Abstract:
A catheter comprising:-a transmission line ( 104, 106, 924, 1202, 1302, 1902, ), wherein the transmission line comprises a plurality of radio frequency traps ( 118, 318, 418, 518, 618, 718, 818, 918, 1018, 1202, 1404, ); and-a cooling line ( 104, 304, 1200, 1900 ) for cooling the plurality of radio frequency traps with a fluid.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a catheter, in particular to catheters for use in magnetic resonance imaging systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Radio frequency ablation catheters are used to ablate or destroy tissues by the use of electrical energy. A radio frequency ablation catheter may be inserted into a vein or artery. The use of X-ray based medical imaging techniques is known for guiding the placement and operation of radio frequency ablation catheters. Radio frequency ablation catheters have been successful in ablating heart tissue that causes rhythm disturbances and for the ablation of tissue in other therapies. 
         [0003]    Modern conventional EP ablation catheters for use with X-ray imaging systems are equipped with tip cooling, mainly to avoid overheating and charring of tissue next to the tip, to avoid blood coagulation, and in consequence to allow for more RF power for deeper and faster ablation. 
         [0004]    U.S. Pat. No. 7,388,378 discloses a device for protecting the conductive parts of an electrical device from current and voltage surges induced by the oscillating magnetic fields of a magnetic resonance imaging system. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention provides for a catheter and a magnetic resonance imaging system in the independent claims. Embodiments are given in the dependent claims. 
         [0006]    X-ray imaging techniques are compatible with catheters that contain wires or transmission lines as X-rays do not induce currents on transmission lines. However, the attenuation of X-rays by hard structures in the body such as bone is larger than in soft tissues. This is a disadvantage of using X-ray imaging equipment for guiding a catheter, because for some uses, such as guiding a radio frequency ablation catheter for ablating soft tissues, imaging soft tissues is useful. In contrast, magnetic resonance imaging is able to effectively image soft tissues. Therefore, it would be advantageous to have a catheter such as a radio frequency ablation catheter that is compatible with magnetic resonance imaging. A difficulty of using magnetic resonance imaging to guide the use of a radio frequency ablation catheter is that the electromagnetic fields generated during the operation of a magnetic resonance imaging system may induce currents in the radio frequency transmission line used to deliver radio frequency power to the tip electrode of a radio frequency ablation catheter. Radio frequency ablation catheters may have a tip electrode, multiple electrodes, or distributed electrodes. It is understood herein that references to tip electrodes or electrodes apply equally to all electrodes of radio frequency ablation catheters. 
         [0007]    Embodiment of the invention may provide a solution to this problem by using radio frequency traps that are distributed along a transmission line. It is understood herein that references and limitations to radio frequency transmission lines also apply to transmission lines. Radio frequency transmission lines are defined herein as transmission lines adapted for transmitting an electrical signal or electrical power at radio frequencies. A transmission line is defined herein as a wire or conductor adapted for transmitting an electrical signal or electrical power. Radio frequency traps suppress induced radio frequency currents and associated tip heating but may heat up themselves. For a radio frequency ablation catheter, the radio frequency traps may be cooled by a liquid that is provided to cool the electrode of the catheter. For this purpose, the traps may be designed such that locations of high electric fields of the trap that may cause radio frequency heating of adjacent lossy dielectrics are mostly confined to within or regions close to the cooling liquid. Heat transfer to the cooling liquid is optimized, while electric fields entering adjacent tissue of a subject are avoided. Hence, direct radio frequency heating of tissue adjacent to the catheter tube is excluded. Similarly, the inductive parts of the radio frequency trap that may be subjected to resistive heating are designed such that heat transfer to the cooling line is optimized and heat transfer to the body tissue is minimized. Hence, generation of excess heat in or near the radio frequency trap is minimized, and residual generated heat is constantly cooled so that the steady-state trap temperature is kept low which prevents radio frequency trap malfunction and secondary tissue heating. 
         [0008]    The invention provides for a catheter comprising a transmission line. The transmission line comprises a plurality of radio frequency traps. The catheter further comprises a cooling line for cooling the plurality of radio frequency traps with a fluid. When catheters are used in an environment which has ambient radio frequency energy such as in a magnetic resonance imaging unit the transmission line may be able to pick up and have induced current on it due to the ambient radio frequency energy. Radio frequency traps may be used to prevent an induced current on the transmission line however the radio frequency energy is concentrated in the radio frequency traps which is eventually converted into heat. The fluid transported by the cooling line is used to distribute or remove heat from the radio frequency traps. This combination leads to a catheter which is safer for use in a magnetic resonance imaging system. 
         [0009]    The entire transmission line may be cooled by the fluid in the cooling line or only a portion of the transmission line may be cooled. This depends upon the design of the radio frequency trap. The portion of the radio frequency trap that dissipates the radio frequency energy will be heated, and this is the portion of the radio frequency trap which is preferably cooled. For instance if discrete capacitors and inductors are used to form the radio frequency trap, then it is advantageous to have the discrete capacitors or inductors within the cooling line or in proximity to the cooling line so that heat can be removed by the cooling line. 
         [0010]    There are many possible types of catheters which may be implemented as embodiments of the invention:
       Diagnostic EP catheters having several wires to connect the electrodes   Actively tracked catheters having one or more wired miniature receive coils for localization in magnetic resonance procedures:   angioplasty catheters for dilation of occluded vessels with or with out stent deployment   valve replacement catheters   catheters for deployment of occluder devices for patent foramen ovale   Intravascular MR imaging catheters with wired internal MR imaging coils   Catheters that require wired sensors for physiological measurements:   pressure catheters for measurement of internal blood pressure   catheters for measurement of internal blood flow   catheters for measurement of internal blood temperature       
 
         [0021]    In another embodiment the catheter is a radio frequency ablation catheter. The transmission line is a radio frequency transmission line. The catheter further comprises an electrode. The cooling line is adapted for transporting the fluid to the electrode. The transmission line comprises a connection end and an electrode end. The connection end is adapted to be connected to a radio frequency generator. The electrode end is connected to the electrode. This embodiment is advantageous because it allows the radio frequency ablation catheter to be used in a high radio frequency field without interfering in the operation of the catheter. For instance radio frequency energy of hundreds of kilohertz may be used for ablating tissue in the vicinity of the electrode. The radio frequency traps may be constructed such that they block a different frequency of radio frequency energy. For instance the radio frequency field generated by a magnetic resonance imaging system could be blocked and prevented from causing an induced current on the transmission line which causes additional heating to the electrode. 
         [0022]    In another embodiment the plurality of radio frequency traps comprises coaxial chokes. In this embodiment the transmission line is a coaxial cable. The outer shielding of the coaxial cable is cut at regular intervals. One end of the shielding is connected to the inner conductor of the coaxial cable and the other end is open or is optionally connected with a capacitor. The sections of the outer conductor of the coaxial cable are shorter than a quarter wavelength of a frequency which is desired to be blocked or trapped. Being shorter than a quarter wavelength prevents induced currents from building up on the outer conductor of the coaxial cable. This prevents an induced current from building up on the inner conductor of the coaxial cable. 
