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Studies in X-ray dynamic speckle imaging
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
posted on 08.02.2017by Irvine, Sarah Catherine
This thesis describes the study of two dynamic speckle systems present in the context of X-ray propagation-based phase contrast imaging. The first application to be examined is that of the rotating random phase screen diffuser, used in phase contrast imaging in order to improve the homogeneity of the sample-incident beam and thus ideally the image quality. The second pertains to the technique of flow measurement from analysis of blood speckle patterns. Whilst speckle is central to both of these, it is in quite different ways: a moving diffuser introduces a temporally
uncorrelated dynamic speckle pattern, the observed effect of which is an incoherently averaged smooth background intensity. Conversely, the time-dependent correlation of dynamic blood speckle patterns is deliberately exploited as a useful tool for the quantitative determination of flow.
In the first instance we present the spatial coherence measurements of the biomedical imaging beamline BL20XU at SPring8, Japan, using a simple, low-cost prism based interferometry method developed by Suzuki (2004). We then study the effect of the addition of a rotating diffuser to the setup. Besides the observed increase in beam-homogeneity due to the diffuser, we show that the diffuser acts to decrease the observed degree of coherence as a fixed percentage of that measured in its absence. We are able to reproduce this effect in simulation via the model of the diffuser as the time-averaged incoherent sum of speckled intensities caused by random phase
perturbations to the X-ray wavefield. Following this key result, we extend our investigation
of diffusers to study their effect on the quality of typical propagation-based phase contrast images, which depend on the high contrast and visibility of Fresnel diffraction fringe features. The deleterious effect of the diffuser on these fringes is demonstrated to be minimisable through the simple expedient of placing the diffuser as close as possible to the effective source of the beamline. This is explained through a discussion of local transverse phase gradients.
In the second application which comprises the majority of this thesis, we begin with a characterising study of the speckle patterns observed in propagation-based X-ray phase contrast images of blood, a phenomenon insufficiently explained in the literature of the time. We adopt a theoretical model which allows us to view blood as a two-material system of randomly positioned red blood cells suspended within a plasma matrix, the projection of which effectively creates a weak random phase perturbation of the X-ray wavefield. Free-space propagation and associated self-interference of this wavefield yields phase contrast in the form of speckle. Its statistical analysis is demonstrated possible through inspection of the magnitude of the Fourier transform of the intensity. The Fourier description of the speckle is well characterised via application of the linear phase Contrast Transfer Function (CTF) formalism for weak objects in the Fresnel regime. The agreement between experiment, simulation and analytical expression is strong, providing an improved understanding of the speckle which may be applied to the velocimetric analysis of dynamic patterns.
In view of the ultimate goal of achieving a high-resolution, accurate blood flow measurement technique capable of full-field multi-component vector analysis for in vivo cardiovascular research, we apply our knowledge of the speckle system to recommend key steps for future analysis of dynamic speckle patterns. We find it necessary to use single-image phase retrieval methods, specifically one based on the transport of intensity equation. For images recorded with typical levels of noise, the TIE-based phase retrieval algorithm requires significant regularisation due to the very low contribution of absorption to the final contrast. We also demonstrate the utility of Fourier mask filters for suppression of unwanted image artefacts. With the flow signal thus optimised, we develop methods for flow reconstruction based on the Abel transform. For a rotationally-symmetric flow, we present the first tomographic reconstructions of axial blood flow within a cylinder; after which we extend the validity of the reconstruction theory to include non-axial flow, culminating in the full, four-dimensional reconstruction of pulsatile flow within a simple in vitro stenotic model. Importantly, this new vector tomographic reconstruction technique may be achieved with single image pairs, which is important for non-steady flow patterns.