Patent Publication Number: US-8116884-B2

Title: Miniature circular mapping catheter

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/891,065, filed Aug. 8, 2007, issued as U.S. Pat. No. 7,860,578 on Dec. 28, 2010, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND ART 
     The present invention relates to apparatus and methods for cardiac ablation and to miniature sensor structures useful in such apparatus and methods. 
     Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or “AF.” Certain cardiac arrhythmias can be treated by deliberately damaging the tissue along a path crossing a route of abnormal conduction, either by surgically cutting the tissue or by applying energy or chemicals to the tissue, so as to form a scar. The scar blocks the abnormal conduction. For example, in treatment of AF, it has been proposed to ablate tissue in a partial or complete loop around a pulmonary vein, within the ostium or opening connecting the vein to the heart, or within the wall of the heart surrounding the ostium. It would be desirable to perform such ablation using a catheter-based device which can be advanced into the heart through the patient&#39;s circulatory system. 
     As described in commonly assigned U.S. Pat. No. 6,635,054, the disclosure of which is incorporated by reference herein, an expansible structure is used as a reflector for directing and focusing ultrasonic waves from an ultrasonic transducer into a region of tissue to be ablated. This arrangement can be used, for example, to treat atrial fibrillation by ablating a circular region of myocardial tissue encircling the ostium of a pulmonary vein. The ablated tissue forms a barrier to abnormal electrical impulses which can be transmitted along the pulmonary veins and, thus, isolates the myocardial tissue of the atrium from the abnormal impulses. As disclosed in commonly assigned U.S. Provisional Patent Application Ser. No. 60/448,804, filed Feb. 20, 2003, and in commonly assigned, co-pending U.S. Published Patent Application No. 2004/0176757 (hereinafter “the &#39;757 application”), issued as U.S. Pat. No. 7,837,676 on Nov. 23, 2010, and PCT International Application No. PCT/US04/05197, the disclosures of which are incorporated by reference herein, a catheter-carried expansible ablation structure as disclosed in the &#39;054 patent can be equipped with a steering mechanism so that the orientation of the expansible structure relative to the heart can be controlled by the physician without relying upon physical engagement with the pulmonary vein or pulmonary vein ostium. 
     It is often desirable to monitor electrical signals propagating within the heart. For example, McGee et al., U.S. Pat. No. 5,860,920, discloses a structure incorporating an elongated element with numerous electrodes disposed along a distal region of the structure. The structure is advanced into the heart within a guide tube or sheath, which is then retracted so as to expose the distal region. In this condition, the distal region, under its own resilience, forms itself into a hoop shape, which can be pressed into engagement with a region of the heart wall as, for example, a region surrounding the bicuspid valve or the mitral valve. The electrodes pick up electrical signals propagating within the heart. The electrodes can be connected to a source of electrical energy, so that the electrical energy applied through the electrodes ablates the cardiac tissue. Swanson et al., U.S. Pat. No. 5,582,609, discloses another loop-forming structure carrying electrodes for electrical ablation. Fuimaono et al., U.S. Pat. No. 6,628,976, discloses a catheter with a similar loop-like structure said to be useful in mapping electrical activity or “wavelets” within a pulmonary vein, coronary sinus or other “tubular structure” prior to treatment of the condition. 
     Marcus et al., U.S. Pat. No. 5,295,484, discloses a catheter carrying both an ultrasonic transducer and electrodes for sensing electrical potentials within the heart. These electrodes can be used to allow the physician to determine whether the arrhythmia has persisted after the ablation process. Also, the aforementioned &#39;054 patent and &#39;054 patent disclose, in certain embodiments, expansible balloon structures having ring-like electrodes thereon for detecting electrical signals within the heart. 
