Optical Coherence Tomography Systems and signal processing in SD-OCT

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1 Optical Coherence Tomography Systems and signal processing in SD-OCT Chandan S.Rawat 1, Vishal S.Gaikwad 2 1 Associate Professor V.E.S.I.T., Mumbai chandansrawat@gmail.com 2 P.G.Student, V.E.S.I.T., Mumbai gaikwadvishal999@gmail.com ABSTRACT Optical Coherence Tomography (OCT) is a high speed image acquisition technique. It is used for high resolution cross-sectional imaging. It can acquire images of a biomedical objects in the scale of micrometer resolution.oct is used for medical and nonmedical 2D/3D imaging applications. Medical applications include retina imaging, identifying tumour boundaries in the mucosa, early detection of lung cancer.non-medical applications are nondestructive evaluation of paints and coating. There are two types of OCT systems Time-Domain OCT(TD-OCT),Fourier Domain OCT(FD-OCT).FD-OCT is further divided into Spectral Domain OCT(SD-OCT) and Swept-Source Domain OCT (SS-OCT).SD-OCT uses broadband light source with spectrometer for detection of back reflected signals.sd-oct offers significant advantages in image acquisition speed over TD-OCT. Keywords: Optical coherence Tomography, OCT, Types of OCT, OCT systems. 1. INTRODUCTION SD-OCT system normally uses broadband light source such as Super Luminescence source diode (SLD).The OCT system normally works on Michelson-Interferometer technique. The power of a broadband source is divided into sample arm and reference arm of the interferometer through the coupler. The light reflected back by the reflectors in the sample is recollected by the sample arm fiber. The light reflected by the reference mirror is recollected by the reference arm fiber. The light from both the sample and reference arms is again coupled back in the fiber coupler and part of it is redirected towards the detector. Due to the finite coherence length of the source, optical interference is observed only when the optical path lengths of the beams reflected by the sample reflector and the reference mirror differ from less than the coherence length [4]. The interference pattern is modulated by an envelope function related to the coherence function of the source. In the FD-OCT, the reference arm is fixed and there is no moving parts are required to obtain the depth scan data. Hence the FD-OCT increases the data acquisition speed when compared to the TD-OCT. Moreover, the FD-OCT provides a better signal-tonoise ratio when compared with the TD-OCT [5]. 2. PRINCIPLE OF OCT SYSTEMS OCT is based on a Michelson interferometry. A Michelson interferometer is illuminated with low coherence light from a broadband light source. In TD-OCT system the reference arm is a movable mirror. By shifting this mirror a depth profile from the sample can be recorded, since interference is only observed if the optical path of a backscattering structure in the sample arm will coincide with the optical path length in the reference arm. Normally, the envelope of the interference signal is taken to form the depth profile (so called A scan) at one position on the sample surface. By adjoining several A-scans taken at adjacent locations on the sample cross-sectional images or whole 3D volumes can be acquired. It is worth noting that the lateral resolution determined by the spot size of the light is decoupled from the axial (depth) resolution which is related to the width of the spectrum of the light source, the broader the spectrum the higher the axial resolution [6]. The setup in Figure 1 belongs to the spectral-domain OCT (SD-OCT) systems. The position of the reference mirror determines the axial measurement location. It is evident that all photons which are backscattered from other depths than the current axial measurement position do not contribute to the interference and are lost. FD-OCT uses a fixed reference mirror and the spectral distribution of the light exiting the interferometer is analyzed. In FD-OCT system variant called spectral-domain OCT. The spectrum is taken with a grating spectrometer (with a CCD line camera) and a depth profile is obtained by calculating the Fourier-transform of the spectrum in the k-space. The acquisition speed of such systems is only limited by the read-out rate of the line sensor, which is routinely in the kilo-hertz range, a scan rate which cannot be achieved with the TD-OCT with movable reference mirror. The gain in sensitivity is more than 20 db for FD-OCT systems when compared to their TD-OCT counterparts [6].

