WO2001070112A1 - High output therapeutic ultrasound transducer - Google Patents

Abstract

A therapeutic ultrasound delivery system, including a catheter body (17); a plurality of axially spaced-apart cylindrical vibrational transducers (14) disposed along a length of the catheter body; a first spring connector (22) wrapped around the outer surfaces of the surfaces of the vibrational transducers; and a second connector (24) disposed in contact with the inner surfaces of the vibrational transducers.

Description

HIGH OUTPUT THERAPEUTIC ULTRASOUND TRANSDUCER

CROSS-REFERENCES TO RELATED APPLICATIONS The present application is a Continuation-in-Part of U.S. Patent Application Ser No. 9/531,037, filed March 20, 2000, the full disclosure which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD The present mvention is related to medical devices and systems, particularly therapeutic ultrasound systems.

BACKGROUND OF THE INVENTION Percutaneously introduced catheters having ultrasound transducers thereon can be used to deliver localized doses of therapeutic ultrasound energy to various sites within a body. Such systems are ideally suited for treating or preventing pathological conditions such as arterial restenosis due to intimal hyperplasia.

To achieve a high level of therapeutic effectiveness, a high amplitude of ultrasound vibration is required. Unfortunately, the acoustic output from a conventional transducer design is typically limited by the inherent properties ofthe piezoelectric material which forms the transducer. Specifically, when operating typical piezoelectric ceramic transducers at high vibrational amplitudes, the ceramic tends to fracture. This transducer failure is caused by the high tensile stresses within the ceramic material during transducer operation, and the problem is exacerbated by the fact that although piezoelectric ceramic materials tend to have high compressive strengths, they have relatively low tensile strengths. A further problem common to existing catheter-based ultrasound systems is that they lack the necessary flexibility to negotiate tortuous paths through body lumens. This is especially true when such systems comprise a plurality of axially spaced apart ultrasound transducers disposed along the length ofthe catheter body. In such cases, the catheter flexibility is unfortunately influenced both by the number and size ofthe conductors that are used to interconnect the various transducers. A further problem common to existing catheter-based ultrasound systems which use a plurality of ultrasound transducers is the difficulty in individually wiring each of these transducers, since a large number of individual wires leading to each ofthe transducers typically results in a rather bulky system.

BACKGROUND OF THE INVENTION The present invention provides ultrasound and other vibrational transducer systems comprising a vibrational transducer, typically an ultrasound transducer, which can be operated at very high vibrational amplitudes without failure. As such, the present invention provides systems to prevent the ultrasound transducer, which preferably comprises a ceramic piezoelectric material, from breaking apart at high amplitude operation. The present ultrasound transducer system is ideally suited for use in a catheter based therapeutic ultrasound energy delivery system.

In a preferred aspect, the present invention comprises a piezoelectric ceramic ultrasound transducer having a restraint received therearound. The restraint is dimensioned or otherwise formed to have a structure which exerts a compressive pre-stress on the piezoelectric ceramic transducer element where the stress can be maintained during the operation ofthe transducer. Advantageously, the compressive pre-stress provided by the restraint operates to prevent tensile failure ofthe ceramic transducer at high acoustic output. In a preferred aspect, the strength ofthe compressive pre-stress provided by the restraint on the transducer is approximately equal to the tensile strength ofthe transducer element. As will be explained, when this occurs, the restrained transducer can provide approximately twice the acoustic output of a comparable un-restrained device before tensile failure occurs.

In one exemplary aspect, the strength ofthe compressive pre-stress provided by the restraint is approximately half-way between the tensile strength and the compressive strength ofthe ceramic transducer material. (Stated another way, the strength ofthe compressive pre-stress provided by the restraint is approximately equal to the average ofthe tensile strength and the compressive strength ofthe ceramic transducer material).

As will be explained, when this occurs, the restrained transducer can be operated at a significantly increased output amplitude without failure. In various preferred aspects, the compressive pre-stress provided by the restraint is just high enough to permit operation ofthe device without tensile failure at an output amplitude determined to be safe and effective for treating or preventing a pathological condition such as arterial restenosis due to intimal hyperplasia. In these preferred aspects, the required thickness and stiffness (as described below) ofthe restraint may be preferably kept to the minimum necessary to meet the acoustic output requirements, thereby minimizing the size ofthe device, and minimizing the requirements ofthe electrical drive circuitry, while maximizing the efficiency ofthe device in converting electric power into acoustic power.

In preferred aspects, the restraint may comprise a tensioned wire or filament(s) which is/are wrapped around the transducer. In other aspects, the restraint may comprise a jacket having an inner diameter which is initially fabricated to be slightly smaller than the outer diameter ofthe transducer. The jacket is then stretched to expand to a larger diameter such that it can just be received over the transducer. The transducer is then inserted within the expanded jacket, and the jacket is then allowed to contract such that it exerts a compressive pre-stress on the transducer. Systems for fabricating the jacket from a shape memory metal such as a nickel Titanium alloy (e.g.: Nitinol™) are also set forth.

The transducer is preferably cylindrically shaped, and may have an optional central longitudinal bore passing therethrough, with the bore defining an inner surface ofthe transducer. In various aspects, the inner and outer surfaces ofthe transducer are covered in whole or in part by an electrode. In alternative aspects, the opposite longitudinal ends ofthe transducer are covered in whole or in part by an electrode. In alternate embodiments ofthe invention, the transducer is formed from a series of alternating annular shaped polymer and piezoelectric ceramic rings, commonly referred to as a piezoelectric stack.

In a preferred aspect ofthe invention, the vibrational mode ofthe transducer is a relatively low frequency "breathing mode", wherein the circumference ofthe cylinder oscillates around a nominal value, and the stress within the ceramic is predominantly in the tangential direction. In this case, tensile stress from the vibration ofthe transducer which may otherwise lead to failure can be balanced by compressive pre-stress in the tangential direction applied by a wrapped jacket type restraint. In an exemplary aspect, the transducer may be made of a PZT-8, (or PZT-4) ceramic material, but other piezoelectric ceramics, electro-strictive ceramic materials, or non- ceramic materials such as piezoelectric crystals may be used as well.

In the aspect ofthe invention in which a wrapped restraint is used, the tensioned member wrapped around the transducer may be a metal wire, metal or polymeric braid, mono-filament polymer, glass fiber, or a bundle of polymer, glass or carbon fibers. Wires may have circular cross sections or be formed as a ribbon or square wire. In various aspects, the wire is placed under tension when initially wrapped around the ultrasound transducer so as to maintain the compressive pre-stress on the transducer. Alternatively, the tension may be introduced after the wrapping is applied using thermal, chemical, mechanical or other type of process.

