The development of functional retinal imaging is of great interest to clinical and experimental ophthalmology, because it should provide more sensitive tools for ocular diseases diagnostic that would go beyond current gold standard of simple evaluation of the static retinal morphology. In this presentation we will review our recent progress in measurements and interpretation of OCT-based optoretinograms (ORG) i.e., the paradigm of using NIR OCT to measure in vivo bleaching-induced changes in retinal morphology (transient changes in volume of individual neurons, or thickness of retinal layers). Specifically, comparison between different instrumentations used to acquire ORGs and between results acquired using clinical (human) and experimental (animal) systems will be presented. Additionally, intensity-based and phase-based ORG extraction framework will be presented. Finally, we will discuss our findings in the context of current understanding of measured process, being a result osmotically driven water movements between the photoreceptors, and other retinal neurons and its surroundings.
In this work we present a retinal imaging system that incorporates full-field (FF) swept-source (SS) optical coherence tomography (OCT) with hardware adaptive optics (AO) correction operating in real time. We demonstrate that this configuration permits resolution of foveal cones, with volume rates adequate to measure light-evoked changes in photoreceptors. With the OCT reference arm blocked, the system is a high speed (kHz) AO flood illumination (FI) camera, supporting future studies of optoretinographic responses through common path interference. The characterization and demonstration of the system performance with in vivo human photoreceptor mosaic are presented.
KEYWORDS: Eye, Angiography, Optical coherence tomography, In vivo imaging, Image quality, Data acquisition, Wavefronts, Wavefront sensors, Visualization, Systems modeling
Accurate and reproducible OCT angiography (OCTA) measurements are highly dependent on the overall phase stability of the sample. Raster-scanning OCT systems are vulnerable to eye motion, which makes phase correlation impossible if the retinal displacement is too large. Numerical methods exist to correct components of phase shifts due to the axial movement, but that due to lateral movement bigger, then imaging spot are not generally correctable. Real-time eye tracking provides a method to reduce the phase shifts caused by lateral eye movement. Here we report the advancements on monitoring ocular metrics during OCTA acquisition and its effects on image quality.
In vivo functional imaging of human photoreceptors is an emerging field, with compelling potential applications in basic science, translational research, and clinical management of ophthalmic disease. Measurement of light-evoked changes in the cone photoreceptors has been successfully demonstrated using adaptive optics (AO) coherent flood illumination (CFI), AO scanning light ophthalmoscopy (SLO), AO optical coherence tomography (OCT), and full-field OCT with digital aberration correction (DAC). While the optical and computational principles of these systems differ greatly, and while these differences manifest in the resulting measurements, we believe that the approaches are all sensitive to light-evoked swelling of the cells. We describe a combined OCT-SLO with AO designed to measure this light-evoked swelling. In addition to OCT measurement of cone responses, we report their simultaneous OCT-SLO measurement as well as OCT measurement of rod photoreceptor function, neither of which, to our knowledge, have been reported before.
Objective optical assessment of photoreceptor function may permit earlier diagnosis of retinal disease than current methods such as perimetry, electrophysiology, and clinical imaging. Recent work with adaptive optics (AO) flood imaging, conventional OCT and phase-sensitive full-field OCT have revealed apparent changes in photoreceptor outer segment (OS) length in response to visible stimuli. In this work, we describe an AO-OCT system designed to measure these stimulus-evoked OS length changes. The OCT subsystem consisted of a Fourier-domain mode-locked laser that acquires A-scans at 1.64MHz and an AO subsystem providing diffraction-limited imaging with a closed-loop correction rate of 20Hz. To our knowledge this is the highest-speed AO-OCT system developed to date. Visible stimuli were delivered using a LED-based Maxwellian view channel incorporated into the system. In a dark-adapted healthy subject, 1-deg square volumetric images were acquired at a rate of 32Hz. Images were acquired for 10s, with a 10ms bleaching stimulus flash with variable intensity. Strip-based registration was used to track individual cones in the volume series, and time series of the resulting depth-resolved complex signal were analyzed. Stimulus-evoked changes in the morphology of OS and RPE were observed in the M-scan amplitude. In the M-scan phase, the difference between the IS/OS and COST was shown to increase in response to the stimulus flash, and the magnitude of the phase change depended upon flash intensity. These results suggest that cone OS elongates in response to visible stimuli, and that the length change scales with stimulus intensity.
We have designed and implemented a dual-mode adaptive optics (AO) imaging system that combines spectral domain optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) for in vivo imaging of the human retina. The system simultaneously acquires SLO frames and OCT B-scans at 60 Hz with an OCT volume acquisition time of 4.2 s. Transverse eye motion measured from the SLO is used to register the OCT B-scans to generate three-dimensional (3-D) volumes. Key optical design considerations include: minimizing system aberrations through the use of off-axis relay telescopes, conjugate pupil plane requirements, and the use of dichroic beam splitters to separate and recombine the OCT and SLO beams around the nonshared horizontal scanning mirrors. To demonstrate system performance, AO-OCT-SLO images and measurements are taken from three normal human subjects ranging in retinal eccentricity from the fovea out to 15-deg temporal and 20-deg superior. Also presented are en face OCT projections generated from the registered 3-D volumes. The ability to acquire high-resolution 3-D images of the human retina in the midperiphery and beyond has clinical importance in diseases, such as retinitis pigmentosa and cone–rod dystrophy.
We present noninvasive, three-dimensional, depth-resolved imaging of human retinal and choroidal blood circulation with a swept-source optical coherence tomography (OCT) system at 1065-nm center wavelength. Motion contrast OCT imaging was performed with the phase-variance OCT angiography method. A Fourier-domain mode-locked light source was used to enable an imaging rate of 1.7 MHz. We experimentally demonstrate the challenges and advantages of wide-field OCT angiography (OCTA). In the discussion, we consider acquisition time, scanning area, scanning density, and their influence on visualization of selected features of the retinal and choroidal vascular networks. The OCTA imaging was performed with a field of view of 16 deg (5 mm×5 mm) and 30 deg (9 mm×9 mm). Data were presented in en face projections generated from single volumes and in en face projection mosaics generated from up to 4 datasets. OCTA imaging at 1.7 MHz A-scan rate was compared with results obtained from a commercial OCTA instrument and with conventional ophthalmic diagnostic methods: fundus photography, fluorescein, and indocyanine green angiography. Comparison of images obtained from all methods is demonstrated using the same eye of a healthy volunteer. For example, imaging of retinal pathology is presented in three cases of advanced age-related macular degeneration.
