Patent Publication Number: US-6658279-B2

Title: Ablation and imaging catheter

Description:
This application is a continuation of application Ser. No. 09/301,020 filed Apr. 28, 1999, now U.S. Pat. No. 6,522,913 issued Feb. 18, 2003, which is a continuation of application Ser. No. 08/379,504, filed Oct. 28, 1996, now U.S. Pat. No. 5,904,651. 
    
    
     FIELD OF THE INVENTION 
     In a general sense, the invention is directed to systems and methods for visualizing interior regions of the human body. In a more particular sense, the invention is directed to systems and methods for mapping or ablating heart tissue for treating cardiac conditions. 
     BACKGROUND OF THE INVENTION 
     Systems and methods for visualizing interior regions of a living body are known. For example, ultrasound systems and methods are shown and described in Yock U.S. Pat. No. 5,313,949 and Webler et al. U.S. Pat. No. 5,485,846. 
     Due to dynamic forces within the body, it can be difficult to stabilize internal imagining devices to consistently generate accurate images having the quality required to prescribe appropriate treatment or therapy. There is often an attendant need to constantly position and reposition the image acquisition element. In addition, tissue and anatomic structures inside the body can contact and occlude the image acquisition element. 
     External imaging modalities are available. Still, these alternative modalities have their own shortcomings. 
     For example, in carrying out endocardial ablation procedures, fluoroscopic imaging is widely used to identify anatomic landmarks within the heart. Fluoroscopic imaging is also widely used to locate the position of the ablation electrode or electrodes relative to the targeted ablation site. It is often difficult to identify these anatomic sites using fluoroscopy. It is also difficult, if not impossible, to use fluoroscopy to ascertain that the desired lesion pattern has been created after ablation. Often, the achievement of desired lesion characteristics must be inferred based upon measurements of applied ablation power, system impedance, tissue temperature, and ablation time. Furthermore, fluoroscopy cannot readily locate the border zones between infarcted tissue and normal tissue, where efficacious ablation zones are believed to reside. 
     SUMMARY OF THE INVENTION 
     The invention provides improved systems and methods that acquire images of interior body regions in conjunction with diagnostic or therapeutic procedures. The systems and methods introduce into the interior body region a catheter tube carrying an imaging element for visualizing tissue. The catheter tube also carries a support structure, which extends beyond the imaging element for contacting surrounding tissue away from the imaging element. The support element stabilizes the imaging element, while the systems and methods operate the imaging element to visualize tissue in the interior body region. The systems and methods resist dislodgment or disorientation of the imaging element, despite the presence of dynamic forces. The support structure also carries a diagnostic or therapeutic component to contact surrounding tissue. In one embodiment, the component comprises a tissue ablation electrode. In another embodiment, the component comprises an electrode to sense electrical events in tissue. 
     The systems and methods make use of the images obtained by the imaging element for one or more purposes, including (i) orienting the diagnostic or therapeutic component within the interior body region; or (ii) characterizing tissue morphology, including infarcted tissue; or (iii) assessing contact between the diagnostic or therapeutic component and the surrounding tissue; or (iv) viewing a lesion pattern after transmitting ablation energy; or (v) identifying thrombus. 
     In a preferred embodiment, a steering mechanism moves the imaging element without moving the support structure. The steering mechanism permits the imaging element to acquire image slices so that accurate displays of interior body regions can be generated for viewing and analysis by the physician. Accurate images allow the physician to prescribe the appropriate treatment or therapy. 
     The invention also provides improved systems and methods that provide enhanced, accurate visualization of interior regions of the heart in connection with the creation of lesions patterns aimed at treating arrhythmias. In a preferred embodiment, the support structure carries one or more electrode elements for contacting heart tissue within the heart. In use, the electrode element is intended to transmit ablation energy to form lesions in heart tissue, transmit pacing energy to heart tissue, or sense electrical impulses to map heart tissue, or all three. 
     In use, the imaging element may visualize tissue surrounding the one or more electrodes on the support structure. In one embodiment, the imaging element comprises an ultrasonic transducer. In another embodiment, the imaging element comprises a fiber optic assembly. The imaging element allows the physician to (i) orient the support structure with respect to a preselected anatomic site within the heart; or (ii) characterize tissue morphology, including infarcted tissue; or (iii) assess contact between an electrode and the endocardium; or (iv) view a lesion pattern; or (v) identifying thrombus before or after an ablation. 
     In another preferred embodiment, systems and methods for treating atrial fibrillation use the support structure to carry a plurality of spaced-apart energy transmitting electrodes. The systems and methods introduce the catheter into a heart atrium to place at least some of the electrodes in contact with heart tissue. The systems and methods simultaneously transmit ablating energy from a source through each electrode to generate an additive heating effect between electrodes that forms a continuous lesion pattern in tissue contacted by the electrodes. The systems and methods also manipulate the imaging element to visualize tissue surrounding the support structure. The systems and methods display the image for use by the physician; for example, to orient the multiple electrode support structure with respect to a preselected anatomic site within the heart; or to assess contact between electrodes and tissue; or to view the continuous lesion pattern after transmitting ablation through the multiple electrode support structure; or to characterize tissue morphology; or to identify thrombus; or any combination of the foregoing uses. 
     In one embodiment, an ablation/imaging catheter includes an elongate tube having a distal tube end that extends within the interior region. A porous electrode structure is mounted to the distal tube end, the porous electrode structure comprising an interior region for receiving a conductive medium. An ultrasonic transducer assembly is housed within the distal tube end. 
     Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a system for visualizing tissue that includes a support structure carrying an imaging probe; 
     FIG. 2 is a side section view of the imaging probe and support structure of FIG. 1 in a collapsed condition within an external slidable sheath; 
     FIG. 3 is a side section view of a portion of a spline that forms a part of the support structure shown in FIG. 1; 
     FIGS. 4A and 4B are side sectional, somewhat diagrammatic views of the deployment of the support structure and imaging probe shown in FIG. 1 within a heart chamber; 
     FIG. 5A is a side section view of the support structure and imaging probe shown in FIG. 1, showing various paths in which the imaging probe can be moved when located within a body region; 
     FIG. 5B is a side view of an alternative embodiment of an imaging probe and a support structure comprising a single spline element; 
     FIG. 6 is an enlarged view of one embodiment of the support structure and imaging probe, in which the imaging probe includes a rotating ultrasonic transducer crystal; 
     FIG. 7 is an enlarged view of another embodiment of the support structure and imaging probe, in which the imaging probe includes a fiber optic assembly; 
     FIG. 8 is a partial side section, perspective, and largely schematic, view of a support structure and imaging probe as shown in FIG. 1, in which the imaging probe is associated with a system to conduct contrast echocardiography to identify potential ablation sites by imaging tissue perfusion; 
     FIG. 9 is a partial side section, largely schematic view of the support structure and imaging probe shown in FIG. 1, including an electro-mechanical axial translator connected to the imaging probe; 
     FIG. 10 is a side section view, somewhat diagrammatic is nature, showing a support structure and imaging probe, in which both the structure and the probe carry electrodes; 
     FIG. 11 is a side section view of a portion of an electrode-carrying spline that forms a part of the support structure shown in FIG. 10; 
     FIG. 12 is a side section view of a heart and a perspective view of the support structure and imaging probe shown in FIG. 10, being used in association with a separate roving mapping, pacing, or ablating electrode; 
     FIG. 13A is a side view, with portions removed, of a support assembly comprising a expanded porous body capable of ionic transfer of ablation energy, which carries an interior imaging probe; 
     FIG. 13B is a side elevation view of the porous body shown in FIG. 13A, with the porous body shown in a collapsed condition for introduction into an interior body region; 
     FIG. 14 is a side view of a support assembly carrying within it the porous body and imaging probe assembly shown in FIGS. 13A and 13B; 
     FIG. 15 is a side view, somewhat diagrammatic in form, showing a support structure that carries within it a movable imaging probe, the support structure also carrying multiple electrodes sized to create long lesion patterns; 
     FIG. 16 is an illustration representative of a typical small tissue lesion pattern; 
     FIG. 17 is an illustration representative of a typical larger tissue lesion pattern; 
     FIG. 18 is an illustration representative of a typical long tissue lesion pattern; 
     FIG. 19 is an illustration representative of a typical complex long tissue lesion pattern; 
     FIG. 20 is an illustration representative of a typical segmented tissue lesion pattern; 
     FIG. 21 is a side section view, somewhat diagrammatic in form, showing a support structure that carries within it an image acquisition element gated according to intracardiac activation sensed by an electrode also carried by the support structure; 
     FIG. 22 is a side section view, somewhat diagrammatic in form, of a support structure that carries within it an image acquisition element, also shown with an enlarged perspective view, comprising a phased transducer array that includes multiple transducers panels scored on different planar sections of a piezoelectric material; 
     FIG. 23 is a side section view of a support structure that carries within it an image acquisition element comprising a phased multiple transducer array carried on flexible spline elements; 
     FIG. 24 is a side section view of a support structure that carries within it an image acquisition element comprising a phased multiple transducer array carried on an expandable-collapsible body; 
     FIG. 25 is a side section view, somewhat diagrammatic in form, of a support structure that carries within it an image acquisition element comprising an optical coherence domain reflectometer; 
     FIG. 26 is a diagrammatic view of a system for identifying the physical characteristics of a support structure using a machine-readable code, to enable the creation of a positioning matrix (shown in FIG. 10) to guide the imaging probe within the structure; 
     FIG. 27 is a diagrammatic view of one implementation of the machine-readable code used to identify the individual physical characteristics of the support structure shown in FIG. 26; and 
     FIG. 28 is a diagrammatic view of another implementation of the machine-readable code used to identify the individual physical characteristics of the support structure shown in FIG.  26 . 
    
    
     The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a system  10 , which embodies features of the invention, for visualizing interior regions of a living body. The invention is well adapted for use inside body lumens, chambers or cavities for either diagnostic or therapeutic purposes. It particularly lends itself to catheter-based procedures, where access to the interior body region is obtained, for example, through the vascular system or alimentary canal, without complex, invasive surgical procedures. 
