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

Visualization of two-dimensional transverse blood flow direction using optical coherence tomography angiography

SIGNIFICANCE

Evaluation of vessel patency and blood flow direction is important in various medical situations, including diagnosis and monitoring of ischemic diseases, and image-guided vascular surgeries. While optical coherence tomography angiography (OCTA) is the most widely used functional extension of optical coherence tomography that visualizes three-dimensional vasculature, inability to provide information of blood flow direction is one of its limitations.

AIM

We demonstrate two-dimensional (2D) transverse blood flow direction imaging in en face OCTA.

APPROACH

A series of triangular beam scans for the fast axis was implemented in the horizontal direction for the first volume scan and in the vertical direction for the following volume scan, and the inter A-line OCTA was performed for the blood flow direction imaging while the stepwise pattern was used for each slow axis scan. The decorrelation differences between the forward and the backward inter A-line OCTA were calculated for the horizontal and the vertical fast axis scans, and the ratio of the horizontal and the vertical decorrelation differences was utilized to show the 2D transverse flow direction information.

RESULTS

OCTA flow direction imaging was verified using flow phantoms with various flow orientations and speeds, and we identified the flow speed range relative to the scan speed for reliable flow direction measurement. We demonstrated the visualization of 2D transverse blood flow orientations in mouse brain vascular networks in vivo.

CONCLUSIONS

The proposed OCTA imaging technique that provides information of 2D transverse flow direction can be utilized in various clinical applications and preclinical studies.

Computationally effective 2D and 3D fast phase unwrapping algorithms and their applications to Doppler optical coherence tomography

We propose a simplification for a robust and easy to implement fast phase unwrapping (FPU) algorithm that is used to solve the phase wrapping problem encountered in various fields of optical imaging and metrology. We show that the number of necessary computations using the algorithm can be reduced compared to its original version. FPU can be easily extended from two to three spatial dimensions. We demonstrate the applicability of the two- and three-dimensional FPU algorithm for Doppler optical coherence tomography (DOCT) in numerical simulations, and in the imaging of a flow phantom and blood flow in the human retina in vivo. We introduce an FPU applicability plot for use as a guide in the selection of the most suitable version of the algorithm depending on the phase noise in the acquired data. This plot uses the circular standard deviation of the wrapped phase distribution as a measure of noise and relates it to the root-mean-square error of the recovered, unwrapped phase.

Short-time series optical coherence tomography angiography and its application to cutaneous microvasculature

We present a new optical coherence tomography (OCT) angiography method for imaging tissue microvasculature in vivo based on the characteristic frequency-domain flow signature in a short time series of a single voxel. The angiography signal is generated by Fourier transforming the OCT signal time series from a given voxel in multiple acquisitions and computing the average magnitude of non-zero (high-pass) frequency components. Larger temporal variations of the OCT signal caused by blood flow result in higher values of the average magnitude in the frequency domain compared to those from static tissue. Weighting of the signal by the inverse of the zero-frequency component (i.e., the sum of the OCT signal time series) improves vessel contrast in flow regions of low OCT signal. The method is demonstrated on a fabricated flow phantom and on human skin in vivo and, at only 5 time points per voxel, shows enhanced vessel contrast in comparison to conventional correlation mapping/speckle decorrelation and speckle variance methods.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Wide dynamic range high-speed three-dimensional quantitative OCT angiography with a hybrid-beam scan

We demonstrate a novel hybrid-beam scanning-based quantitative optical coherence tomography angiography (OCTA) that provides high-speed wide dynamic range blood flow speed imaging. The hybrid-beam scanning scheme enables multiple OCTA image acquisitions with a wide range of multiple time intervals simultaneously providing wide dynamic range blood flow speed imaging independent of the blood vessel orientation, which was quantified over a speed range of 0.6∼104  mm/s through the blood flow phantom experiments. A fully automated high-speed hybrid-beam scanning-based quantitative OCTA system demonstrates visualization of blood flow speeds in various vessels from the main arteries to capillaries in a 4  mm×4  mm area (1024 A-lines × 512 B-scans) in vivo in 20 s, showing its potential as a useful imaging tool for various biomedical applications.

Optical coherence tomography angiography: A comprehensive review of current methods and clinical applications

OCT has revolutionized the practice of ophthalmology over the past 10-20 years. Advances in OCT technology have allowed for the creation of novel OCT-based methods. OCT-Angiography (OCTA) is one such method that has rapidly gained clinical acceptance since it was approved by the FDA in late 2016. OCTA images are based on the variable backscattering of light from the vascular and neurosensory tissue in the retina. Since the intensity and phase of backscattered light from retinal tissue varies based on the intrinsic movement of the tissue (e.g. red blood cells are moving, but neurosensory tissue is static), OCTA images are essentially motion-contrast images. This motion-contrast imaging provides reliable, high resolution, and non-invasive images of the retinal vasculature in an efficient manner. In many cases, these images are approaching histology level resolution. This unprecedented resolution coupled with the simple, fast and non-invasive imaging platform have allowed a host of basic and clinical research applications. OCTA demonstrates many important clinical findings including areas of macular telangiectasia, impaired perfusion, microaneurysms, capillary remodeling, some types of intraretinal fluid, and neovascularization among many others. More importantly, OCTA provides depth-resolved information that has never before been available. Correspondingly, OCTA has been used to evaluate a spectrum of retinal vascular diseases including diabetic retinopathy (DR), retinal venous occlusion (RVO), uveitis, retinal arterial occlusion, and age-related macular degeneration among others. In this review, we will discuss the methods used to create OCTA images, the practical applications of OCTA in light of invasive dye-imaging studies (e.g. fluorescein angiography) and review clinical studies demonstrating the utility of OCTA for research and clinical practice.