         [0023]    A coaxial choke is also known as a coaxial trap and is commonly called a “bazooka balun.” A coaxial choke creates a high impedance for currents flowing on the outside of a coaxial cable. Its original use was to suppress unbalanced currents at feed points of balanced antennas connected to an (unbalanced) coaxial cable (“balun” =balanced-unbalanced). The high impedance for external shield currents occurs at a basic frequency, according to the full wavelength. For magnetic resonance imaging, this is chosen to be the Larmour frequency, suppressing shield currents induced during a magnetic resonance radio frequency transmission. 
         [0024]    In another embodiment, the transmission line comprises an outer shield electrode. The outer shield electrode comprises a plurality of sections that are spaced apart a maximum of a predetermined distance along the transmission line. The plurality of sections is connected electrically by the plurality of radio frequency traps. In an implementation of this embodiment, the outer shield could be the braided shield of a coaxial cable. The braided shield could be cut in to create electrically isolated sections. These electrically isolated sections could then be connected together electrically using the radio frequency traps. For instance a capacitor and an inductor in parallel could be used to connect two adjacent sections of the outer shield. This embodiment could use a single conductor or wire that is shielded by the outer shield from ambient radio frequency energy. Alternatively, multiple conductors or wires could be shielded by the outer shield. The radio frequency traps could be cooled by placing the transmission line within the cooling line, or by placing the components of the radio frequency trap in thermal contact with the cooling line. 
         [0025]    In another embodiment the plurality of radio frequency traps each comprise a capacitor and an inductor connected in parallel. The radio frequency traps are spaced apart a maximum of a predetermined distance along the radio frequency transmission line. 
         [0026]    In another embodiment the plurality of radio frequency traps comprises a printed circuit board. The inductor is formed on the printed circuit board. A coil of wire may be patterned on the printed circuit board. The inductor may be formed on a single layer of a circuit board or it may be formed on multiple layers. 
         [0027]    In another embodiment the radio transmission line is formed on a printed circuit board. The inductor is formed on the printed circuit board. In this embodiment the entire or a substantial length of the transmission line of the catheter is made on a long narrow printed circuit board. 
         [0028]    The invention provides for a catheter. The catheter may be a radio frequency ablation catheter. It is understood herein that references to radio frequency ablation catheters apply equally to other catheters which contain wires or transmission lines. An exception to this is that not all catheters which are embodiments of the invention comprise an electrode or tip electrode. The radio frequency ablation catheter comprises an electrode. The electrode may be a tip electrode. The electrode may be at any position along the position of the radio frequency ablation catheter. The radio frequency ablation catheter may also have multiple electrodes. All references to a tip electrode are understood to be applicable herein to any other electrodes which may be a component of a radio frequency ablation catheter. The radio frequency ablation catheter further comprises a cooling line adapted for transporting a fluid to the tip electrode. The radio frequency ablation catheter further comprises a radio frequency transmission line. The radio frequency transmission line comprises a connection end and an electrode end. The connection end is adapted to be connected to a radio frequency generator. The electrode end is connected to the tip electrode. The radio frequency transmission line comprises a plurality of radio frequency traps. The plurality of radio frequency traps each comprises a capacitor and an inductor connected in parallel. The radio frequency traps are spaced apart a maximum of a predetermined distance along the radio frequency transmission line. The cooling line is adapted for cooling the plurality of radio frequency traps. 
         [0029]    The radio frequency traps are tuned to blocking frequency. When the radio frequency ablation catheter is used in a magnetic resoancance imaging system it is desirable to tune the radio frequency traps to the Larmour frequency of the magnetic resoance imaging system by choosing inductance L and capacitance C such that 
         [0000]    
       
         
           
             
               ω 
               = 
               
                 1 
                 
                   LC 
                 
               
             
             , 
           
         
       
     
         [0000]    where ω is the Larmour frequency. Coil and distributed capacitors are additionally constrained by the dimensions of cooling line. 
         [0030]    Coil design (number of windings, winding density) and choice of capacitance may be determined by mounting test coils onto the cooling tube and/or by choice of an appropriate lumped or distributed capacitors. The resonance frequency of the resulting trap may be measured as follows: The port of a network analyzer is connected to a circular pick-up coil that is used to couple weakly inductively to the coil L. The S11 mode of the network analyzer then shows a minimum of reflected power at the resonance frequency of the RF trap. 
         [0031]    Iteratively, coil parameters and capacitors may be varied to adjust the resonance frequency to ω. 
         [0032]    Fine tuning of the radio frequency trap in the fully assembled state may be achieved by changing the winding density of the coil. For this purpose, the coil should be wound loosely onto the cooling tube during assembly, and only after measurement of the resonance frequency of the RF trap and fine tuning by slight variation of the winding positions, the coil windings may be fixed by an adhesive. 
         [0033]    When the radio frequency ablation catheter is connected to a radio frequency generator, radio frequency electrical power is transmitted to the tip electrode via the radio frequency transmission line. The radio frequency electrical power from the radio frequency generator heats the tissue next to the tip electrode due to the local high current density which causes local ablation of tissue. Tissue which is not adjacent to the tip electrode is heated by the conduction of heat from the region of tissue that is heated. The cooling line transports a fluid to the tip electrode to prevent the tissue directly adjacent to the tip electrode from becoming too hot. Several different varieties may be used. For instance the cooling line may carry a saline solution to the tip which then leaks cooling fluid into tissue adjacent to the tip electrode for the purpose of tip irrigation. Alternatively, a closed loop can be used where preferably a first tube transports a cooling fluid into the tip for the purpose of cooling of the tip, and the cooling line equipped with traps is used to provide the return path for the cooling liquid. Alternatively, the traps may be cooled by the first tube. In all closed loop set-ups no liquid leaks into the body, which allows use of cooling liquids other than saline. 
         [0034]    The radio frequency traps may comprise a capacitor and inductor that are connected in parallel. This allows the frequency trap to be tuned to a narrow frequency band. Essentially the capacitor and inductor form a notch filter. Placing the radio frequency traps along the length of the radio frequency transmission line periodically allows the radio frequency ablation catheter to be used in a region with a varying radio frequency electromagnetic field. The radio frequency traps can be tuned so that they have a high impedance at the resonance frequency of the radio frequency trap and prevent the radio frequency electromagnetic field from inducing a current on the radio frequency transmission line. The radio frequency traps block this current by storing energy within the capacitor and inductor. This stored energy is eventually converted into heat and this is why the cooling line is used for cooling the radio frequency traps. The cooling tube cools both the tip electrode and the radio frequency traps. 
         [0035]    In another embodiment the capacitor is within the cooling line. This embodiment is beneficial because the capacitor is surrounded by coolant and also the capacitor is within the cooling line and further from the catheter wall. The catheter wall is a tube or housing which surrounds the cooling line and the radio frequency transmission line. 
         [0036]    In another embodiment the cooling line has an exterior surface. The capacitor is in contact with the exterior surface. The capacitors which make up the traps are placed in contact with the cooling line in order to transfer heat away from them. 
         [0037]    In another embodiment the inductor comprises a coil. The coil is within the cooling line. The coil of each of the plurality of radio frequency traps is placed within the cooling line. This is advantageous because the fluid which is used to cool the tip electrode is able to cool each of the coils which make up the plurality of radio frequency traps. 