     Despite all of these efforts in the art, however, still further improvement would be desirable. Particularly, providing sensing structures that prevent damage to the tissue and are capable of passing through a lumen smaller than one millimeter is desirable. Providing electrical sensing structures on a balloon-like or other expansible ablation device complicates fabrication of the device and makes it more difficult to make the device collapse to a small diameter for advancing or withdrawing the device through the vascular system. Further, mounting the electrodes on the same catheter as an ultrasonic transducer, as disclosed in the &#39;484 patent, limits placement of the electrodes and the configuration of the transducer array and associated structures. The particular structures shown in the &#39;484 patent, for example, are not well suited to formation of a ring-like lesion or sensing of electrical potentials at numerous locations. Use of a loop-forming sensing element entirely divorced from an ablation device, as contemplated in U.S. Pat. No. 6,628,976, necessarily requires separate steps for placement of such a device which adds both complexity and risk to the procedure. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides apparatus for cardiac treatment which includes a catheter having proximal and distal ends and a lumen, as well as an expansible ablation device mounted at or near the distal end of the catheter. The ablation device has a collapsed condition and an expanded condition, and is operative to apply energy to cardiac tissues in proximity to the device when the device is in the expanded condition. In its expanded condition, the device and catheter define a port open to the exterior of the expansible ablation device on the distal side of the device. Desirably, the ablation device defines a bore extending through the ablation device. The bore has a first end communicating with the lumen and a second end defining the port. 
     Apparatus according to this aspect of the invention desirably also includes an elongated sensor probe which also has proximal and distal ends. The sensor probe includes one or more electrodes disposed adjacent the distal end of the sensor probe. The lumen and the ablation device are constructed and arranged so that the sensor probe can be removably positioned in the passageway, with the distal end of the sensor probe projecting out of the ablation device through the port. The sensor probe has a distal section and a floppy section formed in the distal section. The distal most tip of the sensor probe has a ball formed thereon. The floppy section has a wire core and a polymeric tube covering the wire core. The polymeric tube is made from a soft material such as a thermoplastic elastomer or a polyether block amide and has a low durometer value, for example, 35-72 Shore D. An example of the soft material is Pebax®. The wire and the polymeric tube are in engagement in the area near the ball such that there is no relative motion between them in the area near the ball. 
     A further aspect of the invention provides methods of cardiac ablation which include the steps of advancing an apparatus including a catheter and an expansible ablation device into the subject while the ablation device is in a collapsed condition, until the ablation device is disposed in a chamber of the subject heart, and then expanding the ablation device to an expanded condition. In a method according to this aspect of the invention, the ablation device desirably is positioned in a desired disposition relative to the heart and actuated to apply energy in a loop-like region having a predetermined spatial relationship to the ablation device, and thereby ablate the tissue in this region so as to form a lesion. Methods according to the invention desirably further include the step of advancing a sensing probe with atraumatic tip having a floppy section with a ball formed at the leading edge, through a continuous passageway from the proximal end of the catheter through the ablation device, so that a distal region of the sensing probe projects out of a port on the ablation device and contacts tissue of the subject adjacent the ablation device. In methods according to this aspect of the invention, the ablation device desirably at least partially positions the projecting distal region of the sensing probe relative to the heart. The method desirably further includes the step of detecting electrical signals in the subject using the sensing probe. Methods according to this aspect of the invention afford advantages similar to those discussed above in connection with the apparatus. 
     Yet another aspect of the invention provides a probe having a proximal end and a distal end. A ball is formed at the tip of the distal end. A floppy section is attached to the ball. The floppy section has a wire core and a polymeric tube covering the wire core. The polymeric tube is made from a soft material such as Pebax®. The wire core and the polymeric tube are in engagement in the area near the ball such that there is no relative motion between them in the area near the ball. In its deployed condition, the probe body desirably is hoop shaped. The hoop desirably carries one or more of the functional elements such as the electrodes. 
     Yet another aspect of the invention provides a probe which includes a catheter having a proximal end and a distal end. The distal end when unconstrained form a circular loop and an arcuate stem attached to the circumference of the loop. The arcuate stem has a maximum first radius about same size as the diameter of the loop and a minimum second radius equal to about one fourth of the diameter of the loop. The first radius is measured by projecting the stem in a plane that is perpendicular to the plane of the loop and the second radius is measured by projecting the stem in the plane of the loop. The loop desirably carries one or more of the functional elements such as the electrodes. 
     These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic, partial sectional view of apparatus in accordance with one embodiment of the invention during one stage of a method in accordance with an embodiment of the invention. 
         FIG. 2  is a fragmentary, diagrammatic perspective view depicting components of the apparatus shown in  FIG. 1  at another stage in the method. 