Optical Coherence Tomography for Retinal Surgery: Perioperative Analysis to Real-Time Four-Dimensional Image-Guided Surgery

Magnification of the surgical field using the operating microscope facilitated profound innovations in retinal surgery in the 1970s, such as pars plana vitrectomy. Although surgical instrumentation and illumination techniques are continually developing, the operating microscope for vitreoretinal procedures has remained essentially unchanged and currently limits the surgeon's depth perception and assessment of subtle microanatomy. Optical coherence tomography (OCT) has revolutionized clinical management of retinal pathology, and its introduction into the operating suite may have a similar impact on surgical visualization and treatment. In this article, we review the evolution of OCT for retinal surgery, from perioperative analysis to live volumetric (four-dimensional, 4D) image-guided surgery. We begin by briefly addressing the benefits and limitations of the operating microscope, the progression of OCT technology, and OCT applications in clinical/perioperative retinal imaging. Next, we review intraoperative OCT (iOCT) applications using handheld probes during surgical pauses, two-dimensional (2D) microscope-integrated OCT (MIOCT) of live surgery, and volumetric MIOCT of live surgery. The iOCT discussion focuses on technological advancements, applications during human retinal surgery, translational difficulties and limitations, and future directions.

Optical coherence tomography based angiography [Invited]

Optical coherence tomography (OCT)-based angiography (OCTA) provides in vivo, three-dimensional vascular information by the use of flowing red blood cells as intrinsic contrast agents, enabling the visualization of functional vessel networks within microcirculatory tissue beds non-invasively, without a need of dye injection. Because of these attributes, OCTA has been rapidly translated to clinical ophthalmology within a short period of time in the development. Various OCTA algorithms have been developed to detect the functional micro-vasculatures in vivo by utilizing different components of OCT signals, including phase-signal-based OCTA, intensity-signal-based OCTA and complex-signal-based OCTA. All these algorithms have shown, in one way or another, their clinical values in revealing micro-vasculatures in biological tissues in vivo, identifying abnormal vascular networks or vessel impairment zones in retinal and skin pathologies, detecting vessel patterns and angiogenesis in eyes with age-related macular degeneration and in skin and brain with tumors, and monitoring responses to hypoxia in the brain tissue. The purpose of this paper is to provide a technical oriented overview of the OCTA developments and their potential pre-clinical and clinical applications, and to shed some lights on its future perspectives. Because of its clinical translation to ophthalmology, this review intentionally places a slightly more weight on ophthalmic OCT angiography.

Spectrometer calibration for spectroscopic Fourier domain optical coherence tomography

We propose a simple and robust procedure for Fourier domain optical coherence tomography (FdOCT) that allows to linearize the detected FdOCT spectra to wavenumber domain and, at the same time, to determine the wavelength of light for each point of detected spectrum. We show that in this approach it is possible to use any measurable physical quantity that has linear dependency on wavenumber and can be extracted from spectral fringes. The actual values of the measured quantity have no importance for the algorithm and do not need to be known at any stage of the procedure. As example we calibrate a spectral OCT spectrometer using Doppler frequency. The technique of spectral calibration can be in principle adapted to of all kind of Fourier domain OCT devices.

Comparison of amplitude-decorrelation, speckle-variance and phase-variance OCT angiography methods for imaging the human retina and choroid

We compared the performance of three OCT angiography (OCTA) methods: speckle variance, amplitude decorrelation and phase variance for imaging of the human retina and choroid. Two averaging methods, split spectrum and volume averaging, were compared to assess the quality of the OCTA vascular images. All data were acquired using a swept-source OCT system at 1040 nm central wavelength, operating at 100,000 A-scans/s. We performed a quantitative comparison using a contrast-to-noise (CNR) metric to assess the capability of the three methods to visualize the choriocapillaris layer. For evaluation of the static tissue noise suppression in OCTA images we proposed to calculate CNR between the photoreceptor/RPE complex and the choriocapillaris layer. Finally, we demonstrated that implementation of intensity-based OCT imaging and OCT angiography methods allows for visualization of retinal and choroidal vascular layers known from anatomic studies in retinal preparations. OCT projection imaging of data flattened to selected retinal layers was implemented to visualize retinal and choroidal vasculature. User guided vessel tracing was applied to segment the retinal vasculature. The results were visualized in a form of a skeletonized 3D model.

Tissue thickness calculation in ocular optical coherence tomography

Thickness measurements derived from optical coherence tomography (OCT) images of the eye are a fundamental clinical and research metric, since they provide valuable information regarding the eye's anatomical and physiological characteristics, and can assist in the diagnosis and monitoring of numerous ocular conditions. Despite the importance of these measurements, limited attention has been given to the methods used to estimate thickness in OCT images of the eye. Most current studies employing OCT use an axial thickness metric, but there is evidence that axial thickness measures may be biased by tilt and curvature of the image. In this paper, standard axial thickness calculations are compared with a variety of alternative metrics for estimating tissue thickness. These methods were tested on a data set of wide-field chorio-retinal OCT scans (field of view (FOV) 60° x 25°) to examine their performance across a wide region of interest and to demonstrate the potential effect of curvature of the posterior segment of the eye on the thickness estimates. Similarly, the effect of image tilt was systematically examined with the same range of proposed metrics. The results demonstrate that image tilt and curvature of the posterior segment can affect axial tissue thickness calculations, while alternative metrics, which are not biased by these effects, should be considered. This study demonstrates the need to consider alternative methods to calculate tissue thickness in order to avoid measurement error due to image tilt and curvature.

