Abstract:
Implantable medical devices including an ultrasonic transducer and methods of optimizing an ultrasonic transducer of an implantable medical device are disclosed. The implantable medical device can include a housing, an ultrasonic transducer disposed within an interior of the housing, and a limiting structure configured to constrain deformation of the ultrasonic transducer. The limiting structure can include a separate structure coupled to the housing, or can comprise a resonant portion of the housing itself. During operation, the ultrasonic transducer is configured to communicate at a frequency at or near a resonant frequency of the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/780,992, filed Jul. 20, 2007, now U.S. Pat. No. 7,949,396, which claims the benefit of Provisional Application No. 60/820,055, filed Jul. 21, 2006, both of which are herein incorporated by reference in their entirety for all purposes. 
    
    
     PARTIES TO A JOINT RESEARCH AGREEMENT 
     The claimed invention was made subject to a joint research agreement between Cardiac Pacemakers, Inc. and Remon Medical Technologies Ltd. 
     TECHNICAL FIELD 
     The present invention relates to transducers used in combination with an implantable medical device for wireless communication between the implantable medical device and remote devices implanted in the body. The present invention more particularly relates to ultrasonic transducers used in combination with a metallic cavity implantable medical device. 
     BACKGROUND 
     Implantable medical devices are often used to treat a variety of medical conditions. Examples of implantable medical devices include drug delivery devices, pain management devices, and devices that treat heart arrhythmias. One example of an implantable medical device used to treat heart arrhythmias is a cardiac pacemaker, which is commonly implanted in a patient to treat bradycardia (i.e., abnormally slow heart rate). A pacemaker includes a pulse generator and leads, which form the electrical connection between the pulse generator and the heart. An implantable cardioverter defibrillator (ICD) is used to treat tachycardia (i.e., abnormally rapid heart rate). An ICD also includes a pulse generator and leads that deliver electrical energy to the heart. Pulse generators typically include a metallic housing for a battery and electrical circuitry and a header for connecting the leads to the pulse generator. 
     Implantable medical devices are also useful in the treatment of heart failure. For example, cardiac resynchronization therapy (CRT) (also commonly referred to as biventricular pacing) is an emerging treatment for heart failure, which involves stimulation of both the right and left ventricles to increase hemodynamic efficiency and cardiac output. The treatment of heart failure and heart arrhythmias can be enhanced through the use of remote implanted devices. One example of such a remote device is a pressure sensor located in the vasculature. Communication between the implantable medical device and the remote device can allow the sensor data to be downloaded by a clinician used to modify the therapy delivered by the implantable medical device, or both. There is therefore a need for an implantable medical device that includes a transducer for communication with a remote implanted device. 
     SUMMARY 
     The present invention, according to one embodiment is an implantable medical device comprising a housing and an ultrasonic transducer having a communication frequency coupled to a portion of the housing. The housing resonates at the communication frequency, and a casing is coupled to the housing and disposed over the ultrasonic transducer. The casing is adapted to amplify the deformation of the ultrasonic transducer in a bending mode and transfer the bending moment to the housing. 
     The present invention, according to another embodiment, is an implantable medical device comprising a housing having an upper portion and a lower portion. A first ultrasonic transducer is coupled to a first connection rod and is coaxial with the first connection rod. The first ultrasonic transducer and first connection rod are interposed between the upper and lower portions such that the first ultrasonic transducer is adapted to vibrate the upper and lower portions simultaneously. 
