Patent Publication Number: US-2012029393-A1

Title: Compact ultrasound transducer assembly and methods of making and using the same

Description:
BACKGROUND 
     The invention relates to an ultrasound transducer assembly, and more particularly to a compact ultrasound transducer assembly having adjustable focal depth, and methods of making and using the same. 
     Ultrasound refers to acoustic waves having a frequency above the upper limit of the human audible range (i.e., above 20 kHz). In ultrasonic applications (such as imaging and delivering therapy), acoustic waves are generated by a transducer and directed towards a region of interest. It is often desirable to focus the acoustic waves at different focal depths within a target (such as a patient). The different focal depths may be located within the same region of interest, or may be present in different regions of interest within the target. 
     Conventionally, while providing therapy, therapy transducers are mechanically focused using shaped piezoelectric transducers. Such transducers are typically spherical in shape and are bulky and expensive to design and manufacture due to the complexity involved in shaping and surface-finishing of non-flat piezoelectric materials. The spherical shape is necessary in order to focus the ultrasonic energy at a particular depth. For integrated imaging and therapeutic ultrasound transducers, it is desirable to locate the imaging transducer at the center of the therapy transducer. However, the depth of the sphere of the therapy transducer results in an increased distance between the apex and the contact face of the therapy transducer. Thus, for a spherical shaped transducer, the imaging transducer is located at a relatively long distance from the focal point of the therapy transducer, resulting in sub-optimal image quality. In addition, a spherical single element transducer has a fixed focal depth determined by the transducer geometry, i.e. it is not possible to change the focal depth. In order to vary the focal depth of a spherical transducer, it is possible to use an annular array of transducer elements, however, this greatly increases the cost and complexity of the transducer and system. Finally, the large dome-shaped transducer requires a large, bulky housing that is cumbersome to handle. 
     Therefore, it would be desirable to provide an ultrasound transducer assembly that provides a compact and cost-effective solution for adjusting a focal depth. 
     BRIEF DESCRIPTION 
     In one embodiment, a compact ultrasound transducer assembly is provided. The ultrasound transducer assembly comprises a compact housing, a therapy transducer disposed in the compact housing, and an interchangeable Fresnel lens having a flat surface and a structured surface. The Fresnel lens is disposed in the compact housing so that the flat surface of the Fresnel lens faces the therapy transducer. The structured surface of the Fresnel lens is disposed in a direction away from the therapy transducer, and the Fresnel lens is removably coupled to the therapy transducer, the compact housing, or both. 
     In another embodiment, a compact ultrasound transducer assembly is provided. The ultrasound transducer assembly comprises a housing, a flat therapy transducer disposed in the housing, wherein the therapy transducer comprises a single element, an imaging transducer disposed in the housing, an interchangeable Fresnel lens that is removably coupled to the housing, or the therapy transducer, or both the housing and the therapy transducer, and a fastening structure that removably couples the Fresnel lens to the housing, or the therapy transducer, or both the housing and the therapy transducer. 
     In yet another embodiment, a method of assembling a compact ultrasound transducer assembly is provided. The method comprises providing a compact housing, disposing a therapy transducer in the compact housing, and removably coupling an interchangeable Fresnel lens to the therapy transducer, the compact housing, or both. 