         [0038]    In another embodiment the cooling line has an exterior surface. The inductor comprises a coil. The coil is wrapped around the exterior surface. This embodiment is advantageous because the coil is placed in contact with the cooling line and can be used to efficiently cool the coil. In this way the coils which make up the inductors for each of the plurality of radio frequency traps is cooled. 
         [0039]    In another embodiment the capacitor is a lumped capacitor. A lumped capacitor as used herein is a capacitor where the electrodes and the dielectric layer used to form the capacitor are folded. Capacitors that are used as electrical components for electrical devices are typically lumped capacitors. 
         [0040]    In another embodiment the capacitor is a distributed capacitor. A distributed capacitor as used herein is a capacitor whose electrodes and dielectric layer are not folded. 
         [0041]    An example of a distributed capacitor would be two flat electrodes with a dielectric layer between the two. 
         [0042]    In another embodiment the capacitor comprises a dielectric layer. The cooling line forms the dielectric layer. For instance the cooling line may comprise a dielectric layer. An electrode could be placed on the interior and exterior of the cooling line opposing each other. This would then form a capacitor. 
         [0043]    In another embodiment the capacitor comprises a first electrode. The capacitor further comprises a dielectric layer. The capacitor further comprises a second electrode. The dielectric layer is in contact with the first electrode and the second electrode. The surface area of the first electrode is larger than the surface area of the second electrode. The second electrode is in contact with the cooling line. The plurality of radio frequency traps function by storing energy within the capacitor and the inductor. As a result there can be large electric fields between the first electrode and the second electrode. By having the second electrode smaller than the first electrode and having the second electrode in contact with the cooling line the large electric fields are directed away from the catheter wall. This has the benefit that when the radio frequency ablation catheter is used within a subject, the large electric fields of the capacitors will not cause heating in the subject. 
         [0044]    In another embodiment the first electrode and the second electrode have a curvature that matches the curvature of the cooling line. This embodiment is advantageous because the second electrode is smaller than the first electrode and is in contact with the cooling line. The curved surface further directs the large electric fields to the interior of the cooling line. This further reduces the large electric field of the plurality of radio frequency traps. 
         [0045]    In another embodiment the radio frequency line comprises a conductive tube. The conductive tube may cover the surface of the cooling line or the conductive tube and the cooling line may be the same component. If they are separate components then the cooling line may be a dielectric tube. The radio frequency trap comprises a gap in the conductive tube. If the conductive tube and the cooling line are the same component, then the conductive tube will cover the surface of a dielectric tube. The inductor is connected across the gap in the conductive tube and may be wrapped around the dielectric tube. The capacitor comprises a third electrode. The capacitor further comprises the conductive tube. The third electrode is mounted inside the dielectric tube. The radio frequency line is the conductive tube. The inductor may be formed by a coil of wire wrapped either inside or outside of the dielectric tube across the gap. If the coil is on the inside of the dielectric tube then the coil may needs to go through or around the dielectric tube in order to contact the conductive tube. The third electrode may be implemented in several different ways also. The third electrode could comprise two separate sub-electrodes which are located inside the cooling line. The sub-electrodes would each form a capacitor with the conductive tube on either side of the gap in the conductive tube. The two sub-electrodes could then be electrically connected together. If a separate cooling line and conductive tube are used, there would be no need to make a hole to form the capacitor. Alternatively there could be an electrical connection which goes through the cooling line and attaches to one end of the conductive tube. There would then be a wire which is connected to the third electrode and then the third electrode is connected to the inside of the conductive tube. 
         [0046]    In another embodiment the tip electrode comprises a temperature sensor. This embodiment is particularly advantageous, because the temperature sensor can be used to monitor the temperature of the tip electrode when the radio frequency ablation catheter is in use. If the tip electrode heats more than is expected then this may be an indication that one or more of the radio frequency traps has failed. This is because if the radio frequency traps fail then a current could be induced in the radio frequency transmission line. 
         [0047]    In another aspect the invention provides for a magnetic resonance imaging system. The magnetic resonance imaging system comprises a magnet adapted for generating a magnetic field for orienting magnetic spins of nuclei of a subject located within the imaging volume. The magnetic resonance imaging system further comprises a radio frequency system for acquiring magnetic resonance data. Magnetic resonance data as defined herein as the representation of radio signals acquired during the process of operating a magnetic resonance imaging system. For instance during the operation of a magnetic resonance imaging system gradient fields and radio frequency fields are used to manipulate and control the orientation of magnetic spins of nuclei. When the magnetic spins relax they emit radio frequencies which can be detected using an antenna and recorded. The recording of these radio transmissions from the magnetic spins is the magnetic resonance data. Magnetic resonance data can be transformed using Fourier techniques into images or visualizations of the imaging volume of the subject. The radio frequency system comprises a radio frequency transceiver and a radio frequency coil. It is understood that the radio frequency transceiver could in fact be a separate transmitter and a separate receiver. The radio frequency coil could also be a separate transmit coil and a separate receive coil. The magnetic resonance imaging system further comprises a magnetic field gradient coil for spatial encoding of the magnetic spins of the nuclei within the imaging volume. The magnetic resonance imaging system further comprises a magnetic field gradient coil power supply for supplying current to the magnetic field gradient coil. The magnetic resonance imaging system further comprises a subject electrode adapted for forming an electrical connection with the subject. The magnetic resonance imaging system further comprises a subject support adapted for receiving the subject. The subject electrode may be integrated into the subject support. 
         [0048]    The magnetic resonance imaging system further comprises a radio frequency generator for producing radio frequency power at a first frequency. The radio frequency generator is connected to the subject electrode. The radio frequency generator is adapted for connecting to a radio frequency ablation catheter according to an embodiment of the invention. When the radio frequency ablation catheter is placed within a subject that is in contact with the subject electrode, a complete electrical circuit is formed through the catheter, the subject and then back through the subject electrode. The magnetic resonance imaging system further comprises a computer system adapted for constructing images from the magnetic resonance data and for controlling the operation of the magnetic resonance imaging system. The computer system is adapted for generating magnetic resonance images of the subject when the radio frequency generator is operational. This magnetic resonance imaging system is beneficial, because the magnetic resonance images can be used to guide the use of the radio frequency ablation catheter by a physician or operator. 
         [0049]    In another embodiment the radio frequency generator is adapted for generating radio frequency power at a test frequency. The test frequency would be preferentially a frequency to which the plurality of radio frequency traps is tuned to. The radio frequency generator is adapted for generating the test frequency at lower power than the first frequency. The radio frequency generator comprises a reflected power meter for measurement the reflected power of the test frequency. The radio frequency generator is adapted for detecting a malfunction of at least one of the plurality of radio frequency traps using the reflected power. The radio frequency generator is further adapted for signaling the computer system when the malfunction is detected. One way of measuring the power is by incorporating a network analyzer functionality into the radio frequency generator. Alternatively, the reflected power meter may function by measuring the standing wave ratio at the test frequency. 
