CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of US Provisional Patent Application No. 61/642,129 filed on May 3, 2012 the disclosures of which are hereby incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to an apparatus and method for ultrasound renal denervation and more specifically the controlling of the ultrasound energy field for a more uniform and complete renal denervation.

BACKGROUND OF THE INVENTION

[0003] Recently it has been shown that hypertension may be treated by inactivating conduction of the renal nerves surrounding the renal artery. However, the difficulty of this treatment is locating the renal nerves because of their small size, their varied locations with respect to the renal artery, and the inability of current technology to help with identification and registration of them. The inability to locate the renal nerves makes it difficult to apply a treatment specifically to the renal nerves without damaging the renal artery or other structures in the body. In addition, the locations of the renal nerves and the size of the renal arteries vary from person to person, making it difficult to predict an accurate treatment location. Another difficulty of this treatment is knowing if a sufficient amount of energy has been applied to the renal nerves to discontinue their activity. This is because the positive effects of the treatment are not detectable for a period of time.

[0004] Patent Publication No. WO/2011/053757 (Application Serial No. PCT/US2010/054637)('757), incorporated by reference herein in its entirety, discloses the use of an ultrasound transducer placed in a renal artery that transmits a cylindrical field of unfocused ultrasound energy which propagates through the renal artery to the renal nerves located within the renal artery, on the surface of the renal artery, and adjacent to the renal artery. Patent Publication '757 also discloses an actuator that is adapted to control the transducer to produce a predetermined ultrasound energy level that produces a therapeutically effective dose of ultrasound energy within an impact volume. The impact volume extends from the transducer to the immediate surrounding tissue of the renal artery and the dosage of ultrasound energy within the impact volume affects nerves without damaging renal artery tissue. Because the impact volume is relatively large, and because the tissues throughout the impact volume reach temperatures sufficient to inactivate nerve conduction, the actual locations of the renal nerves do not need to be determined. Therefore, the treatment can be performed without targeting or focusing on the renal nerves and without measuring the temperature of tissues. [0005] However, in order to ensure inactivation of the renal nerves without damaging the renal artery for each patient treated, the predetermined ultrasound energy level needs to be accurate and consistent regardless of the transducer used during the renal denervation procedure. However, the manufacturing tolerances, material and physical properties variation of transducers cause the ultrasound field and power emitted from each transducer to be different. Therefore, it is desirable to have a more accurate and consistent ultrasound field and power emitted from the transducer.

[0006] In addition, some locations of the ultrasound transducer may not resonate at the same frequency as other locations of the ultrasound transducer due to variations in the thickness of the transducer material, variations in the electro-mechanical properties of the transducer material, imperfections in the transducer material, surface, electrodes, and other irregularities in the manufacturing process. This may cause the ultrasound energy field to be non-uniform. A non-uniform field may cause the ultrasound energy to damage portions of the renal artery and/or miss inactivating renal nerves. Therefore, it is desirable to have a more uniform field of ultrasound energy to provide a more effective treatment.

[0007] Patent Publication 757 describes the renal nerves being captured within the impact volume. An additional problem to capturing all the renal nerves in the impact volume is that some arteries are much larger than others. Patent Publication 757 describes measuring the renal artery and increasing the ultrasound dosage level to increase the impact volume depth to ensure that the impact volume encompasses the renal artery. However, it is equally important that the impact volume does not encompass other structures in the body causing damage to those structures. In order to encompass the renal artery and yet protect these collateral structures, it is desirable to control the depth of the ultrasound energy.

SUMMARY OF THE INVENTION

[0008] Before the present apparatus and methods are described, it is to be understood that this disclosure is not limited to the particular apparatus and methodologies described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope.

[0009] The present invention includes an apparatus and method of uniformly inactivating renal nerve conduction (renal denervation) with an ultrasound transducer. Embodiments of the present invention may compensate for imperfections of the ultrasound transducer and the size and location of the renal artery by providing a more uniform ultrasound field, allowing the depth of the impact volume to be controlled, and providing a more accurate and consistent dose of ultrasound energy.

