Abstract:
A catheter introduction apparatus provides an ultrasound assembly for emission of ultrasound energy. In one application the catheter and the ultrasound assembly are introduced percutaneously, and transseptally advanced to the ostium of a pulmonary vein. An anchoring balloon is expanded to center an acoustic lens in the lumen of the pulmonary vein, such that energy is converged circumferentially onto the wall of the pulmonary vein when a transducer is energized. A circumferential ablation lesion is produced in the myocardial sleeve of the pulmonary vein, which effectively blocks electrical propagation between the pulmonary vein and the left atrium.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to methods and apparatus for the medical treatment of disease of the heart. More particularly, this invention relates to a method and apparatus for treating cardiac arrhythmias by ablating in a vicinity of pulmonary venous tissue.  
           [0003]    2. Description of the Related Art  
           [0004]    Tissue ablation from the inner walls of hollow viscera of the body generally, and the vascular system in particular, has been found to be useful in the treatment of various medical conditions. Technological developments in intravascular catheters, manipulative instruments adapted to intravascular catheters, and catheter localization techniques have especially benefited the field of cardiology. Percutaneous transcatheter ablation has been used successfully in the treatment of conduction defects and arrhythmias of various types. Today, atrial tachyarrhythmias are a common application for ablative therapy.  
           [0005]    Various ablative modalities have been employed in the past, such as ablation by direct heating. Energy can be conducted to the target tissue using various modalities, such as ultrasound, laser, resistive heating, and radiofrequency energy.  
           [0006]    One ablative approach is the so-called “maze” technique. In general, the maze procedure attempts to block abnormal conduction patterns in the left atrium by establishing a maze-like pattern of linear lesions in the left atrial wall.  
           [0007]    Atrial arrhythmias are known to be associated with abnormal electrical activity of tissue foci in the vicinity of the pulmonary veins, especially the superior pulmonary veins. Various ablative treatments of such foci have been attempted. For example, the production of linear atrial lesions by radiofrequency ablation, in combination with ablation of suspected arrhythmogenic foci has been performed using transcatheter techniques.  
           [0008]    More recently, circumferential lesions at or near the ostia of the pulmonary veins have been created to treat atrial arrhythmias. U.S. Pat. Nos. 6,012,457 and 6,024,740, both to Lesh, disclose a radially expandable ablation device, which includes a radiofrequency electrode. Using this device, it is proposed to deliver radiofrequency energy to the pulmonary veins in order to establish a circumferential conduction block, thereby electrically isolating the pulmonary veins from the left atrium.  
           [0009]    Radiofrequency ablation using multiple contiguous circumferential points, guided by electro-anatomical mapping is proposed in the document,  Circumferential Radiofrequency Ablation of Pulmonary Vein Ostia: A New Anatomic Approach for Curing Atrial Fibrillation,  Pappone C, Rosanio S, Oreto G, Tocchi M, Gugliotta F, Vicedomini G, Salvati A, Dicandia C, Mazzone P, Santinelli V, Gulletta S, Chierchia S, Circulation 102:2619-2628 (2000). It is emphasized that particular care must be exercised to ensure that the ablation sites are indeed contiguous; otherwise irregular electrical activity in the pulmonary vein may continue to contribute to atrial arrhythmia.  
           [0010]    It has also been proposed to produce circumferential ablative lesions using ultrasound energy delivered via a cylindrical ultrasound transducer through a saline-filled balloon. This technique is described in the document,  First Human Experience With Pulmonary Vein Isolation Using a Through-the-Balloon Circumferential Ultrasound Ablation System for Recurrent Atrial Fibrillation,  Natale A, Pisano E, Shewchik J, Bash D, Fanelli R, M D; Potenza D; Santarelli P; Schweikert R; White R; Saliba W; Kanagaratnam L; Tchou P; Lesh M, Circulation 102:1879-1882 (2000). Ablation times on the order of 2 minutes are reported.  
           [0011]    U.S. Pat. No. 6,117,101 to Diederich et al. discloses a technique for producing circumferential lesions for electrical isolation of the pulmonary veins. Using a balloon catheter, a cylindrical ultrasound transducer is provided on an inner member within a balloon, and emits a radial ultrasound signal that is sonically coupled to the balloon&#39;s outer skin.  