         [0050]    The computer system is further adapted for reducing the generation of radio frequency power by the radio frequency transceiver when the computer system is signaled by the radio frequency generator. The generation of radio frequency power may be also stopped when the computer system is signaled by the radio frequency generator. In this embodiment the test frequency is used to test if the radio frequency traps are functioning. If a trap becomes shorted or open then the impedance at the test frequency may change. As was mentioned before the test frequency could be at the frequency for which the radio frequency traps are tuned. Alternatively the test frequency could also be at a different frequency, for instance a frequency that is higher than the test frequency and the first frequency or at a frequency which is intermediate to the first frequency and the Larmour frequency of a hydrogen atom in the magnet of the magnetic resonance imaging system. If a radio frequency trap fails the impedance and therefore the reflected or transmitted power of the test frequency may change. It is understood herein that a measurement of the reflected power is equivalent to a measurement of the transmitted power at the test frequency. This may indicate a failed radio frequency trap. 
         [0051]    In another embodiment the radio frequency ablation catheter has a tip electrode which comprises a temperature sensor. The radio frequency generator is further adapted for determining a temperature measurement of the tip electrode using the temperature sensor. The computer system is further adapted for receiving the temperature measurement from the radio frequency generator. The computer system is further adapted for reducing the generation of radio frequency power by the radio frequency transceiver when the temperature measurement is above a predetermined safety threshold. If the plurality of radio frequency traps has a trap which fails, this may induce a current in the radio frequency transmission line. This may lead to a heating of the tip electrode. By monitoring the temperature of the tip electrode for an abnormal increase in temperature during operation may allow the detection of failed radio frequency traps. 
         [0052]    In another embodiment, the catheter may comprise sensors to measure the temperature of the cooling liquid. An elevated temperature of the cooling may also be used for detecting the malfunction of a radio frequency trap or otherwise strong radio frequency coupling of the magnetic resonance system to the catheter e.g. also due to incorrect use of the magnetic resonance imaging system or the catheter. 
         [0053]    Moreover, the temperature measured at the tip of the catheter, e.g. the measured temperature of the cooling liquid, can be employed to control various RF functions of the magnetic resonance examination system. In particular the radio frequency power delivered by the radio frequency ablation catheter can be accurately regulated on the basis of the temperature measured at the tip of the catheter. In another aspect, the power level of the RF excitation field of the magnetic resonance examination system can be controlled on the basis of the temperature measured at the tip of the catheter. Thus, the SAR level deposited in/on the patient&#39; body is controlled on the basis of temperature measured at the tip of the catheter. This control of the RF functions of the magnetic resonance examination system on the basis of the temperature measured at the tip of the catheter can be advantageously employed independently of the monitoring of failure of switchable traps in the transmission line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]    In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which: 
           [0055]      FIG. 1  illustrates an embodiment of a radio frequency ablation catheter according to an embodiment of the invention; 
           [0056]      FIG. 2  illustrates a further embodiment of a radio frequency ablation catheter  200  according to an embodiment of the invention; 
           [0057]      FIG. 3  illustrates a section of cooling line with an embodiment of a radio frequency trap according to the invention; 
           [0058]      FIG. 4  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0059]      FIG. 5  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0060]      FIG. 6  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0061]      FIG. 7  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0062]      FIG. 8  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0063]      FIG. 9  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0064]      FIG. 10  illustrates a section of cooling line with a further embodiment of a radio frequency trap according to the invention; 
           [0065]      FIG. 11  shows a functional diagram of a magnetic resonance imaging system according to an embodiment of the invention; 
           [0066]      FIG. 12  shows a section of cooling line of a catheter according to an embodiment of the invention; 
           [0067]      FIG. 13  shows an embodiment according to the invention of a transmission line manufactured using a printed circuit board; 
           [0068]      FIG. 14  shows an embodiment according to the invention of a radio frequency trap constructed on a printed circuit board; 
           [0069]      FIG. 15  shows a further embodiment according to the invention of a radio frequency trap constructed on a printed circuit board; 
           [0070]      FIG. 16  shows a further embodiment according to the invention of a radio frequency trap constructed on a printed circuit board; 
           [0071]      FIG. 17  shows a further embodiment according to the invention of a radio frequency trap constructed on a printed circuit board; 
           [0072]      FIG. 18  shows an embodiment according to the invention of a radio frequency transmission line manufactured using multiple printed circuit boards; 
           [0073]      FIG. 19  shows a section of cooling line according to an embodiment of the invention with a transmission line that has coaxial chokes; 
           [0074]      FIG. 20  shows a section of a catheter according to an embodiment of the invention with a transmission line  1902  that has coaxial chokes; 
           [0075]      FIG. 21  shows a further embodiment according to an invention of a cooling line with a coaxial choke; 
           [0076]      FIG. 22  shows a further embodiment according to the invention of a catheter; and 
           [0077]      FIG. 23  shows a cross sectional view of a catheter according to a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0078]    Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent. 
         [0079]      FIG. 1  illustrates an embodiment of a radio frequency ablation catheter  100  according to an embodiment of the invention. The radio frequency ablation catheter  100  has a catheter wall  102 . Within the catheter wall  102  is a cooling line  104 . The catheter wall  102  may be a tube. The catheter wall  102  is typically 2 mm to 3 mm in diameter. At the end of the catheter wall  102  is a tip electrode. In the embodiment shown in  FIG. 1  there is a radio frequency transmission line  106  which is shown running through the cooling line  104 . The radio frequency transmission line  106  has an electrode end  108  which is connected to the tip electrode  110 . The tip electrode  110  is at the end of the catheter wall  102 . The arrow labeled  112  shows the direction of fluid which is used to cool the tip electrode  110 . In this embodiment the tip electrode  110  has a channel  114  or channels in the tip electrode which allow fluid to exit the tip electrode  110 . The arrows labeled  116  indicate the flow of fluid out of the tip electrode channel  114 . In typical use one liter per hour of fluid, which is typically a saline solution, is used. The radio frequency transmission line  106  also comprises radio frequency traps  118 . The radio frequency traps  118  are shown as being spaced a predetermined distance  120  apart. The radio frequency traps  118  are tuned to a particular blocking frequency. Radio frequency traps are constructed of an inductor and a capacitor in parallel. This produces a so called notch filter. Often the predetermined distance  120  is smaller than a wavelength of an electromagnetic wave at the blocking frequency in the medium to which the radio frequency ablation catheter is placed. For practical purposes a material which could be used to calculate the wavelength could be water because radio frequency ablation catheters are typically used within tissue which is comprised mostly of water. By placing these radio frequency traps  118  at a distance less than a wavelength this impedes the generation of a current on the radio frequency transmission line  106  by an external electromagnetic wave at the blocking frequency. It may be beneficial to place several radio frequency traps  118  within one wavelength. This is because if a single radio frequency trap fails then a current will still not be able to be induced on the radio frequency transmission line  106 . 
         [0080]    In the embodiment shown in  FIG. 1  the radio frequency transmission line  106  and the radio frequency trap  118  are both shown as being within the cooling line  104 . The radio frequency transmission line  106  may be inside or outside of the cooling line  104 . The radio frequency trap  118  may also be inside or outside of the cooling line  104 . If the radio frequency trap  118  is not within the cooling line  104 , then the components which make up the radio frequency trap  118  are preferentially in contact with the cooling line  104 . 
