Shock wave source for extracorporeal lithotripsy

A shock wave source for an extracorporeal lithotripsy system has a number of electro-acoustic transducers arranged in a concave surface, each transducer having an acoustic axis, and the shock wave source having an acoustic axis. The transducers are each pivotally mounted, and a common adjusting element is provided which pivots each of the transducers so that their acoustic axes intersect at a focus, which lies on the acoustic axis of the shock wave source. The common element also permits adjustment of the location of the focus along the shock wave source acoustic axis so as to be more distal or more proximate relative to the shock wave source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is presented to a shock wave source for use in 
extracorporeal lithotripsy and in particular to such a shock wave source 
having a plurality of electro-acoustic transducers arranged along a 
concave surface, which can be driven in a pulsed fashion for generating 
shock waves in a propagation medium disposed between the transducers and a 
patient. 
2. Description of the Prior Art 
In extracorporeal lithotripsy, a shock wave source is pressed against the 
body of a patient, in which a calculus is disposed, with a flexible 
membrane of the shock wave source functioning as a coupling agent. A 
suitable locating system is used to insure that the calculus to be 
disintegrated is located at the focus of the shock wave source. The 
calculus disintegrates into fragments by the action of the shock waves 
emanating from the shock wave source, and these fragments can be 
eliminated in a natural manner. 
A shock wave source is described in German OS 33 19 871, corresponding to 
British Specification 21 40 693, wherein a plurality of electro-acoustic 
transducers are disposed along a concave surface. Each of the transducers 
can be individually driven in a pulsed manner, to generate shock waves in 
a propagation medium disposed between the transducers and the patient. 
Each transducer has an acoustic axis, and the shock wave source as a whole 
also has an acoustic axis. The acoustic axes of the transducers intersect 
at a focus which lies on the acoustic axis of the shock wave source. The 
transducers are arranged on a surface which is a portion of a sphere, so 
that the focus of the shock wave source corresponds to the center of 
curvature of this surface. Consequently, the transducers in this shock 
wave source are positioned a relatively large distance from the body 
surface of the patient, if the calculus to be disintegrated is disposed 
close to the body surface. Because this known shock wave source has a 
constant aperture angle, which is defined by the radius of curvature of 
the surface on which the transducers are arranged and by the diameter 
thereof, the shock waves must be coupled to the body of the patient via an 
extremely small region of the body surface. This results in an undesirably 
high power density at this location of the body surface, which may be 
injurious under certain circumstances. 
If locating of the calculi is undertaken with an ultrasound locating system 
disposed in the center of the spherical surface on which the transducers 
are disposed, further disadvantages result. If the ultrasound locating 
means is disposed so that it can be applied to the body surface of the 
patient with only the interposition of the coupling membrane, as is most 
desirable for obtaining accurate ultrasound images, the ultrasound probe 
occupies a considerable portion of the region of the body surface 
available for coupling of the shock waves during treatment. This means 
that the power density at the remaining portion of the body surface 
available for treatment must be further increased in order to assure 
success of the treatment. If the ultrasound probe is moved away from the 
body surface so that an adequately large region of the body surface is 
available for shock wave treatment, reflections of the ultrasound waves 
emitted by the ultrasound probe will arise at the coupling membrane, 
thereby resulting in image artifacts in the ultrasound image, making 
locating of the calculus to be disintegrated more difficult, or 
impossible. 
Another shock wave source is disclosed in German OS 31 19 295, 
corresponding to U.S. Pat. No. 4,526,168, wherein the transducers are 
driven with a chronologically offset signal so that the shock waves 
emitted from the individual transducers simultaneously arrive at the focus 
of the shock wave source. This known shock wave source has a control unit 
which acts on the drive system for the transducers, so that the 
chronological offset is variable and the focus of the shock wave source 
can thus be displaced along the acoustic axis of the shock wave source. 
