Abstract:
Implantable medical leads and systems utilize reflection points within the lead to control radio frequency current that has been induced onto one or more filars. The radio frequency current may be controlled by the reflection points to block at least some of the radio frequency current from reaching an electrode of the lead and to dissipate at least some of the radio frequency current as heat on the filar. Controlling the radio frequency current thereby reduces the amount that is dissipated into bodily tissue through one or more electrodes of the lead and reduces the likelihood of tissue damage. The reflection points may be created by physical changes such as to material or size in the filar and/or in insulation layers that may be present such as an inner jacket about the filar and an outer jacket formed by the body of the lead.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments are related to implantable medical leads and systems including implantable medical leads that may carry induced radio frequency energy. More particularly, embodiments are related to implantable medical leads and related systems that include reflection points to control the induced radio frequency energy. 
       BACKGROUND 
       [0002]    Implantable medical systems include an implantable medical device connected to an implantable medical lead. The implantable medical device is used to produce stimulation signals for delivery to tissue of a patient and/or to sense physiological signals from the tissue of the patient. The implantable medical lead includes electrical contacts on a proximal end that are connected to electrical connectors within the medical device. Electrodes are present on a distal end of the implantable medical lead to contact the tissue at the stimulation site. Filars are present within the lead to carry electrical signals between the contacts at the proximal end and the electrodes at the distal end. 
         [0003]    The implantable medical leads can present an issue for a patient who may need to undergo a magnetic resonance imaging (MRI) scan. An MRI scan exposes the patient to radio frequency (RE) electromagnetic energy. This RF energy may be collected by the filars in the form of induced RF electrical current during the MRI scan. This RE electrical current may be delivered to the tissue of the patient via the electrodes at the distal end. 
         [0004]    The RF electrical current induced onto the filars presents a serious condition. The electrode is a relatively small amount of surface area such that the RF electrical current from a given electrode is dissipated into a relatively small amount of tissue which may heat the tissue by an excessive amount that causes tissue damage. Furthermore, the electrode may be located adjacent to sensitive tissue such as within the brain or spine where tissue damage from the excessive heating by the induced RF current may have severe consequences. 
       SUMMARY 
       [0005]    Embodiments address issues such as these and others by providing implantable medical leads and implantable medical systems where the implantable medical leads include reflection points that control the radio frequency energy induced onto the filars. The reflection points may be present on the filar(s) or on the insulation layer(s) such as on an inner jacket formed about individual filars or on an outer jacket of the lead. The reflection points may be created in various ways such as changing physical dimensions like the diameter at a given point or by changing the materials that are present at a given point. 
         [0006]    Embodiments provide implantable medical systems and leads. The implantable medical lead includes an outer jacket and a filar located within the outer jacket. The filar includes physical changes that establish multiple radio frequency reflection points located along the length of the filar. The implantable medical lead further includes a proximal contact and a distal electrode, with the filar interconnecting the proximal contact to the distal electrode. 
         [0007]    Embodiments provide other implantable medical systems and leads. The implantable medical lead includes an outer jacket and a filar surrounded by an inner jacket. The filar and inner jacket are located within the outer jacket, and the inner jacket includes physical changes that establish multiple radio frequency reflection points located along the length of the filar. The implantable medical lead further includes a proximal contact and a distal electrode, with the filar interconnecting the proximal contact to the distal electrode. 
         [0008]    Embodiments provide additional implantable medical systems and leads. The implantable medical lead includes an outer jacket and a filar being located within the outer jacket such that the outer jacket includes physical changes that establish multiple radio frequency reflection points located along the length of the filar at non-standard intervals of repetition. The implantable medical lead further includes a proximal contact and a distal electrode, with the filar interconnecting the proximal contact to the distal electrode. 