[0010] In a preferred embodiment of the invention, the apparatus includes a catheter having an ultrasound transducer and a memory element that carries information about the transducer; and an actuator electrically connected to the transducer. The memory element may include an identification code which enables an Internet download of transducer performance specifications peculiar to the particular transducer. Or the memory element may set forth transducer performance specifications to enable manual input thereof into a transducer-control computer or actuator unit. Other possible modes of transducer information handling are set forth hereinafter. Preferably, the memory element directly incorporates, for example, electronically stores, transducer performance information such as transducer frequency, non- uniformity information and transducer power output efficiency. The actuator is adapted to input the transducer's information from the memory element, determine a frequency sweep range and frequency steps (in the case of a discrete rather than continuous variation in applied frequency), determine an output power level, and actuate the transducer using the determined frequency sweep and frequency steps to produce a therapeutically effective dose of ultrasound energy that is sufficient to inactivate conduction of renal nerves throughout an impact volume.

[0011] The method includes inputting (manually, or electronically, for instance, via the Internet or from an electronic memory element) the transducer's frequency and non-uniformity information into an actuator with a processor, determining a frequency sweep range and frequency steps based on the frequency and the non-uniformity information, inserting the transducer in a renal artery, and actuating the transducer using the determined frequency sweep range and frequency steps (or rate of frequency change) to produce a therapeutically effective dose of ultrasound energy that is sufficient to inactivate conduction of renal nerves throughout an impact volume and compensate for varying transducer characteristics.

[0012 ] In some embodiments of the invention, the memory element is an RFID transmitter and the actuator is adapted to read the RFID.

[0013 ] Another embodiment of the present invention includes controlling the depth of the impact volume. In this embodiment, collateral body structures, such as kidney, spine, lumbar artery, urethra, and bowel, around the renal artery are imaged and distances from the transducer to the collateral body structures are determined. If a collateral body structure is not within the impact volume, the fundamental actuation frequency of the transducer is used; however, if a collateral body structure is within the impact volume, a higher harmonic or lower frequency resonance is used. The use of a broad resonance spectrum may include the exclusive use of the single discrete actuation frequency or mixing different discrete actuation frequencies. If there is a partial use of any of the harmonic actuation frequencies, it may be interwoven (e.g., multiplexed or alternated) with the fundamental actuation frequency and the duty cycle may be selected based on the closeness of the collateral structure.

[0014] Another embodiment of the invention includes providing an accurate and consistent dose of ultrasound energy. In this embodiment, the transducer information read from a catheter includes the acoustic characteristics of the transducer. The method further includes measuring the size of the renal artery and using the size of the renal artery and the acoustic characteristics of the transducer to determine a dosage to use when actuating the transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which like reference characters refer to like parts throughout, and in which:

FIG. 1 is a diagrammatic view depicting components of the apparatus in accordance with one embodiment of the present invention.

FIG. 2 is a diagrammatic view depicting the apparatus in accordance with one embodiment of the present invention during operation.

FIG. 3 is a functional, block diagrammatic view depicting portions of a component used in the apparatus.

FIG. 4 is a flow chart depicting the steps used in a method according to one embodiment of the present invention.

FIG. 5 is a flow chart depicting the steps used in a method according to one embodiment of the present invention.

FIG. 6 is a diagrammatic view depicting portions of the apparatus during operation.

FIG. 7 is a flow chart depicting the steps used in a method according to one embodiment of the present invention.

FIG. 8 is a diagrammatic view depicting a portion of the apparatus in conjunction with a renal artery during operation.

FIG. 9a and 9b are fragmentary diagrammatic perspective views depicting a portion of the apparatus.

FIG. 10 is a fragmentary diagrammatic perspective view depicting a portion of the apparatus in conjunction with a renal artery during operation.

