Introduction imaging modality that uses coherent light
Introduction Optical Coherence Tomography (OCT)is an imaging modality that uses coherent light to capture 2D and 3D images of micrometerresolution from within optical scattering media such as biological tissue. Itis used for medical imaging and industrial non-destructive testing. OCT isbased on low-coherence interferometry, generally employing near-infrared light.In laser interferometry which is the conventional interferometry with longcoherence length, interference of light occurs over a distance of meters. InOCT, this interference is shortened to a distance of micrometers, due to theuse of broad bandwidth light sources. The use of relatively long wavelengthlight allows it to penetrate into the scattering medium.
The concept of OCTimaging is based on the measurement of echo time delay and the magnitude ofbackscattered light. OCT enables the real time visualization of the tissue whichmakes it a powerful imaging tool for medical applications. BasicTissue Optics The two basic optical properties ofliving tissues are absorption and scattering.
The absorption coefficient ?a(m?1) and the the scattering coefficient ?s (m?1)are defined as follows:Absorptioncoefficient – theprobability of absorption of a photon at an infinitesimal distance ?d when the photonpropagates over the infinitesimal distance i.e. for an absorption event themean free path is 1/?a Scatteringcoefficient – theprobability of scattering of a photon at an infinitesimal distance ?d when the photonpropagates over the infinitesimal distanceWhenindividual scattered photons are detected in OCT, the relevant light from thesource collected by the interferometer detector travels in the tissue adistance of 2d which is the sum of two distances: the distance in the tissuefrom the source to the point where the light is backscattered or reflected and thedistance from the point where the light is backscattered or reflected back tothe detector. These two distances are equal and the light is attenuated twiceover that distance. Lamber-Beer’slaw describes the light attenuation in a non-scattering media: where I(d)- the intensity at a distance d I0 – the light intensity incident onthe tissueLambert–Beer’s law is used to calculate thetotal attenuation using the total attenuation coefficient ?t.?t= ?a + ?s TheOCT System OCT interferometer system for catheter/endoscopeimaging The objective of the OCT techniqueis to measure the echo time delay of light using interferometric techniques.
Throughthe use of interferometric techniques, the tissue backscattered light signal iscorrelated with the light that has travelled an already known reference pathlength. Since the speed of light is faster than the speed of sound, the use ofinterferometry techniques to determine the echo time delay of light is crucial.Both the magnitude of backscattering light properties and the echo time delaycan be measured using interferometry. A frequently used detection method is theMichelson interferometer which applies a scanning reference delay arm. OCT usesinterferometric techniques to perform high resolution performance of lightechoes. In an OCT system, the fiber-optic coupler of the interferometer isanalogous to an optical beam splitter and divides the input light into areference arm and a measurement arm. A catheter or other image device isconnected to the optical fiber in the measurement arm. The catheter scans thetransverse position of the measurement beam and focuses the beam onto thetissue that is being imaged.
The catheter collects the light echoes asbackscattered by the tissue. The collected light echoes are returned back tothe measurement arm. A retro-reflecting mirror at a calibrated distance isattached in the reference arm.
The echo light of the reference arm is returnedwith a calibrated delay. At the fiber coupler the two echoes of light, one fromthe tissue and one from the reference arm are combined. A high-speedphoto-detector detects the intensity of the interference and finally, theelectronic signal is processed in order to extract a measurement of echo timedelay. Severaldetection methods are used to measure the echo time delay of light. There are twointerferometric techniques used in OCT systems to perform measurements:· Time- Domain OCT (TD-OCT): applies an interferometer with a low-coherence lightsource and scanning reference delay is used· Fourier- Domain OCT (FD-OCT): applies a narrow bandwidth, frequency-swept light sourceand a stationary reference delay interferometer is used Time – Domain OCT (TD-OCT) Interferometer (low-coherence) by the Time – Domain OCTtechnique The Time – Domain OCT (TD-OCT)systems are based on a low-coherence interferometer, which is actually aMichelson-type interferometer. The back scattered light echoes from the tissueare correlated with scattered light.
The light travels a known reference pathdelay. Interference can be observed when light from the reference arm arrivesat the same time as light from the tissue when using a low-coherence lightsource. By detecting the envelope of the modulated interference signal axialinformation can be obtained. As the reference path is scanned, the echoes aremeasured sequentially at different depths. A beam splitter splits the beamfrom a light source into two beams.
The first light beam is directed to thetissue and is backscattered from the structures of tissue at different depths.From the backscattered light beam consisting of multiple echoes, relevantinformation can be derived about the depth or range of the different tissue structures.A reference mirror reflects the second light beam.
The position of thereference mirror in time, thus the reference beam has a variable time delay. Atthe beam splitter the reference beam and the measurement beam are interferedwith. A photodetector detects the output. When the light beam is considered aslight pulse, if the tissue backscattered light pulse and the mirror reflectedlight pulse arrive synchronously within the pulse duration then the two pulseswill coincide. This happens if the two distances, the distance that lighttravels in the interferometer measurement arm when it is backscattered from thetissue and the distance that light travels in the interferometer reference pathare similar. A modulation in intensity is produced when the two light pulsesinterfere.
