Abstract:
A method for position sensing includes inserting a probe having a first probe-electrode and a second probe-electrode into a body of a subject, and coupling body-surface electrodes to a surface of the body. Currents passing between the first probe-electrode and the body-surface electrodes are measured, using first circuitry coupled to at least the first probe-electrode and having a first electrical ground, and position coordinates of the probe are determined responsively to the measured currents. Second circuitry, having a second electrical ground, is coupled to at least the second probe-electrode, and the first electrical ground is isolated from the second electrical ground.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to sensing the position of an object placed within a living body, and specifically to position sensing using impedance measurements. 
       BACKGROUND OF THE INVENTION 
       [0002]    A wide range of medical procedures involves placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Real-time imaging methods are often used to assist medical practitioners in visualizing the object and its surroundings during these procedures. However, in many situations, real-time imaging is not possible or desirable. Instead, systems for obtaining real-time spatial coordinates of the internal object are often utilized. Many such position-sensing systems have been developed or envisioned in the prior art. 
         [0003]    For example, U.S. Pat. No. 5,983,126, to Wittkampf, whose disclosure is incorporated herein by reference, describes a system in which catheter position is detected using electrical impedance methods. U.S. Patent Application Publications 2006/0173251, to Govari et al., and 2007/0038078, to Osadchy, whose disclosures are incorporated herein by reference, describe impedance-based methods for sensing the position of a probe by passing electrical currents through the body between an electrode on the probe and a plurality of locations on a surface of the body. 
       SUMMARY OF THE INVENTION 
       [0004]    Embodiments of the present invention provide efficient means and methods for determining in real-time the position of a probe placed within a living body, based on measurement of currents passing between electrodes on the probe and body surface electrodes. The methods and means described hereinbelow are used to reduce distortion of the measured currents, thus enhancing the accuracy of the position measurements. 
         [0005]    There is therefore provided, in accordance with an embodiment of the present invention, a method for position sensing, including: 
         [0006]    inserting a probe including a first probe-electrode and a second probe-electrode into a body of a subject; 
         [0007]    coupling body-surface electrodes to a surface of the body; 
         [0008]    measuring, using first circuitry coupled to at least the first probe-electrode and having a first electrical ground, currents passing between the first probe-electrode and the body-surface electrodes; 
         [0009]    determining position coordinates of the probe responsively to the measured currents; 
         [0010]    coupling second circuitry, having a second electrical ground, to at least the second probe-electrode; and 
         [0011]    isolating the first electrical ground from the second electrical ground. 
         [0012]    In some embodiments, isolating the first electrical ground further includes coupling the first electrical ground to the second electrical ground via a predetermined inter-ground coupling impedance. 
         [0013]    Typically, the value of the inter-ground coupling impedance is selected to maximize an accuracy of determining the position coordinates. 
         [0014]    In some embodiments, the inter-ground coupling impedance is between 500 and 5000 Ohm. 
         [0015]    Typically, inserting the probe includes passing the probe into a heart of the subject, and coupling the second circuitry includes measuring an electrical activity of the heart using at least the second probe-electrode. 
         [0016]    In some embodiments, both of the first and second probe-electrodes are coupled for use in both determining the position coordinates and measuring the electrical activity. 
         [0017]    In further embodiments, the first circuitry includes a front-end including an isolation transformer having a primary winding and a secondary winding, which is coupled to at least the first probe-electrode. In such embodiments, isolating the first electrical ground from the second electrical ground may include coupling the secondary winding of the isolation transformer to the first electrical ground while the primary winding is coupled to the second electrical ground. 
         [0018]    In some embodiments, measuring the currents includes coupling a front end having an output impedance typically greater than 100,000 Ohm to transmit the currents through at least the first probe-electrode. 