           [0012]    A known drawback in the use of ultrasound energy for cardiac tissue ablation is the difficulty in controlling the local heating of tissue. There are tradeoffs between the clinical desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the ultrasound device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure.  
           [0013]    In consideration of these, and other factors, it is appropriate, in designing a practical ultrasound emitter, to consider the amplitude of the ultrasound signal, the amount of time required for the energy application, the size of the electrode, and the contact area, as well as ease of positioning, withdrawal, and repositioning of the device so as to be able to conveniently produce multiple lesions during the same medical procedure.  
           [0014]    Previous approaches to controlling local heating include the inclusion of thermocouples within the electrode and feedback control, signal modulation, local cooling of the catheter tip, and fluid assisted techniques, for example perfusion of the target tissue during the energy application, using chilled fluids. Typical of the last approach is described by Mulier et al. in U.S. Pat. No. 5,807,395.  
           [0015]    Publications which describe various medical techniques of interest include:  
           [0016]    Scheinman M M, Morady F. Nonpharmacological Approaches to Atrial Fibrillation.  Circulation  2001;103:2120-2125.  
           [0017]    Wang P J, Homoud M K, Link M S, Estes III N A. Alternate Energy Sources for Catheter Ablation.  Curr Cardiol Rep  1999 July; 1(2):165-171.  
           [0018]    Fried N M, Lardo A C, Berger R D, Calkins H, Halperin H R. Linear Lesions in Myocardium Created By Nd:YAG Laser Using Diffusing Optical Fibers: In Vitro and In Vivo Results.  Lasers Surg Med  2000;27(4):295-304.  
           [0019]    Eigler N L, Khorsandi M J, Forrester J S, Fishbein M C, Litvack F. Implantation and Recovery of Temporary Metallic Stents in Canine Coronary Arteries.  J Am Coll Cardiol  1993; 22(4):1207-1213.  
           [0020]    Synthetic Biodegradable Polymers as Medical Devices; by John C. Middleton and Arthur J. Tipton. 1998.  
           [0021]    Keane D, Ruskin J, Linear Atrial Ablation With A Diode Laser And Fiber Optic Catheter.  Circulation  1999; 100:e59-e60.  
           [0022]    Ware D, et al., Slow intramural heating with diffused laser light: A unique method for deep myocardial coagulation.  Circulation ; Mar. 30, 1999; pp. 1630-1636.  
           [0023]    Other medical technologies of interest are described in U.S. Pat. Nos. 5,891,134 to Goble et al., 5,433,708 to Nichols et al., 4,979,948 to Geddes et al., 6,004,269 to Crowley et al., 5,366,490 to Edwards et al., 5,971,983, 6,164,283, and 6,245,064 to Lesh, 6,190,382 to Ormsby et al., 6,251,109 and 6,090,084 to Hassett et al., 5,938,600 to Swartz et al., and 6,064,902 to Haissaguerre et al.  
           [0024]    All of the patents and publications cited in this disclosure are incorporated herein by reference.  
         SUMMARY OF THE INVENTION  
         [0025]    It is therefore a primary object of some aspects of the present invention to provide improved apparatus and method for electrically isolating the pulmonary vein by accomplishing a circumferential conduction block surrounding the pulmonary vein ostium in complishing a circumferential conduction block surrounding the pulmonary vein ostium in a single ablation application of ultrasound energy.  
           [0026]    It is another object of some aspects of the present invention to reduce the time required to perform ultrasonic isolation of the pulmonary veins.  
           [0027]    These and other objects of the present invention are attained by a catheter introduction apparatus that includes an ultrasound assembly for emission of ultrasound energy. In one application, the catheter and the ultrasound assembly are introduced percutaneously, and transseptally advanced to the ostium of a pulmonary vein. An anchor such as an anchoring balloon is expanded to center an acoustic lens in the lumen of the pulmonary vein, such that energy is converged circumferentially onto the wall of the pulmonary vein when a transducer is energized. A circumferential ablation lesion is produced in the myocardial sleeve of the pulmonary vein, which effectively blocks electrical propagation between the pulmonary vein and the left atrium.  
