Patent Publication Number: US-8116846-B2

Title: Intravascular antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims priority to, U.S. application Ser. No. 10/840,549, filed on May 6, 2004, which is issued as U.S. Pat. No. 7,496,397 on Feb. 24, 2009, entitled INTRAVASCULAR ANTENNA, the entire contents of which are incorporated herein by reference. 
     The present application hereby makes reference to U.S. patent application Ser. No. 10/008,380, filed Nov. 13, 2001, entitled IMPEDANCE-MATCHING APPARATUS AND CONSTRUCTION FOR INTRAVASCULAR DEVICE, which is issued as U.S. Pat. No. 7,194,297 on Mar. 20, 2007. The present application also makes reference to pending U.S. patent application Ser. No. 11/954,861, filed Dec. 12, 2007, entitled APPARATUS AND CONSTRUCTION FOR INTRAVASCULAR DEVICE. 
    
    
     BACKGROUND 
     The present invention relates generally to intravascular devices. More particularly, the present invention relates to antennae used in intravascular magnetic resonance imaging (MRI) devices. Intravascular imaging involves generating an image of tissue surrounding an intravascular device. Visualization involves generating an image of a catheter or other intravascular device on another image, or by itself, usually through localized signals from tissue immediately adjacent the device. 
     Imaging, visualization and tracking of catheters and other devices positioned within a body may be achieved by means of a magnetic resonance imaging (MRI) system. Typically, such a magnetic resonance imaging system may be comprised of a magnet, a pulsed magnetic field gradient generator, a transmitter for electromagnetic waves in radio frequency (RF), a radio frequency receiver, and a controller. In a common implementation, an antenna is disposed either on the device to be tracked or on a guidewire or catheter (commonly referred to as an MR catheter) used to assist in the delivery of the device to its destination. In one known implementation, the antenna comprises an electrically conductive coil that is coupled to a pair of elongated electrical conductors that are electrically insulated from each other and that together comprise a transmission line adapted to transmit the detected signal to the RF receiver. 
     In one embodiment, the coil is arranged in a solenoid configuration. The patient is placed into or proximate the magnet and the device is inserted into the patient. The magnetic resonance imaging system generates electromagnetic waves in radio frequency and magnetic field gradient pulses that are transmitted into the patient and that induce a resonant response signal from selected nuclear spins within the patient. This response signal induces current in the coil of electrically conductive wire attached to the device. The coil thus detects the nuclear spins in the vicinity of the coil. The transmission line transmits the detected response signal to the radio frequency receiver, which processes it and then stores it with the controller. This is repeated in three orthogonal directions. The gradients cause the frequency of the detected signal to be directly proportional to the position of the radio-frequency coil along each applied gradient. 
     The position of the radio frequency coil inside the patient may therefore be calculated by processing the data using Fourier transformations so that a positional picture of the coil is achieved. In one implementation this positional picture is superposed with a magnetic resonance image of the region of interest. This picture of the region may be taken and stored at the same time as the positional picture or at any earlier time. 
     In a coil-type antenna such as that described above, it is desirable that the impedance of the antenna coil substantially match the impedance of the transmission line. In traditional impedance matching of MRI coils, shunt-series or series shunt capacitor combinations suffice to tune the coil. In such traditional applications, the capacitors almost never pose a size constraint. 
     However, for intravascular coils, miniaturization of the tuning capacitors is necessary. Discrete components have been employed to construct matching and tuning circuits on intravascular devices. But such components are bulky and are not easily incorporated into the design of the device. Also, placement of the tuning capacitors away from the coil without a reduction in the signal-to-noise ratio (SNR) is desirable. It has been proposed to use open circuit stub transmission lines as a means of fabricating arbitrary or trimmable capacitors and to use short-circuited stubs as tuning inductors. Such probes are tuned by trimming the length of the coaxial cables. However, these circuits still result in a relatively large device that is not ideal for intravascular navigation. Also, the circuits require many connections and the fabrication process is relatively complex. 
     Another problem that arises with intravascular MRI antenna devices is that present day devices have a number of limitations. Such antennas include single loops, and saddle coils. Those antennas are resonant at some frequency, but that frequency is typically in the gigahertz range which is quite high. The antennae are typically small compared to the wavelength of the signal to be detected. Therefore, it is very difficult to tune a conventional coil antenna to optimize for that wavelength. In prior systems, the circuit was tuned using the discrete inductors and capacitors on the coil antenna mentioned above. 
     It can thus be seen that, often, it is desirable to make the antenna as large as possible. However, the diameter of the antenna is limited by the need to access the vessel. Simple expanding loops partially address this limitation, but they can be difficult to deploy in the vessel. 
