Electromagnetic pressure pulse source

An electromagnetic pressure pulse source for generating focused pressure pulses as an electrically conductive membrane and a coil system which drives the membrane by rapidly displacing the membrane from the coil system. The coil system is formed by an annular array having a number of annular zones which can be individually activated to cause the generation of pressure pulses in variable chronological relation to each other, which permits the location of the focus of the resulting shockwave to be adjusted with a range, and/or the diameter of the focus to be changed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to an electromagnetic pressure pulse 
source for generating focused pressure pulses, of the type having an 
electrically conductive membrane which is rapidly displaced by means of a 
coil system. 
2. Description of the Prior Art 
Electromagnetic pressure pulse sources having a membrane driven by a coil 
system as described, for example, in U.S. Pat. No. 4,674,505, are used for 
medical purposes in the treatment of calculosis, pathological bone 
conditions and pathological tissue changes. Such a pressure pulse source 
is normally applied to the body surface of the patient by means of a 
flexible coupling pillow, filled with a liquid medium for acoustic 
coupling. As a consequence of the flexibility of the coupling pillow, the 
spacing of the pressure pulse source from the body surface can be set, 
while maintaining contact between the coupling pillow and the body 
surface, so that the focus of the pressure pulses lies in the zone to be 
treated. The zone to be treated will be at a different depth below the 
surface dependent on the individual treatment case. Because the spacing of 
the focus from the body surface can be varied only to a slight extent in 
this manner, a number of proposals have been suggested to alleviate this 
situation. For pressure pulse sources wherein the pressure pulses are 
focused by an acoustic lens, for example, it is proposed in German OS 37 
35 993 to provide two such acoustic lenses with variable spacing between 
the lenses to displace the focus, or alternatively to provide a lens 
having a variable focal length in the form of a liquid lens (vario lens). 
All solutions heretofore proposed for displacing the focus have significant 
disadvantages associated therewith. In the aforementioned technique of 
displacing the focus by adjusting the distance of the pressure pulse 
source from the body surface of the patient, problems arise if the 
treatment zone lies immediately under the body surface, because of the 
small entry area for the pressure pulses which is then available. This 
results in the skin of the patient, which is sensitive to pain, 
experiencing high stress, such that hematoma can even occur. In the case 
of pressure pulse sources having an ultrasound B-scan applicator arranged 
in a central bore of the pressure pulse source for locating purposes, the 
B-scan applicator must be retracted if the zone to be treated lies close 
to the body surface, since it would otherwise be in the propagation path 
of the pressure pulses. This means that no ultrasound images, or only poor 
ultrasound images, can be produced while charging the patient with 
pressure pulses. Additionally, the necessary mechanism for adjusting the 
pressure pulse source, and possibly the B-scan applicator as well, 
involves considerable outlay. 
The aforementioned pressure pulse source disclosed German OS 37 35 993 also 
has the above disadvantages, because although the mechanism for adjusting 
the pressure pulse source can be eliminated, a mechanism must nonetheless 
be provided for adjusting one of the lenses. This disadvantage can be 
avoided by using a vario lens for displacing the focus, however, a vario 
lens permits the focus to be displaced only slightly, and additionally 
involves a not inconsiderable design outlay and space requirement. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electromagnetic 
pressure pulse source of the type having a coil system and electrically 
conductive membrane wherein the focus can be displaced over a broad range 
in a simple and economic manner. 
It is another object of the present invention to provide such an 
electromagnetic pressure pulse source wherein the focus can be displaced 
without the use of complex mechanical means. 
It is a further object of the present invention to provide such an 
electricmagnetic pressure pulse source wherein an ultrasound applicator 
can be contained in the pressure pulse source and can remain in contact 
with the surface of a subject while the subject is being irradiated by the 
pressure pulses (shockwaves). 
Another object of the present invention is to provide such an 
electromagnetic pressure pulse source wherein the acoustic energy present 
in the region of the surface of the subject is substantially independent 
of the focal position which has been selected. 
The above objects are achieved in accordance with the principles of the 
present invention in an electromagnetic pressure pulse source having 
focused pressure pulses which has an electrically conductive membrane and 
a coil system for driving the membrane by rapid displacement thereof, in 
which the coil system and membrane in combination form an annular array 
having a plurality of annular zones which can be individually activated to 
generate pressure pulses in variable chronological relationship to each 
other. 
Background information relating to annular array technology in the context 
of ultrasound diagnostics is found in the article "Expanding-Aperture 
Annular Array," Dietz et al., Ultrasound Imaging, Vol. 1, No, 1, 1979, 
pages 56ff. The teachings of this article are incorporated herein by 
reference. 