         FIG. 3  is a diagrammatic sectional view taken along line  3 - 3  in  FIG. 2 . 
         FIG. 4  is a diagrammatic perspective view of the components illustrated in  FIG. 2  during another stage in the method. 
         FIG. 5  is a diagrammatic elevational view of a probe in accordance with a second embodiment of the invention. 
         FIG. 6  is a fragmentary diagrammatic sectional view depicting a portion of the probe shown in  FIG. 5 . 
         FIG. 7  is a fragmentary diagrammatic sectional view depicting a further portion of the probe shown in  FIG. 5 . 
         FIG. 8  is a diagrammatic perspective view depicting a portion of the probe shown in  FIG. 5 . 
         FIG. 9  is a diagrammatic elevational view of a probe in accordance with a third embodiment of the invention. 
         FIG. 10  is a schematic view of the atraumatic tip of the third embodiment. 
         FIG. 11  shows an elevational view of the hoop region of the third embodiment. 
         FIG. 12  shows a side view of the hoop region of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As seen in  FIG. 1 , apparatus according to one embodiment of the invention includes an insertable structure  10  incorporating an elongated catheter  12  having a proximal end  14 , which remains outside of the body, and a distal end  16  adapted for insertion into the body of the subject. As used in this disclosure with reference to structures which are advanced into the body of a subject, the “distal” end of such a structure should be taken as the end which is inserted first into the body and which penetrates to the greatest depth within the body, whereas the proximal end is the end of the structure opposite to the distal end. Insertable structure  10  also includes an ablation unit  18  mounted to catheter  12  adjacent distal end  16 . Ablation unit  18  incorporates a reflector balloon  20  and a structural balloon  22  having a common wall  24 . Reflector balloon  20  is linked to an inflation lumen (not shown) in catheter  12 , which extends to the proximal end of catheter  12  and which is connected, during use, to a source of a gas under pressure, such as air or, more preferably, carbon dioxide, as, for example, to a gas-filled hypodermic syringe, so that reflector balloon  20  can be inflated with a gas. Structural balloon  22  is connected through a separate inflation lumen (not shown) to a source of a liquid such as isotonic saline solution, so that structural balloon  22  can be inflated with the liquid. A cylindrical ultrasonic emitter  23  is mounted within structural balloon  22 . Balloons  20  and  22 , and particularly the common wall  24  separating balloons  20  and  22 , are designed so that in their inflated, operative condition illustrated in  FIG. 1 , balloons  20  and  22  are in the form of bodies of revolution about a central or forward-to-rearward axis  26 . Emitter  23  is cylindrical and is coaxial with balloons  20  and  22 . 
     A tube defining a bore  28  extends through structural balloon  22  at central axis  26 . Tube bore  28  communicates with a port  29  on or forward of a forward wall  38  of structural balloon  22 . Tube bore  28  also communicates with a lumen  30  within catheter  12 . Lumen  30  extends to proximal end  14  of catheter  12  and is provided with a suitable fluid connection such as a Luer hub. Tube bore  28  and lumen  30  together form a continuous passageway extending from outlet port  29 , just distal to the ablation device back to proximal end  14  of catheter  12 . The tube defining bore  28  may be formed from a material such as an expanded polymer of the type commonly used in vascular grafts, so that the interior bore  28  of the tube remains patent when the tube is stretched. 
     The common wall  24  separating balloons  20  and  22  forms an active, reflective interface. This active interface desirably has the form of a surface of revolution of a parabolic section around central axis  26 . When balloons  20  and  22  are in their inflated, expanded configuration shown in  FIG. 1 , ultrasonic waves emitted by emitter  23  are directed radially outwardly away from axis  26  and impinge on the parabolic active interface formed by common wall  24 , where it is reflected forwardly and slightly outwardly away from axis  26  and focused so that the ultrasonic waves emitted along various paths mutually reinforce within a ring-like ablation region A, just forward of forward wall  38  of structural balloon  22  encircling axis  26 . The focused ultrasonic waves in this region can effectively ablate myocardial tissue and form a substantial conduction block extending through the heart wall in a relatively short time, typically about a minute or less. The ablation unit  18  can be selectively steered to position it in a desired orientation for ablating the tissue in a desired location. 
