BE1023908B1 - GUIDE FOR MEDICAL TOOLS WITH REFLECTOR FOR ELECTRIC CURRENT INDUCED BY TIME-VARIAN MAGNETIC FIELDS - Google Patents

Abstract

My invention is a new type of implantable biocompatible conductor (electrode), for example for pacemakers and neurostimulators. The conductor has the same desirable mechanical strength and almost as low resistance to direct current as the best available, and is just as easy to manufacture, but hardly warms up in an active typical magnetic resonance scanner (MRI). This prevents tissue damage to the patient during an MRI scan. The electrode consists of a conductive sheath, containing alternating conductive cores made of 2 different materials. These 3 components are intimately connected by cold drawing. The correct choice of materials and the length of the 2 alternating cores leads to the desired properties, without harmful heating due to the induction of electric current, due to the time-varying magnetic fields in a typical MRI scanner.

Description

Title of the invention: Conductor for medical devices with reflector for electric current induced by time-varying magnetic fields.

This invention relates to a medical device conductor with electric current reflector as described in the preamble of the first claim.

Implantable medical devices such as pacemakers and neurostimulators use electrical conductors to deliver an electrical pulse to human tissue.

When patients undergo a magnetic resonance (MR) scan, such implantable devices pose a risk of tissue damage due to the heating of those electrical conductors, due to the current induction caused by the radiofrequency (RF) magnetic field emitted from the MR scanner.

Tissue damage caused by heating of implant parts can be severe and permanent. This is why manufacturers should pay close attention to the correct design of medical devices for safe postoperative MR imaging.

Numerous solutions have been tested and commercially available for reducing or avoiding tissue damage caused by MR scanners. Examples are: RF filter elements, RF shields, RF "chokes" (chokes), resonance traps and coatings. All of these are aimed at mitigating the warming of the conductors.

The invention described below creates a cost-effective solution that avoids the detrimental heating of conductors, without having to sacrifice the mechanical and electrical qualities of the electrical conductor.

This invention proposes the installation of reflectors in the conductor. These reflectors are placed at strategic positions in the conductor and prevent the resonance of standing waves, which is the cause of the undesired heating. Conductors for implantable medical devices as described in the preamble of the first claim are known from, inter alia, US2015 / 057730. US2015 / 057730 describes implantable medical conductors and systems that use reflective points within the conductive body to control the radio frequency current generated on one or more wires. The high-frequency current can be controlled by blocking the reflection points for the purpose of blocking at least a part of the radio frequency current such that it cannot reach an electrode of the line and to allow at least a part of the high-frequency current to be heat is dissipated on the wire. Regulating the radio frequency current thereby reduces the amount dissipated in body tissue by one or more electrodes of the conductor and reduces the risk of tissue damage. The reflection points may be formed by physical changes such as change of material or size of the wire and / or insulating layers that may be present such as an inner sheath around the wire and an outer sheath formed by the body of the conductor. However, the conductor described in US2015 / 057730 has the disadvantage that the impedance at the passage of direct current (DC) or signals with a low frequency remains too high. The conductor described in US2015 / 057730 has the additional disadvantage that a compact bundling and the manufacture of a conductor with a substantially constant diameter cannot be achieved due to the presence of the thickenings on the conductor. US2009 / 149920 describes an implantable medical device for use in an MRI environment. The medical device comprises: a lead comprising an internal electrical conductor coupled to an electrode and at least one outer resistor extending radially over the inner electrical conductor along at least a portion of a length of the conductor. The inner electrical conductor has a first resistor. The resistance of the at least one outer resistance is greater than the first resistance. The outer resistor is adapted to dissipate RF electromagnetic energy received by an MRI device along the length of the conductor. However, the electrode is difficult to produce, certainly if biocompatibility is intended, and its mechanical strength is insufficient.

US2005222659 discloses a conductor intended to be implanted in the body of a patient. The conductor comprises a body and a conductive filling which is located in the body of the conductor and which has a distal end. An electrode is electrically connected to the conductor body and includes a stimulation portion, a coil, and at least one coil of wire wound around the coil and electrically coupled between the stimulation portion and the distal end portion, so that between the distal end portion and the stimulation portion coil is formed. Electric coupling can be obtained by welding, spot welding, laser welding and the like. However, the electrode is difficult to produce, certainly if biocompatibility is intended, and its mechanical strength is insufficient.

Description of the invention

The law requires manufacturers of implantable medical equipment to guarantee the safety of this equipment in the case of post-operative magnetic resonance (MR) imaging in accordance with the ISOTS 10974 (1) standard.

A combination of safety and risk assessments is required. One of the possible risks is tissue damage due to the heating of metal components. This evaluation is described in the ASTM F2182-11a (2) standard.

This evaluation measures the temperature increases of the implant when the MR scanner is switched on.

These measurements must be performed in a worst-case scenario. The permissible temperature increase is 2 ° C at full intensity of the RF magnetic field.

