Patent Publication Number: US-9849273-B2

Title: Power parameters for ultrasonic catheter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/830,145, filed Jul. 2, 2010, now U.S. Pat. No. 8,192,391, which claims the priority benefit of U.S. Provisional Application No. 61/222,959, filed Jul. 3, 2009, the entirety of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to ultrasound systems, and more specifically to ultrasound catheter systems. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic energy had been used to enhance the intravascular delivery and/or effect of various therapeutic compounds. In one system, ultrasound catheters are used to deliver ultrasonic energy and therapeutic compounds to a treatment site within a patient&#39;s vasculature. Such ultrasound catheters can comprise an elongate member configured to be advanced through a patient&#39;s vasculature and an ultrasound assembly that is positioned near a distal end portion of the elongate member. The ultrasound assembly is configured to emit ultrasonic energy. Such ultrasound catheters can include a fluid delivery lumen that is used to deliver the therapeutic compound to the treatment site. In this manner, ultrasonic energy is delivered to the treatment site to enhance the effect and/or delivery of the therapeutic compound. 
     For example, ultrasound catheters have been successfully used to treat human blood vessels that have become occluded by plaque, thrombi, emboli or other substances that reduce the blood carrying capacity of the vessel. See, for example, U.S. Pat. No. 6,001,069. To remove the occlusion, the ultrasound catheter is advanced through the patient&#39;s vasculature to deliver a therapeutic compound containing dissolution compounds directly to the occlusion. To enhance the effect and/or delivery of the therapeutic compound, ultrasonic energy is emitted into the therapeutic compound and/or the surrounding tissue at the treatment site. In other applications, ultrasound catheters are used for other purposes, such as for the delivery and activation of light activated drugs. See, for example, U.S. Pat. No. 6,176,842. 
     SUMMARY OF THE INVENTION 
     While such ultrasound catheters systems have been proven to be successful, there is a general need to continue to improve the effectiveness and speed of such systems. In this manner, treatment and/or hospital time can be reduced. 
     Accordingly, one aspect of the present invention comprises an ultrasound catheter system comprising a catheter having at least ultrasonic element; a control system configured to generate power parameters to drive the ultrasonic element to generate ultrasonic energy. The control system is configured to vary at least one of the power parameters and at least one physiological parameter. In some embodiments, the power parameters include at least peak power. In some embodiments, the peak power is ramped repeatedly ramped between a first minimum value and a first maximum value, while the physiological parameter is repeatedly ramped between a second minimum value and a second maximum value during operation of the catheter. 
     Another aspect of the present invention comprises a method of operating an ultrasonic catheter. In the method, a catheter with at least one ultrasonic element is advanced to a treatment site in a patient&#39;s vascular system or tissue. The at least one ultrasonic element is driven to generate ultrasonic energy. A therapeutic compound is delivered to the treatment site through the catheter. A power parameter and a physiological parameter of the ultrasonic element is varied during the operation. In some embodiments, the power parameter includes at least peak power. The peak power and the physiological parameter was varied by repeatedly ramping the peak power between a first maximum value and a first minimum value and repeatedly ramping the physiological parameter between a second minimum value and a second maximum value. The driving parameters of the ultrasonic element can in some embodiments attain non-linear or pseudo-linear acoustic output, or in other embodiments attain linear acoustic output. 
     Another aspect of the present invention is a control system for an ultrasound catheter. The control system includes control unit configured to vary acoustic parameters of an ultrasonic element of an ultrasonic catheter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the cavitation promoting systems and methods 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 certain features of an example ultrasonic catheter. 
         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 a preferred technique for electrically connecting five groups of ultrasound radiating members to form an ultrasound assembly. 
         FIG. 6  is a schematic wiring diagram illustrating a preferred 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. 11  is a longitudinal cross-sectional view of selected components of an exemplary ultrasound catheter assembly that is particularly well-suited for treatment of cerebral vascular occlusions, and that includes a cavitation promoting surface. 
         FIG. 12  schematically illustrates an example ultrasonic energy pulse profile. 
         FIG. 13  is a chart showing the lysis enhancement factor of a variety of ultrasonic protocols. 
         FIG. 14  is a chart illustrating Peak Power (W) as a function of time according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As used herein, the term “ultrasonic energy” is used broadly, includes its ordinary meaning, and further includes mechanical energy transferred through pressure or compression waves with a frequency greater than about 20 kHz. Ultrasonic energy waves have a frequency between about 500 kHz and about 20 MHz in one example embodiment, between about 1 MHz and about 3 MHz in another example embodiment, of about 3 MHz in another example embodiment, and of about 2 MHz in another example embodiment. As used herein, the term “catheter” is used broadly, includes its ordinary meaning, and further includes an elongate flexible tube configured to be inserted into the body of a patient, such as into a body part, cavity, duct or vessel. As used herein, the term “therapeutic compound” is used broadly, includes its ordinary meaning, and encompasses drugs, medicaments, dissolution compounds, genetic materials, and other substances capable of effecting physiological functions. A mixture comprising such substances is encompassed within this definition of “therapeutic compound”. As used herein, the term “end” is used broadly, includes its ordinary meaning, and further encompasses a region generally, such that “proximal end” includes “proximal region”, and “distal end” includes “distal region”. 
