WO2011109733A1

WO2011109733A1 - Mri compatible transmission line circuit

Info

Publication number: WO2011109733A1
Application number: PCT/US2011/027236
Authority: WO
Inventors: Thomas W. Lloyd; Steven R. Wedan; Charles A. Plowe
Original assignee: Imricor Medical Systems, Inc.
Priority date: 2010-03-04
Filing date: 2011-03-04
Publication date: 2011-09-09

Abstract

An MRI compatible transmission line circuit construct is provided. The construct includes at least one filter component constructed from a transmission line. The at least one filter component may include a plurality of filters positioned along the length of the transmission line that resolve the issue of insufficient attenuation by effectively blocking the RF induced current on the transmission line from entering a connected circuitry component disposed at the distal end of the transmission line circuit.

Description

MR] COMPATIBLE TRANSMISSION LINE CIRCUIT

FIELD OF THE INVENTION

[0001] The invention relates to medical devices used in the magnetic resonance imaging (MRJ) environment and in particular to a method and device for attenuating electromagnetic fields applied to such devices during MRI scanning.

BACKGROUND OF THE INVENTION

[0002] MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.

[0003] MRI uses three fields to image patient anatomy: a large static magnetic field, a time- varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.

[0004] Each of the three fields associated with MRI presents safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that can result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices which have elongated conductive structures such as transmission lines in catheters, sheaths, guidewires, stent or valve delivery systems, ICD leads, pacemaker leads, neurostimulator leads, or the like.

[0005] A variety of MRI techniques are being developed as alternatives to X- ray imaging for guiding interventional procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device and patient tissue can be monitored to improve therapy delivery. Thus, tracking the position of medical devices is useful in interventional procedures. Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the position of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations. Thus, as the field of interventional MRI grows and more patients are catheterized in the MR environment, the need for safe devices in the MRI environment increases.

[0006] The RF-induced heating safety risk associated with transmission lines in the MRI environment results from a coupling between the RF field and the transmission line. In this case several heating related conditions exist.

[0007] One condition exists where RF induced currents in the transmission line may cause Ohmic heating in the transmission line itself and/or the

components connected to the transmission line, and the resultant heat may transfer to the patient. In such cases, it is important to attempt to both reduce the RF induced current present in the transmission line and to limit the current delivered into the connected components. Another condition exists where RF currents in the transmission line couple to conductive structures that contact tissue. In this situation, RF currents induced on the transmission line may be delivered into the tissue through a conductive structure not in direct electrical contact with the transmission line resulting in a high current density in the tissue and associated Joule or Ohmic tissue heating. Also, when the transmission line is connected to circuitry that is tissue contacting, direct injection of the induced current into the tissue may occur resulting in a high current density in the tissue and associated Joule or Ohmic tissue heating. Lastly, RF induced currents on the transmission line may result in increased local specific absorption of RF energy in nearby tissue, thus increasing the tissue's temperature. The foregoing phenomenon is referred to as dielectric heating. Dielectric heating may occur even where no thermal or electrical contact to the tissue exists.

[0008] Methods and devices for attempting to solve the foregoing problem are known. For example, transformers or baluns can be placed in the transmission line to effectively reduce , the electrical length of the line eliminating resonant coupling and limiting the current induced on the line.

[0009] Notwithstanding the foregoing attempts to reduce RF-induced heating, significant issues remain. For example, to effectively reduce the electrical length of the transmission line, three or more transformers are needed. Each of these transformers requires a matching network at each point of contact with the transmission line. Each matching network requires 3-4 surface mount capacitors. Also, size constraints inherent to the design of interventional devices limit the cross sectional area of the transformer. For all these reasons, manufacturing of transmission lines that rely solely on integrated transformers for safety is difficult and time consuming. Due to the size constraints alone, PCB based transformers significantly attenuate the control signal and are excessively fragile and prone to failure. Additionally, if multiple transmission lines are required, transformers placed in close proximity may result in cross coupling between the transmission lines. [0010] Current technologies for reducing RP-induced heating in medical devices, especially those with elongated conductive structures such as transmission lines, are inadequate. Therefore, new transmission line constructs and lead or catheter assemblies are necessary to overcome the problems of manufacturability and robustness associated with existing solutions.

BRIEF SUMMARY OF THE INVENTION

[0011] It is an object of the invention to provide an improved device and method for reducing RF-induced heating of tissue by attenuating the RP current induced in the medical device by MRI. [0012] It is a further object of the invention to provide a novel circuit construction that is MRI compatible and resolves the limitations of the current technology such as manufacturability and robustness.

[0013] It is a further object of the invention to provide a novel circuit construction that maintains physical flexibility, maneuverability and the ability to bend.

[0014] In one embodiment the invention is a circuit adapted to be used with an implantable or interventional lead or catheter assembly. Each circuit includes a plurality of filter components constructed from a single transmission line.

[0015] In one embodiment the filter component may comprise one or more filter(s) or inductive segments positioned along the length of the transmission line providing an electrical impedance sufficient to attenuate induced RP current on the transmission line and resolve the issue of RF induced heating of tissue.

[0016] In one embodiment, the filters may comprise a plurality of multiple inductive segments placed in close proximity such as within approximately 5 centimeters or less for the purpose of providing more attenuation than a single filter alone, while still allowing the device to bend. [0017] In one embodiment, multiple filters placed in close proximity may be formed to create a distributed reactance. For example, two co-radially wound transmission lines may create a distributed reactance. In an alternative

embodiment three or more co-radially wound transmission lines may create a distributed reactance.

