Abstract:
A system and method for opto-acoustic tissue and lesion assessment in real time on one or more of the following tissue characteristics: tissue thickness, lesion progression, lesion width, steam pop, and char formation, system includes an ablation element, laser delivery means, and an acoustic sensor. The invention involves irradiating tissue undergoing ablation treatment to create acoustic waves that have a temporal profile which can be recorded and analyzed by acoustic sampling hardware for reconstructing a cross-sectional aspect of the irradiated tissue. The ablation element (e.g., RF ablation), laser delivery means and acoustic sensor are configured to interact with a tissue surface from a common orientation; that is, these components are each generally facing the tissue surface such that the direction of irradiation and the direction of acoustic detection are generally opposite to each other, where the stress waves induced by the laser-induced heating of the tissue below the surface are reflected back to the tissue surface.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to electrophysiologic catheters, and in particular to laser-optoacoustic electrophysiologic catheters for monitoring tissue and lesion assessment. 
       BACKGROUND 
       [0002]    For certain types of minimally invasive medical procedures, real time information regarding the condition of the treatment site within the body is unavailable. This lack of information inhibits the clinician when employing a catheter to perform a procedure. An example of such procedures is tumor and disease treatment in the liver and prostate. Yet another example of such a procedure is cardiac ablation used to treat atrial fibrillation. This condition in the heart causes abnormal electrical signals to be generated in the endocardial tissue resulting in irregular beating of the heart. 
         [0003]    The most frequent cause of cardiac arrhythmias is an abnormal routing of electricity through the cardiac tissue. In general, most arrhythmias are treated by ablating suspected centers of this electrical misfiring, thereby causing these centers to become inactive. Successful treatment, then, depends on the location of the ablation within the heart as well as the lesion itself. For example, when treating atrial fibrillation, an ablation catheter is maneuvered into the right or left atrium where it is used to create ablation lesions in the heart. These lesions are intended to stop the irregular beating of the heart by creating non-conductive barriers between regions of the atria that halt passage through the heart of the abnormal electrical activity. 
         [0004]    The lesion should be created such that electrical conductivity is halted in the localized region (transmurality), but care should be taken to prevent ablating adjacent tissues. Furthermore, the ablation process can also cause undesirable charring of the tissue and localized coagulation, and can evaporate water in the blood and tissue leading to steam pops. 
         [0005]    Currently, lesions are evaluated following the ablation procedure, by positioning a mapping catheter in the heart where it is used to measure the electrical activity within the atria. This permits the physician to evaluate the newly formed lesions and determine whether they will function to halt conductivity. It if is determined that the lesions were not adequately formed, then additional lesions can be created to further form a line of block against passage of abnormal currents. Clearly, post ablation evaluation is undesirable since correction requires additional medical procedures. Thus, it would be more desirable to evaluate the lesion as it is being formed in the tissue. 
         [0006]    A known method for evaluating lesions as they are formed is to measure electrical impedance. Biochemical differences between ablated and normal tissue can result in changes in electrical impedance between the tissue types. Although impedance is routinely monitored during electrophysiologic therapy, it is not directly related to lesion formation. Measuring impedance merely provides data as to the location of the tissue lesion but does not give qualitative data to evaluate the effectiveness of the lesion. 
         [0007]    Another approach is to measure the electrical conductance between two points of tissue. This process, known as lesion pacing, can also determine the effectiveness of lesion therapy. This technique, however, measures the success or lack thereof from each lesion, and yields no real-time information about the lesion formation. 
         [0008]    In a broader sense, ultrasonic imaging is also known for detecting abnormalities in soft tissue organs with acoustic boundaries. But tissues can be acoustically homogenous and therefore undetectable by ultrasound imaging. Similar limitations are posed by optical imaging based on time-resolved or phase resolved detection of diffusely reflected light pulses or photon density waves. 
         [0009]    Laser optoacoustic technology can offer advantages over the aforementioned technologies. Improvement in sensitivity, spatial resolution and interpretation of images is possible with suitable manipulation of (1) short-pulse laser irradiation to generate transient stress waves under conditions of temporal stress confinement, where such irradiations provide large amplitude of generated stress with profiles resembling that of light distribution in tissues to yield sharp images with accurate localization; (2) time-resolved detection of stress profile for obtaining diagnostic information from the temporal profile of generated stress wave; and (3) use of wide-band piezoelectric detectors to correctly reproduce stress profiles to obtain spatial resolution of tomography. However, application of this technology in vivo, and particularly in vivo endocardial and epicardial applications, has been limited due to various factors, including space constraints and integration of the equipment to provide irradiation and detection of optoacoustic data. 
