Spectral Domain Optical Coherence Tomography (SD ...

"2012 International Conference on Optical Engineering (ICOE)"

Spectral Domain Optical Coherence Tomography (SD-OCT) for Imaging Non-Biological Samples Byomakash Mahapatra, Mary Gisby Poulose, Indrajit Boiragi, Roshan Makkar, K.Chalapathi Optoelectronics Division, Society for Applied Microwave Electronics Engineering and Research (SAMEER) IIT Bombay Campus, Hill Side, Mumbai, India-400076. Abstract- This paper describes the basic technique of spectral domain optical coherence tomography (SD-OCT) imaging for some non-biological samples. Spectral domain optical coherence tomography (SD-OCT) technique is an interferometric technique that provides depth-resolved tissue structure information that encoded by spectral analysis of the interference fringe pattern using a broad band super luminescent diode (SLD) source and high resolution (0.8um) fiber optic spectrometer. The raw data is captured for some biological/non-biological sample with a lateral scanning of 60 steps and axial resolution of 13μm. The data has been processed with MATLAB to form a high resolution structural image. Keywords- Michelson Interferometer, SD-OCT, SLD.

I.

INTRODUCTION

Optical coherence tomography (OCT) has developed to a powerful technique to image biological and nonbiological samples with a high sensitivity and high resolution. The spectral domain optical coherence tomography (SD-OCT) has significant advantages, in terms of acquisition speed and sensitivity, as compared to time domain optical coherence tomography (TDOCT), where we have to change the reference path to get the interference fringes and a limited photon are detected which are in the coherence length. But in SDOCT simultaneously all photon are detected from all depth within the window. The key technological parameters of any (morphological) imaging modality that significantly influence its ultimate clinical and research utility are: axial (depth) resolution, transverse resolution, detection sensitivity, image penetration depth in samples and data acquisition time[1]-[3][8]. A high axial image resolution enables visualization of the

detailed architectural morphology of the different layer in the biological tissue. The transverse resolution of an imaging system is commonly defined as the FWHM of the transverse point-spread function (PSF) and it determine by light directed and collected from the samples. The Imaging depth is mainly determined by the wavelength, optical power of the light source and optical properties, such as absorption and scattering of the sample, and hence strongly depends on the sample being imaged and the imaging wavelengths. In addition to above parameter functional tissue information is playing an increasingly important role as an adjunct diagnostic parameter or source of image contrast. In SD-OCT systems diffraction gratings in spectrometer separate spectral components almost linearly in wavelength (λ) domain, but it is difficult to process data in λ-domain, so it becomes unevenly sampled in k domain with inverse relationship π/λ, and then resample to achieve uniform spacing in k- domain in order to use Fast Fourier Transform (FFT). The accuracy of the re-sampling method is important to the image reconstruction. Traditional resampling methods include linear and cubic B-Spline interpolations. In this paper we report a SD-OCT system setup at our laboratory. We have used a spectrometer and a broadband light source with a bandwidth of 14nm centered at 830 nm, and spectrometer having 0.8μm resolution. Our system achieved a line-scan rate of 1 kHz and an axial resolution of 13 μm. SD-OCT imaging using our system is performed on nonbiological tissues such as foam sheet and check cell. II. THEORETICAL BASIS OF SD- OCT

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There are two distinct methods have been developed to implement SDOCT. The first approach, a spectrometer-based SDOCT also known as Fourier domain OCT (FD-OCT), uses a broadband light source and a low-loss spectrometer to measure the spectral oscillations. The second method, swept-source SDOCT, that has also been termed as optical frequency domain reflectometry (OFDR) [1][3], which employs a rapidly tuned narrowband source. The beam splitter in an SD-OCT interferometer splits a broadband source field into a reference field Er and sample field Es. The sample field focuses through the scanning optics and objective lens to some point below the surface of the tissue. After scattering back from the tissue, the modified sample field Es1 mixes with Er on the surface of the photodetector. Given the assumption that the photodetector captures all of the light from the reference and sample arms, the intensity that impinges on the photodetector from the interference signal at the spectrometer is given by [2][4]: (1) In SDOCT the reference mirror remains fixed having certain path length which has a constant intensity and light reflected from different depths change with respect to the reflectivity of that layer, this can be express as

P(k) is the source power spectral density, RR and Rn are the reflectivity of the reference and nth sample reflector, Zr is the positions of the reference reflector . Zn and Zm are position of sample reflector at and point. The depth information can be retrieve by the inverse Fourier transform of above eq.