[0018] In one embodiment, the novel transmission line circuit construct may include a single coaxial transmission line thereby minimizing the need for bonding points which reduces the possibility of mechanical failure of the line.

[0019] In one embodiment inductive segments may be formed using a coaxial transmission line creating an impedance on the shield and/or inner conductor that attenuate the induced RF current while maintaining the transmission properties of the coaxial transmission line thereby allowing transmission of a differential signal.

[0020] In one embodiment inductance created using a coaxial transmission line may be constructed to reduce transmission line loss at a specific frequency. [0021] In one embodiment inductance created using a coaxial transmission line may be constructed to increase the impedance of the shield and/or inner conductor at a specific frequency.

[0022] In one embodiment inductance created using a coaxial transmission line may be constructed to utilize capacitance between the windings to increase the attenuation provided by the individual filters.

[0023] In one embodiment inductance created using a coaxial transmission line may be constructed to utilize capacitance between the windings to create a resonant LC circuit on the shield and/or inner conductor while maintaining the transmission line properties of the cable. [0024] In one embodiment the inductance created using a coaxial

transmission line may be constructed to produce a phase shift along the shield to lengthen, shorten, or eliminate the resonant length and/or resonant insertion depth of the transmission line. [0025] In one embodiment the coaxial cable may be 32AWG or smaller.

[0026] In one embodiment the coaxial cable may have a thin outer insulation to decrease the space between the windings and increase the impedance per unit length along the shield. [0027] In one embodiment the filter component comprises two filter components. One filter component may be an inductive coupling mechanism, such as a transformer, placed at or near the antenna/transmission line interface that provides additional protection from RF heating of tissue by effectively decoupling the common mode currents on the transmission line from the antenna and blocking the RF induced current on the transmission line from entering the antenna where Ohmic heating of the conductors themselves may be more likely and electromagnetic radiation of energy induced on the transmission line is more efficient.

[0028] The second filter component may comprise one or more filter(s) or inductive segments along the length of the transmission line that resolve(s) the issue of excessive heating of the transformer by significantly attenuating the current induced on the transmission line before it reaches the transformer. The filter(s) may also attenuate the RF current reflected from the transformer thereby resolving the issue of the strong reflected power from the transformer and the associated dielectric heating.

[0029] Alternatively, the decoupling mechanism, or transformer, may be replaced by an LC circuit formed with the coaxial cable.

[0030] In one embodiment the transmission line circuit and integrated components may be constructed to be integrated into a 10 French or smaller catheter.

[0031] In one embodiment the transmission line circuit may be used in an implanted or interventional medical device such as an ablation catheter, diagnostic catheter, sheath, guidewire, stent or valve delivery system, ICD lead, pacemaker lead, neurostimulator lead, or the like.

[0032] In one embodiment a catheter or transmission line assembly includes an elongated body having first and second ends. The elongate body defines a lumen therewithin which receives first and second circuits. First and second circuits each include a transmission line that forms a plurality of filters distributed along a length thereof. A distal antenna located at the distal end of the elongate body is coupled to the second transmission line. The elongate body also includes a proximal antenna at the first end and proximal to the distal antenna. The proximal antenna is electrically coupled to the first transmission line. The second end of the elongate body is operably coupled to electronic controls, either external or internal to the body. In one embodiment, the second end attaches to amplifiers for sensing/receiving MRI tracking signals. One filter formed by each

transmission line may comprise one or more non-resonant filter(s) or inductive segments positioned along the length of the elongate body to attenuate the current induced on the transmission line before it reaches the antenna, LC filter, or inductive coupling mechanism (such as a transformer or the like). A second filter formed by each transmission line may be an LC filter or inductive coupling mechanism at or near the antenna/transmission line interface that increases attenuation and/or provides effective decoupling of common mode currents by effectively blocking the RF induced current on the transmission line from entering the antenna. The inductive filter(s) may also attenuate the RF current reflected from the LC filter/transformer thereby resolving the issue of the strong reflected power from the LC filter/transformer and the associated dielectric heating. [0033] In another embodiment a transmission line assembly includes an elongated body having first and second ends. A plurality of antennas is located at the distal end of the elongate body. The plurality of antennas may include any number of antennas, tracking coils, or the like. The elongate body further defines a lumen therewithin which receives a plurality of circuits. Each individual transmission line comprising the plurality of circuits forms a plurality of filters, or inductive segments, distributed along a length thereof. The second end of the elongate body may be operably coupled to electronic controls (either external or internal to the body) or amplifiers for sensing/receiving MRI tracking signals. Each individual circuit comprising the plurality of transmission lines may also include a resonant or non-resonant LC filter, or alternatively include an inductive coupling mechanism such as a transformer positioned within the lumen of the elongate body at a distal end thereof at or near the antenna/transmission line interface.

[0034] In another embodiment a lead assembly includes an elongate body having a proximal end and a distal end, the elongate body defining a lumen therewithin. The distal end is arranged and configured to sense/receive MRI tracking signals and the proximal end is operably coupled to an electronic control. At least one antenna is located on the elongate body and at least one electrical circuit is in communication with the at least one antenna. The circuit is housed within the elongate body and includes one or more transmission lines that form at least one inductive filter and may include a resonant or non-resonant LC filter or an inductive coupling mechanism. The inductive coupling mechanism is positioned at the distal end of the elongate body proximate to the

antenna/transmission line interface. The circuit may be flexible or rigid.

