Abstract:
Medical devices implanted in a patient can be activated and powered by an RF signal. Unless the medical device is properly oriented with respect to the transmitting antennas enough signal energy may not be received to power that device. However, optimum orientation can not be assured due to constraints on the implantation position. The present transmitting antenna is flat and omnidirectional thereby eliminating the need to properly orient the implanted medical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to apparatus for transmitting a radio frequency signal to a medical device implanted in an animal, and more particularly to cardiac pacing devices in which the radio frequency signal causes the implanted device to deliver energy to cardiac tissue for the purpose of stimulating contractions.  
         [0005]     2. Description of the Related Art  
         [0006]     A remedy for people with slowed or disrupted natural heart beating is to implant a cardiac pacing device. A cardiac pacing device is a small electronic apparatus that stimulates the heart to beat at regular intervals. That device consists of a pulse generator, implanted in the patient&#39;s chest, which produces electrical pulses that stimulate heart contractions. Electrical wires extend from the pulse generator to several electrodes placed adjacent specific muscles of the heart, which when electrically stimulated produce contraction of the adjacent heart chambers.  
         [0007]     It is quite common that the wires extend through arteries or veins which enter the heart so that the electrodes can be placed in the muscle of the heart chamber requiring stimulation. The wires typically extend for some distance through the arteries or veins and may pass through one or two heart valves. In other patients, patch electrodes are placed on the exterior heart surface with wires extending through tissue to the pacing device. With either type of wire placement, it is important that the electrodes be attached to the proper positions on the heart to stimulate the muscles and produce contractions. Thus, it is desirable to properly locate the electrodes for maximum heart stimulation with minimal adverse impact to other physiological functions, such as blood circulation.  
         [0008]     More recently wireless pacing devices have been proposed, such as the one described in U.S. Pat. No. 6,445,953. With this type of device, a radio frequency (RF) signal is transmitted from a conventional pacing circuit to stimulator devices placed on the heart at locations where stimulation is to occur. For example, the stimulator device can be mounted on a stent that is implanted in a blood vessel of the heart. The radio frequency signal activates the stent which applies an electrical stimulation pulse to the heart tissue. Electrical power for stimulating the heart is derived from the energy of the radio frequency signal.  
         [0009]     One of the difficulties in this wireless system is ensuring that a maximum amount of the RF energy is received by the stimulator device. In the case of a stent, the antenna is a coil located on a cylindrical surface and receives the greatest amount of energy from an electromagnetic field oriented in a direction through the turns of the coil. However, since the stent can be implanted in different orientations in the patient&#39;s body and the orientation of the transmitter antenna similarly varied, it is difficult to ensure that the electromagnetic field from the RF signal will be properly oriented with respect to the stent antenna.  
       SUMMARY OF THE INVENTION  
       [0010]     An antenna assembly is provided for transmitting a radio frequency signal to activate a device implanted in an animal. The antenna assembly has a substantially planar structure comprising a first antenna, a second antenna and a third antenna stacked on top of one another. The first antenna emits a first electromagnetic wave that propagates along a first axis, and the second antenna emits a second electromagnetic wave that propagates along a second axis which is substantially orthogonal to the first axis. The third antenna emits a third electromagnetic wave that propagates along a third axis which is substantially orthogonal to the first axis and the second axis.  
         [0011]     Thus electromagnetic waves are emitted omnidirectionally from the antenna assembly and the receiving medical device can derive energy from the electromagnetic waves regardless of the orientation of the medical device to the transmitting apparatus. In a preferred embodiment of the antenna assembly, the first antenna has a first coil section on one side of a first axis of symmetry and a second coil section located on another side of the first axis of symmetry. The second antenna includes a third coil section on one side of a second axis of symmetry and a fourth coil section on another side of the second axis of symmetry; wherein the second axis of symmetry is orthogonal to the first axis of symmetry. Preferably, the signal being transmitted is applied to the first antenna ninety degrees out of phase with the signal applied to the second antenna. This emits a circularly polarized RF signal from the first and second antennas. The third antenna has a conductive single coil section that emits the third electromagnetic wave that propagates orthogonally to the circularly polarized RF 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  depicts a cardiac pacing apparatus implanted in a patient;  
         [0013]      FIG. 2  is an isometric, cut-away view of a blood vessel with a vascular electrode-stent of the cardiac pacing apparatus;  
         [0014]      FIG. 3  is a schematic block diagram of an electrical circuit on the vascular electrode-stent;  
         [0015]      FIG. 4  is a schematic block diagram of the pacing device in  FIG. 1  which incorporates the present invention;  
         [0016]      FIG. 5  is a cross sectional view through an antenna assembly of the pacing device;  
         [0017]      FIG. 6  illustrates a first antenna in the antenna assembly;  
         [0018]      FIG. 7  shows a second antenna in the antenna assembly; and  
         [0019]      FIG. 8  illustrates a third antenna in the antenna assembly.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     With initial reference to  FIG. 1 , an apparatus  10  for applying electrical stimulation to pace a heart  11  comprises a pacing device  12  and one or more vascular electrode-stents  20  and  21  located in arteries or veins  14  through which blood flows to or from the heart muscles. As will be described in greater detail, the pacing device  12  emits a radio frequency signal  16  which produces an electric current in the implanted vascular electrode-stents, thereby stimulating the heart muscle.  