         [0081]      FIG. 2  shows a further embodiment of a radio frequency ablation catheter  200  according to an embodiment of the invention. The design of the radio frequency ablation catheter  200  shown in  FIG. 2  is very similar to the design of the radio frequency ablation catheter  100  shown in  FIG. 1 . The design of the tip electrode  210  and the method of cooling the tip electrode  210  differs from that as shown in  FIG. 1 . There is a tip electrode  210  which is connected to the cooling line  104  and radio frequency transmission line  106  as was shown in  FIG. 1 . The tip electrode  210  also has a channel  214  for the fluid which is used to cool the tip electrode  210 . However in this embodiment the fluid does not exit the tip electrode  210  but returns back within the cavity  216  formed by the catheter wall  102 . The arrow labeled  212  indicates the direction of fluid flow out of the channel of the tip electrode  214  and back through the cavity  216  formed by the catheter wall  102 . The fluid flow out of the channel may also be through an additional line. 
         [0082]    In this embodiment there is also a temperature sensor  202  within the tip electrode  210 . The temperature sensor  202  may be used to monitor the temperature of the tip electrode  210  during operation. An abnormally high temperature of the tip electrode  210  during operation may indicate the failure of one or more radio frequency traps  118 . There is a high impedance line  204  connecting to the sensor temperature  202 . The temperature sensor  202  may be implemented using a temperature sensor such as a thermal couple. The high impedance line  204  may be wire connections which have a sufficiently high impedance that it is not necessary to provide an impedance to block the generation of current on the high impedance line  204 . Alternatively, the temperature sensor  202  may be connected to the radio frequency transmission line  104 . The read-out unit for signals of the temperature sensor  202  would then be equipped with an AC blocking circuit to reject the radio frequency electrical power used for ablation, but not the low frequency signals used for temperature sensing. 
         [0083]      FIG. 3  illustrates a section of cooling line  304  with an embodiment of radio frequency trap  318  according to the invention. An equivalent circuit  302  is also shown. The components in the equivalent circuit  302  are labeled identically with those of the radio frequency trap  318 . Exterior to the cooling line  304  is the radio frequency transmission line  106 . A coil  306  wrapped around the cooling line  304  forms the inductor of the radio frequency trap  318 . Within the cooling line  304  is a lumped capacitor  308 . There are holes  310  in the cooling line to connect the lumped capacitor  308  to the radio frequency transmission line  106 . 
         [0084]      FIG. 4  illustrates a further embodiment of a radio frequency trap  418  that is similar to that shown in  FIG. 3 .  FIG. 4  also has an equivalent circuit diagram  402  which labels components in the same way as is shown for the section of cooling line  304 . There is a section of cooling line  304  shown. In this embodiment the radio frequency transmission line  106  is connected to a coil  406  which is located inside of the cooling line  304 . Connected in parallel with the coil  406  is a lumped capacitor or capacitance  408 . Both the coil  406  and the lumped capacitor  408  are located within the cooling line  304  in this embodiment. 
         [0085]      FIG. 5  illustrates a further embodiment of a radio frequency trap  518 . There is also an equivalent circuit  502  where the components are labeled. In this embodiment the cooling line  304  has the radio frequency transmission line  106  exterior to it. There is a coil  506  wrapped around the cooling line  304 . Interior to the cooling line  304  is a distributed capacitor  508 . The cooling line has holes  310  which allow electrical connection between the radio frequency transmission line  106  and the distributed capacitor  508 . In this example the distributed capacitor  508  is shown as being two electrodes with a dielectric between them. The advantage of this embodiment is that the distributed capacitor  508  has a very large surface area. This aids in cooling the distributed capacitor  508 . 
         [0086]      FIG. 6  shows a further embodiment of a radio frequency trap  618 . In this figure there is an equivalent circuit  602  which shows the components of the radio frequency trap  618 . In this embodiment there is a cooling line  304 . Interior to the cooling line is the radio frequency transmission line  106 . There is a coil  606  and a distributed capacitor  608  which are both within the cooling line. 
         [0087]    In the embodiments shown in  FIG. 3  and  FIG. 5 , the capacitors (lumped or distributed) and all conductive parts causing local high electrical E-fields are fully immersed inside the cooling liquid. This fully excludes high field strength to enter the tissue of the patient. The constant flow through the cooling liquid tube effectively “blurs” the dangerous local specific absorption ration (SAR) hot spot by distributing the heat with the liquid convection or even removes the heat completely from the system in case of the closed-irrigation approach.  FIG. 4  and  FIG. 6  depict variants of the embodiments  FIGS. 3 and 5 . In these embodiments, the radio frequency transmission line  106  is routed inside the cooling tube, which further reduces the overall profile of the assembly. In all embodiments where parts of the radio frequency traps or the ablation cable are routed inside the cooling tube, preferably a biocompatible isolative coating is applied to such parts to achieve biocompatibility and to prevent leakage of radio frequency currents from such parts into the cooling liquid which would lead to minor parasitic radio frequency heating of the cooling liquid inside the cooling tube. 
         [0088]      FIG. 7  shows a further embodiment of a radio frequency trap  718 . There is a side view  700  shown, an equivalent circuit view  702  and a bottom view  704 . The bottom view  704  does not show the cooling line  104 . The radio frequency transmission line  106  is shown as being exterior to the cooling line  104 . There is a coil  106  wrapped around the cooling line  104 . There is a capacitor which is formed by a first electrode  720  and a second electrode  722 . The first electrode  720  has a larger surface area than the second electrode  722 . Additionally the second electrode  722  is adjacent to the cooling line  104 . The first electrode  720  in this embodiment is used as a shield against electric field lines  724 . The radio frequency trap  718  may store large amounts of electromagnetic energy. The use of the first electrode  720  as a shield improves the safety of the radio frequency trap  718 . Alternatively the radio frequency transmission line  106  and/or the coil  706  and/or the first electrode  720  and/or second electrode  722  may be located within the cooling line  104 . 
         [0089]      FIG. 8  shows a similar embodiment to that shown in  FIG. 7 . In  FIG. 8  a side view of a section of cooling line  104  with a radio frequency trap  118  is shown. View  802  shows an equivalent circuit with the components labeled. View  804  shows a bottom view of view  800  but without the cooling line  104  shown. Shown are radio frequency transmission lines  106  which are exterior to the cooling line  104 . There is a coil  806  which is wrapped around the cooling line  104 . Connected in parallel with the coil  806  is a capacitor which is formed by a first electrode  820  and a second electrode  822 . In this embodiment the first electrode  820  and the second electrode  822  are shown as having a curvature which matches that of the cooling line  104 . The actual curvatures shown in the fig. are exaggerated to make them more visible. As with the embodiment shown in  FIG. 7  the radio frequency transmission line  106  and/or the coil  806  and/or the first electrode  820  and/or the second electrode  822  may be located inside of the cooling line  104 . 
