not change with duration of hyperglycemia despite glucose levels that were more than twice times normal.

Conclusions

In early stages of hyperglycemia, retinal microvasculature structure (lumen diameter, capillary anatomy) and function (red blood cell flux, capillary perfusion) were not impaired despite 3 months of chronically elevated blood glucose. These findings suggest that hyperglycemia alone for 3 months does not alter capillary structure or function in profoundly hyperglycemic mice.

Three-dimensional segmentation and depth-encoded visualization of choroidal vasculature using swept-source optical coherence tomography

The choroid provides nutritional support for the retinal pigment epithelium and photoreceptors. Choroidal dysfunction plays a major role in several of the most important causes of vision loss including age-related macular degeneration, myopic degeneration, and pachychoroid diseases such as central serous chorioretinopathy and polypoidal choroidal vasculopathy. We describe an imaging technique using depth-resolved swept-source optical coherence tomography (SS-OCT) that provides full-thickness three-dimensional (3D) visualization of choroidal anatomy including topographical features of individual vessels. Enrolled subjects with different clinical manifestations within the pachychoroid disease spectrum underwent 15 mm × 9 mm volume scans centered on the fovea. A fully automated method segmented the choroidal vessels using their hyporeflective lumens. Binarized choroidal vessels were rendered in a 3D viewer as a vascular network within a choroidal slab. The network of choroidal vessels was color depth-encoded with a reference to the Bruch's membrane segmentation. Topographical features of the choroidal vasculature were characterized and compared with choroidal imaging obtained with indocyanine green angiography (ICGA) from the same subject. The en face SS-OCT projections of the larger choroid vessels closely resembled to that obtained with ICGA, with the automated SS-OCT approach proving additional depth-encoded 3D information. In 16 eyes with pachychoroid disease, the SS-OCT approach added clinically relevant structural details, including choroidal thickness and vessel depth, which the ICGA studies could not provide. Our technique appears to advance the in vivo visualization of the full-thickness choroid, successfully reveals the topographical features of choroidal vasculature, and shows potential for further quantitative analysis when compared with other choroidal imaging techniques. This improved visualization of choroidal vasculature and its 3D structure should provide an insight into choroid-related disease mechanisms as well as their responses to treatment.

Optical Coherence Tomography for Ophthalmology Imaging

Optical coherence tomography (OCT) is a depth-resolved imaging modality, which is able to achieve micrometer-scale resolution within biological tissue noninvasively. In the past 30 years, researchers all around the world had made several essential efforts on techniques relevant to OCT. OCT has become a routine process for eye diseases with different types. In this chapter, the three important stages in the development of OCT are briefly illustrated, including the time domain OCT (TD-OCT), the frequency domain OCT (FD-OCT) and the optical coherence tomography angiography (OCTA). Each of the technique has made great progress for use on living human eye imaging in clinical applications. TD-OCT was first proposed and commercialized, which is able to achieve acceptable 2D depth-resolved cross-sectional images of human retina in vivo. FD-OCT was the upgraded OCT technique compared with TD-OCT. By capturing the coherent signal within the Fourier space, the FD-OCT could improve the image sensitivity compared with TD-OCT, and achieve dozens of kilo hertz imaging speed. OCTA is the newest developments of OCT technique, which is able to visualize the micro vasculature networks of human retina in vivo. With OCTA technique, the newest ophthalmologic OCT system is able to achieve detailed diagnosis for both micro-structure and vasculature abnormalities for clinical applications. The further development of OCT technique on imaging speed, contrast, resolution, field of view, and so on will make OCT to be a more powerful tool for clinical usages.