     In another embodiment, a method for providing therapy to a region of interest is provided. The method comprises directing a first ultrasound beam to a region of interest at a first focal depth using a first interchangeable Fresnel lens, interchanging the first Fresnel lens with a second interchangeable Fresnel lens, wherein the interchangeable lenses comprise a fastening structure that enables the lenses to be readily detached and reattached without substantially interrupting the therapy, and directing another ultrasound beam at a second focal depth using the second interchangeable Fresnel lens. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a cross-sectional view of an examples of an ultrasound transducer assembly comprising a layer of an adhesive material between the therapy transducer and an interchangeable Fresnel lens; 
         FIG. 2  is a cross-sectional view of an example of an ultrasound transducer assembly comprising recesses in the housing for receiving flanges formed in an interchangeable Fresnel lens; 
         FIG. 3  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing having sockets arranged to receive protrusions in an interchangeable Fresnel lens; 
         FIG. 4  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing having arms and an interchangeable Fresnel lens having protrusions; 
         FIG. 5  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing having arms and an interchangeable Fresnel lens disposed on the arms of the housing; 
         FIG. 6  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing having a fixed arm and a partially moveable arm; 
         FIG. 7  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing and an interchangeable Fresnel lens having a coupler that is coupled to both the housing and the interchangeable Fresnel lens; 
         FIG. 8  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a clip for coupling the housing to an interchangeable Fresnel lens; 
         FIG. 9  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing and an interchangeable Fresnel lens having a spring loaded bearing for mounting the Fresnel lens to the housing; 
         FIG. 10  is a cross-sectional view of an example of an ultrasound transducer assembly comprising a housing and an interchangeable Fresnel lens with one or more thumbscrews to fasten the Fresnel lens to the housing; 
         FIG. 11  is a schematic diagram of an example of a method for assembling the ultrasound transducer assembly of the invention; 
         FIG. 12  is a schematic diagram of an example of a method for replacing one interchangeable lens with another; and 
         FIG. 13  is a schematic diagram of an example of an ultrasound system comprising ultrasound transducer assembly of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The ultrasound transducer assemblies of the invention provide a simplified, inexpensive and compact device for delivering ultrasonic therapeutic energy at varying focal depths inside a target. In certain embodiments, the ultrasound transducer assembly comprises a compact ultrasound transducer assembly with a compact housing, a flat therapy transducer disposed in the compact housing and an interchangeable Fresnel lens disposed in the compact housing. The interchangeable Fresnel lens is removably coupled to the therapy transducer, the compact housing, or both. The compact ultrasound transducer assembly may also comprise an imaging transducer. In one embodiment, the therapy transducer may be a high intensity focused ultrasound (HIFU) transducer. In one example, the therapy transducer may operate at a frequency in a range from about 0.1 MHz to about 50 MHz. 
     The region of interest may be a one-dimensional, two-dimensional or a three dimensional region. The region of interest is typically located inside a patient. The region of interest may comprise undesirable tissue, such as diseased tissue or adipose tissue. In one example, providing the therapy may include destroying the undesirable tissue. 
     As used herein, the term “Fresnel lens” means an acoustically directing component through which acoustic energy may be passed to direct, or redirect the acoustic energy, for example, by focusing the acoustic energy to one or more foci. The Fresnel lens may have a flat surface and a structured surface opposite the flat surface. The structured surface may comprise a series of closely spaced grooves that control the refraction of acoustic energy. The acoustic energy from the ultrasound transducer is focused using the series of grooves. The grooves may act like concentric prisms that bend and focus the acoustic energy. The Fresnel lens may be a thin, and relatively flat structure. 
     In certain embodiments, the interchangeable Fresnel lens is disposed in the beam path of an ultrasonic beam propagating from the therapy transducer. The interchangeable Fresnel lens may be disposed in the ultrasound transducer assembly such that the flat surface of the interchangeable Fresnel lens is closer to the therapy transducer than the structured surface. An acoustic coupling medium may be disposed at least in a portion between the therapy transducer and the flat surface of the Fresnel lens to provide enhanced acoustic coupling between the therapy transducer and the Fresnel lens. Non-limiting examples of the acoustic coupling medium may include acoustic gel, or water. The acoustic coupling medium may be in the form of a thin layer of gel or water. While an acoustic coupling medium may be required between the therapy transducer and the Fresnel lens, there may not be a need for an acoustic coupling medium between the housing and the Fresnel lens. The acoustic coupling medium may be pre-disposed on the flat surface, the structured surface or both, of the Fresnel lens, before disposing the Fresnel lens in the ultrasound transducer assembly. For example, an acoustic coupling gel may be disposed on the flat surface of the Fresnel lens prior to coupling the Fresnel lens with the therapy transducer. Alternatively, the acoustic coupling medium may be disposed on the flat surface, the structured surface or both, of the Fresnel lens during or after disposing the Fresnel lens in the ultrasound transducer assembly. 
     The terms “interchangeable Fresnel lens” and “Fresnel lens” may be used interchangeably throughout the application. The interchangeable Fresnel lens may include an acoustic coupling medium disposed on the flat surface, the structured surface, or both. The interchangeable Fresnel lens may focus, shape, or direct the ultrasonic energy to a single focus or to a plurality of foci. The thickness of the interchangeable Fresnel lens may depend on the desired focal depth of the ultrasound beam and the number of grooves. In one embodiment, the thickness of the interchangeable Fresnel lens may be in a range from about 1 mm to about 20 mm. The interchangeable Fresnel lens may be made of a polymer material. In one example, the interchangeable Fresnel lens may be made of a flexible material. The interchangeable Fresnel lens may be formed using inexpensive methods such as but not limited to, casting or injection molding. The interchangeable Fresnel lens may have a low acoustic absorption. 
