Abstract:
A connector assembly for a thin film acoustic receiver array provides a spring support block having a plurality of holes each aligning one helical compression spring which serves as a conduit between a rear surface of the piezoelectrict film and a circuit card. The front surface of the film is supported against the force of the springs using an acoustically transparent material that may also provide matching between water and the piezoelectric film

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     The present invention relates to ultrasonic receiver arrays for use in imaging ultrasonic devices and, in particular, to an improved method of providing electrical connection for such receiver arrays. 
     Ultrasound may be used to characterize living tissue through the attenuation, change in speed of sound, or other modification of ultrasonic energy through the tissue. A device using this approach for quantitative measurement of bone quality, such as may be useful in the study and treatment of osteoporosis, provides an ultrasonic transmitter positioned across from an ultrasonic receiver about a volume which may receive a portion of the body containing bone with high trabecular content. A convenient site for such a measurement is the os calcis of the human heel, which includes substantial trabecular bone structure and minimal intervening soft tissue. 
     It can be desirable to combine the capability of imaging and quantitative measurement to an ultrasonic device, for example, to allow the operator to ensure correct foot location and thus improve repeatability in measurements taken at different times. U.S. Pat. No. 6,027,449, entitled: “Ultrasonometer Employing Distensible Membranes”, assigned to the assignee of the present case and hereby incorporated by reference, describes a method of manufacturing an ultrasound detection array using a thin film of piezoelectric material plated with regularly spaced electrodes. The electrodes are attached to processing circuitry using acoustically transparent Mylar connectors. Such connectors provide extremely high quality connection with minimal acoustic disruption, but can be difficult to manufacture. What is needed is an alternative connection method that provides high reliability, linearity, and stability. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a contact system for film-type piezoelectric material permitting simplified manufacturing. The piezoelectric film is supported on its front face by an acoustically transparent material and a set of springs are sandwiched between the rear face of the piezoelectric film and a circuit board having processing circuitry, to provide electrical connection therebetween. The springs may be pre-assembled in a carrier by vibratory or other automatic assembly techniques and provide for high areal density interconnection with moderate effect on the acoustic signal. 
     Specifically, the present invention provides an ultrasonic array using a piezoelectric sheet having a plurality of electrodes spaced at predetermined array locations on a rear surface of the sheet. A set of electrically independent conductive springs are positioned at the array locations and a circuit card having electrical terminals positioned at the array location on a front side of the circuit card, is placed proximate thereto. A retention frame compresses the array of conductive springs between the piezoelectric sheet and the circuit card to establish electrical communication between the electrodes and terminals. 
     In this way, an acoustically light and readily manufactured connection is made. 
     An acoustically transparent support block may be fastened to a front surface of the piezoelectric material. This block allows the thin film piezoelectric material to resist the pressure of the springs. The block may further provide for impedance matching from water coupling material to the piezoelectric film. In this regard, the support block may have an acoustic impedance between the acoustic impedance of the piezoelectric sheet and the acoustic impedance of water. 
     The circuit card may include at least one multiplexer circuit on the second side of the circuit card opposite the terminals but communicating with the terminals and for selectively collecting at least one communication lead to ones of the terminals. 
     In this way, the high density of connections may be converted to a convenient number of leads and the circuitry for doing so may be displaced from acoustic contact with the piezoelectric film. 
     The device may include a spring support plate positioned between the film and the circuit card having a series of axial holes sized to support the springs in position at the array locations. A means for maintaining an air gap positioned between the spring support plate and the film may be provided. 
     In this way, the springs may be supported to improve manufacturability of the device without interfering with the acoustic properties of the connection. 
     The array locations may be interstices of a rectangular grid separated by less than one-half centimeter. 
     Thus, the present invention can provide extremely high connection densities. 
     The foregoing features and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an imaging/quantitative ultrasonic densitometer suitable for use with the present invention showing an ultrasonic reception unit and ultrasonic transmission unit opposed across a footwell; 
     FIG. 2 is an exploded perspective view of the ultrasonic reception unit of FIG. 1 showing the constituent thin film transducer attached to a coupling plate and compliant water filled bladder, on one side, and attached via a spring array and spring retention plate to a circuit card, on the other side; 
     FIG. 3 is a fragmentary cross-section of the reception unit of FIG. 1 along line  3 — 3  showing the compression of the springs as held by the spring retention plate between the film and the circuit board; 
     FIG. 4 is a perspective view of the fragment of FIG. 3 showing the electrical connection of the multiplexers through plate-through holes of the circuit card; and 
     FIG. 5 is a schematic representation of the densitometer of FIG. 1 showing the control of the transmitter unit and the receiver unit by a microprocessor, which also controls mechanical subsystems and a display. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an imaging/quantitative ultrasonic device  10  includes a housing  12  having a generally upward opening footwell  14  sized to receive a human foot. At the toe end of the footwell  14  on the upper surface of the housing  12  is a display/touch panel  16  allowing data to be entered into or received from an internal computer (not shown in FIG.  1 ). Flanking the footwell  14  near the heel end of the footwell is an ultrasonic transmitter unit  18  and an ultrasonic receiver unit  20  supporting at their opposed surfaces compliant bladders  22  holding a coupling fluid such as water. The bladders  22  serve to communicate ultrasonic energy from the contained transducers of the transmitter unit  18  through a patient&#39;s foot inserted into the footwell  14  and back out to the contained transducer of the receiver unit  20 . 
     Referring now to FIGS. 2,  3  and  4 , the receiver unit  20  may include a piezoelectric sheet  24  of circular outline positioned normal to a transmission axis between the receiver unit  20  and transmitter unit  18 . 