     As expounded herein, ultrasonic energy is often used to enhance the delivery and/or effect of a therapeutic compound. For example, in the context of treating vascular occlusions, ultrasonic energy has been shown to increase enzyme mediated thrombolysis by enhancing the delivery of thrombolytic agents into a thrombus, where such agents lyse the thrombus by degrading the fibrin that forms the thrombus. The thrombolytic activity of the agent is enhanced in the presence of ultrasonic energy in the thrombus. However, it should be appreciated that the invention should not be limited to the mechanism by which the ultrasound enhances treatment unless otherwise stated. In other applications, ultrasonic energy has also been shown to enhance transfection of gene-based drugs into cells, and augment transfer of chemotherapeutic drugs into tumor cells. Ultrasonic energy delivered from within a patient&#39;s body has been found to be capable of producing non-thermal effects that increase biological tissue permeability to therapeutic compounds by up to or greater than an order of magnitude. 
     Use of an ultrasound catheter to deliver ultrasonic energy and a therapeutic compound directly to the treatment site mediates or overcomes many of the disadvantages associated with systemic drug delivery, such as low efficiency, high therapeutic compound use rates, and significant side effects caused by high doses. Local therapeutic compound delivery has been found to be particularly advantageous in the context of thrombolytic therapy, chemotherapy, radiation therapy, and gene therapy, as well as in applications calling for the delivery of proteins and/or therapeutic humanized antibodies. However, it should be appreciated that in certain arrangements the ultrasound catheter can also be used in combination with systemic drug delivery instead or in addition to local drug deliver. In addition, local drug delivery can be accomplished through the use of a separate device (e.g., catheter). 
     As will be described below, the ultrasound catheter can include one or more one or more ultrasound radiating members positioned therein. Such ultrasound radiating members can comprise a transducer (e.g., a PZT transducer), which is configured to convert electrically energy into ultrasonic energy. In such embodiments, the PZT transducer is excited by specific electrical parameters (herein “power parameters” that cause it to vibrate in a way that generates ultrasonic energy). As will be explained below, Applicants have discovered that by non-linearly (e.g., randomly or pseudo randomly) varying one or more of the power parameters the effectiveness of the ultrasound catheter (e.g., the effectiveness of enhancing the removal of a thrombus) can be significantly enhanced. While, for example, U.S. Pat. No. 5,720,710 taught that randomly changing the frequency of the ultrasonic frequency could significantly enhance the remedial effect of the ultrasonic energy, these results with respect to varying the other acoustic parameters were not expected. In addition, because PZT transducers are often configured to be driven and a particularly frequency, varying the other acoustic parameters may have significant advantages over varying the frequency. In addition, varying the electrical parameters may also be used in combination with varying the frequency (e.g., in a manner taught by U.S. Pat. No. 5,720,710). 
     The techniques disclosed herein are compatible with a wide variety of ultrasound catheters, several examples of which are disclosed in USA Patent Application Publication US 2004/0024347 A1 (published Feb. 5, 2004; discloses catheters especially well-suited for use in the peripheral vasculature) and USA Patent Application Publication 2005/0215942 A1 (published Sep. 29, 2005; discloses catheters especially well-suited for use in the cerebral vasculature). Certain of the techniques disclosed herein are compatible with ultrasound catheters that would be unable to generate cavitation at an intravascular treatment site but for the use of such techniques. 
     With reference to the illustrated embodiments,  FIG. 1  illustrates an ultrasonic catheter  10  configured for use in a patient&#39;s vasculature. For example, in certain applications the ultrasonic catheter  10  is used to treat long segment peripheral arterial occlusions, such as those in the vascular system of the leg, while in other applications the ultrasonic catheter  10  is used to treat occlusions in the small vessels of the neurovasculature or other portions of the body (e.g., other distal portions of the vascular system). Thus, the dimensions of the catheter  10  are adjusted based on the particular application for which the catheter  10  is to be used. 
     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  located in the distal region  15  of the catheter  10 . The tubular body  12  and other components of the catheter  10  are manufactured in accordance with a variety of techniques. Suitable materials and dimensions are 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  1800  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, in certain embodiments nickel titanium or stainless steel wires are placed along or incorporated into the tubular body  12  to reduce kinking. 
     The energy delivery section  18  of the tubular body  1200  optionally comprises a material that (a) is thinner than the material comprising the proximal region  1400  of the tubular body  1200 , or (b) has a greater acoustic transparency than the material comprising the proximal region  14  of the tubular body  12 . Thinner materials generally have greater acoustic transparency than thicker materials. Suitable materials for the energy delivery section  1800  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  is formed from the same material or a material of the same thickness as the proximal region  18 . 
     One or more fluid delivery lumens are incorporated into the tubular body  12 . For example, in one embodiment a central lumen passes through the tubular body  12 . The central lumen extends through the length of the tubular body  1200 , and is coupled to a distal exit port  1290  and a proximal access port  1310 . The proximal access port  1310  forms part of the backend hub  1330 , which is attached to the proximal region  14  of the catheter  10 . The backend hub  1330  optionally further comprises cooling fluid fitting  1460 , which is hydraulically connected to a lumen within the tubular body  12 . The backend hub  1330  also optionally comprises a therapeutic compound inlet port  1320 , which is hydraulically connected to a lumen within the tubular body  12 . The therapeutic compound inlet port  1320  is optionally also hydraulically coupled to a source of therapeutic compound via a hub such as a Luer fitting. 
