Abstract:
A water pillow for coupling acoustic energy into tissue. The pillow is configured to conform to a transducer to facilitate coupling of ultrasound energy. The pillow includes a pouch that accommodates the transducer to enable a snug fit between the pillow and the transducer. The pillow includes a liquid inlet and outlet to facilitate liquid circulation for cooling, is biocompatible, has a low attenuation, is conformal to the shape of the transducer, facilitates use of an adjustable pressure to achieve various standoffs, includes an integral pouch to facilitate an interference fit to the transducer, facilitates water circulation for cooling, is sterilizeable, and is disposable. Either the surface of the pillow adapted to conform to a tissue interface, or the surface of the pillow adapted to conform to the transducer, or both, can include pores configured to weep liquid to facilitate acoustic coupling of the pillow with the transducer and/or tissue.

Description:
RELATED APPLICATIONS  
       [0001]     This application is based on a prior copending provisional application, Ser. No. 60/610,451, filed on Sep. 16, 2004, the benefit of the filing date of which is hereby claimed under 35 U.S.C. § 119(e). The present application is further a continuation-in-part application of patent application Ser. No. 10/977,339, filed Oct. 29, 2004, the benefit of the filing date of which are hereby claimed under 35 U.S.C. § 120. 
     
    
     GOVERNMENT RIGHTS  
       [0002]     This invention was funded at least in part with grant No. 2-R42-HD38440-02 from the National Institutes of Health, grant N00014-01-96-0630 from the Department of the Navy, and grant N00014-01-G-0460, also from the Department of the Navy. The U.S. government may have certain rights in this invention. 
     
    
     BACKGROUND  
       [0003]     Ultrasound is widely used for imaging a patient&#39;s internal structures without risk of exposure to potentially harmful radiation, as may occur when using X-rays for imaging. An ultrasound examination is a safe diagnostic procedure that uses high frequency sound waves to produce an image of the internal structures of a patient&#39;s body. Many studies have shown that these sound waves are harmless and may be used with complete safety, even to visualize the fetus in pregnant women, where the use of X-rays would be inappropriate. Furthermore, ultrasound examinations generally require less time than examinations using other imaging techniques, and ultrasound examinations are typically less expensive than examinations using other imaging techniques.  
         [0004]     More recently, the use of high intensity focused ultrasound (HFU) for therapeutic purposes, as opposed to imaging, has received significant attention in the medical community. HIFU therapy employs ultrasound transducers that are capable of delivering 1,000-10,000 W/cm 2  to a focal spot, in contrast to diagnostic imaging ultrasound, where intensity levels are usually below 0.1 W/cm 2 . A portion of the energy from these high intensity sound waves is transferred to a targeted location as thermal energy. The amount of thermal energy thus transferred can be sufficiently intense to cauterize undesired tissue, or to cause necrosis of undesired tissue (by inducing a temperature rise greater than about 70° C.) without actual physical charring of the tissue. Tissue necrosis can also be achieved by mechanical action alone (i.e., by cavitation that results in mechanical disruption of the tissue structure). Further, where the vascular system supplying blood to an internal structure is targeted, HIFU can be used to induce hemostasis. The focal region of this energy transfer can be tightly controlled so as to obtain necrosis of abnormal or undesired tissue in a small target area without damaging adjoining normal tissue. Thus, deep-seated tumors can be destroyed with HIWU without surgical exposure of the tumor site.  
         [0005]     An important component in any type of ultrasound therapy system is the mechanism for coupling the acoustic energy into the tissue. Good acoustic coupler is necessary to efficiently transfer the ultrasound energy from the transducer to the treatment site. The ideal acoustic coupler is a homogenous medium that has low attenuation, and an acoustic impedance similar to that of the tissue being treated. Due to its desirable acoustic transmission characteristics, water has commonly been used as the coupling medium in many therapeutic applications of ultrasound.  
