EP0421286B1 - Piezoelectric transducer - Google Patents

Description

The invention relates to a piezoelectric transducer for generating focused ultrasonic shock waves for use in lithotripsy with the features of the preamble of claim 1.

Piezoelectric transducers are generally known, for example from DE-PS 34 25 992. The use of a coupling medium for coupling the ultrasonic shock waves to the patient's body in such transducers is also well known.

If the known transducers are successfully used in practice in therapy, the problem often arises that the dimensions of the transducer are very large in order to obtain the focus on the energy density required for the disintegration of the calculus to be destroyed.

The energy densities that can be generated with piezoelectric materials are very high, but only a very small part of the available energy is introduced into the coupling medium (water or oil) in practice, since the sound-producing ceramic and the water / oil are acoustically very strong differentiate from each other.

Against the background of the foregoing, it is the object of the present invention to develop a transducer of the type mentioned at the outset in such a way that the energy density of the ultrasonic shock waves generated by it is so large in its focus that it is possible to reduce its dimensions.

This object is achieved by the features of claim 1. Accordingly, the known transducer is designed so that between the transducer elements and the coupling medium, an intermediate medium is provided at least from one layer, the acoustic impedance of which is between that of the ceramic of the transducer elements and that of the coupling medium and that the thickness of the layer is dimensioned such that the relationship d> τ _k .c _LA applies, where τ _{k is} the propagation time of the sound in the piezoceramic of the transducer elements and c _{LA is} the speed of sound in the respective intermediate medium.

In the present case, the thickness of the layer of the intermediate medium cannot be measured on the basis of the wavelength of the ultrasound, since the ultrasound shock waves generated by the transducer have a very wide frequency spectrum. In this respect, an adaptation, as known from US-PS 415 6863, does not provide anything for the present task solution. This is because there is only provision for the thickness of one Potting compound, which has the acoustic impedance of the coupling medium (water), to be dimensioned to a quarter of the wavelength of the sound waves emanating from the individual transducers. In the present case, the requirements for impedance matching are completely different. Here it is not the individual frequency or wavelength that is the basis of all considerations, but the transit time of the sound through the individual transducer element.

If a layer of the intermediate medium is introduced between the active surface of each piezoelectric transducer element and the coupling medium, it must have a certain thickness and a certain acoustic impedance in order to achieve optimal results.

Since it is not a question of resonance tuning in the present case, the damping in the intermediate layers is not of great importance, as long as it does not assume extreme values and the necessary thickness, which is given by the above-mentioned dimensioning regulation, is not exceeded many times over.

The acoustic impedance to be selected depends on the acoustic conditions at the interface between the active transducer elements and the layer of the intermediate medium or on the known sound transmission factors at the interface between two media of different acoustic impedance. In any case, it lies between that of the ceramic of the transducer elements and that of the coupling medium.

The acoustic thickness of the layer of the intermediate medium must be greater than that of the ceramic of the transducer elements.

The effect that more energy gets into the coupling medium can be increased in that several layers of intermediate media are provided between the transducer elements and the coupling medium, the acoustic impedances of which decrease from the first layer on the transducer elements in the direction of radiation of the ultrasonic shock waves.

The sound will always only partially pass each boundary layer because a part is always reflected. This reflection will always be soft, i.e. a phase reversal will occur because the impedance of each intermediate medium is greater than that of the next one or the water. If the reflected portion of the sound then hits the previous boundary layer, it becomes hard, that is to say without a phase reversal, and is reflected and then partially runs into the next layer of an intermediate medium or at the end into the coupling medium.

According to advantageous refinements, the layer or the layers of the intermediate media can each be assigned to one transducer element, uniformly all transducer elements together or mixed partially uniformly together and partially in each case to one transducer element.

Basically, the described construction of the transducer according to the invention can be implemented in the case of self-focusing transducers, for example dome-shaped transducers, but also in the case of planar transducers.

In the latter case it can advantageously be provided that at least a layer of an intermediate medium is designed as an acoustic lens. This layer then takes over the task of focusing the ultrasonic shock waves on the focus of the transducer, so that no additional effort is required.

According to a further advantageous embodiment, in the direction of radiation of the ultrasonic shock waves, the transducer has a layer of an intermediate medium on the transducer elements, which has a surface that electrically connects the transducer elements and faces them. This surface is then connected to one pole of the pulse generator. The first layer is thus used as a common electrode for all transducer elements, which not only significantly reduces the amount of wiring previously required, but also makes the transducer overall more compact and less susceptible to faults.

