Abstract:
Apparatus, consisting of an insertion tube having a distal end configured for insertion into proximity with tissue in a body of a patient and containing a lumen having an electrical conductor for conveying electrical energy to the tissue. The apparatus further includes a conductive cap attached to the distal end of the insertion tube and coupled electrically to the electrical conductor, wherein the conductive cap has an outer surface. In addition there are a multiplicity of optical fibers contained within the insertion tube, each fiber terminating in proximity to the outer surface of the cap, and being configured to convey optical radiation to and from the tissue while the electrical energy is being conveyed to the tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/716,578, entitled “Irrigated Catheter Tip with Temperature Sensor Array,” filed Dec. 17, 2012, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to invasive medical devices, and particularly to probes used in ablating tissue within the body. 
       BACKGROUND 
       [0003]    Minimally-invasive intracardiac ablation is the treatment of choice for various types of arrhythmias. To perform such treatment, the physician typically inserts a catheter through the vascular system into the heart, brings the distal end of the catheter into contact with myocardial tissue in areas of abnormal electrical activity, and then energizes one or more electrodes at or near the distal end in order to create tissue necrosis. 
         [0004]    It has been found that cooling the area of the ablation site reduces tissue charring and thrombus formation. For this purpose, for example, Biosense Webster Inc. (Diamond Bar, Calif.) offers the ThermoCool® irrigated-tip catheter for use with its CARTO® integrated mapping and ablation system. The metal catheter tip, which is energized with radio-frequency (RF) electrical current to ablate the tissue, has a number of peripheral holes, distributed circumferentially around the tip, for irrigation of the treatment site. A pump coupled to the catheter delivers saline solution to the catheter tip, and the solution flows out through the holes during the procedure in order to cool the catheter tip and the tissue. 
         [0005]    U.S. Patent Application Publication 2010/0030209, whose disclosure is incorporated herein by reference, describes a catheter with a perforated tip, which includes an insertion tube, having a distal end for insertion into a body of a subject. A distal tip is fixed to the distal end of the insertion tube and is coupled to apply energy to tissue inside the body. The distal tip has an outer surface with a plurality of perforations through the outer surface, which are distributed circumferentially and longitudinally over the distal tip. A lumen passes through the insertion tube and is coupled to deliver a fluid to the tissue via the perforations. 
         [0006]    Some ablation catheters include sensors for monitoring temperature during the ablation procedure. For example, U.S. Pat. No. 5,957,961, whose disclosure is incorporated herein by reference, describes a catheter having a distal segment carrying at least one electrode extending along the segment and having a number of temperature sensors arranged along the distal segment adjacent the electrode, each providing an output indicative of temperature. The catheter is coupled to a power source, which provides RF energy to the electrode. Temperature processing circuitry is coupled to the temperature sensors and the power source, and controls power output from the power source as a function of the outputs of the temperature sensors. 
         [0007]    As another example, U.S. Pat. No. 6,312,425, whose disclosure is incorporated herein by reference, describes an RF ablation catheter tip electrode with multiple thermal sensors. A tip thermal sensor is located at or near the apex of the distal-end region, and one or more side thermal sensors are located near the surface of the proximal-end region. The electrode is preferably an assembly formed from a hollow dome-shaped shell with a core disposed within the shell. The side thermal sensor wires are electrically connected inside the shell and the core has a longitudinal channel for the side thermal sensor wires welded to the shell. The shell also preferably has a pocket in the apex of the shell, and the end thermal sensor wires pass through the core to the apex of the shell. 
         [0008]    It has also been found that contact between the ablation electrode and the tissue being ablated affects the efficacy of the ablation, so that methods for detection of tissue-electrode contact have been developed. For example, U.S. Pat. No. 6,217,574, whose disclosure is incorporated herein by reference, describes an irrigated split tip electrode catheter. A signal processor activates an RF generator to transmit a low level RF current to each electrode member of the split tip electrode. The signal processor receives signals indicative of the impedance between each electrode member and one or more surface indifferent electrodes and determines which electrode members are associated with the highest impedance. Such electrode members are stated to be those in greatest contact with the myocardium. 
         [0009]    As another example, U.S. Pat. No. 6,391,024, whose disclosure is incorporated herein by reference, describes a method of assessing the adequacy of contact between an ablation electrode and biological tissue. The method measures the impedance between an ablation electrode and a reference electrode at a first and second frequencies. A percentage difference between the first-frequency impedance and the second-frequency impedance is stated to provide an indication of the state of electrode/tissue contact. 
