Miniature ultrasonic transducer for use in the oral cavity

Abstract

The present disclosure describes an ultrasonic sensor for soft tissue imaging, including a piezoelectric element having a front surface and a back surface used for transmitting an ultrasonic signal and receiving an echo ultrasonic signal of a measurement target, and a backing element having a front surface shaped to match the shape of the back surface of the piezoelectric element, the backing element being made of a structured composite material including at least two base materials having significantly different acoustic impedances such that the acoustic attenuation rate of the backing element exceeds 1.2 dB / mm / MHz and one of them is metal.

Description

Technical Field

[0001] The present invention relates to the field of intraoral measurement devices for the medical industry. In particular, but not limited to, the present invention relates to a small ultrasonic transducer used for intraoral imaging applications in the dental industry.

Background Art

[0002] Ultrasonic imaging has been adapted for use in the oral cavity in many implementation methods and has been found to be particularly useful for tasks such as measuring the depth of periodontal pockets. Conditions such as gingivitis can be detected, for example, by sensing the acoustic response of tissues.

[0003] Since ultrasonic imaging does not emit ionizing radiation, it is inherently safer than ionizing methods and can be repeatedly performed if necessary. Ultrasonic imaging can be used to replace or complement various types of X-ray imaging (cone beam computed tomography, i.e., CBCT, panoramic X-ray, or intraoral X-ray imaging), magnetic resonance imaging (MRI), or nuclear medicine examinations.

[0004] Ultrasonic imaging typically may use high-frequency sound waves in the range of 1 to 100 MHz. Since high-frequency sound waves attenuate more than low-frequency sound waves at a given distance, high-frequency sound waves are mainly suitable for imaging surface structures, such as dermatological or dental imaging. For example, high-frequency sound waves are preferably 20 to 50 MHz in periodontal pocket examinations. Conversely, low frequencies are suitable for imaging the deepest structures of the body. Furthermore, the higher the frequency, the higher the resolution of the image.

[0005] An ultrasonic imaging device generally includes one or more transducers that function as an ultrasonic beam transmitter and / or an ultrasonic beam receiver that receives echoes from the transmitted signal. Further, the ultrasonic imaging device may include various processing and display elements used for generating and displaying an image from the acquired signal. The ultrasonic beam transmitter generates an ultrasonic signal from an electrical signal, and conversely, the ultrasonic receiver generates an electrical pulse from a mechanical ultrasonic signal.

[0006] An object in the path of the transmitted ultrasonic signal returns a portion of the ultrasonic energy to a transducer that generates an electrical signal indicative of the detected structure. The transducer may be of different types among single-element transducers or multi-element transducers (such as annular arrays, linear arrays, 2D arrays, etc.). Further, when using a multi-element transducer, the transmission of the ultrasonic signal can be delayed to enable adaptive focusing. Electronic adaptive focusing makes it possible to improve the resolution according to the depth of the imaged organ. Also, the sensitivity of the transducer increases and the image quality of the contrast image is improved.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] Figure 1 is a schematic diagram of a single-element ultrasonic transducer.

[0009] As shown, the ultrasonic transducer 100 includes a piezoelectric element 105, an acoustic material 110 also called a backing element, and a housing 115. It may also include a matching layer (not shown) on the front surface 120 of the piezoelectric element to improve the acoustic lens and acoustic focus. The piezoelectric element typically includes a piezoelectric material disposed between two electrodes connected to a pulse generator and receiver (not shown) via connection lines 125. The piezoelectric material can convert an electrical pulse into an ultrasonic signal when the electrodes are electrically stimulated, and can also convert an ultrasonic signal into an electrical signal. In a computing device (e.g., a personal computer) in signal communication with the ultrasonic transducer, by processing the transmitted ultrasonic signal and the measured echo ultrasonic signal (indicated by the solid line arrow), an image can be generated that characterizes an object that reflects the transmitted ultrasonic signal, such as, for example, gingiva or other oral soft tissues (and possibly the tooth surface), generally indicated by reference numeral 130. The piezoelectric element may be, for example, a poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) piezoelectric film.