     The invention may be used in diverse body regions for diagnosing or treating diseases. For example, various aspects of the invention have application for the diagnosis and treatment of arrhythmia conditions within the heart, such as ventricular tachycardia or atrial fibrillation. The invention also has application in the diagnosis or treatment of intravascular ailments, in association, for example, with angioplasty or atherectomy techniques. Various aspects of the invention also have application for diagnosis or treatment of ailments in the gastrointestinal tract, the prostrate, brain, gall bladder, uterus, and other regions of the body. The invention can also be used in association with systems and methods that are not necessarily catheter-based. The diverse applicability of the invention in these and other fields of use will become apparent. 
     I. Visualization for Diagnostic Purposes 
     The invention makes it possible for a physician to access and visualize or image inter-body regions, to thereby locate and identify abnormalities that may be present. The invention provides a stable platform through which accurate displays of these images can be created for viewing and analysis by the physician. Accurate images enable the physician to prescribe appropriate treatment or therapy. 
     As implemented in the embodiment shown in FIG. 1, the invention provides a system  10  comprising a support structure  20  that carries within it an imaging or visualizing probe  34 . As FIG. 1 shows, the system  10  includes a flexible catheter tube  12  with a proximal end  14  and a distal end  16 . The proximal end  14  carries an attached handle  18 . The distal end  16  carries the support structure  20 . 
     A. The Support Structure 
     The support structure  20  can be constructed in various ways. In one preferred embodiment (illustrated in FIG.  1 ), the structure  20  comprises two or more flexible spline elements  22 . In FIG. 1, the support structure  20  includes eight spline elements  22 . Of course, fewer or more spline elements  22  can be present. For example, FIG. 5A shows the support structure  20  comprising just two, generally oppositely spaced spline elements  22 . As another example, FIG. 5B shows the support structure  20  comprising a single spline element  22 . In FIG. 5B, the distal end  23  of the spline element  22  is attached to a stylet  25 , carried by the catheter tube  12 , which moves the distal end  23  (as shown by arrows  27 ) along the axis of the catheter tube  12  to adjust the curvature of the spline element  22 . 
     As FIG. 3 shows, each spline element  22  preferably comprises a flexible core body  84  enclosed within a flexible, electrically nonconductive sleeve  32 . The sleeve  32  is made of, for example, a polymeric, electrically nonconductive material, like polyethylene or polyurethane. The sleeve  32  is preferable heat shrunk about the core body  84 . 
     The core body  84  is made from resilient, inert wire or plastic. Elastic memory material such as nickel titanium (commercially available as NITINOL™ material) can be used. Resilient injection molded plastic or stainless steel can also be used. Preferably, the core body  84  is a thin, rectilinear strip. The rectilinear cross-section imparts resistance to twisting about the longitudinal axis of the core body  84 , thereby providing structural stability and good bio-mechanical properties. Other cross-sectional configurations, such as cylindrical, can be used, if desired. 
     The core bodies  84  of the spline elements  22  extend longitudinally between a distal hub  24  and a base  26 . The base  26  is carried by the distal end  16  of the catheter tube  12 . As FIG. 1 shows, each core body  84  is preformed with a convex bias, creating a normally open three-dimensional basket structure expanded about a main center axis  89 . 
     As FIG. 2 shows, in the illustrated and preferred embodiment, the system  10  includes an outer sheath  44  carried about the catheter tube  12 . The sheath  44  has an inner diameter that is greater than the outer diameter of the catheter tube  12 . As a result, the sheath  44  slides along the outside of the catheter tube  12 . 
     Forward movement (arrow  43 ) advances the slidable sheath  44  over the support structure  20 . In this position, the slidable sheath  44  compresses and collapses the support structure  20  into a low profile (shown in FIG. 2) for introduction through a vascular or other body passage to the intended interior site. 
     Rearward movement (arrow  45 ) retracts the slidable sheath  44  away from the support structure  20 . This removes the compression force. The freed support structure  20  opens (as FIG. 1 shows) and assumes its three-dimensional shape. 
     (i) Deployment of the Support Assembly 
     The methodology for deploying the support structure  20  of course varies according to the particular inter-body region targeted for access. FIGS. 4A and 4B show a representative deployment technique usable when vascular access to a heart chamber is required. 
     The physician uses an introducer  85 , made from inert plastic materials (e.g., polyester), having a skin-piercing cannula  86 . The cannula  86  establishes percutaneous access into, for example, the femoral artery  88 . The exterior end of the introducer  85  includes a conventional hemostatic valve  90  to block the outflow of blood and other fluids from the access. The valve may take the form of a conventional slotted membrane or conventional shutter valve arrangement (not shown). A valve  90  suitable for use may be commercial procured from B. Braun Medical Company (Bethlehem, Pa.). The introducer  85  includes a flushing port  87  to introduce sterile saline to periodically clean the region of the valve  90 . 
     As FIG. 4A shows, the physician advances a guide sheath  92  through the introducer  85  into the accessed artery  88 . A guide catheter or guide wire (not shown) may be used in association with the guide sheath  92  to aid in directing the guide sheath  92  through the artery  88  toward the heart  94 . It should be noted that the views of the heart  94  and other interior regions of the body in this Specification are not intended to be anatomically accurate in every detail. The Figures show anatomic details in diagrammatic form as necessary to show the features of the invention. 
     The physician observes the advancement of the guide sheath  92  through the artery  88  using fluoroscopic or ultrasound imaging, or the like. The guide sheath  92  can include a radio-opaque compound, such as barium, for this purpose. Alternatively, a radio-opaque marker can be placed at the distal end of the guide sheath  92 . 
     In this way, the physician maneuvers the guide sheath  92  through the artery  88  retrograde past the aortic valve and into the left ventricle  98 . The guide sheath  92  establishes a passageway through the artery  88  into the ventricle  98 , without an invasive open heart surgical procedure. If an alternative access to the left atrium or ventricle is desired (as FIG. 15 shows), a conventional transeptal sheath assembly (not shown) can be used to gain passage through the septum between the left and right atria. Access to the right atrium or ventricle is accomplished in the same manner, but without advancing the transeptal sheath across the atrial septum. 
     As FIG. 4A shows, once the guide sheath  92  is placed in the targeted region, the physician advances the catheter tube  12 , with the support structure  20  confined within the slidable sheath  44 , through the guide sheath  92  and into the targeted region. 
     As FIG. 4B shows, pulling back upon the slidable sheath  44 . (see arrow  45  in FIG. 4B) allows the structure  20  to spring open within the targeted region for use. When deployed for use (as FIG. 4B shows), the shape of the support structure  20  (which, in FIG. 4B, is three-dimensional) holds the spline elements  22  in intimate contact against the surrounding tissue mass. As will be explained in greater detail later (and as FIG. 4B shows), the support structure  20  has an open interior  21 , which surrounds the imaging probe  34 , keeping the tissue mass from contacting it. 
     As FIGS. 1 and 4B show, the geometry of flexible spline elements  22  is radially symmetric about the main axis  89 . That is, the spline elements  22  uniformly radiate from the main axis  89  at generally equal arcuate, or circumferential, intervals. 
     The elements  22  also present a geometry that is axially symmetric along the main axis  89 . That is, when viewed from the side (as FIGS. 1 and 4B show) the proximal and distal regions of the assembled splines  22  have essentially the same curvilinear geometry along the main axis  89 . 
     Of course, if desired, the spline elements  22  can form various other geometries that are either radially asymmetric, or axially asymmetric, or both. In this respect, the axial geometry for the structure  20 , whether symmetric or asymmetric, is selected to best conform to the expected interior contour of the body chamber that the structure  20  will, in use, occupy. For example, the interior contour of a heart ventricle differs from the interior contour of a heart atrium. The ability to provide support structures  20  with differing asymmetric shapes makes it possible to provide one discrete configuration tailored for atrial use and another discrete configuration tailored for ventricular use. Examples of asymmetric arrays of spline structures  20  for use in the heart are shown in copending U.S. application Ser. No. 08/728,698, filed Oct. 28, 1996, entitled “Asymmetric Multiple Electrode Support Structures,” which is incorporated herein by reference. 
     B. The Imaging Probe 
     As FIG. 5A shows, the imaging probe  34  located within the support structure  20  includes a flexible body  36 , which extends through a central bore  38  in the catheter tube  12 . The body  36  has a distal region  40  that projects beyond the distal end  16  of the catheter tube  12  into the interior of the support structure  20 . The body  36  also includes a proximal region  42  that carries an auxiliary handle  46 . Another conventional hemostatic valve  48  is located at the distal end  16  of the catheter tube  12  to block the backflow of fluid through the catheter tube  12  while allowing the passage of the body  36 . 
     The distal body region  40  carries an image acquisition element  50 , which will be called in abbreviated form the IAE. The IAE  50  generates visualizing signals representing an image of the area, and objects and tissues that occupy the area, surrounding the structure  20 . The IAE  50  can be of various constructions. 
     (i) Ultrasonic Imaging 
     In one embodiment (see FIG.  6 ), the IAE  50  comprises an ultrasonic transducer  52 . The transducer  52  forms a part of a conventional ultrasound imaging system  54  generally of the type shown in U.S. Pat. No. 5,313,949. This patent is incorporated herein by reference. 
     The transducer  52  comprises one or more piezoelectric crystals formed of, for example, barium titinate or cinnabar, which is capable of operating at a frequency range of 5 to 20 megahertz. Other types of ultrasonic crystal oscillators can be used. For example, organic electrets such as polyvinylidene difluoride and vinylidene fluoride-trifluoro-ethylene copolymers can also be used. 
     The imaging system  54  includes a transmitter  56  coupled to the transducer crystal  52  (see FIG.  6 ). The transmitter  56  generates voltage pulses (typically in the range of 10 to 150 volts) for excitation of the transducer crystal  52 . The voltage pulses cause the transducer crystal  52  to produce sonic waves. 
