Abstract:
A method includes displaying a position of a distal end of a medical probe that is being navigated in an organ of a patient on a three-dimensional (3D) map of the organ. In response to an event, a plane of interest including the distal end is selected, a real-time Magnetic Resonance Imaging (MRI) slice of the organ is acquired at the selected plane, and the MRI slice is displayed overlaid on the 3D map.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to medical imaging, and particularly to methods and systems for real-time MRI in interventional cardiology. 
     BACKGROUND OF THE INVENTION 
     Magnetic Resonance Imaging (MRI) is commonly used for medical imaging in a variety of applications. MRI processing is typically computationally intensive, and therefore real-time MRI for a large volume is usually feasible at relatively low spatial and temporal resolution. 
     U.S. Patent application publication 2010/0312094, to Guttman, et al., whose disclosure is incorporated herein by reference, describes MRI-guided surgical systems with preset scan planes. During ablation MR thermometry (2-D) can be used to show real-time ablation formation taking a slice along the catheter and showing the temperature profile increasing. It is contemplated that 2D and/or 3D GRE pulse sequences can be used to obtain the MR image data. However, other pulse sequences may also be used. 
     U.S. Pat. No. 8,620,404, to Mistretta, whose disclosure is incorporated herein by reference, describes system and method for generating time-resolved 3D medical images of a subject. The method includes acquiring a time series of two-dimensional (2D) data sets from a portion of the subject using a magnetic resonance imaging (MRI) system and reconstructing the time series of 2D data sets into a 2D time series of images of the subject having a given frame rate. 
     U.S. Patent application publication 2013/0184569, to Strommer, et al., whose disclosure is incorporated herein by reference, describes methods for producing an electrophysiological map of the heart. An example method may include determining a target location and an orientation of a catheter tip, confirming that the tip is located at the target location, measuring the heart parameter value at each of the target locations, and superimposing a plurality of representations of the heart parameter value. 
     U.S. Pat. No. 8,675,996, to Liao, et al., whose disclosure is incorporated herein by reference, describes a method for registering a two-dimensional image of a cardiocirculatory structure and a three-dimensional image of the cardiocirculatory structure. The method includes acquiring a three-dimensional image including the cardiocirculatory structure using a first imaging modality. The acquired three-dimensional image is projected into two-dimensions to produce a two-dimensional projection image of the cardiocirculatory structure. A structure of interest is segmented either from the three-dimensional image prior to projection or from the projection image subsequent to projection. A two-dimensional image of the cardiocirculatory structure is acquired using a second imaging modality. 
     U.S. Pat. No. 8,676,300, to Strommer, et al., whose disclosure is incorporated herein by reference, describes method and system for navigating through an occluded tubular organ. The procedures included injecting a first dye injection into the tubular organ, the first dye approaching a first end of the occluded segment. Multiple first-injection two-dimensional (2D) images of the tubular organ are acquired, each acquired from a different perspective, the first-injection 2D images further acquired with a respective organ timing signal reading. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention that is described herein provides a medical system including an interface and a processor. The interface is configured to communicate with a Magnetic Resonance Imaging (MRI) system. The processor is configured to display a position of a distal end of a medical probe that is being navigated in an organ of a patient on a three-dimensional (3D) map of the organ, and, in response to an event, to select a plane of interest including the distal end, to acquire from the MRI system, via the interface, a real-time MRI slice of the organ at the selected plane, and to display the MRI slice overlaid on the 3D map. 
     In some embodiments, the 3D map of the organ is created by a 3D magnetic position tracking system. In other embodiments, the processor is configured to receive a selection of the plane from a user. In alternative embodiments, the processor is configured to choose the plane automatically in response to the event. In yet another embodiment, the organ includes a heart, and the medical probe includes a cardiac catheter. 