FIG. 11 is a fragmentary diagrammatic perspective view depicting a portion of the apparatus. FIG. 12 is a fragmentary diagrammatic perspective view depicting a portion of the apparatus.

DETAILED DESCRIPTION

[0015] The present invention includes an apparatus and method for providing a more uniform renal denervation by controlling the ultrasound energy field. Referring to FIG. 1 , the apparatus 1 includes an ultrasound transducer 30 adapted for transmission of ultrasound energy. The apparatus 1 also includes an actuator 20 electrically connected to the transducer 30 to control the transducer 30 to transmit a uniform therapeutically effective dose of ultrasound energy that is sufficient to inactivate conduction of renal nerves in an impact volume 11 (FIG. 2). The apparatus 1 may further include a catheter 18, in which the distal end includes the transducer 30 and the balloon 24, and the proximal end is connected to the actuator 20 through a handle 19.

[0016] Referring to FIG. 2, access to the renal nerves, running between the kidney 25 and the brain may be accomplished by first putting a sheath 12 into the femoral artery (not shown) and the aorta 8 and then feeding the ultrasound transducer 30 through the sheath 12 into the renal artery 10. Once the transducer 30 is in the renal artery 10, the actuator 20 initiates the flow of cooling fluids into the balloon 24, which expands the balloon 24 and generally centers the transducer 30 in the renal artery. The balloon 24 may or may not occlude the flow of blood through the renal artery 10. The actuator 20 then energizes the transducer 30 to emit a predetermined ultrasound energy level that produces the therapeutically effective dose of ultrasound energy within an impact volume 11. The impact volume 11 extends from the transducer to encompass the renal artery 10 and immediate surrounding tissues and the dosage of ultrasound energy within the impact volume 11 affects nerves without damaging renal artery 10 tissue. The renal nerves are not viewable via standard medical techniques available to the user but are known to be located in the outer layers of the renal artery 10, on the surface of the renal artery 10, and adjacent to the renal artery 10. The impact volume 11 may be a 360- degree cylindrical ultrasound field that encompasses the renal artery 10 and the renal nerves, but not other structures of the body.

[0017] Referring to FIG. 3, the actuator 20 may include a user interface 40 (also shown in FIG. 2) connected to a CPU 42. The user interface 40 may include a keyboard and/or other input devices such as, a touch screen (also serving as an output), a remote control, a pointer or a joystick. The actuator 20 may also include an interface to the Internet (not shown) for downloading data. Control data and transducer data to the CPU 42 may come from the user interface 40, the Internet, and/or the catheter 18 (through the Digital I/O 47) as described below. The CPU 42 includes a software program, stored in memory, for generating the ultrasound energy level that produces the therapeutically effective dose of ultrasound energy within the impact volume 11. The CPU may optionally be used to perform one, some, or all of the operations described herein.

[0018] The CPU 42 enables the frequency generator 44 to generate a 10 MHz signal, for example, which is amplified by the RF amplifier 49, transmitted through the RF bridge/sensor 51 , transmitted through the patient interface module 46, and transmitted through the catheter 18 by electrical wires 26 to the transducer 30. The CPU 42 also controls the peristaltic pump 37 (also shown in FIG. 2) using the pulse width module 45 and the digital I/O 47 to pump cooling water from the water source 35 (also shown in FIG. 2) through the catheter 18 to the transducer 30 (further explained below). Feedback signals from the peristaltic pump 37 and the RF bridge/sensor 51 are fed back to the CPU 42 through the analog I/O 43 in order for the CPU 42 to monitor the correct operation of the catheter 18. If there is a problem, the CPU 42 will alert the user through the user interface 40 and may cease the operation of the frequency generator 44 and the peristaltic pump 37.