Suitable materials which may be used for either ofthe wrapped or jacket-type restraints described herein include, but are not limited to, high tensile strength elastic material selected from the group consisting of steel, titanium alloys, beryllium copper alloys, nickel, titanium and other shape memory allows (e.g.: Nitinol™), and epoxy impregnated kevlar, glass, polyester or carbon fiber. In one exemplary embodiment ofthe invention, the restraint comprises a 0.001" x 0.003" Beryllium Copper alloy ribbon wire having a tensile strength of 150,000 psi or greater, wrapped around the transducer under 0.25 lbs of tension. In aspects ofthe invention where the restraint comprises a wire or ribbon wire, the restraint may comprise multiple layers of wire or ribbon wrappings using thinner ribbon or smaller wire than would be used for a single layer of wrapped restraint. An advantage of using such smaller diameter wire or thinner ribbon wire would be that reduced bending stress would be experienced during the wrapping process, thereby permitting the wire or ribbon to be tensioned to a higher average stress without breaking. This in turn would allow a higher compressive pre-stress to be applied to the ceramic transducer element using a thinner and less stiff restraint than would instead be required for a single layer wrap ofthe same material.

In those aspects ofthe invention where the restraint comprises a wire, ribbon wire, or other fiber under tension, the wire restraint may be fixed in place on the surface of the transducer by gluing, soldering or welding, with the compressive pre-stress being maintained during the operation ofthe transducer. Such fixation could be continuous or only at spaced apart points or regions along the contact length between the restraint and the transducer.

The use of a beryllium copper alloy wire as the restraint has numerous advantages including its high tensile strength, (typically 150 kpsi or greater), corrosion resistance and conductive properties. A further advantage is that a beryllium copper alloy wire is easily solderable. As such, it may be soldered both to an outer surface ofthe transducer, and between adjacent wraps around the transducer without the need for a special solder tab. In addition, a beryllium copper alloy wire can easily be soldered at temperatures below the Curie temperature ofthe ceramic transducer material, (which is about 300° C for PZT-8 ceramic). Typically as well, a beryllium copper alloy wire has a tensile strength / modulus of elasticity on the order of 190 kpsi/19Mpsi = 1/100. This advantageous ration is similar to that of stainless steel which typically has a tensile strength /modulus of elasticity on the order of 300 kpsi/30Mpsi = 1/100. In the aspects ofthe invention where the restraint comprises a jacket, such jacket may be made from a very high strain limit material having good elastic properties and high tensile strength. Such a jacket could first be formed and then expanded to be slipped over the transducer and then allowed to recover, thereby radially compressing the transducer. If instead fabricated from Nitinol™, the j acket can be formed and then expanded to be slipped over the transducer. If maintained at a sufficiently low temperature, the jacket will maintain its expanded size as it is placed over the transducer. When the temperature is allowed to rise above a critical value the jacket material will contract, thereby applying compressive pre-stress to the transducer. In preferred aspects, a composite polymer is applied over the outside of the restraint. The composite polymer is adapted to dampen longitudinal axis vibrations, to provide an electrical insulating layer and to provide a convenient surface to which an outer jacket ofthe catheter may be attached. Suitable materials for such a composite polymer include, but are not limited to, materials selected from the group consisting of high strength adhesives such as epoxy or cyano-acrylate, and polymers such as heat-shrinkable PVDF, polyester, nylon, Pebax, PVDF or polyethylene.

The present invention also provides methods of generating and delivering high levels of therapeutic ultrasound energy to a patient. In particular, the present invention provides methods of delivering a high output from a therapeutic ultrasound energy delivery system by exerting a compressive pre-stress on a piezoelectric ceramic ultrasound transducer with a restraint wrapped or formed to be disposed around the transducer; and by maintaining the compressive pre-stress on the transducer during the operation ofthe transducer. In various aspects, the exertion of a compressive pre-stress on the ultrasound transducer is achieved by wrapping a tensioned wire or fiber(s) around the transducer. In other aspects, exerting a compressive pre-stress on the ultrasound transducer is achieved by expanding a jacket to a diameter sufficient to be received over the transducer, inserting the transducer into the jacket and allowing the jacket to contract against the outer surface ofthe transducer, or by fabricating the restraint from a shape memory material such as Nitinol™ expanded to fit over the transducer and then shrunk with heat to apply a compressive pre-stress to the transducer. In preferred aspects, the ultrasound transducer is cylindrical in shape and may further comprise a longitudinally extending bore therethrough. When air is disposed within this bore, the ultrasound energy emitted by the transducer will be directed predominately radially outwards, since very little ultrasound energy passes from the dense ceramic transducer into the low density air. Thus, the efficiency ofthe transducer can be enhanced, providing an ideal transducer system for mounting on a catheter.

In various preferred aspects, a plurality of vibrational transducers are provided in the present catheter system. Preferably, such transducers are axially spaced apart along a length ofthe catheter body, hi this plural transducer system aspect ofthe invention, the transducers preferably comprise hollow cylinders (i.e.: a cylinder having a longitudinally extending bore passing therethrough in an axial direction, as described above). These transducers preferably have inner and outer surfaces which are metallic and at which an electric voltage is applied, thereby driving transducer operation. hi accordance with the present invention, the restraint which may be wrapped or otherwise disposed around these transducers may comprise a continuous element extending over a plurality of successive transducers. Preferably, such a restraint extends over two, or more preferably three, or most preferably all ofthe axially spaced apart transducers in the probe or catheter. In preferred aspects, such a restraint may comprise a flexible member which may comprise one or more wires or fibers having a spring or helix shaped or serpentine or zig-zag shaped structure.

In one preferred aspect, the restraint comprises a "spring connector" which is wrapped around (and extends over) a plurality of successive transducers, and exerts an inward compressive force on successive transducers.

As stated above, the preferred restraint may be wrapped around the outer surfaces ofthe successive axially spaced-apart transducers. Such a restraint exerts an inward pre-stress on the outer surfaces ofthe vibrational transducers such that transducer output can be increased, while simultaneously decreasing the likelihood of transducer failure. It is to be understood that reference herein to an outer "spring connector" is not limited, but is instead defined to include any form of flexible restraint which exerts an inward pre-loading on a plurality of axially spaced apart transducers.

In preferred aspects, the inward pre-stress exerted by the restraint received over the outer surfaces ofthe successive transducers is about 25% to 75% ofthe breaking (i.e. tensile) strength ofthe transducers.

The inward pre-stress exerted by the restraint (which may be wrapped or otherwise disposed around the outer surface ofthe transducers) may also be: (a) at least equal to the tensile strength ofthe transducers, (b) greater than the tensile strength ofthe transducers, and less than the average ofthe compressive and tensile strengths ofthe transducers (ie: V way between the compressive and tensile strengths ofthe transducers), or (c) approximately equal to the average ofthe compressive and tensile strengths ofthe transducers (ie: V_> way between the compressive and tensile strengths ofthe transducers).

It is to be understood that these ranges for the inward pre-stress exerted by the restraint wrapped or disposed around successive transducers will be most preferred when an inner connector, and may comprise a spring structure (which is received within the hollow bores ofthe successive transducers) exerts little or no appreciable outward pre-loading on the inner surfaces ofthe transducers. In preferred aspects, such an inner connector may comprise one or more wires or fibers having a spring or helix shaped or serpentine or zig-zag structure. In a most preferred aspect, the inner connector comprises a spring.

Should the inner connector instead exert an outward pre-loading, the range of inward pre-loading exerted by the restraint can be increased accordingly to compensate.

Optionally, the restraint (which is wrapped around the outer surfaces ofthe transducers) can be attached to the outer surfaces ofthe transducers by a variety of techniques. These include, but are not limited to, gluing, soldering and welding.

