Abstract:
Methods and systems are provided for wirelessly powering a medical device in a living subject using external radiofrequency energy. A radiofrequency driving unit outside the subject irradiates the medical device. A passive antenna is positioned outside the subject, generally opposing the driving unit to redirect the field generally toward the device. The reradiating element increases uniformity of the electromagnetic field produced by the driving unit, which reduces local tissue heating in the subject and in personnel attending the subject.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to remotely powering wireless devices. More particularly, this invention relates to minimizing heating of body tissues during exposure to an electromagnetic field while powering a wireless medical device. 
     2. Description of the Related Art 
     Implantable or insertable medical devices are sometimes wirelessly powered by the transmission of radiofrequency (RF) energy from a radiator that is located external to a patient&#39;s body. One or more power coils incorporated in the device receive the radiofrequency energy. For example, some wireless location transponders comprise such power coils, and one or more position sensing coils for receiving externally generated position signals. The transponders typically use the wirelessly received energy to convert the position signals into high frequency signals, and to drive the power coil (or a separate transmission coil) to transmit the high frequency signals to an externally located processing unit, which responsively determines the position and the orientation of the transponder. 
     For example, a transponder and apparatus for operating the transponder employing analog high-frequency signals is described in U.S. Patent Application Publication No. 2003/0120150, entitled “Wireless Position Sensor,” whose disclosure is herein incorporated by reference. The apparatus for operating the transponder includes a plurality of field generators, which generate electromagnetic fields at different respective frequencies in a vicinity of the object, and a radiofrequency driver, which radiates a radiofrequency driving field toward a wireless transponder. The transponder includes at least one sensor coil, in which a signal current flows responsively to the electromagnetic fields, and a power coil, which receives the radiofrequency driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal for communicating information to a receiver or interrogator. In medical applications such transponders, whether analog or digital, typically comprise multiple sensor coils, such as three mutually-orthogonal coils, as described in European Patent EP 0 776 176 to Ben-Haim et al. Position and orientation coordinates of the transponder can thus be determined without ambiguity. 
     These location transponders enable the determination of the position and orientation of an object in the body without the need for any wired connection between the sensing coil and the external processing unit. Such wireless transponders may be implanted in the body of a patient, such as in a bone of the patient, or incorporated into an implantable medical device. However, there is a concern that when the device is being actively powered by a radiofrequency driver, there could be harmful local tissue heating resulting from non-uniformities in the electromagnetic field. 
     In general the deposition of radiofrequency energy in the human body tends to increase the body temperature. A World Health Organization document,  Environmental Health Criteria  137, available on the Internet at the URL “http://www.inchem.org/documents/ehc/ehc/ehc137.htm”, indicates that there exists a threshold specific absorption rate (SAR) of radiofrequency energy for frequencies above about 1 MHz of 1-4 W/kg, above which there is increasing likelihood of adverse health effects. Below about one MHz, standards are based on induced currents in the body, causing shocks and burns. Furthermore, pulsed fields may be of particular concern. In the case of pulsed electromagnetic fields, it has been shown, under a number of conditions, that the thresholds for biological effects at frequencies above several hundred MHz are decreased when the energy is delivered in short (1-10 μs) pulses. A safe limit for such pulses cannot even be identified on the basis of available evidence. It would appear to be prudent to minimize exposure of patients and medical personnel to such fields. 
     SUMMARY OF THE INVENTION 
     According to disclosed embodiments of the invention, methods and systems are provided for wirelessly powering a medical device in a living subject using external radiofrequency energy while minimizing the local deposition of radiofrequency energy in tissues. A radiofrequency driving unit outside the subject irradiates the medical device. A passive antenna is positioned outside the subject, generally opposing the driving unit, which redirects the field generally toward the device. The reradiating element increases uniformity of the electromagnetic field produced by the driving unit, and thereby reduces local tissue heating in the subject and in personnel attending the subject. 
     An embodiment of the invention provides a method for wirelessly powering a medical device that is located in a living subject, which is carried out by generating a radiofrequency energy field at a first position outside the subject, the field extending into the subject to energize the device, and passively reradiating the field from a second position outside the subject generally toward the first position. 