We demonstrate OCT angiography (OCTA) and Doppler OCT imaging of the choroid in the eyes of two
healthy volunteers and in a geographic atrophy case. We show that visualization of specific choroidal layers requires
selection of appropriate OCTA methods. We investigate how imaging speed, B-scan averaging and scanning density
influence visualization of various choroidal vessels. We introduce spatial power spectrum analysis of OCT en face
angiographic projections as a method of quantitative analysis of choroicapillaris morphology. We explore the possibility
of Doppler OCT imaging to provide information about directionality of blood flow in choroidal vessels. To achieve
these goals, we have developed OCT systems utilizing an FDML laser operating at 1.7 MHz sweep rate, at 1060 nm
center wavelength, and with 7.5 μm axial imaging resolution. A correlation mapping OCA method was implemented for
visualization of the vessels. Joint Spectral and Time domain OCT (STdOCT) technique was used for Doppler OCT
imaging.
We tested and compared the capability of multiple optical coherence tomography (OCT) angiography methods: phase variance, amplitude decorrelation and speckle variance, with application of the split spectrum technique, to image the choroiretinal complex of the human eye. To test the possibility of OCT imaging stability improvement we utilized a real-time tracking scanning laser ophthalmoscopy (TSLO) system combined with a swept source OCT setup. In addition, we implemented a post- processing volume averaging method for improved angiographic image quality and reduction of motion artifacts. The OCT system operated at the central wavelength of 1040nm to enable sufficient depth penetration into the choroid. Imaging was performed in the eyes of healthy volunteers and patients diagnosed with age-related macular degeneration.
We describe the design and performance of a recently implemented retinal imaging system for the human eye that combines adaptive optics (AO) with spectral domain optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO). The AO-OCT-SLO system simultaneously acquires SLO frames and OCT B-scans at 60 Hz with an OCT volume acquisition scan rate of 0.24 Hz. The SLO images are used to correct for eye motion during the registration of OCT B-scans. Key optical design considerations are discussed including: minimizing system aberrations through the use of off-axis relay telescopes; choice of telescope magnification based on pupil plane requirements and restrictions; and the use of dichroic beam splitters to separate and re-combine OCT and SLO beams around the nonshared horizontal scanning mirrors. We include an analysis of closed-loop AO correction on a model eye and compare these findings with system performance in vivo. The 2D and 3D OCT scans included in this work demonstrate the ability of this system to laterally and axially resolve individual cone photoreceptors, while the corresponding SLO images show the en face mosaics at the photoreceptor layer showing rods and cones. Images from both healthy and diseased retina are presented.
We present a noninvasive phase-variance (pv)–based motion contrast method for depth-resolved imaging of the human chorioretinal complex microcirculation with a newly developed phase-stabilized high speed (100-kHz A-scans/s) 1-μm swept-source optical coherence tomography (SSOCT) system. Compared to our previous spectral-domain (spectrometer based) pv-spectral domain OCT (SDOCT) system, this system has the advantages of higher sensitivity, reduced fringe wash-out for high blood flow speeds and deeper penetration in choroid. High phase stability SSOCT imaging was achieved by using a computationally efficient phase stabilization approach. This process does not require additional calibration hardware and complex numerical procedures. Our phase stabilization method is simple and can be employed in a variety of SSOCT systems. Examples of vasculature in the chorioretinal complex imaged by pv-SSOCT from normal as well as diseased eyes are presented and compared to retinal images of the same subjects acquired with fluorescein angiography and indocyanine green angiography. Observations of morphology of vascular perfusion in chorioretinal complex visualized by our method are listed.
We demonstrate the feasibility of our newly developed phase stabilized high-speed (100 kHz A-scans/s) 1 μm sweptsource
optical coherence tomography (SSOCT) system with the phase-variance based motion contrast method for
visualization of human chorioretinal complex microcirculation. Compared to our previously reported spectral domain
(spectrometer based) phase-variance (pv)-SDOCT system it has advantages of higher sensitivity, reduced fringe washout
for high blood flow speeds and deeper penetration in choroid. High phase stability SSOCT imaging was achieved by
using a computationally efficient phase stabilization approach. This process does not require additional calibration
hardware and complex numerical procedures. Our phase stabilization method is simple and can be employed in a variety
of SS-OCT systems. Examples of vasculature in the chorioretinal complex imaged by pv-SSOCT is presented and
compared to retinal images of the same volunteers acquired with fluorescein angiography (FA) and indocyanine green
angiography (ICGA).
This article [J. Biomed. Opt.. 18, , 086002 (2013)] was originally published online on 2 August 2013 with an error in the author list. The first author’s name was reversed, and should be “Raju Poddar” as it appears above.
This article was corrected online on 27 August 2013. It appears correctly in print.
A high-speed (100 kHz A-scans/s ) complex conjugate resolved 1 μm swept source optical coherence tomography (SS-OCT) system using coherence revival of the light source is suitable for dense three-dimensional (3-D) imaging of the anterior segment. The short acquisition time helps to minimize the influence of motion artifacts. The extended depth range of the SS-OCT system allows topographic analysis of clinically relevant images of the entire depth of the anterior segment of the eye. Patients with the type 1 Boston Keratoprosthesis (KPro) require evaluation of the full anterior segment depth. Current commercially available OCT systems are not suitable for this application due to limited acquisition speed, resolution, and axial imaging range. Moreover, most commonly used research grade and some clinical OCT systems implement a commercially available SS (Axsun) that offers only 3.7 mm imaging range (in air) in its standard configuration. We describe implementation of a common swept laser with built-in k-clock to allow phase stable imaging in both low range and high range, 3.7 and 11.5 mm in air, respectively, without the need to build an external MZI k-clock. As a result, 3-D morphology of the KPro position with respect to the surrounding tissue could be investigated in vivo both at high resolution and with large depth range to achieve noninvasive and precise evaluation of success of the surgical procedure.