Alternatively, (or in addition to the foregoing attachment techniques) the restraint can be held in a fixed relation to the outer surfaces ofthe transducers by its natural tendency to contract or "re-coil" around the transducers. Specifically, the restraint may comprise a spring (or other shaped) connector which can be unwound such that it increases in diameter to the degree that it can be slipped over the transducers (while in its expanded state). Thereafter, the spring connector can be simply left to naturally "re-coil", such that it contracts around the outer surfaces ofthe transducers, and thereby exerts an inward pre-loading on the transducers. In this aspect, the natural (unexpanded) diameter ofthe spring connector is slightly smaller than the outer diameter ofthe transducers. The use of the present restraint, which may comprise a spring connector disposed around the outer surface ofthe transducers offers many specific advantages, including, but not limited to, the following.

First, the natural tendency ofthe spring to contract operates to exert a desired inward pre-loading force on the transducers, thereby offering the advantages of increased output with reduced likelihood of transducer failure, as explained in reference to the various "restraints" described herein.

Secondly, a single spring connecting several transducers is very easy to install when the catheter system is first assembled. This is due to the fact that the wire spring simply be rotated at one end (while being held at its other end) to unwind it to a diameter sufficient that it can be slipped over the various transducers.

Thirdly, being a single continuous element which wraps around the outer surfaces of successive transducers, the present spring connector provides excellent ease and 5 simplicity in system wiring as it can operate as a single electrical contact wire between the outer surfaces ofthe various transducers.

Fourthly, being a spring which preferably wraps rather firmly around the outer surfaces ofthe spaced-apart transducers, the present spring connector advantageously also holds the transducers apart at preferred axial separation distances, which remain constant

10 over time. hi various preferred aspect ofthe invention, an inner connecting wire is disposed in contact with the inner surfaces of successive transducers. In a most preferred aspect, the inner connecting wire is a spring which is positioned in contact with the inner surfaces ofthe transducers. It is to be understood, however, that in accordance with the

15. present invention, the inner connecting wire need not be in the form of a spring. For example, a simple wire (or wires) can be used to maintain electrical contact between the inner surfaces of successive transducers. However, in the preferred case where the inner connecting wire does comprise a spring, such a spring offers numerous advantages, including, but not limited to, the following. 0 First, a spring electrically connecting the inner surfaces of successive transducers to one another is very easy to install when the catheter system is first assembled. For example, such a wire spring may simply be rotated at one end (while being held at another) to tighten it to a diameter sufficiently small that it can be slipped within the hollow inner bore of successive transducers. After it has been so positioned, it is only necessary to 5 release the wire such that it springs back (i.e.: expands) into a larger diameter state, (thereby gently pushing up against the inner surfaces ofthe transducers).

Secondly, being a single continuous element, such a spring connector provides excellent ease and simplicity in system wiring as can be operated as a single electrical contact wire connecting together the inner surfaces ofthe various transducers.

30

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a cylindrical shaped ultrasound transducer having a wire restraint wrapped therearound.

Fig. 2 is a sectional view taken along lines 2-2 in Fig. 1. Fig. 3 is a perspective view of a cylindrical shaped ultrasound transducer having a restraining jacket received thereover.

Fig. 4 is a sectional view taken along lines 4-4 in Fig. 3.

Fig. 5 is a perspective view of a transducer and restraint received within an outer coating.

Fig. 6 is an illustration of a system for wrapping a tensioned wire around an ultrasound transducer.

Fig. 7A is a sectional view corresponding to Fig. 5, showing electrodes attached to inner and outer surfaces ofthe transducer, with the restraining jacket as shown in Figs. 3 and 4.

Fig. 7B corresponds to Fig 7A, but instead shows an electrode connected to the outer surface ofthe transducer by way of a solder tab.

Fig. 7C corresponds to Fig 5, but instead shows an electrode soldered directly to the restraining wire, as illustrated in Figs. 1 and 2. Fig. 8 illustrates a tool for expanding a jacket such that it can be received over the transducer.

Fig. 9 shows an alternate ultrasound transducer comprising alternating annular piezoelectric and polymer sections.

Fig. 10 shows a stress vs. time plot for an unrestrained transducer. Fig. 11 shows a stress vs. time plot for a restrained transducer, operating at less than optimal output.

Fig. 12 shows a stress vs. time plot for a restrained transducer, operating at optimal output.

Fig. 13 shows a plurality ofthe present transducers mounted to a catheter system for delivering therapeutic ultrasound to a patient.

Fig. 14 is an illustration of a two tubular shaped transducers (shown in sectional view), with a coiled spring positioned in contact with their inner surfaces.

Fig. 15 is an illustration of a two tubular transducers, with a coiled spring positioned in contact with their outer surfaces. Fig. 16 is an illustration similar to Fig. 14, but showing more coils per unit distance within each transducer than between successive transducers.

Fig. 17 is an illustration of an ultrasound catheter system according to the present invention, showing the flexibility ofthe present system. DESCRIPTION OF THE SPECIFIC EMBODIMENTS A problem common to therapeutic ultrasound transducers is that when operating an ultrasound transducer such as a piezoelectric ceramic transducer at a very high output, the transducer will tend to fracture. Accordingly, the therapeutic effectiveness of catheter based ultrasound delivery systems have been somewhat limited since the level of vibrational amplitude of therapeutic ultrasound energy which their transducers are able to emit is limited, especially over prolonged periods of operation.

Referring to Figs. 1 and 2, the present invention provides a system for preventing fracture of a ultrasound transducer, (such as a ceramic ultrasound transducer), when the transducer is operated at a high output. In a first aspect, the present invention provides a system for preventing tensile failure in a transducer 10, by way of a wire 14 which is wrapped tightly around transducer 10. As can be seen, transducer 10 is cylindrical shaped, having an optional longitudinally extending central bore 11 extending therethrough. In various preferred embodiments, transducer 10 has a preferred outer diameter of 0.25 to 0.02 inches, a more preferred outer diameter of 0.175 to 0.03 inches, and a most preferred outer diameter of 0.100 to 0.03 inches.

In various preferred embodiments, transducer 10 has a preferred inner diameter of 0.2 to 0.01 inches, a more preferred inner diameter of 0.125 to 0.015 inches, and a most preferred inner diameter of 0.05 to 0.015 inches. In various preferred embodiments, transducer 10 has a preferred length of 1.0 to 0.01 inches, a more preferred length of 0.750 to 0.010 inches, and a most preferred length of 0.5 to 0.01 inches.

. It is to be understood, however, that the preferred dimensions set forth herein are merely exemplary and that the present invention is not so limited to the dimensions set forth herein.

In preferred aspects, the present system provides a "high output" of therapeutic ultrasound energy, being defined herein as being greater than that used for diagnostic imaging. In a most preferred aspect ofthe present invention, such "high output" is equal to or greater than 1.9 MI (mechanical index). In preferred aspects, the "high output" is achieved with an MI less than that at which cavitation damage occurs.