           [0028]    There is therefore provided, in accordance with an embodiment of the present invention, a method for electrically isolating a left atrium of a heart from a pulmonary vein, including the steps of:  
           [0029]    introducing an ultrasound assembly into said heart proximate an ostium of said pulmonary vein;  
           [0030]    anchoring said ultrasound assembly to said pulmonary vein using an anchor; and  
           [0031]    thereafter conducting ultrasound energy in a path extending from said ultrasound assembly to a circumferential ablation region of said pulmonary vein, said path substantially avoiding said anchor.  
           [0032]    In an embodiment, said step of conducting said ultrasound energy is performed by converging said ultrasound energy into a circumferential line of focus that intersects said ablation region.  
           [0033]    In an embodiment, said anchor includes a balloon, and said step of aligning is performed by expanding said balloon to engage said pulmonary vein.  
           [0034]    In an embodiment, said step of conducting said ultrasound energy is performed in exactly one application.  
           [0035]    In an embodiment, a duration of said one application is less than 300 seconds.  
           [0036]    In an embodiment, said step of introducing is performed by:  
           [0037]    disposing said ultrasound assembly on an intravascular catheter; and  
           [0038]    passing a distal portion of said intravascular catheter through a blood vessel into said heart, wherein said ultrasound assembly is spaced apart from said ablation region, wherein said path has a generally forward direction from said ultrasound assembly toward said ablation region.  
           [0039]    In this case, in an embodiment, said step of passing said distal portion of said intravascular catheter includes activating said ultrasound assembly to apply ultrasound energy to a fossa ovalis of said heart.  
           [0040]    In an embodiment, the method includes the step of adjusting a beam of said ultrasound energy to conform to an anatomy of said ablation region.  
           [0041]    In an embodiment, the method includes the step of conducting an effective amount of energy from said ultrasound assembly to ablate a portion of a fossa ovalis of said heart while performing said step of introducing said ultrasound assembly.  
           [0042]    There is further provided, in accordance with an embodiment of the present invention, an apparatus for electrically isolating a cardiac chamber, including:  
           [0043]    an intravascular catheter having a distal end; an anchor proximate said distal end; and  
           [0044]    an ultrasound transducer assembly disposed external to said anchor for emitting ultrasound energy that is directed to a circumferential ablation region, a path of said ultrasound energy substantially avoiding said anchor.  
           [0045]    In an embodiment, the apparatus includes a sensor disposed in said catheter for detecting cardiac electrical activity. For some application, the apparatus includes a transmitting antenna disposed in said catheter for transmitting signals from said sensor.  
           [0046]    In an embodiment, said anchor includes a balloon.  
           [0047]    In an embodiment, a body section of said ultrasound transducer assembly has a proximal cross section and a distal cross section, said proximal cross section being larger than said distal cross section. For some applications, said body section is a truncated cone, and an inclination angle of said truncated cone is about 20 degrees.  
           [0048]    In an embodiment, said ultrasound transducer assembly includes an omnidirectional lens that focuses a beam of said ultrasound energy circumferentially on said ablation region. For some applications, said ultrasound transducer assembly includes an array of transducer elements, and a control unit for controlling individual ones of said transducer elements to shape said beam.  
           [0049]    In an embodiment, in an operational position said ultrasound transducer assembly is spaced apart from said ablation region.  
           [0050]    In an embodiment, said ultrasound transducer assembly operates at a frequency of between about 3 and 4 MHz.  
           [0051]    In an embodiment, said ultrasound transducer assembly includes:  
           [0052]    a diffraction grating for directing said ultrasound energy that is output from said ultrasound transducer assembly in a desired direction; and  
           [0053]    a transducer layer capable of transducing energy delivered thereto at different frequencies, said transducer layer being disposed within said catheter proximate said diffraction grating.  
           [0054]    In this case, in an embodiment, said diffraction grating is a thin-film disposed on an external surface of said catheter.  
           [0055]    In an embodiment, said ultrasound transducer assembly has a bandwidth that is between about 50% and about 80% of a primary operating frequency thereof.  