         [0021]     Referring to  FIG. 2 , an electrode-stent  20  is placed in the artery or vein  14  of the heart  11 . The body  23  of the electrode-stent  20  has a design similar to well-known expandable vascular stents that are employed to enlarge a restricted blood vessel. Such vascular stents have a generally tubular design that initially is collapsed to a relatively small diameter enabling them to pass freely through an artery or vein of a patient.  
         [0022]     The procedure for implanting the electrode-stent  20  is similar to that used for conventional vascular stents. For example, the balloon at the end of a standard catheter is inserted into the electrode-stent  20  in a collapsed, or reduced diameter, configuration. That assembly then is inserted through an incision in a vein or artery near the skin of a patient and threaded through the vascular system to the appropriate location adjacent the heart  11 . Specifically, the electrode-stent  20  ultimately is positioned in a cardiac artery or vein  14  adjacent to a section of the heart muscle where stimulation should be applied. The balloon of the catheter then is inflated to expand the vascular electrode-stent  20  which expansion also slightly enlarges the artery or vein  14 , as seen in  FIG. 2  which embeds the electrode-stent  20  in the wall of the blood vessel. This slight enlargement of the artery or vein  14  and the tubular design of the electrode-stent allows blood to flow relatively unimpeded through the device. The balloon is deflated, the catheter is removed from the patient, and the incision is closed. The electrode-stent  20  remains in the artery or vein without any wire connecting an electrode to pacing device  12 .  
         [0023]     With reference to  FIGS. 2 and 3 , the vascular electrode-stent  20  has a body  23  on which a signal receiving circuit  22  is mounted. The signal receiving circuit  22  includes an antenna  28 , a radio frequency signal detector  26 , and a stimulator, that is formed by first and second electrodes  24  and  25 , for example. The antenna  28  comprises a coil having a plurality of turns and is connected to an input of the radio frequency signal detector  26  that may be tuned to the frequency (e.g. 27 MHz.) of the RF signal  16  emitted by the pacing device  12 , but does not necessarily have to be a tuned circuit. Upon detecting the radio frequency signal  16 , the detector  26  converts the energy of that signal into a differential voltage pulse that is applied to the first and second electrodes  24  and  25 . Those electrodes form an electric circuit path with the patient&#39;s heart tissue allowing for stimulation of that tissue. Thus, each time the pacing device  12  emits a radio frequency signal  16 , a pulse of electrical voltage is produced in the vicinity of the electrode-stent  20 , thereby stimulating the heart muscle adjacent that electrode.  
         [0024]     Of particular interest to the present invention is the pacing device  12  illustrated in detail in  FIG. 4 . In large part the internal circuitry and operation of the pacing device is similar to that of prior cardiac pacers. However, instead of the pacing signal being applied to stimulation electrodes via wires, a radio frequency signal is produced. The pacing device  12  is powered by a battery (not shown).  
         [0025]     The pacing device  12  contains a local oscillator  40  that produces the radio frequency signal at the predefined frequency (e.g. 27 MHz.) used by the cardiac pacing apparatus  10 . This radio frequency signal is applied to the input of an amplifier  42  which is gated by a trigger signal from a conventional pacing signal generator  41 . The circuitry of the pacing signal generator  41  is the same as that used in prior medical equipment to determine when a heart stimulation pulse is required. The output signal resulting from that determination enables the amplifier  42  to pass a burst of the radio frequency signal from the local oscillator  40 .  