         [0009]    Embodiments provide other implantable medical systems and leads. The implantable medical lead includes an outer jacket and a filar being located within the outer jacket. Discrete circuit elements are electrically coupled to the filar and establish multiple radio frequency reflection points located along the length of the filar at nonstandard intervals of repetition. The implantable medical lead includes a proximal contact and a distal electrode, with the filar interconnecting the proximal contact to the distal electrode. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a patient with an implantable medical system in the presence of RE electromagnetic energy. 
           [0011]      FIG. 2  shows an example of a longitudinal cross-section of an implantable medical lead having reflection points provided by changes in filar diameter, filar material, inner jacket diameter, inner jacket material, outer jacket diameter, and outer jacket material. 
           [0012]      FIG. 3  shows a first lateral cross-section of the implantable medical lead of  FIG. 2  at a reference point. 
           [0013]      FIG. 4  shows a second lateral cross-section of the implantable medical lead of  FIG. 2  at a first reflection point, 
           [0014]      FIG. 5  shows a third lateral cross-section of the implantable medical lead of  FIG. 2  at a second reflection point. 
           [0015]      FIG. 6  shows a fourth lateral cross-section of the implantable medical lead of  FIG. 2  at a third reflection point. 
           [0016]      FIG. 7  shows a fifth lateral cross-section of the implantable medical lead of  FIG. 2  at a fourth reflection point. 
           [0017]      FIG. 8  shows a sixth lateral cross-section of the implantable medical lead of  FIG. 2  at a fifth reflection point. 
           [0018]      FIG. 9  shows a seventh lateral cross-section of the implantable medical lead of  FIG. 2  at a sixth reflection point. 
           [0019]      FIG. 10  shows an eighth lateral cross-section of the implantable medical lead of  FIG. 2  at a seventh reflection point. 
           [0020]      FIG. 11  shows a ninth lateral cross-section of the implantable medical lead of  FIG. 2  at an eighth reflection point. 
           [0021]      FIG. 12  shows a tenth lateral cross-section of the implantable medical lead of  FIG. 2  at a ninth reflection point. 
           [0022]      FIG. 13  shows an eleventh lateral cross-section of the implantable medical lead of  FIG. 2  at a tenth reflection point. 
           [0023]      FIG. 14  shows a longitudinal distribution of reflection points in one example of a lead. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Embodiments provide implantable medical leads and systems that include reflection points along the leads to control RF energy that is induced as RE current onto mars of the leads. The reflection points may be present on the filar or on other elements of the lead such as an inner jacket around the individual filar or an outer jacket that may form the outer body of the lead. The reflection points may be created by physical changes such as a change in diameter of the filar or insulator or a change in the materials that are used for the filar or insulator that produce a change in a characteristic impedance of the lead. 
         [0025]      FIG. 1  shows an environment where an implantable medical system  100  has been implanted within a patient  108 . The implantable medical system  100  includes an implantable medical device (IMD)  102  and an implantable medical lead  104 . The implantable medical lead  104  is connected to the IMD  102  at a proximal end, and the lead  104  extends to a stimulation site where electrodes  106  on the distal end are present to electrically interface with the tissue of the patient  108 . 
         [0026]    The patient  108  is being exposed to RF electromagnetic energy  110 . This RE energy  110  encounters the implantable medical system  100  and may induce RF current onto the lead  106 . However, the lead  106  may include reflection points positioned at various locations to reflect the RF current away from the electrodes and to cause at least some of the RF current to dissipate as heat over the filar(s) present within the lead  104  rather than exiting through the electrodes  106 . 
         [0027]      FIG. 2  shows a longitudinal cross-section of an example of a lead  200  with reflection points. This lead  200  is shown with a single contact  204 , single filar  208 , and single electrode  206 . It will be appreciated that any number of contacts, filars, and electrodes may be present and may benefit from the reflection points. In this particular example, the lead  200  includes a body  202  which may be established by the outer jacket, or the outer jacket may be an outer layer adhered to the body  202 . The body  202  defines a lumen  210  that may be present to receive a stylet that guides the lead  200  during implantation and is removed thereafter. 
