Abstract:
Methods and apparatus for producing synchronously pumped (SP) ultrasonic bursts which can be used to create ultrasound radiation pressure. The emission of an ultrasonic burst by a piezoelement is synchronized with the reflection of another ultrasonic burst by the same piezoelement to create a combined ultrasonic burst with increased amplitude. Repeated synchronized emissions and reflections of ultrasonic bursts lead to resonant growth of a burst amplitude to equal that of a standing wave, but without formation of a nodal structure. In some embodiments, the SP ultrasonic bursts generate shear waves. In some embodiments, the shear waves are resonant shear waves. In some embodiments, the shear waves are formed in a supersonic regime. Shear waves thus formed can be used for various treatments of biological tissues, with or without RF heating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 61/385,163 filed Sep. 22, 2010, which is incorporated herein by reference in its entirety. 
     
    
     FIELD AND BACKGROUND 
       [0002]    Embodiments of the invention disclosed hereinbelow relate in general to ultrasound apparatus amd methods and in particular to generation of ultrasonic bursts and their use in various media. 
         [0003]    Various methods are known for delivering and coupling acoustic (also referred to herein as “ultrasound” or “ultrasonic”) energy to a region of tissue to perform a diagnostic and/or therapeutic and/or cosmetic procedure on a patient&#39;s tissue. Among such procedures are, for example, non-invasive assaying of blood analytes, drug delivery by phonophoresis, lithotripsy, tissue ablation, and lysis of fat cells for cosmetic removal of adipose tissue. 
         [0004]    For many types of therapeutic and/or cosmetic acoustic applications, such as for example lithotripsy, tissue ablation and lysis noted above, sufficient acoustic energy must be delivered to a tissue region to destroy and remove tissue in the region. Generally, the acoustic energy is delivered by focusing at least one beam of relatively intense ultrasound on the region. The high intensity, focused ultrasound, conventionally referred to by the acronym “HIFU”, may be used to generate various thermal and mechanical effects on tissue that include local heating of tissue and/or cavitation that disrupts and destroys the tissue. Tissue raised to and maintained at a temperature above about 42° C. dies rapidly, and mechanical stresses generated by cavitation breach and tear cell membranes of the tissue. 
         [0005]    However, it is often difficult to efficiently treat relatively large volumes of tissue using HIFU. For example, HIFU beams are often focused to relatively small volumes of tissue and can require relatively large dwell times at the focal volumes to destroy tissue therein. Typically, a focal volume of a HIFU beam is substantially contained within a prolate ellipsoid. For a frequency of ultrasound equal to about 200 kHz, which is commonly used in ultrasound tissue treatment, the ellipsoid has a long axis of about 15 mm along a direction of propagation of the beam and a maximum cross section perpendicular to the propagation direction having a diameter of about 7.5 mm. For frequency of about 1 MHz, the long axis is about 3 mm and the cross section diameter is about 1.5 mm. In general, the focal volume has a lateral diameter of approximately 1 wavelength and a length of between about 2-3 wavelengths. Boundaries of the focal volume are assumed to be in regions where acoustic intensity is attenuated by about 6 dB. Treating an extended region of tissue with HIFU, for example to lyse adipose tissue, can therefore often be a relatively tedious task that requires a relatively long time to perform. As a result, various techniques have been proposed and/or used for expanding a useful focal volume of HIFU beams and for electronically and/or mechanically scanning the beams to treat relatively large tissue volumes. 
         [0006]    However, controlling HIFU beams to deliver effective acoustic energy which is spatially relatively homogenous over an extended tissue volume that is a desired target for treatment and which does not adversely affect non-target tissue can be problematic. Often configurations of extended focal volume HIFU beams exhibit “hot spots” that limit therapeutic and/or cosmetic use of the beams. Also, ultrasound propagated into the body so that it is substantially focused in a desired region, generally propagates through and past the focal region and is incident on organs and/or body features for which the ultrasound is not intended. 
