University of Lübeck, Medical Laser Center Lübeck GmbH Optical Coherence Tomography 4. Functional OCT Dr. Gereon Hüttmann / 2009
Doppler OCT (D-OCT) & Polarizationsensitive OCT (PS-OCT) Photonics II, by David Klinger
PS-OCT Introduction Polarization Tissue & Polatization (scattering, birefirgence) Principle of PS-OCT Results
Introduction PS-OCT Octopuses communicating over changing of polarization of light in the skin. -> tissue can change polarization! -> we can use polarization to get additional information about biological sturctures. 4
Polarization describes the orientation of the oscillations of transverse waves in the plane perpendicular to the wave's direction of travel Normal case: unpolarized light direction const. Ampl. change Polarizations: Linear Circular Elliptic direction change Ampl. Const. direction change Ampl. change 5
Tissue & Polatization Two machanisms dominate the changes in the polarization state of light porpagation through biological tissue: scattering Scattering occurs when an incident light wave causes a dipole to vibrate in the direction parallel to the electric field. 6
Tissue & Polatization birefringence light being broken up into the fast (ordinary ray) and slow (extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference. Effect characterized by: the difference in the indices of refraction for the ordinary and extraordinary rays the length of the sample All fibrillar structures, including collagen fibrils, are birefringent objects, i.e.
Principle of PS-OCT 1. M R P BS MP Detector Max. Interference! 8
Principle of PS-OCT 2. M R P BS MP 90 difference No Detector Interference!
Principle of PS-OCT 3. P M R BS Structures change polarisation MP In phase! Birefringance structures like collagen. Detector Interference!
Results increased birefringence relative to normal skin Damage to tissue can change bifringent tissues structure or organization and change its birefringence. Nerve fibers, for example, are bifringent, and so are skin and other connective tissues. OCT images of scar tissue, in vivo. Structural (A) and polarization-sensitive (B) images from a region of scar tissue on the hand, 5 mm wide by 1.2 mm deep. Labeled arrows in (B) indicate clinicallydetermined regions of scar tissue and adjacent normal skin. 11
Results No change of polarization change of polarization -> collagen Structure Picture: heart valve exvivo without cells 12
Result PSOCT has been shown to be an extremely useful technique for real-time monitoring and guiding of medical procedures as well as material diagnostics. Typical applications of PS-OCT vascular imaging of blood vessels plaque detection early detection of caries and other dental applications diagnosis of glaucoma many retinal diseases determination of burn depth in the skin non-invasive detection of strain in industry material 13
D-OCT Introduction Doppler Principle of D-OCT Results
( D-OCT ) Doppler OCT particle motion in the sample will lead to a shift in frequency of the scattered signal. Frequency differences between the reference and target signals will give rise 17 to a drift velocity in the interference pattern.
Doppler f u 0 2cos( ) f Δλ/λ 1 ( k k ) u 2 0 = v/c d 0 Δλ = wavelength shift λ 0 = wavelength if source is not moving v = velocity of source c = speed of light Δf= frequencyshift 18
D-OCT Moving particles (red blood cells) within the sample can cause phase changes in the interference signal. By comparing the phase change of the same pixel between high sequential scans, velocity information can be obtained. 19
FFT of detected Spectrum 1st A-Scan R e Im 2nd A-Scan R e Im Y Y jy Re Im change in polarcoordinates f ( v) r( v) e ( j ( v)) Δφ 20
Results 1mm
Results Negativ Flow -pi Negativ Flow -pi/2 Positiv Flow +pi Positiv Flow +pi/2 1mm
Results 23
D-OCT Disadvantages Requires Knowledge of Scattering Medium Must know indices of refraction for each layer to estimate structure or flowrate accurately Makes measurements in reactive environments difficult Relies on Optical Transmission Flows/structures examined must have sufficient optical transmissivity Limit on probe depth of ~3mm in biological tissues Artefacts Brownian Motion Frequency broadening introduces error in Doppler shift Imager Effect Thermal noise, speckle noise, amplifier noise, movement of scanner 24
D-OCT Advantages Simultaneous structure and flow imaging FD-DOCT can extract structure and flow information from same data Allows correlation of flows with the influence of structures Non-Invasive Near IR light has little impact on flows/structures being probed Non-invasive in vivo imaging subcutaneous blood flow, retinal arteries, developing embryos High resolution Structural ~10µm and Flows ~XXcm/sec Small measurement volume detailed maps of flow field (gradient of flow in blood vessels ) 25