Treatment planning & targeting during shock wave therapy

Shock wave lithotripsy is a non-invasive procedure by which kidney stones are fragmented. Currently, many shock waves are delivered to the body that do not impact the stone, but do result in tissue trauma, mainly caused by cavitation. This work presents a monitoring system to reduce the number of shock waves delivered to the body, by tracking the target, and to detect cavitation throughout the procedure. For the first goal, a circular array, housing twenty-two 0.5 MHz transducers that can be mounted around the aperture of a clinical lithotripter, was used to collect pulse-echo data, from which an acoustic triangulation algorithm was able to locate stones. The accuracy of the system in water was better than 4 mm in the lateral direction and better than 2 mm in the axial direction within ±15 mm of the focus—which is the variation that would be expected clinically. For the second goal, the acoustic pressure field of a shock wave therapy source was measured and its cavitation field was determined. Cavitation was localised using a conventional ultrasound probe and passive acoustic mapping. Bubbles were shown to grow, collapse, and rebound, and the characteristic times were determined. It was found that these parameters increased with the energy level and the pulse repetition frequency and that at the highest energy setting the characteristic time was 288 μs. Finally, the characteristic time of cavitation in an ex vivo pig kidney and for an artificial stone in water due to a clinical shock wave source was determined using passive acoustic mapping. The characteristic times in the pig kidney were shorter than for the stone, e.g. 560 μs in the kidney and 730 μs on the stone at EL 6. No significant correlation between cavitation on a stone that fractured well and a stone that fractured poorly could be determined. The work in this thesis heralds an advance in current lithotripsy monitoring techniques that are based on B-mode and x-ray. Real-time tracking of the kidney stone and cavitation mapping throughout the treatment would limit the renal injury that routinely occurs during lithotripsy.