           [0056]    There is yet further provided, in accordance with an embodiment of the present invention, an apparatus for electrically isolating a cardiac chamber, including:  
           [0057]    an intravascular catheter;  
           [0058]    an anchor disposed proximate a distal end of said catheter; and  
           [0059]    an ultrasound transducer assembly disposed proximal to said anchor for emitting ultrasound energy, a path of said ultrasound energy substantially avoiding said anchor, wherein a body section of said ultrasound transducer assembly has a proximal cross sectional area and a distal cross sectional area, said proximal cross sectional area being larger than said distal cross sectional area, and wherein said ultrasound transducer assembly emits said ultrasound energy as a beam that is focused on an ablation region that substantially surrounds said ultrasound transducer assembly.  
           [0060]    In an embodiment, the apparatus includes a sensor disposed in said catheter for detecting cardiac electrical activity. In this case, in an embodiment, the apparatus includes a transmitting antenna disposed in said catheter for transmitting signals from said sensor.  
           [0061]    In an embodiment, said anchor includes a balloon.  
           [0062]    In an embodiment, said body section is a truncated cone, and an inclination angle of said truncated cone is about 20 degrees.  
           [0063]    In an embodiment, said ultrasound transducer assembly includes an array of transducer elements, and a control unit for controlling individual ones of said transducer elements to shape said beam.  
           [0064]    In an embodiment, in an operational position said ultrasound transducer assembly is spaced apart from said ablation region.  
           [0065]    In an embodiment, said ultrasound transducer assembly operates at a frequency of about 3 and 4 MHz.  
           [0066]    In an embodiment, said ultrasound transducer assembly includes:  
           [0067]    a diffraction grating for directing said ultrasound energy that is output from said ultrasound transducer assembly in a desired direction; and  
           [0068]    a transducer layer capable of transducing energy delivered thereto at different frequencies, said transducer layer being disposed within said catheter proximate said diffraction grating.  
           [0069]    In an embodiment, said diffraction grating is a thin-film disposed on an external surface of said catheter.  
           [0070]    In an embodiment, said ultrasound transducer assembly has a bandwidth that is between about 50% and about 80% of a primary operating frequency thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0071]    For a better understanding of these and other objects of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein:  
         [0072]    [0072]FIG. 1 is a perspective view of a therapeutic catheter that is constructed and operative in accordance with a preferred embodiment of the invention;  
         [0073]    [0073]FIG. 2 is a sectional schematic view of a transducer assembly in an operational position at a pulmonary vein ostium in accordance with a preferred embodiment of the invention;  
         [0074]    [0074]FIG. 3 is a flow chart of a method for electrically isolating pulmonary veins, which is operative in accordance with a preferred embodiment of the invention;  
         [0075]    [0075]FIGS. 4 and 5 schematically illustrates certain aspects of a method of intracardiac catheter access during a first phase of the method shown in FIG. 3; and  
         [0076]    [0076]FIG. 6 schematically illustrates a transducer assembly, in accordance with a preferred embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0077]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well known circuits, control logic, and other apparatus have not been shown in detail in order not to unnecessarily obscure the present invention.  
         [0078]    Turning now to the drawings, reference is made to FIG. 1, which illustrates a medical device that is constructed and operative in accordance with a preferred embodiment of the invention. An intravascular catheter  10  has a proximal end  12  and a distal end  14 . The distal end  14  is provided with at least one seal  16 , and optionally a second seal  18 . The seals  16 ,  18  are preferably inflatable balloons, made from rubber, polyurethane, or a similar elastic material. The catheter  10  has one or more lumens, which conduct fluid for inflating and deflating the seals  16 ,  18 . One of the lumens terminates in a port  20 , and is useful for injection of fluids and withdrawal of blood as may be required during use. Other lumens are provided for passage of guidewires and instruments therethrough. An inflatable anchoring balloon  22 , shown in a deflated condition, is located distal to the seals  16 ,  18 . The catheter  10  also has a coaxial guidewire lumen  24 . Disposed near the tip of the catheter  10 , approximately 1 cm proximal to the anchoring balloon  22 , is an ultrasound transducer assembly  26  which is coaxial with the catheter  10 .  
