Patent Publication Number: US-2011060253-A1

Title: Ultrasonic catheter with axial energy field

Description:
PRIORITY APPLICATION 
     This application is a divisional of U.S. application Ser. No. 10/751,843, filed 5 Jan. 2004, which claims the benefit of U.S. Provisional Application 60/438,141, filed 3 Jan. 2003, the entire disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to ultrasonic catheters, and more specifically to ultrasonic catheters configured to deliver ultrasonic energy and a therapeutic compound to a treatment site. 
     BACKGROUND OF THE INVENTION 
     Several medical applications use ultrasonic energy. For example, U.S. Pat. Nos. 4,821,740, 4,953,565 and 5,007,438 disclose the use of ultrasonic energy to enhance the effect of various therapeutic compounds. An ultrasonic catheter can be used to deliver ultrasonic energy and a therapeutic compound to a treatment site within a patient&#39;s body. Such an ultrasonic catheter typically includes an ultrasound assembly configured to generate ultrasonic energy and a fluid delivery lumen for delivering the therapeutic compound to the treatment site. 
     As taught in U.S. Pat. No. 6,001,069, ultrasonic catheters can be used to treat human blood vessels that have become partially or completely occluded by plaque, thrombi, emboli or other substances that reduce the blood carrying capacity of the vessel. To remove or reduce the occlusion, the ultrasonic catheter is used to deliver solutions containing therapeutic compounds directly to the occlusion site. Ultrasonic energy generated by the ultrasound assembly enhances the effect of the therapeutic compounds. Such a device can be used in the treatment of diseases such as peripheral arterial occlusion or deep vein thrombosis. In such applications, the ultrasonic energy enhances treatment of the occlusion with therapeutic compounds such as urokinase, tissue plasminogen activator (“tPA”), recombinant tissue plasminogen activator (“rtPA”) and the like. Further information on enhancing the effect of a therapeutic compound using ultrasonic energy is provided in U.S. Pat. Nos. 5,318,014, 5,362,309, 5,474,531, 5,628,728, 6,001,069 and 6,210,356. 
     Ultrasonic catheters can also be used to enhance gene therapy at a treatment site within the patient&#39;s body. For example, U.S. Pat. No. 6,135,976 discloses an ultrasonic catheter having one or more expandable sections capable of occluding a section of a body lumen, such as a blood vessel. A gene therapy composition is then delivered to the occluded portion of the vessel through the catheter fluid delivery lumen. Ultrasonic energy generated by the ultrasound assembly is applied to the occluded vessel, thereby enhancing the delivery of a genetic composition into the cells of the occluded vessel. 
     Ultrasonic catheters can also be used to enhance delivery and activation of light activated drugs. For example, U.S. Pat. No. 6,176,842 discloses methods for using an ultrasonic catheter to treat biological tissues by delivering a light activated drug to the biological tissues and exposing the light activated drug to ultrasonic energy. 
     SUMMARY OF THE INVENTION 
     In certain applications, it is desirable to project an axial ultrasonic energy field from the distal end of an ultrasonic catheter. Such a field, also referred to as a “forward-facing” field, is useful in many of the aforementioned applications, including clot dissolution and gene therapy. In particular, a high-power, forward-facing ultrasonic energy field is useful in the dissolution of an aged blood clot located in the coronary vasculature. Thus, a ultrasonic catheter that is capable of producing such an energy field, and that is also compatible with conventional cardiological practice, has been developed. 
     In accordance with the foregoing, in an exemplary embodiment, a catheter system for delivering ultrasonic energy to a treatment site within a body lumen comprises a tubular body. The tubular body has a proximal end, a distal end and an energy delivery section positioned between the proximal end and the distal end. The catheter further comprises an inner core configured for insertion into the tubular body. The inner core comprises a first ultrasound radiating member axially separated from a second ultrasound radiating member by an intermediate flexible joint region. The inner core further comprises an electrically conductive portion configured to allow a voltage difference to be applied to at least one of the ultrasound radiating members. The inner core further comprises a high impedance cap positioned proximal to at least one of the ultrasound radiating members. 
     In another exemplary embodiment, an ultrasound assembly comprises an elongate member having a proximal region and a distal region opposite the proximal region. A high impedance cap is positioned adjacent to the elongate member distal region. The ultrasound assembly further comprises an ultrasound radiating member positioned distal to the high impedance cap. The ultrasound radiating member is configured to generate a distribution of ultrasonic energy that has a greater density in a region axially distal to the ultrasound radiating member than in an annular region surrounding the ultrasound radiating member. 
     In another exemplary embodiment, an apparatus comprises a tubular body having a proximal end, a distal end opposite the proximal end, and a treatment zone located between the distal end and the proximal end. The apparatus further comprises a plurality of fluid delivery lumens defined within the tubular body. The apparatus further comprises an inner core comprising a high impedance cap and at least one ultrasound radiating member positioned distal to the high impedance cap. The apparatus further comprises a plurality of cooling fluid channels defined between at least an inner surface of the tubular body and an outer surface of the inner core. Each cooling fluid channel is positioned generally radially between two fluid delivery lumens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the ultrasonic catheter disclosed herein are illustrated in the accompanying drawings, which are for illustrative purposes only. The drawings comprise the following figures, in which like numerals indicate like parts. 
         FIG. 1  is a schematic illustration of an ultrasonic catheter configured for insertion into large vessels of the human body. 
         FIG. 2  is a cross-sectional view of the ultrasonic catheter of  FIG. 1  taken along line  2 - 2 . 
         FIG. 3  is a schematic illustration of an elongate inner core configured to be positioned within the central lumen of the catheter illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the elongate inner core of  FIG. 3  taken along line  4 - 4 . 
         FIG. 5  is a schematic wiring diagram illustrating an exemplary technique for electrically connecting five groups of ultrasound radiating members to form an ultrasound assembly. 
         FIG. 6  is a schematic wiring diagram illustrating an exemplary technique for electrically connecting one of the groups of  FIG. 5 . 
         FIG. 7A  is a schematic illustration of the ultrasound assembly of  FIG. 5  housed within the inner core of  FIG. 4 . 
         FIG. 7B  is a cross-sectional view of the ultrasound assembly of  FIG. 7A  taken along line  7 B- 7 B. 
         FIG. 7C  is a cross-sectional view of the ultrasound assembly of  FIG. 7A  taken along line  7 C- 7 C. 
         FIG. 7D  is a side view of an ultrasound assembly center wire twisted into a helical configuration. 
         FIG. 8  illustrates the energy delivery section of the inner core of  FIG. 4  positioned within the energy delivery section of the tubular body of  FIG. 2 . 
         FIG. 9  illustrates a wiring diagram for connecting a plurality of temperature sensors with a common wire. 
         FIG. 10  is a block diagram of a feedback control system for use with an ultrasonic catheter. 
         FIG. 11A  is a side view of a treatment site. 
         FIG. 11B  is a side view of the distal end of an ultrasonic catheter positioned at the treatment site of  FIG. 11A . 
         FIG. 11C  is a cross-sectional view of the distal end of the ultrasonic catheter of  FIG. 11B  positioned at the treatment site before a treatment. 
         FIG. 11D  is a cross-sectional view of the distal end of the ultrasonic catheter of  FIG. 11C , wherein an inner core has been inserted into the tubular body to perform a treatment. 
         FIG. 12  is a side view of an ultrasound catheter that is particularly well suited for insertion into small blood vessels of the human body. 
         FIG. 13A  is a cross-sectional view of a distal end of the ultrasound catheter of  FIG. 12 . 
         FIG. 13B  is a cross-sectional view of the ultrasound catheter of  FIG. 12  taken through line  13 B- 13 B of  FIG. 13A . 
         FIG. 14  is a side view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and including solid cylindrical ultrasound radiating members. 
         FIG. 15  is a side view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field including comprising hollow cylindrical ultrasound radiating members. 
         FIG. 16  is a side view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and including a plurality of ultrasound radiating members. 
         FIG. 17A  is a cross-sectional view of a distal end of a catheter guidewire configured to produce a forward-facing ultrasonic energy field and comprising a plurality of ultrasound radiating members with substantially similar dimensions. 
         FIG. 17B  is a cross-sectional view of the catheter guidewire of  FIG. 17A  taken along line  17 B- 17 B. 
         FIG. 18A  is a cross-sectional view of a distal end of a catheter guidewire configured to produce a forward-facing ultrasonic energy field and comprising a plurality of ultrasound radiating members with varying dimensions. 
         FIG. 18B  is a cross-sectional view of the catheter guidewire of  FIG. 18A  taken along line  18 B- 18 B. 