By fashioning the pressure pulse source as an annular array, it is possible 
to generate pressure pulses having differently curved wave fronts by 
suitable selection of the points in time at which the individual annular 
zones are activated, and respectively different positions of the focus of 
the pressure pulses can thus be set. This is undertaken in a purely 
electronic manner, so that all mechanical components are eliminated in 
conjunction with the displacement of the focus. The points in time at 
which the annular zones are to be activated to generate pressure pulses 
for achieving a defined focal position can be easily calculated from the 
average running times which arise between the individual annular zones and 
the focus which has been selected. If all annular zones are simultaneously 
activated to generate pressure pulses, the curvature of the wave front 
thus produced corresponds to that of the annular zones. In all other 
cases, the curvature of the wave front which is generated will deviate 
from that of the annular zones. 
It is not necessary to change the physical spacing of the pressure pulse 
source relative to the surface of the subject to be sound-irradiated in 
order to displace the focus, so that the size of the surface of the 
subject which is to be charged with acoustic energy, and thus the stress 
experienced by that area, are essentially independent of the position of 
the focus which has been selected, which is particularly important in 
medical applications. Even when the zone to be acoustically irradiated, 
and thus the focus of the pressure pulses, lies immediately beneath the 
surface of the subject, a centrally arranged ultrasound B-scan applicator 
can be present and need not be retracted, so that a good image quality is 
always insured, and the mechanical components required for adjusting the 
position of the B-scan applicator can be eliminated. 
For a defined position of the focus, the points in time at which the 
individual annular zones are activated to generate pressure pulses are 
normally selected under the assumption that the pressure pulses emanating 
from the individual annular zones will simultaneously arrive at the focus 
which has been selected. It is also possible, however, to enlarge the 
diameter of the focus so that the pressure present at the focus can be 
lowered by slight deviations of these points in time. The characteristic 
of the focus, i.e. the diameter thereof and the pressure occurring in the 
focus, can thus be adapted to individual applications. Moreover, any 
dependency of the pressure in the focus and/or the diameter of the focus 
on the focus position which has been selected can be compensated. 
Similarly, the aperture and/or the focus diameter as well as the pressure 
occurring in the focus can be influenced by suppressing activation for 
generating pressure pulses of the outermost or innermost annular zone. 
In one embodiment of the invention, the coil system is arranged in a 
seating surface having sections allocated to the individual zones which 
are respectively fixed in space relative to each other. There is thus no 
change in the position of the zones relative to each other in order to 
displace the focus, or to vary its characteristic. 
Annular array technology is also described in German OS 31 19 295 in the 
context of piezoelectric pressure pulse sources for generating focused 
pressure pulses. Such an annular array has not been achieved in practice, 
however, because of serious disadvantages. Known piezoelectric pressure 
pulse sources must have an extremely large diameter, in comparison to 
other types of pressure pulse sources, in order to achieve a defined 
pressure in the focus. This requires an extremely high number of annular 
zones, which in turn requires a significant technical outlay. Moreover, 
the outermost annular zones must be extremely narrow, which creates 
further technological problems because rings which are sufficiently narrow 
and which also have the required electrical strength are extremely 
difficult to manufacture. It has therefore been assumed by those of skill 
in the art that pressure pulse sources for generating focused pressure 
pulses cannot be practically achieved using an annular array technique, 
with justifiable outlay. This assumption is demonstrated in European 
application 0 327 917, wherein an attempt is made to retain the advantages 
of annular array technology while avoiding the disadvantages thereof. 
European Application 0 327 917 discloses a pressure pulse source having a 
plurality of individual transducers arranged in a mosaic pattern, wherein 
the individual transducers are mechanically adjusted for displacement of 
the focus, and can be driven with chronological offset. 
The knowledge which those of skill in the art have relating to annular 
array techniques in piezoelectric technology cannot be automatically 
transferred to the context of an electromagnetic pressure pulse source 
since a person of skill in the art would assume that the portions of the 
coil system which would be allocated to the outer annular zones would have 
such a high inductivity that only a small current would flow when the coil 
system is charged with a high-voltage pulse in the standard manner for 
generating a pressure pulse. Because the pressure which can be achieved is 
approximately proportional to the square of the line density of the 
current flowing through the coil system, measured transversely relative to 
the winding direction, those of skill in the art would assume that 
electromagnetic pressure pulse sources making use of an annular array 
technique would have to have extremely large dimensions in order to 
achieve a defined pressure. This would mean that one of the important 
advantages of electromagnetic pressure pulse sources, namely their compact 
structure, would have to be sacrificed in order to make use of the annular 
array technique. The person of skill in the art would be additionally 
discouraged from pursuing the design of an electromagnetic pressure pulse 
source in an annular array technique because such a person would have 
assumed that the membrane would have to be formed by a plurality of 
separate membrane elements for each of the annular zones, which would 
require an enormous technological outlay. 
