University of Lübeck, Medical Laser Center Lübeck GmbH Optical Coherence Tomography 3. The Art of OCT Dr. Gereon Hüttmann / 2009
System perspective (links clickable) Light sources Superluminescent diodes Semiconductor amplifiers Femtosecond lasers Beam delivery and probes Hand-held probe Catheter Ophthalmoscope Microscope OCT imaging engine Resolution Reference delay scanning Doppler/polarization/spectroscopy Detection Frequency domain Computer control Drive system Real-time display Data management Image & signal processing Motion reduction Speckle reduction Image enhancement Rendering algorithms
Optical Coherence Tomography Base Technology Applications devices Clinical Applications
The Four Families of OCT Sampling Parameter Spectrum SNR n S Coherence function SNR n l / z S c max Time multiplexed (point detector) Swept Source (OFDR) Demanding light source Phase Noise FFT necessary TD-OCT Demanding phase mod. Phase Noise Reduced SNR Space multiplexed (line detector) SD-OCT Good optics needed Limitations due to detector performance FFT necessary Linear OCT Limitations due to detector performance Limited z-range Reduced SNR
Time Domain OCT Faserkoppler Photodiode1 - Photodiode Bandpaßfilter Faserkoppler Phasenmodulator Photodiode2 Lichtquelle Gleichrichter Bandpaßfilter Gleichrichter Tiefpaßfilter Computer Probenarmoptik Tiefpaßfilter Probe Fiber optical interferometer -> stable, flexible Sophisticated signal processing -> measurement over 10 orders of magnitude
The Reference Delay Line Linear translated mirror Multipass translated mirror
Rotation based delay-line
Fiber Streching
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 mm (typically), Pulsed lasers mode-locked Ti:Al2O3 (800 nm), 3 micron axial resolution (or less).
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
TD-OCT Faserkoppler Photodiode1 - Phasenmodulator Photodiode2 Faserkoppler Lichtquelle Bandpaßfilter Gleichrichter Tiefpaßfilter Computer Probenarmoptik Probe
FD-OCT (Spectral Radar) Refenenz SLD Objekt FFT Spektrometer
Signals in FD-OCT I( k) I I 2 I I cos k z S R S R
Phase-resolved FDOCT Sarunic MV, Choma MA, Yang CH, et al. Instantaneous complex conjugate resolved spectral domain and swept-source OCT using 3x3 fiber coup OPTICS EXPRESS 13 (3): 957-967 FEB 7 2005
Removing the Depth Ambiguities
Das Spektrometer Linear image Sensor mit 1024 (1) Pixel, 8µx125µm (2) 180nm Spektralbereich Auflösung 0.1 nm Messtiefe 1 mm FWHM 4 µm 150 (3) A-Scans (4) pro Sekunde SNR = 108 db (7) (9)
Image Sensors Adressierungslogik Adressierungslogik A:Passiv Pixel CMOS Sensor Adressierungslogik B: Active Pixel CMOS Sensor C: CCD Sensor
Signal [electrons] Relative amplitude Spatial Resolution of the Detector CMOS-Sensor LIS 1024 5000000 pixel 518 pixel 519 pixel 520 1,0 4000000 0,8 3000000 0,6 2000000 0,4 1000000 0,2 0 a) 40 60 80 100 120 Lateral position of focus [µm] 0,0 0 1 2 3 4 Spatial frequency [cycles/pixel]
Roll-Off of the Sensitivity
Noise [elektrons] Noise performance of the Detector 100000 10000 NMOS Hamamatsu 3904 Hamamatsu 3903 CMOS (LIS 1024) quantum noise limit 1000 1000000 1E7 1E8 Average charge per pixel [elektrons]
Signalaufbereitung
Spectral Shaping of the Spectrum (White Adjust) Hanning window
Intensity [a.u] Amplitu Spectral Shaping of the Spectrum 4,0 3,5 3,0 Hanning window Tapered Cosine Multi gauss Noisy 120 100 Hanning window Tapered Cosine Multi gauss Noisy 2,5 80 2,0 1,5 60 1,0 40 0,5 0,0 0 200 400 600 800 1000 Wavelength [a.u] 20 0,86 0,88 0,90 0,92 0,94 0,96 0,98 1,00 Pathlengthdifference [mm] Hanning window Tapered cosine window Multi gauss
Transformation to k=2 /
Swept-Source OCT
Swept Soures for OCT 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.
Multimode Operation
Fourier Domain Mode Lock (FDLM) Dispersice Pulse Streching
Dispersion in the Interferometer
4 2 optical path lengths and chromatic dispersion 1 L c - 1 1 Probe Photodiode - 1 1-1 No additional optics
4 3 Optical Path Lengths and Chromatic Dispersion 1 n g d - 1 1 Probe d Photodiode - 1 1-1 Additional optics with n g = n - l dn/dl 1
4 4 Optical Path Lengths and Chromatic Dispersion 1-1 1 Probe d Photodiode - 1 1-1 Correction in the reference arm
4 5 Optical Path Lengths and Chromatic Dispersion 1 2 c L ( c l D d ) 0 chr 2-1 1 Probe d Photodiode - 1 1-1 Group velocity dispersion D chr = 1/c 0 dn g /dl 0
46 Optical Path Lengths and Chromatic Dispersion 1-1 1 Probe d Photodiode - 1 1-1 Correction by introducing GVD in the reference arm
47 Conditions for Balancing the Optical Path and GVD Balancing of optical path length probe n g d i n g reference d i Balancing of group velocity dispersion probe D d D d chr i chr i reference
48 Group Index of Refraction and Group Velocity Dispersion at 1300 nm D Material n chr g [ps/nm/km] Air 1,00 0 Fused Silica 1,46 2,6 BK7 1,52-2,32 Water (20 C) 1,34-19,4 (37 C) 1,34 23,9 BaSF1 1,62-42,3 SF57 1,83-115,9 GRIN-Lens EG27 (NSG) 1,53-47,7 ZD-fiber n. m. 0 NZD-fiber (LEAF) n. m. -24,3
49 n p, n g D chr [psec/nm/ km] Chromatic Dispersion of Fused Silica 1,48 0 1,47 n g -200 Quarz 1,46-400 SF57 1,45 n p -600 1,44 0,8 0,9 1 1,1 1,2 1,3 1,4 Wellenlänge [µm] -80 0 0,8 0,9 1 1,1 1,2 1,3 1,4 Wellenlänge [µm]
index of refrection 50 OCT Endoscope graded index lense (GRIN) radius GRIN lense in mm CCD-camera scanner 3 mm 0 endoscope 270 mm length
51 Compensation of GVD Length of NZD-fiber [mm] NZD fibre length inside the reference = 1,15 m
52 Compensation of GVD Length of NZD-fiber [mm] NZD fibre length inside the reference = 1,15 m
Application Systems Galvanometerscanner Collimator Lens Beamsplitterplate Probe
Micromotor Endoscope Herz PR, Chen Y, Aguirre AD, et al. Micromotor endoscope catheter for in vivo, ultrahigh-resolution optical coherence tomography OPTICS LETTERS 29 (19): 2261-2263 OCT 1 2004