Lecture 23 Optical Coherence Tomography OCT: Basic

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Lecture 23

• Optical Coherence Tomography

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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.

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OCT vs. standard imaging
Standardclinical
Highfrequency
OCT
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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

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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 10-30 µm Time resolution
30fs!!!
Interferometry is used to measure small time
delays of scattered photons
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Good OCT sources have small coherence length and
large bandwidth
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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

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Lateral resolution Decoupled from axial
resolution
ffocal length d lens diameter
Lateral resolution similar to that in a standard
microscope
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Light sources for OCT

• Continuous sources
• 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 bandwidth _at_ 800 nm12 mm coherence length
  (in air).

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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 http//www.rp-photonics.com/
superluminescent_diodes.html
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Light sources for OCT

• Pulsed lasers
• mode-locked TiAl2O3 (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.

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Construction of image
Source of contrast refractive index
variations Image reconstructed by scanning
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Applications in ophthalmology
Normal patient
Patient with impaired vision (20/80) The cause
is a macular hole
Patients other eye (vision 20/25) Impending
macular hole, which can be treated
http//rleweb.mit.edu/Publications/currents/cur11-
2/11-2oct.htm
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Applications in cancer detection
Squamous epithelium
Columnar epithelium crypts
Loss of organization
http//rleweb.mit.edu/Publications/currents/cur11-
2/11-2oct.htm
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Applications in developmental biology
Eyeye eaear mdedulla ggills hheart
iintestine
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Ultra-high resolution OCT
Image through the skin of a living frog
tadpole Resolution 3 mm
http//rleweb.mit.edu/Publications/currents/cur11-
2/11-2oct.htm
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Ultra-high-resolution-OCT versus commercial OCT
W. Drexler et al., Ultrahigh-resolution
ophthalmic optical coherence tomography, Nature
Medicine 7, 502-507 (2001)
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3-D Reconstruction In vivo images of human eye
using spectral-domain OCT
N. A. Nassif et al., Opt. Express 12, 367-376
(2004)