[0035] While multiple embodiments, objects, features and advantages are disclosed, still other embodiments of the invention will become apparent to those skilled in the art from the following detailed description taken together with the accompanying figures, the foregoing being illustrative and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 is a block diagram depicting the basic components of the invention housed within a catheter assembly.

[0037] FIG. 2A is a diagram depicting an embodiment of the invention in which inductive filters are distributed along a transmission line in a spaced apart relationship with an LC filter proximate an antenna. [0038] FIG. 2B is a diagram depicting an embodiment of the invention in which inductive filters are distributed along a transmission line in a spaced apart relationship.

[0039] FIG. 3 A is a sectional view of an exemplary medical device including MR compatible transmission lines forming inductive filters distributed along the transmission line and each transmission line forming a LC filter proximate an antenna.

[0040] FIG. 3B shows a detailed view of the inductive filters of FIG. 3 A.

[0041] FIG. 4A is a schematic view of the exemplary medical device of FIG. 3 with MR compatible transmission lines positioned within the lumen of the transmission line assembly.

[0042] FIG. 4B is a schematic view of an exemplary medical device with MR compatible transmission lines embedded in a jacket surrounding the transmission line assembly. [0043] FIG. 5 depicts an embodiment of the invention in which multiple inductive segments formed from a single transmission line are grouped together and distributed along the transmission line.

[0044] FIG. 6A is a perspective view depicting co-radially wound coaxial transmission lines. [0045] FIG. 6B is a schematic view of the co-radially wound transmission lines of FIG. 6 A positioned inside an exemplary medical device.

[0046] FIG. 6C is a schematic view of the co-radially wound transmission lines of FIG. 6 A embedded in the jacket of the exemplary medical device.

[0047] FIG. 7 is a sectional view of an exemplary medical device including MR compatible transmission lines and a co-radial inductive coupling device. [0048] FIG. 8 is a diagram illustrating one exemplary co-radially wound inductive coupling mechanism that may be utilized in the medical device of FIG. 7.

[0049] FIG. 9 is a diagram illustrating one exemplary flex circuit antenna with an integrated inductive coupling mechanism and associated matching network.

[0050] FIG. 10 is a diagram illustrating one exemplary PCB based inductive coupling mechanism with associated matching networks.

DETAILED DESCRIPTION OF THE INVENTION [0051] In describing the invention herein, reference is made to an exemplary transmission line assembly comprising a catheter. However, as will be appreciated by those skilled in the art the present invention may be used with any implantable medical device. By implantable we mean permanently as with cardiac pacemaker and defibrillator systems and neurostimulator systems; or temporarily implantable such as in interventional procedures and including by way of example ablation catheters, diagnostic catheters, sheaths, guidewires, stent delivery systems, or the like. Further the exemplary transmission line assembly may be used external to the body but still be in close proximity to body tissue such as the skin. Also as used herein, a transmission line is any conductive structure that is in electrical contact with an antenna. Typically, a transmission line is a coaxial cable. However, a transmission line may also be a circuit board trace, a conductive lumen, or any material which includes two conductors arranged to form a transmission line.

[0052] Generally speaking, the invention relates to medical devices requiring a transmission line capable of transmitting a differential mode signal of a specific frequency while simultaneously attenuating common mode signals of the same frequency. However, the invention is not limited to attenuating common mode currents. The invention is also intended to attenuate currents induced on the shield of a coaxial cable. Further the invention may include components that effectively block common mode current from reaching the antenna such as inductive coupling components including transformers and the like.

[0053] FIG. 1 is a block diagram illustrating the transmission line assembly 100 in its simplest form in accordance with the present invention. Transmission line assembly 100 broadly includes elongate body 110 having first 112 and second 114 ends and defining a lumen 116 therewithin. A connected circuitry component 118 is located at the first end 112 of elongate body 110 and is in electrical communication with circuit 120. The connected circuitry component 118 will hereafter be referred to as an antenna but should be understood to include any circuitry used in a medical device including, but not limited to, electrodes, temperature sensors, strain gauges, pressure sensors, tracking coils, antennas, and the like. Lumen 116 houses circuit 120. Circuit 120 includes at least one transmission line 122 forming a plurality of spaced apart filter components 124. Each circuit 120 may be constructed from a single, continuous length of a transmission line material, such as coaxial cable. Alternatively, the circuit 120 may be constructed with discrete filter components and a single transmission line or multiple lengths of non-continuous transmission line connecting the discrete filter components. Alternatively, the circuit 120 may be constructed with one transmission line forming filter components 124 and a separate component forming filter component 126.

