Optical Coherence Tomography in Retinal Diseases Sandeep Saxena
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Optical Coherence Tomography1

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INTRODUCTION
Optical coherence tomography (OCT) is a new technique for high-resolution cross-sectional visualization of retinal structure. Optical coherence tomography achieves 2- or 3-dimensional cross-sectional imaging of retina.
Optical coherence tomography is based on the principle of Michelson interferometry.
Imaging with OCT is analogous to ultrasound B-scan in that distance information is extracted from the time delays of reflected signals. However, the use of optical rather than acoustic waves in OCT provides a much higher (5-10 micron) longitudinal resolution in the retina versus the 100-micron scale for ultrasound. This is due to the fact that the speed of light is nearly a million times faster than the speed of sound.
Use of optical waves also allows a noncontact and noninvasive measurement. The ability to evaluate tissue, in vivo, can have a significant impact on the diagnosis and management of a wide range of retinal diseases.3
Time-domain detection technique measures the echo time delay of backscattered or back reflected light via an interferometer with a mechanically scanning optically referenced path.
Fourier-domain, spectral-domain or frequency-domain detection technique echo time delays of light are measured by Fourier transforming the interference spectrum of the light signal, which requires no mechanical axial scanning and results in an acquisition speed much higher than that of time-domain OCT. Also, this new technology has a higher sensitivity.4
 
STRATUS OCT (CARL ZEISS MEDITEC INC., USA)
This is an advanced imaging device. This instrument is an interferometer that resolves retinal structures by measuring the echo delay time of light (broad bandwidth near-infrared light beam; 820 nm) that is reflected and back scattered from different microstructural features in the retina. The instrument electronically detects, collects, processes and stores the echo delay patterns from the retina. With each scan pass, the instrument captures from 128 to 768 longitudinal (axial) range samples, i.e. A-scans. Each A-scan consists of 1024 data points over 2 mm of depth. Thus the instrument integrates from 131,072 to 786,432 data points to construct a cross-sectional image (tomogram) of retinal anatomy. It displays the tomograms in real time using a false color scale that represents the degree of light backscattering from tissues at different depths in the retina. The system stores the scans, which can be selected for later analysis. The OCT image can be displayed on a gray scale where more highly reflected light is brighter than less highly reflected light. Alternatively, it can be displayed in color whereby different colors correspond to different degrees of reflectivity.5
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Stratus optical coherence tomography
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CIRRUS HIGH-DEFINITION OCT (CARL ZEISS MEDITEC INC., USA)
This instrument is based on spectral-domain technology. It provides high definition cross-sectional images and 3D layer segmentation maps of internal limiting membrane (ILM) and retinal pigment epithelium (RPE). Scanning laser ophthalmoscope with fundus image with overlay of retinal thickness map, 3D retinal thickness map, 3D segmentation of retinal pigment epithelium and internal limiting membrane layers and 3D segmentation of retinal pigment epithelium layer is available. Axial resolution of this instrument is 5 μm with a transverse resolution of 20 μm. Scan speed is 27000 A-scans per second. Fundus imaging is live during scanning.7
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Cirrus high-definition optical coherence tomography
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COPERNICUS SPECTRAL-DOMAIN HIGH-RESOLUTION OCT (OPTOPOL, POLAND)
This is a 3D retina imaging (zooming, rotating, sectioning, surface reconstruction) system. It is based on spectral-domain technology which is 50 times faster than conventional OCT. It has 6 μm axial resolution with 25000 A-scans per second scanning speed, 1050 A-scans per mm and 8200 lines measurement in 0.4 seconds. It creates AVI animations of retina cross-sections.
 