         [0006]     In previous hemostasis studies in which HIWU has been used to arrest bleeding of injured blood vessels and organs, the HIFU transducer was contained within a water-filled, conical, plastic housing with a thin, polyurethane membrane at the tip. This coupler was designed for superficial treatments, since it places the HIFU focus only several millimeters beyond the tip of the water-filled cone. While this coupling method has been useful for hemostasis experiments, it has many drawbacks that would make it impractical for a clinical setting. These disadvantages include requirements for degassing, sterilization, and circulation and containment issues. Due to the limitations of the current HIFU applicators, an alternative coupling medium is desirable.  
         [0007]     Latex condoms have been used as a disposable sheath for rectal and vaginal ultrasound probes, and when filled with water, such sheaths facilitate acoustic coupling. With respect to HIFWU therapy probes, a HIWU transducer can generate significant amounts of heat, which should be dissipated to protect the patient and to prolong the life of the transducer. Latex condoms, and ultrasound probe sheaths specifically intended for such ultrasound coupling, are not designed to facilitate circulation of a cooling liquid.  
         [0008]     It would be desirable to provide a disposable acoustic coupler for use with ultrasound probes, configured to circulate a cooling liquid proximate an ultrasound transducer.  
       SUMMARY  
       [0009]     Disclosed herein is an acoustic coupler that facilitates acoustically coupling an acoustic device (such as an ultrasound imaging transducer or an ultrasound therapy transducer) to a physical mass, where the acoustic device is configured to direct acoustic energy into or through the physical mass. The physical mass is generally biological tissue, although no limitation is implied with respect to the type of physical mass to which the acoustic energy is directed. The acoustic coupler includes a liquid chamber (e.g., a water pillow) having a first surface configured to conform to a transducer, and a second surface configured to conform to the physical mass into which the acoustic energy provided by the transducer is to be directed. The liquid chamber further includes a liquid inlet configured to be coupled to a liquid supply, and a liquid outlet configured to be coupled to a discharge volume. A pump can be used to circulate liquid through the liquid chamber to dissipate heat generated by the transducer. Generally, the liquid chamber is filled with water (or saline solution), although the use of water in this embodiment should not be considered to be a limitation on the concept.  
         [0010]     Preferably, the first surface (configured to conform to the transducer) provides cooling to the front face of the transducer. The acoustic coupler further preferably exhibits one or more of the following characteristics: the acoustic coupler is formed from biocompatible materials, the acoustic coupler exhibits a low attenuation, a liquid pressure in the liquid chamber of the acoustic coupler can be varied to achieve various standoffs, the acoustic coupler is formed from materials that can be sterilized, and the acoustic coupler device can be viewed as disposable.  
         [0011]     In one implementation, the acoustic coupler includes a pouch coupled to the liquid chamber. The pouch defines an open-ended volume configured to receive a transducer, so that the pouch can be used to attach the acoustic coupler to the transducer. Preferably, the pouch is formed from a flexible and elastomeric material, such that the acoustic coupler is attached to the transducer via an interference fit. The dimensions of the pouch can be varied to accommodate specific transducer configurations. In some embodiments, the pouch is configured to encompass the transducer and at least a portion of a handle or probe supporting the transducer.  
         [0012]     The liquid inlet and liquid outlet of the liquid chamber are preferably configured to enhance a circulation of liquid in the liquid chamber. Various different configurations can be empirically tested to determine the effectiveness of a specific configuration. Exemplary configurations include disposing the liquid inlet substantially adjacent to the liquid outlet, as well as disposing the liquid inlet substantially opposed to the liquid outlet. In some embodiments, the liquid inlet and liquid outlet are separated by an acute angle. In some embodiments, the liquid inlet and liquid outlet are separated by an angle of between about 40° and about 100°.  
         [0013]     At least one of the first surface (configured to conform to the front face of the transducer), and the second surface (configured to conform to the physical mass) can include at least one opening to allow liquid from the liquid chamber to wet the surface, thereby enhancing the acoustic coupler between the surface and the transducer and/or the physical mass. Preferably, a surface configured to “weep” liquid to facilitate coupling will include a plurality of small pores that release sufficient liquid to facilitate coupling, without releasing excessive amounts of liquid. Various different agents can be added to the liquid used to inflate the liquid chamber, to be released through such pores. Such agents can include ultrasound contrast agents, therapeutic agents, and sterilization agents.  
