Abstract:
A thin film piezoelectric material employs a metallic backer plate to provide high output, non-resonant ultrasonic transmissions suitable for quantitative ultrasonic measurement and/or imaging.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     --  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     --  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to ultrasonic acoustic transmitters, such as may be used in transmission quantitative ultrasonic imaging and measurements, and in particular to an improved thin film transmitter.  
         [0004]     Transmitting ultrasonic transducers are used, for example, in bone densitometers where ultrasound is transmitted through in vivo bone, most typically the os calcis of the heel, in order to measure trabecular bone. Common measurements made by such densitometers include the speed of sound (SOS) and broadband ultrasonic attenuation (BUA) in the bone. Images of the bone based on these or other measurements may also be provided by the densitometer. Densitometers of this type are described in U.S. Pat. Nos. 5,840,029 and 6,517,487 assigned to the assignee of the present invention and hereby incorporated by reference.  
         [0005]     Ceramic transducers are commonly used as the transmitting ultrasonic transducer in such densitometers because of their high output signals. In this application, the mechanical resonance of the ceramic transducer is adjusted to be near the principal frequency being transmitted. Operation in this “resonant” mode increases the output of the transducer, but can make manufacturing of the transducer difficult because of the high sensitivity of the transducers resonant frequency to variations in the dimensions of the many subcomponents of the transducer. Slight differences in resonant frequencies of the transducers on different machines complicate the effort to provide highly repeatable measurements that are machine independent.  
         [0006]     Thin film polymer piezoelectric materials such as polyvinylidene fluoride (PDVF) may also be used as an ultrasonic transducer as described in U.S. Pat. No. 6,305,060 issued Oct. 23, 2001 and U.S. Pat. No. 6,012,779 issued Jan. 11, 2000 assigned to the assignee of the present invention and hereby incorporated by reference. Application of PVDF to transmitting ultrasonic transducers has been limited because of low output levels.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides an ultrasonic transmitter using a piezoelectric film and suitable for use in transmission quantitative ultrasonic imaging systems. The transducer provides suitable output levels and may operate in a non-resonant mode avoiding some of the difficulties of manufacturing present ceramic transducers.  
         [0008]     Generally, the invention employs a thin metallic backer plate to the piezoelectric film that provides a sharp discontinuity in acoustic impedance at the back surface of the piezoelectric film that increases the acoustic output from the piezoelectric film&#39;s front surface. The low acoustic attenuation of the metal of the metallic plate, which would normally cause reflections off of the metal plate&#39;s back surface to interfere with the wave in the piezoelectric film, is minimized by adjusting the plate thickness to provide a proper phasing of reflections that reduces this interference. The transducer may be essentially non-resonant to reduce coloration of the signal, and the metallic plate may provide an electrode for the piezoelectric film, further simplifying the manufacturing process. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]      FIG. 1  is a perspective exploded view of the ultrasonic transducer of the present invention showing a protective acoustically transparent layer followed by a thin film piezoelectric material, a metallic backer plate and support structure;  
         [0010]      FIG. 2  is a fragmentary, elevational cross section through the transducer of  FIG. 1  showing the layers of the transducer as assembled and the connection of electrodes to opposite sides of the piezoelectric material; and  
         [0011]      FIG. 3  is a simplified diagram of a quantitative ultrasonic densitometer using the transducer of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring now to  FIG. 1 , an ultrasonic transmitting transducer  10  constructed according to the present invention includes a disk-shaped piezoelectric film  12 .  
         [0013]     In the preferred embodiment, the piezoelectric film  12  is a polyvinylidene fluoride film (PVDF) that has been polarized to create piezoelectric properties according to methods well understood in the art. The piezoelectric film  12  includes a tab  14  extending downwardly from an edge of the disk which may be folded rearward as shown.  
         [0014]     A front face  18  of the piezoelectric film  12  and the front side of the tab  14  may be coated with a conductive material such as copper by vacuum metallization or electroplating to create a front electrode  29 . This front electrode  29  may be coated with nickel to reduce corrosion and will provide an essentially continuous conductive surface over the front face  18 , and lower surface of the tab  14  when the tab  14  is folded backward as shown in  FIG. 1 .  
         [0015]     The front face  18  of the piezoelectric film  12  and the front electrode  29  may then be covered by an acoustically transparent protective film  28  such as Teflon to prevent direct contact between water or other acoustic coupling medium and the front electrode  29 .  
         [0016]     A rear face  20  of the piezoelectric film  12  may be similarly coated with conductive material to form an outer annular electrode  22  and an inner disk-shaped electrode  24  centered within the outer annular electrode  22  and electrically isolated from the outer annular electrode  22  by a small ring-shaped gap  26 .  
         [0017]     Attached to the rear face  20  of the piezoelectric film  12  is a metallic backer plate assembly  30  having an outer annular plate  32  of area substantially equal to that of outer annular electrode  22  and a center disk plate  34  corresponding in area to and aligning with the inner disk-shaped electrode  24 . The alignment assures that the center disk plate  34  only touches inner disk-shaped electrode  24  and not outer annular electrode  22  and outer annular plate  32  touches only outer annular electrode  22  and not inner disk-shaped electrode  24 . Bonding between the backer plate assembly  30  and the piezoelectric film  12  may be done through the use of a thin adhesive such as epoxy, a process simplified by the flexible nature of the piezoelectric film  12 . The outer annular electrode  22  and inner disk-shaped electrode  24  are optional and may be replaced simply with the metallic surfaces of the backer plate assembly  30  such that the charge pushed to the PVDF is transferred via the close proximity of the metal backing plate.  