This known shock wave source has a focal distance which is electronically 
variable, i.e., with electronic focusing. This permits the shock wave 
source to the operated with a small focal distance, and thus with a large 
aperture angle, for treatment of calculi lying close to the body surface 
of the patient, so that the power density at the body surface can be held 
within tolerable limits. If an ultrasound locating system is used, this 
can be applied to the body surface of the patient, with only the coupling 
membrane being interposed therebetween, at the same time as treatment, 
without the region of the body surface available for coupling of the shock 
waves being diminished due to the positioning of the ultrasound probe. In 
this known shock wave source, however, the extent of each transducer 
transversely relative to the direction of its acoustic axis cannot exceed 
1/8th of the wave length of the shock wave emitted by the transducer. If 
this condition is not observed, excessive transit time differences will 
result at the focus between those components of the shock wave which are 
respectively emitted from the edge of the transducer and from the center 
of the transducer. Effective focusing would be impossible under those 
conditions. Consequently, this known shock wave source must have an 
extremely high number of relatively small transducers in order to generate 
shock waves having adequate energy and an adequate degree of focusing. 
This results in a complicated structure for the shock wave source itself, 
and also requires a complicated drive system and a complicated control 
system for the transducers. Additionally, sufficient electrical strength 
cannot be guaranteed, with an economic material outlay, when such 
extremely small transducers are used. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a shock wave source 
having a plurality of electro-acoustic transducers, which achieves a lower 
power density at the body surface of the patient being treated, for 
treatment of calculi lying close to the body surface, and having a simple 
structure and the ability to withstand the high electrical and mechanical 
stresses which occur during such treatment. 
The above object is achieved in accordance with the principles of the 
present invention in a shock wave source wherein the transducers are 
individually pivotally mounted so that the acoustic axis of each 
transducer is pivotable in a plane containing the acoustic axis of the 
shock wave source, and having a control element for pivoting the 
transducers in common so that the focus, at which the acoustic axes of the 
transducers intersect, is movable along the acoustic axis of the shock 
wave source from a more proximate to a more distal focal distance, while 
maintaining the acoustic axis of the transducers aligned at the focus. 
In the shock wave source disclosed herein the focus can thus be displaced 
between a more proximate and a more distal focal length, so that the 
aperture angle of the shock wave source is also adjustable. This provides 
the possibility of setting the focal distance, and thus the aperture angle 
of the shock wave source, in accordance with the requirements of a 
particular treatment, so that only a low power density exists at the body 
surface during coupling of the shock waves to the body of the patient. It 
is simultaneously possible to provide an ultrasound locating probe in the 
center of the shock wave source. A complicated structure of the shock wave 
source is avoided because the adjustment of the focal distance is 
undertaken mechanically. Thus relatively large transducers can be used, 
and a complex drive means and an associated control means (electronic) is 
not required. Because relatively large transducers can be used, these 
transducers will have an adequate electrical strength, without the need of 
special measures. 
The shock wave emitted by the individual transducers will simultaneously 
arrive at the focus of the shock wave source when, taking the shape of the 
concave surface on which the transducers are disposed into consideration, 
a focal distance is selected wherein the transit time of the shock waves 
from the individual transducers to the focus is the same for all 
transducers, i.e., a focal distance wherein all of the transducers are 
disposed at the same distance from the focus of the shock wave source. A 
chronologically offset arrival of the shock waves from the individual 
transducers at the focus of the shock wave source, however, is not always 
undesirable, because it has been shown that successful treatment can be 
achieved even with such an offset. 
In a further embodiment of the invention, however, the shock wave source 
can be provided with electronic focusing means for driving the individual 
transducers with chronologically offset signals so that the shock waves 
emanating from the individual transducers simultaneously arrive at the 
focus of the shock wave source. The control means for operating the drive 
system permits variation in the drive of the transducers, and can be 
adapted with the mechanical focusing structure so that the chronological 
offset can be matched to the respective pivoted positions of the 
transducers. Because mechanical focusing is provided in addition to this 
electric focusing, significantly larger transducer elements can be used 
than in the case of shock wave sources exclusively using electronic 
focusing. This permits the drive means and the control means to be 
constructed in a significantly less complex manner, in comparison to such 
systems operating exclusively by electronic focusing. 
In a further embodiment of the invention, each individual transducer may be 
provided with a focusing element, so that each transducer emits focused 
shock waves. Each transducer in this embodiment has a focus, the foci 
coinciding on the acoustic axis of the shock wave source at a focal 
distance from each transducer which corresponds to the mean value of the 
more proximate and the more distal focal lengths of the shock wave source. 
This results in a focus of the shock wave source having an extremely small 
three-dimensional extent. 
Piezo-electric transducers are preferably used as the electro-acoustic 
transducers in the shock wave disclosed herein. 
