OCT Summary

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    10-Oct-2015

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it's explanation about principle of optical coherence tomography. it's based on 2 papers related with basic OCT theory.

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<p> 1</p> <p>OCT Summary2014.4.4 Chap 1,41. Introduction OCTAn optical imaging tool which has been used to cross-sectional images of the internal microstructure in biological tissues in vivo. 2High (High resolution-Micrometer resolution(1-15m) &amp; High speed(high scanning speed-It makes possible to observe live cell) Long wavelength light(low coherence)2-D &amp; 3-D image Disadvantage -&gt; CT MRI . 3. General interference law for partially coherent light I(k) (k-domain=2/:wave# photo current) IFFT -&gt; z(depth information)-domain we get interference signal, its -domain. when it is directly converted from wavelength to wave#(k-domain), the interference signal has nonlinear sampling. It makes axial resolution broadening. So we have to need resampling procedure. (1. spline interpolation method 2. a zero-filling interpolation method) * Interpolation method &amp; </p> <p>TD-OCTFD-OCT2. OCT system main parameter Resolution Penetration depthSensitivity (SNR=power)Image acquisition rate(frame rate)2-(1) Resolution (&amp; Penetration depth) Axial resolution is defined by(1) half of the coherence length(2) center wave length and FWHM(full-width at half-maximum=-3dB bandwidth) of the optical source -&gt;Power/2 bandwidth </p> <p>Lateral resolution is related to spot size focused on in the sample ( spot size center wavelength, beam size, focal length and depth of field(2*Rayleigh length ) -&gt;spot size short focal length large numerical aperture(NA) . The small spot size the depth of focus sample image contrast . -&gt; so, OCT sample (penetration depth ) low NA .</p> <p>The Depth range(Zmax) is limited by the resolution of the spectrometer (or sampling resolution in SS-OCT) -&gt; according to the Nyquist sampling theorem the sampling frequency has to be twice as large as the highest frequency in the spectrum for example, interference signal band width &gt; 2*spectrometer resolution(band width) analog signal digitalize nyquist frequency </p> <p>52-(2) Optical Source An optical source determines performance such as (1) penetration depth (2) resolution (3) image qualityOCT -&gt; the near infrared(NIR) region(700nm~1500nm),called the optical window source . Because it has lower the absorption coefficient of water and blood than the other region.-&gt; Therefore biological tissue information . And the high optical power of the source penetration depth and SNR of the OCT . </p> <p>OCT image resolution source center wavelength FWHM . Axial resolution shorter center wavelength and broader bandwidth source . 2-(2) Side lobe When the optical source has the shape of non-Gaussian, Ideal gaussian shape Fourier transform gaussian shape. real side lobe Optical source sidelobe mainlobe coherence length axial resolution . (axial resloution=coherence length/2) far ghost image image quality . Ghost image? Mirror image, IFFT . Optical source Ghost image side lobe Main beam DC line , 2 ghost image . Source gaussian shape -&gt; power Full-range technique ( )- . IFFT . image depth . </p> <p>2-(3) Signal-to-Noise Ratio, Noise source Shot noise : from current fluctuation (: background light and the dark current in the detector)-&gt;SD TD noise TD . Thermal noise : from random thermal particle motion 1/f noise : a characteristic noise in active devices, arises in the low frequency region Relative intensity noise(RIN,) : fluctuation in optical power from the source and from the mechanical movement of the optical mounts (real fluctuation .-&gt;optical mount power )</p> <p>* noise . </p> <p>2-(3) SNR in TD SNR=signal/noise= the signal power from a perfectly reflecting mirror/the weakest sample reflectivity(signal power noise level ) SNR 1/NEB NEB : the noise equivalent bandwidth of the signal bandpass = the electrical bandwidth(f) for detection of the interferometric signal f = 2*Vs/0 (Vs : reference mirror speed, : optical source bandwidth , 0 : the center wavelength of the optical source ) -&gt; so! NEB= f = 2*Vs/0 , NEBVs-&gt; Vs&amp; NEB SNR 2-(3) SNR in FD SD-OCT system -&gt; SNR (Shot noise ?) SD-OCT SNR depends on M(the # of illuminated detector= pixel# of the CCD camera) Ex)1024pixel CCD camera TD OCT 24dB - SNRsd the exposure timeSS-OCT -&gt; SD-OCT SNR exposure time A-line scan rate fA The optical power and the SNR of OCT penetration depth . -&gt; optical power can make images with deeper penetration depth in both TD&amp;FD. The high optical power increases the SNR -&gt; ! In FD the SNR decreases with depth due to the finite resolution of the spectrometer. -&gt; so! High resolution spectrometer 3. PS-OCT</p> <p>4. Advanced Frequency Domain OCT SD-OCT 800nm wavelength region (1300nm InGaAs line scan camera acquisition )-&gt;Recently we have used 1000nm wavelength source. It has higher image quality than 800nm when we observe under retina area. 1300nm wavelength SS-OCT(using a frequency-swept laser,16kHz and 20kHz) . (1) To obtain high-speed imaging and (2) To enable dual-balanced detection to eliminate the DC autocorrelation noise over SD-OCT(signal vertical, horizontal .)Ultra-high-speed swept source(&gt;200kHz)using the Fourier domain mode locking(FDML) method ( ring laser ) -&gt; mode locking , 4-(1) Experimental setup of the SD-OCT SLD(superluminescent diodes/compact&amp;inexpensive) : 0=1310nm, =FWHM=86nm, power=7mW -&gt; z=(2ln2*02)/(3.14* )=270nm</p> <p>4-(1)-a Diffraction grating </p> <p>4-(1)-bCCD camera 12.5MHz, 12.5Mclocks/1secFor 1024pixel 1line(A-scan) data minimum 266clocks . 