Optical Coherence Tomography

Excerpt

Optical coherence tomography (OCT) is a noninvasive imaging technique that uses visible and infrared electromagnetic waves to provide detailed, cross-sectional images of body tissues. OCT has widespread application in ocular imaging to diagnose and monitor various ophthalmic pathologies in both the anterior and posterior segments. OCT is commonly employed in evaluating and managing vitreoretinal and macular diseases in addition to processes affecting the optic nerve head, including glaucoma.

OCT has evolved considerably since its invention in the early 1990s and the introduction of the first commercial ophthalmological device in 1996. The 3 main types of OCT are time-domain, spectral-domain, and swept-source. These types differ in image acquisition, scanning speed, axial and transverse resolution, and range of imaging.

Time-Domain Optical Coherence Tomography

Time-domain OCT (TD-OCT) is a first-generation technology that uses a low-coherence interferometer to measure the time delay and magnitude of backscattered light from different tissue depths, subsequently constructing two-dimensional images in a manner similar to ultrasound technology. However, TD-OCT measures only one point at a time, and the coherence length of the light source limits depth resolution. TD-OCT typically uses a superluminescent diode with a relatively broad spectrum as its light source, resulting in decreased image resolution compared to newer technologies. TD-OCT has largely been replaced by techniques that offer faster acquisition speeds, higher image resolution, and a better range of imaging.

Spectral-Domain Optical Coherence Tomography

Spectral-domain OCT (SD-OCT) is a second-generation technology with significantly faster acquisition speeds, deeper tissue penetration, and higher image resolution. SD-OCT uses a spectrometer to detect the spectrum of backscattered light, allowing simultaneous measurement of multiple tissue points. Higher-speed data acquisition enables three-dimensional tissue imaging and improved resolution.

SD-OCT uses Fourier transformations to generate high-resolution, cross-sectional images of biological tissues. In SD-OCT, a broadband superluminescent diode source is used to illuminate the tissue. Light reflected from the tissue is detected using a spectrometer that separates the reflected light into its constituent wavelengths. Interference patterns between the reflected light and a reference beam are measured for each wavelength and processed using Fourier transformations to generate high-resolution images.

Due to its improved imaging capabilities, SD-OCT has become the standard for clinical use in ophthalmology, enabling earlier diagnosis and management of conditions such as age-related macular degeneration, diabetic retinopathy, and glaucoma. SD-OCT also has clinical applications in dermatology, cardiology, and gastroenterology.

Enhanced Depth Imaging

Enhanced depth imaging (EDI) in SD-OCT is an imaging modality that places the objective lens in a closer scanning position to the eye, permitting better depth sensitivity and improved visualization of deeper ocular structures, particularly the choroid. EDI-OCT is particularly useful when diagnosing and managing diseases that affect the choroid, such as age-related macular degeneration, choroidal neovascularization, polypoidal choroidal vasculopathy, and central serous chorioretinopathy. EDI-OCT can also provide crucial information about the thickness of the retina and the presence of fluid or swelling in the retina or choroid. Modern SD-OCT and swept-source OCT (SS-OCT) machines are capable of acquiring enhanced depth images.

Swept-Source Optical Coherence Tomography

SS-OCT is an advanced noninvasive medical imaging technique that uses a wavelength-sweeping laser and a single dual-balanced photodetector to capture high-resolution images of the anterior segment, retina, optic nerve, and choroid. The longer wavelength of the light source permits deeper tissue penetration and faster scanning speeds, resulting in excellent widefield visualization of posterior segment eye structures superior to SD-OCT with EDI.

Optical Coherence Tomography Angiography

OCT angiography (OCTA) is a noninvasive imaging method that gives a three-dimensional visualization of blood vessels at different tissue levels within the retina and choroid. The images captured by OCTA provide details far superior to those obtained using conventional fundus fluorescein angiography or indocyanine green angiography; OCTA does not carry the time requirement or risks associated with systemic contrast administration. OCTA has multiple applications in neuro-ophthalmology, including multiple sclerosis, anterior ischemic neuropathy, hereditary optic neuropathy, and glaucoma.