         FIG. 19A  is a cross-sectional view of a distal end of a catheter guidewire configured to produce a forward-facing ultrasonic energy field and comprising a plurality of ultrasound radiating members with rectangular dimensions. 
         FIG. 19B  is a cross-sectional view of the catheter guidewire of  FIG. 19A  taken along line  19 B- 19 B. 
         FIG. 20  is a side view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and comprising a proximal end joint. 
         FIG. 21  is a side view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and comprising a flat distal horn. 
         FIG. 22  is a cross-sectional view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and comprising a blunt distal horn. 
         FIG. 23  is a cross-sectional view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and comprising a short pointed distal horn. 
         FIG. 24  is a cross-sectional view of a distal end of an ultrasonic catheter configured to produce a forward-facing ultrasonic energy field and comprising a long pointed distal horn. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described above, ultrasonic catheters capable of delivering multi-frequency ultrasonic energy and/or forward-facing ultrasonic energy fields to a treatment site within a patient&#39;s vasculature have been developed. Exemplary embodiments of these ultrasonic catheters, including exemplary methods of use, are described herein. 
     The ultrasonic catheters described herein can be used to enhance the therapeutic effects of therapeutic compounds at a treatment site within a patient&#39;s body. As used herein, the term “therapeutic compound” refers broadly, without limitation, to a drug, medicament, dissolution compound, genetic material or any other substance capable of effecting physiological functions. Additionally, any mixture comprising any such substances is encompassed within this definition of “therapeutic compound”, as well as any substance falling within the ordinary meaning of these terms. The enhancement of the effects of therapeutic compounds using ultrasonic energy is described in U.S. Pat. Nos. 5,318,014, 5,362,309, 5,474,531, 5,628,728, 6,001,069, 6,096,000, 6,210,356 and 6,296,619. Specifically, for applications that treat human blood vessels that have become partially or completely occluded by plaque, thrombi, emboli or other substances that reduce the blood carrying capacity of a vessel, suitable therapeutic compounds include, but are not limited to, an aqueous solution containing heparin, urokinase, streptokinase, TPA and BB-10153 (manufactured by British Biotech, Oxford, UK). 
     Certain features and aspects of the ultrasonic catheters disclosed herein may also find utility in applications where the ultrasonic energy itself provides a therapeutic effect. Examples of such therapeutic effects include preventing or reducing stenosis and/or restenosis; tissue ablation, abrasion or disruption; promoting temporary or permanent physiological changes in intracellular or intercellular structures; and rupturing micro-balloons or micro-bubbles for therapeutic compound delivery. Further information about such methods can be found in U.S. Pat. Nos. 5,269,291 and 5,431,663. Further information about using cavitation to produce biological effects can be found in U.S. Pat. RE36,939. 
     The ultrasonic catheters described herein are configured for applying ultrasonic energy over a substantial length of a body lumen, such as, for example, the larger vessels located in the leg. However, it should be appreciated that certain features and aspects of the present invention may be applied to catheters configured to be inserted into the small cerebral vessels, in solid tissues, in duct systems and in body cavities. Such catheters are described in U.S. patent application Ser. No. 10/309,417, entitled “Small Vessel Ultrasound Catheter” and filed Dec. 3, 2002, the entire disclosure of which is hereby incorporated herein by reference. Additional embodiments that may be combined with certain features and aspects of the embodiments described herein are described in U.S. patent application Ser. No. 10/291,891, entitled “Ultrasound Assembly For Use With A Catheter” and filed Nov. 7, 2002, the entire disclosure of which is hereby incorporated herein by reference. 
     For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. It is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     The embodiments disclosed herein are intended to be within the scope of the present invention. These and other embodiments should be apparent based on the following detailed description, which refers to the attached figures. The present invention is not limited to any particular disclosed embodiment, but is limited only by the claims set forth herein. 
     Overview of a Large Vessel Ultrasound Catheter. 
     With initial reference to  FIG. 1 , an ultrasonic catheter  10  configured for use in the large vessels of a patient&#39;s anatomy is schematically illustrated. For example, the ultrasonic catheter  10  illustrated in  FIG. 1  can be used to treat long segment peripheral arterial occlusions, such as those in the vascular system of the leg. 
     As illustrated in  FIG. 1 , the ultrasonic catheter  10  generally comprises a multi-component, elongate flexible tubular body  12  having a proximal region  14  and a distal region  15 . The tubular body  12  includes a flexible energy delivery section  18  and a distal exit port  29  located in the distal region  15  of the catheter  10 . A backend hub  33  is attached to the proximal region  14  of the tubular body  12 , the backend hub  33  comprising a proximal access port  31 , an inlet port  32  and a cooling fluid fitting  46 . The proximal access port  31  can be connected to control circuitry  100  via cable  45 . 
     The tubular body  12  and other components of the catheter  10  can be manufactured in accordance with any of a variety of techniques well known in the catheter manufacturing field. Suitable materials and dimensions can be readily selected based on the natural and anatomical dimensions of the treatment site and on the desired percutaneous access site. 
     For example, in a preferred embodiment the proximal region  14  of the tubular body  12  comprises a material that has sufficient flexibility, kink resistance, rigidity and structural support to push the energy delivery section  18  through the patient&#39;s vasculature to a treatment site. Examples of such materials include, but are not limited to, extruded polytetrafluoroethylene (“PTFE”), polyethylenes (“PE”), polyamides and other similar materials. In certain embodiments, the proximal region  14  of the tubular body  12  is reinforced by braiding, mesh or other constructions to provide increased kink resistance and pushability. For example, nickel titanium or stainless steel wires can be placed along or incorporated into the tubular body  12  to reduce kinking. 
     In an embodiment configured for treating thrombus in the arteries of the leg, the tubular body  12  has an outside diameter between about 0.060 inches and about 0.075 inches. In another embodiment, the tubular body  12  has an outside diameter of about 0.071 inches. In certain embodiments, the tubular body  12  has an axial length of approximately 105 centimeters, although other lengths may by appropriate for other applications. 
     The energy delivery section  18  of the tubular body  12  preferably comprises a material that is thinner than the material comprising the proximal region  14  of the tubular body  12  or a material that has a greater acoustic transparency. Thinner materials generally have greater acoustic transparency than thicker materials. Suitable materials for the energy delivery section  18  include, but are not limited to, high or low density polyethylenes, urethanes, nylons, and the like. In certain modified embodiments, the energy delivery section  18  may be formed from the same material or a material of the same thickness as the proximal region  14 . 
     In certain embodiments, the tubular body  12  is divided into at least three sections of varying stiffness. The first section, which preferably includes the proximal region  14 , has a relatively higher stiffness. The second section, which is located in an intermediate region between the proximal region  14  and the distal region  15  of the tubular body  12 , has a relatively lower stiffness. This configuration further facilitates movement and placement of the catheter  10 . The third section, which preferably includes the energy delivery section  18 , generally has a lower stiffness than the second section. 
       FIG. 2  illustrates a cross section of the tubular body  12  taken along line  2 - 2  in  FIG. 1 . In the embodiment illustrated in  FIG. 2 , three fluid delivery lumens  30  are incorporated into the tubular body  12 . In other embodiments, more or fewer fluid delivery lumens can be incorporated into the tubular body  12 . The arrangement of the fluid delivery lumens  30  preferably provides a hollow central lumen  51  passing through the tubular body  12 . The cross-section of the tubular body  12 , as illustrated in  FIG. 2 , is preferably substantially constant along the length of the catheter  10 . Thus, in such embodiments, substantially the same cross-section is present in both the proximal region  14  and the distal region  15  of the catheter  10 , including the energy delivery section  18 . 
     In certain embodiments, the central lumen  51  has a minimum diameter greater than about 0.030 inches. In another embodiment, the central lumen  51  has a minimum diameter greater than about 0.037 inches. In one preferred embodiment, the fluid delivery lumens  30  have dimensions of about 0.026 inches wide by about 0.0075 inches high, although other dimensions may be used in other applications. 
     As described above, the central lumen  51  preferably extends through the length of the tubular body  12 . As illustrated in  FIG. 1 , the central lumen  51  preferably has a distal exit port  29  and a proximal access port  31 . The proximal access port  31  forms part of the backend hub  33 , which is attached to the proximal region  14  of the catheter  10 . The backend hub  33  preferably further comprises cooling fluid fitting  46 , which is hydraulically connected to the central lumen  51 . The backend hub  33  also preferably comprises a therapeutic compound inlet port  32 , which is in hydraulic connection with the fluid delivery lumens  30 , and which can be hydraulically coupled to a source of therapeutic compound via a hub such as a Luer fitting. 