[0054] Filter component 126 at the antenna/transmission line interface 128 may be an inductive filter or LC circuit to provide additional attenuation of the RF current before it reaches the antenna. In one embodiment, the filter component 126 may be a transformer that is operable to effectively decouple common mode current on the transmission line from the antenna while allowing differential mode currents to pass. Filter components 124 preferably include a plurality of filters or inductive segments that attenuate induced RF current on the transmission line. In embodiments where filter component 126 is a resonant LC circuit or a

transformer, filter components 124 may address excessive heating of the resonant filter or transformer by significantly attenuating the current induced on the wire before it reaches the filter component 126. Filter components 124 may also attenuate current reflected from filter component 126 thereby reducing the associated dielectric heating. Although filter component 126 has been described herein as comprising an LC circuit, resonant LC circuit, or transformer, providing such component at the antenna/transmission line interface 128 may not be necessary. For example, the filter component 126 may be replaced with a filter component that is similar or identical to the filter components 124 without departing from the intended scope of the present invention.

[0055] Any non-magnetic transmission line may be used in constructing the circuit in accordance with the present invention, including those comprising silver plated copper, copper, titanium, titanium alloys, tungsten, gold and combinations of the foregoing. Transmission line 120 may have a bondable insulation that is heat, chemical or adhesively bondable to permit formation of the filters during manufacture with one transmission line. Transmission line 120 may have a Teflon (FEP) coating allowing it to be inserted into an elongated shaft. [0056] FIG. 2A is a schematic diagram depicting an embodiment of the invention. Transmission line assembly 200 broadly includes an elongate body 210 having first 212 and second 214 ends and includes lumen 216 therewithin.

Transmission line or catheter assembly 200 includes first antenna 218 located at the first end 212 of transmission line assembly 200. First antenna 218 may be a distal tracking coil or the like. Lumen 216 houses circuit 220. Circuit 220 includes at least one transmission line 222 forming a plurality of spaced apart filter components 224. Each circuit may be constructed from a single, continuous length of non-magnetic transmission line constructed from silver plated copper, copper, titanium, titanium alloys, tungsten, gold and combinations of the foregoing. This eliminates the necessity for connection points at each end of each filter 224 and thereby improving the mechanical durability of the circuit 220 and reducing the manufacturing cost thereof. Alternatively, each circuit may comprise multiple lengths of transmission lines. As with the embodiment depicted in FIG 1 , transmission line 222 may be bondable such as by heat, chemicals or adhesives, to permit formation of the filters during manufacture with one transmission line. The transmission line may have a Teflon (FEP) coating allowing it to be inserted into an elongated shaft. In the illustrated embodiment, the transmission line assembly 200 may include an LC filter or transformer 226 positioned adjacent and proximal to the antenna/transmission line interface 228. Alternatively, the LC filter or transformer 226 may be replaced by an inductive filter similar or identical to the filters 224 as illustrated in FIG. 2B.

[0057] When present, LC filter or transformer assembly 226 is adapted to increase the common mode impedance between the transmission line and the antenna 218. LC Filter or transformer assembly 226 effectively blocks RF induced current by being constructed such that the inductive and capacitive characteristics of the filter together increase the impedance at the MRI RF frequency of interest for example, approximately 64MHz for a 1.5 Tesla MRI or approximately 128MHz for a 3.0 Tesla MRI. Filtering components 224 distributed along the length of the transmission line attenuate the induced current on the transmission line itself before the current reaches the antenna, LC filter, or transformer assembly 226 thereby avoiding excessive heating of the assemblies 226 and 218. The filtering components 224 together preferentially create at least 1,000 or more Ohms of impedance along the entire circuit 220, for a transmission line of approximately 1 meter. Those of skill in the art will appreciate that the amount of total impedance will necessarily change as the lead length varies. Each filtering component 224 may comprise an inductance formed by transmission line 222 with approximately 20 turns to approximately 45 turns, creating more than approximately 100 Ohms of impedance on the shield and center conductor, when sized to fit in an 8 French catheter assuming an inside diameter of the inductive segment to be 0.045 inches. Fewer turns are necessary to create the same impedance for larger diameter inductive segments. Filtering components 224 may be spaced non-uniformly, such that the segments of transmission line between them each have a different resonant frequency, or substantially uniformly.

[0058] Further, the number of turns in the transmission line 222 can be adjusted to shift the resonant length or insertion depth of the catheter. For example, a change of approximately 20 turns may shift the resonant insertion depth by about 10 centimeters. [0059] Referring now to FIG. 3 A a detailed sectional view of one

embodiment of the invention is illustrated. Transmission line assembly 300 includes elongate body 310 surrounded by j acket 311. Elongate body 310 includes first 312 and second 314 ends and includes lumen 316 therewithin.

Second end 314 is adapted to be connected to electronic controls, internal or external to the patient body, and may include a connector (not shown). Lumen 316 houses circuits 320, 321. Circuits 320, 321 each include one transmission line 322, 323, respectively, located within the lumen 316 of transmission line assembly 300 as seen in FIG. 4A. In an alternative embodiment, transmission lines 322, 323 can be embedded in jacket 311, as seen in FIG. 4B, thereby decreasing the overall diameter of the transmission line assembly 300. Each transmission line 322, 323 comprises a single length of coaxial transmission line, each of which forms a plurality of spaced apart filter components 324, 325, respectively. Filter components 324, 325 comprise filters or inductive segments that are spaced apart along the length of transmission lines 322, 323. Antennas 319, 318 are located on the first end 312 of elongate body 310 and are electrically coupled to the first and second transmission lines 322, 323, respectively. In the illustrated embodiment, first antenna 319 is a proximal tracking coil and second antenna is a distal tracking coil. In one exemplary embodiment the tracking coils are flex circuit tracking coils. However, the antennas 318, 319 may be any type of tracking coil known to those skilled in the art of active tracking within an MRI and specifically include helical antennas. The antenna may be a single tracking coil. Alternatively, the antenna may be a series of tracking coils. Still yet alternatively, the antennas may be antennas placed on either side of the housing. Thus, although the illustrated embodiment is depicted as distal and proximal flex circuit based tracking coils, any of the foregoing antenna embodiments fall within the scope of the invention.