     The present invention, according to yet another embodiment, is a method of optimizing an ultrasonic transducer and a housing of an implantable medical device. The method comprises determining system level requirements for the ultrasonic transducer and selecting an initial ultrasonic transducer based on the system level requirements. A first finite element methods analysis is conducted to verify the feasibility of the initial ultrasonic transducer, and a second finite element methods analysis and water tank experiments are conducted to determine whether the housing and ultrasonic transducer have a desired vibration mode at a targeted ultrasonic communication frequency. The ultrasonic transducer or the design of the housing are optimized based on the results of the first and second finite element methods analysis and water tank experiments. The resonance frequency and amplitude of the optimized ultrasonic transducer are verified using finite element method analysis and water tank experiment. A final ultrasonic transducer and housing design are selected based upon the results of the verifying step. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a combined cutaway and perspective view of an implantable medical device in accordance with one embodiment of the present invention. 
         FIG. 2  is a front view of the inside of the implantable medical device of  FIG. 1  in accordance with one embodiment of the present invention. 
         FIGS. 3A-3B  depict various views of the implantable medical device of  FIG. 2 . 
         FIGS. 4A-4B  are various views of an implantable medical device in accordance with another embodiment of the present invention. 
         FIGS. 5A-5B  are various views of an implantable medical device in accordance with yet another embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of an implantable medical device in accordance with another embodiment of the present invention. 
         FIGS. 7A-7B  are various views of an implantable medical device in accordance with another embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of an implantable medical device in accordance with yet another embodiment of the present invention. 
         FIGS. 9A-9B  are various views of an implantable medical device in accordance with another embodiment of the present invention. 
         FIGS. 10A-10B  are various views of an implantable medical device in accordance with another embodiment of the present invention. 
         FIGS. 11A-11B  are various views of an implantable medical device in accordance with another embodiment of the present invention. 
         FIG. 12  is a cross-sectional view of an implantable medical device in accordance with yet another embodiment of the present invention. 
         FIG. 13  is a cross-sectional view of an implantable medical device in accordance with yet another embodiment of the present invention. 
         FIG. 14  is a flowchart depicting an exemplary method of optimizing an implantable medical device having an acoustic transducer in accordance with the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of an implantable medical device (IMD)  10 . The IMD  10  includes a pulse generator  12  and a cardiac lead  14 . The lead  14  operates to convey electrical signals between the heart  16  and the pulse generator  12 . A proximal end  18  of the lead  14  is coupled to the pulse generator  12  and a distal end  20  is coupled to the heart  16 . The lead  14  includes a lead body  17  extending from the lead proximal end  18  to the lead distal end  20 . 
     The heart  16  includes a right atrium  22 , a right ventricle  24 , and a pulmonary artery  26 . A tricuspid valve  28  is located between and controls the flow of blood from the right atrium  22  and the right ventricle  24 . A pulmonic valve  30  is located between and controls the flow of blood from the right ventricle  24  to the pulmonary artery  26 . The heart  16  also includes a left atrium  32 , a left ventricle  34 , and an aorta  36 . A mitral valve  38  is located between and controls the flow of blood from the left atrium  32  to the left ventricle  34 . An aortic valve  40  is located between and controls the flow of blood from the left ventricle  34  to the aorta  36 . In one embodiment, the IMD  10  includes a plurality of leads  14 . For example, it may include a first lead  14  adapted to convey electrical signals between the pulse generator  12  and the left ventricle  34  and a second lead  14  adapted to convey electrical signals between the pulse generator  12  and the right ventricle  24 . 
     In the embodiment shown in  FIG. 1 , a helical electrode  42  penetrates the endocardium  43  of the right ventricle  24  and is embedded in the myocardium  44  of the heart  16 . When positioned as above, the electrode  42  can be used to sense the electrical activity of the heart  16  or to apply a stimulating pulse to the right ventricle  24 . In other embodiments, the cardiac lead  14  of the present invention can also be implanted in any other portion of the heart  16  as known in the art. For example, it may be implanted in the right atrium  22 , the right ventricle  24 , the pulmonary artery  26 , the left ventricle  34 , or in the coronary veins. In one embodiment, the IMD  10  includes multiple electrodes  42  disposed to sense electrical activity and/or deliver therapy to both the left and right sides of the heart  16 . In one embodiment, the lead  14  can be an epicardial lead where the electrode  42  penetrates the epicardium  45 . 