[0019] The acoustic power level of ultrasound permitted by the ultrasound transducer 30 is dependent on the characteristics of the transducer 30 and the electrical power from the actuator 20 used to excite the transducer 30. Because of the variation of the transducers' 30 electro-mechanical properties and load conditions, the ultrasound emission characteristics of each transducer 30 are different, and the same amount of electrical power from the actuator 20 will not produce the same acoustic power level of ultrasound energy from each transducer 30. In one embodiment of the present invention, the same acoustic power of ultrasound energy from different ultrasound transducers 30 may be accomplished by adjusting the electrical power from the actuator 20 to each ultrasound transducer 30 based on the transducer's characteristics.

[0020] Referring to FIG. 4, the method of adjusting the electrical power from the actuator 20 to the transducer 30 is shown. The method includes the step of connecting a catheter 18 to the actuator 20 (step 50), as known by one skilled in the art. In this embodiment, catheter 18 may include a programmable machine-readable memory element in the handle 19 that stores transducer information that may be read by the CPU 42 through the digital I/O 47. In an alternative embodiment, a human readable element (not shown), such as a bar code or QR code affixed to the catheter handle 19, may be scanned and provided to the CPU 42 through the user interface 40, as known to one skilled in the art. Similarly, the user may take a picture with a mobile device and transmit it to the CPU 42. The transducer information about the specific transducer 30 connected to the catheter 18 may include an ID and other information described below. Alternatively, the readable element may be encoded with a serial number or other information identifying the individual catheter 18, so that the ID and transducer information may be retrieved from a central database accessible through a communication link such as the Internet. The method includes the step of reading the transducer information (step 55), which includes the acoustic characteristics of the transducer 30. The acoustic characteristics of the transducer 30 may include fundamental operating frequency, impedance, and acoustic power output efficiency. The acoustic power output efficiency information may be over a large frequency range. The method includes inserting the transducer 30 into the renal artery (step 60), which may include the method described below. The method may also include measuring the size of the renal artery 10 (step 65), for example as described in Patent Publication '757 in which the transducer 30 transmits and receives an ultrasound ping pulse, the actuator 20 determines the time between the transmitted and received ultrasound ping pulse, and the actuator 20 determines the size of the renal artery 10, as known by one skilled in the art. The size of the renal artery 10 may also be measured by a user with a Doppler imaging ultrasound system, fluoroscopic system, MRI or other imaging modality and manually provided to the actuator 20 through the user interface 40. The size of the renal artery can be a factor in determining the required acoustical power. For example, the required acoustical power for renal artery sizes smaller than 6mm may be half of the acoustical power for renal artery sizes larger than 6mm. Once the size of the renal 10 is determined, the actuator 20 then determines a dosage for the electrical power level based on the acoustic characteristics of the transducer 30 and the renal artery 10 size (step 70). This is determined by the equation:

Electrical Power level = Required acoustical power for the renal artery size ^* Scaling factor

Where: Scaling factor is LC/AC

LC is the lab setup constant for a standardized transducer to produce the required acoustical power for an average renal artery size and AC is the acoustic characteristics of transducer obtained from acoustic scanning with the particular lab setup.

The actuator 20 then actuates the transducer 30 (step 80) in order to produce the required dose of therapeutically effective ultrasound energy that is sufficient to inactivate conduction of renal nerves throughout an impact volume 11 that encompasses the renal artery 10. This is accomplished by providing electrical power for a certain time from the actuator 20 to the transducer 30.

[0021] Similarly, the uniformity (or non-uniformity) of the transducer 30 varies depending on manufacturing tolerances and defects in the material. For example, when the center of the cylindrical transducer 30 is drilled, the center may be shifted causing one side of the transducer 30 to be thicker than the other side. Improving the uniformity of the ultrasound field emitted by the ultrasound transducer 30 over the application time may be accomplished by varying the frequency of the electrical power 26 to the transducer 30. Variations in the transducer 30 may cause the transmitted ultrasound energy field to be non-uniform at a single operating frequency. However, the non-uniformity of the transmitted ultrasound energy field changes when the frequency of the electrical signal 26 into the transducer 30 changes. Therefore, in one embodiment of the present invention, a more uniform ultrasound energy field is accomplished over time by varying the frequency of the electrical signal 26 into the transducer 30. The varying of the frequency may be in a linear progression or may be random.

