Optical Coherence Tomography, or OCT, is a medical imaging technology that uses light to create detailed cross-sectional pictures of tissue. It is especially important in eye care because it can show the layered structure of the retina without surgery or direct contact. Doctors use OCT to detect and monitor conditions such as glaucoma, macular degeneration, and diabetic retinopathy.
Its ability to reveal micrometer-scale details makes it one of the most powerful tools for viewing living tissue safely.
Key Facts
- OCT uses near-infrared, low-coherence light to image tissue layers.
- Interference occurs when light from a sample arm and reference arm recombine.
- Axial resolution is approximately Δz = 2 ln(2) λ0^2 / (π Δλ) for a Gaussian spectrum.
- Greater light bandwidth Δλ gives better depth resolution.
- OCT measures backscattered light intensity as a function of depth to build an A-scan.
- Many adjacent A-scans combine to form a 2D B-scan image of tissue structure.
Vocabulary
- Optical Coherence Tomography
- Optical Coherence Tomography is an imaging method that uses reflected light and interference to create cross-sectional images of microscopic tissue layers.
- Low-coherence light
- Low-coherence light is light with a short coherence length, allowing OCT to select reflections from very small depth ranges.
- Interferometry
- Interferometry is a measurement technique that combines two light waves to reveal information from their interference pattern.
- A-scan
- An A-scan is a one-dimensional OCT depth profile showing reflected light strength from different tissue depths.
- Retina
- The retina is the light-sensitive tissue at the back of the eye that contains layered cells responsible for vision.
Common Mistakes to Avoid
- Confusing OCT with an ordinary camera image. OCT does not mainly record surface color, it measures reflected light from different depths to build a cross-sectional map.
- Assuming stronger light always gives a better image. Too much light can be unsafe for tissue, and image quality also depends on coherence length, bandwidth, focusing, and signal processing.
- Thinking OCT uses X-rays or ultrasound. OCT uses light, usually near-infrared light, so its physics is based on optics and interference rather than ionizing radiation or sound waves.
- Ignoring the role of bandwidth in resolution. A broader light spectrum gives a shorter coherence length, which improves the ability to separate nearby tissue layers in depth.
Practice Questions
- 1 An OCT system has a center wavelength λ0 = 850 nm and a spectral bandwidth Δλ = 50 nm. Using Δz = 2 ln(2) λ0^2 / (π Δλ), estimate the axial resolution in micrometers.
- 2 A retina is scanned with 600 A-scans across a 6.0 mm line. What is the spacing between adjacent A-scans in micrometers?
- 3 Explain why low-coherence light helps OCT distinguish reflections from two nearby retinal layers instead of mixing them into one signal.