OCT imaging for small animals--a swept source platform.
Soon after its appearance in early nineties, OCT--Optical Coherence Tomography--has become a very promising optical imaging technique and new developments and applications confirm its huge potential in living tissue assessment. Based on the light interference phenomenon  it is widely used in the field of ophthalmology as a diagnostic tool by allowing cross-sectional real-time imaging of ocular structures such as the retina .
Currently, two main types of OCT systems can be considered: Time-Domain OCT (TD-OCT) and Fourier Domain OCT (FD-OCT)--that can be further divided in Spectral Domain OCT (SD-OCT) and Swept Source OCT (SS-OCT) as we will see [1,4]. TD-OCT was the first to be developed and its working principle is quite similar to a Michelson interferometer . Light from a low-coherence source is directed into a beamsplitter where it is divided in two paths: reference and sample. Interference fringes occur with backreflected light under the condition that both path lengths match to within the coherence length of the light. OCT interferogram is obtained by moving the position of the reference mirror changing the reference path by a certain amount ([DELTA]z) to match multiple sample optical structure and a depth profile (A-Scan) of the sample can be acquired .
In FD-OCT the reference mirror is fixed and the interference profile is acquired by detecting the output spectrum using a spectrometer and an array detector (often CCD camera) and, then, Fourier transformation. A recent approach for FD-OCT has been the so-called Swept-Source OCT (SS-OCT). In SS-OCT the novelty is the light source--a swept source - that is a tunable narrowband laser that is swept over a broad range of optical frequencies with a sweep frequency of 100 kHz or more. In this configuration, spectrometer and camera, are replaced by an InGaAs dual balanced photodiode detector and a high sample rate digitizer. This type of OCT presents advantages: reduced fringe washout, improved sensitivity with imaging depth, longer image range and higher detection efficiencies [4, 5].
1.Theory. In a simple theoretical formulation let us consider an open-air OCT system with a beamsplitter, reference and sample paths and a Gaussian-shape broadband source. Light coming from the source can be represented by its electric field wave component E(w, t) expressed as a complex exponential :
E (w, t) = A (w) [e.sup.-i(wt + kz)] (eq. 1)
[A(w).bar] is the source amplitude spectrum, [w.bar] the angular frequency and [k.bar] the wavenumber. The second term in the exponential accounts for phase which can be arbitrarily considered zero at input electric field. At the end (detector), phase appears due to different interactions throughout the interferometer both for reference and sample arms as follows, respectively:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (eq. 2)
n represents refraction index, [z.sub.R] and [z.sub.S] the path lengths in reference and sample arms and the factor of two arises from light double pass. Off course the sample is not a simple reflecting surface and has a layered refraction index structure which determines a reflectance, [r.bar], for each layer given by the Fresnel's equations. At the end we can consider that there is a sample response function R(w) given by 
R(w) = [[integral].sup.+[infinity].sub.-[infinity]](w, z)[e.sup.i2n(w,z)wz/c] dz (eq. 3)
[E.sub.R](w, t, [DELTA]z) = E(w, t)[e.sup.-i2an[DELTA]z/c] [E.sub.s](w, t) = E(w, t)R(w) (eq. 4)
where [DELTA]z represents the reference mirror translation distance.
After interference the resultant field into the detector is [E.sub.T]= [E.sub.R] + [E.sub.S] and optical detector are really sensitive to Irradiance, I, which, by the square law, can be written as I =<[E.sub.T][E.sup.*.sub.T] >. After proper calculations it can be shown that I = [I.sub.0] + 2Re<[E.sub.S][E.sup.*.sub.R] >, containing, the second term, the relevant cross-interference information .
3. Implementation. The high speed SS-OCT system developed is represented in fig. 1. It includes a commercially available swept source laser, AXP50125-3 1060nm (Axsun Technologies, USA) with a central wavelength of 1060 nm, bandwidth 110 nm and sweep frequency of 100 kHz; a fast I/O 400 MSPS acquisition board, X5-400M (Innovative Integration, USA) and an InGaAs balanced amplified photodetector, PDB145C (Thorlabs GmbH, Germany). In this balanced mode only cross-interference optical signal is detected rejecting the common-mode or [dc.bar] component and enhancing sensitivity. A fully customized software program has been developed. Data processing and OCT imaging are executed on the graphics processing unit (GPU) through NVIDIAS's Compute Unified Device Architecture (CUDA) technology. To perform B or C-scans over a sample, the laser beam is directed along one and two axes, respectively, by a Scanning Galvo System GVS002 (Thorlabs GmbH, Germany). With current hardware implementation, about 2048 points in sagittal plane (A-scan) are digitalized in 10 [micro]s and a maximum of 200 points in 2 ms are allowed in the transverse plane (B-scan). Consequently, 200 lines in the coronal plane (C-scan) can be obtained in 400 ms.
Balanced detector output signal show the interference fringes signal. Digitalization with equal k-spacing sampling can be achieved by clocking the high speed A/D channel of the acquisition board with the clock (so-called optical clock) provided by the source. The Fourier transform analysis can be directly applied on the acquired data.
Moreover, the laser source also provides a trigger signal which is connected to the SYNC port of the acquisition board. This signal is responsible for starting the I/O module. Fig. 2 represent program acquisition panel for a three layer sample.
The total axial with present optical layout was 500 [micro]m. It could be demonstrated that the position of the Fourier transform peak varies linearly with the distance of the sample, as expected, with 8 um resolution. Sensitivity around 55 dB has already been achieved but can be optically improved.
DISCUSSION AND CONCLUSION
OCT images of small animals retinal structure can give researchers the means to understand physiology, pathology and phenotypes of intact living systems similar to human beings and the instrument will be a valuable tool for research on retinal physiology. It will also be used as a development platform for new OCT instrumentation and methods. Currently, two-dimensional images in amplitude scale are generated. Noise removal, averaging and neighboring scans interpolation algorithms are being tested to improve contrast and image quality. The current developed program provides flexible control of the data acquisition and overall system with high automation. One parameter under consideration is the polarization control. Polarization sensitive evaluation of the interferograms can give additional information about sample birefringence enhancing image quality. Fiber polarization controller has been used with promising results (better sensitivity and SNR) . Improvement of the binomium depth of view--for at least one or two mm--and lateral resolution proceeds as well as image reconstruction algorithm speed.
This work was supported by PEST/C/SAU/UI3282/2013 and also by FEDER, through the Programa Operacional Factores de Competitividade - COMPETE and by National funds through FCT--Fundacao para a Ciencia e Tecnologia in the frame of the project PTDC-SAL-ENB-122128-210.
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Jose P. Domingues (1,2), Susana F. Silva (1), Antonio Miguel Morgado (1,2) and Rui Bernardes (1,3)
(1) IBILI--Institute for Biomedical Imaging and Life Sciences, University of Coimbra, Portugal
(2) Department of Physics, Faculty of Sciences and Technology, University of Coimbra Portugal
(3) Centre of New Technologies for Medicine, AIBILI, Coimbra, Portugal
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|Title Annotation:||optical coherence tomography|
|Author:||Domingues, Jose P.; Silva, Susana F.; Morgado, Antonio Miguel; Bernardes, Rui|
|Publication:||Journal of the Mississippi Academy of Sciences|
|Date:||Apr 1, 2014|
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