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Optical coherence tomography

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  • 1. Abstract—this letter presents behavior of Optical coherencetomography technology in both frequency domain and timedomain, working principles for optical coherence tomography.This paper also includes Properties of light sources to be used asan optical source in optical coherence tomography system andComparison of optical coherence tomography with other imagingmodalities.Index Terms — OCT, coherence, TomographyI. INTRODUCTIONOptical Coherence refers to the ability of light wave toproduce interference pattern [1]. If two light waves broughttogether and they produce no interference pattern then they arecalled incoherent; if they produce interference pattern thenthey are said to be coherent. Tomography refers to imagingcross section of the body to revel underlying medicalconditions [2]. Optical Coherence Tomography is an imagingtechnique which works similar to ultrasound, but instead ofsound waves Oct uses light waves. By using the time delay oflight wave reflected from different paths inside the structure,an Oct system can reconstruct depth profile of samplestructure. By Scanning the light beam laterally across thesurface three dimensional images can be created. Lateralresolution is determined by the spot size of the light beam.Depth resolution depends primarily on the optical bandwidthof the light source. Oct has already established as a standardimaging modality for imaging of the eye.Optical Coherence Tomography is a noncontact,noninvasive imagining technique used to obtain highresolution cross sectional images of the retina and anteriorsegments. Oct has ultrahigh spatial resolution. Oct is threedimensional imaging techniques that measure reflected lightfrom tissue conditions. Oct of retina means doing verticalbiopsy section of the retina. Instead of viewing undermicroscope,we are presented with false color (codingtechnique used to make image more comprehensible) withmicron level resolution and instead of knife light is used.There are two fundamental Oct techniques: opticaldiffraction tomography (ODT), and diffuse opticaltomography. OCT is physically found on ODT and it has vastmajority of applications in biomedical field. DiffuselyPropagating photons are used by DOT. Temporally and/orspatially modulated light is launched into tissue and multiplescattered. Perturbation methods, Back projection methods, andNonlinear optimization methods are used to derivetomographic images from transmitted light. Optical diffractiontomography uses single scattered light and derivestomographic images by the Fourier diffraction projectiontheorem.Fig 1 low coherence inferometryOne method of measuring time delay is to use low coherenceoptical tomography that measures the echo time delay andintensity of backscattered light by interfering it with light thathas travelled a known reference path length and time delay[3]. Measurements are performed using Michelson typeinterferometer (fig 1).Light from source is incident on thebeam splitter, and one of the beams is incident onto the sampleto be imaged, while the second beam travels a reference pathwith a variable path length. The backscattered light from thesample is interfered with reflected light from the referencebeam and detected with photo detector at the output ofinterferometer. In low coherence or short pluses method,interference occurs only when two path length match to withincoherence length of light source.Optical Coherence TomographyK P SiddhapathakInstitute of Technology, Nirma University10BEC097@nirmauni.ac.in
  • 2. Fig 2 is a schematic explaining creation of cross sectionimages in Oct technology. The optical beam is focused andinto the samples being imaged and the intensity of backscattered light and echo time delay is measured to create axialback scattering profile. The incident beam is then scanned inthe transverse direction and axial back scattering profile ismeasured at different transverse position to create twodimensional dataset, which represents the optical backscattering through cross section of the tissue and that isrepresented as a logarithmic gray scale or false colour image[3].Fig 2 construction of cross section imagesII. OCT LIGHT SOURCESA careful selection of light source is important because asthe power of light source increase excess noise increases andno additional sensitivity might be gained. For example largeprobe beam power is not tolerated in ophthalmology. A properselection of wavelength can increase probing depth and reducescattering. The shape of the spectrum determines the structureof the depth point spread function (PSF) [4].A. Coherence propertiesSpatial coherence determines lateral resolution and depthresolution while temporal resolution determines depthresolution of OCT. Multiple transverse mode light source areuseful because if they are used with reduced space coherencein parallel OCT can reduce coherent cross-talk between OCTchannels and thus reduces speckle.The properties of a laser beam depends on parameters likegain bandwidth, resonator characteristic and fluorescencewavelength of laser medium as well as mode volume in thelaser cavity, so there is wide variety of parameter for laser tobe qualified as optical source for OCT.SLD is the most popular light source used in OCT .SLDbehaves like edge emitting laser diode. SLD does not haveoptical feedback or cavity. Super luminescence occurs, whenspontaneous emission of edge emitter laser diode experiencehigher gain due to higher injection current. The spacecoherence of the emitted radiation is high since SLDs areimplemented in waveguide structure. The wavelength isdetermined by material and it’s layering with insemiconductor.B. WavelengthPenetration depth of OCT is determined by emissionwavelength and source power. Absorption of tissue tends todecrease with increase in wavelength.Biological tissue is nothomogeneous material. It is