Abstract:
Thermography of an ablation site is carried out by navigating a probe into contact with target tissue in the heart, obtaining a first position of a position sensor in the probe and acquiring a first magnetic resonance thermometry image of the target tissue. The method is further carried out during ablation by iteratively reading the position sensor to obtain second positions, and acquiring a new magnetic resonance thermometry image of the target tissue when the distance between the first position and one of the second positions is less than a predetermined distance. The images are analyzed to determine the temperature of the target tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application No. 62/305,026, which is herein incorporated by reference. 
     
    
     COPYRIGHT NOTICE 
       [0002]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to devices and methods for transferring non-mechanical forms of energy to or from the body. More particularly, this invention relates to magnetic resonance thermometry during cardiac ablation therapy. 
         [0005]    2. Description of the Related Art 
         [0006]    The meanings of certain acronyms and abbreviations used herein are given in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Acronyms and Abbreviations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 GRE 
                 Gradient-Recalled Echo 
               
               
                   
                 MRI 
                 Magnetic Resonance Imaging 
               
               
                   
                 PRF 
                 Proton Resonance Frequency 
               
               
                   
                 TE 
                 Echo Time 
               
               
                   
                   
               
             
          
         
       
     
         [0007]    Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. Important sources of undesired signals are located in the tissue region along the pulmonary veins of the left atrium and in myocardial tissue associated with cardiac ganglionic plexi. In this condition, after unwanted signals are generated in the pulmonary veins or conducted through the pulmonary veins from other sources, they are conducted into the left atrium where they can initiate or continue arrhythmia. 
         [0008]    Procedures for treating arrhythmia include disrupting the areas causing the arrhythmia by ablation, as well as disrupting the conducting pathway for such signals. Ablation of body tissue using electrical energy is known in the art. The ablation is typically performed by applying alternating currents, for example radiofrequency energy, to one or more ablation electrodes, at a sufficient power to destroy target tissue. Typically, the electrodes are mounted on the distal tip or portion of an invasive probe or catheter, which is inserted into a subject. The distal tip may be tracked in a number of different ways known in the art, for example by measuring magnetic fields generated at the distal tip by coils external to the subject. 
         [0009]    A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure. 
         [0010]    Various techniques have been developed to control local heating of the ablation site. For example commonly assigned U.S. Pat. No. 6,997,924 to Govari et al., which is herein incorporated by reference, describes a technique of limiting heat generated during ablation by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level. This patent, like other examples, relies on measurements of the temperature at the ablation site. Typically, a temperature sensor such as a thermocouple or thermistor, may be mounted on or near the ablation electrodes. 
         [0011]    Magnetic resonance thermometry based on the proton resonance frequency (PRF), has been proposed for intrabody temperature measurements. A phase shift in the PRF is linear over a wide temperature range, and is relatively insensitive to tissue type. PRF-based phase imaging has become a favored technique for MRI thermometry. 
         [0012]    The principles of PRF-based phase imaging are well-known. Briefly, the nuclear shielding effect of electrons in an aqueous medium increases with temperature, leading to reduced local magnetic field strength and hence a reduced PRF. This can be exploited by choosing a gradient-recalled echo (GRE) imaging pulse sequence for acquiring multiple phase images during heating such that the phase differences of the dynamic images are proportional to the echo time (TE). Under these conditions, the temperature change ΔT(t) may be calculated: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                           ΔΦ 
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                             ( 
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                           γ 
                           · 
                           α 
                           · 
                           
                             B 
                             0 
                           
                           · 
                           TE 
                         
                       
                       = 
                       
                         
                           
                             Φ 
                              
                             
                               ( 
                               t 
                               ) 
                             
                           
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                             Φ 
                             0 
                           
                         
                         