         [0010]    Thus, there is a need for an integrated electrophysiologic catheter capable of monitoring tissue and performing lesion assessment, especially for endocardial and epicardial tissue, in real-time using optoacoustic technology for improved sensitivity and spatial resolution. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention recognizes that light delivered in sufficiently short pulse widths is selectively absorbed by tissue elements and surrounding medium (blood), and is converted to heat. This heat produces an acoustic wave which can be detected by an acoustic sensor. A delay in receive time of the acoustic wave is proportional to the distance of elements generating the acoustic wave from the light delivery optics, and can be used to determine tissue thickness. To that end, optoacoustic imaging employs non-resonant acoustic frequencies which result from optical absorption properties of materials within the field of view of the light delivery optics. As such, the signal output has greater sensitivity to materials with different optical absorption properties, such as those between tissue and blood or air. It is therefore possible to obtain high resolution imaging of biological tissue through blood, with an operable range up to several centimeters (as determined by wavelength, optical absorption, and acoustic sensor size). Such imaging can be particularly advantageous during and concurrently with ablation for visualization of lesion formation. 
         [0012]    The present invention is directed to a system and method for opto-acoustic tissue and lesion assessment in real time on one or more of the following tissue characteristics: tissue thickness, lesion progression, lesion width, steam pop, and char formation. The system includes an ablation element, laser delivery means, and an acoustic sensor. These elements work by irradiating tissue undergoing ablation treatment to create acoustic waves that have a temporal profile which can be recorded and analyzed by acoustic sampling hardware for reconstructing a cross-sectional aspect of the irradiated tissue. In accordance with the present invention, the ablation element (e.g., RF ablation), laser delivery means and acoustic sensor are configured to interact with a tissue surface from a common orientation; that is, these components are each generally facing the tissue surface such that the direction of irradiation and the direction of acoustic detection are generally opposite to each other, where the stress waves induced by the laser-induced heating of the tissue below the surface are reflected back to the tissue surface. 
         [0013]    In a more detailed embodiment, the system includes a catheter having an integrated distal tip section that is configured for irradiation and acoustic detection, an electronic scope and a processor. Advantageously, tissue that is heated by the irradiation from the catheter tip section produces an acoustic wave that is detected by a acoustic detector which generates a signal representative of a tissue characteristic which is received by an electronic scope to record a temporal profile of the acoustic wave. The processor uses the temporal profile to reconstruct a cross-sectional aspect of the tissue. 
         [0014]    The present invention is also directed to a catheter for opto-acoustic tissue assessment in real time. In one embodiment, the catheter has a catheter body and a distal tip section that is configured for irradiation and acoustic detection, wherein a tissue is heated by the irradiation to produce an acoustic wave that is detected by an acoustic detector mounted on the tip section and the acoustic detector generates a signal representative of a tissue characteristic. In a more detailed embodiment, the catheter is configured for use with cardiac tissue and the tip section is configured for RF ablation. Moreover, the irradiation emitted by the catheter may be a laser pulse, and the tissue of interest may be a lesion resulting from RF ablation. 
         [0015]    The present invention is also directed to a method of assessing tissue with laser optoacoustic imaging, comprising irradiating tissue from a distal end of a catheter to heat said tissue for producing an acoustic wave, detecting said acoustic wave with an acoustic transducer mounted on the catheter, recording characteristics of the acoustic wave, and analyzing the acoustic wave to assess a tissue characteristic. The analysis performed may include analyzing on a temporal basis, for example, to determine the distance between the tissue generating the acoustic wave and the distal end of the catheter. 
         [0016]    The present invention is designed to use optoacoustic technology in conjunction with RF ablation. To that end, the light used to heat the tissue is generally not affected by the portion of the electromagnetic radiation used for ablation. The spectral window for use in the present invention is about 400 nm to 2000 nm, preferably 700 nm and 1100 nm, determined by the absorption band(s) of the contrast species of interest. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0018]      FIG. 1  illustrates an embodiment of an opto-acoustic ablation system in accordance with the present invention. 
           [0019]      FIG. 4  illustrates another embodiment of an opto-acoustic ablation system in accordance with the present invention. 
           [0020]      FIG. 2A  is a side cross-sectional view of an embodiment of a catheter according to the present invention, including the junction between a catheter body and an intermediate section, taken along a first diameter. 
           [0021]      FIG. 2B  is a side cross-sectional view of an embodiment of a catheter according to the invention, including the junction between a catheter body and an intermediate section, taken along a second diameter generally perpendicular to the first diameter of  FIG. 2A . 
           [0022]      FIG. 3A  is a side cross sectional view of an embodiment of a catheter according to the invention, including a junction between a plastic housing and a tip electrode, taken along a first diameter. 
           [0023]      FIG. 3B  is a side cross-sectional view of an embodiment of a catheter according to the invention, including a junction between a plastic housing and a tip electrode, taken near a second diameter generally perpendicular to the first diameter of  FIG. 3A ; 
           [0024]      FIG. 3C  is a longitudinal cross-sectional view of an embodiment of the intermediate section of  FIGS. 2A and 2B . 
           [0025]      FIG. 3D  is a side cross sectional view an embodiment of a catheter according to the invention, including a junction between a plastic housing and a tip electrode, taken along line  3 D- 3 D in  FIG. 4 . 
           [0026]      FIG. 4  is a longitudinal cross-sectional view of an embodiment of the tip electrode of  FIGS. 3A and 3B . 
           [0027]      FIG. 5  is a distal end view of an embodiment of a tip electrode. 
