Patent Application
20070083244
This invention generally relates to EMI filter assemblies incorporating one or more passive filter elements including feedthrough capacitors and lossy ferrite inductors, or conventional inductors or the like. These EMI filter assemblies are typically used in active implantable medical devices (AIMDs), such as cardiac pacemakers, cardioverter defibrillators, neurostimulators and the like, for decoupling and shielding internal electronic components of the AIMD from undesirable electromagnetic interference (EMI) signals.
Compatibility of cardiac pacemakers, implantable defibrillators and other types of AIMDs with magnetic resonance imaging (MRI) and other types of hospital diagnostic equipment has become a major issue. If one goes to the websites of the major cardiac pacemaker manufacturers in the United States, which include St. Jude Medical, Medtronic and Guidant, one will see that the use of MRI is generally contra-indicated with pacemakers and implantable defibrillators. See also “Safety Aspects of Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertation submitted to the Swiss Federal Institute of Technology Zurich presented by Roger Christoph Luchinger. “Dielectric Properties of Biological Tissues: I. Literature Survey”, by C. Gabriel, S. Gabriel and E. Cortout; “Dielectric Properties of Biological Tissues: II. Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau and C. Gabriel; “Dielectric Properties of Biological Tissues: III. Parametric Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989, all of which are incorporated herein by reference.
However, an extensive review of the literature indicates that MRI is indeed often used with pacemaker patients. The safety and feasibility of MRI in patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. There are a number of papers that indicate that MRI on new generation pacemakers can be conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuable diagnostic tools. An absolute contra-indication for pacemaker patients means that pacemaker and ICD wearers are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the power of the MRI magnetic field, programming the pacemaker to fixed or asynchronous pacing mode (activation of the reed switch), and then careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers after an MRI procedure occurring many days later.
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field which is used to align protons in body tissue. The field strength varies from 0.5 to 1.5 Tesla in most of the currently available MRI units in clinical use. Some of the newer MRI system fields can go as high as 4 to 5 Tesla. This is about 100,000 times the magnetic field strength of the earth. A static magnetic field can induce powerful mechanical forces on any magnetic materials implanted within the patient. This would include certain components within the cardiac pacemaker itself and or lead wire systems. It is not likely (other than sudden system shut down) that the static MRI magnetic field can induce currents into the pacemaker lead wire system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor, or the conductor itself must move within the magnetic field for currents to be induced. The lossy ferrite inductor or toroidal slab concept as described herein is not intended to provide protection against static magnetic fields such as those produced by magnetic resonance imaging.
The second type of field produced by magnetic resonance imaging is the pulsed RF field which is generated by the body coil or head coil. This is used to change the energy state of the protons and illicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the magnetic field is circularly polarized in the actual plane; and (2) the electric field is related to the magnetic field by Maxwell's equations. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic field strength.
The third type of electromagnetic field is the time-varying magnetic gradient fields which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 1 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements.
A particular concern is due to excessive currents which can be induced in implanted lead wires from a medical diagnostic procedure. A typical example would be excessive currents induced due to the radio frequency (RF) pulsed field of an MRI system. These excessive currents can cause heating of the lead wire through high power (I2R) loss or heating in tissue due to excessive current flowing through tissue itself. This situation is not limited solely to magnetic resonance imaging (MRI). There are a number of other medical diagnostic and/or therapy procedures that involve RF fields. This includes diathermy, electrical surgical knives, such as the Bovi knife, RF ablation and the like. Anytime an implanted lead wire system is exposed to high power RF fields, current can be induced in the lead wire system. This is due to three primary mechanisms, which include induced magnetic coupling through bounded loop areas, induced currents through antenna action, or induced currents or voltages from current circulating in body tissue which create associated voltage drops.