     The central lumen  51  is configured to receive an elongate inner core  34  of which a preferred embodiment is illustrated in  FIG. 3 . The elongate inner core  34  preferably comprises a proximal region  36  and a distal region  38 . Proximal hub  37  is fitted on the inner core  34  at one end of the proximal region  36 . One or more ultrasound radiating members are positioned within an inner core energy delivery section  41  located within the distal region  38 . The ultrasound radiating members form an ultrasound assembly  42 , which will be described in greater detail below. 
     As shown in the cross-section illustrated in  FIG. 4 , which is taken along lines  4 - 4  in  FIG. 3 , the inner core  34  preferably has a cylindrical shape, with an outer diameter that permits the inner core  34  to be inserted into the central lumen  51  of the tubular body  12  via the proximal access port  31 . Suitable outer diameters of the inner core  34  include, but are not limited to, about 0.010 inches to about 0.100 inches. In another embodiment, the outer diameter of the inner core  34  is between about 0.020 inches and about 0.080 inches. In yet another embodiment, the inner core  34  has an outer diameter of about 0.035 inches. 
     Still referring to  FIG. 4 , in an exemplary embodiment, the inner core  34  includes a cylindrical outer body  35  that houses the ultrasound assembly  42 . The ultrasound assembly  42  comprises wiring and ultrasound radiating members, described in greater detail in  FIGS. 5 through 7D , such that the ultrasound assembly  42  is capable of radiating ultrasonic energy from the energy delivery section  41  of the inner core  34 . The ultrasound assembly  42  is electrically connected to the backend hub  33 , where the inner core  34  can be connected to control circuitry  100  via cable  45  (illustrated in  FIG. 1 ). Preferably, an electrically insulating potting material  43  fills the inner core  34 , surrounding the ultrasound assembly  42 , thus preventing movement of the ultrasound assembly  42  with respect to the outer body  35 . In one embodiment, the thickness of the outer body  35  is between about 0.0002 inches and 0.010 inches. In another embodiment, the thickness of the outer body  35  is between about 0.0002 inches and 0.005 inches. In yet another embodiment, the thickness of the outer body  35  is about 0.0005 inches. 
     In an exemplary embodiment, the ultrasound assembly  42  comprises a plurality of ultrasound radiating members  40  that are divided into one or more groups G 1 , G 2 , G 3 , . . . G(n). For example,  FIGS. 5 and 6  are schematic wiring diagrams illustrating one technique for connecting five groups of ultrasound radiating members to form the ultrasound assembly  42 . As illustrated in  FIG. 5 , the ultrasound assembly  42  comprises five groups G 1 , G 2 , G 3 , G 4 , G 5  of ultrasound radiating members that are electrically connected to each other. The five groups are also electrically connected to the control circuitry  100 .  FIG. 6  is a schematic wiring diagram illustrating an example group G(n) which comprises a plurality of ultrasound radiating members  40 . 
     As used herein, the terms “ultrasonic energy”, “ultrasound” and “ultrasonic” are broad terms, having their ordinary meanings, and further refer to, without limitation, mechanical energy transferred through longitudinal pressure or compression waves. Ultrasonic energy can be emitted as continuous or pulsed waves, depending on the requirements of a particular application. Additionally, ultrasonic energy can be emitted in waveforms having various shapes, such as sinusoidal waves, triangle waves, square waves, or other wave forms. Ultrasonic energy includes sound waves. In certain embodiments, the ultrasonic energy has a frequency between about 20 kHz and about 20 MHz. For example, in one embodiment, the waves have a frequency between about 500 kHz and about 20 MHz. In another embodiment, the waves have a frequency between about 1 MHz and about 3 MHz. In yet another embodiment, the waves have a frequency of about 2 MHz. The average acoustic power is between about 0.01 watts and 300 watts. In one embodiment, the average acoustic power is about 15 watts. 
     As used herein, the term “ultrasound radiating member” refers to any apparatus capable of producing ultrasonic energy. For example, in one embodiment, an ultrasound radiating member comprises an ultrasonic transducer, which converts electrical energy into ultrasonic energy. A suitable example of an ultrasonic transducer for generating ultrasonic energy from electrical energy includes, but is not limited to, piezoelectric ceramic oscillators. Piezoelectric ceramics typically comprise a crystalline material, such as quartz, that change shape when an electrical current is applied to the material. This change in shape, made oscillatory by an oscillating driving signal, creates ultrasonic sound waves. In other embodiments, ultrasonic energy can be generated by an ultrasonic transducer that is remote from the ultrasound radiating member, and the ultrasonic energy can be transmitted, via, for example, a wire that is coupled to the ultrasound radiating member. 
     Still referring to  FIG. 5 , the control circuitry  100  preferably comprises, among other things, a voltage source  102 . The voltage source  102  comprises a positive terminal  104  and a negative terminal  106 . The negative terminal  106  is connected to common wire  108 , which connects the five groups G 1 -G 5  of ultrasound radiating members  40  in series. The positive terminal  104  is connected to a plurality of lead wires  110 , which each connect to one of the five groups G 1 -G 5  of ultrasound radiating members  40 . Thus, under this configuration, each of the five groups G 1 -G 5 , one of which is illustrated in  FIG. 6 , is connected to the positive terminal  104  via one of the lead wires  110 , and to the negative terminal  106  via the common wire  108 . 
     Referring now to  FIG. 6 , each group G 1 -G 5  comprises a plurality of ultrasound radiating members  40 . Each of the ultrasound radiating members  40  is electrically connected to the common wire  108  and to the lead wire  110  via one of two positive contact wires  112 . Thus, when wired as illustrated, a constant voltage difference will be applied to each ultrasound radiating member  40  in the group. Although the group illustrated in  FIG. 6  comprises twelve ultrasound radiating members  40 , one of ordinary skill in the art will recognize that more or fewer ultrasound radiating members  40  can be included in the group. Likewise, more or fewer than five groups can be included within the ultrasound assembly  42  illustrated in  FIG. 5 . 
       FIG. 7A  illustrates one preferred technique for arranging the components of the ultrasound assembly  42  (as schematically illustrated in  FIG. 5 ) into the inner core  34  (as schematically illustrated in  FIG. 4 ).  FIG. 7A  is a cross-sectional view of the ultrasound assembly  42  taken within group G 1  in  FIG. 5 , as indicated by the presence of four lead wires  110 . For example, if a cross-sectional view of the ultrasound assembly  42  was taken within group G 4  in  FIG. 5 , only one lead wire  110  would be present (that is, the one lead wire connecting group G 5 ). 
     Referring still to  FIG. 7A , the common wire  108  comprises an elongate, flat piece of electrically conductive material in electrical contact with a pair of ultrasound radiating members  40 . Each of the ultrasound radiating members  40  is also in electrical contact with a positive contact wire  112 . Because the common wire  108  is connected to the negative terminal  106 , and the positive contact wire  112  is connected to the positive terminal  104 , a voltage difference can be created across each ultrasound radiating member  40 . Lead wires  110  are preferably separated from the other components of the ultrasound assembly  42 , thus preventing interference with the operation of the ultrasound radiating members  40  as described above. For example, in one preferred embodiment, the inner core  34  is filled with an insulating potting material  43 , thus deterring unwanted electrical contact between the various components of the ultrasound assembly  42 . 
       FIGS. 7B and 7C  illustrate cross sectional views of the inner core  34  of  FIG. 7A  taken along lines  7 B- 7 B and  7 C- 7 C, respectively. As illustrated in  FIG. 7B , the ultrasound radiating members  40  are mounted in pairs along the common wire  108 . The ultrasound radiating members  40  are connected by positive contact wires  112 , such that substantially the same voltage is applied to each ultrasound radiating member  40 . As illustrated in  FIG. 7C , the common wire  108  preferably comprises wide regions  108 W upon which the ultrasound radiating members  40  can be mounted, thus reducing the likelihood that the paired ultrasound radiating members  40  will short together. In certain embodiments, outside the wide regions  108 W, the common wire  108  may have a more conventional, rounded wire shape. 
     In a modified embodiment, such as illustrated in  FIG. 7D , the common wire  108  is twisted to form a helical shape before being fixed within the inner core  34 . In such embodiments, the ultrasound radiating members  40  are oriented in a plurality of radial directions, thus enhancing the radial uniformity of the resulting ultrasonic energy field. 
     The wiring arrangement described above can be modified to allow each group G 1 , G 2 , G 3 , G 4 , G 5  to be independently powered. Specifically, by providing a separate power source within the control system  100  for each group, each group can be individually turned on or off, or can be driven with an individualized power. This provides the advantage of allowing the delivery of ultrasonic energy to be “turned off” in regions of the treatment site where treatment is complete, thus preventing deleterious or unnecessary ultrasonic energy to be applied to the patient. 