It has been surprisingly shown, however, that the membrane in the preferred 
versions of the invention may be a common membrane for several annular 
zones, and in fact may be a common membrane for all annular zones, with 
the coil system for each of the annular zones being a separate coil 
arrangement. When a specific annular zone is activated to generate 
pressure pulses by charging the corresponding coil arrangement with a 
high-voltage pulse, the common membrane is not driven surface-wide (i.e., 
is not driven overall) as would be expected. To the contrary, a pressure 
wave which drives the membrane is only introduced into a localized region 
of the membrane situated in the immediate proximity of the coil 
arrangement which has been charged. The regions of the membrane associated 
with the annular zones which have not been activated to generate a 
pressure pulse remain essentially inactive. In order to further avoid 
against activated annular zones undesirably influencing inactivated 
annular zones, in an embodiment of the invention the common membrane is 
provided with annular expansion beads disposed between neighboring annular 
zones. 
In a further embodiment of the invention, the annular zone and a generator 
for charging the coil arrangements respectively allocated to the annular 
zones with high-voltage pulses are dimensioned so that the pressure of the 
pressure pulses respectively emanating from the annular zones is 
substantially the same for each zone. This has a positive effect on the 
chronological course of the pressure pulse which results in the focus, 
because the pressure pulses emanating from the individual annular zones 
are substantially identically modified with respect to their pulse shape 
on their way to the focus as a result of non-linear compression properties 
of the media traversed by the pressure pulses. In a preferred embodiment, 
the coil arrangements for the respective annular zones are charged with 
high-voltage pulses of the same amplitude by the generator. A significant 
simplification of the generator is achieved as a result, because 
high-voltage pulses of the same amplitude are then required for all 
annular zones. 
A reduction in the number of annular zones, and thus a reduction in the 
dimensions, particularly the diameter, of the pressure pulse source are 
achieved in an embodiment of the invention wherein the membrane and the 
coil arrangements in the respective regions of the annular zones are 
curved around a geometrical focus. Preferably the membrane and the coil 
arrangements are curved around a common geometrical focus in the region of 
an annular zone. As a result of this measure, the coil arrangements of 
even the outer annular zones have only slight inductivity, so that high 
currents flow in the coil arrangements as a consequence of high-voltage 
pulses of a given amplitude, and pressure pulses of a correspondingly high 
pressure can be produced. These advantages are particularly pronounced in 
a preferred embodiment wherein the membrane and the coil arrangements are 
spherically concavely curved in the region of each annular zone, and in 
particular have the same radius of curvature in the region of all annular 
zones. In this embodiment, both the coil arrangements and the membrane are 
arranged on a spherical cap. For annular zones it has been shown that a 
pressure pulse source of such a structure having a diameter of 
approximately 160 mm and a radius of curvature of the spherical cap of 
approximately 160 mm is sufficient to achieve a displacement of the focus 
amounting to a total range of 100 min. With a given high-voltage pulse 
amplitude supplied to the coil arrangements, the pressure achieved in the 
focus is independent of the position of the focus to an extent which is 
not significantly less than that of a conventional electromagnetic 
pressure pulse source having a spherical cap structure as disclosed, for 
example, in German OS 33 12 014. 
A low number of annular zones is also achieved in a further version of the 
invention wherein the pressure pulse source is followed, in the direction 
of pulse propagation, by an acoustic lens, preferably a positive lens. The 
use of such a lens is advantageous with respect to the manufacturing 
outlay, because with such a lens the membrane and the coil system can be 
made planar. The acoustic lens is preferably a liquid lens, because such a 
lens can be constructed with a smaller thickness than solid lenses having 
the same focusing effect. 
In other modifications of the invention, the pressure pulse source may have 
a reflector on which the generated pressure pulses are incident, the 
reflector being curved around a geometrical focus. A relatively low number 
of annular zones is required with this modification, because the reflector 
itself provides a certain focusing effect. A compact structure of the 
pressure pulse source is achieved if the annular zones emit pressure 
pulses in a substantial radial direction, and the reflector reflects the 
pressure pulses substantially in an axial direction. The reflector 
preferably annularly surrounds the pressure pulse source, because a large 
aperture can be thereby achieved. 