                           γ 
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         [0000]    where φ(t) and φ 0  are the image phase at time t and an initial time, respectively; γ is the gyromagnetic ratio of hydrogen (42.58 MHz per Tesla); α is the temperature coefficient of the shielding constant for the shielding effect; and B 0  is the main magnetic field strength. 
         [0013]    However, for moving tissues, such as a beating heart, MRI images in sequence are generally not in registration with one another. Accurate thermometry is not possible, because any observed phase change could be due, at least in part, to a movement in the target being imaged. 
       SUMMARY OF THE INVENTION 
       [0014]    Disclosed embodiments of the invention involve locating a catheter at the tissue ablation site. The catheter has a position sensor at its distal tip, and an electrode on the distal tip is used for ablation, the ablation causing a change of temperature desired to be tracked. The position information obtained from the sensor readings provides an indication that the relevant part of the field of view of the imaging system is in registration with that of a previous image. The indication triggers acquisition of a new image. 
         [0015]    There is provided according to embodiments of the invention a method, which is carried out by inserting a probe into a heart of a living subject. The probe has a position sensor and an ablation electrode disposed on its distal portion. The method is further carried out by navigating the probe into a contacting relationship with target tissue of the heart, activating the ablation electrode, obtaining a first reading of the position sensor to obtain a first position, and acquiring a first magnetic resonance thermometry image of the target tissue at the first position. The method is further carried out iteratively by taking new readings of the position sensor to obtain second positions, acquiring a new magnetic resonance thermometry image of the target tissue when a distance between the first position and one of the second positions is less than a predetermined distance, and analyzing the first magnetic resonance thermometry image and the new magnetic resonance thermometry image to determine the temperature of the target tissue. 
         [0016]    According to one aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image includes determining a phase change therebetween of a proton resonant frequency and calculating the temperature of the target tissue from the phase change. 
         [0017]    According to a further aspect of the method, the position sensor is a magnetic location sensor and the new readings are taken at 10 ms intervals. 
         [0018]    According to still another aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image comprise measuring a proton resonance frequency phase shift and correlating the phase shift with temperature. 
         [0019]    According to an additional aspect of the method, the first magnetic resonance thermometry image and the new magnetic resonance thermometry image are obtained from pulse sequences. 
         [0020]    According to another aspect of the method, the pulse sequences are gradient-recalled echo pulse sequences. 
         [0021]    According to one aspect of the method, measuring a proton resonance frequency phase shift is performed spectroscopically. 
         [0022]    According to a further aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image comprise measuring a proton density spin lattice relaxation time. 
         [0023]    According to an additional aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image comprise measuring a spin-spin relaxation time. 
         [0024]    According to yet another aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image comprise measuring a diffusion coefficient. 
         [0025]    According to still another aspect of the method, acquiring the first magnetic resonance thermometry image and the new magnetic resonance thermometry image comprise measuring a magnetization transfer. 
         [0026]    There is further provided according to embodiments of the invention an apparatus including a flexible probe adapted for insertion into a heart of a patient and having a position sensor in its distal portion. A processor is linked to the position sensor and is configured for sending control signals to a magnetic resonance imager. The processor is cooperative with the magnetic resonance imager for obtaining a first reading of the position sensor to establish a first position when the distal portion is in a contacting relationship with target tissue of the heart, acquiring a first magnetic resonance thermometry image of the heart at the first position, and during ablation thereafter iteratively taking new readings of the position sensor to obtain second positions, acquiring a new magnetic resonance thermometry image of the target tissue when a distance between the first position and one of the second positions is less than a predetermined distance, and analyzing the first magnetic resonance thermometry image and the new magnetic resonance thermometry image to determine a temperature of the target tissue. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]    For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
           [0028]      FIG. 1  is a pictorial illustration of a system for performing cardiac catheterization procedures in accordance with an embodiment of the invention; 
           [0029]      FIG. 2  is a series of diagrams illustrating the acquisition of MRI thermometry images from a moving tissue in accordance with an embodiment of the invention; 
           [0030]      FIG. 3  is a flow chart of a method of determining the temperature of an ablation site, in accordance with an embodiment of the invention; 
           [0031]      FIG. 4  is a composite diagram comprising two MRI thermography images acquired in accordance with an embodiment of the invention; and 
           [0032]      FIG. 5  is a collection of two MRI images that are suitable for MRI thermography. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
         [0034]    Documents incorporated by reference herein are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
       Overview 