     Advantageously, the compact ultrasound transducer assembly enables the same therapy transducer to be used in applications that require focusing acoustic waves at different focal depths. The acoustic waves may be focused at different focal depths by simply replacing one interchangeable Fresnel lens with another. One interchangeable Fresnel lens may be replaced by another using simple techniques, such as but not limited to, simply unscrewing a lens and replacing it. Since a Fresnel lens used in a HIFU transducer may be exposed to potentially damaging effects of high intensity ultrasound, a method for easy replacement of an inexpensive Fresnel lens may provide cost savings for the user, who might otherwise require the transducer assembly to be serviced. Changing the Fresnel lens may also help to reduce the build up of heat on the patient and the Fresnel lens. 
     In addition, since the focal depth of the therapy transducer can be varied by using different Fresnel lenses, that is, since the transducer elements are not relied upon to vary the focal depth, the therapy transducer may be, but is not limited to, a single element transducer. The single element transducer simplifies the electrical system design and also reduces the manufacturing cost for the ultrasound transducer assembly. The single element transducer may use a single transmitting system (e.g. channel). Advantageously, single element transducer simplifies the system design and reduces the cost of the transducer assembly. Also, the compact shape of the therapy transducer and the Fresnel lens reduces the typical bulkiness of a spherical ultrasound transducer. 
     Thermal stresses that develop in the ultrasound transducer assembly may decouple the interchangeable Fresnel lens from the therapy transducer. In one embodiment, a thermally conductive layer may be provided between the therapy transducer and the interchangeable Fresnel lens to alleviate stresses arising in the transducer assembly due to the thermal expansion mismatch between the therapy transducer and the Fresnel lens. Alternatively, the acoustic coupling medium may itself serve the function of a thermally conductive layer. 
     In certain embodiments, an imaging transducer may be disposed in the compact housing, in addition to the therapy transducer. The flat shape of the therapy transducer enables the imaging transducer to be disposed closer to the focal depth of the acoustic waves of the therapy transducer than a spherical ultrasound transducer. The imaging signals are back-scattered from structures in the body, such as adipose tissue, muscular tissue, blood cells, veins or objects within the body (e.g., a catheter or needle), to produce echoes that return to the imaging transducer. The imaging transducer may receive the backscattered waves at different times, depending on the distance into the tissue from which they return and the angle at which they return. The interchangeable Fresnel lens may be coupled to one or more of the housing, the imaging transducer, or the therapy transducer. 
     In certain embodiments, the therapy or imaging transducers may comprise one or more transducer elements, one or more matching layers, and focusing components, such as an acoustic lens. The transducer elements may be arranged in a spaced relationship, such as, but not limited to, an array of transducer elements disposed on a layer, where each of the transducer elements may include a transducer front face and a transducer rear face. The transducer elements may comprise, but are not limited to, a piezoelectrically active material, such as lead zirconate titanate (PZT), lithium niobate, lead titanate, barium titanate, and/or lead metaniobate, or combinations thereof. The piezoelectrically active component of the transducer element may also, or alternatively, comprise one or more of a piezoelectric ceramic, a piezoelectric crystal, piezoelectric plastic, and/or piezoelectric composite materials. In addition to, or instead of, a piezoelectrically active material, transducer may comprise any other materials configured for generating radiation and/or acoustical energy such as capacitively coupled transducers or other acoustic sources. The transducer may also comprise one or more matching and/or backing layers coupled to the piezoelectrically active material. The therapy transducer may also include one or more matching layers disposed adjacent to the front face of the transducer elements, where each of the matching layers may include a matching layer front face and a matching layer rear face. The matching layers facilitate acoustic impedance matching of the differentials that may exist between the high impedance transducer elements and a low impedance patient. 
     Although, the embodiments illustrated hereinafter are described in the context of a medical imaging system, the compact ultrasound transducer assemblies may be adapted for other medical and non-medical applications, such as, for example, industrial borescopes that are used for monitoring and detection. 