In a preferred embodiment of the invention the outermost annular zone has 
an outside diameter in the range of 80 through 200 mm, and three through 
five annular zones are provided with a spherical curvature of the membrane 
and the coil arrangements. The membrane preferably has a radius of 
curvature which is also in a range between 80 and 200 mm, the radius of 
curvature preferably corresponding to the outside diameter of the 
outermost annular zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electromagnetic pressure pulse source constructed in accordance with the 
principles of the present invention is shown in FIG. 1 in the form of a 
shockwave source for medical purposes. In the example of FIG. 1, the 
shockwave source is used for non-invasive disintegration of a kidney stone 
S in the body K of a patient. Although differing in the structure and 
operation of the membrane and coil arrangement, the overall structure of 
the shockwave source shown in FIG. 1 generally corresponds to the 
shockwave source disclosed in German OS 33 12 014. A coil carrier 1, 
consisting of electrically insulating material, has a seating surface 2 
for a coil system generally referenced 3, which is spherically concavely 
curved around a geometrical focus FG of the shockwave source. A one-piece 
membrane 4, which is also spherically concavely curved around the 
geometrical focus FG, is disposed opposite that side of the coil system 3 
facing away from the coil carrier 1. The membrane 4 consists of 
electrically conductive material, for example copper or aluminum. The coil 
system 3 and the membrane 4 are separated from each other by an insulating 
foil 5 of constant thickness. The membrane 4 is clamped at its peripheral 
edge between the coil carrier I and an annular retainer part 6 by means of 
screws, with only the center lines of two such screws being schematically 
indicated by dot-dash lines. A flexible coupling membrane 7, consisting of 
polymeric material, is attached to the retainer part 6. The space defined 
by the membrane 4, the retainer part 6 and the coupling membrane 7 is 
filled with a liquid, acoustic propagation medium, for example water, for 
the shockwaves emanating from the membrane 4. 
In order to insure that the membrane 4, as shown, lies flush against the 
coil system 3 (with the insulating foil 5 interposed therebetween), 
measures not shown in FIG. 1 but disclosed in German OS 33 12 014 can be 
used which allow the water in the housing 6 to be placed under an elevated 
pressure in comparison to the ambient (atmospheric) pressure. 
Alternatively, the space between the membrane 4 and the coil system 3 can 
be evacuatable, as described in European Application 0 188 750. 
The shockwave source has a central bore which extends through the coil 
carrier 1, the coil system 3, the insulating foil 5 and the membrane 4. An 
ultrasound head 8 of, for example, an ultrasound B-scan applicator, as 
part of an ultrasound locating system is received liquid-tight in this 
bore. The ultrasound head 8 is adjustable by a schematically indicated 
adjustment system 9, so as to be movable at least in the direction of the 
center axis M of the shockwave source, which proceeds through the 
geometrical focus FG. The ultrasound head 8 can thus be brought into 
contact with the body surface of the body K, with the coupling membrane 7 
interposed therebetween, in the manner required for good image quality, as 
is shown in FIG. 1. 
In contrast to the shockwave source disclosed German OS 33 12 014, the coil 
system 3 in the inventive embodiment of FIG. 1 is not formed by a single, 
spherically curved coil having helically turns arranged on the seating 
surface 2. Instead, the coil system 3 is formed by four annular coils 3a, 
3b, 3c and 3d arranged concentrically relative to the center axis M of the 
shockwave source. These annular coils have respective terminals 10a 
through 10d and 11a through 11d. The turns of the annular coils 3a through 
3d respectively connecting the associated terminal pairs are helically 
wound on the seating surface 2. The annular coils 3a through 3d are 
connected via the terminals 10a through 10d and 11a through 11d to a 
high-voltage pulse generator 24, schematically shown in a block diagram. 
The pulse generator 24 includes respective high-voltage capacitors Ca 
through Cd for each of the annular coils 3a through 3d. Each annular coil 
3a through 3d also has a triggerable spark gap 12a through 12d allocated 
thereto. The spark gaps 12a through 12d permit the respective high-voltage 
capacitors Ca through Cd to be discharged into the respective annular 
coils 3a through 3d . The high-voltage capacitors Ca through Cd are 
connected to a single charging current source 13 with which the 
high-voltage capacitors Ca through Cd can be charged to a high-voltage, 
for example 20 kV. The trigger electrodes of the spark gaps 12a through 
12d are connected to the output of a trigger pulse generator 16 via 
respective trigger lines 14a through 14d which contain respective pulse 
delay circuits 15a through 15d. The trigger pulse generator 16 has a 
switch 17 and, depending on the position of the switch 17, provides either 
an internally generated periodic sequence of trigger pulses having a 
frequency of, for example, 2 Hz, or a single trigger pulse upon the 
actuation of a key 18 connected to the trigger pulse generator 16, or a 
single trigger pulse when a control pulse is supplied to the trigger pulse 
generator 16 via a line 19. The control pulse on the line 19 may be 
derived in a known manner from a periodic body function of the patient, 
for example from the respiratory cycle. 
