Reference electrode dedicated to catheter tissue proximity estimation

Abstract

The subject matter of the invention is a reference electrode dedicated to catheter tissue proximity estimation. The present disclosure provides a catheter that includes a shaft having a distal end, and an expandable distal end assembly configured for insertion into a cavity of an organ of a patient. An expanded assembly defines an inner volume and includes (i) a proximal base section configured to couple the assembly to a distal end of the shaft, (ii) a plurality of functional electrodes at least partially outside the inner volume and configured to be placed in tissue contact with a wall of the cavity, and (iii) a reference ring electrode located on the proximal base section of the assembly outside the inner volume, where the reference ring electrode is selectively positioned outside the inner volume at the base section, and where the reference ring electrode is configured to couple with each of the plurality of functional electrodes, the ring-shaped electrode is coupled to the plurality of functional electrodes for generating an electric field between each of the plurality of functional electrodes and the ring-shaped electrode.

Description

Technical Field

[0001] The present disclosure relates generally to invasive medical probes, and in particular to estimating the proximity of a catheter to tissue. Background Art

[0002] Previously, a technique for estimating the proximity of an electrode of a catheter to tissue was proposed in the patent literature. For example, U.S. Patent Application Publication 2023 / 0112251 describes a system comprising a catheter and a processor. The catheter includes a distal end assembly coupled to the distal end of the shaft for insertion into a cavity of a patient's organ, the distal end assembly including (i) one or more functional electrodes configured to be placed in contact with the wall tissue of the cavity, and (ii) a reference electrode configured to be placed in the cavity but not in contact with the wall tissue. The reference electrode is disclosed as an electrode positioned within an inner volume defined by the distal end assembly. The reference electrode is selectively placed within the inner volume so that the reference electrode is kept away from the tissue wall. The processor is configured to (i) estimate one or more impedances between one or more functional electrodes and a reference electrode in the functional electrode, and (ii) for at least one functional electrode among the one or more functional electrodes, determine whether the functional electrode is in physical contact with the wall tissue based on impedance.

[0003] The present disclosure will be more fully understood through the following detailed description of the embodiments of the present disclosure in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 is a schematic illustration of a catheter-based electroanatomical (EA) mapping and ablation system according to an example of the present disclosure;

[0005] Figure 2 is a schematic illustration of a basket assembly configured to electrically sense proximity of a functional electrode to cavity wall tissue according to an example of the present disclosure; and

[0006] Figure 3 is a flow chart schematically illustrating a method and algorithm for estimating proximity of a functional electrode to cavity wall tissue according to an example of the present disclosure. Summary of the invention

[0007] The wall tissue of a cavity of an organ of a patient, such as a heart chamber, can be electroanatomically mapped and / or ablated using a catheter having a plurality of functional electrodes mounted at an expandable distal end assembly of the catheter. During a mapping and / or ablation procedure in a heart chamber, a physician can manipulate the expanded distal end assembly to bring the electrodes into contact with the chamber wall to acquire and / or apply an electrical signal.

[0008] The quality of electrical mapping and / or ablation depends on the proximity of the functional electrode to the cavity wall tissue (also referred to in this disclosure as "Tissue Proximity Indication (TPI)"). The proximity scale may use arbitrary units (e.g., an index ranging from 1 to 10) or may be given in physical units of distance.

[0009] The processor can be configured to estimate proximity using the calibrated proximity data by converting the impedance to electrode-tissue proximity. One way to calculate the TPI is to: (i) identify a range of minimum and maximum values ​​recorded over time from the monitored imbalance, (ii) normalize the monitored impedance according to the range, and (iii) relate the normalized range to the scale of the TPI.

[0010] Another option is that the TPI is a binary value, for example, 0 for non-touch and 1 for touch. One way to calculate the binary TPI is to: (i) identify a range of minimum and maximum values ​​recorded over time from the monitored imbalance, (ii) set a threshold value based on the range, and (iii) define a corresponding normalized threshold value based on the normalized range of TPI. For example, the normalized threshold value is 0.7, above which the reported TPI is 1, and below which the reported TPI is 0.

