Lecture 24


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Lecture 24

  1. 1. Lecture 23 Optical Coherence Tomography
  2. 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. 3. 1 mm 1 cm 10 cm Penetration depth (log) 1 µm 10 µm 100 µm 1 mm Resolution (log) OCT Confocal microscopy Ultrasound Standard clinical High frequency OCT vs. standard imaging
  4. 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. 5. 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 – 3x108 m/sec
  6. 6. Good OCT sources have small coherence length and large bandwidth
  7. 7. Axial resolution • The axial resolution is – notice that ∆λ is the 3dB-bandwidth! – The broader the bandwidth the shorter the coherence length and the higher the resolution 2 2 0 02 ln 2 1 2ln 2 0.44c c l λ λ π ν π λ λ = = ≈ ∆ ∆ ∆
  8. 8. Lateral resolution: Decoupled from axial resolution x∆ 2 x∆ 4 f x d λ π   ∆ =  ÷   Lateral resolution ∆z ∆z ∆z High NA Low NA x∆b ∆z Lateral resolution similar to that in a standard microscope f=focal length d= lens diameter
  9. 9. 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 µm (typically), • Example – 25 nm bandwidth @ 800 nm 12 µm coherence length (in air).
  10. 10. 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
  11. 11. 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.
  12. 12. Construction of image Source of contrast: refractive index variations Image reconstructed by scanning
  13. 13. 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
  14. 14. Applications in cancer detection Loss of organization Columnar epithelium: crypts Squamous epithelium http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm
  15. 15. Applications in developmental biology Ey=eye; ea=ear; m=dedulla; g=gills; h=heart; i=intestine
  16. 16. Ultra-high resolution OCT Image through the skin of a living frog tadpole Resolution: 3 µm http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm
  17. 17. mµ mµ Ultra-high-resolution-OCT versus commercial OCT W. Drexler et al., “Ultrahigh-resolution ophthalmic optical coherence tomography”, Nature Medicine 7, 502-507 (2001)
  18. 18. 3-D Reconstruction: In vivo images of human eye using spectral-domain OCT RPE NFL I T N S I S TN N. A. Nassif et al., Opt. Express 12, 367-376 (2004)