Abstract:
A method of monitoring a position of a catheter tip relative to a target tissue. The method includes steps of identifying a wall structure in the target tissue; guiding a catheter towards the wall structure; monitoring a location of the catheter; directing a navigator beam towards the wall structure; aligning the navigator beam with the location of the catheter; monitoring a position of the wall structure using the navigator beam; and determining the position of the wall structure relative to the location of the catheter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/791,987, filed Mar. 15, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to real-time-MRI-guided radiofrequency (RF) cardiac tissue ablation. 
         [0003]    Real-time-MRI-guided radiofrequency (RF) cardiac tissue ablation can be used for treatment of conditions such as cardiac arrhythmias including atrial fibrillation (AF). RF ablation is a minimally-invasive catheter-based procedure in which RF energy is used to create lesions in the atrial wall in a prescribed manner to isolate ectopic electrical activity. 
         [0004]    During RF ablation procedures, accurate knowledge of contact made between catheter tip and the structure to be treated, for example the left atrial (LA) wall, is crucial for success of creating the lesion, for accurate electrocardiogram measurement, and for ensuring patient safety. 
         [0005]    However, the monitoring of tip-tissue contact, which is currently being performed using X-ray or ultrasound technologies, is a known shortcoming of current real-time MRI technology. During RF ablation, commonly used to treat cardiac arrhythmias such as atrial fibrillation, scars are intentionally created in the atrial wall to isolate the sources/triggers of abnormal electrical activity. The success rate of RF ablation is limited by the poor imaging that is currently clinically available, generally using X-ray projection imaging and intracardiac echo (ICE). X-ray imaging has poor soft-tissue contrast and only provides projection views. Ultrasound/ICE catheter positioning is cumbersome and provides only limited wall visualization. Visualization of the catheter tip visualization over a static atrial shell does not provide real-time relative positioning of the catheter tip and the wall. 
         [0006]    Poor imaging guidance results in the inaccurate deposition of RF energy and, consequently, can lead to errors in the intended locations of the scars. This may result in the incomplete electrical isolation of the arrhythmia triggers and, in turn, the recurrence of symptoms. 
       SUMMARY 
       [0007]    Accordingly, disclosed herein is a novel method to monitor the contact made between the catheter tip and the target tissue during real-time-MRI-guided radiofrequency (RF) ablation. 
         [0008]    In one embodiment, the invention provides a method of monitoring a position of a catheter tip relative to a target tissue. The method includes steps of identifying a wall structure in the target tissue; guiding a catheter towards the wall structure; monitoring a location of the catheter; directing a navigator beam towards the wall structure; aligning the navigator beam with the location of the catheter; monitoring a position of the wall structure using the navigator beam; and determining the position of the wall structure relative to the location of the catheter. 
         [0009]    In another embodiment the invention provides a system for monitoring a position of a catheter tip relative to a target tissue. The system includes a magnetic resonance imaging (MRI) system, a catheter including a position tracking system and ablation and electrogram recording electrodes, and a controller in communication with the MRI system and the catheter. The controller is configured to identify a wall structure in the target tissue, monitor a location of the catheter during guidance of the catheter towards the wall structure, direct a navigator beam towards the wall structure, align the navigator beam with the location of the catheter, report a position of the wall structure using the navigator beam, and determine the position of the wall structure relative to the location of the catheter. 
         [0010]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a real-time MRI display with a catheter superimposed on an image of a heart. 
           [0012]      FIG. 2  shows a diagram of how an MRI pencil beam navigator is formed. 
           [0013]      FIG. 3  shows a diagram of one possible geometry of the disclosed system. 
           [0014]      FIG. 4  shows the position of navigator (dotted lines) and right atrial RA wall (arrow) 
           [0015]      FIG. 5  shows an M-mode display of cardiac wall position obtained using a pencil beam navigator over several cardiac cycles, the position being identified using automatic wall detection of RA wall (indicated by the black wavy line). 
           [0016]      FIG. 6   a  shows a conventional cine image frame showing the MRI pencil beam position of the same slice as shown in  FIG. 6   b.    
           [0017]      FIG. 6   b  shows an “M-mode” display obtained from an MRI pencil beam navigator, indicating the position of the atrial wall (“wall”) as a function of time, where the left atrium (“LA”) is at the top of the graph and the lung is at the bottom of the graph. 
           [0018]      FIG. 7   a  shows a conventional cine image frame showing the MRI pencil beam position of the same slice as shown in  FIG. 7   b.    
           [0019]      FIG. 7   b  shows an “M-mode” display obtained from an MRI pencil beam navigator, indicating the position of the atrial wall (“wall”) as a function of time, where the left atrium (“LA”) is at the top of the graph and the lung is at the bottom of the graph. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0021]    During an RF ablation, commonly used to treat cardiac arrhythmias, a catheter is percutaneously inserted into a vein or artery and maneuvered into a heart chamber under the guidance of real-time magnetic resonance imaging (MRI). Once the catheter is in an appropriate position, and electrode at the end of the catheter is used to ablate cardiac tissue. As discussed above, however, improved techniques are required for monitoring a position of the catheter relative to the intended target of the catheter, which is often the atrial wall. 
