Abstract:
Systems and methods for shielding implantable leads from magnetic fields during medical procedures such as magnetic resonance imaging (MRI) are described. In various embodiments, the lead includes an inner conductor that is helically shaped and radially surrounded, at least in part, by one or more outer shielding conductors. The pitch of the inner conductor, and in some cases also the outer conductor, can be varied (e.g., continuously or at certain points) along the length of the lead, forming a plurality of high impedance points along the length of the lead which result in the dissipation of electromagnetic energy at an interrogation frequency of a magnetic resonance imaging device (e.g., 64 MHz, 128 MHz, or the like). In some embodiments, the variance in the pitch of the inner conductor follows a sinusoidal function, a modified square-wave function, or some other repeating pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/992,897, filed on Dec. 6, 2007, which is hereby incorporated by reference in its entirety for all purposes. 
     TECHNICAL FIELD 
     Various embodiments of the present invention generally relate to medical devices and the simultaneous delivery of diagnostic and therapeutic treatments. More specifically, embodiments of present invention relate to medical devices and methods of shielding implantable leads from magnetic fields during medical procedures such as magnetic resonance imaging (MRI). 
     BACKGROUND 
     Magnetic resonance imaging (MRI) is a non-invasive imaging method that utilizes nuclear magnetic resonance techniques to render images within a patient&#39;s body. Typically, MRI systems employ the use of a magnetic coil having a magnetic field strength of between about 0.2 to 3 Teslas. During the procedure, the body tissue is briefly exposed to RF pulses of electromagnetic energy in a plane perpendicular to the magnetic field. The resultant electromagnetic energy from these pulses can be used to image the body tissue by measuring the relaxation properties of the excited atomic nuclei in the tissue. 
     During imaging, the electromagnetic radiation produced by the MRI system may be picked up by implantable device leads used in implantable medical devices such as pacemakers or cardiac defibrillators. This energy may be transferred through the lead to the electrode in contact with the tissue, which may lead to elevated temperatures at the point of contact. The degree of tissue heating is typically related to factors such as the length of the lead, the conductivity or impedance of the lead, and the surface area of the lead electrodes. Exposure to a magnetic field may also induce an undesired voltage in the lead. 
     SUMMARY 
     Systems and methods are described for shielding implantable leads from magnetic fields during medical procedures such as magnetic resonance imaging (MRI). Some embodiments generally relate to a medical device, comprising a lead having a proximal section, a distal section, and a length. In some cases, the proximal section can be coupled to a pulse generator (e.g., a pacemaker, a cardiac defibrillator, monitoring devices such as sensors, and/or the like), the distal section can include an electrode and can be implanted within a heart of a patient. In this configuration, for example, the lead is configured to convey electrical signals between the heart and the pulse generator through an inner conductor. 
     The lead includes, in various embodiments, an inner conductor that is helically shaped. In some embodiments, the inner conductor may be radially surrounded, at least in part, by one or more outer shielding conductors. The pitch of the inner conductor can continuously vary along the length of the lead forming a plurality of high impedance points along the length of the lead to inhibit the absorbed electromagnetic energy from traveling along the length of the lead at an interrogation frequency of a magnetic resonance imaging device (e.g., 64 MHz, 128 MHz, or the like). For example, in one or more embodiments, the variance in the pitch of the inner conductor follows a sinusoidal function, a modified square-wave function, a triangle function, a saw-tooth function, a quadratic function, or some other repeating pattern. In some embodiments, the one or more outer shielding conductors can have a pitch that varies continuously along the length of the lead (e.g., via a sinusoidal function, a modified square-wave function, or some other repeating pattern), or alternatively has a pitch that changes at one or more points along the length of the lead. 