         [0019]    There is also provided, in accordance with an embodiment of the present invention, a medical system, including: 
         [0020]    a probe adapted to be inserted into a body of a subject, the probe including a first probe-electrode and a second probe-electrode; 
         [0021]    a plurality of body-surface electrodes, which are adapted to be fixed to a surface of the body at respective locations; 
         [0022]    first circuitry, coupled to at least the first probe-electrode and configured to measure currents passing between the first probe-electrode and the body-surface electrodes, the first circuitry having a first electrical ground; 
         [0023]    a positioning processor configured to determine position coordinates of the probe responsively to the measured currents; and 
         [0024]    second circuitry, coupled to at least the second probe-electrode and having a second electrical ground, which is isolated from the first electric ground. 
         [0025]    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 
         [0026]      FIG. 1  is a schematic pictorial illustration of a medical system, in accordance with an embodiment of the present invention; 
           [0027]      FIG. 2  is a schematic diagram, showing interaction between electrodes and associated circuitries thereof, in accordance with an embodiment of the present invention; 
           [0028]      FIGS. 3 ,  4 , and  5  are schematic electrical diagrams, illustrating influence of functional electrodes on positioning currents, in accordance with embodiments of the present invention; and 
           [0029]      FIG. 6  is a schematic electrical diagram presenting a typical implementation of a positioning probe front-end, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]      FIG. 1  is a schematic pictorial illustration of a medical system  20 , in accordance with an embodiment of the present invention. System  20  comprises a probe  30 , such as a catheter, which is adapted to be inserted into an internal body cavity, such as a chamber of a heart  40 , of a subject  50 . Typically, the probe is used by a practitioner  60  for one or more medical diagnostic or therapeutic functions, such as intra-cardiac electrocardiography (ECG), mapping electrical potentials in the heart, performing ablation of heart tissue, or other medical functions. In order to facilitate effective application of the medical procedure, system  20  is adapted to determine the position of probe  30  within the body of the subject. The position of the probe, along with other diagnostic and/or therapeutic data, is typically displayed to practitioner  60  on monitor  70 , or presented by means of other suitable media. 
         [0031]    The distal tip of probe  30  comprises a plurality of electrodes  80 ,  82  and  84 , referred to herein as probe-electrodes. The probe-electrodes are connected by wires through the insertion tube of probe  30  to a control unit  100 , which comprises first circuitry adapted to determine the position of the probe within the subject&#39;s body and second circuitry adapted to perform one or more diagnostic or therapeutic functions. The first and second circuitries are referred to herein as the positioning and functional circuitries, respectively, and are shown in detail in the figures that follow. The term functional refers herein to one or more medical diagnostic or therapeutic functions of system  20  (e.g., measurement and mapping of cardiac electrical signals). One or more of the probe-electrodes (referred to herein as the positioning probe-electrodes) are coupled to the positioning circuitry, while one or more of the probe-electrodes (referred to herein as the functional probe-electrodes) are coupled to the functional circuitry. Typically, the same probe-electrodes are utilized both for positioning and for medical diagnostic or therapeutic functions. Therefore, the first and second sets of electrodes typically overlap. However, in some cases, the two sets of electrodes may be disjoint. 
         [0032]    Control unit  100  is further connected by wires through one or more cables  105  to a plurality of body-surface electrodes  110 ,  112 ,  114 ,  116 ,  118 , and  120 , which are coupled to a body-surface (i.e., the skin) of the subject. The body-surface electrodes typically comprise adhesive skin patches. In alternative embodiments of the invention, the body-surface electrodes may vary in number and may take other forms. The body-surface electrodes comprise a set of first body-surface electrodes  110 ,  112 , and  114 , referred herein as positioning body-surface electrodes, which are coupled to the positioning circuitry. The body-surface electrodes may additionally comprise one or more second body-surface electrodes  116 ,  118 , and  120 , referred herein as functional body-surface electrodes, which are connected to the functional circuitry. Typically, the two sets of body-surface electrodes are disjoint, but in some cases, the two sets may overlap. 