Exemplary prior art feedthrough capacitor EMI filters for implantable medical devices are described in U.S. Pat. Nos. 5,333,095; 5,905,627; 5,973,906; 4,424,551; 4,220,813; 5,531,003; 5,867,361; and 6,414,835, the contents of which are incorporated herein. Feedthrough capacitors are very desirable for EMI filters in that they provide a very low impedance over a very broad range of frequencies. The geometry of the feedthrough capacitor is such that it acts as a coaxial device and is relatively free of self-resonances. This is not true of conventional rectangular chip capacitors or any capacitors with a lead wire. In those cases, the series inductance self resonates with the capacitor thereby rendering the capacitor ineffective as an EMI filter above the self-resonant frequency (the capacitor literally becomes inductive). Accordingly, the feedthrough capacitor has become the mainstay EMI filter for all types of implantable medical devices, including pacemakers, ICDs, neurostimulators and the like. The feedthrough capacitor functions by providing a very low impedance to the housing or casing of the implantable medical device. In the case of a cardiac pacemaker, for example, the housing is typically of titanium. The titanium housing forms an equipotential surface which is a very effective electromagnetic shield. For example, when exposed to the radiation energy from a microwave oven, this shield effectively blocks, reflects and absorbs such high frequency energy. The feedthrough capacitor works in concert with this electromagnetic shield by decoupling EMI which is picked up by implanted lead wires and shunting that high frequency energy to the titanium or other conductive housing of the implantable medical device. By shunting such energy, it turns into harmless eddy currents and therefore is dissipated as a very low level of harmless heat. This prevents the EMI energy from reaching the sensitive internal electronic circuits of the AIMD which can cause permanent or temporary malfunction.
However, the presence of the EMI filtered feedthrough capacitor, by definition, reduces the input impedance of the implantable medical device. As an example, again consider the case of a cardiac pacemaker. Without the EMI filtered feedthrough capacitor the input impedance of the pacemaker might be as high as 10,000 ohms at MRI pulsed frequencies. By placing the feedthrough capacitor(s) at the point of lead wire ingress into and out of a cardiac pacemaker, the feedthrough capacitor itself determines the input impedance. This input impedance varies with frequency according with the following formula: Xc=½πfC. Where Xc is equal to the capacitive reactance in ohms, f is the frequency in hertz, and C is the capacitance in farads. A typical capacitor value that is used in prior art feedthrough capacitors for AIMDs is about 4000 picofarads.
By way of further explanation, we will consider the capacitor reactance of this particular capacitor in a 3-Tesla MRI system. A 3-Tesla MRI system has an RF pulse frequency of approximately 128 MHz. Therefore, the capacitive reactance equation becomes ½π(128×106 Hz)(4000×10−12F) or Xc=0.31Ω. If one considers EMI filter protection only, the feedthrough capacitor desirably lowers the input impedance of the cardiac pacemaker from approximately 10,000 ohms all the way down to 0.31 ohms. This effectively shorts or decouples the high frequency EMI associated with the 128 MHz signal to the titanium housing thereby preventing it from getting into the sensitive internal electronic circuits of the AIMD. However, this situation presents a dilemma when the AIMD patient is exposed to medical device procedures, such as MRI. The very powerful RF fields of the MRI system induce voltages (electromotive forces—EMFs) into the implanted lead wire system. The presence of this very low input impedance to the cardiac pacemaker (0.31 Ω) causes very high currents to flow due to Ohms Law. This can cause overheating of the lead wire itself or it can cause excessive current to flow at the point of tissue interface, for example, between pacemaker Tip and Ring electrodes and through the myocardial tissue and, for example, the right ventricle.
Why not use a much lower value of feedthrough capacitor, for example, 400 picofarads? A 400-picofarad feedthrough capacitor would present a 3.1 ohm input impedance to the cardiac pacemaker. This would greatly reduce the current in the associated lead wire system. The problem is that this would make the AIMD vulnerable to high frequency emitters, such as closely held cellular telephones and similar devices that are found in the patient environment. In addition, there are a number of compliance standards for active implantable medical devices. This includes ANSI/AAMI/PC69 (in the United States), CENELEC 45502-2-1 (in Europe) and a pending ISO standard. The ISO Neurostimulator Committee is also considering a draft standard for EMI compliance. In other words, the AIMD manufacturer must provide a high degree of input filtering not only to make the patient safe from environmental emitters, but to also comply with various regulatory EMI standards.
Why must this be done with passive components, such as capacitors, inductors and resistors? The answer is that active electronic filters do not have enough dynamic range in general to stay linear in the presence of very large RF fields such as those produced by medical diagnostic equipment. This is particularly problematic as microchips have become smaller and more dense. It was not very many years ago that it was possible to buy 8-micron microchip technology. However, it is now very difficult to buy even 4-micron technology, with newer microchips in the submicron range. This has many positive aspects which allow microchips to be smaller and pack in more transistors into smaller spaces. However, an undesirable trade off to these ultrathin technologies is that they operate at lower voltages and become increasingly sensitive to a lack of dynamic range or what's known as a limitation on quiescent operating point. In the presence of extremely large RF fields, such as those produced by MRI, such active filters go into a non-linear region. This actually creates more EMI as the incoming EMI signal is distorted which produces many undesirable harmonics and demodulation products.
