Patent Publication Number: US-7916013-B2

Title: RFID detection and identification system for implantable medical devices

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods of identifying implanted medical devices. More specifically, this invention relates to radio frequency identification (RFID) tags for use with medical devices implanted in a patient. 
     There are known in the art various methods for identifying implanted medical devices. One such method is the use of X-ray identification tags encapsulated within header blocks of pacemakers or implantable cardioverter defibrillators (ICD). Such X-ray identification tags can be read on an X-ray of the implanted device and provide information to the physician. The information so provided is limited due to space and typically includes only the manufacturer and model number of the implanted device. It would be beneficial if physicians were able to obtain additional information about an implanted device and/or a patient from an implanted identification tag. Such beneficial information includes, in addition to the manufacturer and model number of the device, the serial number of the device, the treating physician&#39;s name and contact information and, if authorized by the patient, the patient&#39;s name, contact information, medical condition and treatment, and other relevant information. Currently, most active implantable medical device (AIMD) patients carry some sort of identification. This could be in the form of a card carried in the wallet or an ID bracelet indicating, for example, that they are a pacemaker wearer of a certain model and serial number. However, such forms of identification are often not reliable. It is quite common for an elderly patient to be presented at the emergency room (ER) of a hospital without their wallet and without wearing any type of a bracelet. In addition, there have been a number of situations where the patient (due to dementia or Alzheimer&#39;s, etc.) cannot clearly state that he or she even has a pacemaker. Often times the ER physician will palpitate the patient&#39;s chest and feel that there is an implanted device present. If the patient is comatose, has low blood pressure, or is in another form of cardiac distress, this presents a serious dilemma for the ER. At this moment in time, all that the ER knows is that the patient has some sort of an AIMD implant in his or her chest. It could be a pacemaker, a cardioverter defibrillator, or even a vagus nerve stimulator or deep brain stimulator. What happens next is both laborious and time consuming. The ER physician will have various manufacturers&#39; internal programmers transported from the hospital cardiology laboratory down to the ER. ER personnel will then try to interrogate the implantable medical device to see if they can determine what it is. For example, they might first try to use a Medtronic programmer to see if it is a Medtronic pacemaker. Then they might try a St. Jude, a Guidant, and ELA, a Biotronik or one of a number of other programmers that are present. If none of those programmers work, then the ER physician has to consider that it may be a neurostimulator and perhaps go get a Cyberonics or Neuropace programmer. 
     It would be a great advantage and potentially life saving if the ER physician could very quickly identify the type of implant and model number. In certain cases, for example, with a pacemaker patient who is in cardiac distress, with an external programmer they could boost the pacemaker output voltage to properly recapture the heart, obtain a regular sinus rhythm and stabilize blood pressure. All of the lost time running around to find the right programmer, however, generally precludes this. Accordingly, there is a need for a way to rapidly identify the type and model number of an active implantable medical device so that the proper external programmer for it can be rapidly identified and obtained. 
     It is already well known in the prior art that RFID tag implants can be used for animals, for example, for pet tracking. It is also used in the livestock industry. For example, RFID tags can be placed in cattle to identify them and track certain information. There is also a preliminary approval from the FDA for an injectable RFID tag into a human. A problem with this has to do with the fact that none of the current RFID tags have been designed to have long term reliability and biocompatibility within the body fluid environment. 
     Other general methods, none of which are specific to AIMDs, include encapsulating an RFID tag in plastic or placing the RFID tag in a plastic or glass tube with an epoxy infill. However, as will be discussed more fully below, none of these materials provide a truly hermetic seal against body fluids. 
     It is also important to note that lead wire systems generally remain in the human body much longer than the active implantable medical device itself. For example, in the case of a cardiac pacemaker, the cardiac pacemaker batteries tend to last for 5 to 7 years. It is a very difficult surgical procedure to actually remove leads from the heart once they are implanted. This is because the distal TIP of the lead wires tend to become embedded and overgrown by myocardial tissue. It often takes very complex surgical procedures, including open heart surgery, to remove such lead wire systems. When a pacemaker is replaced, the pectoral pocket is simply reopened and a new pacemaker is plugged into the existing lead wire. However, it is also quite common for lead wires to fail for various reasons. They could fail due to breakdown of electrical insulation or they could migrate to an improper position within the heart. In this case, the physician normally snips the lead wires off and abandons them and then installs new lead wires in parallel with the old abandoned leads. Abandoned lead wires can be quite a problem during certain medical diagnostic procedures, such as MRI. It has been demonstrated in the literature that such lead wires can greatly overheat due to the powerful magnetic fields induced during MRI. Accordingly, it is important that there be a way of identifying abandoned leads and the lead type. Accordingly, there is a need to identify such abandoned lead wires to an Emergency Room physician or other medical practitioner who may contemplate performing a medical diagnostic procedure on the patient such as MRI. This is in addition to the need to also identify the make and model number of the active implantable medical device. It is also important to note that certain lead wire systems are evolving to be compatible with a specific type of medical diagnostic procedure. For example, MRI systems vary in static field strength from 0.5 Tesla all the way above 10 Tesla. A very popular MRI system, for example, operates at 3 Tesla and has a pulse RF frequency of 128 MHz. There are specific certain lead wire systems that are involving in the marketplace that would be compatible with only this type of MRI system. In other words, it would be dangerous for a patient with a lead wire designed for 3 Tesla to be exposed to a 1.5 Tesla system. Thus, there is also a need to identify such lead wire systems to Emergency Room and other medical personnel when necessary. For example, a patient that has a lead wire system that has been specifically designed for use with a 3 Tesla MRI system may have several pacemaker replacements over the years. 
