Sensor for acoustic shockwave pulses

A sensor for acoustic shockwave pulses, such as in lithotripsy, includes a piezo-electric measuring membrane disposed between two coupling membranes. The intervening space between the coupling membranes is filled with a coupling medium for transmitting the acoustic shockwave pulses. The measuring membrane is in a defined condition largely independent of external influences so that flows in the coupling medium or different pressure exertions on the two coupling membranes do not influence the measured signal produced by the measuring membrane.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a sensor for measuring acoustic 
shockwave pulses and using a piezo-electric measuring membrane. 
2. Description of the Related Art 
Acoustic shockwave pulse sensors are useful in lithotripsy, for example, in 
disintegating kidney stones through the use of shockwaves. 
A shockwave sensor is disclosed in German Published Application No. 34 37 
976. The shockwave sensor is used for measuring pressure amplitudes of a 
shockwave pulse in a propagation medium, such as in water. An important 
application for such sensor is to measure the pressure at the focus of 
focused shockwaves. The German published application, however, does not 
disclose how the sensor is to be constructed outside of the propagation 
medium, for example, in a lithotripsy application referred to as "dry 
coupling" with the patient. Likewise not discussed are difficulties which 
arise due to incorrect measured values identified by the piezo-electric 
measuring membrane as a result of secondary causes, such as, for example, 
circulation of the propagating medium (i.e. water) by pumps or due to the 
flow of and/or waves in the propagating medium which arise from the 
transmission of the shockwave. 
In German Published Application No. 33 28 051 is disclosed the use of 
shockwave tubes in lithotripsy. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a sensor for acoustic shockwave 
pulses which provides defined measuring conditions independent of where 
the sensor is used. It is thus possible to accurately measure shockwave 
emitted by an independent pulse source with the present sensor. Under 
certain conditions, the sensor is also capable of being coupled directly 
to a source, or on the other hand, is capable of being introduced into a 
test basin or patient tank. In a particular example, the present sensor is 
suitable for dry coupling in conjunction with a lithotriptor. 
These and other objects are achieved in a sensor including a piezo-electric 
measuring membrane disposed between two coupling membranes in which the 
intervening space between the coupling membranes is filled with a coupling 
medium for transmission of the acoustic shockwave pulses. 
When, for example, the present invention is arranged within a propagating 
medium in the transmission path of a lithotriptor, then the disposition of 
the coupling membranes or webs on each side of the measuring membrane 
prevents flows or waves which propagate in the transmission path from 
exerting an influence on the measuring membrane. An advantage is thereby 
realized when the measuring membrane is situated in a closed capsule so 
that waves in the propagating medium which proceed in the direction of the 
membrane plane are kept away from the measuring membrane. 
When, on the other hand, the present sensor is utilized in lithotripsy in 
combination with a shockwave generator by "dry coupling" i.e. without a 
patient tank, then the sensor is situated between a transmitting membrane 
of the shockwave generator and the skin of the patient, or a coupling disk 
connected to the skin of the patient when such disk is provided, so that 
the sensor identifies the measured values of the shockwave. In the case of 
the dry coupling, mutually different forces are exerted on the two 
coupling membranes. These different forces stress the piezo-electric 
measuring membrane and cause inaccurate measurements, and can even result 
in damage to the sensor. If the coupling medium in the sensor is in 
pressure equilibrium at both sides of the measuring membrane so that the 
same static pressures act on both sides of the measuring membrane, then 
the undesirable pre-stressing of the piezo-electric measuring membrane and 
the resulting imprecise measurements and even damage to the sensor are 
avoided. Pressure equilibrium is created in the present sensor by 
providing a conducting opening extending through a partition wall which 
separates the two coupling membranes. Since the measuring membrane is 
mounted on the partition wall, the pressure is equalized at both sides of 
the measuring membrane. 
Further advantages and developments of the invention derive from providing 
a filling opening through which the coupling medium is supplied into the 
sensor. An electrical connection for transmitting electrical signals 
externally of the sensor are also preferrably provided connected to the 
piezo-electric membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a disk-shaped sensor including a ring, or annular-shaped 
housing wall, 1 which includes a partition wall 3 extending radially 
inwardly at a middle of an inside edge of the ring 1. The partition wall 3 
of the illustrated embodiment is parallel to the two opposite end faces of 
the ring 1. The partition wall 3 is provided with a central opening 5 over 
which extends a piezo-electric measuring membrane, or foil, 7. The central 
opening is circular in shape and has a diameter of approximately 40 mm so 
that the conducting area, or sensing area, of the measuring membrane for 
shockwave pulses also has a diameter of approximately 40 mm. The ring or 
housing wall 1 is composed, for example, of a plastic material, such as 
PVC (polyvinyl chloride). The measuring, or sensing, membrane 7 is 
preferrably of a web of PVDF (polyvinylidene fluoride). Although the 
preferred embodiment is circular with a like-shaped measuring membrane, 
other shapes can be used as well. 