     The catheter  100  is configured to have one or more ultrasound radiating members positioned therein. For example, in certain embodiments an ultrasound radiating member is fixed within the energy delivery section  18  of the tubular body, while in other embodiments a plurality of ultrasound radiating members are fixed to an assembly that is passed into the central lumen. In either case, the one or more ultrasound radiating members are electrically coupled to a control system  1100  via cable  1450 . In one embodiment, the outer surface of the energy delivery  18  section can include an cavitation promoting surface configured to enhance/promote cavitation at the treatment site. 
     With reference to  FIG. 2-10 , an exemplary arrangement of the energy delivery section  18  and other portions of the catheter  10  described above. This arrangement is particularly well-suited for treatment of peripheral vascular occlusions. 
       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 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  40  form an ultrasound assembly  42 , which will be described in 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 , the inner core  34  preferably comprises 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 a control system  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 a preferred embodiment, the ultrasound assembly  42  comprises a plurality of ultrasound radiating members  40  that are divided into one or more groups. For example,  FIGS. 5 and 6  are schematic wiring diagrams illustrating one technique for connecting five groups of ultrasound radiating members  40  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  40  that are electrically connected to each other. The five groups are also electrically connected to the control system  100 . 
     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 . The control circuitry can be configured as part of the control system  1100  and can include circuits, control routines, controllers etc configured to vary one or more power parameters used to drive ultrasound radiating members  40 , 
     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  310  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 GI 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  310  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  312 . Because the common wire  108  is connected to the negative terminal  106 , and the positive contact wire  312  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. 
     One of ordinary skill in the art will recognize that 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 focused, more diffuse ultrasonic energy field to the treatment site. 
     In a preferred 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 configuration 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 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 ports  30  to the treatment site. For example, in one embodiment, fluid delivery ports closer to the proximal region of the energy delivery section  18  have smaller diameters then fluid delivery closer to the distal region of the energy delivery section  18 , thereby allowing uniform delivery of fluid across the entire energy delivery section. 
     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 can 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.degree. increments for a three-lumen configuration), thereby providing uniform cooling fluid flow over the inner core  34 . Such a configuration is desirably 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 temp of the inner core energy delivery section  41  within a desired range. 
     In a preferred 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 a preferred 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 making providing the inner core  34  with a length that is less than the length of the tubular body. In other embodiments, a protrusion is formed on the internal side of the tubular body in the distal region  15 , thereby preventing the inner core  34  from passing through the distal exit port. 
     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 inner 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 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  (as illustrated), 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 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  1100  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 (at 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.degree. 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 are preferably operated in a pulsed mode. For example, in one embodiment, the time average electrical power supplied to the ultrasound radiating members is between about 0.001 watts and about 5 watts and can be between about 0.05 watts and about 3 watts. In certain embodiments, the time average electrical power over treatment time is approximately 0.45 watts or 1.2 watts. The duty cycle is between about 0.01% and about 90% and can be between about 0.1% and about 50%. In certain embodiments, the duty ratio is approximately 7.5%, 15% or a variation between 1% and 30%. The pulse averaged electrical power can be between about 0.01 watts and about 20 watts and can be between approximately 0.1 watts and 20 watts. In certain embodiments, the pulse averaged electrical power is approximately 4 watts, 8 watts, 16 watts, or a variation of 1 to 8 watts. As will be described above, the amplitude, pulse width, pulse repetition frequency, average acoustic pressure or any combination of these parameters can be constant or varied during each pulse or over a set of portions. In a non-linear application of acoustic parameters the above ranges can change significantly. Accordingly, the overall time average electrical power over treatment time may stay the same but not real-time average power. 
     In one embodiment, the pulse repetition rate is preferably between about 1 Hz and about 2 kHz and more can be between about 1 Hz and about 50 Hz. In certain preferred embodiments, the pulse repetition rate is approximately 30 Hz, or a variation of about 10 to about 40 Hz. The pulse duration or width is can be between about 0.5 millisecond and about 50 milliseconds and can be between about 0.1 millisecond and about 25 milliseconds. In certain embodiments, the pulse duration is approximately 2.5 milliseconds, 5 or a variation of 1 to 8 milliseconds. In addition, the average acoustic pressure can be between about 0.1 to about 2 MPa or in another embodiment between about 0.5 or about 0.74 to about 1.7 MPa. 
     In one particular embodiment, the transducers are operated at an average power of approximately 0.6 watts, a duty cycle of approximately 7.5%, a pulse repetition rate of about 30 Hz, a pulse average electrical power of approximately 8 watts and a pulse duration of approximately 2.5 milliseconds. 
     The ultrasound radiating member used with the electrical parameters described herein preferably has an acoustic efficiency than about 50% and can be greater than about 75%. The ultrasound radiating member 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 member is preferably between about 0.1 cm and about 0.5 cm. The thickness or diameter of the ultrasound radiating members is preferably between about 0.02 cm and about 0.2 cm. 