[0060] The first and second transmission lines 322, 323 are electrically insulated from one another. Both the first and second transmission lines 322, 323 may include an insulative or non-conductive coating. The insulative coating may be a heat bondable material such as polyurethane, nylon, polyester, polyester- amide, polyester-imide, polyester-amide-imide and combinations of the foregoing. The insulative coating may also be Teflon (FEP), or the like, to allow for easy insertion of the transmission line assembly into an elongated shaft.

Alternatively, only one transmission line may be insulated. The insulation may comprise the bondable material mentioned previously. In addition, circuits 320, 321 , as best seen in FIG. 3B, are further electrically insulated as both transmission lines 322, 323 are wound around non-conductive tube 330 defining a lumen therewithin. Tube 330 may be formed of a silicone material, Teflon, expanded tetrafluoroethylene (eTFE), polytetrafluoroethylene (pTFE), or the like, as described below. Winding the filters 324, 325 or inductive segments around non- conductive tube 330 facilitates construction of the filters. Moreover, non- conductive tube 330 advantageously allows the circuits to maintain flexibility and maneuverability when placed inside an elongate body. Advantageously, other items necessary or desirably used in the surgical or interventional procedure such as fiber optic cables and irrigation lumens may also be passed through the lumen of tube 330.

[0061] Referring to FIG. 3 A, proximal antenna 319 is coupled to the first transmission line 322 with distal antenna 318 located distal to the proximal antenna 319 and coupled to the second transmission line 323 at the first end 312 of transmission line assembly 300. Lumen 316 houses circuits 320, 321 comprising transmission lines 322, 323, respectively. Alternatively, and as best illustrated in FIG. 4B, transmission lines 322, 323 may be embedded wholly or partially in jacket 311. As discussed previously, each transmission line 322, 323 forms a plurality of spaced apart filter components 324, 325 comprising inductive filters. As in previous embodiments, each circuit is optionally constructed from a single, continuous length of non-magnetic transmission line such as silver plated copper, copper, titanium, titanium alloys, tungsten, gold and combinations of the foregoing. However, each circuit may alternatively be constructed from multiple lengths of transmission line or include discrete filter components connected by separate lengths of transmission line. If all filters are formed from one length of transmission line, the transmission line may be adhesively bonded or have a bondable coating to permit formation of the filters during manufacture with one transmission line.

[0062] With further reference to FIG. 3B, each circuit 320, 321 is constructed substantially similarly. Transmission lines 322, 323 are wound over flexible tube 330 which is preferably made from polyimide, polyolefin, pTFE, eTFE, polyetherketone (PEEK) and other similar flexible materials. During manufacture a stiff rod (not shown) is placed inside of flexible tube 330 to provide added support for the assembly process. After manufacture, the rod is removed and the flexible tubing 330 with circuit constructs is placed in elongate body 310.

Although the filters 326, 327 are illustrated as LC filters in FIG. 3 A, they may also be replaced with other suitable filter components such as "standard" inductive filters similar to filters 324, 325 as illustrated in FIG. 3B without departing from the intended scope of the invention.

[0063] As illustrated in FIG. 3B, first transmission line 322 is helically wound around tube 330. Those of skill in the art will appreciate that connecting segments 332 do not necessarily need to comprise a specific numbers of turns around tube 330. Rather, it is important to wind the transmission line in such a manner as to include some slack or "play" thereby allowing the transmission line assembly to maintain its flexibility during use. Inductive sections 324 are formed by coiling transmission line 322 over flexible tube 330. Each inductive section 324 may be formed by helically winding or coiling transmission line 322 approximately forty-five turns, creating approximately 120 ohms, when sized to fit in an 8 French catheter assuming an inside diameter of the inductive segment to be 0.045 inches. Those of skill in the art will appreciate, however, that fewer turns may be necessary to create the same impedance for larger diameter inductive segments. Inductive segments 324 may be spaced non-uniformly, such that the segments of transmission line between them each have a different resonant frequency, or may be placed substantially uniformly. Additionally, the number of turns may be varied to change the resonant insertion depth of the completed circuit. [0064] Second circuit 321 is constructed next and substantially similarly to circuit 320. Those of skill in the art will appreciate that the exemplary

transmission line assembly illustrated in FIGS. 3A and 3B comprises two circuits 320, 321 and two antennas 319, 318. However, any number of circuits and corresponding antennas can be constructed. For example, in one exemplary construct three circuits each comprising a plurality of filters are electrically coupled to three antennas to provide additional information about the device orientation.

[0065] As referenced above and shown in FIGS. 4A and 4B, the circuits can be constructed so that filters may be embedded, partially or wholly, in the catheter jacket.