         [0033]    The positioning circuitry of the control unit is adapted to drive and measure electric currents, referred herein as positioning-currents, between the positioning probe-electrodes and the positioning body-surface electrodes. Responsive to the measured positioning currents, a positioning processor (shown in  FIG. 2 ), which is typically incorporated within control unit  100 , estimates the coordinates the distal end of probe  30  within the body. The positioning processor typically comprises a general-purpose computer processor, which is programmed in software to estimate the probe coordinates according to methods described in the above-cited Patent Application Publications 2006/0173251 and 2007/0038078. Additionally or alternatively, the positioning processor may employ other suitable positioning methods. 
         [0034]    Probe coordinate estimation is typically based on correspondence between positioning currents and respective distances of intra-body paths. For example, we may denote the distances from probe electrode  80  to body-surface electrodes  110 ,  112 , and  114  by D1, D2, and D3, respectively, and denote the positioning currents from probe electrode  80  to body-surface electrodes  110 ,  112 , and  114  by I1, I2, and I3, respectively. According to methods described in the above-cited patent applications, the ratio of distances D1:D2:D3 can be estimated based on the ratio of currents I1:I2:I3. The coordinates of probe electrode  80  can than be derived from the estimated ratio D1:D2:D3. 
         [0035]    Since probe coordinates calculation rely on positioning currents between related electrodes, it is desirable that the positioning currents not be influenced by electrical coupling with non-related electrodes. For example, if the ratio I1:I2:I3 varies due to electrical coupling with functional probe-electrode  82 , the ratio D1:D2:D3 might be incorrectly estimated. Methods according to embodiments of the present invention, which are described hereinbelow, facilitate eliminating or reducing the effect of the functional electrodes on the positioning currents, thus enabling accurate and reliable positioning of probe  30  within the subject&#39;s body. 
         [0036]      FIG. 2  is a schematic diagram, showing interaction between probe-electrodes and body-surface electrodes, and associated circuitries thereof, in accordance with an embodiment of the present invention. As noted above, control unit  100  (not shown explicitly in  FIG. 2 ) comprises positioning circuitry  200 , a positioning processor  205 , and functional circuitry  210 . Positioning circuitry  200  comprises one or more positioning probe front-ends (e.g., front-ends  220  and  222 ), and each positioning probe front-end is coupled to a positioning probe-electrode (e.g., probe-electrodes  80  and  82 ). Positioning probe front-end typically comprises a high impedance driver (such as is shown below in  FIG. 6 ), which drives positioning currents between respective positioning probe electrode and a plurality of positioning body-surface electrodes. For example, positioning probe front-end  220  drives positioning currents I1, I2 and I3 between probe electrode  80  and body-surface electrodes  110 ,  112 , and  114 , respectively. 
         [0037]    The impedance of positioning probe front-end  220  is typically much higher than the impedance of a path through a human body, and therefore positioning probe front-end  220  is approximately a current source. For example, a typical impedance of a path through a human body is 100 Ohm, and the output impedance of a positioning probe front-end is typically higher than 100,000 Ohm. The positioning currents are typically AC currents, for example AC currents in the range of 100-110 kHz. Therefore, the term impedance refers herein to impedance measured over the frequency range of the positioning currents, for example impedance measured over the range of 100-110 kHz. 
         [0038]    Positioning circuitry  200  also comprises current-sensing devices  230 ,  232  and  234 , which are coupled to positioning body-surface electrodes  110 ,  112  and  114 , and measure the respective positioning currents I1, I2, and I3. In alternative embodiments of the invention, the positioning currents may be measured by a single current-sensing device, by means of time multiplexing. 
         [0039]    Based on the positioning currents I1, I2, and I3, positioning processor  205  calculates the coordinates of positioning probe-electrode  80  within body  50 , according to methods described in the above-cited patent applications, or according to other suitable current-based positioning methods. 