     With reference now to  FIG. 11 , the energy delivery section of an ultrasound catheter that is configured for treating small vessels (e.g., for treatment of cerebral vascular occlusions) is shown and that includes an optional cavitation promoting surface  71 . In this embodiment, the catheter includes an inner core  73  that defines a utility lumen  72  configured to pass materials such as a guidewire, a therapeutic compound and/or a cooling fluid. The catheter assembly  70  further includes a distal tip element  74  and a hollow cylindrical ultrasound radiating member  77  that is mounted on the inner core  73 . Certain of these components are optional, and are omitted from alternative embodiments. In an example embodiment, the diameter of the catheter outer body  76  is less than about 5 French, although other dimensions are used in other embodiments. In addition, although only a single ultrasound element is shown, in modified embodiments, more one ultrasound element can b mounted along the lumen  72 . 
     In example embodiments, the ultrasound radiating member  77  illustrated in  FIG. 11  is a tubular piezoceramic transducer that is able to radiate ultrasonic energy in a length mode, a thickness mode, and a circumferential mode. The ultrasound radiating member  77  is capable of generating a peak acoustic pressures that are preferably between about 0.7 MPa and about 10 MPa, and that are more preferably between about 1.2 MPa and about 6 MPa. However such parameters may be different if the catheter includes cavitation promoting surfaces or other modifications. 
     In a modified embodiment, the ultrasound radiating member  77  has a resonant frequency greater than or equal to approximately 1 MHz in the thickness mode. In certain embodiments, the ultrasound radiating member included in an ultrasound catheter optionally includes an electrode, such as a nickel-plated electrode, that enables electrical wires to be soldered thereto. 
     As will be described below, the ultrasound catheter includes one or more one or more ultrasound radiating members positioned therein. Such ultrasound radiating members can comprise a transducer (e.g., a PZT transducer), which is configured to convert electrically energy into ultrasonic energy. In such embodiments, the PZT transducer is excited by specific electrical parameters (herein “power parameters” or “acoustic parameters” that cause it to vibrate in a way that generates ultrasonic energy). As will be explained below, Applicants have discovered that non-linearly varying (e.g., randomly or pseudo randomly) one or more of the power parameters the effectiveness of the ultrasound catheter (e.g., the effectiveness of enhancing the removal of a thrombus) can be significantly enhanced. By non-linearly varying one or more of the power parameters the ultrasound radiating members create nonlinear acoustic pressure, which as described above can increase the effectiveness of the acoustic pressure in enhancing a therapeutic compound. In one application, the effect of nonlinearly varying acoustic pressure has been found by Applicant to enhance enzyme medicated thrombolysis by almost 1.9 times as compared to the application of substantially constant acoustic pressure. Examples of nonlinear variances include, but are not limited to, multi variable variations, variations as a function of a complex equation, sinusoidal variations, exponential variations, random variations, pseudo random variations and/or arbitrary variations. While nonlinear variance is preferred, in other arrangements it is anticipate that one or more of the parameters discussed can be varied in a linear manner either alone or combination with the nonlinear variance. 
       FIG. 12  will be used to explain certain power parameters which can used to drive the ultrasound radiating members. As shown, the members can be driven a series of pulses  2000  having peak power P or amplitude and duration τ. During these pulses  2000 , the ultrasound radiating members as driven at a certain frequency f as described above by the electrical current. The pulses  2000  can be separated by “off” periods  2100 . The cycle period T is defined as the time between pulse initiations, and thus the pulse repetition frequency (“PRF”) is given by T −1 . The duty cycle is defined as the ratio of time of one pulse to the time between pulse initiations τT −1 , and represents the fraction of time that ultrasonic energy is being delivered to the treatment site. The average power delivered in each cycle period is given by PτT −1 . Accordingly, the illustrated embodiment, the ultrasound radiating members are operated using pulses, or modulated electrical drive power instead of continuous drive power 
     In one embodiment, the average power delivered in each cycle period is preferably between about 0.1 watts and about 2.0 watts. In such an embodiment, each cycle period has a different average power value, wherein the average power values for the different cycles vary in a nonlinear fashion. Examples of non-linear variation include, but are not limited to, simple or complex variable or multi-variable equations, varying randomly, pseudo randomly and/or in an arbitrary manner. For instance, in one such modified embodiment, each cycle period has an average power quantity that is randomly or pseudo randomly distributed between a maximum average power quantity and a minimum average power quantity. The average power of each cycle period can be adjusted by manipulating one or more parameters of the waveform in the cycle period, such as, but not limited to, peak power P, reduced power P′, pulse repetition frequency, pulse duration τ, and duty cycle. 
     In another embodiment, the duty cycle is preferably between about 1% and about 50%, is more preferably between about 2% and about 28%. During operation of the catheter, the duty cycle can vary in a nonlinear fashion. For instance, in one such modified embodiment, the duty cycle that is randomly or pseudo randomly distributed between a maximum duty cycle and a minimum duty cycle. For example, in one embodiment, the values for the maximum duty cycle are between about 25% and about 30%, and typical values for the minimum duty cycle are between about 1.5% and about 3.5%. In yet another embodiment, the duty cycle is varied non-linearly from a minimum value of about 2.3% and a maximum value of about 27.3%. In one embodiment, other parameters of the waveform are manipulated such that each cycle period has the same average power, even though the duty cycle for each cycle period is varying in a nonlinear fashion. 