     There is additionally provided, in accordance with an embodiment of the present invention, a method including displaying a position of a distal end of a medical probe that is being navigated in an organ of a patient on a 3D map of the organ. In response to an event, a plane of interest including the distal end is selected, a real-time MRI slice of the organ is acquired at the selected plane, and the MRI slice is displayed overlaid on the 3D map. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic pictorial illustration of an MRI system and a magnetic position tracking system during a minimally invasive cardiac procedure, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic pictorial illustration of a MRI slice overlaid on a three-dimensional (3D) magnetic position tracking map, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flow chart which schematically illustrates a method for acquiring a real-time MRI image and overlaying it with a magnetic position tracking map during an intra-cardiac procedure, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Some minimally invasive procedures use magnetic position tracking maps, such as provided by the Biosense Webster CARTO™ system, to navigate a catheter or other medical probe in a patient&#39;s body. In some events, a physician needs a real-time image of an organ near a catheter&#39;s distal end. MRI is one of the imaging solutions, but 3D MRI requires intensive calculations and therefore usually cannot provide the required resolution in real-time. 
     Embodiments of the present invention that are described herein below provide a method and system to obtain real-time imaging of the vicinity of the catheter&#39;s distal end during navigation, using a 3D magnetic position tracking map. Instead of acquiring a complete 3D MRI model, which is not feasible to perform in real time, the disclosed techniques acquire and display a MRI slice in a selected plane of interest which contains the catheter&#39;s distal end. By settling for an image at a specific plane, the physician can be provided with an overlaid image of an MRI slice on the magnetic position map in real-time. 
     In the context of the present patent application and in the claims, the terms “MRI slice” and “2D MRI slice” refer to a thin MRI slice (e.g., 3 millimeters in thickness) acquired by an MRI system on a specified 2D plane. For all practical purposes such a slice is regarded as two-dimensional, even though it has a finite thickness. 
     The embodiments described herein refer mainly to cardiac catheters and cardiac procedures. Alternative embodiments, however, are applicable for any minimally-invasive medical procedures such as laparoscopy or endoscopy, and are not limited to cardiac applications. 
     System Description 
       FIG. 1  is a schematic pictorial illustration of an MRI system  22  and a magnetic position tracking system  20  during a minimally invasive cardiac procedure, in accordance with an embodiment of the present invention. MRI system  22  is connected to magnetic position tracking system  20  via an interface  56 . Magnetic position tracking system  20  comprises a console  26 , and a catheter  24 , which comprises a distal end  34  as shown in an insert  32  of  FIG. 1 . 
     A cardiologist  42  navigates catheter  24  in a patient&#39;s heart  28 , until distal end  34  reaches the desired location in this organ, and then cardiologist  42  performs the medical treatment using distal end  34 . In other embodiments, the disclosed techniques can be used with procedures that are performed in any other organ, and instead of cardiologist  42 , any suitable human user can use the system. 
     This method of position tracking is implemented, for example, in the CARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.) and is described in detail in U.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, in PCT Patent Publication WO 96/05768, and in U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. 
     Console  26  comprises a processor  58 , a driver circuit  60 , interface  56  to MRI system  22 , input devices  46 , and a display  40 . Driver circuit  60  drives magnetic field generators  36 , which are placed at known positions below a patient&#39;s  30  torso. In response to an event, cardiologist  42  selects (using input devices  46  and a suitable Graphical User Interface (GUI) on screen  40 ) a desired plane, which comprises distal end  34 . In another embodiment, processor  58  selects the desired plane automatically. Processor  58  requests a MRI slice of the selected plane from MRI system  22 , via interface  56 . MRI system  22  acquires the requested slice and sends it, via interface  56 , to processor  58 . 
     Processor  58  creates an overlaid image of a 3D magnetic position tracking map with a MRI slice and displays this image on screen  40 . 
     The configuration of system  20  shown in  FIG. 1  is an example configuration, which is chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable configuration can be used for implementing the system. Certain elements of system  20  can be implemented using hardware, such as using one or more Application-Specific Integrated Circuits (ASICs) or Field-Programmable Gate Arrays (FPGAs) or other device types. Additionally or alternatively, certain elements of system  20  can be implemented using software, or using a combination of hardware and software elements. 
