The use of ultrasound to create tissue hyperthermia to support the treatment of cancer

Abstract

The value of mild hyperthermia in improving the outcome of radiotherapy and chemotherapy treatments is well established. However, clinical applications are currently restricted to accessible tumours, with the application of controlled hyperthermia in solid tumours deep within the body presenting an unresolved problem. Ultrasound is an attractive heating technique because of its ability to create a focus at depth which can be steered around the tumour volume. However, despite considerable research no clinically usable transducers for deep tumour applications have resulted. In this thesis the underlying principles that govern the characteristics of phased array transducers have been examined. The concept of an idealised phased array has been introduced, and analysis of simulated fields from such arrays has enabled a new set of equations to be defined which relate the geometry of the field to the fundamental array design parameters (including the array diameter, radius of curvature and frequency of operation). Further simulations have examined the impact of secondary array design parameters (such as the individual element size, number density and layout geometry) which modify the field from that of the idealised case. Analysis of these has enabled an upper limit to be placed on the element size within any planar array in order to prevent undesirable changes in the characteristics of the focal region. A fifteen element phased array with a random element distribution has been constructed based on the design principles established in the simulation work. Measurements of the inter-element cross-coupling have been made, demonstrating that acoustic coupling dominated for inter-element pitches of less than 8 mm, while electrical coupling dominated at larger inter-element pitches. The field produced by the array in an acoustic tank has been characterised and compared against simulation predictions, showing good agreement in terms of the geometries of the focal region and the grating lobes. However, a number of differences have also been identified. In particular, the focal region was closer to the surface of the physical transducer in the measured fields compared to the simulation results, and there were numerous small high intensity regions between the surface of the transducer and the focus which were absent from the simulated fields. A sensitivity analysis, using a simulated factorial experiment, has been performed to identify the origin of these differences, with the results indicating that the presence of a secondary vibrational mode within the elements of the array was the principal causative factor. Finally, calculations have been performed which demonstrate the feasibility of manufacturing an array suitable for the application of mild hyperthermia in deep tumours based on the array design scheme presented in this thesis. Potential extensions of the array design have also been described which would improve the behaviour of the array under steering and provide further increase in the focal intensity