Quantitative analyses of diameter and running pattern of choroidal vessels in central serous chorioretinopathy by en face images

This study was to investigate the choroidal vessels in eyes with central serous chorioretinopathy (CSC) quantitatively. We studied 41 CSC eyes and their fellow eyes, and 41 normal eyes of 41 age-adjusted individuals. En-face optical coherence tomography image of the top 25% slab of Haller’s layer was analyze. The mean vessel area, vessel length, and vessel diameter were calculated. The running pattern of the vessels was quantified and used to determine the degree of symmetry, the “symmetry index”. The vessel area of CSC eyes was not significantly different from that of fellow eyes but significantly larger than that of normal eyes. The vessel length of CSC eyes was not significantly different from fellow eyes but significantly shorter than that of normal eyes. The mean vessel diameter was larger in CSC eyes than in the fellow eyes and the normal eye. The symmetry index was not significantly different in CSC eyes from that of their fellow eyes but was smaller than that of normal eyes. The quantitative analysis showed that eyes with CSC had larger choroidal vessels and asymmetrical vessels running in Haller’s layer.

Forming Optimal Projection Images from Intra-Retinal Layers Using Curvelet-Based Image Fusion Method

Background

Image fusion is the process of combining the information of several input images into one image. Projection images obtained from three-dimensional (3D) optical coherence tomography (OCT) can show inlier retinal pathology and abnormalities that are not visible in conventional fundus images. In recent years, the projection image is often made by an average on all retina that causes to lose many intraretinal details.

Methods

In this study, we focus on the formation of optimum projection images from retinal layers using Curvelet-based image fusion. The latter consists of three main steps. In the earlier studies, macular spectral 3D data using diffusion map-based OCT were segmented into 12 different boundaries identifying 11 retinal layers in three dimensions. In the second step, projection images are attained using conducting some statistical methods on the space between each pair of boundaries. In the next step, retinal layers are merged using Curvelet transform to make the final projection images.

Results

These images contain integrated retinal depth information as well as an ideal opportunity to better extract retinal features such as vessels and the macula region. Finally, qualitative and quantitative evaluations show the superiority of this method to the average-based and wavelet-based fusion methods. Overall, our method obtains the best results for image fusion in all terms such as entropy (6.7744) and AG (9.5491).

Conclusion

Creating an image with more and detailed information made by the Curvelet-based image fusion has significantly higher contrast. There are also many thin veins in Curvelet-based fused image, which are absent in average-based and wavelet-based fused images.

Noise and bias in optical coherence tomography intensity signal decorrelation

Functional optical coherence tomography (OCT) imaging based on the decorrelation of the intensity signal has been used extensively in angiography and is finding use in flowmetry and therapy monitoring. In this work, we present a rigorous analysis of the autocorrelation function, introduce the concepts of contrast bias, statistical bias and variability, and identify the optimal definition of the second-order autocorrelation function (ACF) g ⁽²⁾ to improve its estimation from limited data. We benchmark different averaging strategies in reducing statistical bias and variability. We also developed an analytical correction for the noise contributions to the decorrelation of the ACF in OCT that extends the signal-to-noise ratio range in which ACF analysis can be used. We demonstrate the use of all the tools developed in the experimental determination of the lateral speckle size depth dependence in a rotational endoscopic probe with low NA, and we show the ability to more accurately determine the rotational speed of an endoscopic probe to implement NURD detection. We finally present g ⁽²⁾-based angiography of the finger nailbed, demonstrating the improved results from noise correction and the optimal bias mitigation strategies.

Forward multiple scattering dominates speckle decorrelation in whole-blood flowmetry using optical coherence tomography

Quantitative blood flow measurements using optical coherence tomography (OCT) have a wide potential range of medical research and clinical applications. Flowmetry based on the temporal dynamics of the OCT signal may have the ability to measure three-dimensional flow profiles regardless of the flow direction. State-of-the-art models describing the OCT signal temporal statistics are based on dynamic light scattering (DLS), a model which is inherently limited to single scattering regimes. DLS methods continue to be applied to OCT despite the knowledge that red blood cells produce strong forward multiple scattering. Here, we postulate that forward multiple scattering is the primary mechanism causing the rate of speckle-decorrelation derived from data acquired in vivo to deviate from the rate of decorrelation determined in phantom experiments. We also postulate that multiple scattering contributions to decorrelation are only present when the sample exhibits velocity field inhomogeneities larger than the scale of a resolution volume and are thus absent in rigid bulk motion. To test these hypotheses, we performed a systematic study of the effects of forward multiple scattering on OCT signal decorrelation with phantom experiments under physiologically relevant flow conditions and relative bulk motion. Our experimental results confirm that the amount of forward multiple scattering affects the proportionality between lateral flow and decorrelation. We propose that multiply scattered light carries information from different locations in the sample and each location imprints scattering dynamics on the scattered light causing increased decorrelation rates. Our analysis confirms that the detection of forward scattered light inside the vessel lumen causes an increase in the rate of decorrelation which results in an overestimation of blood flow velocities at depths as shallow as 40 µm into whole blood for OCT systems with typical numerical apertures used in retinal imaging.

