FR2634089A1 - Ultrasound transducer with improved sensitivity - Google Patents

Abstract

Ultrasound transducer 10 comprising a damping baseplate 20 to which is affixed a piezoelectric structure 30. According to the invention, the said piezoelectric structure 30 is composed of at least two stacked piezoelectric layers 301, 302, made from the same composite piezoelectric material, and whose acoustic impedances Z1, Z2 are equal to the acoustic impedance ZB of the damping baseplate 20. Furthermore, the said ultrasound transducer 10 utilises emission means 401, 402 furnishing a delay delta =e/v between the excitations of two consecutive piezoelectric layers 301, 302, v denoting the speed of propagation in the composite piezoelectric material. Application to medical imaging and lithotrity.

Description

"ULTRASONIC TRANSDUCER WITH IMPROVED SENSITIVITY".

The present invention relates to an ultrasonic transducer comprising a damping soleplate to which an acoustic piezoelectric structure adapted to said damping sole is contiguous, and intended for the purpose of transmitting an ultrasonic beam towards a medium before propagation.
The invention finds a first application in the general field of ultrasonic ultrasound examination of ultrasound media and, more particularly, in the field of medical ultrasound of biological tissues. It also finds another advantageous application in the field of the destruction of concretions inside organs (kidneys, gallbladder), also called lithotripsy.

 In the field of ultrasound, a technical problem that arises for any ultrasound transducer is, in coolie emission in reception, to provide an acoustic signal of short duration, this in order to obtain a great power of resolution in depth. Hence the need to realize a broadband trans actor For this, it is essential to limit as much as possible the reflections on the outsole of damped oaths acoustic waves propagating towards the back of the transducer. In the field of lithotripsy the technical problem is also to avoid the reflection on whether it absorbs part of the energy produced, because the resulting depression induc- tion can cause, by cavitation, haemorrhages - or the destruction of the tissues around the concretion to be destroyed.

 A solution to this general technical problem is given by French Patent Application No. 2,567,394, which describes an ultrasonic transducer comprising a damping soleplate to which a piezoelectric structure consisting of a piezoelectric ceramic layer is joined. In order to adapt acoustically first to the piezoelectric ceramic, and thus to eliminate reflections on said sole, it is made of copper, brass or bronze alloy, which has a similar acoustic impedance of that of the piezoelectric material used (approximately 30 MRayl).

 The ultrasonic transducer known from the state of the art, consisting of an acoustically damping metal pad and an acoustically adapted piezoelectric layer, however, has several disadvantages. First, because of its damping, it offers a low sensitivity since a large part of the acoustic energy produced by the piezoelectric layer is absorbed in the damping base.

The performance of an ultrasonic transducer therefore generally results from a compromise between short wave and energy efficiency. In ultrasound, the use of the signal from a transducer thus damped requires the use of a fairly heavy electronics due to the low signal-to-noise ratio obtained. On the other hand, the use of a copper-based metallic material to produce the mooring sole leads to a much heavier transducer than a transducer having a damping base of epoxy resin for example.

Also, the technical problem to be solved by the object of the present invention is to provide an ultrasonic transducer comprising a damping soleplate to which is attached a piezoelectric structure acoustiquelent adapted to said damping sole, and intended at least for the emission of an ultrasonic beam towards a medium before propagation, ultrasonic transducer which would have a lighter structure and an improved sensitivity without prejudice being brought to its damping qualities
The solution to the technical problem posed consists, according to the present invention, in that said piezo structure
electrical is composed of at least two piezoelectric layers
stacked, of the same thickness, made in the same
composite piezoelectric material, and whose impedances
acoustic are equal to the acoustic impedance of the sole
damping, and in that said ultrasonic transducer
has means of emission providing a delay I = e / v between
the excitati i "s two consecutive piezoelectric layers,
v denoting the velocity of, propagation of longitudinal waves
composite piezoelectric material.

