Patent Description:
Many patients suffer from occluded arteries and other blood vessels which restrict blood flow. Occlusions can be partial occlusions that reduce blood flow through the occluded portion of a blood vessel or total occlusions (e.g., chronic total occlusions) that substantially block blood flow through the occluded blood vessel. In some cases, an occlusion may be or otherwise include a calcified lesion that may impact a physician's ability to place a stent, or conduct balloon angioplasty, for example. The calcified lesion may be treated to soften and weaken the calcified lesion, which can make subsequent treatments such as stenting and balloon angioplasty more effective. A need remains for alternate devices and methods for treating calcified lesions. <CIT> describes a weeping balloon catheter with an ultrasound element. A treatment system is arranged to release microbubbles in proximity to a blood clot. Apertures in a balloon wall extend through the thickness of the balloon wall to allow fluid in an outer balloon cavity to pass through apertures and out of the balloon member.

No surgical methods form part of the invention. This disclosure provides design, material, manufacturing method, and use alternatives for medical devices. For example, the disclosure is directed to an ultrasound catheter that is adapted for placement within a blood vessel having a vessel wall for treating a calcified lesion within or adjacent the vessel wall The ultrasound catheter includes an elongate shaft extending from a distal region to a proximal region, an ultrasound transducer that is disposed relative to the distal region of the elongate shaft and is adapted to impart near-field acoustic pressure waves within the calcified lesion in order to induce fractures in the calcified lesion. An inflatable balloon is disposed about the ultrasound transducer and is coupled to the elongate shaft, the inflatable balloon having a collapsed configuration suitable for advancing the ultrasound catheter through a patient's vasculature and an expanded configuration suitable for anchoring the ultrasound catheter in position relative to a treatment site.

Alternatively or additionally, the inflatable balloon may include a proximal waist and a distal waist, the inflatable balloon secured to the elongate shaft via the proximal waist and the distal waist, with the proximal waist disposed proximal of the ultrasound transducer and the distal waist disposed distal of the ultrasound transducer.

Alternatively or additionally, the inflatable balloon may be configured to be inflated using an inflation fluid, the inflation fluid being a medium through which the ultrasound transducer transmits acoustic pressure waves.

Alternatively or additionally, the inflation fluid may include pre-formed gas bubbles, droplets or other cavitation nuclei that can be excited into resonance, collapse or other cavitation behavior to generate or amplify the acoustic pressure waves impinging upon the calcified lesion.

Alternatively or additionally, the inflation fluid may include gas bubbles or droplets having an average diameter of about <NUM> micrometer to about <NUM> micrometers.

Alternatively or additionally, the inflatable balloon may have an inner surface, and the inner surface of the balloon may include a hydrophilic or hydrophobic treatment.

Alternatively or additionally, a portion of the ultrasound transducer may include a hydrophilic or hydrophobic treatment.

Alternatively or additionally, the inflatable balloon may include an inner surface, and the inner surface of the balloon may include a mechanical or chemical treatment that localizes, traps, collects or nucleates bubbles.

Alternatively or additionally, the inflatable balloon may be a single wall balloon.

Alternatively or additionally, the inflatable balloon may be a double wall balloon, the double wall forming an inner chamber proximate the ultrasound transducer and an outer chamber surrounding the inner chamber.

Alternatively or additionally, the double wall balloon may include an inner wall that is formed of a semipermeable material and an outer wall that is formed of a non-permeable material.

Alternatively or additionally, the ultrasound transducer may be configured to transmit a substantially uniform acoustic pressure over a length of about <NUM> millimeters to about <NUM> millimeters at a radial distance of about <NUM> millimeters to about <NUM> millimeters as measured from a longitudinal central axis of the elongate shaft.

Alternatively or additionally, the ultrasound transducer may include a plurality of individual ultrasound transducers.

Alternatively or additionally, each of the individual ultrasound transducers may be independently electrically driven.

Another example of the disclosure is an ultrasound device that is adapted for placement within a blood vessel having a vessel wall for causing mechanical fractures in a calcified lesion within or adjacent the vessel wall. The ultrasound device includes an elongate shaft extending from a distal region to a proximal region and an ultrasound transducer that is disposed within the distal region of the elongate shaft and is ultrasound transducer adapted to impart unfocused acoustic pressure waves upon the calcified lesion in order to induce fractures in the calcified lesion. An inflatable balloon is disposed about the ultrasound transducer and coupled to the elongate shaft. The ultrasound transducer has an effective length that is at least twice a distance between the ultrasound transducer and the calcified lesion when the ultrasound device is disposed proximate the calcified lesion.