Differentiation of morphotic elements in human blood using optical coherence tomography and a microfluidic setup

We demonstrate a novel optical method for the detection and differentiation between erythrocytes and leukocytes that uses amplitude and phase information provided by optical coherence tomography (OCT). Biological cells can introduce significant phase modulation with substantial scattering anisotropy and dominant forward-scattered light. Such physical properties may favor the use of a trans-illumination imaging technique. However, an epi-illumination mode may be more practical and robust in many applications. This study describes a new way of measuring the phase modulation introduced by flowing microobjects. The novel part of this invention is that it uses the backscattered signal from the substrate located below the flowing/moving objects. The identification of cells is based on phase-sensitive OCT signals. To differentiate single cells, a custom-designed microfluidic device with a highly scattering substrate is introduced. The microchannels are molded in polydimethylsiloxane (PDMS) mixed with titanium dioxide (TiO2) to ensure high scattering properties. The statistical parameters of the measured signal depend on the cells' features, such as their size, shape, and internal structure.

OCT angiography by absolute intensity difference applied to normal and diseased human retinas

We compare four optical coherence tomography techniques for noninvasive visualization of microcapillary network in the human retina and murine cortex. We perform phantom studies to investigate contrast-to-noise ratio for angiographic images obtained with each of the algorithm. We show that the computationally simplest absolute intensity difference angiographic OCT algorithm that bases only on two cross-sectional intensity images may be successfully used in clinical study of healthy eyes and eyes with diabetic maculopathy and branch retinal vein occlusion.

Three-dimensional vascular imaging of proliferative diabetic retinopathy by Doppler optical coherence tomography

PURPOSE

To evaluate the 3-dimensional architecture of neovascularization in proliferative diabetic retinopathy using Doppler optical coherence tomography (OCT).

DESIGN

Prospective, nonrandomized clinical trial.

METHODS

Seventeen eyes of 14 patients with proliferative diabetic retinopathy were prospectively studied. Prototype Doppler OCT was used to evaluate the 3-dimensional vascular architecture at vitreoretinal adhesions.

RESULTS

Proliferative membranes were detected in all eyes with proliferative diabetic retinopathy by standard OCT images. Doppler OCT images detected blood flow by neovascularization of the disc in 12 eyes and neovascularization elsewhere in 11 eyes. Doppler OCT images showed the 3-dimensional extent of new vessels at various stages of neovascularization, and the extent of new vessels could be clearly confirmed at vitreoretinal adhesions.

CONCLUSIONS

Doppler OCT is useful for the detection and evaluation of the 3-dimensional vascular structure of neovascularization, and can assist in the noninvasive assessment of proliferative diabetic retinopathy.

Complex differential variance algorithm for optical coherence tomography angiography

We describe a complex differential variance (CDV) algorithm for optical coherence tomography based angiography. The algorithm exploits both the intensity and phase changes of optical coherence tomography (OCT) signals from flowing blood to achieve high vascular contrast, and also intrinsically reject undesirable phase signals originating from small displacement axial bulk tissue motion and instrument synchronization errors. We present this algorithm within a broader discussion of the properties of OCT signal dynamics. The performance of the algorithm is compared against two other existing algorithms using both phantom measurements and in vivo data. We show that the algorithm provides better contrast for a given number of measurements and equivalent spatial averaging.

Glaucomatous and age-related changes in corneal pulsation shape. The ocular dicrotism

PURPOSE

To ascertain whether the incidence of ocular dicrotic pulse (ODP) increases with age, it is more pronounced in glaucomatous than in normal eyes and whether it is related to cardiovascular activity.

METHODS

261 subjects aged 47 to 78 years were included in the study and classified into four groups: primary open angle glaucoma (POAG), primary angle-closure glaucoma (PACG), glaucoma suspects with glaucomatous optic disc appearance (GODA) and the controls (CG). Additionally, in each group, subjects with ODP were divided into two age subgroups around the median age. A non-contact ultrasonic method was used to measure corneal indentation pulse (CIP) synchronically with the acquisition of electrocardiography (ECG) and blood pulse signals. ODP was assessed from the acquired signals that were numerically processed in a custom written program.

RESULTS

ODP incidence was about 78%, 66%, 66% and 84% for CG, GODA, POAG, and PACG group, respectively. With advancing age, the ODP incidence increased for all subjects (Δ = 12%), the highest being for the PACG and POAG groups (Δ = 30%). GODA group did not show an age-related increase in the incidence of ODP.

CONCLUSIONS

The ocular dicrotism, measured with non-contact ultrasonic method, was found to be a common phenomenon in elderly subjects. The increased ODP incidence in PACG and POAG group may correspond to either higher stiffness of glaucoma eyes, biochemical abnormalities in eye tissues, changes in ocular hemodynamics, may reflect the effect of medications or be a combination of all those factors. The results of GODA group suggest different mechanisms governing their ocular pulse that makes them less susceptible to generating ODP and having decreased predisposition to glaucoma.

