Abstract:
Medical lead systems utilizing electromagnetic bandstop filters are provide which can be utilized in a magnetic resonance imaging (MRI) environment for patients who have implanted medical devices. Such lead systems may be advantageously used in left ventricle cardiac stimulation systems, neuro-stimulation systems, and deep brain electrodes used for the treatment of Parkinson&#39;s disease and other movement disorders. The bandstop filters, which include a tuned parallel capacitor and inductor circuit, are backwards compatible with known implantable deployment systems.

Description:
BACKGROUND OF INVENTION 
       [0001]    Magnetic resonance imaging (MRI) is currently contra-indicated for patients who have implanted medical devices. This is due largely to the patient safety issue that results when the strong electromagnetic fields of an MRI system interact with the antenna-like therapy delivery leads of an active implantable medical device (AIMD). It is well documented that the radio frequency (RF) signals that are generated by the MRI system can couple along the length of a lead body and create induced current loops. These current loops can cause significant heating at points of high current concentration, the most significant of which is the distal tip, where the lead system makes direct contact with body tissue. 
         [0002]    As disclosed in U.S. patent application Ser. No. 11/558,349, filed Nov. 9, 2006, TANK FILTERS PLACED IN SERIES WITH THE LEAD WIRES OR CIRCUITS OF ACTIVE MEDICAL DEVICES TO ENHANCE MRI COMPATIBILITY, and Provisional Patent Application No. 60/968,662, filed Aug. 29, 2007, entitled A CYLINDRICAL BAND STOP FILTER FOR M LEAD SYSTEMS, the contents of which are incorporated herein by reference, a novel method to minimize the expected heating at the distal tip of the lead system is to incorporate a bandstop filter. This bandstop filter is comprised of an inductor and capacitor in parallel, with the entire filter connected in series to the lead system. In such a system, the bandstop filter can be constructed so that its resonant frequency or frequencies coincides with the RF operating frequency of one or more MRI systems. 
         [0003]    RF frequencies are directly related to the MRI machine static magnetic field by the Lamour Relationship. Typical values are 64 MHz for 1.5 T systems, and 128 MHz for 3.0 T systems. At resonance, the impedance of the bandstop filter is quite high which reduces the flow of current at the MRI RF pulsed frequency thereby reducing leadwire and/or electrode heating. Increasing the impedance at the distal tip also greatly reduces the amount of RF current that would flow into body tissue. It has been documented that excess current can cause tissue damage or even tissue necrosis. 
         [0004]    Implementation of this technology in implantable leads is a significant challenge. Bandstop filters for use in lead systems must be biocompatible, not significantly change the electrical performance characteristics of the lead (except within the context of the invention), and must not significantly affect size, weight, or implantability. With increasingly smaller leads being developed to accommodate small vasculature and left ventricular pacing through the coronary sinus, bandstop technology must be equally scalable to match the same demands. 
         [0005]    The present invention satisfies these needs and provides other related advantages. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to medical lead systems which utilize electromagnetic bandstop filters. Such medical lead systems are advantageously utilized in a magnetic resonance imaging (MRI) environment for patients who have implanted medical devices. 
         [0007]    The medical lead system of the present invention includes a lead wire configured for insertion into a venous system. The lead wire includes a terminal pin at one end for connecting the lead system to the implantable medical device. An electrode is provided at an opposite end, which is in contact with biological cells. The bandstop filter associated with the lead wire attenuates current flow through the lead wire at a selected frequency. The bandstop filter comprises a capacitor in parallel with an inductor. The parallel capacitor and inductor are placed in series with the lead wire. Values of capacitance and inductance are selected such that the bandstop filter is resonant at selected MRI frequencies. 
         [0008]    In a preferred form, the lead wire comprises a proximal section extending from the terminal pin, and a reduced-diameter distal section. The bandstop filter is disposed between the proximal and distal sections. Moreover, the bandstop filter may include tines for fixing the bandstop filter in a desired location within the venous system. 