Investigation of temporal vascular effects induced by focused ultrasound treatment with speckle-variance optical coherence tomography

Focused ultrasound (FUS) can be used to locally and temporally enhance vascular permeability, improving the efficiency of drug delivery from the blood vessels into the surrounding tissue. However, it is difficult to evaluate in real time the effect induced by FUS and to noninvasively observe the permeability enhancement. In this study, speckle-variance optical coherence tomography (SVOCT) was implemented for the investigation of temporal effects on vessels induced by FUS treatment. With OCT scanning, the dynamic change in vessels during FUS exposure can be observed and studied. Moreover, the vascular effects induced by FUS treatment with and without the presence of microbubbles were investigated and quantitatively compared. Additionally, 2D and 3D speckle-variance images were used for quantitative observation of blood leakage from vessels due to the permeability enhancement caused by FUS, which could be an indicator that can be used to determine the influence of FUS power exposure. In conclusion, SVOCT can be a useful tool for monitoring FUS treatment in real time, facilitating the dynamic observation of temporal effects and helping to determine the optimal FUS power.

Blood flow velocity quantification using split-spectrum amplitude-decorrelation angiography with optical coherence tomography

The split-spectrum amplitude-decorrelation angiography (SSADA) algorithm was recently developed as a method for imaging blood flow in the human retina without the use of phase information. In order to enable absolute blood velocity quantification, in vitro phantom experiments are performed to correlate the SSADA signal at multiple time scales with various preset velocities. A linear model relating SSADA measurements to absolute flow velocities is derived using the phantom data. The operating range for the linear model is discussed along with its implication for velocity quantification with SSADA in a clinical setting.

Optical coherence tomography in the diagnosis and management of diabetic macular edema: time-domain versus spectral-domain

Optical coherence tomography (OCT) is an important imaging modality in the setting of diabetic macular edema (DME). Its use allows more precise evaluation of retinal pathology in DME, including retinal thickness and edema, vitreomacular interface abnormalities, subretinal fluid, and foveal microstructural changes. Additional advantages include its ability to quantitatively monitor response to treatment of DME by laser, intravitreal pharmacotherapies, and vitreoretinal surgery. OCT measurements are now used in all major clinical studies of DME treatment as critical endpoints. This article presents a review of both time-domain and spectral-domain OCT in the diagnosis and management of DME. The authors discuss the various parameters evaluated by the OCT systems and provide an evidence-based evaluation of their accuracy, significance, reliability, and limitations. As the capability of OCT continues to advance, it appears that its use will play an increasingly important role in the understanding, evaluation, and treatment of DME.

Split-spectrum amplitude-decorrelation angiography with optical coherence tomography

Amplitude decorrelation measurement is sensitive to transverse flow and immune to phase noise in comparison to Doppler and other phase-based approaches. However, the high axial resolution of OCT makes it very sensitive to the pulsatile bulk motion noise in the axial direction. To overcome this limitation, we developed split-spectrum amplitude-decorrelation angiography (SSADA) to improve the signal-to-noise ratio (SNR) of flow detection. The full OCT spectrum was split into several narrower bands. Inter-B-scan decorrelation was computed using the spectral bands separately and then averaged. The SSADA algorithm was tested on in vivo images of the human macula and optic nerve head. It significantly improved both SNR for flow detection and connectivity of microvascular network when compared to other amplitude-decorrelation algorithms.