Changes in cochlear blood flow in mice due to loud sound exposure measured with Doppler optical microangiography and laser Doppler flowmetry

In this work we determined the contributions of loud sound exposure (LSE) on cochlear blood flow (CoBF) in an in vivo anesthetized mouse model. A broadband noise system (20 kHz bandwidth) with an intensity of 119 dB SPL, was used for a period of one hour to produce a loud sound stimulus. Two techniques were used to study the changes in blood flow, a Doppler optical microangiography (DOMAG) system; which can measure the blood flow within individual cochlear vessels, and a laser Doppler flowmetry (LDF) system; which averages the blood flow within a volume (a hemisphere of ~1.5 mm radius) of tissue. Both systems determined that the blood flow within the cochlea is reduced due to the LSE stimulation.

Assessment of the flow velocity of blood cells in a microfluidic device using joint spectral and time domain optical coherence tomography

Although Doppler optical coherence tomography techniques have enabled the imaging of blood flow in mid-sized vessels in biological tissues, the generation of velocity maps of capillary networks remains a challenge. To better understand the origin and information content of the Doppler signal from small vessels and limitations of such measurements, we used joint spectral and time domain optical coherence tomography to monitor the flow in a model, semitransparent microchannel device. The results obtained for Intralipid, whole blood, as well as separated red blood cells indicate that the technique is suitable to record velocity profiles in vitro, in a range of microchannel configurations.

Quantitative lateral and axial flow imaging with optical coherence microscopy and tomography

Optical coherence tomography (OCT) and optical coherence microscopy (OCM) allow the acquisition of quantitative three-dimensional axial flow by estimating the Doppler shift caused by moving scatterers. Measuring the velocity of red blood cells is currently the principal application of these methods. In many biological tissues, blood flow is often perpendicular to the optical axis, creating the need for a quantitative measurement of lateral flow. Previous work has shown that lateral flow can be measured from the Doppler bandwidth, albeit only for simplified optical systems. In this work, we present a generalized model to analyze the influence of relevant OCT/OCM system parameters such as light source spectrum, numerical aperture and beam geometry on the Doppler spectrum. Our analysis results in a general framework relating the mean and variance of the Doppler frequency to the axial and lateral flow velocity components. Based on this model, we present an optimized acquisition protocol and algorithm to reconstruct quantitative measurements of lateral and axial flow from the Doppler spectrum for any given OCT/OCM system. To validate this approach, Doppler spectrum analysis is employed to quantitatively measure flow in a capillary with both extended focus OCM and OCT.

Averaging techniques for OCT imaging

State-of-the-art Fourier-domain optical coherence tomography (OCT) allows for the acquisition of up to millions of spectral fringes per second. This large amount of data can be used to improve the quality of structural tomograms after effective averaging. Here, we compare three OCT image improvement techniques: magnitude averaging, complex averaging, and spectral and time domain OCT (STdOCT). We evaluate the performance for images on both linear and logarithmic intensity scales and discuss their advantages and disadvantages. We propose the use of the STdOCT approach as it offers the best advantages. Applications to in vivo imaging and speckle reduction are presented.

Multi-parametric imaging of murine brain using spectral and time domain optical coherence tomography

Examination of brain functions in small animal models may help improve the diagnosis and treatment of neurological conditions. Transcranial imaging of small rodents' brains poses a major challenge for optical microscopy. Another challenge is to reduce the measurement time. We describe methods and algorithms for three-dimensional assessment of blood flow in the brains of small animals, through the intact skull, using spectral and time domain optical coherence tomography. By introducing a resonant scanner to the optical setup of the optical coherence tomography (OCT) system, we have developed and applied a high-speed spectral OCT technique that allows us to vary the imaging range of flow and to shorten measurement time. Multi-parameter signal analysis enables us to obtain both qualitative and quantitative information about flow velocity from the same set of data.

The use of forward scatter to improve retinal vascular imaging with an adaptive optics scanning laser ophthalmoscope

Retinal vascular diseases are a leading cause of blindness and visual disability. The advent of adaptive optics retinal imaging has enabled us to image the retinal vascular at cellular resolutions, but imaging of the vasculature can be difficult due to the complex nature of the images, including features of many other retinal structures, such as the nerve fiber layer, glial and other cells. In this paper we show that varying the size and centration of the confocal aperture of an adaptive optics scanning laser ophthalmoscope (AOSLO) can increase sensitivity to multiply scattered light, especially light forward scattered from the vasculature and erythrocytes. The resulting technique was tested by imaging regions with different retinal tissue reflectivities as well as within the optic nerve head.

Effects of hypoxia on cochlear blood flow in mice evaluated using Doppler optical microangiography

Reduced cochlear blood flow (CoBF) is a main contributor to hearing loss. Studying CoBF has remained a challenge due to the lack of available tools. Doppler optical microangiography (DOMAG), a method to quantify single-vessel absolute blood flow, and laser Doppler flowmetry (LDF), a method for measuring the relative blood flow within a large volume of tissue, were used for determining the changes in CoBF due to systemic hypoxia in mice. DOMAG determined the change in blood flow in the apical turn (AT) with single-vessel resolution, while LDF averaged the change in the blood flow within a large volume of the cochlea (hemisphere with ∼1 to 1.5 mm radius). Hypoxia was induced by decreasing the concentration of oxygen-inspired gas, so that the oxygen saturation was reduced from >95% to ∼80%. DOMAG determined that during hypoxia the blood flow in two areas of the AT near and far from the helicotrema were increased and decreased, respectively. The LDF detected a decrease in blood flow within a larger volume of the cochlea (several turns averaged together). Therefore, the use of DOMAG as a tool for studying cochlear blood flow due to its ability to determine absolute flow values with single-vessel resolution was proposed.

