NEUROMODULATION LEAD FOR REDUCING INTERACTIONS WITH MRI

Abstract

A lead has at least a first and a second conductor. The first conductor and the second conductor each have an electrically conducting core that is surrounded by an electrical insulator. The electrical insulator of the first conductor if formed from a first material, and the electrical insulator of the second conductor is formed from a second material. The first material differs from the second material.

Claims

1. A lead, comprising: at least a first conductor and a second conductor, said first conductor and said second conductor each having an electrically conducting core and an electrical insulator surrounding said electrically conducting core, said electrical insulator of said first conductor formed of a first material, and said electrical insulator of said second conductor formed of a second material, wherein said first material differing from said second material.

2. The lead according to claim 1, wherein said first material and said second material are chosen such that said first and second conductors respectively comprise a resonance frequency that is different from a frequency used in an magnetic resonance imaging device.

3. The lead according to claim 1, wherein said first material and said second material are selected from the group consisting of: ceramics, polymers, ETFE, PFA, PTFE, polyimides, aluminum oxides, barium titanate, and titanium dioxide.

4. The lead according to claim 1, wherein said first and the second conductors each form a helical coil.

5. The lead according to claim 1, wherein said first and second conductors are two of a plurality of conductors, each of said conductors other than said first and second conductors has said electrically conducting core and a further electrical insulator surrounding said electrically conducting core, said further electrical insulator formed from a material selected from the group consisting of said first material, said second material, and a further material being different from said first and second materials.

6. The lead according to claim 5, further comprising a lead body insulator surrounding each of said conductors.

7. The lead according to claim 6, further comprising at least one further conductor disposed with said plurality of conductors, said at least one further conductor is formed as a non-insulated electrically conducting member, which is electrically insulated with respect to its surrounding by means of said electrical insulator and said further electrical insulator of adjacent ones of said conductors and/or by means of said lead body insulator.

8. The lead according to claim 1, wherein said first and second conductors are co-radial and/or co-axial helical coils.

9. The lead according to claim 5, wherein said plurality of conductors form an inner coil structure and a co-axial outer coil structure surrounding said inner coil structure, wherein each of said inner coil structure and said co-axial outer coil structure contain at least one of said conductors of said plurality of conductors.

10. The lead according to claim 1, wherein the lead is a medical lead.

11. The lead according to claim 1, wherein the lead is an electrode lead having a plurality of electrodes.

12. The lead according to claim 1, wherein the lead is adapted for spinal cord stimulation.

13. A method for producing a lead, which comprises the steps of: providing at least a first and a second conductor, the first conductor and the second conductor each having an electrically conducting core and an electrical insulator surrounding said electrically conducting core; forming the electrical insulator of the first conductor out of a first material; forming the electrical insulator of the second conductor out of a second material, wherein the first material differing from the second material; and surrounding the first and second conductors with an outer lead body insulator.

14. The method according to claim 13, wherein the first material and the second material are chosen such that the first and second conductors comprise a resonance frequency that is different from a given frequency used in a magnetic resonance imaging device.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0032] FIG. 1A is a diagrammatic, side view of a lead comprising co-radial conductors having electrical insulations, wherein each insulation can consist of one of at least two different materials;

[0033] FIG. 1B is an enlarged, partially cut-away sectional view of a section of the lead shown in FIG. 1A;

[0034] FIG. 1C is a sectional view of a conductor shown in FIG. 1B; and

[0035] FIG. 2 is a partially cut-away view showing the lead comprising an outer and a co-axial an inner coil structure, each structure comprising a plurality of co-radial conductors having electrical insulations, wherein each insulation can consist of one of at least two different materials.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Referring now to the figures of the drawings in detail and first, particularly to FIGS. 1A-1C thereof, there is shown a lead 1 according to the present invention which comprises at least a first and a second conductor 12, 13, wherein the first conductor 12 and the second conductor 13 each comprise an electrically conducting core 100 that is surrounded by an electrical insulator 21, 22, 23 (as shown in FIG. 1C), wherein the electrical insulator 22 of the first conductor 12 consists of a first material M1, and whereinanalogouslythe electrical insulator 23 of the second conductor 13 consists of a second material M2, wherein the first material M1 differs from the second material M2.