[0010] Note that the acoustic material or backing element 110 aims to suppress the piezoelectric element 105 (from a mechanical perspective) to reduce parasitic echoes that may interfere with the measurements performed on the surface of the transducer, and attenuate the ultrasonic signal generated at the back of the piezoelectric element (as indicated by the dashed line arrow). To avoid perturbation of the measurements performed on the front of the transducer, the piezoelectric element should receive the back-side echo signal after the front-side echo signal. Also, the back echo signal should be either completely absorbed or have its amplitude reduced as much as possible. From the perspective of the materials commonly used in the manufacture of the backing element, this leads to an increase in the size of the backing element.

[0011] Figure 2 shows an example of the ultrasonic signals transmitted and received by the ultrasonic transducer over time with low attenuation support. To make the explanation clear and concise, for example, by briefly exciting the piezoelectric element between the two electrodes of the ultrasonic transducer 100 in FIG. 1 with an electrical pulse, a short acoustic signal is generated at time t eAssume that it is transmitted in one direction. The transmitted signal indicated by reference numeral 200 is reflected by the object to be imaged, for example, the gingiva and other oral soft tissues (and possibly the tooth surface) 130 in FIG. 1. The echo acoustic signal indicated by reference numeral 205 is received by the transducer at time point t re . Depending on the object to be imaged, multiple echoes forming a one-way radio frequency (RF) line or echo line may be generated at different times. As shown in the figure, additional parasitic signals (reference numeral 210) generated from the ultrasonic signal transmitted at the back of the piezoelectric element are reflected within the backing element, for example, within the backing element 110 in FIG. 1. The parasitic signal 210 contributes to the RF line and is received by the transducer at time point t rp . As shown in the figure, the characteristics of the backing element are such that the signal reverberated within the backing element reaches the piezoelectric element much earlier than the signal to be measured (i.e., the signal reverberated by the object to be imaged) in order to avoid perturbation of the measurement.

[0012] To improve measurement accuracy, the acquisition of echo signals is generally performed over an acquisition time window indicated by reference numeral 215, which is defined as a function of the characteristics of the transducer, the object to be imaged, and the estimated distance between the transducer and the object to be managed. By appropriately setting the acquisition window and using a transducer with an efficient backing element, the influence of parasitic echoes is minimized. Note, however, that multiple parasitic echoes often overlap the echo line of interest within the acquisition time window 215.

[0013] Such ultrasonic transducers are generally efficient, but specific challenges related to intraoral ultrasonic imaging relate to the design of a probe that can be used for aligning the entire intraoral structure, i.e., along an axis perpendicular to both the buccal and lingual surfaces of each tooth in the mouth, from the perspective of the size of the transducer. In fact, from the perspective of efficiency, the ultrasonic transducer must face the area to be imaged.

[0014] Therefore, it is necessary to improve the ultrasonic transducer in order to improve the ultrasonic imaging device for the gingiva and other oral soft tissues (possibly the tooth surface) to be smaller and less expensive.

Means for Solving the Problem

[0015] The present invention is contrived to solve one or more of the above problems.

[0016] From the above viewpoints, a small ultrasonic transducer adapted for ultrasonic imaging of the gingiva and other oral soft tissues (and possibly the tooth surface) is provided.

[0017] According to one aspect of the present invention, a piezoelectric element having a front surface and a back surface used for transmitting an ultrasonic signal and receiving an echo ultrasonic signal of a measurement target; a backing element having a front surface shaped to conform to the shape of the back surface of the piezoelectric element, the backing element being made of a structured composite material including at least two base materials, one of which is metal, and the at least two base materials having significantly different acoustic impedances so that the acoustic attenuation rate of the backing element exceeds 1.2 dB / mm / Mhz. An ultrasonic transducer for soft tissue imaging including the backing element is provided.