     Identification of abandoned lead wires in a patient is also quite important. It has been shown in the past that abandoned lead wires can over heat during MRI procedures. This is particularly true of cardiac lead wires. Lead wires are abandoned for a variety of reasons. Sometimes lead wires will fail or lose contact, for example with the myocardial tissue of the right ventricle. It is a very difficult procedure for a surgeon to remove abandoned lead wires. Such procedures often involve open heart surgery. The reason for this is that after leads have been in place for a long time they tend to become overgrown and encapsulated with myocardial tissue. When a physician encounters one or more defective lead wires it is easier to clip them off and leave them hanging in the pectoral pocket and insert brand new lead wires through the venous system into the right ventricle and in parallel with the old abandoned lead or leads. However, such abandoned lead wires that are not terminated can lead to over heating during MRI procedures. The ANSI/AAMI PC69 task force recently did a study by going to various medical centers around the United States and tracing actual patient X-rays. This data was published at the annual Heart Rhythm Society in New Orleans in May 2005. Reference: Heart Rhythm 2005 abstract tracking number 05-AB-2928-HRS. Therefore, it is a feature of the present invention that the novel hermetically sealed RFID chip with fixation device can be used to attach to one or more abandoned leads in the pectoral pocket. This is very useful whether or not the patient receives a new pacemaker or AIMD, implant or not. That is, if we have a patient that has reverted to normal sinus rhythm and no longer needs a pacemaker and has abandoned leads, the radiology department can quickly tell through the RFID scan whether or not abandoned lead wires are present. As mentioned, this is extremely important to prevent inadvertent MRI on such a patient. In the past, it has been shown that abandoned leads can heat up so much that ablation of cardiac tissue and even perforation of cardiac walls can occur. It is, therefore, a feature of the present invention that both the lead wire system and the AIMD can be separately identified. 
     Accordingly, there is a need for an improved medical identification tag that can store additional information about an implanted device and/or a patient, without unduly increasing the size of the identification tag or jeopardizing the operation of the implanted device or the health of the patient, while providing a better hermetic seal. 
     The present invention meets these needs by providing an RFID tag that can be enclosed within an AIMD or introduced into a patient&#39;s body adjacent to an AIMD. The RFID tag of the present invention is capable of storing information about the medical device, the physician, and the patient, as described above. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system for identifying implants within a patient, comprising an implantable medical device, a radio frequency identification (RFID) tag having an antenna and being associated with the implantable medical device, the RFID tag containing information relating to the patient and/or the implantable medical device, and an interrogator capable of communicating with the RFID tag. 
     Such implantable medical devices may include active implantable medical devices (AIMD) such as a cardiac pacemaker, an implantable defibrillator, a congestive heart failure device, a hearing implant, a cochlear implant, a neurostimulator, a drug pump, a ventricular assist device, an insulin pump, a spinal cord stimulator, an implantable sensing system, a deep brain stimulator, an artificial heart, an incontinence device, a vagus nerve stimulator, a bone growth stimulator, a gastric pacemaker, a Bion, or a prosthetic device and component parts thereof, including lead wires or abandoned lead wires. The active implantable medical device may include a non-metallic header block in which the RFID tag is implanted. 
     The present invention optionally includes a biocompatible and hermetically sealed container in which the RFID tag is disposed. The container may comprise a housing, and an encapsulant made of a thermal-setting polymer or a silicone material within the housing surrounding at least a portion of the RFID tag. The housing is typically manufactured of ceramic, glass, porcelain, sapphire and composites thereof, or specialty polymer composites. Further, a desiccant, also known as a moisture getter, may be disposed within the housing adjacent to the RFID tag. The container may further include a biocompatible end cap hermetically sealed to the housing. The container may also include a fixation hole for affixing the container to body tissue and an optional X-ray identification tag. 
     The RFID tag may be read-only or readable/writable. The interrogator may be a reader/writer device and may be in communication with a computer or computer network. 
     The present invention is also direct to a process for manufacturing the radio frequency identification (RFID) device for identifying the implant within a patient. The process comprises the steps of: 
     associating an RFID tag with an active implantable medical device; 
     storing information relating to the patient or the active implantable medical device on the RFID tag; and 
     implanting the RFID tag in the patient. 
     The process may further comprise the step of embedding the RFID tag in a header block of the active implantable medical device, or encasing the RFID tag in a biocompatible and hermetically sealed container including a ceramic housing and an encapsulant within the housing surrounding at least a portion of the RFID tag. The encapsulant may be comprised of a thermal-setting polymer or a silicone material. An end cap may be hermetically sealed to the housing. The container may also include a fixation hole for affixing the container to body tissue and an X-ray identification tag. 
     These and other aspects of the invention will be apparent to one skilled in the art in light of the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the invention. In such drawings: 
         FIG. 1  is a depiction of a patient with an AIMD fitted with an RFID tag of the present invention and an external interrogator/reader. 
         FIG. 2  is an isometric view of a typical AIMD fitted with a biocompatible enclosed RFID tag of the present invention. 
         FIG. 3  is an isometric view that isolates the header block of the AIMD shown in  FIG. 2  and a close-up view of the embedded RFID tag. 
         FIG. 4  is an isometric view of a biocompatible and hermetically sealed container in accordance with the present invention. 