     The present invention addresses at least one, and possibly more, of these, problems and offers advantages over the prior art. 
     SUMMARY 
     The present invention relates to an elongated intravascular device adapted to be advanced through a vessel of a body. The present invention further includes an antenna which is disposed on an inflatable member such that the antenna can be increased or decreased in size to more accurately tune the systems in which it is employed. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial block diagram of an illustrative magnetic resonance imaging and intravascular guidance system in which embodiments of the present invention can be employed. 
         FIG. 2  is a schematic diagram of an impedance-matching circuit that is known in the art. 
         FIGS. 3A and 3B  illustrate an antenna disposed on an inflatable member in the contracted and expanded or deployed positions, respectively. 
         FIGS. 4A and 4B  illustrate another type of antenna on an inflatable member in the retracted and expanded or deployed positions, respectively. 
         FIG. 5  illustrates yet another type of antenna disposed on an inflatable member. 
         FIGS. 6A and 6B  illustrate-one embodiment of an antenna printed on a flex circuit. 
         FIGS. 6C and 6D  illustrate the flex circuit shown in  FIGS. 6A and 6B  disposed on an inflatable member in the retracted and deployed positions, respectively. 
         FIG. 6E  is a cross sectional view of the flex circuit and inflatable member shown in  FIG. 6D . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a partial block diagram of an illustrative magnetic resonance imaging, visualization and intravascular guidance system in which embodiments of the present invention could be employed. In  FIG. 1 , subject  100  on support table  110  is placed in a homogeneous magnetic field generated by magnetic field generator  120 . Magnetic field generator  120  typically comprises a cylindrical magnet adapted to receive subject  100 . Magnetic field gradient generator  130  creates magnetic field gradients of predetermined strength in three mutually orthogonal directions at predetermined times. Magnetic field gradient generator  130  is illustratively comprised of a set of cylindrical coils concentrically positioned within magnetic field generator  120 . A region of subject  100  into which a device  150 , shown as a catheter, is inserted, is located in the approximate center of the bore of magnet  120 . 
     RF source  140  radiates pulsed radio frequency energy into subject  100  and the MR active sample within device  150  at predetermined times and with sufficient power at a predetermined frequency to nutate nuclear magnetic spins in a fashion well known to those skilled in the art. The nutation of the spins causes them to resonate at the Larmor frequency. The Larmor frequency for each spin is directly proportional to the strength of the magnetic field experienced by the spin. This field strength is the sum of the static magnetic field generated by magnetic field generator  120  and the local field generated by magnetic field gradient generator  130 . In an illustrative embodiment, RF source  140  is a cylindrical external coil that surrounds the region of interest of subject  100 . Such an external coil can have a diameter sufficient to encompass the entire subject  100 . Other geometries, such, as smaller cylinders specifically designed for imaging the head or an extremity can be used instead. Non-cylindrical external coils such as surface coils may alternatively be used. 
     Device  150  is inserted into subject  100  by an operator. Device  150  may be a guide wire, a catheter, an ablation device or a similar recanalization device. Device  150  includes an RF antenna which detects MR signals generated in both the subject and the device  150  itself in response to the radio frequency field created by RF source  140 . Since the internal device antenna is small, the region of sensitivity is also small. Consequently, the detected signals have Larmor frequencies which arise only from the strength of the magnetic field in the proximate vicinity of the antenna. The signals detected by the device antenna are sent to imaging, visualization and tracking controller unit  170  via conductor  180 . 
     In an illustrative embodiment, external RF receiver  160  is a cylindrical external coil that surrounds the region of interest of subject  100 . Such an external coil can have a diameter sufficient to encompass the entire subject  100 . Other geometries, such as smaller cylinders specifically designed for imaging the head or an extremity can be used instead. Non-cylindrical external coils, such as surface coils, may alternatively be used. External RF receiver  160  can share some or all of its structure with RF source  140  or can have a structure entirely independent of RF source  140 . The region of sensitivity of RF receiver  160  is larger than that of the device antenna and can encompass the entire subject  100  or a specific region of subject  100 . However, the resolution which can be obtained from external RF receiver  160  is less than that which can be achieved with the device antenna. 
     The RF signals detected by external RF receiver  160  are sent to imaging, visualization and tracking controller unit  170  where they are analyzed together with the RF signals detected by the device antenna. A number of embodiments which are employed to address the difficulties in making connections to the antenna and to address fabrication process complexity are described in pending U.S. application Ser. No. 11/954,861 entitled APPARATUS AND CONSTRUCTION FOR INTRAVASCULAR DEVICE, filed on Dec. 12, 2007. That application also discusses a number of embodiments showing alternating layers of conductors and dielectric materials to construct components and circuits that can be used to tune a circuit that includes the intravascular device or to match impedances among components or segments of such a circuit. 