[0066] Referring now to FIG. 5, another embodiment of the invention is shown. In this exemplary circuit 520, multiple, small filters 524 are grouped together to form a plurality of inductive sections 540 positioned in a spaced apart relationship along the length of transmission line 522. This grouping of filters collectively increases the impedance of each filter and reduces the current along the transmission line 522. As in other embodiments the filter component at the antenna/transmission line interface 528 may include an LC filter, resonant LC filter, or transformer (in place of a single inductive segments or group of inductive segments) that is adapted to effectively block RF induced current from entering the antenna. Groups 540 of filters 524 distributed along the length of transmission line 522 attenuate the induced current on the transmission line itself before the current reaches the distal end of the transmission line assembly 500.

[0067] Referring now to FIGS. 6A-6C, an alternative embodiment 600 of the invention is shown. As can be seen in FIG. 6A two transmission lines 640, 650 are provided and wound in a co-radial fashion. Each of the transmission lines includes a center conductor 670 and a shield 680. The co-radially wound transmission lines 640, 650 share a common magnetic flux channel in the center of the windings, such that common mode RF present on both transmission lines will tend to cancel and thus be attenuated. This co-radial approach may be expanded to more than two transmission lines and may comprise any number of co-radially wound transmission lines.

[0068] Referring to FIG. 6B, transmission line assembly 600 includes elongate body 610 surrounded by jacket 611. Elongate body 610 includes first 612 and second 614 ends and includes lumen 616 therewithin. Second end 614 is adapted to be connected to electronics, internal or external to the patient body, and may include a connector (not shown). Lumen 616 houses co-radially wound transmission lines 640, 650. In an alternative embodiment, best shown in FIG. 6C, co-radially wound transmission lines 640, 650 may be embedded in jacket 611. Each co-radially wound transmission line 640, 650 comprises a single length of transmission line thereby eliminating the need for bonding points and reducing the possibility of mechanical failure of the line. The transmission lines 640, 650 are wound in the same direction and the coils have the same diameter. When the transmission line assembly is exposed to an RF field, as during an MRJ scan, the co-radially wound transmission lines 640, 650 tend to block higher frequency common mode RF current from being transmitted along the length of an individual transmission line. Thus, in operation the co-radially wound

transmission lines 640, 650 may act like inductive filters to attenuate the induced current on the transmission line itself before the current reaches the antenna. Each co-radially wound transmission line 640, 650 may have an equal or unequal number of turns. Preferably, however, the transmission lines 640, 650 include an equal number of turns to minimize the amount of RF leakage from the coil, such leakage resulting in less effective RF current blocking. In the embodiment shown in FIG. 6B and 6C, the co-radially wound transmission lines 640, 650 extend substantially along the entire length of the transmission line assembly. In other embodiments (not shown) the co-radial transmission lines may extend only along a portion of the body.

[0069] In the exemplary coiled configuration, first and second transmission lines are electrically insulated from one another. Both the first and second transmission lines 640, 650 may include an insulative or non-conductive coating. The insulative coating may be formed of a polyurethane material, nylon, polyester, polyester-amide, polyester-imide, polyester-amide-imide, silicone material, Teflon (FEP), expanded tetrafiuoroethylene (eTFE),

Polytetrafluoroethylene (pTFE), and the like. One benefit of using a coating such as Teflon is that the transmission line assembly may be easily inserted into jacket 611 during manufacture. Alternatively, only one transmission line may be insulated. In any case, transmission lines should be electrically isolated from each other.

[0070] As in previous embodiments, each co-radially wound transmission line 640, 650 is constructed from a single, continuous length of non-magnetic wire such as silver plated copper, copper, titanium, titanium alloys, tungsten, gold and combinations of the foregoing. If each transmission line is constructed from one continuous length, the transmission line may be adhesively bonded or have a bondable coating to permit formation of the filters during manufacture with one wire or cable. Alternatively, several lengths of non-continuous transmission line may be used and still fall within the intended scope of the invention. In such case the transmission lines may be cast in silicone and heat-treated in certain location to ensure that the line does not shift.

[0071] As with other embodiments, transmission lines 640, 650 are co- radially wound over a length of flexible tubing 340 made from polyimide, polyolefin, pTFE, eTFE, polyetherketone (PEK) and other similar flexible materials. The choice between utilizing co-radially wound transmission lines versus discrete inductive segments on each transmission line depends on several factors. Co-radially wound transmission lines can be implemented in a smaller diameter device, since one transmission line never needs to pass over or under another transmission line. However, the impedance of the discrete inductor approach may be more predictable and is not as dependent on length or bend of the device.

[0072] In one embodiment, the transmission line may be a 42 AWG Pico Coax transmission line for a circuit that is approximately one meter in length. Numerical modeling such as for example Finite Difference Time Domain (FDTD) or Method of Moments may be used to approximate the expected current for a particular device. The length of transmission line being used and the expected trajectory in the patient determines the desired total impedance across the circuit. Thus, for any particular length of transmission line the appropriate gauge may then be selected.

[0073] FIG. 7 is a detailed sectional view of yet another embodiment of the invention that is similar to the embodiment of FIG. 3 A. The transmission line assembly 700 of FIG. 7 includes elongate body 710 surrounded by jacket 711. Elongate body 710 includes first 712 and second 714 ends and includes lumen 716 therewithin. Second end 714 is adapted to be connected to electronic controls, internal or external to the patient body, and may include a connector (not shown). Lumen 716 houses circuits 720, 721. Circuits 720, 721 each include one transmission line 722, 723, respectively, located within the lumen 716. In an alternative embodiment, transmission lines 722, 723 can be embedded in jacket 711 to decrease the overall diameter of the transmission line assembly 700 as previously discussed. Each transmission line 722, 723 comprises a single length of coaxial transmission line, each of which forms a plurality of spaced apart filter components 724, 725, respectively. Filter components 724, 725 comprise filters or inductors that are spaced apart along the length of transmission lines 722, 723. Antennas 719, 718 are located on the first end 712 of elongate body 710 and are electrically coupled to the first and second transmission lines 722, 723, respectively.