Three-dimensional retinal and choroidal capillary imaging by power Doppler optical coherence angiography with adaptive optics

Retinal and choroidal vascular imaging is a key to the better understanding and diagnosis of eye diseases. To achieve comprehensive three-dimensional capillary imaging, we used an enhanced vascular imaging technique, so called adaptive optics optical coherence angiography (AO-OCA). AO-OCA enables in vivo high-resolution and high-contrast micro-vascular imaging by detecting Doppler frequency shifts. Using this technique, the retinal and choroidal vasculatures of healthy subjects were imaged. The results show that both intensity and Doppler power images have sufficient contrast to discriminate almost all vasculatures from the static tissue. However, the choriocapillaris, pre-arterioles, and post-venules in the Sattler layer were more contrasted by the Doppler technique. In conclusion, AO-OCA enables three-dimensional capillary imaging, and is especially useful for the detection of the choriocapillaris and choroidal capillary network.

Label-free imaging of blood vessel morphology with capillary resolution using optical microangiography

Several tissue pathologies are correlated with changes in the blood vessel morphology and microcirculation that supplies the tissue. Optical coherence tomography (OCT) is an imaging technique that enables acquiring non-invasive three-dimensional images of biological structures with micrometer resolution. Optical microangiography (OMAG) is a method of processing OCT data which enables visualizing the three-dimensional blood vessel morphology within biological tissues. OMAG has high spatial resolution which allows visualizing single capillary vessels, and does not require the use of contrast agents. The intrinsic optical signals backscattered by the moving blood cells inside blood vessels are used as the contrast for which OMAG images are based on. In this paper, we discuss a brief review of the OMAG theory, and present some examples of applications for this technique.

Logarithmic intensity and speckle-based motion contrast methods for human retinal vasculature visualization using swept source optical coherence tomography

We formulate a theory to show that the statistics of OCT signal amplitude and intensity are highly dependent on the sample reflectivity strength, motion, and noise power. Our theoretical and experimental results depict the lack of speckle amplitude and intensity contrasts to differentiate regions of motion from static areas. Two logarithmic intensity-based contrasts, logarithmic intensity variance (LOGIV) and differential logarithmic intensity variance (DLOGIV), are proposed for serving as surrogate markers for motion with enhanced sensitivity. Our findings demonstrate a good agreement between the theoretical and experimental results for logarithmic intensity-based contrasts. Logarithmic intensity-based motion and speckle-based contrast methods are validated and compared for in vivo human retinal vasculature visualization using high-speed swept-source optical coherence tomography (SS-OCT) at 1060 nm. The vasculature was identified as regions of motion by creating LOGIV and DLOGIV tomograms: multiple B-scans were collected of individual slices through the retina and the variance of logarithmic intensities and differences of logarithmic intensities were calculated. Both methods captured the small vessels and the meshwork of capillaries associated with the inner retina in en face images over 4 mm(2) in a normal subject.

Differential phase-contrast, swept-source optical coherence tomography at 1060 nm for in vivo human retinal and choroidal vasculature visualization

Human retinal and choroidal vasculature was visualized by a differential phase-contrast (DPC) method using high-speed, swept-source optical coherence tomography (SS-OCT) at 1060 nm. The vasculature was recognized as regions of motion by creating differential phase-variance (DPV) tomograms: multiple B-scans of individual slices through the retina were collected and the variance of the phase differences was calculated. DPV captured the small vessels and the meshwork of capillaries associated with the inner retina in en-face images over 4 mm(2). The swept-source laser at 1060 nm offered the needed phase sensitivity to perform DPV and generated en-face images that capture motion in the inner choroidal layer exceeding the capabilities of previous spectrometer-based instruments. In comparison with the power Doppler phase-shift method, DPV provided better visualization of the foveal avascular zone in en-face images.

Tensor total variation approach to optical coherence tomography reconstruction for improved visualization of retinal microvasculature

A novel optical coherence tomography (OCT) reconstruction approach is introduced for improved visualization of inner-retina capillaries in retinal OCT tomograms. The proposed method utilizes a minimization framework based on a tensor total variation (TTV) energy functional, to enforce capillary structural characteristics in the spatial domain. By accounting for structure tensor characteristics, the TTV reconstruction method allows for contrast enhancement of capillary structural characteristics. The novel TTV method was tested on high resolution OCT images acquired in-vivo from the foveal region of the retina of a healthy human subject. Experimental results demonstrate significant contrast and visibility enhancement of the inner retina capillaries in the retinal OCT tomograms, achieved by use of the TTV reconstruction method. Therefore, the TTV method has a strong potential for improved disease progression analysis based on the study of disease-induced changes in the inner retina vasculature.

Motion-insensitive optical coherence tomography based micro-angiography

An improved image processing procedure for suppressing the phase noise due to a motion artifact acquired during optical coherence tomography scanning and effectively illustrating the blood vessel distribution in a living tissue is demonstrated. This new processing procedure and the widely used procedure for micro-angiography application are based on the selection of high-frequency components in the spatial-frequency spectrum of B-mode scanning (x-space), which are contributed from the image portions of moving objects. However, by switching the processing order between the x-space and k-space, the new processing procedure shows the superior function of effectively suppressing the phase noise due to a motion artifact. After the blood vessel positions are precisely acquired based on the new processing procedure, the projected blood flow speed can be more accurately calibrated based on a previously reported method. The demonstrated new procedure is useful for clinical micro-angiography application, in which a stepping motor of generating motion artifacts is usually used in the scanning probe.

