Patent Publication Number: US-7725160-B2

Title: Tunable MRI enhancing device

Description:
REFERENCE TO COPENDING APPLICATION 
   Reference is made to the following related application: IMPEDANCE-MATCHING APPARATUS AND CONSTRUCTION FOR INTRAVASCULAR DEVICE, filed Nov. 13, 2001, Ser. No. 10/008,380 and assigned the same assignee as the present invention. 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to intravascular devices. More particularly, the present invention relates to tunable MRI enhancing devices, such as filters and balloon catheters. 
   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 transmitting 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. 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, in the past, an active MRI self-expanding stent design has been developed. In this design, a surface mountable capacitor was mounted to a stent to produce an LC circuit using the stent as a coil. However, in order to match the resonating frequency of the circuit to the Larmor frequency, the surface mount capacitor had to be tuned prior to implementation. The resonant frequency of such a circuit depends on the diameter of the expanded stent, and is thus not known prior to implementation. Therefore, any tuning procedure prior to implementation carries the risk of mis-tuning. Also, as discussed above, the addition of a surface mount capacitor increases the thickness of the stent. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an MRI enhancing device deployable in a body. In one embodiment, the MRI enhancing device is formed of a wire loop and a capacitor that is at least partially formed by the wire used in forming the wire loop. 
   In another embodiment, the resonant frequency is tunable even when the device is deployed in the body. This can be done using a dielectric material that can be selectively moved between plates of the capacitor to change the capacitance. 
   The invention can be implemented in a variety of devices, such as balloon catheters and filters, for example. 

   
     BRIEF DESCRIPTION OF THE 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 illustration of a system for enhancing an MRI signal. 
       FIGS. 3 and 4  illustrate portions of an intravascular distal protection filter. 
       FIGS. 5A and 5B  illustrate a distal protection filter in conjunction with a guidewire formed as an LC circuit. 
       FIGS. 6A-6C  illustrate another embodiment of a distal protection filter employed as an LC circuit for enhancing MRI signals. 
       FIG. 7  is another embodiment of a coil. 
       FIG. 8  is another embodiment of a coil. 
       FIGS. 9A and 9B  illustrate a balloon catheter, a portion of which forms an LC circuit for enhancing MRI signals. 
   

   DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     FIG. 1  is a partial block diagram of an illustrative magnetic resonance imaging 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, a filter, an ablation device or a similar recanalization or other 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 and tracking controller unit  170  via conductor  180 . 
   External RF receiver  160  also detects RF signals emitted by the subject in response to the radio frequency field created by RF source  140 . 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 and tracking controller unit  170  where they are analyzed together with the RF signals detected by the device antenna. 
   The position of device  150  is determined in imaging and tracking controller unit  170  and is displayed on display means  180 . In an illustrative embodiment, the position of device  150  is displayed on display means  180  by superposition of a graphic symbol on a conventional MR image obtained by external RF receiver  160 . Alternatively, images may be acquired with external RF receiver  160  prior to initiating tracking and a symbol representing the location of the tracked device can be superimposed on the previously acquired image. Alternative embodiments of the invention display the position of the device numerically or as a graphic symbol without reference to a diagnostic image. 
   When performing MRI, tuning the resonant frequency of an implanted antenna (or coil) to the Larmor frequency of the surrounding protons enhances their MR visibility. Using a receiver coil outside the body, as illustrated with respect to  160  in  FIG. 1 , the resonating circuit inside the body induces currents in the receiver coil  160  outside the body, and by this configuration, the MR signal from the area directly surrounding the implanted device can be enhanced. This is better illustrated with respect to  FIG. 2 . 
   As shown in  FIG. 2 , coil  200  is implanted within subject  100 . Receiver coil  160  resides outside of subject  100 . The magnetic field lines (shown generally at  202 ) of implanted coil  200  pass through receiver coil  160  positioned outside of subject  100 . Of course, as discussed above, receiver coil  160  is connected to further electronics to enable visualization. Thus, resonant coil  200  induces currents in receiver coil  160  which enhance the MR signal in the area directly surrounding coil  200 . As discussed above, it is desirable to match the resonating frequency of the resonant coil  200  to the Larmor frequency (63.6 Megahertz at 1.5 Tesla, or 42.4 Megahertz per Tesla). 
     FIGS. 3 and 4  illustrate portions of a known distal vascular protection filter  204 . It can be seen that filter  204  is deployed by a core guidewire  206  that is attached to a filter portion  208  and includes a radioopaque tip  210 . The filter  208  is carried on guidewire  214  such that it can be advanced distal of a lesion or obstruction in a patient&#39;s vessel. To be delivered, filter  208  is illustratively placed within a delivery catheter in a radially collapsed insertion position and advanced distal of the obstruction or lesion. Once the tip of the delivery catheter is past the lesion, filter  204  is removed from the delivery catheter such that filter  208  expands in the vessel. 
   In one embodiment, filter  208  is mounted on a hypotube or sleeve  212  and is positioned between a proximal stop  214  and a distal stop  216 . Stops  214  and  216  may be sufficiently close to one another and to sleeve  212  such that filter  208  has very little perceptible longitudinal movement over guidewire  206 . However, stops  214  and  216  allow guidewire  206  to be rotated within sleeve  212  without disturbing the position of filter  208 . 
     FIG. 3  also illustrates that the proximal facing end of filter  208  is connected to a resilient loop  218 , such as a Nitinol loop, which forms a proximally facing mouth of filter  208 . Loop  218  can be slightly proximally sloping and include an articulation region (shown at  220  in  FIG. 4 ) that is a thinned area of the wire forming loop  218 . This allows loop  218  to collapse and fit within the delivery catheter. 
   A filter sac  222  is also connected to the resilient loop  218 . Sac  222  may illustratively have a number of perforations at its distal end to allow blood or other liquids to pass through the filter sac while capturing any debris that may be dislodged in the vessel. 
   Sac  222  is illustratively formed of a polymer membrane that is connected to loop  218  and guidewire  206  (or to a hypotube or sleeve  212 ) by either laser welding, soldering, adhesive, or another suitable connection mechanism. 
   For removal, a removal catheter (which may be the same as the delivery catheter) is illustratively advanced over guidewire  206  and filter  208  is drawn within the removal catheter. For example, pulling the filter  208  toward the distal end of the removal catheter will illustratively cause loop  218  to collapse. As the filter sac  222  and loop  218  are collapsed, the debris within the filter is illustratively captured within sac  222  and the removal catheter. 
   As discussed above, it is becoming standard procedure to place a filter device, such as that shown in  FIGS. 3 and 4 , distal to a lesion during an angioplasty procedure (or other similar procedure) in order to capture loose particles. It can be seen from  FIG. 3  that the shape of resilient loop  218  lends itself to act as a resonant coil in magnetic resonance imaging. 
   For effective implementation of such a resonating coil, the self-inductance of the coil must be calculated, and a capacitor must be connected to the circuit to create a resonance at the Larmor frequency. The self-inductance of a circular loop of round wire is:
 
 L=μ   0   a (ln(8 a/R )−1.75)  Eq. 1
         Where μ 0 =4π 10 −7  H/m.   R=the radius of the wire; and   a=the radius of the loop.       

   In accordance with one embodiment, “a” is between approximately 1.75 and 2.75 millimeter and the diameter of the round wire used (2R) is 40 micrometers. Thus, using equation 1, the self-inductance L is 1.056e −8 H which equals 10.5 nH. For a larger size filter, with a=2.75, the self-inductance L=18 nH. 
   Next, the capacitance of the capacitor used in the resonating circuit must be determined. The resonating frequency of an LC circuit is: 
   
     
       
         
           
             
               