     Processor  58  typically comprises a general-purpose computer, which is programmed in software to carry out the functions described herein. The software may be downloaded to the computer in an electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     Real-Time Overlay of a MRI Slice on 3D Map 
     Minimally-invasive procedures require external imaging since the physician cannot see the probe during its navigation and treatment. In embodiments of the present invention processor  58  uses the magnetic position tracking capability of system  20  to produce and display a 3D map of the patient heart, overlaid with an image of distal end  34 , so cardiologist  42  knows the exact location and orientation of distal end  34  with respect to heart  28 . 
     During the navigation and treatment process, cardiologist  42  may need images of the pertinent organ around distal end  42 , in real time. In case of MRI, acquisition of a full volume 3D MRI image takes a long time as it requires volumetric scanning and intensive calculations. Other imaging techniques, such as X-RAY fluoroscopy, may acquire an image faster than 3D MRI, but there are cases where MRI is needed for the particular treatment and in order to minimize undesired radiation. The embodiments described herein fulfill the need for real-time MRI during minimally-invasive procedures in cardiology, and is also suitable for other minimally-invasive medical procedures. 
     In case an MRI image is needed in the vicinity of the catheter&#39;s distal end, cardiologist  42  selects a pertinent plane within patient&#39;s heart  28 , which includes distal end  34 . Cardiologist  42  defines the desired plane by using input devices  46  and a suitable GUI on screen  40 , and processor  58  converts the selected plane into a request for MRI system  22 . In response to such an event, processor  58  sends a request to MRI system  22 , via interface  56 , to acquire a MRI slice of the desired plane, which comprises distal end  34 . 
     Since a MRI slice covers a relatively small area and does not require intensive calculations, it can be acquired in real-time. MRI system  22  acquires the requested slice and sends it back to processor  58  via interface  56 . Processor  58  creates an overlaid image of the 3D magnetic position tracking map with the recently-acquired MRI slice and displays it on screen  40 . The overlaid image provides cardiologist  42  a real-time, up-to-date, high-resolution view of the tissue in the vicinity of distal end  34 , for the navigation and therapeutic ablation procedures. In an alternative embodiment processor  58  selects the desired plane automatically, e.g., in response to a specific event, or periodically. 
       FIG. 2  is a schematic pictorial illustration of a MRI slice overlaid on a 3D magnetic position tracking map, in accordance with an embodiment of the present invention. MRI system  22  and magnetic position tracking systems  20  generate a MRI slice  44  and a position tracking map  33 , respectively. To produce this overlaid image, as described above, cardiologist  42  selects a plane of interest comprising distal end  34  in patient&#39;s heart  28 , and processor  58  commands MRI system  22 , via interface  56 , to acquire a MRI slice  44 . MRI system  22  creates MRI slice  44  and sends it to processor  58  via interface  56 . Processor  58  displays the overlaid image of slice  44  on 3D position tracking map  33  on screen  40 . 
     The overlaid image provides cardiologist  42  a real-time high-resolution input for the navigation and treatment procedures. In an alternative embodiment processor  58  selects the desired plane automatically. 
       FIG. 3  is a flow chart that schematically illustrates a method for acquiring a real-time MRI image and overlaying it with a magnetic position tracking map during an intra-cardiac procedure, in accordance with an embodiment of the present invention. In this example, the method is divided into a preparation stage and a real-time procedure stage. In other embodiments, however, the method can comprise the real-time procedure stage without the preparation stage as, once the MRI scanner and CARTO system are installed and co-registered, the inserted catheter is inherently already registered with the MRI frame of reference. 
     The method begins at a 3D MRI acquisition step  200 , when the MRI system acquires a 3D image in patient&#39;s heart  28 . At a catheter insertion step  210 , cardiologist  42  inserts catheter  24  to the patient&#39;s heart and magnetic position tracking system  20  creates a magnetic position tracking map in the area of the distal end location. At a registration step  220 , the system performs registration to create an overlaid image between the 3D MRI image, which was acquired at 3D MRI acquisition step  200 , and the 3D magnetic position tracking map, which was acquired at catheter insertion step  210 . This overlaid image helps cardiologist  42  to plan the medical procedure and to navigate distal end  34  to the target locations in patient&#39;s heart  28 . 