Another example of the disclosure is a method of treating a calcified lesion that is within or proximate a vessel wall forming part of a blood vessel. An ultrasound catheter is advanced through a patient's vasculature until reaching a desired treatment site proximate the calcified lesion, the ultrasound catheter including an ultrasound transducer secured relative to an inner shaft and an inflatable balloon secured to the inner shaft and disposed about the ultrasound transducer. The inflatable balloon is inflated with an inflation fluid to secure the ultrasound catheter in position proximate the calcified lesion. The ultrasound transducer is driven to produce near-field acoustic pressure waves within a thickness of the vessel wall and the calcified lesion in order to induce fractures within the calcified lesion. The inflatable balloon is deflated to permit repositioning or removal of the ultrasound catheter.

Alternatively or additionally, inflating the inflatable balloon with inflation fluid may include inflating the inflatable balloon with an inflation fluid that includes cavitation nuclei having an average diameter of about <NUM> micrometer to about <NUM> micrometers.

Alternatively or additionally, the inflatable balloon may include an inner chamber and an outer chamber, and inflating the inflatable balloon with an inflation fluid may include inflating the inner chamber and the outer chamber at different pressures in order to drive dissolved gasses out of solution.

Alternatively or additionally, the method may further include periodically changing a pressure of the inflatable balloon in order to provide a pulsatile mechanical pressure to the vessel wall.

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:.

Many patients suffer from occluded arteries, other blood vessels, and/or occluded ducts or other body lumens which may restrict bodily fluid (e.g. blood, bile, etc.) flow. Occlusions can be partial occlusions that reduce blood flow through the occluded portion of a blood vessel or total occlusions (e.g., chronic total occlusions) that substantially block blood flow through the occluded blood vessel. Revascularization techniques include using a variety of devices to pass through the occlusion to create or enlarge an opening through the occlusion. In some cases, lesions such as calcified lesions may create problems for revascularization techniques, and it may be beneficial to treat the calcified lesions in order to soften them and make them more malleable.

In some cases, for example, ultrasound may be used to treat vascular lesions, such as fibrotic and calcified lesions, at various states of disease progression, ranging from soft plaques to severely calcified lesions. Vascular lesions that may best lend themselves to being treated with ultrasound-based devices include irregular, severely calcified plaques that are located within and adjacent to vessel walls, and lesions that are more or less rigid and thus may be susceptible to being mechanically fatigued to failure. For example, ultrasound-based devices may be used to produce standing wave pressure patterns within the thickness of the lesion, bending moments at the ends of the lesion, and/or resonance along the length of the lesion. In some cases, the high frequency mechanical action of ultrasound may also be effective in treating earlier state vascular lesions, including fibrotic and soft plaques. In some cases, an ultrasound device may apply a treatment of unfocused, near-field ultrasound waves to treat vascular lesions.

An intravascular device such as an ultrasound catheter may be placed within a blood vessel in order to treat a vascular lesion that is within or adjacent to a vessel wall of the blood vessel. <FIG> is a schematic view of an ultrasound catheter <NUM> placed proximate a calcified lesion <NUM>. The ultrasound catheter <NUM> includes an ultrasound transducer <NUM> disposed relative to an elongate shaft <NUM>. In some cases, the ultrasound transducer <NUM> may include a piezoelectric material, which transmits acoustic pressure in response to an applied voltage. The ultrasound transducer <NUM> may be driven at one or more frequencies in the range of about <NUM> kilohertz (kHz) to about <NUM> megahertz (MHz). The ultrasound transducer <NUM> may be a single ultrasound transducer, or the ultrasound transducer <NUM> may include a series of ultrasound transducers that may be operated to effectively function as a single ultrasound transducer, providing the desired acoustic pressure over the desired treatment area. The acoustic pressure applied may range from tens of kiloPascals (kPa) to in excess of ten megaPascals (MPa).

