Abstract:
A preferred embodiment includes an MRI compatible catheter that maintains a desired temperature during an MRI procedure. Specifically, the catheter includes a lumen containing sensor wire that is surrounded by an insulating material. This material limits the transfer of heat to the outside of the catheter as the sensor wire heats during an MRI procedure.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/876,522 filed Dec. 22, 2006 entitled An MRI Compatible Temperature-Sensing Catheter which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The measurement of temperature remains an important part of many different medical treatments. Often, catheters are used for temperature measurement, such as a ventricular catheters placed in the brain. Thermistor or thermocouple based temperature sensors are typically used in temperature sensing catheters. 
         [0003]    While these measurement catheters are acceptable for many medical purposes, their use within an MRI machine can prove difficult. For example, the wires connected to a temperature sensor increase in temperature when placed in an MRI machine. As the heat from these wires increase, the inner and outer surfaces of the catheter increase in temperature, causing potentially serious injury to the patient. 
         [0004]    MRI procedures are typically 15 minutes and typically use a 1.5-3T magnet. During this time, the MR/rf field continually heats the wire based on the strength of the magnet and the length of the wire. FDA guidelines state that the temperature of the exterior of the catheter not rise more than 2° C. during a 15-minute scan. 
         [0005]    Designs used to reduce heating of the wires by those skilled in the art include placing coils and capacitors at either end of the wire to neutralize induced currents and insulating the wire with an electrical insulator of sufficient quality to avoid current pathways between wires. 
         [0006]    One specific catheter design example can be seen in U.S. Pat. No. 6,829,509, the contents of which are incorporates by reference. This MRI compatible catheter reduces RF heating with incorporated circuitry and materials that address eddy currents and the transfer of electromagnetic energy from one wire to another. 
         [0007]    The inclusion of electronics in a catheter is appropriate for particularly expensive devices, especially when an electrode at the distal end of the catheter might be in contact with the body, e.g. pacemaker leads. However, the cost and size of the electronic components used in designs that reduce heating effect are not appropriate for use in some catheters, such as a ventricular catheter used in the brain. 
       SUMMARY OF THE INVENTION 
       [0008]    Four main catheter embodiments are described according to the present invention that can be used with MRI machines of different magnet strengths. One preferred embodiment reduces heat transfer from the wire to the catheter by stringing the wire through an insulating tube that is then placed in a dedicated lumen. A second preferred embodiment provides a different insulator than the first design. A third preferred embodiment removes heat by a fluid circuit that passes either a fluid or gas (e.g., water or air) past the sensor wire. A fourth preferred embodiment has a removable temperature sensor probe. The sensor is removed prior to an MRI and then replaced. 
         [0009]    One objective of each design is to prevent the sensor wires in the catheter from raising the temperature of the catheter body to an unacceptable temperature. 
         [0010]    As will be discussed, one preferred embodiment of the present invention includes a dedicated catheter lumen through which the sensor wires pass. The lumen is larger than the lumen used to receive uninsulated wires in the present catheters. Some currently available catheters have a diameter of about 0.02″. In a catheter designed to receive a temperature probe, a 0.03″ lumen is preferred. 
         [0011]    Concerning the removable probe embodiment, the probe can be alternatively placed in a manifold attached to a skull bolt. The manifold includes a passage for a catheter (e.g., ventricular catheter) and provides channels through which parameter probes (such as temperature, oxygen, pressure or flow probes) can be passed into the brain. A channel provided expressly for a temperature probe includes a lumen with a closed distal end to allow an unsterile probe to be reinserted after an MRI. 
         [0012]    The two embodiments that rely upon insulation require no attention from the hospital staff. The water or air-cooled design requires the staff to provide an air pump or water source such as an IV bag or liquid pump. The removable probe design requires the staff to remove and replace the probe from the catheter. The most appropriate option of the four designs depends upon the strength of the magnet used in the MRI machine. 
         [0013]    The first three embodiments of the present invention, insulation, air-cooling or liquid cooling, do nothing to reduce the heating effect of the MRI. They instead reduce heat transfer from the wire to the catheter. 
