Characterising the effect of a novel optical delay line and light sources on the resolvability of time domain optical coherence tomography
Date of Award
Doctor of Philosophy
School of Engineering
Faculty of Health, Engineering and Science
Associate Professor Steven Hinckley
Dr Graham Wild
Optical coherence tomography (OCT) is a reflection based imaging technique, with numerous applications, initially in medical diagnostics. Unlike ultrasound, another reflection imaging technique, OCT relies on light scattering in samples, low coherence near- infrared interferometry, and graphical image construction, to acquire 2D and 3D reflectogram images. OCT has resolution an order of magnitude greater than ultrasound, but its depth penetration is less than a few millimetres.
Axial resolution is a critical parameter in determining whether OCT can be used to resolve specific features in a sample image. Typically, measures of resolution have been attributed only to the light source’s characteristics, including its coherence length and the FWHM of its frequency spectrum’s inverse FT, or auto-correlogram. The need to cost effectively visualize the OCT-system-generated auto-correlograms, and OCT cross-correlograms (A-scan) (produced using many different OCT light sources), has necessitated the extrinsic-evolution of an OCT simulation model, presented in this thesis. This research indicated that empirical resolution in vivo is also strongly dependent on the optical characteristics of the tissue, including surface reflection. When the surface reflection is removed from the A-scan, the minimum stratum depth that can be resolved for the sub-surface strata, is significantly enhanced. Furthermore, this subtraction enhances the stratum depth resolution, so that it approaches more closely the light source’s resolution limit, compared to A-scans without the subtraction.
The time domain OCT’s optical delay line (ODL) and light source components were also reviewed to determine their affordability and functionality for engineering a portable, high resolution, but simple OCT modality. To this end, a stationary ODL, using a transmissive optical light valve array and a stepped mirrored structure (SMS), was characterised in ‘proof of principle’ experiments. Unfortunately, fabrication of the SMS, using four affordable techniques, proved unsuccessful; however, more promising techniques, based on the theoretical developments in this thesis, are envisaged. A stochastic pseudo-genetic algorithm (GAM), similar, but not exactly analogous to theoretical genetic intrinsic-evolution, was characterised and used to backwards-fit the solution set of strata depths and reflectivities of a selected OCT A-scan. Unfortunately, its speed in Matlab was not timely enough for e-medical application – though in c this GAM could run faster. This slowness is due to the GAM’s stochastic nature, prompting future investigation of other GAMs, which will eliminate the stochastic element so that more timely results can be achieved.
LCSH Subject Headings
Optical coherence tomography.
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Jansz, P. V. (2015). Characterising the effect of a novel optical delay line and light sources on the resolvability of time domain optical coherence tomography. https://ro.ecu.edu.au/theses/1755