         [0010]    As yet another example, U.S. Pat. No. 6,730,077, whose disclosure is incorporated herein by reference, describes a cryocatheter for treatment of tissue. A signal conductor extends through the catheter to the catheter tip and connects to a thermally and electrically conductive shell or cap that applies an RF current to the region of tissue contacted by the tip. A tissue impedance path between the signal lead and a surface electrode mounted on the patient&#39;s skin is monitored to develop a quantitative measure of tissue contact at the distal tip, 
         [0011]    Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
       SUMMARY 
       [0012]    An embodiment of the present invention provides apparatus, consisting of: 
         [0013]    an insertion tube having a distal end configured for insertion into proximity with tissue in a body of a patient and containing a lumen having an electrical conductor for conveying electrical energy to the tissue; 
         [0014]    a conductive cap attached to the distal end of the insertion tube and coupled electrically to the electrical conductor, wherein the conductive cap has an outer surface; and
       a multiplicity of optical fibers contained within the insertion tube, each fiber terminating in proximity to the outer surface of the cap, and being configured to convey optical radiation to and from the tissue while the electrical energy is being conveyed to the tissue.       
 
         [0016]    Typically the apparatus includes a plurality of temperature sensors, which are mounted within the conductive cap in thermal communication with the outer surface. 
         [0017]    In a disclosed embodiment the outer surface is penetrated by multiple apertures, and the conductive cap defines an inner cavity in fluid communication with the lumen of the insertion tube so as to permit irrigation fluid from the lumen to flow out of the cap through the apertures. 
         [0018]    The cap may have a side wall having a multiplicity of longitudinal bores therein, and the fiber optics are inserted into the bores. The apparatus may also have a multiplicity of windows, transparent to the optical radiation, located in the outer surface of the cap and connected to respective longitudinal bores so as to seal the bores from penetration of fluid into the bores. Typically, at least one of the windows is formed from at least one of a transparent epoxy and a glue. The apparatus may include a scattering agent mixed with at least one of the windows. Typically, at least one of the windows is formed from at least one of optical grade flat material and optical grade lensed material. 
         [0019]    In a further disclosed embodiment a given optical fiber selected from the multiplicity of optical fibers consists of a single optical fiber. Alternatively or additionally, a given optical fiber selected from the multiplicity of optical fibers consists of an optical fiber bundle. 
         [0020]    In a yet further disclosed embodiment the apparatus has an optical module configured to determine contact of the conductive cap with the tissue in response to measuring a first level of the optical radiation conveyed via a given optical fiber to the tissue and a second level of the optical radiation conveyed via the given optical fiber from the tissue. 
         [0021]    In an alternative embodiment the apparatus includes an optical module configured to determine a characteristic of the tissue in response to measuring a first level of the optical radiation conveyed via a first optical fiber to the tissue and a second level of the optical radiation conveyed via a second optical fiber from the tissue. Typically, the characteristic includes a wall thickness of the tissue. 
         [0022]    In a further alternative embodiment the apparatus includes a power generator, coupled to provide the electrical energy to the conductive cap so as to ablate the tissue, and an optical module configured to determine a change in a level of the optical radiation while the tissue is ablating. 
         [0023]    There is further provided, according to an embodiment of the present invention, a method, including: 
         [0024]    inserting a distal end of an insertion tube into proximity with tissue in a body of a patient; 
         [0025]    forming a lumen, including an electrical conductor for conveying electrical energy to the tissue, within the insertion tube; 
         [0026]    attaching a conductive cap, having an outer surface, to the distal end of the insertion tube; 
         [0027]    coupling the conductive cap electrically to the electrical conductor; and 
         [0028]    locating a multiplicity of optical fibers within the insertion tube, each fiber terminating in proximity to the outer surface of the cap, and being configured to convey optical radiation to and from the tissue while the electrical energy is being conveyed to the tissue. 