RTVUE-100 FOURIER-DOMAIN OCT (OPTOVUE INC., ITALY)
This instrument is a Fourier domain/spectral domain 3D scan. It has 5 μm axial resolution, transverse resolution of 15 μm with 26000 A-scans per second scanning speed, 2564096 A-scans per frame. Inner and outer retinal thickness map and internal limiting membrane/retinal pigment epithelium elevation map are available.9
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Copernicus spectral-domain high-resolution optical coherence tomography
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SPECTRALIS HRA+OCT (HEIDELBERG ENGINEERING, GERMANY)
Optical coherence tomography images and simultaneous recording of fluorescein and ICG angiography, digital infrared and blue (“red-free”) reflectance images are obtained with a novel cSLO/OCT imaging system. The optical and technical principles of confocal scanning laser ophthalmoscopy (HRA2, Heidelberg Engineering, Heidelberg, Germany) uses an optically pumped solid state laser (OPSL) source to generate the blue light excitation wavelength of 488 nm for fluorescein angiography, red free and autofluorescence images. Diode laser sources of 790 and 815 nm wavelength are used for ICG and infrared reflectance recordings, respectively. Full emission spectra are recorded via a polarization filter to obtain blue and infrared reflectance images. With regard to the OCT, 40,000 A-scans are acquired per second with a 7 μm optical depth resolution and a 14 μm lateral optical resolution. The new operation software (ART – “Automatic Real Time” – Module, Heidelberg Engineering, Germany) is able to track eye movements in real-time based on the cSLO images. The software then computes and compensates for movements between the B-scan images, caused by position changes of the eye.11
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Spectralis spectral-domain high-resolution cSLO/OCT
(Carsten H. Meyer, MD, Germany).
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ULTRA-HIGH RESOLUTION OPTICAL COHERENCE TOMOGRAPHY
Ultra-high resolution optical coherence tomography (UHR OCT) is a recently developed improvement of the well-established OCT technology enabling unprecedented in vivo subcellular as well as intraretinal visualization. Ophthalmic UHR OCT exceeds standard resolution OCT by obtaining superior axial image resolution of 3 μm and therefore enables enhanced visualization of intraretinal layers and has the potential to perform noninvasive optical biopsy of the human retina, i.e. visualization of intraretinal morphology in retinal pathologies approaching the level of that achieved with histopathology.
This quantum leap in imaging and visualization performance is achieved by employing state-of-the-art ultrabroad bandwidth light source instead of superlumines-cent diodes. The ultimate availability of this UHR OCT technology strongly depends on the availability of such ultrabroad bandwidth light sources that are suitable for OCT applications. Recently reported, cost-effective approaches for broad bandwidth light sources mainly take advantage of the lower power demand with ultra-high resolution OCT imaging. Limiting factors of these systems are relative small bandwidths for ultralow-pump-threshold KLM Titanium: sapphire lasers and strongly modulated spectra of Cr3+-ion lasers, thus not perfectly suitable for OCT applications.13
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Ultra-high Resolution Optical Coherence Tomography
Horizontal image of a normal human macula (bottom) with two-fold magnification (top). ILM: internal limiting membrane; NFL: nerve fiber layer; GCL: ganglion cell layer; IPL, OPL: inner and outer plexiform layer; INL, ONL: inner and outer nuclear layer; HF: Henle's fiber layer; ELM: external limiting membrane; IS, OS PR: inner and outer segment of photoreceptor layer; RPE: retinal pigment epithe-lium. Arrows indicate location of total PR (red PR), IS PR (black IS) and OS PR (black OS) layer thickness measurement
(Wolfgang Drexler, PhD., Austria)
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ANATOMIC ARCHITECTURE OF THE RETINA
The spectral-domain OCT (SD-OCT) scan of the retina shows the anatomic architecture of the retina correlating with distinct bands. There is still some controversy as to the accurate terminology of these bands with histological correlation in the outer retina. To present morphological alterations, the following assumptions have been made: as a plausible morphological substrate of the 1st hyperreflective band (1) is the external limiting membrane, the 2nd band (2) appears to reflect the interface of the inner and outer segments of the photoreceptor layer, the 3rd band (3) is assumed to represent the outer segment—retinal pigment epithelium (RPE) inter digitation and the 4th band (4) may reflect the RPE/Bruch's membrane complex.
It has been speculated that the separation of the 3rd and the 4th hyperreflective band, which is not always visible, is due to multiple scattering on large nonspherical particles (e.g. melanosomes) within the retinal pigment epithelium.15
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Spectral-domain optical coherence tomography
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OPTICAL COHERENCE TOMOGRAPHY IMAGES OF THE INDIVIDUAL INTRARETINAL LAYERS
Optical coherence tomography images of the individual intraretinal layers can also be generated. Quantitative mapping of retinal layers may be documented in the form of various maps. Peeling and layer separation in 3D imaging are becoming elegant options. Visualization of separate layers of 3D images may be utilized to give a novel perspective.
False color coding is used to highlight thickness of various layers.
Retinal thickness map, retinal nerve fiber layer thickness map, retinal pigment epithelium deformation map and inner segment (IS)/outer segment (OS)-RPE deformation map can be documented very well.17
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Stratus Optical Coherence Tomography
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Retinal thickness map on spectral-domain optical coherence tomography
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Retinal nerve fiber layer thickness map on spectral-domain optical coherence tomography
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Inner segment/outer segment to retinal pigment epithelium thickness map on spectral-domain optical coherence tomography
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Retinal pigment epithelium deformation map on spectral-domain optical coherence tomography
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Spectral-Domain Optical Coherence Tomography. Variations in retinal pigment epithelium deformation, retinal nerve fiber layer thickness and retinal thickness are depicted in the form of graphs.
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