     In various embodiments, the lead can include a low impedance insulation near the proximal section and the distal section of the lead. During an MRI procedure, the low impedance insulation can cause the electromagnetic energy produced by the MRI device to be dissipated along the body tissue adjacent to the length of the lead to reduce energy transfer of the electromagnetic energy to the electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an illustrative medical device having a lead implanted within the body of a patient; 
         FIG. 2  is a schematic view showing a simplified equivalence circuit for the lead of  FIG. 1 ; 
         FIG. 3  is a longitudinal cross-sectional view showing the distal portion of an uncoiled lead in accordance with an illustrative embodiment; 
         FIG. 4  is a transverse cross-sectional view of the lead of  FIG. 3 ; 
         FIG. 5  is a view showing the lead of  FIGS. 3-4  having a helix configuration; 
         FIG. 6  is a schematic view showing an equivalence circuit for the lead of  FIG. 5 ; 
         FIG. 7  is a graph showing the impedance magnitude of the lead versus RF frequency in an MRI environment; 
         FIG. 8A  is a view showing a lead having a variable coil conductor pitch; 
         FIG. 8B  shows one illustrative embodiment where high impedance frequency dependent points are created along the length of the lead; 
         FIG. 9  shows two graphs of the impedance of a variable coil conductor pitch verses the length of the lead at two different MRI frequencies; 
         FIG. 10  is a view showing a variable pitch lead in accordance with an illustrative embodiment; 
         FIG. 11  is a graph showing the pitch of a lead following a sinusoidal function; and 
         FIG. 12  is a graph showing the pitch of a lead following a modified square-wave function. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic view of an illustrative medical device  12  equipped with a lead implanted within the body of a patient. In the illustrative embodiment depicted, the medical device  12  comprises a pulse generator implanted within the body. The medical device can be coupled to a lead  14  deployed in the patient&#39;s heart  16 . The heart  16  includes a right atrium  18 , a right ventricle  20 , a left atrium  22 , and a left ventricle  24 . The pulse generator  12  can be implanted subcutaneously within the body, typically at a location such as in the patient&#39;s chest or abdomen, although other implantation locations are possible. 
     A proximal portion  26  of the lead  14  can be coupled to or formed integrally with the pulse generator  12 . A distal portion  28  of the lead  14 , in turn, can be implanted within a desired location within the heart  16  such as the right ventricle  20 , as shown. Although the illustrative embodiment depicts only a single lead  14  inserted into the patient&#39;s heart  16 , it should be understood, however, that multiple leads can be utilized so as to electrically stimulate other areas of the heart  16 . In some embodiments, for example, the distal portion of a second lead (not shown) may be implanted in the right atrium  18 . In addition, or in lieu, another lead may be implanted at the left side of the heart  16  (e.g., in the coronary veins) to stimulate the left side of the heart  16 . Other types of leads such as epicardial leads may also be utilized in addition to, or in lieu of, the lead  14  depicted in  FIG. 1 . 
     During operation, the lead  14  can be configured to convey electrical signals between the heart  16  and the pulse generator  12 . For example, in those embodiments where the pulse generator  12  is a pacemaker, the lead  14  can be utilized to deliver electrical therapeutic stimulus for pacing the heart  16 . In those embodiments where the pulse generator  12  is an implantable cardiac defibrillator, the lead  14  can be utilized to deliver electric shocks to the heart  16  in response to an event such as a heart attack. In some embodiments, the pulse generator  12  includes both pacing and defibrillation capabilities. 
       FIG. 2  is a schematic view showing a simplified equivalence circuit  30  for the lead  14  representing the RF energy picked up at the lead  14  resulting from electromagnetic energy produced by an MRI scanner. During imaging, the length L of the lead  14  functions similar to an antenna, receiving the RF electromagnetic energy that is transmitted into the body from the MRI scanner. Voltage source  34  in  FIG. 2  represents the resultant voltage received by (or picked up by) the lead  14  from the RF electromagnetic energy produced by the MRI scanner. The RF energy picked-up by the lead  14  may result, for example, from the magnetic field produced during magnetic resonance imaging or from RF interference from another device location inside or outside of the patient&#39;s body. 
     The Zl parameter  32  in the equivalence circuit  30  represents the equivalent impedance exhibited by the lead  14  at the RF frequency in the MRI scanner. The impedance value Zl  32  may represent, for example, the parallel inductance and capacitance components exhibited by the lead  14  at an RF frequency of 64 MHz for a 1.5 T MRI scanner or 128 MHZ for a 3.0 T MRI scanner. The magnetic field strength of an MRI scanner typically ranges from 0.2-3 Tesla. Accordingly, the range of the RF frequency would be between 8.53 to 128 MHz. However, there are other MRI scanners that operate at other magnetic field strengths, such as, but not limited to, 5, 7, 9, or even 12 Tesla. The MRI frequency would be equal to approximately 42.58 MHz per Tesla. 