       [0035]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing cardiac catheterization procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system  10  typically comprises a general purpose or embedded computer processor, which is programmed with suitable software for carrying out the functions described hereinbelow. Thus, although portions of the system  10  shown in  FIG. 1  and other drawing figures herein are shown as comprising a number of separate functional blocks, these blocks are not necessarily separate physical entities, but rather may represent, for example, different computing tasks or data objects stored in a memory that is accessible to the processor. These tasks may be carried out in software running on a single processor, or on multiple processors. Alternatively or additionally, the system  10  may comprise a digital signal processor or hard-wired logic. 
         [0036]    The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall at an ablation target site. Electrical activation maps, anatomic positional information, i.e., of the distal portion of the catheter, and other functional images may then be prepared using a processor  22  located in a console  24 , according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765, which is capable of producing electroanatomic maps of the heart as required for the ablation. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
         [0037]    Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating (or cooling) it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias. 
         [0038]    The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a positioning processor  22 , located in the console  24 . 
         [0039]    Ablation energy and electrical signals can be conveyed to and from the heart  12  through the catheter tip and/or one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
         [0040]    Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. 
         [0041]    The console  24  typically contains one or more ablation power connections. The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, freezing technique and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
         [0042]    The positioning processor  22  is an element of a positioning subsystem in the system  10  that measures location and orientation coordinates of the catheter  14 . 
         [0043]    In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils  28 . The positioning subsystem may employ impedance measurement, as taught, for example in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
         [0044]    A MRI imaging device  37  is linked to a control processor  47 , which may be located in the console  24 . An operator may select or override automatic operation to control the operation of the MRI imaging device  37 , for example by revising imaging parameters. The control processor  47  may communicate with the MRI imaging device  37  via a cable  51  to enable and disable the MRI imaging device  37  to acquire image data. An optional display monitor  49 , linked to the control processor  47 , allows the operator to view images produced by the MRI imaging device  37 . When the display monitor  49  is not included, the images may still be viewed on a monitor  29 , either via a split screen or in alternation with other images. 
         [0045]    As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . The processor  22  is typically a computer with appropriate signal processing circuits. The processor  22  is coupled to drive the monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14  and the MRI imaging device  37 , including signals generated by the above-noted sensors and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14 , analyze the electrical signals from the electrodes and generate desired electroanatomic maps. The above-described arrangement works well when a coordinate system is shared between system components, e.g., a combined CARTO-MRI system. This is especially useful when ablating the atria, as its wall is very thin, and it is necessary to define its boundaries. Despite advances in image processing, edge detection of the endocardial wall continues to be challenging, and conventionally requires manual analysis of sequential slice images. However, with a trackable, MRI-compatible, indwelling catheter that touches the endocardial wall and measures contact-force, manual analysis can be avoided. 
         [0046]    Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
       Image Acquisition 
       [0047]    Reference is now made to  FIG. 2 , which is a series of diagrams  53 ,  55 ,  57 ,  59  illustrating the acquisition of MRI thermometry images from a moving tissue in accordance with an embodiment of the invention. An area being imaged by MRI imaging device  37  ( FIG. 1 ) is indicated by a circle  61 . In diagram  53  a point of interest, shown here as an ablation site  63  of a heart is marked by “X” at the center of the circle  61 . The distal end of a probe  65  has an ablation electrode  67  in contact with the ablation site  63 . The position of the circle  61  is constant. 
         [0048]    From the readings, the location of the ablation site  63  with respect to a frame of reference  69  can be determined from readings of a position sensor  71 , which is typically a magnetic location sensor. The ablation site  63  is aligned with the “X” of circle  61  at time T 0  and has coordinates (x 0 , y 0 , z 0 ). An MRI thermometry image (IMAGE 0 ) is acquired at time T 0 . However, at times T 1 , T 2 , cardiorespiratory motions have displaced the ablation site  63  and probe  65  with respect to the circle  61  as shown in diagrams  55 ,  59 . At times T 1 , T 2  the ablation site  63  has coordinates (x 1 , y 1 , z 1 ), (x 2 , y 2 , z 2 ) respectively, which differ from the coordinates (x 0 , y 0 , z 0 ). At time T n . The ablation site  63  has coordinates (x n , y n , z n ), which are recognized by the system as being essentially identical to the coordinates (x 0 , y 0 , z 0 ), i.e., a distance measure between two sets of coordinates that is less than a pre-defined limit δ. For example, the Euclidian distance 
         [0000]      √{square root over (( x   n   −x   r ) 2 +( y   n   −y   r ) 2 +( z   n   −z   r ) 2 )}&lt;δ,   (2)
 
