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Optical coherence tomography (OCT) is an imaging technique that uses light waves to capture micrometer-resolution, cross-sectional images from within biological tissues. OCT devices work by measuring the echo time delay and magnitude of backreflected or backscattered light. This allows reconstruction of 2D or 3D images from optical scattering from internal tissue microstructures. OCT has become an important biomedical imaging technique due to its high resolution, non-invasive nature, and ability to provide microstructure visualization of tissues. In this article, we will explore the principles behind OCT, developments in OCT technology, and clinical applications of OCT devices.
How OCT devices work
Optical Coherence Tomography Devices work on the principle of low-coherence interferometry, specifically using low-coherence light sources with coherence lengths short enough to allow optical sectioning but long enough to achieve micrometer-scale resolution. A broadband light source, usually a superluminescent diode or femtosecond laser, is split into a sample arm and reference arm by a fiber-optic coupler. Light from the sample arm encounters internal sample structures, while light in the reference arm encounters a reference mirror. Backscattered light from both arms recombines and interferes, generating an interference signal that contains depth profile information of internal tissue structures along the optical axis. By measuring this interference signal over time using a spectrometer and photodetector, a 1D depth profile, known as an A-scan, can be reconstructed. Lateral resolution is achieved by transverse scanning of the focused beam over the sample. Multiple A-scans can be combined to produce a 2D cross-sectional image, known as a B-scan. Three-dimensional volumetric data sets, known as C-scans, can be generated by acquiring multiple B-scans.
Advances in OCT technology
Since its invention in the early 1990s, OCT technology has advanced rapidly. Early time-domain OCT devices used mechanically translating reference mirrors for depth scanning, limiting acquisition speeds to around 400 A-scans/second. The development of Fourier domain OCT, also known as spectral domain OCT, helped overcome this limitation. Instead of measuring interference signal amplitude, spectral domain OCT measures the interference signal wavelength spectrum, allowing the entire depth profile to be acquired simultaneously with a spectrometer and linear detector array. This increased acquisition speeds tremendously to around 20,000-100,000 A-scans/second. More recently, swept-source OCT using wavelength-swept lasers has been developed, combining benefits of both time and spectral domain approaches with acquisition speeds exceeding 1 million A-scans/second.
Other technological advances include the development of extended focal depth and full-range OCT to overcome limited imaging depth, adaptive optics to correct for aberrations in the eye, and three-dimensional ultrahigh speed OCT for real-time volumetric retinal imaging. Dual-modality systems combining OCT with other imaging techniques like fluorescence microscopy, ultrasound, and optical microscopy have also expanded capabilities. Miniaturization of OCT probes and handheld and smartphone-integrated OCT devices have improved access and portability. With continuous technological developments, higher speed, resolution, and imaging ranges are allowing OCT to visualize ever finer biological structures and find new applications.
Clinical applications of OCT devices
The high resolution and non-invasive nature of OCT imaging have made it an indispensable tool in ophthalmology. Commercially available ophthalmic OCT devices are used routinely in diagnosis and management of retinal diseases like age-related macular degeneration, glaucoma, diabetic retinopathy, and retinal detachments. Clinical applications include assessment of macular thickness, foveal contour, retinal nerve fiber layer thickness, vitreoretinal interface abnormalities, and surveillance after surgical procedures. Handheld OCT devices allow bedside and point-of-care retinal imaging without complex equipment.
Outside of ophthalmology, OCT has grown significantly in dermatology for high-resolution skin imaging. Devices are used for non-invasive diagnostics of skin cancers, scar assessment, wound healing, and monitoring of conditions like psoriasis and atopic dermatitis. In cardiology, OCT provides micrometer-scale resolution of coronary plaques, helping assess vulnerability in patients with signs of cardiac ischemia. It is also an important tool in gastroenterology for characterization of esophageal, gastric and intestinal wall structures, and detection of early neoplasia. Other clinical applications include imaging of airways in pulmonology, identification of layer structures in histopathology, and intravascular imaging for guidance in minimally invasive surgery. Thewide range of structural information accessible non-invasively has cemented OCT as a versatile clinical and research imaging modality.
Conclusions
Over the past few decades, OCT has evolved rapidly from a technology demonstration to a mainstream clinical imaging technique. Technological advances have dramatically increased acquisition speeds and resolution capabilities. Miniaturization and integration with other modalities are expanding OCT’s clinical utility. The non-invasive, high resolution visualization of microscopic tissue architecture provides clinicians with critical structural information across many medical specialties. Continued developments promise to enhance disease diagnosis, treatment monitoring, and exploration of new biomarkers. As the first optical coherence tomography devices become commercially available in the early 1990s, few would have predicted the widespread clinical impact and utility OCT devices have shown today across ophthalmology, dermatology, cardiology and many other fields. OCT remains a highly active area of research and technological development, keeping its promise as a powerful tool for biomedical imaging.
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1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it
