| Summary: | High Intensity Focused Ultrasound (HIFU) is rapidly emerging as a non-invasive treatment for soft tissue tumours. HIFU is used to generate small regions of high intensity sound at the focus of geometrically or electronically focused transducers. These intensities are used to raise the tissue temperature rapidly to form a region of coagulative necrosis, known as a lesion. Clinical use of HIFU has shown that the technique is safe and is capable of ablating tumours. However, the technique is still in its infancy and, consequently, appropriate methods of treatment delivery, planning and monitoring are under development. In this thesis, ways ofimproving clinical treatment delivery are investigated. During HIFU treatments of the liver, it is desirable to avoid rib exposure. The high ultrasound absorption in bone means that it can heat up, leading to damage of the periosteum and overlying skin. A solution to this problem has been identified as the use a linearly segmented transducer with 10 independent elements. The acoustic field from a number of active drive element configurations of the transducer was investigated with theoretical modelling. In corresponding experiments, the acoustic field and the temperature rise on a rib phantom were measured. A novel method of visualising the heating pattern i~ the beam, namely using a temperature sensitive thermo-chromic liquid crystal, was investigated. The ability to shape the HIFU beam to spare the ribs was demonstrated, allowing the sound axis to be directed approximately a centimetre closer to the rib edge. The use of a diagnostic ultrasound scanner for measurements of ultrasound attenuation of tissue (BAE) and for thermometry (BTl) in HIFU using ultrasound backscatter is investigated. Methods of data acquisition and processing for both techniques are described together with their limitations. These techniques were used to determine the correct HIFU drive power in a lesioning experiment aiming to improve reproducibility of single lesion formation in an ex-vivo liver tissue model. Some evidence that BAE and BTl can be used as tools to improve the reproducibility of single lesion formation has been obtainedwith a reduction in the variance oflesion size and shape. In conclusion, a number of potential ways of improving HIFU treatment delivery have been demonstrated. The techniques for rib sparing, BAE and BTl are ready to be tested in a situation more representative of the clinical scenario, where multiple tissue layers, backscatter inhomogeneity and tissue motion are likely to pose more significant limits on their application.
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