         [0009]    As illustrated, the lead wire may comprise an epicardial lead, a split-cylinder cuff electrode, self-sizing nerve cuffs, a multiple cuff nerve electrode, a multiple bandstop filter array, or a deep brain probe. In each instance, the bandstop filter is incorporated therein. 
         [0010]    More particularly, in one form, the multiple bandstop filter array comprises a plurality of bandstop filter chips disposed on a substrate. At least one of the bandstop filter chips may be thick-film deposited onto the substrate. Where the lead wire comprises a deep brain probe, a flexible conductor may be disposed between the bandstop filter and the electrode. 
         [0011]    It will be appreciated that such lead systems may be advantageously used in left ventricle cardiac stimulation systems, neuro-stimulation systems, and in connection with deep brain electrodes used for the treatment of Parkinson&#39;s Disease and other movement disorders. The bandstop filters, which include a tuned parallel capacitor and conductor circuit, are backwards compatible with known implantable deployment systems. 
         [0012]    Other features and advantages of the present invention will become apparent from the following more detailed description, taken in connection with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a diagrammatic representation of the human heart, showing a left ventricular endocardial lead system embodying the present invention. 
           [0014]      FIG. 1A  is a table showing the relationship between French sizes and millimeters and inches. 
           [0015]      FIG. 2  is an enlarged perspective view of the lead system of  FIG. 1 . 
           [0016]      FIG. 2A  is an enlarged view of the distal lead taken generally of the area indicated by the line  2 A in  FIG. 2 . 
           [0017]      FIG. 3  is a diagrammatic representation of the human heart, showing epicardial leadwire attachment to the outside of the left ventricle in accordance with the present invention. 
           [0018]      FIG. 4  is a cross-sectional view of an epicardial lead embodying the bandstop filter of the present invention taken generally along the line  4 - 4  in  FIG. 3 . 
           [0019]      FIG. 5  is an alternative epicardial lead tip taken generally along the line  4 - 4  in  FIG. 3 . 
           [0020]      FIG. 6  illustrates a split cylinder cuff electrode designed to wrap around a nerve. 
           [0021]      FIG. 7  illustrates a self-sizing helical cuff coil including the bandstop filter chip of the present invention. 
           [0022]      FIG. 8  is a sectional view taken generally along the line  8 - 8  in  FIG. 7 . 
           [0023]      FIG. 9  illustrates a nerve cuff employing a multiplicity of electrodes and bandstop filter chips for a large nerve trunk, in accordance with the present invention. 
           [0024]      FIG. 10  illustrates one methodology of putting multiple bandstop filter chips in series with the lead wires of the multiple cuff electrode of  FIG. 9 . 
           [0025]      FIG. 11  illustrates a multi-conductor lead body connected to a multiple tank filter chip array that has multiple electrodes, in accordance with the present invention. 
           [0026]      FIG. 12  is an exposed perspective view of one of many possible variations of the multiple bandstop filter array of  FIG. 10 . 
           [0027]      FIG. 13  is a diagrammatic, side cross-sectional view of the human head showing the placement of a deep brain probe and electrode embodying the bandstop filter of the present invention. 
           [0028]      FIG. 14  is a diagrammatic, front cross-sectional view of the human head showing use of multiple deep brain probes. 
           [0029]      FIG. 15  is an enlarged sectional view taken generally of the area indicated by the line  15  in  FIG. 14 . 
           [0030]      FIG. 16  is a view similar to  FIG. 15 , illustrating an alternative probe and electrode arrangement. 
           [0031]      FIG. 17  is a sectional view similar to  FIG. 15 , except that the probe containing the bandstop filter chip is embedded under the skull and then the skull bore hole is covered with a bone encapsulant. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    The present invention addresses three primary topics which include: (1) left ventricle cardiac stimulation with a proximal bandstop filter; (2) neurostimulation with various types of neuro-cuffs employing a bandstop filter; and (3) deep brain electrodes that are used for treatment of Parkinson&#39;s Disease and other movement disorders such as tremor and dystonia, depression and neurological disorders. 
