Iwona Gorczynska

Ultrahigh speed Spectral / Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second

We demonstrate ultrahigh speed spectral / Fourier domain optical coherence tomography (OCT) using an ultrahigh speed CMOS line scan camera at rates of 70,000 - 312,500 axial scans per second. Several design configurations are characterized to illustrate trade-offs between acquisition speed, resolution, imaging range, sensitivity and sensitivity roll-off performance. Ultrahigh resolution OCT with 2.5 - 3.0 micron axial image resolution is demonstrated at approximately 100,000 axial scans per second. A high resolution spectrometer design improves sensitivity roll-off and imaging range performance, trading off imaging speed to 70,000 axial scans per second. Ultrahigh speed imaging at >300,000 axial scans per second with standard image resolution is also demonstrated. Ophthalmic OCT imaging of the normal human retina is investigated. The high acquisition speeds enable dense raster scanning to acquire densely sampled volumetric three dimensional OCT (3D-OCT) data sets of the macula and optic disc with minimal motion artifacts. Imaging with approximately 8 - 9 micron axial resolution at 250,000 axial scans per second, a 512 x 512 x 400 voxel volumetric 3D-OCT data set can be acquired in only approximately 1.3 seconds. Orthogonal registration scans are used to register OCT raster scans and remove residual axial eye motion, resulting in 3D-OCT data sets which preserve retinal topography. Rapid repetitive imaging over small volumes can visualize small retinal features without motion induced distortions and enables volume registration to remove eye motion. Cone photoreceptors in some regions of the retina can be visualized without adaptive optics or active eye tracking. Rapid repetitive imaging of 3D volumes also provides dynamic volumetric information (4D-OCT) which is shown to enhance visualization of retinal capillaries and should enable functional imaging. Improvements in the speed and performance of 3D-OCT volumetric imaging promise to enable earlier diagnosis and improved monitoring of disease progression and response to therapy in ophthalmology, as well as have a wide range of research and clinical applications in other areas.

Ultrahigh-speed optical coherence tomography for three-dimensional and en face imaging of the retina and optic nerve head.

PURPOSE To demonstrate ultrahigh-speed optical coherence tomography (OCT) imaging of the retina and optic nerve head at 249,000 axial scans per second and a wavelength of 1060 nm. To investigate methods for visualization of the retina, choroid, and optic nerve using high-density sampling enabled by improved imaging speed. METHODS A swept-source OCT retinal imaging system operating at a speed of 249,000 axial scans per second was developed. Imaging of the retina, choroid, and optic nerve were performed. Display methods such as speckle reduction, slicing along arbitrary planes, en face visualization of reflectance from specific retinal layers, and image compounding were investigated. RESULTS High-definition and three-dimensional (3D) imaging of the normal retina and optic nerve head were performed. Increased light penetration at 1060 nm enabled improved visualization of the choroid, lamina cribrosa, and sclera. OCT fundus images and 3D visualizations were generated with higher pixel density and less motion artifacts than standard spectral/Fourier domain OCT. En face images enabled visualization of the porous structure of the lamina cribrosa, nerve fiber layer, choroid, photoreceptors, RPE, and capillaries of the inner retina. CONCLUSIONS Ultrahigh-speed OCT imaging of the retina and optic nerve head at 249,000 axial scans per second is possible. The improvement of approximately 5 to 10x in imaging speed over commercial spectral/Fourier domain OCT technology enables higher density raster scan protocols and improved performance of en face visualization methods. The combination of the longer wavelength and ultrahigh imaging speed enables excellent visualization of the choroid, sclera, and lamina cribrosa.