     The apparatus further includes an elongated sensor probe  72  ( FIG. 2 ) having a proximal end  74  and a distal end  76 . Probe  72  includes an elongated resilient body having a diameter smaller than the inside diameter of lumen  30 . A plurality of electrodes  80  ( FIGS. 2 and 4 ) extend around resilient probe body  78  in an operative region  82  adjacent distal end  76  of probe  72 , also referred to as a “hoop region.” The probe body  78  also includes a juncture region  84  disposed proximal to operative region  82  and a main or base region  86  extending from juncture region  84  to proximal end  74  of probe  72 . A connector  88  is provided at proximal end  74  of probe  72 . As best seen in  FIG. 3 , resilient body  78  includes a dielectric layer  90  and conductors  92  extending within dielectric layer  90  from electrodes  80  to connector  88  ( FIG. 2 ). Probe body  78  also includes a resilient metallic element  94  extending along the length of probe body  78 . Element  94  may be a super elastic material such as an alloy of nickel and titanium, commonly referred as nitinol. The particular arrangement depicted in  FIG. 3 , with conductors  92  disposed around resilient element  94 , is not essential. For example, conductors  92  may be grouped to one side of elastic element  94 . Probe  72  can be placed into the continuous passageway defined by the catheter lumen  30  and bore  28 , and can be removed therefrom. 
     In its free or unconstrained condition, resilient body  78  assumes the shape depicted in  FIG. 4 . In this shape, hoop region forms a generally circular hoop  82  in a plane transverse to an axis  96  defined by the distal-most part of base region  86 . Hoop  82  encircles axis  96  and is substantially coaxial therewith. The connecting or transition region  84  extends outwardly from axis  96  and also slopes slightly in the forward or distal direction. As discussed below, the probe body is in this unconstrained state when deployed within the heart, and accordingly, this condition is also referred to herein as the “deployed” condition of the probe body. The distal portion of the hoop region may be of an atraumatic design similar to one shown in  FIG. 10  and described in detail hereafter. In a variation of the embodiment shown in  FIG. 4 , the hoop region and juncture region  84  may have the shape described in detail hereafter and depicted in  FIGS. 11 and 12 . 
     A sensor probe in accordance with a second embodiment of the invention has a composite body  200  ( FIG. 5 ) including a tubular metallic shaft section  202  which may be formed from a stainless steel tube of the type commonly used to form hypodermic needles. Shaft section  202  defines an interior bore  203  ( FIG. 6 ). Desirably, shaft section  202  has an outside diameter D s  ( FIG. 6 ) on the order of 1.25 mm or less, more desirably about 1 mm (0.040 inches) or less, and most preferably about 0.9 mm (0.035 inches). Preferably, shaft section  202  extends throughout the majority of the length of probe  200 . For example, shaft section  202  may be on the order of 140 cm (55 inches) long. 
     A distal section  206  is mounted to the distal end  204  of shaft section  202 . Distal section  206  includes a wire core  210  ( FIG. 7 ) with a dielectric, biologically inert polymeric covering  212  overlying the core. The core desirably is formed from a metal such as a nickel-titanium alloy which can be formed to a preselected shape and which will tend to return to the preselected shape when unconstrained. A plurality of electrodes  216  overlie covering  212  in a portion of the length of distal section  206 . As best seen in  FIG. 8 , this portion  208  of distal section  206  in its free or unconstrained condition forms a hoop lying in a plane transverse to the axis defined by the remainder of distal section  206 . In the free or unconstrained condition, distal  206  section may extend about 5 cm (2 inches) proximally from the plane of the hoop. Distal section  206  is formed to this hoop shape and tends to return to this shape when distal section  206  is unconstrained. However, distal section  206  is quite flexible, and hence, can be constrained to a straight or gently curving shape, so as to be advanced through the passageway defined by the catheters and ablation devices discussed above. Shaft section  202  is flexible enough to pass through gently curving portions of the catheter but is considerably stiffer than distal section  206 . 
