Optical Coherence Tomography System 166 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Evaluation of a Constructed Optical Coherence Tomography System Sohad. Elwakeela*, Nafie. ALmusletb, Yahia. Badr a ldienc, Khalid.Haround a,bInstitute of Laser, Sudan University of Science and Technology, P.O.Box:407, Khartoum, Sudan cNational Institute of Laser Enhanced Science, Cairo University, Egypt dFacility of Radiobiological and Imagine Science, Alzaaim Alazahari University, Khartoum, Sudan aEmail: sohadelwakeel@gmail.com Abstract The objective of this work is to design and construct an optical coherence tomography (OCT) system, beside the comparison of the efficiency of this system with other tomography systems, like Photoacoustic system, to evaluate its operation. Michelson interferometer was designed; which represent the heart of the system. Two types of laser sources were used; they were He-Ne laser with wavelength of 632.8 nm and semiconductor laser with wavelengths of (700) nm. Performance of the constructed system was completed by receiving the interference fringes by an optical detector, which was connected to the display unit. Digital oscilloscope, with Fourier transformation, was used to display the signal information in frequency domain. Also CCD camera was used to give two & three dimensional images for the studied samples. The constructed system can be used for many purposes, for examples: measurement of the thickness and determination of the optical properties for different samples. The samples studied by the constructed OCT system were: glass slides, polymer\ pyrex\ carbon coating, layers of onion. To prove the good performance of the constructed OCT system the results were compared with other results gained by a Photoacoustic system for the polymer sample. The comparison proved that the constructed OCT system is operated efficiently and correctly. Keywords: Tomography Imaging; FT; Optical properties (:). ------------------------------------------------------------------------ * Corresponding author. http://asrjetsjournal.org/ American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 167 1. Introduction Nowadays, a broad variety of imaging techniques are becoming available [1]. Tomography imaging techniques, such as: X-ray computed tomography, magnetic resonance imaging, and ultrasound imaging have found widespread applications in medicine. Each of these techniques measure a different physical property and has a resolution and penetration range that prove advantageous for specific applications [2]. Optical coherence tomography (OCT ) is a non-invasive imaging test [3,4,5]. It has the ability to peer inside the body, tissue absorption and scattering coefficients are derived from the characteristics of scattered light transmitted and / or re-emitted by the body. Three basic optical tomography approaches have been developed [6]. Diffraction tomography, (ii) Diffusing photon tomography [7]. (iii) Optical coherence tomography (OCT) [8]. The most straight forward optical technique is by diffraction tomography. Diffraction tomography is the part of the general problem of inverse scattering. A basic mathematical theorem used to perform the reconstruction of the object structure is the Fourier diffraction [9]. In this work, the approach of optical coherence tomography (OCT) was chosen. Optical Coherence Tomography (OCT) is an imaging technology measuring the backscattering properties of tissue, or material. Recently, extensions of OCT technology, including; Doppler flow and polarization sensitive image, have been developed that permit spatially resolved imaging of velocity or birefringence [10]. OCT has potential to go inside the tissue or material and give us the information from the reflected light. OCT performs imaging by measuring the echo time delay and intensity of backscattered light from internal microstructure in the tissue or material [11]. OCT combines the principle of ultrasound with the imaging performance of a microscope, where as ultrasound produced images from backscattered sound, OCT uses light. Cross-sectional images [12,13] are generated by measuring the echo time delay and intensity of light that is reflected or backscattered from internal structure in tissue, or material. Because the velocity of light is extremely high, the echo time delay can not be measured directly. Instead, it is necessary to use correlation or interferometery techniques [14]. In an OCT system the spectrum of the source is very important as it determines the maximum resolution of the image. 