[0074] As further illustrated in FIG. 7, the LC filters 326, 327 of the transmission line assembly 300 of FIG. 3 A have been replaced with inductive coupling components 726, 727. In one exemplary embodiment, the inductive coupling components 726, 727 may comprise co-radial inductive coupling mechanisms or transformers. A diagram illustrating one such inductive coupling component is illustrated in FIG. 8.

[0075] The co-radial inductive coupling mechanisms or transformers 726, 727 may or may not include a matching network at one or both ports. Particularly, matching networks are not required when, for example, the transformer is constructed such that it is inherently tuned. That is, the insertion loss allows for sufficient transmission of differential signals without excessive loss at the MR operating frequency. [0076] As illustrated in FIG. 8, a co-radial transformer 726 may be constructed by winding two wires side by side and in the same orientation over flexible tube 730 or a tube of similar construction that is placed over tube 730. A first port of the transformer 726 may be connected the transmission line and may or may not pass through a matching network. A second port of the transformer 726 may be connected to the input of the antenna or the matching network of the antenna 719.

[0077] FIGS. 9 and 10 are diagrams illustrating an exemplary alternative inductive coupling mechanism in accordance with the invention. Particularly, a PCB based inductive coupling mechanism or transformer 826 may be created by forming two overlapping loops 842 and 844 that allow for inductive coupling therebetween. The first loop 842 may be connected to the inputs 843 for the transmission line through a matching network 846 and the second loop 844 may be connected to the inputs 847 of the antenna 818 either directly as illustrated in FIG. 9, or indirectly as illustrated in FIG. 10 by passing the second loop 844 through a matching network 848 before entering the antenna 818. Integrating the inductive coupling mechanism into the connected circuitry reduces the number of connection points, which make the entire structure more robust and can reduce loss.

[0078] The transformer is illustrated in FIG. 9 as being integrated into the antemia 818 merely for purposes of example and not limitation. Furthermore, the first loop 842 is shown to be larger than the second loop 844 merely for purposes of example and not limitation. Thus, the size and orientation of the loops are presented for illustrative purposes only and are not intended to limit the actual structure and design of the transformer. [0079] As will be appreciated by those skilled in the art, FIGS. 7-10 are presented to generally illustrate the use of an inductive coupling mechanism in conjunction with any of the transmission line configurations discussed herein. Further, co-radial and PCB based transformers represent only two types of coupling mechanisms that may be used in accordance with the invention. Thus, numerous configurations other than those illustrated in FIGS. 7-10 are

contemplated and within the intended scope of the present invention.

[0080] 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.

Claims

We claim:

1. A transmission line assembly comprising:

an elongate body having a proximal end and a distal end, said elongate body defining a lumen therewithal, the distal end arranged and configured to house a connected circuitry component and the proximal end operably coupled to an electronic control;

at least one connected circuitry component located on the elongate body; and

at least one electrical circuit in communication with said at least one connected circuitry component, said circuit housed within said elongate body and comprising one or more transmission lines, said one or more transmission lines forming at least one filter.

2. The transmission line assembly of claim 1 wherein said at least one connected circuitry component comprises a plurality of antennas.

3. The transmission line assembly of claim 1 wherein said one or more transmission lines comprises one or more single, continuous lengths of

transmission line.

4. The transmission line assembly of claim 1 wherein said at least one filter is constructed from a separate length of transmission line that is not continuous with the remaining length of transmission line.

5. The transmission line assembly of claim 1 wherein said one or more transmission lines comprises multiple lengths of non-continuous transmission line.

6. The transmission line assembly of claim 1 wherein said at least one filter comprises a plurality of filters positioned in a spaced apart relationship along a length of said electrical circuit.

7. The transmission line assembly of claim 1 wherein said at least one electrical circuit comprises a plurality of circuits and said at least one connected circuitry component comprises a plurality of antennas wherein each one of said plurality of circuits is electrically coupled to a separate antenna.

8. The transmission line assembly of claim 1 wherein said elongate body comprises a catheter.

9. The transmission line assembly of claim 8 wherein said catheter is 10 French or less.

10. The transmission line assembly of claim 1 wherein said transmission line assembly is used in an implantable medical device.

11. The transmission line assembly of claim 1 wherein said transmission line assembly is used in a non-implantable medical device.

12. The transmission line assembly of claim 1 wherein said at least one connected circuitry component comprises a circuit board trace.

13. The transmission line assembly of claim 1 wherein said transmission line comprises a conductive lumen.

14. The transmission line assembly of claim 1 wherein said transmission line is selected from the group consisting of silver plated copper, copper, titanium, titanium alloys, tungsten, gold and combinations of the foregoing.

15. The transmission line assembly of claim 1 wherein the one or more transmission lines includes an insulative coating bondable by heat, chemical or adhesive means.

16. The transmission line assembly of claim 1 wherein said at least one filter effectively blocks RF induced current from exiting said transmission line assembly.