46990lines/sec (line rate) (B-scan) 1image 512 lines -&gt; 1image/10.9ms, 46990/512=91/1sec(frame rate) </p> <p>x-galvo , trigger T current linear galvo mirrorcurrent linear B-scan Focus spot . </p> <p>y-galvo 3D image scan </p> <p>4-(1)-c wavelength calibration We tilted the reference mirror and obtained the spectrum with an optical spectrum analyzer(OSA)-&gt;reference mirror reference mirror the intensity profile . </p> <p>4-(1)-c wavelength calibration </p> <p>Grating equation- Simulation results 2.Regression equation</p> <p>(=Wave length, n=pixel number) 3d order polynomial fitted results4-(1)-c I(k) (k-domain=2/:wave# photo current) IFFT -&gt; z(depth information)-domain we get interference signal, its -domain. when it is directly converted from wavelength to wave#, the interference signal has nonlinear sampling. It makes axial resolution broadening. So we have to need resampling procedure. (1. spline interpolation method 2. a zero-filling interpolation method) * Interpolation method &amp; 4-(1)-d System performances The full wavelength range of our spectrometer = 150nm pixel 0.145nm . 150/1024 -&gt; spectral resolutionThe depth range = 2.9mm The sensitivity of SD-OCT = 108.7dB </p> <p>4-(1)-dAxial resolution (14.05m/8.8m in air y)-&gt; this difference can be attributed to the errors in the linear rescaling procedure from -domain to k-domain(the high axial pixel size)Frame rate 91fps/15fps (frame rate) the real frame rate calculation time for linear interpolation and fourier transformation was needed. Solution : The multi-thread programming method ( , , </p> <p>4-(2) Swept-Source OCTAdvantageThe rise time of a photodiode is faster than the CCD chips. -&gt;improved Data acquisition speed Interference signal grating detector. Simpler system OCT image mirror image . ? ~? A dual balanced detector ( photo diode 2 . . real Signal , noise .)-&gt; 800nm range optical source . Water absorption eye imaging . </p> <p>Disadvantage Narrow wavelength scanning range FWHM resolution . -&gt; semiconductor optical amplifier(SOA)4-(2)-a External-Line Cavity Swept Source at 850nmOur swept source OCT has the optical wavelength selection filter with Littman configuration Disadvantage of Original Littman configuration -&gt; the output beam is the zeroth-order reflection from the grating at grazing incidence because the diffraction grating works as output coupling in addition to wavelength selection-&gt; so! OCT system alignment too sensitive. </p> <p>Our system -&gt; 1x2 coupler(30:70). -&gt; PC(polarization controller) Gaussian profile . Polarization sensitivity of the Intracavity componet fiber wavelength-dependent loss</p> <p>4-(2)-b The optical spectra of the swept source outputSS in Ref(ring-cavity)Our SS(line-cavity)Center wavelength815-870nm850nmFWHN0.085nm0.068nmTuning range55nm68nmSweeping speed43.2kHz(polygon scanner)200Hz(galvometer)</p> <p>Table. Specification our swept source compared with other groups swept sourceFig (a) 0.063nm with a fixed scanning mirror spectral width (b) center wave length 849nm(c) Optical source (d) t swept source wavelength4-(2)-c Images using the swept-source OCT1000samples/an A-scan Reference arm ND(neutral density/-20dB) filter -&gt; to reduce source intensity noise (RIN?)-&gt; SNR + the sample attenuation constant(40dB) -&gt; ? 1000 point sample-length -&gt; 1024 poing sample-length by zero-padding -&gt;rescaled to the linear frequency domain by using a simple calibration method -&gt;IFFT-&gt; A-scan 512data point </p> <p>1.03mm depth position/z=5.5m TerminologyCornea Iris Lens Optical nerve blind spot Retina Pupil Chroid Sclera </p> <p>Eye OCT images, anterior &amp; posteriorOptical coherence tomography : principles and applicationA.F. Fercher2014.4.7~ </p> <p>1. IntroductionTwo fundamental optical tomography technique. (1) Diffuse optical tomography (DOT)(2) Optical diffraction tomography (ODT)-&gt; OCT is physically founded on ODT </p> <p>* ODT uses single scattered light and derives tomography images by the Fourier diffraction projection theorem. </p> <p>Ballistic photon : photon may travel without scattering Near-ballistic photon : scattering ballistic path 281.1. Basic schemesLCI is based on the occurrence of fringes if the optical path lengths of reference and sample beams coincide within the coherence gate, which is of the size of the so-called round trip coherence length : lc -&gt; resolutionSome out standing properties of OCT (1) depth resolution is decoupled from transverse resolution(2) depth