         [0029]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0030]      FIG. 1  is a schematic, pictorial illustration of a system for intracardiac ablation, in accordance with an embodiment of the present invention; 
           [0031]      FIG. 2A  is a schematic, sectional view of a catheter tip, in accordance with an embodiment of the present invention; 
           [0032]      FIG. 2B  is a schematic, cross-sectional view of the catheter tip of  FIG. 2A ; 
           [0033]      FIG. 3  is a schematic, sectional view of a catheter tip, in accordance with another embodiment of the present invention; 
           [0034]      FIG. 4A  is a schematic, pictorial illustration of a catheter cap, in accordance with yet another embodiment of the present invention; 
           [0035]      FIG. 4B  is a schematic end view of the catheter cap of  FIG. 4A ; 
           [0036]      FIG. 4C  is a schematic, sectional view of the catheter cap of  FIGS. 4A and 4B ; and 
           [0037]      FIGS. 5A-5E  are schematic illustrations of a catheter cap, and of light paths to/from the cap, according to an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0038]    Intracardiac ablation procedures are characterized by rapid temperature changes and non-uniform temperature distribution in the tissue and its vicinity. Therefore, the temperature measured by a sensor at the tip of an ablation catheter may not accurately reflect the actual, current temperature in the tissue. Furthermore, when a temperature sensor in a catheter is washed by irrigation fluid, the temperature reading will reflect the fluid temperature, which is generally far cooler than the tissue temperature outside the catheter. 
         [0039]    Some embodiments of the present invention that are described hereinbelow provide irrigated ablation electrodes with embedded temperature sensors that provide accurate tissue temperature assessment. Such electrodes typically comprise a conductive cap, which is attached to the distal tip of the insertion tube of an invasive probe, such as a cardiac catheter. A cooling fluid flows out through an array of perforations in the electrode to irrigate the tissue under treatment. 
         [0040]    The temperature sensors are mounted at different locations in proximity to the outer surface of the electrode. The electrode is constructed so that the sensors are in proximity to and thermal communication with the outer surface, and are thermally insulated from, rather than immersed in, the cooling fluid within the probe. The sensors thus provide multiple temperature readings that are substantially independent of the cooling fluid temperature, at different locations on the tip electrode. 
         [0041]    Typically, the sensor that gives the highest temperature reading is the one that is in contact with the tissue being ablated, and the temperature measured by this sensor varies linearly with the actual tissue temperature. (Flow of the cooling fluid through the perforations in the electrode is generally lowest in areas that are in firm contact with the tissue, and the sensors in these areas typically give the highest temperature readings.) The reading from this hottest sensor may thus be used in particular to monitor the tissue temperature and control the applied power and duration of the ablation procedure in order to obtain the desired therapeutic result without excessive tissue damage. Alternatively or additionally, the temperature readings of the multiple sensors can be combined and interpolated to give a map of temperature over the area of the catheter tip. 
         [0042]    Some embodiments of the present invention incorporate a multiplicity of optical fibers terminating in proximity to the outer surface of the conductive cap. The optical fibers may transmit optical radiation to the tissue being ablated, and also receive returning optical radiation from the tissue. Measurements of levels of returning radiation at all or some of the fiber optics enable embodiments of the present invention to determine if the conductive cap is in contact with the tissue, as well as to characterize the tissue being irradiated by the radiation. 
         [0043]    Although the disclosed embodiments relate specifically to intracardiac catheters and ablation procedures, the principles of the present invention may similarly be applied, mutatis mutandis, to probes of other types, for use in substantially any sort of invasive thermal treatment. 
         [0044]      FIG. 1  is a schematic pictorial illustration of a system  20  for cardiac ablation treatment, in accordance with an embodiment of the present invention. An operator (such as an interventional cardiologist) inserts a catheter  22  via the vascular system of a patient  26  into a chamber of the patient&#39;s heart  24 . For example, to treat atrial fibrillation, the operator may advance the catheter into the left atrium and bring a distal end  30  of the catheter into contact with myocardial tissue that is to be monitored and/or ablated. 
         [0045]    Catheter  22  is connected at its proximal end to a console  32 , which is controlled by operator  28  to apply and monitor the desired treatment. Console  32  comprises an RF energy generator  34 , which supplies electrical power via catheter  22  to distal end  30  in order to ablate the target tissue. Monitoring circuitry  36  tracks the temperature of the tissue at distal end  30  by processing the outputs of temperature sensors in the distal end, as described below. An irrigation pump  38  supplies a cooling fluid, such as saline solution, through catheter  22  to irrigate distal end  30 . In addition, an optical module  40  provides optical radiation, typically from, but not limited to, a laser, an incandescent lamp, an arc lamp, or a light emitting diode (LED), for transmission from distal end  30  to the target tissue. The module receives and analyzes optical radiation returning from the target tissue and acquired at the distal end, as described below. 
         [0046]    On the basis of information provided by monitoring circuitry  36  and optics module  40 , console  32  may control the power applied by RF energy generator  34  and/or the flow of fluid provided by pump  38 , either automatically or in response to inputs by operator  28 . 
