Page 27 - Spring 2006

Looking to the future
Considerable progress is being made in understanding the mecha- nisms of shock wave action, and in finding ways to improve how lithotrip- sy is performed. Still, it is clear that the problem of adverse effects has not been solved, and ironic that the first lithotripter design is still the “gold stan- dard.” Several areas of investigation promise to yield further advances, and refinement of protocols is ongoing in order to reduce the dose of shock waves needed to break stones, to improve coupling between the shock source and the patient, to explore the use of dual pulses to enhance stone breakage (and perhaps cancel out the adverse effects of cavitation on tissue), and to improve on imaging, targeting and real-time monitoring of stone breakage and tis- sue injury. These challenges leave open many avenues for future basic and translational research. Here we high- light several areas of active research in the field: computational modeling to optimize lithotripter design, image guidance to improve targeting and real- time monitoring, and the use of shock waves for orthopedic therapy.
SWL stands to gain from numeri- cal modeling and simulation: Numerical modeling and simulation can help explain mechanisms of shock wave action, investigate parameter spaces, and predict outcomes. Shock- capturing numerical simulations of the Euler equations, for example, have led to new insights into the mechanisms by which tensile pressures are generated in
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the focal region. Combined with
models for the nucleation and dynam- ics of bubble clouds, these Euler simu- lations have also been used to confirm the “bubble shielding” hypothesis that is the basis of the rate effect discussed previously (Fig. 10). Models are also being developed to calculate the forces
Fig. 12. Computed interaction of a shock wave with a cylindrical air bubble in water. Time progresses from left to right as a Mach 1.02 shock wave propagates from right to left. The shock initiates bubble collapse and a reentrant jet forms. When the reentrant jet collides with the distal side of the bubble, a secondary shock wave is formed.
generated from the collapse of a bubble cloud and to track the growth of cracks in model stones. A key benefit of simu- lations (especially when used in con- junction with experiments and clinical trials) is that it can be easier to isolate and control different physical effects (e.g. number of cavitation nuclei). An ultimate goal of modeling and simula- tion is in the design of more effective (and less injurious) lithotripters. For example, dual-head and piezoelectric- array based devices have more degrees of freedom than classical lithotripters; simulations provide a means by which different designs can be rapidly assessed. Simulations may determine optimal shock wave shape and delivery rate to maximize the force of impact of the bubble cloud (Fig. 11)44 and indi- vidual bubbles (Fig. 12) to create the highest stresses within the stone. One might speculate on ways in which mature numerical tools may be inte- grated with imaging feedback to aid in treatment planning, perhaps even in real-time. For example, based on initial images of the stone, the beam width could be calculated and set. Then based on feedback that the stone has broken, a new beam width might be selected.
Imaging feedback could improve treatment: Imaging feedback coupled with the knowledge of how to adapt treatment has great potential. Researchers are currently probing x-ray computerized tomography (CT)
Fig. 13. B-mode ultrasound image of a pig kidney
before (top) and during (bottom) SWL. The fluid col-
lecting system in the center of the kidney lights up on
the image with what appear to be bubbles.
Transmission loss through the bubble cloud may
effect treatment and the image provides the urologist
feedback on the cloud. (Reprinted with permission
from World Federation of Ultrasound in Medicine
13 and Biology. )
images used to diagnose stones for
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information on stone fragility. Stones with little chance of breaking might be screened from SWL treat- ment. Such images could also be input to one of the models described above to
Fig. 11. Comparison of measured26 and simulated bubble cluster collapse44 at the distal face of a cylindrical model stone. The model can capture the complex cloud behav- ior around a stone, and potentially be used to test parameters, for example to maximize the pressure generated in the stone at collapse. Reprinted with permission from Journal of Endourology and IEEE. (© IEEE)
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