         FIG. 5  is a vertical cross-section of the biocompatible and hermetically sealed container taken along line  5 - 5  of  FIG. 4 . 
         FIG. 6A  is a horizontal cross-section of the biocompatible and hermetically sealed container taken along line  6 - 6  of  FIG. 4 . 
         FIG. 6B  is a horizontal cross-section of a square-shaped alternative of the biocompatible and hermetically sealed container taken along line  6 - 6  of  FIG. 4 . 
         FIG. 6C  is a horizontal cross-section of a rectangular alternative of the biocompatible and hermetically sealed container taken along line  6 - 6  of  FIG. 4 . 
         FIG. 6D  is a horizontal cross-section of an elliptical or oval alternative of the biocompatible and hermetically sealed container taken along line  6 - 6  of  FIG. 4 . 
         FIG. 7  is a vertical cross-section of an alternative construction of the biocompatible and hermetically sealed container of the present invention. 
         FIG. 8  is a vertical cross-section of another alternative construction of the biocompatible and hermetically sealed container of the present invention. 
         FIG. 9  is a vertical cross-section of yet another alternative construction of the biocompatible and hermetically sealed container of the present invention. 
         FIG. 10  illustrates the assembly of the biocompatible and hermetically sealed container of the present invention including an X-ray identification tag. 
         FIG. 11  is an isometric view of an alternative tissue fixation end cap for use with the biocompatible and hermetically sealed container of the present invention. 
         FIG. 12  is a block diagram depicting operation of a system including the RFID tag of the present invention. 
         FIG. 13  is a top view of an RFID tag and antenna of the present invention. 
         FIG. 14  is a block diagram depicting operations of an alternative system including an RFID tag of the present invention. 
         FIG. 15  is a block diagram depicting operation of another alternative system including an RFID tag of the present invention. 
         FIG. 16  is a block diagram depicting operation of yet another alternative system including an RFID tag of the present invention. 
         FIG. 17  is an isometric view of an alternative embodiment of the biocompatible and hermetically sealed container of the present invention. 
         FIG. 17A  is an isometric view of another alternative end cap for use with the biocompatible and hermetically sealed container of the present invention. 
         FIG. 18  is a cross-sectional view of a large needle syringe and biocompatible and hermetically sealed container of the present invention. 
         FIG. 19  an enlarged cross-sectional view of the encapsulated RFID tag in the biocompatible and hermetically sealed container depicted in  FIG. 18 ; 
         FIG. 20  is a fragmented sectional view of a prior art unipolar hermetic terminal typically used in active implantable medical devices. 
         FIG. 21  is an enlarged, partially fragmented perspective view of the feedthrough capacitor shown in  FIG. 20 . 
         FIG. 22  is a schematic electrical diagram of the coaxial feedthrough capacitor of  FIG. 20 . 
         FIG. 23  illustrates various EMI attenuation curves for several different multi-element EMI filters. 
         FIG. 24  is a perspective view of a quadpolar feedthrough capacitor combined with a lossy ferrite inductor slab. 
         FIG. 25  is an enlarged sectional view taken generally along the line  25 - 25  of  FIG. 24 . 
         FIG. 26  is a sectional view similar to  FIG. 25  illustrating a quadpolar feedthrough filter terminal constructed in an LL configuration. 
         FIG. 27  is an electrical schematic diagram of the feedthrough terminal illustrated in  FIG. 26 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to a radio frequency identification (RFID) system for use with active implantable medical devices (AIMDs) and an associated RFID tag. Specifically, the RFID system comprises an RFID tag implanted in a patient&#39;s body and associated with an implanted AIMD, and an interrogator in communication with the RFID tag. 
       FIG. 1  is an outline drawing of an adult male pacemaker patient with an AIMD  10 .  FIG. 2  is an isometric view of an AIMD or pacemaker  10  that has been fitted with an RFID tag  12  of the present invention. In a preferred embodiment, as shown in  FIG. 3 , the RFID tag  12  comprises a substrate  22 , an antenna or coil  14  and an RFID chip  16 . These components of the RFID tag  12  are well known in the art. RFID standards are evolving worldwide at various frequencies. For example, a 915 MHz protocol is generally evolving to be used for retail goods and inventory control. However, due to the high frequency, the 915 MHz protocols are not very useful for human implants. The reason for this is that humans are largely water and 915 MHz fields are greatly affected by the presence of water. The preferred embodiment is another RFID protocol which operates at 13.56 MHz which is ideal for an implantable RFID tag. The 13.56 MHz lower frequency will readily penetrate and communicate with the tag instead of reflecting off of the skin surface or being absorbed. There are other lower frequency RFID systems, for example, in the 130 kHz range which would also be suitable. In alternate embodiments, the RFID tag  12  may be enclosed in a biocompatible and hermetically sealed container  40  as shown in  FIG. 4  and as will be described more fully below. 
       FIG. 1  shows a dashed ellipse which indicates one potential location for an AIMD  10 . The location shown in  FIG. 1  is typical of a right or left pectoral muscle implant. Right and left pectoral muscle implants are typical for a cardiac pacemaker or implantable cardioverter defibrillator (ICD). The right and left pectoral muscle region is chosen due to the easy access to the subclavian veins for insertion of lead wires and electrodes down into the heart. The present invention may also find application in AIMDs such as, an implantable defibrillator, a congestive heart failure device, a hearing implant, a cochlear implant, a neurostimulator, a drug pump, a ventricular assist device, a drug pump, a spinal cord stimulator, an implantable sensing system, a deep brain stimulator, an artificial heart, an incontinence device, a vagus nerve stimulator, a bone growth stimulator, a gastric pacemaker, or a prosthetic device. 