We present a new design for a reflective afocal AO-OCT retinal imaging system. The optical performance of this instrument is compared to our previous multimodal AO-OCT/AO-SLO retinal imaging system. The feasibility of new instrumentation for improved visualization of microscopic retinal structures will be discussed. Examples of images acquired with this new AO-OCT instrument will be presented.
We present in vivo noninvasive retinal and choroidal perfusion maps with phase-variance optical coherence tomography (pvOCT). We acquired a pvOCT volumetric data set of a normal subject and visualized blood circulation in the retina and the choroid. En face projection views of the retina as well as the choroid were generated from a manually segmented volumetric data set. In addition, the processed pvOCT images were compared to current standard imaging modalities used for retinal and choroidal vasculature visualization in clinical settings, including fluorescein angiography (FA) and indocyanine green angiography (ICGA).
We present a high-speed complex conjugate resolved 1 μm swept source optical coherence tomography [SS-OCT] system using coherence revival of the light source for clinical imaging of the anterior segment of the eye. High-speed of 100,000 A-scans/sec and 1 μm imaging window of OCT permits dense 3D imaging of the anterior segment, minimizing the influence of motion artifacts and deep penetration of images for topographic analysis. The swept laser performance with internal clocking was adapted to achieve extended imaging depth requirements. The feasibility of our instrument for visualization of the anterior segment of patients with the Boston Keratoprosthesis (KPro) was discussed. The relations between of the KPro and the surrounding tissue were also demonstrated.
We acquired in vivo images of the human optic nerve head (ONH) using an adaptive optics—optical coherence tomography (AO-OCT) system. In order to improve imaging of the lamina cribrosa in the ONH with high lateral resolution and sensitivity, we implemented a complex conjugate artifact-free Fourier domain OCT (Fd-OCT) acquisition scheme with a reference arm-based phase shifting method. This allowed positioning of the lamina cribrosa structures near the zero path length difference where AO-OCT imaging achieves highest sensitivity. Implementation of our complex conjugate artifact removal (CCR) method required constant phase shifts between consecutive axial scans (A-scans), generated by continuous beam path-length changes from offsetting the pivot point of the scanning mirror placed in the reference arm. Fourier transform along the transverse axis and a filtering algorithm allowed reconstruction of CCR AO-OCT images. The suppression ratio of the mirror artifact was approximately 22 dB (at 18,000 A-scans per second acquisition speed) with a paperboard test target and an optimum phase-shift value. Finally, we reconstructed the three-dimensional structure of human ONH with enhanced depth range and sensitivity using CCR AO-OCT.
Recent progress in retinal image acquisition techniques, including optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO), combined with improved performance of adaptive optics (AO) instrumentation, has resulted in improvement in the quality of in vivo images of cellular structures in the outer layers of the human retina. Despite the significant progress in imaging cone and rod photoreceptor mosaics, visualization of cellular structures in the
inner retina has been achieved only with extrinsic contrast agents that have not been approved for use with humans. In this paper we describe the main limiting factors in visualizing inner retinal cells and the methods we implemented to reduce their effects on images acquired with AO-OCT. These include improving the system point spread function (AO performance), monitoring of motion artifacts (retinal motion tracking), and speckle pattern reduction (temporal and spatial averaging). Results of imaging inner retinal morphology and the improvement offered by the new UC Davis AOOCT
system with spatio-temporal image averaging are presented.
KEYWORDS: Optical coherence tomography, Retina, Image segmentation, Visualization, Capillaries, Image processing, Doppler tomography, Data acquisition, In vivo imaging, Data processing
We evaluate methods to visualize human retinal micro-circulation in vivo by standard intensity-based optical coherence
tomography (OCT), speckle-variance optical coherence tomography (svOCT), and phase-variance optical coherence
tomography (pvOCT). En face projection views created from the same volumetric data set of the human retina using all
three data processing methods are created and compared. Additionally we used support vector machine (SVM) based
semi-automatic segmentation to generate en face projection views of individual retinal layers. The layers include: first,
the whole inner retina (from the nerve fiber layer to the outer nuclear layer), and second, from the ganglion cell layer to
the outer nuclear layer. Finally, we compare the retinal vasculature images processed from the three OCT techniques and
fluorescein angiography (FA).
Nowadays in ophthalmologic practice several commercial instruments are available to image patient retinas in vivo.
Many modern fundus cameras and confocal scanning laser ophthalmoscopes allow acquisition of two dimensional en
face images of the retina with both back reflected as well as fluorescent light. Additionally, optical coherence
tomography systems allow non-invasive probing of three-dimensional retinal morphology. For all of these instruments
the available lateral resolution is limited by optical quality of the human eye used as the imaging objective. To improve
lateral resolution and achieve diffraction-limited imaging, adaptive optics (AO) can be implemented with any of these
imaging systems to correct both static and dynamic aberrations inherent in human eyes. Most of the wavefront correctors
used previously in AO systems have limited dynamic range and an insufficient number of actuators to achieve
diffraction-limited correction of most human eyes. Thus, additional corrections were necessary, either by trial lenses or
additional deformable mirrors (DMs). The UC Davis AO flood-illuminated fundus camera system described in this
paper has been previously used to acquire in vivo images of the photoreceptor mosaic and for psychophysical studies on
normal and diseased retinas. These results were acquired using a DM manufactured by Litton ITEK (DM109), which has
109 actuators arranged in a hexagonal array below a continuous front-surface mirror. It has an approximate surface
actuator stroke of ±2μm. Here we present results with a new hi-speed magnetic DM manufactured by ALPAO (DM97,
voice coil technology), which has 97 actuators and similar inter-actuator stroke (>3μm, mirror surface) but much higher
low-order aberration correction (defocus stroke of at least ±30μm) than the previous one. In this paper we report results
of testing performance of the ALPAO DM for the correction of human eye aberrations. Additionally changes made to
our AO flood illuminated system are presented along with images of the model eye retina and in-vivo human retina
acquired with this system.