         [0026]     The output from the amplifier  42  is connected to the input of a signal splitter  43  which divides the radio frequency signal into three signal portions of equal power. Each signal portion is transmitted to an antenna assembly  44  through a separate transmission line  45 ,  46  and  47 , such as individual coaxial cables having a center conductor and a shield conductor. The first transmission line  45  is longer than the second transmission line  46 , so that the respective signals at their antenna ends are ninety degrees out of phase.  
         [0027]     Because the shield conductors are grounded only at the end proximate the signal splitter  43 , the antenna end is not at ground potential due to the inductance of the shield conductor. This could form standing waves in the transmission lines  45 - 47  which dissipate energy that otherwise would be transmitted to the antenna assembly  44 . As a consequence, each transmission lines  45 - 47  is provided with a separate balun  48 ,  49  or  50  in the antenna assembly  44 . The baluns  48 - 50  separate the grounds of the transmission lines, thus providing a high impedance at the antenna end of the shields to attenuate any standing waves. For example, the balun may be formed by a helix of a coaxial transmission line of five turns with a capacitor connected across the shield conductor at the first and last turn, however other types of baluns can be used. The balun is a LC parallel resonator tuned to the frequency of the RF signal. After passing through the baluns  48 - 50 , each transmission line  45 ,  46  or  47  is coupled by a matching circuit  51 ,  52  or  53  to one of three antennas  56 ,  57  or  58 , respectively, to match the amplifier output impedance and the transmission line impedance to the input impedance of the respective antenna.  
         [0028]     Referring to  FIG. 5 , the three planar antennas  56 ,  57  or  58  of antenna assembly  44  are stacked one on top of the other in a multi-layer, laminated structure, that is circular with a diameter of 15-17 cm, for example. The first and second antennas  56  and  57  are formed by conductive stripes on opposite sides of a first substrate  54 , while the third antenna  58  formed by a conductive stripes on a remote surface of a second substrate  55  that abuts the second antenna  57 . However the second substrate  55  could be on the opposite side of the first substrate  54  thus abutting the first antenna  56 . Each substrate is an electrically non-conductive material of a type conventionally used for rigid or flexible printed circuit boards and the conductive stripes are metal that is adhered to one or more surfaces of the substrate. The use of flexible substrates allows a externally worn antenna assembly  44  to bend slightly to conform to the outer surface of a patient&#39;s chest.  
         [0029]     The conductive pattern of the first antenna  56  is illustrated in  FIG. 6  and comprises a coil having first and second coil sections  59  and  60  located symmetrically on opposite sides of a first axis of symmetry  65 . For example, each coil section  59  and  60  has two turns with each turn formed by a generally semicircular lobe and a linear conductor that is parallel to the first axis of symmetry  65 . Coil sections with greater number of turns or even a single turn may also be used in given applications of the antenna assembly. Specifically, the first coil section  59  comprises a first conductive lobe  61  extending outward on one side of the first axis of symmetry  65  with a first end  66  and a second end  67  adjacent that axis, but spaced there from. A second lobe  62  of the first coil section  59  extends within and spaced from the first lobe  61  and has a third end  68  near the first end  66  and a fourth end  69  adjacent the second end  67  of the first lobe. A first linear conductor  76  connects the first end  66  of the first lobe  61  to the fourth end  69  of the second lobe  62 , and a second linear conductor  78  connects the second end  67  of the first lobe to a first node  74 . The first node  74  is on the first axis of symmetry  65  adjacent the first end  66  of the first lobe  61 .  
         [0030]     The second coil section  60  is formed by a third lobe  63  that extends outward on the opposite side of the first axis of symmetry  65  from the first coil section  59  and has a fifth end  70  adjacent to, but spaced from the first end  66  of the first lobe  61 . The third lobe  63  has a sixth end  71 . The fourth lobe  64  of the second coil section  60  is within the third lobe  63  and has a seventh end  72  adjacent the fifth end  70  of the third lobe and has an eighth end  73  that is adjacent to the sixth end  71 . A third linear conductor  80  connects the sixth end  71  of the third lobe  63  to the first node  74 . A fourth linear conductor  82  connects the eighth end  73  of the fourth lobe  64  to the fifth end of  70  of the third lobe  63 .  
