Project Goal: To investigate the feasibility of developing a safe, fast and effective non-invasive treatment for stones in the human body using ultrasonically generated cavitation bubble clouds.

Relevance of the Research: Kidney stones occur in approximately 5% of the UK’s population and are a particularly unpleasant condition, causing severe pain, bleeding and blockage of the urethra.  Stones up to 5mm in diameter have a 90% chance of passing spontaneously through urination. Stones larger than 5mm, or smaller ones that cause obstruction or infection must be urgently treated, which may require invasive surgery. As an alternative to surgery, a non-invasive approach called extracorporeal shockwave lithotripsy (ESWL) has been used since 1980.  ESWL shatters the stone using multiple shock waves from a special transducer.  The resulting sharp fragments of stone can cause pain or be too big to pass out of the body, leading to a need for surgical intervention. Furthermore the shock waves often damage surrounding tissues.  Cavitation bubbles have been a major problem in hydraulic machinery and ships’ propellers for many years as they can erode solid metal into powder very fast if measures are not taken to prevent them.  The power of cavitation is potentially the basis of a much improved non-invasive procedure for the treatment of stones in the human body.

The Approach: To avoid potential ethical issues with the use of human kidney stones, the project team decided to initially use chalk for the practical tests and subsequently move onto synthetic human stone material. The work was focussed on bridging the gap between the novel theoretical concept and future clinical trials on animals by studying “stone crushing” in the laboratory.  Initial interest centred on generating the most erosive bubble cloud at the point where the stone is to be eroded.  The specific objectives were:

Research Outputs:

A high-precision (up to 40 nm resolution) computerised 3-D test rig was constructed.  The acoustic intensity at the focal point where the stone was eroded was measured using an extra-fine needle hydrophone (0.075 mm diameter). The measured field was suitably focused for generating the required erosive cavitation bubble cloud.  The system had a high accuracy (better than 1mm), minimising potential damage to surrounding tissues.

Computer modelling of the system revealed that modulating the HIFU frequency accelerates the growth of the cavitation bubbles and their subsequent collapse when they release the erosion energy.  In simple terms the HIFU transducer frequency follows the resonant frequency of the cavitation bubbles   Compared with a constant HIFU frequency, the bubble growth rate is significantly increased.

A realistic equipment-patient scenario was simulated using “Tissue Mimicking Materials” (TMM) to represent the various layers of the human body between the skin and the kidney stones – see Figure 3.   In total 4 layers were simulated (skin, fat, muscle and kidney) using realistic acoustic impedances. The approach brought  together both numerical modelling techniques (using PZFlex) and experimental studies to optimise the system’s parameters to maximise the stone erosion rate without causing lasting damage to the tissues between the transducer and the target stone.  Figure 4 shows the experimental TMM test piece used in practical tests.

A novel model based on “front tracking” methods for simulating the dynamics of bubble clouds with a high void fraction, and strong bubble-bubble interaction has been developed.  “Virtual Grid-Based” (VGB) front tracking accounts for bubble coalescence and splitting during DNS (Direct Numerical Simulation) and provides much higher accuracy.

Additional studies have extended the simulations into the higher frequency regions (MHz frequencies) employed by the HIFU transducers and examined the thermal effects of bubble oscillations in liquids.

Publications: The project team has published seven conference papers and six refereed journal papers to date and further publications are in preparation.  The work also featured in February 2010 in “The Engineer” magazine - http://www.theengineer.co.uk/news/news-analysis/marine-medicine/1001077.article

Notable Impacts:

This project has generated sufficient information to prove the feasibility of the concept.

The adventurous nature of the project has been instrumental in attracting world experts in cavitation to attend the WIMRC International Cavitation Forum which to date has been held in July 2006, July 2008 and July 2011.  This is now an established event and will be a lasting legacy of this particular WIMRC funded research project.

Next Steps / Technology Transfer: To partner with a medical equipment manufacturer to create a working prototype, to enable clinical trials on animals to be performed prior to seeking ethical approval for trials on human patients.

Interested?  For further information and to discuss possible future collaboration please contact Professor Shengcai Li, School of Engineering, University of Warwick, CV4 7AL.

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