     In certain embodiments, a fastening structure removably couples the interchangeable Fresnel lens to the therapy transducer, the compact housing, or both. The fastening structure may comprise a screw, key, latch, notch, socket, protrusion, removable adhesive, snap-fit, self-locking fit, clip, hook, pin, flange, coupling arm, deformable material, spring-loaded bearing, or combinations thereof. 
     The removable adhesive layer or a deformable material, such as a deformable substrate, may be disposed between the therapy transducer and the Fresnel lens. In one example, the fastening structure may be configured to act as an acoustic coupling medium. For example, the adhesive layer or the deformable substrate may be made of a material that can enhance the acoustic coupling between the therapy transducer and the Fresnel lens. The adhesive or deformable substrate may have low acoustic attenuation. The adhesive material or the deformable material may be present in the form of a continuous layer or as a combination of a plurality of discrete spots. In another example, a portion or the entire housing may be made of a deformable material. In this example, the housing may elastically or plastically deform upon receiving the Fresnel lens. The elastic or plastic deformation area in the housing material may only be a few microns in size. This deformation may provide a sealing between the housing and the Fresnel lens to hold the Fresnel lens in place. 
     The deformable substrate may be a flexible substrate. The material of the deformable substrate may be configured to undergo elastic or plastic deformation when receiving the Fresnel lens. In certain embodiments, the material for the deformable substrate may be such that a recess can be formed in the deformable substrate. The recess may be dimensioned to receive the interchangeable Fresnel lens. In some embodiments, the deformable substrate may include polymers. The material of the deformable substrate may be chosen based on the ease of forming the desired recess shape in the substrate material. The material of the deformable substrate transmits a majority of the ultrasound energy from the transducer to the Fresnel lens. In one embodiment, the deformable substrate may include soft polymers. Soft polymers refer to elastomer type materials such as, but not limited to, polydimethylsiloxane, a copolymer of hexafluoropropylene (HFP) and vinylidene fluoride (VDF or VF 2 ), a terpolymer of tetrafluoroethylene (TFE), vinylidene fluoride (VDF), and hexafluoropropylene (HFP), perfluoromethylvinylether (PMVE), nitrile rubber, polyethylene, polymethylpentene, and thermoplastic elastomers such as ELASTRON® and THERMOLAST®. 
     An embodiment of the compact ultrasound transducer assembly of the invention is generally shown and referred to in  FIG. 1  as assembly  10 . The assembly  10  comprises a therapy transducer  12 , an imaging transducer  14 , a shaft  15  for the imaging transducer  14 , and an interchangeable Fresnel lens  16  disposed in a compact housing  17 . The therapy transducer  12  is substantially flat in shape. The therapy transducer  12  may be a single element transducer. The therapy transducer  12  may generate high intensity focused ultrasound (HIFU). That is, the therapy transducer  12  may be a HIFU transducer. The interchangeable Fresnel lens  16  comprises a flat surface  18  that is disposed closer to the therapy transducer  12 . The structured surface  20  of the Fresnel lens  16  may be disposed away from the therapy transducer. F represents the focal depth of the HIFU focal spot  22  of the therapy transducer  12  in the presence of the Fresnel lens  16 . The focal depth F may be changed by replacing the Fresnel lens  16  with another Fresnel lens. The imaged field or ultrasound beam is represented by  24 . Due to the flat shape of the therapy transducer  12 , the imaging transducer  14  is disposed closer to the HIFU focal spot  22  than conventional spherical shaped therapy transducers, thereby improving the quality of the acquired image. 
     In this embodiment, a layer  26  of the adhesive material is used between the therapy transducer  12  and the Fresnel lens  16 . The adhesive layer  26  may be a patterned or a continuous layer. The adhesive layer  26  may be a dual sided adhesive layer that has adhesive on both the sides. The side  28  of the adhesive layer may have a permanent adhesive, whereas the side  30  may have an adhesive that allows the Fresnel lens to be decoupled from the adhesive layer without having to remove the adhesive layer  26 . Alternatively, the adhesive layer  26  may be a removable adhesive layer  26  that is part of the Fresnel lens  16 . The side  30  of the adhesive layer  26  may be permanently coupled to the Fresnel lens and the side  28  may have a removable adhesive. Suitable temporary adhesives for removable side of the adhesive layer  26  may include epoxy based adhesives, thermoset adhesives, acrylate based adhesives, silicone-based adhesives, elastomer based adhesives, or any combinations thereof. Suitable permanent adhesives may be used on the other side. 