                 
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   Since the Larmor frequency for a 1.5 T MR tube is 63.6 MHz, the capacitance C must be 0.60 nF (1.34 nF for 1.0 Tesla). 
   In order to minimize the dimension of the capacitor, a material having a very high dielectric constant ∈, may be desired because the capacitance is directly related to ∈. 
   In general, dielectric materials are insulators used for their exceptional dielectric properties. When a material is introduced between two plates of a capacitor, the total charge stored in the capacitor changes. The change depends on the ability of the material to polarize under an electric field. The dielectric constant, or permitivity, of a material determines the change in charge storage. For high capacity applications, a high dielectric constant is desirable. Since the dielectric constant depends on the polarization in the material, Ferroelectric materials may be desirable. High dielectric constant materials are also referred to as high K materials. A high permitivity piezoelectric ceramic is referred to under the designation TRSHK1, which has been developed specifically for biomedical ultrasound applications by TRS ceramics (of the State College of Pennsylvania) such material has a dielectric constant exceeding 6,000. Of course, other materials can be used as well. 
   For a simple plate capacitor, the capacitance is calculated as follows:
 
 C=∈   r ·∈ 0   ·A/d   Eq. 3
         Where ∈ 0 =8.855×10 −12 ;   A=the area of the plates of the capacitor;   d=the distance between the plates; and   ∈ r =the dielectric constants of the material between the plates (where dimensions are in meters).       