     At a navigation step  230 , cardiologist  42  navigates catheter  24  to the target location in the patient&#39;s heart using the 3D position tracking map and the 3D MRI image. During the navigation, cardiologist  42  may need an updated local MRI image near the distal end of catheter  24 . At a decision step  240 , cardiologist  42  decides to acquire an MRI image for improved navigation or treatment reasons. 
     The need for additional real-time images may be a result of unexpected events during the procedure, such as obstacles encountered during catheter  24  navigation, or to verify that a specific treatment is performed in the target location. Further alternatively, any other suitable event may warrant an acquisition of a MRI slice. 
     If a slice is not needed, the method loops back to navigation step  230  above, in which cardiologist  42  continues to navigate catheter  24 . If decision step  240  concludes that a MRI slice is needed, then the method proceeds to a plane definition step  250 . At plane definition step  250 , cardiologist  42  examines the pertinent organ on display  40 , and uses input devices  46  and a suitable GUI on display  40  to select the desired plane, which comprises catheter&#39;s distal end  34 . 
     At a slice acquisition step  260 , processor  58  sends a request to MRI system  22 , via interface  56 , to acquire a MRI slice of the plane selected at plane definition step  250  above. The request specifies the pertinent plane to MRI system  22 , using any suitable convention (e.g. plane equations in some common coordinate system). In some embodiments processor  58  also indicates the position coordinates of distal end  34  to MRI system  22 . In response to the request, MRI system  22  acquires the requested MRI slice and sends it to processor  58  via interface  56 . 
     Processor  58  receives MRI slice  44  and performs registration between MRI slice  44  and 3D magnetic position tracking map  33 . At a display step  270 , processor  58  displays the overlaid image between MRI slice  44  and 3D magnetic position tracking map  33  on display  40 . 
     If applicable, cardiologist  42  continues navigation or treatment, as described in navigation step  230  and can request additional real-time MRI images for the same plane, as described in plane definition step  250 , or for other planes in the vicinity of distal end  34 . 
       FIG. 3  shows a specific flow of operations; however the techniques described herein are not limited to this specific flow. In other embodiments the flow may exclude the preparation stage (steps  200 ,  210 , and  220 ) and start, for example, directly with the real-time stage (at step  230 ). In another embodiment the plane selection can be done automatically by the system, in specific events or on a periodic basis. 
     The disclosed techniques can be used in various applications, such as the following six examples: 
     (1) Acquiring a thin slice of the inter-atrial septum (fossa ovalis) for safely performing a transseptal procedure (crossing the inter-atrial septum from the right atrium to the left atrium). 
     (2) Acquiring a thin slice of the Pulmonary Vein os for preplanning and execution of a Pulmonary Vein Isolation procedure. 
     (3) Acquiring a thin slice of an Atrio-Ventricular (Tricuspid or Mitral Valves) or Ventriculo-Atrial Valve (Pulmonary or Aortic Valve) for safe crossing, planning and performing of catheter-based repair or replacement of a cardiac valve. 
     (4) Acquiring a thin slice of the posterior Left Atrium for depiction of the Esophagus, its course and distance from a planned ablation point or line. Re-acquiring the same slice after completing the ablation procedure to rule out immediate post ablation Esophageal damages (edema, ulceration, perforation). 
     (5) Acquiring a thin slice to depict the Right Phrenic Nerve and distance of the nerve from a planned ablation point or line. 
     (6) Acquiring a sequence of thin slices to monitor and assess lesion formation all through an ablation. 
     Although the embodiments described herein mainly address cardiology, the methods and systems described herein can also be used in other minimally invasive applications, such as endoscopy and laparoscopy. 
     Although the embodiments described herein mainly address therapeutic cardiac ablation procedures like treatment of Atrial-Fibrillation, the methods and systems described herein can also be used in other applications. For example, the methods and/or systems can be used for guided needle biopsies, deployment of hepato-billiary stents, exclusion of Abdominal Aortic Aneurysm via stent, and modulation of the Autonomic Nervous System via ablation. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and 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 which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.