Characterization of Outer Retinal Morphology with High-Speed, Ultrahigh-Resolution Optical Coherence Tomography

PURPOSE To visualize, quantitatively assess, and interpret outer retinal morphology by using high-speed, ultrahigh-resolution (UHR) OCT. METHODS Retinal imaging was performed in the ophthalmic clinic in a cross-section of 43 normal subjects with a 3.5-microm, axial-resolution, high-speed, UHR OCT prototype instrument, using a radial scan pattern (24 images, 1500 axial scans). Outer retinal layers were automatically segmented and measured. High-definition imaging was performed with a 2.8-microm axial-resolution, high-speed, UHR OCT research prototype instrument, to visualize the finer features in the outer retina. RESULTS Quantitative maps of outer retinal layers showed clear differences between the cone-dominated fovea and the rod-dominated parafovea and perifovea, indicating that photoreceptor morphology can explain the appearance of the outer retina in high-speed, UHR OCT images. Finer, scattering bands were visualized in the outer retina using high-definition imaging and were interpreted by comparison to known anatomy. CONCLUSIONS High-speed UHR OCT enables quantification of scattering layers in the outer retina. An interpretation of these features is presented and supported by quantitative measurements in normal subjects and comparison with known anatomy. The thick scattering region of the outer retina previously attributed to the retinal pigment epithelium (RPE) is shown to consist of distinct scattering bands corresponding to the photoreceptor outer segment tips, RPE, and Bruchs membrane. These results may advance understanding of the outer retinal appearance in OCT images. The normative measurements may also aid in future investigations of outer retinal changes in age-related macular degeneration and other diseases.

Quantitative cerebral blood flow with Optical Coherence Tomography

Absolute measurements of cerebral blood flow (CBF) are an important endpoint in studies of cerebral pathophysiology. Currently no accepted method exists for in vivo longitudinal monitoring of CBF with high resolution in rats and mice. Using three-dimensional Doppler Optical Coherence Tomography and cranial window preparations, we present methods and algorithms for regional CBF measurements in the rat cortex. Towards this end, we develop and validate a quantitative statistical model to describe the effect of static tissue on velocity sensitivity. This model is used to design scanning protocols and algorithms for sensitive 3D flow measurements and angiography of the cortex. We also introduce a method of absolute flow calculation that does not require explicit knowledge of vessel angles. We show that OCT estimates of absolute CBF values in rats agree with prior measures by autoradiography, suggesting that Doppler OCT can perform absolute flow measurements in animal models.

Anterior segment imaging with Spectral OCT system using a high-speed CMOS camera

We describe a new ultrahigh speed Spectral OCT instrument making use of a CMOS camera and demonstrate high quality in vivo imaging of the anterior segment of the human eye. The high flexibility of the designed imaging system allows a wide range of imaging protocols. Two- and three-dimensional high quality OCT images of the cornea, the anterior chamber and the crystalline lens are presented. A high acquisition rate, up to 135,000 A-scans/second enables three-dimensional reconstruction of the anterior segment during lenticular accommodation, blinking and pupillary reaction to light stimulus. We demonstrate OCT tomographic real time imaging of the lens dynamics during accommodation and high quality OCT cross-sectional images of the entire anterior segment of the eye from the cornea up to posterior part of the crystalline lens.

Comparison of Spectral / Fourier Domain Optical Coherence Tomography Instruments for Assessment of Normal Macular Thickness

Purpose: The purpose of this study was to report normal macular thickness measurements and assess reproducibility of retinal thickness measurements acquired by a time-domain optical coherence tomography (OCT) (Stratus, Carl Zeiss Meditec, Inc., Dublin, CA) and three commercially available spectral/Fourier domain OCT instruments (Cirrus HD-OCT, Carl Zeiss Meditec, Inc.; RTVue-100, Optovue, Inc., Fremont, CA; 3D OCT-1000, Topcon, Inc., Paramus, NJ). Methods: Forty randomly selected eyes of 40 normal, healthy volunteers were imaged. Subjects were scanned twice during 1 visit and a subset of 25 was scanned again within 8 weeks. Retinal thickness measurements were automatically generated by OCT software and recorded after manual correction. Regression and Bland–Altman plots were used to compare agreement between instruments. Reproducibility was analyzed by using intraclass correlation coefficients, and incidence of artifacts was determined. Results: Macular thickness measurements were found to have high reproducibility across all instruments with intraclass correlation coefficients values ranging 84.8% to 94.9% for Stratus OCT, 92.6% to 97.3% for Cirrus Cube, 76.4% to 93.7% for RTVue MM5, 61.1% to 96.8% for MM6, 93.1% to 97.9% for 3D OCT-1000 radial, and 31.5% to 97.5% for 3D macular scans. Incidence of artifacts was higher in spectral/Fourier domain instruments, ranging from 28.75% to 53.16%, compared with 17.46% in Stratus OCT. No significant age or sex trends were found in the measurements. Conclusion: Commercial spectral/Fourier domain OCT instruments provide higher speed and axial resolution than the Stratus OCT, although they vary greatly in scanning protocols and are currently limited in their analysis functions. Further development of segmentation algorithms and quantitative features are needed to assist clinicians in objective use of these newer instruments to manage diseases.

