1. Introductionto Optical Coherence TomographyOpticalcoherence tomography (OCT)is an imaging technique that uses coherent light to capture cross-sectional image of theoptical scattering media (biological tissues) with micrometer-resolution.It is used for medical imaging andindustrial nondestructive testing (NDT). Long wave lengthlight is used to penetrate through scattering medium. Optical CoherentTomography is a type of Optical Tomographic Techniques.
OCT is similar toultrasound imaging, but the different is here we use light waves instead ofsound waves. OCT performs high-resolution,cross-sectional tomographic imaging using backscattered or backreflected lightin internal microstructure in material and biological systems. Imaging can beperformed in situ and in real time.OCT imaging is widely used in biomedical and clinical applications. OCT wasinitially used in the imaging in the eye. OCT enablesthe noncontact, non-invasive imaging of the anterior eye as well as imaging ofmorphologic features of the human retina including the fovea and optic disc.
Nowadays, advanced in technology enables OCT to perform imaging innon-transparent tissues and leads to wide range of applications. Imaging depthis limited by the optical attenuation from scattering and absorption intissues. When compared to ultra sound imaging, imaging depth is limited toultrasound. But resolution of OCT is few times greater than ultrasound. Eventhoughthere is a limit to depth of penetration in OCT, it can provide images oftissue pathology in vivo with spatialresolution of few micrometers.
The dominant application of OCT is currentlyophthalmology. The basis of OCT islow-coherence interferometry. In conventional interferometry with long coherence length, forexample laser interferometry, interference of light occurs over a distance ofmeters. In OCT, this interference is limits within the range of micrometers,owing to the use of broad-bandwidth light sources.
Light in an OCTsystem is broken into two arms—a sample arm (containing the item of interest)and a reference arm (usually a mirror). The combination of reflected light fromthe sample arm and reference light from the reference arm gives rise to aninterference pattern, but only if light from both arms have traveled the”same” optical distance (“same” meaning a difference ofless than a coherence length). By scanning the mirror in the reference arm, areflectivity profile of the sample can be obtained (this is time domain OCT).
Areas of the sample that reflect back a lot of light will create greaterinterference than areas that don’t. Any light that is outside the shortcoherence length will not interfere.16 This reflectivity profile,called an A-scan, contains information about thespatial dimensions and location of structures within the item of interest. Across-sectional tomograph (B-scan) may be achieved by laterallycombining a series of these axial depth scans (A-scan). A face imaging at anacquired depth is possible depending on the imaging engine used.
2. Principle of Operations The functionprinciple of OCT imaging is light interference. Hence, a light interferencesetup is a core requirement of any OCT system. There are many types ofinterference configurations. In OCT system, the light from a low coherencesource is split into two paths by a coupler directing it along two differentarms of an interferometer. One arm is a reference arm and other is a samplearm. When the light exits the fiber end of either arm, it is shaped byvarious optical components such as mirror and lens to control specific beamparameters such as shape, depth of focus and the intensity distribution of thelight. In the reference arm, the light is back-reflected by a reference mirrorand it returns into the interference system.
Likewise, the same process happensin the sample arm. The returning light from both arms recombine at the couplerand generate an interference pattern and it is recorded by the detector. For a particular position of thereference mirror, the light propagating in the reference arm travels a certainoptical distance and forms the corresponding interference pattern only withlight that travelled the same optical distance along the sample arm, includingthe portion of the distance travelled inside the sample. Therefore, when thereference mirror is translated along the propagation direction of light, fordifferent positions of the mirror, the returning reference generatesinterference patterns with light backscattered from corresponding depths withinthe sample. From this, the dependence on depth of the intensity of lightbackscattered from below the sample surface can be measured.depth scan or an A-scan is knownas the OCT signal recorded by the detector during a complete travel of thereference mirror. To form an OCT image, the sample beam has to be translatedacross the sample surface and also A-scan should be recorded at each positionof the beam. A set of consecutive A-scans that are obtained forming an OCTimage is called the B-Scan.
Generally, the sample itself affects the maximumpossible depth that can be probed by an OCT imaging system. Low-coherence semiconductor Super – LuminescentDiode (SLD) is the light source in an OCT system. The characteristics of theSLD are an important design parameter since the axial resolution, also known asthe depth resolution or the coherence gate, is the coherence length of thesource, an intrinsic parameter of the source which is inversely proportional toits spectral bandwidth. When the spectral distribution of the SLD is Gaussian,the axial resolution ?z is given as: Where:? – Central wavelength of SLD?? – Bandwidth of SLDTherefore, broadband opticalsources are required in order to achieve high axial resolution. Axialresolutions in the micron and sub-micron range can be achieved by using sourceswith very large spectral band such as femto-second pulsed lasers or white-lightsources such as halogen lamps.The axial and transverseresolutions are independent in OCT imaging. The transverse resolution isdetermined by the minimum spot size of the focused probing beam, a parameterwhich is inversely proportional to the numerical aperture (NA) of the focusinglens: In relation (2), d is the spot size of the probing beam as it isprojected on the objective lens and f is the focal length ofthe objective lens, i.
e. the lens or lens system located at the end of thesample arm. An important detail that needs mentioning is that there is atrade-off between transverse resolution and depth of field. A high numericalaperture features a great focusing power translating into excellent transverseresolution, i.e. small diameter of the focused beam, with a corresponding shortdepth of field. Meanwhile, a low NA offers a greater diameterof the beam at the focal point but a large depth of field. Most OCT imaging isperformed with a low NA lens in order to ensure a depth offield of the order of millimetres, much longer than the coherence length of thesource.