         [0047]    System  20  may be based on the above-mentioned CARTO system, for example, which provides extensive facilities to support navigation and control of catheter  22 . These system facilities, however, including details of the monitoring and control functions of monitoring circuitry  36  and console  32  generally, are beyond the scope of the present patent application. 
         [0048]      FIGS. 2A and 2B  schematically illustrate distal end  30  of catheter  22 , in accordance with an embodiment of the present invention.  FIG. 2A  is a sectional view along the length of the catheter, while  FIG. 2B  is a cross-sectional view along the cut IIB-IIB that is marked in  FIG. 2A . An insertion tube  42  extends along the length of the catheter and is connected at its distal end to a conductive cap  44 . Typically, insertion tube  42  comprises a flexible, biocompatible polymer, while cap  44  comprises a biocompatible metal suitable to serve as an ablation electrode, such as gold or platinum, for example. Cap  44  is perforated by an array of irrigation apertures  46 , which is open from the outer surface of the cap into an inner cavity  58  within the cap. For typical intracardiac ablation applications, the diameter of cap  44  may be about 2.5 mm, with a wall thickness of about 0.2 mm and apertures  46  of diameter 0.1-0.2 mm. The above dimensions and materials are described by way of example, however, and other suitable materials, with features of larger or smaller dimensions, may similarly be used. 
         [0049]    Cavity  58  is in fluid communication with a lumen  54 , which runs through the length of insertion tube  42 . Lumen  54  is coupled at its proximal end to irrigation pump  38 , and thus conveys irrigation fluid to cavity  58 , from which the fluid flows out through apertures  46 . An electrical conductor  56  conveys electrical energy from RF generator  34 , through insertion tube  42 , to cap  44 , and thus energizes the cap to ablate myocardial tissue with which the cap is in contact. During ablation, the fluid flowing out through apertures  46  irrigates the tissue under treatment. 
         [0050]    Temperature sensors  48  are mounted within conductive cap  44  at locations that are arrayed around the distal tip of the catheter, both axially and circumferentially. In this example, cap  44  contains six sensors, with one group in a distal location, close to the tip, and the other group in a slightly more proximal location. This distribution is shown only by way of example, however, and greater or smaller numbers of sensors may be mounted in any suitable locations within the cap. Sensors  48  may comprise thermocouples, thermistors, or any other suitable type of miniature temperature sensor. These sensors are connected by leads  52  running through the length of insertion tube  42  to provide temperature signals to monitoring circuitry  36 . 
         [0051]    Temperature sensors  48  are mounted within ribs  50  inside cap  44 . The ribs are typically an integral part of cap  44  and may be made from the same material as the outer surface of the cap or from some other suitable type of metal, which is physically and thermally bonded to the cap. The diameter of the ribs may be a few tenths of a millimeter in the present example. The integral construction of ribs  50  with cap  44  causes sensors  48  to be in thermal communication with the outer surface of the cap, i.e., the temperature inside ribs  50  closely tracks the temperature of the outer surface. The ribs are thick enough to thermally insulate these sensors from the irrigation fluid in cavity  58 . As a result, temperature sensors  48  measure the true temperature of the outer surface of cap  44 , which most accurately reflects the temperature of the tissue with which the cap is in contact. 
         [0052]    Typically, distal end  30  contains other functional components, which are outside the scope of the present disclosure and are therefore omitted for the sake of simplicity. For example, the distal end of the catheter may contain steering wires, as well as sensors of other types, such as a position sensor and/or a contact force sensor. A catheter containing sensors of these sorts is described, for example, in U.S. Patent Application Publication 2009/0138007, whose disclosure is incorporated herein by reference. 
         [0053]      FIG. 3  is a schematic sectional view of distal end  30 , in accordance with another embodiment of the present invention. Elements of this embodiment that are similar to corresponding elements in the embodiment of  FIGS. 2A and 2B  are marked with the same indicator numbers. In the embodiment of  FIG. 3 , a conductive, perforated cap  64 , attached to the distal end of insertion tube  42 , is designed to have very low thermal capacity, and sensors  48  are held in contact with cap  64 . As a result of this configuration, the temperature of cap  64  more closely track changes in the actual tissue temperature, and sensors  48  more closely track the temperature of the outer surface of cap  64 . Sensors  48  thus provide a more accurate, timely indication of changes in the temperature of the tissue with which cap  64  is in contact. 