Polarization sensitive optical coherence tomography in the human eye

Optical coherence tomography (OCT) has become a well established imaging tool in ophthalmology. The unprecedented depth resolution that is provided by this technique yields valuable information on different ocular tissues ranging from the anterior to the posterior eye segment. Polarization sensitive OCT (PS-OCT) extends the concept of OCT and utilizes the information that is carried by polarized light to obtain additional information on the tissue. Several structures in the eye (e.g. cornea, retinal nerve fiber layer, retinal pigment epithelium) alter the polarization state of the light and show therefore a tissue specific contrast in PS-OCT images. First this review outlines the basic concepts of polarization changing light-tissue interactions and gives a short introduction in PS-OCT instruments for ophthalmic imaging. In a second part a variety of different applications of this technique are presented in ocular imaging that are ranging from the anterior to the posterior eye segment. Finally the benefits of the method for imaging different diseases as, e.g., age related macula degeneration (AMD) or glaucoma is demonstrated.

Total retinal blood flow measurement with ultrahigh speed swept source/Fourier domain OCT

Doppler OCT provides depth-resolved information on flow in biological tissues. In this article, we demonstrate ultrahigh speed swept source/Fourier domain OCT for visualization and quantitative assessment of retinal blood flow. Using swept laser technology, the system operated in the 1050-nm wavelength range at a high axial scan rate of 200 kHz. The rapid imaging speed not only enables volumetric imaging with high axial scan densities, but also enables measurement of high flow velocities in the central retinal vessels. Deep penetration in the optic nerve and lamina cribrosa was achieved by imaging at 1-µm wavelengths. By analyzing en-face images extracted from 3D Doppler data sets, absolute flow in single vessels as well as total retinal blood flow was measured using a simple and robust protocol that does not require measurement of Doppler angles. The results from measurements in healthy eyes suggest that ultrahigh speed swept source/Fourier domain OCT could be a promising technique for volumetric imaging of retinal vasculature and quantitation of retinal blood flow in a wide range of retinal diseases.

Total retinal blood flow measurement with ultrahigh speed swept source/Fourier domain OCT

Doppler OCT provides depth-resolved information on flow in biological tissues. In this article, we demonstrate ultrahigh speed swept source/Fourier domain OCT for visualization and quantitative assessment of retinal blood flow. Using swept laser technology, the system operated in the 1050-nm wavelength range at a high axial scan rate of 200 kHz. The rapid imaging speed not only enables volumetric imaging with high axial scan densities, but also enables measurement of high flow velocities in the central retinal vessels. Deep penetration in the optic nerve and lamina cribrosa was achieved by imaging at 1-µm wavelengths. By analyzing en-face images extracted from 3D Doppler data sets, absolute flow in single vessels as well as total retinal blood flow was measured using a simple and robust protocol that does not require measurement of Doppler angles. The results from measurements in healthy eyes suggest that ultrahigh speed swept source/Fourier domain OCT could be a promising technique for volumetric imaging of retinal vasculature and quantitation of retinal blood flow in a wide range of retinal diseases.

In vivo volumetric imaging of human retinal circulation with phase-variance optical coherence tomography

We present in vivo volumetric images of human retinal micro-circulation using Fourier-domain optical coherence tomography (Fd-OCT) with the phase-variance based motion contrast method. Currently fundus fluorescein angiography (FA) is the standard technique in clinical settings for visualizing blood circulation of the retina. High contrast imaging of retinal vasculature is achieved by injection of a fluorescein dye into the systemic circulation. We previously reported phase-variance optical coherence tomography (pvOCT) as an alternative and non-invasive technique to image human retinal capillaries. In contrast to FA, pvOCT allows not only noninvasive visualization of a two-dimensional retinal perfusion map but also volumetric morphology of retinal microvasculature with high sensitivity. In this paper we report high-speed acquisition at 125 kHz A-scans with pvOCT to reduce motion artifacts and increase the scanning area when compared with previous reports. Two scanning schemes with different sampling densities and scanning areas are evaluated to find optimal parameters for high acquisition speed in vivo imaging. In order to evaluate this technique, we compare pvOCT capillary imaging at 3x3 mm(2) and 1.5x1.5 mm(2) with fundus FA for a normal human subject. Additionally, a volumetric view of retinal capillaries and a stitched image acquired with ten 3x3 mm(2) pvOCT sub-volumes are presented. Visualization of retinal vasculature with pvOCT has potential for diagnosis of retinal vascular diseases.

Segmentation of Doppler optical coherence tomography signatures using a support-vector machine

When processing Doppler optical coherence tomography images, there is a need to segment the Doppler signatures of the vessels. This can be used for visualization, for finding the center point of the flow areas or to facilitate the quantitative analysis of the vessel flow. We propose the use of a support-vector machine classifier in order to segment the flow. It uses the phase values of the Doppler image as well as texture information. We show that superior results compared to conventional simple threshold-based methods can be achieved in conditions of significant phase noise, which inhibit the use of a simple threshold of the phase values.

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.