     Zb  38  may represent the impedance of tissue at the point of lead contact. Zc  36 , in turn, may represent the capacitive coupling of lead to surrounding tissue along the length of the lead, which is a path for the high frequency current (energy) to leak into the surrounding tissue at the RF frequency of the MRI scanner. Minimizing the (absorbed) energy (represented by source Vi  34 ) reduces the energy that will get transferred to the tissue at the point of lead contact to tissue. 
     The circuit representation in  FIG. 2  and the associated equation described below are for the purpose of illustrating the concept of lead heating in an MRI environment. At frequencies where the wavelength of voltage (current) is close to the size of circuit, a simple lumped sum system (like the circuit illustrated in  FIG. 2 ) may not accurately model the behavior of the lead in the MRI environment. Consequently, in those circumstances where a lumped system does not accurately model the behavior, a distributed system should be used along with Maxwell&#39;s equation for a proper mathematical description of the circuit. A distributed model is a model of a system where the circuit components (e.g., resistors, inductors, capacitors, etc) are distributed across the circuit geometries. In distributed circuits, the voltage at a node (in this case the lead) is not constant and is represented by a wave. In some cases, the approximating distributed model can be created by cascading lumped element equivalent components of small sections of the circuit. 
     As can be further seen in  FIG. 2 , the lead  14  has some amount of leakage  36  into the surrounding tissue at the RF frequency. As further indicated by  38 , there is also an impedance at the point of contact (e.g., within heart  16 ) of the lead electrode to the surrounding body tissue. The resulting voltage Vb delivered to the body tissue is related by the following formula:
 
 Vb=Vi Zbe /( Zbe+Zl ), where  Zbe=Zb  in parallel with  Zc.  
 
     The temperature at the tip of the lead  14  where contact is typically made in the surrounding tissue is related to the power dissipated at  38  (i.e., at “Zb”), which, in turn, is related to the square of Vb. To minimize temperature rises resulting from the power dissipated at  38 , it is thus desirable to minimize Vi ( 34 ) and Zc ( 38 ) while also maximizing Zl ( 32 ). In some embodiments, the impedance Zl ( 32 ) of the lead  14  can be increased at the RF frequency of the MRI scanner, which aids in reducing the power dissipating into the surrounding body tissue at  38 . 
     In some embodiments, the impedance of the lead  14  can be increased by adding impedance to the lead and/or by a suitable construction technique. For example, the impedance of the lead  14  can be increased by increasing the diameter of the conductor coil, although other configurations are possible. For a helical lead construction, if the resonance frequency of the lead is above the RF frequency of the MRI, then impedance exhibited by helical coil acts as an inductor. For an inductor, increasing the cross section of the coil area increases the inductance and, as a result, increases the impedance of the lead  14 . 
     In certain embodiments, the lead  14  can be detuned so as to prevent resonance within the lead  14 . For the illustrative embodiment shown in  FIG. 1 , for example, the lead  14  functions as an antenna having a resonance frequency at length L=integer×λ/2. In some embodiments, the length of the lead  14  can be chosen so as to avoid resonance within the lead  14 . In various embodiments, the length of the lead  14  can be between 40 cm and 100 cm. 
     However, leads can be designed in such a way that is not uniform. Variation in construction of a lead can result in variations of the characteristic impedance of the lead along the length. For example, in some embodiments, the variation could be the pitch of the coil used in lead construction. One benefit of having a variable pitch coil design in accordance with some embodiments is that the variable pitch coil provides robustness in providing high impedance points along the length of the lead with respect to lead stretching and compression. Stretching and compression of the lead can be caused, for example, by manufacturing tolerances, handling, usage (e.g., pulling, bending, etc), as well as other factors. 
     Based on the theory of transmission lines, mismatch in the characteristic impedance can result in reflections of electromagnetic waves. The incident and reflected waves (from the mismatch impedance points) adds vectorially. Consequently, some embodiments adjust the pattern repetition distance to be less than a quarter wavelength if the pattern of lead construction is repeated. At quarter wave length segments, the incident and reflected waves are 180 degrees out of phase and therefore subtract from each other reducing the magnitude of the electromagnetic waves. According to various embodiments, the quarter wave length can be in the range of 10 cm to 25 cm. 
     In some embodiments, shielding can be added to the lead  14  to reduce the amount of electromagnetic energy picked-up from the lead  14 . For example, the energy picked up from the shielding can be coupled to the patient&#39;s body along the length L of the lead  14 , preventing it from coupling to the lead tip. 
