Optical Coherence Tomography (OCT)
Learning Objectives (OCT)
● [LO1] To describe the basic principles behind OCT
● [LO2] To label an OCT scan showing the layers of the retina
● [LO3] To know the applications and limitations of OCT in clinical practice
● [LO4] To identify common ocular conditions from OCT scans
1. Introduction to OCT
[LO1]
● OCT developed in the 1990s as a non-invasive imaging technique for ocular structures.
● Often no contact with the eye; however, pupil dilation may be required to improve
image quality, especially in media opacities or small pupils.
● Provides high-resolution, in vivo cross-sectional images of the retina, optic nerve
head and anterior segment.
Key idea: OCT is essentially a “light-based ultrasound” – it uses light instead of sound to
generate depth-resolved images.
2. Basic Physical Principles of OCT
[LO1]
2.1 Interferometry and Comparison with Ultrasound
[LO1]
● OCT is based on interferometry – analysis of interference patterns created when light
reflected from tissue is combined with a reference beam.
● Analogy to ultrasound:
○ Ultrasound: sends sound waves into tissue, measures time delay and intensity
of returning echoes.
, ○ OCT: sends long-wavelength light into tissue and measures the interference
between incident and reflectedlight.
● Speeds:
○ Speed of sound in water ~ 1480 m/s.
○ Speed of light ~ 3 × 10⁸ m/s → time-of-flight can’t be directly measured, hence
interferometry is used instead.
2.2 Backscatter and Contrast Formation
[LO1]
● Different retinal and ocular layers backscatter and reflect light to varying degrees due
to:
○ Differences in refractive index between tissues.
○ Structural variations (e.g. presence of pigments, nerve fibres, vasculature).
● These differences in backscatter create contrast in the OCT image.
● The reflected light pattern is captured and processed by software to create:
○ 2D cross-sections (B-scans)
○ 3D volumetric reconstructions
3. OCT Image Formation and Terminology
[LO1]
3.1 A-scans and B-scans
[LO1]
● A-scan: a single depth profile (axial reflectivity vs. depth) at one point in the tissue.
● B-scan: hundreds to thousands of A-scans aligned side-by-side to form a 2D
cross-sectional image.
● Macular imaging may involve ~30,000 A-scans for a detailed volume scan.
● Multiple B-scans can be combined to generate a 3D dataset of the retina or optic nerve
head.
3.2 Resolution and Penetration
[LO1]
● Typical axial resolution: 2–10 μm (roughly the size of a red blood cell, ~7 μm).
Learning Objectives (OCT)
● [LO1] To describe the basic principles behind OCT
● [LO2] To label an OCT scan showing the layers of the retina
● [LO3] To know the applications and limitations of OCT in clinical practice
● [LO4] To identify common ocular conditions from OCT scans
1. Introduction to OCT
[LO1]
● OCT developed in the 1990s as a non-invasive imaging technique for ocular structures.
● Often no contact with the eye; however, pupil dilation may be required to improve
image quality, especially in media opacities or small pupils.
● Provides high-resolution, in vivo cross-sectional images of the retina, optic nerve
head and anterior segment.
Key idea: OCT is essentially a “light-based ultrasound” – it uses light instead of sound to
generate depth-resolved images.
2. Basic Physical Principles of OCT
[LO1]
2.1 Interferometry and Comparison with Ultrasound
[LO1]
● OCT is based on interferometry – analysis of interference patterns created when light
reflected from tissue is combined with a reference beam.
● Analogy to ultrasound:
○ Ultrasound: sends sound waves into tissue, measures time delay and intensity
of returning echoes.
, ○ OCT: sends long-wavelength light into tissue and measures the interference
between incident and reflectedlight.
● Speeds:
○ Speed of sound in water ~ 1480 m/s.
○ Speed of light ~ 3 × 10⁸ m/s → time-of-flight can’t be directly measured, hence
interferometry is used instead.
2.2 Backscatter and Contrast Formation
[LO1]
● Different retinal and ocular layers backscatter and reflect light to varying degrees due
to:
○ Differences in refractive index between tissues.
○ Structural variations (e.g. presence of pigments, nerve fibres, vasculature).
● These differences in backscatter create contrast in the OCT image.
● The reflected light pattern is captured and processed by software to create:
○ 2D cross-sections (B-scans)
○ 3D volumetric reconstructions
3. OCT Image Formation and Terminology
[LO1]
3.1 A-scans and B-scans
[LO1]
● A-scan: a single depth profile (axial reflectivity vs. depth) at one point in the tissue.
● B-scan: hundreds to thousands of A-scans aligned side-by-side to form a 2D
cross-sectional image.
● Macular imaging may involve ~30,000 A-scans for a detailed volume scan.
● Multiple B-scans can be combined to generate a 3D dataset of the retina or optic nerve
head.
3.2 Resolution and Penetration
[LO1]
● Typical axial resolution: 2–10 μm (roughly the size of a red blood cell, ~7 μm).