In preferred aspects, the present "high output" therapeutic ultrasound system is operated at an exemplary frequency range of equal to, or greater than, 500 KHz, and less than, or equal to, 3 MHz. Preferably, wire 14 is pretensioned when initially wrapped around transducer 10 such that wire 14 exerts a compressive pre-stress on transducer 10. Wire 14 may be made of any suitable material selected from the group with mechanical properties exhibited by steel, titanium alloys, beryllium copper alloys, Nitinol™. Wire 14 may alternatively comprise a ribbon wire, or square wire, or a multi-strand wire. Wire 14 may alternatively comprise a high tensile strength elastic material such as epoxy-impregnated polyester, kevlar, glass or carbon fiber, in either a mono-filament or multi-filament form.

In a preferred aspect, the tensile stress in wire 14 is about 100 kpsi or higher. In one exemplary aspect ofthe invention, the wire is a 0.001" x 0.0003" Beryllium-Copper (BeCu) alloy ribbon wire under 0.3 lbs. tension, and transducer 10 is made of a PZT-8 ceramic having a 0.050" outer diameter, a 0.010" thickness wall, and a 0.315" length. In this exemplary aspect, the compressive pre-stress applied to the ceramic by the wrapped ribbon restraint is approximately 10 kpsi, which is comparable to the reported static tensile strength of PZT-8 ceramic at 11 kpsi, and significantly greater than the reported dynamic tensile strength of 5 kpsi.

Wire 14 is adapted to provide a compressive pre-stress on transducer 10, wherein the pre-stress is preferably maintained during the operation of transducer 10 by the resilience ofthe restraining wire.

In a preferred aspect, the compressive pre-stress exerted by wire 14 on transducer 10 is approximately equal to, or greater than, the tensile strength ofthe transducer. As will be explained, when the compressive pre-stress exerted on transducer 10 is approximately equal to the tensile strength of transducer 10, a doubling of output amplitude of transducer 10 is provided. In this preferred aspect ofthe invention, the stiffness of wire restraint 14 (or jacket 12) needed to provide this compressive pre-stress is only about 1/7 the stiffness ofthe transducer 10, therefore it does not appreciably restrain the motion of transducer 10, as follows.

The relationship between the stiffness of restraint 12 or 14 and the transducer 10 is established by considering that the modulus of elasticity "Y" of restraint 12 or 14 multiplied by the cross-sectional area of restraint 12 or 14, divided by the modulus of elasticity "Y" of transducer 10 multiplied by the cross-sectional area of transducer 10.

For example, using the BeCu ribbon at 19 Mpsi as wire 14, and PZT-8 ceramic as transducer 10, the modulus of elasticity "Y" ofthe BeCu ribbon is approximately 1.4 times the modulus of elasticity ofthe PZT-8 ceramic at 13 Mpsi, when the cross-sectional area of the BeCu ribbon is only about 1/10 that ofthe ceramic (1 ml ribbon thickness vs. 10 ml. transducer wall thickness). The relative stiffness ofthe restraint versus the transducer is then: stiffness _{restraint =} 7_restraint « A_{rest =} 19 * 1 ^ 1 stiffness _tiΑmduceτ ϊ _b__msducer • A_tansducer 13 * 10 /

In one exemplary aspect ofthe invention, the compressive pre-stress exerted by wire 14 on transducer 10 is approximately half-way between the compressive and tensile strengths of transducer 10, (e: at the average ofthe compressive and tensile strengths of transducer) thereby providing the highest possible output without failure, (as will be explained). To ensure that wire 14 provides a compressive pre-stress on transducer 10, it is also important to ensure that wire 14 does not simply unwrap, thereby losing its contact from the outer surface 13 of transducer 10. Accordingly, wire 14 is preferably glued or soldered against outer surface 13 of transducer 10. Alternatively, adjacent wraps of wire 14 may be soldered, welded, or glued together with wire 14 being secured to the outer surface 13 of transducer 10 by friction.

In one embodiment, wire 14 is welded, soldered, or glued to transducer 10 or to adjacent wraps of wire 14 only at opposite transducer ends 15 and 17. An advantage of welding wire 14 only at ends 15 and 17 is that this avoids relieving the stress in wire 14 due to heating or melting. As such, a circumferential weld near each of ends 15 and 17 may be used to distribute the stress on the weld, with only a few turns of wire 14 near ends 15 and 17 being under reduced stress, with the (unheated) center turns of wire 14 exerting the compressive pre-stress on transducer 10. Alternatively, in another embodiment, wire 14 is welded or adhesively attached along the entire length of transducer 10 between ends 13 and 15. Wire 14 may optionally be a ribbon wire, which has the advantage of distributing stress favorably over surface 13 of transducer 10, with the entire width ofthe ribbon in contact with the ceramic transducer 10, instead of just a narrow strip where a round wire would be in tangential contact with the cylindrical transducer surface. Furthermore, since a ribbon wire provides the maximum amount of metal in a minimum profile, a ribbon wire permits the maximum restraint with minimum increase in the overall dimension ofthe restrained transducer. Furthermore, due to its narrow dimension in the radial direction, ribbon wire would experience much lower bending strain during the wrapping process as compared a round wire of comparable cross-sectional area per unit length. Another advantage of ribbon wire is that it is resistant to stress relief during the welding process in which wire 14 is attached to outer surface 13, since the actual weld would only occupy a portion ofthe ribbon width leaving a large remaining portion to sustain tensile stress while the welding takes place. In preferred aspects, wire 14 is selected from a material with an elongation at failure of greater than wire diameter / transducer radius, having the highest possible tensile strength. Alternatively, ribbon wire 14 is selected from a material with elongation at failure of greater than wire thickness/ transducer radius, having the highest possible tensile strength. In either case, the lowest possible modulus is desired so that there is a minimum of restraint exerted on transducer 10. Examples of such materials include Beryllium Copper (BeCu) alloy 172, with various tempers having tensile strengths of 100-240 kpsi and elongation of 1-10% , or various stainless steel alloys, or high strength Titanium alloys.

In a preferred aspect, wire 14 is wrapped over itself such that a multi-layer restraint is provided. An advantage of wrapping smaller diameter wire is that it will exhibit a lower bending stress, as compared to a larger diameter wire wrapped around the transducer. In one preferred aspect, opposite ends 15 and 17 of transducer 10 may be electroded. Alternatively, in another preferred aspect, an inner surface 19 and outer surface 13 may instead be electroded.

In an alternate embodiment ofthe invention, the restraint used to exert a compressive pre-stress on the transducer comprises a jacket received over the transducer. Referring to Figs. 3 and 4, transducer 10 is shown surrounded by a restraint jacket 12 which is slipped thereover and exerts a compressive pre-stress, similar to that exerted by wire 14, as was described above.

Jacket 12 may preferably be formed to maintain a compressive pre-stress on transducer 10 in a number of ways. In a first aspect, jacket 12 is initially formed with an inner diameter slightly less than the outer diameter of transducer 10. Thereafter, jacket 12 is stretched radially by mechanical or thermal means to expand its inner diameter to a dimension such that it can just be slipped over transducer 10, with transducer 10 received therein as shown in Figs. 3 and 4. After jacket 12 has been slipped over transducer 10, jacket 12 will then be released such that it naturally contracts somewhat around outer surface 13 of transducer 10. Consequently, jacket 12 exerts, and maintains, a compressive pre-stress on transducer 10 during its operation.