     As the transmitter  56  supplies voltage pulses to the transducer crystal  52 , a motor  58  rotates the transducer crystal  52  (being linked by the flexible drive shaft  53 , which passes through a bore in the tube  36 ). The transmission of voltage pulses (and, thus, the sonic waves) and the rotation of the transducer crystal  52  are synchronized by a timing and control element  60 . Typically, the motor  58  rotates the transducer crystal  52  in the range of 500 to 2000 rpm, depending upon the frame rate of the image desired. The rotating transducer crystal  52  thereby projects the sonic waves in a 360° pattern into the interior of the chamber or cavity that surrounds it. 
     Tissue, including tissue forming anatomic structures, such as heart valves (which is generally designated T in the Figures), and internal tissue structures, and deposits or lesions on the tissue, scanned by the rotating transducer crystal  52  will scatter the sonic waves. The support structure  20  also scatters the sonic waves. The scattered waves return to the rotating transducer crystal  52 . The transducer crystal  52  converts the scattered waves into electrical signals. The imaging system  54  includes a receiver  57 , which amplifies these electrical signals. The imaging system  54  digitally processes the signals, synchronized by the timing and control element  60  to the rotation of the transducer crystal  52 , using known display algorithms; for example, conventional radar (PPI) algorithms. These algorithms are based upon the direct relationship that elapsed time (Δt) between pulse emission and return echo has to the distance (d) of the tissue from the transducer, expressed as follows:        d   =       Δ                 t       2      v                       
     where ν is the speed of sound in the surrounding media. 
     The digitally processed signals are supplied to a display unit  59 . The display unit  59  comprises a screen, which can be, for example, a CRT monitor. The display screen  59  shows an ultrasound image or profile in the desired format, which depicts the tissue and anatomic structures scanned by the transducer crystal  52 . The display screen  59  can provide a single or multi-dimensional echocardiograph or a non-imaging A-mode display. A control console (not shown) may be provided to allow selection by the physician of the desired display format. 
     Alternatively, the ultrasonic transducer crystal  52  can be operated in conventional fashion without rotation, as shown in U.S. Pat. Nos. 4,697,595, or 4,706,681, or 5,358,148. Each of these patents is incorporated herein by reference. 
     (ii) Fiber optic Imaging 
     In another embodiment (see FIG.  7 ), the IAE  50  comprises a fiber optic assembly  62 , which permits direct visualization of tissue. Various types of fiber optic assemblies  62  can be used. 
     The illustrated embodiment employs a fiber optic assembly  62  of the type shown in U.S. Pat. No. 4,976,710, which is incorporated herein by reference. The assembly  62  includes a transparent balloon  64  carried at the end of the body  36 . In use, the balloon  64  is inflated with a transparent gas or liquid, thereby providing a viewing window that shields the fiber optic channels  66  and  68  from blood contact. 
     The channels includes an incoming optical fiber channel  66 , which passes through the body  36 . The channel  66  is coupled to an exterior source  70  of light. The channel  66  conveys lights from the source  70  to illuminate the tissue region around the balloon  64 . 
     The channels also include an outgoing optical fiber channel  68 , which also passes through the body  36 . The channel  68  is coupled to an eye piece  72 , which can be carried, for example, on the handle  46 . Using the eye piece  72 , the physician can directly view the illuminated region. 
     (iii) Other Imaging 
     The IAE  50  can incorporate other image acquisition techniques. For example, the IAE  50  can comprise an apparatus for obtaining an image through optical coherence tomography (OCT). Image acquisition using OCT is described in Huang et al., “Optical Coherence Tomography,”  Science,  254, Nov. 22, 1991, pp 1178-1181. A type of OCT imaging device, called an optical coherence domain reflectometer (OCDR) is disclosed in Swanson U.S. Pat. No. 5,321,501, which is incorporated herein by reference. The OCDR is capable of electronically performing two- and three-dimensional image scans over an extended longitudinal or depth range with sharp focus and high resolution and sensitivity over the range. 
     As shown in FIG. 25, the IAE  50  comprises the distal end  220  of an optic fiber path  222 . The distal end  220  is embedded within an inner sheath  224 , which is carried within an outer sheath  226 . The outer sheath  226  extends in the distal body region  40 , within the support structure  20 . 
     The inner sheath  224  includes a lens  228 , to which the distal fiber path end  220  is optically coupled. The inner sheath  224  terminates in an angled mirror surface  230 , which extends beyond the end of the outer sheath  226 . The surface  230  reflects optical energy along a path that is generally perpendicular to the axis of the distal end  220 . 
     A motor  232  rotates the inner sheath  224  within the outer sheath  226  (arrow  237 ). The lens  228  and the mirror surface  230  rotate with the inner sheath  224 , scanning about the axis of rotation. A second motor  234  laterally moves the outer sheath  226  (arrows  236 ) to scan along the axis of rotation). 
     A source  238  of optical energy is coupled to the optic fiber path  222  through an optical coupler  240 . The source  238  generates optical energy of short coherence length, preferably less than 10 micrometers. The source  238  may, for example, be a light emitting diode, super luminescent diode, or other white light source of suitable wavelength, or a short-pulse laser. 
     A reference optical reflector  242  is also coupled by an optic fiber path  244  to the optical coupler  240 . The optical coupler  240  splits optical energy from the source  238  through the optic fiber path  222  to the distal optic path end  220  and through the optic fiber path  244  to the optical reflector  242 . 
     The optical energy supplied to the distal optic path end  220  is transmitted by the lens  228  for reflection by the surface  230  toward tissue T. The scanned tissue T (including anatomic structures, other internal tissue topographic features, and deposits or lesions on the tissue) reflects the optic energy, as will the surrounding support structure  20 . The reflected optic energy returns via the optic path  222  to the optical coupler  240 . 
     The optical energy supplied to the reference optical reflector  242  is reflected back to the optical coupler  240  by a corner-cube retro-reflector  246  and an end mirror  250  (as phantom lines  239  depict). The corner-cube retro-reflector  246  is mounted on a mechanism  248 , which reciprocates the corner-cube retro-reflector  246  toward and away from the optical path  244  and an end mirror  250  (as arrows  241  depict). The mechanism  248  preferable moves the corner-cube retro-reflector  246  at a uniform, relatively high velocity (for example, greater than 1 cm/sec), causing Doppler shift modulation used to perform heterodyne detection. 
     The length or extent of movement of the corner-cube retro-reflector  246  caused by the mechanism  248  is at least slightly greater than half the scanning depth desired. The total length of the optical path  222  between the optical coupler  240  up to the desired scanning depth point is also substantially equal to the total length of the optical path  244  between the optical coupler  240  and the end mirror  250 . Movement of the corner-cube retro-reflector  246  will cause periodic differences in the reflected path lengths  222  and  244 . 
     Reflections received from the optical path  222  (from the lens  228 ) and the optical path  244  (from the end mirror  250 ) are received by the optical coupler  240 . The optical coupler  240  combines the reflected optical signals. Due to movement of the corner-cube retro-reflector  246 , the combined signals have interference fringes for reflections in which the difference in the reflected path lengths is less than the source coherence length. Due to movement of the corner-cube retro-reflector  246 , the combined signals also have an instantaneous modulating frequency. 
     The combined output is coupled via fiber optic path  252  to a signal processor  254 . The signal processor  254  converts the optical output of the coupler  240  to voltage-varying electrical signals, which are demodulated and analyzed by a microprocessor to provide an image output to a display device  256 . 
     Further details of image acquisition and processing using OCDR are not essential to an understanding of the invention, but can be found in the above-cited Swanson U.S. Pat. No. 5,321,501. 
     C. Manipulating the Imaging Probe 
     Regardless of the particular construction of the IAE  50 , the support structure  20  positioned about the distal region of the probe  34  remains substantially in contact against surrounding tissue mass T as the IAE  50  operates to acquire the desired image or profile (see FIGS. 5 to  8 ). The support structure  20  serves to stabilize the IAE  50  and keep tissue T from contacting and possible occluding the IAE  50 . 
     Stabilizing the IAE  50  is particularly helpful when the geometry of surrounding body chamber or passage  100  is dynamically changing, such as the interior of a heart chamber during systole and diastole. The IAE  50  is thereby allowed to visualize tissue and anatomic structures T, without the attendant need for constant positioning and repositioning. The structure  20  thus makes possible the generation of accurate images of the targeted body region by the IAE  50 . 
     (i) Manual 
     In a preferred embodiment (see FIG.  5 A), the physician can move the IAE  50  within the structure  20  forward and rearward (respectively, arrows  101  and  103  in FIG. 5A) by pushing or pulling upon the auxiliary handle  46 . By torquing the handle  46  (arrows  105  in FIG.  5 A), the physician may also manually rotate the IAE  50  within the structure  20 . 
     The illustrated and preferred embodiment further includes a mechanism  74  for deflecting, or steering, the distal region  40  of the body  36 , and with it the IAE  50 , transverse of the axis  89  (as depicted in phantom lines  40  in FIG.  5 A). 
     The construction of the steering mechanism  74  can vary. In the illustrated embodiment, the steering mechanism  74  is of the type shown in U.S. Pat. No. 5,336,182, which is incorporated by reference. The steering mechanism  74  of this construction includes an actuator  76  in the auxiliary handle  46 . In the illustrated embodiment, the actuator  76  takes the form of a cam wheel rotated by means of an external steering lever  78 . The cam wheel  76  holds the proximal ends of right and left steering wires  80 . The steering wires  80  extend from the cam wheel  76  and through the body  36 . The steering wires  80  connect to the left and right sides of a resilient bendable wire  82  or spring present within the distal region  40 . Rotation of the cam wheel  76  places tension on steering wires  80  to deflect the distal region  40  of the body  36 , and, with it, the IAE  50  (as shown by arrows  107  in FIG.  5 A). 
     Thus, the physician can manually move the IAE  50  with respect to the structure  20  in three principal directions. First, the IAE  50  can be moved along the axis  86  of the structure  20  by pushing and pulling on the auxiliary handle  46  (arrows  101  and  103 ). Second, the IAE  50  can be moved rotationally about the axis  86  of the structure  20  by torquing the auxiliary handle  46  (arrows  105 ). Third, the IAE  50  can be moved in a direction normal to the axis  86  of the structure  20  by operating the steering mechanism  74  (arrows  107 ). 