         [0054]    As illustrated in  FIG. 3 , cap  64  contains an inner wall  60 , which is not perforated, in close proximity and parallel to the cap. Lumen  54  supplies irrigation fluid to a cavity  66  that is formed between cap  64  and wall  60 , and the irrigation fluid exits this cavity through apertures  46  in cap  64 . Typically, cap  64  and wall comprise thin shells of metallic material and are held apart by small metallic spacers  62 , around which the fluid is able to flow within cavity  66 . These spacers may be distributed within cap in any suitable arrangement, for example in pairs (like the pair shown in  FIG. 3 ) of axially-spaced sensors at different circumferential locations. Spacers  62  also hold temperature sensors  48  in thermal communication with the outer surface of cap  64 , while insulating the sensors from the surrounding irrigation fluid in cavity  66 . Even without the insulating effect of spacers  62 , the effect of the irrigation fluid temperature on sensors  48  in this embodiment is minimal due to the small volume of cavity (relative to cavity  58  in the preceding embodiment, for example). 
         [0055]    In a configuration suitable for intracardiac ablation, cap  64  has an outer diameter of about 2.5 mm and a similar length. The thickness of both cap  64  and wall  60  is about 100 μm, while apertures  46  have a diameter in the range of 25-100 μm. Although cap  64  and wall  60  are very thin, the mechanical integrity of the entire structure is maintained by connecting the cap and wall together with spacers  62 . 
         [0056]      FIGS. 4A-4C  schematically illustrate a catheter cap  70 , in accordance with yet another embodiment of the present invention. Cap  70  may be used at distal end  30  of catheter  22  in place of the caps shown in the preceding embodiments.  FIG. 4A  is a schematic, pictorial illustration of cap  70 , while  FIG. 4B  is a schematic end view showing the interior of the cap, and  FIG. 4C  is a sectional view taken along the line IVC-IVC in  FIG. 4B . 
         [0057]    Cap  70  comprises a side wall  74  that is relatively thick, on the order of 0.4 mm thick, in order to provide the desired thermal insulation between temperature sensors  48  and the irrigation fluid inside a central cavity  76  of the tip. As in the preceding embodiments, the irrigation fluid exits cavity  76  through apertures  46 . Sensors  48  are mounted in hollow tubes  78 , which are filled with a suitable glue, such as epoxy and fitted into longitudinal bores  72  in side wall  74 . Tubes  78  may comprise a suitable plastic material, such as polyimide, and may be held in place by a suitable glue, such as epoxy. This arrangement provides an array of six sensors as in the preceding embodiments, with possible advantages of greater ease of manufacture and durability. 
         [0058]      FIGS. 5A-5D  schematically illustrate a catheter cap  100 , and  FIG. 5E  schematically illustrates paths taken by light to/from windows in the cap, in accordance with an alternative embodiment of the present invention. Apart from the differences described below, the operation of cap  100  is generally similar to that of cap  70  ( FIGS. 4A ,  4 B, and  4 C), and elements indicated by the same reference numerals in both caps  70  and  100  are generally similar in construction and in operation.  FIG. 5A  is a schematic, perspective illustration of cap  100 ,  FIG. 5B  is a schematic end view showing the interior of the cap,  FIG. 5C  is a schematic sectional view taken along the line VC-VC in  FIG. 5B , and  FIG. 5D  is a schematic sectional view taken along the line VD-VD in  FIG. 5B . 
         [0059]    In addition to the three longitudinal bores  72  described above for cap  70 , wherein sensors  48  mounted in tubes  78  are fitted, in cap  100  three through longitudinal bores  102 , and three blind longitudinal bores  106  are formed in side wall  74 . As is illustrated in  FIG. 5B , the three sets of bores  72 ,  102 , and  106  may be distributed symmetrically around an axis  110  of cap  100 . However, the bores are not necessarily distributed symmetrically around axis  110 . 
         [0060]    Each through longitudinal bore  102  terminates in a an opening  114  in the outer surface of wall  74 , and a transparent window  116  is placed in the opening. A fiber optic  118  is inserted into each of the through bores. There is a respective opening  120 , in the outer surface of wall  74 , to each blind bore  106 , and a transparent window  124  is placed in each opening  120 . A fiber optic  128  is inserted into each of the blind bores. Windows  116  and  124  act as seals preventing fluid external to the outer surface of cap  70  from penetrating into the bores containing the fiber optics. Windows  116  and  124  may be formed by filling openings  114  and  120  with an optically transparent glue or epoxy. In some embodiments the material of the windows may be filled with a scattering agent to diffuse light passing through the windows. Alternatively, the windows may be formed from an optical quality flat or lensed material, and may be secured to their openings with glue. 