Thus, on emission, the piezoelectric layers
are excited one after the other starting with the diaper
the farthest from the front of the transducer with, for
each piezoelectric layer, a delay law 5 excitedly
which takes into account the path traveled by the acoustic wave
generated by the layer that precedes it. In this way, the
force signal emitted by a layer is brought into phase with
precedents, so that at the front of the
transducer, the pressure force emitted is the resultant of the
forces generated by each piezoelectric layer.
therefore, at the output, a damped ultrasound signal, wide
band, the amplitude of which is multiplied by a factor equal to
number of piezoelectric layers used in the structure
re. In addition, the damping base, consisting of
a material exhibits the same acoustic impedance as the first piezoelectric layer directly attached to it, which prevents a wave of stationary waves from developing.
loppe at the inter, ieu'r;'This layer by multiple reflections at the interfaces. Since the first piezoelectric layer also plays the damping base layer for the first layer, the fact of taking for it an identical material leads to the second piezoelectric layer in the same acoustic conditions as the first one. The same reasoning extends iteratively when one passes, eventually, from. the second to a third layer, then to the following ones.

 It should therefore be emphasized that, in view of the cooperative effect of the piezoelectric layers in the production of a more intense ultrasonic signal, the invention synergistically combines a cooperative impedance matching effect providing ideal damping.

 On the other hand, the use1 for producing the piezoelectric layers of a composite material whose acoustic impedance is of the order of 9 mMayl makes it possible to envisage forming the damping base in a charged epoxy resin. whose acoustic impedance can easily reach 9 MRayl. This with the advantage that the ultrasonic transducer thus obtained is much lighter than that described in the state of the art mentioned above using copper. It should also be pointed out that the size and the weight of the ultrasonic transducer according to the invention can be further reduced in relation to the arrangements leading, for the same surface area, to an increase in transmission sensitivity, which is very appreciable in the field of lithotrithy, especially, given the congestion of current systems.

 Composite materials have many other advantages over conventional piezoelectric ceramics. First, the composites offer a much better acoustic adaptation with the human body whose impedance is of the order of 1.5 HRayl. On the other hand, their dielectric constant being lower, composites have a higher electrical impedance than ceramics. They have, moreover, a better coefficient of piezoelectric coupling and thus a better energetic efficiency. Finally, mechanically more flexible, composites can more easily dampen transversal deformations particularly undesirable in the case of ultrasonic transducers dedicated to lithotripsy.

 When the ultrasonic transducer is to be used ultrasound, it is expected that said structure is also intended for the reception of ultrasound signals returned to the transducer, and that it has receiving means providing a delay 6 = e / v between the detections signals received by two consecutive piezoelectric layers. In this way, the cooperative effect that occurs in transmission also occurs on reception, the resulting reception signal being equal to the sum of the individual signals reFus by each of the piezoelectric layers.

 Finally, in the case where the acoustic impedance of the composite materials used is very different from that of the scanning medium, it is envisaged that the last piezoelectric layer of said structure comprises a quarter-wave adaptation layer with the medium beforehand. spread.

The following description, with reference to the accompanying drawings given by way of non-limiting examples, will make it clear what the invention consists of and how it can be achieved.
Figure 1 is a diagram of a first embodiment of the invention, especially for ultrasound.

 Fig. 2 is a diagram of a second embodiment of the invention suitable for use in lithotrithy.

Fig. 1 shows an ultrasound transducer 10 for ultrasound application, comprising an epoxy resin damping pad 20 loaded with tungsten powder, of minimal, etc. . having an acoustic impedance ZB of the order of 9 MRayl. On the flange 20 is joined a piezoelectric structure 30 composed, in the example of Figure i, two piezoelectric layers 301, 302 stacked, the same thickness and made of the same composite material.This new type of material is constituted in a known manner, a piezoelectric ceramic 31 of lead zirconate-titanate (P; T) or of lead metanob formerly, in which st is inserted a piezoelectric polymer 32 such as polyvinylidemide difluoride. obtained to the advantage, inter alia, that the piezoelectric layers 301, 302 provide an acoustic impedance Z1,
Z2 also of the order of 9 MRayl, which confers a very satisfactory acoustic adaptation of the layers with the sole 20 damping.