     To address the above-described problem, an illustrative embodiment set out in detail in the co-pending referenced application employs alternating layers of conductors and dielectric materials to construct components and circuits that can be used to tune a circuit of the intravascular device or to match impedances among components or segments of such a circuit.  FIG. 2  is a schematic diagram of an impedance-matching circuit  200  that is known in the art. Impedance-matching circuit  200  includes transmission lines  202 ,  204 , capacitances  206 ,  208 ,  210 , and inductive coil  212 . This circuit, with nodes  214 ,  216 ,  218 ,  220  and  222  is used in impedance matching. 
     Another problem associated with intravascular devices for use in the system shown in  FIG. 1  arises with respect to employing a suitable antenna within the patient.  FIG. 3A  is a side view of an intravascular device  300  in accordance with one illustrative embodiment of the present invention. Intravascular device  300  illustratively includes an inflatable portion  302  supported by an elongate member (such as a catheter or guidewire)  304 . In the embodiment shown in  FIG. 3A , a conductor  306 , with associated connection pads  308  are disposed on the inflatable member  302  to collectively form antenna portion  310 . 
     In one illustrative embodiment, intravascular device  300 , along with antenna  310 , functions as an antenna in a system such as that shown in  FIG. 1 , to receive RF signals and transmit the signals back to a receiver/controller. In an alternative embodiment, intravascular device  300  performs functions in addition to acting as an antenna. For example, in one embodiment, device  300  can also serve as a guidewire used to assist in the delivery of another intravascular device to an intravascular location. In yet another embodiment, device  300  can serve as an ablation device used to disintegrate an occlusion in a vessel. In still another embodiment, device  300  is deployed as a dilatation device or other device supported by a catheter, and device  300  can be formed integrally with catheter  304  or separately therefrom. 
       FIG. 3A  shows device  300  in its radially collapsed or retracted, delivery position and  FIG. 3B  shows device  300  in a radially expanded, or deployed position. In  FIG. 3A , inflatable member  302  is deflated, and can be twisted or folded over on itself. When inflatable member  302  is inflated to the position shown in  FIG. 3B , conductor  306  expands in the direction indicated by arrows  312  and conductor  306  forms a single turn antenna on the surface of inflatable member  302 . When expandable member  302  is deflated, antenna  310  moves in a direction opposite arrows  312  and thus acts to recoil to the position shown in  FIG. 3A , either by folding or slightly twisting the material forming expandable member  302 . 
     Connection pads  308  can be coupled to one or more conductors forming a transmission line extending proximally to the system shown in  FIG. 1 , across the dielectric balloon material. This can act as a variable tuning capacitor in the circuit since the capacitance may change as the balloon pressure is changed. 
     In one illustrative embodiment, inflatable member  302  is formed of a non-compliant material, such as polyethylene terephthalate (PET). This tends to avoid sheer failure at an interface between conductor  306  and connection pads  308  and the material forming inflatable member  302 . 
     Conductor  306  and pads  308  can be printed on the surface of inflatable member  302  using any number of a variety of techniques. For example, lithography or transfer printing can create the patterns. For the conductive portions of antenna  310 , gold may be one illustrative conductor for the circuits. In addition to gold, other materials can be used for the conductive portions of antenna  310  as well. For example, conductive polymers (polymers loaded with conductive particles) or other materials can used as well. 
     Inflation and flexing of inflatable member  302  can create additional stresses on the interface between inflatable member  302  and antenna  310 . In addition, folding of inflatable member  302  can create sheer stress between the balloon and the conductors, and can also create tensile stresses in the conductors themselves. Thus, in one illustrative embodiment, the conductors  306  and  308  are not disposed over folding portions of inflatable member  302 . In another embodiment, a tie layer  314  is used between the conductive portions of antenna  310  and the surface of inflatable member  302 . The tie layer exhibits flexible, elastic characteristics that minimize sheer stress at the interface between the two materials. The tie layer can be any suitable form of material or coating that exhibits desired elastic behavior to reduce stresses seen at the interface between antenna  310  and expandable member  302  to a desirable level. In another embodiment, areas of the antenna are configured (such as with increased flexibility) to be more easily folded or bent without fatiguing and without exhibiting unduly increased stress. 
