Abstract:
A thin film piezoelectric material employs an array of metallic backer plates to provide high output, non-resonant ultrasonic transmission and reception suitable for ultrasonic measurement and/or imaging.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/713,417 filed Nov. 14, 2003, hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to ultrasonic acoustic combination receivers and transmitters, such as may be used in quantitative ultrasonic imaging and measurements, and in particular to an improved thin film receiver/transmitter. 
   Quantitative ultrasonic imaging is used, for example, in bone densitometry 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. 
   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. Significant differences in transmission frequencies can affect quantitative measurements such as assessments of bone density. 
   Thin film polymer piezoelectric materials such as polyvinylidene fluoride (PVDF) may also be used as a receiving 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 
   The present invention provides an ultrasonic transmitter and receiver using a piezoelectric film and suitable for use in 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. The non-resonant mode also allows rapid sequential transmission and reception of ultrasonic signals from local targets (for example, in medical imaging) without interference from transducer ringing. 
   Generally, the invention employs a set of thin metallic backer electrodes attached to the piezoelectric film that provides a sharp discontinuity in acoustic impedance at the back surface of the piezoelectric film to increase the acoustic output from the piezoelectric film&#39;s front surface during transmission. During reception, each of the backer electrodes operates independently to provide spatial discrimination necessary for most quantitative applications. During transmission, the backer electrodes operate in unison, for example, as a ground plane. The metallic backer electrodes may be copper adhered to a printed circuit board further simplifying the manufacturing process. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       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 electrode and support structure; 
       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 
       FIG. 3  is a block diagram of a quantitative ultrasonic apparatus using the transducer of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an ultrasonic transmitting and receiving transducer  10  constructed according to the present invention includes a disk-shaped piezoelectric film  12 . In the preferred embodiment, the piezoelectric film  12  may be a polyvinylidene fluoride film (PVDF) that has been polarized to create piezoelectric properties according to methods well understood in the art. 
   A front face  18  of the piezoelectric film  12  is preferably coated with a thin flexible layer of conductive material such as copper. This front electrode  29  may be coated with nickel to reduce corrosion. These materials may be applied by vacuum metallization or electroplating or other methods and creates a front electrode  29  which is continuous. The electrode may also be sub-divided into multiple elements such as to allow individual stimulus to various parts of the assembly. Devices organized in this manner would be capable of generating a focused or otherwise directed sound beam. 
   The front face  18  of the piezoelectric film  12  and the front electrode  29  may be covered by an acoustically transparent protective film  28  such as Teflon to prevent direct contact between water or other acoustic coupling medium (providing a path between the ultrasonic transmitting and receiving transducer  10  and an imaged object such as a bone or organ of a patient). 
   Referring also to  FIG. 2 , a rear face  20  of the piezoelectric film  12  abuts a series of backer electrodes  30  supported in the preferred embodiment on a printed circuit board  32 . Each of the backer electrodes  30  in the preferred embodiment are squares, disks, or other shapes as an application may require of copper approximately 0.025 inches thick arranged in vertical columns and horizontal rows or other pattern and spaced apart to allow mutual electrical isolation over the area of the piezoelectric film  12 . This thickness is thicker than the 20 mil copper cladding normally obtainable on standard printed circuit board material and is preferably much less than ¼ wavelength of the relevant ultrasonic transmission frequency and less than 0.050 inches thick. The spacing of the squares of copper partially define the fundamental resolution of the ultrasonic transmitting and receiving transducer  10  when receiving, and may be varied accordingly. 
   In the preferred embodiment, the backer electrodes  30  abut the rear face  20  of the piezoelectric film  12  with or without intervening conductive material. In this case, the backer electrodes  30  capacitively couple to the rear face  20  of the piezoelectric film  12 . However, it will be recognized that in an alternative embodiment, a conductive paste or epoxy or the like may be used. 
   The metal of the backer electrodes  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. 
