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Optical Coherence
Tomography
INTRODUCTION
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Medical imaging modality with
penetration depths
1-10um resolution and 1-2mm
High-resolution, sub- surface, non-invasive or minimally invasive
internal body imaging technique for structural and quantitative
imaging
OCT is analogous to ultrasound imaging
PRINCIPLE AND
INSTRUMENTATION
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Based on principle of low coherence interferometry
Imaging is performed by measuring the echo time delay and
intensity of back-reflected or backscattered light
Measurements are performed using a Michelson
interferometer with a low coherence length light source
OCT SYSTEMS USING MICHELSON’S
INTERFEROMETER
TYPES OF OCT SYSTEMS
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Time Domain (TD) OCT Systems
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Spectral Domain (SD) OCT systems
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Swept Source (SS) OCT Systems
TIME DOMAIN (TD) OCT SYSTEMS
SPECTRAL DOMAIN (OCT) SYSTEMS
ADVANTAGES OF SD-OCT OVER
TD-OCT
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Permits faster acquisition of the entire depth profile(A-scans)
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Video-rate imaging is possible
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High-speed acquisition without any moving parts minimizes
any distortion in the OCT images due to motion in the sample
Entire depth profile (A scan)is measured from a single spectrum
with no mechanical scanning of the reference path
SWEPT SOURCE (SS) OCT SYSTEMS
BASELINE SIGNAL PROCESSING
CHAIN IN OCT SYSTEMS
BACKGROUND SUBTRACTION
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The background is subtracted from the acquired data
To eliminate the reference power term, the reference
spectrum from only the reference arm is detected and
subtracted from the interference spectrum
Variations due to fixed pattern noise in the line scan camera
and variations in power spectral densities of source can be
suppressed.
RE-SAMPLING
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In SD-OCT systems, spectrometers measure optical intensity as
a function of wavelength
In order to apply the Fast Fourier Transform (FFT)
reconstructing the axial scan as a function of depth,
the spectrum should be evenly sampled in k-space
Therefore, the spectrometer output must be transformed from
the wavelength to the frequency space
IMAGE FORMATION (FFT)
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The basic operation to get the depth resolved A-scan from the
interference fringes
The structural image is obtained by taking the magnitude
of the complex FFT output
Each FFT creates a particular A-scan
By moving the galvanometer in x direction ,the successive Ascan line is created
By moving the galvanometer in both x-y direction, a full 3D
volume can be generated
MAGNITUDE COMPUTATION
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FFT output is a complex number
The structural information is contained in the magnitude of
the FFT output
The function provide 15.5 bits of accuracy
LOG COMPRESSION
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16 bit value provides 96 dB dynamic range
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Human visualization range is about 40-60dB
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16 bit data is compressed using a non-linear function to reduce
the dynamic range for visualization
Log function is a common non-linearity used in OCT
Two approximations of log compression to map the 16-bit
input to 8-bit data for display
Linear approximation
Quadratic approximation
ADVANTAGES OF OCT
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Depth resolution is independent of the sample beam
aperture
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High depth and traversal resolution
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Contact-free and non-invasive operation
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Coherence gate can substantially improve the probing depth
in scattering media
APPLICATIONS OF OCT
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Ophthalmology
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Dentistry
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Dermatology
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Gastroenterology
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Intra-Operating Surgery
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Cancer Diagnosis
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Non-medical OCT Applications
CONCLUSION
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OCT is a new imaging modality somewhat comparable to
ultrasound in that it provides structural information without
ionizing radiation.
The depth of penetration is low compared to ultrasound.
Polarization sensitive and spectroscopic imaging mode allow
additional information regarding the biological tissues to be
imaged.
OCT has been used both in vivo and ex vivo.
It has also been used non-invasively as well as in minimally
invasive in vivo imaging.
REFERENCE
*1. D. C. Adler, T. H. Ko, P. R. Herz, and J. G. Fujimoto, Optical
Coherence Tomography Contrast
*Enhancement Using Spectroscopic Analysis with Spectral AutoCorrelation, Optics Express, pp..
*2. Algorithms for Optical Coherence Tomography on
TMS320C64x+ TI DSP (SPRABB7)
*3. M. Brezinsky, Optical Coherence Tomography, Elsevier, 2006.
*Optical Coherence Tomography Using a Single Line Scan Camera,
Optics Express, pp. 2421-2431