     By coordinating push-pull and torquing movement of the handle  46  with operation of the steering lever  78 , the physician can manually move the IAE  50  in virtually any direction and along any path within the structure  20 . The IAE  50  can thereby image tissue locations either in contact with the exterior surface of the structure  20  or laying outside the reach of the structure  20  itself. 
     (ii) Automated (Acquiring Image Slices) 
     FIG. 9 shows an electromechanical system  102  for manipulating the IAE  50  within the structure  20 . The system  102  synchronizes the imaging rate of the IAE  50  with movement of the IAE  50  within the structure  20 . The system allows the physician to use the structure  20  to accurately acquire a set of image slices, which can be processed in an automated fashion for display. 
     The details of the system  102  can vary. As shown in FIG. 9, the system  102  includes a longitudinal position translator  104  mechanically coupled to the probe handle  46 . The translator  104  includes a stepper motor  106  that incrementally moves an axial screw ill attached to the handle  46 . The motor  106  rotates the screw  111  to move the IAE  50  at a specified axial translation rate within the structure  20 , either forward (arrows  101 ) or rearward (arrows  103 ). As FIG. 9 shows, during axial translation, the distal body region  40  carrying the IAE  50  is preferably maintained in a generally straight configuration, without transverse deflection. By synchronizing the axial translation of the IAE  50  within the structure  20  with the imaging rate of the IAE  50 , the system  102  provides as output axially spaced, data sample slices of the region surrounding the IAE  50 . 
     For example, the use of an axial translator  104  of the general type shown in FIG. 4 in combination with a rotating transducer crystal  52  of the type shown in FIG. 6 is described in U.S. Pat. No. 5,485,846, which is incorporated herein by reference. By rotating the transducer crystal  52  in synchrony with the axial translation rate of the translator  104 , the system  102  provides axially spaced, 360° data sample slices of the region perpendicular to the transducer crystal  52 . Conventional signal processing techniques are used to reconstruct the data slices taken at specified intervals along the axis into three-dimensional images for display. This technique is well suited for acquiring images inside blood vessels or other body regions having a known, relatively stable geometry. 
     When used to acquire images inside a beating heart chamber, the stepper motor  106  is preferable gated by a gating circuit  190  (see FIG. 9) to the QRS of an electrocardiogram taken simultaneously with image gathering, for example, by using a surface electrode  188  shown in FIG.  9 . The gating circuit  190  is also synchronized with the imaging system  54  (as described in greater detail in conjunction with FIG.  6 ), so that the data image slices are recorded in axial increments at either end-diastolic or end-systolic points of the heart beat. When imaging an atrium, the data slice recordings are preferably gated to the p-wave. When imaging a ventricle, the imaging is preferably gated to the r-wave. 
     Alternatively, the circuit  190  is gated to the timing of local intracardiac electrogram activation. In this arrangement (see FIG.  21 ), the flexible body  36 , which carries the transducer  54  within the structure  20 , also carries an electrode  184  to sense electrograms in the region of the structure  20 . The sensed electrograms are conveyed to the circuit  190  to gate the stepper motor  106 , as before described. When imaging an atrium, the data slice recordings are gated to the atrial intracardiac electrogram activation. Likewise, when imaging a ventricle, the data slice recordings are gated to the ventricular intracardiac electrogram activation. 
     As FIG. 21 shows, the body  36  carrying the transducer  54  and the electrode  184  is preferably confined for movement within a straight, generally rigid sheath  186 . The sheath  186  guides the body  36  along a known, stable reference axis  183 . 
     The sheath  186  is also preferably constructed of an ultrasonically transparent material, like polyethylene. The transducer  54  and electrode  184  move in tandem within the confines of the sheath  186  (as shown by arrows  187  and  189  in FIG. 21) in response to the gated action of the stepper motor  106 . Because the sheath  186  is ultrasonically transparent, the transducer  54  can remain within the confines of the sheath  186  while acquiring images. Nonlinearities in image reconstruction caused by deflection of the transducer outside of the axis  183 , as would occur should the transducer  54  move beyond the sheath  186 , are avoided. The acquired data image slices, position-gated by the electrograms while maintained along a known, stable reference axis  183 , are generated for accurate reconstruction into the desired three-dimensional image. 
     Alternatively, a catheter tracking system as described in Smith et al. U.S. Pat. No. 5,515,853, may be used to track the location and orientation of the IAE  50  during movement. Another system that can be used for this purpose is disclosed in copending U.S. patent application Ser. No. 08/717,153, filed Sep. 20, 1996 and entitled “Enhanced Accuracy of 3-Dimensional Intraluminal Ultrasound (ILUS) Image Reconstruction, “naming Harm TenHoff as an inventor. 
     (iii) Localized Guidance 
     The structure  20  itself can establish a localized position-coordinate matrix about the IAE  50 . The matrix makes it possible to ascertain and thereby guide the relative position of the IAE  50  within the structure  20  (and thus within the targeted body cavity), to image specific regions within the targeted body cavity. 
     In this embodiment (see FIG.  10 ), the IAE  50  carries an electrode  31  for transmitting electrical energy. Likewise, each spline  22  carries an array of multiple electrodes  30  for transmitting electrical energy. 
     In the illustrated embodiment (see FIG.  11 ), the electrodes  30  are supported about the core body  84  on the flexible, electrically nonconductive sleeve  32 , already described. The electrodes  30  are electrically coupled by wires (not shown), which extend beneath the sleeve  32  through the catheter tube  12  to external connectors  32 , which the handle  18  carries (see FIG.  1 ). 
     In the illustrated embodiment, each electrode  30  comprises a solid ring of conductive material, like platinum, which is pressure fitted about the sleeve  32 . Alternatively, the electrodes  30  comprise a conductive material, like platinum-iridium or gold, coated upon the sleeve  32  using conventional coating techniques or an ion beam assisted deposition (IBAD) process. Still alternatively, the electrodes  30  comprise spaced apart lengths of closely wound, spiral coils wrapped about the sleeve  32 . The coils are made of electrically conducting material, like copper alloy, platinum, or stainless steel. The electrically conducting material of the coils can be further coated with platinum-iridium or gold to improve its conduction properties and biocompatibility. Further details of the use of coiled electrodes are found in U.S. Pat. No. 5,545,193 entitled “Helically Wound Radio-Frequency Emitting Electrodes for Creating Lesions in Body Tissue,” which is incorporated herein by reference. 
     In yet another alternative embodiment, the electrodes  30  can be formed as part of a ribbon cable circuit assembly, as shown in pending U.S. application Ser. No. 08/206,414, filed Mar. 4, 1994, which is incorporated herein by reference. 
     In this arrangement (see FIG.  10 ), a micro-processor controlled guidance element  108  is electrically coupled to the electrodes  30  on the structure  20  and the electrode  31  carried by the IAE  50 . The element  108  conditions the electrodes  30  on the structure  20  and the IAE electrode  31  to generate an electric field (shown in phantom lines  113  in FIG. 10) within the structure  20 , while also sensing electrode electric potentials in the electric field. More particularly, the element  108  commands a transmitting electrode, which can be either the IAE electrode  31  or at least one of the electrodes  30  in the structure  20 , to transmit electrical energy. The element  108  commands a sensing electrode, which also can be either the IAE electrode  31  or at least one of the electrodes  30  on the structure  20 , to sense electrical energy emitted by the emitting electrode. 
     The element  108  generates an output by analyzing spatial variations in the electrical potentials within the field  113 , which change based upon the relative position of the IAE electrode  31  relative to electrode  30  on the structure  20 . The variations can comprise variations in phase, variations in amplitude, or both. Alternatively, the element  108  generates an output by analyzing spatial variations in impedances between the transmitting and sensing electrodes. The output locates the IAE  50  within the space defined by the structure  20 , in terms of its position relative to the position of the multiple electrodes  30  on the structure  20 . 
     The element  108  includes an output display device  110  (e.g., a CRT, LED display, or a printer), which presents the position-identifying output in a real-time format most useful to the physician for remotely guiding the IAE  50  within the structure  20 . 
     Further details of establishing a localized coordinate matrix within a multiple electrode structure for the purpose of locating and guiding the movable electrode within the structure are found in copending patent application Ser. No. 08/320,301, filed Oct. 11, 1994 and entitled “Systems and Methods for Guiding Movable Electrode Elements Within Multiple Electrode Structures.” This application is incorporated herein by reference. 
     In a preferred embodiment (see. FIG.  26 ), structure  20  carries an identification component  270 . The identification component  270  carries an assigned identification code XYZ. The code XYZ identifies the shape and size of the structure  20  and the distribution of electrodes  30  carried by the structure  20 , in terms of the number of electrodes and their spatial arrangement on the structure  20 . The structure-specific information contained in the code XYZ aids the element  108  in creating a positioning matrix using the electrodes  30 , to help guide the IAE  50  within the structure  20 . 
     In the illustrated embodiment (see FIG.  26 ), the coded component  270  is located within the handle  46  attached to the proximal end  14  of the catheter tube  12  that carries the structure  20 . However, the component  270  could be located elsewhere in relation the structure  20 . 
     The coded component  270  is electrically coupled to an external interpreter  278  when the structure  20  is coupled to the element  108  for use. The interpreter  278  inputs the code XYZ that the coded component  270  contains. The interpreter  278  electronically compares the input code XYZ to, for example, a preestablished master table  280  of codes contained in memory. The master table  280  lists, for each code XYZ, the structure-specific information required to create the positioning matrix to guide the IAE  50  within the structure  20 . 
     The element  108  preferably includes functional algorithms  288  which set guidance parameters based upon the code XYZ. These guidance parameters are used by the signal processing component  274  of the element in analyzing the spatial variations of the electric field created within the structure  20  to guide the IAE  150 . The guidance parameters are also used to create the position-identifying output displayed on the device  110 . 
     Because knowledge of the physical characteristic of the structure  20  and the spatial relationship of the electrodes  30  is important in setting accurate guidance parameters, the algorithms  288  preferably disable the guidance signal processing component  274  in the absence of a recognizable code XYX. Thus, only structures  20  possessing a coded component  270  carrying the appropriate identification code XYZ can be used in association with the element  108  to guide the IAE  50 . 