     The embodiments described above, and illustrated in  FIGS. 5 through 7 , illustrate a plurality of ultrasound radiating members grouped spatially. That is, in such embodiments, all of the ultrasound radiating members within a certain group are positioned adjacent to each other, such that when a single group is activated, ultrasonic energy is delivered at a specific length of the ultrasound assembly. However, in modified embodiments, the ultrasound radiating members of a certain group may be spaced apart from each other, such that the ultrasound radiating members within a certain group are not positioned adjacent to each other. In such embodiments, when a single group is activated, ultrasonic energy can be delivered from a larger, spaced apart portion of the energy delivery section. Such modified embodiments may be advantageous in applications wherein it is desired to deliver a less focussed, more diffuse ultrasonic energy field to the treatment site. 
     In an exemplary embodiment, the ultrasound radiating members  40  comprise rectangular lead zirconate titanate (“PZT”) ultrasound transducers that have dimensions of about 0.017 inches by about 0.010 inches by about 0.080 inches. In other embodiments, other configurations may be used. For example, disc-shaped ultrasound radiating members  40  can be used in other embodiments. In a preferred embodiment, the common wire  108  comprises copper, and is about 0.005 inches thick, although other electrically conductive materials and other dimensions can be used in other embodiments. Lead wires  110  are preferably 36-gauge electrical conductors, while positive contact wires  112  are preferably 42-gauge electrical conductors. However, one of ordinary skill in the art will recognize that other wire gauges can be used in other embodiments. 
     As described above, suitable frequencies for the ultrasound radiating member  40  include, but are not limited to, from about 20 kHz to about 20 MHz. In one embodiment, the frequency is between about 500 kHz and 20 MHz, and in another embodiment the frequency is between about 1 MHz and 3 MHz. In yet another embodiment, the ultrasound radiating members  40  are operated with a frequency of about 2 MHz. 
       FIG. 8  illustrates the inner core  34  positioned within the tubular body  12 . Details of the ultrasound assembly  42 , provided in  FIG. 7A , are omitted for clarity. As described above, the inner core  34  can be slid within the central lumen  51  of the tubular body  12 , thereby allowing the inner core energy delivery section  41  to be positioned within the tubular body energy delivery section  18 . For example, in a preferred embodiment, the materials comprising the inner core energy delivery section  41 , the tubular body energy delivery section  18 , and the potting material  43  all comprise materials having a similar acoustic impedance, thereby minimizing ultrasonic energy losses across material interfaces. 
       FIG. 8  further illustrates placement of fluid delivery ports  58  within the tubular body energy delivery section  18 . As illustrated, holes or slits are formed from the fluid delivery lumen  30  through the tubular body  12 , thereby permitting fluid flow from the fluid delivery lumen  30  to the treatment site. Thus, a source of therapeutic compound coupled to the inlet port  32  provides a hydraulic pressure which drives the therapeutic compound through the fluid delivery lumens  30  and out the fluid delivery ports  58 . 
     By evenly spacing the fluid delivery lumens  30  around the circumference of the tubular body  12 , as illustrated in  FIG. 8 , a substantially even flow of therapeutic compound around the circumference of the tubular body  12  can be achieved. In addition, the size, location and geometry of the fluid delivery ports  58  can be selected to provide uniform fluid flow from the fluid delivery lumen  30  to the treatment site. For example, in one embodiment, fluid delivery ports  58  closer to the proximal region of the energy delivery section  18  have smaller diameters than fluid delivery ports  58  closer to the distal region of the energy delivery section  18 , thereby allowing uniform delivery of fluid across the entire energy delivery section  18 . 
     For example, in one embodiment in which the fluid delivery ports  58  have similar sizes along the length of the tubular body  12 , the fluid delivery ports  58  have a diameter between about 0.0005 inches to about 0.0050 inches. In another embodiment in which the size of the fluid delivery ports  58  changes along the length of the tubular body  12 , the fluid delivery ports  58  have a diameter between about 0.001 inches to about 0.005 inches in the proximal region of the energy delivery section  18 , and between about 0.005 inches to 0.0020 inches in the distal region of the energy delivery section  18 . The increase in size between adjacent fluid delivery ports  58  depends on the material comprising the tubular body  12 , and on the size of the fluid delivery lumen  30 . The fluid delivery ports  58  can be created in the tubular body  12  by punching, drilling, burning or ablating (such as with a laser), or by any other suitable method. Therapeutic compound flow along the length of the tubular body  12  can also be increased by increasing the density of the fluid delivery ports  58  toward the distal region  15  of the tubular body  12 . 
     It should be appreciated that it may be desirable to provide non-uniform fluid flow from the fluid delivery ports  58  to the treatment site. In such embodiment, the size, location and geometry of the fluid delivery ports  58  can be selected to provide such non-uniform fluid flow. 
     Referring still to  FIG. 8 , placement of the inner core  34  within the tubular body  12  further defines cooling fluid lumens  44 . Cooling fluid lumens  44  are formed between an outer surface  39  of the inner core  34  and an inner surface  16  of the tubular body  12 . In certain embodiments, a cooling fluid is introduced through the proximal access port  31  such that cooling fluid flow is produced through cooling fluid lumens  44  and out distal exit port  29  (see  FIG. 1 ). The cooling fluid lumens  44  are preferably evenly spaced around the circumference of the tubular body  12  (that is, at approximately 120° increments for a three-lumen configuration), thereby providing uniform cooling fluid flow over the inner core  34 . Such a configuration is desired to remove unwanted thermal energy at the treatment site. As will be explained below, the flow rate of the cooling fluid and the power to the ultrasound assembly  42  can be adjusted to maintain the temperature of the inner core energy delivery section  41  within a desired range. 
     In an exemplary embodiment, the inner core  34  can be rotated or moved within the tubular body  12 . Specifically, movement of the inner core  34  can be accomplished by maneuvering the proximal hub  37  while holding the backend hub  33  stationary. The inner core outer body  35  is at least partially constructed from a material that provides enough structural support to permit movement of the inner core  34  within the tubular body  12  without kinking of the tubular body  12 . Additionally, the inner core outer body  35  preferably comprises a material having the ability to transmit torque. Suitable materials for the inner core outer body  35  include, but are not limited to, polyimides, polyesters, polyurethanes, thermoplastic elastomers and braided polyimides. 
     In an exemplary embodiment, the fluid delivery lumens  30  and the cooling fluid lumens  44  are open at the distal end of the tubular body  12 , thereby allowing the therapeutic compound and the cooling fluid to pass into the patient&#39;s vasculature at the distal exit port. Or, if desired, the fluid delivery lumens  30  can be selectively occluded at the distal end of the tubular body  12 , thereby providing additional hydraulic pressure to drive the therapeutic compound out of the fluid delivery ports  58 . In either configuration, the inner core  34  can prevented from passing through the distal exit port by configuring the inner core  34  to have a length that is less than the length of the tubular body  12 . In other embodiments, a protrusion is formed on the inner surface  16  of the tubular body  12  in the distal region  15 , thereby preventing the inner core  34  from passing through the distal exit port  29 . 
     In still other embodiments, the catheter  10  further comprises an occlusion device (not shown) positioned at the distal exit port  29 . The occlusion device preferably has a reduced inner diameter that can accommodate a guidewire, but that is less than the outer diameter of the central lumen  51 . Thus, the inner core  34  is prevented from extending through the occlusion device and out the distal exit port  29 . For example, suitable inner diameters for the occlusion device include, but are not limited to, about 0.005 inches to about 0.050 inches. In other embodiments, the occlusion device has a closed end, thus preventing cooling fluid from leaving the catheter  10 , and instead recirculating to the proximal region  14  of the tubular body  12 . These and other cooling fluid flow configurations permit the power provided to the ultrasound assembly  42  to be increased in proportion to the cooling fluid flow rate. Additionally, certain cooling fluid flow configurations can reduce exposure of the patient&#39;s body to cooling fluids. 
     In certain embodiments, as illustrated in  FIG. 8 , the tubular body  12  further comprises one or more temperature sensors  20 , which are preferably located within the energy delivery section  18 . In such embodiments, the proximal region  14  of the tubular body  12  includes a temperature sensor lead wire (not shown) which can be incorporated into cable  45  (illustrated in  FIG. 1 ). Suitable temperature sensors include, but are not limited to, temperature sensing diodes, thermistors, thermocouples, resistance temperature detectors (“RTDs”) and fiber optic temperature sensors which use thermalchromic liquid crystals. Suitable temperature sensor  20  geometries include, but are not limited to, a point, a patch or a stripe. The temperature sensors  20  can be positioned within one or more of the fluid delivery lumens  30 , and/or within one or more of the cooling fluid lumens  44 . 
