Published on 09/05/2015 by admin
Filed under Opthalmology
Last modified 22/04/2025
This article have been viewed 1175 times
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Optical coherence tomography (OCT) is a medical diagnostic imaging technology that captures micron resolution three-dimensional images. It is based on the principle of optical reflectometry, which involves the measurement of light back-scattering through transparent or semi-transparent media such as biological tissues. It achieves this by measuring the intensity and the echo time delay of light that is scattered from the tissues of interest. Light from a broadband light source is broken into two arms, a reference arm and a sample arm that is reflected back from structures at various depths within the posterior pole of the eye.
There are two main ways in which the backscattered light can be detected:
▶ Time domain (TD) detection
▶ Fourier domain (FD) detection – which is further broken down into:
• Spectral domain (SD)
• Swept source (SS)
In time domain OCT scanning, light from the reference arm and light reflected back from the sample undergo interference, and the interference over time is used to generate an ‘A-scan’ depth resolved image of the retina at a single point. Moving the sample and the light source with respect to each other generates multiple A-scans that are combined into a cross-sectional linear image called the B-scan or ‘line scan’. Scanning speeds of TD-OCTs are typically around 400 A-scans/second. The primary commercially available TD-OCT device is the Stratus OCTTM made by Carl Zeiss Meditech.
In this technology, the spectral interference pattern between the reference beam and the sample beam is dispersed by a spectrometer and collected simultaneously with an array detector. This simultaneous collection allows for much faster scanning speeds than the traditional time domain devices where a mechanically moving interferometer gathers the data over time. An A-scan is then generated using an inverse Fourier transform on the simultaneously gathered data. Commercially available SD-OCT devices have scanning rates of 18,000–70,000 A-scans/second.
Higher scan speeds in the SD-OCT faster acquisition time, which minimizes the chance of eye movements during acquisition, especially in patients with poor fixation. Both hardware and software enhancements permit precise image registration which allows for more reliable comparison between visits. Faster acquisition speeds also mean a higher sampling density of the macula, minimizing the chances of missing pathology. The higher speeds allow for the production of three-dimensional OCT scans. The broader light sources of SD-OCT devices achieve a higher axial resolution than TD-OCT, allowing better visualization of retinal anatomy. Commercially available SD-OCT devices include: the Cirrus OCT made by Carl Zeiss Meditech, the Spectralis OCT made by Heidelberg Engineering, 3D-OCT 1000 (Topcon), Bioptigen SD OCT (Bioptogen) and the RT-Vue (Optovue).
In swept source (or optical frequency domain) OCT scanning, the light source is rapidly swept in wavelength and the spectral interference pattern is detected on a single or small number of receivers as a function of time. The spectral interference patterns obtained as a function of time then undergo a reverse Fourier transform to generate an A-scan image. Higher scanning speeds allow for denser sampling and better registration. The swept source OCT also has less sensitivity roll-off with depth, allowing better visualization of structures deep to the retina. At present, swept source OCT is not widely available commercially with the DRI-OCT 1 (Topcon) being the only commercially available device.
Handbook of Retinal OCT Optical Coherence Tomography
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