         [0040]    Functional circuitry  210  of the control unit comprises one or more functional probe front-ends  240  and  242 , which are coupled to functional probe-electrodes  80  and  82 , respectively. Functional circuitry  210  may also comprise one or more functional body-surface front-ends (e.g., a body-surface front-end  250 ), which are coupled to functional body-surface electrodes (e.g., functional body-surface electrode  120 ). In cases in which functional circuitry  210  comprises ECG circuitry, the body-surface electrode attached to the right leg of the subject serves typically as a common reference for differential ECG measurements. In those cases, the right leg electrode is typically coupled to the ground of the ECG circuitry via impedance on the order of 10,000 Ohm. 
         [0041]    Usually, grounds of distinct circuits of an electrical system are highly coupled, since all circuits of the same system are typically fed, directly or indirectly, by the same power source (e.g., the mains). Furthermore, it is a common practice to connect the grounds of all circuits of an electric system to one common ground. For example, all circuits that are implemented on the same printed circuit board (PCB) are typically connected to the same one or more ground layers of the PCB, all ground layers of all PCBs are typically connected to the system chassis, and the system chassis is typically connected to the mains ground. 
         [0042]    However, in embodiments of the present invention, positioning circuitry  200  and functional circuitry  210  are connected to distinct grounds  260  and  270 , respectively, and ground  260  is deliberately isolated from ground  270 . Typically, ground  270  is implemented as one or more ground layers of one or more PCBs, which are connected to a system common ground, for example to the system chassis; while ground  260  is implemented as a dedicated return path, which is isolated from the respective PCBs ground layers, and from the system common ground. Isolating of ground  260 , according to embodiments of the present invention, is employed in order to maintain the validity and accuracy of the positioning process, as elaborated hereinbelow. 
         [0043]    In some embodiments of the present invention, isolated grounds  260  and  270  are further coupled by an inter-ground coupling impedance  280  (e.g. a capacitor), in order to eliminate or reduce the effect of parasitic couplings on the validity and accuracy of the positioning process, as elaborated further below. 
         [0044]      FIGS. 3 ,  4 , and  5  are schematic electric diagrams, illustrating the potential influence of functional electrodes on positioning currents, in accordance with embodiments of the present invention.  FIG. 3  illustrates the effect of functional probe-electrode  82  on positioning currents I1, I2, and I3, which flow between positioning probe-electrode  80  and body-surface electrodes  110 ,  112 , and  114 , respectively (on the assumption that there is coupling between grounds  260  and  270 ). Positioning currents I1, I2, and I3 are driven by front-end  220 , and are measured by measurement devices  230 ,  232 , and  234 , respectively. We denote the intra-body distances between probe-electrode  80  and body-surface electrodes  110 ,  112 , and  114 , by D1, D2, and D3, respectively, as noted above. (D1, D2, and D3 are not shown in the figure). The respective intra-body paths between the probe-electrode and body-surface electrode are denoted by P 1 , P 2 , and P 3 , and the respective intra-body impedances by Z 1 , Z 2 , and Z 3 . Positioning currents I1, I2, and I3 are essentially proportional to respective impedances Z 1 , Z 2 , and Z 3 , which are dependent on respective distances D1, D2, and D3, and this dependency provides the basis for the operation of the positioning processor. 
         [0045]    Since functional probe-electrode  82  is also located within the body of the subject, there are inevitable electric coupling paths between electrode  82  and paths P 1 , P 2 , and P 3 .  FIG. 3  shows a coupling path Z 4  between functional probe electrode  82  and an intermediate point  300  on path P 1 . Similarly, there are coupling paths between probe electrode  82  and intermediate points on paths P 2  and P 3 . Intermediate point  300  is illustrated in  FIG. 3  as breaking Z 1  into two impedances Z 1   a  and Z 1   b , wherein Z 1   a +Z 1   b =Z 1 . This illustration is a simplified model of a typically much more complicated model, but it is helpful in illustrating the effect of the coupling between the functional probe electrodes and the positioning currents. 