[0037] Furthermore, the lead 1 can comprises a lead body insulator 30 surrounding each individual conductor 12, 13 for providing further insulation and protection of the single conductors 12, 13.

[0038] Now, in order to avoid RF heating during MRI, conductors 11, 12, . . . , 18 of lead 1 need inductance L and capacitance C. One way to incorporate inductance and capacitance into the lead 1 is to coil the conductors 11, 12 along the length of the lead 1 as shown in FIG. 1B, which turns each conductor 11, 12 into a coil (or solenoid structure) with winding to winding parasitic capacitance. The LC combination creates a filter that blocks the high frequency MRI induced RF signals, while allowing the low frequency biological signals to pass unimpeded. Most SCS leads available today use straight conductors, which have very little inductance.

[0039] Particularly, the lead shown in FIG. 1A can be a co-radial SCS lead 1 with 8 electrodes 40 on a distal end of the lead 1, wherein each of the electrodes 40 is connected via one of the conductors 11, 18 to a connector contact 50 arranged on the proximal end of the lead 1. FIG. 1B shows the coiled conductors 11, 18. In FIG. 1B the three conductors 11, 12 and 14 are coated with an electrical insulator 21, 22, 24 consisting of a first material M1, while the five remaining conductors 13, 15, 16, 17, 18 are coated with an electrical insulator 23, 25, 26, 27, 28 consisting of a second material M2 that differs from the first material M1.

[0040] In addition to adding inductance and capacitance, resonance at MRI RF frequencies needs to be avoided in leads designed for MRI labeling. RF frequencies for MRI devices are e.g. 64 MHz for 1.5 T machines, and 128 MHz for 3T machines. If a conductor 11, 12 on the lead 1 shown in FIG. 1B resonates at MRI RF frequencies then the impedance of the lead 1 drops dramatically at these frequencies and very little energy is dissipated in the lead 1. The result is much more MRI induced current flowing through the lead 1 which results in much more RF heating at the electrodes 40. Avoiding resonance is particularly challenging for leads 1, particularly SCS leads 1, which typically have 8 different conductors 11, 18 connected to 8 different electrodes 40. Each conductor 11, 18 has its own resonance frequency which is distinct from the resonance frequency of the other conductors because each electrode 40 is typically located at a different distance along the lead 1, and hence the conductor 11, 18 connected to each electrode 40 has a different length. So, an 8 conductor lead 1 is much more challenging to design to avoid resonance than a typical 2 conductor cardiac lead because there are 4 times as many conductors that need to be configured to avoid resonance. Worse yet, some SCS leads incorporate paddle electrode arrays with 16 or more separate conductors connecting 16 or more different electrodes. These large number of electrodes 30 multiply the odds that at least one conductor 11, 18 connected to one electrode 40 will be in resonance at MRI frequencies.

[0041] To avoid such resonance frequencies, an embodiment of the present invention particularly uses coiled conductors 11, 18 for the lead 1, particularly an SCS lead 1, with at least two different materials M1, M2 as electrical insulator 21, 28 on the individual conductor core/wires 100 as shown e.g. in FIG. 1B. The coiled conductors 11, 18 give inductance because the coiling creates a solenoid structure, and the coiling also gives capacitance due to increased conductor length, which is capacitive coupled to neighboring windings. The different insulators 21, 28 allow each conductor's self-resonance to be selected during the design phase to avoid MRI frequencies. Some conductors have material M1 as insulator material and some conductors have material M2 as insulator material. The inductance of a solenoid is approximated by the well-known equation

[00001]$L={}_0.Math.\frac{N^2.Math.A}{l},$

the capacitance is approximated by

[00002]$C=\frac{{\mathrm{.Math.}}_0.Math.{}_r.Math.A}{d},$

and the resonance frequency is approximated by

[00003]$f=\frac{1}{2.Math..Math.\sqrt{\mathrm{LC}}}.$

[0042] The dielectric constant .sub.r is a function of the insulation material M1, M2 surrounding the respective conductor 11, 18. By changing the insulation material .sub.r changes, and hence the resonance frequency changes for the respective conductor M1, M2. The preferred materials M1, M2 for insulating the individual lead conductors 11, 18 are e.g. ETFE, PFA, and PTFE. The dielectric constant .sub.r for these materials are shown in Table 1 below. As can be seen ETFE, has a dielectric constant about 25% greater than PTFE or PFA. Therefore, in one embodiment ETFE is used on at least one conductor 11, 18, while PFA or PTFE are used on the other conductors.