Volumetric and quantitative imaging of retinal blood flow in rats with optical microangiography

In this paper, we present methods for 3D visualization and quantitative measurements of retinal blood flow in rats by the use of optical microangiography imaging technique (OMAG). We use ultrahigh sensitive OMAG to provide high-quality 3D RBF perfusion maps in the rat eye, from which the Doppler angle, as well as the diameters of blood vessels, are evaluated. Estimation of flow velocity (i.e. axial flow velocity) is achieved by the use of Doppler OMAG, which has its origins in phase-resolved Doppler optical coherence tomography. The measurements of the Doppler angle, vessel size, and the axial velocity lead to the quantitative assessment of the absolute flow velocity and the blood flow rate in selected retinal vessels. We demonstrate the feasibility of OMAG to provide 3D microangiograms and quantitative assessment of retinal blood flow in a rat model subjected to raised intra-ocular pressure (IOP). We show that OMAG is capable of monitoring the longitudinal response of absolute blood velocity and flow rate of retinal blood vessels to increased IOP in the rat, demonstrating its usefulness for ophthalmological research.

MRI of retinal and choroidal blood flow with laminar resolution

The retina is nourished by two distinct circulations: the retinal vessels within the inner retina and the choroidal vessels behind the neural retina. The outer nuclear layer and the inner and outer segments of the photoreceptors in between are avascular. The aim of this study was to determine whether arterial spin labeling MRI could provide sufficient resolution to differentiate between quantitative retinal blood flow (rBF) and choroidal blood flow (chBF), and whether this technique is sufficiently sensitive to detect vascular-specific blood flow (BF) changes modulated by anesthetics. Arterial spin labeling MRI was performed at 42 × 42 × 400 µm(3) in the mouse retina at 7 T, and was used to investigate the effects of isoflurane and ketamine/xylazine anesthesia on rBF and chBF. MRI yielded unambiguous differentiation of rBF, chBF and the avascular layer in between. Under isoflurane, chBF was 7.7 ± 2.1 mL/g/min and rBF was 1.3 ± 0.44 mL/g/min (mean ± SD, n = 7, p < 0.01). Under ketamine/xylazine anesthesia in the same animals, chBF was 4.3 ± 1.9 mL/g/min and rBF was 0.88 ± 0.22 mL/g/min (p < 0.01). Under ketamine/xylazine anesthesia, rBF was lower by 29% (P < 0.01) and chBF by 42% (P < 0.01) relative to isoflurane. This study demonstrates, for the first time, the quantitative imaging of rBF and chBF in vivo, providing a new method to study basal values and alterations of rBF and chBF.

Comprehensive in vivo micro-vascular imaging of the human eye by dual-beam-scan Doppler optical coherence angiography

Comprehensive angiography provides insight into the diagnosis of vascular-related diseases. However, complex microvascular networks of unstable in vivo organs such as the eye require micron-scale resolution in three dimensions and a high sampling rate to access a wide area as maintaining the high resolution. Here, we introduce dual-beam-scan Doppler optical coherence angiography (OCA) as a label-free comprehensive ophthalmic angiography that satisfies theses requirements. In addition to high resolution and high imaging speed, high sensitivity to motion for detecting tiny blood flow of microvessels is achieved by detecting two time-delayed signals with scanning of two probing beams separated on a sample. We present in vivo three-dimensional imaging of the microvasculature of the posterior part of the human eye. The demonstrated results show that this technique may be used for comprehensive ophthalmic angiography to evaluate the vasculature of the posterior human eye and to diagnose variety of vascular diseases.

Stable absolute flow estimation with Doppler OCT based on virtual circumpapillary scans

Doppler optical coherence tomography has the capability to measure blood flow quantitatively and in vivo. As only the axial component of the velocity can be assessed, the measurements have to be corrected for the angle of the vessels. We present a novel approach to extract quantitative flow data from circumpapillary scans in vivo on the human retina by registering the circular scan to a reference volume scan and extracting the angle directly from the volume. In addition, we perform phase unwrapping and interpolation of the flow under the assumption of a parabolic flow profile. We demonstrate the repeatability of the methods by applying it to different retinal vessels, achieving coefficients of variation of the average velocity of 3 to 8%. Results on the pulsatility and resistance index are also presented.

Multi-megahertz OCT: High quality 3D imaging at 20 million A-scans and 4.5 GVoxels per second

We present ultra high speed optical coherence tomography (OCT) with multi-megahertz line rates and investigate the achievable image quality. The presented system is a swept source OCT setup using a Fourier domain mode locked (FDML) laser. Three different FDML-based swept laser sources with sweep rates of 1, 2.6 and 5.2MHz are compared. Imaging with 4 spots in parallel quadruples the effective speed, enabling depth scan rates as high as 20.8 million lines per second. Each setup provides at least 98dB sensitivity and approximately 10microm resolution in tissue. High quality 2D and 3D imaging of biological samples is demonstrated at full scan speed. A discussion about how to best specify OCT imaging speed is included. The connection between voxel rate, line rate, frame rate and hardware performance of the OCT setup such as sample rate, analog bandwidth, coherence length, acquisition dead-time and scanner duty cycle is provided. Finally, suitable averaging protocols to further increase image quality are discussed.