     In one embodiment, the imaging transducer  14  may be built into a central region of the therapy transducer  12 . In this embodiment, the imaging transducer  14  may be permanently fixed to the central region of the therapy transducer. The imaging transducer may image through the central region of the Fresnel lens and may be designed so as to avoid distortions in the steering and focusing of the imaging beams. In another embodiment, imaging transducer  14  may be removably inserted into an opening in the therapy transducer  12 . In this embodiment, the imaging transducer  14  may extend into the shaft  15 . If the imaging transducer  14  extends past the surface of the therapy transducer  12 , the sides of the imaging transducer  14  may obstruct part of the therapy beam  23 . Conversely, if the imaging transducer  14  does not extend beyond the therapy transducer  12  (and lies flush with the therapy transducer  12 ), the opening in the Fresnel lens  16 , to accommodate the shaft  15 , is designed so that the opening does not obstruct part of the imaging field of view  24  of the imaging transducer  14 . An acoustic coupling medium may be disposed between the imaging transducer  24  and the cavity for the shaft  15 . 
     Although not illustrated, the hole for the shaft  15  may extend to the structured surface  20  of the Fresnel lens  16 . In this case, the Fresnel lens  16  may have a minimal impact on the focusing and steering of the ultrasound beam from the imaging transducer in that region. An insert may be disposed in the central region of the Fresnel lens  16 , where the insert may be made of an acoustically neutral material, and the imaging transducer may image through this acoustically neutral material. 
     In one embodiment, the therapy transducer  12  may rotate or oscillate about an axis. In this embodiment, for the ease of assembly, the shaft  15  for the imaging transducer  14  may coincide with the central axis of the therapy transducer  12 . 
     In one embodiment, the compact housing comprises a receiving notch or a detent. The Fresnel lens comprises a ledge, flange, pin, or projection that engages the housing receiving detent or socket and holds the Fresnel lens. The Fresnel lens may be partially movable between a first position where the Fresnel lens is fastened to the housing and a second position where the Fresnel lens is removable from the housing. 
       FIG. 2  illustrates another embodiment of the compact ultrasound transducer assembly  40  where the housing  42  comprises recesses  44  for receiving flanges  46  formed in the Fresnel lens  48 . The therapy transducer  49  comprises a focal depth F and the imaging transducer  50  comprises field of view  52 . Although not illustrated, an acoustic coupling medium may be disposed between the flat surface  58  of the Fresnel lens  48  and the therapy transducer  49 . The Fresnel lens  48  further comprises a layer  54  of an acoustic coupling medium present on the structured surface  56  of the Fresnel lens  48 . The layer  54  of the coupling medium may be enclosed by a thin membrane  55 . In one embodiment, an acoustic coupling medium may be applied to the thin membrane  55  or the layer  54  (if the thin membrane  55  is not present) to avoid air at the interface between the ultrasound transducer assembly  40  and the patient. 
     If the grooves on the structured surface  56  are deep and have sharp edges, the layer  54  may have sufficient thickness to avoid any discomfort to a patient that may be otherwise caused due to the sharp grooves. That is, the layer  54  may be used to smoothen the structured surface  56  so that there is no discomfort caused to the patient due to the sharp grooves of the structured surface  56 . 
     Alternatively, if the structured surface  56  of the Fresnel lens comprises grooves that are shallow, and the edges of the grooves are not very sharp, the layer  54  of the coupling medium may not be required on the structured surface  56 . However, to enhance acoustic coupling between the patient and the ultrasound transducer assembly and to avoid air at the interface, an acoustic coupling medium may be applied to the structured surface  56  prior to providing treatment to the patient. The acoustic coupling medium so applied may form a thin conformal coating on the grooves of the structured surface  56 , and may have a relatively low thickness as compared to the layer  54  of the acoustic coupling medium. In one example, the acoustic coupling medium may be applied immediately prior to providing treatment to the patient. 