A clamp ring 8 holds the measuring membrane 7 in place over the central 
opening 5 so that the measuring membrane 7 extends across the central 
opening 5 without creases. The clamp ring 8, in one example, is of plastic 
material and is held by conventional plastic screws 8a extending through 
the clamp ring 8 and screwed into the partition wall 3. Other fastening 
means can also be used in place thereof. 
A first coupling membrane, or web, 9 is clamped at a first upper end face 
of the ring 1 by a retaining ring 11, the retaining ring 11 being held by 
a fastening means 11a. The coupling membrane 9 is fabricated of rubber 
such as silicone or EPDM (ethylene-propylene-diene monomer) rubber hving a 
thickness of between 1 and 2 mm, inclusive, and a diameter of 
approximately 120 mm. The coupling membrane 9 is clamped between the end 
face of the ring 1 and the retaining ring 11 so that it extends across the 
extent of the inside diameter of the ring 1 in a slightly stretched 
condition and without creases. 
A disk-shaped, first intervening space 13 extending from the inside edges 
of the ring 1 is defined between the first coupling membrane 9, on one 
side, and the partition wall 3 and measuring membrane 7, on the other 
side. The intervening space 13 is filled with a coupling medium 14 which 
transmits acoustic shockwaves. In one example, the coupling medium 14 is 
degasified distilled water. In another example, oils, such as castor oil, 
can also be used. 
The configuration of first coupling membrane 9, the first retaining ring 11 
and the first intervening space 13 are provided in mirror image at a lower 
side of the sensor, as well. In particular, a second coupling membrane 15 
also made of rubber is fixed against a lower end face of the ring 1 one by 
a second retaining ring 17 held by fastening means 17a. Both the fastening 
means 11a and the fastening means 17a are plastic screws, although other 
fastening means can be used as well. A second intervening chamber, or 
space 19, is formed having similar dimensions as the upper space 13. The 
second intervening space 19 is also filled with the coupling medium 14. In 
one embodiment the intervening spaces 13 and 19 have diameters of, for 
example, approximately 100 mm. Thus, the coupling membranes 9 and 15 each 
have an effective surface of like diameter. In one embodiment, at least 
one of the coupling membranes 9 and 15 is formed of an optically 
transparent material to permit observation of the measuring membrane 7 
from outside the sensor. 
The first intervening space 13 is connected to the second intervening space 
19 by one or more conducting openings extending through the partition wall 
3. The conducting openings 21 enable the pressure to equalize between the 
first and second intervening spaces 13 and 19. It is thereby assured that 
the measuring membrane 7 is unstressed, and when the partition is midway 
between the coupling membranes, the measuring membrane 7 is likewise 
arranged roughly midway between the coupling membranes 9 and 15 since the 
same static pressure appears at both sides thereof. 
The described sensor can be used as a whole in the transmission path of a 
shockwave generator of a lithotriptor without disturbing the measuring 
membrane despite flows and waves which occur in the transmission path. The 
sensor can also be placed between a coupling membrane of the lithotriptor 
generator (not shown) and a patient. 
FIG. 2 shows one possible procedure for electrically contacting the 
measuring membrane 7. The measuring membrane 7 extends beyond an outer 
edge of the clamp ring 8 and is electrically connected by a clamp 
connection 25 which, for example, is formed by a pressure plate 
resiliently held in place by a screw, A solder lug 27 is fastened to the 
clamp 25. A bipolar instrument lead 29 shown in FIG. 2 as a coaxial 
conductor, is soldered to the solder lug 27 and is conducted through a 
radial opening 30 in the ring 1. Care must be taken to ensure the 
existence of a water-tight seal between the ring 1 and the bipolar 
instrument line 29. It is of course understood that an additional 
electrical connection to the measuring membrane 7 is to be made as well. 
Referring now to FIG. 3, identical components are provided with the same 
reference characters as in FIG. 1 and 2. Approximately in the center of 
the ring 1, an introduction opening 31 is provided with a short pipe or 
conduit 33 which is connected to the ring 1 in water-tight fashion. The 
pipe for conduit 33 is connectable to a hose (not shown) through which the 
coupling agent or medium 14 is supplied into the sensor. After filling the 
intervening spaces or chambers 13 and 19 with the coupling medium 14, the 
hose is removed and a cover cap 35 is placed over the conduit 33. The 
cover cap 35 closes the introduction opening 31 so that the coupling 
medium 14 is not mixed with substances outside the sensor. 
Although other modifications and changes may be suggested by those skilled 
in the art, it is the intention of the inventors to embody within the 
patent warranted hereon all changes and modifications as reasonably and 
properly come within the scope of their contribution to the art.