           [0028]      FIG. 5A  is a distal end view of another embodiment of a tip electrode. 
           [0029]      FIG. 7A  is a side cross-sectional view of an embodiment of an irrigated catheter according to the present invention, including the junction between a catheter body and an intermediate section, taken along a first diameter. 
           [0030]      FIG. 7B  is a side cross-sectional view of an embodiment of an irrigated catheter according to the invention, including the junction between a catheter body and an intermediate section, taken along a second diameter generally perpendicular to the first diameter of  FIG. 7A . 
           [0031]      FIG. 8  is a side cross sectional view of an embodiment of a catheter according to the invention, including a junction between a plastic housing and an intermediate section. 
           [0032]      FIG. 10  is a longitudinal cross-sectional view of an embodiment of the tip electrode of  FIGS. 7A and 7B . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  illustrates an embodiment of a system S for laser optoacoustic monitoring to provide real time assessment of lesion formation, tissue status, and tissue morphology. Tissue T is subjected to RF ablation by an ablation element  200  that is energized by an ablation energy source  202  to form lesion  217 . A laser delivery means  204  irradiates the lesion  217  and surrounding tissue within its field of view  215  to stimulate pressure waves  219  (with different delay times T 1 , T 2 . Tn) which are detected by acoustic transducers  208  for imaging the lesion against the surrounding tissue. The laser delivery means can include a fiber optic cable housed in a catheter that is equipped solely or primarily for irradiation, or an integrated catheter as described further below. As understood by one of ordinary skill in the art, the imaging provided by the present invention is based on contrast provided by differential absorption. To that end, a pulsed laser light source  206  drives the laser delivery means  204  to slightly but quickly heat the tissue and the lesion within an irradiation field of view of the laser delivery demeans. This heating causes microscopic expansion in the lesion and surrounding tissue, which have different optical absorptions, to generate the pressure waves  219  with discernible stress profiles that propagate outwardly. An acoustic sensor  208  detects the emitted pressure waves, including the time delays T 1 -Tn, and converts the stress profile into electrical signals that are received by acoustic sampling hardware  210  for reconstructing a cross-sectional representation of the lesion. And, where the delay in the receive time of the acoustic waves is proportional to the distance of the sources generating the acoustic waves from the laser delivery means  204 , the detected signals can be used to determine tissue thickness, lesion progression, lesion width, and other assessment features in real time. Moreover, by employing non-resonant acoustic frequencies which are the result of optical absorption properties of the various materials within the irradiation field of view of the laser delivery means, the resulting signal tends to have a much higher sensitivity to materials with different optical absorption properties, such as those between tissue in various states of ablation, tissue and blood. 
         [0034]    In a more detailed embodiment of  FIG. 1A , a catheter-based system S for real-time laser optoacoustic monitoring is illustrated. Endo- or epi-cardial tissue T is subjected to RF ablation by a catheter  10  having a tip section  36  adapted for RF ablation in creating a lesion  17 . To that end, the catheter tip section  36  has an integrated structure (see  FIG. 1B ) from which radiation  15  is emitted to heat the lesion  17  and surrounding tissue and stimulate pressure waves  19  (with different delay times T 1 , T 2  . . . Tn) which are detected by acoustic transducers  13  for imaging the lesion against the surrounding tissue. A light source  100  provides pulsed irradiation that is delivered to the catheter tip  36  to slightly but quickly heat the tissue and the lesion within an irradiation field of view of the tip section  36 . Similarly, this heating causes microscopic expansion in the lesion and surrounding tissue, which have different optical absorptions, to generate pressure waves with discernible stress profiles that propagate outwardly. The transducers  13 , which may include piezoelectric transducers, mechanical transducers or interferometric optical sensors, detect the time, magnitude and shape of the arriving pressure waves and convert the stress profile into electrical signals that are received by an electronic tracer or scope device  102 , for example, a digital oscilloscope that functions as an analog to digital converter and records the amplitude and temporal profile of the laser-induced stress waves. Signals from the electronic device  102 , for example, digitized signals from the digital oscilloscope, are then analyzed by a computer  104  to reconstruct an image or representation  108  of the lesion on a graphical display  106 . And, again, where the delay in the receive time of the acoustic waves is proportional to the distance of the sources generating the acoustic waves from the irradiation source, the detected signals can be used to determine tissue thickness, lesion progression, lesion width, and other assessment features in real time. 
         [0035]    In accordance with the present invention, the stress detection of the illustrated embodiments of  FIGS. 1 and 1A  are accomplished in a reflection mode. And, in particular, with the catheter-based system of  FIG. 1A , by integrating irradiation emission and optical detection in the catheter tip section  36 , the stress waves detected have been reflected back toward the tissue surface that received the irradiation, where emphasis is made on high spatial resolution. 
