Sound bullets for enhanced biomedical ultrasound systems

Ultrasound is used in many applications, including medical imaging, non-destructive evaluation, therapeutic ultrasound etc. In all these cases, there is usually a need for the formation of images or the creation of a focal region. Current methods for the generation of optimal acoustic fields generally rely on a linear process within the transducer. This linear transduction process influences the resultant properties in terms of spatial resolution and maximum intensity, noting that there are fundamental limits on the spatial resolution and power densities that can be achieved in such focal regions. In recent work in the area of acoustics, it has been demonstrated that a new type of acoustic signal can be generated via non-linear effects in chains of particles, which act as a kind of waveguide. These are based on the propagation of solitary waves. These have been studies at low frequencies, but this study will look at the posibility of using these new structures for use in biomedical ultrasound. Materials that support solitary waves are not used in standard ultrasonic work; little has been published on their use, despite the fact that a step change in performance may be possible. In this proposal, such waves will be generated within ultrasonic sources containing multiple solitary wave chains, at frequencies in the 500 kHz - 5 MHz range. To our knowledge, this has not been investigated before. Arrays are also possible, where each chain forms a single element. Because the chains would be primarily coupled along their length, but not laterally between each chains, issues arising from mechanical cross-coupling might be avoided. Pre-compression of each chain would alter the propagation velocity within it, so that beam-steering/focussing to be created. The propagation charaistics also change with signal amplitude, leading to the possibility of an acoustic diode. These new innovations would have applications in such areas as ultrasound-enhanced drug delivery, High Intensity Focussed Ultrasound (HIFU) for the treatment of tumours, and harmonic imaging.

Impact to cancer treatment: Prostate cancer is the most commonly diagnosed cancer in men and has limited treatment options due to its location within the body. In addition, upper abdominal cancer in the UK, is associated with more than 50% mortality. For up to 90% of them, surgery is not an option, whilst chemotherapy offers poor outcomes. There is a clear need for research into alternative therapy options. The present proposal aims to tackle this important area using new approaches to ultrasound, in both imaging and therapeutic modes.

Impact through areas of application: (i) The use of high intensity focussed ultrasound (HIFU) is an attractive option for tumour destruction. HIFU is a minimally invasive surgical procedure that is used to treat cancers in the prostate, liver, and kidneys. The proposed research has the potential to deliver the next generation of HIFU therapy devices. (ii) Enhanced drug delivery is another important area of treatment for cancerous tumours, which will also be tackled in our work. This approach uses ultrasound to locate and rupture microbubbles that can deliver drugs to the site of interest, rather than to the whole body. This obviously has a high level of potential impact. The creation of more localised and intense ultrasonic focal regions is the goal. (iii) Harmonic imaging: Solitary-wave materials are likely to form acoustic diodes. This property, if confirmed, will be a very exciting breakthrough. The use of chain-like structures, which would naturally be frequency-selective as a function of ultrasonic amplitude, could lead to real advances in harmonic imaging for the medical diagnosis of tumours.

Impact through alignment to EPSRC priorities: The proposed research aligns closely with the Healthcare Technologies Theme, as identified within the research Portfolio at EPSRC. Three priority areas are addressed by this research: (i) Diagnostics/medical imaging - Sound bullets have the promise of improved image resolution performance in diagnostic ultrasound imaging. (ii) Techniques for biomedical understanding/non-linear systems - Frequency-selective acoustic diode structures are possible, of great interest in non-linear harmonic imaging. (iii) Therapeutic technologies: There are applications to both enhanced drug delivery within the human body, and to High Intensity Focused Ultrasound (HIFU) for the treatment of tumours.

Other forms of Impact: These will arrive through explotitation, links to industry and publication/dissemination via journal papers and conference presentations.