Page 26 - Spring 2006

Fig. 10. Numerical simulations34 were used to predict shock focusing and bubble cloud dynamics in an electrohydraulic (Dornier HM3) lithotripter. The simulations revealed that at high pulse rate frequency (PRF) and bubble numbers (N, bubbles/cm3), there is a strong shielding effect–a decrease in peak pressures obtained at the focus (left)—and an associated decrease of energy released by collapsing cavitation bubbles (right). Bars indicate an estimate of the uncertainty in the numerical results.
short delay. In the other machine, the pulses may be triggered
simultaneously or alternated at rates up to 4 Hz. There are
many potential advantages of a dual pulse lithotripter, but we
contend that the most persuasive relates to the rate effect dis-
cussed previously. When dual sources are triggered alternate-
ly, the rate of each source can be halved without increasing
the length of treatment. One manufacturer suggests running
both sources at the standard rate in order to complete a treat-
ment session in half the time. Improvement relies on the
assumption that the two shock wave paths are sufficiently
different that bubbles generated by one source are not affect-
ed by the shock waves from the other source. In addition, if
the confocal sources are triggered non-simultaneously then
only half the total number of shock waves will be triggered
along either one of the tissue paths. Lastly, an optimal inter-
pulse delay could intensify cavitation17 or enhance internal
waves within the stone. Timing a second shock wave to accel-
erate the collapse of cavitation bubbles excited by the first
shock wave was the goal of the first dual pulse lithotripter,
introduced at a meeting of the Acoustical Society of America
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Thus a scientific basis exists for the dual-pulse design, but some technical hurdles to clinical implementation remain. Challenges include timing variability (between the two sources or by different travel paths) that can lead to a change in the location of peak pressure within the body, and greater potential for tissue injury due to larger volumes of exposed tissue. Finally, coupling is doubly difficult with a dual-source dry-head lithotripter. All these effects could offset the potential benefits of dual-pulse lithotripsy, but are problems that should be reasonable to solve.
Additional benefits result from simultaneous trig-
in 1996.
gering of the sources.
ous pulses meeting at the stone means the peak pressure of each source could be halved, sparing tissue damage along each path. Also, the additive pressure field is broader along the center line of the sources than the transverse field of either single source, perhaps yielding the benefit of addition- al squeezing.
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The additive effect of the simultane-
their target. It takes more pulses to break the stone, and this increases exposure of the kidney to shock waves, increasing the potential for tissue trauma. Interestingly, the attempt to make lithotripsy totally anesthesia-free never caught on with physicians. Urologists prefer to treat patients who are under conscious sedation, as they are less likely to move in reaction to pain and, thereby, affect targeting—and out- comes with patients under sedation are significantly better
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its ups and downs. Research reminded the SWL communi- ty of the importance of fundamental acoustics and led to a new understanding of stone-comminution mechanisms, such as squeezing and shear. The knowledge gained has led to significant changes in the lithotripter industry. Innovations in technology
Basic research has inspired the introduction of two new clinical lithotripters. One is the wide-focus lithotripter, and the other is the dual-pulse machine. Both employ novel designs that show a dramatic departure from the more incre- mental changes that have characterized the lithotripter industry in previous years.
The theoretical merits of a broader focus were recog-
nized by W. Eisenmenger and form the foundation of his the-
ory of “quasi-static squeezing.” The Eisenmenger broad-
focus, low-pressure lithotripter also operates at the slowest
rate of any clinical devices (0.3 Hz). This machine was devel-
oped and introduced for clinical use in China. Preliminary
data and reported clinical data on safety and efficacy are
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Two dual-pulse, or two-source lithotripters have been approved by FDA. Both consist of dual confocal shock sources arranged at roughly 90 degrees. In one machine, the pulses are triggered non-simultaneously at a proprietary
than with patients who are not sedated.
Thus, the evolution of the modern lithotripter has seen
The concept of a broad focal zone appears to be gaining acceptance, and two of the three lithotripter man- ufacturers with the largest world-wide market share have since offered modified machines that allow the urologist to broaden the focus of their machines.
encouraging.
24 Acoustics Today, April 2006