Piezoelectric device with reduced negative waves, and use of said device for extracorporeal lithotrity or for destroying particular tissues

The invention relates to a device generating ultrasonic waves focused in a ocal point. Said generator device comprises a plurality of transducer elements arranged externally on a support means for focusing into a focal point, such as a circular dome, and is characterized in that the transducer elements are divided into at least two groups of transducer elements, of which the resonance frequency differs in such a way that the principal positive ultrasonic waves produced by each group add up together and the positive and negative secondary ultrasonic waves are at least partly cancelled, thus reducing significantly the negative waves. This device can be advantageously used for extracorporeal lithotripsy.

The present invention essentially relates to a piezoelectric device with 
reduced negative waves, and to the use of such device for extracorporeal 
lithotripse or for the destruction of particular tissues. 
BACKGROUND OF THE INVENTION 
Conventionally, a piezoelectric generator is essentially composed of one, 
or several or a very large number of piezoelectric elements preferably 
arranged on a circular dome permitting the focusing of all the minute 
waves created by each elementary particle constituting the transducer or 
transducers (see for example the document "Ultrasonics", vol. 5, of April 
1967, pages 105-112; P. P. LELE). 
Another device for generating ultrasonic waves focused in one point has 
also been described, which device comprises a plurality of transducer 
elements of piezoelectric type arranged externally on a circular dome, as 
well as its use for therapeutical purposes, for example for producing 
controlled ocular lesions (see article by COLEMAN, in the American Journal 
of Ophthalmology 86:185-192, 1978). COLEMAN also describes the paraxial 
presence of a diagnosis transducer. 
Experience has proved that the quality of the focusing or in other words, 
the volume of the focal spot, will be smaller as the filling coefficient 
is near to 1 and as the opening defined by the angle of the circular dome 
is high. 
It is further known that the shape of the wave at the focal point is 
identical or near-identical and in any case closely dependent on the shape 
of the wave emitted at the level of the piezoelectric transducer. In the 
case of a transducer with one face in contact with the water and the other 
face in contact with the air (the simplest mounting possible and the most 
used), the generated wave, after an electric pulse, is in the form of a 
damped sine wave presenting positive and negative waves. 
It has been found that negative waves are dangerous for the tissues because 
they can induce cavitation effects. (see article by COLEMAN et al. in 
"Ultrasound in Med. & Biol., Vol. 13, No. 2, pages 69=14 76, 1987). 
The prior art has already proposed to create waves called single-pole waves 
(document DE U.S. Pat. No. 34 25 992). 
When the transducer is subjected to a very short pulse, it delivers a 
series of pulses, the front face emitting straight polarity waves while 
the rear face emits reversed polarity waves. The resultant emitted wave is 
of course the sum of those waves which is therefore now positive, now 
negative. 
The object of the solution proposed in document DE U.S. Pat. No. 34 25 992 
is to "separate" such positive and negatives waves. To do this, the rear 
face is cut irregularly in such a way that the waves reflected by said 
rear face are not focused or very incorrectly focused. The result is that 
the ratio of the positive wave to the negative wave increases as the focal 
point gets closer. 
It should be noted that, with said solution, the second positive wave will 
also be incorrectly focused since it is due to the reflection of the first 
wave onto the rear face of the transducer. 
It is also known to adapt the piezoelectric transducer by placing on the 
front face a material having an impedance ranging between the transducer 
impedance and that of the water used as transmission medium for 
transmitting the ultrasonic wave to the target situated at the focal point 
(see Vol. I, Part A of Physical Acoustics by MASON, Academic Press). 
It is found then, firstly that the signal duration is very short, and 
secondly that the reached maximum pressure value is much higher as can be 
seen when comparing FIG. 1a (emission of non-adapted ultrasonic waves) and 
FIG. 1b (emission of ultrasonic waves by an adapted transducer). 
Therefore, with the single-polar system such as proposed in DE U.S. Pat. 
No. 34 25 992, as only the first wave is focused and as regretfully said 
wave is always weak comparatively to the second and third waves, the ratio 
of the focused wave between an adapted transducer and a non-adapted 
transducer is in the region of a factor 10, but it is easy to realize that 
in the case of a non-adapted transducer, of which the rear face is not 
irregularly cut, the system generates a negative wave of amplitude equal 
to the positive wave. 