As can be seen in the example of <FIG>, the ultrasound transducer <NUM> produces an ultrasound field <NUM> that includes a near field region <NUM> and a far field region <NUM>. In the near field region <NUM>, dynamic acoustic pressures may be cyclically applied to the calcified lesion <NUM>. As used in this application, the near field region <NUM> refers to a region in close proximity radially to a surface of the ultrasound transducer <NUM>, for example, the region extending outward from the transducer surface to a radial distance less than or equal to a length of the ultrasound transducer <NUM>, wherein the acoustic pressure waves transmitted by the ultrasound transducer <NUM> are unfocused and can be controlled to be substantially uniform upon the calcified lesion <NUM>. In some cases, the ultrasound catheter <NUM> may include additional structure, such as an inflatable balloon as will be discussed with respect to subsequent drawings.

In some cases, for example, the ultrasound transducer <NUM> may be configured to impart a uniform or substantially uniform acoustic pressure along the length of the calcified lesion <NUM>. In cardiac vessel disease states, vascular lesions may span a length of <NUM> millimeters (mm) to <NUM> in vessels that are <NUM> to <NUM> in diameter. In peripheral vessel disease states, vascular lesions may span a length of up to <NUM> in vessels up to <NUM> in diameter. Depending on the therapeutic applications, the ultrasound transducer <NUM> may be configured to impart a uniform or substantially uniform acoustic pressure over a length of about <NUM> to about <NUM> at a radial distance of about <NUM> to about <NUM> as measured from a central axis L extending through the elongate shaft <NUM>. While not illustrated, one can appreciate that multiple ultrasound transducers <NUM> may be configured upon a catheter to extend the effective therapeutic length, such as up to a length of <NUM>.

To impart a uniform or substantially uniform acoustic pressure in the near field <NUM>, the ultrasound transducer <NUM> may have a length that is multiple times larger than a diameter of the ultrasound catheter <NUM>. In some cases, the ultrasound transducer <NUM> may have a length that is at least as long as a length of the calcified lesion <NUM>, in some cases, to generate a uniform or substantially uniform acoustic pressure over a length of about <NUM> to about <NUM>.

In some instances, the ultrasound transducer <NUM>, may be a single ultrasound transducer or a series of ultrasound transducers or transducer elements driven in such a way as to effectively act as a single ultrasound transducer. <FIG> provide illustrative but non-limiting examples of how the ultrasound transducer <NUM> may be controlled. In <FIG>, a single ultrasound transducer <NUM> is electrically coupled to an electronic source <NUM> via wires 28a, 28b. <FIG> shows an ultrasound transducer <NUM> and an ultrasound transducer <NUM>. The ultrasound transducer <NUM> is electrically coupled to an electronic source <NUM> via wires 34a, 34b and the ultrasound transducer <NUM> is electrically coupled to the electronic source <NUM> via wires 36a, 36b. In this case, the ultrasound transducer <NUM> and the ultrasound transducer <NUM> are driven with the same frequency and output from the electronic source <NUM>. <FIG> shows an ultrasound transducer <NUM> and an ultrasound transducer <NUM>. The ultrasound transducer <NUM> is electrically coupled to an electronic source <NUM> via wires 46a, 46b. The ultrasound transducer <NUM> is electrically coupled to an electronic source <NUM> via wires 50a, 50b. In this case, the ultrasound transducers <NUM>, <NUM> are independently driven with the electronic sources <NUM>, <NUM>, respectively, and amplitude and phase control may be applied to increase the uniformity of the acoustic pressure imparted to the calcified lesion <NUM>. While <FIG> each show a pair of ultrasound transducers <NUM>, <NUM> and <NUM>, <NUM>, it will be appreciated that this is merely illustrative, as any number of distinct ultrasound transducers may be utilized.

<FIG> is a schematic view of a distal portion of an ultrasound catheter <NUM> that includes an elongate shaft <NUM> that terminates at a distal end <NUM>. In some cases, the distal end <NUM> may include an atraumatic tip, for example. An inflatable balloon <NUM> is secured relative to the elongate shaft <NUM>. In some cases, as illustrated, the inflatable balloon <NUM> includes a proximal waist <NUM> and a distal waist <NUM>, and is secured to the elongate shaft via the proximal waist <NUM> and the distal waist <NUM>. The inflatable balloon <NUM> may be formed of any suitable polymeric material, and may for example be compliant or non-compliant, i.e., the inflatable balloon <NUM> may have an inflated size and shape that is locked in, or the inflatable balloon <NUM> may have an inflated size and shape that varies upon inflation pressure. An ultrasound transducer <NUM> may be secured relative to the elongate shaft <NUM>. In some cases, the inflatable balloon <NUM> may be sized such that the proximal waist <NUM> is disposed proximal of the ultrasound transducer <NUM> and the distal waist <NUM> is disposed distal of the ultrasound transducer <NUM>.