       FIG. 3  illustrates a housing  60  having sockets  62  arranged to receive protrusions  64 , such as but not limited to circular, square or rectangular protrusions, of a Fresnel lens  66 . A therapy transducer  68 , an imaging transducer  70 , and a shaft  72  are also disposed in the housing  60 . The sockets  62  comprise shoulder  74 , onto which the Fresnel lens  66  is snapped by gently deflecting the Fresnel lens  66  over the shoulder, so that the protrusions  64  are disposed into the sockets  62  and rest on the shoulder  74 . To decouple the Fresnel lens  66  from the sockets  62 , one of the protrusions  64  may be further pushed inside the corresponding socket  62 , and the other protrusion may be guided out of the socket. One or both of the sockets  62  may be long enough to allow the protrusions  64  of the Fresnel lens  66  to slide within the sockets  62 . 
       FIG. 4  illustrates an embodiment where a housing  80  comprises a bracket in which the two arms  82  and  84  of the bracket may have the same or different lengths. A therapy transducer  86 , an imaging transducer  88  and a shaft  90  for the imaging transducer  88  are disposed within the housing  80 . 
     A Fresnel lens  92  may have small protrusions  94  to enable the Fresnel lens  92  to be disposed within the housing  80 . Alternatively, as illustrated in  FIG. 5  a Fresnel lens  96  may be disposed on the arms  82  and  84  of the housing. In this embodiment, the Fresnel lens  96  may not include protrusions  94 . 
       FIG. 6  illustrates a compact ultrasound transducer assembly  100  having a housing  102  for disposing a therapy transducer  104 , an imaging transducer  106 , a shaft  108  for the imaging transducer  106 , and a Fresnel lens  110 . The housing  102  comprises a fixed arm  112  and a partially moveable arm  114 . The arm  114  is hinged at one end  116 . The arm  114  further comprises a latch, represented by a line  118 . The latch extends between the arm  114  and leg  120  of the housing  102 . The latch  118  is configured to be decoupled from the leg  120  to allow the arm  114  to rotate about the hinge  122 , as represented by a curved arrow  124  and a dashed rectangle  126 . The arm  114  may be released to the position  126  when the Fresnel lens  110  needs to be replaced. After replacing the Fresnel lens  110 , the arm  114  may be moved to the original position (solid rectangle  114 ), and the arm  114  may be secured in its place using the latch  118 . The Fresnel lens  110  may or may not have small projections. Also, in one embodiment, both the arms  112  and  114  may be moveable. 
       FIG. 7  illustrates a compact ultrasound transducer assembly  128  having a housing  130  for a therapy transducer  132 , an imaging transducer  106 , and a shaft  108  for the imaging transducer  106 . The housing  130  and a Fresnel lens  134  comprise a coupler that is coupled to both the housing  130  and the Fresnel lens  134 . The coupler comprises a snap having male couplers  138  and female couplers  140 . The male couplers  138  are formed on the Fresnel lens  134  and the female couplers  140  are formed on the housing  130 . Alternatively, male couplers  138  may be formed on the housing  130  and the female couplers  140  may be formed on the Fresnel lens  134 . Also, other means for removably coupling the housing  130  and the Fresnel lens  134  may be used in place of male and female couplers  138  and  140 . For example, couplers  138  and  140  may comprise threaded screws and corresponding threads in the housing, a dowel and pin assembly, a lock and key assembly, a post and clasp assembly, or a hook and loop fastener. 
       FIG. 8  illustrates a compact ultrasound transducer assembly  140  having a housing  142  for a therapy transducer  144 , an imaging transducer  106 , and a shaft  108  for the imaging transducer  106 . A clip  146  is used to couple the housing  142  to the Fresnel lens  148  by snapping over the end of the housing  142  and the perimeter of Fresnel lens  148 . Alternatively, the clip  146  may be snapped through a groove in the housing  142 . The clip  146  may have a “U” or semicircular shape, and the clip  146  has flanges  150  that engage the Fresnel lens  148 . The Fresnel lens  148  may have grooves to receive the flanges  150 . The clip  146  may also engage the Fresnel lens  148  by pushing it toward the Fresnel lens  148  and in a direction transverse to a centerline of the Fresnel lens  148  as represented by arrows  152 . The Fresnel lens  148  may be removed by pulling the clip in a direction opposite to that shown by the arrows  152 . The clip  146  may be made of a material that retains its resilience after being repeatedly subjected to expansion (while removing the Fresnel lens  148 ). Such materials include, but are not limited to, spring steel and stainless steel. 