     As shown in  FIG. 1 , a remote device  46  is located in the pulmonary artery  26 . Alternatively, the remote device  46  could be located in the right ventricle  24 , the aorta  36 , or any other location in or near the heart  16  or vasculature. The remote device  46  shown in  FIG. 1  comprises a pressure sensor. The remote device  46  shown in  FIG. 1  can be used to measure pressure in the pulmonary artery  26 . In one embodiment, the remote device  46  measures end-diastolic pressure in the pulmonary artery  26 . The sensed pressure can be used to predict decompensation of a heart failure patient or to optimize pacing or defibrillation therapy. One example of a pressure sensor  46  adapted to measure pressure is disclosed in U.S. Pat. No. 6,764,446 to Wolinsky et al. 
     While the IMD  10  shown in  FIG. 1  comprises a cardiac pacemaker, in other embodiments, the IMD  10  could comprise any other medical device suitable for implantation in the body. For example, the IMD  10  could comprise a drug delivery device or a pain management device. The remote device  46  can comprise any type of chronically implanted device or remote sensor adapted to deliver therapy or monitor biological functions. The remote device  46  can be located anywhere in the body adapted for sensing a desired biological parameter or delivering therapy. For example, the remote device  46  could comprise a volume sensor or sense any other cardiac parameter, such as maximum or minimum pressure, or calculate a cardiac parameter derivative, such as the slope of the pressure. In other embodiments, the remote device  46  could comprise a glucose level monitor, a pulmonary sound sensor, a satellite pacing device, or any other remote sensing or therapy-delivering device. A plurality of remote devices  46  could be implanted throughout the body and in wireless communication with each other and with an IMD  10 . 
       FIG. 2  depicts a front view of the inside of the pulse generator  12 . The pulse generator  12  includes a housing  48  and a header  50 . An acoustic transducer  52  is attached to the inside of the housing  48  and is electrically connected to control circuitry (not shown). The acoustic transducer  52  can be used as a sensor, an actuator, or as both a sensor and an actuator.  FIGS. 3A-3B  depict cross-sectional views of the housing  48 . The acoustic transducer  52  includes electrodes  54  and can be coupled to the inside of the housing  48  by an insulating bonding layer  55 . In the embodiment shown in  FIG. 2 , the acoustic transducer  52  has a circular shape, but the acoustic transducer could take any other shape, such as rectangular, beam-shaped, circular, annular, or triangular. 
     In one embodiment, the acoustic transducer  52  comprises a piezoelectric material. Piezoelectric materials adapted for use in the acoustic transducer  52  include piezo polymer, piezo crystal, or piezo ceramic materials. In one embodiment, the acoustic transducer  52  can comprise a polyvinylidine difluoride (PVDF) material. In another embodiment, the acoustic transducer  52  can comprise a lead zirconate titanate (PZT) material. In yet another embodiment, the acoustic transducer can comprise a piezo single crystal material, such as lead magnesium niobate-lead titanate (PMN-PT). In other embodiments, the acoustic transducer  52  can comprise a cMUT transducer. In one embodiment where a PZT material is used, the thickness of the PZT material is approximately equivalent to the thickness of the housing  48 . In one embodiment, the acoustic transducer  52  comprises PZT5A material, has a diameter of 25.4 millimeters or less, and has a thickness of 3 millimeters or less. 
     As shown in  FIGS. 3A-3B , one electrode  54  is connected to an AC voltage source and the other electrode  54  is connected to ground. (The thickness of the electrodes  54  in the Figures is not shown to scale.) The AC voltage can be applied to the acoustic transducer  52  to cause it to vibrate at a desired frequency. Alternatively, both electrodes  54  could be driven simultaneously by an H-bridge, as is known to one of skill in the art. In one embodiment, the acoustic transducer  52  has a mechanical resonance of greater than approximately 20 kiloHertz. In another embodiment, the acoustic transducer  52  has a mechanical resonance at a frequency of approximately 40 kiloHertz. In yet another embodiment, the acoustic transducer  52  can operate in an electrically resonant mode. 