         [0079]    Reference is now made to FIG. 2, which is a sectional schematic view of the transducer assembly  26  in an operational position at a pulmonary vein ostium  28  in accordance with a preferred embodiment of the invention. The disclosure of FIG. 2 should be read in conjunction with FIG. 1. The catheter  10  has been slidably inserted over a guidewire  30  (through guidewire lumen  24 ), which was previously introduced into a pulmonary vein lumen  32 . The anchoring balloon  22  is expanded and fixes the apparatus in position. The transducer assembly  26  is disposed proximate the ostium  28 , external to the anchoring balloon  22 . It will be noted that the transducer assembly  26  is not in direct contact with either the anchoring balloon  22 , nor with target tissue  34  to be ablated, which is located near the ostium  28 . Advantageously, the placement of the transducer assembly  26  outside the anchoring balloon  22  allows for simplicity of construction, and for direct application of ultrasound energy to the target tissue  34 , thereby avoiding distortion and loss of precision in energy delivery that might occur if the energy passed through the wall of the balloon. Moreover, as is further disclosed hereinbelow, the use of ultrasonic beam focusing techniques eliminates the difficulty of physically conforming the transducer to the wall of the pulmonary vein, as is required by conventional techniques, which often required multiple versions of the catheter  10 , each dimensioned to one of many anatomic variations of the structures near the target ablation zone. Since direct contact between the transducer assembly  26  and the target tissue  34  is eliminated according to this embodiment of the present invention, it is also not required that the transducer assembly  26  vary sectionally in stiffness, a requirement which was disclosed, for example, in the above-noted U.S. Pat. No. 6,117,101. Variation in stiffness was required in order to assure stable engagement with the pulmonary vein.  
         [0080]    The transducer assembly  26  has a lumen  36  for passage therethrough of the guidewire  30 . A body section  38  is preferably shaped as a truncated cone, preferably having an inclination angle  40  of approximately 20 degrees. Thus, the cross section of a proximal portion of the body section  38  is larger than the cross section of its distal portion. A piezoelectric element  42  of known type, such as a ceramic, is present within the body section  38 .  
         [0081]    The transducer assembly  26  functions as an omnidirectional ultrasonic lens, forming a generally forward-directed circumferential beam  44 , indicated by dashed lines in FIG. 2. The beam  44  converges onto the target tissue  34 . The piezoelectric element  42  may be realized as an array of transducers, which can be tuned, under control of a control unit  46 , so as to shape the beam  44  as may be required for a particular ablation procedure, in order to adapt the beam to the local anatomy. This can be done in a known manner, for example by operating elements of the array out of phase with one another. The transducer assembly  26  is connected by a cable  48  to a suitable power source  50  and to the control unit  46 .  
         [0082]    Preferably the transducer assembly  26  is 4.0 mm in length, and has an OD of 2.6 mm. The transducer assembly  26  is quarter-wave impedance matched, using air-backing material within the body section  38 . It preferably operates at an excitation frequency of 3-4 MHz, and has a focal depth of 15 mm. Typical driving power is 30-40W.  
         [0083]    Structures suitable for the components of the transducer assembly  26  are disclosed, for example, in U.S. Pat. No. 6,296,619, and the above-noted U.S. Pat. No. 6,117,101, which are incorporated herein by reference. It is also possible to construct the transducer assembly  26  as a thin-film polymer wrapped about the outer surface of the catheter  10 .  
         [0084]    Preferably, the active sites to be ablated are identified using the location and mapping system disclosed in commonly assigned U.S. Pat. No. 5,840,025, which is herein incorporated by reference. Certain components of the location and mapping system are incorporated into the distal end  14  of the catheter  10 , namely a sensor  52 , which is a mapping electrode, and a transmitting antenna  54 , which can be a dipole antenna. The sensor  52  detects local electrical activity of the heart, and the antenna  54  transmits signals to a plurality of receiving antennae (not shown) which are placed on the body surface of a patient during use. The distal end  14  can be radio-opaque, in order to facilitate its localization by conventional radiographic techniques, alternatively or in addition to the system disclosed in the above-noted U.S. Pat. No. 5,840,025.  
         [0085]    In embodiments in which the system disclosed in the above-noted U.S. Pat. No. 5,840,025 is not used, the sensor  52  performs conventional monitoring of local electrical activity, and the antenna  54  can then be omitted.  