High-speed, high-resolution optical coherence tomography retinal imaging with a frequency-swept laser at 850 nm

High-speed, high-resolution optical coherence tomography (OCT) imaging of the human retina is demonstrated using a frequency-swept laser at 850 nm. A compact external cavity semiconductor laser design, optimized for swept-source ophthalmic OCT, is described. The laser enables an effective 16 kHz sweep rate with >10 mm coherence length and a tuning range of approximately 35 nm full width at half-maximum, yielding an axial resolution of <7 micro m in tissue.

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.

Efficient reduction of speckle noise in Optical Coherence Tomography

Speckle pattern, which is inherent in coherence imaging, influences significantly axial and transversal resolution of Optical Coherence Tomography (OCT) instruments. The well known speckle removal techniques are either sensitive to sample motion, require sophisticated and expensive sample tracking systems, or involve sophisticated numerical procedures. As a result, their applicability to in vivo real-time imaging is limited. In this work, we propose to average multiple A-scans collected in a fully controlled way to reduce the speckle contrast. This procedure involves non-coherent averaging of OCT A-scans acquired from adjacent locations on the sample. The technique exploits scanning protocol with fast beam deflection in the direction perpendicular to lateral dimension of the cross-sectional image. Such scanning protocol reduces the time interval between A-scans to be averaged to the repetition time of the acquisition system. Consequently, the averaging algorithm is immune to bulk motion of an investigated sample, does not require any sophisticated data processing to align cross-sectional images, and allows for precise control of lateral shift of the scanning beam on the object. The technique is tested with standard Spectral OCT system with an extra resonant scanner used for rapid beam deflection in the lateral direction. Ultrahigh speed CMOS camera serves as a detector and acquires 200,000 spectra per second. A dedicated A-scan generation algorithm allows for real-time display of images with reduced speckle contrast at 6 frames/second. This technique is applied to in vivo imaging of anterior and posterior segments of the human eye and human skin.

Spectral optical coherence tomography: a novel technique for cornea imaging.

Purpose: Spectral optical coherence tomography (SOCT) is a new, noninvasive, noncontact, high-resolution technique that provides cross-sectional images of the objects that weakly absorb and scatter light. SOCT, because of very short acquisition time and high sensitivity, is capable of providing tomograms of substantially better quality than the conventional OCT. The aim of this paper is to show the application of the SOCT to cross-sectional imaging of the cornea and its pathologies. Methods: Eleven eyes with different corneal pathologies were examined with a slit lamp and the use of a prototype SOCT instrument constructed in the Institute of Physics, Nicolaus Copernicus University, Toruń, Poland. Results: Our SOCT system provides high-resolution (4 μm axial, 10 μm transversal) tomograms composed of 3000-5000 A-scans with an acquisition time of 120-200 ms. The quality of the images is adequate for detailed cross-sectional evaluation of various corneal pathologies. Objective assessment of the localization, size, shape, and light-scattering properties of the changed tissue is possible. Corneal and epithelial thickness and the depth and width of lesions can be estimated. Conclusion: SOCT technique allows acquiring clinically valuable cross-sectional optical biopsy of the cornea and its pathologies.