         [0036]    With reference to  FIGS. 2A and 2B , an embodiment of a catheter  10  for use with the system S in accordance with the present invention comprises an elongated catheter body  12  having proximal and distal ends, a deflectable intermediate section  14  (uni or b-directionally) at the distal end of the catheter body  12 , and a tip section  36  at the distal end of the intermediate section, and a control handle  16  at the proximal end of the catheter body  12 . In accordance with the present invention, the tip section  36  incorporates an integrated design that provides both irradiation of the tissue of interest and detection of stress waves emanating therefrom. 
         [0037]    The catheter body  12  comprises an elongated tubular construction having a single, axial or central lumen  18 . The catheter body  12  is flexible, i.e., bendable, but substantially non-compressible along its length. The catheter body  12  can be of any suitable construction and made of any suitable material. A construction comprises an outer wall  22  made of an extruded plastic. The outer wall  22  may comprise an imbedded braided mesh of stainless steel or the like to increase torsional stiffness of the catheter body  12  so that, when the control handle  16  is rotated, the catheter body  12 , the intermediate section  14  and the tip electrode  36  of the catheter  10  will rotate in a corresponding manner. 
         [0038]    Extending through the single lumen  18  of the catheter body  12  are components, for example, an electrode lead wire  40  and thermocouple wires  41  and  45  protected by a sheath  39 , a fiber optic cable  43 , transducer lead wires  55 , a compression coil  44  through which a puller wire  42  extends, and an electromagnetic sensor cable  74 . A single lumen catheter body can be preferred over a multi-lumen body because it has been found that the single lumen body permits better tip control when rotating the catheter. The single lumen permits the various aforementioned components to float freely within the catheter body. If such components were restricted within multiple lumens, they tend to build up energy when the handle is rotated, resulting in the catheter body having a tendency to rotate back if, for example, the handle is released, or if bent around a curve, to flip over, either of which are undesirable performance characteristics. 
         [0039]    The outer diameter of the catheter body  12  is not critical, but is preferably no more than about 8 french, more preferably 7 french. Likewise the thickness of the outer wall  22  is not critical, but is thin enough so that the central lumen  18  can accommodate the aforementioned components. The inner surface of the outer wall  22  may be lined with a stiffening tube  20 , which can be made of any suitable material, such as polyimide or nylon. The stiffening tube  20 , along with the braided outer wall  22 , provides improved torsional stability while at the same time minimizing the wall thickness of the catheter, thus maximizing the diameter of the central lumen  18 . The outer diameter of the stiffening tube  20  is about the same as or slightly smaller than the inner diameter of the outer wall  22 . Polyimide tubing may be preferred for the stiffening tube  20  because it may be very thin walled while still providing very good stiffness. This maximizes the diameter of the central lumen  18  without sacrificing strength and stiffness. 
         [0040]    Referring also to  FIGS. 3A ,  3 B and  3 C, the intermediate section  14  comprises a shorter section of tubing  19  having multiple lumens. The tubing  19  is made of a suitable non-toxic material that is preferably more flexible than the catheter body  12 . A suitable material for the tubing  19  is non-braided polyurethane. The outer diameter of the intermediate section  14 , like that of the catheter body  12 , is preferably no greater than about 8 french, more preferably 7 french. The size and number of the lumens is not critical. In an embodiment, the intermediate section  14  has an outer diameter of about 7 french (0.092 inch). The tubing has a first off-axis lumen  30 , a second off-axis lumen  32 , a third off-axis lumen  34  and a fourth off-axis lumen  35 , that are generally about the same size, each having a diameter of from about 0.032 inch to about 0.038 inch, preferably 0.036 inch. In the illustrated embodiment, the puller wire  42  extends through the first lumen  30 , and an optical waveguide, e.g., the fiber optic cable  43 , and the transducer lead wires  55  extend through the second lumen  32 . The electrode lead wire  40  extends through the third lumen  34 . The thermocouple wires  41  and  45  also extend through the third lumen  34 , and an electromagnetic sensor cable  74  extend through the fourth lumen  35 . 
         [0041]    As best shown in  FIGS. 2A and 2B , the catheter body  12  in one embodiment is attached to the intermediate section  14  by means of an outer circumferential notch  24  configured in the proximal end of the tubing  19  that receives the inner surface of the outer wall  22  of the catheter body  12 . The intermediate section  14  and catheter body  12  are attached by glue or the like. Before the intermediate section  14  and catheter body  12  are attached, the stiffening tube  20  is inserted into the catheter body  12 . The distal end of the stiffening tube  20  is fixedly attached near the distal end of the catheter body  12  by forming a glue joint  23  with polyurethane glue or the like. Preferably a small distance, e.g., about 3 mm, is provided between the distal end of the catheter body  12  and the distal end of the stiffening tube  20  to permit room for the catheter body  12  to receive the notch  24  of the intermediate section  14 . If no compression coil is used, a force is applied to the proximal end of the stiffening tube  20 , and, while the stiffening tube  20  is under compression, a first glue joint (not shown) is made between the stiffening tube  20  and the outer wall  22  by a fast drying glue, e.g., cyanoacrylate. Thereafter a second glue joint  26  is formed between the proximal ends of the stiffening tube  20  and outer wall  22  using a slower drying but stronger glue, e.g., polyurethane. 