     Referring once again to  FIG. 1 , one can see an interrogator  20 , also known as a hand held scanner or reader. The interrogator  20  transmits an electromagnetic field pulse  26  which is intercepted by the antenna  14  that is part of the implanted RFID tag  12 . The implanted RFID tag  12  is generally passive. That means that it does not have its own self-contained source of energy such as a battery. The electromagnetic field  26  that comes from the interrogator  20  resonates with the antenna  14  and RFID chip  16  providing energy for the RFID chip  16  to generate a signal and the antenna  14  to emit a return pulse  28 . This pulse  28  is picked up by an antenna  14  in the interrogator  20 . The pulse  28  contains digital modulation. As previously described, this digital modulation can contain information such as the model number of the patient&#39;s AIMD, the serial number of the AIMD, the manufacturer of the lead wire system, the name of patient&#39;s physician, and contact information for the physician. In addition, if the patient authorizes, the digital pulse can also contain the patient&#39;s name, the patient&#39;s medical condition, the patient&#39;s address and telephone number, and other pertinent information. 
       FIG. 2  is an isometric view of a typical AIMD  10 , such as a cardiac pacemaker. Cardiac pacemakers typically have a metallic housing  18  which can be of titanium, stainless steel or the like. This metallic housing  18  is laser welded shut and generally contains a hermetic feedthrough terminal  30  for passage of lead wires  32  into the interior of the metallic housing  18 . Said hermetic feedthrough terminals  30  are well known in the art and are generally laser welded into the metallic housing  18  of the implantable medical device. The lead wires  32  as shown in  FIG. 2 , are generally routed to connectors  34 . The connectors  34  provide a convenient location to plug in the lead wires  32  which are routed to the heart for pacing and biologic sensing. The connector assembly  30 ,  32 ,  34  is generally encapsulated within a molded non-metallic, i.e., plastic or ceramic, header block  36 , as shown. Usually, this header block  36  is of clear casting materials which are well known in the art. Opaque thermal setting or chemically setting materials may also be used. 
     Referring once again to  FIG. 2 , there is an RFID tag  12  which has been cast into the header block  36 . Not shown are suitable fixtures used to position the connectors  34  and RFID tag  12  during the casting of the header block  36 . The RFID tag  12  shown in  FIG. 2  may be enclosed within a biocompatible and hermetically sealed container  40  as mentioned above and as will be described below. 
       FIG. 3  isolates the header block  36  of  FIG. 2  with an RFID tag  12  embedded within the header block  36 . In this case, the RFID tag  12  is not enclosed within a biocompatible and hermetically sealed container  40 . As shown in  FIG. 3A , the RFID tag  12  a substrate  22 , an antenna  14 , and an RFID chip  16 . The substrate  22  may comprise single or multiple layers. The antenna  14  is for both receiving electromagnetic energy to power the RFID chip  16  and for retransmitting a digital pulse. These devices are well known in the art. 
       FIGS. 2 and 3  both show a non-hermetically sealed RFID tag  12  which is encapsulated within the molded header block of an AIMD such as a cardiac pacemaker. Such molded header blocks are common in the industry and are designated by ISO Standards IS-1, DF-1 or IS-4 or the equivalent. These header blocks  36  typically contain a connector system so that the medical practitioner can plug in lead wires for example those that would run from the pacemaker into the chambers of the heart. Referring to  FIG. 2  one can see that this header block material is a solid encapsulated material such as an epoxy, thermal setting polymer or the like. In general such materials are not considered truly hermetic and will have leak rates varying from 10 −5  to 10 −6  cubic centimeters per second. Accordingly, if such active implantable medical device as shown in  FIG. 2  were implanted for long periods of time, then body fluids would eventually, due to the bulk permeability of the header block  36  material reach the electronic circuits of the RFID tag  12 . Body fluids are comprised primarily of water and dissolved salts including sodium, chlorine, potassium, calcium and the like. These are ionic and if they reach the surfaces of the RFID tag  12  it will readily short it out. Thus, in the preferred embodiment as will be described herein, the RFID tag  12  will be hermetically sealed. However, a short term medical implant device placement of the RFID chip within the header block  36  would be acceptable. For example, the average life of most cardiac pacemakers is five to seven years. The lead wires are left in place while pacemakers are replaced as their batteries deplete. Accordingly, in the present invention it would be acceptable to place a non-hermetically sealed RFID tag  12  into an encapsulated header block as shown in  FIG. 3  as long as this was not designed for a long term implant. Long term implants would include cochlear implants, certain neurostimulators or Bions which could be in the human body for forty years or longer, and the like. 
     The hermetic seal characteristics of the header block assembly  36  depend upon the ability of the molding or plastic materials of the header block  36  to prevent body fluids from penetrating to the RFID tag  12 . Penetration of body fluids over time to the RFID tag  12  may cause degradation of insulation resistance, or short circuits. Accordingly, hermetically encapsulating the RFID tag  12 , as will be described below, is the preferred embodiment. 