         [0086]    Reference is now made to FIG. 3, which is a flow chart of a method for electrically isolating pulmonary veins, which is operative in accordance with a preferred embodiment of the invention. The description of FIG. 3 should be read in conjunction with FIG. 1.  
         [0087]    In initial step  56 , routine preparation of a subject (not shown) and equipment are accomplished. This includes attachment of various monitoring and grounding leads, as may be required for electrophysiological monitoring of the procedure, and for the operation of the above-noted location and mapping system.  
         [0088]    Next, at step  58 , a series of events begins, ultimately leading to the positioning of the catheter  10  and the transducer assembly  26  at the ostium of a pulmonary vein. Step  58  is conventional. In a preferred approach, the venous system is accessed using the well-known Seldinger technique, in which an introducer sheath is positioned in a peripheral vein, typically a femoral vein. A guiding sheath is introduced through the introducer sheath, and is advanced via the inferior vena cava into the right atrium. Then, using a Brockenbrough needle, the fossa ovalis of the interatrial septum is punctured, and the puncture dilated if necessary. The Brockenbrough needle is withdrawn, and the guiding sheath placed in the left atrium. Alternatively, the ablation catheter is energized as it contacts the interatrial septum, usually at the fossa ovalis, in order to facilitate passage through the septum. Ablation of septal tissue eases the passage of the catheter through the septum, reduces the amount of hardware used, and shortens the procedure, as it is not necessary to pass a dilator through the fossa ovalis. It is also possible to access the left atrium via the superior vena cava, or to use a retrograde intra-arterial technique.  
         [0089]    Next, in step  60  a guidewire is advanced through the guiding sheath, through the left atrial chamber, and into a pulmonary vein.  
         [0090]    The order in which the specific pulmonary veins are visited and treated is arbitrary, but it is preferable to concentrate first on the two superior pulmonary veins, in which the muscular sleeves are more prominent than in the inferior pulmonary veins. Thereafter the inferior pulmonary veins may be isolated. Typically, an ablation procedure involves the isolation of all four pulmonary veins.  
         [0091]    Reference is now made to FIG. 4, which schematically illustrates certain aspects of the method of electrical pulmonary vein isolation in accordance with a preferred embodiment of the invention. The description of FIG. 4 should be read in conjunction with FIG. 3. FIG. 4 represents the status at the completion of step  60  (FIG. 3). A cutaway view of a left atrial chamber  62  includes a right superior pulmonary vein  64  and a left superior pulmonary vein  66 , whose ostium  68  is indicated. The view of FIG. 4 also includes a right inferior pulmonary vein  70 , and a left inferior pulmonary vein  72 . A conventional guiding sheath  74  has a distal end  76  which has been positioned on the left atrial side of an interatrial septum  78 . A conventional guidewire  80  extends through the lumen of the guiding sheath  74 , into the lumen of the left superior pulmonary vein  66 . It will be understood that while the guidewire  80  is shown in relation to the left superior pulmonary vein  66 , the technique is equally applicable to the other pulmonary veins.  
         [0092]    Referring again to FIG. 3, at step  82 , the guiding sheath is withdrawn, and an ablation catheter is slidably tracked over the guidewire, using the guidewire lumen of the catheter. The catheter is advanced into the left atrium. While maneuvering the catheter in the heart, its position is preferably monitored by the location and mapping system disclosed in the above-noted U.S. Pat. No. 5,840,025, or alternatively by conventional imaging modalities. The tip of the catheter is located at the ostium of a pulmonary vein.  
         [0093]    Reference is now made to FIG. 5, which schematically illustrates certain aspects of the method of electrical pulmonary vein isolation in accordance with a preferred embodiment of the invention. The description of FIG. 5 should be read in conjunction with FIGS. 3 and 4. FIG. 5 represents the status at the completion of step  82  (FIG. 3). Structures in FIG. 5 which are identical to corresponding structures in FIG. 4 have been given like reference numerals. The shaft of the catheter  10  extends through the interatrial septum  78 . The anchoring balloon  22  and the transducer assembly  26  lie across the ostium  68  of the left superior pulmonary vein  66 , and the principal axis of the transducer assembly  26  is substantially coaxial with the left superior pulmonary vein  66 . During placement, the anchoring balloon  22  is deflated.  