     The coded component  270  can be variously constructed. It can, for example, take the form of an integrated circuit  284  (see FIG.  27 ), which expresses in digital form the code XYZ for input in ROM chips, EPROM chips, RAM chips, resistors, capacitors, programmed logic devices (PLD&#39;s), or diodes. Examples of catheter identification techniques of this type are shown in Jackson et al. U.S. Pat. No. 5,383,874, which is incorporated herein by reference. 
     Alternatively, the coded component  270  can comprise separate electrical elements  286  (see FIG.  28 ), each one of which expressing a individual characteristic. For example, the electrical elements  286  can comprise resistors (R 1  to R 4 ), comprising different resistance values, coupled in parallel. The interpreter  278  measures the resistance value of each resistor R 1  to R 4 . The resistance value of the first resistor R 1  expresses in preestablished code, for example, the number of electrodes on the structure. The resistance value of the second resistor R 2  expresses in preestablished code, for example, the distribution of electrodes on the structure. The resistance value of the third resistor R 3  expresses in preestablished code, for example, the size of the structure. The resistance value of the fourth resistor R 4  expresses in preestablished code, for example, the shape of the structure. 
     Alternatively, the electrodes  30 / 31  can define passive markers that, in use, do not transmit or sense electrical energy. The markers are detected by the physician using, for example, external fluoroscopy, magnetic imaging, or x-ray to establish the location of the structure  20  and the IAE  50 . 
     D. Multiple Phased Transducer Arrays 
     The stability and support that the structure  20  provides the IAE  50  is well suited for use in association with an IAE  50  having one or more phased array transducer assemblies. The stability and support provided by the structure  20  make it possible to accommodate diverse numbers and locations of phased array transducers in close proximity to tissue, to further enhance the resolution and accuracy of images created by the IAE  50 . 
     In one embodiment, as FIG. 22 shows, the structure  20  carries an IAE  50  comprising a phased array  192  of ultrasonic transducers of the type shown, for example, in Shaulov U.S. Pat. No. 4,671,293, which is incorporated herein by reference. As FIG. 22 shows, the array  192  includes two groups  194  and  196  of electrodes. The electrode groups  194  and  196  are differently partitioned by channels  206  on opposite faces or planar sectors  194 ′ and  196 ′ of a piezoelectric material  198 . The channels  206  cut through the electrode surfaces partially into and through the piezoelectric material  198  to prevent mechanical and electrical coupling of the elements. 
     The channels  206  on the planar section  194 ′ create spaced transducer elements  202   a ,  202   b ,  202   c , etc. Likewise, the channels  206  on the planar section  196 ′ create spaced transducer elements  204   a ,  204   b ,  204   c , etc. 
     The electrode groups  194  and  196  are alternatively pulsed by a conventional phase array circuit  200 . During one pulse cycle, the electrode element group  194  is grounded, while the transducer elements  204   a ,  204   b ,  204   c , etc. on the other planar section  196 ′ are simultaneously pulsed, with the phase relationship of the stimulation among the transducer elements  204   a ,  204   b ,  204   c , etc. set to create a desired beam angle, acquiring an image along the one planar sector  196 ′. During the next pulse cycle, the other electrode element group  196  is grounded, while the transducer elements  202   a ,  202   b ,  202   c , etc. on the other planar section  194 ′ are likewise simultaneously pulsed, acquiring another image along the planar sector  194 ′. Further details, not essential to the invention, are provided in Haykin,  Adaptive Filter Theory , Prentice-Hall, Inc. (1991), pp. 60 to 65. 
     The signals received by the transducer groups  202   a ,  202   b ,  202   c , etc. and  204   a ,  204   b ,  204   c , etc., when pulsed, are processed into amplitude, phase, frequency, and time response components. The processed signals are compared to known configurations with varying transducers activated to produce and measure the desired waveform. When signals from combinations of transducers are processed, a composite image is produced. 
     The phased array  192  shown in FIG. 22 permits the real time imaging of two different planar sectors, which can be at any angle with respect to each other. 
     FIGS. 23 and 24 show other embodiments of an IAE  50  comprising a phased array of transducers carried within the structure  20 . 
     In the embodiment shown in FIG. 23, the IAE  50  comprises an array of flexible spline elements  208  having a known geometry. The spline elements  208  are carried within the support structure  20 , which itself comprises a larger diameter array of flexible spline elements  22 , as previously discussed in conjunction with FIG.  1 . Each flexible spline element  208  carries a grouping of multiple ultrasonic transducers  210 . 
     Collapsing the outer structure  20  of spline elements  22  by advancing the sheath  44  (previously described and shown in FIGS. 1 and 2) also collapses the inner IAE structure of spline elements  208 . The mutually collapsed geometry presents a low profile allowing joint introduction of the structures  22  and  208  into the desired body region. 
     In the embodiment shown in FIG. 24, the IAE  50  comprises an expandable-collapsible body  212  carried within the support structure  20 . Again, the structure  20  is shown as comprising the array of flexible spline elements  22 . Like the flexible spline elements  208  shown in FIG. 23, the exterior surface of the body  212  carries an array of multiple ultrasonic transducers  210 . 
     An interior lumen  214  within the body  216  carrying the IAE  50  conducts a fluid under pressure into the interior of the body  212  (as shown by arrows  213  in FIG. 24) to inflate it into a known expanded geometry for use. In the absence of the fluid, the body  212  assumes a collapsed geometry (not shown). The advanced sheath  44  envelopes the collapsed body  212 , along with the outer structure  20 , for introduction into the desired body region. 
     In the illustrated embodiment, the ultrasonic transducers  210  are placed upon the spline elements  208  or expandable body  212  (which will be collectively called the “substrate”) by depositing desired transducer materials or composites thereof onto the substrate. Ion beam assisted deposition, vapor deposition, sputtering, or other methods can be used for this purpose. 
     To create a spaced apart array of transducers  210 , a masking material is placed on the substrate to keep regions free of the deposited material. Removal of the masking material after deposition of the transducer materials provides the spaced apart array on the substrate. Alternatively, an etching process may be used to selectively remove sectors of the transducer material from the substrate to form the desired spaced apart array. The size of each deposited transducer  210  and the density of the overall array of transducers  210  should be balanced against the flexibility desired for the substrate, as conventional transducer material tends to be inherently stiffer than the underlying substrate. 
     Alternatively, transducers  210  can be attached in a preformed state by adhesives or the like to the spline elements  208  or flexible body  212 . Again, the size of each attached transducer  210  and the density of the overall array of transducers  210  should be balanced against the flexibility desired for the substrate. 
     Signal wires may be coupled to the transducers  210  in various ways after or during deposition or attachment; for example by soldering, or by adhesive, or by being deposited over. Various other ways to couple signal wires to solid or deposited surfaces on an expandable-collapsible body are discussed in copending patent application Ser. No. 08/629,363, entitled “Enhanced Electrical Connections for Electrode Structures,” filed Apr. 8, 1996, which is incorporated herein by reference. 
     The signal wires may be bundled together for passage through the associated catheter tube  12 , or housed in ribbon cables for the same purpose in the manner disclosed in Kordis U.S. Pat. No. 5,499,981, which is incorporated herein by reference. 
     It should be appreciated that the multiple ultrasonic transducers  210  could be supported on other types of bodies within the structure  20 . For example, non-collapsible hemispherical or cylindrical bodies, having fixed predetermined geometries, could occupy the interior of the structure  20  for the purpose of supporting phased arrays of ultrasonic transducers  210 . Alternatively, the signal wires and transducers may be braided into a desired three-dimensional structure. The braided structure may further be laminated to produce an inflatable balloon-like structure. The dimensions of these alternative transducer support bodies can vary, subject to the requirement of accommodating introduction and deployment in an interior body region. 
     Other examples of phased arrays of multiple transducers are found, for example, in Griffith et al. U.S. Pat. No. 4,841,977 and Proudian et al. U.S. Pat. No. 4,917,097. 
     Phased arrays of multiple transducers may be used in association with gating techniques, described above in conjunction with FIG. 9, to lessen the image acquisition time. In the dynamic environment of the heart, gating may be used to synchronize the phased acquisition of multiple plane images with the QRS or intracardiac electrogram activation, particularly if it is desired to analyze the images over more than one heart beat. 
     E. Visualization During Cardiac Mapping Procedures 
     (i) Electrical Activity Sensing 
     As just shown (see FIG. 10) and described, the structure  20  can carry an array of electrodes  30  for the purpose of guiding the IAE  50 . These same electrodes  30  can also serve to sense electrical impulses in tissue, like myocardial tissue. This sensing function in heart tissue is commonly called “mapping.” 
     As FIG. 10 shows, when deployed for use inside a heart chamber, the support structure  20  holds the electrodes  30  in contact against the endocardium. The electrodes sense the electrical impulses within the myocardium that control heart function. In this arrangement the element  108  includes or constitutes an external signal processor made, for example, by Prucka. Engineering, Inc. (Houston, Tex.). The processed signals are analyzed to locate aberrant conductive pathways and identify foci. The foci point to potential ablation sites. 
     Alternatively, or in combination with mapping, the electrodes  30  on the support structure  20  can be used to derive an electrical characteristic, such as impedance, in heart tissue for the purpose of characterizing tissue and locating aberrant conductive pathways. Systems and methods for deriving an electrical characteristic of tissue for this purpose are disclosed, for example, in Panescu et al U.S. Pat. No. 5,494,042, which is incorporated herein by reference. An electrical characteristic is derived by transmitting electrical energy from one or more electrodes into tissue and sensing the resulting flow of electrical energy through the tissue. 