       FIG. 4  is an isometric view of a biocompatible and hermetically sealed container  40  in accordance with the present invention. This hermetically sealed container  40  is designed to encase the RFID tag  12 . Since the RFID tag  12  is generally constructed of materials that are not long term biocompatible and body fluid resistant, it is important to prevent body fluids from reaching the RFID tag  12 . Even if the RFID tag  12  is embedded deeply within a molded polymer header block  36  as illustrated in  FIG. 3 , when such a device is implanted into body tissue for many years (cochlear implants may last forty years or longer), moisture can slowly penetrate due to the bulk permeability of the polymer material of the header block  36 . In the art, this is known as the leak rate or hermeticity of a device. Generally speaking, adjunct sealants, polymers and the like are not considered truly hermetic. A leak rate of 10 −9  cubic centimeters per second or slower is required to assure that moisture will not penetrate to sensitive electronics over long periods of time. In order to achieve such low leak rates, generally glass seals or gold brazed ceramic seals are required. It is well known that brazed ceramic seals are generally superior to fused or compression glass seals. 
     The marginal hermeticity of certain glass seals is demonstrated by antique marine floats that were used to hold fishing nets. These generally consisted of hollow glass spheres or balls which were filled with air. Now that many years have passed, many of these hollow glass spheres are partially filled with water. This is an example of how water can penetrate through glass given enough time due to the bulk permeability of the glass itself. Dense ceramic materials, such as alumina, generally do not allow this water penetration. 
     Prior art RFID chips that are used for both animal and sometimes for human implant have a serious deficiency in that they are not truly hermetically sealed. These devices often use a cylindrical glass cup which is filled with epoxy or other type polymer materials such as silicone or the like. A deficiency with such seals as mentioned above is, that over long periods of time, moisture will slowly penetrate and reach sensitive electronic circuits. When moisture reaches electronic circuits under low bias voltage conditions, dendrites and tin whiskers can form thereby shorting out or reducing insulation resistance to electronic components. There is another problem of great concern and that is not all of the materials that are used within the RFID chip itself (for example within the ASIC electronics) are biocompatible. Therefore, moisture intrusion over long periods of time can lead to issues with toxicity to surrounding tissues as these non-biocompatible materials leach out. Accordingly, it is the preferred embodiment of the present invention that the RFID chip be completely hermetically sealed with a maximum leak rate of 1×10 −7  cubic centimeters per second. As used herein “hermetically sealed” means a leak rate of 10 −7  cubic centimeters per second or slower. In fact, in the preferred embodiment as described in  FIGS. 4-10  a maximum leak rate of no more than 1×10 −12  cubic centimeters per second is ideal. This is in sharp contrast to prior art polymer fill systems which achieve at most a leak rate of around 1×10 −5  cubic centimeters per second, and are not considered hermetic seals in accordance with the present invention. 
     Referring now back to  FIGS. 4 and 5 , the RFID tag  12  has been placed inside the biocompatible and hermetically sealed container  40 . This sealed container  40  has an extruded, machined, or pressed ceramic housing  42 . It is not possible to make the entire sealed container  40  out of a metal such as titanium because this would shield the RFID tag  12  from the electromagnetic field from the interrogator  20 . In other words, if the RFID tag  12  was placed inside the titanium housing of an AIMD  10 , this would shield the radio frequency pulses. This would completely prevent the RFID tag  12  from receiving energy or sending out any pulses. Accordingly, the ceramic housing  42  as indicated in  FIGS. 4 and 5 , allows electromagnetic fields to freely pass to and from the RFID tag  12 . 
     The ceramic housing  42  as shown in  FIG. 5 , is formed by ceramic manufacturing operations that are well known in the art. This generally consists of taking pure alumina ceramic powders, formulating them with a binder system and pressing them into the desired shape. This is then fired or sintered at very high temperature which makes a very hard structure. In a preferred embodiment, the housing  42  is hermetically sealed using an end cap  44  that covers an open end of the housing  42 . In  FIG. 5 , the end cap  44  is constructed from titanium but may also be ceramic. The ceramic housing  42  is first selectively metallized using a sputtering technique. A preferred methodology would be to sputter a titanium-molybdenum composition  46  which is suitable for wetting a gold braze joint  48 . There are also a number of other methods of providing metallization on ceramic tubes, which are well known in the art and would provide a suitable surface for gold brazing. The gold brazed joint  48  is used to make a metallurgical hermetic connection between the end cap  44  and the ceramic housing  42 . 
     Referring once again to  FIG. 5 , the RFID tag  12  is in an encapsulant  50  so that it will not rattle around or vibrate inside the overall sealed container  40 . Such encapsulant  50  can be of a variety of non-conductive materials, including thermal-setting nonconductive polymers, silicones and the like. There is also a desiccant material  51  that is placed inside the device as a moisture getter. Some background is needed in order to better understand this. In a relatively large implantable medical device such as a cardiac pacemaker, there is a significant amount of open air space inside of the device. This is typically backfilled with dry nitrogen or the like. Because of the relatively large amount of open air space, the hermetic terminal for ingress and egress of lead wires through the device can have a leak rate of from 10 −7  to 10 −9  cubic centimeters per second. This allows a certain amount of moisture to penetrate over a period of years. In other words, when a small amount of moisture enters into a relatively large available space, droplets or moisture thin films will not typically be formed. The moisture will disburse and will gradually raise what is called the residual moisture (humidity) level inside the device. The residual moisture level typically starts at zero and will slowly climb over the life of the device to around 8%. However, in a relatively tiny hermetically sealed space as shown in the hermetically sealed enclosure of  FIG. 5  there is much less available free air space. Accordingly, the hermetic seal that is formed with gold braze  48  in the enclosure in  FIG. 5  preferably would have a lower leak rate. In the preferred embodiment, it is anticipated that these devices will be tested to a leak rate of no more than 1×10 −12  cubic centimeters per second. This means that much less moisture will penetrate the device and there will be much less chance for a moisture thin film or droplet to form on the sensitive electronic circuits. The desiccant material  51  has been added as a safety mechanism such that hermetic terminals having a leak rate in the approximate range of 1×10 −7  to 1×10 −9  cubic centimeters per second can be safely used. That is any residual moisture over a long period of time tending to enter the same space as the hermetically sealed RFID tag  12  would be entrapped with the desiccant material  51  and have very little chance to form a moisture thin film or droplet which could lead to dendrite growth or failure of the electronic circuits. Desiccants are generally well known in the prior art and can include anhydrous magnesium and calcium sulfate. Also activated silica gels are commonly used. Other acceptable desiccants include molecular sieves, montmorillonite clay activated carbons and synthetic sodium aluminosilicate. All of these desiccants have a very strong affinity for water and also absorb moisture mounting to more than 20% of their original weight. 
