The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "State-of-the-art pacemakers, ICDs and other cardiac rhythm management devices (CRMDs) can be equipped with radio-frequency (RF) communication devices for communicating with external systems such as bedside monitors or external diagnostics systems. In particular, RF communication devices have been developed to utilize Medical Implant Communication Service (MICS)-band radio transmissions or Medical Device Radiocommunications Service (MedRadio)-band transmissions. (MedRadio maintains the spectrum previously allocated for MICS (402-405 MHz) while adding additional adjacent spectrum (401-402 MHz and 405-406 MHz).) Herein, the term 'MICS/MedRadio' will be used for the sake of completeness and generality to refer to MICS, MedRadio or both.) RF capable devices use an antenna within the header or adjacent header for receiving or transmitting RF signals. However, problems arise in designing such antennas due to the increasing miniaturization of CRMDs and their components.
"In particular, there can be a loss of RF communication performance due to the reduction in size of the header and the device case (also called the housing or the 'can') of the CRMD. As technology improves, the sizes of the implantable devices continue to shrink but the laws of physics regarding RF communications do not change. Since about 2005, at least some CRMD designers have employed a shorted loop antenna for RF communications. However, RF computer simulations indicate that a further reduction in device size would diminish antenna performance below acceptable levels. Accordingly, there is a need to provide improved antenna designs for use with CRMDs, especially relatively small devices.
"In this regard, there are many challenges to designing a well performing antenna for use within an implantable medical device. One issue is the significant amount of attenuation inherent to the system since the RF signal travels through the lossy human body. Another problem is that the size of the antenna is limited by the size of the header (at least for devices where the antenna is to be fitted inside the header.) Ideally, the antenna should have a length equal to a quarter wavelength of the operating frequency (which is typically near 400 MHz), but it is difficult to design an antenna that fits within a device header while achieving that length. Hence, for antennas to be housed in the device header, the quarter wavelength constraint can result in an antenna much smaller than needed for optimum performance. Another issue is that the antenna should have an input impedance that is the complex conjugate of impedance of the internal circuitry of the device so maximum power transfer can take place. If the impedance of the antenna is too low or too high, additional mismatch losses will occur, which will decrease signal power.
"FIG. 1 illustrates an antenna 2 that attempts to meet these requirements using a folded monopole design commonly known as an 'Inverted L antenna' for use within the header 4 of an exemplary CRMD 6. The Inverted L is a monopole that ideally should be sized to a quarter wavelength of its operating frequency with a 90-degree bend to resemble a downward facing L. The antenna can fit within a fairly small header volume but suffers from very low input impedance. Also, this antenna is best suited for higher gigahertz (GHz) frequency applications where the necessary antenna length for resonance is relatively short. At 400 MHz, implementing an Inverted L antenna becomes impractical for implantable device purposes, as this would require a very long antenna that would not fit within the header. To solve the impedance issue, an extra branch 7 can be connected to the Inverted L and shunted to ground. This topology, shown in FIG. 2, is known as the 'Inverted F antenna.' (An Inverted F antenna design is discussed, for example, in U.S. Pat. No. 7,047,076 to Li et al., entitled 'Inverted-F Antenna Configuration for an Implantable Medical Device.') The extra shunt connection provides a larger input impedance for matching purposes but the Inverted F still suffers from lack of adequate length for practical applications wherein the antenna must fit within the header of a relatively small CRMD.
"Accordingly, there is a need to provide an improved antenna, particularly for MICS/MedRadio applications, that addresses these and other issues. It is to this end that aspects of the invention are generally directed."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In accordance with an exemplary embodiment, an implantable medical device is provided for implant within a patient wherein the device includes RF communication components installed within a case of the device. An antenna having an inverted E shape is coupled to the RF communication components. The inverted E shape of the antenna incorporates many of the aforementioned benefits of an inverted F antenna while providing, in preferred embodiments, for capacitive loading to allow for tuning of impedance and resonance frequency. In an illustrative embodiment, the inverted E antenna is installed within a header mounted to an exterior of the case of the implantable device. The case provides a ground plane for the antenna. The antenna has three branches extending from a main horizontal arm that forms the base of the inverted 'E.' The three branches include: a first 'capacitive branch' connecting one end of the main arm to the case via a capacitive load; a second 'RF signal feed branch' connecting a middle portion of the main arm to the internal RF components of the device via a feedthrough (or feedthru) in the case; and a third 'inductive branch' connecting the other end of the main arm to the case to provide a shunt to ground. Note that, at MICS frequencies, a shunt (depending on its dimensions) may behave like a small inductor and hence the third branch that is shunted to ground is referred to as the inductive branch. At resonance, the capacitive loading of the first branch cancels the inductance of the third branch to provide optimal radiation efficiency as well as to provide a real impedance with no reactive (i.e. imaginary) component.