     The proximal end of distal section  206  abuts distal end  204  of shaft section  202  and is bonded to shaft section  202 . Desirably, wire core  210  extends into bore  203  of shaft section  202  a short distance from this abut joint. A plurality of fine insulated wires  220  are disposed within bore  203  of shaft section  202 . These wires  220  are electrically connected to electrodes  216  on distal section  206 . The probe body also includes a proximal section  222  and a transition section  224  extending from proximal section  222  to proximal end  226  of shaft section  202 . The proximal end section may include a relatively stiff polymeric tube having an interior bore (not shown). Transition section  224  may include a polymeric tube having stiffness intermediate between that of the proximal end section and shaft section  202 , this tube also having an interior bore. The interior bores of the transition section  224  and proximal section  222  may communicate with the bore of shaft section  202 . Alternatively, the metallic tube forming shaft section  202  may extend through the interior bores of transition section  224  and proximal section  222 . In either arrangement, wires  220  may extend all the way to the proximal end of proximal end section  222 . An electrical connector  230  is connected to these wires and, hence, to electrodes  216 . An atraumatic tip may be formed at the end region of distal section  206 . The details of construction of the atraumatic tip are shown in  FIG. 10  and discussed in detail hereafter. In a variation of the second embodiment, distal section  206  may have the shape described in detail hereafter and depicted in  FIGS. 11 and 12 . 
     A sensor probe  300  in accordance with a third embodiment of the invention is shown in  FIG. 9 . Sensor probe  300  has a proximal section  302  and a distal section  304 . Proximal section  302  is approximately 90 percent of the length of sensor probe  300 . Proximal section  302  includes a stainless steel hypotube coated with Teflon. The stainless steel hypotube provides pushability to the sensor probe  300  and transfers maximum force from a handle  306  to the opposite end of sensor probe  300 . A core wire (not shown) runs through the proximal twelve inches of the hypotube to prevent it from collapsing during procedural manipulation. A plurality of electrodes (not shown) extend around a portion of a nitinol wire  312  ( FIG. 10 ) that forms a loop  324  ( FIG. 11 ). The electrodes are electrically insulated from nitinol wire  312 . One example of a manner in which the electrodes may be insulated from nitinol wire  312  may be seen in  FIG. 3  where the nitinol wire is designated by numeral  94 . As seen in  FIG. 3  insulated wires  92  are routed along probe body  78  through the dielectric layer  90  and exit dielectric layer  90  through small holes (not shown) cut into dielectric layer  90 . The insulation is removed from the portion of the insulated wire  92  extending out from dielectric layer  90 . The exposed wire is wrapped around probe body  78  and knotted, providing for a secure connection to probe body  78 . A slightly oversized electrode made from Platinum or Pt10Ir is positioned directly over the wrapped wire then is mechanically swaged into position so that it becomes immovable and provides a seal to prevent blood leakage into probe body  78 . The electrodes are wired to an electrical connector (not shown) that may be located in handle  306 . 
     Distal section  304  consists of a loop section  308  and an intermediate section  310 . Intermediate section  310  includes a polyimide shaft that is approximately 10.5 centimeters long. The polyimide shaft is resistant to compression and at the same time flexible. The polyimide shaft when pushed through a narrow pathway does not increase frictional resistance. The polyimide shaft is connected to the distal end of the hypotube and the proximal end of loop section  308 . The soft Pebax® tubing  320  ( FIG. 10 ) and Polyimide tubing are bonded using a lap joint design. The distal end of the Polyimide tubing may be hot necked to accept Pebax® tubing  320 . Pebax® tubing  320  is also necked to thin out the wall. The necked length of Pebax® tube  320  is cut to 5 mm. The necked length of the Polyimide tubing is cut to 7 mm. The dies used to neck each tube are configured to enable the necked Pebax® tube  320  to slip fit over the necked section of the Polyimide. Pebax® tubing  320  is slipped over the necked part of the Polyimide and this is heat bonded. This heat bonding process is accomplished in two steps using a progressively tighter Teflon sleeve to ensure a strong bond and to reduce the profile of the bond to match the tubing. The proximal portion of the Polyimide is bonded to the hypotube by first inserting it over a tubing to give it an appropriate shape and thereafter inserting the shaped polyimide tubing over the wire bundle. The part over the wire bundle is inserted into the actual hypotube and heated again to mold the joint. Then, this joint is adhesive bonded by applying cyanoacrylate adhesive (marketed by Locktite Corporation) between the hypotube and the molded polyimide to form a lap joint. The Pebax® tube  320  is of a very soft durometer. For example, Pebax® tube  320  may have durometer range of 35-72 shore D. More preferably, the Pebax® tube may have 70 shore D durometer value that allows for maximum flexibility and shapes nicely over the narrow loop radii to create a soft atraumatic tip as discussed hereafter. A single piece of Pebax® tubing may be used to make the distal soft segment spanning from the proximal end bonded to the Polyimide shaft and ending at the distal end where it is fused to the ball section  314  of the nitinol wire  312 . 