       FIGS. 6A-6D  show cross-sectional views of various alternative shapes for the ceramic housing  42  and end cap  44  previously described in  FIG. 5 .  FIG. 6A  is a round cross-section, which is identical to that previously shown in  FIG. 5 . An alternative square cross-section is shown in  FIG. 6B . A rectangular cross-section is shown in  FIG. 6C . An elliptical or oval cross-section is shown in  FIG. 6D . All the configurations and others will be apparent to those skilled in the art. 
       FIG. 7  is a very similar biocompatible and hermetic sealed container  40  as previously described in  FIGS. 5 and 6 ; however, in this case, the ceramic housing  42  is open at both ends and two end caps  44  hermetically seal the container  40 . The reason for this is that the ceramic housing  42  may be extruded in a continuous operation and then blade cut. This could make the ceramic housing  42  much less expensive than the closed end housing  42  previously shown in  FIG. 5 . A negative of the assembly as described in  FIG. 7  is that there are two end caps  44  which must be gold brazed or welded  48  to the ceramic housing  42 . Accordingly, there must be two circumferential or perimeter metallized bands  46  of the ceramic housing  42  so that the gold braze  48  will wet and form a hermetic seal. It is a matter of manufacturing cost trade-offs whether to use the single end cap  44  assembly as described in  FIG. 5  or the dual end cap  44  assembly as shown in  FIG. 7 . 
     It should also be mentioned that the end caps  44  may be of titanium, stainless steel, tantalum, niobium or other suitable biocompatible metallic material. There are also a number of ceramic materials that may be used for the end cap  44 , including alumina ceramic and the like. However, in order to form the gold braze joint  48 , a ceramic end cap  44  may also have to be selectively metallized  46  by sputtering, plating, vapor deposition or the like. There are also a number of alternative materials that may be used for the hermetic housings  42  as described herein. These include all ceramic, glasses, sapphire, porcelain, polymer composites and the like. 
       FIG. 8  is an alternative method of installation of an end cap  44  wherein the end cap  44  is placed inside of the ceramic housing  42 .  FIG. 9  is yet another method of having a step titanium end cap  44  with a gold braze joint  48  between the butt ends of the ceramic housing  42  and the step of the end cap  44 . Referring once again to  FIG. 8 , one can see that there is a novel hole  58  convenient for placing a suture. This could be used to affix the hermetically sealed RFID tag to any point within the human body. This suture hole  58  can also be used to affix the RFID tag to an active or abandoned lead wire system. This is important for the purposes of identifying the type of lead wire system and its compatibility with certain medical diagnostic procedures, such as certain types of MRI systems. 
       FIG. 10  is an exploded view of the sealed container  40  of  FIGS. 4 and 5 . The RFID tag  12  is positioned for insertion into the ceramic housing  42 . After the RFID tag  12  is inserted and encapsulated, a gold braze pre-form  48   a  is positioned near the joint of the end cap  44  and the ceramic housing  42  as shown. An optional X-ray identification tag  52  may also be affixed to the sealed container  40  with more gold braze pre-forms  54 , as shown. The gold braze pre-forms  48   a  and  54  are re-flowed in a vacuum brazing furnace. When the assembly is placed into the vacuum brazing furnace, the gold braze pre-form  48   a  seals the end cap  44  to the ceramic housing  42  and the one or more gold braze pre-forms  54  attach the X-ray identification tag  52  to the ceramic housing  42 . Low temperature brazes are preferred so as not to cause thermal damage to the RFID tag. As previously described, the ceramic housing  42  is selectively metallized  46  using sputtering or equivalent techniques prior to placement in the vacuum brazing furnace so that the gold braze pre-forms  48   a  and  54  will wet to the ceramic tube  42 . Suitable low temperature brazes include Ti—Cu—SiI, Cu—SiI and the like. 
     X-ray identification tags  52  are well known in the art for encapsulating with pacemaker and ICD header blocks. The reason for the X-ray identification tag  52  is so that a physician can read a patient chest X-ray and obtain valuable information such as pacemaker model number and manufacturer. Having a redundant identification system like this is desirable in the very unlikely event that the RFID tag  12  should fail to operate. 