The delay times ta through td of the respective pulse delay circuits 15a 
through 15d can be set via respective control lines 20a through 20d. The 
control lines 20a through 20d are connected to a control unit 21 having 
two adjustment knobs 22 and 23. The adjustment knob 22 serves the purpose 
of displacing the acoustic focus between the positions FN and FF along the 
center axis M of the shockwave source, the position FN being located 
closer to the shockwave source than the geometrical focus FG and the 
position FF being farther from the shockwave source than the geometrical 
focus FG. The adjustment knob 23 serves the purpose of varying the 
diameter of the acoustic focus. As used herein, the term "acoustic focus" 
is that region surrounding the location of maximum pressure which is 
limited by the -6dB isobar. The acoustic focus is therefore that region 
wherein the pressure is at least half the maximum occurring pressure. As 
used herein, the diameter of the acoustic focus means the maximum diameter 
thereof in a plane proceeding at a right angle relative to the center axis 
M of the shockwave source. 
When one of the spark gaps 12a through 12d is triggered, the corresponding 
high-voltage capacitor Ca through Cd discharges rapidly into the 
corresponding annular coil 3a through 3d . The pulse-like current which is 
thereby caused to flow through the respective annular coil 3a through 3d 
has an associated magnetic field. As a consequence of this magnetic field, 
eddy currents are induced in that annular region of the membrane 4 
disposed opposite the energized coil 3. The direction of the eddy currents 
in the annular region of the membrane is opposite to the direction of the 
current flowing in the energized annular coil. The eddy currents thus have 
an associated magnetic field which is oppositely directed to the magnetic 
field associated with the current flowing in the energized coil. Repelling 
forces are thereby generated between the energized coil and the associated 
annular region of the membrane 4, which cause a pressure pulse to be 
initiated into the water adjacent the annular region of the membrane 4. 
This pressure pulse is in the form of an annular wave front which is 
spherically curved, substantially around the geometrical focus FG. The 
pressure pulse intensifies in its propagation path through the water and 
through the body tissue of the patient to gradually form a shockwave. As 
used herein, a "shockwave" means a pressure pulse having an extremely 
steep rising (leading) edge. The term "shockwave" will always be used for 
simplicity, regardless of whether a generated pressure pulse has already 
intensified to form a shockwave. The shockwave source of FIG. 1 thus has 
four annular zones Za through Zd which can be activated to independently 
generate shockwaves relative to each other. For the geometrical focus FG, 
the margin rays of the pressure pulses of the respective annular zones Za 
through Zd are indicated with dot-dash lines. The sections of the seating 
surface which carry the annular coils 3a through 3d associated with the 
zones Za through Zd are stationary relative to each other. 
The control unit 21 is constructed so that the delay times ta through td 
are of identical size when the adjustment knob 22 is, for example, in a 
middle setting position. When the adjustment knob 22 is brought to this 
position and a trigger pulse from the trigger pulse generator 16 is 
supplied to the pulse delay circuits 15a through 15d the annular zones Za 
through Zd are thus simultaneously activated to generate shockwaves. As a 
consequence of the spherical curvature of the membrane 4 and of the 
annular coils 3a through 3d the resulting shockwaves simultaneously arrive 
in the acoustic focus forming in the immediate proximity of the 
geometrical focus FG, having the effect of a single shockwave being formed 
at that location. The control unit 21 is further fashioned so that when 
the adjustment knob 22 is set at one extreme position, the delay times ta 
through td are set so that a trigger pulse for activating the outermost 
annular zone Za occurs first, followed by triggering of pulses in the 
annular zones Zb and Zd, Zc and the innermost annular zone Zd. The delay 
times ta through td are matched to each other so that the respective 
shockwaves emanating from the individual annular zones Za through Zd 
simultaneously arrive in the position FN of the acoustic focus. When the 
adjustment knob 22 is set at the opposite extreme setting position, the 
delay times ta through td are set so that the innermost annular zone Zd is 
first triggered and is thus first activated to generate a shockwave, 
followed by the zones Zc, Zb and the outermost zone Za. Again, the delay 
times ta through td are selected so that the shockwaves emanating from the 
respective annular zones Za through Zd simultaneously arrive in the 
position FF of the acoustic focus. Between these two extreme positions of 
the adjustment knob 22, the delay times ta through td are varied so that 
the acoustic focus can be shifted between the extreme positions FN and FF 
with infinite variation, the above-explained special case arising when all 
delay times ta through td are identical. The delay times ta through td are 
set for each position of the acoustic focus so that, as stated above, the 
respective shockwaves emanating from the individual annular zones Za 
through Zd simultaneously arrive in the acoustic focus which has been 
selected. 