Jacket 12 may preferably be fabricated from a high tensile strength elastic material, including any ofthe exemplary materials set forth above with respect to wire 14. Alternatively, jacket 12 may be fabricated from a shape memory metal such as Nitinol™. In this aspect ofthe invention, a change in temperature will alter the size of jacket 12 such that it constricts around transducer 10 after having been received thereover. For example, a Nitinol™ alloy can be chosen to be Martensitic at the temperature of liquid nitrogen, and super-elastic in the temperature range from room temperature to body temperature and slightly above. The Nitinol™ alloy would be austenitic at elevated temperatures. Such a material can be fabricated as a thin wall tube with inner diameter slightly less than that ofthe transducer. For example, the ceramic transducer could have an outer diameter of 0.050" with a 0.010" wall thickness and a 0.315" length. The Nitinol™ tube could be fabricated with an inner diameter of 0.048" and a wall thickness of 0.002". When the Nitinol™ is cooled to liquid nitrogen temperature (~ -200°C) the Nitinol™ becomes Martensitic and is relatively easily expanded to an inner diameter of 0.052", allowing it to be slipped over the outside ofthe ceramic transducer. When the Nitinol™ warms up to room temperature, it becomes super-elastic, and it attempts to recover to its original fabricated dimensions. The recovery is limited by the ceramic, but the super-elastic alloy applies a compressive pre-stress to the ceramic, thereby preventing premature tensile failure ofthe ceramic.

When using either jacket 12 or wire 14 as the restraint on transducer 10, such restraint will preferably have a high tensile strength so that only a thin layer ofthe restraint material will be adequate, yet also have to have a low stiffness such that it would not unduly restrain the ceramic transducer 10.

When using either a wire restraint (Figs. 1 and 2) or a jacket restraint (Figs. 3 and 4), the restraint is preferably received within an outer coating 16, as shown in Fig. 5. Outer coating 16 may preferably comprise a composite polymer, which operates to dampen longitudinal vibrations and provide an electrical insulating layer. In an exemplary aspect, outer coating 16 comprises a high strength thin wall polymer such as 0.001" thick polyester or nylon polymer, attached to jacket 12 by a high strength adhesive, preferably having at least 500 psi shear strength.

The present invention also sets forth systems for wrapping wire 14 around transducer 10 such that wire 14 remains in tension. Referring to Fig. 6, two strands of wire 14 are shown being wrapped simultaneously around transducer 10 as transducer 10 is rotated in direction R. In this system, a pair of equal weights Wl and W2 keep wire 14 under tension as wire 14 passes over pulleys PI and P2. Since Wl and W2 are equal, the wires 14 will not produce any net bending stress on the transducer 10 which could cause it to break during the manufacturing process. Alternatively, weight W2, pulleys PI and P2 and one wire 14 may be eliminated to simplify the wrapping fixture. In this case, the transducer 10 must be strong enough to resist the bending stress created by the tensioned wrapping wire 14.

Longitudinally extending bore 11, as seen in Figs. 1 to 5, may preferably be air filled. Advantages of an air-filled bore include the fact that ultrasound energy can not be transmitted thereacross. Instead, all ofthe ultrasound energy emitted by transducer 10 will advantageously be reflected off of inner surface 19, and directed radially outwardly, thereby increasing the therapeutic effectiveness of transducer 10. Another advantage of air-filled bore 11 is that it can be used for passage of a guidewire therethrough. Fig. 7 A shows an embodiment ofthe present invention in which jacket 12 is made of Nitinol™, with an electrical lead 22 passing under outer covering 16 and through a hole 9 passing through jacket 12 such that an electrical lead 22 may be attached to electroded outer surface 13 of transducer 10. Similarly, an electrical lead 24 is attached to the inner surface 19 of transducer 10 as shown. Fig. 7B shows electrical lead 22 connected to electroded outer surface 13 by way of a solder tab 18. Fig. 7C shows electrical lead 22 soldered directly to electrically conductive wire 14, which is in direct contact with electroded outer surface 13 of transducer 10.

In a preferred aspect, wire 14 is soldered at ends 15 and 17 to prevent unwrapping from transducer 10. The outer electrode connection may be made by soldering directly to wire 14. As such, transducer 10 can be wrapped all the way from end-to-end with no unwrapped segment required for lead attachment.

Fig. 8 illustrates a tool for expanding jacket 12 such that it can be received over transducer 10. The tool comprises a split mandrel 20 and a tapered conical wedge 21. Conical wedge 21 is inserted into a bore passing through split mandrel 20 such that jacket 12 can be expanded. In a preferred aspect, jacket 12 is made of Nitinol™, and the insertion of wedge 21 into mandrel 20 is preferably done at a cool temperature such that when Nitinol™ jacket 12 returns to a warmer temperature, it will tend to retract radially inwards. In an exemplary aspect, Nitinol™ jacket 12 will have a thickness of approximately 0.002", offering an improved compromise in terms of strength and low restraint. In preferred aspects, transducer 10 will be operated at a low temperature rise.

Such low temperature rise can be achieved by maintaining a low duty cycle, or alternatively by providing a cooling flow such as a saline infusion over transducer 10 during its operation. Preferably, a temperature rise of less than 5° C will be achieved. Preferably, the fluid could be introduced through an annular space between transducer 10 and a polyimide guidewire sleeve. Temperature monitoring by a catheter mounted thermistor or thermocouple can also be used.

Referring to Fig. 9, an alternate transducer system is provided with transducer 30 comprising alternating annular sections of PZT ceramic 32 and polymer 34. Transducer 30 is ideally suited to avoiding longitudinal failure. In accordance with the present invention, transducer 30 may be substituted for transducer 10 in any ofthe above described embodiments ofthe present invention. For example, transducer 30 is preferably restrained by a wire 14 wrapped therearound, or a jacket 12 slipped thereover, the restraint used in turn being received within outer covering 16, as described. As stated above, the strength ofthe compressive pre-stress provided by wire

14 or jacket 12 on transducer 10 is at least approximately equal to the tensile strength ofthe transducer material and more preferably, approximately equal to the average ofthe tensile and compressive strengths ofthe material, (ie: at a value V% way between the tensile and compressive strengths ofthe material). This is explained as follows. Referring to Fig. 10, a stress vs. time plot for an unrestrained transducer is shown. Acoustic vibrations in the fransducer are characterized by oscillation in the sfress. In a conventional transducer, without a pre-stress, the stress oscillates around zero, alternating between compressive (positive) stress and tensile (negative) sfress.

Since piezo-electric ceramic materials typically have much higher compressive strengths compared to their tensile strengths, compressive pre-stress pennits higher acoustic amplitude without subjecting the ceramic to tensile sfress beyond its limit. Specifically, the tensile sfrength ofthe transducer material is shown by line 50 and the compressive strength ofthe transducer material is shown by line 52. (As can be seen, line 50 is closer to zero than line 52, thus indicating that the transducer is more likely to fail in tension than in compression). If the stress during one ofthe cycles of oscillation exceeds the tensile strength ofthe ceramic, then the transducer will fracture. Accordingly, when operating an unrestrained transducer, the maximum tensile stresses will equal the maximum compressive stresses. Accordingly, the maximum peak-to-peak amplitude ofthe oscillations in the stress (i.e.: the difference between lines 50 and 70) will be double the tensile sfrength (i.e.: the difference between zero and line 50) ofthe transducer material.