     In another embodiment, the peak power P delivered to the treatment site is preferably between about 0.1 watts and about 20 watts, is more preferably between about 5 watts and about 20 watts, and is most preferably between about 8 watts and about 16 watts. Within the ranges, during operation of the catheter, the peak power P can vary in a nonlinear fashion. For instance, in one such modified embodiment, each cycle period has a peak power quantity that is randomly or pseudo randomly distributed between a maximum peak power P max  and a minimum peak power P min . Typical values for the maximum peak power P max  are between about 6.8 watts and about 8.8 watts, and typical values for the minimum peak power P min  are between about 0.1 watts and about 1.0 watts. In another embodiment, the peak power is varied non-linearly between a maximum peak power P max  of about 7.8 watts and a minimum peak power P min  of about 0.5 watts. In one embodiment, other parameters of the waveform are manipulated such that each cycle period has the same average power, even though the peak power P for each cycle period is varying in a nonlinear fashion. 
     In another embodiment, the effect of a therapeutic compound is optionally enhanced by using a certain pulse repetition frequency PRF and/or a certain pulse duration τ. In one example embodiment, the PRF is preferably between about 5 Hz and about 150 Hz, is more preferably between about 10 Hz and about 50 Hz, and is most preferably between about 20 Hz and about 40 Hz. In one embodiment, the PRF remains substantially constant during the course of a treatment. However, in certain modified embodiments the PRF is non-linearly varied during the course of a treatment within the ranges described above. For example, in one such modified embodiment the PRF is varied linearly during the course of the treatment, while in another such modified embodiment the PRF is varied nonlinearly during the course of the treatment. Examples of nonlinear variances include, but are not limited to, sinusoidal variations, exponential variations, and random variations. For instance, in an example embodiment the PRF is varied randomly between a maximum PRF and a minimum PRF during the course of a treatment. Typical values for the maximum PRF are between about 28 Hz and about 48 Hz, and typical values for the minimum PRF are between about 5 Hz and about 15 Hz. In another embodiment, the maximum PRF is about 38 Hz and the minimum is about 10 Hz. In one embodiment, the pulse repetition interval is varied between about 25 to about 100 ms. 
     The pulse amplitude, pulse width and pulse repetition frequency during each pulse can also be constant or varied in a non-linear fashion as described herein. Other parameters are used in other embodiments depending on the particular application. 
     In one example embodiment, the pulse duration τ is preferably between about 1 millisecond and about 50 milliseconds, is more preferably between about 1 millisecond and about 25 milliseconds, and is most preferably between about 2.5 milliseconds and about 5 milliseconds. In a modified embodiment, each cycle period has a different pulse duration τ, wherein the pulse duration values vary in a nonlinear fashion with the ranges described above. For instance, in one such modified embodiment, each cycle period has a pulse duration quantity that is randomly distributed between a maximum pulse duration τ max  and a minimum pulse duration τ min . Typical values for the maximum pulse duration τ max  are between about 6 milliseconds and about 10 milliseconds (and in one embodiment about 8 milliseconds), and typical values for the minimum pulse duration τ min  are between about 0.1 milliseconds and about 2.0 milliseconds (and in one embodiment 1 millisecond), In one embodiment, other parameters of the waveform are manipulated such that each cycle period has the same average power, even though the pulse duration τ for each cycle period is varying in a nonlinear fashion. In other embodiments, the average power can be varied non-linearly. 
     In addition, the average acoustic pressure can also non-linearly varied as described above between about 0.1 to about 2 MPa or in another embodiment between about 0.5 or about 0.74 to about 1.7 MPa. 
     The control system  1100  can be configured to vary one or more of the power parameters as discussed above. Accordingly, the control system  1100  can include any of a variety of control routines, control circuits, etc. so as to vary the power parameters described above. As mentioned above, the control parameters can be varied in combination with other operating parameters (e.g., frequency) of the ultrasound radiating member and/or catheter. Alternatively, the power parameters may be varied using a software package that controls the operation of the ultrasound radiating members. It should also be appreciated that one, two, three or all of the parameters (and subsets thereof) can be non-linearly varied at the same time or by themselves. 
     A study to investigate the effect of a variety of randomization protocols on clot lysis was conducted. The randomization protocols involved non-linearly varying peak power, pulse width, pulse repetition frequency, or combinations of the above. The randomization protocols were tested using a time average power of either about 0.45 W or about 0.90 W, and were compared to a standard Neurowave E11 protocol. 
     Clots were prepared by adding 1 mL of citrated human pooled plasma to a polystyrene culture tube. Clotting was initiated by the addition of 100 μL of 0.2M calcium chloride and 100 μL of 12.5 U/ml bovine thrombin. Fixtures equipped with drug delivery lumens and an ultrasonic catheter were inserted into the clot, thereby allowing the clot to form around the fixtures. Clots were allowed to incubate for 10 minutes in a 37 degrees C. water bath before initiating the clot lysis procedure. 
     Clot lysis was initiated by delivering rt-PA to the clot via the drug delivery lumens. A total of 0.08 mL of 5000 U/mL rt-PA solution was delivered to the clot over a period of 5 minutes at a rate of 0.96 mL/hr. 
     After drug delivery was completed, the clot was subjected to 5 minutes of ultrasound exposure, and 25 minutes of additional incubation time subsequent to the ultrasound treatment. The clots were then removed from the polystyrene culture tubes and pressed between filter paper to remove serum from the clots before the clots were weighed. 
     The acoustic protocols tested are summarized in Table 1 provided below. “PW” represents pulse width and “PRF” represents pulse repetition frequency. Ranges indicate that the parameter was varied randomly within the range shown. For example, for the R3P-d protocol, peak power was varied from 1.6 to 7.9 W, pulse width was varied from 1.16 to 8.16 ms, and pulse repetition frequency was varied from 10 to 40 Hz. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Description of acoustic protocols 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Average 
                   