     The IAE  50  carried within the multiple electrode structure  20  greatly assists the physician in mapping or characterizing tissue, whether in the heart or elsewhere in the body, by locating the electrodes  30  in the desired orientation with respect to selected anatomic sites. For example, when used within the heart, the physician can manipulate the IAE  50  in the manners previously described to visual identify the coronary sinus, heart valves, superior and inferior vena cava, the fossa ovalis, the pulmonary veins, and other key anatomic sites in the heart. Relying upon the visual information obtained by the IAE  50 , the physician can then orient the multiple electrode structure  20  with respect to one or more of these anatomic sites. Once properly oriented, the physician can further visualize with the IAE  50 , to assure that all or a desired number of the electrodes  30  carried by the structure  20  are in intimate contact with tissue required for good signal transmission or good signal acquisition. 
     As FIG. 12 shows, the IAE  50  can also be used to help visually steer a separate mapping electrode  112 , carried on its own catheter tube  121 , outside or within the support structure  20  into the desired location in contact with heart tissue. If the roving electrode  112  is present within the confines of the support structure  20 , the structure  20  also serves to stabilize the electrode  112 . The guidance processing element  108  as previously described (see FIG. 10) can be used in association with the structure  20  to electronically home the external mapping electrode  112  to a desired location within the structure  20 . 
     (ii) Contrast Echocardiography 
     FIG. 8 shows a system  170  that includes the structure  20  carrying an IAE  50  to identify perfusion patterns in myocardial tissue and, thereby, diagnose potential ablation sites within the heart. In this embodiment, the IAE  50  carried within the structure  20  comprises a rotating ultrasonic transducer  52  of the type previously described in conjunction with FIG.  6 . The system  170  shown in FIG. 8 also preferably includes an electro-mechanical system  102  for incrementally moving the transducer  52  within the structure  20  to obtain axially spaced, data sample slices of the region surrounding the transducer  52 . The details of this the system  102  have been previously described in conjunction with FIG.  9 . The electro-mechanical system  102  may also be gated to the QRS of an electrocardiogram or to intracardiac electrogram activation to acquire images at either end-diastolic or end-systolic points of the heart cycle, in the manner also previously described in conjunction with FIG. 9 or  21 . 
     The system  170  shown in FIG. 8 includes a separate catheter  172 . The catheter  172  includes an interior lumen  174 , which is coupled to a source of an echoluscient contrast media  176 . The catheter  172  injects the media  176  into the blood stream. 
     The echoluscient contrast media  176  used may vary. In a preferred embodiment, the media  176  comprises sonicated albumin microbubbles, or their equivalent, having a diameter smaller than red blood cells (which are typically about 8 μm). 
     When carried within the blood stream, the microbubbles in the media  176  are perfused into tissue, just as the blood components that accompany them. The microbubbles in the media  176 , perfused into tissue, strongly scatter ultrasonic waves. They appear ultrasonically “bright” in contrast to the less ultrasonically “bright” cellular components of blood also perfused into tissue. The physician is thereby able to accurately observe the patterns of perfusion of the media  176  into tissue. The more volume of media  176  perfused into tissue, the brighter the ultrasonic image, and vice versa. 
     Myocardial tissue that has been infarcted has significantly lower perfusion characteristics than healthy myocardial tissue. See, for example, Nath et al., “Effects of Radiofrequency Catheter Ablation on Regional Myocardial Blood Flow,”  Circulation,  1994; 89: 2667-2672; and Villaneuva et al., “Assessment of Risk Area During Coronary Occlusion and Infarct Size After Reperfusion with Myocardial Contrast Echocardiography Using Left and Right Atrial Injections of Contrast,”  Circulation,  1993; 88:596-604). 
     As FIG. 8 shows, the catheter  172  is preferably maneuvered percutaneously into a selected coronary vessel. The contrast media  176  is injected through the catheter lumen  174  into the vessel, and thus into the vascular system near the heart. 
     If the selected vessel is the coronary artery, the media  176  is distributed throughout the regions of the heart perfused by the coronary artery, increasing the resolution and contrast in a selected localized region. More global distribution of contrast media  176  can be obtained by selecting an injection site in one of the heart chambers or in the pulmonary artery. 
     For example, if myocardial tissue in the basil or posterio-lateral aspect of the left ventricle is slated for diagnosis, the catheter  172  is preferably maneuvered to inject the media  176  into the circumflex coronary artery branch of the left main artery. If myocardial tissue in the anterior aspect of the right or left ventricles is slated for diagnosis, the catheter  172  is preferably maneuvered to inject the media  176  into the left anterior descending (LAD) coronary artery branch of the left main artery. If myocardial tissue in the free wall of the right ventricle or the posterior ventricular septum is slated for diagnosis, the catheter  172  is preferably maneuvered to inject the media  176  into the right coronary artery. 
     Alternatively, the media  176  can be injected directly into the left atrium or left ventricle. In this arrangement, the body  36  carrying the transducer  52  can also include an interior lumen  178  to convey the media  176 . This approach may be easier and potentially less traumatic than injection directly into the coronary artery. However, a portion of the media  176  will still be dispersed past the coronary arteries and through the systemic arterial system, thereby resulting in a poorer resolution per given volume of media  176  injected. Therefore, a larger volume of media  176  should be injected directly into the left atrium or ventricle to obtain contrast in myocardial tissue comparable to a smaller volume of media  176  injected directly into a coronary artery, as described above. 
     Furthermore, contrast media  176  may be injected systemically into the femoral vein. Again, with this approach, significant portions of the media  176  will be disbursed within the circulatory system, and, in particular, into the lungs. As just discussed, a larger volume of media  176  should be injected systemically into the femoral vein to obtain contrast in myocardial tissue comparable to a smaller volume of media  176  injected directly into a coronary artery. 
     The system  170  includes a receiver and processor  180  and display device  182 , as earlier described in conjunction with FIG.  6 . In synchrony with the axial translation system  102 , the receiver and processor  180  preferably creates a three-dimensional image for display on the device  182 . Alternatively, an echocardiographic image may be created for display without using the axial translation system  102 . 
     The contrast media  176  highlights the differences in perfusion in myocardial tissue surrounding the structure  20 . Regions of infarcted tissue are visually characterized, as they are not well perfused with blood and appear in negative contrast to the healthy tissue regions that are well perfused. The same visually characterized, negative contrast regions of infarcted tissue may also form part of the pathways of slow conduction of electrical impulses. These slow conduction pathways may be a substrate for ventricular tachycardia and therefore candidates for cardiac ablation. These candidate regions of slow conduction pathways will, in the presence of the contrast media  186 , appear on the ultrasonic device  182  as zones of negative contrast, being significantly less ultrasonically “bright” than well perfused tissue regions. The candidate regions of slow conduction will typically have infarcted tissue interspersed with well perfused tissue. The candidate regions will therefore appear ultrasonically “mottled”, with patchy regions of darker contrast interspersed with lighter contrast. The mottled zones will appear contiguous to negative contrast areas. The image resolution of the device  182  should preferably be fine enough to discern among mottled zones, light contrast zones, and dark contrast zones. 
     The support structure  20  maintains the transducer  54  in a stable, substantially unobstructed viewing position near the targeted tissue region. The transducer  54  thereby generates ultrasonic images of the differences in perfusion of the media  176  throughout the imaged heart tissue. The system  170  therefore make possible the accurate characterization of tissue for identifying potential ablation sites using contrast echocardiography. 
     In addition to identifying candidate ablation sites, the stable, unobstructed perfusion images that the system  170  provides, also make it possible to discern the lesion characteristic required to treat the arrhythmia. The perfusion pattern may indicate a localized, contained mottled contrast area, suited for treatment by creating an equally localized, small surface area lesion. Alternatively, the perfusion pattern may indicate a larger or deeper mottled contrast area, or a mottled contrast area that is elongated or a random complex of different, intersecting geometries. These instances give rise to the need for corresponding larger or deeper lesion patterns, or long or intersecting legion patterns, or lesion patterns otherwise having geometries tailored to the geometry of the mottled contrast area. 
     The stable, unobstructed perfusion images that the system  170  provides also make it possible to characterize tissue substrates associated with polymorphic ventricular tachycardia. The system  170  makes it possible to characterized these regions using echocardiography during normal sinus rhythm. Conventional mapping of electrical events requires induction of sometimes hemodynamically unstable rhythms to locate and ablate substrates associated with polymorphic ventricular tachycardia. 
     The stable, unobstructed perfusion images that the system  170  provides also make it possible to discern intermediate contrast zones between “bright” (well perfused tissue) images and negative contrast (not well perfused, infarcted tissue) images. These intermediate contrast zones also delineate the infarcted tissue border. Once identified, tissue ablation can be conducted with the objective of ablating tissue within the border zone, to eliminate the potential for ventricular tachycardia substrates. 
     The system  170  may characterize tissue morphology based upon echocardiography to locate potential ablation sites in other ways. For example, the system  170  may image based upon ultrasonic frequency domain analyses. For example, the intensity of the second harmonics can be used to identify tissue morphologies such as scar tissue, ischemic tissue, infarcted tissue, and healthy tissue as a function of tissue elasticity. Frequency domain analyses like second harmonics may be used without the injection of contrast media  170  to characterize tissue for ablation purposes. 
     The system  170  for carrying out contrast echocardiography may also incorporate an IAE  50  comprising multiple transducers and using phased array techniques to enhance the perfusion images, as previously described in conjunction with FIGS. 22 to  24 . 
     FIG. 8 shows the system  170  being used in association with intracardiac echocardiography. It should also be appreciated that the echocardiography can be used to characterize tissue morphology, and thereby identify potential ablation sites, using external ultrasound transducers located outside the body. 
     It should also be appreciated that the system  170  can be used as an adjunct to other echography procedures; for example, transesophageal or transthoracic echography. 
     The analysis of tissue perfusion patterns to characterize myocardial tissue to locate potential ablation sites can also be accomplished using external imaging techniques other than echography. For example, magnetic resonance imaging (MRI) can be used. Using MRI, an isotope, such as gadolinium-chelate, is injected to serve as the contrast material. As another example, computerized tomography (CT) scanning can be used. Using CT, iodine radiopaque compounds, such as renografin, can be injected to serve as the contrast material. As another example, nuclear imaging using thallium as the contrast material can be used. Using any of these alternative imaging techniques, slow conduction pathways in myocardial tissue will, in the presence of the appropriate contrast media, appear as zones of negative or mottled contrast. As before discussed, the image resolution of the alternative technique should preferably be fine enough to discern among mottled zones, light contrast zones, and dark contrast zones. The alternative imaging techniques, like echography, can also be used to discern intermediate contrast zones, which delineate infarcted tissue borders. 