         [0014]     A system configured to be used with such acoustic couplers preferably includes a liquid supply, a pump configured to circulate the liquid, and a cooling unit configured to thermally condition the liquid. A degassing unit, such as a second pump, is preferably included to remove gas bubbles from the liquid. In some embodiments, a liquid line coupling the liquid supply to the liquid inlet is larger than a liquid line coupling the liquid outlet to the liquid supply, to increase a flow resistance on the outlet of the liquid chamber relative to its inlet.  
         [0015]     A related method for acoustically coupling a transducer to a physical mass while providing cooling to the transducer includes the steps of positioning a liquid chamber between the physical mass and the transducer, introducing a liquid into the liquid chamber, such that a first surface of the liquid chamber conforms to the transducer and a second surface of the liquid chamber conforms to the physical mass, and introducing additional liquid into the liquid chamber, thereby establishing a circulating flow of liquid that absorbs heat from the transducer, providing cooling to the transducer. Additional method steps relate to releasing a portion of liquid disposed within the liquid chamber from at least one of the first surface and the second surface to facilitate acoustically coupling the surface to the transducer and/or physical mass. Yet another method step relates to securing the liquid chamber to the transducer using a pouch defining an open-ended volume configured to achieve a snug fit with the transducer.  
         [0016]     This Summary has been provided to introduce a few concepts in a simplified form that are further described in detail below in the Description. However, this Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     DRAWINGS  
       [0017]     Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0018]      FIG. 1A  schematically illustrates a first embodiment of an acoustic coupler including a liquid chamber configured to acoustically couple a transducer to a physical mass, and to circulate a cooling liquid to cool the transducer, as well as a pouch configured to attach the acoustic coupler to the transducer;  
         [0019]      FIG. 1B  schematically illustrates an exemplary acoustic device including a transducer and a handle;  
         [0020]      FIG. 1C  schematically illustrates an acoustic coupler in  FIG. 1A  that is attached to an acoustic device of FIGURE lB;  
         [0021]      FIGURE 1D  is an enlarged view of the liquid chamber portion of the acoustic coupler of  FIG. 1A , illustrating that the liquid chamber includes a first surface configured to conform to the transducer, and a second surface configured to conform to a physical mass into which acoustic energy from the transducer is to be directed;  
         [0022]      FIG. 2A  schematically illustrates a second embodiment of an acoustic coupler configured to be attached to a different type of acoustic device;  FIG. 2B  schematically illustrates another exemplary acoustic device including a transducer and a handle;  
         [0023]      FIG. 2C  schematically illustrates the acoustic coupler of  FIG. 2A  attached to the exemplary acoustic device of  FIG. 2B ;  
         [0024]      FIGS. 2D and 2E  are views of the liquid chamber portion of the acoustic coupler of  FIG. 2A , illustrating that an opening or pores can be provided in a wall of the liquid chamber to enable liquid from the liquid chamber to be released, to enhance an acoustic coupler between the liquid chamber and at least one of the transducer and the physical mass;  
         [0025]      FIG. 2F  schematically illustrates an alternative configuration for a liquid inlet and a liquid outlet portion of the liquid chamber of  FIG. 2A , wherein an angle of about 100° is defined between the liquid inlet and liquid outlet;  
         [0026]      FIG. 2G  schematically illustrates an alternative configuration for a liquid inlet and a liquid outlet portion of the liquid chamber of  FIG. 2A , wherein an angle of about 40° is defined between the liquid inlet and liquid outlet;  
         [0027]      FIG. 2H  is an image of the acoustic coupler of  FIG. 2A  attached to the acoustic device of  FIG. 2B , illustrating how a liquid pressure in the liquid chamber can be varied to achieve a different standoff;  