                   
                   
               
               
                 Acoustic Protocol 
                 Power 
                 Peak Power 
                 PW 
                 PRF 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Neurowave E11 
                 0.45 W 
                 5.3 
                 W 
                 2.8 
                 ms 
                 30 
                 Hz 
               
               
                 (E11-S) 
               
               
                 R3P-d (R1.4) 
                 0.45 W 
                 1.6-7.9 
                 W 
                 1.16-8.16 
                 ms 
                 10-40 
                 Hz 
               
               
                 R1P-f (R5.5) 
                 0.45 W 
                 3.75 
                 W 
                 0.31-19.53 
                 ms 
                 30 
                 Hz 
               
               
                 R1P-g (R5.6) 
                 0.90 W 
                 3.75 
                 W 
                 0.62-39.07 
                 ms 
                 30 
                 Hz 
               
               
                 R2P-a (R6.0) 
                 0.45 W 
                 1.6-7.9 
                 W 
                 0.54-9.8 
                 ms 
                 30 
                 Hz 
               
               
                 R2P-b (R6.1) 
                 0.90 W 
                 1.6-7.9 
                 W 
                 1.09-19.6 
                 ms 
                 30 
                 Hz 
               
               
                   
               
            
           
         
       
     
     The randomization protocols were compared to the fixed parameter Neurowave E11 protocol as described in Table 1. Lysis enhancement factor (LEF %) was calculated using the following formula: 
               L   ⁢           ⁢   E   ⁢           ⁢   F   ⁢           ⁢   %     =       (       (         W   c     -     W     lus   i             W   c     -     W   l         )     -   1     )     ×   100           
The variables in the above equation are:
 
W c  [mg]: Average clot weight of the negative control samples (no treatment).
 
W 1  [mg]: Average clot weight from positive control group (drug treatment only).
 
W lus  [mg]: Average clot weight from each individual ultrasound treatment group.
 
       FIG. 13  shows the LEF % for the protocols tested. The results indicate that varying peak power and pulse width simultaneously in the randomization protocol give significantly better lysis enhancement in the test environment than varying either parameter alone or when they are varied together with pulse repetition frequency. In addition, higher peak powers generally yielded improved lysis response. It should be appreciated that the Lysis enhancement factor is only one measure of the efficacy of the treatment and that the methods and technique described above may have additional and/or different efficacy benefits in situ. 
     In one embodiment, one way of implementing a randomization protocol is to generate and execute a plurality of ultrasonic cycle profiles, where each ultrasonic cycle profile can have randomly generated power parameter values. As previously mentioned, power parameters include, but are not limited to, peak power, pulse width, pulse repetition frequency and pulse repetition interval. Generally, for each power parameter, a random number generator, for example, can be used to select a value within a bounded range determined by the operator. Examples of suitable ranges are described above. For example, one ultrasonic cycle profile can have a randomly selected peak power value, while the other power parameters are non-randomly selected. Another ultrasonic cycle profile may have a plurality of randomly selected power parameters values, such as peak power and pulse width. This process can be used to generate the desired number of ultrasonic cycle profiles. 
     Each ultrasonic cycle profile can be run for a profile execution time. For example, if the profile execution time is approximately 5 seconds, each ultrasonic cycle profile will be run for approximately 5 seconds before the next ultrasonic cycle profile is run. In some embodiments, the profile execution time is less than about 5 seconds. For example, in some embodiments the profile execution time is between about one second and about 30 seconds. In some embodiments, the profile execution time is less than about one second. In some embodiments, the profile execution time is increased so that accurate measurements can be taken of the executed power parameters. In some embodiments, the profile execution time itself can be selected randomly from a predetermined range. 
     In some embodiments, it is desirable to deliver a particular time averaged power. Because the power parameters may be randomized, it may take the execution of a plurality of ultrasonic cycle profiles before the time averaged power approaches an asymptotic value. In some embodiments, the execution of about 40 to about 50 ultrasonic cycle profiles is required for the time averaged power to become asymptotic. In other embodiments, less than about 40 ultrasonic cycle profiles are required, while in yet other embodiments, more than about 50 ultrasonic cycle profiles are required. In some embodiments, ultrasonic cycle profiles are executed until the time average power approaches an asymptotic value. For example, if the profile execution time is 5 seconds and the overall execution time is 30 minutes, 360 ultrasonic cycle profiles will be executed, which in some embodiments is sufficient for the time average power to approach an asymptotic value. 
     Many of the above-described parameters relate to the electrical input parameters of the ultrasonic elements of the catheter. Varying these electrical parameters results in varying the acoustic output of the catheter. Accordingly, the desired affect of non-linearly or randomly varying the acoustic parameters can also be described directly. 
     For example, acoustic parameters of the ultrasound catheter that can be useful to control, by varying the parameter non-linearly or randomly or by holding the parameter constant, include, for example, peak rarefactional pressure, p r . In a sound wave, a positive acoustic pressure corresponds to compression, and a negative acoustic pressure corresponds to rarefaction. Therefore, the peak value of the rarefactional acoustic pressure can be important for safety reasons because it is one of the factors responsible for inertial cavitation. By controlling the magnitude of the peak rarefactional pressure, inertial cavitation can be induced, stopped, prevented or reduced. Peak rarefactional pressure can range from about 0.1 MPa to about 2.5 MPa, or from about 0.9 MPa to about 2.1 MPa, or about 1.6 MPa. The peak rarefactional pressure generated by an ultrasound catheter can be measured in an acoustic tank using a hydrophone. 
     Another parameter is spatial peak pulse-average intensity, I SPPA , which is defined as the value of the pulse-average intensity at the point in the acoustic field where the pulse-average intensity is a maximum or is a local maximum within a specified region. Spatial peak pulse-average intensity can range from about 1 W/cm 2  to about 200 W/cm 2 , or about 20 W/cm 2  to about 140 W/cm 2 , or about 86 W/cm 2 . For an ultrasound pulse that is a sinusoidal waveform having constant acoustic pressure amplitude, the spatial-peak pulse-average intensity can be calculated from the peak-rarefactional acoustic pressure as: 
     
