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 Standard clinical High frequency OCT vs. standard imaging 1 mm 1 cm 10 cm Penetration depth (log) 1 m 10 m 100 m 1 mm Resolution (log) Confocal microscopy Ultrasound
OCT in non-invasive diagnostics
diagnosing retinal diseases.
early detection of skin cancers.
vulnerable plaque detection.
Endoscopy (fiber-optic devices)
Doppler OCT (blood flow)
spectroscopic OCT (absorption, high speed)
Polarization Sensitive-OCT (birefringence).
OCT: Principle of operation OCT is analogous to ultrasound imaging Uses infrared light instead of sound Interferometry is used to measure small time delays of scattered photons Human skin 5 mm wide x 1.6 mm deep SpatialResolution: 10-30 μm Time resolution: 30fs!!! Speed of sound ~ 1480 m/sec (in water) Speed of light – 3x10 8 m/sec
Good OCT sources have small coherence length and large bandwidth
The axial resolution is
notice that is the 3dB-bandwidth!
The broader the bandwidth the shorter the coherence length and the higher the resolution
Lateral resolution: Decoupled from axial resolution Lateral resolution similar to that in a standard microscope f=focal length d= lens diameter Lateral resolution z z z High NA Low NA b z
L ight source s for OCT
800 nm (SLD),
1300 nm (SLD, LED),
1550 nm, (LED, fiber),
power: 1 to 10 mW (c.w.) is sufficient,
10 to 15 m (typically),
25 nm bandwidth @ 800 nm 12 m 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
mode-locked Ti:Al2O3 (800 nm),
3 micron axial resolution (or less).
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 Loss of organization Columnar epithelium: crypts Squamous epithelium 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 m http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm
Ultra-high-resolution-OCT versus commercial OCT 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 N. A. Nassif et al ., Opt. Express 12, 367-376 (2004) RPE NFL I T N S I S T N