         [0028]      FIG. 3  is a functional block diagram of a system including an acoustic coupler, such as illustrated in  FIGS. 1A and 2A , a circulation pump, a liquid reservoir, a cooling unit, a flow meter, and a degassing unit;  
         [0029]      FIG. 4A  schematically illustrates another exemplary acoustic device, including a generally spoon shaped transducer, and a handle;  
         [0030]      FIG. 4B  illustrates additional details for the structure of the acoustic device of  FIG. 4A ;  
         [0031]      FIG. 4C  is an image of the acoustic device of  FIG. 4A  and yet another embodiment of an acoustic coupler;  
         [0032]      FIG. 4D  is an image of the acoustic coupler configured to be used with the acoustic device of  FIG. 4A ;  
         [0033]      FIG. 4E  is an image of the acoustic coupler of  FIG. 4D  attached to the acoustic device of  FIG. 4A ;  FIG. 5A  schematically illustrates yet another embodiment of an acoustic coupler being used to acoustically couple the transducer of an acoustic device to tissue;  
         [0034]      FIG. 5B  is an enlarged view of the acoustic coupler of  FIG. 5A , illustrating that the pouch portion of the acoustic coupler is disposed in a central portion of the acoustic coupler, such that when the acoustic coupler is attached to an acoustic device, the acoustic coupler substantially encompasses the acoustic device, proximate to the transducer;  
         [0035]      FIG. 6  is a chart illustrating that exemplary acoustic couplers as described herein can achieve satisfactory cooling of a HIFU transducer; and  
         [0036]      FIG. 7  graphically illustrates the percentage of heat removed by an acoustic coupler including a liquid chamber, as a function of a focal depth of the transducer. 
     
    
     DESCRIPTION  
       [0000]     Figures and Disclosed Embodiments Are Not Limiting  
         [0037]     Exemplary embodiments are illustrated in referenced Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive.  
         [0038]      FIG. 1A  schematically illustrates a first embodiment of an acoustic coupler  10  including a liquid chamber  12  configured to acoustically couple a transducer to a physical mass and to circulate a cooling liquid to cool the transducer, as well as a pouch  18  configured to attach the acoustic coupler to the transducer. Liquid chamber  12  includes a liquid inlet  14  and a liquid outlet  16 . Liquid, such as water or a saline solution, can be introduced into liquid chamber  12  via liquid inlet  14 . Note that pouch  18  defines an open-ended volume that is sized and shaped to accommodate a transducer. Preferably, pouch  18  is formed from a flexible (elastomeric) material so that acoustic coupler  10  can be attached to a transducer (or an acoustic device including a transducer) by an interference fit between the acoustic coupler and the device.  
         [0039]      FIG. 1B  schematically illustrates an exemplary acoustic device  19  including a transducer  20 , a handle  22 , and a lead  21  that couples the transducer to a power supply (not shown). In general, transducer  20  will be a HIFU therapy transducer configured to emit ultrasound waves sufficiently energetic to induce a therapeutic effect at a treatment site. The terms “therapeutic transducer,” “HIFU transducer,” and “high intensity transducer,” as used herein, all refer to a transducer that is capable of being energized to produce ultrasonic waves that are much more energetic than the ultrasonic pulses produced by an imaging transducer and which can be focused or directed onto a discrete location, such as a treatment site in a target area. Such transducers generally generate more heat during use than ultrasound imaging transducers. Thus, HIFU transducers have a greater need for cooling than do imaging transducers. However, while the acoustic couplers disclosed herein are expected to be particularly beneficial when used with a HIFU transducer, it should be recognized that these acoustic couplers are not limited for use in connection with any specific transducer. Furthermore, it should be recognized that the acoustic couplers disclosed herein can be configured to be used with many different shapes of transducers, and with many different acoustic devices incorporating transducers.  