       FIG. 10  highlights the details of the atraumatic tip of loop section  308  of sensor probe  300 . The core section has nitinol wire  312  having a diameter of approximately 0.012 inch. The distal end portion of nitinol wire  312  has four sections—a ball section  314 , a reduced section  316 , a tapered section  318  and a main section  319 . Ball section  314  is the distal most section and is approximately 0.04 inch long. In this section the diameter of the nitinol wire is approximately 0.012 inch. Reduced section  316  is located between ball section  314  and tapered section  318 . Reduced section  316  is approximately 0.12 inch in length and approximately 0.005 inch diameter. One end of tapered section  318  is connected to reduced section  316  and has a diameter equal to the reduced section  316  where they are connected. The opposing end of tapered section  318  has larger diameter equal to the diameter of main section  319  and is connected with main section  319 . The diameter of the main section is 0.012 inches. A Pebax® tubing  320  covers nitinol wire  312  in distal section  308 . Approximately six millimeters of Pebax® tubing  320  covering the distal end portion of nitinol wire  312  is heated in place around nitinol wire  312  at 250° F. for 30 seconds resulting in heat setting in this section. The tubing near the ball section  314  is bonded/melted at 450 deg F. for about 10-15 seconds. This results in Pebax® tubing shrinking in onto the nitinol wire  312  and taking the shape of the underlying nitinol wire  312 . The Pebax® tubing physically locks to nitinol wire  312  at reduced section  316 , ball section  314  and tapered section  318 . Moreover, the Pebax® tubing shrinks into tight frictional engagement with main section  319 . The fusing of nitinol wire  312  and Pebax® tubing locks nitinol wire  312  in place relative to the Pebax® tubing and results in a unitized construction. Such construction creates a relatively stable structure that precludes independent movement of outer layers with respect to inner nitinol wire  312  and thereby allows the sensor probe to easily pass through a tight bore. A ball  322  is formed at the very distal tip of ball section  314 . Ball  322  is formed from a UV cured adhesive such as Dymax 206-CTH-T, Dymax 203A-CTH-T, or Loctite 3311. Loop section  308  may be formed to loop shape by heat setting in one of the last steps of the assembly procedure. The ball  322  along with the taper formed Pebax® tubing  320  and tapered nitinol wire  312  provide a highly atraumatic tip. An atraumatic tip described above may also be provided for the sensor probes shown in  FIGS. 2 and 5  and described in detail previously. 
     Loop section  308  has loop  324  ( FIG. 11 ). Loop  324  is formed by nitinol wire  312  arcuately bending the wire  312  in two planes perpendicular to each other. Loop  324  lies in a plane perpendicular to the longitudinal axis of nitinol wire  312 . To form is loop  324 , the distal portion of nitinol wire  312  starts to turn away from the longitudinal axis in an arcuate manner. The radius of an arc  326  formed by nitinol wire  312  and as projected in a plane is equal to the diameter of loop  324 . The plane on which arc  326  is projected passes through the center of loop  324 , runs along the longitudinal axis and is angularly located with respect to loop  324  such that the measured radius is maximum it can be. Upon reaching the plane in which loop  324  is to lay, nitinol wire  312  bends to form loop  324 . The diameter of loop  324  may vary between 15 and 25 millimeters. Smaller or larger loop diameter may also be employed. Nitinol wire  312 , in forming loop  312 , also bends arcuately as projected in the plane of loop  324 . The radius of arc  328  may be approximately between 5 and 7 millimeters. The radius of arc  328  is one fourth of the diameter of loop  324 . The straight part of the tip is approximately 0.197 inch long and forms the atraumatic tip. The point where the maximum loop diameter is first achieved and the point where the loop ends (i.e., where the atraumatic tip is attached to the loop) form a 30-degree angle with respect to the centre of the loop as seen in end view of loop  324 . The end of arc  328  that is at the center of loop  324  is projecting radially out from the center of loop  324 . The end of arc  328  that is at the circumference of loop  324  is tangential to loop  324 . The tip of nitinol wire  312  turns slightly more than 360 degrees thereby forming a slight overlap in loop  324 . The overlap is approximately 0.197 inches. The radius of arcs  326  and  328  are such that the loop structure is compact, sufficiently stiff to hold circular shape during use, but nonetheless capable of being straightened such that the sensor probe can pass through a lumen as small as 0.038 inches or smaller. 