The inflatable balloon <NUM> may be inflated using any suitable inflation fluid. Examples include water, saline (e.g., <NUM>% sodium chloride), and a mixture of saline and a radiopaque contrast agent (e.g., a <NUM>/<NUM> mixture). In some cases, the inflation fluid may be chosen for how acoustic energy transmits through the inflation fluid. It will be appreciated that by selecting a particular fluid with which to inflate the inflatable balloon <NUM>, one is able to control the efficiency of acoustic energy transmission through the fluid and to the calcified lesion <NUM> (<FIG>). In one example, the inflation fluid may be chosen to have a specific characteristic acoustic impedance to serve as an acoustic matching between the ultrasound transducer <NUM> and the vessel wall. In another example, the inflation fluid may be chosen to have a specific characteristic acoustic impedance to serve as an acoustic matching to minimize transmission loss across a wall of the inflatable balloon <NUM>. In another example, the inflation fluid may be chosen to have a specific sound velocity in order to modify the near field behavior of the ultrasound transducer <NUM>.

In some cases, the inflation fluid may include dissolved gas, gas bubbles, stabilized microbubbles, droplets, commercial ultrasound contrast agent, or other cavitation nuclei which may be excited into resonance, collapse and cavitation behavior by the ultrasound transducer <NUM> to generate or significantly amplify the acoustic pressure waves impinging upon the calcified lesion. As shown for example in <FIG>, the inflation fluid may include a saline solution with an abundant population of cavitation nuclei <NUM> in suspension. In some cases, the cavitation nuclei <NUM> may be configured, chemically or otherwise, to preferentially migrate towards an inner wall <NUM> of the inflatable balloon <NUM>. The cavitation nuclei <NUM> in the inflation fluid may, for example, have an average diameter that ranges from about <NUM> micrometer to about <NUM> micrometers. In some cases, the size distribution and type of cavitation nuclei <NUM> may be optimally matched to the sound field frequency and pressure transmitted by the ultrasound transducer <NUM>. In some cases, the size distribution and type of cavitation nuclei <NUM> may be selected based at least in part upon a collapse pressure and penetration depth selected for a particular treatment target. As an example, <NUM> micrometer diameter, air filled microbubbles may be selected as an optimal cavitation nuclei for an ultrasound catheter with an ultrasound transducer <NUM> operating at <NUM> megahertz (MHz). The cavitation nuclei <NUM> may be replenished by injection through the catheter into the inflatable balloon <NUM>, by sonication from the ultrasound transducer <NUM>, as well as by other methods. In some cases, the inner wall <NUM> of the inflatable balloon <NUM>, a surface of the ultrasound transducer <NUM>, and/or one or more surfaces of the elongate shaft <NUM>, may include a surface treatment such as a hydrophilic or hydrophobic coating or mechanical patterning to localize, trap, collect, or nucleate the cavitation nuclei <NUM>. In some cases, the surface treatments may be selected to drive the cavitation nuclei <NUM> to locations where they will be most effective in amplifying the pressure waves impinging on the calcified lesion. In other cases, the surface treatment(s) may be selected to drive cavitation nuclei <NUM> away from a surface of the ultrasound transducer <NUM>.

<FIG> show illustrative but non-limiting examples of surface treatment that may be used. In some cases, as shown for example in <FIG>, the ultrasound catheter <NUM> may include a hydrophilic surface treatment <NUM> on the inner wall <NUM> of the inflatable balloon <NUM> and/or a hydrophobic surface treatment <NUM> on the ultrasound transducer <NUM> and the elongate shaft <NUM>. Examples of suitable hydrophilic treatments include polyarylene oxides, polyvinylpyrolidones, polyvinylalcohols, hydroxyl alkyl cellulosics. Examples of suitable hydrophobic treatments include silicone based coatings and fluoropolymers. <FIG> illustrate various patterns that may be applied to localize cavitation nuclei <NUM>. <FIG> is a schematic illustration of an inflatable balloon having a mechanical pattern formed on a surface of an inflatable balloon <NUM>. In some cases, as shown for the example in <FIG>, an ultrasound catheter <NUM> may include a hydrophilic surface treatment on the inner wall <NUM> of the inflatable balloon <NUM>. The inflatable balloon <NUM> includes a series of features that are arranged along the inner wall <NUM> of the inflatable balloon wall to localize cavitation nuclei <NUM>. In <FIG>, an inflatable balloon <NUM> includes a series of features <NUM> that are arranged circumferentially with uniform axial spacing to localize cavitation nuclei <NUM> in circular bands along an inner surface <NUM> of the inflatable balloon <NUM>. In <FIG>, an inflatable balloon <NUM> includes a series of features (not illustrated) that are arranged to localize cavitation nuclei <NUM> uniformly spaced in both radial and axial directions upon the inner surface of the inflatable balloon <NUM>. It will be appreciated that the features described may be mixed and matched, for example, and may be formed in a variety of processes including mechanically removing material from the balloon wall and depositing material on the balloon wall.