         [0028]    The body  202  forming the outer jacket may be made of various materials. Examples include elasthane, silicone, other polymers and the like. Likewise, the filar  208  may be made of various materials such as MP35N®, alloy, platinum, silver cored MP35N® alloy, and the like. 
         [0029]    Additionally, the lead  200  may include an inner jacket that is not shown in  FIG. 2  but adheres to the outer surface of the filar  204  and isolates the filar  204  from the body  202 . The inner jacket may also be made of various materials. Examples include ethylene tetrafluoroethylene (ETFE), other polymers, and the like. 
         [0030]    The physical parameters including the dimensions and the types of material used for each of the components within the lead  200  such as the outer jacket of the body  202 , the inner jacket, and the filar  206  contribute to the characteristic impedance of the filar  202 . To create a reflection point, the characteristic impedance is altered at a given location where the reflection point is desired. To alter the characteristic impedance and thereby create the reflection point, a physical change is present in either a dimension or a material for the particular component. Examples of such physical changes are present in this example of the lead  200 , where cross-section  3 - 3  shown in  FIG. 3  is a reference point showing the normal construction where a reflection point is not present. Cross-sections  4 - 4  through  13 - 13  show examples of some of the other reflection points that are present in this example of the lead  200 . 
         [0031]      FIG. 3  shows a lateral cross-section taken through  3 - 3  of  FIG. 2 . Here, the elements of the lead  200  including the body  202  forming the outer jacket, the filar  212 , and the inner jacket  208  on the filar  212  are normal in that this represents the configuration of the areas of the lead where no reflection point is present. In the example of  FIG. 2 , this cross-section taken through  3 - 3  is in a normal portion near the proximal end of the lead  200 . It will be appreciated that normal portions such as this that may extend for significant portions of the lead may appear at any point along the lead from proximal tip to distal tip. 
         [0032]      FIG. 4  shows a lateral cross-section taken through  4 - 4  of  FIG. 2  where a reflection point is present. Here, the body  202  forming the outer jacket includes additional material  202 ′ creating a larger diameter over a particular length of the lead  200 . The additional material  202 ′ may be the same or different material than the body  202 . The other elements including the filar  212  and inner jacket  214  have not changed. The change in diameter results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that rather than increasing the diameter, the body  202 ′ forming the outer jacket may have a reduced diameter to create a reflection point. 
         [0033]    An alternative to the layer  202 ′ being an additional jacket material,  202  may represent a floating electrode. In this case, the floating electrode may be attached to the body  202  in the same manner as the electrode  206  that is used for stimulation, but the floating electrode is not connected to a filar  212 . The floating electrode presents a physical change to the outer jacket that creates a change in the characteristic impedance of the filar  212  such that the presence of the floating electrode creates a reflection point. In some examples, the floating electrode present at any given reflection point may be capacitively coupled to one or more of the filars within the lead  200 . 
         [0034]      FIG. 5  shows a lateral cross-section taken through  5 - 5  of  FIG. 2  where a reflection point is present. Here, the body  202  forming the outer jacket includes one or more types of dopant materials. In this particular example, two dopant materials  216  and  218  are present and create a change in the conductance of the outer jacket over a particular length of the lead  200 . Examples of dopant materials include metals such as titanium, stainless steel, platinum, and the like. The other elements including the filar  212  and inner jacket  214  have not changed. The change in material results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. 
         [0035]      FIG. 6  shows a lateral cross-section taken through  6 - 6  of  FIG. 2  where a reflection point is present. Here, the body  202  forming the outer jacket is normal in size and material. However, the inner jacket  214  includes additional material  214 ′ creating a larger diameter over a particular length of the lead  200 . The additional material  214 ′ may be the same or different material than the material of the inner jacket  214 . The other elements including the filar  212 . and body  202  forming the outer jacket have not changed. The change in diameter results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that rather than increasing the diameter, the inner jacket  214 ′ may have a reduced diameter to create a reflection point. 