       FIG. 9  illustrates a compact ultrasound transducer assembly  141  having a housing  143  for a therapy transducer  145 , an imaging transducer  106 , and a shaft  108  for the imaging transducer  106 . The housing  143  comprises recesses  147  and  149 . Although, in the illustrated embodiment, the recess  147  is shown as a cylindrical recess, and the recess  149  is shown as a spherical recess  149 , however, these recesses  147  and  149  may have any other suitable shapes. The cylindrical recess  147  is configured to receive protrusion  151  of an interchangeable Fresnel lens  153 . The spherical recess  149  is configured to receive a spring-loaded spherical bearing  155  of the Fresnel lens  153 . As illustrated in the enlarged view in the dashed circle  157 , the spring-loaded spherical bearing  155  comprises a spring  159  that is coupled to the Fresnel lens  153 . During installation of the Fresnel lens  153  in the transducer assembly  141 , the bearing  155  is compressed into the housing  143  upon encountering the shoulder  161  of the housing  143 . Upon installation of Fresnel lens  153  in the housing  143  the bearing  155  is disposed into the spherical recess  149  of the housing  143 . The spring loaded bearing  155  may be at one or more positions around the perimeter of the Fresnel lens  153 . Although not illustrated, a lever arm may be provided in the housing  143  to ease the removal of the Fresnel lens  153 . 
       FIG. 10  illustrates a compact ultrasound transducer assembly  154  having a housing  156  for a therapy transducer  158 , an imaging transducer  106 , and a shaft  108  for the imaging transducer  106 . The housing  156  comprises two or more recesses  163  in the housing  156 , and recesses  165  in an interchangeable Fresnel lens  167 . One or both of the recesses  163  and  165  may be threaded. Screws, such as thumbscrews  169  may be inserted in the recesses  163  and  165  to couple Fresnel lens  167  to the housing  156 . Although not illustrated, the thumbscrews  169  may be replaced with capture pins or clips that are coupled to the housing  156 . 
       FIG. 11  is a schematic drawing of a method for assembling a compact ultrasound transducer assembly  160 . In this embodiment, a housing  162  comprises passageways  164  and threads/notches  166 . The passageways  164  may be used for releasing excess air, acoustic coupling medium, or excess adhesive, from the transducer assembly. A therapy transducer  168  and an imaging transducer  170  are disposed in the housing  162 . The imaging transducer  170  is coupled to a shaft  172 . The therapy transducer  168  may also be coupled to the shaft  172 . Shaft  172  may also be coupled to a drive motor to rotate the imaging and/or therapy transducers  168  and  170 . 
     Assembly  160  comprises Fresnel lens  174  having a channel  175  through which the shaft  172  passes. The Fresnel lens  174  is coupled to a structure  176  having pins/screws  178  for coupling the Fresnel lens to the housing  162 . Other fastening structures may be used for coupling the Fresnel lens  174  to the housing  162 . When the Fresnel lens  174  is coupled to the housing  162 , the pins/screws  178  are received by the threads/notches  166 . The structure  176  may be made of a glass, ceramic, metal, cermets, or polymers, such as but not limited to, thermoplastic elastomers, hard polymers, such as but not limited to, polyether ether ketone (PEEK), polypropylene, poly(methyl methacrylate) (PMMA), polyethelene, olefin copolymers, modified ethylene-tetrafluoroethylene) fluoropolymer (ETFE), polyetherimide, polyvinyl chloride (PVC), and the like. 
     The shaft  172  extends through the Fresnel lens  174  and may be used for advancing or withdrawing the imaging transducer  170  through the therapy transducer  168  and the Fresnel lens  174 . 
     The Fresnel lens  174  comprises acoustic coupling medium  177  disposed on the structured surface of the Fresnel lens  174 . An acoustic coupling medium  179  (such as gel) may be disposed on the structure  176  and the Fresnel lens  174 , along with the structure  176 , may be coupled to the housing  162 . The excess acoustic coupling medium  178  may be released through the passageways  164  and removed simply by wiping off the excess. The acoustic coupling medium  179  provides acoustic coupling between the Fresnel lens  174  and the therapy transducer  168 . 
