Generation of Two Successive Shock Waves Focused to a Common Focal Point

Abstract

A generator of two successive shock waves focused on a common focal point has been developed. Cylindrical pressure waves created by multichannel electrical discharges on two cylindrical composite anodes are focused by a metallic parabolic reflector-cathode. Near the common focus, the waves are transformed into strong shock waves. The anodes are energized from separate power supplies. This allows us to vary the time interval between the discharges and stagger the waves' arrival to the focal point. Schlieren photographs of the focal region show that mutual interaction of the two waves results in generation of a large number of secondary short wavelength shocks. Measurements of the shock waveforms at the focus demonstrate that the second (i.e., later arriving) wave is strongly attenuate due to the medium inhomogeneity produced by the first wave. Localized injury of a rabbit's liver induced by the shock waves has been demonstrated by the method of magnetic resonance imaging. Histological examination of the liver samples taken from the injured region revealed a very sharp boundary between the injured and healthy tissues

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GENERATION OF TWO SUCCESSIVE SHOCK WAVES FOCUSED TO
A COMMON FOCAL POINT

P. Sunkaξ ξ, V. Stelmashuk, V. Babicky, M. Clupek
Institute of Plasma Physics, Academy of Sciences of the Czech Republic
Za Slovankou 3, 182 21 Prague 8, Czech Republic

J. Benes, P. Pouckova
Institute of Biophysics, First Faculty of Medicine, Charles University, Prague
U nemocnice 2, 120 08 Prague 2, Czech Republic

J. Kaspar, M. Bodnar
Institute for Biomedical Engineering, Czech Technical University,
Studnickova 7, 120 00 Prague 2, Czech Republic
∗ ∗


Abstract

Generator of two successive shock waves focused to a
common focal point has been developed. Cylindrical
pressure waves created by multichannel electrical
discharges on two cylindrical composite anodes are
focused by a metallic parabolic reflector and near the
focus they are transformed to strong shock waves. The
anodes are energized from separate power supplies and
time delay between the discharges can be varied.
Schlieren photos of the focal region demonstrated that
interaction of the two waves results in creation of a large
number of secondary short wavelength shocks. Strong
attenuation of the second wave has bee shown by
measurements of the shock waveforms at the focus.
Localized injury of rabbit’s liver induced by the shock
waves has been demonstrated by the method of magnetic
resonance imaging. Histological analysis of the liver
samples taken from the injured region revealed that the
boundary between the injured and health tissues is very
sharp.

I. INTRODUCTION

Interaction of focused shock waves with soft tissues has
been studied as a possible method for an extracorporeal
treatment of some kind of cancers. Localized action of the
shock waves in an acoustically homogeneous medium is
generally attributed to cavitations produced by the
rarefaction wave. Collapsing cavitations create strong
secondary shock waves of tens of nanosecond in duration
that damage cell membranes. Local thermal effects
accompanying cavitations collapse and production of
chemical radicals may also play some role in the cells

∗ Work supported by the Grant Agency of the Czech Republic under the contract No. 202/05/0685 and partly supported
by Ministry of Education, Youth and Sports of the Czech Republic under the contract No. MSM6840770012
ξ email: sunka@ipp.cas.cz
killing. To enhance the effect of cavitations, gas bubbles
or cavitation nucleation agents have been injected into the
studied tissues and only in such cases reduction of tumor
growth has been observed [1,2].
During the last years we have developed a novel type of
generator of focused shock waves [3,4]. A cylindrical
pressure wave is produced by a multi-channel discharge at
a “composite anode”. The anode consists of a stainless
steel cylindrical electrode (∅ 60x100 mm) coated by a
thin layer (d = 0.2 – 0.3 mm) of porous ceramics
deposited on the anode surface by the technology of
plasma spraying. We have observed that in a highly
conducting water solution (σ = 5 – 20 mS/cm) a large
number of short (less than 1 mm long) discharge channels
are created and they are distributed almost homogenously
on the anode surface. Each discharge channel expands and
creates semi-spherical pressure wave and by superposition
of all of the waves a cylindrical pressure wave
propagating from the anode is formed. The cylindrical
pressure wave is focused by a metallic parabolic reflector
(cathode) and near the focus it is transformed into a strong
shock wave. Amplitude of the pressure wave has reached
100 MPa. The rarefaction wave in amplitude of 20 – 25
MPa produced cavitations.
In the present experiments we have divided the anode to
two parts energized from separate pulse power supplies.
In such arrangement we are able to generate either one
shock wave, or two successive shocks focused to a
common focal point. Varying the time delay between the
two discharges we are able to study interaction of the
second wave with an inhomogeniety and cavitations
created by the first wave. The aim of the experiments is to
localize action of the shock waves propagating in an
originally acoustically homogeneous medium.
0-7803-9189-6/05/$20.00 ©2005 IEEE.1433

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II. EXPERIMENTAL SETUP

Arrangement of the present experiment is schematically
shown in Fig.1. A cylindrical composite anode consists of
two parts (A1 - ∅ 60x70 mm, A2 - ∅ 77x25 mm) energi-
zed from separate pulse power supplies that can be
switched on with a different time delay. If they are
switched on at the same time, the shock wave from the
larger diameter anode reaches the focus 5 µs before the
wave from the smaller diameter anode. Pulse power
supplies consist of low inductive capacitors (0,5 µF and
1µF respectively) charged up to 30 kV and switched to
the discharges by triggered spark gaps. The generator can
produce either a single shock wave, or two successive
waves focused to a common focal point. The generator
has been placed in a tank where the conductive liquid was
separated from the experimental volume containing
distilled water by an acoustically transparent membrane.