         [0061]    In a disclosed embodiment each fiber optic  118  or each fiber optic  128  is a single fiber optic, typically having a diameter of approximately 175 μm. In an alternative disclosed embodiment each fiber optic  118  or each fiber optic  128  comprises a bundle of substantially similar fiber optics, typically having a bundle diameter also of approximately 175 μm. Implementing the fiber optics as bundles increases the flexibility of cap  100  with respect to more proximal regions of catheter  22 . 
         [0062]    Such an increase in flexibility is advantageous if cap  100  is connected to the more proximal regions of the catheter by a spring whose deflections are measured for the purpose of measuring a force on the cap, since the increased flexibility means there is little or no change in the spring deflection for a given force. A spring which may be used to join the cap  100  to the more proximal regions of the catheter is described in U.S. patent application Ser. No. 12/627,327, to Beeckler et al., whose disclosure is incorporated herein by reference. 
         [0063]    Optical module  40  is configured to be able to provide optical radiation to any one of fiber optics  118  and  128 , for transmission from any of the associated windows  116  and  124  so as to irradiate tissue in proximity to cap  100 . Simultaneously, the module is able to acquire, via any or all of the windows, radiation returning from the irradiated tissue. 
         [0064]    The array of windows  116  and  124 , and their associated fiber optics, enables embodiments of the present invention to employ a number of different methods, using optical radiation, for determining characteristics of the irradiated tissue, as well as the proximity of cap  100 , or a region of the cap, with respect to the tissue. By way of example three such methods are described below, but those having ordinary skill in the art will be aware of other methods, and all such methods are included within the scope of the present invention. 
         [0065]    A first method detects contact of any one of windows  116  or  124 , and consequently of the catheter, with tissue. Optical radiation, of a known intensity, is transmitted through each fiber optic, so as to exit from the optic&#39;s window. The intensity of the radiation returning to the window is measured while cap  100  is not in contact with tissue, typically while the cap is in the blood of heart  24 . Optical monitor  40  may use these intensities as reference values of the optical radiation. For any given window, a change in the value from the window&#39;s reference value, as measured by the module, may be taken to indicate that the window is in contact with tissue. 
         [0066]    A second method measures characteristics of tissue being irradiated by the optical radiation. As illustrated in  FIG. 5E , for all six windows  116 ,  124  there are a total of 21 different paths, comprising 6 paths  150  where radiation from a given window returns to that window, and 15 paths  160  where radiation from a given window returns to a different window. The change of optical radiation for a given path or group of paths depends on characteristics of tissue in the path or group of paths, so that measurements of the change in all of the paths provide information related to characteristics of the tissue in proximity to cap  100 . 
         [0067]    For example, the change in all of the paths may be measured by sequentially transmitting, in a time multiplexed manner, optical radiation from each of windows  116  and  124 , and measuring the returning radiation. A first transmission from a first window in such a sequence provides values for five paths  160  plus a return path  150  to the first window, a second transmission from a second window provides values for four new paths  160  plus a return path  150  to the second window, . . . a fifth transmission from a fifth window provides values for two new paths (a path  160  to the sixth window, and a return path  150  to the fifth window). A sixth and final transmission from a sixth window provides one return path  150  through the sixth window. 
         [0068]    Optical module  40  may measure the changes of all the paths, and, typically using a calibration procedure, may derive from the changes optical characteristics of tissue within the paths. Such characteristics may include an overall level of ablation of tissue, or an amount and/or type of necrotic tissue, in the paths. 
         [0069]    A third method uses changes of levels of optical radiation returning to windows  116  and/or  124 , such as are described in the two methods, to make an estimate of the wall thickness of tissue being illuminated by the optical radiation. 
         [0070]    Although a number of particular implementation examples have been shown and described above, alternative implementations of the principles embodied in these examples will be apparent to those skilled in the art after reading the foregoing description and are considered to be within the scope of the present invention. 
         [0071]    For example, in one embodiment, temperature sensors  48  may not be installed in wall  74 , and only fiber optics  118  are incorporated into the wall. Such an embodiment enables determination of tissue contact with the cap, and/or characterization of the tissue in proximity to the cap, by methods described above. 
         [0072]    It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.