Lecture 23
Optical Coherence Tomography
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.
OCT vs. standard
imaging
Resolution (log)
Standard
clinical
1 mm
Ultrasound
100 mm
10 mm
High
frequency
Confocal
microscopy
OCT
1 mm
Penetration depth (log)
1 mm
1 cm
10 cm
OCT in non-invasive
diagnostics
• Ophthalmology
– diagnosing retinal diseases.
• Dermatology
– skin diseases,
– early detection of skin
cancers.
•
Functional imaging
–
–
–
–
Doppler OCT (blood flow)
spectroscopic OCT (absorption, high
speed)
optical properties
Polarization Sensitive-OCT
(birefringence).
• Cardio-vascular diseases
– vulnerable plaque detection.
• Endoscopy (fiber-optic
devices)
– gastroenterology
– gynecology
• Embryology/Developmental
biology
• Guided surgery
– delicate procedures
• brain surgery,
• knee surgery
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
Good OCT sources have small coherence length and large bandwidth
Axial resolution
• The axial resolution is
l02
2c ln 2 1
2 ln 2 l02
lc 

 0.44
 D
 Dl
Dl
– notice that Dl is the 3dB-bandwidth!
– The broader the bandwidth the shorter the
coherence length and the higher the
resolution
Lateral resolution: Decoupled
from axial resolution
Low NA
Lateral resolution
2Dx
Dz
Dx
Dz
High NA
4l  f 
Dx 
  d 
f=focal length
d= lens diameter
b
Dz
Lateral resolution similar to that in a standard microscope
Dx
Dz
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 @ 800 nm
12 mm coherence length (in air).
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
Light sources for OCT
• Pulsed lasers
– 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.
Construction of image
Source of contrast:
refractive index variations
Image reconstructed by
scanning
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
http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm
Applications in cancer detection
Squamous epithelium
Columnar epithelium: crypts
Loss of organization
http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm
Applications in developmental biology
Ey=eye; ea=ear; m=dedulla; g=gills; h=heart; i=intestine
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
Ultra-high-resolution-OCT
versus commercial OCT
mm
mm
W. Drexler et al., “Ultrahigh-resolution ophthalmic optical coherence
tomography”, Nature Medicine 7, 502-507 (2001)
3-D Reconstruction: In vivo images of human eye
using spectral-domain OCT
I
N
I
RPE
NFL
N
S
T
T
S
N. A. Nassif et al., Opt. Express 12, 367-376 (2004)