[0011] The system measures the impedance between a pair of electrodes, one of which is a reference electrode in the blood pool and the other is a functional electrode. Based on this measurement, the processor runs an algorithm to determine the proximity of the functional electrode to the tissue. Minimum impedance occurs when the functional electrode is well within the blood pool of the heart chamber, and maximum impedance occurs when the functional electrode is in full contact (e.g., engaged with a threshold pressure). Impedance readings are continuous between these two extremes.

[0012] In some known methods, the reference electrode is positioned within the inner volume of the expandable distal assembly as described above. The inventors of the present case have discovered that improved sensitivity can be achieved by directing a larger portion of the electric field outward through a volume that may potentially include a portion of the tissue wall. To this end, they propose positioning the reference electrode at a position outside the inner volume of the expandable distal assembly where the reference electrode will be spaced apart from any contact with the tissue.

[0013] The example of the present disclosure described herein provides an annular electrode, which is selectively positioned at the base of an expandable assembly and adjacent to the expandable portion of a distal assembly. The base of the expandable assembly, as a part of the assembly, is an element of the distal end of the shaft of the catheter that the assembly is coupled to, that is, the annular reference electrode is at the distal side of the shaft. Since the annular reference electrode is at the base of the expandable assembly, the annular reference electrode does not usually touch the tissue wall when the expandable assembly is in an expanded state. In addition, the annular reference electrode has a diameter of about 2-5mm, while the distal end assembly is spherical in its expanded state, with a diameter of about 20-30mm. Due to this significant difference in diameter, positioning the reference electrode at the base of the distal end assembly makes the assembly act as a spacer separating the reference electrode from the tissue.

[0014] The path of the potential between such annular reference electrode and the functional electrode is sensitive to tissue proximity because both the multiple functional electrodes and the reference annular electrode are outside the inner volume defined by the expandable assembly. Therefore, proximity is based on monitoring the increase in impedance of each functional electrode over time.

[0015] Additionally, and primarily for basket-shaped catheters, by coating the inwardly facing sides of the splines with an insulating coating, the electric field path through the inwardly facing sides of the functional electrodes may be eliminated and / or reduced, such as Figure 2 Described in .

[0016] In an example, the additional guard ring protrudes further than the reference ring electrode and is configured to act as a spacer that prevents contact between the reference ring and the tissue wall. In this way, the guard ring reduces contact situations, such as near ridges of cardiac chamber anatomy.

[0017] Since in some examples, the distal tip assembly is essentially a single piece made of nitinol tube, the base section, which is the proximal section of the distal tip assembly, is conductive. In these examples, the reference ring electrode is electrically insulated from the nitinol base.

[0018] Finally, despite the above efforts, sometimes the reference electrode may touch the tissue. This can be identified by comparing the electrical signals from all functional electrodes. If all signals from all functional electrodes indicate a touch (e.g., all signals show an increase in impedance at the same time), it is most likely caused by the reference electrode touching the tissue, because some functional electrodes should always be immersed in the blood pool. Therefore, in this case, the system discards the reading.

[0019] System Description

[0020] Figure 11 is a schematic illustration of a catheter-based electroanatomical (EA) mapping and ablation system 10 according to an example of the present disclosure. The system 10 is configured to determine whether a given functional electrode 26 of a plurality of functional electrodes 26 of a basket catheter 14 is in full contact (or proximity) with tissue or is submerged in a blood pool 33 of a cardiac chamber, for example, prior to performing diagnosis and / or ablation.