17. The transmission line assembly of claim 1 wherein said at least one filter attenuates induced current on said transmission line before said current reaches said at least one connected circuitry component.

18. The transmission line assembly of claim 1 wherein said at least one filter is constructed such that inductive and capacitive characteristics resonate to create a maximal impedance at approximately 64 MHz for a 1.5 Tesla MRI.

19. The transmission line assembly of claim 1 wherein said at least one filter is constructed such that inductive and capacitive characteristics resonate to create a maximal impedance at approximately 128 MHz for a 3.0 Tesla MRI.

20. The transmission line assembly of claim 1 wherein said at least one filter is formed with between approximately 20 and approximately 45 turns of

transmission line.

21. The transmission line assembly of claim 6 wherein said spaced apart relationship comprises uniform or non-uniform spacing.

22. The transmission line assembly of claim 8 wherein said catheter includes a catheter jacket.

23. The transmission line assembly of claim 22 wherein said circuit is partially or wholly embedded in said catheter jacket.

24. The transmission line assembly of claim 2 wherein said antennas comprise printed circuit boards.

25. The transmission line assembly of claim 2 wherein said antennas comprise solenoid antennas.

26. The transmission line assembly of claim 2 wherein said antennas are positioned in a parallel relationship on either side of said elongate body.

27. The transmission line assembly of claim 15 wherein said insulative coating is selected from the group consisting of fluorinated ethylene propylene, polyurethane, nylon, polyester, polyester-amide, polyester-imide, polyester- amide-imide and combinations of the foregoing.

28. The transmission line assembly of claim 1 wherein said electrical circuit is housed entirely within the lumen of said elongate body.

29. The transmission line assembly of claim 1 further comprising a flexible tube around which said at least one filter is helically wound.

30. The transmission line assembly of claim 1 wherein said at least one filter is constructed from a plurality of inductive windings.

31. The transmission line assembly of claim 1 wherein the one or more transmission lines includes a Teflon or FEP coating.

32. A transmission line assembly comprising:

an elongate body having a proximal end and a distal end, said elongate body defining a lumen therewithin, the distal end arranged and configured to house a connected circuitry component and the proximal end operably coupled to an electronic control;

at least first and second connected circuitry components located within the elongate body;

at least first and second transmission lines co-radially wound;

at least first and second filters formed from said at least first and second transmission lines and in electrical communication with said first and second connected circuitry components.

33. The transmission line assembly of claim 32 wherein said at least first and second connected circuitry components comprise a plurality of antennas.

34. The transmission line assembly of claim 32 wherein said at least first and second transmission lines comprise a plurality of co-radially wound transmission lines.

35. A method of constructing a transmission line assembly including two or more electrical circuits for use in implantable and non-implantable medical devices comprising:

providing a first length of transmission line to form a first electrical circuit; helically winding said first length of transmission line over a length of flexible tubing from a first end of said circuit to a second end of said circuit; forming a plurality of filters from said first length of transmission line in a spaced apart relationship along said flexible tubing;

providing a second length of transmission line to form a second electrical circuit;

helically winding said second length of transmission line over said length of flexible tubing from a first end of said circuit to a second end of said circuit; and

forming a plurality of filters from said second length of transmission line in a spaced apart relationship along said flexible tubing.

36. The method of claim 35 further comprising providing a third length of transmission line to form a third electrical circuit;

helically winding said third length of transmission line over said length of flexible tubing from a first end of said circuit to a second end of said circuit; and forming a plurality of filters from said third length of transmission line in a spaced apart relationship along said flexible tubing.

37. The method of claim 35 wherein said first end is a proximal end and said second end is a distal end.

38. The method of claim 35 wherein said first end is distal end and said second end is a proximal end.

39. The transmission line assembly of claim 6 wherein said plurality of filters comprise multiple filters that are grouped together in a spaced apart relationship.

40. The transmission line assembly of claim 6 wherein said plurality of filters create at least 1,000 or more Ohms of impedance along said electrical circuit given a transmission line assembly length of one meter.

41. The transmission line assembly of claim 1 wherein the circuit is flexible.

42. The transmission line assembly of claim 1 wherein the circuit is rigid.

43. The transmission line assembly of claims 1 , 32 and 35 wherein the transmission line assembly is MRI compatible.

44. The transmission line assembly of claim 1 wherein said at least one filter is selected from the group consisting of an LC filter, a resonant LC filter, and a transformer.

45. The transmission line assembly of claim 1 wherein the at least one filter is designed to pass differential mode currents and attenuate or block common mode currents.

46. The transmission line assembly of claim 1 wherein the at least one filter is designed to pass differential mode currents and attenuate or block shield currents.

47. The transmission line assembly of claim 1 wherein a transformer is placed adjacent to an interface between the one or more transmission lines and the at least one connected circuitry component for blocking common mode and shield currents from entering the at least one connected circuitry component.

48. The transmission line assembly of claim 47 wherein the transformer at the interface is a co-radial transformer.

49. The transmission line assembly of claim 47 wherein the transformer is formed by two overlapping PCB based loops.

50. The transmission line assembly of claim 47 wherein the transformer is integrated into the at least one connected circuitry component.

51. The transmission line assembly of claim 1 wherein said at least one filter utilizes capacitance between adjacent sections of the transmission line to increase the impedance provided by the filter.