     In a method according to one aspect of the present invention, ablation device  18  is positioned within a chamber of the heart as, for example, within the left atrium LA of a subject to be treated. A guide sheath (not shown) is advanced through the venous system into the right atrium and through the septum separating the right atrium and left atrium, so that the guide sheath provides access to the left atrium. Typically, the apparatus is advanced through the guide sheath with balloons  20  and  22  in a deflated, collapsed condition. This operation may be performed by first advancing a guide wire (not shown) into the heart, and then advancing insertable structure  10 , with balloons  20  and  22  in a deflated condition, over the guide wire, and through the guide sheath. During this operation, probe  72  is not present in tube bore  28  and lumen  30 . The guide wire passes through tube bore  28  and through lumen  30 . A guide sheath also may be used during the insertion process. 
     When ablation device  18  is disposed inside the heart chamber, the physician manipulates device  18  to vary the orientation of ablation device  18 , and hence the orientation of forward-to-rearward axis  26 , until device  18  is positioned in the desired spatial relationship to the heart, with axis  26  extending generally normal to the surface of the heart surrounding the ostium OS of a pulmonary vein PV. 
     The physician may verify the proper disposition of ablation device  18  relative to the heart by injecting a fluid contrast medium through the continuous passageway defined by lumen  30  and tube bore  28  and out through port  29  on the distal or forward side of ablation device  18 . Depending upon the pressure with which the contrast medium is injected, some portion of the contrast medium may pass into the pulmonary vein and other portions may remain within the left atrium. While the contrast medium is present, the subject is imaged using an imaging modality which will show the contrast medium as, for example, conventional x-ray or fluoroscopic imaging. 
     With the ablation apparatus properly positioned for ablation, the physician may actuate ultrasonic emitter  23 , as by actuating an electrical energy source (not shown) connected to emitter  23  by conductors in catheter  12  (also not shown). Ultrasonic emitter  23  directs ultrasonic energy onto wall  24  between balloons  20  and  22 , where the energy is reflected in a forward direction F and focused into the ring-like ablation region A. The focused ultrasonic energy heats and ablates the myocardial tissue in this region, thereby converting this tissue into scar tissue which is not capable of conducting electrical impulses. 