[0021] The system 10 includes one or more catheters that are inserted percutaneously through the patient's vascular system into a cavity or vascular structure of the heart 12 by a physician 24. Typically, a delivery sheath catheter is inserted into the left atrium or right atrium near a desired location in the heart 12. One or more catheters may then be sequentially inserted into the delivery sheath catheter to reach the desired location. The one or more catheters may include a catheter dedicated to sensing intracardiac electrogram (IEGM) signals, a catheter dedicated to ablation, and / or a catheter dedicated to both sensing and ablation. An exemplary basket-shaped catheter 14 configured for sensing IEGM is shown herein. As shown in inset 45, the physician 24 brings a basket-type expandable distal tip assembly 28 (hereinafter also referred to as "expandable distal tip assembly 28") mounted on the shaft 44 of the catheter 14 into contact with the heart wall to sense a target site in the heart 12. For ablation, the physician 24 similarly brings the distal end of the ablation catheter to the target site for ablation.

[0022] As shown in inset 65, the catheter 14 is an exemplary catheter including one and preferably multiple functional electrodes 26, which are optionally distributed on multiple splines 22 at the expandable distal tip assembly 28 and are configured to sense IEGM signals. The catheter 14 additionally includes a proximal position sensor 29 (e.g., a TAS 29 including three EMCs) embedded in the distal end 46 of the shaft 44 near the expandable distal tip assembly 28 to track the position of the distal end of the expandable distal tip assembly 28. Optionally and preferably, the position sensor 29 is a magnetically based position sensor that includes a magnetic coil for sensing a three-dimensional (3D) position. The distal end 46 of the shaft 44 may include an amplification circuit that is configured to amplify the output of the three EMCs from the sensor 29.

[0023] The magnetic position sensor 29 operates with an external position mat 25, which includes a plurality of magnetic coils 32 configured to generate a magnetic field in a predefined workspace. Using operation with the external position mat 25 (each EMC uses a different frequency), the processor can determine the position of each EMC 29 on the coordinate system of the position tracking system.

[0024] Details of magnetic-based position sensing technology are described in U.S. Patents 5,5391,199, 5,443,489, 5,558,091, 6,172,499, 6,239,724, 6,332,089, 6,484,118, 6,618,612, 6,690,963, 6,788,967, and 6,892,091.

[0025] The system 10 includes one or more electrode patches 38 positioned in contact with the skin of the patient 23 to establish a position reference for impedance-based tracking of the location pad 25 and the functional electrodes 26. For impedance-based tracking, current is directed toward the electrodes 26 and sensed at the electrode skin patches 38 so that the position of each electrode can be triangulated via the electrode patches 38. The real-time orientation of the expandable distal tip assembly 28 of the catheter 14 can be calculated based on the tracked position of the electrodes 26. The relative orientation is represented by the angle formed between the distal tip 46 and the longitudinal axis 42 of the expandable assembly 28 (to the distal edge 16 of the assembly).

[0026] Details of impedance-based position tracking techniques are described in U.S. Patent Nos. 7,536,218, 7,756,576, 7,848,787, 7,869,865, and 8,456,182.

[0027] The catheter 14 is configured to acquire an electrical signal indicative of the proximity of any given functional electrode 26 to the wall tissue of the heart 12. To this end, the signal generator 35 is configured to generate an AC signal between the reference ring electrode 17 and each of the functional electrodes 26. The processor measures the corresponding impedance between each functional electrode 26 and the reference ring electrode 17, which is located on the base 37 of the expandable distal tip assembly 28 and outside the internal volume 77 defined by the splines of the assembly 28. Having an electrical path between each functional electrode 26 and the reference ring electrode 17 to the assembly 28 improves the sensitivity of the measurement to tissue proximity. The reference ring electrode 17 is positioned on the base of the assembly 28 at a position that avoids contact with the tissue wall when the distal tip assembly 28 is in the expanded state, as shown in FIG. Figure 2 Further described in .

[0028] Recorder 11 displays electrograms 21 captured using surface ECG electrodes 18 and intracardiac electrograms (IEGMs) captured using functional electrodes 26 of catheter 14. Recorder 11 may include pacing capabilities for pacing the cardiac rhythm and / or may be electrically connected to a separate pacemaker.