2 3. TIME DOMAIN OCT (TD-OCT) Figure 1 Schematic of SD-OCT [6] Conventional TD-OCT is the first generation OCT system where, the reference arm is moved to perform the depth scan as shown in Figure 2. To obtain 3D image data of the sample, a beam is required to scan across the surface of the sample using a scanner mirror. During the transverse scanning, the reference mirror performs the depth scanning (A-scan) on the sample and a photo detector records the interference signal for further processing. For each depth location, the reference mirror is moved over a defined range to perform the axial scan while the scanner mirror is held immobile. The transverse scanning is commonly performed by using a Galvo-mirror scanner or by using a micro-electro-mechanical-system (MEMS) mirror scanner. Since the depth scan data is collected by moving the reference mirror with a time-varying location, this method is called time domain optical coherence tomography (TD-OCT) [7]. Figure 2 Schematic of TD-OCT [6] 4. SWEPT SOURCE DOMAIN OCT(SS-OCT) In this configuration, the depth scans (A-scan) are performed by sweeping the individual wavelength spectrum coming from the broadband source. A photo detector is used to record the interference signal and further processed to retrieve the depth profile. A similar lateral scanning (B-scan) like a TD-OCT or a SD-OCT is performed in the transverse direction of the sample. The schematic of the SS-OCT configuration is shown in Figure 3.With recent research advances, OCT imaging of optically scattering non transparent media is possible, thus enabling a wide variety of application in biological and non biological sample imaging. One of most important advances for imaging in optically scattering samples was the use of broadband source with a longer coherence length light source where optical scattering is reduced. The use of ultrashort pulse like femtosecond laser technology enables OCT imaging at unprecedented resolutions and can achieve a resolution up to 3 µm [7].

3 Figure 3 Schematic of SS-OCT [6] 5. SPECTROMETER BASED OCT SYSTEM The experimental setup of Spectrometer based SD-OCT system uses a broadband light source, usually a super-luminescent diode (SLD). A 2 2 fiber-optic coupler forms the Michelson interferometer. In the detector arm, a low-loss spectrometer measures the spectrum of the interference pattern of light returned from the reference and sample arms of the interferometer. The grating-based spectrometer measures the interference signal as a function of wavelength λ [8]. Usually, the spectral data is rescaled and resampled evenly in k-space, before it is Fourier transformed to get the sample depth profile or A-scan. High spatial resolution a desirable feature of an OCT system requires the use of a light source with wider spectral width. The larger depth scan range would require a higher number of spectrometer s linear array (CCD camera) pixels. 5.1 Signal processing in SD-OCT Signal processing is the main part of SD-OCT system, since it gives brief idea about the sample structure and also help to remove noise from the signal. Though the first generation of OCT systems was based on time domain systems, the popularity of frequency domain systems (whether spectral or swept source) are increasing due to their speed of acquisition and due to improved sensitivity. In an SD-OCT system, the depth encoded spectral signal is acquired from the entire depth by the grating-based spectrometer. Here, the scanner arm performs the transverse scan while the spectrally encoded A-scan data is acquired by the spectrometer as a function of wavelength (λ). Data processing in the SD-OCT is based on discrete Fourier transformation (DFT) of the interference signal. First, the acquired signal is resampled from the wavelength to the wave number space k=2π/λ, and then inverse fourier transform operation is performed to generate axial profile of the sample. Flow chart of the SD-OCT signal processing is shown in Figure 4[9]. Figure 4 SD-OCT axial scan generation [9]

