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UBC Theses and Dissertations

Spectral domain optical coherence tomography system design : sensitivity fall-off and processing speed enhancement Chan, Kenny K. H.


Spectral domain optical coherence tomography (SD-OCT) is an imaging modality that provides cross-sectional images with micrometer resolution. One major drawback of SD-OCT, however, is the depth dependent sensitivity fall-off by which image quality rapidly degrades in regions corresponding to deeper locations of the sample. This disadvantage is due to the finite spectral resolution of the hardware as well as the software reconstruction method that is used. SD-OCT employs a broadband light source for illumination and a spectrometer for signal detection. This system uses diffraction grating to separate spectral components by wavelengths (λ), which are then detected by a CCD array. The sensitivity fall-off is dependent on the spot size shining on the CCD, with respect to the pixel size of the CCD array. This hardware contribution to the fall-off can be minimized by careful design of the spectrometer. The software reconstruction is based mainly on the discrete Fourier transform (DFT) of the measured spectral data, which can be performed quickly using the widely accepted fast Fourier transform (FFT) algorithm, provided that the input is sampled uniformly in the wavenumber (k) domain. Due to the inverse relationship between k and λ, the data must be resampled to achieve a uniform spacing in k. Accuracy of the resampling method is important for the reconstruction, since the performance of the interpolation algorithm tends to degrade as the signal approaches the Nyquist sampling rate. This also causes a sensitivity fall-off for signals originating at greater depths, which corresponds to a higher modulation fringe frequency in the k domain. The goal of this thesis is to outline the development of a real-time SD-OCT imaging system that can deliver high quality images. The aim is to solve two major problems of current state-of-the-art SD-OCT systems, namely the depth dependent sensitivity fall-off and the image reconstruction time limitation. An SD-OCT system is demonstrated using a new reconstruction approach based on non-uniform fast Fourier transform (NUFFT). Using parallel computing techniques, our system can produce high quality images at over 100 frames per second with less than 12.5dB sensitivity fall-off over the full imaging range of 1.7mm.

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