     In one embodiment, the acoustic transducer  52  is adapted to generate and receive acoustic waves having a frequency greater than approximately 20 kiloHertz, has a transmit sensitivity greater than approximately 100 Pascals per Volt at 0.25 meters of water or transmitting voltage response (TVR) greater than approximately 148 decibels (dB) referenced to (re) 1 microPascal per Volt at 1 meter of water, has a receive sensitivity greater than approximately 0.5 milliVolt per Pascal or free-field voltage sensitivity (FFVS) greater than −186 dB re 1 Volt per microPascal, and has a total static capacitance less than or equal to approximately 20 nanoFarads. In another embodiment, the acoustic transducer  52  is adapted to generate and receive acoustic waves having a frequency of approximately 40 kiloHertz, has a transmit sensitivity greater than approximately 200 Pascals per Volt at 0.25 meters of water or TVR greater than approximately 154 decibels re 1 microPascal per Volt at 1 meter of water, has a receive sensitivity greater than approximately 0.5 milliVolts per Pascal or FFVS greater than −186 dB re 1 Volt per microPascal, and a total static capacitance less than or equal to approximately 8 nanoFarads. 
     The acoustic transducer  52  can be used for wireless communication between the IMD  10  and the remote device  46 . As shown in  FIG. 3B , acoustic signals are transmitted from the IMD  10  to the remote device  46  by applying an AC voltage or a charge change to the acoustic transducer  52  so that the acoustic transducer  52  deforms and the pulse generator housing  48  vibrates in response to the deformation. Acoustic signals sent from the remote device  46  are received by the acoustic transducer  52  when an impinging acoustic wave results in mechanical vibration of the housing  48 , thus causing a voltage change or a charge density change in the acoustic transducer  52 , which is detected by control circuitry (not shown). 
       FIGS. 4A-4B  depict an embodiment of the present invention where a casing  62  encloses the acoustic transducer  52 . The casing  62  serves two functions. First, it bends when the acoustic transducer  52  deforms, thereby applying a bending moment to the housing  48 . Second, it mechanically amplifies the deformation of the acoustic transducer  52 , particularly when the housing  48  has a resonant mode at the desired frequency. In one embodiment, the casing  62  has a diameter of greater than 25 millimeters, a height of less than 4 millimeters, a top thickness of between 0.2 and 1 millimeter, and a wall thickness of between 3 to 6 millimeters. In one embodiment, the acoustic transducer  52  is attached to the casing  62  and there may be a gap or space between the acoustic transducer  52  and the housing  48 . In another embodiment, the acoustic transducer  52  is attached to the housing  48  and there may be a gap between the acoustic transducer  52  and the casing  62 . 
       FIGS. 5A-5B  and  6  depict alternative embodiments of an IMD  10  having an acoustic transducer  52 . As shown in  FIGS. 5A-5B , the housing  48  includes annular regions  64  having a thinner cross-section than a substantial portion of the housing  48 . The regions  64  shown in  FIG. 5A  comprise a “bull&#39;s eye” but alternatively could have any other shape, including a plurality of rectangles or circles. As shown in  FIG. 5B , the acoustic transducer  52  is adjacent to the regions  64 , thereby allowing for increased vibration, movement, and/or deformation of the housing  48 . In one embodiment, the thickness of the regions  64  is approximately 0.12 millimeter.  FIG. 6  depicts an alternative housing  48  where the region  64  takes the form of a corrugated or wavy region of the housing  48  located underneath the acoustic transducer  52 . The gaps  66  shown in  FIGS. 5A-5B  and  6  can contain air, nitrogen, some other gas, or vacuum. As shown,  FIGS. 5A-5B  and  6  include the casing  62  described with respect to  FIGS. 4A-4B , but in alternative embodiments, the casing  62  need not be present. 
       FIGS. 7A-7B  depict an alternative embodiment of an IMD  10  having an acoustic transducer  52 . In this embodiment, the acoustic transducer  52  has an annular shape. The acoustic transducer  52  acts as a limiting structure and defines the resonance characteristics of the region  56  by establishing boundary conditions for the region  56 . The resonance characteristics of the region  56  enhance the performance of the acoustic transducer  52 . When acoustic waves having the same frequency as the resonant frequency of the region  56  impact the region  56 , the region  56  vibrates, resulting in deformation of the acoustic transducer  52 . This deformation results in a voltage or a charge change in the acoustic transducer  52 , which is detected by the control circuitry. Driving the acoustic transducer  52  using an AC voltage or an H-bridge at the resonant frequency results in periodic deformation of the acoustic transducer  52 . This deformation causes the region  56  to vibrate at the resonant frequency, thereby transmitting an acoustic wave from the region  56  at the desired frequency. 
     The dimensions of the acoustic transducer  52  can be determined using the following formula from Blevins, “Formulas for Natural Frequencies and Mode Shapes”, ISBN 1-57524-184-6, herein incorporated by reference in its entirety: 
     