4 6. ADVANTAGES OF SD-OCT The biomedical community has seen a continuous string of successful efforts at increasing the spectral bandwidth of optical sources. The increased bandwidth in turn leads to decreased coherence length and enables the acquisition of ultra high axial resolution OCT images [9]. In a TD-OCT system, increasing source bandwidth decreases the SNR as it requires increased electronic detection bandwidth. In order to maintain the same SNR, one has to either decrease the A-scan rate or depth scan range or increase the sample illumination intensity. As a matter of fact both high A-scan rate and larger depth scan range are highly desired features in OCT systems. At the same time, in many cases such as in ophthalmic imaging, the incident optical power cannot be increased beyond the American National Standards Institute (ANSI) limits. This problem can be resolved by using a more sensitive technique. Fortunately, both spectrometer and swept source-based SD-OCT systems show significant sensitivity advantage over the TD-OCT. Therefore, SD-OCT systems may be used to perform OCT imaging at higher speed and (or) scan depth. As mentioned earlier, swept source SD-OCT acquires interferometric data from a sample as the wavelength of tunable laser is adjusted linearly in time. As wider optical wavelength sweep range will deliver higher depth resolution and faster wavelength tuning of the source will permit rapid data acquisition, the design and functionality of tunable source plays important role in the performance of swept source-based SD-OCT system. In recent years, various laser designs in 1300 nm band have been demonstrated for swept source SDOCT. Demonstrated tunable light sources are based on galvanometer scanning mirror [12], polygon rotating mirror [13], and tunable Fabry-Perot (FP) cavity [14]. Using a single polygon mirror scanner, Oh et al. reported a dual laser cavity swept source with 145 nm tuning range around 1310 nm and at 20 khz repetition rate [15]. The peak and average optical power were limited to 1.8 and 1.35 mw, respectively. The output optical power of these systems was relatively low. Recently, R. Huber et al. demonstrated a swept source with 120 nm tunable bandwidth centered at 1310 nm [16] based on the use of a tunable FP cavity. This swept source is capable of delivering approximately 45 mw output optical power at 20 khz sweep rate. Motions in the sample or probe can cause signal fading, decrease in axial and (or) transversal resolution and reduction in SNR which are manifested as image distortion and blurring. In TDOCT, probe or sample related motion affects only the image pixel for which the signal is being acquired. If the signal acquisition time per pixel is very small, the degradation in SNR and spatial resolution due to motion may be negligible. However, motion-induced image degradation in SD-OCT systems in which signal is acquired over time from various depths of interest inside the sample may be significant. The problem can be alleviated by increasing the OCT imaging speed a focus of recently reported work. In spectrometer- based SD-OCT systems, the motional artifacts can be further reduced by decreasing the CCD illumination time [19]. Spectroscopic OCT is an enhancement of standard OCT and provides spatially resolved spectroscopic information of the sample [20]. This added information about depth resolved absorption can lead to enhanced image contrast allowing differentiation of tissue pathologies. In TD-OCT, the interferometry data corresponding to each pixel is processed to retrieve the spectroscopic information. In the case of SDOCT, there is a direct access to the spectroscopic information. This shows that SD-OCT systems can lead to a simple and fast way to investigate depth resolved spectral signatures of a biological sample. The swept source SD-OCT system has many advantages, such as reduced excess noise, no SNR drop-off, and heterodyne detection capability, over spectrometer-based SD-OCT system. However, all reported implementations of swept sources have been realized in the 1300 nm band. In some cases, it might be preferable to do the imaging in other wavelength bands. For instance, it was reported that 1040 nm light is more suitable for retinal imaging because it will enhance penetration into the choroids [21]. Similarly, most of the molecular dyes such as indocyanine green (ICG) have absorption bands in the 800 nm regime. Therefore, to get the advantages of swept source SDOCT, it would be highly desired to realize high-speed, broadly tunable swept sources in other wavelength bands. Another worthy research direction is to investigate ways to resolve the problem of SNR drop-off in spectrometer-based SD-OCT systems that severely limits the scan depth range to approximately 2 mm or less. Until now, there is no efficient method reported to significantly improve the SNR drop-off. References [1] James G. Fujimoto, Stephen A.Boopart. Optical Coherence Tomography: An Emerging Technology for Biomedical Imaging and Optical Biopsy Neoplasia, Vol.No.-2, Nos.1-2, January-April 2000, pp [2] C.K.Hitzenberger,T Lasser, W Drexler, Optical Coherence Tomography Principals and Applications.A.F.Fercher,.Institute of physics publishing Rep. Prog. Phys. 66 (2003) [3] A. F. Fercher, C. K. Hitzenberger, and. R. Leitgeb Performance of Fourier domain vs. time domain optical coherence tomography. 21 April 2003 / Vol. 11, No. 8 / optics express 889 [4] Part of Proceedings of SPIE Photonics in Dermatology and Plastic Surgery. Vol. 8207A Photonics in Dermatology and Plastic Surgery, Jan-2012.

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