constituted of cells maintained inlattice, so called extracellular matrix. This matrix is composedof bundles of structure proteins such ascollagen and elastin,and is partly filled with hydrated gel. Therefore scatteringratio is in the range of 100-1000, however scattering decreasesmonotonically as wavelength increases.C. Spectral StructureThe important parameter of the coherence function can beaffected by manipulation of the spectrum:width, smoothnessand position. Spectrometric depth resolution as well as OCTdepth resolution is defined by spectral width of the lightsource. Spectral width of light source is defined as the widthoccupied by the light source in terms of λ. The width of thecoherence function is ruled by the Fourier uncertainty relation,which says that the product of variance of Fourier transformpair reaches its minimum Gaussian functions.Therefore, in themajority of cases a Gaussian power spectrum is aimed byspectral shaping.Three basic possibilities have been used forcoherence function shaping:time-domain spectral shaping,digitally correcting the spectrum of the depth-scans, andfrequency-domain spectral shaping. Coherence function alsodepends on Fourier shift and theorem says that shift of timefunction is equivalent to a phase shift of its Fouriercomponents.III. TIME DOMAIN OCTFig3: Schematic of Time domain OCT systemFigure3 shows a schematic diagram of basic fiber based
  • 3. TDOCT system[5]. The light from broadband source is splitinto two parts, the reference and sample arm by Michelsoninterferometer. The mirror which can be scanned in the axialdirection terminates the reference arm. In the sample arm thelight is weakly focused into samples. The interference signalbetween the backscattered sample and reflected referencewave is then recorded. The axial optical sectioning ability ofthe technique is due to following reason: Because the light isemitted from broadband source which has large range ofoptical wavelength, strong interference is only detected whenlight from reference and sample arm travel same opticaldistance. Coherent interference is observed only when opticalpath length differs by less than the coherence length of thelight source, a quantity that inversely proportional to itsbandwidth. Axial scanning of reference arm reflector isequivalent to performing optical sectioning of samples,allowing for the generation of map of optical reflectivityversus depth. Transverse scanning of the sample is achievedvia rotation of a sample galvanometer mirror.IV. SPECTRAL DOMAIN OCTFig 4: Schematic of Spectral Domain OCTFig 4 shows schematic of spectral domain setup of opticalcoherence tomography[5]. Most of the components areidentical to the setup of time domain technology. The maindifference is in spectral domain optical coherence tomographyand time domain optical coherence tomography is that inSDOCT system the reference arm length is fixed.The outputof interferometer is analyzed with spectrometer (spectraldomain), Instead of obtaining depth information of the sampleby scanning reference arm length. The measured spectrum ofinterferometer output contains same information as an axialscan of the reference arm. From the interferometer outputspectrum via Fourier transform, the map of optical reflectivityversus depth is obtained. The practical implementation ofspectral domain optical coherence tomography is shown in fig4 is also commonly referred to as a frequency domainOCT.Another variant of SDOCT uses wavelength tunablelaser to rapidly sweep through the range of wavelength,allowing the spectrum of the interferometer output to berecorded sequentially using signal detector.This technique iscalled Swept-Source OCT (SSOCT) and it is particularlyinteresting for OCT systems operating at wavelengths longerthan 1 micrometer, where expensive InGaAs image sensorswould be required for FDOCT.V. COMPARISON OF OCT WITH OTHER IMAGING MODALITIESFig 5 Comparison of OCT resolution and imaging depths to those ofalternative techniquesUsing different criteria like resolution, imaging depth,acquisition time, Complexity, and sample intrusiveness, OCTcan be directly compared with different techniques[5]. Fromthe fig 5 Imaging depth of OCT is typically limited to 1mmwhich is less than that of ultra sound, high resolutioncomputed tomography, and magnetic resonance imaging, butits resolution is greater. The comparison is reserved withrespect to confocal microscopy. Like ultrasound, theacquisition time of OCT is short enough to supporttomographic imaging at video rates, making it much tolerantto subject motion than either CT or MRI. It does not requirephysical contact with sample and may be used with in airfilled hollow organs.OCT uses non-ionizing radiation atbiologically safe levels, allowing much longer exposure time,and its level of complexity is closer to ultrasound than toComputed Tomography or Magnetic Resonance Imaging,allowing for the realization of low-cost portable scanners. Thepoint-scanning nature of OCT technology allows it to beimplemented in fiber optics, which makes endoscopic andcatheter-based imaging possible.VI. ConclusionOptical coherence tomography is new technology forperforming high resolution cross section images. Specificadvantages of OCT are high depth and transverseresolution,the fact, that its depth resolution is decoupled fromtransverse resolution, high probing depth in scattering media,contact-free and non-invasive operation, and the possibility tocreate various function dependent image contrastingmethods.Advances in OCT technology have made it possibleto apply OCT in a wide variety of applications but medicalapplications are still dominatingREFERENCES[1] http://skullsinthestars.com/2008/09/03/optics-basics-coherence[2] http://www.wisegeek.com/what-is-tomography.htm
  • 4. [3] James G. Fujimoto “Optical Coherence Tomography”[4] A F Fercher, W Drexler, C K Hitzenberger and T Lasser “OpticalCoherence Tomography –principles and application”[5] http:/obel.ee.uwa.edu.au/research/oct/intro