         [0040]      FIGURE 1C  schematically illustrates acoustic coupler  10  of  FIG. 1A  attached to acoustic device  19  of  FIGURE 1B . Note that transducer  20  is encompassed by the open-ended volume of pouch  18 . As illustrated in FIGURE ID, when acoustic coupler  10  is properly positioned relative to acoustic device  19 , and liquid chamber  12  is filled with a liquid, a surface  24  of liquid chamber  12  substantially conforms to transducer  20 . It should be recognized that there exists significant variation in acoustic devices incorporating transducers. For example, in some acoustic devices (particularly devices configured for HIFU therapy), a front face of the transducer is exposed. When acoustic coupler  10  is used with an acoustic device including a transducer whose front face is exposed, surface  24  will generally conform to the front face of the transducer. Some acoustic devices include a transducer to which an optional lens  20   a  (see  FIGURE 1D ) has been attached. For example, aluminum lenses are sometimes employed to achieve better focusing of a HIFU transducer. When acoustic coupler  10  is used with an acoustic device including a transducer to which such a lens has been coupled, surface  24  will generally conform to the front face of the lens. Because such lenses normally exhibit good heat transfer properties, cooling the lens will also cool the underlying transducer. Furthermore, some acoustic devices incorporate a relatively thin housing to enclose a transducer. When acoustic coupler  10  is used with an acoustic device in which the transducer is covered by such a housing, surface  24  will generally conform to the housing, covering the transducer. Again, such housings are generally formed from a relatively thin material, so that cooling the housing proximate the transducer will also cool the underlying transducer. Therefore, it should be recognized that in respect to the following disclosure and the claims that follow, where terms such as “substantially conforms to a transducer,” “substantially conforms to the transducer,” “conforms to a transducer,” and “conforms to the transducer” are employed, such language is intended to encompass the ability of the acoustic coupler to conform to a lens that is coupled to the transducer, as well as conforming to a housing encapsulating the transducer.  
         [0041]     Referring once again to  FIGURE 1D , liquid chamber  12  further includes a surface  26  configured to conform to a physical mass  28  into which acoustic energy from the transducer is to be directed. Either or both surfaces  24  and surface  26  can be coated with mineral oil (or some other coupling medium or coupling gel) to ensure adequate acoustic coupling is achieved.  
         [0042]     As noted above, the acoustic couplers disclosed herein preferably exhibit one or more of the following characteristics: biocompatibility, low attenuation, variability of liquid pressure in the liquid chamber enabling different standoffs to be achieved, sterilizability, and disposability. The acoustic couplers disclosed herein can be formed of a polymer material, such as polyurethane. Such a material is biocompatible, sterilizable, has a low attenuation, is flexible (so that the material readily conforms to a physical mass as well as to a transducer, or lens or housing as noted above), and is able to achieve an interference fit with an acoustic device (where dimensions of the pouch have been selected to accommodate a particular form factor for an acoustic device). In some embodiments, the pouch is oversized, so that a wider variation of form factors can be accommodated. In such embodiments, additional attachment mechanisms can be employed, such as straps, couplings, or elastic bands, to secure the acoustic coupler to the acoustic device.  
         [0043]      FIG. 2A  schematically illustrates a second embodiment of an acoustic coupler configured to be attached to a different type of acoustic device. Acoustic coupler  10   a  includes a liquid chamber  12   a , a liquid inlet  14   a , a liquid outlet  16   a , and a pouch  18   a . Once again, pouch  18   a  defines an open-ended volume configured to receive a transducer. Note that the size and shape of pouch  18   a  of  FIG. 2A  is different than the size and shape of pouch  18  in  FIG. 1A , because acoustic coupler  10   a  ( FIG. 2A ) is configured to be used with a different acoustic device than is acoustic coupler  10  ( FIG. 1A ).  
         [0044]      FIG. 2B  schematically illustrates an exemplary acoustic device  19   a  including a transducer  20   a , a handle  22   a , and a lead  21   a  that couples the transducer to a power supply (not shown). Once again, transducer  20   a  will likely be a HIFU therapy transducer, although this exemplary use of the acoustic couplers with HIFU transducers is not intended to represent a limitation.  
         [0045]      FIG. 2C  schematically illustrates acoustic coupler  10   a  of  FIG. 2A  attached to acoustic device  19   a  of  FIG. 2B . Note that transducer  20   a  is substantially encompassed by the open-ended volume of pouch  18   a . As illustrated in  FIG. 2D , acoustic coupler  10   a  includes a surface  24   a  configured to conform to a transducer (or to a lens attached to a transducer, or to a housing enclosing a transducer, generally as described above). Acoustic coupler  10   a  further includes a surface  26   a  configured to conform to a physical mass (not shown) into which acoustic energy from the transducer is to be directed. It should be noted that pouch  18   a  not only encompasses transducer  20   a , but also a portion of handle  22   a  supporting transducer  20   a . Preferably, pouch  18   a  has dimensions selected to accommodate the form factor of acoustic device  19   a , so that an interference fit is achieved when transducer  20   a  and the portion of handle  22   a  supporting transducer  20   a  are introduced into pouch  18   a . As noted above, some embodiments will incorporate an oversized pouch, to accommodate a wider range of acoustic device form factors, and then use other attachments to secure the acoustic coupler to the acoustic device.  
