Page 20 - Spring 2006

PROGRESS IN LITHOTRIPSY RESEARCH
Michael R. Bailey
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington Seattle, Washington 98105
James A. McAteer
Department of Anatomy and Cell Biology, Indiana University School of Medicine Indianapolis, Indiana 46202
Yuri A. Pishchalnikov
Department of Anatomy and Cell Biology, Indiana University School of Medicine Indianapolis, Indiana 46202
Mark F. Hamilton
Department of Mechanical Engineering and Applied Research Laboratories, The University of Texas at Austin Austin, Texas 78713-8029
Tim Colonius
Department of Mechanical Engineering, California Institute of Technology Pasadena, California 91125
Introduction
Shock wave lithotripsy (SWL) for the non-invasive treat- ment of kidney stones was introduced in the United States in 1984. SWL virtually eliminated the need for open surgery to remove kidney stones, and it did not take long for physicians and patients to endorse this revolutionary technology. Early reports told of the efficient removal of even the most troublesome stones without apparent complica- tions, and SWL quickly became the “treatment modality of choice.” It was not long, however, before concerned physi- cians began to report the occurrence of adverse effects in SWL,1 particularly involving vascular trauma and including cases of severe hemorrhage in the kidney and acute renal fail- ure—significant side effects of serious consequence. Researchers quickly recognized the challenge and opportuni- ty to determine the mechanisms of shock wave action in lithotripsy, and in 1988, the Acoustical Society of America held the first in a series of popular sessions devoted to the topic of shock waves in medicine. The goal of the inaugural session was to improve the fundamental understanding of lithotripsy—to bring better devices and treatments to patients. The goal of this paper is to report on progress in this effort.
Background
2
Historically, stone disease (urolithiasis) has accounted for
seven to ten of every 1000 hospital admissions in the United
States,3 and currently, treatment approaches $2 billion annu-
4
ally. The introduction of SWL revolutionized the treatment
of symptomatic stones. In SWL, shock waves are focused through the body wall, to target stones in the kidney or other sites within the urinary tract, Fig. 1(a). Generally, 2000–4000 shock waves are administered at a rate between 0.5 and 2 Hz. Lithotripters produce shock pulses such as those shown in Fig. 2. A roughly 1 μs duration, positive pressure spike is fol- lowed by a ~5 μs duration, negative pressure trough. Peak amplitudes range from 15–150 MPa. Even with a recent surge
Roughly 10% of all people suffer from kidney stones.
in the popularity of more invasive surgical methods such as using tools built into catheters that can be threaded up the urinary tract (ureteroscopy), or gaining access to the interior of the kidney through a narrow (~1 cm diameter) channel established through the body wall (percutaneous nephros- tolithotomy)—and acousticians’ intuition that applying a sequence of high-pressure shock waves is an extreme thera- py—SWL remains the most common treatment for sympto-
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source, a method of acoustically coupling shock waves to the patient, and an imaging system for targeting. In the first lithotripter the patient (under anesthesia) reclined in a water bath, which acoustically coupled the shock waves to the body. In subsequent systems—dry-head lithotripters—the shock
matic kidney stones.
Lithotripters have three main components, a shock wave
Fig. 1. Three technologies for shock wave sources. Electrohydraulic (spark source and reflector) is shown at top with the shock wave focused in a cross- sectional view of a patient’s abdomen. The drawing, also, illustrates the con- cept of a dry treatment head in which the shock source is enclosed in a water- filled cushion that must be wetted to the patient with a coupling gel or fluid. The electromagnetic lithotripter (cen- ter) utilizes a high electrical current through a coil to displace a metal plate to generate the acoustic wave that is then focused. The piezoelectric lithotripter (bottom) uses a focused arrangement of piezoceramic elements. (Reprinted with permission from
49 Acoustical Physics. )
18 Acoustics Today, April 2006