[0018] The small ultrasonic transducer according to the present invention enables the imaging of the entire oral cavity structure, that is, the transducer can be easily incorporated into an intraoral probe that can be used for alignment along an axis perpendicular to both the buccal and lingual surfaces of each tooth in the mouth. In particular, the depth of the periodontal pocket can be measured by ultrasonic imaging using the small ultrasonic transducer of the present invention.

[0019] In one embodiment, the acoustic impedance of the backing element is greater than 25 MPa·s / m.

[0020] In one embodiment, the acoustic impedance of the other of the at least two base materials is less than or equal to half of the acoustic impedance of the one base material.

[0021] In one embodiment, the one substrate includes sintered metal spheres, such as sintered bronze.

[0022] In one embodiment, the diameter of the metal spheres is from half to twice the wavelength value of the transmitted ultrasonic signal.

[0023] In one embodiment, the other substrate is tin.

[0024] In one embodiment, the piezoelectric element is of the P(VDF-TrFE) type.

[0025] In one embodiment, the thickness of the backing element is less than 3 mm, preferably less than 2 mm, along the longitudinal axis of the transducer.

[0026] In one embodiment, the shape of the back surface of the backing element on the side opposite to the back surface of the piezoelectric element is concave, convex, or conical.

[0027] According to another aspect of the present invention, there is provided a method for manufacturing the ultrasonic transducer described above, the method comprising: - obtaining a porous piece made of the one substrate; - filling the pores of the porous piece with the other substrate.

[0028] In one embodiment, the method further includes machining the filled porous material to shape the backing element, coating the front surface of the backing element with nickel, and / or selecting the one substrate, and the one substrate is selected as a function of the target thickness, acoustic impedance, and / or acoustic attenuation rate of the backing element.

[0029] According to still another aspect of the present invention, there is provided an intraoral dental probe including the ultrasonic transducer described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Multiple embodiments of the present invention will be described by way of example only with reference to the following drawings.

Figure 1

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Figure 6

Mode for Carrying Out the Invention

[0031] Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings, but the same reference numerals in each drawing refer to the same elements of the structure.

[0032] In the following drawings and text, similar elements are denoted by similar reference numerals, and similar descriptions regarding the arrangement or interaction of elements and elements already described are omitted. Terms such as "first", "second", etc. do not necessarily indicate an order or priority relationship in use, and may be used simply to more clearly distinguish one element from another unless otherwise specified.

[0033] In the context of the present disclosure, terms such as "viewer", "operator", and "user" are equivalent and refer to viewers, technicians, and other individuals who acquire, view, and operate ultrasonic images such as intraoral images on a display monitor. "Instructions from the operator", "instructions from the user", or "instructions from the viewer" are obtained from explicit commands input by the viewer, for example, by clicking a button on an ultrasonic probe or system hardware, or by using a computer mouse, or by using touch screen or keyboard input.

[0034] In the context of the present disclosure, expressions such as "in a signal communication state" indicate that two or more devices and / or elements can communicate with each other via a signal passing through some kind of signal path. The signal communication may be wired or wireless. The signal may be a communication, power, data, or energy signal. The signal path may include physical, electrical, magnetic, electromagnetic, optical, wired, and / or wireless connections between a first device and / or element and a second device and / or element. The signal path may also include additional devices and / or elements between the first device and / or element and the second device and / or element.

[0035] The term "subject" refers to the gums and other intraoral soft tissues (and possibly the tooth surfaces) of the patient being imaged and is considered equivalent to the "imaging target" of the corresponding imaging system as an optical term.