   Referring now to  FIG. 2 , the backer electrodes  30  may be attached to a front face of a printed circuit board  32  at the sites of conductive plate-through holes  34  in the printed circuit board  32 . This attachment may be by conventional soldering techniques. The use of separate backer electrodes soldered to the printed circuit board  32  overcomes limitations on standard copper cladding thickness in commercial clad printed circuit boards. The metal of the metallic backer electrodes  30  has an acoustic impedance different from that of the substrate of the printed circuit board  32  (e.g. fiberglass epoxy) minimizing acoustic transmission through this interface as will be understood to those of ordinary skill in the art. 
   The plate-through holes  34  may connect via conductive traces  36  in multiple layers of the printed circuit board  32  to integrated circuits  40  attached to the rear surface of the printed circuit board  32 . The integrated circuits  40  provide input signal processing such as multiplexing, and amplification as will be described. 
   Referring now to  FIG. 3  in the preferred embodiment for use in a ultrasonic imaging machine  38 , the controller  50 , operating in a transmission mode, activates a signal generator  54  to provide a high voltage electrical signal applied through a switch  59  to the electrode  29  to stimulate the piezoelectric film  12 . The signal generator  54  may, for example, provide a 500 KHz wide band pulse referenced to a fixed crystal oscillator. The switch  59  is a solid-state switch controlled by the controller  50  to alternately connect the electrode  29  to either the signal generator  54  or to ground or a functionally similar source of constant voltage. Alternatively, multiple generators could be used to generate focused or otherwise controlled transmit waves. Contact with electrode(s)  29  may be made through thin wires or flexible circuit elements passing from the circuit card to the front face of the piezoelectric film  12 . 
   The voltage of the signal generator  54 , when applied with respect to the virtual ground of the backer electrodes  30 , produces a transmitted ultrasonic signal  60 . 
   When so energized, the piezoelectric film  12  will direct the transmitted ultrasonic signal  60  generally along a longitudinal axis  15  perpendicular to the front face  18  of the piezoelectric film  12 . Most of the signal directed along longitudinal axis  15  toward the rear face  20  is reflected at the boundary between the piezoelectric film  12  and the backer electrodes  30  which have distinctly different acoustic impedances. While the inventor does not wish to be bound by a particular theory, it is believed that the small signal passing into the backer electrodes  30  is reflected at the interface between the backer electrodes  30  and the printed circuit board  32 . 
   Immediately after transmission of the transmitted ultrasonic signal  60 , the controller  50  changes the switch  59  to connect the electrode  29  to ground or other constant voltage reference. 
   Each backer electrode  30  is connected to a separate transconductance amplifier  42  operating so that the input of the amplifier  42  connected to the backer electrode  30  is at a virtual ground. The output from each of the amplifiers  42  may then be received by a controller  50  providing for the necessary sampling and digitization of the amplifier output signals. The controller  50  may then execute a stored program to process these signals according to methods well known in the art to produce a B-mode ultrasonic image and/or a quantitative measurement of an imaged object then presented on a display console  52 . 
   In typical B-mode operation, the transmitted ultrasonic signal  60  from the ultrasonic transmitting and receiving transducer  10  will proceed to a target  62  in front of the ultrasonic transmitting and receiving transducer  10  to produce an echo ultrasonic signal  64  returning to the ultrasonic transmitting and receiving transducer  10 . When the echo ultrasonic signal  64  strikes the piezoelectric film  12 , piezoelectric voltages may be detected at the backer electrodes  30  to be received by the amplifiers  42  and forwarded to the controller  50 . 
   When the target is relatively close to the transducer  10 , it is important that vibrations of the piezoelectric film  12  from the transmission of transmitted ultrasonic signal  60  have died out prior to receipt of echo ultrasonic signal  64 . This is practical because of the non-resonant operation of the piezoelectric film  12  relative to conventional ceramic transducers. 
   The ultrasonic transmitting and receiving transducer  10  is essentially non-resonant at ultrasonic frequencies as defined both by center frequency and Q and has a lower construction cost than a ceramic device. The ultrasonic transmitting and receiving transducer  10  can have an operating bandwidth of 3 MHz or more compared to a 300 KHz bandwidth achievable with ceramic transducers. 
   Because of the low resonance of the ultrasonic transmitting and receiving transducer  10 , the output wave is not colored by 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 beam profile produced by the ultrasonic transmitting and receiving transducer  10 . 
   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.