       FIG. 3  is a schematic view showing the distal portion  28  of the uncoiled lead  14  of  FIG. 1  in greater detail.  FIG. 3  may represent, for example, a longitudinal cross-sectional view of the lead  14  prior to being subjected to a winding step to impart the helical shape to the lead  14 , as discussed further herein. In some embodiments, and as shown in  FIG. 3 , the lead  14  includes an inner conductor  40  having a proximal end (not shown) and a distal end. The distal end of the conductor can be configured to contact the surrounding tissue (e.g., through electrode  44 ) to provide therapeutic energy to a desired treatment site such as the patient&#39;s heart. 
     In some embodiments, and as further shown in conjunction with a transverse cross-sectional view of the lead in  FIG. 4 , the lead  14  further includes an outer shield  42  that extends along all or a portion of the length of the lead  14 , and is configured to at least in part radially surround the inner conductor  40  to prevent RF electromagnetic waves generated by an MRI scanner from interfering with the electrical signals delivered through the lead  14 . Although only a single outer shielding conductor  42  is shown in the embodiment of  FIGS. 3-4 , multiple outer shields may be provided in other embodiments. Similarly, other embodiments include multiple leads with one or more outer shielding conductors  42 . 
     The lead  14  further includes a number of layers of insulating material disposed about the outer shielding conductor  42  and the inner conductor  40 . A first layer  46  of insulating material, for example, may insulate the inner conductor  40  from the outer shielding conductor  42 . A second layer  48  of insulating material, in turn, may insulate the outer shielding conductor  42  from the body tissue surrounding the lead  14 . Some embodiments use thinner insulation with the highest dielectric material that can be used in implants. Some embodiments can use a low impedance insulation made from a material with a relative dielectric constant of five to ten times the dielectric constant of the inner conductor. 
     The shield by itself typically exhibits high impedance along the length of the lead compared to a low impedance along a cross-section of the lead. This high impedance can be achieved, for example, by inductance from coiling the lead wire.  FIG. 5  is a view showing the illustrative uncoiled lead  14  of  FIGS. 3-4  having a helix configuration along its length. If the turns of the lead  14  as shown in  FIG. 5 , and consequently, turns of the outer shielding conductor  42 , touch the adjacent ones, a short circuit is created and the effect of inductance is eliminated. In some embodiments, second insulating layer  48  can prevent the turns of the coil shield from coming in contact with each other when uncoiled lead  14  of  FIGS. 3-4  is placed in a helix configuration as shown in  FIG. 5 . In some embodiments, the highest pitch for a given diameter of the lead that can produce a coil that has an acceptable mechanical characteristic that is desirable. 
     Another reason for having insulation  48  in some embodiments is to control the “turn by turn capacitance” of the lead. If fluid, such as blood, penetrates inside the lead, an increase in the capacitance between coil winding turns can result. The relative dielectric constant of water or blood is about 80 compared to 1 for air. Since capacitance is proportional to relative dielectric constant, this results in a significant shift in resonance frequency. 
     For example assume that the resonance frequency of the lead is about 100 MHz. In this case, the inductance of the lead would be dominant since the frequency of the RF waveform is 64 MHz for 1.5 T MRI. If blood penetrates between winding turns, a shift in the resonance frequency of the lead from about 100 MHz to about 11 MHz would result. This resonance frequency is now lower than the operating frequency of the MRI (64 MHz), in which case the turn by turn capacitance is dominant. In this case, RF energy instead of traveling through inductance created by coil windings, travels through the turn by turn capacitance and lead impedance is reduced. Note that the further the operating frequency is from the resonance frequency, the lower the impedance of the lead (see, e.g.,  FIG. 7 ). 
     In some embodiments, lead  14  can have a low impedance path to surrounding tissue at or near the electrode  44 , but a high impedance (e.g., trace impedance) along the length of the lead. For example, in the embodiments shown in  FIG. 3 , a lower impedance at or near the electrode  44  can be created by terminating the outer conductor  42  and/or any insulating material before the electrode  44 . More specifically,  FIG. 3  shows the wire integrated with the shielding. 
     According to some embodiments, a thin insulation can be used to provide a high capacitance between the electrode  44  and surrounding tissue. The thin insulation can also provide a low impedance path for MRI induced RF energy. However, the thin insulation generally does not effect the impedance for therapy delivery that typically contains energy with low frequency contents. In certain embodiments, the impedance of the inner conductor can be designed to be similar to the impedance of the one or more outer shielding conductors  42 . However, in other embodiments, the impedance of the inner conductor can be different from the impedance of the one or more outer shielding conductors  42 . 
