Post on 28-Mar-2015
An Alternative Method of Extending Imaging Window of
Fourier Domain – Optical Coherence Tomography by Using a Complex
Conjugate Removal Technique
Amine Bouchti
University California Davis
Research Advisor: John S. Werner
Research Supervisor: Robert J
Zawadzki
Optical Coherence Tomography (OCT)
– An interferometric technique that offers, in vivo, cross-sectional views of biological microstructure tissue such as the human retina.
– The depth structure of the sample is reconstructed from backscattered light by Fourier domain OCT (FD-OCT).
– The images are generated by scanning the incident light beam at different axial depths and transverse positions.
What is Fourier Domain ?
• Fourier domain is the analysis of functions or signal with respect to frequency.
• It relates to the Fourier transform by decomposing a function into a finite number of frequencies.
• Fourier transform functions are complex ( they have Amplitude and phase.) In standard FD-OCT only the magnitude of Fourier transform is displayed.
Position(z)
Main Applications of OCT• The axial resolution of OCT in retinal tissue is about 1-
15 µm, which is 10 to 100 times better than ultrasound or MRI.
• It enables visualization of the internal architectural morphology of the retina noninvasively; in real time and provides a 3 dimensional view of the retina.
• OCT can detect and diagnose early stages of disease before physical symptoms and irreversible vision loss can occur.
OCT apparatus
• Schematic of the OCT System
SLD: Superluminescent diode source.
PC:polarization controllers.
NDF: Neural density filter.
FI: Faraday isolator.
M: Mirror.
DG: Diffraction grating.
CCD: CCD cameras.
FD-OCT Signal Processing
Amplitude Vs. pixelsAmplitude Vs. Pixels
•Di[km] .S[ Km].(RR+Rs+2√(RRRs) cos(2∆xkm +I))
Subtract DC
Di[km] .S[ Km] 2√(RRRs) cos(2∆xkm +I)
•Fourier transform: Di[Xn] = Di[km]e-
(j2(kmXn))
Di[xn] S[xn] 2√(RRRS)((xn+∆x)+(xn-∆x))
Amplitude Vs. Pixels
Di[km] .S[ Km] 2√(RRRs) cos(2∆xkm +I)
Fourier Transform
Amplitude Vs.Position
• In Standard FD-OCT only half of the imaging window can be used.
• Due to a reflection at + ∆X that cannot be distinguished from a reflection at -∆X. This is called Complex Conjugate Artifact.
Limitations
Goal
How can this artifact be removed?
Setting up andaligning thespectrometer
Simultaneousdetecting from both
cameras
Connecting thefibers and calculatingthe coupler splitting
ratios
Minimizedispersion
Lab viewDrive
the system Process
data
3X3fiber coupler
2spectrometers
Creat a complexinterferometric signal
Solution
The Complex FD-OCT Schematic
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
1
2
3x3 algorithms
•Signal 1 ( I1) -DC: Di[km] .S[ Km] 2√(RRRs) cos(2∆xkm +I)
•Signal 2 (I2) -DC: Di[km] .S[ Km] 2√(RRRs) cos(2∆xkm +I)
•Complex equation: real part + j imaginary part
If we assume that I1= the real part
the imaginary part can be obtained by the following equation:
I1cos(∆) - I2
Iim=
Sin(∆)
Graph of , I2/ I1 Graph of ∆Graph of CCD1Vs. CCD2
Results
• The removal of the complex conjugate artifact
Did it Work?
Single CCD ImageHalf screenSingle CCD Image
Full screen
Single CCD ImageFull screen
Two CCD ImageFull screen
Acknowledgments
•Center for Adaptive Optics, a National
Science Foundation Science and technology
Center(STC), AST-987683.
•UC Davis medical Center.
References
• University of California Davis Medical center, http://vsri.ucdavis.edu/.• Marinko V.Sarunie, Michael A. Choma, Changhuei Yang, Joseph A. Izatt,
“Instantaneous complex conjugate resolved spectra domain and swept-source OCT using 3x3 fiber coupler,”Opt.Express 13,957- 967 (2005)
• Michael A. Choma, Changhuei Yang, Joseph A. Izatt, “Instantaneous quadrature low-coherence interferometry with 3x3 fiber-optic couplers,” Opt.Lett.28 , 2162-2164 (2003)
• N.A. Nassif, B. Cense, B.H. Park, M.C. Pierce, S.H. Yun, B.E. Bouma,G.J.Tearney, “In vivo high-resolution video-rate spectral-domain optical coherence tomography of the human retina and optic nerve,” Opt.Express 12,367- 376 (2004)