Optical coherence tomography (OCT) is an imaging technique that uses light to capture micrometer-resolution, two- and three-dimensional images from within biological tissues. It measures the echo time delay and intensity of backscattered light to produce digital cross-sectional images. OCT has been widely used in ophthalmology for imaging the retina and anterior segment since the 1990s. Technological advancements have vastly improved image resolution, scan speed, and visualization capabilities of modern OCT devices.

Optical Coherence Tomography Device Workings

OCT uses low-coherence near-infrared light, usually from a superluminescent diode or femtosecond laser source. The light entering the eye is split into a sample arm and reference arm. The sample arm light interacts with the tissue and returns a backscattered signal. The reference arm light reflects off a mirror and returns as well. When these two signals are combined, their interference produces an interferogram that can be Fourier transformed into depth profiles along the optical axis. By scanning the beam laterally over the tissue, cross-sectional images are constructed. Sweeping the reference arm allows 3D imaging with micrometer axial resolutions.

Spectral Domain vs Time Domain OCT

Early OCT systems used Time-Domain (TD-OCT) detection that measured echo time delays by physically moving the reference mirror. Though they provided high resolution images, TD-OCT devices were slow with acquisition speeds of only 400-600 A-scans per second. More recent systems employ Spectral Domain (SD-OCT) detection that uses a fixed reference arm and measures echo time delays through Fourier domain analysis of the interferogram spectrum. SD-OCT has significantly faster acquisition speeds up to 100,000 A-scans/sec allowing ultra-high resolution 3D retinal and anterior segment imaging.

Clinical Applications of Optical Coherence Tomography Devices

Retinal Disorders - OCT is indispensable for diagnosing and monitoring retinal diseases. Cross-sectional imaging can visualize layers of the retina and differentiate subtle abnormalities in conditions like age-related macular degeneration, diabetic retinopathy, and retinal vein occlusions. 3D imaging captures the progression of pathology over time.

Glaucoma - OCT imaging of the peripapillary retinal nerve fiber layer and macula helps diagnose and monitor glaucoma. Quantification of layer thickness provides an objective measure of glaucomatous damage. Some devices also enable angle imaging of the anterior segment for gonioscopy.

Cataracts - Anterior segment OCT allows high resolution imaging of the crystalline lens and aids in preoperative evaluation and intraoperative planning for cataract surgery. Lens thickness and opacity measurements guide surgical decisions.

Corneal Diseases - Cross-sectional corneal imaging reveals pathology of the stroma, epithelium and endothelium in conditions like keratoconus, dystrophies, and infections. Quantification of layer thickness helps monitor disease progression and treatment response.

Advances in Optical Coherence Tomography Devices

Angiography - Optical coherence tomography angiography (OCTA) uses motion contrast imaging methods to visualize the retinal and choroidal vasculature without dye injection. It can detect microvascular abnormalities in diseases affecting the circulation like diabetic retinopathy and age-related macular degeneration.

En Face Imaging - Sweeping illumination beams laterally rather than depth-wise generates en face retinal maps displaying wider fields of view. These enhance visualization of peripheral retina pathology.

Ultrahigh Resolution - Next generation Fourier-domain OCT systems achieve axial resolutions approaching 3 microns, enabling clearer single cell resolution of the outer retina and choroid. Cellular features captured provide deeper understanding of pathophysiology.

Integrated Imaging - Multi-modality platforms integrate OCT with other modalities like Fundus autofluorescence, Near-infrared reflectance and Scanning laser ophthalmoscopy. Registered structural and functional information aid comprehensive disease evaluation.

In summary, optical coherence tomography has revolutionized ophthalmic imaging through its ability to non-invasively capture micrometer scale cross-sectional and 3D views of ocular tissues. Constant technological upgrades are enhancing its resolution, speed, functionality and integration potential. As a prominent area of translational research, OCT will likely play a key role in advancing our understanding, diagnosis and management of ocular diseases.

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