A New Acoustic Lens Design for Electromagnetic Shock Wave Lithotripters

 

Nathan Smith - nathan.b.smith@duke.edu

Dept. of Mechanical Engineering and Materials Science, Duke Univ.

144 Hudson Hall, Box 90300, Durham, NC 27708

 

W. Neal Simmons, Georgy N. Sankin, Pei Zhong

Dept. of Mechanical Engineering and Materials Science, Duke Univ.

Durham, NC 27708

 

Popular version of paper 1pBB11

Presented Monday afternoon, April 19, 2010

159th ASA Meeting, Baltimore, MD.

 

 

Extracorporeal shock wave lithotripsy (ESWL) is a clinically preferred technique for treatment of kidney stone disease involving the use of focused acoustic shock waves to obliterate stones inside a patient. A key advantage ESWL has over other stone treatment methods is its noninvasiveness, i.e. surgery is unnecessary. Many different designs exist to convert low-energy density acoustic pulses (over a large surface area) outside a patient into high-energy density focused shock waves (over a small area) inside a patient; one clinically relevant model known as the electromagnetic (EM) lithotripter uses, in essence, a loudspeaker fitted with a lens for acoustic focusing. Other clinical lithotripters employ electrohydraulic (EH) and piezoelectric technologies.

 

The 1^st-generation lithotripter, known as the Dornier HM-3, utilizes EH technology for shock wave generation and a truncated ellipsoidal reflector for wave focusing. The HM-3 machine is characterized by its low peak pressure and wide focal width. Development of the subsequent 2^nd- and 3^rd-generation machines erroneously centered on narrowing the lithotripter focal width and increasing peak pressure to maximize stone targeting and destructive capacity. Unfortunately, patient respiration and stone spreading in the kidney collecting system can produce elusive and widespread stone targets. Furthermore, conventional fluoroscopic (x-ray) imaging does not allow for visualization and targeting of smaller stones. Currently, ESWL researchers and urologists are again noticing the benefit in stone fragmentation efficiency using wide focal width lithotripters.

Another desirable characteristic of the Dornier HM-3, as well as other EH lithotripters, is its unique wave shape. Figure 1 illustrates how EH lithotripters produce strong compressive shock waves and long trailing tensile regions, whereas EM and piezoelectric lithotripters produce shock wave profiles with ringing of compression and tension phases, similar to a damped oscillator.

 

 

Figure 1. (A) Representative electrohydraulic (EH) lithotripter waveform. (B) Representative electromagnetic (EM) lithotripter waveform demonstrating secondary compression from ringing (circled).

 

The impact of ringing is on bubble activity, a known mechanism of stone fragmentation. Tension from the trailing region of the lithotripter pulse can induce sub-micron-sized gas pockets in the urine to grow into millimeter-sized bubbles. After the passage of the lithotripter pulse these bubble collapse, often generating highly localized pressure on the order of the lithotripter shock wave. Compression ringing in the urine can act to lessen the growth of bubbles, thereby lessening their collapse impact on the surface of stones.

 

A novel acoustic lens design applicable to EM lithotripters has been developed as a means to simultaneously increase bubble activity in the focal region through reduction of secondary compression while broadening the lithotripter focal width. Through a ring cut along the periphery of the acoustic lens, the effective aperture of the lithotripter is reduced, which inversely affects its focal size. The lens cut also acts to delay a portion of the acoustic pulse as it approaches the focusing lens, as can be seen in Figure 2.

 

 

 

Figure 2. (A) EM lithotripter cross-section demonstrating pulse superposition. (B) Theoretical representation of pulse superposition as component waveforms. (C) The theoretical combined superposed waveform.

 

This delayed pulse superposes on the unmodified shock wave in such a way that it creates destructive interference and flattens the ringing present in its shock wave profile. Figure 3 shows actual waveforms from this destructive interference compared to the original lens design as well as an energy density plot to illustrate the beam widening effect.

 

 

Figure 3. (A) Comparison between original and new lens waveforms at the focus and (B) 4 mm away in the focal plane to demonstrate actual reduction of second compression. (C) Energy density comparison showing widened beam with new lens.

 

In vitro phantom tests and in vivo animal studies in a swine model have shown promising results with the new lens design. Experiments were conducted with both the original and new lenses at equivalent focal acoustic energies in a radius encompassing most clinical kidney stones treated with ESWL as well as the phantom kidney stones used for this study. With a nearly 50% wider beam diameter than the original source, the EM lithotripter with new lens produced statistically higher stone fragmentation efficiency for clinically relevant doses in the swine model as well as for idealized in vitro conditions detailed in Figure 4.

 

 

Figure 4. (A) Fragmentation efficiency in a swine model for 500, 1000 and 2000 shock doses. (B) Fragmentation efficiency in a flat-base tube holder at 2 different positions in the focal plane (focus and 10 mm from focus) and 2 different shock doses (500 and 2000).]

 

Lens optimization and fragmentation efficiency assessment phases have concluded, and the new lens for EM lithotripters is presently undergoing evaluation for tissue injury potential in comparison with the original lens. Before long, the new lens will be implemented and evaluated clinically.