 The ultrasonic transducer 10 is intended, in particular, for the emission of an ultrasonic beam to a medium before 70 of propagation. For this purpose, it has a transmission stage 401 comprising, in a conventional manner, a sequencer composed of an oscillator and a frequency divider which controls at the frequency of recurrence of 5 kHz a generator whose electrical signals d excitation are sent, on the one hand, to the piezoelectric layer 301t and, on the other hand, to the piezoelectric layer 302 via a delay line 402 which provides a delay 6 = e / v between the excitations of the layer 302 and of the layer 301, v being the speed of propagation of the longitudinal waves of the composite piezoelectric material. Thus, because of the phase shift 6, the resulting acoustic force at the output is twice the acoustic force produced by each of the piezoelectric layers, whence a gain of 6dB. The electrical contacts between the piezoelectric layers 301, 302 (grounding), between the layers and the damping base 20 and a possible acoustic impedance matching layer 60 with the propagation medium before 70 are carried out by silver and epoxy paste electrodes, for example.

 In echography, the piezoelectric structure 30 is also intended to receive the echographic signals returned to the transducer 10. In order to reproduce, on reception, the advantageously cumulative effect obtained in transmission, the ultrasonic transducer 10 has, as well as 1, a delay line 2 providing a delay 6 = e / v between the ultrasound signals produced by the layers 301, 302, of an adder 51 making the sum of the signals thus put in phase, and a receiving and processing stage 501 comprising a high frequency amplifier including gain compensation as a function of depth, a logarithmic compression amplifier, a storage and scan conversion device, and a display device. in the case of a structure with two piezoelectric layers, the gain obtained in transmission and in reception is 12 dB.

 FIG. 2 shows an ultrasonic transducer 10 more especially intended for lithotripsy, in the sense that the cushioning sole 22 has a spherical-shaped spherical face 21 of center 0, the point of focus of the ultrasonic energy. from the transducer. The piezoelectric layers 30t, 30z are connected, wedged in FIG. 11 to transmission means 401, 402, not shown. The transducer of FIG. 2 offers the particular advantage that, due to the elasticity of the piezoelectric polymer 32 constituting the piezoelectric layers, the mechanical deformations, directed substantially towards the axis of the sphere, and capable of causing a detachment of the layers of the face 21 of consolation, are wellabsorbes, which is not the case when the layers are made of pure ceramic, much more rigorous

Claims (4)

1. Ultrasonic transducer (10) comprising a damping sole (20) to which a piezoelectric structure (30) acoustically adapted to said damping base is contiguous, and intended at least for the emission of a beam ultrasound to a propagation medium (70), characterized in that said piezoelectric structure (30) is composed of at least two stacked piezoelectric layers (301, 302) of the same thickness e, made of a same composite piezoelectric material, and the acoustic impedances (Z1, Z2) are equal to the acoustic impedance (ZB) of the damping pad (20), and in that the said ultrasonic transducer (10) has means (401, 402) for transmitting a delay 6 = e / v between the excitations of two consecutive piezoelectric layers (301, 302), v denoting the propagation velocity of the longitudinal waves of the composite piezoelectric material. 2. ultrasonic transducer according to claim 1, characterized in that said structure (30) is also intended for receiving ultrasound signals returned to the transducer, and in that it has receiving means (50150251) providing a delay 6 = e / v between the detections of the signals received by two consecutive piezoelectric layers (301, 302). 3. ultrasonic transducer according to claim 1, characterized in that the sole (20) of damping has a face (21) of spherical fit. 4. ultrasonic transducer according to any one of claims i to 3, characterized in that the last piezoelectric layer (302) of said structure (30) comprises a quarter-wave adaptation layer (60) with the front medium ( 70) of propagation.