         [0033]      FIG. 1  is a diagrammatic representation of a human heart  20  which includes right and left subclavian veins  22  and  24  respectively, the superior vena cava  26  and the aorta  28 . A lead wire  30 , which is typically routed from a biventricular cardiac pacemaker or a biventricular implantable cardioverter defibrillator (ICD) (which are not shown), is routed through a catheter  31  and directed, in this case, through the left subclavian vein  24  and then down through the superior vena cava  26  and into the coronary sinus  32 . The leadwire  30  must first enter the coronary sinus ostium  34  where the implanting physician selects the correct location. The coronary sinus  32  is actually divided into two zones: the first part (on the left) is known as the coronary sinus  32 ; and the second part (on the right) is called the great cardiac vein  36 . The great cardiac vein  36  wraps around the back of the left ventricle. The bandstop filter  38  of the present invention is intended to be placed ideally near the end of the great cardiac vein  36  where it breaks into several venous branches. These branches are called the posterior branch, the lateral branch  40  and the anterior branch  42 . A more comprehensive name, for example, would be the interventricular branch. 
         [0034]    Referring once again to  FIG. 1 , one can also see the right ventricle  44  and the right atrium  46 . Also shown are the left atrium  48  and the left ventricle  50 . The ideal location for a proximal bandstop filter  38  is shown. An ideal length for the proximal bandstop filter  38  would be between 5 and 7.5 mm in length. At this particular location, at the end of the great cardiac vein  36 , cardiac motion is relatively small and fibrotic tissue will tend to encapsulate the bandstop filter  38  and its lead wires  30  and thereby attach it/fixate it in position in this relatively low motion region. This is a particular advantage to the present invention, in that the lead  30  will remain highly reliable and resistant to breakage. Because of the relatively large diameter of the coronary sinus  32  and the great cardiac vein  36 , this portion of the lead wire system, including the bandstop filter  38 , can be of much larger diameter (for example, 7 or 8 French). Beyond this point, where the great cardiac vein  36  branches, the venous systems become much smaller. In general, these branches are below 6 French in diameter and ideal electrode sizes go all the way down to 3 French.  FIG. 1A  shows the relationship between French size, millimeters and inches. Since left ventricular pacing is important for cardiac resynchronization and treatment of congestive heart failure, it is a feature of the present invention that a lead wire reduction occurs at the point of egress of the bandstop filter  38  allowing insertion of electrodes into the small diameter venous system in the proper position outside the left ventricle  50 . 
         [0035]    The primary benefit of locating the bandstop filter  38  in the coronary sinus  32  and/or great cardiac vein  36  is that the diameter of the bandstop filter  38  itself can be larger making it much easier to manufacture. The distal portion  52  of the lead  30  from the bandstop filter  38  is smaller (3 to 6 French size) for easier employment and navigation into the branch veins of the left ventricle  50 . Secondary benefits beyond the diameter of the bandstop filter  38  include the length of the bandstop filter. Entering into and navigating the coronary sinus  32  and great cardiac vein  36  generally involve larger bend radii compared to accessing and navigating the branch vessels. Therefore the portion of the lead  52  that traverses through and resides in the branch vessels must be very small and very flexible, not having a stiff section longer than approximately 1.5 mm as a rule of thumb. Rigid sections of the lead  30  measuring longer than 1.5 mm can impede the ability to navigate around the tight corners and bends of the branch vessels. In the coronary sinus  32  and great cardiac vein  36 , however, there is substantially more latitude, and stiff sections of the lead could approach 5 mm or even 7.5 mm without drastically impeding deliverability. A secondary benefit of locating the bandstop filter  38  in the coronary sinus  32  or the great cardiac vein  36  has to do with MRI image artifacts. Although the image artifact will be quite small due to avoiding the use of ferromagnetic materials, it is still beneficial to locate the bandstop filter  38  away from the coronary arteries, ventricular wall motion or other anatomies/physiologies/pathologies of most interest. If a bandstop filter  38  is located in the coronary sinus  32 , however, it could generate small artifact in the vicinity of the valves. Another benefit of having the bandstop filter  38  located in the coronary sinus  32  or the great cardiac vein  36  is that its rigidness provides a foundation on which fixation fixtures may be more strategically utilized. For example, one or more tines could originate from the region of the lead where the bandstop filter  38  resides. Additionally, rigidness of the bandstop filter  38  makes the tines more effective in their engagement of the vessel walls. Alternatively, a rigid portion of the lead  30 , skillfully navigated beyond a corner or bifurcation, can function as a fixation mechanism that proves difficult or requires skill to track the lead. 