<FIG> is a schematic view of a distal portion of an ultrasound catheter <NUM> that includes an elongate shaft <NUM> that terminates at a distal end <NUM>. In some cases, the distal end <NUM> may be an atraumatic tip, for example. An inflatable balloon <NUM> is secured relative to the elongate shaft <NUM>. In some cases, the inflatable balloon <NUM> includes a proximal waist <NUM> and a distal waist <NUM>, by which the inflatable balloon <NUM> is secured to the elongate shaft <NUM>. The inflatable balloon <NUM> may be formed of any suitable polymeric material, and may for example be compliant or non-compliant, i.e., the inflatable balloon <NUM> may have an inflated size and shape that is locked in, or the inflatable balloon <NUM> may have an inflated size and shape that varies upon inflation pressure. An ultrasound transducer <NUM> may be secured relative to the elongate shaft <NUM>. In some cases, the inflatable balloon <NUM> may be sized such that the proximal waist <NUM> is disposed proximal of the ultrasound transducer <NUM> and the distal waist <NUM> is disposed distal of the ultrasound transducer <NUM>.

In some cases, the inflatable balloon <NUM> may be a double wall inflatable balloon, having an inner wall <NUM> and an outer wall <NUM>. An inner chamber <NUM> is defined within the inner wall <NUM>, and an outer chamber <NUM> is defined between the inner wall <NUM> and the outer wall <NUM>. In some cases, the inner wall <NUM> may be made of a semipermeable material and the outer wall <NUM> may be made of a non-permeable material. If both the inner chamber <NUM> and the outer chamber <NUM> contain a volume of fluid, and a relatively higher pressure is applied to the inner chamber <NUM> and a relatively lower pressure is applied to the outer chamber <NUM>, the pressure differential may drive gas out of solution in the inner chamber <NUM> to form gas bubbles or other cavitation nuclei in the outer chamber <NUM>.

In some cases, the outer chamber <NUM> may contain a volume of gas-saturated fluid, and reducing the pressure in the inner chamber <NUM> to create a pressure differential may drive gas out of solution in the outer chamber <NUM>. In some cases, the inner chamber <NUM> may initially contain a volume of gas and the outer chamber <NUM> may initially contain a volume of fluid (liquid). A relatively higher pressure may be applied to the inner chamber <NUM> and a relatively lower pressure may be applied to the outer chamber <NUM>. The pressure differential drives gas out of solution in the inner chamber <NUM> to form nuclei in the outer chamber <NUM>. The gas in the inner chamber <NUM> may be replaced with a fluid prior to operating the ultrasound transducer <NUM>.

In some cases, as shown in <FIG>, the inner wall <NUM> may include a mechanical patterning <NUM> such that injection of a gas-saturated fluid into the outer chamber <NUM> causes bubble generation at the sites of the mechanical patterning <NUM>. In some cases, chemical patterning or chemical surface treatments may be used to influence where and how bubbles nucleate. In some cases, the pressure in the inflatable balloon <NUM> (or the inflatable balloon <NUM>) may be periodically fluctuated to create a pulsatile mechanical action by the inflatable balloon <NUM> (or <NUM>).