       FIG. 11  is a novel end cap  44  that is formed with a fixation hole comprising a post  56  and a loop  58 . This end cap  44  is designed so that a surgeon can put a suture or stitch through the loop  58  and affix the container  40  to body tissue. This is very important in cases where a container  40  is to be implanted adjacent to a prosthetic device or outside of the AIMD  10 . Certain AIMDs  10 , such as deep brain or neurostimulators, are simply too small or do not have a header block  36  into which to encapsulate or capture the container  40 . In this case, during surgery, a loop  58  as shown in  FIG. 11  allows a convenient location for the physician to stitch and fixate the container  40 . The hole feature  58  as shown in  FIG. 11 , can be used to stitch or fix any of the containers of the present invention to body tissue, such as muscle tissue, a ligament, a rib or the like. As previously mentioned, feature  58  can also be used to affix any of the embodiments of the present invention to active or abandoned lead wire systems for AIMDs. 
     In most cases, the container  40  is about the size of two grains of rice. Accordingly, if the container  40  were simply placed into the body without fixation, it could migrate through muscle or other tissues. This would make it very difficult to locate for purpose of use or if it was later desired to remove it. 
       FIGS. 12 ,  13  and  14  depict block diagrams of the RFID system in operation. As described above, the RFID tag  12  consists of a substrate  22 , an RFID chip  16 , and an antenna  14 . The interrogator  20  with associated antenna  24  discharges electromagnetic energy  26  to the antenna  14  of the RFID tag  12 , which powers up the RFID chip  16  and allows it to produce the electromagnetic return signal  28 , as shown. The electromagnetic return signal  28  is detected by the interrogator  20  and presented as a digital code sequence. The RFID tag  12  may be read-only (RO) or read/write (RW). With an RW RFID tag  12 , a physician may use an external programmer or interrogator  20  to write additional patient information to the RFID tag  12 . This additional information may include patient name, patient address, medical condition, and so on. In the case of an RO RFID tag  12 , the RFID tag  12  would be installed at the time of AIMD manufacture and would designate manufacturer, model number and other key information. However, an RO RFID tag  12  would not be later programmable and could not include added important information such as patient name, doctor name, patient diagnosis and so forth. The interrogator  20  may comprise programmer or programmer/reader, which would permit direct display of all of the information contained on the RFID tag  12 . 
       FIG. 15  illustrates a very similar system as previously described in  FIGS. 12 ,  13  and  14  except that the interrogator  20  is designed to be integrated with a computer system  60  which may be linked to the worldwide web. In this case, a unique digital number transmitted by the RFID tag  12  may be entered into the computer system  60 . The computer system  60  maintains a database of important information that is all keyed to the digital information transmitted by the RFID tag  12 . In this way, the physician or emergency room personnel may obtain the digital code from the RFID tag  12  which enters automatically (or manually) into the computer system  60  to immediately get a download, including all of the information required as to the model and serial number of the AIMD, lead wire system, patient and physician information, and patient history when available. The RFID tag could also access the new American College of Cardiology National Cardiovascular Data Registry (ACC-NCDR). ACC-NCDR is a comprehensive cardiac and date repository for three national registries: the CathPCI Registry, the CarotidStent Registry, and the ICD Registry. The ICD Registry was developed in partnership with the Heart Rhythm Society and is designed for participation by hospitals. It collects detailed information on ICD implantations and has as one of its missions helping hospitals meet regulatory requirements and Medicare requirements. 
       FIG. 16  illustrates a system very similar to that described in  FIG. 15  except that the output of the interrogator  20  would go to an antenna and processor  38  which are designed to be linked directly to a laptop computer  62 . This could also be done by USB or equivalent cable interface network  72 . The laptop computer  62  may contain a full database by model numbers and serial numbers of medical implantable devices. A drawback to this type of system is that it would be very difficult to keep updated with current patient and physician information. 
       FIG. 17  is an isometric view of the RFID tag  12  that was previously described in  FIGS. 4 and 5 , but has been modified in accordance with the end cap  44  described in  FIG. 11 . The titanium end cap  44  includes a loop  58  to fix in body tissue or affix to an active or abandoned lead wire set. The metallization  46  on the ceramic housing  42  and the braze  48  forms a hermetic seal. The style of post  56  and loop  58  depicted is just one type one with ordinary skill in the art will recognize. As an alternative,  FIG. 17A  shows another embodiment. It will be obvious to those skilled in the art that loops  58  may also be placed directly on the ceramic housing  42  itself. 
       FIG. 18  illustrates a large needle syringe  70  designed for injecting the RFID tag container  40  directly into body tissue. In this case, the sealed container  40  has an end cap  44  that is designed to make a smooth transition from the ceramic housing  42  to the end cap  44 . This makes the container  40  suitable for injection into body tissue. As previously mentioned, a negative to this approach is that the container  40  may tend to migrate over time within the body tissue. 
       FIG. 19  is an exploded view taken from  FIG. 18  illustrating a cross-section of the container  40 . The titanium end cap  44  has been butted onto and brazed  48  to the ceramic tube  42  such that it forms a smooth outer surface. 
       FIG. 20  illustrates a prior art unipolar hermetic terminal  80  typically used in active implantable medical devices. Hermetic terminals consist of an alumina insulator  82  which is gold brazed  84  to a ferrule  86 . In turn, the ferrule is typically laser welded  88  to the titanium housing  90  of an active implantable medical device. There is also a hermetic seal  92  that is formed between the alumina insulator  82  and the lead wire  94 . This is typically also done by gold brazing, glass sealing or the like. There is also a prior art ceramic feedthrough capacitor  96  shown co-bonded to the hermetic terminal subassembly. Such feedthrough capacitors  96  are well known in the prior art for decoupling and shielding against undesirable electromagnetic interference (EMI) signals, such as those produced by cellular telephones, microwave ovens and the like. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,905,627; 6,275,369; 6,566,978 and 6,765,779. 