         [0022]    Disclosed herein are methods and systems for visualizing and monitoring the contact made between the target tissue, for example the wall of a beating heart, and the electrodes at the distal end of the catheter. It is necessary to monitor this tip-tissue contact in order to (a) effectively deliver RF energy by ensuring contact between the heat source and the target, and (b) better evaluate the quality of an ablation from the electrogram. The quality of the ablation is judged by estimating the post-ablation reduction of the signal measured by the electrodes at the catheter tip. However, poor imaging makes it hard to determine if the reason for a reduced signal is a successful ablation or the lack of contact between the catheter tip and the target tissue. Accordingly, disclosed are methods and systems for using a pencil beam navigator, positioned at the catheter tip and perpendicular to the atrial wall, in order to automatically detect wall location at a high resolution. Combining this with real-time catheter tracking permits for real-time monitoring of tip-tissue contact. 
         [0023]    The disclosed methods and systems use a navigator beam (such as an MRI pencil-beam navigator, an MRI technique in which a column-shaped volume is excited and a 1D signal is acquired) combined with a real-time-MRI sequence which is capable of tracking the 3D curve of the catheter in real-time. Using this apparatus, the position of the target tissue relative to the catheter tip is determined by automatically positioning the navigator beam so that it is aligned perpendicular to the target tissue (such as the heart wall, the approximate location of which is known from a pre-procedure MRI scan) and concentric with the axis of the catheter tip (the location of which is known from real-time tracking). 
         [0024]    The location of the heart wall is determined from the navigator signal by visualization of the raw signal or automatic detection by image processing. In various alternative embodiments, improved monitoring may be obtained by using a 2D echoplanar projection readout of the excited pencil-beam volume instead of the 1D projection proposed above. Improved monitoring may also be obtained by enhancing blood-tissue contrast by modifying the imaging parameters to conform to balanced-steady-state-free-precession. 
         [0025]    While the state-of-the-art real-time MRI technology allows for the imaging of an MR slice, whose position can be interactively modified, at a temporal resolution of about 4 frames/second, this is insufficient to monitor tip-tissue contact. The advantages of the disclosed methods and systems include that they (1) achieve a higher temporal resolution, of up to about 50 frames/second, (2) avoid the need for the operator to explicitly reposition the imaging slice, (3) are able to monitor to the local volume around the target rather than merely a thin slice through it, and (4) provide a dedicated visualization of the tip-tissue interface (e.g. through an M-mode-like display). 
         [0026]    In various embodiments, the higher temporal resolution achieved using the disclosed methods and systems includes rates of up to about 5 frames/second, up to about 10 frames/second, up to about 20 frames/second, up to about 30 frames/second, or up to about 40 frames/second. The term ‘frames’ is used broadly to indicate various observation points that provide an indication of a position of a structure such as a heart wall at a particular time period. 
         [0027]    The disclosed methods and systems are applicable to various real-time MRI guidance technologies that make use of a catheter. These include a variety of minimally-invasive procedures including ablation procedures for cardiac arrhythmias and the repair of congenital and other mechanical heart defects. 
         [0028]    Various embodiments of the methods and systems disclosed herein may be implemented on an MRI system to provide features including real-time sequences, combination with catheter tracking, and automated placement and detection of structures. 
         [0029]    Under the current real-time MRI paradigm, as shown in  FIG. 1 , a single slice, whose location and orientation are modified interactively by the operator, is imaged continuously with a spatial resolution of about 2 mm and a frame rate of 3 to 4 frames per second. In addition, a 3D static shell of the atrium, extracted from an MR angiography scan acquired at the start of the procedure, and a diagram of the catheter, tracked in real-time using four tracking coils located on the catheter shaft, are displayed. The temporal and spatial resolution provided by this display is sufficient while the operator navigates the device towards the target but during the actual ablation of a target it would be beneficial for the operator to be able to switch to an imaging mode in which visualization is provided solely by a static shell, the diagram of the moving catheter, and the real-time pencil beam navigator display. 
         [0030]      FIG. 2  provides a diagram showing how an MRI pencil beam navigator is formed. A spiral gradient (pulse sequence indicated by ‘Gx’, ‘Gy’) is played concurrently with a shaped RF pulse (‘RF’), as shown in the top left of  FIG. 2 , to produce a circular excitation profile, followed by a readout and a spoiler gradient. This produces a narrow ‘pencil’ beam as shown in the top right of  FIG. 2 . 
         [0031]    In one embodiment, the pencil-beam navigator is implemented using a 2D RF excitation, in which a spiral gradient is played concurrently with an RF pulse. 
         [0032]    The gradient determines the k-space traversal of the beam and the RF amplitude determines the shape of the excitation region. Factors that should be taken into consideration when adjusting the parameters include the fact that the pulse length is limited by T2—relaxation effects and that there is a tradeoff between the sharpness of the profile vs. producing an alias ring artifact (bottom of  FIG. 2 ). In general the diameter of the pencil beam according to embodiments of the invention is in the range of 2 mm to 20 mm, and in one particular embodiment is 5 mm. 