     According to an aspect of the method, the second position generally opposes the first position across the subject. 
     According to another aspect of the method, the device is a transponder having position sensors that obtain power from the field. 
     In one aspect of the method, the field is reradiated by exactly one passive antenna at the second position. 
     According to a further aspect of the method, the passive antenna includes a single coil of wire. 
     According to yet another aspect of the method, the field has a frequency of 13.6 MHz and the passive antenna has a capacitance of about 100 pF. 
     In an additional aspect of the method, the field is resonated at the second position. 
     One aspect of the method includes shielding a portion of the subject from the field, the shielded portion excluding the device. 
     An embodiment of the invention provides an apparatus for wirelessly powering a medical device. The device is located in a living subject and is energized by external radiofrequency energy. A radiofrequency driving unit disposed at a first position outside the subject for generates a radiofrequency energy field that extends into the subject to irradiate the device. A reradiating element is disposed in the field at a second position outside the subject to redirect the field generally toward the device. 
     According to an additional aspect of the apparatus, the device is a transponder having position sensors that derive power from the field. 
     According to still another aspect of the apparatus, the reradiating element is exactly one passive antenna. 
     According to aspect of the apparatus, the passive antenna includes a single coil of wire. 
     According to a further aspect of the apparatus, the passive antenna is resonant at a frequency of the field. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a pictorial illustration of a system for wirelessly energizing a medical device in accordance with a disclosed embodiment of the invention; 
         FIG. 2  shows exemplary field strength curves produced by the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention; 
         FIG. 3  is a finite element model of a human knee shown in slight perspective on an operating table, in which antennae are shown, in accordance with a disclosed embodiment of the invention; 
         FIG. 4  is an end view of a finite element model similar to the finite element model shown in  FIG. 3 , over which a radiation pattern is superimposed, in accordance with a disclosed embodiment of the invention; 
         FIG. 5  shows a finite element model similar to  FIG. 4  with a superimposed radiation pattern, in which antennae are active, in accordance with a disclosed embodiment of the invention; and 
         FIG. 6  is a pictorial illustration of a system for wirelessly powering a medical device that includes a protective shield, in accordance with an alternate embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known circuits, and control logic have not been shown in detail in order not to obscure the present invention unnecessarily. 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for wirelessly energizing a medical device in accordance with a disclosed embodiment of the invention. The system  10  comprises a power-driving unit  12  disposed external to a subject  14  and an implantable or insertable wireless medical device  16 . The medical device  16  is typically incorporated in a catheter (not shown) or implanted in the subject  14 . The medical device  16  comprises at least one power coil  18 , for receiving energy transmitted by the power-driving unit  12 . For applications in which the medical device  16  functions as a wireless location transponder, the system  10  typically further comprises one or more position signal generators  20 , which generate position signals received by at least one position sensing coil  22  incorporated in the medical device  16 . A control unit  24  controls and energizes the position signal generators  20  and the power driving unit  12 . 
     A transponder, which is suitable for use as the medical device  16 , and which transmits digital high-frequency signals is described in U.S. Patent Application Publication No. 2005/0099290 entitled, “Digital Wireless Position Sensor,” whose disclosure is herein incorporated by reference. 
     In order to efficiently transmit power to the medical device  16 , the power driving unit  12  is typically located near or in contact with external tissue of the subject  14 , in a vicinity of the medical device  16 . The power-driving unit  12  generates a radiofrequency signal, typically having a frequency in the megahertz range (e.g., 13.6 MHz), to drive the power coil  18  and thereby power the medical device  16 . The strength of a RF field  26  generated by the power driving unit  12  typically drops off rapidly as the distance from the power driving unit  12  increases. Therefore, a relatively high power level (e.g., between about 12 W/kg and about 20 W/kg) is typically necessary in order to provide sufficient field strength at the medical device  16 , which is typically positioned several centimeters to several tens of centimeters from the power driving unit  12 , depending on the specific application. Such a strong field may undesirably heat tissue of the subject  14  in the vicinity of the power-driving unit  12 , and tissues of the physician performing the procedure and ancillary medical personnel (not shown). 