The individual pulses arrive simultaneously at the selected focus location, 
however, only when the adjustment knob 23 is set at one extreme position. 
The control unit 21 is constructed so that, as the adjustment knob 23 is 
adjusted in the direction of its other extreme position, the delay times 
ta through td increasingly deviate from those delay times for which the 
shockwaves emanating from the annular zones Za through Zd would 
simultaneously arrive at the selected acoustic focus. The maximum 
deviation in the case of the described exemplary embodiment is to .+-.100% 
of the delay times ta through td required for the simultaneous arrival of 
the shockwaves at the selected acoustic focus. Once a location for the 
acoustic focus has been selected, a maximum pressure and a smallest focus 
diameter will result in the case of the simultaneous arrival of all 
shockwaves at the selected acoustic focus. With increasing deviation of 
the delay times ta through td, the maximum pressure is increasingly 
reduced and the diameter of the acoustic focus is increasingly enlarged 
from the case which would occur with identical delay times. This permits 
the maximum pressure and the acoustic focus diameter to be adapted to 
individual requirements. For deviations in the delay times ta through td 
by .+-.100%, a reduction in the maximum pressure by approximately 50% 
occurs, and an enlargement in the diameter of the acoustic focus by 
approximately 100% occurs. In the exemplary embodiment described in FIG. 
1, the control unit 21 is constructed so that, given positions of the 
adjustment knob 23 deviating from the extreme position causing 
simultaneous arrival of the shockwaves, the delay times ta and tc are 
varied so that the shockwaves emanating from the annular zones Za and Zc 
simultaneously arrive at a point which is at a greater distance from the 
shockwave source than the acoustic focus set by means of the adjustment 
knob 22 alone. The delay times tb and td are set by the control unit 21 so 
that the shockwaves emanating from the annular zones Zb and Zd 
simultaneously arrive at a point which is closer to the shockwave source 
than the acoustic focus which has been set by the adjustment knob 22 
alone. The extent to which these points lie outside the acoustic focus in 
one or the other direction increases as the position of the adjustment 
knob 23 increasing deviates from the one extreme position causing 
simultaneous arrival. 
As can be seen in FIG. 1, the outermost and innermost boundary rays of the 
pressure pulses for the cases of focusing onto FN and FF are shown in dot 
and dash lines, from which is apparent that displacement of the focus has 
substantially no influence on the size of the area of the body surface of 
the patient charged with acoustic energy. Thus pain or hematoma will not 
occur even if stones lying immediately under the body surface are treated. 
As can also be seen in FIG. 1, the ultrasound head 8 can remain in 
engagement with the surface of the body K (with the coupling membrane 7 
interposed therebetween) even given the shortest possible focal distance 
FN, without the ultrasound head 8 being situated in the propagation path 
of the shockwaves. 
The annular zones Za through Zd, and the corresponding annular coils 3a 
through 3d are dimensioned so that, taking the capacitances and the 
charging voltages of the capacitors Ca through Cd into consideration (both 
the charging voltages and the capacitances are identical for each zone in 
the case of the exemplary embodiment), the shockwaves each achieve the 
same pressure in the unfocused condition, i.e., in the immediate proximity 
of the membrane 4. This is the case in the pressure pulse source shown in 
FIG. 1, for example, when the outside diameter D of the outermost annular 
zone Za and the radius of curvature R of the membrane 4 each amount to 160 
mm, the radii r0 through r4 are respectively 30 mm, 45 mm, 61 mm, 63 mm 
and 80 mm, and the annular coils 3a through 3d are respectively formed by 
eight turns of a I mm diameter wire, 9 turns of a 1.5 mm diameter wire, 12 
turns of a 1.5 mm diameter wire, and 14 turns of a 1 mm diameter wire. 
Given such dimensioning, the resulting inductances of the annular coils 3a 
through 3d are on the order of magnitude of a few .mu.H, as in the case of 
conventional pressure pulse sources. This means that, given an overall 
capacitance of the high-voltage capacitors Ca through Cd corresponding to 
the capacitance which is standard in conventional practice, currents 
having approximately the same amplitude as in conventional pressure pulse 
sources will flow in the coils. The different wire diameters are not shown 
in FIG. 1, and the wire diameter of the annular coils 3a through 3d as 
well as the thicknesses of the membrane 4 and the insulating foil 5 are 
shown exaggerated for clarity. With the aforementioned dimensioning, the 
focus of the shockwaves can be displaced by a total of 100 mm without 
producing a noteworthy loss in pressure. The focus position FN having the 
smallest distance from the shockwave source has a distance of 
approximately 54 mm from the geometrical focus FG. 