We summarize the performance of an AO-OCT system with reference arm phase shifting for complex conjugate artifactfree
imaging of in vivo retinal structures. As a complex conjugate artifact removal (CCR) method we used a previously
reported technique requiring constant phase shifts between consecutive A-scans. In our system these shifts were
generated by continuous beam path-length changes from offsetting the pivot point of the scanning mirror placed in the
system reference arm. In order to reconstruct the complex spectral fringe pattern we used Fourier transformation along
the transverse axis and a filtering algorithm. The suppression ratio of mirror complex artifact images was assessed based
on acquired in vivo CCR AO-OCT images. Finally, potential problems and limitations connected with this acquisition
scheme and data processing algorithms are discussed.
KEYWORDS: Optical coherence tomography, Visualization, Retina, In vivo imaging, Data acquisition, Phase contrast, Angiography, Imaging systems, Data processing, CMOS sensors
We present high-speed Fourier-domain optical coherence tomography (Fd-OCT) with the phase variance based motion
contrast method for visualizing retinal micro-circulation in vivo. This technique allows non-invasive visualization of a
two-dimensional retinal perfusion map and concurrent volumetric morphology of retinal microvasculature with high
sensitivity. The high-speed acquisition rate at 125kHz A-scans enables reduction of motion artifacts with increased
scanning area if compared to previously reported results. Several scanning schemes with different sampling densities and
scanning areas are evaluated to find optimal parameters for in vivo imaging. In order to evaluate this technique, we
compare OCT micro-capillary imaging using the phase variance technique with fundus fluorescein angiography (FA).
Additionally, volumetric visualization of blood flow for a normal subject is presented.
Eye movements present during acquisition of a retinal image with optical coherence tomography (OCT) introduce
motion artifacts into the image, complicating analysis and registration. This effect is especially pronounced in highresolution
data sets acquired with adaptive optics (AO)-OCT instruments. Several retinal tracking systems have been
introduced to correct retinal motion during data acquisition. We present a method for correcting motion artifacts in AOOCT
volume data after acquisition using simultaneously captured adaptive optics-scanning laser ophthalmoscope (AOSLO)
images. We extract transverse eye motion data from the AO-SLO images, assign a motion adjustment vector to
each AO-OCT A-scan, and re-sample from the scattered data back onto a regular grid. The corrected volume data
improve the accuracy of quantitative analyses of microscopic structures.
Single spectrometer-based complex conjugate artifact removal methods are evaluated for in vivo imaging with complementary metal-oxide semiconductor line scan camera based high-speed Fourier-domain optical coherence tomography (FD-OCT) at 100,000 axial scans per second. Performance of three different phase-shifting methods with the same OCT engine is evaluated using modified data acquisition schemes, depending on the requirements of each phase-shifting technique. The suppression ratio of complex conjugate artifact images using a paperboard is assessed for all tested methods. Several other characteristics, including a list of additional hardware requirements (beyond standard FD-OCT components) and data acquisition schemes for each of the methods is presented. In vivo full-range images of human fingerpad and nail are shown and compared with standard FD-OCT images. Additionally, a complex-conjugate-free human retinal volume acquired at the speed of 100,000 A-scans/s is presented.
We describe results of retinal imaging with a novel instrument that combines adaptive optics - Fourier-domain optical
coherence tomography (AO-OCT) with an adaptive optics scanning laser ophthalmoscope (AO-SLO). One of the
benefits of combining Fd-OCT with SLO includes automatic co-registration between the two imaging modalities and the
potential for correcting lateral and transversal eye motion resulting in motion artifact-free volumetric retinal imaging.
Additionally this allows for direct comparison between retinal structures that can be imaged with both modalities (e.g.,
photoreceptor mosaics or microvasculature maps). This dual imaging modality could provide insight into some retinal
properties that could not be accessed by a single imaging system. Additionally, extension of OCT and SLO beyond
structural imaging may open new avenues for diagnostics and testing in ophthalmology. In particular, non-invasive
vasculature mapping with these modalities holds promise of replacing fluorescein angiography in vascular identification.
Several new improvements of our system are described, including results of testing a novel 97-actuator deformable
mirror and AO-SLO light intensity modulation.
An accurate solid eye model (with volumetric retinal morphology) has many applications in the field of ophthalmology, including evaluation of ophthalmic instruments and optometry/ophthalmology training. We present a method that uses volumetric OCT retinal data sets to produce an anatomically correct representation
of three-dimensional (3D) retinal layers. This information is exported to a laser scan system to re-create it within solid eye retinal morphology of the eye used in OCT testing. The solid optical model eye is constructed from PMMA acrylic, with equivalent optical power to that of the human eye (~58D).
Additionally we tested a water bath eye model from Eyetech Ltd. with a customized retina consisting of five layers of ~60 μm thick biaxial polypropylene film and hot melt rubber adhesive.
We evaluated several, previously published, complex conjugate artifact removal methods and algorithms that have been
proposed for Fourier domain optical coherence tomography (Fd-OCT). To ensure comparable conditions, only one OCT
system was used, but with modified data acquisition schemes, depending on the requirements of each method/algorithm.
This limited our evaluation to single spectrometer based Fd-OCT approaches. The suppression ratio of complex
conjugate artifact images using a paperboard is assessed for all tested methods. Several other metrics are also used for
comparison, including a list of additional hardware requirements (beyond standard Fd-OCT components) and data
acquisition schemes. Finally, in vivo human finger pad and nail images are presented for comparison to the standard Fd-
OCT images and full-range images.
Adaptive optics (AO) and optical coherence tomography (OCT) are powerful imaging modalities that, when
combined, can provide high-resolution (3.5 μm isotropic), 3-D images of the retina. The AO-OCT system at
UC Davis has demonstrated the utility of this technology for microscopic, volumetric, in vivo retinal imaging.