     In various embodiments, the pitch of a helix is the width of one complete helix turn, measured along the helix axis. In accordance with various embodiments, the inner conductor  40  can have an average pitch between, and including, five mils (i.e., 5/1000 of inch) and fifty mils (i.e., 50/1000 of inch). In some embodiments, inner conductor  40  may have a maximum pitch of approximately five mils, while in other embodiments the maximum pitch may be more or less. The variation of the pitch in various embodiments can also be a function of wire diameter. For example, if the wire diameter is 3 mils, then the minimum pitch can be slightly larger than 3 mils. If the wire lead is insulated and turn by turn capacitance is not large, then the minimum pitch can be 3 mils in some embodiments. 
       FIG. 6  is a schematic view showing an equivalence circuit  50  for the coiled helix-shape lead  14  of  FIG. 5 . As shown in  FIG. 6 , the inductance of a helix lead in conjunction with the capacitance between its winding turns forms a resonance circuit  50  along the length of the lead. The RF electromagnetic energy received during an MRI procedure produces a voltage  52 . The lead  14  has an equivalent inductance and capacitance within the circuit  50 , as indicated generally by “Z_lead(f)”. The point of electrode contact to tissue has an equivalent impedance, as indicated by  54 . The impedance “Z_lead(f)” for the lead  14  increases in magnitude at the RF frequency of the MRI scanner. This can be seen, for example, in  FIG. 7 , which shows a dramatic increase in the impedance magnitude at resonance frequency “F 0 ”. 
     In certain embodiments, the pitch of the lead and/or lead shielding can be varied along its length to alter the impedance characteristics of the lead and/or shielding in a desired manner. The pitch can vary continuously or at fixed points. For example, the uncoiled lead  56  in  FIG. 8A  can be formed in a helix-like shape with a pitch that can vary to provide one or more high impedance frequency dependent points.  FIG. 8B  shows one illustrative embodiment where high impedance frequency dependent points  58 ,  60 ,  62  are created along the length of the lead  56 , reducing the RF pickup energy in addition to reducing the transmission of the RF pickup energy into the inner conductor. In some embodiments, an illustrative shielding  64  having a variable pitch can also be placed about the lead  56 , as further shown in  FIG. 8A . In other embodiments, however, the lead can include a shielding having a fixed pitch, shielding only along a portion of the lead, or a lead having no outer shielding at all. 
     In some embodiments, the lead pitch pattern is repeated several times along the length of the lead  56  such that the pitch pattern covers the lead length of less than ¼ of the wavelength of the highest frequency of interest. For example, for a lead  56  subjected to an RF MRI frequency of 64 MHz, the lead pitch pattern may be repeated approximately every 117 centimeters, which corresponds to a ¼ wavelength at this frequency. During an MRI scan, this detuning of the lead  56  prevents the lead  56  from approaching the antenna resonance length, thus minimizing the RF energy picked-up from the lead  56 . 
       FIG. 9  shows two graphs of the impedance of a variable-pitch lead versus the length of the lead at two different RF frequencies, F 1  and F 2 . As shown in  FIG. 9 , the locations along the length of the lead where a relatively high impedance occurs depends on the surrounding pick-up frequency F 1 , F 2  in the environment. Thus, for frequency F 1  shown in the top graph, the location along the length of the lead where an increase of impedance occurs is different than for frequency F 2  shown in the bottom graph. This can be seen by a shift in the impedance peaks in the top and bottom graphs shown in  FIG. 9 . 
       FIG. 10  is a view showing a variable pitch lead  66  in accordance with an illustrative embodiment. As shown in  FIG. 10 , the lead  66  may include a first section  68 , a second section  70 , and a third section  72 . The variance of pitch along each section  68 ,  70 ,  72  may follow a particular function. As can be further seen in  FIG. 11 , for example, the pitch of the lead  66  may follow a function that varies sinusoidally along the length of the lead  66 . The variance in pitch may follow other functions such as a square wave or other such function. In one alternative embodiment depicted in  FIG. 12 , for example, the pitch of lead  66  may follow a modified square-wave function. Other pitch configurations are possible, however. 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.