         [0094]    Referring again to FIG. 3, at step  84  the transducer assembly  26  is positioned such that when it is energized, the circumferential focus of the ultrasound beam intersects the pulmonary vein in which the target tissue is located. Positioning is preferably accomplished by inflating the anchoring balloon  22  so that it expands to fill the lumen of the ostium  68 . The anchoring balloon  22  is then in circumferential contact with the intima of the pulmonary vein. The distal end  14  of the catheter  10  and the transducer assembly  26  are thus forced into a central position with respect to the lumen of the ostium  68 . Perfusion through one of the catheter ports may be employed during step  84  to minimize stasis of blood in the region.  
         [0095]    In step  86 , once the position of the transducer assembly  26  is confirmed, the transducer assembly  26  is energized, and ultrasound energy converges in a circumferential pattern to the target tissue. Local heating caused by absorption of the ultrasound energy results in ablation of the target tissue. The path taken by the ultrasound energy extends directly from the transducer assembly  26  to the target tissue, and does not pass through the anchoring balloon  22 .  
         [0096]    Referring again to FIG. 3, the transfer of ultrasound energy from the transducer assembly  26  to the pulmonary vein in step  86  occurs in a single, relatively short application. The energy application is preferably controlled in response to continuous electrophysiological monitoring, an end point being reached when conduction block is confirmed across the line of ablation. For some applications, feedback techniques known in the art, e.g., on-site temperature measurements, are used to regulate the application of energy to the tissue.  
         [0097]    Upon completion of the ablation, in step  88  the anchoring balloon  22  is deflated. The distal end  14  of the catheter  10  is withdrawn into the left atrial chamber. The guidewire  80  is also withdrawn from the pulmonary vein.  
         [0098]    Next, at decision step  90 , a test is made to determine if more pulmonary veins remain to be electrically isolated. If the determination is affirmative then control proceeds to step  92 , where the next pulmonary vein is selected. Control then returns to step  60 .  
         [0099]    If the determination at decision step  90  is negative, then control proceeds to final step  94 . The anchoring balloon is deflated, and the entire apparatus withdrawn from the patient. The procedure thereupon terminates.  
         [0100]    Reference is now made to FIG. 6, which schematically illustrates a transducer assembly  100  that is constructed and operative in accordance with an alternate embodiment of the invention. The transducer assembly  100  is incorporated in a segment of a catheter shaft  102 . A diffraction grating lens  104  is formed as a thin-film layer on the outside of the catheter shaft  102 , using known techniques. The catheter shaft  102  is sonolucent, at least in the segment occupied by the transducer assembly  100 . A wide band ultrasound transducer  106  opposes the diffraction grating lens  104  within the catheter shaft  102 . A sensor  108  positioned near the transducer assembly  100  has the same function as the sensor  52  (FIG. 1).  
         [0101]    The diffraction grating lens  104  enables control over the direction of the ultrasound beam that is emitted from the transducer assembly  100 . By appropriately changing the frequency of the ultrasound generator, the ultrasound beam can be steered in various directions, as indicated by two representative directions  110 ,  112 .  
         [0102]    For example, an ultrasound transducer having a bandwidth that is 50% of its primary operating frequency of 8 MHz can vary the diffraction angle by more than 60 degrees as the output beam frequency varies over the operating bandwidth.  
         [0103]    The embodiment of FIG. 6 has the advantage of a low profile, which does not interfere with its introduction into the pulmonary vein ostium, and it is capable of directing an ultrasound beam in a desired direction toward an ablation zone.  
         [0104]    Preferably the ultrasound beam is transmitted as a continuous wave at an output of approximately 50-60 watts. Typically the input power is 80 watts or less. As the transducer assembly  100  includes a diffraction lens, the natural focal point of the ultrasound beam is given by the formula  
         D   =         d   2        f       4                 c         ,                         
 
         [0105]    where d is the transducer diameter, c is the speed of sound and f is the frequency. The focal point is preferably 1-2 cm away from the sensor  108 .  
         [0106]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art which would occur to persons skilled in the art upon reading the foregoing description.