[0036] According to some embodiments of the present invention, a backing element made of a structured composite material having a high acoustic impedance is used to improve the sensitivity of an ultrasonic transducer while causing a scattering effect on the ultrasonic signal transmitted from the back surface of the piezoelectric element. Such a scattering effect mainly includes the steps of splitting the ultrasonic signal into a plurality of ultrasonic sub-signals and then splitting the plurality of ultrasonic sub-signals. Due to this scattering effect, both the amplitude and coherence of the ultrasonic sub-signal reflected back to the piezoelectric element can be reduced. The backing element has a high acoustic impedance compared to the acoustic impedance of the piezoelectric element and is suitable for intraoral ultrasonic examinations in order to improve the sensitivity of the transducer (for example, a suitable supporting acoustic impedance is greater than 25 MPa·s / m or MRayl, more preferably greater than 30 MPa·s / m, for a piezoelectric element acoustic impedance on the order of 4.5 MPa·s / m). The acoustic impedance of the backing element is preferably at least five times that of the piezoelectric element. The frequency behavior of the backing element with respect to the vibrator element is such that the center frequency of the vibrator is preferably 20 to 50 MHz. Also, in order to avoid or reduce the back echo caused by the generation of mechanical waves, the backing element has a high (for example, greater than 1.2 dB / mm / MHz) attenuation rate.

[0037] In other words, the backing element is heavy enough (i.e., has a high acoustic impedance with respect to the acoustic impedance of the piezoelectric element) to enable strong energy transmission from the piezoelectric element to the tissue of the imaging target, includes a number of internal diffraction surfaces to attenuate the back effect of mechanical wave generation, and has a size suitable for intraoral ultrasonic examinations. For the sake of convenience of explanation, the thickness of the backing element may be less than 3 mm, preferably less than 2 mm, along the longitudinal axis of the piezoelectric element. Further for the sake of convenience of explanation, the diameter may be less than 1 cm, for example 6 mm.

[0038] It should be noted here that an ultrasonic transducer based on the piezoelectric copolymer P(VDF-TrFE) has an effective thickness coupling coefficient (k t) is low, and it is known that the sensitivity is relatively low. The acoustic impedance of P(VDF-TrFE) is typically on the order of 4.5 MPa·s / m (MRayl). The piezoelectric copolymer P(VDF-TrFE) material is suitable for high-frequency ultrasonic applications and can be easily adapted compared to PZT-type ferroelectric ceramics (due to its flexibility).

[0039] When combined with a P(VDF-TrFE)-type piezoelectric element, using a backing element (light support) with an acoustic impedance less than or near 4.5 MPa·s / m leads to prioritizing the transducer's bandwidth (120%) at the expense of sensitivity. This sensitivity can be significantly improved by using a support with a high acoustic impedance (heavy support). When the acoustic impedance is 80 MPa·s / m, the sensitivity can be improved by 7 dB compared to a conventional configuration including air support with a relative bandwidth of 60%. The inventors determined that it is appropriate to use a backing element with a high acoustic impedance. For the sake of explanation, the inventors determined that a backing element with an acoustic impedance of 32 MPa·s / m can improve the sensitivity by 3 dB with a relative bandwidth of 72% compared to a reference configuration including air support. This result represents an excellent trade-off between sensitivity and bandwidth in imaging applications.

[0040] It should also be noted that the acoustic impedance value of the backing element material must be high. For example, it should be easily achievable to form a front cup to geometrically focus the transducer. The inventors recognize that certain metal substrates, such as sintered bronze (i.e., an alloy of copper and tin), have compatible properties and a high acoustic impedance and can be used as a support material for piezoelectric polymers.

[0041] Also, the material of the backing element should have a thickness that conforms to the specifications of the probe (for example, the thickness of the backing element is less than 3 mm, preferably less than 2 mm) to avoid back echoes, and should have a high acoustic attenuation rate. According to some embodiments of the present invention, such a high acoustic attenuation value can be achieved by using the scattering effect of a multiphase medium, for example, a multiphase medium based on bronze, tin, and air that forms a controlled microporous material. An efficient scattering effect may be obtained when using a metal spherical substrate, and the sphere (or particle) size is on the same order as the wavelength value of the excitation signal (for example, within the range of half to twice the wavelength value of the excitation signal). For example, bronze grains with an average size of 100 μm may be used to attenuate ultrasonic signals having wavelengths within the range [80 μm; 220 μm] corresponding to the frequency range [20 MHz; 50 MHz]. The presence of tin and / or air in the bronze spherical structure increases the acoustic impedance mismatch, thus reducing interference.