Figure 1. Experimental setup

Waveforms of the discharge voltage and current have
been recorded by a digital oscilloscope using capacitive
voltage divider and Rogowski coil, respectively. Schlieren
photography was used for visualization of pressure field
in the focal region. Laser beam lasting 15 ns was of 60
mm in diameter. The laser light scattered on cavitations
was photographed perpendicularly to the laser beam axis
to identify spatial distribution of the cavitations. Temporal
courses of the focused shock waves have been measured
by polyvinilidene fluoride (PVDF) shock gauges model
S_25TCB, produced by
PIEZOTECH S.A. Thickness of the active element was of
0.025 mm and it ensured a temporal resolution better than
20 ns. Probes with the active area 1; 4 and 9 mm
been used. Sensitivity of the probes, as measured by the
the French company
2 have
producer, is in the range of 22.5 – 24.5 pC/N. Injury of
soft tissues of experimental animals induced by the shock
waves have been identified by the magnetic resonance
imaging examinations. Histological analysis of the injured
tissues have been done afterwards.

III.
EXPERIMENTAL RESULTS

Schlieren photos have been taken at different times after
the first discharge started and pictures for single and
double shock waves have been compared. An example of
such comparison is shown in the Fig. 2a, 2b. The waves
propagate from the right to the left in these figures. The
Fig. 2a shows propagation of a single wave in the position
of 13 mm behind the focus. In the Fig. 2b the second
wave is in the same position as the first wave in the 2a,
however, the first wave (not seen in the picture) is 25 mm
ahead the second one. Interaction of the two waves results
in generation of many secondary spherical shocks due to
collapsing cavitations. Pictures of the scattered light show
that due to the second wave the individual bubbles
produced by the first wave (Fig. 3a) fuse to a cloud of
collapsing cavitations (Fig.3b). Waveforms of the two
interacting waves measured at the focus for different time
delay between the discharges are shown in the Fig. 4. It is
apparent that with the increasing time delay between the
discharges the second shock wave is strongly attenuated.
Experiments are in progress to localize the region of the
strong damping of the second shock.



Figure 2. Schlieren photos of the focal region taken for a
single wave - 2a and for two interacting shocks showing
creation of secondary spherical shocks due to the
collapsing cavitations - 2b.

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Figure 3. Photos of laser light scattered on cavitations
taken in the direction perpendicular to the laser beam axis.
3a - Individual bubbles produced by a single wave,
3b - fusion of the individual bubbles to a cloud by the
second shock. The shocks propagate from the top to the
bottom in these pictures.


Figure 4. Waveforms of the shock waves measured at the
focus for different time delay between discharges. With
the increasing delay the second shock is heavy dumped.

Interaction of the focused shock waves with soft
animal tissues has been studied on laboratory rabbits. The
rabbits have been anesthetized depilated on the abdomen
and exposed to 1200 double shocks with a fix time delay
between the waves of 5 µs. Focus of the shock waves has
been targeted to the rabbit’s liver. Next day and 6 days
after the exposure the rabbit was examined by the method
of magnetic resonance imaging. Thereafter, it was
sacrificed and histological analysis of the injured samples
of the liver has been performed. An example of the MRI
scan is shown in Fig. 5. Injury of the liver tissue and
partially also of the front wall of the stomach is apparent.
The injury extends 15 mm corresponding roughly to the
breath excursions. Histology of the injured liver tissue
done 6 days after exposure is shown in Fig. 6. The injured
necrotic tissue is sharply separated from the surrounding
health tissue. A large number of leukocytes presented
demonstrate the immunity reaction to the injury.





Figure 5. An example of MRI scans of the laboratory
rabbit taken the next day after exposure to 1200 shocks.
The rabbit lay on its abdomen in the tunnel. Shocks
propagated from the bottom in this picture. Localized
injury of the liver tissue and of the front wall of the
stomach is apparent




Figure 6. Histology of the injured rabbit liver tissues
done 6 days after exposure to shock waves shows that the
boundary between the injured and health tissues is very
sharp. The central part of the necrotic tissue is almost
totally damaged. Presence of a large number of leucocytes
at the boundary of the injured tissue demonstrates
immunology response of the rabbit to the injury.


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IV.
CONCLUSIONS

Generator of two successive shock waves focused to a
common focal point has been developed. Interaction of
the second wave with the inhomogeniety created in water
by the first wave results in strong attenuation of the
second wave and in generation of a large number of
secondary short wavelength shocks.
Local injury of the rabbit’s liver induced by the shock
waves has been demonstrated by the method of magnetic
resonance imaging. Very sharp boundary between the
injured and health tissues have been shown by
histological analysis of them.


V. REFERENCES

[1] Miller DL, Song JM: Lithotripter shock waves with
cavitation nucleation agents produce tumor growth
reduction and gene transfer in vivo,
Ultrasound Med Biol 28 (10): 1343-1348 Oct 2002

[2] Yu TH, Wang GY, Hu K, et al.: A microbubble agent
improves the therapeutic efficiency of high intensity
focused ultrasound: a rabbit kidney study,
Urological Research 32 (1): 14-19 Feb 2004

[3] P. Sunka: Pulse electrical discharges in water and
their applications,
Physics of Plasmas, Vol. 8: 2587-2594, May 2001.

[4] P. Sunka, V. Babicky, M. Clupek, J. Benes, P.
Pouckova:
Localized Damage of Tissues Induced by Focused Shock
Waves,
IEEE Trans. on Plasma Science, 32,1609-1613, Aug.2004


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