In other terms, even with this solution, the negative wave is still present 
with an intensity such that it is capable of inducing cavitation effects 
dangerous for the tissues situated near to either the concretion or to the 
particular tissues to be destroyed. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to solve the aforesaid 
new technical problem by proposing a solution capable of reducing or 
eliminating completely the amplitude of the negative wave which is 
inherently present in the sine wave produced by any piezoelectric 
transducer. 
It is also the object of the present invention to solve the new technical 
problem which arises by proposing a solution capable not only of reducing 
or eliminating completely the negative wave inherent in the sine wave 
produced by a piezoelectric transducer, but also of keeping to the same 
level the height of the peaks of the emitted wave, hence the pressure peak 
value comparatively to the prior art techniques, and in particular to the 
techniques of reduction of negative wave amplitude. 
Such new technical problems have been solved satisfactorily for the first 
time by the present invention, on an industrial scale. 
To this end, the present invention proposes a device for generating 
ultrasonic waves focused in a point having reduced negative waves, which 
device comprises a plurality of transducer elements of piezoelectric type, 
arranged externally on support means for focussing into a focal point, 
such as for example a circular dome device wherein said transducing 
elements are divided in at least two groups of transducer elements of 
which the resonance frequency differs in such a way that the principal 
positive ultrasonic waves produced by each group are added together, and 
the negative and positive secondary ultrasonic waves are at least partly 
cancelled out. 
According to one particularly advantageous embodiment of the invention, the 
second group of transducer elements, or the other groups of transducer 
elements, has a resonance frequency equal or substantially equal to a 
multiple or a submultiple of the resonance frequency of the transducer 
elements of the first group. 
According to a particular embodiment, the resonance frequency of the second 
group of transducer elements is equal or substantially equal to twice the 
resonance frequency of the first group of transducer elements. 
According to another particularly advantageous embodiment, the transducer 
elements of each group are arranged in alternate fashion, so that the 
total surface occupied by the elements of each group is substantially 
identical. 
According to yet another particular embodiment of the invention, all the 
transducers elements of the different groups are at the same distance, 
called focal distance, from the focal point. When the support means is a 
circular dome, the focal distance is preferably equal to the dome radius 
of curvature. 
In the simpler case where the focal distance is the same for all the 
transducer elements of the different groups, a coinciding of the maximum 
pressure peaks is reached between the groups which have different 
resonance frequencies by an electronic ordering of each group of given 
frequency, this being achieved by sending pulses or a front of delayed or 
advanced voltage depending on the group being ordered with respect to the 
reference group, the time difference (delay or advance) being given by the 
following formula: 
EQU .DELTA.t=.lambda./4 V, 
in which: 
V is the ultrasonic speed inside the medium (generally water), is the 
wavelength of the ordered group. 
In the case where the support means is flat, it is possible to effect an 
electronic focusing as described in document DE-A-31 19 295. 
According to yet another particular embodiment, when the support means is a 
circular dome, the distance set between the front face of the transducer 
elements of one given group and the focusing point, is different from the 
same distance to the focusing point of the other groups of different 
resonance frequency, the electric drive of all the groups is simultaneous 
so that the maximum pressure peaks coincide. 
For example, the focal distance difference between the first group and the 
second group is about one quarter of a wavelength. Consequently, this 
compensation corresponds to the time compensation (.DELTA.t) when the 
focal distance is constant. 
Moreover, when the resonance frequency of a given group is lower than the 
resonance frequency of the first group or reference group, the distance 
between the transducers and the focus point of said given group is greater 
than the distance between the transducers and the focus point of the 
reference group, this meaning that the transducer elements of said given 
group are offset with respect to the transducer elements of the reference 
group. 
Similarly, if the resonance frequency is higher for the transducer elements 
of the given group, the distance between the transducers and the focal 
point is smaller than that between said focal point and the reference 
group of transducer elements, meaning that the transducer elements of said 
given group of higher resonance frequency are placed in front with respect 
to the transducer elements of the reference group. 
According to another particularly advantageous embodiment, the device 
comprises a third group of transducer elements of which the resonance 
frequency is equal or substantially equal to four times the resonance 
frequency of the first group of transducer elements. 