resolution in the histological 1m range is possible (3) the interferometric technique provides highly dynamic range and sensitivity(&gt;100dB)(4) it is important to note that in medical terms LCI and OCT are non-invasive techniques that yield in vivo data </p> <p>Two-photon interferometry has just been shown to have the potential of still higher sensitivity. -&gt; enhancement of depth resolution by a factor of two, and of cancellation of dispersion. (quantum-OCT)291.2. Mathematical treatmentMutual coherence function : correlation of electric fields at different position and also at difference time Complex degree of coherence(mutual coherence normalized)</p> <p>U(t) the photoelectric signal LCI signal photo detector . The envelope of the LCI signal is generated by rectification of the photoelectric ac signal followed by low-pass filtering/ amplitude and phase of the photoelectric ac signal are determined using lock in amplifier. </p> <p>302.1. Single scattering and optical tomographyOCT deviate in several respects from that more known CT concept:(1) DOT uses highly diffracted and scattered radiation (straight ray propagation can only be assumed for a fraction of the photons) (2) OCT images are synthesized from a series of adjacent interferometric depth-scans performed by a straight propagating low-coherence probing beam(decoupling of transversal resolution from depth resolution)(3) OCT uses backscattering Using additional wavelengths gives access to additional depth components. -&gt; wave length diversity is used in LCI. In LCI light scattered back in the opposite direction of the illuminating beam is used for depth localization of light re-emitting sites. </p> <p>2.1. Single scattering and optical tomography </p> <p>2.2. Multiple scattered sample lightOnly single scattered light is useful to derive structural information about the underlying scattering potential. However! Multiple scattering in samples is important!Absorption coeff., scattering coeff., concentration of particles in solution, probing depth, SNR-&gt; LCI and OCT probing depths are not only dependent on absorption and scattering coefficient but also scattering anisotropy(), aperture of the sample lens, and, in particular, on the sample distribution between lens and coherence gate. -&gt; transport theory 2.3. Probing beam Single scattered light detected by the photo detector will be limited to photons scattered at the coherent probe volume. Probe volume = the depth of the coherence gateIt is the path length of single and double scattered photons that determines the probing depth Probing depth the # of photons multiple scattering detected by the photodetector -&gt; time-coherence &amp; space coherence-&gt; incoherent diffuse photon </p> <p>2.4. Sensitivity Lower power -&gt; receiver noise sensitivity High power -&gt; excess noise sensitivity . </p> <p>2.5. Speckle(noise)The sample wave is the sum of many wavelets arising at backscattering sites within the coherent volume. -&gt; random phase Ideally, standard LCI and OCT only relate with the interferogram GSR (heterodyne or related techniques)2.5. Resolution </p> <p>OCT depth resolutionThe FWHM diameterDepth of focus4. Low-coherence interferometry and OCT Fourier-domain OCT As(K) backscattered field amplitude is obtained either by spectral interferometry techniques or by wavelength tuning technique. Fs(z) FT{As(K)} Spectrometric measurement two basic technique (1) the correlation technique (2) the phase shifting techniqueDepth field of view Form birefringence caused by linear and circular anisotropic proteins, such as collagen fibres which build up ECM, can be found in many biological tissues.-&gt; Form birefringence caused by nerve fibres() oriented in parallel bundles is used. ( Retina nerve fibre layer/NFL Glaucoma/ -&gt; It is based on retardation caused by birefringence of the peripapillary retina )</p> <p>5.1. Polarization-sensitive OCT </p> <p>Sample reflectivity, envelopes retardance5.2. Doppler OCT Doppler effect , () . , . + , /=/c(c ) .[ ]()Optical Doppler techniques are based on the interference phenomenon if light scattered by moving particles interferes with a reference beam. The resulting interferogram beats at the Doppler angular frequency wD=K*vs ( K : the scattering vector, vs= the velocity of the moving particles) -&gt; . Blood velocity measurement at the fundus() of human eye </p> <p>5.2. Doppler OCT Fourier-transforming the fringe dataSequential scan processing the most severe limitation of the fringe data techniques-&gt; limited velocity sensitivity techniques to overcome this are(1) phase resolved DOCT using sequential depth-scans -&gt; this technique decouples velocity resolution from spatial resolution Doppler shift sequential depth-scans sequential frame scans sample beam phase . </p> <p>In DOCT the detection bandwidth must be chosen to match the much wider width corresponding to the range of velocities with the consequence of degrading the SNR. To overcome that problem, a frequency tracking band-pass filter based on a modified phase-lock loop has been suggested. </p> <p>5.3. Wavelength-dependent OCTTerminol...</p>