A variety of polymeric materials may be used in manufacturing the ultrasound catheters <NUM>, <NUM>, <NUM> described herein. Some examples of suitable polymers may include polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), polyoxymethylene (POM, for example, DELRIN® available from DuPont), polyether block ester, polyurethane (for example, Polyurethane 85A), polypropylene (PP), polyvinylchloride (PVC), polyether-ester (for example, ARNITEL® available from DSM Engineering Plastics), ether or ester based copolymers (for example, butylene/poly(alkylene ether) phthalate and/or other polyester elastomers such as HYTREL® available from DuPont), polyamide (for example, DURETHAN® available from Bayer or CRISTAMID® available from Elf Atochem), elastomeric polyamides, block polyamide/ethers, polyether block amide (PEBA, for example available under the trade name PEBAX®), ethylene vinyl acetate copolymers (EVA), silicones, polyethylene (PE), Marlex high-density polyethylene, Marlex low-density polyethylene, linear low density polyethylene (for example REXELL®), polyester, polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polytrimethylene terephthalate, polyethylene naphthalate (PEN), polyetheretherketone (PEEK), polyimide (PI), polyetherimide (PEI), polyphenylene sulfide (PPS), polyphenylene oxide (PPO), poly paraphenylene terephthalamide (for example, KEVLAR®), polysulfone, nylon, nylon-<NUM> (such as GRILAMID® available from EMS American Grilon), perfluoro(propyl vinyl ether) (PFA), ethylene vinyl alcohol, polyolefin, polystyrene, epoxy, polyvinylidene chloride (PVdC), poly(styrene-b-isobutylene-b-styrene) (for example, SIBS and/or SIBS 50A), polycarbonates, ionomers, biocompatible polymers, other suitable materials, or mixtures, combinations, copolymers thereof, polymer/metal composites, and the like. In some embodiments the polymeric materials may include a liquid crystal polymer (LCP). For example, the mixture can contain up to about <NUM> percent LCP.

In some cases, the ultrasound catheters <NUM>, <NUM>, <NUM> may include a lubricious, a hydrophilic, a hydrophobic, a protective, or other type of coating. Hydrophobic coatings such as fluoropolymers provide a dry lubricity which improves device handling and device exchanges. Lubricious coatings improve steerability and improve lesion crossing capability. Suitable lubricious polymers are well known in the art and may include silicone and the like, hydrophilic polymers such as high-density polyethylene (HDPE), polytetrafluoroethylene (PTFE), polyarylene oxides, polyvinylpyrolidones, polyvinylalcohols, hydroxy alkyl cellulosics, algins, saccharides, caprolactones, and the like, and mixtures and combinations thereof. Hydrophilic polymers may be blended among themselves or with formulated amounts of water insoluble compounds (including some polymers) to yield coatings with suitable lubricity, bonding, and solubility. Some other examples of such coatings and materials and methods used to create such coatings can be found in <CIT> and <CIT>.

The devices described herein may be formed, for example, by coating, extrusion, co-extrusion, interrupted layer co-extrusion (ILC), or fusing several segments end-to-end. The layer may have a uniform stiffness or a gradual reduction in stiffness from the proximal end to the distal end thereof. The gradual reduction in stiffness may be continuous as by ILC or may be stepped as by fusing together separate extruded tubular segments. The outer layer may be impregnated with a radiopaque filler material to facilitate radiographic visualization. Those skilled in the art will recognize that these materials can vary widely without deviating from the scope of the present invention.

Claim 1:
An ultrasound catheter (<NUM>, <NUM>, <NUM>) adapted for placement within a blood vessel having a vessel wall, the ultrasound catheter for treating a calcified lesion (<NUM>) within or adjacent the vessel wall, the ultrasound catheter comprising:
an elongate shaft (<NUM>, <NUM>, <NUM>) extending from a distal region (<NUM>) to a proximal region;
an ultrasound transducer (<NUM>, <NUM>) disposed relative to the distal region of the elongate shaft, the ultrasound transducer adapted to impart near-field acoustic pressure waves within the calcified lesion (<NUM>) in order to induce fractures in the calcified lesion; and
a double walled inflatable balloon (<NUM>, <NUM>, <NUM>, <NUM>) disposed about the ultrasound transducer (<NUM>) and coupled to the elongate shaft (<NUM>, <NUM>), the double walled inflatable balloon (<NUM>, <NUM>, <NUM>, <NUM>) having a collapsed configuration suitable for advancing the ultrasound catheter through a patient's vasculature and an expanded configuration suitable for anchoring the ultrasound catheter in position relative to a treatment site,
wherein the double walled inflatable balloon (<NUM>, <NUM>, <NUM>, <NUM>) is constructed with an inner wall (<NUM>) and an outer wall (<NUM>) made of a non-permeable material and is configured to be inflated using an inflation fluid, the inflation fluid having cavitation nuclei therein, the inflation fluid being a medium through which the ultrasound transducer transmits acoustic pressure waves.