       FIG. 9  illustrates one embodiment for electrically connecting the temperature sensors  20 . In such embodiments, each temperature sensor  20  is coupled to a common wire  61  and is associated with an individual return wire  62 . Accordingly, n+1 wires can be used to independently sense the temperature at n distinct temperature sensors  20 . The temperature at a particular temperature sensor  20  can be determined by closing a switch  64  to complete a circuit between that thermocouple&#39;s individual return wire  62  and the common wire  61 . In embodiments wherein the temperature sensors  20  comprise thermocouples, the temperature can be calculated from the voltage in the circuit using, for example, a sensing circuit  63 , which can be located within the external control circuitry  100 . 
     In other embodiments, each temperature sensor  20  is independently wired. In such embodiments, 2n wires pass through the tubular body  12  to independently sense the temperature at n independent temperature sensors  20 . In still other embodiments, the flexibility of the tubular body  12  can be improved by using fiber optic based temperature sensors  20 . In such embodiments, flexibility can be improved because only n fiber optic members are used to sense the temperature at n independent temperature sensors  20 . 
       FIG. 10  illustrates one embodiment of a feedback control system  68  that can be used with the catheter  10 . The feedback control system  68  can be integrated into the control system that is connected to the inner core  34  via cable  45  (as illustrated in  FIG. 1 ). The feedback control system  68  allows the temperature at each temperature sensor  20  to be monitored and allows the output power of the energy source  70  to be adjusted accordingly. A physician can, if desired, override the closed or open loop system. 
     The feedback control system  68  preferably comprises an energy source  70 , power circuits  72  and a power calculation device  74  that is coupled to the ultrasound radiating members  40 . A temperature measurement device  76  is coupled to the temperature sensors  20  in the tubular body  12 . A processing unit  78  is coupled to the power calculation device  74 , the power circuits  72  and a user interface and display  80 . 
     In operation, the temperature at each temperature sensor  20  is determined by the temperature measurement device  76 . The processing unit  78  receives each determined temperature from the temperature measurement device  76 . The determined temperature can then be displayed to the user at the user interface and display  80 . 
     The processing unit  78  comprises logic for generating a temperature control signal. The temperature control signal is proportional to the difference between the measured temperature and a desired temperature. The desired temperature can be determined by the user (set at the user interface and display  80 ) or can be preset within the processing unit  78 . 
     The temperature control signal is received by the power circuits  72 . The power circuits  72  are preferably configured to adjust the power level, voltage, phase and/or current of the electrical energy supplied to the ultrasound radiating members  40  from the energy source  70 . For example, when the temperature control signal is above a particular level, the power supplied to a particular group of ultrasound radiating members  40  is preferably reduced in response to that temperature control signal. Similarly, when the temperature control signal is below a particular level, the power supplied to a particular group of ultrasound radiating members  40  is preferably increased in response to that temperature control signal. After each power adjustment, the processing unit  78  preferably monitors the temperature sensors  20  and produces another temperature control signal which is received by the power circuits  72 . 
     The processing unit  78  preferably further comprises safety control logic. The safety control logic detects when the temperature at a temperature sensor  20  has exceeded a safety threshold. The processing unit  78  can then provide a temperature control signal which causes the power circuits  72  to stop the delivery of energy from the energy source  70  to that particular group of ultrasound radiating members  40 . 
     Because, in certain embodiments, the ultrasound radiating members  40  are mobile relative to the temperature sensors  20 , it can be unclear which group of ultrasound radiating members  40  should have a power, voltage, phase and/or current level adjustment. Consequently, each group of ultrasound radiating member  40  can be identically adjusted in certain embodiments. In a modified embodiment, the power, voltage, phase, and/or current supplied to each group of ultrasound radiating members  40  is adjusted in response to the temperature sensor  20  which indicates the highest temperature. Making voltage, phase and/or current adjustments in response to the temperature sensed by the temperature sensor  20  indicating the highest temperature can reduce overheating of the treatment site. 
     The processing unit  78  also receives a power signal from a power calculation device  74 . The power signal can be used to determine the power being received by each group of ultrasound radiating members  40 . The determined power can then be displayed to the user on the user interface and display  80 . 
     As described above, the feedback control system  68  can be configured to maintain tissue adjacent to the energy delivery section  18  below a desired temperature. For example, it is generally desirable to prevent tissue at a treatment site from increasing more than 6° C. As described above, the ultrasound radiating members  40  can be electrically connected such that each group of ultrasound radiating members  40  generates an independent output. In certain embodiments, the output from the power circuit maintains a selected energy for each group of ultrasound radiating members  40  for a selected length of time. 
     The processing unit  78  can comprise a digital or analog controller, such as for example a computer with software. When the processing unit  78  is a computer it can include a central processing unit (“CPU”) coupled through a system bus. As is well known in the art, the user interface and display  80  can comprise a mouse, a keyboard, a disk drive, a display monitor, a nonvolatile memory system, or any another. Also preferably coupled to the bus is a program memory and a data memory. 
     In lieu of the series of power adjustments described above, a profile of the power to be delivered to each group of ultrasound radiating members  40  can be incorporated into the processing unit  78 , such that a preset amount of ultrasonic energy to be delivered is pre-profiled. In such embodiments, the power delivered to each group of ultrasound radiating members  40  can then be adjusted according to the preset profiles. 
     The ultrasound radiating members  40  can be operated in a pulsed mode. For example, in one embodiment, the time average power supplied to the ultrasound radiating members  40  is preferably between about 0.1 watts and 2 watts and more preferably between about 0.5 watts and 1.5 watts. In certain preferred embodiments, the time average power is approximately 0.6 watts or 1.2 watts. The duty cycle is preferably between about 1% and 50% and more preferably between about 5% and 25%. In certain preferred embodiments, the duty ratio is approximately 7.5% or 15%. The pulse averaged power is preferably between about 0.1 watts and 20 watts and more preferably between approximately 5 watts and 20 watts. In certain preferred embodiments, the pulse averaged power is approximately 8 watts and 16 watts. The amplitude during each pulse can be constant or varied. 
     In one embodiment, the pulse repetition rate is preferably between about 5 Hz and 150 Hz and more preferably between about 10 Hz and 50 Hz. In certain preferred embodiments, the pulse repetition rate is approximately 30 Hz. The pulse duration is preferably between about 1 millisecond and 50 milliseconds and more preferably between about 1 millisecond and 25 milliseconds. In certain preferred embodiments, the pulse duration is approximately 2.5 milliseconds or 5 milliseconds. 
     In one particular embodiment, the ultrasound radiating members  40  are operated at an average power of approximately 0.6 watts, a duty cycle of approximately 7.5%, a pulse repetition rate of 30 Hz, a pulse average electrical power of approximately 8 watts and a pulse duration of approximately 2.5 milliseconds. 
     The ultrasound radiating members  40  used with the electrical parameters described herein preferably has an acoustic efficiency greater than 50% and more preferably greater than 75%. The ultrasound radiating members  40  can be formed a variety of shapes, such as, cylindrical (solid or hollow), flat, bar, triangular, and the like. The length of the ultrasound radiating members  40  is preferably between about 0.1 cm and about 0.5 cm. The thickness or diameter of the ultrasound radiating members  40  is preferably between about 0.02 cm and about 0.2 cm. 
       FIGS. 11A through 11D  illustrate an exemplary method for using the ultrasonic catheter  10 . As illustrated in  FIG. 11A , a guidewire  84  similar to a guidewire used in typical angioplasty procedures is directed through a patient&#39;s vessels  86  to a treatment site  88  which includes a clot  90 . The guidewire  84  is directed through the clot  90 . Suitable vessels  86  include, but are not limited to, the large periphery and the small cerebral blood vessels of the body. Additionally, as mentioned above, the ultrasonic catheter  10  also has utility in various imaging applications or in applications for treating and/or diagnosing other diseases in other body parts. 
     As illustrated in  FIG. 11B , the tubular body  12  is slid over and is advanced along the guidewire  84  using conventional over-the-guidewire techniques. The tubular body  12  is advanced until the energy delivery section  18  of the tubular body  12  is positioned at the clot  90 . In certain embodiments, radiopaque markers (not shown) are positioned along the energy delivery section  18  of the tubular body  12  to aid in the positioning of the tubular body  12  within the treatment site  88 . 
     As illustrated in  FIG. 11C , the guidewire  84  is then withdrawn from the tubular body  12  by pulling the guidewire  84  from the proximal region  14  of the catheter  10  while holding the tubular body  12  stationary. This leaves the tubular body  12  positioned at the treatment site  88 . 