       
         
           
             f 
             = 
             
               
                 
                   λ 
                   2 
                 
                 
                   2 
                   · 
                   π 
                   · 
                   
                     a 
                     2 
                   
                 
               
               ⁢ 
               
                 
                   
                     E 
                     · 
                     
                       h 
                       2 
                     
                   
                   
                     12 
                     · 
                     γ 
                     · 
                     
                       ( 
                       
                         1 
                         - 
                         
                           v 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     As used in the above equation, a is the plate radius, h is the plate thickness, E is Young&#39;s modulus, ν is Poisson&#39;s ratio, ρ is the density, γ is the mass per unit area or ρ*h, and λ is a dimensionless frequency parameter dependent on the mode shape that can be found in Blevins. 
     In one embodiment, the acoustic transducer  52  comprises a PZT material and defines a region  56  having a mechanical resonance of greater than approximately 20 kiloHertz. In one embodiment where the housing  48  comprises titanium and λ=3.19 for mode 00, E=116 GigaPascals, ν=0.3, h=0.3 millimeters, and ρ=4500 kg/m 3 , and a=4.2 millimeters, for an annular piezoelectric transducer  52  with an inner radius of 4.2 millimeters, an outer radius of 8.4 millimeters, and a thickness of 2 millimeters, the natural frequency f of the first mode of the region  56  is at 40 kHz. The acoustic transducer  52  can be bonded to the housing  48  using epoxy or medical adhesive. Blevins provides additional mode resonant frequency formulas for additional shapes and boundary conditions. 
     In the embodiment shown in  FIG. 8 , the acoustic transducer  52  is mechanically bonded to a non-active limiting structure  58 . As shown, the limiting structure  58  has an annular shape and defines the resonant region  56 . Use of the non-active limiting structure  58  improves both the transmit and receive sensitivity of the acoustic transducer  52  when the resonant region  56  which has the same resonant frequency as the acoustic transducer  52 . The non-active limiting structure  58  transfers deformation between the acoustic transducer  52  and the housing  48 . For example, when the acoustic transducer  52  is in a receive mode, an impinging acoustic wave causes the resonant region  56  to vibrate, and the resulting deformation is transferred to the acoustic transducer  52  through the non-active limiting structure  58 . When the acoustic transducer  52  is in a transmit mode, actuation of the acoustic transducer  52  causes it to vibrate at the resonant frequency, which is then transferred to the housing  48  by the non-active limiting structure  58 , thus causing the resonant region  56  to vibrate at the resonant frequency. The non-active limiting structure  58  can be comprised of titanium, aluminum, stainless steel, ceramic material, or any other rigid material. The gap  60  between the acoustic transducer  52  and the resonant region  56  can be filled with air, nitrogen, some other gas, or vacuum. 
       FIGS. 9A-9B  depict an alternative embodiment of the IMD  10 . As shown in  FIG. 9A , the limiting structure  58  is located in a corner  70  of the housing  48 , has an approximately semicircular shape, and defines a resonant region  56 . The acoustic transducer  52  is bonded to the housing  48  and extends from the limiting structure  58  into the resonant region  56 . In one embodiment, the length of the acoustic transducer  52  is determined by the strain/stress profile of the resonant region  56 . In one embodiment, the deformation of the acoustic transducer  52  is constrained by the limiting structure  58  and the acoustic transducer  52  has a length of no more than half of the radius of the resonant region  56 . As shown in the cross-sectional view of  FIG. 9B , the limiting structure  58  does not extend to the rear wall  72  of the housing, but in an alternative embodiment, the limiting structure  58  could extend to the rear wall  72 . In yet another alternative embodiment, the limiting structure  58  could be located on the outside of the housing  48 . In one embodiment, the limiting structure  58  could take the shape of an annular ring located on the outside of the housing  48 . 