"In one particular example, the capacitive branch of the antenna includes a capacitor (or other means for providing capacitance) mounted in series between the main arm and a distal end of the branch that is coupled to the case of the device. During device design, the value of the capacitor can be selected in conjunction with other antenna design parameters to substantially cancel any inductance provided by the antenna or to achieve other goals. In particular, by properly selecting the capacitance, the resonant frequency of the antenna can be set to the operating frequency of the device to provide both very good impedance and very good performance without having to change the length or height of the antenna. In another example, the capacitive branch includes a parallel plate mounted via an epoxy dielectric (or other suitable material) to the case of the device so that the plate, the epoxy and the adjacent portion of the case collectively form a plate capacitor. During device design, the capacitance can be set by selecting the size of the plate, the distance from the plate to the case and the electrical characteristics of the dielectric epoxy. In yet another example, the capacitive branch includes a discoidal capacitor mounted within the case via a secondary feedthrough. During design, the value of the discoidal capacitor can be selected in conjunction with other antenna design parameters so as to substantially cancel any inductance or to achieve other goals. In some examples, the antenna might contain non-metallic elements (such as highly conductive fluids.)
"In the illustrative embodiment, the middle RF signal feed branch of the antenna is coupled to the internal RF communication components within the case via the primary feedthrough. In one particular example, the primary feedthrough includes an inner conducting pin, an outer conductor grounded to the case and a dielectric material separating the inner pin from the outer conductor. The inner pin is connected to a terminal of the internal RF components. As to the inductive branch of the antenna, it is mounted directly to the case for providing the shunt to ground. The overall inductance of the antenna can be set during device design by selecting the length of the inductive branch relative to the lengths of the other branches of the antenna.
"With this inverted E configuration, the impedance and resonance frequency of the antenna can thereby be set easily during design to preferred or optimal values by selecting the capacitance provided by the first branch, the inductance provided by the third branch and the location of the middle RF signal feed branch relative to the first and third branches. Indeed, any change to the length and cross-sectional area of the antenna can be seen as a change in inductance, which can be canceled out with a corresponding change in the capacitor. Thus, if the latest model of the implantable device is made smaller (requiring a smaller antenna), suitable adjustments to the design of the inverted E type antenna can be made to maintain preferred or optimal impedance values. That is, impedance can be tuned to match device circuitry. In some examples, the antenna is configured to provide an impedance of about 50 ohms with substantially no reactive components. Hence, the inverted E-shaped antenna and its components allow for great flexibility during device design to achieve operational or performance goals. Also, by allowing for a generally smaller antenna, the header can be made smaller, thus making the overall device smaller and lighter. The antenna may be used either for transmitting or for receiving RF signals. That is, by virtue of the reciprocity theorem, the antenna is equally effective at receiving and transmitting signals. Implantable devices incorporating the antenna may be implemented using a bi-directional half duplex protocol to accommodate both reception and transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
"The above and further features, advantages and benefits of the invention will be apparent upon consideration of the present description taken in conjunction with the accompanying drawings, in which:
"FIG. 1 illustrates a conventional inverted L antenna mounted within the header of an implantable medical device;
"FIG. 2 illustrates a conventional inverted F antenna mounted within the header of an implantable medical device;
"FIG. 3 illustrates pertinent components of an implantable medical system having an CRMD equipped for MICS/MedRadio communication and incorporating an inverted E antenna (mounted within a header of the device) that includes capacitive and inductive loading;
"FIG. 4 illustrates the inverted E antenna of the CRMD of FIG. 3 mounted within the header of an implantable medical device;
"FIG. 5 is a schematic of the inverted E antenna of FIG. 4;
"FIG. 6 illustrates an alternative embodiment of the inverted E antenna of FIG. 4 wherein the branches are curved;
"FIG. 7 illustrates another embodiment of the inverted E antenna of FIG. 4 wherein an capacitor integrated plate is employed;
"FIG. 8 illustrates yet another embodiment of the inverted E antenna of FIG. 4 wherein a discoidal capacitor is employed;
"FIG. 9 illustrates exemplary techniques pertaining to designing and using the inverted E antenna of FIGS. 3-8;
"FIG. 10 is a simplified, partly cutaway view, illustrating the CRMD of FIG. 3 along with a set of leads implanted on or in the heart of the patient; and
"FIG. 11 is a functional block diagram of the CRMD of FIG. 10, illustrating basic circuit elements that provide cardioversion, defibrillation and/or pacing stimulation in the heart, as well as components for MICS/MedRadio communication."
For additional information on this patent application, see: Li, Perry; Mouchawar, Gabriel A.; Amely-Velez, Jorge; Imani, Reza. Inverted E Antenna with Capacitance Loading for Use with an Implantable Medical Device. Filed
Keywords for this news article include: Implantable, Cardio Device,
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