EP0134346B1

EP0134346B1 - Ultrasonic transducers

Info

Publication number: EP0134346B1
Application number: EP19830304934
Authority: EP
Inventors: Henry Peter Beerman
Original assignee: Analogic Corp
Current assignee: Analogic Corp
Priority date: 1983-08-25
Filing date: 1983-08-25
Publication date: 1989-05-31
Also published as: DE3379990D1; EP0134346A1

Description

This invention relates to ultrasonic transducers.
Ultrasonic transducers, employed for example for medical diagnostic purposes, are known in which the transducer is focussed for an intended focal length. Such transducers generally include a spherically curved ceramic piezoelectric element supported on an acoustic backing material, or a flat piezoelectric element supported on an acoustic backing material with an acoustic lens disposed on the front surface of the flat element to provide the intended focussing. These known transducers are operative for only a single focal length, and a different transducer must be constructed for each focal length of interest.
A known ultrasonic transducer is described in FR-A-2316608 and GB A1539569. The transducer comprises a plurality of piezoelectric ultrasonic transducer elements distributed in circular formation on an arcuate support member. The transducer elements may have flat or curved radiation surfaces. The target point is positioned at the centre of a circle along an arc of which the transducers are arranged in succession.
It is an object of the present invention to be able to provide a plurality of different focal lengths within a single ultrasonic transducer.
According to the invention, it is proposed an ultrasonic transducer comprising a plurality of piezoelectric sections disposed along a portion of a cylindrical concave surface, rear electrode- means on the rear surface of the piezoelectric means and front electrode means on the front surface of the piezoelectric means, and means for supporting the piezoelectric sections, characterised in that the concavity of the cylindrical concave surface is in the form of a spiral of length L and in that each piezoelectric section has along the spiral a different focal length.
The transducer thus comprises a piezoelectric element having a cylindrical spiral or generally cylindrical spiral surface with respective sections or zones of the cylindrical spiral providing respective different focal lengths. Preferably, the piezoelectric element is a plastic piezoelectric film, such as polyvinylidene fluoride (PVF₂), disposed on a support member providing the cylindrical spiral surface. The sections may each have a corresponding focus lying in a common plane disposed transversely to the spiral surface. The curved surface of the spiral provides focussing in one dimension, along the length of the spiral.
Focussing in the orthogonal dimension may be provided by a Fresnel zone pattern on the front surface of each section of the piezoelectric film. The zone pattern is formed by electrodes on the front surface of the film extending across the width of the film. The front electrodes of the several sections are electrically connected in series or parallel, or in a series-parallel combination, depending upon the capacitance and reaction required for specific applications. The electrode pattern for each section terminates in a respective electrical terminal for coupling to excitation or reception circuitry. A rear electrode is provided on the back surface of the film, typically in the form of a continuous conductive layer with a common terminal for all sections. The Fresnel pattern can be eliminated and replaced by a continuous electrode for each zone on the front surface of the spiral film in applications where ultrasonic focussing is desired in only one dimension in order to provide a line focus.
Embodiments of the invention are described by way of example, with reference to the drawings, in which:

Figure 1 is a pictorial view of a multiple focus ultrasonic transducer in accordance with the invention;
Figure 2 is a side elevation view of the transducer of Figure 1;
Figure 3 is a front view of the transducer of Figure 1;
Figure 4 is an exploded pictorial view of the piezoelectric film and backing;
Figure 5 is a cutaway pictorial view illustrating the electrode pattern on one section of the spiral surface;
Figure 6 is a side view of an alternative embodiment of the novel transducer employing two piezoelectric elements; and
Figure 7 is a diagrammatic side view of the piezoelectric element illustrating the multiple foci.