         [0036]      FIG. 7  shows a lateral cross-section taken through  7 - 7  of  FIG. 2  where a reflection point is present. Here, the body  202  forming the outer jacket is normal in size and material. However, the inner jacket  214 ′ includes at least one type of dopant material. In this particular example, two dopant materials  220  and  222  are present and create a change in the conductance of the outer jacket over a particular length of the lead  200 . Examples of these dopant materials also include titanium, stainless steel, platinum, and the like. The other elements including the filar  212  and body  202  forming the outer jacket have not changed. The change in materials results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. 
         [0037]      FIG. 8  shows a lateral cross-section taken through  8 - 8  of  FIG. 2  where a reflection point is present. Here, both the body  202  forming the outer jacket and the inner jacket  214  are normal in diameter and material. However, the filar  212 ′ has a reduced diameter, such as by creating a crimp and the inner jacket  214  may fill in the area of reduced diameter. 
         [0038]    The change in diameter of the filar  212 ′ at the area of this cross-section results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that rather than reducing the diameter, the filar  212 ′ may have an increased diameter to create a reflection point such as by welding additional material onto the filar  212 ′. 
         [0039]      FIG. 9  shows a lateral cross-section taken through  9 - 9  of  FIG. 2  where a reflection point is present. Here, both the body  202  forming the outer jacket and the inner jacket  214  are normal in diameter and material. However, the filar  212  has a change in material by the addition of a material  212 ″ adjacent to the material  212 , such as by welding a different material  212 ″ onto the existing filar  212 . It should be noted that both materials define the surface, which may provide a more effective reflection point than using an approach with a core considering that the RE induced current is primarily on the surface due to the skin effect. If the filar diameter is to be maintained as shown in  FIG. 9 , the existing filar material may have a reduced filar diameter while the new filar material increases the filar diameter back to the normal level. The change in material of the filar  212 ″ at the area of this cross-section results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. 
         [0040]      FIGS. 4-9  have shown examples where a single parameter such as size or material has changed to produce a reflection point.  FIGS. 10-13  show examples where multiple parameters have changed to produce for each of the reflection points. 
         [0041]      FIG. 10  shows a lateral cross-section taken through  10 - 10  of  FIG. 2  where a reflection point is present. Here, the inner jacket  214  is normal in diameter and material. However, the filar  212 ″ has a change in material while the diameter may be the same or different, such as by welding a different material onto the existing filar  212 . Additionally, the body  202  forming the outer jacket has additional material  202 ′ that increases the diameter of the body  202 . The combination of the change in material of the filar  212 ″ as well as the diameter of the outer jacket  202 ′ at the area of this cross-section results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that the multiple parameters relating to the filar  212  and the body  202  forming the outer jacket may have additionally or alternatively included other changes such as a reduced diameter of the body  202 ′ forming the outer jacket, a change to the diameter of the filar  212 , and/or a change to the material of the body  202 ,  202 ′ forming the outer jacket. 
         [0042]      FIG. 11  shows a lateral cross-section taken through  11 - 11  of  FIG. 2  where a reflection point is present. Here, the body  202  forming the outer jacket includes one or more types of dopant materials. In this particular example, one dopant material  216  is present. Additionally, the inner jacket  214  includes additional material  214 ′ increasing the diameter of the inner jacket. The combination of the change in material of the body  202  forming the outer jacket and the change in diameter of the inner jacket  214  results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that the multiple parameters relating to the body  202  forming the outer jacket and the inner jacket may have additionally or alternatively included other changes such as a change in diameter of the body  202  forming the outer jacket, a reduced diameter of the inner jacket  214 , and/or a change in the material of the inner jacket  214 . 
