Increasing the FOV of OCTA images while keeping the acquisition time moderate requires high A-scan rates. Therefore, OCTA images appear to be noisier. Deep learning methods can be used for noise reduction. In OCTA volumes small vessels with an orientation perpendicular to the image plane are often removed by deep learning denoising algorithms, due to their small appearance.
To overcome this a 3-dimensional Unet was developed to utilize volumetric information. With the knowledge of also the third dimension, the algorithm is able to distinguish between noise and vessel contrast and is therefore less likely to remove vessels.
We present a flexible OCT engine for acquiring full eye-length, anterior and posterior segment B-scans, as well as 4D live volumes with an effective A-scan rate of up to 2MHz. It is enabled by a MEMS tunable VCSEL with flexible A-scan rates, broad spectral bandwidth and a long instantaneous coherence length. Our GPU based, custom reconstruction and rendering software is able to process and display live volume series at rates of up to 17 volumes per second. We show B-scans and volume series of model eyes.
Cataract surgery is the most frequently performed surgical procedure in all of Ophthalmology. During this process of surgically removing the lens the inner layer of the cornea needs for protection from permanent damage and the anterior chamber demands stabilization, which is provided by Ophthalmic Viscosurgical Devices (OVDs). We therefore present an automatic pipeline to determine the thickness of OVDs in enucleated porcine eyes via semantic segmentation. For the evaluation of the pipeline we segmented 100 volume scans of prepared porcine eyes. This versatile pipeline can be applied for segmentation of anterior segment Optical Coherence Tomography scans of the anterior segment.
We demonstrate imaging of amyloid-beta plaques in ex-vivo Alzheimer’s disease brain tissue using a 1060 nm swept source optical coherence tomography setup. This instrument enabled a global investigation of the brain tissue with a large field of view of 8 mm x 8 mm as well as sequential high resolution imaging. Amyloid-beta (A-β) plaques were identified as highly scattering features. Results were in good agreement with immunohistochemically stained images gained by histology.
Tunable laser sources with sweep-rates higher than 1MHz recently became commercially available. Today’s commercial ophthalmic OCT systems use sweep-rates in the 100-200kHz regime. These much faster laser sources can be used to either significantly reduce the imaging time or significantly increase the field of view (FOV). In this study we investigate the clinical value of OCT with MHz-rate swept source lasers. We implemented a versatile ophthalmic OCT system using a Frequency-Domain-Mode-Locked (FDML) laser with a sweep-rate of 1.7MHz, to address a variety of ophthalmic OCT imaging applications, exhibiting large imaging depth for wide field retinal OCT and OCT angiography (OCTA) with a field of view of up to 90 degrees, as well as for anterior segment imaging, and microscopic OCTA of the choriocapillaris with repetition rates of more than 1kHz.
Side lobe artifacts on point spread functions can be traced back to (1) fringe visibility variation across the spectrum, (2) errors in sampling instances, and (3) window functions. We demonstrate signal processing methods for correcting for all three of these issues. These methods require a system calibration step. If the systems slowly age, the recalibration step could be performed in the field with a fixtured target.
The choriocappilaris layer is considered to be one of the first retinal layers affected in age related macular degeneration and other retinal diseases [1]. Imaging this fine vascular layer below the retinal pigment epithelium (RPE) with optical coherence tomography angiography (OCTA) has been very challenging due to the high scattering and absorption of the RPE. In OCTA higher inter B-scan times increase the sensitivity for slow motion and hence improve the contrast of fine vessels. However, it was demonstrated that for the choriocappilaris the opposite is the case [2]. Novel swept source laser technology enables acquiring images at A-scan-rates of 1.7 million A-scans per second, which is approx. 17 times faster than the fastest commercially available OCT devices [3, 4]. OCTA images of the human macula with different inter B-scan times were acquired and compared. The same prototype system was also used to acquire single shot wide-field OCTA images with up to 60 degree field of view.
We exploit the intrinsic phase stability of akinetic swept source optical coherence tomography to demonstrate digital defocus correction in-vivo at a center wavelength of 1060nm. The high speed of 500kHz enables digital adaptive optics (AO) correction across a field of view of 1.8x1.5deg, currently limited by the employed galvo scanners. The source operates in a previously presented dual resolution mode OCT system (wide field >40deg, AO >3deg) with hardware based adaptive optics. The latter allows to efficiently combine hardware and digital AO, and to further optimize the AO imaging results. We demonstrate the digitally assisted AO performance for both structural imaging as well as for OCT angiography imaging across the full retina down to the choriocapillaris.
