Optical Coherence Tomography in Medical Imaging

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Medical imaging modality with 1-10 um resolution and 1-2 mm 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. Optical Coherence Tomography is 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.

Types Of OCT Systems

  • Time Domain (TD) OCT Systems
  • Spectral Domain (SD) OCT systems
  • Swept Source (SS) OCT Systems

Advantages Of Spectral Domain OCT Over Time Domain OCT System

  • 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


Baseline Signal Processing chain In OCT Systems


Basic OCT system schematic

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.

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

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

  • 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
  • Human visualization range is about 40-60 dB
  • 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 and Quadratic approximation

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
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