In a further embodiment of the invention, the transducer may be combined in 
groups, each group containing a plurality of transducers which are 
arranged in an annulus, having a center axis corresponding to the acoustic 
axis of the shock wave source. This provides the advantage that the 
transit time of the shock waves emanating from each of the transducers in 
a group to the focus of the shock wave source is the same. If electronic 
focusing of the shock wave source is provided in this embodiment, the 
structure (circuitry) of the drive means and the control means is further 
simplified, since the transducers in a group can be driven in common 
simultaneously. Moreover, the arrangement of the transducers in groups 
enables a simplified structure of the mechanical pivoting elements, 
because all of the transducers in a group will assume the same pivoted 
position with respect to the acoustic axis of the shock wave source. 
A simple structural arrangement of the shock wave source is achieved in an 
embodiment of the invention wherein the transducers are all mounted on a 
common holder, each transducer being pivotable around an axis disposed at 
a right angle relative to the plane containing the acoustic axis of that 
transducer and the acoustic axis of the shock wave source. 
In a further embodiment of the invention, the means for pivoting the 
individual transducers includes a plurality of levers corresponding in 
number to the number of transducers, and an actuation element for the 
levers. Each lever has one end connected rigidly to a transducer, and an 
opposite end engaging the actuation element. The actuation element acts on 
the levers to displace the levers and pivot the transducers. If the 
transducers are combined in groups, the pivoting means can be further 
simplified by providing one actuation element for each group of 
transducers, this actuation element engaging the respective levers 
attached to all of the transducers of a group. A further structural 
simplification is achieved in the use of a common means for operating each 
of the actuation elements. 
In a further embodiment of the invention, an ultrasound locating system for 
identifying the position of the calculus to be disintegrated is disposed 
in the center of the concave surface of which the transducers are 
disposed. 
For increasing the ability of the shock wave source, to withstand 
electrical and mechanical stresses the shock source wave may be provided 
with an elastically resilient separating membrane disposed between the 
propagation medium and the transducers, with the transducers having their 
respective shock wave-emitting surfaces disposed against this membrane. 
The transducers may then be surrounded by an electrically insulating 
fluid, which does not mix with the propagation medium due to the presence 
of the separation membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawings, a shock wave source constructed in accordance 
with the principles of the present invention includes a plurality of 
piezo-electric transducers 1 arranged on a concave, dynamically balanced 
surface. This surface is shown planar (i.e., without distortion) in FIG. 2 
for clarity. The surface is a portion of a sphere, as indicated with 
dot-dash lines in FIG. 1. The piezo-electric transducers 1 are contained 
in a housing 2, having an exit aperture 3 for the shock wave generated by 
the transducers 1. The exit aperture 3 is closed by a flexible membrane 4. 
The volume bounded by the housing 2 and the membrane 4 is filled with a 
fluid, for example water, as a propagation medium for the shock waves. 
During treatment of a calculus in a patient the membrane 4 is pressed 
against the patient, and acoustically couples the shock wave source to the 
patient. 
Each transducer 1 has an acoustic axis A, along which the shock waves 
generated by that transducer propagate. The shock waves emitting from the 
transducers 1 converge at a focus F of the shock wave source, which lies 
on an acoustic axis B of the shock wave source. The acoustic axes A 
intersect at the focus F. Due to the spherical shape of the surface on 
which the transducers 1 are arranged, the acoustic axis B of the shock 
wave source corresponds to the center axis of the spherical surface. 
The transducers 1 are connected to a drive system 6 (schematically 
indicated) via lines 5. The drive system 6 drives the transducers 1 with 
voltage pulses for generating shock waves. 
Each transducer 1 has a carrier 7 by which it is pivotally mounted by a pin 
8 to spaced arms 9 of a common holder 10. The holder 10 is connected to 
the housing 2. The transducers may be arranged or combined into two 
groups. The transducers 1 in each group are arranged in the form of an 
annulus, with the inner group containing six transducers 1 and the outer 
group containing twelve transducers 1, as shown in FIG. 2. The transducers 
in each group have the same dimensions, and the center axis of each 
annulus corresponds to the acoustical axis B of the shock wave source. 