         [0090]      FIG. 9  illustrates a radio frequency trap  918  according to an embodiment of the invention. The equivalent circuit  902  is also shown and components are also labeled in this equivalent circuit diagram  902 . In  FIG. 9  a cooling line  304  is shown. The cooling line  304  may comprise a dielectric. In this embodiment the radio frequency transmission line is a conductive tube  924 . The radio frequency trap  918  is formed around a gap  932  in the conductive tube  924 . Between the two sections of the conductive tube  924  is a coil  906  which forms the inductor of the radio frequency trap  918 . The capacitor is formed by a third electrode  926  and a fourth electrode  928 . These electrodes are mounted inside of the cooling line  304 . The third electrode  926  and the fourth electrode  928  are mounted on opposite ends of the gap  932  in the conductive tube  924 . The third electrode  926  and the fourth electrode  928  are capacitively coupled to a section of the conductive tube  924 . The third electrode  926  and a fourth electrode  928  are connected by a wire  930  or some other conductor. The third electrode  926  and the fourth electrode  928  could also be mounted on the outside of the conductive tube  924 . However it is advantageous to have the third electrode  926  and the fourth electrode  928  within the cooling line  304 . This is because the conductive tube  924  shields any high electric fields which may be around the edges of the third electrode  926  or the fourth electrode  928 . 
         [0091]      FIG. 10  shows a similar embodiment to that shown in  FIG. 9 .  FIG. 10  shows an embodiment of a radio frequency trap  918  according to an embodiment of the invention. Equivalent circuit  1002  is also shown. Components are also labeled on the equivalent circuit diagram  1002 . In  FIG. 10  a cooling line  304  is shown. The radio frequency transmission line is a conductive tube  924 . As with the embodiment in  FIG. 9 , there is a gap  932  in the conductive tube  924  where the radio frequency trap  1018  is formed. A coil  1006  connects the two ends of the gap  932  electrically. This coil  1006  forms the inductor of the radio frequency trap  1018 . In this embodiment there is a third electrode  1028  which is located inside the cooling line  304  and is under the conductive tube  924  at one end of the gap  932 . The third electrode  1028  is capacitively coupled to a section of the conductive tube  924 . A wire  1030  then connects the third electrode  1028  to the conductive tube  924  at the other end of the gap  932 . The wire  1030  is connected to the conductive tube  924  through a hole  1010  in the cooling line  304 . 
         [0092]    In the embodiments shown in  FIG. 9  and  FIG. 10 , instead of using a separate radio frequency transmission line and an additional cooling line  304 , as normally done in standard ablation catheters, a conducting tube  924  serving both as RF ablation line and as cooling liquid supply. The cooling line  304  and the conductive tube  924  may be the same component. In this case there may be a section of non-conducting or a dielectric tube mounted between the electrodes  926 ,  928 ,  1028  and the conductive tube  924  which extends across the gap  932  in the conductive tube  924  of the radio frequency trap  918 ,  1018 . Alternatively, there may be a separate cooling tube which is inside of the conducting tube  924 . For the embodiment where a section of non-conducting tubing is used, the coil  906 ,  1006  of radio frequency trap is wound to the non-conductive junction-tubing. 
         [0093]    In  FIG. 9 , this junction tubing is equipped with two wire  930  connected electrodes  926 ,  928  on its inner wall, located next to the ends of the tube. The conducting tubes  924  provide some capacitive overlap with those internal plates, which form a distributed capacitor. Symmetric embodiments with two such distributed capacitors are possible as well as an asymmetric version with only a single capacitor at one end of the trap. The common mode currents induced by the incident radio frequency field of the magnetic resonance system on this conductor tube are suppressed by special miniature coaxial radio frequency traps, which are designed such that almost no electric fields leak into the outside of the catheter, especially not into the adjacent tissue. Thus, eventual heating is confined within the trap. Due to the high thermal coupling of the design of this trap in conjunction with the conductive cooling tube, the trap heat can be efficiently dissipated and distributed. Thus local hot spots are completely eliminated and the ablation line becomes radio frequency safe. 
         [0094]    In an alternative embodiment such a radio frequency trap can also be designed to be directly integrated into a regular ablation cable. Again, the design is such that the electric fields are confined within the trap and do not leak into the adjacent tissue, so that direct tissue heating is avoided. The standard plastic cooling tube of an irrigated-tip catheter is then used as support for an inductive coil element and serves to cool the radio frequency traps. 
         [0095]    In  FIG. 11  an embodiment of a magnetic resonance imaging system according to the invention is illustrated. The magnetic resonance imaging system  1100  has a magnet  1102 . The magnet  1102  may be a superconducting magnet, a permanent magnet, an electromagnet, or a combination of any of the previous three and is for generating a magnetic field for aligning the spins of nuclei of a subject  1112  within an imaging volume  1114 . Inside the bore of the magnet there is also a set of magnetic field gradient coils. The term magnetic field gradient coil refers to one or a collection of coils used for spatial encoding of the magnetic spins of nuclei within the imaging volume. The magnetic field gradient coil  1104  is connected to a magnetic field gradient coil power supply  1106 . 
         [0096]    Also within the bore of the magnet is a radio frequency coil  1108  which is connected to a radio frequency transceiver  1110 . The radio frequency coil  1108  and the radio frequency transceiver  1110  form a radio frequency system which is used for acquiring magnetic resonance data. Also shown in the fig. is a radio frequency ablation catheter  1120 . There is a connection  1122  between the radio frequency ablation catheter  1120  and a radio frequency generator  1118 . The radio frequency generator  1118  is also shown as being connected to a subject electrode  1116 . The subject electrode  1116  forms an electrical connection between the subject  1112  and the radio frequency generator  1118 . In this embodiment the subject electrode  1116  also functions as a subject support. At the end of the radio frequency ablation catheter  1120  is shown the tip electrode  1124 . When the radio frequency generator  1118  supplies the radio frequency catheter  1120  with radio frequency power, a heating zone  1126  within the subject  1112  is heated. 
         [0097]    The radio frequency transceiver  1110 , the magnetic field gradient power supply  1106 , and the radio frequency generator  1118  are all connected to a hardware interface  1130  of a computer system  1128 . The computer system  1128  further comprises a microprocessor  1132  for executing machine executable instructions. The microprocessor is connected to computer storage  1136 . The computer storage is storage which is adapted for storing machine executable instructions or machine readable data. Examples of computer storage are but are not limited to a hard drive, a floppy disk, flash memory, or other storage medium. The microprocessor  1132  is also connected to and able to send instructions to a user interface  1134 . The user interface  1134  comprises components for receiving input data from an operator and also for displaying information or graphics for an operator. For example the user interface may comprise a keyboard and a mouse. 
