Introduction to Optical Coherence Tomography
New to OCT? Optical Coherence Tomography is a long-winded term to describe an imaging system very similar to ultrasound, that uses light instead of sound. Low intensity near infrared light is directed at the patient, and the small amount of light that is reflected is captured by the system’s detectors and produced into an image that shows the internal detail of the structure or tissue of interest.
In contrast to ultrasound, OCT imaging does not require any physical contact with the patient. Bioptigen Envisu SDOIS uses very low power light that is perceived as red by the patient, but is generally not perceived as bright or uncomfortable.
Another difference between ultrasound and OCT is the level of detail that can be seen. Because lightwaves are shorter than soundwaves, OCT generally shows much finer detail than can be seen with ultrasound. On the other hand, ultrasound can be used to see deeper into tissue, and can frequently see into structures that are optically opaque.
OCT is used to see details of the cornea or the retina, where ultrasound will see structures hidden by the iris.
Coherence in Optical Coherence Tomography
Optical Coherence Tomography is based on optical interference, as in holography. All OCT systems have a reference arm and a sample arm. The sample arm delivers light to the patient, and collects the light scattered from the patient. This sample light is mixed with light that is reflected from the reference arm of the system.
Coherence refers to a property of light that enhances the detection of the mixing of the sample and reference light. If the sample and reference light are coherent, the mixing produces an interference pattern that is detected in the system and converted to an image. The sample and reference light are incoherent, they will not produce an interference pattern that can be converted to an image.
OCT systems are designed so that the reference light and sample light are only coherent when the path lengths are very closely matched. Generally, the reference arm position is changed to image different subjects, such as the retina (back of the eye) and the cornea (front of the eye).
Tomography in Optical Coherence Tomography
OCT is a good technique for creating cross-sectional (tomographic) or volumetric pictures of an object. Such a picture is produced by scanning light across the subject to be imaged. The image of the subject that is produced along the depth direction is termed an A-scan. Each A-scan provides information about the reflective or scattering properties of the subject as a function of depth at one position of the scanned beam.
A cross-sectional image is produced by assembling a collection of neighboring A-scans. This is the tomography in OCT. Typically, we think of a B-scan as being an image of a planar slice into the subject, as if we had used a scalpel to cut into tissue. But a B-scan does not have to be a planar image. It is common to take an image along a circle as well, and this cross-sectional view is then an annular scan around a point of interest in the tissue.
A volumetric image is constructed from a collection of B-scans. There are three major types of volumetric images used in OCT imaging:
- Rectangular, or raster volume scan: A series of parallel B-scans
- Radial volume scan: A series of B-scans at regular angular intervals
- Annular volume scan: A series of B-scans forming concentric rings
Each of these scan types is useful in particular circumstances. As examples, rectangular volumes are most frequently used in imaging the macula. Rectangular or annular scans are often used to image in the vicinity of the optic nerve head. Radial scans are most useful for imaging the cornea. These are examples only. Other scan patterns are possible and useful, and no particular scan pattern is mandatory to image a particular structure.
Time Domain, Fourier Domain and Spectral Domain Optical Coherence Tomography
Optical Coherence Tomography originated in measurement techniques first developed to measure optical fibers for telecommunications. The first generation of OCT is now referred to as Time Domain Optical Coherence Tomography, or TDOCT. A-scans are generated in Time Domain OCT by scanning the reference mirror back and forth across a range of positions that match different depths in the subject tissue. TDOCT is relatively simple, but can only acquire A-scans as fast as the reference mirror can scan.
Fourier Domain Optical Coherence Tomography (FDOCT) refers to a second generation of imaging techniques that do not require a scanning reference mirror, and can thus be much faster. FDOCT operates on a principle of spectral interferometry and Fourier transforms. Rather than collecting signals related to different positions in depth by scanning the reference mirror, FDOCT operates by collecting signals related to different wavelengths (colors) of light, and using a mathematical relationship to generate an image. Fourier Domain OCT is more complex than Time Domain OCT, but can operate more than 10-times faster.
Spectral Domain Optical Coherence Tomography (SDOCT) is a particular implementation of Fourier Domain OCT that collects all of the wavelengths of light at the same time using a specially designed spectrometer. Through judicious design of the spectrometer, selection of the light source, and implementation of mathematical techniques, SDOCT systems can be tailored to provide exceptional images for a wide range of research and clinical applications.
Principles of Optical Coherence Tomography
Representative B-scan of finger
Representative volume reconstruction of finger
Low-Coherence Interferometry – Time Domain technology
Low-Coherence Interferometry – Fourier Domain technology
Representative A-scan of finger