Each transducer 1 has a lever 11 associated therewith. One end of each 
lever 11 is connected to the carrier of the associated transducer 1. The 
opposite end of each lever 11 in a respective group of transducers 1 
engages an annular control element 12 or 13, respectively associated with 
the inner or outer group of transducers 1. Each of the control elements 12 
and 13 has a conical engagement surface 14 or 15, on which the opposite 
ends of the levers 11 reside. The center axes of the engagement surfaces 
14 and 15 correspond to the acoustic B of the shock wave source. The 
control elements 12 and 13 are rigidly connected to each other by a 
coupler 16. A rod 17 is attached to the coupler 16, and is conducted to 
the exterior of the shock wave source through a wall of the housing 2 in, 
liquid-tight fashion and engages a schematically indicated displacement 
unit 29. The rod 17 is longitudinally displaceable by the unit 29, with 
the longitudinal axis of the rod 17 being parallel to the acoustic axis B 
of the shock wave source. The inner control element 12 has bore which 
receives a tubular projection 18 of the holder 10, and along which the 
control elements 12 and 13 are longitudinally displaceable. By actuating 
the rod 17, the control elements 12 and 13 are displaced in the direction 
of the acoustic axis B of the shock wave source. The respective engagement 
surfaces 14 and 15 of the control elements 12 and 13 thereby interact with 
the ends of the levers 11. The surfaces 14 and 15 are maintained in 
engagement with the ends of the levers 11 by annular rubber springs or 
riders 19 and 20, which ride on the respective surfaces 14 and 15. The 
interaction of the engagement surfaces 14 and 15 with the levers 11 pivots 
all of the transducers 1 in common, with the transducers in each group 
being pivoted by different amounts due to the differently inclined 
surfaces 14 and 15. 
Each transducer 1 pivots around an axis by means of the pin 8, this axis 
being disposed at a right angle with respect to a plane containing the 
acoustic axis of the respective transducer 1, and the acoustic B of the 
shock wave source. The acoustic axis A of each transducer 1 is thus 
pivotable in a plane which contains the acoustic axis B of the shock waver 
source. As shown in FIG. 1, the levers 11 are attached to the associated 
transducers 1 so that the acoustic axes A of the all of the transducers 1, 
as stated above, intersect at the focus F of the shock wave source. The 
conical angles of the engagement surfaces 14 and 15 of the control 
elements 12 and 13 are selected such that the acoustic axes A of the 
transducers 1 intersect in a focus F on the acoustic axis B of the shock 
wave source for every position of the control elements 12 and 13 which can 
be obtained by the rod 17. The focus F is thus adjustable with infinite 
variation along the acoustic axis B of the shock wave source between a 
more proximal focal length f.sub.1 and a more distal focal length f.sub.2 
of the shock wave source. 
The drive system 6 includes two drive units 21 and 22, the drive unit 21 
driving the transducers 1 of the inner group, and the drive unit 22 
driving the transducers 1 of the outer group. Because the shock waves from 
the transducers 1 of the inner and outer groups respectively cover paths 
of different lengths to the focus F, depending upon the selected focal 
length, a control unit 23 is provided, which is connected to the drive 
units 21 and 22, and which is supplied with the output signal of a 
schematically indicated path generator 24, connected to the rod 17. The 
output signal of the path generator 24 represents a measure for the 
selected focal length on the basis on which the control unit 23 actuates 
the drive units 21 and 22 with a chronological offset, such that the shock 
waves emitted from the transducers 1 of the two groups simultaneously 
arrive at the selected focus F of the shock wave source. Because the 
transducers of a group are set at the same distance from the focus F of 
the shock wave source, only two drive units 21 and 22 are required. 
As can be seen in FIG. 1, each transducer 1 may be provided with an 
acoustic lens 25 so that it emits focussed shock waves. All of the 
acoustic lens 25 have the same focal length f.sub.3, which is selected to 
correspond to the mean value of the more proximal and the more distal 
focal lengths f.sub.1 and f.sub.2 of the shock wave source. This is 
schematically shown in FIG. 1 with the focus F' and the focal length 
f.sub.3 being shown for a transducer 1. As a consequence of the focussed 
shock waves emitted by the transducers 1, the shock wave source will have 
a focal zone which is tightly spatially limited. 
As can be seen in FIG. 2, the individual transducers 1 may each be in the 
shape of a circular disk. As shown with dashed lines for one of the 
transducers 1, the transducers may alternatively have a hexagonal shape, 
which permits a large emitting area to be achieved, given the same area of 
the spherical surface. The transducer groups may be defined by annuli of 
different radii, with all transducers in a group being disposed on the 
same annulus. If the acoustic axis A of every other transducer 1 of the 
outer group is disposed in the same plane as the acoustic axis of the 
transducer 1 adjacent thereto in the inner group, as shown in FIG. 2, it 
is also possible to radially displace the remaining transducers of the 
outer group inwardly, as shown in dashed lines in FIG. 2. The transducers 
1 which are radially offset inwardly then form a third group, which must 
be driven with a further chronological offset using an additional drive 
unit (not shown) within the drive system 6. 