       FIG. 21  is a partial cutaway view showing the details of the prior art feedthrough capacitor  96  as previously illustrated in  FIG. 20 . One can see that it has internally embedded electrode plate sets  98  and  100 . Electrode plate set  100  is known as the ground electrode plate set and is coupled to the capacitor&#39;s outside diameter metallization  102 . The active electrode plate set  98  is electrically connected to the capacitor inside diameter metallization  104 . 
       FIG. 22  is a schematic diagram of the prior art feedthrough capacitor  96  illustrated in  FIGS. 20 and 21 . 
     The present invention resides in RFID readers and systems in order to interrogate and identify an active implantable medical device. In order for the RFID field to be able to read a tag embedded within the human body, it must generate a very powerful yet relatively low frequency field. As previously described, the preferred embodiment is a 13.56 MHz HF reader. Such readers are most effective when held within 10 centimeters of the implant. In general, these are 3 to 6-watt effective radiated power (ERP) devices. In comparison, a cellular telephone which produces a very powerful near field is only a 0.6 to 2-watt ERP devices. Accordingly, the patient with an active implantable medical device is subjected to a very powerful digitally pulsed RFID reader field. Accordingly, it is a feature of the present invention that the AIMD have very robust shielding and filtering against the electromagnetic interference that is being produced by the RFID reader itself. This is in order to assure that the electronics of the AIMD are not subjected to temporary or permanent malfunction. Instances of pacemaker inhibition, microprocessor reset or even permanent damage to device electronics have all been documented in the past due to EMI. Accordingly, there is a need in combination with the present invention for the AIMD to be particularly robust so it will be resistant to the fields produced by the RFID reader. 
     ANSI/AAMI Standard PC69 defines electromagnetic compatibility test requirements for pacemakers and implantable defibrillators. It specifically has a radiated dipole test with a mandatory requirement that the AIMD be resistant when the dipole has 40 milliwatts of net input power. There is also an optional or voluntary test level which is at 8 watts (and 2 watts at certain higher frequencies). PC69 currently covers the frequency range from 450 MHz to 3 GHz which is, of course, above the range of the preferred embodiment 13.56 MHz RFID readers. Because of this, AIMDs tend to use relatively low value feedthrough capacitors as illustrated in  FIGS. 20 and 21 . Such feedthrough capacitance values, for example, can be as low as 300 picofarads and still comply with the mandatory 40-milliwatt level. However, recent testing at Mount Sinai Medical Institute in Miami indicates that pacemakers that do not have a feedthrough capacitor EMI filter to comply with the optional 8-watt level can respond to the signals from RFID readers. Periods of noise sensing, inhibition and misbeats were documented in pacemakers out to a distance of 21 centimeters. This is the distance between the pacemaker placed in a saline tank and a portable RFID reader. 
     Accordingly, it would be preferable to use much higher value feedthrough capacitors than shown in  FIGS. 20 ,  21  and  22 . Unfortunately, it is impractical to indefinitely raise the amount of capacitance value for the feedthrough capacitor. This is because too much capacitance can seriously load down the output of the AIMD. In addition, there is usually insufficient space inside of the AIMD to place too large of a capacitor. Also, large values of capacitance can cause excessive currents to flow in implanted lead wires during MRI procedures. 
     A better way to approach this is illustrated in  FIG. 23  and is more fully described in co-pending patent application Ser. No. 11/097,999, and U.S. Pat. No. 6,999,818, the contents of which are incorporated herein. Such describe the advantages of using multi-element EMI filters. Referring to  FIG. 23 , one can see that the prior art feedthrough capacitors “C” have an attenuation slope shown as C. The average attenuation slope rate for this is only 20 dB per decade. By adding additional series elements, such as inductive and resistive elements, one can greatly increase the attenuation slope rate of the EMI filter. For example, referring to the L 1  or L 2  curve of  FIG. 23 , one can see that the attenuation slope rate has increased to 40 dB per decade. This makes for a much more efficient EMI filter. Calling attention to the LL 1  or LL 2  curve, one can see that the attenuation slope rate has gone up dramatically. In this case, it is 80 dB per decade. This is a much more efficient use of the volume and weight available inside of an implantable medical device. 
       FIGS. 24 and 25  illustrate a quadpolar feedthrough capacitor  96  which is combined with a lossy ferrite inductor slab  106 . This allows the designer to use a relatively low value of capacitance such that it does not load down the output of the AIMD or degrade biologic sensing signals, but at the same time by adding the inductor element, offers a filter with a very high degree of RF immunity. In this way, one can comply with the optional 8-watt level of PC69 and provide immunity to closely held RFID tag readers while not overloading the AIMD circuitry. Too much capacitance on the output of the AIMD also tends to lower its input impedance at MRI RF pulsed frequencies. Accordingly, it is important that the capacitance value also be kept low for this reason. 
       FIGS. 26 and 27 , show a terminal  80  in an LL 1  configuration. Referring once again to  FIG. 23 , one can see that this has an attenuation slope rate of 80 dB per decade which is extremely robust. In point of fact, in combination with the present invention such that the AIMD be resistant to RFID readers in the preferred embodiment, the EMI filter circuit would be modified to be of the L, T or LL configuration. 
     Although various embodiments have been described in detail for purposes of illustration, various modifications may be made without departing from the scope and spirit of the invention.