1. Optical Coherence Tomography
Lecture 23
Optical Coherence Tomography

2. OCT: Basic Principles
Three-dimensional imaging technique with high spatial resolution and large penetration depth even in highly scattering media
Based on measurements of the reflected light from tissue discontinuities
e.g. the epidermis-dermis junction.
Based on interferometry
interference between the reflected light and the reference beam is used as a coherence gate to isolate light from specific depth.

3. OCT vs. standard imaging
1 mm
10 mm
100 mm
Resolution (log)
Ultrasound
Standard clinical
High frequency
OCT
Confocal microscopy
1 mm
1 cm
10 cm
Penetration depth (log)

4. OCT in non-invasive diagnostics
Ophthalmology
diagnosing retinal diseases.
Dermatology
skin diseases,
early detection of skin cancers.
Cardio-vascular diseases
vulnerable plaque detection.
Endoscopy (fiber-optic devices)
gastroenterology
gynecology
Embryology/Developmental biology
Functional imaging
Doppler OCT (blood flow)
spectroscopic OCT (absorption, high speed)
optical properties
Polarization Sensitive-OCT (birefringence).
Guided surgery
delicate procedures
brain surgery,
knee surgery

5. OCT: Principle of operation
OCT is analogous to ultrasound imaging
Uses infrared light instead of sound
Speed of sound ~ 1480 m/sec (in water)
Speed of light – 3x108 m/sec
Human skin
5 mm wide x 1.6 mm deep
SpatialResolution: μm
Time resolution: 30fs!!!
Interferometry
is used to measure
small time delays
of scattered photons

23. Good OCT sources have small coherence length and large bandwidth

24. Axial resolution The axial resolution is
notice that Dl is the 3dB-bandwidth!
The broader the bandwidth the shorter the coherence length and the higher the resolution

26. Lateral resolution: Decoupled from axial resolution
Low NA
Lateral resolution
High NA Dz b Dz
f=focal length
d= lens diameter
Lateral resolution similar to that in a standard microscope

27. Light sources for OCT Continuous sources Example
SLD/LED/superfluorescent fibers,
center wavelength;
800 nm (SLD),
1300 nm (SLD, LED),
1550 nm, (LED, fiber),
power: 1 to 10 mW (c.w.) is sufficient,
coherence length;
10 to 15 mm (typically),
Example
25 nm 800 nm 12 mm coherence length (in air).

28. Superluminescent diodes (SLDs)
Definition: broadband semiconductor light sources based on superluminescence
(Acronym: SLD)
Superluminescent diodes (also sometimes called superluminescence diodes or superluminescent LEDs) are optoelectronic semiconductor devices which are emitting broadband optical radiation based on superluminescence. They are similar to laser diodes, containing an electrically driven p-n junction and an optical waveguide, but lack optical feedback, so that no laser action can occur. Optical feedback, which could lead to the formation of cavity modes and thus to pronounced structures in the spectrum and/or to spectral narrowing, is suppressed by means of tilting the output facet relative to the waveguide, and can be suppressed further with anti-reflection coatings.
Superluminescence: amplified spontaneous emission

29. Light sources for OCT Pulsed lasers Scanning sources
mode-locked Ti:Al2O3 (800 nm),
3 micron axial resolution (or less).
Scanning sources
tune narrow-width wavelength over entire spectrum,
resolution similar to other sources,
advantage that reference arm is not scanned,
advantage that fast scanning is feasible.

30. Construction of image Source of contrast: refractive index variations
Image reconstructed by scanning

31. Applications in ophthalmology
Normal patient
Patient with impaired vision (20/80):
The cause is a macular hole
Patient’s other eye (vision 20/25):
Impending macular hole, which can be treated

32. Applications in cancer detection
Squamous epithelium
Columnar epithelium: crypts
Loss of organization

33. Applications in developmental biology
Ey=eye; ea=ear; m=dedulla; g=gills; h=heart; i=intestine

34. Ultra-high resolution OCT
Image through the skin of a living frog tadpole
Resolution: 3 mm

35. Ultra-high-resolution-OCT versus commercial OCT
W. Drexler et al., “Ultrahigh-resolution ophthalmic optical coherence
tomography”, Nature Medicine 7, (2001)

36. 3-D Reconstruction: In vivo images of human eye using spectral-domain OCT
RPE
NFL I T N S I S T N
N. A. Nassif et al., Opt. Express 12, (2004)