[0029] The system 10 may include an ablation energy generator 50 adapted to conduct ablation energy to a subset of the plurality of electrodes 26 at a distal assembly 28 of a catheter 14 configured for ablation. The energy generated by the ablation energy generator 50 may include, but is not limited to, radio frequency (RF) energy or pulsed field ablation (PFA) energy (including monopolar or bipolar high voltage DC pulses that may be used to achieve irreversible electroporation (IRE)), or a combination thereof.

[0030] The patient interface unit (PIU) 30 is configured to establish electrical communications between catheters, electrophysiology equipment, a power source, and a workstation 55 for controlling the operation of the system 10. The electrophysiology equipment of the system 10 may include, for example, a plurality of catheters, a location pad 25, surface ECG electrodes 18, an electrode patch 38, an ablation energy generator 50, and a recorder 11. Optionally and preferably, the PIU 30 additionally includes processing capabilities for enabling real-time calculations of catheter position and for performing ECG calculations.

[0031] The workstation 55 includes a memory 57, a processor unit 56 having memory or storage loaded with appropriate operating software, and user interface capabilities. The workstation 55 can provide multiple functions, optionally including: (i) three-dimensional (3D) modeling of the endocardial anatomy and rendering the model or anatomical map 20 for display on the display device 27; (ii) displaying the activation sequence (or other data) compiled from the recorded electrograms 21 on the display device 27 as representative visual markers or images superimposed on the rendered anatomical map 20; (iii) displaying the real-time position and orientation of multiple catheters within the cardiac chamber; and (iv) displaying areas of interest (such as where ablation energy has been applied) on the display device 27. A commercial product embodying elements of the system 10 can be CARTO TM 3 System was purchased from Biosense Webster, Inc., 31A Technology Drive, Irvine, CA 92618.

[0032] Although Figure 1 A basket-shaped assembly is described, but the disclosed technology can be applied mutatis mutandis to an expandable balloon assembly having an expandable membrane, wherein the functional electrodes are disposed on the membrane.

[0033] Estimation of Touch Proximity (TP) of Functional Electrodes of Scalable Components

[0034] Figure 2 2 is a schematic illustration of a basket catheter assembly 281 configured for electrical measurement of proximity of a functional electrode 226 to cavity wall tissue according to an example of the present disclosure. The basket assembly 281 may be used to implement the above Figure 1As shown, the assembly 281 is a portion of the catheter 214, which also includes a shaft 244 having a distal end 246. The distal end assembly 228 includes a proximal base 227, which is configured to couple the assembly to the distal end 246 of the shaft 244.

[0035] The basket assembly 281 is implemented as an expandable frame including a plurality of splines 222, wherein the functional electrodes 226 are coupled to the splines. The splines 22 are each electrically insulated from the environment by an insulating layer 262 over most of their area.

[0036] When expanded, such as Figure 2 As shown, the expandable distal tip assembly 281 defines an inner volume 277. At the base of the assembly inside the volume 277, the far-field electrode 223 is used to remove the far-field signal from the IEGM signal acquired by the electrode 226.

[0037] The plurality of functional electrodes 226 are at least partially outside the inner volume and are configured to be placed in contact with the wall tissue of the cavity. Figure 1 The annular electrode 217 is located on the proximal base 227 of the expandable distal tip assembly 281 and outside the inner volume 277. The position of the annular electrode 217 on the base 227 is set to avoid contact with the tissue wall when the distal tip assembly 281 is in the expanded state.

[0038] exist Figure 2 In the example of , the reference ring electrode 217 is a ring mounted on the outer periphery of the proximal base segment 227. The reference ring electrode 217 can be disposed on an insulating layer (not shown), for example, when the base segment 227 is conductive (e.g., made of Nitinol).