         [0036]      FIG. 2  is an enlarged perspective view of the lead wire system  30  taken from  FIG. 1 . One can see that there is a guide wire  54  which is common in the prior art for inserting into position prior to sliding the lead wire system  30  down over it. A terminal pin  56  is designed to plug into the implantable medical device, such as a pacemaker or ICD. The bandstop filter  38  is shown at the point where the lead wire  30  would be reduced from 6-9 French down to 3-6 French. Optional fixation tines  57  are shown which may be affixed to the bandstop filter  38 . By way of reference, the French scale is related to both diameter in millimeters (mm) or inches. For example, 7 French is 2.3 mm (0.092 inch) in diameter and 3 French is only 1 mm in diameter (0.039 inch). The length (l) of the reduced diameter lead wire section  52  can be adjusted in accordance with the branch vein into which the lead system is being inserted in the desired location of the electrodes  58 . Below the electrodes  58  is the other end of the guide wire  54 . Once the electrodes  58  are in the proper position and the system has been tested, the guide wire  54  is typically removed. A particular advantage of the lead system  30  as shown in  FIG. 2  is that no new deployment instruments or catheters are required. In other words, this system that includes the bandstop filter  38  is backwards compatible with all known deployment systems. It is also very important that the lead wire system  30  is designed to be extracted in the case of a broken lead, defective lead or infected lead. The lead wire system illustrated in  FIGS. 2 and 2A , is also backwards compatible with current (prior art) mechanical and laser lead extraction technologies. 
         [0037]      FIG. 3  is a diagrammatic representation of the human heart similar to that illustrated in  FIG. 1 . However, in this case, external (epicardial) electrodes  62  are attached outside and to the left ventricle  50  by means of epicardial leads  60 . A sutureless myocardial lead  60 ,  64  is shown affixed to the outside of the left ventricle. This methodology is well known and generally involves an insertion between the ribs outside of the heart and a screwdriver type feature to affix the sutureless epicardial lead tip electrode  62  in place. Epicardial leads may also have a suture feature which is also well known in the art, which can have a helical or other configuration type tip. It will be obvious to those skilled in the art that the present invention can be extended to any type of external (epicardial) electrode  62  or satellite pacer affixed to the outside of the heart, particularly outside of the left ventricle. 
         [0038]      FIG. 4  is a cross-sectional view taken generally along line  4 - 4  of  FIG. 3 , illustrating an epicardial lead electrode assembly  62  which includes a bandstop filter  70 . In the prior art, the epicardial lead electrode assembly  62  is typically over-molded with silicone rubber  66 . The assembly shown in  FIG. 4  is self-affixing to the myocardial tissue by a helical electrode structure  68 . Typically this electrode is affixed into the myocardium by 3½ mechanical turns and is made of platinum-iridium alloy or equivalent biocompatible material. The helical electrode tip  68  is affixed into the myocardial tissue by a screwdriver type turning surgical tool. The bandstop filter  70 , as illustrated in  FIG. 4 , is taken generally from FIG. 42 of U.S. patent application Ser. No. 11/558,349, filed Nov. 9, 2006. It will be obvious to those skilled in the art that almost all of the other novel bandstop filter embodiments that are disclosed in U.S. patent application Ser. No. 11/558,349 can be incorporated into the epicardial lead  62  in  FIG. 4 . For example the bandstop filters shown in U.S. patent application Ser. No. 11/558,349, FIGS. 35, 37, 42, 58, 65, 69, 70, 80, 85, 87, 94, 115, 118, 128, 130, 132, 133, 141, 142, 149, 151, 156, 157 are all readily adaptable into the bandstop filter  38  and  70  as illustrated in  FIGS. 12 and 4  (any of the designs in U.S. Patent Application No. 60/968,662, filed Aug. 29, 2007, are also applicable). 
