Copy of Development of methods for characterising and testing clinical High Intensity Focused Ultrasound (HIFU) systems

Lead Research Organisation: University of Oxford
Department Name: Engineering Science


The clinical application of high intensity focused ultrasound (HIFU) to the treatment of soft tissue cancers of, for example, the liver, kidney and prostate, is a young and rapidly expanding field. To date more than 30,000 patients have been treated world wide. Successful treatment is achieved when the temperature of the tumour is raised to levels at which instantaneous cell death occurs. The focused beam ensures that only the tissue being targeted is heated, whilst surrounding tissue remains unharmed. Safe and effective use of HIFU requires that validated methods for measurement and testing of clinical devices should be made available as soon as possible. These issues have not been addressed to date in any systematic fashion. Clinical HIFU systems are currently assessed on an ad hoc basis by individual clinical departments and manufacturers, using methods, many of which are unpublished. There is, therefore, an urgent need to produce standard registration and testing equipment and methodology that allows users to characterize clinical HIFU systems for the purposes of checking safety and reproducibility of a machine's output, comparing different devices or commissioning new systems. The programme of work proposed is a mixture of adaptation and extension of existing and emerging techniques to meet the requirements of this new medical technology, and the development of novel methods specifically for this application.The overall aim of this project is to improve the efficacy, safety and range of applicability of clinical HIFU treatments by:A. providing validated methods for: * ultrasonic field characterisation using pressure field mapping and acoustic power measurement techniques; * HIFU system performance testing and quality assurance using novel thermal and cavitation mapping methods * patient exposure monitoring by means of electrical impedance measurements and real time acoustic power measurement;B. establishing a world leading HIFU characterisation facility at the Institute of Cancer Research (ICR);C. disseminating the successful methods, protocols and equipment to a wider user base through: * scientific publication; * contribution to written National and International Standards; * commercial exploitation.
Description High Intensity Focused Ultrasound (HIFU) is rapidly emerging as a viable non-invasive alternative to surgery for the treatment of deep-seated tumours. 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 (the creation and excitation of acousticallly driven bubbles under the effect of the ultrasound field), however, offers the potential for rapid, pre- treatment spatial calibration without the need for time consuming scanning routines. In the present project, the concept of cavitation-based HIFU QA was validated for the first time, with various techniques developed to enable its implementation. A novel cavitation test object made of low-cost, non-toxic materials (agar and talc) was first manufactured and shown to enable highly reproducible cavitation activity over a wide range of HIFU exposure conditions. An array of needle hydrophones was first used to produce two-dimensional maps of cavitation activity and to demonstrate that a growing cavitation region is discernible at increasing insonation amplitudes. Extending the study to include a diagnostic linear array provided improved spatial resolution, enabling accurate, two-dimensional field characterisation within the novel caviation test object. Performing this procedure in multiple focal planes was shown to provide a practical means of characterising a clinical HIFU transducer using hardware that is readily available in the clinic. An alternative array geometry was also investigated 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 was adopted. A model was developed to predict the effect of the multiple acoustic layers of the array on its receive sensitivity spectrum and the results were compared to both a finite element solution and experimental data to inform the eventual array design. An ideal passive array for cavitation-based calibration of HIFU transducers was 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 cavitation sources and could thus enable rapid three-dimensional visualization of the field produced by clinical HIFU transducers. The novel sensor and associated passive caviation mapping reconstruction methods, in combination with the cavitation test object, were found to satisfy most of the original project objectives for rapid HIFU field visualization and calibration in a clinical environment. Further improvements are needed, particularly in terms of minimizing cross-talk across the multiple receiving elements of the two-dimensional array of cavitation detectors. Nevertheless, the results achieved provide a strong foundation for future work into cavitation-based HIFU QA.
Exploitation Route See Exploitation Routes Passive cavitation mapping is being offered as a QA technique to clinical HIFU manufacturers and clinical HIFU unit around the world. The technique is also being investigated further by the National Physical Laboratory, who is currently marketing a single-element cavitation sensor and is considering the development of a multi-element cavitation sensor building on the expertise developed as part of this project.
Sectors Healthcare