Optical Coherence Tomography 1 Optical Coherence Tomography

2 INTRODUCTION Medical imaging modality with 1-10um resolution and 1-2mm penetration depths 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 3 PRINCIPLE AND INSTRUMENTATION 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 4 OCT SYSTEMS USING MICHELSON’S INTERFEROMETER

TYPES OF OCT SYSTEMS Time Domain (TD) OCT Systems 5 TYPES OF OCT SYSTEMS Time Domain (TD) OCT Systems Spectral Domain (SD) OCT systems Swept Source (SS) OCT Systems

TIME DOMAIN (TD) OCT SYSTEMS 6

SPECTRAL DOMAIN (OCT) SYSTEMS 7 SPECTRAL DOMAIN (OCT) SYSTEMS

ADVANTAGES OF SD-OCT OVER TD-OCT 8 ADVANTAGES OF SD-OCT OVER TD-OCT Permits faster acquisition of the entire depth profile(A-scans) Video-rate imaging is possible 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 9 SWEPT SOURCE (SS) OCT SYSTEMS

BASELINE SIGNAL PROCESSING CHAIN IN OCT SYSTEMS 1010 BASELINE SIGNAL PROCESSING CHAIN IN OCT SYSTEMS

BACKGROUND SUBTRACTION 1111 BACKGROUND SUBTRACTION 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.

1212 RE-SAMPLING 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

1313 IMAGE FORMATION (FFT) 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 A-scan line is created By moving the galvanometer in both x-y direction, a full 3D volume can be generated

MAGNITUDE COMPUTATION 1414 MAGNITUDE COMPUTATION 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 16 bit value provides 96 dB dynamic range 1515 LOG COMPRESSION 16 bit value provides 96 dB dynamic range Human visualization range is about 40-60dB 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

1616 ADVANTAGES OF OCT Depth resolution is independent of the sample beam aperture High depth and traversal resolution Contact-free and non-invasive operation Coherence gate can substantially improve the probing depth in scattering media

APPLICATIONS OF OCT Ophthalmology Dentistry Dermatology 1717 APPLICATIONS OF OCT Ophthalmology Dentistry Dermatology Gastroenterology Intra-Operating Surgery Cancer Diagnosis Non-medical OCT Applications

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1919 CONCLUSION 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.

2020 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 Auto- Correlation, 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