[0022] In order to determine the range of non-uniformity the transducer 30 may be characterized after the manufacturing process by driving the transducer 30 over its frequency range and sensing the transmitted ultrasound field with a hydrophone. The signal from the hydrophone produces a power density over the frequency range of the transducer 30 and based on this power density, the fundamental frequency and the range of the frequency sweep is determined. The fundamental frequency is the frequency that corresponds to the frequency with the highest power density output (or a small variation of this frequency) and the frequency sweep covers the range of frequencies around the fundamental frequency that produce, for example, at least 90 percent of the highest power density. The fundamental actuation frequency of the transducer 30 and the range of the non-uniformity may be encoded in the machine- readable or human readable element. One skilled in the art would understand that the encoded information may be a code that identifies the fundamental frequency, i.e. the fundamental frequency plus 10 KHz or two times the fundamental frequency, etc. The non-uniformity information may include a frequency sweep range or a sequence of different frequencies to be used, or output power densities at particular frequencies, etc.

[0023 ] For example, using a transducer 30 with a fundamental actuation frequency of 10 MHz, the electrical signal 26 may have a 9 MHz signal that is adjusted by the CPU 42, 10 KHz every 100 msec. This produces ultrasound energy with a frequency sweep between 9 and 11 MHz, which will ensure that all areas of the transducer 30 emit a comparable amount of ultrasound energy over the application time. The frequencies and step sizes are examples only, because the fundamental actuation frequency typically varies somewhat depending on manufacturing tolerances of the transducer 30. For example variations in the thickness of the transducer 30 cause the fundamental actuation frequency to shift.

[0024] Referring to FIG. 5, the method of improving the uniformity of the ultrasound field over the application time is shown. The method includes the step of connecting the catheter 18 to the actuator 20 (step 100). As described above, the catheter 18 may include a programmable memory element that is programmed with transducer information. The transducer information may include the fundamental actuation frequency of the transducer 30 and the transducer 30 non-uniformity range information, which are read by the CPU 42 (step 105). The CPU 42 determines the range of the frequency sweep based on the fundamental actuation frequency and the range of the non-uniformity (step 110). For example, the fundamental frequency may be 9.5 MHz and the range of the non-uniformity may be +-1MHz. The CPU 42 then determines the frequency steps to be used for the application (step 115). This may be simply dividing the frequency range into four or more steps. Four steps would help minimize the non-uniformity due to a boring shift in the center of the transducer. More steps would further minimize the non- uniformity, however the steps need to be long enough in duration to excite the transducer 30. Another arrangement may be that the step containing the fundamental frequency may be a longer duration than the other frequency steps, or that size of the steps are dependent on the power densities in each step range. Next, the transducer is inserted into the renal artery (step 120) and the electrical signal is fed to the transducer at the determined frequency range with the frequency varying based on the determined steps (step 125).

[0025] As described in Patent Publication 757 the impact volume 11 is where nerves and solid tissues are heated to 42°C or more for at least several seconds however all of the solid tissues, including the intima of the renal artery 10, remain well below 65°C. Thus, throughout the impact volume 11 , the solid tissues are brought to a temperature sufficient to inactivate nerve conduction but below that which causes rapid necrosis of the tissues. The selected shape and length of the ultrasoundxtransducer 30, and the frequency, acoustic power and application time of the ultrasound energy produce the required impact volume 11. In order to inactivate the renal nerves, the depth of the impact volume 11 needs to be large enough to reach the renal nerves yet small enough so that other structures of the body are not affected. If all renal arteries and body structures were the same, a consistent treatment could be achieved with a single predetermined impact volume 11. However, not all renal arteries and body structures are the same, and therefore, it may be desirable to adjust the acoustic power, application time, and/or frequency of the ultrasound energy to optimize the impact volume 11 for a particular body structure. As described in Patent Publication 757, the acoustic power (or the application time) may be adjusted to compensate for larger renal arteries 10.