Fig. 11 shows a sfress vs. time plot for a transducer with a restraint wrapped therearound. In this aspect ofthe invention, the compressive pre-stress (labeled as distance "B"), (ie: the difference between zero and line 54) is equal to the tensile sfrength (labeled as distance "A"), (i.e.: the difference between zero and line 50) ofthe transducer material. Thus* line 54 is at the same level as line 70. As can be seen, the application of such a compressive pre-stress to the transducer results in a doubling ofthe maximum peak-to-peak amplitude of oscillation in the stress relative to that of a comparable unrestrained transducer, (i.e.: the difference between line 56 and zero is twice the difference between line 54 and zero). Fig. 12 shows a stress vs. time plot for a transducer with a restraint wrapped therearound, operating at optimal output. In this aspect ofthe invention, the compressive pre- stress applied by the restraint (line 58) is set to be positioned at an average (ie: ^lA way between) the tensile strength (line 50) and the compressive strength (line 52) ofthe transducer material. As can be seen, the application of such a compressive pre-stress on the transducer effectively maximizes the peak-to-peak amplitude ofthe oscillation in the stress to a level conesponding to the difference between compressive sfrength (line 52) and the tensile sfrength (line 50).

Accordingly, in prefened aspects ofthe invention, the compressive pre-stress applied to the transducer by the restraint is at least equal to, and preferably greater than, the tensile strength ofthe transducer. More preferably, the compressive pre-stress applied to the transducer by the restraint is of an amplitude greater than the tensile strength ofthe material and not exceeding an average value (ie: a value Vz way between) the tensile and compressive strengths ofthe material. In an optimal aspect ofthe invention, the compressive pre-stress is equal to the average ofthe tensile and compressive strengths ofthe material. In another prefened aspect ofthe invention, the compressive pre-stress applied to the transducer is sufficient to pennit reliable operation at the desired acoustic output amplitude, without permitting tensile failure ofthe ceramic and without requiring an unnecessarily stiff or bulky restraint.

As such, Figs. 11 and 12 provide illustrations of how compressive pre-stress permits higher amplitude acoustic vibrations without stress exceeding the tensile strength limit ofthe ceramic compressive strength of ceramic.

Lastly, Fig. 13 is an illustration of a plurality ofthe present cylindrically shaped high output ultrasound transducers 10, with wrapped wire restraint 14 thereover, as previously described herein, mounted along a flexible catheter 60 with spacers 62 disposed therebetween. Spacers 62 may be formed from a flexible polymer material so as to permit catheter 60 to flex between the rigid transducer (10) segments. Outer covering 16 may preferably be formed from a flexible polymer which bonds to jacket 12, and provides a smooth outer surface for catheter 16. A plurality of optional bushings 64 are disposed between fransducers 10 and spacers 62, forming an air gap 65 adjacent the inner surface 66 defining lumen 67 through which guide wire 68 passes, as shown. In a prefened aspect, the guidewire lumen 67 is lubricious and flexible and contains guidewire 68 and has a fluid (such as saline) passing therethrough to provide cooling for transducers 10. Air gap 65 operates to direct the ultrasound energy emitted by transducers 10 radially outwardly, by inhibiting radially inward ultrasound emissions. A prefened material for guidewire lumen 67 is high density polyethylene.

Figs. 14 to 17 show an aspect ofthe invention in which a plurality of axially spaced-apart transducers are used, with coiled springs wrapped around their inner and outer surfaces. As explained herein with reference to other embodiments, the present invention can be used to provide therapeutic ultrasound delivery to a patient. It is to be understood that although the structure ofthe outer restraint as illustrated herein is that of a "spring connector", the present invention is not so limited. Rather, other shapes of connectors can be used, including serpentine, zig-zag and various helical structures, all keeping within the scope ofthe present invention. Referring first to Fig. 17, a catheter 100 having a plurality of hollow cylindrical ultrasound transducers 110 which are spaced apart in an axial direction along the length ofthe catheter body is shown. Close-up views of successive transducers 110 are shown in Figs. 14 to 16 (with the catheter body removed for ease of illustration).

Referring next to Fig. 15, a first spring connector 120 is wrapped around the outer surfaces of successive vibrational transducers 110. In accordance with the present invention, first spring comiector 120 exerts an inward pre-loading on each of transducers 110. In prefened aspects, the strength of this inward pre-loading is about 25% to 75% ofthe breaking (ie: tensile) strength ofthe transducers. In alternate prefened aspects, the strength of this inward pre-loading is: (a) at least equal to the tensile strength ofthe transducers; (b) greater than the tensile strength ofthe fransducers, and less than ^lA way between the compressive and tensile strengths ofthe fransducers; or (c) approximately Vi way between the compressive and tensile strengths ofthe transducers. In this aspect ofthe invention, first spring connector 120 is a form of "restraint" (as described herein), functioning similar to wire 14 or jacket 12. Referring next to Fig. 14, a second connector 140 is disposed in contact with the inner surfaces of successive vibrational fransducers 110. Preferably, second connector 140 comprises a spring (as illustrated), but it need not comprise a spring. For example, it may comprise a simple electrical lead similar to lead 24 in Fig. 7A. In various aspects, each of first spring connector 120 and second coimector 140 may be connected to respective outer and inner surfaces of fransducers 110 by techniques including gluing, soldering, welding or bonding. Additionally, the natural tendency of a spring to "re-coil" or "spring back" into position after it has been deformed may be used to connect the first spring connector 120 and second connector 140 to the transducer surfaces, as follows.

First, the outer spring connector 120 can be unwound such that it expands in diameter, and then be slipped over the fransducers, and allowed to contract, tightening around the transducers. Specifically, the wrapping of first spring connector 120 around the outer surfaces of transducers 110 may be accomplished by unwinding first spring connector 102 such that expands in diameter; slipping first spring connector 120 over the outer surfaces of transducers 110 when first spring connector 120 is in an expanded state; and then allowing first spring connector 120 to contract around the outer surfaces of transducers 110, such that first spring connector exerts an inward pre-stress on the outer surfaces ofthe vibrational transducers.

Secondly, second connector 140 can be tightly wound such that it shrinks in diameter, and then be slipped through the bores through the respective transducers, and then be allowed to expand, tightening against the inner surface ofthe bore through the respective transducers. Specifically, the positioning of second spring comiector 140 in contact with the inner surfaces of vibrational transducers 110 may be accomplished by winding second spring connector 140 such that contracts in diameter; slipping second spring connector 140 through the inner surfaces of successive transducers 110 when second spring connector 140 is in a contracted state; and then allowing second spring connector 140 to expand such that it contacts the inner surfaces of transducers 110. In various prefened aspects, each ofthe first spring connector 120 and second connector 140 may comprise singular or multifiliar wraps. Such multifiliar wraps offer the advantages of increased electrical current carrying capacity with increasing overall stiffness. In various prefened aspects, each ofthe first spring connector 120 and second connector 140 may be spring having a varying pitch (along its length). Such a varying pitch spring is illustrated in Fig. 16 which shows a tight or nanow pitch for second connector 140 in region 140a (i.e.: within transducer 110) and a lose or wide pitch for second connector 140 in region 140b (i.e.: between two fransducers 110). (It is to be understood that spring connector 120 may also have a similar varying pitch in which its coils are spaced closer together when passing over the surface of fransducer 110 and are spaced farther apart in regions between successive transducers).