         [0014]    The first two embodiments house the wire in an insulator with properties sufficient to limit the heat transfer from the wire to the catheter so the rise in the temperature of the body preferably does not exceed 4° C., and more preferably 2° C. A third embodiment removes heat by injecting a fluid, either gas or water, past the wire. The fourth embodiment places the sensor and its wires in a removable probe. The probe is removed from the catheter prior to an MRI procedure and reinserted in the catheter after the procedure. The fluid-cooling design and removable probe designs are best suited to be used in a MR machine with a strong magnet and a catheter with long sensor wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates an insulating sleeve on sensor wires according to a preferred embodiment of the present invention; 
           [0016]      FIG. 2  illustrates a string of insulating beads on sensor wires according to a preferred embodiment of the present invention; 
           [0017]      FIGS. 3   a  and  3   b  illustrate a fluid-cooled catheter design according to a preferred embodiment of the present invention; 
           [0018]      FIG. 4  illustrates a removable temperature probe according to a preferred embodiment of the present invention; 
           [0019]      FIG. 5  illustrates a catheter with a guide tube designed to receive a temperature probe according to a preferred embodiment of the present invention; 
           [0020]      FIG. 6A  illustrates a manifold system within a patient, having multiple lumens for receiving a probe and a catheter according to a preferred embodiment of the present invention; 
           [0021]      FIG. 6B  illustrates the manifold system of  FIG. 6A  out of the patient; and 
           [0022]      FIGS. 7A ,  7 B and  7 C illustrate various views of the manifold system of  FIGS. 6A and 6B   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    As previously described in this specification, the RF field generated by an MRI machine will heat the wires of a typical catheter. Preferably, the temperature rise of the exterior of an MRI compatible catheter during a typical 15 minute scan time will preferably not exceed 4° C., and more preferably 2° C. The heat generated by the wire is defined by the strength of the magnet and the length of the wire. Longer wires get hotter than shorter wires. 
         [0024]    The preferred embodiments of the present invention allow a temperature-sensing catheter to be exposed to the field of an MR machine. Since catheter length and therefore the length of the sensor wires can vary considerably from one catheter to another, different preferred embodiments may be appropriate with different catheter designs. 
         [0025]    Generally, the first two preferred embodiments provide an insulator between the wire and the catheter. The third preferred embodiment removes the heat generated by passing a cooling fluid by the wire during the scan. The fourth preferred embodiment removes the wire from the catheter during an MRI scan. 
         [0026]    Note that the catheters in which a temperature sensor is placed may have more than one lumen for other medical purposes and procedures. For simplicity, the drawings omit extra lumens. 
         [0027]    Also, note that the term wire will be used to describe the two wires of a temperature sensor. While the probe of the present invention is specifically referred to as a temperature probe, it should be understood that the present invention is particularly applicable to any sensor type that includes conducting wires (e.g., an oxygen probe). Thus, the probe as described in the specification may be any probe/sensor that uses electrically conductive wires. 
         [0028]    Insulating Sleeve 
         [0029]    Referring to  FIG. 1 , a catheter  10  with a wire lumen  12  is formed by a wall  14  that encircles the lumen. A sensor wire  18  is positioned within an insulating tube  16  that insulates the wire  18  from the lumen wall  14 . 
         [0030]    Preferably, the insulating tube  16  is flexible enough not to overly stiffen the catheter  10 , can withstand the temperature of the wire and includes a low coefficient of heat transfer. One example insulating material is polyimide, which is ductile, has a particularly low coefficient of heat transfer and is available for use in thin-wall tubes. 
         [0031]    The thermocouple and thermistor wires are preferably 0.005″ in diameter and form a twisted wire set including electrical insulation of about 0.014″ in diameter. The diameter of the insulating tube  16 , affects the overall catheter diameter. 
         [0032]    Preferably, the tube  16  has an ID of 0.020″ and an OD of 0.026″ which balances the size of the lumen  12  and the efficacy of the insulation. If the catheter lumen wall is 0.004″, the tube  16  of these dimensions provides a wall thickness of 0.007″. If used in a ventricular catheter, the example insulating tube  16  will reduce the heat transferred from the wire  18  to the catheter  10  by over two thirds. 
         [0033]    The basis of the heat reduction of the previous example is as follows. Present silicone catheters typically have a wall thickness about 0.02″. The conductivity of polyimide is ⅙ that of silicone. The 0.007″ wall of tube  16  therefore has about the same insulating quality as a 0.042″ silicone tube wall. When the insulating value of the silicone wall is added to that of the polyimide wall of the tube  16 , the insulating effect is equal that of a silicone wall 0.062″ thick or about 3 times that of a silicone tube alone. Over time, the temperature of the wire  18  will increase due to the reduction in heat transfer from wire  18  to the wall of the catheter  10 . The higher temperature of the wire  18  will reduce the net effect of the insulating tube  16 , so improvement may be less than a ⅓ reduction. The lumen diameter required to receive the 0.026″ OD insulating tube  16  is about 0.03″. Such a lumen is about twice as large as a lumen required to string a wire set without a thermal insulation sleeve or tube  16 . 