Accordingly, there is a need for passive EMI filters which provide a high degree of EMI filtering protection to the AIMD electronics while at the same time limiting the current in the implanted lead wire system. The present invention meets these needs and provides other related advantages.
The present invention resides in a process for tuning an EMI filter for an active implantable medical device (AIMD), wherein the EMI filter has a capacitor and an inductor/resistor element. The novel process of the present invention comprises the steps of: (1) evaluating input impedance of the AIMD; (2) configuring the physical relationship of the capacitor and the inductor/resistor element of the EMI filter based on the evaluated input impedance of the AIMD; (3) iteratively selecting component values for the capacitor and the inductor/resistor elements of the EMI filter; and (4) analyzing the impedance characteristics of the selected components through circuit simulation to assess (a) whether the impedance of the EMI filter has been raised sufficiently to reduce undesirable RF currents that would flow during medical diagnostic procedures, and (b) if the AIMD is adequately protected against environmental emitters and complies with regulatory requirements. The novel process of the present invention further includes the steps of (5) building a prototype of the AIMD comprising an EMI filter having selected components that have been assessed to be acceptable; (6) testing the prototype to determine whether the impedance of the EMI filter has been raised sufficiently to reduce undesirable currents that would flow during medical diagnostic procedures; and (7) testing the prototype to determine if the AIMD is adequately protected against environmental emitters and complies with regulatory requirements.
The steps of iteratively selecting and analyzing may be repeated (a) if the impedance of the EMI filter has not been raised sufficiently to reduce undesirable currents that would flow during medical diagnostic procedures, or (b) if the AIMD does not adequately protect against environmental emitters or comply with regulatory requirements. Further, the configuring, iteratively selecting and analyzing steps may be repeated if the prototype fails either of the testing steps. The configuring step may include the step of utilizing an inductive/resistive element located at a point of lead wire ingress and egress from the AIMD followed by the capacitor, where the capacitance value of said capacitor is minimized to reduce RF currents in a lead wire system of the AIMD.
The novel process may further include the step of optimizing component values of the capacitor and the inductor/resistor elements of the EMI filter such that an acceptable level of attenuation is achieved with the lowest possible value of feedthrough capacitance.
The evaluating step may include the steps of utilizing a network analyzer, a sophisticated materials analyzer or spectrum analyzer to look back into the terminal of the AIMD where its implantable leads would normally connect, and performing impedance measurements at RF frequencies of interest.
One or more passive series inductive/resistive elements may be utilized to create a multi-element EMI filter having acceptable attenuation to protect a patient from electromagnetic interference (EMI). The one or more passive series elements may comprise an inductor, a resistor, a combined inductive/resistance element, an air wound inductor, a chip inductor, a wire wound resister, a composition resistor, or a toroidal or solenad inductor with a ferromagnetic material core. The passive series element may alternatively comprise a lead wire through the capacitor or a lossy ferrite conductor slab.
The capacitor and the passive series inductive and/or resistive elements may be combined to form an L, PI, T, LL, 5-element or N-element device.
Further, the iteratively selecting step includes the step of selecting a capacitor with a very low value of capacitance and selecting the maximum value of the inductor/resistor element that would physically fit the geometry available inside the package of the AIMD. The analyzing step may include the step of utilizing a network or spectrum analyzer to analyze the impedance of lead wire systems associated with the AIMD.
Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The accompanying drawings illustrate the invention. In such drawings:
With reference to
As can be seen in
In
There is another implication to these curves of
The tuning process of the present invention involves selecting the proper combinations of much lower value feedthrough capacitors in combination with the series lossy ferrite and/or inductor elements such as to achieve equivalent high frequency EMI attenuation performance. It doesn't even really matter how well the inductor performs in the presence of the MRI field. What is meant by this is that the simple fact that using a feedthrough capacitor that has a lower capacitance value will raise the input impedance of the AIMD. This by itself will greatly reduce the amount of circulating RF current in the AIMD implanted lead wire system. However, in the case of an MRI application, there is a main static field, which can vary anywhere from 0.5 to several Teslas. There are research systems currently in development that even go above 5-Teslas. This main static field can saturate the ferrite core or the magnetic core of most inductive elements. This is not true for an air wound inductor. However, the problem here is that the amount of inductance is very low for the amount of volume required to wind it. In contrast, the lossy ferrite elements and/or iron core inductor elements as described herein, need not work very well (or at all) in the actual bore of an MRI system. This is because when the higher level of EMI filtering is needed, for example, when the patient is in the presence of a microwave oven or a cellular telephone, the lossy ferrite or inductor element will not saturate and will work properly. As shown in
On the other hand, while said patient is under medical supervision undergoing a medical diagnostic or therapy procedure, the same level of EMI filtering is not required. What is more important in this case is that the implanted lead wire system not be subjected to excessive currents which could cause it to overheat and permanently damage surrounding tissues. For example, with cardiac pacemaker patients, it has been demonstrated that after an MRI procedure, surrounding tissue changes and even ablation can occur in the area of the distal Tip. What this means is that there has been some damage to the myocardial tissue. This can result in an increase in the pacemaker capture level (pacing threshold voltage) or even a complete loss of capture. An increase in capture level means that the pacemaker patient would need a much higher voltage output from the pacemaker after the MRI procedure to properly beat the heart as opposed to prior to the procedure. It is highly undesirable and very worrisome for capture level to increase. Not only does increased capture level shorten battery life, but there are concerns about the damage to tissue and the resulting pathology. In certain cases, complete loss of capture has occurred which required life saving procedures followed by implantation of a new pacemaker and associated lead system.
Most, if not all, of these AIMDs have associated implanted lead wire systems. Accordingly, there is a concern about overheating literally all of these devices in the presence of medical diagnostic and therapy procedures that involve high levels of RF energy.
It is useful to refer to a 50-ohm system for the purpose of analyzing and tuning the novel passive EMI filter circuit as described herein. There are various circuit simulators, such as P-Spice, which are very useful for this purpose. Using a typical prior art feedthrough capacitor value of 4000 picofarads, we will now analyze the attenuation in decibels (dB) at the various ANSI/AAMI PC69 frequencies defined as 450 MHz to 3 GHz. The following table of values is the 50-ohm attenuation for a 4000-picofarad feedthrough capacitor:
As previously discussed and calculated, the capacitive reactance associated with these high levels of attenuation is 0.31 ohms, which establishes a very low input impedance for the cardiac pacemaker. Now let's perform the same calculations using a T-circuit EMI filter as shown in
Referring to
As one can see, there is a compromise with a lower level of insertion loss at the MRI pulsed frequencies, but a high level of attenuation performance in the cellular phone frequency range of 950 MHz and above. By reducing the capacitance value by a factor of 10, one also reduces the current in the induced lead wire system by a factor of 10 due to Ohms Law. Remembering that power is an I2R effect, this reduces the power dissipation in the lead wire of the associated body tissue by a factor of 102 or 100. This results in a very significant reduction in heating in certain sections of the implanted lead wire.
As one can see, this is an iterative or tuning process where one works to reduce the capacitance value as low as possible and still provide compliance to the various regulatory standards and also sufficient attenuation against the RF frequency of the particular piece of medical diagnostic or therapy equipment.
It will be obvious to one skilled in the art that similar procedures apply to every one of the circuits that are described above. This included the L, the PI, the T, the LL, the 5-element and the N element circuit configurations. Each one requires a separate set of calculations and verification (validation) measurements.
Accordingly, it is highly desirable to reduce the capacitance value of the feedthrough capacitor element(s) as much as possible while at the same time providing sufficient EMI filter performance.
With reference specifically to
Block B shows the step of configuring the physical relationship of the capacitor and the inductor/resistor element of the EMI filter, based on the evaluated input impedance of the AIMD. In the case where the circuits of the implantable medical device look like a very high impedance inherently at 64 megahertz, then one could desirably select an L-Section filter with the capacitor oriented toward the electronics and the lossy ferrite slab or inductor oriented towards the body fluid side of the device (Block C). In the case where the input impedance was medium, for example in the area of 100 to 500 ohms, then a T-Section filter would be more optimum. The second inductor pointing towards the electronics would tend to raise the impedance thereby making the EMI filter circuit tuning more optimum (Block D). In the third case where the internal impedance of the AIMD electronics were quite low (for example: below 50 ohms), then the desirable circuit configuration would be a double L type of filter (Block E). Of course, the L, T and LL circuit configurations are only examples as five element and n-element filters are also possible.