       FIGS. 4A and 4B  show another device  400  which is similar to device  300  shown in  FIGS. 3A and 3B , except that the antenna  410  forms a multiple turn antenna, rather than a single turn antenna. When inflatable member  302  is deflated, connection pads  408  move in the directions indicated by arrows  412  thus causing antenna  410  to coil about inflatable member  302  by folding inflatable member  302 , or by slightly twisting it. However, when inflatable member  302  is expanded to the position shown in  FIG. 4B , connection pads  408  move in the direction indicated by arrows  414  to cause the diameters of the turns in the antenna  410  to increase. 
       FIG. 5  illustrates yet another type of antenna which can be disposed on the surface of inflatable member  302 .  FIG. 5  is similar to the embodiments shown in  FIGS. 3A-4B , and some items are similarly numbered. However, the antenna disposed on inflatable member  302  in  FIG. 5  is a fractal antenna. As used herein, a fractal antenna is an antenna which takes one of two main geometric types (deterministic, and random, or chaotic). Random fractals may appear as random walks, dendrites, or similar to lightning bolts. Deterministic fractals apply a generator on successive size scales. For purpose of the present discussion, deterministic fractals are deemed to have a finite number of (and at least 2) self-similar iterations. 
     Fractal antennas can be formed much smaller than conventional antennas, with the same performance results. Similarly, fractal antennas are “self-loading” so fewer coils and capacitors are required to make them resonant. Some fractal antennas are available from Fractal Antenna Systems, Inc. of Maiden, Mass. 
     Thus,  FIG. 5  illustrates a device  500 , similar to devices  300  and  400 , except that it employs a fractal antenna  502 . Of course, the fractal antenna  502  can be deterministic or random, with any suitable number of iterations, as desired. Thus, one embodiment of the invention utilizes a “complex” antenna shape that is more geometrically complex than simple loop or saddle loop antenna configurations. For instance, these complex shapes have more geometric features (e.g., angles, shapes or properties) than a simple loop or saddle loop and may illustratively be put on the inflatable member using a technique, or in a configuration, that facilitates folding or collapsing of the inflatable member without breaking or rendering electrically discontinuous, the antenna. This can be done, for example, by printing the complex antenna shape on the inflatable member, by using a tie layer (described below), by making fold regions in the antenna shape that are of increased resilience or otherwise less susceptible to damage from folding or collapsing the inflatable member, or simply by positioning the complex antenna shape on the inflatable member so it need not be folded. 
       FIGS. 6A-6E  illustrate another embodiment of the present invention.  FIG. 6A  shows that in one illustrative embodiment, an antenna  610 , which includes a conductive portion  612  and connection pads  614 , is disposed on a flexible circuit material  616 . In one illustrative embodiment, antenna  610  is printed on flex circuit substrate  616  which is formed from a suitable material, such as polyimide. 
     In one illustrative embodiment, substrate  616  and antenna  610  are biased into a wrapped conformation (or scrolled shape) such as that shown in  FIG. 6B . Flex circuit substrate  616 , with antenna  610  disposed thereon, is then placed on an inflatable member  618 , as shown in  FIG. 6C .  FIG. 6C  shows that inflatable member  618  is similar to inflatable member  302 , and can be supported by a catheter  304 .  FIG. 6C  also shows that inflatable member  618  is in the collapsed or uninflated position. 
     In one illustrative embodiment, flex circuit substrate  616  is attached only at one place (e.g., an end adjacent pad  614  or at an intermediate point) to the surface of inflatable member  618 . Thus, as inflatable member  618  is inflated to the position shown in  FIG. 6D , the attached, portion of substrate  616  remains fixed to the surface of inflatable member  616  while the remainder is allowed to move or slide relative to the surface of inflatable member  618 . 
     This effectively causes the ends of substrate  616  to move relative to one another in the direction indicated by arrow  620  in  FIG. 6C  to the position shown in  FIG. 6D . Of course, this causes antenna  610  (which is shown as a single loop antenna but could be formed as any other antenna as well) to increase in its deployed diameter. 
       FIG. 6E  shows a cross section of one embodiment of the device illustrated in  FIGS. 6A-6D .  FIG. 6E  shows conductor  612  disposed on substrate  616  which is, itself, attached to the surface of inflatable member  618 . The flex circuit substrate  616  can be attached using an adhesive, directly onto the surface of inflatable member  618  or it can be attached to a tie layer  622  disposed at the interface of inflatable member  618  and substrate  616 . The flex circuits are commercially available and generally known. The tie layer may illustratively be any desirable tie layer, such as polyvinyl acetate, low durometer nylon, or polyester, etc. Other tie layers can be used as well. 
     It can thus be seen that the present invention provides unique devices for use in antennas in MRI systems. Such devices provide significant advantages over prior devices. The antenna in accordance with the present invention can take substantially any form such as a monopole, dipole, solenoid, fractal, etc. The present invention is contemplated to cover all antenna configurations. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.