Experimental and theoretical studies of ultrasound computed tomography
The poor lateral resolution and qualitative nature of conventional real-time B-scanning has prompted interest in techniques in which a wide aperture is synthesised by measuring ultrasound over 180 or 360 degrees. This thesis investigates one group of such techniques known as Ultrasound Computed Tomography (UCT). After a brief introduction to conventional ultrasound imaging and the principles of Computed Tomography, a comprehensive review of the literature on UCT is presented. Consideration is made of the assumptions and limitations inherent in these techniques and the consequences for their practical application. It is concluded that combined reflection and transmission UCT offers the best promise for the realisation of high quality images of soft tissue. The potential resolution of conventional B-scanning is compared, theoretically, with that of UCT by consideration of point spread functions derived from linear imaging models. The assumptions required for the derivation of these models, and the consequences for other aspects of image quality of their failure, are explored in detail. The design and construction is described of a prototype system to perform reflection UCT using a commercial linear array rotated in a water bath. Results are presented of studies on phantoms and on soft tissues. The limitations of this system are analyzed, and ways of improving the images, with consequences for the development of another system, discussed. The use of acoustic speed images, obtained from transmission UCT, to compensate for artifacts in the equivalent reflection UCT images is explored theoretically. The design and construction of a system for combined reflection and transmission UCT imaging, again based on linear arrays, is described. Results are presented of acoustic speed images, and of acoustic speed compensation of reflection images, of phantoms and in-vitro targets. Using conclusions drawn from the two systems already investigated, suggestions are made for the design of an optimum clinically acceptable system for UCT imaging of suitably accessible soft tissues such as the breast.