       FIGS. 10A-10B  depict yet another alternative embodiment of the present invention. In this embodiment, the acoustic transducer  52  is located in the corner  70  of the housing  48 . The checkerboard pattern shown in  FIG. 10A  represents the mode shape of the housing  48  at approximately 40 kiloHertz. The regions  73  represent regions of the housing  48  that are moving in the Z-axis. These regions  73  can be moving either in a positive or negative direction along the Z-axis. The dotted regions  73  can be moving in a positive direction along the Z-axis while the undotted regions can be moving in a negative direction along the Z-axis. The lines  74  of the checkerboard pattern represent the nodal regions, or lines where the housing is motionless with respect to the Z-axis. In the embodiment shown, the corner  70  acts as a limiting structure and defines a resonant region  56 , but in other embodiments, the transducer  52  could be located in a region  73  where the nodal lines  74  create a resonant region  56 . As shown, the acoustic transducer  52  is bonded to the top face  76  and is located in the resonant region  56 . In one embodiment, the acoustic transducer  52  is located in a region of maximum stress and strain. In other embodiments, the shape of the housing  48  itself can be changed to obtain a desired frequency characteristic. In other embodiments, the housing  48  could be embossed or include a “dimple” to obtain a desired frequency characteristic. 
       FIGS. 11A-11B  show another alternative embodiment of the IMD  10  of the present invention. In this embodiment, the header  50  acts as a limiting structure on the acoustic transducer  52 . An aperture  78  is located in the header  50  and defines a resonant region  56 . The acoustic transducer  52  extends into the resonant region  56  and is bonded to the inside of the housing  48 . 
       FIG. 12  shows a cross-sectional view of an alternative embodiment of the IMD  10  of the present invention. The IMD  10  includes a housing  48  having an upper portion  48   a  and lower portion  48   b . A connection rod  84  is coupled to the upper portion  48   a  and a second connection rod  84  is coupled to the lower portion  48   b . An acoustic transducer  52  is interposed between the two connection rods  84 . The connection rods  84  have a bell shape in  FIG. 12 , but could have any other shape. When the acoustic transducer  52  of  FIG. 12  is actuated, the portions  48   a ,  48   b  will vibrate, propagating acoustic waves in two directions. The thickness of the acoustic transducer  52  can be adjusted according to the desirable actuation displacement of the housing  48  for generation of acoustic waves. 
       FIG. 13  shows a cross-sectional view of an alternative embodiment where an acoustic transducer  52  is coupled to each portion  48   a ,  48   b  and two connection rods  84  are interposed between the acoustic transducers  52 . The structure depicted in  FIG. 13  also allows acoustic waves to propagate in two directions simultaneously. The symmetrical design of  FIG. 13  allows for easier design and manufacture of the acoustic transducers  52  and connection rods  84 . 