1.1 The general requirements of sources of OCT imaging are: [15] 1- Emission in near IR 2- Short coherence length 3- High irradiance 1.2 Applications of OCT American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 168 Since OCT has a much higher resolution[16,17] compared to other imaging modalities, OCT has emerged as a promising medical diagnostic imaging technology for non invasive in situ cross-sectional imaging of biological tissues and materials [18, 19]. it has significant potential in medical diagnosis [20]. The ability to image internal structures without the need for mechanical probing makes this technique very powerful for medical applications [21]. Its applications in ophthalmology, dermatology, endoscope, cardiology, vascular morphology, gastroterology, dentistry, and embryology have been demonstrated by several groups [22]. 2. Material and methods OCT system constructed for the aim of this work consists of five parts: two laser sources (He-Ne of 632.8 nm, 4 mW, class III, CW diode lasers 700 nm, ≤ 4.25 mW, class III. Constructed Michelson interferometer, detector (optical detector, photomultiplier detector), digital oscilloscope (150 MHz), computer, printer (RS-232 C thermal printer) and CCD (LBA-100A) camera. Different samples were used, to prove that the constructed system can be used for different purposes successfully. The used samples were: polymer (polycorbolen), carbon coating (SPI 5006 supplies made in U.S.A, conductive carbon point), glass (microscope slices, with thickness of 1mm and 1.2mm), onion, Pyrex (slide with thickness of 0.1 mm). 2.1 OCT experiment procedure was done in steps as fallows 1-Michelson interferometer was constructed. 2- Interference pattern of the object was obtained. 3- Photodetector was used to convert an optical signal into an electrical signal. 4- The detector output was displayed as voltage value when it is connected to a digital oscilloscope. 5- Computer or printer was connected serially with digital oscilloscope to display, or print, the detector output signal. 6- Two and three dimensional images of the object were obtained; when the interference fringes were receive by CCD camera. 7- The constructed OCT system was used with different samples for multi purposes. Information like depth, optical properties were determined from the digital oscilloscope signal. To describe the operation of the optical coherence tomography system, the signal should be followed in each stage. First, laser light incident on the beam splitter, which reflects half of the incident light to the reference mirror which was fixed, and the other half of the incident light was transmitted to the object (the sample) through the concave lens in order to concentrate the light when it passed or reflected from the object. Figure (1) shows the block diagram of the constructed system. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 169 Figure 1: block diagram of the constructed (OCT) system Then the two beams were reflected or backscattered again to the beam splitter, and interference fringes were observed after some adjustment done by screws on the fixed mirror. This step was done for all the samples and to scan the sample depth, a micrometer screw was used. The performed pattern was received by the detector to convert it into voltage, which was displayed on the digital oscilloscope. Many steps were done to determine the optical properties for all studied samples. The intensity of that signal was represented (plotted) against scanning distance; the obtained graph was used to determine penetration depth and absorption coefficient. From absorption coefficient, all other optical properties can be calculated. Figures (2) and (3) show the constructed optical coherence tomography system with different detection devices. Figure 2: Figure 3: The constructed OCT system The constructed OCT system using photomultiplier detector using CCD camera to determine 2-D & 3D images At the end, the constructed OCT system results were compared with Photoacoustic system to evaluate its operation. 3. Results and discussions The experimental results were represented in tables and figures. Figures (4) to (9) show the experimental results recorded by digital oscilloscope. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 170 Figure 4: the signal recorded by Figure 5: the signal recorded by digital oscilloscope for glass 1.0mm digital oscilloscope for glass 1.2mm Figure 6: the signal recorded by digital Figure 7: the signal recorded by digital oscilloscope for pyrex 0.1mm oscilloscope for polymer Figure 8: the signal recorded by digital Figure 9: the signal recorded by digital oscilloscope for carbon coating oscilloscope for onion American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 171 From the signal recorded by digital oscilloscope for studied samples we notice: glass has highest peak, while onion has the lowest one and that mean onion absorb most of the light and glass has high reflectivity. Polymer has a short period (1.68µs), which lead to high frequency. SNR (backscattered power divided by the noise equivalent bandwidth of the detection) for the recorded signals is very low for carbon and onion, while glass and pyrex