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.

1 mm 1 cm 10 cm

Penetration depth (log)

1 m

10 m

100 m

1 mm

Resolution (log)

OCTConfocalmicroscopy

Ultrasound

Standardclinical

Highfrequency

OCT vs. standard imaging

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

OCT: Principle of operationOCT is analogous to ultrasound imagingUses infrared light instead of sound

Interferometryis used to measure small time delaysof scattered photons

Human skin5 mm wide x 1.6 mm deep SpatialResolution: 10-30 μmTime resolution: 30fs!!!

Speed of sound ~ 1480 m/sec (in water)

Speed of light – 3x108 m/sec

Good OCT sources have small coherence length and large bandwidth

Axial resolution

• The axial resolution is

– notice that is the 3dB-bandwidth!– The broader the bandwidth the shorter the

coherence length and the higher the resolution

2 20 02 ln 2 1 2ln 2

0.44c

cl

Lateral resolution: Decoupled from axial resolution

x

2 x4 f

xd

Lateral resolution

z

z

z

High NA

Low NA

xb z

Lateral resolution similar to that in a standard microscope

f=focal lengthd= lens diameter

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 m (typically),

• Example– 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

• 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

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 tadpoleResolution: 3 m

http://rleweb.mit.edu/Publications/currents/cur11-2/11-2oct.htm

m

m

Ultra-high-resolution-OCT versus commercial OCT

W. Drexler et al., “Ultrahigh-resolution ophthalmic optical coherencetomography”, Nature Medicine 7, 502-507 (2001)

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)