Patent Publication Number: US-7914525-B2

Title: Bioimpedance measurement system and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Divisional of U.S. Utility patent application Ser. No. 11/283,057, filed Nov. 18, 2005, entitled BIOIMPEDANCE MEASUREMENT SYSTEM AND METHOD, the entirety of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for treating tissue using cooled medical devices using electrical impedance measurements with the device. 
     BACKGROUND OF THE INVENTION 
     Effectiveness of cryotreatment of endocardial tissue is significantly affected by the contact of the catheter tip or thermally transmissive region to the tissue. Ex-vivo studies show a correlation between the lesion sizes created and the tip or thermally-transmissive region to tissue contact quality. A larger lesion size can be achieved with the same device by improving the tip to tissue pressure or contact. Various methods have been used to assess tip or thermally-transmissive region contact, using RF catheters and/or ultrasound imaging. However, none of these methods has proved entirely satisfactory. 
     The problem extends to all areas of tissue treatment wherein the tissue undergoes some change or “physiological event” during the course of treatment. In addition to contact quality assessment, in treatment devices that employ fluid flows, detection and containment of leaks is a critical problem, especially in the operation of cryogenic devices for therapeutic purposes, lest a leak of coolant enter the body and thereby cause significant harm. Known catheters which employ inflatable balloons often inflate the balloons to relatively high pressures that exceed the ambient pressure in a blood vessel or body lumen. However, to contain the coolant, these catheters generally employ thicker balloons, dual-layered balloons, mechanically rigid cooling chambers, and other similar unitary construction containment mechanisms. These techniques however, lack robustness, in that if the unitary balloon, cooling chamber, or other form of containment develops a crack, leak, rupture, or other critical structural integrity failure, coolant may egress from the catheter. To minimize the amount and duration of any such leaks, it is desirable to use a fluid detection system that detects a gas or liquid expulsion or egress from the catheter shaft and signals a control unit to halt the flow of cryogenic fluid. 
     Furthermore, since many treatment systems and methods are applied in internal body lumens, organs or other unobservable tissue regions, the orientation and attitude of the device structure relative to the tissue is of significant importance in ensuring the effective and efficient treatment of tissue. This applies to many tissue treatment systems, both surgical and non-surgical, using a variety of modalities, including cooling through cryotreatment, heat or electrically induced heating, ultrasound, microwave, and RF, to name a few. 
     This collection of problems may be resolved in part by developing a specialized transducer suitable for the “body” environment in which it operates. For many physiological events, there is no specialized transducer. The events in question include changes in the natural state of tissue, such as temperature, dielectric or conductivity changes, structural changes to the cells and cell matrix, dimensional changes, or changes in the operation of, or interplay between, tissue regions and/or foreign bodies, such as blood flow in an artery having a treatment device inserted therein. 
     All of these changes may be correlated to, or affected by, relative changes in the bioelectrical impedance of the tissue region. 
     It would be desirable to provide an apparatus and method of assessing lesion quality, monitoring and detecting any occurrences of fluid egress, determining blood vessel occlusion, determining tissue composition as well as assessing the quality of the contact between the tip or thermally-transmissive region of a cryogenic device and the tissue to be treated. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides methods and systems for detecting fluid egress, assessing lesion quality, determining tissue composition or structure, and providing tissue contact assessment. 
     In an exemplary embodiment, a method is provided for detecting fluid egress including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway between the distal end portion and the proximal end portion, and the shaft has a plurality of electrodes, applying an electrical current between at least two of the plurality of electrodes, measuring impedance voltage between the at least two of the plurality of electrodes and, processing the measured impedance voltage resulting from the applied electrical current to determine if fluid egress is present. 
     The processing step of the method for detecting fluid egress may include the steps of establishing a normal impedance voltage range, monitoring to determine if the impedance voltage varies outside of the impedance voltage range, and generating a signal if the impedance voltage measurement varies outside of the impedance voltage range. A control unit, a microprocessor, an impedance-measuring device or the like may perform the processing step. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
     In another exemplary embodiment, a method is provided for accessing lesion quality including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes a first electrode and a second electrode, and measuring a baseline impedance, activating the catheter such that the treatment portion cools the tissue, applying an electrical current between the first and second electrodes, and processing the measured impedance voltage caused by the applied electrical current to determine the amount of treated tissue after each activation of the catheter. 
     The processing step of the method for accessing lesion quality may be performed by a control unit, a microprocessor, an impedance measuring device or the like. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
     In still another exemplary embodiment, a method is provided for accessing tissue composition including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes a first electrode and a second electrode, activating the catheter such that the treatment portion cools the tissue, applying an electrical current between the first and second electrodes, measuring a impedance voltage between the first and second electrodes, and processing the measured impedance caused by the applied electrical current, establishing a normal impedance range for a tissue type, monitoring the impedance to determine if the impedance varies into a tissue type impedance range, and generating an impedance signal that can be processed to identify the tissue type impedance range. 
     The processing step of the method for accessing tissue composition may be performed by a control unit, a microprocessor, an impedance measuring device or the like. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
     In still another exemplary embodiment, a method is provided for accessing tissue composition including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes a first electrode and a second electrode, activating the catheter such that the treatment portion cools the tissue, applying an electrical current between the first and second electrodes, measuring a impedance voltage between the first and second electrodes, and processing the measured impedance caused by the applied electrical current, delivering coolant to the treatment tip, measuring a second impedance voltage between the first and second electrodes, processing the measured impedance voltage caused by the applied electrical current to determine a delta impedance from the first and second impedances, and determining if the delta impedance has reached a maximum value. 
     The processing step of the method for accessing tissue composition may be performed by a control unit, a microprocessor, an impedance measuring device or the like. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a catheter system having an expandable thermally-transmissive region; 
         FIG. 2  illustrates an embodiment of a shaft of the catheter system of  FIG. 1 ; 
         FIG. 2A  illustrates an embodiment of the catheter system used with a pair of excitation electrodes positioned on a patient&#39;s body; 
         FIG. 3  illustrates a catheter system having a non-expandable thermally-transmissive region; 
         FIG. 3A  illustrates a catheter system having measurement electrodes on the inside of a guidewire lumen; 
         FIG. 3B  illustrates a cutaway view of the guidewire lumen of  FIG. 3A ; 
         FIG. 4A  illustrates an embodiment of a catheter in a deflected configuration and positioned near a treatment site; 
         FIG. 4B  illustrates an embodiment of the catheter tip of  FIG. 4A  having four electrodes in its thermally-transmissive region; 
         FIG. 4C  illustrates an embodiment the catheter tip of  FIG. 4A  having eight electrodes in its thermally-transmissive region; 
         FIG. 5  illustrates an embodiment of a catheter for detecting fluid egress from a catheter; 
         FIG. 6  illustrates an embodiment of a fluid egress algorithm; 
         FIG. 7  illustrates an embodiment of a pulmonary vein occlusion/fluid egress/ice coverage verification algorithm; 
         FIG. 8  illustrates an embodiment of a ice coverage/lesion quality measurement algorithm; 
         FIG. 9  illustrates a graph of the general impedance Z(t) with respect to time; and 
         FIG. 10  illustrates a graph of the general impedance Z(t) with an impedance spike due to a fluid egress. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system and method for detecting fluid egress including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes at least four electrodes, a first pair of electrodes and a second pair of electrodes, applying an electrical current between the first pair of electrodes, measuring a impedance voltage between the second pair of electrodes, and processing the measured impedance voltage caused by the applied electrical current to determine if fluid egress is present. 
     The number and location of the electrodes will affect the system measurement sensitivity. For example, as the distance between the pair of measurement electrodes is decreased, the system measurement sensitivity is increased. On the other hand, as the distance between the pair of measurement electrodes and the pair of excitation current electrodes is decreased, the system measurement sensitivity is decreased. In another embodiment, where a catheter having a single pair of electrodes for both measuring impedance and providing the excitation current, the system measurement sensitivity is also decreased. 
     The processing step of the method for detecting fluid egress may include the steps of establishing a normal impedance voltage range, monitoring to determine if the impedance voltage varies outside of the impedance voltage range, and generating a signal if the impedance voltage measurement varies outside of the impedance voltage range. A control unit, a microprocessor, an impedance-measuring device or the like may perform the processing step. In another embodiment of the method, the treatment portion of the catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having one of each pair of electrodes located near the distal side of the cooling chamber, and one of each pair of electrodes located near the proximal side of the cooling chamber. 
     In addition, a system and method for accessing lesion quality including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes a first electrode and a second electrode, activating the catheter such that the treatment portion cools the tissue, applying an electrical current between the first and second electrodes, measuring a impedance voltage between the first and second electrodes, and processing the measured impedance voltage caused by the applied electrical current to determine the amount of treated tissue after each activation of the catheter. 
     The processing step of the method for accessing lesion quality may be performed by a control unit, a microprocessor, an impedance measuring device or the like. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
     In addition, a system and method for accessing tissue composition including the steps of positioning a catheter at a tissue treatment site, where the catheter includes a shaft, which has a proximal end portion and a distal end portion, wherein the proximal end portion and the distal end portion define at least one fluid pathway therebetween, and the shaft has a treatment portion that includes a first electrode and a second electrode, activating the catheter such that the treatment portion cools the tissue, applying an electrical current between the first and second electrodes, measuring a impedance voltage between the first and second electrodes, and processing the measured impedance caused by the applied electrical current, establishing a normal impedance range for a tissue type, monitoring the impedance to determine if the impedance varies into a tissue type impedance range, and generating a signal when the impedance varies into the identified tissue type impedance range. 
     The processing step of the method for accessing tissue composition may be performed by a control unit, a microprocessor, an impedance measuring device or the like. In another embodiment of the method, the treatment portion of catheter may include a cooling chamber in fluid communication with the at least one fluid pathway and having the first electrode located near the distal side of the cooling chamber, and the second electrode located near the proximal side of the cooling chamber. 
     As many treatment systems and methods are applied in internal body lumens, organs or other unobservable tissue regions, the orientation and attitude of the device structure relative to the tissue is of significant importance in ensuring the effective and efficient treatment of tissue. This applies to many tissue treatment systems, both surgical and non-surgical, using a variety of modalities, including cooling through cryotreatment, heat or electrically induced heating, ultrasound, microwave, and RF, to name a few. 
     Many of these events include changes in the natural state of tissue, such as temperature, dielectric or conductivity changes, structural changes to the cells and cell matrix, dimensional changes, or changes in the operation of, or interplay between, tissue regions and/or foreign bodies, such as blood flow in an artery having a treatment device inserted therein. All of these changes may be correlated to, or affected by, relative changes in the bio-impedance of the tissue region. 
     When using the term impedance, we are referring to the generally accepted definition of the term: a complex ratio of sinusoidal voltage to current in an electric circuit or component, except that as used herein, impedance shall apply to any region or space through which some electrical field is applied and current flows. The impedance, Z, may be expressed as a complex number, Z=R+jX, wherein R is the resistance in real number ohms, X is the reactance in imaginary number ohms, and j is a multiplier that is the positive square root of negative one (−1). 
     Resistance, denoted R, is a measure of the extent to which a substance opposes the movement of electrons among its atoms. The more easily the atoms give up and/or accept electrons, the lower the resistance. Reactance denoted X, is an expression of the extent to which an electronic component, circuit, or system stores and releases energy as the current and voltage fluctuate with each AC (alternating current) cycle. Reactance is expressed in imaginary number ohms. It is observed for AC, but not for DC (direct current). When AC passes through a component that contains reactance, energy might be stored and released in the form of a magnetic field, in which case the reactance is inductive (denoted +jX L ); or energy might be stored and released in the form of an electric field, in which case the reactance is capacitive (denoted −jX C ). The impedance Z may be positive or negative depending on whether the phase of the current lags or leads on that of the voltage. Impedance, sometimes called “apparent resistance”, is different from general resistance, in that impedance applies only to AC; however resistance R applies to both AC and DC, and is expressed in positive real number ohms. 
     As mentioned in the background section, the collection of problems may be resolved in part by developing a specialized transducer suitable for the “body” environment in which it operates. However, for many physiological events, there is no specialized transducer. The events in question include changes in the natural state of tissue, such as temperature, dielectric or conductivity changes, structural changes to the cells and cell matrix, dimensional changes, or changes in the operation of, or interplay between, tissue regions and/or foreign bodies, such as blood flow in an artery having a treatment device inserted therein. Using simple transducers, such as electrodes properly positioned in the tissue, the impedance between them is measured, which may depend on seasonal variations, blood flow, cardiac activity, respired volume, nervous activity, galvanic skin reflex, blood pressure, and salivation, to name a few. In some cases the measured impedance may be dissected into its resistive and reactive components. In other cases the total impedance may be measured, with or without resolution into its components, which may contain sufficient information on the physiological event, especially when measured relative to some reference or “baseline” impedance prior to the physiological event. 
     Additionally, during the operation of a medical device in a therapeutic procedure, such as in a blood vessel, the heart or other body organ, the medical user desires to establish a stable and uniform contact between the tip or thermally-transmissive region of the cryogenic device and the tissue to be treated (e.g., ablated). In those instances where the contact between the tip or thermally-transmissive region of the cryogenic device and the tissue to be treated is non-uniform or instable, the resulting ablation or lesion may be less than optimal. It is desirable for the medical professional to assess the state of the contact between the tip or thermally-transmissive region of the cryogenic device and the tissue to be treated, so that appropriate adjustments can be made to re-position the cryogenic device to obtain a more optimal contact and thus a more effective treatment. 
     In view of the proceeding, the present invention advantageously provides methods and systems for detecting fluid egress, assessing lesion quality, determining tissue composition and structure, determining ice coverage of catheter tip as well as providing tissue contact assessment. 
       FIG. 1  illustrates an exemplary system  30  for performing cryogenic ablation. The system  30  includes an elongate, highly flexible ablation catheter  34  that is suitable for passage through the vasculature. The ablation catheter  34  includes a catheter body  36  having a distal end  37  with a thermally conductive region  38  at or proximal to the distal end  37 . The distal end  37  and the thermally conductive region  38  are shown magnified and are described in greater detail below. The catheter body  36  has a proximal end  40  that is mated to a handle  42  that can include an element such as a lever  44  or knob for manipulating the catheter body  36  and the thermally conductive region  38 . In the exemplary embodiment, a pull wire  46  with a proximal end and a distal end has its distal end anchored to the catheter at or near the distal end  37 . The proximal end of the pull wire  46  is anchored to an element such as a cam  48  in communication with and responsive to the lever  44 . The handle  42  can further include circuitry  50  for identification and/or use in controlling of the ablation catheter  34  or another component of the system  30 . 
     Continuing to refer to  FIG. 1 , the handle  42  can also include connectors that are matable directly to a cryogenic fluid supply/exhaust and control unit or indirectly by way of one or more umbilicals. In the system illustrated, the handle  42  is provided with a first connector  54  that is matable with a co-axial fluid umbilical (not shown) and a second connector  56  that is matable with an electrical umbilical (not shown) that can further include an accessory box (not shown). In the exemplary system the fluid supply and exhaust, as well as various control mechanisms for the system are housed in a single console  52 . In addition to providing an exhaust function for the ablation catheter fluid supply, the console  52  can also recover and/or re-circulate the cooling fluid. The handle  42  is provided with a fitting  58  for receiving a guide wire (not shown) that is passed into a guide wire lumen  60 . During balloon inflation, contrast solution may be injected through the catheter&#39;s inner guide wire lumen  60  and into the pulmonary vein. 
     Still referring to  FIG. 1 , the thermally conductive region  38  is shown as a double balloon having a first membrane (e.g., inner balloon)  62  contained or enclosed within a second membrane (e.g., outer balloon)  64 , thereby defining an interface or junction  57  between the first and second membranes. The second membrane  64  provides a safeguard to prevent fluid from leaking out of the cooling chamber  55  and into surrounding tissue should the first membrane  62 , and therefore the cooling chamber  55 , rupture or develop a leak. The junction  57  between the first and second membranes  62 ,  64  may be substantially under a vacuum, such that the first and second membranes  62 ,  64  are generally in contact with each other, with little or no open space between them. A coolant supply tube  66  in fluid communication with the coolant supply in the console  52  is provided to release coolant from one or more openings in the tube within the inner balloon  62  in response to console commands and other control input. A vacuum pump in the console  52  creates a low-pressure environment in one or more lumens within the catheter body  36  so that coolant is drawn into the lumen(s), away from the inner balloon  62 , and towards the proximal end of the catheter body. The vacuum pump is also in fluid communication with the interface or junction  57  of the inner and the outer balloons  62 ,  64  so that any fluid that leaks from the inner balloon  62  is contained and aspirated. 
     Still referring to  FIG. 1 , the handle  42  includes one or more pressure sensors  68  to monitor the fluid pressure within one or both of the balloons, the blood detection devices  70  and the pressure relief valves  72 . When coolant is released into the inner balloon  62 , the inner and the outer balloon  64  expand to a predetermined shape to present an ablation surface, wherein the temperature of the ablation surface is determined by the material properties of the specific coolant selected for use, such as nitrous oxide, along with the pressure within the inner balloon  62  and the coolant flow rate. 
       FIG. 2  illustrates an embodiment of a shaft or catheter body  36  of the balloon catheter system  34  of  FIG. 1 . The catheter body  36  includes a mounting section  59  in communication with the proximal end of thermally conductive element  38 . The inner balloon  62  and outer balloon  64  are bonded to the mounting section  59 . In this embodiment, the inner balloon  62  and outer balloon  64  are bonded at different locations, which are defined as the inner balloon bond joint  63  and the outer bond joint  65 , however they may be bonded at the same bond joint. Additionally, several sensors are identified including a temperature sensor  61  (e.g., thermocouple wire), leak detectors  67 ,  69  (e.g., leak detection wires) and electrodes  86 ,  88 ,  90  and  92 . In this embodiment, contact assessment, lesion quality, fluid egress and/or tip ice coverage may be provided by using a first pair of electrodes ( 86 ,  88 ); providing the excitation current  107  of well-selected amplitude (e.g., in the range of 0.2 mA to 5 mA) and frequency (e.g., in the range of 250 Hz to 500 kHz) to create a current field and measuring the differential impedance voltage as produced across a second pair of electrodes ( 90 ,  92 ). 
     In another embodiment of the catheter having a single pair of electrodes (e.g.,  90  and  92 ) with one of the pair of electrodes located on the distal side of the thermally-transmissive region  38  (e.g., a single balloon), and the other of the pair located on the proximal side of the thermally-transmissive region  38 , an excitation current  107  of well-selected amplitude and frequency is applied between the two electrodes to create a current field and measure the differential impedance voltage as produced across the same electrodes to determine tissue contact assessment, lesion quality and/or blood occlusion assessment. The processing algorithms and related aspects will be discussed in more detail below. 
     In another embodiment, as illustrated in  FIG. 2A , a pair of excitation current electrodes ( 86 ,  88 ) are located on a patient&#39;s body and create an electrical field (i.e., polarize the patient&#39;s body), and the pair of measurement electrodes ( 90 ,  92 ) are located on the catheter  34 . The tissue contact assessment, lesion quality and fluid egress aspects can be determined by applying an excitation current  107  of a well-selected amplitude and frequency to create a current field and measuring the differential impedance voltage as produced across the pair of electrodes ( 90 ,  92 ). 
     After applying an excitation current to the two electrodes  90 ,  92 , the impedance voltage can be measured by the impedance measurement system  106  (as shown in  FIG. 3 ). The impedance measurement signal is then processed using a signal processor  108  (as shown in  FIG. 3 ), which extracts relevant data from a specific frequency range to correlate the impedance change to, for example, occlusion of a pulmonary vein. The signal processor  108  may be a standalone unit or it may be part of the control unit  52 , the impedance measurement system  106  or another portion of the catheter system. The electrical impedance of the tissue is much higher than the impedance of the blood, so measuring the impedance between the first electrode  90  and the second electrode  92  would indicate the efficacy of a thermally conductive element to tissue contact. With high measurement sensitivity, the system should be able to quantify the contact quality. The impedance measurement system  106  provides information about the baseline impedance that may change as the balloon  38  occludes a vessel, such as a pulmonary vein (PV). As the balloon will occlude or stop the blood flow between the proximal side and the distal side of the balloon, the impedance at a defined frequency will increase, which provides an indication of the quality of the contact between the balloon  38  and the treatment tissue. 
       FIG. 3  illustrates another embodiment of the thermally conductive region  38  of catheter  34  positioned near a treatment tissue site, such as the heart. In this embodiment the thermally conductive region  38  is shown as having a thermally conductive non-balloon element. Although this embodiment is shown with a single thermally conductive element (e.g., tip  94 ), the thermally conductive region  38  may have two or more thermally conductive elements. An excitation current of well-selected amplitude and frequency is applied to the electrodes  90 ,  92  and tip electrode  94 ; the impedance (voltages) can be measured by the impedance measurement system  106  (e.g., between the tip electrode  94  and the electrode  90 ). Once the excitation current is applied to the electrodes it will produce the electrical current lines  102 , which indicate overall field strength. The excitation field provides for or enables the polarization of the tissue or treatment area of the patient. The shape and density of the current lines  102  will characteristically result from the number and placement of the electrodes. The number and placement of the electrodes will determine the overall system sensitivity. Normally, a greater sensitivity is required to perform fluid egress detection as opposed to tissue contact assessment or tip ice formation. 
     In general, the detection of catheter fluid egress and of catheter tissue contact assessment may be determined using the same catheter and electrode configurations. The process for determining fluid egress and tissue contact assessment, typically may be determine by the selection of the excitation current applied to the catheter system. For example, if a gas bubble leak occurs in the catheter, a low frequency excitation current (e.g., in the range of 250 Hz to 100 kHz) can improve the detection of the gas bubble leak since the low frequency signal will not penetrate the gas bubble and the gas bubble will interrupt the electrical lines  102 , and thus cause a spike in the measured impedance Z. On the other hand, if a high frequency excitation current (e.g., in the range of 20 kHz to 500 kHz) is applied, the high frequency excitation current will penetrate the gas bubble and therefore the gas bubble will not interrupt the electrical lines  102 , causing the bubble to go undetected. Therefore, there are certain circumstances where additional electrodes may be necessary to improve the sensitivity of the overall detection system and process to improve leak detection/fluid egress. 
       FIGS. 3A and 3B  illustrate another embodiment of the thermally conductive region  38  of catheter  34 . In this embodiment, the location of the measurement electrodes ( 87  and  91 ) is inside the guide wire lumen  60 . In this embodiment, the inner member  62  and outer member  64  are connected to the catheter shaft  36  and define the cooling chamber  55 . By locating the, measurement electrodes ( 87  and  91 ) on the inside of guide wire lumen  60 , a fluid leak  101  from the catheter shaft  36  or guide wire lumen  60  may be detected. 
       FIG. 4A  illustrates another embodiment of the thermally conductive region  38  of catheter  34  positioned near a treatment tissue site, such as in a pulmonary vein. In this embodiment the thermally conductive region  38  is shown as having a thermally conductive non-balloon element with a plurality of electrodes  90 ,  91 ,  92 ,  94 , etc., wherein the thermally conductive region  38  is in a spiral or coiled configuration. Each of the plurality of electrodes may be monitored by the impedance measurement system  106 , which can provide information about each electrode&#39;s baseline impedance that will vary as the thermally conductive region  38  contacts the targeted treatment tissue. The impedance measurement system  106  may use an impedance multiplexer to measure the impedance (voltages) between the electrodes of the thermally conductive region  38  by scanning the electrodes and recording all the impedance voltages. For example, an excitation current may be applied between electrodes  94  and  92  and the impedance voltage may be measured between electrodes  90  and  91 . Next an excitation current may be applied between electrodes  90  and  93  and the impedance voltage may be measured between electrodes  91  and  92 . This process may continue until impedance measurements are calculated for various combinations of electrodes. 
     The measured impedance voltages may be processed by using a signal processor  108  that can extract relevant data from a specific frequency range to correlate the impedance change for each electrode to that electrode&#39;s contact with the target treatment tissue. The impedance associated with those electrodes in contact with the tissue will be higher than those that are surrounded by the blood pool. Accordingly, the electrodes with the highest impedance will be the ones in best contact with the target treatment tissue, and as a result should provide the orientation of the catheter tip to the treatment tissue site. 
       FIG. 4B  illustrates an embodiment of the thermally conductive region  38  of catheter  34  having four electrodes  90 ,  91 ,  92  and  94 .  FIG. 4C  illustrates another embodiment of the thermally conductive region  38  of catheter  34  having eight electrodes  90 ,  91 ,  92 ,  93 ,  94 ,  95 ,  96  and  97 . The number of electrodes controls the accuracy of the contract assessment of the catheter&#39;s thermally conductive region  38 . As the number of electrodes placed on the catheter&#39;s thermally conductive region  38  increases, the more accurate the contact assessment measurement. In addition, besides providing contact assessment, this system, as well as all the other system embodiments, can also provide enhanced assessment of lesion quality and/or size. For example, by measuring the electrical impedance of tissue prior to a cryogenic treatment, and then measuring the electrical impedance of that tissue subsequent to cryogenic treatment, it is possible to quantify the amount of treated tissue for a particular treatment session or sessions. 
     Depending on the rate of change of the impedance, the cooling profile may be adjusted. For example, a cooling profile may be developed for an optimal treatment regime, where the preset impedances (e.g., Z1, Z2, Z3, and Z4) are desired at corresponding times (e.g., T1, T2, T3 and T4). As a specific time in the treatment regime is reached, the impedance is determined from a measured impedance voltage, and that impedance is compared to a preset impedance (e.g., Z1, Z2, Z3, and Z4). Depending on the measured impedance, the cooling profile may be adjusted to increase or decrease the cooling power of the catheter, thereby providing increased treatment regime control. 
     The catheter, as illustrated in  FIG. 5  can be used for detecting gas or liquid egress  104  from the catheter. After applying a low frequency electrical current (e.g., in the range of 250 Hz to 100 kHz) to the two electrodes  90 ,  92  the impedance can be measured by the impedance measurement system  106 . The impedance measurement signal is then processed using a signal processor  108  that can extract relevant data from a specific frequency range to correlate the impedance change, if any, due to a gas egress into the blood stream. The application of the low frequency electrical current causes an electrical field  110  to form, which basically encloses or encompasses the thermally transmissive region  38 . This electrical field  110  may have similar functionality to the expandable membrane  64 , which is to provide leak detection in the event that the chamber or inside balloon  62  were to rupture, crack or leak. Thus, the electrical field  110  may be called a “virtual balloon” capable of detecting a fluid egress or expulsion  104  from the catheter  34  and generating a signal for automatic shutdown of the catheter system  30 . Of course, unlike the leak detection of the expandable membrane  64 , which measures ruptures or leaks internal to the catheter  34 , the leak detection of electrical field  110  is external to the catheter  34 , and found in the external fluid and tissue being treated. In those situations where fluid egress is present, a signal is generated to stop the flow of cryogenic fluid to the catheter and evacuate all fluid from the catheter. 
     In an alternative embodiment, additional electrodes  86  and  88  may be placed on the shaft of the catheter treatment section (similar to those shown in  FIG. 2 ) to increase the sensitivity of the detection system, and thus provide for lower amplitude and/or lower frequency excitation signals to be used. 
     In conjunction with the various electrode configurations described above, there are various processing algorithms that may be employed. As illustrated in  FIGS. 6 ,  7  and  8 , processing algorithms for determining fluid egress, pulmonary vein occlusion/fluid egress verification and tip ice coverage/lesion quality measurement are provided. 
     Referring to  FIG. 6 , an exemplary fluid egress algorithm is illustrated. The process starts at step  600  and a baseline impedance Z 0  is measured across the measurement electrodes (step  610 ). If the start button for the treatment cycle has not been activated, then another baseline line impedance Z 0  may be measured. Upon the activation of the start button (step  620 ), the refrigerant is delivered to the catheter (step  630 ). After delivery of the refrigerant (coolant) (step  630 ), the impedance Z(t) (step  640 ) is measured and a delta impedance (D) Z(t) is calculated wherein the delta Z is the impedance Z at time t minus the baseline impedance Z 0  (step  650 ), DZ(t)=Z(t)−Z 0 . In general, the baseline impedance Z 0  will increase at a linear rate as freezing of the treatment tissue occurs. For example, if Z 0  is first measured to be 20 ohms, and after application of a treatment cycle, the impedance Z is 25 ohms, then delta impedance DZ(t) is 5 ohms. However, if a fluid leak should occur, then the speed of the change with increase rapidly and cause a sudden spike in the impedance Z(t). The spike on the impedance graph shown in  FIG. 10  illustrates this situation. In step  660 , the signal changes will typical stop and the first derivative dZ(t)/dt is measured. If the dZ(t)/dt is greater than the threshold value, which is a predetermined value for each catheter, then the freezing cycle is halted (step  670 ). Otherwise, the system checks to determine if the treatment cycle has reached the end of the freezing cycle (step  680 ). If so, then the freezing is stopped (step  685 ) and the system is vented (step  690 ). After the system is vented (step  690 ), a notification is generated to the user (step  690 ). 
     Referring to  FIG. 7 , an exemplary balloon catheter controller algorithm for measuring pulmonary vein occlusion/fluid egress/ice coverage is illustrated. Steps  700  through  720  relate to assessing the quality of a pulmonary vein (PV) occlusion. The process commences at step  700  and a low frequency excitation current may be applied across a pair of electrodes (step  705 ). The impedance Z(t) is measured (step  710 ) and the catheter is repositioned until the impedance Z(t) is at a maximum, where the Z max  may be displayed and serves to indicate that a best possible pulmonary vein occlusion has occurred. Upon the activation of the start button (step  725 ), a high frequency excitation current may be applied across a pair of electrodes (step  730 ), and the baseline line impedance Z 0  may be measured (step  735 ). The change to a high frequency excitation current facilitates the measurement of the bio-impedance across a treatment tissue site. 
     After delivery of a refrigerant (coolant) to the catheter (step  740 ), the impedance Z(t) (step  745 ) is measured and a delta impedance (D) Z(t) is calculated wherein the delta Z is the impedance Z at time t minus the baseline impedance Z 0 . (step  735 ) DZ(t)=Z(t)−Z 0 . In step  745 , the signal changes will typical stop and the first derivative dZ(t)/dt is calculated. If the dZ(t)/dt is greater than the threshold value, which is a predetermined value for each catheter, then the freezing cycle is halted (step  750 ). Otherwise, the system continues the freezing treatment (step  755 ) and checks to determine if the impedance Z(t) is at a maximum value (step  760 ). A graph of the general impedance Z(t) with respect to time is illustrated in  FIG. 9 . If the impedance Z(t) is at a maximum value, full ice coverage of catheter treatment tip has occurred and the PV has been occluded. If Z(t) is not at a maximum value, then the effectiveness of the treatment is undetermined (step  770 ). If the treatment cycle has reached the end of the freeing cycle (step  775 ), then the freezing is stopped (step  780 ) and the system is vented (step  785 ). After the system is vented (step  785 ), a notification is generated and sent to the user (step  790 ). Otherwise, the system will measure a new impedance Z(t) (step  745 ) and calculate a delta impedance (D) Z(t) wherein the delta Z is the impedance Z at time t minus the baseline impedance Z 0 . In step  745 , the signal changes will typical stop and the first derivative dZ(t)/dt is calculated. The process continues as discussed in steps  745  through  790  until the selected treatment is completed. 
     Referring to  FIG. 8 , an exemplary catheter controller algorithm for measuring catheter tip ice coverage/lesion quality is illustrated. The process commences at step  800  and a low frequency excitation current may be applied across a pair of electrodes (step  805 ). A baseline impedance Z 01  is measured (step  810 ). Upon the activation of the start button (step  815 ), the impedance Z(t) is measured. The delta impedance DZ(t) is calculated wherein the delta Z is the impedance Z at time t minus the first baseline impedance Z 01  (step  825 ). DZ(t)=Z(t)−Z 01 . The value of delta impedance DZ(t) may be displayed (step  830 ). A refrigerant (coolant) is delivered to the catheter (step  835 ). The value of DZ(t) is processed to determine if DZ(t) has reached a saturated condition (step  840 ). If so, the catheter tip is covered with ice (step  845 ) and the time to saturation is compared to the time threshold (step  850 ). If not, a recommendation that the freezing procedure be halted (step  855 ), the catheter be repositioned (step  860 ) and the process restarted (step  800 ) is generated. If time to saturation is greater than the time threshold, a recommendation that the freezing procedure be halted (step  855 ), the catheter be repositioned (step  860 ) and the process restarted (step  800 ) is generated. If the time to saturation is less than the time threshold, then maximum freezing capability is requested (step  865 ). 
     If the treatment cycle has reached the end of the freeing cycle (step  870 ), then the freezing is stopped (step  875 ) and the temperature of the cooling segment is measured and compared to a temperature threshold (e.g., +1 degree C.) to determine if the catheter tip has warmed sufficiently to be removed from the tissue treatment site (step  880 ). If the temperature of the cooling segment is less than the temperature threshold (e.g., +1 degree C.), the catheter usually remains in its current position, and another temperature reading is taken. If the temperature of the cooling segment is greater than the temperature threshold (e.g., +1 degree C.), a new baseline impedance Z 02  is measured (step  885 ) and the lesion quality may be calculated (step  890 ) by the equation: lesion quality=K*(Z 02 −Z 01 ); where K is a constant multiplier determined from in vitro testing or finite element model calculations and may have a specific value per different catheter type (for e.g., a 6 mm long tip will usually have a different K than a 4 mm long tip). 
     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 herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.