[0026] Referring to FIG. 6, shown is the impact volume 11 impinging on a collateral body structure 28 that is much closer to the renal artery 10 than body structures typical found in patients. The collateral body structure 28 may be the bowels, ureter, or suprarenal glands. In this situation, the collateral body structure 28 is susceptible to damage from the ultrasound energy required for the renal nerve deactivation. One embodiment of the present invention is to increase the frequency of the ultrasound energy to compensate for the closeness of the collateral body structure 28. An increased frequency of the ultrasound energy allows the depth of the impact volume 11 to be shallower. This is because tissue absorbs higher frequency ultrasound energy more readily and therefore, as the ultrasound energy travels through the renal artery 10, it is more readily absorbed and less energy is available to reach the collateral body structure 28 beyond the renal artery 10. The depth of the impact volume 11 is frequency dependent and can be estimated according to the following equation (Sinelnikov et al., 2009, The mechanism of lesion formation by ultrasound ablation catheter. Acoustical Physics Volume 55, 4, 1-12):

Depth = d(f) where f is frequency

Typically, d(f)~ e * a = const > 0

[0027] The present invention accomplishes the adjustment of the frequency of the ultrasound energy to compensate for differences in body structures by modulating the higher harmonics of the transducer 30 for a portion of the application time. Single layer transducers typically emit ultrasound energy at several different resonant frequencies, for example harmonics: f, 2f, 3f, 4f

and lower resonances: f/2, 3f/4 Where:f = the fundamental frequency of the transducer, defined here as a single transducer thickness frequency that produces maximum ultrasound power at given electrical load conditions (constant voltage into 50 Ohms load) produced by actuator system.

[0028] Typically, the power level of each of the lower or higher resonance frequencies is lower than the power level of the first harmonic. Therefore, when using the multiple frequencies, the overall electrical power produced by the actuator may be increased.

[0029] The depth (D) of the impact volume 11 can be described by the bio heat transfer equation (Pennes H.H. Analysis of tissue and arterial blood temperatures in the resting human forearm // J. Appl. Physiol. 1948. V. 1. P. 93-122.) Using the bio-heat transfer equation, it can be seen that increasing the proportion of time that the higher frequency harmonics of the transducer 30 are modulated, or increasing the proportion of the acoustic power level of the higher frequency harmonics of the transducer 30, the depth of the impact volume 11 can be shortened while transmitting a sufficient amount of total energy into the smaller impact volume to achieve renal denervation.

[0030] Referring to FIG. 7, the method of shortening the depth of the impact volume 11 is shown. The method includes inserting the transducer 30 into the renal artery 10 (step 150) and imaging the structures 28 around the renal artery 10 (step 155). This is accomplished by using a Doppler imaging ultrasound system, fluoroscopic system, MRI or other imaging modality. The user then determines if the collateral structures 28 are within the impact volume 11 (step 160). If there are no collateral structures 28 within the impact volume 11 , the fundamental actuation frequency is used (step170); however if there is a collateral structure 28 within the impact depth 11 , then a harmonic frequency is used (step 165). Depending on the closeness of the collateral structure 28, the harmonic and (or) lower resonance frequencies can be used exclusively or can be interwoven with the fundamental frequency. When the harmonic and (or) lower resonance frequencies are interwoven the harmonic or lower resonance frequency of the combined transducer-energization signal may have a 50% duty cycle or some other duty cycle depending on the closeness of the collateral structure 28. In addition, the CPU 42 through the digital I/O 47 and the RF amplifier 49 may adjust the power level of the harmonic and (or) lower resonance frequency. Once the frequency is selected, the electrical signal 26 is fed to the transducer 30 at the selected frequency(ies) (step 175). In an alternative embodiment, the transducer 30 can be formed from a stacked structure of transducer layers, where each layer generates a distinct frequency of ultrasound energy. By activating certain layers of different thicknesses, or a combination thereof, the output frequency can be varied.