[0039] The proximal base section 227 also includes a mechanical guard ring 231 that protrudes outwardly from the proximal base, ie, further outwardly than the reference electrode 227, to prevent the reference electrode 227 from contacting the wall tissue.

[0040] Processor 56 estimates the proximity of a given electrode 226 to wall tissue (wall tissue not shown) based on the impedance signal between electrode 226 and reference electrode 217. The processor may use the impedance value itself or an increase in impedance value relative to a baseline impedance measured when both electrodes are deep inside the blood pool.

[0041] Further improvements in measurement accuracy can be achieved by minimizing the portion of the electrode in the blood that is in contact with tissue by applying an inner electrically insulating coating 241 to the electrode 226. The coating 241 can be a type of polymer or an additional dielectric layer (eg, silicon nitride).

[0042] Although Figure 2A basket-shaped assembly is described, but the disclosed technology can be applied mutatis mutandis to an expandable balloon assembly having an expandable membrane, wherein the functional electrodes are disposed on the membrane.

[0043] Method for estimating touch proximity (TP) of electrodes of a scalable component

[0044] Figure 3 is a flow chart schematically illustrating a method and algorithm for estimating the proximity of a functional electrode to cavity wall tissue according to an example of the present disclosure. The algorithm according to the present embodiment performs a process that begins with identifying the expanded basket assembly 281 in the blood pool 33 inside the heart chamber of the heart 12 at the basket line positioning step 302. This identification can be done using fluoroscopic fluoroscopy or contact force sensing to verify that no mechanical interaction of the basket assembly 281 with the wall tissue of the heart chamber occurs at least part of the time.

[0045] At the electric field generation step 303 , the signal generator 35 generates an AC electric field between the reference ring electrode 217 and the outer surface of each of the functional electrodes 226 .

[0046] Then, at a baseline impedance monitoring step 304 , the system 10 monitors the impedance between each of the functional electrodes 226 and the reference ring electrode 217 .

[0047] At impedance range identification step 306, the processor identifies the impedance range of contact and non-contact for the accumulated monitoring values ​​when the basket occasionally contacts the cavity wall tissue. The range can be determined based on the aggregate output from all functional electrodes 226. Clinically, at or after this stage, the physician 24 can further attempt to make at least a subset of the functional electrodes 226 contact the tissue, for example, over the entire lateral circumference of the assembly 281, as in the treatment of atrial fibrillation in a pulmonary vein isolation procedure.

[0048] Using the monitored impedance and the identified impedance range, the processor calculates the TPI for each functional electrode or a group of such electrodes at a TPI calculation step 308. One way to calculate the TPI is to normalize the impedance measurement to the range detected by finding the lowest and highest impedance values ​​recorded over time, and relate the normalized range to the scale of the TPI (e.g., convert the normalized range to the scale of the TPI).

[0049] Finally, at a TPI reporting step 310, the system 10 reports the TPI of each or a group of functional electrodes 226 to indicate the contact and / or proximity of these functional electrodes with the cardiac chamber wall tissue.

[0050] Figure 3The example flow chart shown is selected purely for conceptual clarity. The present embodiment also includes additional steps of the algorithm, such as obtaining an intracardiac electrocardiogram, which have been intentionally omitted from the disclosure herein to provide a more simplified flow chart. In addition, other steps such as temperature measurement and application of irrigation have been omitted for clarity of presentation. DETAILED DESCRIPTION

[0052] Example 1

[0053] A catheter (14) includes a shaft (44) and an expandable distal tip assembly (28). The shaft has a distal tip (44) configured for insertion into a cavity of an organ (12) of a patient (23). The expandable distal end assembly (28) defines an inner volume (77) when expanded, and the distal end assembly (28) includes: (i) a proximal base section (37) configured to couple the distal end assembly (28) to the distal end (46) of the shaft (44), (ii) a plurality of functional electrodes (26) at least partially outside the inner volume (77) and configured to be placed in contact with the wall tissue of the cavity, and (iii) a reference ring electrode (17) located outside the inner volume (77) on the proximal base section (37) of the distal end assembly (28), wherein the reference ring electrode (17) is selectively positioned outside the inner volume (77) at the base section of the distal end assembly, and wherein the reference ring electrode (17) is configured to couple to each of the plurality of functional electrodes (26) for generating an electric field between each of the plurality of functional electrodes (26) and the ring electrode (17).