Signal power decrease due to fringe washout as an extension of the limited Doppler flow measurement range in spectral domain optical coherence tomography

The recently introduced new phase-dependent Doppler model for spectral domain optical coherence tomography (SD OCT) has shown that the simple linear relation between the Doppler phase shift and the axial velocity component of an obliquely moving sample is not valid. Additionally, for nearly transverse sample motion with high velocities the phase shift will approach a constant value. Consequently, for small Doppler angles the velocity measurement range of the phase-resolved Doppler analysis is limited in SD OCT. Since these undesirable small Doppler angles can not be prevented, for example, in the in vivo 3-D measurement, we introduce a novel method extending the limited velocity detection range taking the signal power decrease due to fringe washout in SD OCT into account. The signal damping of an obliquely moving sample is presented as a function of the axial and transverse displacement by a universally valid contour plot and does not correspond simply to the sum of the axial and transverse effect. A quantitative combination of the Doppler analysis and the signal-damping method is presented with a flow phantom model. The practicability of this new combined method is presented for the blood flow of the saphenous artery in the in vivo mouse model.

High-speed optical coherence tomography: basics and applications

In the past decade we have observed a rapid development of ultrahigh-speed optical coherence tomography (OCT) instruments, which currently enable performing cross-sectional in vivo imaging of biological samples with speeds of more than 100,000 A-scans/s. This progress in OCT technology has been achieved by the development of Fourier-domain detection techniques. Introduction of high-speed imaging capabilities lifts the primary limitation of early OCT technology by giving access to in vivo three-dimensional volumetric reconstructions on large scales within reasonable time constraints. As result, novel tools can be created that add new perspective for existing OCT applications and open new fields of research in biomedical imaging. Especially promising is the capability of performing functional imaging, which shows a potential to enable the differentiation of tissue pathologies via metabolic properties or functional responses. In this contribution the fundamental limitations and advantages of time-domain and Fourier-domain interferometric detection methods are discussed. Additionally the progress of high-speed OCT instruments and their impact on imaging applications is reviewed. Finally new perspectives on functional imaging with the use of state-of-the-art high-speed OCT technology are demonstrated.

Ultrahigh sensitive optical microangiography for in vivo imaging of microcirculations within human skin tissue beds

In this paper, we demonstrate for the first time that the detailed cutaneous blood flow at capillary level within dermis of human skin can be imaged by optical micro-angiography (OMAG) technique. A novel scanning protocol, i.e. fast B scan mode is used to achieve the capillary flow imaging. We employ a 1310nm system to scan the skin tissue at an imaging rate of 300 frames per second, which requires only ~5 sec to complete one 3D imaging of capillary blood flow within skin. The technique is sensitive enough to image the very slow blood flows at ~4 microm/sec. The promising results show a great potential of OMAG's role in the diagnosis, treatment and management of human skin diseases.

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.

Volumetric microvascular imaging of human retina using optical coherence tomography with a novel motion contrast technique

Phase variance-based motion contrast imaging is demonstrated using a spectral domain optical coherence tomography system for the in vivo human retina. This contrast technique spatially identifies locations of motion within the retina primarily associated with vasculature. Histogram-based noise analysis of the motion contrast images was used to reduce the motion noise created by transverse eye motion. En face summation images created from the 3D motion contrast data are presented with segmentation of selected retinal layers to provide non-invasive vascular visualization comparable to currently used invasive angiographic imaging. This motion contrast technique has demonstrated the ability to visualize resolution-limited vasculature independent of vessel orientation and flow velocity.

Transverse motion as a source of noise and reduced correlation of the Doppler phase shift in spectral domain OCT

Recently, a new phase-resolved Doppler model was presented for spectral domain optical coherence tomography (SD OCT) showing that the linear relation between the axial velocity component of the obliquely moving sample and the phase difference of consecutive A-Scans does not hold true in the presence of a transverse velocity component which is neglected in the widely-used classic Doppler analysis. Besides taking note of the new non-proportional relationship of phase shift and oblique sample motion, it is essential to consider the correlation of the phase shift and its specific characteristic at certain Doppler angles for designing Doppler experiments with SD OCT. A correlation quotient is introduced to quantify the correlation of the backscattering signal in consecutive A-Scans as a function of the oblique sample motion. It was found that at certain velocities and Doppler angles no correlation of the phases of sequential A-Scans exists, even though the signal does not vanish. To indicate how the noise of the Doppler phase shift behaves for oblique movement, the standard deviation is determined as a function of the correlation quotient and the number of complex Doppler data averaged. The detailed theoretical model is validated by using a flow phantom model consisting of a 1% Intralipid flow through a 310 microm capillary. Finally, a short discussion of the presented results and the consequence for performing Doppler experiments is given.

Flow velocity estimation by complex ambiguity free joint Spectral and Time domain Optical Coherence Tomography

We show that recently introduced joint Spectral and Time domain Optical Coherence Tomography (STdOCT) can be used for simultaneous complex ambiguity removal and functional Spectral OCT images. This permits to take advantage of higher sensitivity achievable near the zero-path delay. The technique can be used with all Spectral OCT systems that are equipped with an optical delay line (ODL) and provide oversampled scanning patterns. High sensitivity provided by STdOCT allows this technique to be used in Spectral OCT setups with acquisition speed of 100,000 lines/s. We show that different imaging ranges and velocity ranges can be achieved by switching on/off the ODL and a small modification in the processing algorithm. Additionally, the relatively small computational burden of the technique allows for fast computations in the range of less than 5 minutes for 3D data set. We present application of proposed technique to full-range two- and three-dimensional imaging. Morphological and Doppler tomograms of human retina in-vivo are shown. Finally, we identify and discuss artifacts of the technique.