Abstract:
At least one electrode lead outside the body and leading between an RF ablation power source and the unshielded probes in the patient is shielded to substantially eliminate artifacts during concurrent electronic imaging and RF ablation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/074,367 filed Jun. 20, 2006 hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with United States government support awarded by the following agencies:
       NIH CA108869       
 
         [0004]    The United States government has certain rights to this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0005]    The present invention relates to medical procedures involving an exposed radio frequency electrode, such as RF ablation, and, in particular, to a method of providing electronic medical imaging during such radio frequency procedures. 
         [0006]    Electronic medical imaging, for example, digital radiography, computed tomography (CT), ultrasound imaging, and magnetic resonance imaging (MRI), employ sophisticated electronic sensors and computational systems to produce superior images of in vivo tissue. 
         [0007]    In an example computed tomography (CT) system, as known in the art, typically include an x-ray source collimated to form a beam (either a cone beam or a fan beam) extending along an axis through an imaged object to be received by an x-ray detector array. The x-ray source and detector array are oriented to lie within the x-y plane of a Cartesian coordinate system, termed the “imaging plane”. 
         [0008]    The x-ray source and detector array may be rotated together on a gantry within the imaging plane, around the imaged object, and hence around the z-axis of the Cartesian coordinate system. Rotation of the gantry changes the angle at which the fan beam intersects the imaged object, termed the “gantry” angle. 
         [0009]    The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a ray path extending from the x-ray source to that particular detector element. At each gantry angle, a projection is acquired comprised of intensity signals from each of the detector elements. The gantry is then rotated to a new gantry angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set. 
         [0010]    The tomographic projection set may be reconstructed mathematically, for example using the “filtered back projection” algorithm, to yield a cross-sectional image of the imaged object viewable perpendicular to the direction of the x-ray beams. The ability to reconstruct a cross-sectional image from the edge-wise projections relies on a mathematically balanced augmentation and cancellation among the different projections of the projection set. Imaging conditions that upset this balance create severe image artifacts in the forms of streaks and stars that obscure clinical information. Generally, the cause of the artifacts is not easily deduced from observation of the artifacts themselves. 
         [0011]    Common sources of artifacts include: (1) under-sampling of the x-ray signal, (2) failure to obtain projections over sufficient angular range, (3) axial or irregular movement of the patient, x-ray tube, or x-ray detector, (4) partial volumes imaged at only some angles, (5) “beam-hardening” caused by different attenuation of high and low frequency x-rays, and (6) radio opaque structures in the region of interest that create strong “shadows”. 
         [0012]    Radio frequency thermal ablation uses metallic electrodes inserted into tissue, for example a tumor, to produce electrical heating of the tissue to destroy the tumor. Desirably, such ablation may be performed during imaging of the tissue in order to monitor the size of the ablated region. Computed tomography or other electronic medical imaging techniques would be well suited to such monitoring of ablation; however, current experience using CT during radio frequency thermal ablation, for example, is that the ablation process produces severe and obscuring streak artifacts. Similar artifacts have been discovered in digital radiography and Doppler ultrasound images. 
         [0013]    One study of these artifacts, described in: Brennan, “CT Artifact Introduced by a Radio Frequency Ablation”, AJR 2006; 186:S284-S286 (2006), noted that the artifacts are linked to the application of power during ablation and speculated that electromagnetic cross talk was interfering with CT data acquisition in some undetermined way. This paper suggests that the artifacts may be the result of the omission of beam-hardening corrections in fast CT fluoroscopy making the CT system more susceptible to interference or thinner detectors being more susceptible to interference. Discouragingly, however, this study found that these severe artifacts, precluding useful examination of the ablation zone and procedural monitoring, were not appreciably changed with changes in ablation current but could be decreased only by stopping the ablation process or by increasing x-ray tube current substantially. 
       SUMMARY OF THE INVENTION 
       [0014]    The present inventors have determined that the artifacts produced during radio frequency ablation or other similar procedures, such as, cardiac ablation, electrocautery, varicose vein ablation, are electromagnetic interference transmitted from unshielded ablation leads, but more importantly, that even though the ablation electrode must be exposed to tissue and thus unshielded, and further even though substantial current is conducted through the unshielded patient during the ablation process, shielding of only the portion of the cables leading to the electrode outside of the body appears to be sufficient to substantially eliminate artifact generation even for substantial ablation currents. 
         [0015]    Specifically then, the present invention provides a method of monitoring radio frequency ablation comprising placing at least one electrode into conductive contact with tissue of a patient in a region to be ablated and applying a radio frequency electrical signal to the electrode to ablate the tissue in the region, the electrode receiving the radio frequency electrical signal from a remote radio frequency generator via a first conductor connecting between the electrode and the remote radio frequency generator. An electrical return from the tissue to the remote radio frequency generator is provided via a second conductor providing a return path from the patient to the remote radio frequency generator wherein the first and second conductors are shielded by placement within or integrated into a conductive electrical shield for substantially the entire length of the conductor between the patient and the remote radio frequency generator. Concurrently with the application of the radio frequency electrical signal, an electronic medical image of the region being ablated is acquired. 