     The piezoelectric sheet  24  is divided into a number of transducer elements  26  defined by electrodes  28  placed on opposite surfaces of the piezoelectric sheet  24 . Rear electrodes  28 b are deposited by vacuum metallization and may be squares centered at the interstices of a rectangular grid to fall in rectilinear rows and columns. A solid continuous electrode  28 a is positioned on the opposite side of the piezoelectric sheet  24 . The center of each rear electrode  28 b is separated from its neighbor by less than one-half centimeter and the front electrode  28 a is connected to a common reference voltage. 
     The piezoelectric sheet  24  may be constructed polyvinylidene fluoride (PVDF). In manufacture, the piezoelectric sheet  24  is polarized to create its piezoelectric properties by heating and cooling the sheet in the presence of a polarizing electric field according to methods well understood in the art. In the preferred embodiment, the entire sheet is thus polarized, however it may be advantageous to ‘spot polarize’ the sheet where only the areas under the metalization are piezoelectric providing for better cross talk isolation according to polarization methods well known in the art. Mechanical forces operating on the piezoelectric sheet  24  create a voltage between electrodes  28   a  and  28   b.    
     Attached to the front of the piezoelectric sheet  24  in the direction of received ultrasonic energy is a matching plate  30  constructed of an acoustically transmitting material, such as a polyester, having a speed of sound near that of water and the piezoelectric sheet  24  to provide for improved matching between the two. The thickness of the matching plate  30  is arbitrary but chosen to be many times the operating wave length of the ultrasound so as to delay any reverberation effects that may occur due to acoustic impedance mismatches, and to be sufficiently thick so as to withstand reasonable pressure from water on its front side, as will be described, mechanical shock to which the imaging/quantitative ultrasonic device  10  may be subjected, and the combined pressure of connector springs, also to be described. In the preferred embodiment, the matching plate  30  is generally planar, however, lens shaped plates providing a focusing of acoustic energy may also be used. 
     Referring again to FIG. 2, the piezoelectric sheet  24  and matching plate  30  are attached together with an adhesive and fit within a retainer ring  32  that provides a point of attachment for the receiver unit  20  to the housing  12 . The retainer ring  32  also provides a flange on its front surface holding a compliant silicon bladder  33  filled with water to provide a coupling path for ultrasonic energy from the heel of the patient through the matching plate  30  to the piezoelectric sheet  24 . Ports in the retainer ring  32  (not shown) allow inflation of the bladder before use and deflation of the bladder for storage. 
     Referring still to FIGS. 2 and 4, a spring holder  36  is positioned behind the piezoelectric sheet  24  opposite the matching plate  30 . The spring holder  36  is comprised of an insulating disk such as a plastic and having a plurality of axial holes  38 , each aligned with one electrode  28   b , and each hole sized to hold a helical compression springs  40 . 
     The springs  40  may be loaded into the holes  38  of the spring holder  36  by a vibratory feeder or other assembly technique and held in position for assembly by the introduction of a volatile liquid such as alcohol, which acts to retain the springs  40  by surface tension. Each spring  40  is otherwise free to move axially within the holes  38 . 
     Behind the spring holder  36  is a circuit board  42  which may be an epoxy glass material well known in the art. The front surface of the circuit board  42  has a number of terminal pads being part of plate through holes  44  passing through the circuit board  42 . Each of the plate through holes  44  aligns with one of the axial holes  38  and with an electrode  28   b  so that the spring  40  may provide a path from electrode  28   b  to a plate through hole  44 . 
     The circuit board  42  is held adjacent to the piezoelectric sheet  24  by the retainer ring  32  in a manner such that there is an air space between the front surface of the spring holder  36  and the rear surface of the piezoelectric sheet  24  so as to reduce the conduction of ultrasonic energy out of the piezoelectric sheet  24  into the spring holder  36 . Springs  40 , while not as light as aluminized Mylar, provide an acceptably reduced conduction of ultrasonic energy away from piezoelectric sheet  24 . 
     The plate through hole  44  provides a conduit, shown in FIG. 3, conducting electrical energy to the rear side of the circuit board  42  where it may be connected to the lead of a multiplexer  50 , the latter soldered onto a terminal or trace on the rear of the printed circuit board according to techniques well known in the art. Referring to FIG. 4, the multiplexers  50  allow selective connection of one or more transducer element  26  at a time to an output lead  52 . This selective connecting may read, in a scanning process, the voltage at each electrode  28   b.    
     Referring now to FIG. 5, an imaging/quantitative ultrasonic device  10  incorporating the receiver unit  20  provides an internal bus  46  allowing a computer  48  having a processor  50  and memory  53  to communicate both with the transmitter unit  18  and the receiver unit  20 . In this way, the transmitted wave may be controlled according to a program held in memory  53  and the received wave may be processed according to the program in memory  53 . The bus  46  also communicates with the display/touch panel  16  which allows inputting of data to the computer  48  and outputting data from the computer  48  during execution of the program in memory  53 . The bus  46  also allows communication between the computer  48  and the mechanical subsystems  54  such as pumps for inflating the bladders  33  prior to use or deflating the bladders  33  for storage. 
     During operation of the program held in memory  53 , the computer  48  energizes the ultrasonic transmitter unit  18  to produce a generally planar wave  62  for imaging purposes. The computer  48  scans the multiplexers  50  through the transducer elements  26  of the receiver unit  20  to collect and process image data. This image data may consist of attenuation data such as broadband ultrasonic attenuation (BUA) or speed of sound measurements (SOS), a combination of both, or some other acoustic parameter, mapped to a gray scale value and a spatial location in the image corresponding to the location of each transducer element  26  in the ultrasonic receiver unit  20 . The image may be displayed on the display/touch panel  16 . 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.