[0042] Some measurements have shown that a backing element with a thickness of 1.7 mm made of sintered bronze grains with an average particle size of 100 μm has a high acoustic attenuation rate, and the space between the sintered grains is partially filled with tin (that is, the space between the sintered grains remains filled with air). As a result of measurements on different samples, the average value of the acoustic attenuation rate of an ultrasonic signal with a frequency of 25 MHz was 34.7 dB / mm.

[0043] Figure 3 shows an example of a backing element of an ultrasonic transducer according to some embodiments of the present invention. As shown, the backing element 300 has a cylindrical shape with one concave surface conforming to the shape of a piezoelectric element (not shown). The backing element 300 is made of sintered bronze, and the gaps between the bronze grains are filled with tin and air. Such a structure provides a scattering effect for ultrasonic signals that arises from the structure itself and also from the ratio of acoustic impedance between bronze and tin and air. As described above, the thickness of the backing element 300 may be less than 3 mm, preferably less than 2 mm, along the longitudinal axis (reference number 305) of the piezoelectric element. Also, the diameter of the backing element 300 may be less than 1 cm, for example, 6 mm.

[0044] Figure 4 is a cross-sectional view taken along the longitudinal plane of the backing element shown in Figure 3, showing a structure including sintered bronze grains 400, air 405, and tin 410. The figure also shows a concave-shaped portion 415 suitable for receiving a piezoelectric element. Of course, other structures and materials may be used.

[0045] The side of the backing element opposite to the side for receiving the piezoelectric element may have a shape different from that shown, so as to reflect the received ultrasonic signal in a direction other than the longitudinal direction of the backing element. As a result, it is observed that the scattering effect of the backing element increases as shown in Figure 5.

[0046] Figure 5 shows a plurality of examples of the shape of the back surface of the backing element on the side opposite to the side for receiving the piezoelectric element (represented by a dashed line).

[0047] As shown, such a shape may be any of concave (a), convex (b), and conical (c). Also, a combination of different shapes may be used (not shown).

[0048] Figure 6 shows an example of the steps for manufacturing a backing element according to some embodiments of the present invention.

[0049] As shown, the first step (step 600) is aimed at obtaining a first base material having a first acoustic impedance such as sintered bronze (dense bronze with an acoustic impedance of about 40 - 45 MPa·s / m). For the sake of convenience of explanation, such a sintered bronze material is available from Ames (e.g., AmesPores (registered trademark)). According to some embodiments, the first base material includes bronze spherical particles that are compressed and then sintered in a mold. The mold may be configured in consideration of the shape of the piezoelectric element to be attached to the backing element. Alternatively, the obtained material is machined later to match the shape of the front surface to that of the piezoelectric element.

[0050] Sintering may include heating the compressed bronze spherical particles without melting the particles (in the case of bronze, the melting temperature varies in the range of 700 °C to 1300 °C depending on the alloy composition). Under the influence of heat, the grains weld to each other and aggregate the particles. As a result, a metal part with a controllable porosity level is obtained. According to some embodiments, a sintered bronze thin sheet with an average particle size of 100 μm, an outer diameter of 8 mm, and a thickness of 10 mm is obtained. These can be obtained directly from sintered bronze cylinders that are cut (at this stage or later) to obtain the thin sheets. Also according to some embodiments, the porosity of the obtained thin sheets is 35%, the average pore diameter is 53 μm (the maximum pore diameter is 139 μm), and the particle size and pore diameter of these thin sheets thus match the wavelength values of the ultrasonic signals that attenuate (for example, about 80 μm to 220 μm).

