Optical Coherence Tomography (OCT) is an imaging technology that creates detailed cross-sectional images without the need for invasive procedures. It analyses the interference signals from the object under study and a nearby reference point. OCT is frequently employed in the medical field for diagnosing diseases and monitoring treatment progress, offering real-time visuals of organ tissues. This system provides impressive depth resolutions, typically ranging from 5 to 10 µm, enabling non-invasive optical biopsies of living tissues.

It is a type of imaging similar to ultrasound, but it uses scattered light waves, typically in the near-IR to IR range, to create images of specific tissue areas. To achieve sensitivity and selectivity in capturing the scattered light from a particular tissue site, OCT depends on the interference between the scattered light and a reference beam. This imaging technique is especially well-suited for highly scattering materials, such as hard tissues. 

Interference occurs when the wavefronts of two light sources overlap and have a clear phase relation. This coherence, or phase relation, is maintained within a certain distance called the coherence length. When both the reference beam and the light reflected from a scattering site come from the same light source, they create a distinct interference pattern only if the difference in their path lengths falls within this coherence length.

Working of OCT

The source emits a beam of light that is directed onto the sample. Some of the light gets scattered or reflected back from different depths within the sample. This is due to variations in the tissue's composition and structure. The reference mirror reflects some of the light from the source, creating a reference beam. The scattered light from the sample and the reference beam combine and enter the detector. The detector measures the time it takes for the light to travel to different depths in the sample and back to the detector. It also measures the intensity of the light. By comparing the reference beam with the light that has traveled through the sample, OCT can calculate the distance each of the reflected light has traveled. This information allows OCT to create a cross-sectional image of the sample. As the source beam and detector move across the sample, multiple cross-sectional images are captured, and they are combined to create a detailed three-dimensional image of the sample's internal structure.

High and Low Coherent Light Source in OCT

In optical coherence tomography, adjusting the reference beam's position creates a difference in the path lengths traveled by the light reflected from the reference mirror and the scattered photons from the sample being examined. 

High coherence light source:

When dealing with a fully coherent light source, like a high-coherence laser, the interference between the reference beam and the back-scattered beam can be maintained even when the reference mirror is moved over a significant path-length range. This setup does not provide selectivity for back-scattering from a specific depth within the sample.

Low coherence light source:

In the case of a low-coherence source with a short coherence length, interference patterns between the reference and back-scattered beams only occur when their path difference falls within the coherence length. In a 3D scattering medium, this means that at any given position of the reference mirror, only a specific depth range (determined by the coherence length) of back-scattered light will exhibit interference. By scanning the reference mirror, depth discrimination can be achieved, and the resulting interference patterns carry information about the sample's refractive index variations, enabling the creation of an optical image.

In OCT, the axial (depth) resolution depends on the coherence length of the light source. A shorter coherence length results in better depth resolution. To achieve this, a bright but incoherent light source like a superluminescent diode (SLD) or a laser with poor coherence, such as a femtosecond laser source with a broad bandwidth is employed.

Fiber-Based OCT

In this setup, a dual-core fiber is employed. One core of the fiber carries the broad-band light source and divides it into two paths: the sample probe and the reference. The second core of the fiber gathers both the back-scattered signal and the reflected reference signal, combining them to generate the interference pattern.

Advantages of OCT

• High resolution imaging with resolutions of 4-20 µm 
• Real-time imaging is possible
• Fiber-based design allows integration with small catheters/endoscopes

Clinical Benefits of OCT

OCT systems based on optical fibers are compact, portable, do not require physical contact, and can be integrated with laser spectroscopy and Doppler velocimetry.

Applications of OCT

Optical Coherence Tomography has found numerous applications in various fields, including medicine, ophthalmology, and materials science. It is extensively used in ophthalmology to visualize the layers of the retina. OCT helps in diagnosing and monitoring conditions such as macular degeneration, diabetic retinopathy, and glaucoma. It is used for imaging the anterior segment of the eye, including the anterior chamber, iris, and angle structures. This is valuable in glaucoma assessment and evaluation of cataracts. It is employed to visualize blood vessels from within, allowing cardiologists to assess arterial plaques, stents, and vessel luminal dimensions. This also aids in guiding interventional procedures like angioplasty. OCT can provide high-resolution images of the skin layers. It helps dermatologists diagnose and monitor skin conditions such as skin cancer, psoriasis, and eczema.

This technique is used to visualize the gastrointestinal tract's mucosal and submucosal layers, aiding in the detection and staging of conditions like Barrett's esophagus and colorectal cancer. It is used to study the layers of the retina and optic nerve head, which can provide insights into neurodegenerative diseases like multiple sclerosis and Alzheimer's disease. OCT is used to assess dental tissues, including enamel, dentin, and periodontal structures. It aids in diagnosing dental caries, assessing tooth restoration quality, and evaluating periodontal disease. OCT is used to examine the internal structures of materials and coatings, making it valuable for quality control and research in industries such as aerospace, electronics, and manufacturing.

OCT assists in visualizing the anatomy of the ear, nose, and throat, aiding in the diagnosis and treatment of conditions such as chronic sinusitis, vocal cord disorders, and ear diseases. It can be used to examine the microstructure of tumors and assess their margins during surgery, helping surgeons remove cancerous tissue more precisely. OCT is widely used in research for studying various biological and physical phenomena. It allows for non-destructive imaging and provides valuable insights into complex systems.

It is used in cardiology to visualize the inside of blood vessels, providing detailed images that can aid in the diagnosis and treatment of cardiovascular diseases. OCT can be used to inspect and measure various materials, including semiconductor wafers, coatings, and layered structures, making it valuable for quality control and process optimization in manufacturing.