Advantages of varying the spring coil pitch (for either of spring comiector 120 or second connector 140) include, but are not limited to, the following. First, a greater percentage ofthe spring coil can be positioned in direct contact with the outer or inner surface ofthe transducer. This increases the effectiveness ofthe electrical contact made by the spring coil to the transducer.

Secondly, the greater pitch between fransducers results in less electrical resistance and therefore less energy loss due to heating. The present catheter 100 can be used for delivering vibrational energy to a patient. This can be accomplished as follows. Catheter 100 can be introduced into a patient, with first spring connector 120 disposed in contact with outer surfaces of vibrational transducers 110, (exerting an inward pre-loading on the vibrational transducers 110), and with second connector 140 disposed in contact with inner surfaces of transducers 110. Thereafter, transducers 110 can be energized to deliver vibrational energy to the patient.

In optional prefened aspects, transducers 110 are operated at a Mechanical Index (MI) of at least 1.9; and at a frequency of at least 500 KHz, but not exceeding 3 MHz.

In optional prefened aspects, the inner bores of transducers 110 may be cooled with a cooling flow, which may optionally comprise saline.

Claims

WHAT IS CLAIMED IS: 1. A therapeutic ultrasound energy delivery system, comprising: a probe for contact on or in a target location in a patient's body; a vibrational fransducer disposed on the probe; and a restraint disposed around the transducer, wherein the restraint exerts an compressive pre-sfress on the transducer.

2. The therapeutic ultrasound energy delivery system of claim 1 , wherein the compressive pre-stress on the transducer is at least equal to the tensile strength ofthe transducer.

3. The therapeutic ultrasound energy delivery system of claim 1, wherein the compressive pre-sfress on the transducer is greater than the tensile sfrength of the fransducer, and less than the average ofthe compressive and tensile strengths ofthe transducer.

4. The therapeutic ultrasound energy delivery system of claim 1, wherein the compressive pre-stress on the transducer is approximately equal to the average ofthe compressive and tensile strengths ofthe transducer.

5. The therapeutic ultrasound energy delivery system of claim 1, wherein the transducer is cylindrical shaped.

6. The therapeutic ultrasound energy delivery system of claim 1, wherein, the restraint maintains the compressive pre-stress during operation ofthe transducer.

7. The system of claim 1, wherein the restraint comprises: a j acket slipped over the transducer.

8. The system of claim 7, wherein the j acket is stretched to an expanded diameter to be received over the transducer.

9. The system of claim 7, wherein the j acket is formed of a shape memory metal.

10. The system of claim 1 , wherein the restraint comprises: a wire wrapped around the fransducer.

11. The system of claim 1, wherein the fransducer is selected from the group consisting of a piezoelectric ceramic, an elecfrostrictive ceramic and a piezoelectric crystal.

12. The system of claim 10, wherein the wire is a monofilament or multifilament polymer.

13. The system of claim 10, wherein the wire is wrapped around the fransducer under tension.

14. The system of claim 10, wherein the wire is a ribbon wire.

15. The system of claim 10, wherein the wire is soldered to the ultrasound transducer.

16. The system of claim 10, wherein the wire is welded to the ultrasound transducer.

17. The system of claim 10, wherein the wire is glued to the ultrasound transducer.

18. The system of claim 16, wherein the wire is welded to opposite longitudinal ends ofthe transducer.

19. The system of claim 16, wherein the wire is welded to the transducer along the length ofthe fransducer.

20. The system of claim 1, wherein the transducer is made of PZT-8 or PZT-4 ceramic material.

21. The system of claim 1 , wherein the restraint is made of a high tensile strength elastic material selected from the group consisting of steel, titanium alloys, beryllium copper alloys, Nitinol™, and epoxy impregnated kevlar, polyester or carbon fiber.

22. The system of claim 1, further comprising: a composite polymer outer covering disposed around the outside ofthe restraint.

23. The system of claim 22, wherein the composite polymer outer covering comprises a combination of materials selected from the group consisting of a high sfrength epoxy, cyano-acrylate, polyester, PVDF, Pebax, nylon or polyethylene.

24. The system of claim 1, wherein the ultrasound transducer has a central air-filled bore passing longitudinally therethrough.

25. The system of claim 24, further comprising: a first electrode disposed on the outer surface ofthe transducer; and a second electrode is disposed on the inner surface ofthe transducer.

26. The system of claim 1, further comprising: first and second electrodes respectively disposed on opposite longitudinal ends ofthe transducer.

27. The system of claim 1, wherein the transducer comprises a series of alternating annular shaped polymer and piezoelectric ceramic rings.

28. The system of claim 27, further comprising first and second electrodes attached to inner and outer surfaces ofthe transducer.

29. The system of claim 27, further comprising electrodes attached to opposite longitudinal ends of each ofthe piezoelectric ceramic rings.

30. A method for delivering vibrational energy to a patent, comprising: introducing a vibrational transducer to the patient; energizing the transducer to deliver vibrational energy to the patient, wherein the transducer is constrained by a restraint which provides a compressive pre- stress which permits the transducer to operate at a higher acoustic output than would have been achievable without the restraint.

31. The method of claim 30, wherein the compressive pre-stress on the transducer is at least equal to the tensile strength ofthe fransducer.

32. The method of claim 30, wherein the compressive pre-stress on the fransducer is greater than the tensile strength ofthe fransducer, and less than the average ofthe compressive and tensile strengths ofthe transducer.

33. The method of claim 30, wherein the compressive pre-stress on the transducer is approximately equal to the average ofthe compressive and tensile strengths ofthe transducer.

34. The method of claim 30, wherein exerting the compressive pre- sfress is achieved by wrapping a tensioned wire around the transducer.

35. The method of claim 30, wherein exerting a compressive pre-stress on the ultrasound transducer is achieved by stretching a jacket to a diameter sufficient to be received over the transducer and inserting the transducer into the jacket.

36. The method of claim 30, further comprising: providing a composite polymer outer cover wrapped around the restraint.

37. The method of claim 30, further comprising: operating the transducer with first and second electrodes respectively disposed on opposite longitudinal ends ofthe fransducer.

38. The method of claim 30, wherein the transducer has a central air- filled bore passing longitudinally therethrough defining an inner surface ofthe transducer, further comprising: operating the transducer with first and second electrodes respectively disposed on the inner and outer surfaces ofthe transducer.

39. The method of claim 30, further comprising: cooling an imier bore in the fransducer with a fluid flow.

40. The method of claim 39, wherein the fluid flow is a saline infusion.

41. A therapeutic ultrasound catheter system comprising: a catheter; a plurality of vibrational transducers disposed along the length of the catheter; a restraint disposed around each transducer, wherein each restraint exerts a compressive pre-stress on one ofthe transducers; and a plurality of spacers disposed between each of the successive vibrational transducers.

