Abstract and Introduction
Purpose of review Optical coherence tomography (OCT) has revolutionized the clinical practice of ophthalmology. It is a noninvasive imaging technique that provides high-resolution, cross-sectional images of the retina, retinal nerve fiber layer and the optic nerve head. This review discusses the present applications of the commercially available spectral-domain OCT (SD-OCT) systems in the diagnosis and management of retinal diseases, with particular emphasis on choroidal imaging. Future directions of OCT technology and their potential clinical uses are discussed.
Recent findings Analysis of the choroidal thickness in healthy eyes and disease states such as age-related macular degeneration, central serous chorioretinopathy, diabetic retinopathy and inherited retinal dystrophies has been successfully achieved using SD-OCT devices with software improvements. Future OCT innovations such as longer-wavelength OCT systems including the swept-source technology, along with Doppler OCT and en-face imaging, may improve the detection of subtle microstructural changes in chorioretinal diseases by improving imaging of the choroid.
Summary Advances in OCT technology provide for better understanding of pathogenesis, improved monitoring of progression and assistance in quantifying response to treatment modalities in diseases of the posterior segment of the eye. Further improvements in both hardware and software technologies should further advance the clinician's ability to assess and manage chorioretinal diseases.
Optical coherence tomography (OCT) has evolved over the past decade as one of the most important ancillary tests in ophthalmic practice. It is a noninvasive imaging technique and provides high-resolution, cross-sectional images of the retina, the retinal nerve fiber layer (RNFL) and the optic nerve head. With axial resolution in the 5–7 μm range, it provides close to an in-vivo 'optical biopsy' of the retina. 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. 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.[1,2]
At 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). 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. In 2006, the first commercially available spectral-domain (Fourier domain) OCT (SD-OCT) system was introduced. SD-OCT employs detection of the light echoes simultaneously by measuring the interference spectrum, using an interferometer with a high-speed spectrometer. This technique achieves scan rates of 20 000–52 000 A-scans per second and a resolution of 5–7 μm in tissue.[3,4]
Although 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. New innovations in SD-OCT hardware and software now allow for accurate choroidal thickness measurements. In addition, choroidal morphological changes on OCT are now being appreciated. As a result, choroidal imaging is an emerging area of research.
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. 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. 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.
SD-OCT systems can image the choroid, however, using techniques such as image averaging and enhanced depth imaging (EDI). 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.[5,6] When multiple images are averaged, the software reduces the 'speckle.' This sharpens the continuity and enhances the retinal and choroidal features [Fig. 1]. 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[5–10] [Fig. 2].
Optical coherence tomography (OCT) images obtained using Cirrus high definition OCT (HD-OCT) system, showing increasing signal quality with the technique of image averaging. (a) A single B-scan showing low signal and increased noise. The choroid–sclera interface cannot be visualized. (b) Five B-scans averaged together to improve the signal. Note that the choroid–sclera interface is still not clearly visualized. (c) Twenty B-scans averaged together. Note the improvement in signal quality, with a fairly distinct delineation of the choroid–sclera interface (red arrows).
Optical coherence tomography (OCT) images showing increasing signal quality with the technique of enhanced depth imaging (EDI) and longer-wavelength sweeping laser light source. (a) OCT image obtained using Cirrus HD-OCT system with 20 B-scans averaged together, but without EDI. Note that choroid–sclera interface is visualized only slightly toward the far temporal and nasal aspects (red arrows). (b) OCT image obtained using Cirrus HD-OCT system with 20 B-scans averaged together and EDI. Note the improvement in the visualization of choroid–sclera interface (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 that the signal quality is markedly improved, and the choroid–sclera interface can be visualized throughout the line scan (red arrows). Also note that within the same acquisition time (as for Cirrus HD-OCT), the SS-OCT was able to average 80 B-scans, further increasing signal quality.
Apart from the commercially available systems, prototype OCT systems have contributed to an ever-growing body of research studies in this field. These include, but are not limited to, the ultra high-resolution OCT (UHR-OCT), SD-OCT systems employing a longer-wavelength light source permitting deeper tissue penetration, and swept-source OCT (SS-OCT) systems.[12,13] UHR-OCT uses broadband light sources to achieve 3 μm resolution in tissue. SS-OCT uses another form of Fourier domain detection to measure light echoes. It employs a tunable frequency swept laser light source, which sequentially emits various frequencies in time, and the interference spectrum is measured by photodetectors instead of a spectrometer. This increases the signal quality in deep tissue by elimination of the sensitivity of a spectrometer to higher frequency modulation as with SD-OCT,[12,14–18] thereby improving the visualization of the choroid [Fig. 2].
This review discusses the present applications of the various commercially available SD-OCT systems in the diagnosis and management of retinal diseases, with particular emphasis on choroidal imaging. Further, future directions of OCT technology, specifically the innovations in OCT technology with the research prototype OCT systems, their potential clinical uses, and benefits for choroidal imaging are discussed.
Curr Opin Ophthalmol. 2013;24(3):213-221. © 2013 Lippincott Williams & Wilkins