[0051] Such a first substrate is preferably a structured, i.e., a material that is arranged according to a predetermined geometric structure and is porous. Also, the first material is preferably homogeneous, i.e., it means that the density of the first substrate is substantially constant within different parts of the thin sheet. Such a structured and homogeneous material may be referred to as a structured material skeleton.

[0052] These thin sheets can be used directly as backing elements (possibly after machining, polishing, etc. for sizing). However, due to the porosity, it has been found that it is difficult to directly bond a piezoelectric element, particularly a P(VDF-TrFE) piezoelectric element, onto such thin sheets. Therefore, it is preferably carried out (step 605) to impregnate the thin sheets, or one or more portions of the thin sheets, with a second substrate.

[0053] The second substrate should be selected not only as a function of its own properties but also as a function of the properties of the first substrate. In particular, the second substrate should be selected as a function of its own acoustic impedance and as a function of the acoustic impedance of the first substrate. According to some embodiments, the difference in acoustic impedance between the second substrate and the first substrate is suitable for causing a diffraction effect while maintaining the acoustic impedance of the backing element higher compared to the impedance of the piezoelectric element. For the sake of convenience of explanation, the acoustic impedance of the first substrate may be about twice that of the second substrate. Further for illustrative purposes, tin may be selected as the second substrate because it is a ductile metal with a low melting point (232 °C) and can be impregnated with a material having a high melting point such as bronze (890 °C). The penetration of tin occurs at a temperature around 400 °C and can be caused using a soldering iron. Also, the acoustic impedance of tin is significantly different from that of bronze and is about 20 MPa.s / m, which is about half of the impedance value of bronze (40 MPa.s / m), leading to a composition showing a significant scattering effect.

[0054] According to certain embodiments, impregnating the first substrate with the second substrate involves introducing a sintered bronze flake or cylinder into a tin plating flux (e.g., LOCTITE HYDX-20 liquid flux) and degassing the assembly for a predetermined time, e.g., 5 minutes. The flux cleans the material and improves the adhesion of tin. Then, tin can be introduced into the pores of the bronze flake or cylinder by using, for example, a soldering iron placed on the surface of the bronze flake or cylinder so that tin can penetrate into the bronze flake or cylinder by capillary action. The tin used may be a lead-free alloy (e.g., SAC305 with 96% Sn and 4% Ag) having a melting temperature of 232 °C.

[0055] According to another embodiment, the holes of the bronze flakes or cylinders are partially or completely filled with an epoxy resin. For this purpose, the bronze flakes or cylinders (which will be cut later to obtain the flakes) may be covered with an adhesive containing an opening and immersed in the epoxy resin under vacuum conditions. While the flakes or cylinders are being vacuum degassed, the air contained within the flakes or cylinders is replaced with the epoxy resin. Then, to polymerize the epoxy resin, the assembly is heated, for example, at 65 °C for 1 hour.

[0056] Next, the flakes (possibly obtained from the cylinders) are machined. For the sake of convenience of explanation, machining may include adjusting the size of the flakes to the size of the backing element (e.g., reducing the thickness and / or diameter of the flakes), adapting the front surface of the flakes to the shape of the piezoelectric element to which the backing element is to be attached, cleaning, for example, by means of abrasive products, and polishing the backing element. Machining may also include coating the backing element, for example, coating the front surface of the backing element (i.e., the surface on which the piezoelectric element is to be placed) with nickel so that whiskers, which may generate parasitic signals, are not formed over time.

[0057] Optionally, the obtained material (e.g., bronze flakes or cylinders) can be further pressed with an element having a convex portion (e.g., a sphere) to form a concave-shaped portion on the surface to which the piezoelectric element is to be assembled, and the concave shape is adapted to focus the ultrasonic beam (note that the P(VDF-TrFE) piezoelectric element is flexible).

[0058] Also, by filling the gaps between the bronze grains with tin or other materials that solidify after addition, at least at the location where the piezoelectric element is to be assembled, it has been found that the quality of the surface to which the piezoelectric element is to be assembled is improved, and thus the adhesion of the piezoelectric element to the backing element is improved without affecting the electroacoustic characteristics of the transducer.