The current system uses an AOptix bimorph deformable mirror (DM) for low-order, high-stroke correction and
a 140-actuator Boston Micromachines DM for high-order correction. Developments to improve performance or
functionality of the instrument are on-going. Based on previous work in system characterization we have focused
on improved AO control. We present preliminary results and remaining challenges for a newly implemented
Fourier transform reconstructor (FTR). The previously reported error budget analysis is also reviewed and
updated, with consideration of how to improve both the amount of residual error and the robustness of the
system. Careful characterization of the AO system will lead to improved performance and inform the design of
future systems.
Imaging the structure and correlating it with the biochemical content of the retina holds promise for fundamental
research and for clinical applications. Optical coherence tomography (OCT) is commonly used to image the 3D
structure of the retina and while the added functionality of biochemical analysis afforded by Raman scattering
could provide critical molecular signatures for clinicians and researchers, there are many technical challenges to
combining these imaging modalities. We present an ex vivo OCT microscope combined with Raman spectroscopy
capable of collecting morphological and molecular information about a sample simultaneously. The combined
instrument will be used to investigate remaining technical challenges to combine these imaging modalities, such
as the laser power levels needed to achieve a Raman signal above the noise level without damaging the sample.
We describe and compare two volume visualization methods for Optical Coherence Tomography (OCT) retinal
data sets. One of these methods is CPU-slicing, which is previously reported and used in our visualization engine.
The other is GPU-ray casting. Several metrics including image quality, performance, hardware limitations and
perception are used to grade the abilities of each method. We also discuss how to combine these methods to
make a scalable volume visualization system that supports advanced lighting and dynamic volumetric shadowing
techniques on a broad range of hardware. The feasibility of each visualization method for clinical application as
well as potential further improvements are discussed.
We describe a novel instrument that combines adaptive optics - Fourier-domain optical coherence tomography (AO-OCT) with an adaptive optics scanning laser ophthalmoscope (AO-SLO). Both systems share a common AO sub-system and vertical scanner to permit simultaneous acquisition of retinal images from both OCT and SLO. One of the benefits of combining OCT with SLO includes automatic co-registration between the two imaging modalities and potential for correcting lateral and transversal eye motion resulting in motion artifact-free volumetric retinal imaging. Results of using this system for eye model imaging are presented. Feasibility for clinical application is briefly discussed as well as potential further improvements of the current system.
Recent developments in adaptive optics - optical coherence tomography (AO-OCT) allow for ultra-high isotropic resolution imaging of the
in-vivo retina, offering unprecedented insight into its volumetric microscopic and cellular structures. In addition to this promising achievement, the clinical impact and application of this technology still needs to be explored. This includes assessment of limitations and challenges for existing as well as future AO-OCT systems,
especially in the context of potential transfer of this technology from an optical bench to a portable imaging system. To
address these issues we will describe our current AO-UHR-OCT focusing on its sub-components, as well as application for clinical imaging. Additionally, we describe some directions for future development of our AO-OCT instrument that would improve its clinical utility including: new compact AO-OCT design, new improved AO sub-system (extreme AO), and new generations of Fourier-domain-OCT.
Ultra-high isotropic resolution imaging of retinal structures was made possible with an adaptive optics system using dual deformable mirrors and a Fourier-domain optical coherence tomography (Fd-OCT) system with correction for longitudinal chromatic aberration. This system was used to image microscopic retinal structures of healthy as well as diseased retinas in vivo. The improved resolution and contrast enhanced visualization of morphological structures in the retina can be clearly seen. The benefits of this instrument are apparent from comparison of new images with those acquired using a previous generation AO-OCT instrument. Big change in the appearance of speckle field (reduction in speckle size) can be observed as well. Additionally, further improvements in volumetric data acquisition and image representation will be discussed. This includes creation of large Field of View (FOV) AO-OCT volume from multiple sub-volumes and its visualization. Also techniques and results of reducing speckle contrast by averaging multiple B-scans will be presented.
Advances in Fourier-domain optical coherence tomography (Fd-OCT) permit visualization of three-dimensional morphology of in-vivo retinal structures in a way that promises to revolutionize clinical and experimental imaging of the retina. The relevance of these advances will be further increased by the recent introduction of several commercial Fd-OCT instruments that can be used in clinical practice. However, due to some inherent limitations of current Fd-OCT technology (e.g., lack of spectroscopic information, inability to measure fluorescent signals), it is important to co-register Fd-OCT data with images obtained by other clinical imaging modalities such as fundus cameras and fluorescence angiography to create a more complete interpretation and representation of structures imaged. The co-registration between different imaging platforms becomes even more important if small retinal changes are monitored for early detection and treatment. Despite advances in volume acquisition speed with FD-OCT, eye/head motion artifacts can be still seen on acquired data. Additionally high-sampling density, large field-of-view (FOV) Fd-OCT volumes may also be needed for comparison with conventional imaging. In standard Fd-OCT systems, higher sampling density and larger imaging FOV (with constant sampling densities) lead to longer acquisition time which further increases eye/head motion artifacts. To overcome those problems, we tested 3D stitching of multiple, smaller retinal volumes which can be acquired in a less time (reduction of motion artifacts) and/or when stitched create a larger FOV representation of the retina. Custom visualization software that makes possible manual co-registration and simultaneous visualization of volumetric Fd-OCT data sets is described. Volumetric visualizations of healthy retinas with corresponding fundus pictures are presented followed by examples of retinal volumes of high sampling density that are created from multiple "standard" Fd-OCT volumes.