         [0042]    If desired, a spacer can be located within the catheter body between the distal end of the stiffening tube and the proximal end of the tip electrode. The spacer provides a transition in flexibility at the junction of the catheter body and intermediate section, which allows this junction to bend smoothly without folding or kinking. A catheter having such a spacer is described in U.S. patent application Ser. No. 08/924,616, entitled “Steerable Direct Myocardial Revascularization Catheter”, the entire disclosure of which is incorporated herein by reference. 
         [0043]    As illustrated in  FIGS. 3A and 3B , the tip section  36  extends from the distal end of the intermediate section  14 . In the illustrated embodiment, the tip electrode has a diameter about the same as the outer diameter of the tubing  19  of the intermediate section  14 . The intermediate section  14  and the tip electrode are attached by glue  27  or the like applied circumferentially around a junction of the tubing  19  and the tip electrode  36 . Moreover, the components extending between the intermediate section  14  and the tip electrode, e.g., the lead wire  40 , the transducer lead wires  55 , the thermocouple wires  41  and  45 , and the puller wire  42 , help keep the tip electrode on the intermediate section. 
         [0044]    In the illustrated embodiment, the tip section  36  has a generally hollow distal portion. The tip electrode comprises a shell  38  of generally uniform thickness and a press-fit alignment member or plug  59  positioned at or near the proximal end of the shell to seal the hollow distal portion. The shell and the plug are formed from any suitable material that is both thermally and electrically conductive which allows for radio frequency ablation using an RF generator. Such suitable materials include, without limitation, platinum, gold alloy, or palladium alloy. A tip electrode and method for manufacturing same are disclosed in application Ser. No. 11/058,434, filed Feb. 14, 2005, and application Ser. No. 11/453,188; filed Jun. 13, 2006, the entire disclosures of which are hereby incorporated by reference. 
         [0045]    The tip section  36  is energized for RF ablation by the lead wire  40  that extends through the third lumen  34  of intermediate section  14 , the central lumen  18  of the catheter body  12 , and the control handle  16 , and terminates at its proximal end in an input jack  75  that may be plugged into an appropriate monitor (not shown). The portion of the lead wire  40  extending through the central lumen  18  of the catheter body  12 , control handle  16  and distal end of the intermediate section  14  is enclosed within the protective sheath  39 , which can be made of any suitable material, preferably Teflon®. The protective sheath  39  is anchored at its distal end to the distal end of the intermediate section  14  by gluing it in the lumen  34  with polyurethane glue or the like. The lead wire  40  is attached to the tip electrode  36  by any conventional technique. In the illustrated embodiment, connection of the lead wire  40  to the tip section  36  is accomplished, for example, by welding the distal end of the lead wire  40  into a first blind hole  31  ( FIG. 3D ) in the alignment member  59  of the tip electrode  36 . 
         [0046]    A temperature sensing means is provided for the tip electrode  36  in the disclosed embodiment. Any conventional temperature sensing means, e.g., a thermocouple or thermistor, may be used. With reference to  FIGS. 3A and 3B , a suitable temperature sensing means for the tip section  36  comprises a thermocouple formed by a wire pair. One wire of the wire pair is the copper wire  41 , e.g., a number “40” copper wire. The other wire of the wire pair is the constantan wire  45 , which gives support and strength to the wire pair. The wires  41  and  45  of the wire pair are electrically isolated from each other except at their distal ends where they contact and are twisted together, covered with a short piece of plastic tubing  63 , e.g., polyimide, and covered with epoxy. The plastic tubing  63  is then attached in a second blind hole  33  of the tip electrode  36  ( FIG. 3B ), by epoxy or the like. The wires  41  and  45  extend through the third lumen  34  in the intermediate section  14 . Within the catheter body  12  the wires  41  and  45  extend through the central lumen  18  within the protective sheath  39  along with the lead wire  40 . The wires  41  and  45  then extend out through the control handle  16  and to a connector connectable to a temperature monitor (not shown). Alternatively, the temperature sensing means may be a thermistor. A suitable thermistor for use in the present invention is Model No. AB6N2-GC14KA143T/37C sold by Thermometrics (New Jersey). 
         [0047]    Referring to  FIGS. 2A ,  3 A and  3 D, the puller wire  42  as part of a means for deflecting the catheter extends through the catheter body  12 , is anchored at its proximal end to the control handle  16 , and is anchored at its distal end to the tip electrode  36 . The puller wire is made of any suitable metal, such as stainless steel or Nitinol, and is preferably coated with Teflon® or the like. The coating imparts lubricity to the puller wire. The puller wire preferably has a diameter ranging from about 0.006 to about 0.010 inches. 
         [0048]    The compression coil  44  is situated within the catheter body  12  in surrounding relation to the puller wire. The compression coil  44  extends from the proximal end of the catheter body  12  to the proximal end of the intermediate section  14  ( FIG. 2A ). The compression coil is made of any suitable metal, preferably stainless steel, and is tightly wound on itself to provide flexibility, i.e., bending, but to resist compression. The inner diameter of the compression coil is preferably slightly larger than the diameter of the puller wire  42 . The Teflon® coating on the puller wire allows it to slide freely within the compression coil. If desired, particularly if the lead wire  40  is not enclosed by a protective sheath, the outer surface of the compression coils can be covered by a flexible, non-conductive sheath, e.g., made of polyimide tubing, to prevent contact between the compression coils and any other wires within the catheter body  12 . 