TABLE-US-00001 TABLE 1 ETFE PFA PTFE Dielectric Constant 2.6 2.1 2.1

[0043] In one embodiment 8 different conductors connected to 8 different electrodes are all wound co-radially as shown in FIG. 1B.

[0044] FIG. 2 shows a further embodiment of the lead 1 according to the present invention also using two different insulator materials M1, M2. Here, two conductors 11, 12 of an outer coil 4 structure comprised of 5 conductors 11, 12, 13, 14, 15 in form of co-radial helical coils comprise electrical insulators 21, 22 formed out of a first material M1. Further, the lead comprises an inner coil structure 3 surrounded by the co-axial outer coil structure 4, wherein the inner coil structure 3 is comprised of three conductors 16, 17, 18 in form of co-radial helical coils, wherein one conductor 17 is formed out of the first Material M1. The other conductors 13, 14, 15, and 16, 18 comprise insulators 23, 24, 25, 26, 28 made out of a different second material M2.

[0045] Here, in the second embodiment, a co-axial/co-radial design is used for the conductors 11, 18. In this embodiment there are two (or even more) layers of coiled conductors, namely an inner layer or coil structure 3 and an outer layer (or coil structure) 4. This construction is more complicated than the co-radial design shown in FIGS. 1A-1C, but it has several advantages. First, the outer coil 4 has less conductors than the co-radial design shown in FIGS. 1A-1C (since some of the conductors are moved to the inner coil 3), and less conductors means that each conductor has a finer pitch. The finer pitch results in a much greater inductance (inductance is proportional to the square of the pitch), which results in a better MRI induce RF current rejection. Second, the conductors 16, 18 in the inner coil structure 3 are shielded by the outer coil structure 4. Third, there is greater average capacitance between conductors in this design since the average distance between conductors 11, 18 is reduced. The reduced average distance is because of the finer pitch of both the inner and outer coil structure 3, 4 compared to the co-radial coil of FIGS. 1A-1C, and the nested arrangement of the two coil structures 3, 4 which causes coupling between the coils structures 3, 4. The greater average capacitance results in a better (lower cut off frequency) low pass (LC) filter. In addition, the inductive and capacitive coupling mechanisms result in more current sharing between the conductors at MRI frequencies. This helps average out the MRI induced currents on each conductor, eliminating peaks. One example implementation of this co-axial/co-radial design shown in FIG. 2 is detailed in Table 2 below.

[0046] In another embodiment at least one conductor (e.g. of the conductors 11, 18 shown in FIG. 1A-1C or 2) has no separate insulation around it at all (insulation to the body 30 is still provided by the lead body insulator/tube 30 which is typically silicone or polyurethane. This embodiment not only changes the resonance frequency of the conductor(s) with no separate insulation, but also allows all the conductors 11, 18 to be packaged closer together, resulting in a tighter winding pitch and higher overall inductance for each conductor. Particularly, at most, every other conductor can be without insulation.

[0047] Particularly, having different insulation materials M1, M2 on different filars of the inner and/or outer coil structure allows fine tuning of the resonant frequency of the lead 1 for each electrode 40. If a certain lead length leads to one or more electrodes 40 being in electrical resonance at MRI frequencies, then the insulation material M1, M2 can be changed on that particular filar/conductor to shift the resonance away from MRI frequencies. Furthermore, the present invention allows for visual identification between conductors during the manufacturing process (e.g. to make sure that the appropriate conductor gets welded to the appropriate contact). Particularly, colorants can be added to one or more of the coatings to make it readily apparent which conductor is which. Particularly, in the above-described embodiment in which the insulation is removed altogether on one or more filars/conductors, the pitch can be increased which increases the inductance and improves MRI performance. In one embodiment of this, only every other conductor is insulated.

[0048] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.