Cavitational methods for characterising and testing clinical high intensity focused ultrasound systems

High Intensity Focused Ultrasound (HIFU) is rapidly emerging as a viable alternative to more invasive methods for routine treatment of malignant tissue. Key to the success of HIFU therapy is the development of standardised Quality Assessment (QA) procedures, designed to regularly assess the safety and efficacy of clinical HIFU transducers. Current HIFU QA procedures offer either accurate spatial calibration at the expense of impractically long acquisition times, or more rapid characterisation that provides only a spatially averaged measure of performance. Exploitation of acoustically induced inertial cavitation, however, offers the potential for rapid, pre-treatment spatial calibration without the need for time consuming scanning routines. In this thesis, the concept of cavitation-based HIFU QA is validated for the first time, with various techniques developed to enable its implementation. To begin with, two-dimensional mapping using an array of needle hydrophones establishes that a growing cavitation region may be reconstructed at increasing insonation amplitudes. Extending the study to include a diagnostic linear array provides improved spatial resolution, enabling accurate, two-dimensional field characterisation within a novel, microparticle-seeded test medium that is shown to maximise the magnitude and spatial uniformity of cavitation occurrences. Performing this procedure in multiple focal planes provides a practical means of characterising a clinical HIFU transducer using readily available hardware. An alternative array geometry is proposed for rapid, three-dimensional field characterisation. To allow accurate placement of a large number of elements within such an array, a novel, multiple-layer Printed Circuit Board (PCB) construction method is adopted. A model is developed to predict the effect of the multiple acoustic layers of the array on its receive sensitivity spectrum and the results contrasted with both a finite element solution and experimental data to inform the eventual array design. An ideal passive array is thence specified through a combined experimental and computational approach, taking care to optimise the element size, number and distribution. This array configuration has the potential to enable reconstruction of a wide array of sources. The second generation prototype fulfils most of the requirements but has highlighted issues with crosstalk. Nevertheless, the results presented thus far provide a strong foundation for future work into cavitation- based HIFU QA.EThOS - Electronic Theses Online ServiceGBUnited Kingdo