It is clear from anyone skilled in the art that the sum of the waves 
emitted by the transducer groups of different resonance frequency will 
give a resulting wave with a reduced amount of negative waves due to said 
combination, the principal positive waves adding up together while the 
negative waves are cancelled to a maximum, particularly by the secondary 
positive waves. It will be worth noting that the total sum of the 
principal positive waves produces a pressure peak identical to that 
produced in the conventional case in which only one resonance frequency is 
used for all the transducer elements. 
The result is an effect which is totally unexpected and nonobvious to 
anyone skilled in the art who wants to destroy concretions lithotripsy or 
special tissues (surgical operations), or even to an expert in ultrasonic 
techniques.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring first to FIGS. 2, 2a and 2b, these show a device for generating 
ultrasonic waves focused in one focal point, with a reduced amount of 
negative waves according to the present invention. 
Said generator device designated by general reference 1 is of a type which 
is well known in ultrasonic generator technique, as partly summarized in 
the introductory part of the present description. 
The illustrated generator device 1 comprises a plurality of piezoelectric 
type transducer elements 2, 4, 6, arranged externally on a support means 8 
for focusing in one focal point 10. Said support means 8 is, in the 
illustrated case, a circular dome. This structure as well as the means for 
transmitting the pulses to the transducer elements, being known of anyone 
skilled in the art, will not be further described. 
According to the present invention, the device is characterized in that the 
transducer elements are divided in at least two groups of transducer 
elements, i.e. as illustrated by way of example, a first group of 
transducer elements 2, a second group of transducer elements 4 and a third 
group of transducer elements 6, the resonance frequencies of which differ 
in such a way that the principal positive ultrasonic waves produced by 
each group are adding up and the secondary ultrasonic (positive or 
negative waves) are at least partly cancelled. 
According to a particularly advantageous embodiment of the invention, the 
second group of transducer elements 4, or the other groups of transducer 
elements (4, 6), have a resonance frequency which is equal or 
substantially equal to a multiple or a submultiple of the resonance 
frequency of the transducer elements 2 of the first group, particularly to 
a multiple of said frequency. 
According to another preferred embodiment, the resonance frequency of the 
second group of transducer elements 4 is equal or substantially equal to 
twice the resonance frequency F.sub.o of the first group of transducer 
elements 2. 
According to another illustrated particular embodiment, the transducer 
elements of each group are arranged in alternated fashion, so that the 
total surface occupied by the transducer elements of each group is 
substantially identical, as can be seen clearly from FIGS. 2, 2a and 2b. 
Taking for example FIG. 2a which illustrates a variant embodiment with 
three groups of transducer elements of different resonance frequency 
arranged in alternated fashion and designated respectively, the first 
group of transducer elements 2, the second group of transducer elements 4 
and the third group of transducer elements 6. In the case of a third group 
of elements such as 6 being present, the resonance frequency of said 
transducer elements 6 is equal or substantially equal to four times the 
resonance frequency F.sub.o of the first group. 
By way of example, said resonance frequency F.sub.o of the first group of 
transducer elements 2 is equal to 0.5 MHz. 
Also in FIG. 2b, which illustrates another variant embodiment with two 
groups of transducer elements of different resonance frequency, the center 
position is for example occupied by a transducer element 2 of the first 
group whereas each peripheral sector is occupied alternately by transducer 
elements of the first group (element 2) and by transducer elements of the 
second group (element 4). 
Also, to obtain different resonance frequencies, the thickness of the 
transducer elements 2 of the first group is twice or substantially twice 
the thickness of the transducer elements 4 of the second group and, in the 
case of the presence of a third group of transducer elements 6, it is 
equal or substantially equal to four times the thickness of the transducer 
elements 6 of the third group. 
According to a simpler embodiment, all the transducer elements (2, 4, 6) of 
the different groups are at the same distance from the focal point 10, 
called focal distance. 
When the support means is a circular dome, as illustrated in FIG. 2, the 
focal distance is preferably equal to the radius of curvature of the dome. 
In the simplest case where the focal distance is the same for all the 
transducer elements (2, 4, 6) of the different groups, a coinciding of the 
maximum pressure peaks is obtained between the different groups having 
different resonance frequencies, by an electronic ordering on each group 
of given frequency, this being achieved by sending pulses or a front of 
voltage which is either delayed or advanced depending on the group being 
ordered with respect to the reference group, the time difference (delay or 
advance) being given by the formula: 
EQU .DELTA.t=.lambda./4 V 
in which: 
V is the ultrasonic speed inside the medium filling the dome, and generally 
constituted by water, 
is the wavelength of the ordered group. 