         [0034]    Insulating Beads on a Wire 
         [0035]      FIG. 2  illustrates another preferred embodiment of an insulating arrangement according to the present invention in which short insulating tubes, segments or beads  20  are placed along the wire  18 . Preferably, the beads  20  are longitudinally spaced along the wire at intervals that keep the catheter wall  12  from contacting the bare wire  18  if the catheter  10  is bent to the extent that it forms a 0.25″ radius. 
         [0036]    In one specific example, beads  20  have a length of 0.05″, are composed of polyimide and are fixed on the wire  18  at intervals of 0.150″. The air gap between the beads has a lower K factor than polyimide and thus reduces overall heat transfer. In cal/(cm)(sec)(° C.), the conductivity of air is 6.9×10 −7  vs. 3.7×10 −4  for polyimide, a three orders of magnitude difference. This construction can be used in an MRI with a stronger magnet and/or a catheter with a longer wire than the preferred embodiment of  FIG. 1 . 
         [0037]    Fluid Cooled Catheter 
         [0038]    In another preferred embodiment, a cooling fluid or gas (e.g., water or air) is passed over or adjacent the wire.  FIG. 3   a  illustrates a two-lumen construction in which an inflow lumen  22  delivers (e.g. via a pump) a cooling fluid through a crossover  24  to an outflow lumen  26  and ultimately to an exit in a transition tube  36 . The wire  18  is located in the outflow lumen  26  and is thereby cooled by the cooling fluid. The wire  18  is fixed to a transition tube  36  by an adhesive  35 . Both the lumens are closed where needed by a plug  28 . The inlet lumen has a luer connector  34  on its proximal end that mates with a similar connector on a fluid source line. A transition tube  36  carries the wire out of the catheter to a connector, not shown. The cooling fluid exits out of a transition tube. The transition tube has a luer connector  34  that can mate with a take-away line, not shown. Thus, the cooling fluid can be delivered over the wire  18  during an MRI procedure and thereby maintain the temperature of the catheter. 
         [0039]      FIG. 3   b  illustrates a single fluid lumen embodiment that uses a fluid delivery tube  38  that extends the length of the wire lumen and delivers fluid to the bottom of the lumen. The fluid cools the wire  18  as it flows out the lumen. A transition tube  36  has a Y  40  that separates the wire  18  from the outflow fluid. The cooling fluid exits the lumen through an outflow line  42  that has a luer fitting  30 . The wire  18  exits the upper leg  41  of the Y where the wire  18  is bonded to an interior of the lumen of the catheter by an adhesive  34 . 
         [0040]    Removable Temperature Probe 
         [0041]      FIG. 4  illustrates a preferred embodiment of a removable temperature probe  1 . Preferably, the temperature probe  1  can be removed prior to an MRI procedure, thereby eliminating any temperature increase of the wires  18 . Alternately, any of the previously described insulation or cooling embodiments (such as those seen in  FIGS. 1-3   b ) can be used with the wire  18  of probe  1 , allowing the user to choose an appropriate probe  1  for a specific MRI machine. The removable temperature probe  1  allows the device to be used in any strength MRI machine if the other components of the catheter  10  (e.g., the catheter of  FIG. 5 ) are non-magnetic. 
         [0042]    The wire  18  of the probe  1  is located in a probe housing  44 . The distal end of the housing is a closed hemisphere. The wire  18  passes through a probe connector  30  (e.g., a luer fitting) and is held in place by an adhesive. 
         [0043]      FIG. 5  illustrates a catheter  10  with a lumen and transition tube  36  designed to receive the temperature probe  1 . Both of the lumen ends are closed with a plug  28 . The transition tube  36  has a transition tube connector  34  that mates with the probe connector  30  on the temperature probe  1 . A guide tube  46  is preferably attached to the proximal end of the transition tube  36 . The curvature of the guide tube  46  and its low friction properties enable the temperature probe  1  to be inserted into the lumen of the catheter  10 . Inserting the probe into a 0.025″ diameter lumen without the attributes of the guide tube may be problematic as the bend radius required to enter a 0.025″ lumen is quite small. When the probe is fully inserted in the catheter, the guide tube connector  34  and the probe connector  30  are joined. 
         [0044]    Preferably, the probe housing  44  is composed of polyimide, as it is available in very thin wall tubes and has a low coefficient of friction. In one example, the polyimide probe housing  44  is 0.01″ ID×0.015″ OD. The lumen diameter of the catheter is 0.03″. A guide tube  46  is integrated into the provided port. It serves two functions. The entry of the probe  1  into the catheter  10  requires that the probe make a very sharp turn into a very small lumen of the catheter  10 . The curved section of the guide tube  46  causes the probe  1  to curve as it makes the transition from the guide tube to the catheter lumen. The guide tube  46  also provides a surface with a low coefficient of friction. Catheter materials are somewhat grabby and can make the insertion of the probe into a small diameter lumen difficult. 