     As illustrated in  FIG. 11D , the inner core  34  is then inserted into the tubular body  12  until the ultrasound assembly is positioned at least partially within the energy delivery section  18  of the tubular body  12 . Once the inner core  34  is properly positioned, the ultrasound assembly  42  is activated to deliver ultrasonic energy through the energy delivery section  18  to the clot  90 . As described above, in one embodiment, suitable ultrasonic energy is delivered with a frequency between about 20 kHz and about 20 MHz. 
     In a certain embodiment, the ultrasound assembly  42  comprises sixty ultrasound radiating members  40  spaced over a length between approximately 30 cm and 50 cm. In such embodiments, the catheter  10  can be used to treat an elongate clot  90  without requiring movement of or repositioning of the catheter  10  during the treatment. However, it will be appreciated that in modified embodiments the inner core  34  can be moved or rotated within the tubular body  12  during the treatment. Such movement can be accomplished by maneuvering the proximal hub  37  of the inner core  34  while holding the backend hub  33  stationary. 
     Referring again to  FIG. 11D , arrows  48  indicate that a cooling fluid flows through the cooling fluid lumen  44  and out the distal exit port  29 . Likewise, arrows  49  indicate that a therapeutic compound flows through the fluid delivery lumen  30  and out the fluid delivery ports  58  to the treatment site  88 . 
     The cooling fluid can be delivered before, after, during or intermittently with the delivery of ultrasonic energy. Similarly, the therapeutic compound can be delivered before, after, during or intermittently with the delivery of ultrasonic energy. Consequently, the steps illustrated in  FIGS. 11A  through  11 D can be performed in a variety of different orders than as described above. In an exemplary embodiment, the therapeutic compound and ultrasonic energy are applied until the clot  90  is partially or entirely dissolved. Once the clot  90  has been dissolved to the desired degree, the tubular body  12  and the inner core  34  are withdrawn from the treatment site  88 . 
     Overview of a Small Vessel Ultrasound Catheter 
     Over the years, numerous types of ultrasound catheters have been proposed for various therapeutic purposes. However, none of the existing ultrasound catheters is well adapted for effective use within small blood vessels in the distal anatomy. For example, in one primary shortcoming, the region of the catheter on which the ultrasound assembly is located (typically along the distal end portion) is relatively rigid and therefore lacks the flexibility necessary for navigation through difficult regions of the distal anatomy. Furthermore, it has been found that it is very difficult to manufacture an ultrasound catheter having a sufficiently small diameter for use in small vessels while providing adequate pushability and torqueability. Still further, it has been found that the distal tip of an ultrasound catheter can easily damage the fragile vessels of the distal anatomy during advancement through the patient&#39;s vasculature. 
     Accordingly, an urgent need exists for an improved ultrasound catheter that is capable of safely and effectively navigating small blood vessels. It is also desirable that such a device be capable of delivering adequate ultrasound energy to achieve the desired therapeutic purpose. It is also desirable that such a device be capable of accessing a treatment site in fragile distal vessels in a manner that is safe for the patient and that is not unduly cumbersome. The present invention addresses these needs. 
     The advancement of an ultrasound catheter through a blood vessel to a treatment site can be difficult and dangerous, particularly when the treatment site is located within a small vessel in the distal region of a patient&#39;s vasculature. To reach the treatment site, it is often necessary to navigate a tortuous path around difficult bends and turns. During advancement through the vasculature, bending resistance along the distal end portion of the catheter can severely limit the ability of the catheter to make the necessary turns. Moreover, as the catheter is advanced, the distal tip of the catheter is often in contact with the inner wall of the blood vessel. The stiffness and rigidity of the distal tip of the catheter may lead to significant trauma or damage to the tissue along the inner wall of the blood vessel. As a result, advancement of an ultrasound catheter through small blood vessels can be extremely hazardous. Therefore, a need exists for an improved ultrasound catheter design that allows a physician to more easily navigate difficult turns in small blood vessels while minimizing trauma and/or damage along the inner walls of the blood vessels. To address this need, preferred embodiments of the present invention described herein provide an ultrasound catheter that is well suited for use in the treatment of small blood vessels or other body lumens having a small inner diameter. 
     As used herein, the term “ultrasound energy” is a broad term and is used in its ordinary sense and means, without limitation, mechanical energy transferred through pressure or compression waves with a frequency greater than about 20 kHz. In one embodiment, the waves of the ultrasound energy have a frequency between about 500 kHz and 20 MHz and in another embodiment between about 1 MHz and 3 MHz. In yet another embodiment, the waves of the ultrasound energy have a frequency of about 3 MHz. 
     As used herein, the term “catheter” is a broad term and is used in its ordinary sense and means, without limitation, an elongate flexible tube configured to be inserted into the body of a patient, such as, for example, a body cavity, duct or vessel. 
     Referring now to  FIGS. 12 through 13B , for purposes of illustration, preferred embodiments of the present invention provide an ultrasound catheter  1100  that is particularly well suited for use within small vessels of the distal anatomy, such as, for example, in the remote, small diameter, neurovasculature in the brain. 
     As shown in  FIGS. 12 and 13A , the ultrasound catheter  1100  generally comprises a multi-component tubular body  1102  having a proximal end  1104  and a distal end  1106 . The tubular body  1102  and other components of the catheter  1100  can be manufactured in accordance with any of a variety of techniques well know in the catheter manufacturing field. As discussed in more detail below, suitable material dimensions can be readily selected taking into account the natural and anatomical dimensions of the treatment site and of the desired percutaneous access site. 
     Preferably, the tubular body  1102  can be divided into at least three sections of varying stiffness. The first section, which preferably includes the proximal end  1104 , is generally more stiff than a second section, which lies between the proximal end  1104  and the distal end  1106  of the catheter. This arrangement facilitates the movement and placement of the catheter  1102  within small vessels. The third section, which includes ultrasound radiating element  1124 , is generally stiffer than the second section due to the presence of the ultrasound radiating element  1124 . 
     In each of the embodiments described herein, the assembled ultrasound catheter preferably has sufficient structural integrity, or “pushability,” to permit the catheter to be advanced through a patient&#39;s vasculature to a treatment site without buckling or kinking. In addition, the catheter has the ability to transmit torque, such that the distal portion can be rotated into a desired orientation after insertion into a patient by applying torque to the proximal end. 
     The elongate flexible tubular body  1102  comprises an outer sheath  1108  (see  FIG. 13A ) that is positioned upon an inner core  1110 . In an embodiment particularly well suited for small vessels, the outer sheath  1108  comprises extruded PEBAX, PTFE, PEEK, PE, polymides, braided polymides and/or other similar materials. The distal end portion of the outer sheath  1108  is adapted for advancement through vessels having a very small diameter, such as those in the neurovasculature of the brain. Preferably, the distal end portion of the outer sheath  1108  has an outer diameter between about 2 and 5 French. More preferably, the distal end portion of the outer sheath  1108  has an outer diameter of about 2.8 French. In one preferred embodiment, the outer sheath  1108  has an axial length of approximately 150 centimeters. 
     In other embodiments, the outer sheath  1108  can be formed from a braided tubing formed of, by way of example, high or low density polyethylenes, urethanes, nylons, and the like. Such an embodiment enhances the flexibility of the tubular body  1102 . For enhanced pushability and torqueability, the outer sheath  1108  may be formed with a variable stiffness from the proximal to the distal end. To achieve this, a stiffening member may be included along the proximal end of the tubular body  1102 . 
     The inner core  1110  defines, at least in part, a delivery lumen  1112 , which preferably extends longitudinally along the entire length of the catheter  1100 . The delivery lumen  1112  has a distal exit port  1114  and a proximal access port  1116 . Referring again to  FIG. 12 , the proximal access port  1116  is defined by drug inlet port  1117  of a back end hub  1118 , which is attached to the proximal end  1104  of the other sheath  1108 . The illustrated back end hub  1118  is preferably attached to a control box connector  1120 , the utility of which will be described in more detail below. 
     The delivery lumen  1112  is preferably configured to receive a guide wire (not shown). Preferably, the guidewire has a diameter of approximately 0.008 to 0.012 inches. More preferably, the guidewire has a diameter of about 0.010 inches. The inner core  1110  is preferably formed from polymide or a similar material which, in some embodiments, can be braided to increase the flexibility of the tubular body  1102 . 
     With particular reference to  FIGS. 13A and 13B , the distal end  1106  of the catheter  1102  preferably includes the ultrasound radiating element  1124 . In the illustrated embodiment, the ultrasound radiating element  1124  comprises an ultrasound transducer, which converts, for example, electrical energy into ultrasound energy. In a modified embodiment, the ultrasound energy can be generated by an ultrasound transducer that is remote from the ultrasound radiating element  1124  and the ultrasound energy can be transmitted via, for example, a wire to the ultrasound radiating element  1124 . 