         [0018]     The outer annular electrode  22  and a center disk plate  34  of the metallic backer plate assembly  30  is preferably 0.025 inch stainless steel. The metal of the backer plate assembly  30  has an acoustic impedance substantially different from the material of the piezoelectric film  12  to reduce but not eliminate acoustic coupling between the two. In an alternative embodiment, the outer annular electrode  22  and a center disk plate  34  of the metallic backer plate assembly  30  may be composed of many electrically independent electrodes covering generally the regions of the outer annular electrode  22  and a center disk plate  34 , for phased array operation.  
         [0019]     The piezoelectric film  12  and the backer plate assembly  30  may all be held within an injection molded housing  40  having a mounting base  42  supporting a disk-shaped support plate  41 . The disk-shaped support plate  41  may abut a rear face of the backer plate assembly  30  to support the same while exposing the front face  18  of the piezoelectric film  12  under the protective film  28 . The injection molded housing  40  may be constructed of a thermoplastic having an acoustic impedance differing substantially from the metal of the backer plate assembly  30  minimizing acoustic transmission through this interface as will be understood to those of ordinary skill in the art.  
         [0020]     The disk-shaped support plate  41  has two holes  44  and  46  through its surface and opening at its rear surface to allow electrode leads  50  and  48  to pass through the housing  40  to electrically contact center disk plate  34  and outer annular plate  32 , respectively. Center disk plate  34  and outer annular plate  32  may capacitively couple to the outer annular electrode  22  and inner disk-shaped electrode  24 , respectively, or may be joined to the outer annular electrode  22  and inner disk-shaped electrode  24 , respectively, using a conductive paste or epoxy or the like.  
         [0021]     When the piezoelectric film  12  and backer plate assembly  30  is mounted on the housing  40 , the tab  14  of the piezoelectric film  12  passes through a slot  52  between the disk-shaped support plate  41  and the mounting base  42  to permit the attachment of electrode lead  54  to the tab  14  at the rear of the housing  40 .  
         [0022]     When energized, the piezoelectric film  12  will create an acoustic signal  16  directed generally along a longitudinal axis perpendicular to a front face  18  of the piezoelectric film  12 . While the inventor does not wish to be bound by a particular theory, it is believed that a similar signal will be generated in the opposite direction to be reflected by the backer plate assembly  30  as a result of the difference of acoustic properties of the PDVF and stainless steel. The small amount of acoustic energy conducted coupled into the stainless steel of the backer plate assembly  30  is largely reflected at the interface between the backer plate assembly  30  and the housing  40 , and the thickness of the backer plate assembly  30  is adjusted so that the phase of the returning signal (part of which will be coupled into the piezoelectric film  12 ), adds constructively to the existing signal.  
         [0023]     The resulting transducer is essentially non-resonant at ultrasonic frequencies (as defined both by center frequency and Q) and has lower construction costs than a ceramic device. Because of the low resonance of the ultrasonic transmitting transducer  10 , the output wave is not colored by the resonant characteristics providing improved device-to-device consistency. Although the present inventors do not wish to be bound by a particular theory, they believe that the thin film piezoelectric film  12  has an additional advantage over ceramic as a transmitter in that it provides very little lateral mode wave such as improves the beam profile produced by ultrasonic transmitting transducer  10  in the region of the outer annular plate  32 . The resulting ultrasonic transmitting transducer  10  can have an operating bandwidth of 3 MHz or more, compared to a 300 KHz bandwidth achievable with ceramic transducers.  
         [0024]     Referring now to  FIG. 3 , an ultrasonic densitometer  60  employing the ultrasonic transmitting transducer  10  of the present invention may provide an oscillator  62  operating in two modes, one for quantitative ultrasound and one for imaging. In a quantitative ultrasound mode, the oscillator  62  may provide a 500 kilohertz pulse energizing only the central section of the piezoelectric film  12  associated with the inner disk-shaped electrode  24  to produce a relatively narrow beam  64  through the os calcis  66  of the human heel  68 . That narrow beam  64  may be detected by a receiving transducer  70  providing a signal amplified by an amplifier  72  and passed to a processing computer  74  for quantitative measurements.  
         [0025]     In imaging mode, the oscillator  62  may be operated at a substantially higher frequency, for example, one megahertz and both the piezoelectric film  12  associated with inner disk-shaped electrode  24  and associated with outer annular electrode  22  may be energized through switch  76  to provide a broad area beam  78  to be detected by the receiving transducer  70  to provide an image that may be generated on a display screen  80  of the computer  74 .  
         [0026]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. For example, although the invention has been described in the context of a transmitter only, it will be appreciated that it can be used as part of a transceiver that provides for transmission and reception of ultrasonic signals. Accordingly, the word transmitter in the claims should not operate to preclude use as both a transmitter and receiver.