         [0045]    In operation, the catheter  10  is implanted within a patient and the removable probe  1  is placed within the catheter  10 , coupling the transition tube connector  34  and the probe connector  30 . Prior to an MRI procedure, the transition tube connector  34  and the probe connector  30  are uncoupled and the probe  1  is removed. Once the MRI procedure has been performed, the probe  1  can be reintroduced into the catheter  10  and the transition tube connector  34  and the probe connector  30  can be coupled once more. 
         [0046]      FIGS. 6A through 7C  illustrate a preferred embodiment similar to the previously described preferred embodiment, further including a manifold  52  that snaps into or couples to a bolt  54  via biased locking tabs  59 . The bolt  54  includes a portion implantable within a patient (e.g., a patient&#39;s skull) and a passage therethrough. The manifold  52  includes multiple passages on its proximal end that feed into a multi-lumen extension  53  connected on its distal end. In other words, the manifold  52  “funnels” or directs different probes, tubes or other devices into the multi-lumen extension  53  via individual passages. Preferably, the manifold  52  also includes a catheter passage that allows a catheter to pass through the manifold  52 , and ultimately into the patient, but not into the multi-lumen extension  53 . In this respect, a probe  1  can be advanced into a patient without being sterilized since the multi-lumen extension  53  provides a barrier between the patient and the probe  1 . 
         [0047]    The multi-lumen extension  53  is an elongated tubular member that may be a separate extension that couples directly to a distal side of the manifold  52  or optionally is unitary with the manifold  52 . Alternately the multi-lumen extension  53  may be a distal part of a catheter  10  (i.e., similar to the catheter shown in  FIG. 5 ). 
         [0048]    Preferably, a Touhy-Borst fitting holds a catheter  10  (e.g., a ventricular catheter shown in  FIGS. 6A and 6B ) to the manifold  52  and therefore to bolt  54 , allowing the catheter  10  to pass into the patient. The manifold  52  preferably includes several pigtails (e.g., flexible tubes, one of which is shown as pigtail  58 ) that each connect to a passage on the proximal end of the manifold  52 . The temperature probe  1  can be inserted into the brain through one pigtail and other sensors, such as an oxygen sensor can be inserted into other pigtails. For the sake of clarity, only pigtail  58  is shown in  FIG. 6A-7C . This pigtail  58  includes a probe connector  30  (e.g., a luer connection) on its proximal end. Alternately, the probe  1  may be passed directly into the manifold  52  without the use of a pigtail  58 . 
         [0049]    The temperature probe  1  is inserted into a pigtail  58  and is secured by the probe connector  30 . The distal end of the probe  1  passes through a passage of the manifold  52  and into a lumen  55  of the multi-lumen extension  53 . When fully inserted, the temperature sensor of the probe  1  resides in the patient (e.g., a patient&#39;s brain) but is separated from actual contact with the patient by the closed multi-lumen extension  53 . The temperature probe can be inserted and removed as needed. Preferably, the lumen  55  is closed at a distal end (i.e., the end inserted into the patient), however the lumen  55  may also be open within the multi-lumen extension  53 . 
         [0050]    Since the pigtail  58  is connected to the manifold  52 , any pushing, pulling or other force exerted on the pigtail  58  or probe  1  will be transferred to the manifold  52  and the bolt  54 , instead of to the catheter  10 , as may occur in the preferred embodiment of  FIGS. 4 and 5 . 
         [0051]    In operation, the bolt  54  is implanted within the patient (e.g., into patient&#39;s skull  60  in  FIG. 6A ). Next, the catheter  10  is inserted through the Touhy-Borst of the manifold  52 , through the bolt  54  and into the patient (e.g., the brain). The manifold  52  is then connected to the bolt  54 . The probe  1  is inserted into the pigtail  58 , through the manifold  52  and into the lumen  55  of the multi-lumen extension  53 . The probe connector  30  is coupled to the probe  1  to maintain the position of the probe  1 . Prior to an MRI procedure, the probe connector  30  is uncoupled from the probe  1  and the probe  1  is removed from the catheter  10 . Once the MRI procedure is complete, the probe  1  can be reintroduced into the pigtail  58  as previously described. 
         [0052]    Alternately, the pigtail  58 , the manifold  52  and the multi-lumen extension  53  are composed of an insulating material as previously described in this specification (e.g., polyimide). In this respect, the probe  1  can be left within the patient during an MRI procedure. 
         [0053]    The first two embodiments of  FIGS. 1 and 2  are convenient in that they require no involvement from the MRI staff. The third and fourth embodiments may require that the staff prepare the catheter prior to the MRI but may be less expensive than the first two embodiments. 
         [0054]    Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.