     II. Visualization for Therapeutic Purposes 
     The foregoing description of the structure  20  and associated IAE  50  exemplify use in the performance of general diagnostic functions, to accurately locate and identify abnormalities that may be present in body cavities or in electrical activities within tissue. The structure  20  and associated IAE  50  can also aid in providing therapeutic functions, alone or in combination with these and other diagnostic functions. 
     The following exemplifies this use in the context of treating cardiac arrhythmias. However, it will be appreciated that there are diverse applications where the invention can serve therapeutic functions or both diagnostic and therapeutic functions. 
     A. Lesion Formation 
     Once a potential ablation site has been identified by mapping (typically, in the ventricle), or by reference to an anatomic landmark within the heart (typically, in the atrium), or by deriving an electrical characteristic, the physician deploys an ablation element to the site. While various types of ablation energy can be used, in the preferred implementation, the ablation electrode transmits radio frequency energy conveyed from an external generator (not shown). The ablation element can takes various forms, depending upon the type of lesion required, which, in turn, depends upon the therapeutic effect desired. 
     (i) Smaller Lesions 
     Typically, lesions that are characterized as “small and shallow” have a depth of about 0.5 cm, a width of about 10 mm, and a lesion volume of up to 0.2 cm 3 . FIG. 16 exemplifies the geometry for a typical “small” lesion  118 . These lesions are desired in the sinus node for sinus node modifications, or along the A-V groove for various accessory pathway ablations, or along the slow zone of the tricuspid isthmus for atrial flutter (AFL) or AV node slow pathways ablations. For this purpose, a physician will typically deploy an electrode having approximately an 8F diameter and a 4 mm length to transmit radio frequency energy to create small and shallow lesions in myocardial tissue. 
     This type of ablation electrode can be used in association with the support structure  20 , even when the catheter tube bore is occupied by the imaging probe  34 . In this arrangement (see FIG.  12 ), the physician separately deploys the ablation electrode as a “roving” electrode  112  outside the support structure  20 . The physician then steers the external electrode  112  into the confines of the support structure  20  for ablation (such an electrode  112  can also perform an auxiliary mapping function, as already described). Usually, the electrode  112  is preferably operated in a uni-polar mode during ablation, in which the radio frequency ablation energy transmitted by the electrode  112  is returned through an indifferent patch electrode  114  externally attached to the skin of the patient. 
     The support structure  20  serves to stabilize the external “roving” ablation electrode  112  within a confined region of the heart. The IAE  50  can be used in this arrangement to help visually navigate the roving ablation electrode  112  into the desired location in contact with heart tissue. The guidance processing element  108  as previously described (see FIG. 10) can also be used in association with the structure  20  to electronically home the roving ablation electrode  112  to the desired ablation site contacting the support structure  20 . 
     Alternatively (as FIGS. 5 and 10 show), the electrode  31  that the IAE  50  carries can comprise an ablation electrode, in the manner shown in U.S. Pat. No. 5,385,148, which is incorporated herein by reference. The exterior diameter of the IAE  50  (with electrode  31 ) is preferably larger than the interior diameter of the catheter tube bore  38  (see FIG.  5 A). Thus, while the IAE  50  (and electrode  31 ) can be freely moved within the structure  20  in the manner already described, it cannot be withdrawn into the catheter tube bore. 
     In this arrangement, the slidable sheath  44  that encloses the structure  20  during deployment (see FIG.  2 ), also encloses the IAE  50  and ablation element  31  within the collapsed structure  20 . Further details of a structure integrating a movable element within a multiple electrode support structure can be found in U.S. Pat. No. 5,476,495, which is incorporated herein by reference. 
     As before explained, the guidance processing element  108  (FIG. 10) can also create a position-identifying output in a real-time format most useful to the physician for guiding the ablation electrode  31  carried by the IAE  50  within the structure  20  toward a potential site identified for ablation. 
     In an alternative embodiment, the exterior diameter of the IAE  50  (with electrode  31 ) is smaller than the interior diameter of the catheter tube bore  38 . The IAE  50  and the entire imaging probe  34  can thereby be withdrawn through the catheter tube bore  38  from the catheter tube  12 . In this arrangement, the catheter tube  12  carrying the multiple electrode support structure  20  and the imaging probe  34  comprise separately deployed components. The imaging probe  34  is deployed through the catheter tube  12  only when the visualization function is required. When the imaging probe  34  is withdrawn, the catheter tube bore  38  is open to provide passage for other components; for example, the separate mapping or ablation electrode  112  shown in FIG.  12 . In this arrangement, the imaging probe  34  can be switched in situ with the mapping or ablation electrode  112 , without altering the position of the structure  20 . 
     (ii) Larger Lesions 
     The elimination of ventricular tachycardia (VT) substrates is thought to require significantly larger and deeper lesions, with a penetration depth greater than 1.5 cm, a width of more than 2.0 cm, with a lesion volume of at least 1 cm 3 . There also remains the need to create lesions having relatively large surface areas with shallow depths. FIG. 17 exemplifies the geometry of a typical larger surface area lesion  120 , compared to the geometry of the smaller lesion  118  shown in FIG.  16 . 
     FIGS. 13A and 13B show an alternative embodiment of the invention, which provides a composite structure  122  carrying an imaging probe  124  and an ablation element  126 , which is capable of providing larger lesions. The composite structure  122  (like structure  20  shown in FIG. 1) is carried at the distal end of a flexible catheter tube  12 . The proximal end of the catheter tube carries an attached handle  18  for manipulating the composite structure in the manners previously described. 
     The composite structure  122  comprises an expandable-collapsible hollow body  128  made from a porous transparent thermoplastic or elastomeric material. The size of the pores  129  in the body  128  are exaggerated for the purpose of illustration in FIG.  13 A. The entire body  128  may be porous, or the body  128  may include a discrete porous region. 
     The body  128  carries within it an interior electrode  130 , which is formed of an electrically conductive material that has both a relatively high electrical conductivity and a relatively high thermal conductivity. Materials possessing these characteristics include gold, platinum, platinum/iridium, among others. Noble metals are preferred. An insulated signal wire  132  is coupled to the electrode  130 , which electrically couples the electrode  130  to an external radio frequency generator  134 . 
     An interior lumen  136  within the catheter tube  12  conducts an electrically conductive liquid  140  under pressure from an external source  138  into the hollow interior of the expandable-collapsible body  128 . As FIG. 13A shows, the electrically conductive liquid  140  inflates the body  128  to an enlarged, or expanded, geometry. As will be explained later, it is this expanded geometry that makes possible the formation of the larger lesions desired. As FIG. 13B shows, in the absence of the fluid  140 , the expandable-collapsible body  128  assumes a collapsed, low profile. It is this low profile that permits straightforward introduction of the structure  122  into the body. 
     When radio frequency energy is transmitted by the interior electrode  130 , the electrically conductive liquid  140  within the body  128  establishes an electrically conductive path. The pores of the porous body  128  establish ionic transport of ablation energy from the electrode  130 , through the electrically conductive liquid  140 , to tissue outside the body. The paths of ionic transport are designated by arrows  142  in FIG.  13 A. 
     Preferably, the liquid  140  possesses a low resistivity to decrease ohmic loses, and thus ohmic heating effects, within the body  128 . The composition of the electrically conductive liquid  140  can vary. In the illustrated and preferred embodiment, the liquid  140  comprises a hypertonic saline solution, having a sodium chloride concentration at or near saturation, which is about 9% weight by volume. Hypertonic saline solution has a low resistivity of only about 5 ohm·cm, compared to blood resistivity of about 150 ohm·cm and myocardial tissue resistivity of about 500 ohm·cm. 
     Alternatively, the composition of the electrically conductive liquid  140  can comprise a hypertonic potassium chloride solution. This medium, while promoting the desired ionic transfer, requires closer monitoring of the rate at which ionic transport  142  occurs through the pores, to prevent potassium overload. When hypertonic potassium chloride solution is used, it is preferred to keep the ionic transport rate below about 10 mEq/min. The imaging probe  124  is also located within the body  128 . As before described, the probe  124  includes a flexible body  36 , which extends through a central bore  38  and a hemostatic valve (not shown) at the distal end of the catheter tube  12 . The body  36  has a distal region  40  that projects beyond the distal end  16  of the catheter tube  12  into the interior of the support structure  20 . The distal body region  40  carries an IAE  150 , which is sealed from the surrounding liquid  140 , for example, within a housing. Like IAE  50  before described, the IAE  150  generates visualizing signals representing an image of objects surrounding the body  128 . 
     As before explained in conjunction with FIG. 5A, the IAE  150  is preferably carried for forward and rearward movement by pushing or pulling upon the body  36 . The IAE  150  is also preferably movable transverse of the body axis by the provision of a steering mechanism  76  in the distal region  40 , as already described. 
     The IAE  150  can be variously constructed, depending upon the transparency of the body  128  to imaging energy. 
     For example, if the body  128  is transparent to optical energy, the IAE  150  can comprise a fiber optic channel, as already generally described (see FIG. 7 or FIG.  25 ). Regenerated cellulose membrane materials, typically used for blood oxygenation, dialysis, or ultrafiltration, can be made to be optically transparent. Regenerated cellulose is electrically non-conductive; however, the pores of this material (typically having a diameter smaller than about 0.1 μm) allow effective ionic transport  142  in response to the applied RF field. At the same time, the relatively small pores prevent transfer of macromolecules through the body  128 , so that pressure driven liquid perfusion through the pores  129  is less likely to accompany the ionic transport  142 , unless relatively high pressure conditions develop within the body  128 . 
     Regenerated cellulose is also transparent to ultrasonic energy. The IAE  50  can thus alternatively comprise an ultrasonic transducer crystal, as also already described (see FIG.  6 ). 