We describe a compact MEMS-based adaptive optics (AO) optical coherence tomography (OCT)
system with improved AO performance and ease of clinical use. A typical AO system consists of a
Shack-Hartmann wavefront sensor and a deformable mirror that measures and corrects the ocular
and system aberrations. Because of limitations on current deformable mirror technologies, the
amount of real-time ocular-aberration compensation is restricted and small in previous AO-OCT
instruments. In this instrument, we incorporate an optical apparatus to correct the spectacle
aberrations of the patients such as myopia, hyperopia and astigmatism. This eliminates the tedious
process of using trial lenses in clinical imaging. Different amount of spectacle aberration
compensation was achieved by motorized stages and automated with the AO computer for ease of
clinical use. In addition, the compact AO-OCT was optimized to have minimum system aberrations
to reduce AO registration errors and improve AO performance.
Adaptive optics (AO) and optical coherence tomography (OCT) are powerful imaging modalities that, when
combined, can provide high-resolution, 3-D images of the retina. The AO-OCT system at UC Davis has been
under development for 2 years and has demonstrated the utility of this technology for microscopic, volumetric, in
vivo retinal imaging. The current system uses a bimorph deformable mirror (DM) made by AOptix Technologies,
Inc. for low-order, high-stroke correction and a 140-actuator mirco-electrical-mechanical-system (MEMS) DM
made by Boston Micromachines Corporation for high-order correction. We present our on-going characterization
of AO system performance. The AO-OCT system typically has residual wavefront error of 100 nm rms. The
correctable error in the system is dominated by low-order error that we believe is introduced by aliasing in the
control loop. Careful characterization of the AO system will lead to improved performance and inform the design
of future systems.
Recent developments in Fourier domain—optical coherence tomography (Fd-OCT) have increased the acquisition speed of current ophthalmic Fd-OCT instruments sufficiently to allow the acquisition of volumetric data sets of human retinas in a clinical setting. The large size and three-dimensional (3D) nature of these data sets require that intelligent data processing, visualization, and analysis tools are used to take full advantage of the available information. Therefore, we have combined methods from volume visualization, and data analysis in support of better visualization and diagnosis of Fd-OCT retinal volumes. Custom-designed 3D visualization and analysis software is used to view retinal volumes reconstructed from registered B-scans. We use a support vector machine (SVM) to perform semiautomatic segmentation of retinal layers and structures for subsequent analysis including a comparison of measured layer thicknesses. We have modified the SVM to gracefully handle OCT speckle noise by treating it as a characteristic of the volumetric data. Our software has been tested successfully in clinical settings for its efficacy in assessing 3D retinal structures in healthy as well as diseased cases. Our tool facilitates diagnosis and treatment monitoring of retinal diseases.
The ability to obtain true three-dimensional (3D) morphology of the retinal structures is essential for future clinical and
experimental studies. It becomes especially critical if the measurements acquired with different instruments need to be
compared, or precise volumetric data are needed for monitoring and treatment of retinal disease. On the other hand, it is
well understood that optical coherence tomography (OCT) images are distorted by several factors. Only limited work has
been performed to eliminate these problems in ophthalmic retinal imaging, perhaps because they are less evident in the
more common 2D representation mode of time-domain OCT. With recent progress in imaging speed of Fourier domain -
OCT (Fd-OCT) techniques, however, 3D OCT imaging is more frequently being used, thereby exposing problems that
have been ignored previously. In this paper we propose possible solutions to minimize and compensate for artifacts
caused by subject eye and head motion, and distortions caused by the geometry of the scanning optics. The first is
corrected by cross-correlation based B-scan registration techniques; the second is corrected by incorporating the
geometry of the scanning beam into custom volume rendering software. Retinal volumes of optical nerve head (ONH)
and foveal regions of healthy volunteer, with and without corrections, are presented. Finally, some common factors that
may lead to increased distortions of the ophthalmic OCT image such as refractive error or position of the subject's head
are discussed.
Two deformable mirrors (2DM) were used in an adaptive optics - optical coherence tomography (AO-OCT) system to
image in vivo microscopic retinal structures of healthy and diseased retinas. As a result, multiple morphological
structures not previously seen in vivo have been visualized. Among those presented are three-dimensional
representations of the fovea and optic nerve head (ONH), revealing cellular structures and micro-vasculature. Drusen in
macular degeneration and photoreceptor dystrophies are also presented. Different methods for displaying volumetric
AO-OCT data to facilitate visualization of certain morphological details are compared.
Adaptive Optics (AO) have been increasingly combined with a variety of ophthalmic instruments over the last decade to
provide cellular-level, in-vivo images of the eye. The use of MEMS deformable mirrors in these instruments has recently
been demonstrated to reduce system size and cost while improving performance. However, currently available MEMS
mirrors lack the required range of motion for correcting large ocular aberrations, such as defocus and astigmatism.
In order to address this problem, we have developed an AO system architecture that uses two deformable mirrors, in a
woofer / tweeter arrangement, with a bimorph mirror as the woofer and a MEMS mirror as the tweeter. This setup
provides several advantages, including extended aberration correction range, due to the large stroke of the bimorph
mirror, high order aberration correction using the MEMS mirror, and additionally, the ability to 'focus' through the
retina.
This AO system architecture is currently being used in four instruments, including an Optical Coherence Tomography
(OCT) system and a retinal flood-illuminated imaging system at the UC Davis Medical Center, a Scanning Laser
Ophthalmoscope (SLO) at the Doheny Eye Institute, and an OCT system at Indiana University. The design, operation
and evaluation of this type of AO system architecture will be presented.
Ultrahigh axial resolution in adaptive optics - optical coherence tomography (AO-OCT) is fundamentally limited by the
intrinsic chromatic aberrations of the human eye. Variation in refractive index of the ocular media with wavelength
causes the spectral content of broadband light sources to focus at different depths in the retina for light entering the eye
and at the imaging detector for light exiting. This effect has not been previously reported for ultrahigh-resolution OCT
(without AO) likely because the effect is masked by the relatively long depth of focus dictated by the small pupils used
in these systems. With AO, the pupil size is much larger and depth of focus substantially narrower. As such the
chromatic aberrations of the eye can counteract the lateral resolution benefit of AO when used with broadband light
sources. To more fully tap the potential of AO-OCT, compensation of the eye's chromatic and monochromatic
aberrations must occur concurrently. One solution is to insert an achromatizing lens in front of the eye whose chromatic
aberrations are equal but opposite in sign to that of the eye. In this paper we evaluate the efficacy of a novel design that
uses a custom achromatizing lens placed near the fiber collimating optic. AO-OCT images are acquired on several
subjects with and without the achromatizing lens and in combination with two light sources of different spectral width.