     The physician may detect electrical signals within the pulmonary vein or pulmonary vein ostium by inserting a sensor probe into the subject through the continuous passageway defined by lumen  30  and tube bore  28 . Any one of the sensor probes described previously may be used. A method of using the sensor probes is described hereafter with reference to sensor probe shown in  FIG. 2 , however, the method is also applicable to the sensor probes shown in  FIGS. 5 and 9  and any variations thereof. When using the sensor probe of  FIG. 2  the physician manually straightens the hoop region and transition portion  84  as these are inserted through the proximal end of the catheter. The probe body  78  has sufficient flexibility so that it can be advanced distally through the passageway. As the probe body  78  advances through catheter  12 , the curvature of probe body  78  conforms to the existing curvature of catheter  12 . As probe body  78  continues to advance, it reaches the condition shown in  FIG. 2 . The probe does not tend to buckle and jam as it is threaded through the passageway of the catheter. During threading, of course, the distal end portion is not in the hoop-shape shown, but instead is straight or slightly curved to match the curvature of the passageway in the catheter. As the physician continues to advance the sensor probe  72 , the hoop region, distal end  76 , and transition region pass distally out of port  26 , the atraumatic tip leads the way. Since the atraumatic tip is the part of the hoop region that first contacts tissue, any potential tissue damage such as perforation of heart wall due to being poked by a sharp edge is prevented. Upon exiting port  26 , the hoop region and transition region spring back to their unconstrained or deployed shape, as depicted in  FIG. 4 . This places hoop  82  concentric with axis  96  defined by main region  86  of probe body  78 . However, because main region  86  is disposed within tube bore  28  of ablation device  18 , main region  86  is coaxial with axis  26  of ablation device  18 . Thus, hoop  82  tends to deploy in a plane perpendicular to axis  26  of the ablation device, with hoop  82  concentric with axis  26 . As mentioned above, during placement of ablation device  18 , the physician has already positioned this axis  26  in alignment with the pulmonary vein ostium and has positioned this axis  26  generally normal to the plane of the heart tissue encircling the ostium. Thus, hoop  82  tends to deploy in a location as shown in  FIG. 1 , with hoop  82  lodged within the pulmonary vein ostium, or (depending upon the diameter of the pulmonary vein) in the pulmonary vein itself, and with hoop  82  lying in the plane transverse to the axis of the pulmonary vein and the axis of the pulmonary vein ostium. All of this is accomplished without substantial manipulation by the physician to aim or locate hoop  82 . Stated another way, ablation device  18  and catheter  12  act as an introducer structure which directs the distal portion of sensor probe  72  into alignment with the pulmonary vein ostium. Thus, placement of sensor probe  72  can be accomplished readily. 
     Although catheter  12  and ablation device  18  act to introduce and aim the hoop region of the sensor, the hoop region is not rigidly mounted to ablation device  18  or catheter  12 , and hence, is not rigidly positioned by these devices. Transition region has some flexibility, so that hoop  82  can be displaced or tilted somewhat from perfect coaxial alignment with ablation device  18 . This allows the hoop region to engage the tissues substantially around the pulmonary vein or ostium, even where these anatomical features are not perfectly aligned with the axis of ablation device  18 . Also, hoop  82  has some flexibility, and accordingly can conform to these structures, even where the same are not perfectly circular. 
     With hoop  82  engaged with the tissues, electrodes  80  on hoop  82  will also be engaged with the tissues and hence will receive electrical signals propagating within the tissues. The physician can monitor these electrical signals using a conventional signal detection system  99  ( FIG. 1 ) connected to a connector  88  and hence connected to electrodes  80  through conductors  92  ( FIG. 3 ) of sensor probe  72 . If these electrical signals indicate that abnormal conduction is continuing to occur, the physician can actuate the ablation device again. Sensing probe  72  need not be removed from the device during such further ablation. Alternatively, sensing probe  72  may be removed and other procedures, such as injection of additional contrast medium to confirm the desired disposition of ablation device  18 , may be performed. In a further variant, sensor probe  72  may be introduced and placed as described above before actuation of ablation device  18 , most typically after correct placement of ablation device  18  has been confirmed, as by use of the contrast medium technique discussed above. 
     In a further variant, ablation device  18  can be repositioned to a new position as partially depicted in broken lines at  18 ′ in  FIG. 1 . Sensor probe  72  may be retracted into catheter  12  and ablation device  18 , or may be entirely removed during the repositioning step. The same steps as discussed above may be repeated. The ability to retract or entirely remove sensor probe  72  facilitates repositioning. For example, contrast medium may be injected again to confirm the moved position of the apparatus. Also, because sensor probe  72  does not project from the apparatus during the repositioning step, it does not interfere with repositioning. 
     Numerous variations and combinations of the features discussed above can be utilized without departing from the present invention. The probe and method of probe deployment discussed above may be used for purposes other than sensing electrical signals in the context of an ablation process. For example, the probe can be used in a cardiac mapping operation, distinct from an ablation process. In a further variant, the functional elements of the probe (the sensing electrodes) may be used as ablation electrodes; or may be replaced by functional elements other than electrodes as, for example, discrete ultrasonic transducers or the like for an ablation process; or by sensors other than electrodes as, for example, chemical sensors. Further, although the present invention is particularly useful in performing procedures within the heart, it can be applied to performing procedures within other internal organs of a human or animal subject, or indeed, to performing procedures within a cavity of an inanimate subject.