[0054] Example 2

[0055] A catheter (14) according to Example 1, wherein the reference ring electrode (17) is mounted on the outer periphery of the proximal base segment (37), and wherein the reference ring electrode (17) is electrically insulated from the base segment (37).

[0056] Example 3

[0057] A catheter (14) according to any one of Examples 1 and 2, wherein the distal end assembly (28) is a basket-shaped assembly (281) having an expandable frame, the expandable frame comprising a plurality of splines (22), and wherein the functional electrode (26) is coupled to the splines (22).

[0058] Example 4

[0059] A catheter (14, 214) according to any one of embodiments 1 to 3, and including a far-field electrode (223) located within the inner volume (77, 277).

[0060] Example 5

[0061] A catheter (14) according to any one of Examples 1 to 3, wherein the distal tip assembly is a balloon assembly having an expandable membrane, and wherein the functional electrode is disposed on the membrane.

[0062] Example 6

[0063] According to the catheter (14, 214) described in any one of Examples 1 to 5, the catheter also includes a mechanical protection ring (231) which is located proximal to the reference ring electrode (17, 217) and protrudes further outward from the proximal base section (37, 227) than the reference ring electrode, and is configured to serve as a spacer to prevent contact between the reference ring (17, 217) and the tissue wall.

[0064] Example 7

[0065] A system (10) includes a catheter (14), a signal generator (35), an interface (30), and a processor (56). The catheter (14) includes a shaft (44) and an expandable distal tip assembly (28). The shaft has a distal tip (44) configured to be inserted into a cavity of an organ (12) of a patient (23). The expandable distal tip assembly (28) defines an internal volume (77) when expanded, and the distal tip assembly (28) includes: (i) a proximal base section (37) configured to couple the distal tip assembly (28) to a distal end (46) of the shaft (44), (ii) a plurality of functional electrodes (26) at least partially outside the internal volume (77) and configured to be placed in contact with wall tissue of the cavity, and (iii) a plurality of electrodes (26) located at the distal tip assembly (28). A reference ring electrode (17) on the proximal base section (37) outside the inner volume (77), wherein the reference ring electrode (17) is selectively positioned outside the inner volume (77) at the base section of the distal tip assembly, and wherein the reference ring electrode (17) is configured to couple with each of the plurality of functional electrodes (26) for generating an electric field between each of the plurality of functional electrodes (26) and the ring electrode (17). The signal generator (35) is configured to generate an AC signal between the reference ring electrode (17) and each of the functional electrodes (26). The interface (30) is configured to receive electrical readings between the plurality of functional electrodes (26) and the reference ring electrode (17). The processor (56) is configured to (i) estimate the corresponding impedance between the functional electrode (26) and the reference ring electrode (17) based on the electrical reading, and (ii) estimate the proximity of the functional electrode (26) to the wall tissue for at least one given functional electrode (26) among the plurality of functional electrodes (26) based on the impedance.

[0066] Example 8

[0067] A system (10) according to Example 7, wherein the signal generator (35) is configured to generate the AC signal between the reference ring electrode (17, 217) and each functional electrode in the functional electrodes (26, 226) by generating an electric field between the reference ring electrode (17, 217) and the outer surface of each functional electrode in the functional electrodes (26, 226).

[0068] Example 9

[0069] A system (10) according to any of Examples 7 and 8, wherein the processor (56) is configured to determine that the reference ring electrode (17, 217) is in physical contact with the wall tissue by determining that the measured impedances among the multiple functional electrodes (26, 226) are all above a given threshold.