         [0007]    For example, adipose tissue generally resides in the subcutaneous layer of the skin and is located in a region from about a few mm to a few tens of mm below the skin surface. In procedures for tissue ablation and lysis of fat cells for cosmetic removal of adipose tissue, ultrasound focused to fat tissue below the skin may propagate beyond the adipose tissue, and impinge on and damage internal organs and body features lying below the subcutaneous layer. If the ultrasound is being used to treat belly fat, the ultrasound may, for example, be incident on the liver. If the ultrasound is used to treat cellulites in the hip region, the ultrasound may be incident on and reflected from bone tissue below the skin. The reflected ultrasound can interfere with the ultrasound propagated into the body to treat the cellulites and generate a standing acoustic wave having intensity at or near the skin surface that can damage the skin. 
         [0008]      FIG. 1   a  shows schematically a cross section of a perspective view of an ultrasound treatment apparatus  50  (UTA) as disclosed in PCT/IB2010/052624. UTA  50  comprises a vacuum vessel  52  for drawing up and holding a region of tissue region  40  to be treated with ultrasound, and an array of ultrasound transducers  30  for generating the ultrasound. In an embodiment, transducers  30  are interfaced to tissue region  40  by protective buffer layers  132 . Buffer layers  132  have thickness equal to about a quarter wavelength of ultrasound generated by transducers  30  and acoustic impedance substantially equal to about that of tissue  40 . Optionally, vacuum vessel  52  may have other shapes and/or the transducers may be arranged in different arrangements. UTA  50  comprises a “shape adapter” that distorts the shape of tissue drawn up into vacuum vessel  52  so that the drawn up tissue makes substantially continuous and intimate contact with buffers  132 . In an embodiment, the shape adapter comprises a “plunger”  60  having a stem  62  that protrudes into vacuum vessel  52  through an aperture  53  and may be mounted with a plunger head  64 . Optionally, stem  62  comprises a pipe  66  formed having spiracles  67  through which air may be aspirated from vessel  52  to provide partial vacuum for drawing skin into the vessel. The pipe may be sealed in aperture  53  to reduce, leakage of air between the pipe and walls of aperture so that a suitable partial vacuum for drawing up skin can be created in vessel  52 . Arrows  68  schematically represent flow of air aspirated from vessel  52 . Optionally, plunger head  64  may be an oblate, “pumpkin shaped”, body. In some embodiments, the position of plunger  60  may be fixed and located so that when tissue  40  is drawn up into vessel  52 , plunger head  64  deforms the drawn up tissue, as shown in  FIG. 1 , so that it spreads laterally and makes close, continuous contact with buffers  132 . 
         [0009]    In some embodiments, plunger  60  may be configured so that it is moveable “up” and “down” to adjust depth to which plunger head  64  intrudes into vessel  52 . Optionally, plunger head  64  may be moved up and down by moving stem  62  along its length in aperture  53 . Sealing of stem  62  in aperture  53  to maintain suitable reduced leakage of air between the stem and aperture may be provided using any of various methods and materials known in the art. For example, stem  62  might be sealed against air leaks using a configuration of o-rings and/or vacuum greases. 
         [0010]    It was noted that shape adapters may be different from plunger  60 , having for example a cylindrical or annular shaped plunger head. Alternatively, a plunger head may comprise a plurality of component elements such as an array of parallel cylinders that are pressed to skin drawn into vacuum vessel  52 . Plunger head  64  may be formed from an elastic membrane and displacement of the plunger head may be accomplished by filling the plunger head with a gas or liquid to expand it, or by removing gas or liquid to cause the plunger head to contract. 
