Page 22 - Spring 2006

Fig. 3. Acoustic output of representative clinical lithotripters. This table charts the focal volume (F2 volume) and peak positive pressure (MPa) of a sampling of the greater than 50 models of lithotripters that have been used in the United States. Values are for power settings (or kV) used for a typical patient treatment. Since focal volume and pressure are dependent on power or charging potential at the shock source, there is no metric for equivalence that allows the direct comparison of the absolute values of these data. Still, is can be seen that as used in the clinical setting, most lithotripters developed since the HM3 generate greater pressures focused to a smaller focal zone. Clinical outcomes with these machines have not been as good as with the HM3. The Xi Xin/Eisenmenger lithotripter has the largest focal volume and is used at the lowest
10 pressure (and lowest rate). (Reprinted with permission from Martin Dunitz Limited, London. )
cern is the frequency and severity of adverse effects seen with
narrow focus lithotripters. Kidney hematoma rates as high as
12% have been reported for narrow focus machines, com-
pared to an occurrence of less than 1% observed with the
original clinical lithotripter [10 and references within].
Perforation of the colon and rupture of the spleen have
7,10
Shock wave-induced cavitation can occur in tissue, and cavitation is the likely mechanism of vascular trauma: Cavitation—in short, bubble action—is capable of generating impressive force (Fig. 4)11 and is understood to play a critical role in stone comminution. It has been shown that cavitation
12
13
Reports voicing concern for the higher occur- rence of side effects with these machines have caught the attention of urologists and the lithotripsy community, and there is now an increased awareness of the potential for injury with such narrow focus lithotripters.
occurred.
Other studies have demonstrated that cavitation occurs within the urine in the kidney in fewer than 100 shock waves and in the tissue itself after many more shock waves (preliminary evi-
occurs in the kidney of patients during lithotripsy.
A variety of studies have shown that cavitation is involved in vessel rupture in SWL. For example, fitting the ellipsoidal reflector of a Dornier HM3 lithotripter with an acoustically pressure- release material produced effectively time-inverted wave- forms (due to amplitude inversion on reflection, focusing, and nonlinear wave propagation) in which the amplitude of the positive pressure (compression) phase was not affected,
dence indicates approximately 1000 shock waves).
Side effects are increased with narrow focus, high
amplitude lithotripters: Most current lithotripters are EMLs
that generate high amplitude (upwards of 100 MPa) shock
pulses delivered to a relatively narrow (4-8 mm) focal vol-
ume—defined by the half maximum of the pressure field
10
(Fig. 3).
These devices have proven to be less effective than
lithotripters with broader focal volumes. Stone-free rates are
9
lower, and re-treatment rates are higher. A significant con-
Fig. 4. High-speed photograph of a 1-mm diameter bubble collapsing in response
11
to acoustic excitation.
the bubble that damages the metal surface. The process has been responsible for erosion of the tip of ship propellers. Photograph reprinted with permission from L. Crum and IEEE. (© IEEE)
20 Acoustics Today, April 2006
The surface on the lower side leads to a water jet through