This advantageous development can be implemented in a simple manner in that the first layer is solid and metallic. Aluminum, for example, is suitable for this purpose, the acoustic impedance of which corresponds to the conditions mentioned.

In the case of a planar transducer, this embodiment can advantageously be developed further in that the layer is constructed as a solid, acoustic lens. This then again takes on the task of focusing the ultrasonic shock waves on the converter focus.

Each transducer element has a so-called backing, the acoustic impedance is at least as large as that of the ceramic of the individual transducer elements. This measure ensures an almost reflection-free termination of the transducer elements, so that unwanted negative tensile impulses for lithotripsy are limited to a practically possible minimum. The backings can be designed in such a way that the sound coming from the ceramic is scattered so that it does not in the focus of the converter, which can be achieved, for example, by roughening the back of the backings or by shaping it into a cone, for example.

However, all transducer elements can also be provided with a common backing for their reflection-free termination.

In all of the above-mentioned designs of the transducer, the energy density of the ultrasonic shock waves in the transducer focus compared to previous transducers has been increased by "passive" measures through the better coupling of the ultrasonic shock waves into the coupling medium, that is, through the better utilization of the energy generated by the transducer elements. However, in addition to this measure, some of the described embodiments also allow the energy density in the converter focus to be increased by “active” measures. This relates in particular to the control of the converter elements by means of higher voltages. Up to now, this was not easily possible primarily due to safety aspects, but also with regard to the converter's service life.

Accordingly, it is provided in an advantageous development of the transducer that the transducer elements with the electrically conductive carrier by means of electrically conductive fasteners are clamped, the carrier being connected to the other pole of the pulse generator. This makes it possible to control the converter elements with higher voltages without the converter elements bursting out of their anchoring, which would result in irreparable damage.

A higher controllability with higher voltages, whereby the output power of the converter is actively increased, can be achieved in the embodiments of the converter described above, in which the first layer of an intermediate medium on the converter elements is solid and metallic and thus serves as an electrode that the space outlined by the first layer, the common backing, or the support is liquid-tight and gas-tight by means of electrically non-conductive side walls, and that this space is filled with a highly insulating medium. A gas, oil or also a solid insulator can be considered as a highly insulating medium.

It is also possible to design the transducer in such a way that an electrically conductive first layer forms the carrier, which is connected to one pole of the pulse generator, and that this carrier encloses a liquid-tight and gas-tight space with a housing, which is sealed with a highly insulating Medium is filled. This also results in a relative increase in the energy density of the ultrasonic shock waves generated by the transducer in focus due to a higher radiation power on the one hand and a better coupling of the energy into the coupling medium on the other hand.

Another embodiment of the converter, in which active and If the focus is passively provided for increasing the energy density, the result is if the first layer consists of a highly insulating potting material which also fills the spaces between the transducer elements. Here, the first layer takes on both the task of impedance matching and the task of lateral electrical insulation of the converter elements from one another, as a result of which the converter can be controlled with higher voltages than before.

Polyurethane epoxy mixtures or silicones are particularly suitable as potting material.

Figur 1

ein erstes Ausführungsbeispiel des Wandlers,

Figur 2

ein zweites Ausführungsbeispiel,

Figur 3

ein drittes Ausführungsbeispiel,

Figur 4

ein viertes Ausführungsbeispiel,

Figur 5

ein fünftes Ausführungsbeispiel,

Figur 6

ein sechstes Ausführungsbeispiel,

Figur 7

ein siebentes Ausführungsbeispiel,

Figur 8

ein achtes Ausführungsbeispiel,

und

Figur 9

ein neuntes Ausführungsbeispiel,

jeweils in schematischer Schnittansicht.The invention is explained in more detail with the aid of a few exemplary embodiments. Here shows:

Figure 1

a first embodiment of the converter,

Figure 2

a second embodiment,

Figure 3

a third embodiment,

Figure 4

a fourth embodiment,

Figure 5

a fifth embodiment,

Figure 6

a sixth embodiment,

Figure 7

a seventh embodiment,

Figure 8

an eighth embodiment,

and

Figure 9

a ninth embodiment,

each in a schematic sectional view.

In the drawings, identical parts are provided with the same reference symbols.

FIG. 1 shows a dome-shaped and thus self-focusing transducer which bundles the generated ultrasonic shock wave from the piezoelectric transducer elements onto the focus 15 via a coupling medium 20. The transducer elements 2 are attached to a carrier 8 with their active surface.