Anyone skilled in the art can thus know directly how to proceed with the 
electronic ordering of each group of frequency, in order to obtain the 
coinciding of the maximum pressure peak between the different groups at 
focal point 10. 
For example, if the resonance frequency F.sub.o of the first group of 
piezoelectric elements 2 is 0.5 MHz and if the resonance frequency of the 
second group of piezoelectric elements 4 is twice that measurement, i.e. 1 
MHz, the electronic ordering of the second group will be obtained with a 
delay of about one quarter of period T, i.e. 250 nsecs. 
According to another possible embodiment, particularly when the support 
means is constituted by a circular dome, as illustrated in FIG. 2, the set 
distance between the front face of the transducer elements of the given 
group and focal point 10 is different from the same distance to said focal 
point of the other groups of different resonance frequency, this making it 
possible to achieve simultaneous electric or electronic ordering of all 
the groups so that the maximum pressure peaks coincide. 
For example, the difference between the focal distances of the first and 
second groups is about one quarter of wavelength. 
Such compensation is found to correspond to a time compensation (.DELTA.t) 
when the focal distance is constant. 
It is therefore possible either to place all the transducer elements at an 
identical distance form the focal point 10 with a different electric 
ordering between the groups, while being synchronized in order to obtain 
coinciding maximum pressure peaks, or to modify the focal distance of the 
different groups, thereby performing a simultaneous ordering of all the 
transducer elements of the different groups. 
Another suitable possibility according to the invention consists in using a 
flat support means as described in document DE-A-31 19 295, the focal 
distance being then different for transducer elements which may be of the 
same group, and in this case an electronic focusing is achieved as 
described hereinabove, this being perfectly obvious to anyone skilled in 
ultrasonic technology. 
With that combination of transducer elements of different resonance 
frequency, it is possible to reduce significantly the emission of negative 
waves while significantly increasing the pressure peak of the principal 
positive wave as illustrated in FIGS. 3a and 3b with respect to FIGS. 1a 
and 1b, explained hereafter. 
For simplification purposes, FIG. 3a diagrammatically illustrates the wave 
emitted in time by the first group of transducer elements 2, the 
represented curve being also referenced 2, and by the second group of 
transducer elements 4, the curve then being referenced 4. It is observed 
that the principal positive wave 2p emitted by the first group of 
transducer elements 2 and the principal positive wave 4p emitted by the 
second group of transducer elements 4 have coinciding peaks, and 
consequently they will add up together to give a resultant principal 
positive wave, as illustrated in FIG. 3b and referenced 2p+4p. 
On the contrary, the secondary positive waves and the secondary negative 
waves emitted by each group of transDucer elements 2, 4 are going to be at 
least partly cancelled, thus giving the curve of resultant wave of FIG. 
3b. 
It is found, as a result, that the negative wave has been significantly 
reduced. This effect will be amplified by the presence of the third group 
of transducer elements 6. 
The amplitude of the negative wave will thus have been reduced to a level 
situated below the threshold at which said negative wave causes a 
cavitation effect capable of destroying or damaging the cells of tissues 
situated close to the target to be destroyed, such as a concretion, or a 
diseased tissue such as a tumor, contrary to what could be achieved with 
the generating device according to the prior art and comprising only one 
group of transducer elements with the same resonance frequency. 
Moreover, and this is also a particularly unexpected and non-obvious effect 
for anyone skilled in the art, the resultant wave obtained according to 
the invention has a maximum pressure peak which is at least equal to the 
pressure peak obtained according to the prior art. The efficiency of 
destruction of the target present in the focus point is thus further 
improved by the practical presence of a single pressure peak, without any 
sensitive negative part. 
Unquestionably, all the aforesaid determinant technical advantages have 
been obtained according to the invention. 
The present invention obviously includes all the means which are the 
technical equivalents of the means described hereinabove. 
For example, the piezoelectric transducer elements may be of any type, such 
as based on conventional titanate-zirconate or of any constitution 
enabling them to fulfill the required function. Similarly, the dimensions 
of the individual transducer elements as well as the radius of the 
circular dome can vary within a wide range. It is not even necessary for 
the transducer elements to be placed on a circular dome as focusing can be 
achieved electronically as taught for example in document DE-A-3 119 295, 
by changing the times of electric ordering of each group of transducer 
elements.