     In order to increase the uniformity of the field  26 , the system  10  further comprises a passive antenna  28 , which typically comprises at least one coil or loop  30 . For example, the antenna  28  may comprise a single 80 cm loop typically with about 100 pF capacitance. However, the capacitance may vary, so long as the loop is configured so as to resonate at the frequency of the field developed by the power-driving unit  12 . The antenna  28  is positioned on the side of the subject  14  opposite the side on which the power-driving unit  12  is positioned, typically between about 1 and about 1.5 meters from the power-driving unit  12 . The antenna  28  is typically entirely passive; it thus does not require a power source or coupling to a control unit. The antenna  28  re-radiates a portion of the field&#39;s energy. As a result, the field  26  is generally relatively stronger in the vicinity of the antenna  28  and of the medical device  16 , and relatively weaker in the vicinity of the power-driving unit  12 , than would be the case in the absence of the antenna  28 . 
     Reference is now made to  FIG. 2 , which is a graph showing theoretical exemplary field strength curves, in accordance with a disclosed embodiment of the invention. In the theoretical example illustrated, a curve  32  represents the strength of the field  26  ( FIG. 1 ), when the antenna  28  is not employed, at distances between 0 m and 1.5 m from the power-driving unit  12 , in a generally upward direction from the power-driving unit  12  and through the subject  14 . As can be seen, the strength drops off rapidly as the distance from the driving unit increases. A curve  34  represents the strength of the field  26 , when the antenna  28  is deployed at 1.5 m from the power-driving unit  12 . The re-radiation from the antenna  28  substantially flattens the curve, resulting in a more uniform field distribution. 
     Example 
     Reference is now made to  FIG. 3 , which is a finite element model  36  of a human knee  38  shown in slight perspective on an operating table, in accordance with a disclosed embodiment of the invention. Muscle conductivity of 0.6 Seim was assumed for the models in this Example. A power-driving element  40  is disposed beneath the knee  38 . Passive re-radiating antennae  42 ,  44  are situated above the knee  38 . 
     Reference is now made to  FIG. 4 , which is an end view of a finite element model  46  in accordance with a disclosed embodiment of the invention, similar to the finite element model  36  ( FIG. 3 ), in which the antennae  42 ,  44  are absent. A simulated radiation pattern created by a driving element  48  is shown. An area  50  of intense RF radiation is indicated, overlapping an operative site  52 . 
     Reference is now made to  FIG. 5 , which is a view of the finite element model  46 , in which the antennae  42 ,  44  ( FIG. 3 ) are now active in a simulation, in accordance with a disclosed embodiment of the invention. The perspective of  FIG. 5  differs somewhat from  FIG. 4 , and most of the finite element model has been removed to better illustrate the radiation pattern. Instead, a rectangle  54  outlines the location of the knee component of the finite element model  46 . The region of most intense RF radiation is indicated by an area  56 , which is considerably reduced in size when compared to the area  50  ( FIG. 4 ). Only a relatively small portion of the operative site in the lower portion of the rectangle  54  is occupied by the area  56 . 
     Alternate Embodiment 
     Reference is now made to  FIG. 6 , which is a pictorial illustration of a system for wirelessly powering a medical device that includes a protective shield  58 , in accordance with a disclosed embodiment of the invention. The shield  58 , which comprises a material that blocks RF energy (e.g., aluminum foil, copper shields, brass, iron), is coupled to a ground  60  and placed between the power driving unit  12  and tissue of the subject  14  that need not be exposed to the field  26  ( FIG. 1 ). In the example shown in  FIG. 6 , the medical device  16  has been implanted or inserted into a left leg  62  of the subject  14 , and the shield  58  is configured to protect a right leg  64  from the field  26 . Configurations for protecting other areas of the subject&#39;s body, and the physician (not shown) performing a medical procedure while powering the medical device  16 , will be readily apparent to those skilled in the art. The shield  58  may be employed additionally or alternatively to the antenna  28  ( FIG. 1 ). 
     The field created in the arrangement of  FIG. 6  is not uniform. Nevertheless, addition of a reradiating antenna tends to decrease non-uniformities, as the effect of the field is relatively unchanged far from the antenna, and the field is reduced closer to the antenna. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.