The combination of the achromatizing lens and broadband light source yielded the sharpest images of the retina and the
smallest speckle.
Adaptive Optics - Optical Coherence Tomography (AO-OCT) has demonstrated a promising improvement in lateral
resolution for retinal imaging compared to standard OCT. Recent developments in Fourier-domain OCT technology
allow AO-OCT instruments to acquire three-dimensional (3D) retinal structures with high speed and high "volumetric"
resolution (in all three dimensions). One of the most important factors (besides acquisition speed) that will determine the
true potential of this technique is its ability to achieve diffraction-limited lateral resolution (~3 μm) while operating in
the ultrahigh axial resolution range (~3 μm) offered by OCT. Theoretical studies have shown that the eye's chromatic
aberrations may drastically reduce volumetric resolution. This is a critical finding because for "standard" stand alone
ultrahigh OCT, increasing the spectral bandwidth of the light source improves axial resolution without compromising
lateral resolution. To study the effects of spectral bandwidth on AO-OCT systems for retinal imaging two different light
sources offering 6 and 3 μm axial resolution were tested. This comparison was based on both AO correcting system
performance as well as the quality of corresponding OCT images.
We report first observations of the three-dimensional morphology of cone photoreceptors in the living human retina.
Images were acquired with a high-speed adaptive optics (AO) spectral domain optical coherence tomography (SD-OCT)
camera. The AO system consists of a Shack-Hartmann wavefront sensor and bimorph deformable mirror (AOptix) that
measure and correct the ocular and system aberrations at a closed-loop rate of 12 Hz. Unlike previous AO-OCT and AOSLO
instruments, the bimorph mirror was strategically positioned between the XY mechanical scanners and the subject's
eye so as to avoid beam distortion at the pupil plane, which is created when the mirror compensates for the refractive
error of the eye. This new configuration is evaluated empirically and with commercial ray tracing software. The SDOCT
system consists of a superluminescent diode and a 512 pixel line scan charge-coupled device (CCD) that acquires
75,000 A-scans/sec. This rate is more than two times faster than that previously reported. Retina motion artifiacts were
minimized by quickly acquiring small volume images of the retina with and without AO compensation. Camera
sensitivity was sufficient to detect reflections from all major retinal layers. The distribution of bright spots observed
within C-scans at the inner segment / outer segment (IS/OS) junction and at the posterior tips of the OS were found to be
highly correlated with one another and with the expected cone spacing. No correlation was found between the IS/OS
junction and either the plexiform layers or the layers immediately behind the OS posterior tips.
KEYWORDS: Image segmentation, Optical coherence tomography, 3D image processing, Eye, 3D acquisition, Visualization, Retina, 3D metrology, Data acquisition, 3D visualizations
The acquisition speed of current FD-OCT (Fourier Domain - Optical Coherence Tomography) instruments allows rapid
screening of three-dimensional (3D) volumes of human retinas in clinical settings. To take advantage of this ability
requires software used by physicians to be capable of displaying and accessing volumetric data as well as supporting
post processing in order to access important quantitative information such as thickness maps and segmented volumes.
We describe our clinical FD-OCT system used to acquire 3D data from the human retina over the macula and optic
nerve head. B-scans are registered to remove motion artifacts and post-processed with customized 3D visualization and
analysis software. Our analysis software includes standard 3D visualization techniques along with a machine learning
support vector machine (SVM) algorithm that allows a user to semi-automatically segment different retinal structures
and layers. Our program makes possible measurements of the retinal layer thickness as well as volumes of structures of
interest, despite the presence of noise and structural deformations associated with retinal pathology. Our software has
been tested successfully in clinical settings for its efficacy in assessing 3D retinal structures in healthy as well as
diseased cases. Our tool facilitates diagnosis and treatment monitoring of retinal diseases.
Adaptive optics-optical coherence tomography (AO-OCT) has the potential to improve lateral resolution for OCT retinal
imaging. Several reports have already described the successful combination of AO with a scanning confocal Fourier-domain
OCT instrument to permit real-time three-dimensional (3D) imaging with high resolution (in all three
dimensions). One of the key components that sets the performance limit of AO is the wavefront corrector. Several
different wavefront correctors have been used in AO-OCT systems so far. In this paper we compare two commercially
available wavefront correctors: an AOptix Bimorph deformable mirror (DM) and a Boston Micromachines Micro-
Electro Mechanical System (MEMS) DM (used for the first time in an AO-OCT system). To simplify the analysis, we
tested their performance for the correction of low-amplitude high-order aberrations (with minimal defocus and
astigmatism). Results were obtained with an AO-OCT instrument constructed at UC Davis that combines state-of-the-art
Fourier-domain OCT and an AO design to allow simultaneous testing of both mirrors without the need to modify the
optical system.