Referring to Figures 1 and 2, there is shown an ultrasonic transducer constructed in accordance with the invention, which comprises a piezoelectric film 10 supported on a support or backing 12 of acoustic damping material and having a concave surface 14 of varying radius of curvature and uniform width and length. A filler maetrial 16 for acoustic damping is disposed rearwardly of support 12, the entire assembly being contained within a housing 18.
As seen in Figure 1 and Figure 3, the piezoelectric film 10 is divided into adjacent sections which are formed by strips of substantially constant width along the length, L, of the concave surface 14, each section having a different respective focal length. Referring to Figure 7, section 10a has a focus at 0₁, section 10b has a focus at O2, section 10c has a focus at 0₃, and section 10d has a focus at 0₄. The focal points 0₁ to 0₄ lie along an axis 20 which is the optical axis of the transducer. The sections can have continuously increasing radius of curvature to provide part of a true spiral, or each section can have constant or substantially constant radius of curvature to approximate a spiral path.
Each section of the film 10 has a Fresnel zone pattern thereon across the width, w, of the film surface to provide focussing in the width dimension as shown in Figures 1 and 3. Focussing in the longitudinal direction of the spiral is provided by the curved surfaces of the sections. The Fresnel zone pattern for each section is slightly different to that of the others to account for the different focal lengths. The Fresnel pattern for each section is provided by conductive strips 22 formed on the front surface of the film 10, the front electrodes being electrically interconnected to provide an intended capacitance and reactance. A rear electrode 24 is provided on the rear surface of the film 10 in the form of a continuous conductive layer providing a common electrode for the sections.
The Fresnel zone pattern for one section is illustrated in Figure 5. The pattern includes a plurality of electrode areas symmetrically disposed about a centre line, each of the electrode areas being of predetermined width and spaced from adjacent electrode areas by a predetermined distance. The centre line of each electrode area lies at a distance d from the centre line of the Fresnel pattern and can be found by
where n is an integer 0, 1, 2, etc. for each successful electrode area; a is the mean focal length for the particular section of the curved surface; and A is the wavelength per cycle.
The width of each electrode area Ad can be obtained by substituting n±0.25 for the integer n in equation 1. The centre of each area between the electrode areas can be found by substituting (2n+1)/2 for the integer n in equation 1.
If the number of electrode areas is relatively small, equation 1 reduces to
As an example, for a frequency f of 1 MHz, a focal length of 10 centimeters, and a sound velocity v in water of 1.5x10⁵ centimeters per second, the wavelength X is equal to v/' f=(₁.₅x1₀ ⁵)/10⁶=0.15 centimeters per cycle. Thus, the centre of the electrode areas in the section under discussion are expressed as follows:
For purposes of the above example, the section is considered as having a constant radius, and therefore constant focal length, throughout its extent. Since the surface is actually a portion of a cylindrical spiral which has a slightly varying focal length throughout its zone length, the location of the electrode areas should be calculated for the mean focal length for the zone. Alternatively, the electrode areas can be calculated separately for the end portions of a zone to accommodate the focal length variations.
For each section of the spiral, the electrode areas are electrically connected in series or parallel, or in a series-parallel combination to provide an intended capacitance to achieve a reactance of predetermined value, typically in the range of 25-50 ohms. Each section has a respective electrical terminal 25 (Figure 5) for connection to electronic circuitry for energizing the transducer for transmission for receiving and processing signals produced in response to received ultrasonic energy. The rear electrode is common to all sections and has a common terminal which serves as the second terminal for all sections. In the illustrated embodiment, the piezoelectric film is polyvinylidene fluoride (PVF₂), and the electrodes are formed of a nickel-chrome alloy. The electrodes are provided on the film in any known manner, such as by vacuum sputtering.
The polyvinylidene fluoride has a broadband frequency response, and therefore the thickness of the film is not as critical as with typical PZT materials which have a much narrower band frequency response. For a frequency constant of about 50.8 KHz-cm (20 KHz-inches), the film operative at 1 MHz can have a thickness of about 250-500 microns. For a dielectric constant K of 13, the capacitance C for each square centimetre of the electrode area of a Fresnel pattern is
where e' is the permittivity of free space (0.088x 10-'2) and where t is the film thickness in centimetre. For a film thickness of 250 microns, the capacitance C is equal to 46 picofarads per square centimetre. For a reactance X_. of 50 ohms, the capacitance is
For each section or zone in which the electrode areas are connected in parallel, the total electrode area is 3185 picofarads/46 picofarads per square centimetre, which equals 69 square centimetres.
In the event that focussing in two orthogonal axes is not needed, the Fresnel pattern can be eliminated, and the front electrode provided by a continuous electrode film formed on each section of the front surface of the piezoelectric material, each front electrode having a respective electrical terminal. In this version, a line focus would be provided by each section of the spiral surface, as distinguished from a point focus provided in the embodiment described above.
Another embodiment is shown in Figure 6 in which a piezoelectric film 10 is supported on a ceramic piezoelectric material 30 such as PZT (lead zirconate titanate). Both piezoelectric materials are disposed in a portion of a cylindrical spiral path, as in the above embodiment. This dual layer structure is supported on an acoustic damping backing material, as in the above embodiment, and can otherwise be similarly housed. In typical fabrication, the PZT material 30 is bent into the spiral configuration while in its plastic state prior to firing, and after firing, it will retain its spiral shape. the piezoelectric film 10 can then be bonded to the PZT material. Front and rear electrodes are provided for each piezoelectric layer, the electrode areas being connected to respective terminals. The Fresnel electrode pattern can be provided for each zone on the front surface of the film, and on the rear surface of the PZT layer, with a common electrode layer interposed between the rear surface of the film and the front surface of the PZT material. Alternatively, each piezoelectric layer can have the Fresnel pattern for each zone on its front surface, and a rear electrode on its rear surface, with an electrically insulating spacer provided between the front electrodes of the PZT material and the rear electrode of the film material to maintain electrical isolation between the two transducers.
The polyvinylidene fluoride film is more effective for ultrasonic reception than for transmission, while the PZT material is superior for transmission rather than reception. Thus, in the composite structure illustrated in Figure 6, the PZT layer is energized with an appropriate driving signal for transmitting ultrasonic energy in a focussed manner to an object under study, and the film layer is operative to receive energy preferentially focussed onto the respective section or zone of the film to generate output signals representative of received ultrasonic energy.
The novel transducer finds particular application as an immersion transducer for medical diagnostic purposes. The immersion transducer is placed in a vessel containing water or other liquid, the transducer being spaced from the subject by the interposed liquid. Ultrasonic energy is coupled via the liquid from the transducer to the subject, which is also immersed in the liquid. Alternatively, a thin layer of liquid or gel can be employed to couple the transducer directly to living tissue.
The invention is also useful in other frequency applications. For example, the transducer can be employed for sonar, in which case the transducer dimensions would be appropriately scaled up to accommodate the lower frequencies employed for sonar work. For medical diagnostic purposes, frequencies are typically in the range of 1-10 MHz, while sonar is operative at about 30 KHz.
The scope of the invention is not to be limited except as indicated in the appended claims.

