University of Lübeck, Medical Laser Center Lübeck GmbH Optical Coherence Tomography. Theory Dr. Gereon Hüttmann / 009
What is OCT? ( for the MD )
Lichtquelle Probe Detektor Display OCT is Ultrasound with Light! OCT measures the time-of-flight distribution of light, which is reflected from tissue, similar to ultrasound When color-coded, a set of A-scans can be combined to a sectional image of the tissue (B-scan). Z-resolution: 4-15 µm Depth range: 1-1 mm Scan rate: up to 7 Mega A- scans/s or 3.5 Giga voxel/s
A-Scan B-Scan C -Scan
Resolution Absorption, reduced scattering coefficient [cm -1 ] 10 6 10 5 10 4 103 Proteins (Cornea) Melanin (RPE) 10 Water 1 m 100 mm 10 mm 1 mm 100 µm 10 µm 1 µm 100 nm 100 nm 101 10 0 10-1 10- OCT Microscopy 10-3 Diffuse optical imaging Ultrasound 1 µm 10 µm 100 µm 1 mm 10 mm 100 mm 1 m Maximal imaging depth Blood (HbO ) 0,1 0, 0,5 1,0,0 5,0 10,0 Wavelength [µm] MRT Scattering of tissue
Comparision with other Imaging Modalities
7 Comparision of Ultrasound with OCT frequenc axial resolution depth e 50-70 7,5 MHz 10 µm mm 10 MHz 158 µm 35 mm MHz 7 µm 8 mm 30 MHz 5 µm 4 mm 50 MHz 31 µm mm 75 MHz 1 µm 1,5 mm 100 MHz 16 µm 1 mm OCT wavelength: 1,3 µm 830 nm resolution: 15 µm, 5µm depth: 1 mm
What is OCT? ( for an engineer in biomedical imaging )
Method for 3-D Imaging! 1. Tomographic imaging and reconstruction - needs 360 access to sample Detector Tissue. Confocal imaging - lateral and axial resolution are linked; high axial resolution only with large focusing angle - only light from one plane is detected x 3. Time-of-Flight/Interferometry Lichtquelle Detektor Δx Δz - high depth resolution independent of the focus angle - all plane are detected simultaneously Probe Display
What is OCT? ( for the physicist )
Superposition of Two Light Waves -4-4 x1 x 0.5 1 1.5 I I 1 I ) cos( ), ( ), ( ), ( ), ( ), ( ), ( 1 1 1 1 1 x x t t t I I I I I t r t r t r t r t r t r I t t r t r I 1 ), ( ), ( ) ( ) ( 1 1 1 t x k i t x k i e I e I
z z z S R Interferomet ry Laser sample E I exp i k z ; k / S S S Ik ( ) Spectrum I( z) Intensity g k 0 k / I( k) I I I I cosk z S R S R z 0 z
z z z S R Lamp, LED, SLD sample E I exp i k z ; k / S S S Ik ( ) S SD, FD-OCT I( z) TD-OCT g F{I}(x) k 0 I( k) I I I I S( k k )cos k z S R S R FFT 0 k k / z 0 z Demodulation g I( z) I( k) dk I I I I z cos k z S R S R 0
Optical Coherence-Tomography (OCT) OCT measures the time-of-flight distribution of light, which is reflected from tissue, by interferometry Time Domain OCT (TD-OCT) The tissue structure is sampled by changing the length of the reference arm and thereby moving the coherence gate trough the tissue From Optopol Technology S. A. Fourier Domain OCT (FD-OCT) Each tissue structure results in specific modulation frequency on the spectrum measured at the output of the interferometer. The large the path lengths difference of the interferometer arms, the higher the frequency.
Spectrum and Correlation Function are Linked (Wiener-Kinchine theorem) the temporal coherence envelope function G(t) is related to the power spectral function S(n) through G(t) = FT{S(n)}
Depth Resolution
Axial resolution
Lateral resolution: Decoupled from axial resolution Low NA x x z Lateral resolution 4 f x d Depth of focus b z z x bzr High NA b x z Depth resolution c ln 1 ln 0 0 z 0.44 n
Depth gating
Signal-to-Noise-Ratio I 1. 1.0 0.8 0.6 0.4 0. 40 0 0 0 40 S I R N z
Multiple Lateral Scatters Scatters in different distance generate their own coherent signal. If signals from scatters overlap the interfere. A random distribution of scatters will generate a random intensity distribution. (speckles). Speckles!
Speckle
Multiple Lateral Scatters A laterally extended scattering objects generates a random modulation of the intensity on the detector, which destroys the contrast of the detected interference pattern. Detection aperture has to be restricted to one speckle!
Figures of Merit for an OCT-Device TD-OCT FD-OCT Depth resolution Measurable depth range z max z R Signal to noise ratio z / z n OCT max n (number of detected photons) t P / h Sample Sensitivity S z z max / OCT 1 n S 1 n Dynamic range DR 1 FWC Effective imaging depth
What is OCT for me? (. after 15 years experience with OCT )
1. Confocal imaging at a depth resolution which is not NA limited, by interferometric measurement of the optical path lengths.. Use heterodyn gain to achieve quantum noise limited sensitivity (100 db) 3. Measurement all tissue structures of one A-scan in parallel with high speed (4,000,000 A-scans/Second) 4. Measurements of longitudinal movements with subwavelength accuracy (e.g. flow, elastography) The price to pay: coherent (speckle) noise!