         [0033]      FIG. 3  shows a diagram of a system for performing real-time MRI-guided RF ablation according to embodiments of the invention. Shown are the atrial wall, the catheter, and the pencil beam. The position of the catheter may be monitored using a variety of techniques; in the embodiment shown in  FIG. 3 , the position and approximate shape of the catheter is tracked using information provided by several tracking coils (four in the illustrated embodiment) attached to various portions of the catheter. 
         [0034]    The positioning of the pencil-beam, such that it extends through the catheter tip and approximately perpendicular to the atrial wall as shown in  FIG. 3 , is possible due to the real-time tracking of the catheter and the acquisition of a 3D MR angiography scan at the start of the procedure. The pencil beam is adjusted so that it is concentric with the distal end of the catheter and approximately perpendicular to the structure in question, e.g. the atrial wall. The pencil beam can then provide accurate (e.g. 1-2 mm resolution) information about the position of the wall or other structure at a relatively high frequency (e.g. in a range of 5-100 Hz) which can be combined with information about the position of the catheter tip to help a user guide the catheter and to know if and when the catheter is about to touch the wall or other structure. 
         [0035]      FIGS. 4 and 5  demonstrate the use of the pencil-beam navigator (generated using TE/TR=3.5/25 ms, 8° flip angle, providing 1.5 mm readout resolution) to detect the location of the right atrial wall in a pig in a 3 Tesla scanner.  FIG. 4  shows the position of the pencil-beam overlaid on a slice of the MR angiography image, where the 5-mm radius pencil-beam is indicated by the dotted lines and intersects the wall of the right atrium, which is indicated by the arrow.  FIG. 5  shows the signal from the navigator in an M-mode display which plots distance along the pencil-beam versus time. The navigator is acquired continuously over more than three heartbeats at about 40 frames per second (40 Hz). The automatic detection of the wall, denoted by the horizontal wavy line, is performed by finding the center of the edge in the known neighborhood of the atrial wall. The vertical dashed lines denote the R-wave detected from the concurrently acquired ECG signal and are seen to match the periodicity of motion of the wall.  FIGS. 6   a ,  6   b ,  7   a , and  7   b  show results of in vivo canine experiments in which the position of the left atrial wall-lung interface is monitored using a 5 mm cylindrical pencil beam navigator. Continuous acquisition was performed over a period of several heartbeats. The scale of the M-mode display data is superimposed on the cine image to provide an indication of how far the cardiac wall traverses in each cardiac cycle. 
         [0036]    The disclosed MRI-based methods and systems provide several benefits over current X-ray and ultrasonic technologies for obtaining position information during RF ablation and similar procedures. Compared to X-ray methods, using MRI eliminates exposure to ionizing radiation. In addition, the disclosed methods and systems provide better catheter navigation as a result of real-time catheter tracking, better soft-tissue imaging, and the ability to generate arbitrary views. The disclosed methods and systems also provide improved ablation verification and provide information about the state of the ablated tissue using T1 and T2 relaxation times, and MRI thermometry. Finally, improved accuracy associated with use of the disclosed methods and systems provides reduced collateral structural damage. 
         [0037]    Accordingly, disclosed are methods and systems for detecting atrial wall motion in vivo, using a pencil-beam navigator in order to monitor tip-tissue contact during real-time-MRI-guided RF ablation. In various embodiments, the disclosed methods and systems may be incorporated into a real-time, interactive sequence and combined with existing catheter tracking methods in order to monitor tip-tissue contact in real-time. Further optimizations, such as of the length and profile of the RF pulse, the automatic positioning of the pencil-beam, the relative timing of the acquisition of catheter tracking data, and the most effective visualization of the information, may also be integrated with the disclosed methods and systems. In still further embodiments, a 2D echoplanar projection readout of the excited pencil-beam volume may be used instead of, or in addition to, 1D projections as disclosed herein. Other embodiments may employ bSSFP-like, instead of GRE-like, blood-tissue contrast by adjusting the gradient moments accordingly. In various embodiments the disclosed methods and systems may include fully automated detection of the target structure (e.g. atrial wall). Further, various embodiments may incorporate a priori information, such as a pre-acquired atrial shell, into the placement of the navigator beam and the detection of the wall or other structure. 
         [0038]    In various embodiments, the invention includes systems for carrying out the procedures disclosed herein. The system may include an MRI system, a catheter, and a controller. The controller may be contained in one or several units and may include a computing system having a microprocessor for executing software instructions as well as input and output capabilities including a keyboard, mouse, and display, and may be coupled to one or more other computers via a network such as a local network and/or a global network such as the Internet. 
         [0039]    Thus, the invention provides, among other things, methods and systems for real-time-MRI-guided radiofrequency (RF) cardiac tissue ablation. Various features and advantages of the invention are set forth in the following claims.