         [0046]     Referring once again to  FIG. 2D , surface  26   a  can optionally include a least one opening configured to enable a liquid in liquid chamber  12   a  to wet surface  26   a , thereby enhancing the acoustic coupler between surface  26   a  and the physical mass. While not specifically shown, it should be recognized that such openings can also be beneficially incorporated into surface  24   a , to similarly enhance the acoustic coupler between surface  24   a  and the transducer (or a lens covering the transducer, or a housing covering the transducer, generally as described above). Preferably, the size, shape, and location of such openings are selected to generate a thin layer of liquid on the surface without releasing substantially more liquid than is required to facilitate coupling. In some specific applications, it may be desirable to continually flush the physical mass to which the acoustic device is being coupled with a liquid. (For example, an acoustic device configured to provide emergency treatment in field conditions may incorporate openings providing sufficient liquid flow to continually flush the physical mass.) In such embodiments, the openings will be sized, shaped, and oriented to achieve the desired flow.  FIG. 2E  schematically illustrates a liquid chamber  12   b , which includes a plurality of pores (generally indicated by an arrow  30   a ) configured to wet a surface  26   b  with liquid flowing from liquid chamber  12   b . Particularly in embodiments where the wall or surface of the liquid chamber configured to couple with the physical mass toward which the acoustic device is directing acoustic energy includes at least one opening, that opening can be used to deliver a plurality of different agents to the surface. Such agents can include ultrasound contrast agents, therapeutic agents, and sterilization agents. Particularly with respect to the acoustic device configured to provide emergency treatment under field conditions noted above, disinfectant agents can be introduced into the liquid to prevent potential infections.  
         [0047]     Referring once again to acoustic coupler  10  of  FIG. 1A , note that liquid inlet  14  and liquid outlet  16  are substantially opposite each other (i.e., an angle of about 180° is defined between them). In contrast, liquid inlet  14   a  and liquid outlet  16   a  of acoustic coupler  10   a  ( FIG. 2A ) are substantially adjacent (i.e., they are separated by an angle of about 0°). It should be recognized that the relative orientations of the liquid inlet and the liquid outlet can effect a circulation of the liquid within the liquid chamber, which in turn, can affect the ability of the circulating liquid to dissipate heat generated by the transducer. The liquid inlet and liquid outlet of the liquid chamber are preferably configured to enhance a circulation of liquid in the liquid chamber. Various different configurations can be empirically tested to determine the effectiveness of a specific configuration. Exemplary configurations include disposing the liquid inlet substantially adjacent to the liquid outlet (as exemplified by acoustic coupler  10   a  of  FIG. 2A ), as well as disposing the liquid inlet substantially opposite to the liquid outlet (as exemplified by acoustic coupler  10  of  FIG. 1A ). In some embodiments, the liquid inlet and liquid outlet are separated by an acute angle, as is schematically illustrated in  FIG. 2D  by a liquid inlet  13   a  and a liquid outlet  15   a , where an acute angle  17   a  separates the liquid inlet from the liquid outlet. In some embodiments, the liquid inlet and liquid outlet are separated by an angle of between about 40° to about 100°.  FIG. 2F  schematically illustrates an angle  17   b  of about 100°, while  FIG. 2G  schematically illustrates an angle  17   c  of about 40°, either of which can be used as a separation angle between the liquid inlet and the liquid outlet (as well as any angle in between). Empirical studies with acoustic coupler  10  and acoustic coupler  10   a  have indicated that acoustic couplers having a liquid inlet that is substantially opposite to the liquid outlet exhibit higher heat removal rates. However, the difference between the heat removal rate for acoustic coupler  10  and acoustic coupler  10   a  was not significant for small, relatively low-power transducers, and only becomes significant when relatively larger transducers, generating more heat, are employed.  