We present retinal photoreceptor imaging with a line-field parallel spectral domain OCT modality, utilizing a commercially available 2D CMOS detector array operating at and imaging speed of 500 B-scans/s. Our results demonstrate for the first time in vivo structural and functional retinal assessment with a line-field OCT setup providing sufficient sensitivity, lateral and axial resolution and 3D acquisition rates in order to resolve individual photoreceptor cells. The phase stability of the system is manifested by the high phase-correlation across the lateral FOV on the level of individual photoreceptors. The setup comprises a Michelson interferometer illuminated by a broadband light source, where a line-focus is formed via a cylindrical lens and the back-propagated light from sample and reference arm is detected by a 2D array after passing a diffraction grating. The spot size of the line-focus on the retina is 5μm, which corresponds to a PSF of 50μm and an oversampling factor of 3.6 at the detector plane, respectively. A full 3D stack was recorded in only 0.8 s. We show representative enface images, tomograms and phase-difference maps of cone photoreceptors with a lateral FOV close to 2°. The high-speed capability and the phase stability due to parallel illumination and detection may potentially lead to novel structural and functional diagnostic tools on a cellular and microvascular imaging level. Furthermore, the presented system enables competitive imaging results as compared to respective point scanning modalities and facilitates utilizing software based digital aberration correction algorithms for achieving 3D isotropic resolution across the full FOV.
We present retinal photoreceptor imaging with a line-field parallel spectral domain OCT modality, utilizing a commercially available 2D CMOS detector array operating at and imaging speed of 500 B-scans/s. Our results demonstrate for the first time in vivo structural and functional retinal assessment with a line-field OCT setup providing sufficient sensitivity, lateral and axial resolution and 3D acquisition rates in order to resolve individual photoreceptor cells. The setup comprises a Michelson interferometer illuminated by a broadband light source, where a line-focus is formed via a cylindrical lens and the back-propagated light from sample and reference arm is detected by a 2D array after passing a diffraction grating. The spot size of the line-focus on the retina is 5μm, which corresponds to a PSF of 50μm and an oversampling factor of 3.6 at the detector plane, respectively. A full 3D stack was recorded in only 0.8 s. We show representative enface images, tomograms and phase-difference maps of cone photoreceptors with a lateral FOV close to 2°. The high-speed capability and the phase stability due to parallel illumination and detection may potentially lead to novel structural and functional diagnostic tools on a cellular and microvascular imaging level. Furthermore, the presented system enables competitive imaging results as compared to respective point scanning modalities and facilitates utilizing software based digital aberration correction algorithms for achieving 3D isotropic resolution across the full FOV.
KEYWORDS: Motion measurement, Eye, Optical coherence tomography, Tissues, Tissue optics, Phase measurement, Retina, In vivo imaging, Linear filtering, Data acquisition
We use phase-sensitive optical coherence tomography to measure relative motions within the human eye. From a sequence of tomograms, the phase difference between successive tomograms reveals the local axial motion of the tissue at every location within the image. The pulsation of the retina and of the lamina cribrosa amounts to, at most, a few micrometers per second, while the bulk velocity of the eye, even with the head resting in an ophthalmic instrument, is a few orders of magnitude faster. The bulk velocity changes continuously as the tomograms are acquired, whereas localized motions appear at acquisition times determined by the repeated scan of the tomogram. This difference in timing allows the bulk motion to be separated from any localized motions within a temporal bandwidth below the tomogram frame rate. In the human eye, this reveals a map of relative motions with a precision of a few micrometers per second.
We use phase-sensitive optical coherence tomography (OCT) to measure the deformation of the optic nerve head during
the pulse cycle, motivated by the possibility that these deformations might be indicative of the progression of glaucoma.
