4.2 Optical coherence tomography (OCT)

Optical coherence tomography (OCT) is a game-changing imaging technique that lets us peek inside tissues without cutting them open. It uses light to create super detailed cross-sectional images, giving doctors a non-invasive way to spot tiny changes in organs like eyes and skin.

OCT works by splitting light into two paths and measuring how it bounces back from tissues. This clever trick lets it map out layers and structures with incredible precision, making it a go-to tool for diagnosing eye diseases and skin conditions.

OCT System Principles

Components and Setup

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• Optical coherence tomography (OCT) is a non-invasive imaging technique that uses low-coherence light to capture high-resolution, cross-sectional images of biological tissues
• OCT systems typically consist of a low-coherence light source (superluminescent diode or femtosecond laser), a beam splitter, a reference arm, a sample arm, and a detector
• The low-coherence light source emits a broadband light signal, which is split into two paths by the beam splitter: the reference arm and the sample arm
• The reference arm contains a mirror that reflects the light back to the beam splitter, while the sample arm directs the light onto the biological tissue being imaged

Image Formation and Resolution

• Backscattered light from the tissue interferes with the light from the reference arm, creating an interference pattern that is detected by the photodetector
• The interference pattern contains information about the depth and reflectivity of the tissue structures, which is used to reconstruct a cross-sectional image
• The axial resolution of OCT is determined by the coherence length of the light source, with shorter coherence lengths providing higher resolution
• Typical axial resolutions range from 1-15 micrometers, depending on the light source used

Low Coherence Interferometry in OCT

Principles of Low Coherence Interferometry

• Low coherence interferometry (LCI) is the fundamental principle behind OCT imaging, enabling high-resolution depth-resolved imaging of biological tissues
• In LCI, a low-coherence light source is used to generate a broadband light signal with a short coherence length, typically in the range of micrometers
• The short coherence length allows for high axial resolution in OCT imaging, as interference between the reference and sample arm light occurs only when their path lengths are closely matched within the coherence length
• By scanning the reference arm mirror, the interference pattern changes, allowing depth-resolved information to be obtained from the sample

Axial and Transverse Resolution

• The interference signal is strongest when the path length difference between the reference and sample arms is within the coherence length of the light source, enabling precise localization of reflective structures within the tissue
• LCI enables OCT to achieve high axial resolution, typically in the range of 1-15 micrometers, depending on the light source used
• The transverse resolution of OCT is determined by the focusing optics in the sample arm and is typically in the range of 10-30 micrometers
• Higher numerical aperture focusing optics can improve transverse resolution but at the cost of reduced depth of field

OCT Applications in Biomedical Imaging

Ophthalmology

• OCT has found widespread application in ophthalmology for non-invasive imaging of the retina, cornea, and anterior segment of the eye
• Retinal OCT enables visualization of the layered structure of the retina, including the retinal nerve fiber layer, photoreceptors, and retinal pigment epithelium, aiding in the diagnosis and monitoring of conditions such as glaucoma, age-related macular degeneration, and diabetic retinopathy
• Anterior segment OCT allows for detailed imaging of the cornea, iris, and lens, useful in the assessment of corneal thickness, keratoconus, and angle-closure glaucoma
• OCT angiography (OCTA) enables non-invasive visualization of retinal and choroidal vasculature without the need for contrast agents

Dermatology and Other Applications

• In dermatology, OCT is used for non-invasive imaging of the skin, providing cross-sectional views of the epidermis, dermis, and superficial blood vessels
• Dermatological OCT assists in the diagnosis and monitoring of skin conditions such as skin cancer (basal cell carcinoma and melanoma), psoriasis, and atopic dermatitis
• OCT can also be used to assess the efficacy of dermatological treatments and monitor wound healing processes
• Other biomedical applications of OCT include imaging of the coronary arteries (intravascular OCT), gastrointestinal tract (endoscopic OCT), and oral cavity (dental OCT)
• Functional extensions of OCT, such as Doppler OCT and polarization-sensitive OCT, enable visualization of blood flow and tissue birefringence, respectively, providing additional diagnostic information

OCT Advantages vs Other Modalities

Advantages of OCT

• High spatial resolution: OCT offers micrometer-scale resolution, enabling visualization of fine tissue structures that may not be resolved by other imaging modalities such as ultrasound or MRI
• Non-invasive and non-ionizing: OCT uses low-power, near-infrared light, making it a safe and non-invasive imaging technique without the risks associated with ionizing radiation (X-rays) or contrast agents
• Real-time imaging: OCT can acquire images in real-time, allowing for immediate visualization and assessment of tissue structures during diagnostic procedures or surgical interventions
• Depth-resolved imaging: OCT provides cross-sectional images of tissue, enabling visualization of subsurface structures and layers that may not be accessible by surface imaging techniques such as microscopy

Limitations of OCT

• Limited imaging depth: OCT has a typical imaging depth of 1-3 mm in biological tissues due to light scattering and absorption, which limits its ability to image deeper structures compared to modalities like ultrasound or MRI
• Reduced image contrast: OCT relies on the intrinsic optical properties of tissues for image contrast, which may result in reduced contrast between different tissue types compared to modalities that use exogenous contrast agents (MRI with gadolinium contrast)
• Motion artifacts: OCT is sensitive to motion artifacts caused by patient movement or physiological processes (breathing, heartbeat), which can degrade image quality and require motion correction techniques
• Difficulty imaging through opaque or highly scattering tissues: OCT imaging can be hindered by opaque or highly scattering tissues, such as bones or heavily pigmented skin, limiting its application in certain anatomical regions