     Other porous materials, which are either optically transparent or otherwise transparent to the selected imaging energy, can be used for the body  128 . Candidate materials having pore sizes larger than regenerated cellulous material, such as nylon, polycarbonate, polyvinylidene fluoride (PTFE), polyethersulfone, modified acrylic copolymers, and cellulose acetate, are typically used for blood microfiltration and oxygenation. Porous or microporous materials may also be fabricated by weaving a material (such as nylon, polyester, polyethylene, polypropylene, fluorocarbon, fine diameter stainless steel, or other fiber) into a mesh having the desired pore size and porosity. These materials permit effective passage of ions in response to the applied RF field. However, as many of these materials possess larger pore diameters, pressure driven liquid perfusion, and the attendant transport of macromolecules through the pores, are also more likely to occur at normal inflation pressures for the body  128 . Considerations of overall porosity, perfusion rates, and lodgment of blood cells within the pores of the body  128  must be taken more into account as pore size increase. 
     Low or essentially no liquid perfusion through the porous body  128  is preferred. Limited or essentially no liquid perfusion through the porous body  128  is beneficial for several reasons. First, it limits salt or water overloading, caused by transport of the hypertonic solution into the blood pool. This is especially true, should the hypertonic solution include potassium chloride, as observed above. Furthermore, limited or essentially no liquid perfusion through the porous body  128  allows ionic transport  142  to occur without disruption. When undisturbed by attendant liquid perfusion, ionic transport  142  creates a continuous virtual electrode at the body  128 -tissue interface. The virtual electrode efficiently transfers RF energy without need for an electrically conductive metal surface. 
     As shown in FIG. 13A, the porous body  128  serves a dual purpose. Like the structure  20 , the porous body  128  keeps open the interior chamber or passages within the patient&#39;s body targeted for imaging, while at the same time keeping tissue T away from potential occluding contact with the IAE  150 . The body  128  also helps to stabilize the position of the IAE  50 . In these ways, the body  128 , like the support structure  20 , provides a substantially stationary platform for visualizing tissue and anatomic structures for diagnostic purposes, making possible the creation of an accurate image of the targeted body cavity. 
     Furthermore, through the ionic transfer  142  of the RF field generated within the body  128 , the porous body  128  also serves the therapeutic function as a tissue ablation element. The use of a porous body  128 , expanded after introduction to an enlarged diameter (see FIG.  13 A), makes possible the creation of larger lesions in a controlled fashion to ablate epicardial, endocardial, or intramural VT substrates. By also controlling the porosity, and thus the electrical resistivity of the body  128 , the physician can significantly influence the depth of the lesion. The use of a low-resistivity body  128  results in deeper lesions, and vice versa. 
     Further details of the use of porous bodies to deliver ablation energy through ionic transport are found in copending patent application Ser. No. 08/631,356, filed Apr. 12, 1996 and entitled “Tissue Heating and Ablation Systems and Methods Using Electrode Structures With Distally Oriented Porous Regions,” which is incorporated herein by reference. 
     In an alternative embodiment, the porous body  128  and IAE  150  can themselves occupy the interior of a multiple spline support structure  146 , as shown in FIG.  14 . In this arrangement, the exterior multiple spline structure  146  provides added stabilization and protection for the porous body and IAE  150 . As shown in FIG. 14, the multiple spline support structure  146  may also carry an array of electrodes  148 . These electrodes  148  can be used for mapping or characterizing tissue or for guidance of the interior porous ablation body and IAE  150 , in the manners previously described. 
     (iii) Long Lesions 
     Atrial geometry, atrial anisotropy, and histopathologic changes in the left or right atria can, alone or together, form anatomic obstacles. The obstacles can disrupt the normally uniform propagation of electrical impulses in the atria, resulting in abnormal, irregular heart rhythm, called atrial fibrillation. 
     U.S. patent application Ser. No. 08/566,291, filed Dec. 1, 1995, and entitled “Systems and Methods for Creating Complex Lesion Patterns in Body Tissue” discloses catheter-based systems and methods that create complex long lesion patterns in myocardial tissue. In purpose and effect, the systems and methods emulate the open heart maze procedure, but do not require costly and expensive open heart surgery. These systems and methods can be used to perform other curative procedures in the heart as well. 
     The multiple spline support structure  152  shown in FIG. 15 is well suited for therapeutic use in the atrial regions of the heart. In FIG. 15, a transeptal deployment is shown, from the right atrium (RA), through the septum (S), into the left atrium (LA), where the support structure  152  is located for use. 
     The longitudinal splines  154  carry an array of electrodes  156 . The electrodes  156  serve as transmitters of ablation energy. An IAE  50 , as previously described, is movably carried within the interior of the structure  152 . 
     The electrodes  156  are preferably operated in a uni-polar mode, in which the radio frequency ablation energy transmitted by the electrodes  156  is returned through an indifferent patch electrode  158  externally attached to the skin of the patient. Alternatively, the electrodes  156  can be operated in a bi-polar mode, in which ablation energy emitted by one or more electrodes  156  is returned an adjacent electrode  156  on the spline  154 . 
     The size and spacing of the electrodes  156  shown in FIG. 15 are purposely set for creating continuous, long lesion patterns in tissue. FIG. 18 shows a representative long, continuous lesion pattern  160 , which is suited to treat atrial fibrillation. Continuous, long lesion patterns  160  are formed due to additive heating effects when RF ablation energy is applied in a uni-polar mode simultaneously to the adjacent electrodes  156 , provided the size and spacing requirements are observed. The additive heating effects cause the lesion pattern  160  to span adjacent, spaced apart electrodes  156 , creating the desired elongated geometry, shown in FIG.  18 . The additive heating effects will also occur when the electrodes  156  are operated simultaneously in a bipolar mode between electrodes  156 , again provided the size and spacing requirements are observed. 
     The additive heating effects between spaced apart electrodes  156  intensify the desired therapeutic heating of tissue contacted by the electrodes  156 . The additive effects heat the tissue at and between the adjacent electrodes  156  to higher temperatures than the electrodes  156  would otherwise heat the tissue, if conditioned to individually transit energy to the tissue, or if spaced apart enough to prevent additive heating effects. 
     When the spacing between the electrodes  156  is equal to or less than about 3 times the smallest of the diameters of the electrodes  156 , the simultaneous emission of energy by the electrodes  156 , either bipolar between the segments or unipolar to the indifferent patch electrode, creates the elongated continuous lesion pattern  160  shown in FIG. 18 due to the additive heating effects. Conversely, when the spacing between the electrodes  156  is greater than about 5 times the smallest of the diameters of the electrodes  156 , the simultaneous emission of energy by the electrodes  156 , either bipolar between segments or unipolar to the indifferent patch electrode, does not generate additive heating effects. Instead, the simultaneous emission of energy by the electrodes  156  creates an elongated segmented, or interrupted, lesion pattern  162  in the contacted tissue area, as shown in FIG.  20 . 
     Alternatively, when the spacing between the electrodes  156  along the contacted tissue area is equal to or less than about 2 times the longest of the lengths of the electrodes  156 , the simultaneous application of energy by the electrodes  156 , either bipolar between electrodes  156  or unipolar to the indifferent patch electrode, also creates an elongated continuous lesion pattern  160  (FIG. 18) due to additive heating effects. Conversely, when the spacing between the electrodes  156  along the contacted tissue area is greater than about 3 times the longest of the lengths of the electrodes  156 , the simultaneous application of energy, either bipolar between electrodes  156  or unipolar to the indifferent patch electrode, creates an elongated segmented, or interrupted, lesion pattern  162  (FIG.  20 ). 
     In an alternative embodiment (see FIG.  15 ), the assembly includes periodic bridge splines  164 . The bridge splines  164  are soldered or otherwise fastened to the adjacent longitudinal splines  154 . The bridge splines  164  carry electrodes  166 , or are otherwise made to transmit ablation energy by exposure of electrically conductive material. Upon transmission of ablation energy, the bridge splines  166  create long transverse lesion patterns  168  (see FIG. 19) that span across the long longitudinal lesion patterns  160  created by the adjacent splines  154 . The transverse lesions  168  link the longitudinal lesions  160  to create complex lesion patterns that emulate the patterns formed by incisions during the surgical maze procedure. 
     Further details of the creation of complex long lesion patterns in the treatment of atrial fibrillation are found in copending U.S. application Ser. No. 08/566,291, filed Dec. 1, 1995, and entitled “Systems and Methods for Creating Complex Lesion Patterns in Body Tissue,” which is incorporated herein by reference. 
     B. Lesion visualization 
     The IAE  50 / 150  associated with the structures shown permits the physician to visually inspect the lesion pattern during or after ablation to confirm that the desired pattern and depth have been created. By manipulating the IAE  50 / 150  in the manner described above during or after ablation, the physician can view the lesions from different directions, to assure that the lesion geometry and depth conforms to expectations. The IAE  50 / 150  can also inspect a long lesion pattern (like patterns  160  or  168  in FIG. 19) during or after ablation for gaps or interruptions, which could, if present, provide unwanted pathways for aberrant electrical pulses. Contrast echocardiography, employing contrast media (as earlier described in conjunction with FIG.  8 ), may also be used to identify gaps in long lesions during or after their formation. Since perfusion through thermally destroyed tissue is significantly less than in other tissue, gaps in long lesion patterns (i.e., tissue that has not been thermally destroyed) will, in the presence of contrast media, appear ultrasonically “brighter” than tissue in the lesion area. Ablation of these gaps, once identified by the IAE  50 / 150 , completes the long lesion pattern to assure that the intended therapeutic result is achieved. 
     The IAE  50 / 150  can also help the physician measure the width, length, and depth of the lesion pattern. Using the IAE  50 / 150 , the physician can directly measure these physical lesion characteristics, instead of or as an adjunct to predicting such characteristics from measurements of applied power, impedance, tissue temperature, and ablation time. 
     The IAE  50 / 150  can further help the physician characterize tissue morphology. Using the IAE  50 / 150 , the physician can visualize border regions between healthy and infarcted tissue, alone or in combination with electrical impulse sensing with the electrodes  156 . 
     Various features of the invention are set forth in the following claims.