42. A method of treating arterial restenosis, comprising: inserting the catheter system of claim 41 into a patient's artery; and emitting ultrasound energy from the plurality of vibrational transducers.

43. The method of claim 30, wherein the vibrational transducer is operated at a Mechanical Index (MI) of at least 1.9.

44. The method of claim 30, wherein the vibrational transducer is operated at a frequency of at least 500 KHz.

45. The method of claim 30, wherein the vibrational transducer is operated at a frequency not exceeding 3 MHz.

46. A therapeutic ultrasound delivery system, comprising: a catheter body; a plurality of axially spaced-apart hollow cylindrical vibrational fransducers disposed along a length ofthe catheter body; an outer restraint disposed around the outer surfaces of the vibrational transducers, the outer restraint exerting an inward pre-stress on the outer surfaces ofthe vibrational transducers; and an inner connector disposed in contact with the inner surfaces ofthe vibrational transducers.

47. The system of claim 46, wherein the outer restraint is attached to the outer surface ofthe vibrational fransducers by one ofthe group consisting of gluing, soldering, welding or bonding.

48. The system of claim 46, wherein the inner connector is attached to the inner surface ofthe vibrational transducers by one ofthe group consisting of gluing, soldering, welding or bonding.

49. The system of claim 46, wherein the inner and outer surfaces ofthe vibrational transducers are metallic.

50. The system of claim 46, wherein at least one ofthe outer restraint and inner connector comprises a single filament wrap.

51. The system of claim 46, wherein at least one of the outer restraint and inner connector comprises a multifiliment wrap.

52. The system of claim 46, wherein the outer restraint exerts an inward pre-stress equal to about 25% to 75% ofthe breaking strength ofthe transducers.

53. The system of claim 46, wherein the outer restraint exerts an inward pre-stress at least equal to the tensile strength ofthe transducers.

54. The system of claim 46, wherein the outer restraint exerts an inward pre-stress that is greater than the tensile strength ofthe transducers, and less than the average ofthe compressive and tensile strengths ofthe transducers.

55. The system of claim 46, wherein the outer restraint exerts an inward pre-stress that is approximately equal to the average ofthe compressive and tensile strengths ofthe transducers.

56. The system of claim 46, wherein the vibrational transducers are made from the group consisting of a piezoelectric ceramic, an elecfrostrictive ceramic and a piezoelectric crystal.

57. The system of claim 56, wherein the vibrational transducers are made from the group consisting of PZT-8 and PZT-4 ceramic material.

58. The system of claim 46, wherein the outer restraint is a spring.

59. The system of claim 46, wherein the inner connector is a spring.

60. The system of claim 59, wherein the innerconnector pushes gently outwardly against the inner surfaces ofthe vibrational fransducers.

61. The system of claim 58 , wherein the pitch of the spring is less in regions adjacent each ofthe individual fransducers than in regions between each ofthe individual transducers.

62. The system of claim 59, wherein the pitch ofthe spring is less in regions within the inner bores of each of the individual transducers than in regions between each ofthe individual transducers.

63. The system of claim 46, wherein the outer restraint is made from a material having a Young's Modulus in the range of 10,000,000 psi to 70,000,000.

64. The system of claim 46, wherein the outer restraint is made from a material having a tensile strength in the range of 50,000 psi to 400,000.

65. The system of claim 46, wherein the outer restraint is made from a material having electrical conduction properties in the range of 9 to 100 ohms per mihft.

66. The system of claim 46, wherein at least one ofthe outer restraint or inner connector is made from a material selected from the group consisting of beryllium copper alloys, steel alloys, aluminum alloys, nickel alloys, nickel titanium alloys and tungsten alloys.

67. The system of claim 46, wherein the outer restraint is disposed around the outer surfaces of two successive vibrational transducers.

68. The system of claim 46, wherein the outer restraint is disposed around the outer surfaces of three successive vibrational fransducers.

69. The system of claim 46, wherein the outer restraint is disposed around the outer surfaces of all ofthe fransducers in the catheter body.

70. A method of assembling a system for delivering vibrational energy to a patient, comprising: providing a catheter body; providing a plurality of axially spaced apart hollow cylindrical vibrational transducers disposed along a length ofthe catheter body; connecting together the plurality of hollow cylindrical vibrational transducers within the catheter body by: wrapping a first spring connector around the outer surfaces ofthe vibrational fransducers wherein the first spring connector exerts an inward pre-stress on the outer surfaces ofthe vibrational fransducers; and positioning a second spring connector in contact with the inner surfaces ofthe vibrational fransducers.

71. The method of claim 70, wherein wrapping the first spring connector around the outer surfaces ofthe vibrational transducers comprises: unwinding the first spring connector such that expands in diameter; slipping the first spring connector over the outer surfaces ofthe vibrational transducers when the first spring connector is in an expanded state; and allowing the first spring connector to contract around the outer surfaces of the vibrational transducers, such that the first spring connector exerts an inward pre-sfress on the outer surfaces ofthe vibrational transducers.

72. The method of claim 70, wherein positioning the second spring connector in contact with the inner surfaces ofthe vibrational transducers comprises: winding the second spring connector such that contracts in diameter; slipping the second spring connector through the inner surfaces ofthe vibrational transducers when the second spring connector is in a contracted state; and allowing the second spring connector to expand such that it contacts the inner surfaces ofthe vibrational transducers.

73. The method of claim 70, further comprising: attaching the first spring connector to the outer surfaces ofthe vibrational transducers by one ofthe group consisting of gluing, soldering, welding or bonding.

74. The method of claim 70, further comprising: attaching the second spring connector to the inner surfaces ofthe vibrational fransducers by one ofthe group consisting of gluing, soldering, welding or bonding.

75. The method of claim 70, wherein the first spring coimector exerts an inward pre-stress equal to about 25% to 75% ofthe breaking strength ofthe transducers.

76. The method of claim 70, wherein the first spring connector exerts an inward pre-sfress equal at least equal to the tensile strength ofthe fransducer.

77. The method of claim 70, wherein the first spring connector exerts an inward pre-stress that is greater than the tensile strength ofthe transducer, and less than the average ofthe compressive and tensile strengths ofthe transducer.

78. The method of claim 70, wherein the first spring connector exerts an inward pre-stress that is approximately equal to the average ofthe compressive and tensile strengths ofthe fransducer.

79. A method for delivering vibrational energy to a patent, comprising: introducing a catheter having a plurality of axially spaced apart hollow cylindrical vibrational transducers disposed therealong into a patient, wherein an outer restraint is disposed in contact with outer surfaces ofthe vibrational transducers and exerts an inward pre-loading on the vibrational transducers, and wherein an inner coimector is disposed in contact with inner surfaces ofthe vibrational transducers; and energizing the vibrational fransducers to deliver vibrational energy to the patient.

80. The method of claim 79, further comprising: cooling an inner bore in the transducer with a fluid flow.

81. The method of claim 80, wherein the fluid flow is a saline infusion.

82. The method of claim 79, wherein the vibrational transducer is operated at a Mechanical Index (MI) of at least 1.9.

83. The method of claim 79, wherein the vibrational fransducer is operated at a frequency of at least 500 KHz.

84. The method of claim 79, wherein the vibrational fransducer is operated at a frequency not exceeding 3 MHz.