The problem of determining optimal measurement setups that improve the resolution of reconstructed images is well known in several inverse problem applications such as magnetic tomography, electrical impedance tomography, and electrical resistivity tomography. In electrical resistivity tomography, for example, several optimisation strategies have been proposed, their aim being to collect "optimal datasets", which when inverted , should give tomographic resolutions close to that obtained from “comprehensive datasets" comprising all possible linearly independent measurements. While these optimisation strategies are useful, there has been no conceptual framework developed for finding optimal measurement setups within the context of inverse problems. Electrical resistivity tomography is an ill-posed inverse problem, and components of the inversion process influence the nature of reconstructions obtained. Such a framework would incorporate within its optimisation process important components of the inversion process. This research describes the development of sl1ch a meta inverse framework. The framework incorporates within its optimisation process important ingredients of an inversion process such as choice of regularisation parameter, nature and size of data error, and a priori knowledge on solutions. As a first example, 'the framework is implemented for an acoustic source reconstruction linear problem, the aim being to find optimal acoustic receiver locations at which best reconstructions of the acoustic source strength can be obtained. To improve the quality of acoustic source strength reconstructions, a framework adaption algorithm is developed for use with the meta inverse framework The adaption algorithm uses successive measurements to improve the quality of reconstructions. Numerical results from implementing the meta inverse framework illustrate its success at finding optimal locations at which best reconstructions of the acoustic source strength can be obtained. The results also show that the framework adaption algorithm can be successfully implemented to improve the quality of reconstructions. To implement the meta inverse framework for electrical resistivity tomography, new forward and inverse solvers were developed. The forward solver is based on the finite integration technique (FIT), the inverse solver is called the domain search algorithm. The FIT solver generates the 'simulated data for an assumed resistivity model of the subsurface; the domain search inverse solver searches to find a resistivity model that gives an acceptable fit to the simulated data. Numerical results from implementing these solvers show that they are successful in simulating and reconstructing the resistivity distribution in electrical resistivity tomography. To find optimal electrode locations from which best reconstructions can be obtained for a 2D 'resistivity tomography problem, the meta inverse framework is incorporated into the forward and inverse solvers. Numerical results from implementing these algorithms show that the meta inverse framework is successful in finding optimal electrode locations at which best reconstructions can be obtained.

No description available.

No description available.

No description available.

No description available.

Fast Field-Cycling (FFC) MRI allows the magnetic field to be switched during an imaging scan. FFC-MRI can be used to exploit a characteristic of contrast agents, i.e. the variation of its spin-lattice relaxation time (T1) or rate (R1= 1/T1) with magnetic field in order to increase contrast. Contrast agents play an essential role in MRI, allowing improved diagnosis and delineation of diseased tissue. However, the R1, and hence the effectiveness of contrast agents, varies significantly with magnetic field. Thus, Fast Field-Cycling (FFC) MRI can be used to take advantage of this variation to improve image contrast, allowing more sensitive detection of the agent. In this project new contrast agents, developed by a collaborating group (Invento S.r.l., Italy) were investigated for use with FFC-MRI. R1 dispersion curves of samples containing a range of contrast agents were first obtained using both a commercial relaxometer and a home-built whole-body FFC-MRI system, and the accuracy of the home-built FFC-MRI system was verified. The magnetisation behaviour of these samples during field-cycling pulse sequences was modelled in order to predict the pulse sequence parameters necessary for maximum T1 contrast. Images were obtained, using a number of novel imaging techniques developed on the home-built whole-body FFC-MRI system, and also, using standard T1 weighted imaging on a 3 T Philips clinical MRI scanner. A new FFC-MRI imaging method, ΔR1 mapping was employed to show an increase in contrast using a novel Mn2+ based liposomal contrast agent compared with T1 weighted images at 5 mT, 59 mT and 3 T. The low concentrations of Mn2+ based liposomal contrast agents used with ΔR1 mapping indicate suitability for molecular imaging

This thesis describes the search for a practical skin dosimetry method for cardiac catheterization procedures, and the application of an optimisation strategy in barium enema imaging. Kodak EDR2 film was characterised across the range of exposure conditions used in the cardiac catheterization laboratory. Its dose-response curve was modelled using a novel equation, and overall uncertainty in film response was estimated. The film saturated at 1 Gy, limiting its usefulness for skin dosimetry. Its performance was found to be strongly dependent on beam filtration, an aspect that had not previously been studied. The film was then used to measure skin doses to patients undergoing coronary angiograms and angioplasties. For angiograms, all skin doses were well below 1 Gy. For angioplasties, 23% of films showed localised saturation, indicating peak skin doses of at least 1 Gy. Dose-area-product was shown to be a poor predictor of high peak skin dose. A mathematical model was developed and software written, to calculate patient skin dose maps from exposure and projection data stored in the image files. This offered a practical method for assessing the magnitude and approximate location of the peak skin dose. Accuracy was limited by a lack of information regarding fluoroscopic exposures, couch position and beam limitation. After including an estimated contribution from fluoroscopy, the model successfully identified those patients whose skin doses exceeded 1 Gy. Following a baseline survey of local barium enema practice, several dose reduction methods were considered. It was decided to introduce copper filtration. 0.1 mm copper reduced mean patient DAP by 37%, without any measurable difference in contrast detail detectability. A detailed phantom study determined the optimal copper thickness as 0.3 mm. This reduced mean patient DAP by 55%, relative to the baseline survey. A visual grading analysis study showed no significant difference in clinical image quality.

Cone-beam computed tomography (CBCT) is an imaging technique used in conjunction with radiation therapy. CBCT is used to verify the position of tumours just prior to radiation treatment session. The accuracy of the radiation treatment of thoracic and upper abdominal tumours is heavily affected by respiratory movement. Blurring artefacts, due to the movement during a CBCT scanning, cause misregistration between the CBCT image and the planning image. There has been growing interest in the use of motion-compensated CBCT for correcting the breathing-induced artefacts. A wide range of iterative reconstruction methods have been developed for CBCT imaging. The direct motion compensation technique has been applied to algebraic reconstruction technique (ART), simultaneous ART (SART), ordered-subset SART (OS-SART) and conjugate gradient least squares (CGLS). In this thesis a dual modality imaging of electrical impedance tomography (EIT) and CBCT is proposed for the first time. This novel dual modality imaging uses the advantages of high temporal resolution of EIT imaging and high spatial resolution of the CBCT method. The main objective of this study is to combine CBCT with EIT imaging system for motion-compensated CBCT using experimental and computational phantoms. The EIT images were used for extracting motion for a motion-compensated CBCT imaging system. A simple motion extraction technique is used for extracting motion data from the low spatial resolution EIT images. This motion data is suitable for input into the direct motion-compensated CBCT. The performance of iterative algorithms for motion compensation was also studied. The dual modality CBCT-EIT is verified using experimental EIT system and computational CBCT phantom data.

Thesis (MPH.) -- University of Limpopo, 2012