       FIG. 12  is a schematic drawing of a method for quickly replacing one Fresnel lens with another without significantly interrupting a therapy procedure. The therapy beam  181  is directed to a region of interest at a focal length F using a first interchangeable Fresnel lens  180 . The imaging field  179  includes the HIFU focus of the therapy beam  181 . Subsequently, the first interchangeable Fresnel lens  180  is replaced with a second interchangeable Fresnel lens  182  having a second focal length F′, which is different from the first focal length F. A second therapy beam  195  is directed to same or different region of interest at the second focal depth F′ using the second interchangeable Fresnel lens  182 . The imaging field  197  includes the HIFU focus of the therapy beam  195 . The first and second Fresnel lenses  180  and  182  have acoustic coupling media  184  and  186 , respectively, predisposed on the structured surfaces of the Fresnel lenses  180  and  182 . A therapy transducer  190 , an imaging transducer  192 , and a shaft  194  are disposed in a housing  188 . In this embodiment, the housing  188  comprises slots  196  for coupling the Fresnel lenses  180  and  182  to the housing. 
     The Fresnel lenses  180  and  182  may be installed and removed by hand or with simple tools. A suitable removal tool would be capable of securely holding the housing  188  while allowing the user to decouple the Fresnel lens  180  from the housing  188 . Such an assembly tool would be capable of securely holding the housing  188  while allowing the user to push the Fresnel lens  182  in the housing  188 . In one embodiment, the housing  188  may be slightly expandable to detach the Fresnel lens from the housing  188 . 
       FIG. 13  is a schematic drawing of an example of a compact ultrasound system  200  having an ultrasound transducer assembly  202  that can provide therapy to the region of interest  204 , and optionally, display images of the region of interest  204 . The ultrasound transducer assembly  202  comprises a compact housing  206  for a therapy transducer  208 , an imaging transducer  210 , and an interchangeable Fresnel lens  212 . In one example, the therapy transducer  208  may be used for ablating tissues, such as diseased tissues, adipose tissues, or non-adipose tissues. The imaging transducer  210  may image the region of interest  204  before applying the therapy, after applying the therapy, or while applying the therapy. 
     In addition to, or instead of, a piezoelectrically active material, transducers  208  and  210  may comprise other materials suitable for generating radiation and/or acoustic energy such as capacitively coupled transducers or other acoustic sources. Transducers  208  and  210  may also comprise one or more matching and/or backing layers coupled to the piezoelectrically active material. Transducers  208  and  210  may also comprise single or multiple damping elements. 
     A subsystem  215  may comprise a therapy module  214  for controlling the delivery of therapy to the treatment locations based on one or more therapy parameters or information from the diagnostic module. The therapy module  214  is connected to a user interface  220 , such as a mouse or keyboard, and controls operation of the transducer. The therapy module  214  is configured to receive inputs from a user using the user interface  220 . The therapy module  214  may receive imaging and/or therapy commands from the user through a user interface  220  for applying therapy to the region of interest  204 . 
     The therapy module  214  may be implemented utilizing any combination of dedicated hardware boards, digital signal processors, and processors. While the therapy module  214  is configured to deliver a therapy to the treatment locations based on one or more therapy parameters or information from the diagnostic module, the diagnostic module  216  is configured to control the imaging transducer  210  to obtain diagnostic ultrasound signals from the region of interest  204 . 
     A processing unit processes the acquired ultrasound information (e.g., RF signal data or IQ data pairs) and prepares frames of ultrasound information for display device  218 . The display device  218  may comprise one or more monitors that may present patient information, such as diagnostic and therapeutic ultrasound images, to the user for review, diagnosis, analysis, and/or treatment. The display device  218  may automatically display, for example, a (two dimensional) 2D, (three dimensional) 3D, or (four dimensional) 4D ultrasound data set stored in the memory or currently being acquired. This stored data set may also be displayed with a graphical representation (e.g., an outline of a treatment space or a marker within the treatment space). 
     The flat therapy transducer with low profile Fresnel lens allows the imaging transducer to be located closer to the HIFU focus, improving image quality. In addition, the flat therapy transducer (e.g., HIFU) is much simpler and less expensive to manufacture than a spherical transducer. Also, the transducer assembly employing the interchangeable Fresnel lens is relatively simple and economical to manufacture, and may be manufactured using a batched and/or automated process. 
     Ease in replacing a Fresnel lens with another enables the user (such as the technician or physician operating the device) to replace the Fresnel lens in a time efficient fashion, and allows the physician to use a single transducer with different Fresnel lenses to accommodate a wide variety of patients and procedures. Since a primary cause of failure in a HIFU device is due to the HIFU energy damaging the materials in the acoustic path, inexpensively replacing one Fresnel lens with another avoids costly service and repairs to the transducer assembly. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the scope of the invention.