In the exemplary embodiment shown, the carrier 8 is identical to the first layer 3, the thickness of which depends on the relationship d> τ _k . c _{LA is} dimensioned, where τ _{k is} the transit time of the sound in the piezoceramic of the transducer elements 2 and c _{LA is} the speed of sound in the layer 3.

A further layer 4 of an intermediate medium serving for impedance matching is applied to layer 3, the acoustic impedance of which lies between that of layer 3 and that of coupling medium 20. The above relationship applies correspondingly to the thickness of layer 4, where c _{LA is} the speed of sound in layer 4.

The layer 3 or the carrier 8 is solid and metallic, that is to say electrically conductive. It serves as a common electrode for all converter elements 2 and is accordingly connected to one pole of a pulse generator 7. The other pole of the generator 7 is connected via a wiring 11 at the rear end of the converter elements 2 via electrically conductive individual backings 6. The conical shape of the backings 6 causes sound coming from the back to be scattered in such a way that it is not focused in the focus 15.

Aluminum is considered as the material for the layer 3 or the carrier 8 if water is the coupling medium 20 is used.

The formation of the first layer 3 as a solid support 8 enables it to enclose a liquid and gas-tight space with a housing 21, which is filled with a highly insulating medium 18. The medium 18 prevents a jump of sparks at the individual converter elements 2 at a high voltage applied to the elements 2. Accordingly, this converter can be controlled with a voltage which enables a significantly higher output compared to known converters.

FIG. 2 shows an embodiment of a dome-shaped transducer in which the transducer elements 2 are braced on the back with electrically conductive individual backings 6 and with an electrically conductive carrier 8 by means of screws 9.

Two layers 3 and 4 of intermediate media are applied to the converter elements 2 in order to adapt the impedance to the coupling medium (not shown).

The first layer 3 is electrically conductive. It is used to supply the voltage from the pulse generator 7 to the converter elements 2. The other pole of the generator 7 is connected to the converter elements 2 via the carrier 8, screws 9 and backings 6.

FIG. 3 shows a planar transducer in which the transducer elements with the individual backings 6 are clamped to the carrier 8 by screws 9. The adaptation of the acoustic impedance is achieved here by three layers 3, 4 and 5 of intermediate media on the transducer elements 2 of course, the conditions mentioned above for their acoustic impedances are met.

Layer 5 is assigned to all transducer elements 2 together here. It is also designed as an acoustic lens which, together with the first matching layer (3), focuses the emitted ultrasonic shock waves.
FIG. 4 also shows a planar transducer, in which three layers 3, 4 and 5 of intermediate media are applied to the transducer elements 2, as already explained in connection with the exemplary embodiment according to FIG. 3, in the radiation direction of the ultrasonic shock waves. Here, the middle layer 4 is provided as a common layer and designed as a focusing acoustic lens.

In the present case, electrically non-conductive side walls 16, the common carrier 8 and the layer 4 outline a liquid and gas-tight space which is filled with a highly insulating medium 18.

A similar embodiment is shown in the rest of FIG. 5. Here, however, all layers 3, 4 and 5 are provided in common for all transducer elements 2, of which layers 4 and 5 have a lens function.

In the exemplary embodiment shown in FIG. 6, the converter elements have a common backing 14, which also closes the space outlined by the first layer 3 and the electrically non-conductive side walls 16, in which a highly insulating medium 18 is located. The back of the backing 14 is designed so that sound reflected from it is no longer focused in the focus of the transducer. All layers 3 to 6 are common for All transducer elements are provided, layers 4 and 5 being designed as lenses for focusing the ultrasonic shock waves.

As FIG. 7 shows, the use of a common backing 14 is also possible with a dome-shaped converter. Here, the layers 3 and 4 of the intermediate media are each assigned to a converter element 2.

FIG. 8 shows an extreme case in which the piezoceramic material 2 is in one piece. This is completed on the back by a backing 14. The impedance matching is done by two layers 3 and 4 of coupling media.

Finally, FIG. 9 shows a particularly preferred embodiment of the converter. Only one layer 3 of an intermediate medium is shown here. Layer 3 consists of a highly insulating potting material, for which, for example, polyurethanes, epoxy mixtures or silicones can be used.

The potting material has an acoustic impedance which again lies between that of the ceramic of the transducer elements 2 and that of the coupling medium 20. The spaces 22 between the individual transducer elements 2 are filled with it.