This happens when the light pulses coincide. The modulation inintensity is measured by a photodetector. The approach in order to measure thetime delays of light echoes coming from different structures of the tissue andfrom different depths is to scan mechanically the position of the referencemirror so that the time delay of the reference light pulse varies continuously.In conclusion, the key idea of the Time – Domain interferometer is to measurethe time delays of optical echoes sequentially and it is done by scanning areference path, so that different echo delays are measured at different times.
Fourier – Domain OCT (FD-OCT) Interferometer by the Fourier – Domain OCT technique Fourier – Domain OCT (FD-OCT)systems use a narrow bandwidth light source. This light source is frequencyswept in time and the reference arm in the interferometer that is used isstable. The backscattered tissue light echoes are interfered with scatteredlight. This light travels an already known reference path delay.
The two light beams:the one coming from the tissue and the other traveling the reference path havedifferent frequencies. This is because of the fact that the light coming fromthe tissue is delayed in time when compared to the light traveling thereference path. The interference varies according to the differences infrequency. By applying the Fourier transform in the signal of the detector,axial scan information is obtained. In the Fourier – Domain detection, all theechoes of light are measured at once instead of sequentially as in Time -Domain detection. Thus, the imaging speed of the Fourier – Domain detection ishigher than the imaging speed of the Time – Domain detection. This is the mostimportant difference between the Fourier – Domain detection and the Time –Domain detection.
OCT ImageResolution The high resolutions of OCT images playan important role in its clinical use. Different mechanisms determine theresolution of an OCT image in the axial and in the traverse directions. Thefocused spot size of the optical beam determines the traverse resolution.
Theresolution of the measurement for the echo time delay determines the resolutionof the image in the axial direction. In fact, the light source used for themeasurement determines this axial resolution. If a low-coherence light sourceis used to measure the echo time-delay, the coherence length of the sourcedetermines the axial resolution. If a frequency light is used then the axialresolution is determined by the tuning range of the light. The axial resolutionfor standard OCT systems is 10-15 ?m.An OCT image of the human retina with high resolution provides better differentiationof intra-retinal layers. Thus, more detailed features of the photoreceptorlayer can be extracted which enables the diagnosis of an early disease. For anintravascular OCT image, improvements to increase transverse pixel density and improvementsto increase the speed of imaging are very important.
In an OCT image, the number ofaxial scans determines the number of pixels in the transverse direction. Theimage acquisition time increases proportionately to the number of transversepixels. By increasing the transverse pixel density, the image quality can beimproved. High-definition images are produced by high axial scan density. In theseimages, the improvement of visualization is apparent. FD-OCT systems useFourier – Domain detection methods that enable higher axial scan densitiesproviding high quality OCT images by reducing the examination time. Applicationsof Optical Coherence Tomography OCT was initially applied forimaging in ophthalmology. Advances in OCT technology have made it possible touse OCT in a wide variety of applications with the medical applications still beingthe dominating ones.
Speci?cadvantages of OCT compared to alternative optical techniques are •depth resolution is independent of the sample beam aperture•the coherence gate can substantially improve the probing depth in scatteringmediaTheadvantages of OCT compared to non-optical imaging modalities are •high depth and transversal resolution•contact-free and non-invasive operation and the possibility to create• function dependent image contrast- Related contrasting techniques are based on Doppler frequency shift,polarization andwavelength-dependent backscattering Themain disadvantage of OCT compared to alternative imaging modalities in medicineis its limited penetration depth in scattering media. An OCT image from a coronary artery acquired using anFD-OCT system Ophthalmology OCT is heavily used byophthalmologists to obtain high resolution images of the eye’s anteriorsegment and retina. Because of its cross-sectional capabilities, OCT provides astraightforward method of assessing axonal integrity in multiple sclerosis and glaucoma. OCT is also well suited to assess macular degeneration and is considered the new standard for the assessmentof diabetic macular oedema. More recently, ophthalmic OCT devices have beenengineered to perform angiography and have been used to assess retinalmicrovasculature pathology in diseases such as glaucoma and diabeticretinopathy.
Cardiology In the context of cardiology, OCTis used to image coronary arteries in order to visualize vessel walllumen morphology and microstructure at a resolution ten times higher than the otherexisting modalities such as intravascular ultrasounds and X-ray angiography.For this kind of application – Intracoronary Optical Coherence Tomography,approximately 1 mm in diameter fiber-optics catheters are used to accessartery lumen through semi-invasive interventions, that is percutaneouscoronary intervention.The higher imaging speedof FD-OCT enabled the widespread adoption of this imaging technology forcoronary artery imaging. Recent developments of intravascular OCT included thecombination with other optical imaging modalities. OCT has been combinedwith fluorescence molecular imaging to enhance its capability todetect molecular/functional and tissue morphological information at the sametime.
Similarly,combination with near-infrared spectroscopy has been also demonstrated. Oncology Endoscopic OCT has been applied tothe detection and diagnosis of cancer and precancerouslesions such as Barrett’s esophagus and esophageal dysplasia.