[0070] Example 10

[0071] A system (10) according to any one of embodiments 7 to 9, wherein the processor (56) is configured to estimate the proximity using calibrated proximity data that converts between impedance and electrode-tissue proximity.

[0072] Embodiment 11

[0073] A method comprising inserting an expandable distal tip assembly (28) of a catheter (14) into a cavity of an organ (12) of a patient (23), wherein the distal tip assembly (28) defines an internal volume (77) when expanded, the assembly comprising: (i) a proximal base section (37) configured to couple the distal tip assembly to a distal end (46) of a shaft (44) of the catheter, (ii) a plurality of functional electrodes (26) at least partially outside of the internal volume (77) and configured to be placed in contact with wall tissue of the cavity, and (iii)

[0074] A reference ring electrode (17) located on the proximal base section (37) of the distal tip assembly (28) outside the inner volume (77), wherein the reference ring electrode is selectively positioned outside the inner volume at the base section of the distal tip assembly, and wherein the reference ring electrode (17) is configured to couple with each of the plurality of functional electrodes (26) for generating an electric field between each of the plurality of functional electrodes and the ring electrode (17). An AC signal is generated between the reference ring electrode (17) and each of the functional electrodes (26). A resulting electrical reading between the plurality of the functional electrodes (26) and the reference ring electrode (17) is received. A corresponding impedance between the functional electrode (26) and the reference ring electrode (17) is estimated based on the electrical reading. Based on the impedance, for at least one given functional electrode (26) among the plurality of functional electrodes, the proximity of the functional electrode to the wall tissue is estimated.

[0075] Although the embodiments described herein are primarily directed to cardiac diagnostic applications, the methods and systems described herein can also be used in other medical applications.

[0076] It should be understood that the above embodiments are cited by way of example, and the present disclosure is not limited to what is specifically shown and described above. On the contrary, the scope of the present disclosure includes combinations and sub-combinations of the various features described above, as well as variations and modifications thereof, which will occur to those skilled in the art upon reading the above description, and which are not disclosed in the prior art.

Claims

1. A catheter, comprising: a shaft having a distal end configured for insertion into a cavity of an organ of a patient; and An expandable distal tip assembly, the expandable distal tip assembly defining an internal volume when expanded, the distal tip assembly comprising: a proximal base section configured to couple the distal tip assembly to a distal end of the shaft; a plurality of functional electrodes at least partially exterior to the inner volume and configured to be placed in contact with wall tissue of the cavity; and A reference ring electrode located on the proximal base section of the distal end assembly outside of the inner volume, wherein the reference ring electrode is selectively positioned on the base section of the distal end assembly outside of the inner volume, and wherein the reference ring electrode is configured to couple with each of the plurality of functional electrodes for generating an electric field between each of the plurality of functional electrodes and the ring electrode.

2. The catheter according to claim 1, wherein The reference ring electrode is mounted on an outer periphery of the proximal base segment, and wherein the reference ring electrode is electrically insulated from the base segment.

3. The catheter according to claim 1, wherein: The distal tip assembly is a basket-shaped assembly having an expandable frame including a plurality of splines, and wherein the functional electrodes are coupled to the splines.

4. The catheter of claim 1 and comprising a far-field electrode located within the inner volume.

5. The catheter according to claim 1, wherein The distal tip assembly is a balloon assembly having an expandable membrane, and wherein the functional electrode is disposed on the membrane.

6. The catheter of claim 1 , further comprising a mechanical protection ring located proximal to the reference ring electrode and protruding further outward from the proximal base segment than the reference ring electrode, and configured to act as a spacer to prevent contact between the reference ring and a tissue wall.