The purpose of this study was to evaluate the performance of a bimorph deformable mirror from AOptix, inserted into an adaptive optics system designed for in-vivo retinal imaging at high resolution. We wanted to determine its suitability as a wavefront corrector for vision science and ophthalmological instrumentation. We presented results obtained in a closed-loop system, and compared them with previous open-loop performance measurements. Our goal was to obtain precise wavefront reconstruction with rapid convergence of the control algorithm. The quality of the reconstruction was expressed in terms of root-mean-squared wavefront residual error (RMS), and number of frames required to perform compensation. Our instrument used a Hartmann-Shack sensor for the wavefront measurements. We also determined the precision and ability of the deformable mirror to compensate the most common types of aberrations present in the human eye (defocus, cylinder, astigmatism and coma), and the quality of its correction, in terms of maximum amplitude of the corrected wavefront. In addition to wavefront correction, we had also used the closed-loop system to generate an arbitrary aberration pattern by entering the desired Hartmann-Shack centroid locations as input to the AO controller. These centroid locations were computed in Matlab for a user-defined aberration pattern, allowing us to test the ability of the DM to generate and compensate for various aberrations. We conclude that this device, in combination with another DM based on Micro-Electro Mechanical Systems (MEMS) technology, may provide better compensation of the higher-order ocular wavefront aberrations of the human eye
A design for a high-resolution scanning instrument is presented for in vivo imaging of the human eye at the cellular scale. This system combines adaptive optics technology with a scanning laser ophthalmoscope (SLO) to image structures with high lateral (~2 μm) resolution. In this system, the ocular wavefront aberrations that reduce the resolution of conventional SLOs are detected by a Hartmann-Shack wavefront sensor, and compensated with two deformable mirrors in a closed-loop for dynamic correction and feedback control. A laser beam is scanned across the retina and the reflected light is captured by a photodiode, yielding a two-dimensional image of the retina at any depth. The quantity of back-scattered light from the retina is small (0.001% of reflection) and requires the elimination of all parasite reflections. As an in vivo measurement, faint cellular reflections must be detected with a low-energy source, a supraluminescent laser diode, and with brief exposures to avoid artifacts from eye movements. The current design attempts to optimize trade-offs between improved wavefront measurement and compensation of the optical aberrations by fractioning the light coming to the wavefront sensor, better sensitivity by increasing the input light energy or the exposure time and the response speed of the system. This instrument design is expected to provide sufficient resolution for visualizing photoreceptors and ganglion cells, and therefore, may be useful in diagnosing and monitoring the progression of retinal pathologies such as glaucoma or aged-related macular degeneration.
We have combined Fourier-domain optical coherence tomography (OCT) with a closed-loop Adaptive Optics (AO) system. The AO-OCT instrument has been used for in vivo retinal imaging. High-lateral resolution of our AO-OCT system allows visualization of the microscopic retinal structures not accessible by standard OCT instruments.
KEYWORDS: Optical coherence tomography, Image quality, In vivo imaging, Signal to noise ratio, Mirrors, Spectroscopy, Charge-coupled devices, Retina, Eye, Imaging systems
We built a Fourier domain optical coherence tomography (FD-OCT) system using a line scan CCD camera that allows real time data display and acquisition. This instrument is able to produce 2D B-scans as well as 3D data sets with human subjects in vivo in clinical settings. In this paper we analyze the influence of varying exposure times of the CCD detector on image quality. Sensitivity values derived from theoretical predictions have been compared with measurements (obtained with mirrors and neutral density filters placed in both interferometer arms). The results of these experiments, discussion about differences between sensitivity values, potential sources of discrepancies, and recommendations for optimal exposure times will be described in this paper. A short discussion of observed artifacts as well as possible ways to remove them is presented. The influence of relative retinal position with respect to reference mirror position will also be described.
The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.
Adaptive optics (AO), a mature technology developed for astronomy to compensate for the effects of atmospheric turbulence, can also be used to correct the aberrations of the eye. The classic phoropter is used by ophthalmologists and optometrists to estimate and correct the lower-order aberrations of the eye, defocus and astigmatism, in order to derive a vision correction prescription for their patients. An adaptive optics phoropter measures and corrects the aberrations in the human eye using adaptive optics techniques, which are capable of dealing with both the standard low-order aberrations and higher-order aberrations, including coma and spherical aberration. High-order aberrations have been shown to degrade visual performance for clinical subjects in initial
investigations. An adaptive optics phoropter has been designed and constructed based on a Shack-Hartmann sensor to measure the aberrations of the eye, and a liquid crystal spatial light modulator to compensate for them. This system should produce near diffraction-limited optical image quality at the retina, which will enable investigation of the psychophysical limits of human vision. This paper describes the characterization and operation of the AO phoropter with results from human subject testing.
I want to begin by thanking each of you for attending. This session is very, very exciting to me, and I know that it willbe exciting and rewarding to you as well. We will begin by acknowledging Dr. All an Rodrigues and Dr. Danny Rich. We are the progenitors of this session. I believe that it is incumbent upon us to prepare ourselves, our companies or organizations for tomorrow by staying abreast of the state of-the-art, changes in technology, science, and trends, in our ever-changing world.
Color vision is inseparable from spatial vision. Chromatic and achromatic aspects of visual experience together subserve our perception of the forms of objects. This view is supported by physiological studies demonstrating that both color and luminance are carried along with form information on the same optic nerve fibers, albeit at different spatial scales. These scale differences can be summarized by contrast sensitivity functions measured with chromatic and achromatic spatial sinusoids, and may be illustrated by digitally filtered images that separate achromatic and chromatic variations. Analyses of the chromatic content of natural images also demonstrate a close link with the chromatic and spatial tuning of neural pathways. While characteristic properties of natural scenes can predict general characteristics of visual coding, color can vary widely across individual images, and thus could not be represented optimally by a fixed visual system. However, color coding is not fixed, but rather adjusts to both the average color and distribution of colors in scenes through processes of adaptation. Such adjustments may support color constancy and coding efficiency, and may also optimize detection and discrimination of colors that are novel in an image. Finally, the spatial properties of color-coding mechanisms are essential to our perception of figure and ground. Chromatic (border) contrast enhances the difference between figure and ground, while homogenization of object surfaces is facilitated by short- and long-range processes of assimilation and color spreading.
We discuss the design and implementation of a low-cost, high-resolution adaptive optics test-bed for vision research. It is well known that high-order aberrations in the human eye reduce optical resolution and limit visual acuity. However, the effects of aberration-free eyesight on vision are only now beginning to be studied using adaptive optics to sense and correct the aberrations in the eye. We are developing a high-resolution adaptive optics system for this purpose using a Hamamatsu Parallel Aligned Nematic Liquid Crystal Spatial Light Modulator. Phase-wrapping is used to extend the effective stroke of the device, and the wavefront sensing and wavefront correction are done at different wavelengths. Issues associated with these techniques will be discussed.
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