Note that the signal quality is markedly improved, and the choroid–sclera interface can be visualized throughout the line scan (red arrows). (c) OCT image obtained using a swept-source OCT (SS-OCT) system centered at a wavelength of 1050 nm, and an imaging speed of 100 000 A-scans per second. Note the improvement in the visualization of choroid–sclera interface (red arrows). (b) OCT image obtained using Cirrus HD-OCT system with 20 B-scans averaged together and EDI. Note that choroid–sclera interface is visualized only slightly toward the far temporal and nasal aspects (red arrows). (a) OCT image obtained using Cirrus HD-OCT system with 20 B-scans averaged together, but without EDI. Optical coherence tomography (OCT) images showing increasing signal quality with the technique of enhanced depth imaging (EDI) and longer-wavelength sweeping laser light source. Along with image averaging, EDI involves setting the choroid adjacent to the zero delay line, which allows enhanced visualization of choroid up to the sclera, by taking advantage of the sensitivity roll-off characteristic of SD-OCT systems. When multiple images are averaged, the software reduces the ‘speckle.’ This sharpens the continuity and enhances the retinal and choroidal features. Image averaging involves obtaining multiple B-scans from the same retinal location that are then averaged together to increase the signal-to-noise ratio, typically in proportion to the square root of the number of images averaged. SD-OCT systems can image the choroid, however, using techniques such as image averaging and enhanced depth imaging (EDI). Also, the pixel density of TD-OCT, which is limited by the number of axial scans in the OCT image, makes visualization of the choroid difficult. In addition, because of the relatively low signal-to-noise ratio of TD-OCT, the signal and image information from the deeper layers of the choroid is not of high enough quality to see precise morphological details. The choroid cannot be well visualized using the Stratus OCT, as the retinal pigment epithelium (RPE) is highly light scattering, resulting in attenuation of the relatively weak reflection signal from the choroid. As a result, choroidal imaging is an emerging area of research. In addition, choroidal morphological changes on OCT are now being appreciated. New innovations in SD-OCT hardware and software now allow for accurate choroidal thickness measurements. Īlthough OCT is used extensively for clinical decision making and monitoring of many posterior segment diseases based on macular, optic nerve and RNFL imaging, until recently, the choroid was not able to be clearly imaged with this technique. This technique achieves scan rates of 20 000–52 000 A-scans per second and a resolution of 5–7 μm in tissue. SD-OCT employs detection of the light echoes simultaneously by measuring the interference spectrum, using an interferometer with a high-speed spectrometer. In 2006, the first commercially available spectral-domain (Fourier domain) OCT (SD-OCT) system was introduced. Time-domain OCT (TD-OCT) systems featured scan rates of 400 A-scans per second with an axial resolution of 8–10 μm in tissue. Īt its advent, time-domain detection was the technique employed by commercially available OCT systems such as the Stratus OCT (Carl Zeiss Meditec, Inc, Dublin, CA). The light waves that are backscattered from the retina, interfere with the reference beam, and this interference pattern is used to measure the light echoes versus the depth profile of the tissue in vivo. OCT employs light from a broadband light source, which is divided into a reference and a sample beam, to obtain a reflectivity versus depth profile of the retina. With axial resolution in the 5–7 μm range, it provides close to an in-vivo ‘optical biopsy’ of the retina. It is a noninvasive imaging technique and provides highresolution, cross-sectional images of the retina, the retinal nerve fiber layer (RNFL) and the optic nerve head. Optical coherence tomography (OCT) has evolved over the past decade as one of the most important ancillary tests in ophthalmic practice.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |