a. Field of the Invention
This invention relates to a system and method for assessing the degree of coupling between an electrode and tissue in a body.  In particular, the instant invention relates to a system and method for assessing the degree of electrical coupling between electrodes on an diagnostic and/or therapeutic medical device such as a mapping or ablation catheter and tissue, such as cardiac tissue. 
b. Background Art
Electrodes are used on a variety of diagnostic and/or therapeutic medical devices.  For example, electrodes may be used on cardiac mapping catheters to generate an image of the internal geometry of a heart and electrical potentials within the tissue.  Electrodes are also used on ablation catheters to create tissue necrosis in cardiac tissue to correct conditions such as atrial arrhythmia (including, but not limited to, ectopic atrial tachycardia, atrial fibrillation, and atrial flutter).  Arrhythmia can create a variety of dangerous conditions including irregular heart rates, loss of synchronous atrioventricular contractions and stasis of blood flow which can lead to a variety of ailments and even death.  It is believed that the primary cause of atrial arrhythmia is stray electrical signals within the left or right atrium of the heart.  The ablation catheter imparts ablative energy (e.g., radiofrequency energy, cryoablation, lasers, chemicals, high-intensity focused ultrasound, etc.)  to cardiac tissue to create a lesion in the cardiac tissue.  This lesion disrupts undesirable electrical pathways and thereby limits or prevents stray electrical signals that lead to arrhythmias. 
The safety and effectiveness of many of diagnostic and/or therapeutic devices is often determined in part by the proximity of the device and the electrodes to the target tissue.  In mapping catheters, the distance between the electrodes and the target tissue affects the strength of the electrical signal and the identity of the mapping location.  The safety and effectiveness of ablation lesions is determined in part by the proximity of the ablation electrode to target tissue and the effective application of energy to that tissue.  If the electrode is too far from the tissue or has insufficient contact with the tissue, the lesions created may not be effective.  On the other hand, if the catheter tip containing the electrode contacts the tissue with excessive force, the catheter tip may perforate or otherwise damage the tissue (e.g., by overheating).  It is therefore beneficial to assess the quality of contact between the electrode and the tissue. 
Contact between a catheter electrode and tissue has typically been determined using one or more of the following methods: clinician sense, fluoroscopic imaging, intracardiac echo (ICE), atrial electrograms (typically bipolar D-2), pacing thresholds, evaluation of lesion size at necropsy and measurement of temperature change at the energy delivery site.  Each of these methods has disadvantages, however. 
Although a clinician can evaluate contact based on tactile feedback from the catheter and prior experience, the determination depends largely on the experience of the clinician and is also subject to change based on variations in the mechanical properties of catheters used by the clinician.  The determination is particularly difficult when using catheters that are relatively long (such as those used to enter the left atria of the heart). 
Because fluoroscopic images are two-dimensional projections and blood and myocardium attenuate x-rays similarly, it is difficult to quantify the degree of contact and to detect when the catheter tip is not in contact with the tissue.  Fluoroscopic imaging also exposes the patient and clinician to radiation. 
Intracardiac echo is time consuming and it is also difficult to align the echo beam with the ablation catheter.  Further, intracardiac echo does not always permit the clinician to confidently assess the degree of contact and can generate unacceptable levels of false positives and false negatives in assessing whether the electrode is in contact with tissue. 
Atrial electrograms do not always correlate well to tissue contact and are also prone to false negatives and positives.  Pacing thresholds also do not always correlate well with tissue contact and pacing thresholds are time-consuming and also prone to false positives and false negatives because tissue excitability may vary in hearts with arrhythmia.  Evaluating lesion size at necropsy is seldom available in human subjects, provides limited information (few data points) and, further, it is often difficult to evaluate the depth and volume of lesions in the left and right atria.  Finally, temperature measurements provide limited information (few data points) and are difficult to evaluate in the case of irrigated catheters. 
A more recent method of assessing contact between the catheter electrode and tissue is the use of force sensors incorporated into the catheter to measure contact force between the catheter tip and tissue.  Contact force, however, does not directly measure how well electrical energy is coupled between the catheter electrode and tissue.  Particularly for radio-frequency (RF) ablation catheters, a measure of electrical coupling may be more relevant to ablation safety and efficacy in different types of tissue and in different types of catheter tip to tissue surface alignment (e.g., perpendicular versus parallel orientation).  The use of force sensors also requires significant structural adjustments and technological advances for use in conventional ablation catheters. 
Contact between the catheter electrode and tissue has also been evaluated by measuring impedance between the catheter electrode and an electrode disposed on the a patient's skin.  During radio frequency (RF) ablation, the generation of RF energy is controlled by an ablation generator.  The ablation generator displays a measure of the magnitude of impedance (Z).  This measurement, however, does not correlate well with the more localized contact between the catheter electrode and tissue because it measures the impedance provided not just by the local target tissue, but the entire impedance from the electrode to a cutaneous return electrode through various body tissues and fluids.  Furthermore, generator reported impedance is usually infrequently obtained and at low resolution (about 1Ω).  It is also not readily available to the clinician in a format that allows easy interpretation and correlation to tissue contact. 
In priority U.S. patent application Ser.  No.  12/253,637, a system and method are provided for determining a degree of coupling between a catheter electrode and tissue in which an electrical coupling index (ECI) is generated as an indicator of impedance at the interface of the electrode and the target tissue.  Because the coupling index is indicative of impedance at the interface of the electrode and target tissue, the inventive system and method provide a better assessment of coupling between the electrode and the tissue and the index permits a better assessment of the degree of coupling between an electrode and tissue as compared to prior art systems.  The index, however, is subject to variability based on changes to properties associated with patient bodies (e.g. differences in body temperature among patients) and components of the system (e.g., differences resulting from the use of different ablation catheters). 
The inventors herein have recognized a need for a system and method for determining a degree of coupling between a catheter electrode and tissue that will minimize and/or eliminate one or more of the above-identified deficiencies.