         [0098]    The user interface  1134  may also comprise a computer display for displaying information and graphics. The user interface may comprise a display  1134  for displaying magnetic resonance images and also plain images a physician or operator can use while guiding the radio frequency ablation catheter  1120  in the subject  1112 . The computer system  1128  also comprises computer memory  1138 . The computer memory contains machine readable data and machine executable instructions for use by the microprocessor  1132 . Stored within the memory  1138  is a computer program product  1140 . The computer program product comprises a catheter control module  1142 . The catheter control module  1142  comprises machine executable instructions which allow the microprocessor  1132  to control the functionality of the radio frequency generator  1118 . The catheter control module  1142  may also control specialized instructions for controlling the operation and ensuring the safety of the radio frequency ablation catheter  1120 . For instance if the tip electrode  1124  has a temperature sensor the catheter control module  1142  may contain specialized machine executable instructions which determine if the tip electrode  1124  is abnormally warm due to currents induced in the radio frequency transmission line by the acquisition of magnetic resonance imaging data. For instance during the guiding of the radio frequency ablation catheter, when ablation is not being performed, the acquisition of magnetic resonance imaging data may cause heating of the tip electrode. Secondly, during the use of the radio frequency ablation catheter to ablate tissue, the acquisition of magnetic resonance imaging data may induce currents in the radio frequency transmission line that lead to additional tip electrode  1124  heating. If this additional heating of the tip electrode  1124  in either of these two cases exceeds a predetermined safety threshold, magnetic resonance imaging may be stopped 
         [0099]    Similarly if the radio frequency generator  1118  contains a reflected power meter for measuring the reflected power of a test frequency that is injected into the radio frequency ablation catheter  1120  there may be specialized code within the catheter control module  1142  which allows microprocessor  1132  to determine if there is a failure of the radio frequency ablation catheter  1120 . The computer program product also comprises a magnetic resonance imaging control module  1144  for controlling the functionality of the magnetic resonance imaging system  1100 . The computer program product  1140  also comprises an image reconstruction module  1146 . The image reconstruction module  1146  contains machine executable instructions for reconstructing magnetic resonance data into magnetic resonance images. 
         [0100]    In practice the radio frequency generator  1118  will typically generate radio frequency power at approximately 500 kHz to produce ablation in the subject in the heating zone  126  of the tip electrode. The frequency of the radio frequency traps depends upon the strength of the magnetic field and type of atomic spin which is being measure. For instance, in a 1.5 Tesla field the nuclei of Hydrogen atoms have a resonance frequency of approximately 64 MHz. The large difference in frequency between the Larmour frequency and the frequency used for ablation allows the radio frequency traps to effective filter at the Larmour frequency without a large attenuation at the frequency used to produce ablation. 
         [0101]      FIG. 12  shows a section of cooling line  1200  according to an embodiment of the invention. Within the cooling line  1200  is the transmission line  1202 . The transmission line  1202  is connected to radio frequency traps  1204 . In this embodiment the radio frequency trap  1204  and the transmission line  1202  are both located within the cooling line  1200 . By being located within the cooling line  1200  the radio frequency traps  1204  are able to be cooled by forcing a fluid through the cooling line  1200 . In this example the radio frequency traps  1204  are constructed on a printed circuit board. 
         [0102]      FIG. 13  shows an embodiment of a transmission line according to an embodiment of the invention manufactured using a printed circuit board. A section of cooling line  1200  is also shown in this figure. Instead of having a transmission line which is connected to individual radio frequency traps, the transmission line  1302  and the radio frequency traps are both connected together on the same piece of printed circuit board. The printed circuit board is thin enough so that it is flexible and is bendable. During use as the catheter is manipulated, the printed circuit board is able to twist and bend within the catheter allowing a full range of motion for the catheter. 
         [0103]      FIG. 14  shows an embodiment of a radio frequency trap constructed on a printed circuit board  1410 . There is a capacitor  1406  connected in parallel with a coil  1408  patterned on the surface of the printed circuit board  1410 . In this example the radio frequency trap  1404  is constructed on a single side of the printed circuit board  1410 . 
         [0104]      FIG. 15  shows an alternative embodiment of a radio frequency trap  1504  constructed on a printed circuit board  1410 . Again a capacitor  1406  is shown in parallel with a coil  1508 . In this embodiment the coil  1508  has more than one turn. In order to connect the capacitor  1406  and the coil  1508  a trace  1512  on the opposing side of the printed circuit board  1410  is used. 
         [0105]      FIG. 16  shows an alternative embodiment of a radio frequency trap  1604  constructed on a printed circuit board  1410 . Again a capacitor  1406  is shown in parallel with a coil  1608 . The coil  1608  is formed on two layers of the printed circuit board  1410 . The section of the coil  1608  is formed on the same side of the printed circuit board as the capacitor  1406 . The dashed line  1612  indicates section of the coil formed on the opposing side of the printed circuit board  1410 . Forming a portion of the printed circuit board on the opposing side allows a coil  1608  to be formed with a larger number of turns. 
         [0106]      FIG. 17  shows an alternative embodiment of a radio frequency trap  1704  formed on a printed circuit board  1410 . In this embodiment a coil  1708  is formed on one side of the printed circuit board  1410 . Instead of using a discreet capacitor, capacitive electrodes are formed on opposing sides of the printed circuit board  1410 . The printed circuit board  1410  forms the dielectric of the capacitor. The electrode labeled  1714  forms one electrode of the capacitor and the dashed line  1716  indicates an electrode formed on the opposing side of the printed circuit board  1410 . In the embodiment shown in  FIG. 17  both the capacitor and the coil  1708  are formed on the printed circuit board  1410 . 
         [0107]      FIG. 18  shows a section of cooling line  1200  with an alternative embodiment of a printed circuit board transmission line. The transmission line is formed by sections of individual printed circuit boards  1800 . A detailed view of one of the printed circuit boards is shown. Each printed circuit board  1800  has a coil  1802  or inductor which is formed on the surface of the printed circuit board. Additionally each printed circuit board has an electrode  1804  which is also formed on the surface of the printed circuit board  1800 . Capacitors are then formed by placing a dielectric layer  1806  between two printed circuit boards  1800 . The resonant frequency of the radio frequency trap can be adjusted by adjusting the amount of overlap between two adjacent capacitive electrodes  1804 . The sections of printed circuit board  1800  are connected together to form the transmission line for the catheter. There is a through contact  1808  for forming electrical contact between adjacent printed circuit boards  1800 . 
         [0108]      FIG. 19  shows a section of cooling line  1900  with a transmission line  1902  that has coaxial chokes  1914 . The transmission line  1902  is located within the cooling line  1900 . The arrows  1912  indicate fluid flow through the cooling line  1900 . The coaxial choke  1914  is formed by having a coaxial outer shield  1904  that surrounds the transmission line  1902 . There is dielectric material  1906  between the outer shield  1904  and the transmission line  1902 . The outer shield is broken into sections. At one end there is a connection  1908  between the outer shield  1904  and the transmission line  1902 . The other end of the outer shield  1904  is either not connected to the transmission line  1902  or is connected through a capacitor  1910 . When the length of the outer shield  1904  is less than a quarter wavelength of the incident electromagnetic radiation then there will be a very small or no current induced in the transmission line  1902 . In this embodiment the outer shield  1904  is cooled by fluid which flows  1912  through the cooling line  1900 . 
         [0109]      FIG. 20  shows a section of catheter  200  according to an embodiment of the invention. Within the catheter is a section of cooling line  1900 . The arrows  1912  indicate fluid flow through the cooling line  1900 . Within the cooling line  1900  is an embodiment of a transmission line. In this embodiment there are multiple transmission lines  2002 . The multiple transmission lines  2002  are protected from ambient electromagnetic fields by coaxial chokes  1914 . In this example the coaxial choke  1914  comprises an outer shield  1904  and an inner shield  2004 . The inner shield  2004  is a tube through which the multiple transmission lines  2002  run. Surrounding the inner shield  2004  is a dielectric layer  1906 . Surrounding the dielectric layer  1906  is the outer shield  1904 . The outer shield  1904  is connected to the inner shield  2004  at point labeled  2008 . This is where the connection between the outer shield  1904  and the inner shield  2004  is formed. The other end of the outer shield is either not connected to the inner shield  2004  or is connected to the inner shield through a capacitor  1910 . In such a catheter  2000 , the structure of the coaxial choke  1914  is repeated periodically. The outer shield  1904  of the coaxial choke  1914  is cooled by fluid flow  1912  through the cooling line  1900 . 