         [0049]    As shown in  FIG. 2A , the compression coil  44  is anchored at its proximal end to the proximal end of the stiffening tube  20  in the catheter body  12  by glue joint  50  and at its distal end to the intermediate section  14  by glue joint  51 . Both glue joints  50  and  51  preferably comprise polyurethane glue or the like. The glue may be applied by means of a syringe or the like through a hole made between the outer surface of the catheter body  12  and the central lumen  18 . Such a hole may be formed, for example, by a needle or the like that punctures the outer wall  22  of the catheter body  12  and the stiffening tube  20  which is heated sufficiently to form a permanent hole. The glue is then introduced through the hole to the outer surface of the compression coil  44  and wicks around the outer circumference to form a glue joint about the entire circumference of the compression coil. 
         [0050]    With reference to  FIGS. 2A ,  3 A and  3 C, the puller wire  42  extends into the first lumen  30  of the intermediate section  14 . The puller wire  42  is anchored at its distal end to the tip electrode  36  within the third blind hole  73  in the alignment member  59 , as shown in  FIG. 3D . A method for anchoring the puller wire  42  within the tip electrode  36  is by crimping metal tubing  46  to the distal end of the puller wire  42  and soldering the metal tubing  46  inside the blind hole  73 . Anchoring the puller wire  42  within the alignment member  59  provides additional support, reducing the likelihood that the tip electrode  36  will fall off. Alternatively, the puller wire  42  can be attached to the side of the tubing  19  of the intermediate section  14  as understood by one of ordinary skill in the art. Within the first lumen  30  of the intermediate section  14 , the puller wire  42  extends through a plastic, preferably Teflon®, sheath  81 , which prevents the puller wire  42  from cutting into the wall of the intermediate section  14  when the intermediate section is deflected. 
         [0051]    Longitudinal movement of the puller wire  42  relative to the catheter body  12 , which results in deflection of the tip electrode  36 , is accomplished by suitable manipulation of the control handle  16 . A suitable control handle is described in U.S. Pat. No. 6,602,242, the entire disclosure of which is hereby incorporated by reference. 
         [0052]    In the illustrated embodiment of  FIGS. 3A ,  3 B and  3 D, the tip section  36  carries an electromagnetic sensor  72 . The electromagnetic sensor  72  is connected to the electromagnetic sensor cable  74 , which extends through a passage  75  ( FIG. 4 ) in the alignment member  39 , the third lumen  35  of the tip electrode section  36  through the central lumen  18  of the catheter body  12 , and into the control handle  16 . As shown in  FIG. 1 , the electromagnetic sensor cable  74  then extends out the proximal end of the control handle  16  within an umbilical cord  78  to a sensor control module  75  that houses a circuit board (not shown). Alternatively, the circuit board can be housed within the control handle  16 , for example, as described in U.S. patent application Ser. No. 08/924,616, entitled “Steerable Direct Myocardial Revascularization Catheter”, the entire disclosure of which is incorporated herein by reference. The electromagnetic sensor cable  74  comprises multiple wires encased within a plastic covered sheath. In the sensor control module  75 , the wires of the electromagnetic sensor cable  74  are connected to the circuit board. The circuit board amplifies the signal received from the electromagnetic sensor  72  and transmits it to a computer in a form understandable by the computer by means of the sensor connector  77  at the proximal end of the sensor control module  75 , as shown in  FIG. 1 . Because the catheter can be designed for single use only, the circuit board may contain an EPROM chip which shuts down the circuit board approximately 24 hours after the catheter has been used. This prevents the catheter, or at least the electromagnetic sensor, from being used twice. Suitable electromagnetic sensors for use with the present invention are described, for example, in U.S. Pat. Nos. 5,558,091, 5,443,489, 5,480,422, 5,546,951, 5,568,809, and 5,391,199 and International Publication No. WO 95/02995, the disclosures of which are incorporated herein by reference. An electromagnetic mapping sensor  72  may have a length of from about 6 mm to about 7 mm and a diameter of about 1.3 mm. 
         [0053]    In accordance with a feature of the present invention, the catheter  10  is adapted to facilitate optoacoustically-based real-time assessment of ablation tissue characteristics, including without limitation, tissue thickness, lesion progression, lesion width, and other assessment features in real time. These assessments are accomplished by employing non-resonant acoustic frequencies which are the result of optical absorption properties of the various tissue elements within the irradiation field of view of the catheter tip section. The catheter  10  therefore allows real-time assessment of lesion formation, tissue status and tissue morphology. 
