A practical system for the application of ultrasound computed tomography to medical imaging

This work outlines the progress we have made towards the realisation of practical systems for medical reflection and transmission ultrasound computed tomography (UCT) imaging. Our system for generating reflection-mode UCT images uses a conventional pulse-echo scanner for data acquisition, but shows considerable improvement in resolution and contrast compared to conventional equipment. This is demonstrated by examples of conventional pulse-echo and reflection UCT images of phantoms and in-vitro tissues. However, we have found that the image quality obtainable from reflection UCT is significantly limited when there are large acoustic speed variations (ie >5%) within the target, a situation known to occur within the breast. We have also built a practical transmission-mode UCT system which again utilises standard pulse-echo equipment. This system has been shown to be capable of generating accurate quantitative images of the acoustic speed distributions of phantoms. Unfortunately, acoustic speed images of real (in-vitro) targets show significant geometric distortion which limits the practical application of transmission UCT for direct diagnosis by itself. However we have shown that even these distorted acoustic speed images can be used to provide worthwhile compensation for the effects of varying acoustic speed on reflection UCT images obtained from the same target. The combination of reflection and transmission UCT to provide images with acoustic speed compensation shows considerable improvements in resolution, as demonstrated by examples of phantoms and in-vitro tissues