One terminal of each of the high-voltage capacitors Ca through Cd is at 
grounded potential. The high-voltage capacitors Ca through Cd are 
connected via coaxial lines (not shown in FIG. 1) to the annular coils 3a 
through 3d (as well as via the respective spark gaps 12a through 12d ) so 
that a high difference in potential occurs only between the annular coils 
3b and 3c. Increased insulation must therefore only be provided between 
the annular coils 3b and 3c, as indicated in FIG. 1 by the somewhat larger 
spacing between these coils. The spaces between the annular coils 3d as 
well as between their respective turns, moreover, are filled with an 
insulating casting resin in a known manner, but not shown in the drawings. 
For conducting treatment, the shockwave source and the body K of the 
patient are first positioned relative to each other so that the calculus 
to be disintegrated is located on the center axis M of the shockwave 
source. This occurs using the ultrasound locating system to which the 
ultrasound head 8 is connected via a line 8a. A line-shaped mark is mixed 
into the ultrasound image displayed on a picture screen (not shown) in a 
known manner, the position of the mark corresponding to that of the center 
axis M. Subsequently, the acoustic focus is displaced by actuating the 
adjustment knob 22 so that it coincides with the mark, as indicated in 
FIG. 1 by the designation for the acoustic focus F. The position of the 
acoustic focus can be checked in the ultrasound image with reference to a 
mark whose position changes upon actuation of the adjustment knob 22 in 
accord with the displacement of the acoustic focus. Corresponding data are 
supplied to the ultrasound locating system by the control unit 21 via a 
line 21a. Because the position of the mark identifying the acoustic focus 
is dependent on the position of the ultrasound head along the center axis 
M, data identifying the position of the ultrasound head on the center axis 
M are supplied to the ultrasound locating system from the adjustment unit 
9 via a line 9a. When the acoustic focus has been set in the described 
manner, the stone S is disintegrated into fragments with a series of 
shockwaves, until the fragments are so small that they can be eliminated 
naturally. 
A further embodiment of a shockwave source constructed in accordance with 
the principles of the present invention is shown in FIG. 2 wherein 
elements identical or similar to those already discussed in connection 
with FIG. 1 are provided with the same reference symbols. The primary 
difference between the embodiments of FIG. 1 and FIG. 2 is that the 
embodiment of FIG. 2 has a seating surface 2 which is planar, disposed at 
one end of a tubular housing 30 to which the coil carrier 1 is attached by 
screws or suitable fasteners indicated with dot-dash lines. Consequently, 
the coil system formed by the annular coils 3a through 3d the membrane 4 
and the insulating foil 5 are also planar. As a substitute for the lack of 
the spherical curvature, the shockwave source of FIG. 2 has an acoustic 
positive lens in the form of a plano-convex liquid lens 25 disposed in the 
path of shockwave propagation toward the subject. The liquid lens has an 
entry wall 26 consisting of polymethyipentene (TPX), an exit wall 27 
consisting of polytetrafluorethylene, and contains a lens liquid 28 
enclosed between the entry wall 26 and exit wall 27. The lens liquid 28 is 
a fluorocarbon liquid, for example Flutec PP3.RTM. or Fluorinert 
FC75.RTM.. 
Because the housing 30, closed at its opposite end by the coupling membrane 
7, contains water as the propagation medium for the shockwaves, and the 
speed of sound is lower in the lens fluid 28 than in water, the 
plano-convex shape of the liquid lens 25 effects a focusing of the 
shockwaves onto the geometrical focus FG which lies on the center axis M 
of the shockwave source, when the annular zones Za through Zd are 
simultaneously activated to generate a pressure pulse. Due to the 
simultaneous activation of the annular zones Za through Zd, a single 
planar shockwave arrives at the liquid lens 25. By actuating the 
adjustment knobs 22 and/or 23 of the high-voltage pulse generator 24 (not 
shown again in FIG. 2) in the manner described above in connection with 
the embodiment of FIG. 1, the focus of the shockwaves can be shifted with 
infinite variation between the positions FN and FF, or a variation in the 
pressure and the diameter of the acoustic focus can be effected. 