[0031] In another aspect of the invention, in order to place the transducer 30 in the renal artery 10, the transducer 30 is fed through the femoral artery and the aorta 8 and then into the renal artery, which branches off the aorta 8 at an angle that may be as much as or more than 90 degrees. Often a guide wire is used to steer a catheter which has a central lumen into an artery. A guide wire, made of a flexible metal construction, is inserted into an artery 10 and the catheter with a central lumen is slipped over the wire. However, in this situation the lumen is in contact with the blood and the transducer, through which the lumen goes through, possibly causes the blood to heat up. In the present invention, the transducer 30 is a closed transducer 30 so that the blood is not heated. When the transducer 30 is closed, a guide wire cannot be used to steer the transducer 30 into the renal artery 10 because the transducer 30 cannot be advanced over the guide wire. Referring to FIG. 8, the closed transducer 30 may be steered into the renal artery 10 by a specially formed catheter 18. In this embodiment of the present invention, the catheter 18 is formed from a shape memory alloy, polymers or metals, such as copper-aluminum-nickel, and nickel-titanium alloys, PEEK polymers, or a combination of alloys, polymers, or metals in a construction which allow the catheter 18 to maintain a shape when molded by the user to the angle that is needed to enter the renal artery 10. The shape memory material allows the catheter 18 to be straightened during insertion into the sheath 12, and upon exiting the sheath 12 at the sheath's distal end, to automatically bend into the renal artery 10. Prior to the procedure, using a Doppler imaging ultrasound system, fluoroscopy or other imaging methods, the user determines the angle that the renal artery 10 comes off the aorta 8. The user then bends the catheter 18 (changing shape of the section with embedded shape memory material) to match this angle. In a process of catheter shaping, the catheter can be activated with radio frequency to achieve temperature or other electro-magnetic induced shape fixation. Next the user feeds the transducer 30 through the sheath 12, which causes the catheter 18 to almost straighten and upon exiting the sheath 12 the transducer 30 begins to bend into the renal artery 10, shown by 30' and 18'. The catheter construction and shape memory material allows the catheter 18 to retain the bend shape after going through the sheath. Alternatively, a manufacturer may produce multiple stock catheters 18 each with a distal end bend of a respective angle and the user may select the one with the appropriate angle.

[0032] In another aspect of the invention, in order to provide improved centering of the ultrasound transducer 30 in the renal artery 10, the present invention may include balloon bubbles 16, as shown in FIG. 9a. The balloon bubbles 16 may be distal and/or proximal to the transducer 30. The balloon bubbles 16 are formed from a membrane of thin Kraton or similar high performance elastomer, such as silicon, latex, silicon rubber, fluoro elastomers, saturated and unsaturated rubbers, and polyethylene. The membrane is constrained by a rigid catheter frame 14, as shown in FIG. 9b. The membrane lies flat while the catheter 18 is maneuvered into the renal artery 10. Once the catheter 18 is positioned in a renal artery 10, cooling water is pumped into the membrane and the pressure from the water causes the membranes to expand through the rigid catheter frame 14 into the balloon bubbles 16, as shown in FIG 9a. The cooling water may be pumped into the membrane as described in Patent Publication 757.

[0033] The maximum balloon pressure can be estimated as follows:

Balloon pressure <= 2 ^* t ^* TS/d,

where t is material thickness of the membrane, TS is material tensile strength of the membrane, and

d is balloon diameter.

For example, for t = 20 microns, d = 5 mm, TS = 20 MPa, the balloon pressure is p <= 23 Psi. One can operate below this pressure by regulating the fluid flow through a catheter 18. This is typically accomplished through catheter 18 design and control of the inlet and outlet pressure for a circulating fluid. In a static case the pressure in the balloon 24 is equal to the inlet pressure. In case of the fluid flow through a catheter 18 and a balloon, the pressure inside a balloon depends on the shaft construction and input pressure. For a catheter 18 design where the inlet and the outlet lumens or pathways are substantially different, the Poiseuille equation can be easily applied to estimate a balloon pressure.