An ultrasound locating probe 26, which is a part of an ultrasound locating 
system 30 for identifying the position of the calculus to be 
disintegrated, may be arranged in the center of the spherical surface, the 
focus F of the shock wave source being aligned with the calculus by means 
of the probe 26, and its associated ultrasound locating system. The 
ultrasound probe 26 is received in the tubular projection 18 of the holder 
10, and thus extends along the acoustic axis B of the shock wave source. 
The probe 26 is longitudinally displaceable in the bore of the projection 
18, so that after the shock wave source has been applied to the body of 
the patient to be treated, the probe 26 can be brought to a position 
against the body surface of the patient, with only the membrane 4 being 
interposed therebetween. For displacing the probe 26, a slide 27 is 
attached thereto, which is conducted to the exterior of the shock wave 
source through the housing 2 in liquid-tight fashion. The slide 27 is 
longitudinally displaceable as indicated by the double arrow. Electrical 
lines (not shown) connecting the probe 26 to the remainder of the 
ultrasound locating system (not shown) may be located in the interior of 
the slide 27. 
After alignment of the focus F of the shock wave source has been undertaken 
using the ultrasound locating system, a further adjustment can be 
undertaken by using the piezo-electric transducers 1 to receive a trial 
shock wave reflected from the calculus to be disintegrated. The received, 
reflected shock waves can then be analyzed in terms of amplitude. The 
focal length of the shock wave source can then be further adjusted by 
displacing the rod 17 so that the reflected components of the shock waves 
have a maximum amplitude, indicating that the focal length is optimally 
set for this particular treatment. Using this optimization for setting the 
focal length, shock waves having a reduced amplitude than would otherwise 
be used can be emitted by the shock wave source. 
In the embodiment shown in FIG. 3 wherein components identical to those 
already discussed are provided with the same reference numerals, an 
elastically resilient separating membrane 28 is provided between the 
coupling membrane 4 and the transducers 1. The outer edge of the 
separating membrane 28 is secured to the inside wall of the housing 2, and 
its inner edge is secured to the tubular projection 18 of the holder 10, 
both edges being secured in liquid-tight fashion, for example by gluing. 
The volume bounded by the housing 2 and the coupling membrane 4 is thus 
subdivided into two volumes by the separating membrane 28 and by the 
projection 18 of the holder 10. The volume between the coupling membrane 4 
and the separating membrane 28 is filled with a fluid, for example, water, 
serving as a propagation medium for the shock waves. The remaining volume, 
in which the transducers 1 are disposed, is filled with an electrically 
insulating fluid, for example insulating oil so that an electrically and 
mechanically durable shock wave source is achieved. 
To insure introduction of the shock waves emitted from the transducers 1 
into the propagation medium via the separating membrane 28 with optimally 
low losses, the surface of each transducer 1 from which the shock waves 
are emitted is applied snugly against the separating membrane 28. In the 
embodiment shown in FIG. 3, these surfaces are the outer surfaces of the 
respective acoustic lenses 25 which face toward the focus F of the shock 
wave source. To insure that no spaces are present between the separating 
membrane 28 and the shock wave-emitting surfaces of the transducers 1, 
even when the transducers 1 are pivoted, the shock wave-emitting surfaces 
of the transducers 1 may be glued to the separating membrane 28. 
Alternatively, if the separating membrane 28 exhibits the necessary 
elasticity, as well as a suitable shaping, gluing can be omitted. It is 
also possible to seat the separating membrane 28 against the shock 
wave-emitting surfaces of the transducers 1 by generating a higher liquid 
pressure in the volume containing the propagation medium than in the 
volume containing the transducers 1. 
To further avoid acoustic losses, the separating membrane 28 consists of a 
material having an acoustic impedance substantially corresponding to that 
of the propagation medium. If water is used as the propagation medium, a 
suitable material for the separating membrane 28 is EPDM rubber. 
The remaining details of the embodiment shown in FIG. 3 correspond in 
structure and operation to the embodiment of FIG. 1 already described. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.