The most commonly used MR scanners (this is 90%) are equipped with permanent magnets with a field force of 1.5 or 3 Tesla. Magnetic resonance imaging requires an induced radio frequency (RF) magnetic field on top of the static magnetic field. The frequency must be 64 MHz or 128 MHz with a field force of 1.5 Tesla resp. 3 Tesla. These frequencies are determined by the magnetic spin ratio of hydrogen atoms of 42.5 MHz / Tesla.

Electric conductors that are subjected to such radio frequencies of the magnetic field tend to heat up due to electrical flow (induction law of Faraday). The heating is maximum when the conductor length is half the wavelength of the induced magnetic field. (') A harmonized ISO standard is at the project stage: Assessment of the Safety of Magnetic Resonance Imaging for Patients with an Active Implantable Medical Device. The current standard is ASTM F2503 - 13 Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment (2 JStandard Test Method for Measurement of Radio Frequency Induced Heating On or Near Passive Implants During Magnetic Resonance Imaging.

This is because of the occurrence of "standing" waves at the main frequency of the RF magnetic field.

A critical length for implanted conductors is therefore set at 30 cm resp. 15 cm. With modern pacemakers and neurostimulators, the length of the conductors is within this range. (The distance from the subclavicular cavity to the heart or head is approximately 30 cm).

The theory of standing waves is well known and shows that the maximum heating occurs at the ends of the conductor. (The voltage is maximum at the ends in case of resonance of the induced current).

The present invention reduces the effective length of the conductors of the inductive current path and thus reduces the heating. It is based on the use of a composite conductor with the same mechanical and electrical qualities as existing conductors.

The properties of the conductor are supposed to be in accordance with the theory of a transmission line.

The following two paragraphs describe how the new conductor (subject of the patent application) minimizes the impedance in transmission of low-frequency signals, and mitigates the induced harmful heating during an MR scan.

Impedance with low-frequency signals.

The conductor (Fig. 1) is designed such that a low impedance occurs when DC (DC) or low-frequency signals pass through (specifically <100 kHz). This objective is obtained by assembling a composite wire consisting of an outer jacket or envelope sleeve (Figure 1 tag 1), made of a highly resistant alloy and a core wire inside (Figure 1 tag 2). The two components are merged concentrically intimately by "cold drawing of a filled tube" (tube filled cold drawing).

A good example is the composite wire consisting of a silver core wire (fig.3 tag 2) wrapped with a super alloy known as MP35 alloy (fig. 3 tag 1). This is a NiCoCr alloy with exceptional mechanical properties and fatigue resistance.

A conductor filament made from MP35 alloy with a typical diameter has a low frequency impedance of approximately 100 Ohm / m.

The low-frequency impedance is considerably reduced by introducing a highly conductive silver (Ag) core wire to approximately 5 Ohm / m (FIG. 3-tag 2), with the same typical diameter.

This drastically lower resistance significantly extends the life of the batteries of medical devices.

The technique for manufacturing this composite wire is called "cold pulling of a filled tube". More specifically, a hollow tube of MP35 is fed with a silver wire, and cold drawn through a die. As a result, the tube and core wire are connected in solidarity.

This composite shows an optimum balance between mechanical and electrical properties. In particular, this composite has a low impedance at the passage of low-frequency signals. Pacemakers and neurostimulators send typical pulsing signals to the tissues. A useful bandwidth for transmitting therapeutic pulses while maintaining a high signal fidelity is around 100 kHz.

The impedance at low frequency is mainly determined by the ohmic resistance of the conductor. Variations in capacitance or induction due to the winding or braiding of the conductor are negligible at these low frequencies. It follows that the direct current (DC = direct current) resistance of the conductor is a measure of the impedance pre-low frequency signals.

Impedance for induced currents through RF magnetic fields from MR scanners.

The impedance of conductors at higher frequencies, in particular for the frequencies of 64 MHz and 128 MHz, can no longer be measured via the ohmic resistor. Capacitive and inductive effects are no longer negligible here. The effects of wave propagation and resonance must be taken into account.

The invention proposed here consists of a composite conductor with the addition of RF reflectors built into the core wire (Fig. 1 tag 3). These reflectors are short segments that fit into the core wire. These segments have a significant deviating resistance compared to the resistance of the highly conductive core wire. Thus, the induced RF current must propagate according to a path consisting of concatenated segments with different impedances. Seen electrically, this cord is assembled through the enveloping shaft tube into one macro-structure.

The successive discontinuities in the core wire (fig. 2 tag 3) partially reflect the incoming electric wave.

The reflection coefficient is represented by the following formula (see Figure 2):

This formula shows that the reflection is directly proportional to the variation in the impedance.

In the newly designed conductor, the RF reflection particles (Fig. 1 tag 3), due to their significantly higher impedance than the highly conductive core wire, cause an important reflection of the electric wave induced by the RF magnetic field.

