Lecture 23 Optical Coherence Tomography OCT Basic Principles

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 – 3 x 108 m/sec Human skin 5 mm wide x 1. 6 mm deep Spatial. Resolution: 10 -30 μm Time resolution: 30 fs!!! 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 – notice that Dl is the 3 d. B-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 High NA Dz Dz f=focal length d= lens diameter b Dz Lateral resolution similar to that in a standard microscope 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 m. W (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: Al 2 O 3 (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/cur 11 -2/11 -2 oct. htm

Applications in cancer detection Squamous epithelium Columnar epithelium: crypts Loss of organization http: //rleweb. mit. edu/Publications/currents/cur 11 -2/11 -2 oct. 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/cur 11 -2/11 -2 oct. 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 I N RPE I NFL S N T T S N. A. Nassif et al. , Opt. Express 12, 367 -376 (2004)