     In the embodiment illustrated in  FIGS. 13A and 13B , the ultrasound radiating element  1124  is configured as a hollow cylinder. As such, the inner core  1110  can extend through the lumen of the ultrasound radiating element  1124 . The ultrasound radiating element  1124  can be secured to the inner core  1110  in any suitable manner, such as with an adhesive. A potting material may also be used to further secure the mounting of the ultrasound radiating element along the central core. 
     In other embodiments, the ultrasound radiating element  1124  can be configured with a different shape. For example, the ultrasound radiating element may take the form of a solid rod, a disk, a solid rectangle or a thin block. Still further, the ultrasound radiating element  1124  may comprise a plurality of smaller ultrasound radiating elements. The illustrated arrangement is the generally preferred configuration because it provides for enhanced cooling of the ultrasound radiating element  1124 . For example, in one preferred embodiment, a drug solution can be delivered through the delivery lumen  1112 . As the drug solution passes through the lumen of the ultrasound radiating element, the drug solution may advantageously provide a heat sink for removing excess heat generated by the ultrasound radiating element  1124 . In another embodiment, a return path can be formed in the space  1138  between the outer sheath and the inner core such that coolant from a coolant system can be directed through the space  1138 . 
     The ultrasound radiating element  1124  is preferably selected to produce ultrasound energy in a frequency range that is well suited for the particular application. Suitable frequencies of ultrasound energy for the applications described herein include, but are not limited to, from about 20 kHz to about 20 MHz. In one embodiment, the frequency is between about 500 kHz and 20 MHz and in another embodiment from about 1 MHz and about 3 MHz. In yet another embodiment, the ultrasound energy has a frequency of about 3 MHz. 
     As mentioned above, in the illustrated embodiment, ultrasound energy is generated from electrical power supplied to the ultrasound radiating element  1124 . The electrical power can be supplied through the controller box connector  1120 , which is connected to a pair wires  1126 ,  1128  that extend through the catheter body  1102 . The electrical wires  1126 ,  1128  can be secured to the inner core  1110 , lay along the inner core  1110  and/or extend freely in the space between the inner core  1110  and the outer sheath  1108 . In the illustrated arrangement, the first wire  1126  is connected to the hollow center of the ultrasound radiating element  1124  while the second wire  1128  is connected to the outer periphery of the ultrasound radiating element  1124 . The ultrasound radiating element  1124  is preferably, but is not limited to, a transducer formed of a piezolectic ceramic oscillator or a similar material. 
     With continued reference to  FIGS. 13A and 13B , the distal end  1104  of the catheter  1100  preferably includes a sleeve  1130 , which is generally positioned about the ultrasound radiating element  1124 . The sleeve  1130  is preferably constructed from a material that readily transmits ultrasound energy. Suitable materials for the sleeve  1130  include, but are not limited to, polyolefins, polyimides, polyester and other materials having a relatively low impedance to ultrasound energy. Low ultrasound impedance materials are materials that readily transmit ultrasound energy with minimal absorption of the ultrasound energy. The proximal end of the sleeve  1130  can be attached to the outer sheath  1108  with an adhesive  1132 . To improve the bonding of the adhesive  1132  to the outer sheath  1108 , a shoulder  1127  or notch may be formed in the outer sheath for attachment of the adhesive thereto. Preferably, the outer sheath  1108  and the sleeve  1130  have substantially the same outer diameter. 
     In a similar manner, the distal end of the sleeve  1130  can be attached to a tip  1134 . In the illustrated arrangement, the tip  1134  is also attached to the distal end of the inner core  1110 . Preferably, the tip is between about 0.5 and 4.0 millimeters in length. More preferably, the tip is about 2.0 millimeters in length. As illustrated, the tip is preferably rounded in shape to reduce trauma or damage to tissue along the inner wall of a blood vessel or other body structure during advancement toward a treatment site. 
     With continued reference to  FIG. 13B , the catheter  1100  preferably includes at least one temperature sensor  1136  along the distal end  1106 . The temperature sensor  1136  is preferably located on or near the ultrasound radiating element  1124 . Suitable temperature sensors include but are not limited to, diodes, thermistors, thermocouples, resistance temperature detectors (RTDs), and fiber optic temperature sensors that used thermalchromic liquid crystals. The temperature sensor is preferably operatively connected to a control box (not shown) through a control wire, which extends through the catheter body  1102  and back end hub  1118  and is operatively connected to a control box through the control box connector  1120 . The control box preferably includes a feedback control system having the ability to monitor and control the power, voltage, current and phase supplied to the ultrasound radiating element. In this manner, the temperature along the relevant region of the catheter can be monitored and controlled for optimal performance. Details of the control box can be found in Assignee&#39;s co-pending provisional application entitled CONTROL POD FOR ULTRASONIC CATHETER, Application Ser. No. 60/336,630, filed Dec. 3, 2001, which is incorporated by reference in its entirety. 
     In one exemplary application of the ultrasound catheter  1100  described above, the apparatus may be used to remove a thrombotic occlusion from a small blood vessel. In one preferred method of use, a free end of a guidewire is percutaneously inserted into the patient&#39;s vasculature at a suitable first puncture site. The guidewire is advanced through the vasculature toward a treatment site wherein the blood vessel is occluded by the thrombus. The guidewire wire is preferably then directed through the thrombus. 
     After advancing the guidewire to the treatment site, the catheter  1100  is thereafter percutaneously inserted into the vasculature through the first puncture site and is advanced along the guidewire towards the treatment site using traditional over-the-guidewire techniques. The catheter  1100  is advanced until the distal end  1106  of the catheter  1100  is positioned at or within the occlusion. The distal end  1106  of the catheter  1100  may include one or more radiopaque markers (not shown) to aid in positioning the distal end  1106  within the treatment site. 
     After placing the catheter, the guidewire can then be withdrawn from the delivery lumen  1112 . A drug solution source (not shown), such as a syringe with a Luer fitting, is attached to the drug inlet port  1117  and the controller box connector  1120  is connected to the control box. As such, the drug solution can be delivered through the delivery lumen  1112  and out the distal access port  1114  to the thrombus. Suitable drug solutions for treating a thrombus include, but are not limited to, an aqueous solution containing heparin, urokinase, streptokinase, and/or tissue plasminogen activator (TPA). 
     The ultrasound radiating element  1124  is activated to emit ultrasound energy from the distal end  1106  of the catheter  1100 . As mentioned above, suitable frequencies for the ultrasound radiating element  1124  include, but are not limited to, from about 20 kHz to about 20 MHz. In one embodiment, the frequency is between about 500 kHz and 20 MHz and in another embodiment between about 1 MHz and 3 MHz. In yet another embodiment, the ultrasound energy is emitted at a frequency of about 3 MHz. The drug solution and ultrasound energy are applied until the thrombus is partially or entirely dissolved. Once the thrombus has been dissolved to the desired degree, the catheter  1100  is withdrawn from the treatment site. 
     Overview of Ultrasonic Catheter with Axial Energy Field. 
     As described above, in certain applications, it is desirable to project an axial (or “forward-facing”) ultrasonic energy field from the distal end of an ultrasonic catheter. For example, a high-power, forward-facing ultrasonic energy field is useful in the dissolution of aged blood clot located in the coronary vasculature. 
       FIG. 14  illustrates a distal end of an ultrasonic assembly  2000  configured to produce a forward-facing ultrasonic energy field. Such an ultrasonic assembly  2000  can be used, for example, with certain of the above-described catheters by passing the ultrasonic assembly  2000  through the catheter central lumen. 
     The ultrasonic assembly  2000  illustrated in  FIG. 14  comprises a series of substantially cylindrical ultrasound radiating members  2040  with flat ends, wherein electrodes  2100  are mounted on each of the ultrasound radiating member flat ends. The ultrasound radiating members  2040  have a positive electrode  2100 (+) and a negative electrode  2100 (−). The ultrasonic assembly  2000  further comprises a proximal cap  2200  positioned adjacent the most proximal of the ultrasound radiating members  2040 . Preferably, the proximal cap  2200  has a relatively high acoustic impedance compared to the other materials surrounding the ultrasound radiating members, such as the tubular body and the patient&#39;s vasculature. In one embodiment, the proximal cap may be made of such materials as copper, stainless steel, or plated copper or stainless steel. The cap  2200  preferably has an impedance in the range of about 35-110 MRayle and more preferably about 40-50 MRayl. This configuration causes the radiated ultrasonic energy field to be substantially forward-facing (that is, in the distal direction) along the axis of the ultrasonic assembly  2000 . 