7. A system, comprising: A catheter, the catheter comprising: a shaft having a distal end configured for insertion into a cavity of an organ of a patient; and An expandable distal tip assembly, the expandable distal tip assembly defining an internal volume when expanded, the distal tip assembly comprising: a proximal base section configured to couple the distal tip assembly to a distal end of the shaft; a plurality of functional electrodes at least partially exterior to the inner volume and configured to be placed in contact with wall tissue of the cavity; and a reference ring electrode located on the proximal base section of the distal tip assembly outside of the inner volume, wherein the reference ring electrode is selectively positioned at the base section of the distal tip assembly outside of the inner volume, and wherein the reference ring electrode is configured to couple with each of the plurality of functional electrodes for generating an electric field between each of the plurality of functional electrodes and the ring electrode; a signal generator configured to generate an AC signal between the reference ring electrode and each of the functional electrodes; an interface configured to receive electrical readings between a plurality of said functional electrodes and a reference ring electrode; and A processor, the processor being configured to: estimating the respective impedances between the functional electrode and the reference ring electrode based on the electrical readings; and Based on the impedance, for at least one given functional electrode among the plurality of functional electrodes, a proximity of the functional electrode to the wall tissue is estimated.

8. The system according to claim 7, wherein: The signal generator is configured to generate the AC signal between the reference ring electrode and each of the functional electrodes by generating an electric field between the reference ring electrode and an outer surface of each of the functional electrodes.

9. The system according to claim 7, wherein: The processor is configured to determine that the reference ring electrode is in physical contact with the wall tissue by determining that measured impedances among the plurality of functional electrodes are all above a given threshold.

10. The system according to claim 7, wherein: The processor is configured to estimate the proximity using calibrated proximity data converted between impedance and electrode-tissue proximity.

11. A method comprising: Inserting an expandable distal tip assembly of a catheter into a cavity of an organ of a patient, wherein the distal tip assembly defines an internal volume when expanded, the assembly comprising: a proximal base segment configured to couple the distal tip assembly to a distal end of a shaft of the catheter; a plurality of functional electrodes at least partially exterior to the inner volume and configured to be placed in contact with wall tissue of the cavity; and a reference ring electrode located on the proximal base section of the distal tip assembly outside of the inner volume, wherein the reference ring electrode is selectively positioned at the base section of the distal tip assembly outside of the inner volume, and wherein the reference ring electrode is configured to couple with each of the plurality of functional electrodes for generating an electric field between each of the plurality of functional electrodes and the ring electrode; and to generate an AC signal between the reference ring electrode and each of the functional electrodes; receiving resulting electrical readings between a plurality of said functional electrodes and said reference ring electrode; estimating the respective impedances between the functional electrode and the reference ring electrode based on the electrical readings; and Based on the impedance, for at least one given functional electrode among the plurality of functional electrodes, a proximity of the functional electrode to the wall tissue is estimated.

12. The method according to claim 11, wherein: Generating the AC signal between the reference ring electrode and each of the functional electrodes includes generating an electric field between the reference ring electrode and an outer surface of each of the functional electrodes.

13. The method according to claim 11, wherein: Determining that the reference ring electrode is in physical contact with the wall tissue includes determining that measured impedances among the plurality of functional electrodes are all above a given threshold.

14. The method according to claim 11, wherein: Estimating the proximity includes using calibrated proximity data that converts between impedance and electrode-tissue proximity.

15. The method according to claim 11, wherein: The reference ring electrode is mounted on an outer periphery of the proximal base segment, and wherein the reference ring electrode is electrically insulated from the base segment.

16. The method according to claim 11, wherein: The distal tip assembly is a basket-shaped assembly having an expandable frame including a plurality of splines, and wherein the functional electrodes are coupled to the splines.

17. The method according to claim 11, wherein: The distal tip assembly is a balloon assembly having an expandable membrane, and wherein the functional electrode is disposed on the membrane.

18. The method of claim 11 and including using a mechanical guard ring located proximal to the reference ring electrode and protruding further outward from the proximal base section than the reference ring electrode to prevent contact between the reference ring and a tissue wall.

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