         [0110]      FIG. 21  shows a further embodiment of a section of cooling line  2102  with a coaxial choke. In this embodiment, the transmission line and cooling line  2102  are combined. The fluid  2104  flows  1912  through the transmission line  2102 . The transmission line  2102  in this embodiment is a hollow tube. As with the embodiments shown in  FIGS. 19 and 20  a coaxial choke  1914  is formed by an outer shield  1904  which surrounds an inner conductor which is in this case the transmission line  2102 . In this embodiment the structure of the coaxial choke  1914  is repeated periodically along the length of the transmission line  2102 . One end of the outer shield  1904  is connected to the transmission line  2102  at the point labeled  2108 . The other end of the outer shield  1904  is either not connected to the transmission line  2102  or is connected to the transmission line through a capacitor  1910 . The space between the outer shield  1904  and the transmission line  2102  may either be filled with a dielectric material  1906  or it may be an air gap. In this embodiment the transmission line  2102  is cooled by the fluid  2104 . 
         [0111]      FIG. 22  shows an alternative embodiment of a catheter according to an embodiment of the invention.  FIG. 22  shows a cross sectional view of the catheter. The outer wall  2000  of the catheter is shown. In this embodiment there is an inner wall  2200 . Between the wall of the catheter  2000  and the inner wall  2200  is a region  2202  for fluid flow. The cooling line is the region between the inner wall  2200  and the catheter wall  2000 . The arrow labeled  2204  indicates fluid flow through the cooling line. The catheter has an inner cavity  2208  within the inner wall  2200 . The circle labeled  2206  indicates a possible location of the transmission line and associated radio frequency traps. In this embodiment the outer shell of the catheter  2000  is cooled so that any heat from the radio frequency traps is carried away before reaching the patient. The transmission line and radio frequency traps may be of any form as was described previously. 
         [0112]      FIG. 23  shows a cross sectional view of a short section of a further embodiment of a catheter according to the invention. Shown is the catheter wall  2300 . Within the catheter wall  2300  is a section of the cooling line  2302 . The arrows  2304  indicate fluid flow within the cooling line  2302 . In the embodiment shown in this figure, there is a transmission line  2306  which is shielded by sections of an outer shield  2308 . The sections of the outer shield  2308  are connected together by radio frequency traps  2310 . The outer shield  2308  is isolated from the transmission line  2306  by a dielectric layer  2312  or other material such as air. 
       LIST OF REFERENCE NUMERALS: 
       [0000]    
       
           100  Radio frequency ablation catheter 
           102  Catheter wall 
           104  Cooling line 
           106  Radio frequency transmission line 
           108  Electrode end of radio frequency transmission line p 0  ∠Tip electrode 
           112  Arrow indicating flow of fluid to tip electrode 
           114  Channel in tip electrode 
           116  Arrow indicating flow of fluid out of tip electrode channel 
           118  Radio frequency trap 
           120  Predetermined distance between adjacent radio frequency traps 
           200  Radio frequency ablation catheter 
           202  Temperature sensor 
           204  High impedance line 
           210  Tip electrode 
           212  Arrow indicating fluid flow out of tip electrode channel 
           214  Channel in tip electrode 
           216  Cavity formed by catheter wall 
           302  Equivalent circuit 
           304  Cooling line 
           306  Coil 
           308  Lumped capacitor 
           310  Holes in cooling line 
           318  Radio frequency trap 
           402  Equivalent circuit 
           406  Coil 
           408  Lumped capacitor 
           418  Radio frequency trap 
           502  Equivalent circuit 
           506  Coil 
           508  Distributed capacitor 
           518  Radio frequency trap 
           602  Equivalent circuit 
           606  Coil 
           618  Radio frequency trap 
           700  Side view 
           702  Equivalent circuit 
           704  Bottom view without cooling line 
           706  coil 
           718  Radio frequency trap 
           720  First electrode 
           722  Second electrode 
           724  Electric field lines 
           800  Side view 
           802  Equivalent circuit 
           804  Bottom view without cooling line 
           806  coil 
           818  Radio frequency trap 
           820  First electrode 
           822  Second electrode 
           902  Equivalent circuit 
           906  Coil 
           918  Radio frequency trap 
           924  Conductive tube 
           926  Third electrode 
           928  Fourth electrode 
           930  Wire 
           932  Gap in conductive tube 
           1002  Equivalent circuit 
           1010  Hole 
           1018  Radio frequency trap 
           1028  Third electrode 
           1030  Wire 
           1100  Magnetic resoance imaging system 
           1102  Magnet 
           1104  Magnetic field gradient coil 
           1106  Magnetic field gradient power supply 
           1108  Radio frequency coil 
           1110  Radio frequency transceiver 
           1112  Subject 
           1114  Imaging zone 
           1116  Subject electrode 
           1118  Radio frequency generator 
           1120  Radio frequency ablation catheter 
           1122  Connection 
           1124  Tip electrode 
           1126  Heating zone 
           1128  Computer system 
           1130  Hardware interface 
           1132  Microprocessor 
           1134  User interface 
           1136  Computer storage 
           1138  Compute memory 
           1140  Computer program product 
           1142  Catheter control module 
           1144  Magnetic resonance imaging control module 
           1146  Image reconstruction module 
           1148  Magnetic resonance data 
           1150  Magnet resonance image 
           1200  Cooling line 
           1202  Transmission line 
           1204  Radio frequency trap 
           1302  Transmission line 
           1404  Radio frequency trap 
           1406  Capcacitor 
           1408  Coil 
           1410  Printed circuit board 
           1508  Coil 
           1512  Circuit board trace on opposing side of printed circuit board 
           1800  Printed circuit board 
           1802  Coil 
           1804  Capacitive electrode 
           1806  Dielectric layer 
           1900  Cooling line 
           1902  Transmission line 
           1904  Outer shield 
           1906  Dielectric 
           1908  Connection between outer shield and transmission line 
           1910  Capacitor 
           1912  Fluid flow through cooling line 
           1914  Coaxial choke 
           2000  Wall of catheter 
           2002  Multiple transmission lines 
           2004  Inner shield 
           2008  Connection between outer shield and inner shield 
           2102  Combined transmission line and cooling line 
           2104  Fluid 
           2108  Connection between outer shield and transmission line 
           2200  Inner wall 
           2202  Region for fluid flow 
           2204  Cross indicating fluid flow 
           2206  Location of transmission line and radio frequency traps 
           2208  Inner cavity 
           2300  Catheter wall 
           2302  Cooling line 
           2304  Arrow indicating fluid flow 
           2306  Transmission line 
           2308  Outer shield 
           2310  Radio frequency trap 
           2312  Dielectric layer