Variable velocity range imaging of the choroid with dual-beam optical coherence angiography

In this study, we present dual-beam Doppler optical coherence angiography with variable beam separation. Altering beam distance, independently of the scanning protocol, provides a flexible way to select the velocity range of detectable blood flow. This system utilized a one-micrometer wavelength light source to visualize deep into the posterior eye, i.e., the choroid. Two-dimensional choroidal vasculature maps of a human subject acquired with different beam separations, and hence with several velocity ranges, are presented. Combining these maps yields a semi-quantitative visualization of axial velocity of the choroidal circulation. The proposed technique may be useful for identifying choroidal abnormalities that occur in pathological conditions of the eye.

Enhanced imaging of choroidal vasculature by high-penetration and dual-velocity optical coherence angiography

Dual-beam-scan Doppler optical coherence angiography (DB-OCA) with a 1-μm-wavelength probe is demonstrated for improved in vivo choroidal angiograms of the human eye. This method utilizes two scanning beams with spatial and temporal separation on the retina, and provides two measurable velocity ranges. The method achieves higher sensitivity to very low velocity flows than conventional Doppler optical coherence tomography. Moreover, longer wavelengths allowing greater penetration, enhanced visualization of choroidal vessels is verified with en-face projection images of the Doppler shift squared. Specifically, better choroidal vasculature visibility is achieved at a wavelength of 1 μm than at 840 nm.

Real-time bulk-motion-correction free Doppler variance optical coherence tomography for choroidal capillary vasculature imaging

In this paper, we analyze the retinal and choroidal blood vasculature in the posterior segment of the human eye with optimized color Doppler and Doppler variance optical coherence tomography. Depth-resolved structure, color Doppler and Doppler variance images are compared. Blood vessels down to the capillary level were detected and visualized with the optimized optical coherence color Doppler and Doppler variance method. For in-vivo imaging of human eyes, bulk-motion induced bulk phase must be identified and removed before using the color Doppler method. It was found that the Doppler variance method is not sensitive to bulk-motion and the method can be used without correcting the bulk-motion when the sample-movement-induced velocity changes gradually. Real-time processing and displaying of the structure and blood vessel images are very interesting and is demonstrated using a dual quad-core Central Processing Unit (CPU) workstation. High resolution images of choroidal capillary of the vasculature network with phased-resolved color Doppler and Doppler variance are shown.

Image chorioretinal vasculature in albino rats using photoacoustic ophthalmoscopy

We imaged the microvascular network in both the retina and the choroid in an albino rat eye using photoacoustic ophthalmoscopy guided by optical coherence tomography. Relying on optical absorption and ultrasonic detection, photoacoustic ophthalmoscopy can image both retinal and choroidal vessel networks with high contrast.

Spectral-domain optical coherence tomography: a comparison of modern high-resolution retinal imaging systems

PURPOSE

To provide a review of commercially available spectral-domain optical coherence tomography (SD OCT) systems in clinical ophthalmology.

DESIGN

Perspective.

METHODS

Review of current manufacturer information, selected articles from the literature, and the authors' clinical experience.

RESULTS

Because the premise of SD OCT technology is the nonproprietary mathematical formula of Fourier transformation, 9 different SD OCT systems currently are or soon will be commercially available. Also demonstrated are Cirrus en face C-scan visualization of photoreceptor attenuation resulting from acute zonal occult outer retinopathy and Spectral OCT/scanning laser ophthalmoscopy microperimetric analysis of a macular caldera lesion resulting from North Carolina macular dystrophy.

CONCLUSIONS

Advances in high-resolution imaging of the anterior and posterior segment have revealed new in vivo details of anatomic, physiologic, and pathologic data for the practice of ophthalmology. Compared with time-domain OCT, SD OCT tends to derive increased retinal thickness and decreased nerve fiber layer thickness measurements. This is likely because of increased depth of resolution and greater volume of data acquired with each scan. Interdevice comparison is not practical because of differences in individual segment boundary algorithms. Improvements in photoreceptor inner segment-outer segment layer visualization should aid our understanding of visual dysfunction in a variety of retinal pathologic features. As the technology develops, SD OCT will continue to provide new insights about ocular structure and disease.