         [0039]      FIG. 5  is very similar to the epicardial lead electrode assembly  62  described in  FIG. 4 , except that it has a novel ring structure  72  associated with a bandstop filter chip  70 . This epicardial bipolar electrode also has a bandstop filter chip  70 ′ in series with its helical tip electrode  68 . Not shown, but obvious to those skilled in the art, is that a screwdriver type head mechanism can be added for convenient adapting to prior art deployment instruments. As previously mentioned, any of the cylindrical bandstop filter chips as described in U.S. Patent Application No. 60/968,662, filed Aug. 29, 2007, can also readily adapted to any of the novel bandstop filter applications as described herein. 
         [0040]      FIG. 6  illustrates a split cylinder cuff electrode  74  embodying two electrodes (Anode (a) and Cathode (c)). This is designed to be inserted by a physician around a nerve. It is a bipolar system typically consisting of a 6-8 French diameter lead body  76 . A double bandstop filter chip  78  (two discrete bandstop filter chips in parallel) in accordance with the present invention is located as shown. In general, the cuff  74  is sized to match the diameter of the nerve which passes through its center. The lead body  76 , after the double bandstop filter  78 , is of a reduced diameter, generally in the 3-4 French range. Not shown is a closing suture which is typically used to draw the cuff together after it&#39;s installed. 
         [0041]      FIG. 7  illustrates helical nerve cuffs  80  which are self-sizing. These incorporate electrode foils  82  which are well known in the prior art. The lead body  84  is attached to a bandstop filter  86  of the present invention. This can be unipolar or bipolar as shown. The electrode foil  82  can either be etched or stamped and then the termination point where the conductor attaches to the foil is either prepared or fabricated. This is the location to where the bandstop filter  86  is electrically attached and incorporated. The conductors and the foil  82  on the bandstop filter  86  are laid into a split mold and assembled and then silicone is injected into the mold.  FIG. 7  illustrates one leg of a bipolar or multipolar lead. Obviously, a bandstop filter  86  would be required for each electrode foil in the multipolar configuration. 
         [0042]      FIG. 8  is an enlarged sectional view taken generally along line  8 - 8  of  FIG. 7 . 
         [0043]      FIG. 9  illustrates a larger multiple cuff nerve electrode  88  for current steering in a large nerve trunk. Various electrodes can be stimulated by trial and error to obtain the optimal result. For example, for pain control, one can try various electrodes and various types of electrical stimulation by trial and error until pain is minimized or eliminated. The multiple parallel filter electrodes  90 , similar to that described in  FIG. 4 , can be placed in conjunction with each one of the conductors  92  as shown. 
         [0044]      FIG. 10  illustrates an alternative in that a multiple bandstop filter array  94  is shown in series with the lead body  96 . This can in turn be connected to the cuff electrode  88  of  FIG. 9  or to the multiple cuff electrodes  74 , 80  illustrated in  FIGS. 6 and 7 . It can also be adapted to the multiple single electrodes illustrated in  FIG. 11 . The multiple bandstop chip as illustrated in  FIGS. 10 and 11  can be made in a variety of ways utilizing the technology disclosed in U.S. patent application Ser. Nos. 11/558,349, and 60/968,662, by putting the devices on a substrate next to each other on each lead wire. The structure shown in  FIGS. 10 and 11  can be over-molded with silicone or the like to provide reliable mechanical attachment and protection from body fluids. 
         [0045]      FIG. 12  is taken generally along the line  12 - 12  from  FIG. 10 , and illustrates one of many ways to form a multiple array  98  of bandstop filter chips. One can see that there is a thin substrate  100  which can be of alumina or any other substrate material known in the prior art. Different types of bandstop filters  102   a - f  (MRI chips) are shown by way of illustrating that there are many ways to construct this multiple array  98 . Referring to drawings from U.S. patent application Ser. No. 11/558,349, the MRI chips  102   a - f  are similar to FIGS. 80-85 and FIG. 136 (which is thick film deposited right on the substrate). Those skilled in the art will realize that imbedded or MEMs components can also be used to form the multiple bandstop MRI filter array  98 . In addition, multilayer substrates may be used to increase packaging density. Wirebond pads  104 , leadwires  106  and  106 ′ and electrical connections  108  (typically laser welds or gold wirebonds), are also shown. The bandstop filter chips  102   a - f  have end terminations  110  as shown. There are electrical connections from these end terminations  110  to the circuit traces  112 . The entire assembly is desirably over coated, over molded (with silicone or the like), or glass encapsulated to enhance mechanical strength and biocompatibility. 