Block F is the process of interatively selecting component values starting with the lowest possible value of capacitance. For example, in prior art pacemakers a typical feedthrough capacitor's value as previously mentioned is 4000 picofarads. In Block F, one would start with a very low value of capacitance (for example, 100 picofarads and then select the maximum value of lossy ferrite inductor slab values that would physically fit the geometry that is available inside the package of the cardiac pacemaker. One would then use a circuit simulator to evaluate the amount of attenuation given the impedance of the cardiac pacemaker that had been previously measured in Block A and then also source impedance in the implanted lead wire system. This step requires using a network analyzer to analyze the impedance for the particular lead wire system (Block G). It will be noted that a unipolar lead has a different impedance than bi-polar leads and spiral leads. Therefore, for the particular active implantable medical devices contemplated, Block G would be the step of taking the lead wire system that is designed to work in conjunction with the AIMD and analyze its impedance characteristic. This is important for the entire circuit simulation as performed in Block F will be more accurate. The circuit simulation that is performed in Block F will allow one to determine the new input impedance for the AIMD and assess whether or not the impedance has been raised sufficiently so that undesirable RF currents that would flow during medical diagnostic procedures and associated heating has been reduced to acceptable levels. At the same time, the circuit simulation will assess the EMI filter attenuation at various high-frequencies to make sure that the AIMD is still adequately protected against environmental emitters (such as cellular phones) and in addition will comply with the various regulatory requirements.
One would first start with an assessment of the current that is induced in the lead wire system (Block H). If the current is found to be too high then one would attempt to reduce the capacitance value even further and use that volume to further increase the amount of lossy ferrite/slab inductance. In the case where the current was very low, one would use the opposite procedures. We could then increase the capacitance value until the current was just able to produce an acceptable level of lead wire heating. This is done at the same time while we're also looking at the EMI attenuation of the filter (Block I). In the case where the EMI attenuation was too low, one would either have to raise the value of the feedthrough capacitor or increase the series of inductive resistance elements until there was an acceptable level of EMI filter attenuation. If the EMI filter performance was too high, this would be an indication that one could further reduce the value of feedthrough capacitor thereby minimizing the current. As a general rule of thumb, the tuning procedure is optimized when you reach an acceptable level of EMI filter attenuation and have the lowest possible value of feedthrough capacitance or capacitors that are consistent with that value. At this point, the design is finalized.
In Block J, one then builds prototypes of the finalized design and then submits them for testing of two types. Testing in Block K is EMI testing in accordance with ANSI/AAMI/PC69 or equivalent standard. Testing in Block L is to expose the system with its associated lead wires in an MRI bore in a Gel Tank and use optical or equivalent Fizo measuring equipment to actually measure the heating in the lead wire system. If both of these levels are acceptable, then the design is deemed qualified. Decision Block M indicates that if both the Blocks K and L testing (in other words for MRI and EMI are both passed) then the product is done and ready for FDA regulatory approval. In the event that the device failed the EMI testing then we would go back up and re-design the filter portion wherein we may have to increase the capacitance value and increase the series inductive elements until we reach an acceptable level of EMI. If we fail the acceptable heating requirement during the MRI board test, then we have to go up and re-design for a lower level of current which would require a lower level of capacitance be designed in. The requirement for this tuning process is that the circuit prediction analysis shown in Block F is not entirely perfect and this is due to the complication as a complex situation involving non-linear impedance interactions and field interactions and various coupling mechanisms that are involved. In the case where both Blocks K and L testing fail, this means that there is a serious design issue which would require re-design of the pacemaker to allow more space. What this means is that we would need more physical room to put in higher levels of inductance or series resistance so that we could further lower the capacitance value.
Referring now to
Accordingly, it should be apparent that the present invention provides a tuning process for a EMI filter manufactured with passive components for active implantable medical devices wherein in the preferred embodiment:
The process may be modified:
Although a particular embodiment of the invention has been described in detail for purposes of illustration, modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 60724479 | Oct 2005 | US |