     In certain embodiments, as also illustrated in  FIG. 14 , the ultrasonic assembly further comprises one or more joints  2300  positioned between the individual ultrasound radiating members  2040 . In this embodiment, the joints  2300  comprise an electrically insulating material to provide electrical insulation between successive ultrasound radiating members  2040 . In an exemplary embodiment, the joints  2300  comprise a material having acoustic transmission properties (such as, for example, acoustic impedance and acoustic transmission velocity) that closely match the acoustic properties of the adjacent ultrasound radiating members  2040 , while still providing additional flexibility to the ultrasonic assembly  2000 . Non limiting examples of such materials are epotek 377 and hysol 2039/3561. Such materials preferably have an impedance in the range of about 1-30 MRayl and more preferably about 2-8 MRayl. 
     Although one orientation of electrode polarities is illustrated in  FIG. 14 , other electrode polarity orientations may be more appropriate in other embodiments. For example, a modified electrode polarity orientation is illustrated in  FIG. 15 . In this embodiment, the joints  2300  may be formed from an conductive material such that the two transducers  2040  form a “sandwich”, “paired” design, sharing a common electrode as described above with reference to  FIG. 7B . 
       FIG. 15  further illustrates that an ultrasonic assembly  2000  having a hollow core  2110  can be used in modified embodiments. Such embodiments are advantageous in applications where something is to be passed through the hollow core  2110  of the ultrasonic assembly  2000 , such as for example, a guidewire, a cooling fluid or a therapeutic compound. The hollow core  2110  can be constructed in the ultrasound radiating members  2040 , the electrodes  2100  and the joints  2300  using conventional techniques, such as for example, drilling or laser cutting. As mentioned above, in this embodiment, the joint  2300  may be made a conductive material. However, the hollow ultrasound radiating members illustrated in  FIG. 15  can be used with the electrical configuration illustrated in  FIG. 14 . Additional information relating to ultrasound radiating members with hollow cores can be found in U.S. patent application Ser. No. 10/684,845, filed 14 Oct. 2003, the entire contents of which are hereby incorporated herein by reference. 
       FIG. 16  illustrates that the ultrasonic assembly  2000  illustrated in  FIGS. 14 and 15  can be expanded to include more than two ultrasound radiating members  2040 . This configuration facilitates formation of a forward-facing, or axial, energy field. In particular, ultrasonic energy generated by the ultrasound radiating members is focussed in the distal direction due to the presence of the high impedance cap  2200  at the proximal end of the ultrasound radiating members  2040 . In such embodiments, the ultrasonic energy density in a region axially distal to the assembly  2000  is greater than the ultrasonic energy density in an annular region surrounding the assembly  2000 . 
     Still referring to the exemplary embodiment illustrated in  FIG. 16 , joints  2300  are preferably positioned between each of the ultrasound radiating members  2040 , thereby maintaining the overall flexibility of the ultrasonic assembly  2000 . In addition, in an exemplary embodiment, electrodes (not shown for clarity) can be positioned on each flat end of each ultrasound radiating member, with adjacent electrodes separated by joints  2300 . Thus, in certain embodiments, joints  2300  may comprise an electrically insulating material such as  FIG. 14  or an electrically conducting material as in  FIG. 15 . 
     The ultrasonic assemblies described herein can be incorporated for use with both catheters and guidewires. For example,  FIG. 17A  illustrates a guidewire-mounted ultrasonic assembly configured to produce a forward-facing ultrasonic energy field. Specifically, a plurality of cylindrical ultrasound radiating members  2040  are mounted to the distal tip  2410  of a guidewire  2420 . In an exemplary embodiment, the distal tip  2410  of the guidewire  2420  comprises a material with relatively high acoustic impedance (e.g., the high impedance materials described above), thus producing a substantially forward-facing ultrasonic energy field. Electrodes and joints (both not shown for clarity) can be positioned between the ultrasound radiating members  2040  as described above, and as required for particular applications. In such embodiments, the joints may be conductive or insulating depending on the desired electrical arrangement. In the embodiment illustrated in  FIG. 17A , the ultrasound radiating members  2040  have substantially similar dimensions, as can be seen in the cross-sectional view of the end of the ultrasonic assembly shown in  FIG. 17B . 
       FIG. 18A  illustrates a modified embodiment of a guidewire-mounted ultrasonic assembly configured to produce a forward-facing ultrasonic energy field. In such embodiments, the ultrasound radiating members  2040  have decreasing dimensions toward the distal end of the assembly, thus providing a tapered tip.  FIG. 18B  is a cross-sectional view of the catheter guidewire of  FIG. 18A  taken along line  18 B- 18 B. A tapered tip provides increased guidewire maneuverability, which is advantageous in certain applications. From this discussion, one of ordinary skill in the art will recognize that, in general, the guidewire  2420  need not have dimensions that match the dimensions of the ultrasound radiating members  2040 . Again, as with the previous embodiments, the joints may be conductive or insulating. 
       FIG. 19A  illustrates a modified embodiment of a guidewire-mounted ultrasonic assembly configured to produce a forward-facing ultrasonic energy field. In such embodiments, the ultrasound radiating members  2040  have rectangular dimensions, thus providing a square tip. In addition,  FIG. 19A  illustrates that, in such rectangular-dimensioned ultrasound assemblies, the electrodes  2100  can be positioned generally perpendicular to the joints  2030 .  FIG. 19B  is a cross-sectional view of the catheter guidewire of  FIG. 19A  taken along line  19 B- 19 B. In this configuration, a ultrasound radiating member group comprises two ultrasound radiating members separated by an electrode  2100 . A plurality of ultrasound radiating member groups can be axially spaced along the guidewire  2420 , with the groups being separated by the joints  2300 . 
     Rectangular ultrasound radiating members can provide a reduced fabrication cost, which is advantageous in certain applications. In other embodiments, ultrasound radiating members having other cross-sectional shapes, such as other polygons or ovals, can be mounted to the distal tip  2410  of the guidewire  2420 . 
     As described above, the various ultrasound assemblies illustrated in  FIGS. 17A through 19B  can be used with catheters as well as guidewires. Specifically, the ultrasound assemblies described above can be mounted directly on a catheter, or can be mounted on an inner core configured to be passed through a catheter central lumen. By positioning a high acoustic impedance material adjacent to the most proximal ultrasound radiating member, a substantially forward-facing ultrasonic energy field can be produced. 
     By manipulating the dimensions of the ultrasound radiating members comprising the ultrasonic assemblies described above, the resonant frequency of a particular ultrasonic assembly can be determined. Likewise, the resonant frequency of an ultrasonic assembly may be dependent on other factors, such as number of ultrasound radiating members. Thus, if a particular application requires a particular frequency of ultrasonic energy to be applied to the treatment site, the dimensions and number of ultrasound radiating members present within the ultrasonic assembly can be adjusted accordingly. 
     In certain embodiments, such as illustrated in  FIG. 20 , a joint  2300  is positioned between the most proximal ultrasound radiating member  2040 ′ and the relatively high acoustic impedance material  2200 . Such a configuration further increases flexibility, and therefore maneuverability, of the ultrasonic assembly. Again, the joints may be conductive or insulating depending upon the desired electrical configuration. 
     In modified embodiments, such as illustrated in  FIG. 21 , a horn  2250  comprising an acoustically matched material (e.g., a material having a similar acoustic property as the transducer) mounted to the distal tip of the catheter or guidewire. e.g., stainless steel and titanium Such a configuration would further enhance the production of a forward-facing ultrasonic energy field from the ultrasonic assembly. The characteristics of the ultrasonic energy field produced can be further manipulated by adjusting the dimensions and shape of the horn  2250 . For example, appropriate shapes for horn  2250  include flat (illustrated in  FIG. 21 ), blunt (illustrated in  FIG. 22 ), short pointed (illustrated in  FIG. 23 ) and long pointed (illustrated in  FIG. 24 ). The addition of the horn  2250  is particularly advantageous with the sandwich type configurations described with reference, for example, to  FIG. 15 . 
     The various embodiments of the ultrasonic assemblies described herein offer several advantages. For example, the ultrasonic assemblies described herein can be operated at a reduced operational frequency as compared to conventional ultrasonic assemblies having a comparable size. Lower operational frequencies advantageously increase the therapeutic effect of the ultrasonic energy. Additionally, the ultrasonic assemblies described herein can be operated with increased power output and widened forward dispersion as compared to conventional ultrasonic assemblies. Furthermore, the highly directional nature of the ultrasonic energy field produced by the ultrasonic assemblies described herein is advantageous in certain applications that require a targeted or focussed delivery of ultrasonic energy to the treatment site. 
     SCOPE OF THE INVENTION 
     While the foregoing detailed description discloses several embodiments of the present invention, it should be understood that this disclosure is illustrative only and is not limiting of the present invention. It should be appreciated that the specific configurations and operations disclosed can differ from those described above, and that the methods described herein can be used in contexts other than treatment of vascular diseases.