Scanning protocols dedicated to smart velocity ranging in spectral OCT

We introduce a new type of scanning protocols, called segmented protocols, which enable extracting multi-range flow velocity information from a single Spectral OCT data set. The protocols are evaluated using a well defined flow in a glass capillary. As an example of in vivo studies, we demonstrate two- and three-dimensional imaging of the retinal vascular system in the eyes of healthy volunteers. The flow velocity detection is performed using a method of Joint Spectral and Time domain OCT. Velocity ranging is demonstrated in imaging of retinal vasculature in the macular region and in the optic disk area characterized by different flow velocity values. Additionally, an enhanced visualization of retinal capillary network is presented in the close proximity to macula.

Three-dimensional quantitative imaging of retinal and choroidal blood flow velocity using joint Spectral and Time domain Optical Coherence Tomography

Recently, joint Spectral and Time domain Optical Coherence Tomography (joint STdOCT) has been proposed to measure ocular blood flow velocity. Limitations of CCD technology allowed only for two-dimensional imaging at that time. In this paper we demonstrate fast three-dimensional STdOCT based on ultrahigh speed CMOS camera. Proposed method is straightforward, fully automatic and does not require any advanced image processing techniques. Three-dimensional distributions of axial velocity components of the blood in human eye vasculature are presented: in retinal and, for the first time, in choroidal layer. Different factors that affect quality of velocity images are discussed. Additionally, the quantitative measurement allows to observe a new interesting optical phenomenon - random Doppler shift in OCT signals that forms a vascular pattern at the depth of sclera.

Spectral domain optical coherence tomographic imaging of geographic atrophy

BACKGROUND AND OBJECTIVE

To compare images of geographic atrophy (GA) obtained using spectral domain optical coherence tomography (SD-OCT) with images obtained using fundus autofluorescence (FAF).

PATIENTS AND METHODS

Five eyes from patients with dry AMD were imaged using SD-OCT and FAF, and the size and shape of the GA were compared.

RESULTS

GA appears bright on SD-OCT compared with the surrounding areas with an intact retinal pigment epithelium because of increased reflectivity from the underlying choroid. SD-OCT and FAF both identified GA reproducibly, and measurement of the area of GA is comparable between the two methods with a mean difference of 2.7% of the total area.

CONCLUSION

SD-OCT can identify and quantitate areas of GA. The size and shape of these areas correlate well to the areas of GA seen on autofluorescence images; however, SD-OCT imaging also provides important cross-sectional anatomic information.

Investigation of post-glaucoma-surgery structures by three-dimensional and polarization sensitive anterior eye segment optical coherence tomography

A sequential case series of post-glaucoma-surgery structures examined by three-dimensional corneal and anterior eye segment optical coherence tomography (3D-CASOCT) and 3D polarization sensitive CASOCT (PS-CASOCT) is presented. A total of 5 patients who underwent glaucoma surgery were included in this study. Of these, 1, 1, and 3 patient underwent trabeculotomy, laser iridotomy, and trabeculectomy respectively. One patient each who had undergone trabeculotomy or laser iridotomy was examined using a prototype 3D-CASOCT. This prototype is based on swept-source OCT technology, uses a probe beam with a center wavelength of 1.31 microm, and has an axial resolution of 11.6 microm and a scanning speed of 20,000 A lines/s. All 3 patients who underwent trabeculectomy were examined by PS-CASOCT, which has similar specifications to those of 3DCASOCT, measures the depth-resolved birefringence of a specimen, and yields conventional OCT images. Detailed 3D visualization of the incision site of trabeculotomy and the ablation site of laser iridotomy was achieved using 3D-CASOCT. PS-CASOCT revealed, in addition to the structural details, the birefringent properties of the tissues of the trabeculectomy bleb. Some blebs showed abnormal birefringence in the conjunctiva and in a remnant fluid pool. This may indicate the existence of fibrosis in these regions. Both 3D-CASOCT and PS-CASOCT provide clinically significant information for the postoperative assessment of structures created during glaucoma surgery. Interactive 3D datasets of all cases are provided for interactive clinical review. Complex raw 3D OCT volumes are also provided as a reference dataset for the development of PS-OCT algorithms.