         [0054]    As shown in  FIGS. 2A ,  3 A and  3 B, an optical waveguide, e.g., the fiber optic cable  43  is provided in the catheter to emit irradiation at the distal end, whereby the light selectively absorbed by the lesion and surrounding tissue (solid and fluid medium) is converted to heat which produces an acoustic wave detectable by the transducers  13  integrated in the tip section  36 . The fiber optic cable  43  transmits light from the light source  100  ( FIG. 1 ) to the tip electrode  36 . The fiber optic extends through the lumen  18  of the catheter body  12 , through the second lumen  32  of the intermediate section  14  and into the tip section  36  where the distal end of the cable  43  is fixedly mounted in an on-axis irradiation opening  80  which is located generally at the most distal location along the longitudinal axis of the tip section  36  for on-axis transmission at the tip section. The fiber optic cable  43  may be any suitable optical wave guide wherein light introduced at one of the cable is guided to the other end of the cable with minimal loss. The cable  43  may be a single fiber optic cable or fiber bundles. It may be single mode (also known as mono-mode or uni-mode), multi-mode (with step index or graded index) or plastic optical fiber (POF), depending on a variety of factors, including but not limited to transmission rate, bandwidth of transmission, spectral width of transmission, distance of transmission, diameter of cable, cost, optical signal distortion tolerance and signal attenuation, etc. 
         [0055]    There are additional off-axis openings  83  formed in the tip section  36  in which the transducers  13  are mounted. In the illustrated embodiment of  FIGS. 3 ,  3 A,  3 B and  5 , there are three openings  83  for three respective transducers  13  that are arranged generally equi-spaced from each other and from the opening  80 , and generally equi-angular about the opening  80 , equally offset from each other at about 120 degrees. It is understood by one of ordinary skill in the art that the number and arrangement of the irradiation opening  80  and transducer openings  83  may be varied as appropriate or desired. For example, there can be off-axis irradiation openings  80 ′ and/or additional transducer openings  83  ( FIG. 5 ). The total number of openings  80 ′ and  83  may range between about 3 to 6, where an embodiment with four openings could be arranged at about a 90 degree offset angle, five openings and transducers at about a 72 degree offset angle, or six openings and transducers at about a 60 degree offset angle. 
         [0056]    In the illustrated embodiment, the openings  80  and  83  are sized to receive the cable  43  and the transducers  13  in a generally snug-fit fashion. However, in an alternative embodiment as illustrated in  FIGS. 7A and 7B , the openings  80 ,  80 ′ are sized larger to allow fluid (e.g. saline) to flow pass the distal end of the cable(s)  43  and reach outside the tip electrode for cooling the tip electrode and ablation site and/or enabling larger and deeper lesions. Additional openings  87 , as shown in  FIG. 5A , may be formed in the shell to allow further irrigation of the tip electrode. The fluid is fed into the chamber  49  by an irrigation means, as shown in  FIG. 7B , that include a tube segment  48  extending from the distal end of the fourth lumen  35  of the intermediate section  14  and a passage  76  in the plug  59  ( FIG. 10 ). The distal end of the segment  48  is anchored in the passage  76  and the proximal end is anchored in the fourth lumen  35  by polyurethane glue or the like. Accordingly, the passage  76  is generally aligned with the fourth lumen  35  of the intermediate section  14 . The segment  48 , like the puller wires  42 , provides additional support for the tip electrode. The irrigation tube segment  48  is in communication with a proximal infusion tube segment (not shown) that extends through the central lumen  18  of the catheter body  12  and terminates in the proximal end of the fourth lumen  35  of the intermediate section  14 . The proximal end of the first infusion tube segment extends through the control handle  16  and terminates in a luer hub  90  ( FIG. 1 ) or the like at a location proximal to the control handle. In practice, fluid may be injected by a pump (not shown) into the infusion tube segment through the luer hub  90 , through the infusion tube segment  48 , into the chamber  49  in the tip electrode  36 , and out the openings  80 . The infusion tube segments may be made of any suitable material, and is preferably made of polyimide tubing. A suitable infusion tube segment has an outer diameter of from about 0.32 inch to about 0.036 inch and an inner diameter of from about 0.28 inch to about 0.032 inch. 
         [0057]    The pump can maintain the fluid at a positive pressure differential relative to outside the chamber  49  so as to provide a constant unimpeded flow or seepage of fluid outwardly from the chamber  49  which continuously seeps out from the openings  80 . 
         [0058]    In the illustrated embodiment of  FIGS. 7A ,  7 B and  8 , a housing  21  extends between the intermediate section  14  and the tip electrode  36  so that the electromagnetic sensor  72  can remain near the tip electrode and remain dry. The housing  21  (e.g., a plastic tube member) is attached to the tubing  19  of the intermediate section by creating a circumferential notch  37  in the distal end of the tubing  19 , placing the proximal end of the housing  21  on the distal end of the tubing  19 , and filling the notch  37  with glue. The distal end of the housing  21  and the tip electrode  36  are attached by glue at a seam  69 . All the components extending into or through the alignment member  59  help keep the tip electrode  36  attached to the housing  21 . 