A spectral-domain OCT system acquired 100k A-scans per second, with measurements from a pulse-oximeter recorded
simultaneously, correlating OCT data to the subject’s pulse. Data acquisition lasted for 2 seconds, to cover at least two
pulse cycles. A frame-rate of 200–400 B-scans per second results in a sufficient degree of correlated speckle between
successive frames that the phase-differences between fames can be extracted. Bulk motion of the entire eye changes the
phase by several full cycles between frames, but this does not severely hinder extracting the smaller phase-changes due
to differential motion within a frame. The central cup moves about 5 μm/s relative to the retinal-pigment-epithelium
edge, with tissue adjacent to blood vessels showing larger motion.
Retinal and choroidal vascular imaging is an important diagnostic benefit for ocular diseases such as age-related macular degeneration. The current gold standard for vessel visualization is fluorescence angiography. We present a potential non-invasive alternative to image blood vessels based on functional Fourier domain optical coherence tomography (OCT). For OCT to compete with the field of view and resolution of angiography while maintaining motion artifacts to a minimum, ultrahigh-speed imaging has to be introduced. We employ Fourier domain mode locking swept source technology that offers high quality imaging at an A-scan rate of up to 1.68 MHz. We present retinal angiogram over ∼ 48 deg acquired in a few seconds in a single recording without the need of image stitching. OCT at 1060 nm allows for high penetration in the choroid and efficient separate characterization of the retinal and choroidal vascularization.
We demonstrate an ultra high speed fiber based polarization sensitive spectral domain optical coherence
tomography system, using two ultra high speed CMOS line scan cameras. With this system an A-scan rate of up to
128 kHz was achieved. The system is based on polarization maintaining fibers and retrieves the backscattered
intensity, retardation and optic axis orientation with only one A-scan per measurement location. This high speed data
acquisition enables averaging of several acquired B-scans of intensity, retardation, optic axis orientation, and Stokes
vectors, which strongly reduces speckle noise. We discuss different averaging techniques and compare the results in
healthy human retinas.
We present a method to contrast the blood flow of retinal vessels from the
surrounding static tissue. It is based on the extinction of the interference
fringes by phase shifts of π. If moving particles within the sample
introduce an additional phase shift, the signal from these particles will, in
contrast to the signal from static tissue, not be attenuated. We demonstrate
different variants of this method, where we introduce phase shifts during
single A-scans, consecutive A-scans and consecutive B-scans. We show
proof-of-principle measurements with a piezo mirror, as well as in-vivo
measurements of the human retina for the different phase shifting schemes.
We demonstrate its capability to contrast a wide range of perfused retinal
vessels; from large vessels present in the optic nerve head region to the
capillary network surrounding the fovea.
We present a method for the automated extraction of Doppler OCT flow information by using a support vector machine
that combines different features for classification. We employ histogram equalization that makes it possible to
distinguish vessels from bulk tissue by texture analysis. This method is particularly applicable to settings with significant
phase noise as it is more robust to multiple scattering components than simple threshold-based methods.
We present a single spectrometer functional spectral domain optical coherence tomography
system, which allows for encoding additional information within the spatial frequencies. The
method is based on a differentiation between orthogonal polarization channels through spatial
modulation introduced by an electro-optic modulator. This method is used to perform Ultrahigh-
speed retinal polarization sensitive optical coherence tomography (PSOCT). With this
setup, we realized for the first time polarization sensitive OCT measurements of the human
retina in-vivo, with camera line rates of up to 160.000 A-scans per second. Compared to
PSOCT systems, operating at traditional line rates, this significantly improves patient comfort
during the measurements and gives the possibility to resolve microscopic retinal details, while
still gaining information of the polarization characteristics of the tissue. In this proceeding we
present preliminary results acquired with this ultra-high speed PSOCT system.
We propose an algorithm to extract the angles of vessels for the correction of flow measurements in circumpapillary
Doppler OCT scans. Firstly, we register a volume to two reference scans in order to determine the physiologically
correct structure of the volume. Then, vessels are segmented in the volume and the angles are calculated and stored in a
look-up table. After having registered the circular scan to the volume by using the projection along the z-axis, the angles
can be extracted from the look-up table. Repeatability measurements of flow parameters on 5 vessels of a healthy subject
are presented.