Multidimensional en-face OCT imaging of the retina

Fast T-scanning (transverse scanning, en-face) was used to build B-scan or C-scan optical coherence tomography (OCT) images of the retina. Several unique signature patterns of en-face (coronal) are reviewed in conjunction with associated confocal images of the fundus and B-scan OCT images. Benefits in combining T-scan OCT with confocal imaging to generate pairs of OCT and confocal images similar to those generated by scanning laser ophthalmoscopy (SLO) are discussed in comparison with the spectral OCT systems. The multichannel potential of the OCT/SLO system is demonstrated with the addition of a third hardware channel which acquires and generates indocyanine green (ICG) fluorescence images. The OCT, confocal SLO and ICG fluorescence images are simultaneously presented in a two or a three screen format. A fourth channel which displays a live mix of frames of the ICG sequence superimposed on the corresponding coronal OCT slices for immediate multidimensional comparison, is also included. OSA ISP software is employed to illustrate the synergy between the simultaneously provided perspectives. This synergy promotes interpretation of information by enhancing diagnostic comparisons and facilitates internal correction of movement artifacts within C-scan and B-scan OCT images using information provided by the SLO channel.

Visualization of 3-D high speed ultrahigh resolution optical coherence tomographic data identifies structures visible in 2D frames

Optical coherence tomography has allowed unprecedented visualization of ocular structures, but the identity of some visible objects within slices remains unknown. This study reconstructs a number of those objects in 3D space, allowing their identification by observation of their 3D morphology. In the case mottling deep within image slices through the optic disc, C-mode imaging provided visualization of the appearance and distribution of laminar pores. In the case of white spots and streaks sometimes observed in image slices through the cornea, C-mode imaging contoured to the path of those white spots allowed their visual identification as nerves extending radially into the cornea from the limbus. White spots observed in ultra-high resolution retinal image slices were identified as blood within retinal capillaries. C-mode contour-corrected imaging of three dimensional structures provided the identification of previously unidentified structures visible in cross-sectional image slices.

Automated retinal shadow compensation of optical coherence tomography images

We present an automated numerical method of compensating for retinal shadows in the choroid. In this method, signal extinction caused by retinal vessels is estimated by subtracting median A-scans obtained from beneath the retinal vessels and A-scans from the surrounding area. Adding the obtained offset vector to A-scans from beneath the retinal vessels allows compensating for shadows in the choroid. In vivo imaging of the human eye was performed by 840-nm-band standard resolution spectral domain optical coherence tomography (SD-OCT), and choroidal vasculature projection images were calculated. Removal of retinal shadows distinctly improved the readability of choroidal images.

In vivo volumetric imaging of vascular perfusion within human retina and choroids with optical micro-angiography

Optical micro-angiography (OMAG), based on Fourier domain optical coherence tomography (OCT), is a recently developed imaging modality that provides dynamic blood flow imaging within microcirculation tissue beds in vivo. This paper presents its first application in imaging the blood circulations in posterior chamber of human eye. To eliminate/minimize the motion artifacts in OMAG flow image caused by the inevitable subject movement, we describe a method to compensate the bulk tissue motion by use of phase changes in sequential OCT A scan signals. By use of a fast OMAG/OCT imaging system at ~840nm wavelength band, we show that OMAG is capable of providing volumetric vasculatural images in retina and choroids, down to capillary level imaging resolution, within approximately 10 s. The depth-resolved volumetric views of the separate retina and choroid vasculatures are also presented. In the end of this paper, we provide a comparison of the OMAG results with those from Doppler OCT and optical coherence angiography.