         [0031]     The first node  74  is a short conductive element which partially fills the gap between the first end  66  of the first lobe  61  and the fifth end  70  of the third lobe  63 . A second node  84 , formed by another short conductive element, is located on the first axis of symmetry  65  adjacent to the first node  74  between the third and seventh ends  68  and  72  of the second and fourth lobes  62  and  63 , respectively. The third end  68  of the second lobe  62  and the seventh end  72  of the fourth lobe  64  are electrically connected by insulated jumpers  85  and  86  to the second node  84 . The first and second nodes  74  and  84  provide terminals for coupling the first transmission line  45  to the first and second coil sections  59  and  60  of the first antenna  56 .  
         [0032]     The first antenna  56  is connected by the first matching circuit  51  to the first transmission line  45 . Specifically, the first matching circuit  51  has a first, or impedance matching, capacitor  92  which couples the center conductor of that transmission line to the second node  84  of the antenna. The first transmission line  45  also has a shield conductor that is connected directly to the first node  74  and a second, or tuning, capacitor  94  is connected between the first and second nodes  74  and  84 . The coupling of the first transmission line to the first and second nodes  74  and  84 , applies the radio frequency signal from the signal splitter  43  to the first antenna  56 . This results in the first antenna  56  emitting an electromagnetic field B in the direction indicated by the arrow at the center of the antenna.  
         [0033]     The conductive pattern of the second antenna  57  is illustrated in  FIG. 7  and is identical to that of the first antenna  56  except that it is rotated ninety degrees on the first substrate  54 . The second antenna  57  comprises third and fourth coil sections  98  and  99  opposite sides of a second axis of symmetry  105 . For example, each of these coil sections  98  and  99  has two turns with each turn formed by a generally semicircular lobe and a linear conductor parallel to the second axis of symmetry  105 . Specifically the third coil section  98  comprises a fifth conductive lobe  101  extending outward on one side of the second axis of symmetry  105  with a ninth end  106  and a tenth end  107  adjacent that axis, but spaced there from. A sixth lobe  102  of the third coil section  98  extends within and spaced from the fifth lobe  101  and has an eleventh end  108  near the ninth end  106  and a twelfth end  109  adjacent the tenth end  107  of the fifth lobe. A fifth linear conductor  116  connects the ninth end  106  of the fifth lobe  101  to the twelfth end  109  of the sixth lobe  102 , and a sixth linear conductor  118  connects the tenth end  107  of the fifth lobe to a third node  114 . The third node  114  is on the second axis of symmetry  105  adjacent the ninth end  106  of the fifth lobe  101 .  
         [0034]     The fourth coil section  99  is formed by a seventh lobe  103  that extends outward on the opposite side of the second axis of symmetry  105  from the third coil and has a thirteenth end  110  adjacent to, but spaced from ninth end  106  of the fifth lobe  101 . The seventh lobe  103  has a fourteenth end  111 . The eighth lobe  104  of the fourth coil section  99  is within the seventh lobe  103  and has a fifteenth end  112  adjacent the thirteenth end  110  of the seventh lobe and a sixteenth end  113  that is adjacent to the fourteenth end  111 . A seventh linear conductor  120  connects the fourteenth end  111  of the seventh lobe  103  to the third node  114 . An eighth linear conductor  122  connects the sixteenth end  113  of the eighth lobe  104  to the thirteenth end  110  of the seventh lobe  103 .  
         [0035]     The third node  114  is a short conductive element between the ninth and thirteenth ends  106  and  110  of the fifth and the seventh lobes  101  and  103 , respectively. A fourth node  124  is a short conductive element located on the second axis of symmetry  105  adjacent to the third node  114  between the eleventh and fifteenth end ends  108  and  112  of the sixth and eighth lobes  102  and  104 , respectively. The eleventh end  108  of the sixth lobe  102  and the fifteenth end  112  of the eighth lobe  104  are electrically connected by insulated jumpers  125  and  126  to the fourth node  124 . The third and fourth nodes  114  and  124  provide terminals for coupling the second transmission line  46  to the third and fourth coil sections  98  and  99  of the second antenna  57 .  