Claims

1. An ultrasonic transducer comprising a plurality of piezoelectric sections (10a, 10b, 10c, 10d; 30) disposed along a portion of a cylindrical concave surface (14), means (12) for supporting the piezoelectric sections, rear electrode means (24) on the rear surfaces of the piezoelectric sections and front electrode means (22) on the front surfaces of the piezoelectric sections, characterised in that the concavity of the cylindrical concave surface is in the form of a spiral of length L and in that each piezoelectric section has along the spiral a different focal length.

2. A transducer according to Claim 1, wherein the foci (0₁, 02,...04) of the piezoelectric sections are disposed forwardly of the concave side of the transducer along a common axis.

3. A transducer according to Claim 2, wherein the front electrode means (22) presents for each piezoelectric section a Fresnel pattern zone across the spiral surface, wherein the length of each zone is substantially greater than its width which is disposed along the length L of the spiral surface, and each section (10a...10d) has a Fresnel pattern zone focal length corresponding to the focal length of the respective associated section (10a...10d) of the spiral surface (10).

4. A transducer according to Claim 3, wherein the front electrode means (22) for each section comprises an array of spaced electrode areas.

5. A transducer according to Claim 4, wherein the electrode areas of each section are electrically interconnected to provide a predetermined capacitance and reactance.

6. A transducer according to Claim 5, wherein the Fresnel zone pattern for each section is of different width and spacing to provide a respective focal length.

7. A transducer according to any one of Claims 3 to 6, wherein the Fresnel zone pattern for each section is symmetrically disposed about the longitudinal centre line of the portion of cylindrical spiral surface.

8. A transducer according to any preceding Claim, wherein the piezoelectric means (10) is a piezoelectric film.

9. A transducer according to Claim 8, wherein the piezoelectric film is made of polyvinylidene fluoride.

10. A transducer according to any preceding Claim, wherein the supporting means (12) includes acoustic damping material (16).

11. A transducer according to Claim 10, wherein the supporting means (12) has a curved surface on which the piezoelectric means is disposed, and which conforms with the curvature of the piezoelectric means (10).

12. A transducer according to Claim 11, wherein the piezoelectric means (10) has a uniform width along the length of the portion of cylindrical spiral surface.