         [0000]    where (x r , y r , z r ) are coordinates of a reference point corresponding to the ablation site on a reference image. 
         [0049]    The recognition triggers acquisition of a second MRI thermometry image (IMAGE 1 ). Currently available sensors are capable of reporting a position every 10 ms. Suitable parameters for the MRI image are TE 40 ms, slice thickness 3 mm and flip angle 60 degrees. The MRI thermometry images may be triggered at every beat to acquire a new slice during the time interval T 0 &lt;T&lt;Tn. 
         [0050]    Reference is now made to  FIG. 3 , which is a flow chart of a method of determining the temperature of an ablation site, in accordance with an embodiment of the invention. The procedure is described with respect to the exemplary system  10  ( FIG. 1 ), but may be performed with other system configurations. The process steps are shown in a particular linear sequence in  FIG. 3  for clarity of presentation. However, it will be evident that many of them can be performed in parallel, asynchronously, or in different orders. Those skilled in the art will also appreciate that a process could alternatively be represented as a number of inter-related states or events, e.g., in a state diagram. Moreover, not all illustrated process steps may be required to implement the method. 
         [0051]    At initial step  73  the heart is catheterized conventionally with a probe having a position sensor and an ablation electrode in its distal portion. The ablation electrode is brought into contact with a target location using known methods. 
         [0052]    Next, at step  75  a reference image including the target area is acquired on the MRI imaging device  37 . Ambient body temperature is assumed for purposes of correlating the PRF phase in the reference image with temperature. 
         [0053]    Next, at step  77  the position sensor  71  is read and reference coordinates (x r , y r , z r ) of the sensor and the target location are determined on the reference image. 
         [0054]    Next, at step  79  the ablation electrode is activated to begin ablation of the target tissue. 
         [0055]    MRI thermometry images such as PRF-based phase images are acquired during the procedure from time to time. One suitable pulse sequence for the images is a gradient-recalled echo pulse sequence with the above-noted MRI parameters. A pause occurs at delay step  81  where it is determined if a predetermined delay interval has expired. If the MRI thermometry images are to be acquired continually, then the delay interval is set to zero. 
         [0056]    Next, at step  83  a reading is obtained from the position sensor  71  and its coordinates (x n , y n , z n ) determined. 
         [0057]    Next, at decision step  85 , it is determined if the distance between the coordinates obtained at step  77  and step  83  is smaller than a predetermined value δ, i.e., the inequality (2) shown above is satisfied. If the determination at decision step  85  is negative, then control returns to step  83  and another reading is obtained from the position sensor  71 . 
         [0058]    If the determination at decision step  85  is affirmative, then control proceeds to step  87 . A new MRI thermometry image is acquired. 
         [0059]    Then, at step  89  the temperature at the ablation site is determined on the new image, typically by PRF-based phase temperature mapping, and deriving the temperature of the ablation site from the frequency phase shift, using the principles of equation (1) given above. Control then returns to delay step  81  to begin a new waiting period. 
       Alternate Embodiments 
       [0060]    The PRF shift with temperature can be measured spectroscopically, using a reference substance such as a lipid, which is temperature independent. Alternatively, there are a number of temperature-dependent characteristics in magnetic resonance imaging, for example, proton density spin lattice relaxation time; spin-spin relaxation time; diffusion coefficient and magnetization transfer. Shifts in the measurements of these characteristics may be determined in the analysis of the MRI images in step  89  ( FIG. 3 ). 
       EXAMPLE 
       [0061]    Reference is now made to  FIG. 4 , which is a composite diagram comprising two MRI thermography images  91 ,  93  showing typical results in a prospective cardiac ablation procedure in accordance with an embodiment of the invention. An increase in temperature  95  at ablation site  97  is noted in the later image  93 , compared to temperature  99  in the earlier image  91 . The operator can react to the measured temperatures  95 ,  99  by adjusting the power and/or duration of the ablation procedure as is known in the art. 
         [0062]    Reference is now made to  FIG. 5 , which is a collection of two MRI images  101 ,  103  that are suitable for MRI thermography according to an embodiment of the invention. The images  101 ,  103  were obtained from data acquired in one slice in accordance with an embodiment of the invention. Images  101 ,  103  are an amplitude and a phase image, respectively. Ablation site  105  is indicated on both images. 
         [0063]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.