         [0046]    If ground  260  of the positioning circuitry were coupled to ground  270  of the functional circuitry as in systems known in the art, there would be undesired currents that flow between positioning probe-electrode  80  and ground  260 , via functional probe-electrode  82 . The undesired currents change the desired positioning currents, and degrade the accuracy of the positioning process. For example, if front-end  220  comprises a current source, the undesired currents reduce the desired positioning currents, but each current is typically reduced by a different amount due to geometrical and physiological factors. Consequently, the ratio of currents I1:I2:I3 changes, thus reducing the accuracy of the position measurement. 
         [0047]    However, according to an embodiment of the present invention, ground  260  is isolated from ground  270 , and therefore undesired currents cannot flow via functional probe-electrode  82 . As a result, the positioning currents are not changed, and the validity and accuracy of the positioning process are maintained, regardless of the functional probe-electrodes. 
         [0048]      FIG. 4  illustrates an embodiment in which the functional circuitry comprises ECG circuitry, and front-end  250  is coupled to electrode  120 , which is placed on the subject&#39;s right leg. Since right-leg ECG electrode  120  and positioning electrodes  110 ,  112 , and  114  are all coupled to the skin, there is inevitable electrical coupling between electrode  120  and electrodes  110 ,  112 , and  114 . (For the sake of simplicity, only the coupling with electrode  110  is shown in the figure.) Consequently, there is a parasitic electrical coupling between path P 1  and ground  260 , via functional probe-electrodes  82  and  84 , functional probe front-ends  240  and  242 , right-leg front-end  250 , and right-leg electrode  120 . (Similar coupling mechanisms, not shown in the figure, apply also to other paths, e.g., paths P 2  and P 3 ). 
         [0049]    However, since the impedance of right-leg front-end  250  is typically about 10,000 Ohm, the impedance of the parasitic coupling through right-leg electrode  120  is always above 10,000 Ohm, regardless of the numbers of functional probe electrodes. Consider, for example, the case of forty functional probe-electrodes, and assume that the impedance of each functional probe front-end is about 10,000 Ohm. The collective impedance of the forty functional probe electrodes and their associated front-ends is 10,000/40=250 Ohm. This low collective impedance could interfere significantly with the positioning process. However, since the impedance of right-leg front-end  250  is 10,000 Ohm, and grounds  260  and  270  are isolated from one another, the overall parasitic impedance is as high as 10,250 Ohm, and has minor effect on the positioning process. 
         [0050]      FIG. 5  presents an embodiment of the present invention in which grounds  260  and  270 , which are deliberately isolated from each other, are further coupled by predetermined inter-ground coupling impedance  280 , typically implemented by a capacitor. The goal of the inter-ground coupling impedance is to reduce the influence of possible parasitic coupling, as explained hereinbelow. 
         [0051]    In typical configurations of system  20 , there might be parasitic couplings between ground  270  and positioning body-surface electrodes  110 ,  112  and  114 . Two such couplings, denoted by Z 10  and Z 12 , are shown in the figure. Additionally, there might be parasitic couplings between ground  270  and measurement devices  230 ,  232 , and  234 . Two such coupling, denoted by Z 20  and Z 22 , are shown in the figure. Parasitic couplings such as Z 20  and Z 22  might be caused, for example, by parasitic capacitance between the positioning circuitry (e.g., measurement devices  230  and  232 ) and the one or more ground layers of the PCBs. The parasitic couplings modify the readings of the measurement devices, and therefore degrade the accuracy of the positioning process. For example, parasitic couplings Z 10  and Z 20  (Z 12  and Z 22 ) enable flow of parasitic current from positioning probe-electrode  80 , via functional probe-electrode  82 , to measurement device  230  ( 232 ), respectively, and thus increase the reading of the measurement device and degrade the accuracy of the ratio I1:I2:I3. 
         [0052]    In the embodiment shown in  FIG. 5 , the undesired effect of parasitic couplings, such as Z 10 , Z 11 , Z 20 , and Z 21 , is reduced by introducing inter-ground coupling impedance  280  between grounds  260  and  270 . The coupling impedance  280  is typically selected to be considerably lower than the values of the parasitic impedances Z 10 , Z 11 , Z 20 , and Z 21 . Consequently, most of the parasitic currents flow through inter-ground coupling impedance  280 , and the parasitic currents trough the measurements devices are reduced accordingly. 
         [0053]    On the other hand, inter-ground coupling impedance  280  should be selected to be considerably higher than intra-body impedances Z 1 , Z 2 , and Z 3 , in order to maintain the benefit of the isolation between grounds  260  and  270 , as elaborated above. 
         [0054]    In a typical system, the best value of the inter-ground coupling impedance to yield maximal accuracy of the positioning process, may be determined empirically. Determining the value is facilitated by the fact that the overall accuracy of the positioning process is typically a concave function of the coupling impedance. For example, in a typical system, the total parasitic coupling is about 5,000 Ohm, and the typical intra-body impedances are about 100 Ohm. For such system, the value for the inter-ground coupling impedance should typically be about 1,000 Ohm. 
         [0055]      FIGS. 3 ,  4 , and  5  address the potential influence of functional probe-electrodes (e.g., probe-electrode  82 ) on positioning currents flowing through a positioning probe-electrode (e.g., probe-electrode  80 ). In principle, there might also be similar undesired influence of a first positioning probe-electrode on the positioning currents flowing from a second positioning probe-electrode. 
         [0056]    Returning to  FIG. 2 , functional probe-electrode  82  is also a positioning probe-electrode, and is also coupled to positioning probe front-end  222 . Therefore, undesired parasitic currents might in principle flow from positioning probe-electrode  80 , via positioning probe-electrode  82  and positioning front-end  222 , to ground  260 . However, since the impedance of positioning-probe front-end  222  is typically much higher than intra-body impedances Z 1 , Z 2 , and Z 3  (shown in  FIG. 3 ), the effect of coupling between positioning probe-electrodes is minor. For example, the impedance of positioning probe front-end  222  is typically above 100,000 Ohm, while the values of intra-body impedances Z 1 , Z 2 , and Z 3  are typically about 100 Ohm. 
         [0057]      FIG. 6  is a schematic electrical diagram, showing a typical implementation of positioning probe front-end  220 , in accordance with an embodiment of the present invention. Front-end  220  typically comprises an isolation transformer  400 , whose primary winding is fed by an operational amplifier  410  driven by an AC source  420 , and whose secondary winding is coupled to positioning probe-electrode  80  and to ground  260  via resistors  430  and  440 , respectively. The impedance of resistors  430  and  440  is typically much higher than that of the subject&#39;s body. For example, the impedance of resistors  430  and  440  is typically on the order of 60,000 Ohm, while the typical impedance of the human body is typically on the order of 100 Ohm. The primary winding of transformer  400  is coupled to common ground  270 , while the secondary winding is coupled to positioning circuitry ground  260 , which is isolated from common ground  270 . Consequently, front-end  220  introduces no galvanic coupling between grounds  260  and  270 . There might be some parasitic capacitance between the secondary winding of transformer  400  and ground  270 , but the high value of resistor  440  reduces the effect of such parasitic capacitance, and front-end  220  maintains the isolation between grounds  260  and  270 . Due to this isolation, the functional probe-electrodes do not affect the positioning currents flowing from the positioning probe-electrodes, and the accuracy of the positioning process is maintained. 
         [0058]    Isolation transformer  400  can be further adapted to step up the voltage produced by amplifier  410  to a level suitable for driving the positioning currents, by appropriate selection of the ratio between the windings. As a typical example, transformer  400  may be adapted to step up the primary voltage by a factor of five, from 20 Volts to 100 Volts. 
         [0059]    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 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.