     FIGS. 5A and 5B  illustrate one embodiment of implementing an MR enhancing LC circuit in a filter such as that shown in  FIGS. 3 and 4 .  FIG. 5A  illustrates core wire  206  connected to loop  218  as shown in  FIG. 3 . However, instead of welding loop  218  to guidewire  206  to form the loop, only one end  350  of loop  218  is directly connected to guidewire  206 . The other end  352  of loop  218  is separated from end  350  by a ceramic (or other dielectric) material. Thus, ends  350  and  352  form two plates of a capacitor which result in an active electric circuit of a coil (loop  218 ) in series with the capacitance created by plates  350  and  352 . 
     FIG. 5B  illustrates one embodiment of forming the capacitor. The ends  350  and  352  of the wire forming loop  218  are ground to form the hemispherical shapes shown in  FIG. 5B , with substantially flat surfaces  356  and  358  facing one another. The dielectric layer  353  is placed between the flattened faces  356  and  358 . 
   In the exemplary embodiment discussed herein, the diameter of the wire (which can be Nitinol) forming loop  218  is 40 micrometers. By grinding away half of the wire at ends  350  and  352 , this leaves a space between ends  350  and  352  of 40 micrometers. Thus, a flat piece of suitable dielectric material having a thickness of 40 micrometers is placed between the faces  356  and  358  of ends  350  and  352  of the wire. By making the length of ends  350  and  352  approximately 1.12 millimeters (0.044 inches) long, this results in a capacitor having a capacitance of 0.6 nF. In accordance with one embodiment, the capacitor is covered or surrounded by a non-conducting material  360  in order to create a mechanical bond. 
   As is known within MRI, standing RF waves can occur in long metallic sections resulting in heating effects. Therefore, it is desirable that guidewire  206  be formed of metallic sections which are less than 12 centimeters in length (a half wavelength) to avoid standing waves. Similarly, however, guidewire  206  can be formed of a non-conducting material such as glass, or a polymer, or any other suitable material. 
   The capacitor of this embodiment saves significant space over discrete capacitors in prior systems. The capacitance of the capacitor can be trimmed by simply starting with a long length of capacitor and removing pieces of it either by a grinding process, laser ablation, etc., until the resonance frequency matches the Larmor frequency. However, as discussed in the background section, it can be desirable to tune the resonance frequency of the circuit continuously while in the body within the MRI scanning system. 
     FIGS. 6A-6C  illustrate one embodiment in which the circuit can be tuned within the body.  FIGS. 6A-6C  are similar to the embodiment shown in  FIGS. 5A and 5B . However, instead of loop  218  simply being connected to a core wire, it is illustratively connected to a hypotube or sleeve  370 . As shown in  FIG. 6B , ends  350  and  352  are flattened to form a capacitor. Also, a dielectric material  353  is inserted between the plates of the capacitor. 
   However, as shown in  FIG. 6B , an opening  372  is left between the plates of the capacitor in the region containing dielectric material  353 . A dielectric wire  374  is illustratively slidably disposed in opening  372 . Wire  374  illustratively extends proximally to a region where it is accessible by the user. Thus, wire  374  can be slidably moved within hypotube  370  and opening  372 . By advancing or retracting wire  374  distally into or proximally out of opening  372  in the capacitor formed by ends  350  and  352 , the capacitance of the capacitor can be changed because this action introduces more or less dielectric material (a greater or lesser length of wire  374 ) in between the capacitor plates. Of course, it may be desirable that the hypotube  370  be made out of sections of metal with polymer tube connectors, or that it simply be made out of a polymer material, again in order to avoid standing wave heating affects. 
     FIG. 6C  is similar to  FIG. 6B  except that the opening  372  is just large enough for wire  374  to slide through. Thus, more dielectric material  353  is positioned between the plates of the capacitor. 
   A number of other embodiments are contemplated as well. For example, as shown in  FIG. 7 , instead of having a single wire loop  218 , a bundle of wire loops  218  can be used. All of these wire loops can be used to form the opening of the distal protection device. However, by forming the loop out of a bundle of thinner wires, this increases their inductance which allows a reduction in the physical size of the capacitor since less capacitance is required for the resonant LC circuit. 
   Similarly, as shown in  FIG. 8 , instead of simply using the loop which forms the mouth of the filter as the coil, the coil can be extended. In the embodiment shown in  FIG. 8 , an additional wire (such as a very thin strand of spiral gold or platinum  380 ) is connected to, and extends the coil  218 . Strand  380  spirals about the filter sac  222  and terminates with its end  382  located proximally to form one of the plates of the capacitor. 
   In addition,  FIGS. 9A and 9B  illustrate how the present invention can be implemented in a balloon device.  FIG. 9A  illustrates a balloon  400  formed of conventional balloon material and connected to an inflation catheter  402  having a primary inflation lumen  403  therein. A conductive coil  404  is embedded in, or attached to, the balloon material. A pair of leads  406  are connected to the ends of coil or loop  404  and extend proximally. 
     FIG. 9B  illustrates that leads  406  are disposed closely adjacent to one another to form a capacitance therebetween. Also, a lumen  408  is positioned between leads  406 . A ceramic (or other dielectric) wire  410  can thus be slidably moved within lumen  408  to change the amount of dielectric material between leads  406 . This changes the capacitance developed between leads  406 . 
   Since the coil will expand during expansion of balloon  400 , the self-inductance of the coil will change. Expanding the balloon increases the self-inductance of the coil. As this happens, the physician can withdraw the ceramic wire  410  in order to reduce the capacitance associated with leads  406 . Thus, the LC circuit can be continuously tuned. 
   Also, since the resonant frequency of the circuit depends on the coil surface, the physician is able to recognize when the balloon has obtained a certain diameter. For example, wire  410  may illustratively be connected to a scale marked on the proximal hub side of balloon catheter  402 . The resonant frequency of the LC combination shown in  FIGS. 9A and 9B  is measured prior to implantation (such as during manufacturing) as a function of the position of wire  410  relative to the scale and the diameter of the balloon. In other words, for every balloon diameter, there is one position for wire  410  to enable a resonating frequency equal to the Larmor frequency. Thus, when the physician expands the balloon, the physician then slides wire  410  until the resonating frequency is located. The physician can then find the precise diameter of the balloon simply by looking at the position of wire  410  relative to the scale on catheter  402 . 
   It should be noted that the present invention can also be used in an embodiment in which the tunable circuit is connected to a transmission line (such as a coaxial cable) incorporated into the structure of the guidewire (e.g., guidewire  206 ). A number of embodiments showing such a transmission line are set out in the patent application referred to above. Briefly, elongate conductors separated by dielectric layers and connected to a coil form an impedance matched circuit for efficient transmission of the antenna signal proximally to external processing circuitry. The conductors can be incorporated into the guidewire, a catheter or other elongate member. They can be formed by electroplating, embedding conductive material, or otherwise. Connections can be made by welding, using electrically conductive adhesive or epoxy, or in another manner. Tuning the circuit for impedance matching can be done as well. 
   It can thus be seen that the present invention incorporates an active LC circuit into an intravascular filter and balloon catheter for use in MRI applications. The present invention also incorporates the design of a capacitor into the functional construction of the wire connection connecting the wire loop to the guidewire or other placement device. The present invention also makes the resonant frequency selectively variable during the MRI procedure. 
   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.