[0034] In another aspect of the present invention, provided is a method of centering the transducer 30 within the artery by incorporating an asymmetrical guide wire lumen within the transducer 30. Normally a guide wire 22 is placed symmetrically at the center of the transducer 30. Referring to FIG. 10, in this embodiment of the present invention, the guide wire 22 is placed through the transducer 30 somewhat away from the center of the transducer 30. This allows the user to rotate the transducer 30 when it is too close to one side of the renal artery 10 and bring it closer to the center 31 of the renal artery 10, as shown by transducer 30'.

[0035] The transducer 30 may be enclosed in a non-circular shaped balloon 24, shown in FIG 1. In its inflated condition, the balloon 24 may engage with the wall of the renal artery 10 and cooling fluid circulated through the balloon 24 may cool the intima of the renal artery 10 and/or the transducer 30. The balloon 24 may be sized to occlude the blood flow through the renal artery or may be sized to allow the blood to continue to flow through the renal artery. In order to provide cooling fluid to the balloon 24, the peristaltic pump 37 and the water source 35 (in FIG. 2 and 3) are connected to an input lumen 4 within the catheter 18. Referring to FIG. 11 , in order to provide a water pressure inside the balloon 24 that is large enough so that the balloon 24 can withstand compression from the renal artery 10 yet small enough that the balloon 24 does not over expand and burst, one embodiment of the present invention uses an asymmetric flow of water. In this embodiment, a single inlet lumen 4 brings cooling fluid to the balloon 24, but two outlet lumens 6 bring cooling fluid from the balloon 24. Although this embodiment doubles the outlet flow of cooling fluid, which is typically water, by using two outlet lumens 6, one skilled in the art would understand that the asymmetric flow passages of cooling fluids may be accomplished a number of different ways, for example, a single outlet lumen 6 of twice the size of the inlet lumen 4 may be used. In addition, the asymmetry of the water flow passages may be three or more times greater. As an example, in a catheter where the accumulative area of outlet flow is about 3 times the area of inlet flow, the result is a pressure differential predominantly occurring over inlet length from 30 - 40 Psi at inlet down to 5-6 Psi in catheter balloon. The rest of the pressure differential gradually goes to zero over the larger accumulative cross section outlet water passages length.

[0036] Referring to FIG. 12, in another aspect of the present invention, cooling fluid (e.g. water) from the inlet lumen 4 in the catheter 18 flows through an inner tube 32 of the transducer 30. The inner tube 32 is closed at its distal end by a bead of glue 38 and the cooled fluid is released through a strategically located distal opening 34 in the inner tube 32. The opening 34 is strategically located at the distal end of the transducer 30 away from the outlet lumens 6 in the catheter 18 at the proximal end of the transducer 30. This allows a unidirection uniform flow of the cooling water, or other fluid, across the transducer 30 and between the backing tube 36 and the transducer 30 to the outlet lumen 6, as depicted by the arrows.

[0037] Furthermore, various embodiments described herein or portions thereof can be combined without departing from the present invention. Each embodiment may be incorporated into the apparatus 1 or all of the embodiments may be incorporated into the apparatus 1. For example, the uniformity of the ultrasound field may cause the electrical signal 26 to have a frequency sweep over a certain range, where the center frequency of the range is a harmonic and (or) lower resonance frequency of the fundamental frequency in order to achieve a shallower impact volume 11. Another variation may be that frequency sweep in the electrical signal 26 to the transducer 30 may be achieved via a continuous change rather than in steps. In addition, the acoustic power level from the transducer 30 may be adjusted for each frequency step in the range. This may be accomplished by changing the scaling factor described above in accordance with the variation in the frequency of the electrical signal fed to the transducer.