         [0016]    It is thus one feature of at least one embodiment of the invention to exploit the recognition of the disproportionate influence of electrical interference from the leads to permit artifact free imaging during the ablation process. 
         [0017]    The electrical shield may be a first and second conductive tube separating the first and second conductors respectively, each tube providing a high-frequency path to ground. 
         [0018]    It is thus one feature of at least one embodiment of the invention to provide a system that permits greatest freedom in placement of the electrode and the return conductors. 
         [0019]    The tubes may be conductive braids. 
         [0020]    It is thus one feature of at least one embodiment of the invention to provide leads that are flexible and thus amenable to use with the patient in an imaging device. 
         [0021]    The first and second conductors are center conductors of standard coaxial cables and the first and second conductive tubes are coaxial shields surrounding the center conductors separated by an electrical dielectric providing a characteristic impedance of the coaxial cable. 
         [0022]    It is thus one feature of at least one embodiment of the invention to permit the use of standard cabling that is commercially available. 
         [0023]    Alternatively, the electrical shield may provide a conductive tube surrounding the first conductor and wherein the second conductor is a portion of the conductive tube. 
         [0024]    It is thus one feature of at least one embodiment of the invention to provide a configuration with fewer leads reducing the possibility of entanglement and a decreasing their radiative length. 
         [0025]    The electrical shield comprises a conductive tube surrounding the first and second conductors. The first and second conductors may be twisted about each other. 
         [0026]    It is thus one feature of at least one embodiment of the invention to promote interference cancellation by exploiting the countervailing current flows in the first and second conductors. 
         [0027]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]      FIG. 1  is a representation of a CT system, in phantom, showing the internal x-ray tube and detector, and a radio frequency ablation system positioned near the CT system for concurrent imaging and ablation; 
           [0029]      FIG. 2  is a cross-sectional view through a patient in the CT system showing insertion of an ablation electrode for ablation of a tumor and the relative location of the detector during a portion of the imaging process; 
           [0030]      FIG. 3  is a schematic representation of a first shielding technique; 
           [0031]      FIG. 4  is a schematic representation of a second, balanced, shielding technique; 
           [0032]      FIG. 5  is a schematic representation of a third reduced conductor shielding technique; 
           [0033]      FIGS. 6   a  and  6   b  are pictorial representations of x-ray CT images showing the streak artifacts generated without the present invention and their substantial reduction when the present invention is employed; 
           [0034]      FIGS. 7   a  and  7   b  are pictorial representations of digital x-ray images showing the streak artifacts generated without the present invention and their substantial reduction when the present invention is employed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    Referring now to  FIG. 1 , a CT machine  10  includes a gantry  12  having a bore  16  extending along an axis  15  to receive a patient (not shown) supported on a patient table  18  extending along the axis  15 . Inside the gantry  12 , an x-ray tube  20  may project an x-ray beam  17  to an x-ray detector  22  opposed across the bore  16 . The x-ray tube  20  and detector  22  may orbit about the bore  16  (on a rotation axis aligned with axis  15 ) to obtain projections through the patient on the patient table  18  at a variety of angles in a plane perpendicular to axis  15 . 
         [0036]    A radio frequency ablation system  24  may be positioned near the bore  16  to provide a source of radio frequency power through a generator  26  connected to leads  28 . In a “monopolar” mode, one lead  28  is connected to a radio frequency probe  30  and the other to a conductive ground pad  32 . Generally, the invention is equally applicable to a bipolar system where current flows between two probes  30  and  30 ′ or portions of a single probe having two mutually insulated portions (not shown in  FIG. 1 ). 
         [0037]    Referring now to  FIGS. 2 and 3 , the probe  30  may be inserted into an organ  34  within a patient  36  positioned within the bore  16  for imaging. The probe  30  may have a small cross-section and a pointed tip to be inserted by piercing the skin so that a proximal end  38  of the probe  30  may embed in or near a tumor at an ablation region  40 . The outer surface of the probe  30  in the ablation region  40  is electrically conductive to form an ohmic contact with the region  40  for the introduction of electrical current into the region  40  and heating thereof. 
         [0038]    A distal end  42  of the probe  30  extends out of the patient  36  to be connected to a center conductor  46  of a shielded cable  44  providing outer shield  48  coaxially around the center conductor  46 . The center conductor  46  of the shielded cable  44  provides electrical communication between the probe  30  and the generator  26 . At the generator  26 , the center conductor  46  is connected to a radio frequency power source  47 , for example, providing 500 kHz radio frequency power and, in any case, radio frequency electrical power less than 300 MHz. 