Commercial OCT systems typically use transverse resolutions of20–25 ?m.3. Quality AssuranceQuality Index(QI) is an OCT image quality parameterthat is used to assess the quality of the image. QI can be calculated based onimage histogram information using a software program.
As a first step to findthe Quality Index, four new parameters were identified based on reflectivityvalues of the pixels that comprise each image. · Low: the first (lowest) percentileof all recorded reflectivity values in a given image.· Noise: the reflectivity value thatcorresponds to the 75th percentile of all recorded reflectivity values in agiven image. (Signal values up to this point in the scan were consideredextraneous signal from sources other than the retina).· Saturation: the reflectivity valuerepresenting the 99th percentile of all recorded reflectivity values in a givenimage.· Middle: the mean value of noise andsaturation.Quality Index is comprising of twocomponents. They are Intensity Ratio(IR) and Tissue Signal Ratio (TSR).
Thesetwo components are calculated by the above four parameters. Intensity Ratio (IR) IR is analogous to signal tonoise ratio (SNR) provided by the manufacturer. The manufacturer’s SNR is themaximum SNR value among all a?scans. IR is calculated by information on histogramthat takes entire image into account. SNR calculation requires preprocessedsignal data that are proprietary and not available to OCT users. We thereforecalculated an SNR equivalent, IR, using OCT raw data that any user can export.
(2) Tissue signalratio (TSR): This calculates the ratio of the number of highlyreflective pixels versus those with lower reflectivity. The numerator is thenumber of pixels between the “Middle” and “Saturation” intensity values. Thedenominator is the number of pixels between the “Noise” and “Middle” intensityvalues.Our new parameter, QI, was then constructed as the productof the IR and the TSR described above: Ultrahigh resolutionis obtained in the ophthalmic imaging using OCT. The axial resolution isapproximately 3m, which is determined by measuring the full-width half maximumof isolated reflections from structures in the retina.
Index of refractionnormalizes the axial dimension of the OCTimage to convert optical delay into geometrical distances. The transverseresolution is determined by aperture of the pupil, which is approximately 15 ?min all of the retinal OCT images. Ultrahigh-resolution OCT enables visualization of the foveal andoptic disc contour, as well as internal architectural morphology of the retinaand choroid that is not resolvable with conventional resolution OCT. Ultrahigh-resolutionOCT images can be processed using segmentation algorithms to identify andquantitatively measure intraretinal structure relevant for the early diagnosisand monitoring of ophthalmologic diseases. In addition to the retina,ultrahigh-resolution OCT can also image the anterior segment with unprecedentedresolution. In this case, the transverse resolution can be improvedsignificantly because the pupil does not limit the numerical aperture for beamfocusing.
An in vivo ultrahigh-resolution OCTimage of the cornea of a normal human subject with approximately 2 ?m axial and6 ?m transverse resolutions is shown in Fig. 4. The cornealepithelium, Bowman’s layer, intrastromal morphology (for example, corneallamellae) can clearly be differentiated. Retinal nervefiber layer (RNFL) thickness, a measure of glaucoma progression, can bemeasured in images acquired by spectral domain optical coherence tomography(OCT). The accuracy of RNFL thickness estimation, however, is affected by thequality of the OCT images. In this paper, a new parameter, signal deviation(SD), which is based on the standard deviation of the intensities in OCTimages, is introduced for objective assessment of OCT image quality.
Two otherobjective assessment parameters, signal to noise ratio (SNR) and signalstrength (SS), are also calculated for each OCT image. The results of theobjective assessment are compared with subjective assessment. In the subjectiveassessment, one OCT expert graded the image quality according to a three-level scale(good, fair, and poor).
The OCT B-scan images of the retina from six subjectsare evaluated by both objective and subjective assessment. From the comparison,we demonstrate that the objective assessment successfully differentiatesbetween the acceptable quality images (good and fair images) and poor qualityOCT images as graded by OCT experts. We evaluate the performance of theobjective assessment under different quality assessment parameters anddemonstrate that SD is the best at distinguishing between fair and good qualityimages.
The accuracy of RNFL thickness estimation is improved significantlyafter poor quality OCT images are rejected by automated objective assessmentusing the SD, SNR, and SS.