         [0036]     The second antenna  57  is connected by the second matching circuit  52  to the second transmission line  46 . Specifically, the second matching circuit  52  has one capacitor  128  which couple the center conductor of that transmission line to the fourth node  124  of the second antenna. The second transmission line  46  also has a shield conductor that is coupled directly to the third node  114  and another capacitor  129  is connected between the third and fourth node  114  and  124 . The coupling of the second transmission line  46  to the third and fourth nodes  114  and  124  applies the radio frequency signal from the signal splitter  43  to the second antenna  57 . This results in the second antenna  57  emitting an electromagnetic field B in the direction indicated by the arrow at the center of the antenna, which is orthogonal to the direction of the electromagnetic field generated by the first antenna  56 .  
         [0037]     With reference to  FIG. 8 , the third antenna  58  comprises a single coil  130  with two turns formed by a pair of concentric, annular conductors  131  and  132  which preferably are circular. It should be understood that the third antenna  58  may have fewer or more turns depending on the particular application of the antenna assembly  44 . The first annular conductor  131  has a gap, thereby forming seventeenth and eighteenth ends  134  and  135 . The second annular conductor  132  is within the first annular conductor  131  and also has a gap, thereby creating nineteenth and twentieth ends  136  and  137 . A bridging conductor  138  connects the eighteenth end  135  to the nineteenth end  136 .  
         [0038]     The third antenna  58  is connected to the third transmission line  47  by the third matching circuit  53 , The center conductor of the third transmission line  47  is connected by a capacitor  141  to the seventeenth end  134  of the first annular conductor  132 . The shield conductor of the third transmission line  47  is connected directly to the twentieth end  137 . Another capacitor  142  is connected across the seventeenth end  134  and the twentieth end  137 . The third antenna  58  emits an electromagnetic field B in a direction perpendicular to the plane of the drawing.  
         [0039]     The radii of each lobe  101 - 104  of the second antenna  57  are different than the radii of the lobes  61 - 63  of the first antenna  56 , so that the respective conductors do not lie over one another in the layered antenna assembly  44  as evident in  FIG. 5 . Similarly, the radii of the first and second annular conductors  131  and  132  of the third antenna  58  are shown different than the radii of the lobes in the other two antennas  56  and  57 , so that its conductive pattern does not lie over either of the conductors of the antenna lobes. Offsetting the conductive elements of each of three planar antennas  56 - 58  in this manner, reduces the capacitive coupling between adjacent antennas. Alternatively, it may be possible that the conductive patterns of the first and third antennas lie over each other as the intermediate circuit board layers provide a substantial dielectric to impede detrimental capacitive coupling.  
         [0040]     Referring again to  FIG. 4 , when the pacing signal generator  41  determines that a stimulation pulse is required, the amplifier  42  is triggered to pass a burst of the radio frequency signal from the local oscillator  40 . The signal splitter  43  then divides the signal into three separate portions for the three antennas  56 - 58 . The signal portions are applied through one combination of a balun  44 - 46  and a matching circuit  50 - 52  to the respective antenna  56 - 58  within assembly  44 . Because the first transmission line  45  feeding the first antenna  56  is longer than the second transmission line  46  for the second antenna  57 , the signal portion to the first antenna  56  is ninety degrees out of phase with respect to the signal portion that is applied to the second antenna  57 . As a result, a circular polarized RF field, parallel to the plane of the antenna assembly  44 , is generated by the first and second antennas  56  and  57 . The circular polarized field in this plane is important in order to achieve an omnidirectional distribution of the RF field. If the first and second antennas  56  and  57  receive signals that were in phase, a linear field would be generated in a direction forty-five degrees with respect to the orientation of each antenna which would be same as would be achieved by a single antenna oriented in that direction.  
         [0041]     The third signal portion, fed through the third balun  46  and the third matching circuit  52 , is applied to the third antenna  58 . The circular design of this antenna emits a radio frequency wave that propagates in a direction that is orthogonal to the circular polarized field produced by the other two antennas  56  and  57 . As a result of the orientation of each of these emitted RF fields, the antenna assembly  44  forms an omnidirectional field which induces voltage into the circuitry on the implanted stent, independent of the orientation of the stent with respect to the transmitter antenna assembly  44 .  
         [0042]     The foregoing description was primarily directed to a preferred embodiment of the invention. Even though some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. For example, although the invention has been described in the context of a cardiac pacing device, the novel antenna may be used with devices for electrically stimulating other organs of the body, such as the brain for seizure control. The present antenna may also be used to communicate with sensing devices implanted in an animal. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.