         [0039]    One end of the shield  48  (conveniently at the generator  26 ) is connected to ground (or any low impedance voltage reference) to create a constant potential shield around conductor  46  reducing the radiation of electromagnetic energy. The shield  48  may be connected to a metallic housing of the generator  26  providing an enclosed Faraday shield for the generator  26  which may also be grounded or connected to any low impedance voltage reference. Grounding for this purpose refers to a low impedance connection at the frequency of the generator  26  that need not be ohmic. 
         [0040]    Similarly, a ground pad  32 , providing a broad area of contact to the patient  36 , may be attached to a center conductor  52  of a separate shielded cable  50  providing a coaxial outer shield  54 . As is understood in the art, the ground pad  32  provides a broad area of contact to the skin of the patient  36  allowing electrical flow between the probe  30  and the ground pad  32  without significant heating near the ground pad  32  during high heating and ablation in the region  40 . 
         [0041]    The center conductor  52  of the shielded cable  50  provides electrical communication between the ground pad  32  and the generator  26 , at which the center conductor  52  joins to the generator ground and the shield  54  is connected to a ground or another point of low impedance constant voltage. 
         [0042]    This configuration may be used in the monopolar mode, described above, with a probe  30  and ground pad  32 , or (as shown) used in a bipolar mode with a first probe  30  and similar second probe  30 ′ both placed within the patient  36  with current flowing between them to create the ablation region  40  as described below. 
         [0043]    Referring now to  FIG. 4 , in an alternative balanced shielding configuration, the conductors  46  and  52  may be placed in close proximity (for example, loosely twisted) so that their countervailing currents tend to cancel. They are then together placed in surrounding coaxial shield  60  attached to a source of constant potential with respect to the patient  36  as described above with respect to shields  54  and  48 . This configuration may be used in the monopolar mode, described above, with a probe  30  and ground pad  32 , or (as shown) used in a bipolar mode with a first probe  30  and similar second probe  30 ′ both placed within the patient  36  with current flowing between them to create the ablation region  40 . The probes  30  and  30 ′ may be needle probes as described above or umbrella probes or other types of percutaneous electrodes. 
         [0044]    Referring now to  FIG. 5 , in an alternative reduced lead shielding configuration, the conductor  46  is placed in coaxial shield  60  attached to a source of constant potential with respect to the patient  36  as described above with respect to shields  54  and  48 . In this case, the conductor  52  may be integrated with the shield  60  to reduce the need for a separate conductor  52  along the shielded length of the conductor  46 . This configuration may be used in the monopolar mode, described above, with a probe  30  and ground pad  32 , or (as shown) used in a bipolar mode with a first probe  30  and similar second probe  30 ′ both placed within the patient  36  with current flowing between them to create the ablation region  40 . 
         [0045]    The probes  30  and  30 ′ may be needle probes as described above or umbrella probes or other types of percutaneous electrodes. 
         [0046]    The conductors  46  and  52  may be separated from the shields  48 ,  54  or  60  by means of an intervening electrical insulator  49 . In the embodiments of  FIGS. 3 and 5 , the shield  60  and the conductors  46  may be realized by a standard coaxial cable such as provides a radio frequency transmission line RG type cables or maybe a standard shielded cable without transmission line properties. The shields  48 ,  54  and  60  may be braided conductive wire to provide flexibility to the cables  44 . 
         [0047]    Referring now to  FIG. 6   a,  an example CT image  61  shows streak artifacts  62  emanating from an arbitrary point removed from the ablation of probe  30  reminiscent of beam-hardening artifacts caused by a spectral change in x-ray energy as it passes through a patient. Shielding of the conductors  46  and  52  in the embodiment of  FIG. 3  produces the image of  FIG. 6   b  in which the artifacts are substantially eliminated. 
         [0048]    Referring now to  FIG. 7   a,  an example digital x-ray image shows horizontal banding. Shielding of the conductors  46  and  52  in the embodiment of  FIG. 3  produces the image of  FIG. 6   b  in which the banding is substantially eliminated. 
         [0049]    While the present invention has been described in the context of computed tomography and digital radiography, it will be understood that the same technique may be applicable to other imaging modalities including not only computed tomography, ultrasound, magnetic resonance imaging, and the like, but also, for example, electrically sensitive computer monitors attached to devices such as endoscopes and the like. The fact that limited shielding of cables leading to an unshielded patient can provide pronounced attenuation of electrical interference may also be useful for non-imaging applications such as the acquisition of ECG signals etc. The shielding techniques described could include non-tubular shielding arrangements such as tightly twisted pair were shielding is provided by close proximity of counteracting currents. Further, it will be understood that other devices applying electrical energy to the body when concurrent imaging must be conducted using electronic imaging devices, may benefit from the present invention. Such devices may include, for example, those providing for cardiac ablation, electrocautery, varicose vein ablation, etc. 
         [0050]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.