       
         
           
             
               I 
               SPPA 
             
             = 
             
               
                 
                   p 
                   r 
                   2 
                 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ρ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   c 
                 
               
               × 
               
                 10 
                 
                   - 
                   4 
                 
               
             
           
         
       
     
     where: 
     p r  is the peak rarefactional acoustic pressure (Pa) 
     ρ is the density of the medium (kg/m 3 ) 
     c is the speed of sound in the medium (m/s) 
     Symbol: I SPPA    
     Unit: Watt per square-centimeter, W/cm 2    
     NOTE: The 10 −4  multiplication factor converts units of I SPPA  to W/cm 2 . If this factor is left out, the units of I SPPA  are W/m 2 . 
     Another parameter is spatial peak time-average intensity, I SPTA , which is defined as the value of the temporal-average intensity at the point in the acoustic field where the pulse-average intensity is a maximum or is a local maximum within a specified region. Spatial peak time-average intensity can range from about 0.1 W/cm 2  to about 50 W/cm 2 , or about 0.5 W/cm 2  to about 40 W/cm 2 , or about 7 W/cm 2 . The spatial-peak temporal-average intensity can be calculated from the spatial-peak pulse-average intensity as:
 
 I   SPTA   =I   SPPA ×DC÷100
 
     where: 
     I SPPA  is the spatial-peak pulse-average intensity (W/cm 2 ) 
     DC is the duty cycle (%) 
     Symbol: I SPTA    
     Unit: Watt per square-centimeter, W/cm 2    
     In addition to the acoustic and electrical parameters described above, it can also be desirable to focus on non-linearly or randomly varying physiological parameters. For example, the mechanical index, MI is a relative indicator of the potential for mechanical bioeffects, particularly cavitation. Scientific evidence suggests that mechanical bioeffects, like cavitation, are a threshold phenomenon, occurring only when a certain level of output is exceeded. The potential for mechanical effects increases as peak rarefactional pressure increases, but decreases as ultrasound frequency increases. The mechanical index accounts for both rarefactional pressure and frequency. The higher the index reading, the larger the potential for mechanical bioeffects. In addition, the occurrence of cavitation is also highly dependent on properties of the medium such as viscosity, temperature, and dissolved gas content. The mechanical index can range from about 0.1 to about 3, or about 0.5 to about 2, or about 0.7 to about 1.6, or about 1.3. Mechanical index can be calculated by dividing the peak rarefactional pressure (in MPa) by the square root of the frequency (in MHz): 
     
       
         
           
             
               M 
               ⁢ 
               
                   
               
               ⁢ 
               I 
             
             = 
             
               
                 p 
                 r 
               
               
                 f 
                 
                   1 
                   / 
                   2 
                 
               
             
           
         
       
     
     where: 
     p r  is the peak rarefactional pressure (MPa) 
     f is the frequency (MHz) 
     Symbol: MI 
     Unit: None 
     Another parameter, which can be considered a physiological parameter, is the soft tissue thermal index, TIS, which is a quantity related to calculated or estimated maximum temperature rise in an ultrasound field under certain defined assumptions. The thermal index is the ratio of total acoustic power to the acoustic power required to raise tissue temperature by 1° C. under defined assumptions. The thermal index is a relative indicator of temperature increase. It is based on a model for which 1 W of ultrasound energy raises the temperature 1° C. However, in general, a TIS value of 1 should not be taken literally to mean an actual increase in temperature of 1° C. The actual increase in temperature in the patient is influenced by a number of factors such as tissue type, blood perfusion, and exposure time. The soft tissue thermal index can range from about 0.1 to about 25, or from about 0.2 to about 13, or about 3. 
     The formula for calculating the soft tissue thermal index varies slightly depending on the whether the beam area (the area on a specified surface, normal to the direction of ultrasound propagation, in which the acoustic intensity is greater than some specified fraction of the maximum value in that surface at the transducer face) is less than or greater than 1 cm 2 . The interaction between acoustic beam dimensions and the cooling effect of perfusion determines the position of maximum temperature increase. A perfusion rate characterized by a perfusion length of 1 cm is assumed. This translates to a situation where for beam area less than 1 cm 2 , output power is the relevant parameter governing temperature increase, and for beam area greater than 1 cm 2 , acoustic intensity is the relevant parameter governing temperature increase. For a beam area at the transducer output face less than 1 cm 2 , the soft tissue thermal index is calculated as: 
     
       
         
           
             TIS 
             = 
             
               
                 
                   W 
                   TA 
                 
                 × 
                 f 
               
               210 
             
           
         
