Fig. 1 Principle of an acoustic diode Fig. 1.1 Principle of an acoustiv diode
Fig. 2 Effects of an acoustic diode on a) acoustic emission, b) tc and c) maximum bubble expansion
In the simplest form, an acoustic diode (AD) consists of two acoustically transparent membranes, which are held at the periphery by an aluminum ring (Fig. 1). To establish surface contact between the membranes, a partial vacuum is applied to the cavity formed by the membranes and the supporting ring. Because of the existing mechanical contact, the leading compressive wave of the LSW can pass through almost unaltered. Yet, the ensuing tensile wav e of the LSW will pull the membranes apart, thus creating a large acoustic impedance mismatch that will effectively truncate the transmission of the ensuing part of the tensile wave. Depending on the adhesion and/or cohesion strength between the membranes , a controllable threshold for truncating the negative pressure of the LSW could be achieved (Riedlinger R, Acoustic diode, U.S. Patent 4618796, 1989).
We have constructed a prototype of an AD and tested its effect on cavitation produced by the Dornier XL-1 lithotripter. The membranes of the AD were made of plastic membrane 114 mm in thickness. With a partial vacuum of 1.228 Psi inside the acoustic diode, tc of the cavitation bubble clusters was found to be significantly reduced (41.6% at 20 KV, Fig. 2a and Fig. 2b). High-speed images confirmed that with the AD, the maximum bubble expansion was significantly reduced both in terms of bubble size and density (Fig. 2c).
Fig. 1 Principle of an acoustic diode
Fig. 1.1 Principle of an acoustiv diode
Fig. 2 Effects of an acoustic diode on a) acoustic emission,
b) tc and c) maximum bubble expansion
To evaluate the effect of the AD on tissue injury, we conducted experiments
on bubble oscillation in the 200 mm hollow fiber
vessel phantoms, using the Dornier XL-1 lithotripter. During the course
of the experiment, high-speed imaging system was used to monitor the presence
of intraluminal bubbles inside the fiber. As shown in Fig.
3a, the number of shocks n eeded to produce the rupture (Nr) was found
to increase from 10 using the standard reflector to 51 with the addition
of the AD. High-speed images (Fig 3c) show that the
AD significantly reduced the dilation of the fiber wall. Stone fragmentation
re sults (see
Fig. 3b) show that 14.5% reduction on
weight loss with the addition of the AD. These results demonstrate the
potential of applying the AD to upgrade the existing clinical lithotripters
to reduce tissue injury
Fig. 3 Effects of an acoustic diode on a) does-dependent
rupyure, b) stone fragmentation, c) bubble expension inside 200 micron
hollow fiber.
Considering the modi fication of a LSW to suppress bubble expansion
and thus reduce tissue injury, we evaluated the effect of a truncated shock
wave on bubble dynamics. As shown in Fig. 4, when P-LSW
is truncated from -10 MPa to -0.5 MPa , the corresponding value of Rmax
and Pw2 would drop monotonically from 1100 mm
to 120 mm and from 41 MPa to 4 MPa, respectively.
This significant reduction in the maximum expansion of LSW-induced bubbles
could greatly reduce the potential for vascular injury in vivo.
Fig. 4 Effects of truncating the tensile pressure of LSW
on maximum bubble expansion and shock wave emission.