         [0046]      FIG. 13  is a diagrammatic side cross-sectional view of the human head showing the skull  114  and the brain  116 . A burr hole  118  is drilled through the skull  114  for placement of deep brain probe  120  with associated electrodes  122 . One can see that there is a lead wire body  124  which has been tunneled up underneath the skin and attaches to the deep brain probe  120 . One or more bandstop filter chips of the present invention are located inside of the skull burr hole  118 . 
         [0047]      FIG. 14  is a diagrammatic cross-sectional front view of the human head, showing that there can be multiple deep brain probes  120  . . .  120   n  placed as previously described in connection with  FIG. 13 . In a preferred embodiment, the top of the deep brain probe  120  and associated bandstop filter would be flush with the top of the skull  114 . The lead wire  124  is generally connected to a pulse generator or transmitter which is either implanted or can sit outside the skin. There can also be a receiver which sits on the skin. The deep brain probe  120  can also have a nail head or nail shank. 
         [0048]      FIG. 15  is an elongated sectional view of the area indicated by line  15  in  FIG. 14 , of the deep brain probe  120 . Shown is the location of the bandstop filter  126 , the skin  128  which covers the skull  114 , the lead wire  124 , the burr hole  118 , dura layer  130  and the brain  116 . At the end of the deep brain probe  120  are electrodes  132 . 
         [0049]      FIG. 16  illustrates an alternative view wherein a highly flexible region  134  is connected between the bandstop filter  126  and the electrodes  132 . This flexible section  134  accounts for the relative motion between the skull  114  and the brain  116 . 
         [0050]      FIG. 17  illustrates an alternate configuration taken from  FIG. 15  wherein the top of the bandstop filter  126  is below the skull bone  114 . The burr hole  118  is covered with a bone encapsulant  136 . It will be obvious to those skilled in the art that the deep brain probe  120  of the present invention can be placed in various locations that are convenient for the physician/surgeon. 
         [0051]    In all of the previously described embodiments, it is important that the bandstop filter be as close to the electrode-to-tissue interface as possible. The reason for this is that lead wire systems act as transmission lines in that they have series inductance and resistance and also stray capacitance from lead to lead. This tends to cause them to decouple into various loops at MRI pulsed frequencies. It is for this reason, for example in a cardiac pacemaker, that placing a bandstop filter at the housing of the cardiac pacemaker will not provide adequate cooling at the end of, for example, a 52 cm lead wire whose electrode tip is inside, for example, the right ventricle. The impedance of the lead wire would tend to decouple the bandstop filter by presenting a very high impedance at the MRI RF pulsed frequencies. Accordingly, it is a feature of the present invention that the bandstop filter be placed in relatively close proximity to the delivery electrodes as illustrated in the accompanying drawings. 
         [0052]    This principle varies with the RF pulsed frequency of the MRI machine. For example, a 0.5 Tesla machine has an RF pulsed frequency of 21 MHz. In this case, the wavelength is long enough where the bandstop filter could be a considerable distance away from the delivery electrode and still be quite effective. However, when one gets up around 3 Tesla with an RF pulsed frequency of 128 MHz, then the bandstop filter must be much closer to the delivery electrode. This is because the impedance along the series lead wire tend to increase at higher frequencies. In the present invention, in all cases, the bandstop filter is no more than 15 cm away from the delivery electrode. This will provide effective reduction in current flow at the MRI pulsed frequency thereby providing effective cooling at the distal electrode tips. 
         [0053]    Although several embodiments of the invention have been described in detail for purposes of illustration, various modifications of each 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.