       
     
     where: 
     W TA  is the temporal-average acoustic power (mW) 
     f is the frequency (MHz) 
     Symbol: TIS 
     Unit: None 
     As with the electrical parameters noted above, the above-described acoustic and physiological parameters (either alone or in combinations) can be non-linearly varied within the ranges described above. Examples of nonlinear variances include, but are not limited to, multi variable variations, variations as a function of a complex equation, sinusoidal variations, exponential variations, random variations, pseudo random variations and/or arbitrary variations. While nonlinear variance is preferred, in other arrangements it is anticipate that one or more of the parameters discussed can be varied in a linear manner either alone or combination with the nonlinear variance. 
     In addition, although many embodiments have been described in the context of an intravascular catheter it should be appreciated that the non-linear application of one or more power parameters can also be applied to non-intravascular catheters or devices and/or non catheter applications. For example, the non-linear varying of one or more power parameters may also find utility in applications in which the ultrasound is applied through an external (with respect to the body or with respect to the vascular system). In particular, the discussion above can be applied to external ultrasound application in which the ultrasound source is external to the patient and/or treatment site. It is also anticipated that the methods and techniques described herein can be applied to non-vascular applications. In addition, in some embodiments, the therapeutic affects of the ultrasound can be utilized alone without a therapeutic compound. 
     Peak Power and Physiological Parameter Variation 
       FIG. 14  illustrates an embodiment in which peak power is varied in specific manner. Specifically, in this embodiment, peak power is repeatedly increased and decreased over time. As shown, the peak power can be increased and then decreased (e.g., ramped up to a peak value than ramped down to a minimum value) in a linear manner. However, in modified embodiments, the peak power can be increased to a specific value and then decreased to a specific value in a non-linear or pseudo-linear manner (e.g., a sinusoidal, curved, or non-linear or complex profile). In the illustrated embodiment, the peak power is ramped up and down by moving through discrete peak power values labeled A-D. However, more or less values can be used or a substantially continuous ramping can also be used. In some embodiments, the maximum and minimum peak values between which the peak power is ramped, can be changed and/or varied over time. In some embodiments, the each of the maximum and the minimum values remains constant. 
     In one embodiment, the peak power varies from about 0.1 Watts to about 30 Watts and in another embodiment from about 1.5 Watts to about 8 Watts. Within these ranges, in one embodiment, the peak power can have between about 1 to 5 discrete values and in another embodiment 2 specific values. 
     In some embodiments, while the peak power is ramped up and down, the other power parameters can remain constant, substantially constant and/or varied (e.g., as described above). For example, Table 2 shows the power parameters for one embodiment in which peak powers ramped between about 1.5 and about 7.88 W. During this ramping, pulse width (PW) and pulse repetition frequency (PRF) are varied. In this embodiment, pulse width and pulse repetition frequency were varied in a manner to maintain pulse repetition (PRF) is 20-40 Hz, in other embodiments the pulse repetition can be maintained within 15-45 Hz. These pulse repetition frequency ranges are presently preferred by Applicant. In one embodiment, the pulse length (pulse width, PW) can be adjusted to each selected pulse repetition frequency to ensure that temporal average power over treatment time and resulting thermal index (heat generation) remains within a clinically acceptable range. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Example Power Protocol 
               
            
           
           
               
               
               
            
               
                 Peak Power 
                 PW 
                 PRF 
               
               
                 (W) 
                 (mSec) 
                 (Hz) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 5 
                 6.86 
                 26 
               
               
                 2.5 
                 5 
                 24 
               
               
                 1.5 
                 8.06 
                 21 
               
               
                 2.5 
                 5 
                 27 
               
               
                 5 
                 6.86 
                 21 
               
               
                 7.88 
                 4 
                 24 
               
               
                 5 
                 6.86 
                 26 
               
               
                 2.5 
                 5 
                 24 
               
               
                 1.5 
                 8.06 
                 21 
               
               
                 2.5 
                 5 
                 27 
               
               
                 5 
                 6.86 
                 21 
               
               
                 7.88 
                 4 
                 24 
               
               
                   
               
            
           
         
       
     
     Similarly, the physiological parameter described above can also be varied in the same manner as varying the peak power. In some embodiments, the physiological parameter can be ramped up- and down-wards between a minimum value and a maximum value. The maximum and the minimum values may not be the same as the maximum and the minimum values for the peak power ramping. In some embodiments, the ramping of the physiological parameter may also be done in a linear manner, and in other embodiments, the ramping can be done in a non-linear or pseudo-linear manner (e.g., a sinusoidal, curved, or non-linear or complex profile). As with the peak power, the physiological parameter can be ramped up and down by moving through discrete physiological parameter values. In some embodiments, the maximum and minimum values between which the physiological parameter is ramped can be changed and/or varied over time. In some embodiments, each of the maximum and the minimum values remains constant. 
     In some embodiments, both the peak power and at least one physiological parameter can be varied in any of the manner described above at the same time. For example, both the peak power and the physiological parameter may be ramped up and down in a linear manner or in a non-linear or pseudo-linear manner at the same time. However, in some embodiments, the peak power may be ramped in a non-linear manner while the physiological parameter is ramped in a linear manner. In other embodiments, the peak power may be ramped in a linear manner while the physiological parameter is ramped in a non-linear manner. 
     Varying peak power and/or physiological parameter as described above has particular advantages. For example, Applicants believe that ramping peak power and/or physiological parameter creates acoustic “momentum” that advantageously results in radiation force transfer to media such as effectively accelerating acoustic streaming, which can enhance the therapeutic effects (described above) of the ultrasound. 
     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 occlusions.