         [0043]      FIG. 12  shows a lateral cross-section taken through  12 - 12  of  FIG. 2  where a reflection point is present, Here, the inner jacket  214 ″ includes one or more dopant materials. in this particular example, one dopant material  220  is present. Additionally, the filar  212 ′ has a reduced diameter. The combination of the change in material of the inner jacket  214 ″ and the change in diameter of the filar  212 ′ results in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that the multiple parameters relating to the inner jacket and filar may have additionally or alternatively included other changes such as a change in diameter of the inner jacket  214 ″, a change in diameter of the inner jacket  214 ″, and/or a change in the material of the inner jacket  214 . 
         [0044]      FIG. 13  shows a lateral cross-section taken through  13 - 13  of  FIG. 2  where a reflection point is present. Here, the inner jacket  214 ″ includes one or more dopant materials. In this particular example, one dopant material  222  is present. The inner jacket  214 ″ also includes additional material  214 ′ increasing the diameter of the inner jacket. Additionally, the filar  212 ′ has a reduced diameter while also including a different material  212 ″ welded onto the existing reduced diameter portion  212 ′. The body  202  firming the outer jacket includes additional material  202 ″ that increases the diameter of the body  202  forming the outer jacket. Furthermore, the additional material  202 ″ is doped with one or more dopants, in this case a single dopant type  218 . The combination of the change in materials and sizes of the inner jacket  214 ′,  214 ″, the filar  212 ′,  212 ″, and the body  202 ,  202 ″ forming the outer jacket result in a change to the characteristic impedance of the filar  202  thereby producing a reflection point. It will be appreciated that the multiple parameters relating to the body forming the outer jacket, the inner jacket and the filar may have additionally or alternatively included other changes as well. 
         [0045]      FIG. 14  shows an example of longitudinal distribution  300  of reflection points  302  along the length of a lead. It can be seen that some reflection points are created by a change only to a single element, the outer jacket ( 1 ), the inner jacket ( 2 ), or the filar ( 3 ). It can be seen that some reflection points are created by a change to two elements, the outer jacket ( 1 ) and the inner jacket ( 2 ), the outer jacket ( 1 ) and the filar ( 3 ), or the inner jacket ( 2 ) and the filar ( 3 ). Additionally, it can be seen that some reflection points are created by a change to all three elements, the outer jacket (II), the inner jacket ( 2 ), and the filar ( 3 ), 
         [0046]    In  FIG. 14  it can further be seen that in this example there is a nonstandard interval of repetition. In other words, the spacing from one reflection point to the next reflection point varies. It can also be seen in this example that there is a nonstandard interval of repetition from a reflection point involving a change to a particular element to the next reflection point involving the same element. This nonstandard interval of repetition may assist in reflecting the RF current away from the electrode and in dissipating the RF current as heat in the filar(s), 
         [0047]    While the examples of  FIGS. 2-13  show a single filar and a single inner jacket, multiple filars may be present and each filar may have a dedicated inner jacket. The physical change to the filar and/or the inner jacket may vary from filar to filar. For instance, a given reflection point involving the inner jacket ( 2 ) and/or the filar ( 3 ) as shown in  FIG. 14  may pertain to one filar, multiple filars, or all filars present within a lead. For instance, some of those reflection points of  FIG. 14  may pertain to one filar while others may pertain to multiple filars. 
         [0048]    The reflection points discussed herein may also be created by the presence of discrete circuit elements such as resistors, capacitors, and/or inductors that are electrically coupled to the filar(s). Thus, any or all of the reflection points  302  illustrated in  FIG. 14  may be discrete circuit elements with nonstandard intervals of repetition rather than physical changes to the filar, inner jacket, or outer jacket. For instance, there may be series resistors in-line along the filars ( 1 ), series inductors in-line along the filars ( 2 ), and/or capacitors that are parallel from the filar to floating electrodes that may have a large surface area in comparison to the stimulation electrodes and/or may be located in less critical tissue ( 3 ) that are present to vary the characteristic impedance while not adversely affecting the delivery of stimulation signals or sensed signals through the filars. 
         [0049]    While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.