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Wide-bandwidth Lithotripsy Hydrophone

E. Carr Everbach

Swarthmore College Department of Engineering

Background

Students at Swarthmore College, a private four-year college near Philadelphia with Quaker roots, have the opportunity to participate in advanced research since Swarthmore professors have strong research interests but no graduate students. Acoustics is an attractive field because of its inherent accessibility to undergraduates, and biomedical acoustics further attracts Engineering majors with interests in medicine or biomechanics. Development of practical devices helps students learn the importance of engineering design and integrates the electrical and mechanical concepts the students are learning in their coursework. For this reason, student participation in research is considered an important component of the educational process at Swarthmore.

In the medical procedure known as extracorporeal lithotripsy,1 a high-amplitude acoustic shock wave is created by a device known as a lithotripter (from the Latin roots for "stone" and "break into fragments") and focused onto a patient's kidney stone, which is thereby pulverized. The procedure is considered much less traumatic than abdominal surgery, and cheaper, since it can be performed on an outpatient basis. The precise mechanism by which the converging acoustic shock wave breaks the stone is still a matter of active research in acoustics. However, stone breakage is thought to be governed by either the internal reflection of the compressional portion of the acoustic pulse (which becomes a tension wave) from the posterior of the stone, or the action of microscopic gas bubbles collapsing onto the stone surface in response to the rarefactional portion of the acoustic pulse.2 Both mechanisms may operate simultaneously or in combination, but each is dependent upon the temporal and spatial characteristics of the lithotripter pulse.

Lithotripters of various designs produce acoustic shock waves using different techniques. Often an initial shock wave is created by an underwater explosion driven by a high-voltage spark discharge.3 The spark-gap electrodes are located at one focus of a hemi-ellipsoidal metal bowl so that the spherically- expanding shock wave reflects from the bowl walls and converges at the opposite focus (Fig. 1). This second focus is placed (via ultrasound imaging or fluoroscopy) so as to coincide with the patient's kidney stone. Acoustic pulses can also be created via an electromagnetic device that is analogous to a loudspeaker, or via a piezoelectric crystal (or crystal array), but in these cases the waves produced are nearly sinusoidal and not shock waves.3 By focusing them with an acoustic lens or by placing the crystals on a spherical backing surface, the converging waves steepen and form shock waves en route to the stone. The "shape" of the lithotripter pulse varies from one lithotripter to another but shares the common characteristics of a rapid

1996 ASEE Annual Conference Proceedings

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Everbach, E. C. (1996, June), Wide Bandwidth Lithotripsy Hydrophone Paper presented at 1996 Annual Conference, Washington, District of Columbia. 10.18260/1-2--6401