KEYWORDS: Retina, Optical coherence tomography, In vivo imaging, Signal to noise ratio, Eye, Reflectivity, Signal detection, Solids, Tissues, Heterodyning
An experimental design for noninvasive assessment of neural retinal tissue function with enhanced sensitivity is presented. By matching the response detection to a defined flicker frequency stimulus similar to heterodyne detection, the response signal will be shifted out of the low-frequency noise and the specificity of response detection will be strongly enhanced. Optimal measurement parameters are discussed, such as the function and timing of the response function to a single flash stimulus. The results indicate responses on the order of 200 ms that have been probed with our frequency-encoded approach using 5 Hz flickering. Preliminary results indicate the feasibility of our measurement concept to assess small changes in reflectivity with enhanced sensitivity. A functional tomogram for response localization and quantification is introduced.
We developed a high speed Doppler tomography system together with flow extraction algorithms that provide
a flexible tool to assess retinal perfusion. The aim of the present study is to stimulate perfusion by flickering
with light of adjustable color and to measure changes depending on light frequency and flicker location. We
observed relative changes in arterial flow velocity during flicker stimulation up to 50%. We found in arteries
close to the optic nerve head the highest flicker response at a frequency of 8Hz. We believe that a multimodal
functional imaging concept is of high value for an accurate and early diagnosis and understanding of
retinal pathologies and pathogenesis.
We present a polarization sensitive spectral domain optical coherence tomography system that is capable to retrieve with
a single camera both retardation and optical axis orientation. The method is based on a differentiation between
orthogonal polarization channels through spatial modulation introduced by an electro-optic modulator. Proof-of principle
using a chromatic quarter wave plate designed for 1300nm as a sample and acquisitions of a piece of borealis, a highly
birefringend plastic is provided. Results of the method for in-vivo imaging of a fingertip are presented.
Novel CMOS detector technology allows for high-speed volumetric tissue
imaging with Fourier domain optical coherence tomography. Acquisition
speeds of 200kHz reveal comprehensive image details due to the virtual lack of
motion artifacts. We applied this system to the retina and achieved high
resolution imaging with 5μm x 3μm transverse and axial resolution. Such
resolution allows observing microscopic details such as photoreceptor cone
mosaic, nerve fiber bundles, and the capillary bed without applying adaptive
optics instrumentation. Fast image series reveal perfusion dynamics of full
retinal volumes and allow for tracking of individual photoreceptors to study
their dynamics. We observed that for in-vivo imaging speed is crucial for
reproducible maps of microscopic tissue details.
We present a method to separate blood flow information from static tissue in Doppler Fourier domain optical
coherence tomograms (D-FDOCT). Histograms of the Doppler tomograms are used to differentiate between
pixels containing information about blood flow and pixels representing static tissue. By setting pixels within
a certain histogram range to 0 only the blood flow information remains. This approach is demonstrated on
different retinal D-FDOCT volume scans taken with a high speed CMOS based FDOCT system. The advantage
of the presented approach is the small post processing effort together with the direct availability of quantitative
Doppler flow maps.
Probing the retina with flicker light of defined frequencies allowed to offset the detection for intrinsic signals from proband motion artifacts as well as blood flow. In addition the fast imaging sequence capability of FDOCT is promising for the assessment of fast physiologic changes within retinal structures. For the present study two measurement protocols are evaluated: first, taking fast tomogram series across a flickered region, and then constructing via frequency analysis and bandpass filtering a functional OCT tomogram similar to fMRI. The second protocol consists of a fast local A-scan series at 17kHz rate with 1Hz flicker. 'Light-on' time is 250ms. 'Lights off' time is 750ms. 500ms before 'light-on' is used for calculating the baseline. Finally the average over 5 cycles is taken. A clear negative response is found at the outer photoreceptor segment for both 'light-on' and 'light-off' edge. The response appears to be stronger for the 'light off' edge. The shape of the responses is analysed and might eventually be used in linear regression models to enhance the sensitivity of our fOCT approach.
Doppler OCT systems allow nowadays to visualize quantitative and qualitative angiographic maps of retinal tissue. We equipped the instrument with a pulse oximeter and recorded the pulse synchronously with the resonant Doppler flow data. Recombination of tomograms according to the heart beat cycles yields full volumes for each cycle instant. We believe such multi-dimensional functional information and the ability to monitor dynamic processes over time to open exciting perspectives that ultimately contribute to a better understanding of retinal physiology and patho-physiology in-vivo.
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