         [0048]      FIG. 2H  is an image of acoustic coupler  10   a  of  FIG. 2A  attached to acoustic device  19   a  of  FIG. 2B , illustrating how a liquid pressure in liquid chamber  12   a  can be varied to achieve a different standoff. When a first liquid pressure is used, liquid chamber  12   a  inflates to a maximum height indicated by a line  23 . Increasing the amount of liquid forced into the liquid chamber increases the pressure and causes liquid chamber  12   a  to inflate further (generally as indicated by a curve  25 ), so that it exhibits a different maximum height, generally as indicated by a line  27 . Variation of the liquid pressure in the liquid chamber can thus be used to achieve different standoffs.  
         [0049]      FIG. 3  is a functional block diagram of a system  32  including an acoustic coupler  10 b (such as illustrated in  FIGS. 1A and 2A ), a circulation pump  36 , a liquid reservoir  34 , a cooling unit  35 , a flow meter  37 , a temperature sensor  39 , and an optional (but preferred) degassing unit  38 . The function of circulation pump  36  is to provide a motive force for establishing a circulating liquid within the liquid chamber of acoustic coupler  10 b. The function of liquid reservoir  34  is to provide a supply of the circulatory liquid, such as water or saline solution, and to receive cooled liquid from cooling unit  35 . Note cooling unit  35  can be beneficially implemented using a combination fan and heat exchanger. Those of ordinary skill in the art will recognize that many types of cooling units can be employed, such as thermoelectric coolers and other types of electromechanical chillers.  
         [0050]     It should be understood that the relative position of the cooling unit is not important. For example,  FIG. 3  indicates that the cooling unit is coupled in fluid communication with the liquid outlet of the acoustic coupler, such that the liquid returning from the acoustic coupler is cooled before being returned to the liquid supply for recirculation. An alternative configuration would be to position the cooling unit between the liquid supply and the liquid inlet of the acoustic coupler, such that the liquid is cooled before it is introduced into the liquid volume, rather than being cooled after it has exited the liquid volume.  
         [0051]     Temperature sensor  39  can be disposed in a number of different locations. A temperature sensor can be introduced into the liquid chamber of acoustic coupler  10 b, or into one or both of the liquid inlet and liquid outlet of acoustic coupler  10 b. The purpose of the temperature sensor is to monitor the temperature of the circulating liquid, to determine if additional cooling or additional chilled liquid flow are required to sufficiently cool the transducer. Flow meter  37  can be implemented using one or more valves configured to enable a flow rate to be varied, and preferably includes a meter providing an indication of a current flow rate, as well as the one or more valves to control it. In an empirical system developed to test the concept disclosed herein, degassing unit  38  was implemented using an additional pump. Those of ordinary skill in the art will recognize that other conventional degassing techniques can be employed. In the empirical system noted above, a flow resistance on the liquid outlet of the acoustic coupler was increased relative to that of the liquid inlet, by providing a larger liquid line coupling the liquid inlet of the acoustic coupler to the liquid supply than was provided to couple the liquid outlet of the acoustic coupler to the cooling unit.  
         [0052]      FIG. 4A  schematically illustrates another exemplary acoustic device  40 , which includes a generally spoon shaped transducer  42  and a handle  44 . Transducer  42  is a phased array transducer including  11  different transducer elements, six of which have complete annuli, and five of which have truncated annuli. Transducer  42  exhibits a focal range of about 3-6 cm.  
         [0053]      FIG. 4B  illustrates additional details of transducer  42 , clearly showing the plurality of different emitter elements that are included therein. Generally spoon-shaped transducer  42  includes  11  discrete emitter elements, all equal in area, each element being separated from its neighbors by about 0.3 mm. Six of the emitter elements have complete annuli, and five emitter elements have truncated annuli. The overall transducer dimensions are about 35 mm×60 mm. Generally spoon-shaped transducer  42  has a center frequency of around 3 MHz, a focal length of about 3-6 cm, a geometric focus of about 5 cm, and a maximum focal intensity of about 3000 W/cm 2 .  