have the biggest value. Table 1: illustrates the samples results calculated from the above figures (4-9). ΔLB(μm) spatial resolution in longitudinal direction R R(μm) Longitudinal resolution (depth resolution) |Samples 0.2068 0.2758 Glass 1.0mm 0.2735 0.3647 Glass 1.2mm 0.4172 0.556279 Pyrex 0.1mm 0.1164 0.152973 Polymer 0.128 0.17064 Carbon 0.1059 0.1412 Onion According to the above table, longitudinal resolution is higher than transverse resolution for all samples. Pyrex has the biggest resolution (longitudinal resolution 0.55627 µm, transverse resolution 0.4172 µm) and the minimum for Polymer. From the above results we noticed that; the constructed optical coherence tomography system (OCT) can be used to determine the depth resolution and transverse resolution [19] in longitudinal direction with good performance for different samples, beside that the constructed OCT is easy to use and efficient in performance. From the variation of the intensity with distance, in order to obtain a spectral tomography of the objects [23,24], optical properties can be determined by fitting linear or logarithm function. Figures (10-14) show this variation, and table (2) illustrates the optical properties for different samples. Figure 10: intensity of the backscattered light Figure 11: intensity of the backscattered light as as a function of depth for glass 1mm a function of depth for glass 1.2mm 0 20 40 60 80 100 120 140 160 1112131415161718191101111121131141151161171181191201211221231241251261271281291 Xμm I ( a. u) 0 20 40 60 80 100 120 140 160 1 11 21 31 41 51 61 71 81 91 Xμm I (a.u) American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 172 Figure 12: intensity of the backscattered Figure 13: intensity of the backscattered light light as a function of depth for pyrex as a function of depth for polymer Figure 14: intensity of the backscattered light as a function of depth for carbon coating Carbon and onion have low backscattered signal and glass, Pyrex have a bigger value due to low SNR, while polymer has value between them. Table 2: Samples optical properties Object type Penetration depth (mm) Absorption coefficient (mm-1) Attenuation coefficient (mm- 1) Scattering coefficient (mm- 1) Glass1mm 0.037 20.382 27.02702703 6.645027027 Glass1.2 0.070 13.860 14.286 0.426 Pyrex 0.060 13.750 16.667 2.917 Polymer 0.065 13.860 15.385 1.525 Carbon 0.024 36.474 41.667 5.193 0 20 40 60 80 100 1 11 21 31 41 51 61 71 X μm I ( a. u) 0 50 100 150 200 250 300 350 400 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Xµm I(a .u ) American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 173 Object type Reduced scattering Coefficient (mm-1) Reduced attenuation coefficient (mm-1) Reduced penetration depth (mm-1) Glass1mm 13.290045 33.672045 0.029698 Glass1.2 0.8514 14.7114 0.65 Pyrex 5.834 19.584 0.051062 Polymer 3.049 16.909 0.05914 Carbon 10.385 46.859 0.02134 From the above tables we can notice that the constructed (OCT) can be used for determination of the optical properties [24] of the different samples successfully. To prove the good performance of the constructed OCT, system the results were compared with another results gained by a Photoacoustic system for the polymer and carbon coating samples. Figure (15) shows the recorded photoacoustic signals, which were generated after laser excitation of the polymer and carbon coating. Figure 15: photoacoustic response for polymer and carbon The optical absorption coefficient of the samples can be determine from photoacoustic signals (PA) by fitting the logarithm of the (PA) signal amplitude as a linear function of time[25] as in (OCT). From the above results and discussions one can notice that both techniques (Constructed OCT & PA) have the ability to gain spectrum for different samples, beside that, samples optical properties can be determined by using both techniques. 4. Concolutions From the obtained results one can conclude that: 1- The constructed (OCTS) is a demand for gaining insight into functional parameters of tissues or materials. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 51, No 1, pp 166-175 174 2- It has potential to extract all optical properties. 3- The results showed that the tomographic system can be used as diagnostic systems with high resolution. 4- (OCTS) tomographic technique can be used to get two & three dimensional image information of internal structure of the object 5- The technique has a potential to determine the thickness and the optical properties for different samples. 6- The constructed technique has no ionizing effect compared with another tomography imaging (nuclear rays). 5. Recommendations The constructed Optical coherence Tomography system can be modified to be used with invivo measurements by using optical fiber, sources with partial coherence like Superluminescent diode (SLD). 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