If the length of the insulated segments (fig. 1 tag 2) is considerably smaller than a quarter of this wave, then it appears to be impossible to generate resonant standing waves in the conductor at the frequencies of 64 MHz and 128 MHz.

It is true that the effective length of the conductor that is eligible for induction is limited to Lsegm (Fig. 3 - C). The absence of a resonance possibility translates into a no longer measurable temperature increase of the conductor due to Faraday induction.

In practice it can be assumed that my invention must be safely applicable for 1.5 Tesla and 3 Tesla scanners. The characteristic length L segm must then be more than less than 7.5 cm in an industrial design (= 'A wavelength at 128 MHz in saline).

The length of the reflector must be a feasible practical compromise between manufacturability and the total DC resistance of the conductor. This resistance can easily be calculated on the basis of an equivalent network of resistors connected in parallel and in series. When Lrefl «Lsegm and the casing shaft tube conducts electrically, the incorporation of the RF reflectors will only marginally influence the overall DC resistance of the conductor.

Note on the "skin effect": it is scientifically known that the flow of the signals with frequency 64 MHz and 128 MHz is strongly concentrated in a thin outer shell of the conductor.

This effect also occurs in this newly invented conductor and is described by the formulas below:

(See https://en.wikipedia.Org/wiki/Skin_effect#Resistance)

Because the resistance of the enveloping shaft is an order of magnitude higher than that of the core wire, the electric current concentrates in the outer shell of the core wire. The fraction that still runs through the outer shell of the enveloping shaft tube is negligible.

Because of the skin effect and provided that p-core «p - outside house, the outside house (fig. 1 tag 1) plays the role of an insulator at high frequencies, such as 64 MHz and higher.

In the attached figures, "Lsegm" is the effective length of the conductive segment for Faradayan induction.

Application in practice

The invention proposed here can be produced by an industrial process of "cold drawing of a filled tube".

A core wire is fed into an outer shaft casing (the tube), the core wire consisting of a succession of wire pieces of highly conductive material, regularly interrupted by short segments of a material with lower conductivity. This assembly is finally pulled through a mold with a smaller diameter, so that the tube and core wire form one solidary whole.

A practical composite conductor is obtained by combining a NiCOC super alloy (MP35 ™) shaft shell with a silver (Ag) core wire with a 50% cross-sectional area interrupted by reflectors made from the same MP35 alloy. The reflectors must have a length of 1 cm and be inserted every 5 cm.

Originality

There is no known product of a conductor with built-in reflectors that is designed in such a way that there is hardly any temperature increase due to the RF magnetic field of scanners.

A known solution to mitigate the heat effect is to apply special coatings to the conductor, which either shield the conductor from the RF magnetic field, or thermally isolate it from the enveloping tissue.

Another experimental technique consists of adding components that change the sensitivity to induction. These components change the impedance of the conductor to prevent resonance. They are either capacitive or inductive, the so-called RF filters or RF chokes. This design is still under development and still faces problems of bio-compatibility and robustness.

The great advantage of my invention is that it can be produced with existing production techniques for medical applications, including the winding of the wire in a helical spring, which makes my invention cost-efficient. The proposed conductor can also obtain the same strength characteristics as the existing conventional wires, with the same electrical qualities.

Claims (9)

CONCLUSIONS A medical implantable device conductor with electric current reflector induced by time-varying magnetic fields generated by a magnetic resonance scanner, the conductor comprising an electrically conductive core wire and a sheathing, the electrically conductive core wire consisting of segments made of a first material and at least one second segment of a second material different from the first, the segments made of the second material serving as a reflector for electric waves induced by the time-varying magnetic fields, characterized by limited heating-up phenomena at current passage generated by the time-varying magnetic fields, because segments of the first and second material are arranged alternately in the core wire and because the length of the segments made of the first material is shorter than a quarter of the wavelength of the time-variant magnet ische fields. The conductor of claim 1, wherein the core wire is a concatenation of segments of a material with a higher conductivity and segments of a material with a lower conductivity. The conductor of claim 1, wherein the segments of said first and second material are arranged (concatenated) in an alternate manner along the length of the wire. The conductor of claim 3, wherein the sheath and core segments are solidified by cold drawing. The conductor of claim 1, wherein said materials are biocompatible. The conductor according to any of the preceding claims, wherein the core wire is a wire made of segments made of silver, alternated with segments made of a NiCoCr alloy, and the sheath is made of a NiCoCr alloy, preferably an MP35 alloy. 7. A conductor filament made of segments made of silver, interspersed with segments made of a NiCoCr alloy, the filament having a typical low-frequency impedance of approximately 100 Ohm / m. An implant comprising a conductor according to any one of the preceding claims. A method for producing a conductor according to any of claims 1 to 7, wherein a core wire is fed into a sheath, the core wire consisting of a succession of wire pieces of highly conductive material, regularly interrupted by short segments from a material with lower conductivity, and wherein the core wire in the sheath is pulled through a die, the die having a smaller diameter than the sheath, so that the tube and core wire are consolidated into a whole.