     The embodiments shown in  FIGS. 12 and 13  increase the sensitivity of the acoustic transducer or transducers  52 . The acoustic transducer or transducers  52  can comprise a piezoelectric material such as PZT or PVDF. In  FIGS. 12 and 13 , the portions  48   a ,  48   b  can apply a pre-stress to the acoustic transducer or transducers  52 . In one embodiment, the acoustic transducer or transducers  52  and connection rods  84  are resonant in the thickness mode. In another embodiment, the acoustic transducer or transducers  52  and connection rods  84  are resonant in a radial mode. In one embodiment, the combined thickness of the connection rod  84  and the acoustic transducer  52  is approximately half of the wavelength of the communication frequency. In one embodiment, the combined thickness of the connection rod  84  and the acoustic transducer  52  is between 6 and 7 millimeters. In an alternative embodiment, a single connection rod  84  and a single acoustic transducer  52  are interposed between the portions  48   a ,  48   b . In one embodiment, the single acoustic transducer  52  comprises a PZT material 1 centimeter in diameter and 1 millimeter thick. The connection rod  84  has a height of about 6 millimeters, a minimum diameter of 1 centimeter, a maximum diameter of 2.5 centimeters, and a taper beginning at a height of approximately 3 millimeters. In one embodiment, the thickness of the housing  48  is between 0.3 to 2 millimeters and the width of the housing  48  is between about 2.5 to 5 centimeters. 
       FIG. 14  depicts an exemplary method  200  for optimizing an acoustic transducer  52  and an IMD  10  for wireless communication with a remote device  46 . System level requirements such as the power budget, transducer sensitivity, mechanical size, material selection, and the vibration mode of the metallic housing  48  are determined (block  210 ). An initial acoustic transducer  52  is selected based on the system level requirements (block  220 ). A Finite Element Methods (FEM) analysis, as is known to those of skill in the art, is performed to verify the feasibility of the initial transducer in simplified geometries (block  230 ). In this embodiment, verifying the feasibility includes determining whether the acoustic transducer  52  system level attributes fall within an acceptable range for the system level requirements. FEM and water tank experiments are used to determine whether the metallic housing  48  and acoustic transducer  52  have the desired vibration mode at the targeted ultrasonic communication frequency (block  240 ). The design can be optimized by varying the design of the housing  48 , incorporating a casing  62 , modifying the design  62  of the casing, modifying the characteristics of the acoustic transducer  52 , including the dimensions, or any combination thereof (block  250 ). 
     Once the design is further refined, the underwater resonance frequency and amplitude of the acoustic transducer  52  can be verified through Finite Element Method models and water tank experiments (block  260 ). The experiments can be conducted in a water tank using a hydrophone and can utilize a scanning laser vibrometer (SLV). One such SLV can be obtained from Polytec GmbH, Polytec-Platz 1-7, D-76337 Waldbronn, Germany. The design can again be optimized by varying the parameters such as housing  48  design, acoustic transducer  52  design, etc. (block  250 ). This optimization is repeated until the desired resonance characteristics are obtained and a final acoustic transducer design is reached (block  270 ). 
     The invention has been described with respect to implantable medical devices such as pacemakers and defibrillators, but could be adapted for use in any other implantable medical device, such as an insulin pump, neurostimulator, drug delivery system, pain management system, heart or lung sound sensor, or any other implantable medical device. The remote device  46  can comprise any type of chronically implanted device or remote sensor adapted to deliver therapy or monitor biological functions, such as pressure sensor, glucose level monitor, a pulmonary sound sensor, volume sensor, satellite pacing device, or any other remote sensing or therapy-delivering device, and can be located anywhere in the body adapted for sensing a desired biological parameter or delivering therapy. A plurality of remote devices  46  could be implanted throughout the body and in wireless communication with each other and with an IMD  10 . 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.