(5) SD-OCT has the advantage that it has a very high axial image resolution, which depends upon the coherence length of the light source ( c) inversely proportional to the spectral bandwidth (Δλ). Mathematically it can expressed as [4][8]: Ζ=

To achieve a high axial resolution i.e around10-15μm superluminescent(SLD) diode source is used here transverse resolution can be is directly depend upon the focusing spot size and inversely proportional to the focusing angle or numerical aperture of the incident light source and is mathematically represented as: (7) Having depth of focus Z can be represented as (8)

(2) (3) For SDOCT the interferogram is measured as a function of optical wave number, k. Here the spectral interferogram is acquired using a spectrometer in the detection arm of the interferometer. The measured photocurrent signal generated by n reflectors is given as [3]

(4) Where I (k) is the detector photocurrent and K=2

;

(6)

A high value of the numerical aperture (NA) decreases the transverse resolution values and depth of focus Z. So for imaging a sample, we had to compromise in the lateral resolution by taking a low numerical aperture of focusing to achieve a greater depth profile. III.

METERIAL AND METHODS

A. Data Capturing In this experimental set up we used a 830nm SLD source of BLM-S830 high power series with a output of 20mW and spectral width of 14nm with a spectrometer of BLUE-WAVE (Stellernet) having spectral range 200-1150 nm and a 16bit digitizer which is get input through a single mode fiber .For lateral scanning in this SD-OCT system have used XYZ linear stages system having a position resolution of 0.048μm

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for a full scale range of 25mm, which varies position of the sample laterally over the full scale range in all three directions, the interference light together with light from the reference arm focus on the diffraction grating which physically separate the different frequencies of light. The light is then collected by the CCD detector which produces a raw data varying in the wavelength domain (λ). Figure: 3 Basic step for signal processing using MATLAB.

IV.

RESULTS

We are in the first phase of experiment so far we have taken images of some non-biological sample such foam sheet, cheeck cell and focused on a fixed point at different depth scans to get a M-mode image which have a variation of intensity in depth direction as shown in figure 4 and 5.In the second phase of this experiment we gone through a lateral scanning with XYZ linear stages of 50 step for total range of 25mm so each step input it varies a distance of 0.5mm in XYZ direction over the sample, with a axial resolution of 13μm we get a B-mode image for the plane mirror as shown in figure5.

Figure: 1 Schematic diagram of SD -OCT

Figure2: spectral data obtain from spectrometer in scope mode

B. Signal Processing The spectrometer captures signal in a variation of wavelength λ space and need to convert from λ space to k-space before implementing of FFT. So for getting structural image representation the raw data is processed through different stages such as background subtraction, re-sampling, image formation, image enhancement, and dispersion compensation[2][3] . The first two processes are performing by linear interpolation with zero pending and then performed FFT. Each FFT give a particular A-scan for the corresponding fringe pattern and by integration of a number of A-scan we get a 2-D image after software based dispersion compensation. The basic MATLAB process for the OCT signal processing given below in fig.3

(a) (b) Figure: 4 (a) A-scan image of foam sheet which show the depth profile. (b)M-mode image constructed from multiple A-scans

(a) (b) Figure: 5(a) A-scan image of cheek cell which show the depth profile, (b). M-mode image constructed from multiple Ascans of cheek cell.

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[7]

resolution imaging using echoes of light,”optics and photonics news January2000. Joel S. Schuman MD FACS,” Spectral domain optical coherence tomography for glaucoma, ” Trans Am

Ophthalmol Soc , Vol 106 , 2008. [8] [9] (a)

(b)

wolfgange Drexler, James G. Fujmoto,” A Text book on optical coherence tomography,springer,2008 K. Divakar Rao*, Y. Verma, H. S. Patel and P. K. Gupta,” Non-invasive ophthalmic imaging of adult zebra fish eye using optical coherence tomography, “current science, vol 90, no. 11, June 2006.

(c) Figure: 6 a. A-scan image of mirror which show the depth profile (b) 2-D image for with XYZ linear stage at two difference coherence point (c)Edge detected image which show the outer edge for the Bmode image.

So from the first experiment imaging of foam sheet with an axial scan of 13μm we get a depth profile is calculated to be 0.46mm. V. CONCLUSION Spectral domain OCT is a new imaging technology use for vivo imaging of biological and non biological sample with a μm resolution. In this paper we have describe about SD-OCT system development which includes basic theory, signal preprocessing .The present research gives some basic results of nonbiological sample and shown the imaging capabilities of SD-OCT system .

VI. REFERENCES [1]

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Joseph M. Schmitt, ’’optical coherence tomography ,a review ,” IEEE journal of selected topic on quantumelectronics,vol.5,no.4,july/august1999. Peng Li, Yonghong He, Hui Ma,” Spectral-Domain Optical Coherence Tomography and Applications for Biological Imaging,” 0-7803-9774-6/06,IEEE,2006 Zahid yaqoob,Jigange wu,changhui yang,”spectral domain optical coherance tomography:a better octimaging, ” biotechnique,Dec 2005. Murtaza Ali and Renuka Parlapalli, Signal Processing Overview of Optical Coherence Tomography Systems for Medical Imaging, “Texas instrument, sprabbr,june2010. James G. FUJIMOTO, ”optical coherence tomography ,” C. R. Acad. Sci. Paris, t. 2, Série IV, p. 1099–1111, Applied physics, 2001. James. Fujimoto, wolfgang Drexler, Uwe morgner, Franz kartner and Erich ippen, ”Optical coherence tomography high

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