[0059] Next, the backing element and the piezoelectric element are probably assembled together with the acoustic lens (step 615) and placed in the housing. For the sake of convenience of explanation, the piezoelectric element may be adhered to the backing element using an epoxy resin as an adhesive. Note that the adhesive layer must be thin enough not to interfere with the expected operation of the transducer.

[0060] Although the present disclosure has been described above with reference to some specific embodiments (e.g., single-element transducers), the present invention is not limited to these specific embodiments, and modifications within the scope of the present invention will be apparent to those skilled in the art. In particular, the present invention can be used for multi-element transducers such as linear transducers, circular arrays of transducers, or 2D arrays of transducers. The shape of the backing element is suitably adapted accordingly (e.g., it may be in the shape of a parallelepiped). Also, the shape of the surface of the backing element to which the piezoelectric element is adhered with an adhesive may be different from the surface of the piezoelectric element adhered to the backing element with an adhesive (e.g., one surface may be square while the other surface may be circular). Similarly, the shape of the sintered bronze material for obtaining the backing element may be non-cylindrical.

[0061] By referring to the above exemplary embodiments, still more changes and modifications will be suggested to those skilled in the art, but these are for illustrative purposes only and are not intended to limit the scope of the present invention which is defined only by the appended claims. In particular, different features from different embodiments may be appropriately interchanged.

[0062] In the claims, the term "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of things. The mere fact that different features are recited in mutually different dependent claims does not mean that these features cannot be used advantageously in combination.

Claims

1. A piezoelectric element having a front and a back, used for transmitting ultrasonic signals and receiving echo ultrasonic signals of the object to be measured. An ultrasonic transducer for soft tissue imaging, comprising a backing element having a front surface shaped to match the shape of the back surface of a piezoelectric element, comprising at least two substrates, one of which is metal, and the backing element being made of a structured composite material, wherein the at least two substrates have significantly different acoustic impedances, so that the acoustic attenuation rate of the backing element exceeds 1.2 dB / mm / MHz.

2. The ultrasonic transducer according to claim 1, wherein the acoustic impedance of the backing element is greater than 25 MPa. s / m.

3. The ultrasonic transducer according to claim 1, wherein the acoustic impedance of the other of the at least two substrates is half or less of the acoustic impedance of the one substrate.

4. The ultrasonic transducer according to claim 1, wherein one of the substrates includes a sintered metal sphere.

5. The ultrasonic transducer according to claim 4, wherein the diameter of the metal sphere is half to twice the wavelength of the emitted ultrasonic signal.

6. The ultrasonic transducer according to claim 1, wherein one of the base materials is sintered bronze.

7. The ultrasonic transducer according to claim 1, wherein the other base material is tin.

8. The ultrasonic transducer according to claim 1, wherein the piezoelectric element is of type P(VDF-TrFE).

9. The ultrasonic transducer according to claim 1, wherein the thickness of the backing element is less than 3 mm, preferably less than 2 mm, along the longitudinal axis of the transducer.

10. The ultrasonic transducer according to claim 1, wherein the shape of the back surface of the backing element facing the back surface of the piezoelectric element is concave, convex, or conical.

11. A method for manufacturing an ultrasonic transducer according to claim 1, The steps include obtaining a porous piece made of the aforementioned one substrate, A method comprising the step of filling the pores of the porous piece with the other substrate.

12. The method according to claim 11, further comprising the step of machining the filled porous material in order to shape the backing element.

13. The method according to claim 11, further comprising the step of coating the front surface of the backing element with nickel.

14. The method according to claim 11, further comprising the step of selecting one of the substrates, wherein the one substrate is selected as a function of the target thickness of the backing element, acoustic impedance, and / or acoustic attenuation rate.

15. A dental intraoral probe comprising an ultrasonic transducer according to any one of claims 1 to 10.

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