         [0054]      FIG. 4C  is an image of acoustic device  40  of  FIG. 4A , and yet another embodiment of an acoustic coupler. An ultrasound imaging probe  54  is coupled to acoustic device  40  (a HIFU therapy probe) via a mechanical coupling  57 , to facilitate simultaneous imaging and therapy. An image plane  54   a  is provided by imaging probe  54 , and acoustic device  40  provides a highly focused acoustic beam  42   a . An acoustic coupler  46  has been attached to transducer  42 . Acoustic coupler  46  includes a liquid inlet  50 , a liquid outlet  52 , a liquid chamber  49 , and an open-ended pouch  48  (having a size and shape selected to correspond to a size and shape of transducer  42 ). Acoustic coupler  46  is attached to transducer  42  via an interference fit provided by pouch  48 .  
         [0055]     In an empirical study, acoustic coupler  46  was fashioned out of polyurethane, and a pump circulated water over the face of transducer  42  at a rate of approximately 60 ml/min. That rate was determined empirically to avoid over-inflation of the liquid chamber, and ensure that the transducer temperature did not rise above 40° C. Water was selected as the cooling medium, due to its ease of handling and its effectiveness as a transducer coolant.  
         [0056]      FIG. 4D  is an image of acoustic coupler  46  before being attached to transducer  42 , more clearly showing liquid inlet  50 , liquid outlet  52 , and pouch  48 .  FIG. 4E  is an image of acoustic coupler  46  of  FIG. 4D  attached to acoustic device  40  of  FIG. 4A , illustrating that liquid chamber  49  includes a surface  58  configured to conform to transducer  42 , and a surface  56  configured to conform to a physical mass into which transducer  42  emits highly focused beam  42   a.    
         [0057]      FIG. 5A  schematically illustrates yet another embodiment of an acoustic coupler being used to acoustically couple a transducer of an acoustic device to tissue. An acoustic device  80 , including a therapeutic transducer  82 , has been introduced into a body cavity and is positioned such that a highly focused acoustic beam  88 , generated by transducer  82 , can provide therapy to a target  86  in tissue  63 . Prior to introducing acoustic device  80  into the body cavity, an acoustic coupler  60  is attached to the acoustic device. Acoustic coupler  60  similarly includes a liquid chamber  62 , a pouch  64 , a liquid inlet  66 , and a liquid outlet  68 . Significantly, dimensions of pouch  64  have been selected such that substantially the entire mass of a distal end of acoustic device  80  (including transducer  82 ) can be introduced into pouch  64 . As is illustrated by  FIG. 5B , acoustic coupler  60  includes a surface  70 , which both defines the open-ended volume of pouch  64 , and is configured to conform to the shape and size of transducer  82 . Acoustic coupler  60  also includes a surface  72 , which both defines an outer extent of the acoustic coupler and is configured to conform to tissue  63 . Pouch  64  can be configured to achieve an interference fit with the distal end of acoustic device  80 . Particularly when used as indicated in  FIGURE 5A , inflation of liquid chamber  62  should secure the acoustic coupler to the acoustic device, as well as securing the combination of the acoustic coupler and the acoustic device in the body cavity, even when the dimensions of the pouch are not sufficient to achieve an interference fit.  
         [0058]      FIG. 6  is a chart illustrating the performance of empirical acoustic couplers generally consistent with acoustic coupler  10  of  FIG. 1A  (labeled “opposite” in  FIG. 6 ) and acoustic coupler  10   a  of  FIG. 2A  (labeled “adjacent” in  FIG. 6 ). Both configurations of acoustic couplers were able to remove the heat generated by either a 3.5 MHz transducer or a 5.0 MHz transducer.  
         [0059]      FIG. 7  graphically illustrates the percentage of heat removed by an acoustic coupler (including a liquid chamber) relative to a focal depth of the transducer, indicating that increasing flow rates enable higher percentages of heat to be removed.  
         [0060]     Although the present invention has been described in connection with the preferred form of practicing it and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made to the present invention within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.