Due to the insulation, this converter can be controlled with higher voltages than known converters. In addition, it has the advantage that the transducer elements 2 are embedded in the potting compound with absolute water protection, which results in an outstanding immunity to interference by the transducer.

Claims (17)

1. Piezoelectric transducer for producing focused ultrasound shock waves for application in lithotripsy, it being possible for the ultrasound shock waves thereof, which are emitted in pulses, to be transferred to the body of a patient via a coupling medium (20), comprising a plurality of individual ceramic piezoelectric transducer elements (2) connected to the poles of a pulse generator (7), which transducer elements (2) are fixed on a support (8) in mosaic fashion and electrically insulated from one another laterally, the acoustic sealing of the transducer elements being essentially free of reflection, characterised in that an intermediate medium of at least one layer (3, 4, 5) is provided between the transducer elements (2) and the coupling medium (20), acoustic impedance thereof being between that of the ceramic of the transducer elements (2) and that of the coupling medium (20), and in that the thickness of the layer (3, 4, 5) has dimensions such that the relationship d > τ_k · c_LA applies, τ_k being the propagation time of the sound in the piezoelectric ceramic of the transducer elements (2) and c_LA being the sound velocity in the particular intermediate medium.
2. Piezoelectric transducer according to claim 1, characterised in that several layers (3, 4, 5) of intermediate media are provided between the transducer elements (2) and the coupling medium (20), the acoustic impedances thereof decreasing from the first layer (3) on the transducer elements (2), seen in the emission direction of the ultrasound shock waves, to the coupling medium (20).
3. Piezoelectric transducer according to claim 1 or 2, characterised in that the layer or layers (3, 4, 5) of intermediate media is or are assigned in each case to one transducer element (2).
4. Piezoelectric transducer according to claim 1 or 2, characterised in that the layer or layers (3, 4, 5) is or are assigned uniformly to all transducer elements (2) together.
5. Piezoelectric transducer according to claim 1 or 2, characterised in that the layers (3, 4, 5) are assigned partly uniformly together and partly in each case to one transducer element (2).
6. Piezoelectric transducer according to one of claims 1 to 5, characterised in that at least one intermediate medium (3, 4, 5) is designed as an acoustic lens.
7. Piezoelectric transducer according to one of claims 1 to 6, characterised in that layer (3) on the transducer elements (2), first in the emission direction of the ultrasound shock waves, has a surface connecting the transducer elements (2) electrically with one another and facing the latter, which surface is connected to one pole of the pulse generator (7).
8. Piezoelectric transducer according to one of claims 1 to 7, characterised in that the first layer (3) on the transducer elements (2) is solid and metallic.
9. Piezoelectric transducer according to claim 8, characterised in that a layer (3) is designed as a solid acoustic lens.
10. Piezoelectric transducer according to one of claims 1 to 9, characterised in that each transducer element has a backing (6), the acoustic impedance thereof being at least as large as that of the ceramic of the transducer elements (2).
11. Piezoelectric transducer according to claim 10, characterised in that the backings (6, 14) are designed such that the sound originating from the rear side of the backing is scattered so that it is not bundled at the focal point.
12. Piezoelectric transducer according to one of claims 1 to 9, characterised in that a common backing (14) for all transducer elements (2) is designed for them to have reflection-free sealing.
13. Piezoelectric transducer according to one of claims 1 to 12, characterised in that the transducer elements (2) are braced by means of electrically conductive attachment means (9) to the electrically conductive support (8), and in that the support (8) is connected to the other pole of the pulse generator (7).
14. Piezoelectric transducer according to claim 7, 8 or 9 in conjunction with claim 13, characterised in that the space defined by a layer (3, 4), the common backing (14), or the support (8), is sealed to be liquid-tight and gas-tight by means of electrically non-conductive side walls (16), and in that this space is filled with a highly insulating medium (18).
15. Piezoelectric transducer according to one of claims 1 to 12, characterised in that an electrically conductive first layer (3) forms the support (8) which is connected to one pole of the pulse generator (7), and in that this support (8) with a housing (21) encloses a space sealed to be liquid-tight and gas-tight and which is filled with a highly insulating medium (18).
16. Piezoelectric transducer according to one of claims 1 to 12, characterised in that the first layer (3) consists of a highly insulating sealing material which also fills the gaps (22) between the transducer elements (2).
17. Piezoelectric transducer according to claim 16, characterised in that the sealing material consists of polyurethanes, epoxy mixtures or silicones.

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