         [0059]    As shown in  FIG. 11 , the catheter may also be adapted for electrophysiologic mapping by providing ring electrodes  25  (unipolar or bipolar) proximal the tip electrode. In the illustrated embodiment, the ring electrodes are mounted on the intermediate section  14 . The tip electrode and the ring electrodes are each connected to a separate lead wire  40 . The lead wires  40  for the ring electrodes like the lead wire for the tip electrode extend through the lumen  34  of tip section  14 , the central lumen  18  of the catheter body  12 , and the control handle  16 , and terminate at their proximal end in an input jack (not shown) that may be plugged into an appropriate signal processing unit (not shown) and source of RF energy (not shown). The distal end of the protective sheath  39  is proximal of the most proximal ring electrode thereby allowing the lead wire to connect to the ring electrode. 
         [0060]    The lead wires  40  are attached to the ring electrode  25  by any conventional technique, for example, by making a hole through the side wall of the tubing  19 . A lead wire  40  is then drawn through the hole and the ends of the lead wire  44  are then stripped of any coating and soldered or welded to the underside of the ring electrode  25 , which is then slid into position over the hole and fixed in place with polyurethane glue or the like. The plurality, position and spacing of the ring electrode  59  are not critical. If desired, additional ring electrodes may be used and can be positioned over the flexible tubing  19  of the intermediate section  14  or the plastic housing  21  in a similar manner. 
         [0061]    It is understood by one of ordinary skill in the art that any desired aspects of the different embodiments described herein may be incorporated within a catheter tip section so as to suit the needs and desires in a particular use and application. For example, the embodiment of  FIGS. 7A ,  7 B and  8  need not include irrigation, but the EM sensor  72  can nevertheless be housed outside of the chamber  49 , in tubing  21 , especially if there is insufficient space in the chamber  49  to contain both the EM sensor  72 , the fiber optic cable  43  and the transducer lead wires  55 . 
         [0062]    The present invention includes a method of monitoring epi- or endo-cardial tissue with laser optoacoustic imaging. With reference to  FIGS. 1 and 1A , the method includes irradiating cardiac tissue from a distal end of a catheter to heat said tissue for producing an acoustic wave, detecting said acoustic wave with an acoustic transducer mounted on said distal end of the catheter, recording characteristics of the acoustic wave; and analyzing the acoustic wave to assess a tissue characteristic. In particular, the method includes the use of a catheter having an integrated distal end with both irradiation and detection capabilities, such that the acoustic detection is performed in a reflection mode which emphasizes high spatial resolution of the measured image. 
         [0063]    The irradiation is selectively absorbed by the lesion and surrounding tissue and converted to heat. The heat produces an acoustic wave which is detected by the acoustic transducer or sensor. The delay in the receive time (or temporal profile) of the acoustic wave is proportional to the distance of the tissue elements generating the acoustic wave from the irradiating distal end of the catheter and is used to assess and determine in real time a variety of tissue characteristics, including the reconstruction of a cross-sectional image of the tissue monitored. The tissue characteristics that can be assessed, monitored or determined from the present invention includes, without limitation, tissue thickness, lesion progression, lesion width, and other assessment features. Tissue thickness can be determined in real time and up to a few centimeters (for example, ______ cm) with generally very high resolution. The method of the present invention employs non-resonant acoustic frequencies which are the result of optical absorption properties of the various tissue elements within the irradiation field of view of the catheter. The resulting signal typically has a much higher sensitivity to tissue elements with different optical absorption properties, such as those between tissue in various states of ablation, tissue and blood, etc. 
         [0064]    The foregoing descriptions are directed to a catheter that is configured both for RF ablation and optoacoustic-based assessment from the distal end, the present invention is not limited to such a therapeutic catheter. Accordingly, the present invention also contemplates a diagnostic catheter that provides irradiation and acoustic detection from the distal end on a tissue of interest while a separate therapeutic catheter performs RF ablation on that tissue. The catheter may be nonirrigated or irrigated. 
         [0065]    The present invention can be utilized to determine lesion boundaries in real-time as a lesion is being created in cardiac tissue via RF, ultrasound, laser cryotherapy, high intensity focused ultrasound (HIFU), or laser. In addition, the present invention can also determine tissue dimensions (thickness) in real time, concurrent with the lesion formation. The present invention can then be used to determine the progression of a lesion towards the distal tissue margin, and indicate transmurality when the lesion progresses through the entire tissue thickness. Moreover, in epi/endocardial applications, the present invention can also use a combination of catheters to allow a full range of detection combinations (inside, outside). 
         [0066]    The preceding description has been presented with reference to presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structure may be practiced without meaningfully departing from the principal, spirit and scope of this invention. For example, laser pulses can be delivered via optical fibers, hollow or liquid waveguides, or free-space optics. All wavelengths transmittable by the delivery optics can be used for this technique. A variety of acoustic sensors can be used, including piezoelectric transducers, mechanical transducers, or interferometric optical sensors. The ablation element can include a variety of energy sources, including RF, ultrasound, cryotherapy, HIFU, or laser. 
         [0067]    Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and illustrated in the accompanying drawings, but rather should be read consistent with and as support to the following claims which are to have their fullest and fair scope.