The liquid lens 25 may alternatively be of a biconvex shape. The use of the 
liquid lens 25 offers the advantage of lower thickness in comparison to a 
plano-concave or biconcave solid lens which would, for example, consist of 
polystryol. The use of a liquid lens 25, however, can introduce 
non-linearities in the event of transmission at extremely large acoustic 
powers, because of the highly non-linear compression properties of the 
lens fluid 28. 
In the embodiment of FIG. 2, the membrane 4 includes expansion beads 29a 
through 29c disposed between the annular zones Za through Zd which 
increase the elasticity of the membrane 4 and thus prevent a premature 
failure due to elevated mechanical stresses. As shown in FIG. 2, further 
expansion beads can be provided at the outer edge of the annular zone Za 
and at the inner edge of the annular zone Zd. 
The pressure pulse source shown in the embodiment of FIG. 3 is constructed 
according to the principles of a LARS source (large aperture ring-shaped 
sound source) as generally described in German OS 38 35 318. In this 
embodiment, a radially outwardly emitting, tubular membrane 35 is used as 
the displaceable membrane, which is driven by a coil system 37 disposed 
inside the membrane 35 and wound helically around a tubular coil carrier 
36. The membrane 35 and the coil system 37 are separated from each other 
by an insulating foil 38. The coil system 37 is subdivided into four 
tubular coils 39a through 39d disposed in axial succession on the coil 
carrier 36. The coils 39a through 39d are connected to a high-voltage 
pulse generator 24 (as shown in FIG. 1, but not shown in FIG. 3) via 
terminals 40a through 40d and 41a through 41d in a manner analogous to 
that shown in FIG. 1. 
When the tubular coils 39a through 39d are charged, the respective regions 
of the membrane 35 surrounding the charged coil expands, resulting in the 
introduction of a shockwave into the water contained in the shockwave 
generator as a propagation medium. A total of four annular zones Za 
through Zd are thus present, which can be activated to emit radially 
propagating shockwaves. The shockwaves are incident on a reflector 42 
annularly surrounding the membrane 35, having a reflector face produced by 
the rotation of a section of a parabola P, indicated with dot-dashed 
lines, and having a focal point coinciding with the geometrical focus FG 
of the arrangement, which lies on the center axis M of the shockwave 
source. The vertex SCH of the parabola P lies on a straight line which 
intersects the center axis M at a right angle. When all four annular zones 
Za through Zd are simultaneously activated to generate a shockwave, a 
shockwave having a cylindrical wave front is introduced into the water, 
which is then focused by the reflector 42 onto the focal point of the 
parabola P. By actuating the adjustment knobs 22 and 23 of the 
high-voltage pulse generator 21, the acoustic focus can be displaced 
between the positions FF and FN, or the pressure and the diameter of the 
acoustic focus can be varied, as described above. 
The ultrasound head 8 is arranged in a central bore of the coil carrier 36 
so as to be longitudinally displaceable therein. The ultrasound head 8 has 
associated adjustment elements as described above in connection with FIG. 
1, but which are not shown in FIG. 3. 
All of the above exemplary embodiments have in common the advantage that 
adjustability of the acoustic focus is achieved over a broad range, for 
example, of 100 mm, and to achieve this result the shockwave source 
constructed as an annular array need only have four annular zones. The 
additional outlay over conventional shockwave sources is maintained within 
reasonable limits because a common membrane can be employed for all 
annular zones. It is only necessary to undertake a subdivision of the coil 
system into a plurality of annular or tubular coils corresponding in 
number to the plurality of annular zones, to provide a plurality of 
smaller capacitors corresponding in number to the plurality of annular 
zones instead of a large capacitor, and to provide a plurality of spark 
gaps corresponding in number to the plurality of annular zones instead of 
one spark gap. Relatively uncomplicated electronics in the form of the 
above-described control unit and pulse delay circuits can be used. For 
applications wherein a reduction in the quality of the focusing and/or of 
the adjustment range of the acoustic focus are acceptable, three annular 
zones may be adequate. 
In each of the above embodiments, physical focusing of the shockwaves is 
employed, by means of the spherical curvature in the embodiment of FIG. 1, 
the acoustic lens in the embodiment of FIG. 2, and the curvature of the 
reflector in the embodiment of FIG. 3. It is still within the inventive 
concept disclosed herein, however, to undertake focusing exclusively by 
electronic means, however, this would require an increased number of 
annular zones to obtain the same acoustic focus adjustability and the same 
quality of the focus as are achieved in the embodiments specifically shown 
in FIGS. 1, 2 and 3. 
The above exemplary embodiments have been set forth in the context of a 
pressure pulse source for the disintegration of calculi. It will be 
understood by those of skill in the art, however, that the principles and 
structure disclosed herein can be used for other medical purposes as well 
as for non-medical purposes. 
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.