         [0011]    As described in PCT/IB2010/052624, plunger head  64  is repetitively moved during illumination of tissue region  40  with ultrasound to change a force applied by the plunger head on the tissue region and thereby change the position of mass points in the region relative to ultrasound transmitted by transducers  30 . To change the position of mass points, vacuum vessel  52  may be perturbed at a suitable frequency. The spatial shifting of mass points in tissue region  40  aids in homogenizing the effects of the ultrasound in the tissue region. Optionally, repetitive motion of plunger head  64  may be performed at a frequency substantially equal to a mechanical relaxation time and/or resonant frequency of tissue region  40 . Mechanical resonant and relaxation time frequencies of skin tissue range from about 300 Hz to about 10 kHz. By mechanically vibrating tissue region  40  at a resonant frequency, motion of mass points in the tissue region tend to be relatively large and effects such as tissue heating by the motion are amplified. By mechanically vibrating tissue region  40  at a relaxation time frequency, cavitation effects in the tissue region are also amplified. 
         [0012]    Ultrasonic shear waves are used in non-destructive testing (NDT) and diagnostics of different materials. The use of an acoustic radiation force to remotely generate low-frequency shear waves in viscoelastic media (tissue, rubber like media, etc) is also known. Shear waves generation by pulsed radiation force created by HIFU has been proposed for diagnostic and imaging methods in tissue (see e.g. Bercoff J.; et al. Ultrasound Medicine and Biology. 2003, vol. 13, pp. 143-152 and Bercoff J. ey al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. 2004, vol. 51, pp. 396-409). However, shear waves cannot propagate via fluids that prohibit use of immersion methods. 
         [0013]    The main problems in using shear waves remotely generated by HIFU for NDT purposes (elasticity estimation, shear modulus measurements, etc.) relate to difficulties of introducing and focusing HIFU in rubber like media, as well as to providing acoustic contact between a HIFU transducer and tested objects. There is therefore a need for, and it would be advantageous to have, apparatus and methods of producing ultrasound radiation pressure and shear waves in a more efficient way for various NDT and tissue treatment purposes. 
       SUMMARY 
       [0014]    Embodiments disclosed herein provide methods and apparatus for producing Synchronously Pumped (SP) ultrasonic waves which can be used to create ultrasound radiation pressure. Synchronous pumping is obtained by synchronizing the emission, by a piezoelement, of an ultrasonic burst wave (also referred to simply as “ultrasonic burst” or just “burst”) with opposite polarity to a burst reflected by the same piezoelement, such that the opposite polarity-emitted and reflected bursts combine to form a burst with increased amplitude. Repeats of this action lead to resonance growth of the burst amplitude to equal that of a standing wave but without formation of a nodal structure. 
         [0015]    Some embodiments disclosed herein provide methods and apparatus for producing shear waves in a SP regime. Other embodiments provide methods and apparatus for producing shear wave resonance (SWR) in a SP regime. Yet other embodiments provide methods and apparatus for producing supersonic shear waves (SSW) in a SP regime. Yet other embodiments provide apparatus which include radio frequency (RF) electrodes. 
         [0016]    SP ultrasonic waves as disclosed herein may be used to create ultrasonic pressure in lossy viscoelastic media (e.g. polymers or resins) or in solids having high attenuation (e.g. composites). Methods and apparatus disclosed herein can be used for non-destructive testing (NDT) or diagnostics (e.g. to measure shear elasticity) in media mentioned above, i.e. in polymers, resins or composites). Alternatively, methods and apparatus disclosed herein can be be used in biological media (e.g. biological tissue) for example for treating relatively large regions of tissue with SP ultrasonic waves. In apparatus also including RF electrodes, methods for providing SP and SWR or SSW in the SP regime may also provide RF heating for “combined treatements” of biological tissue. Such combined treatments may include lypolytic, therapeutic and/or cosmetic treatments of tissue. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Non-limiting examples of embodiments disclosed herein are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
           [0018]      FIG. 1   a  shows schematically a cross section of a perspective view of an ultrasound treatment apparatus (UTA) as disclosed in PCT/IB2010/052624; 
           [0019]      FIG. 1   b  shows a cylindrical piezoelement C; 
           [0020]      FIG. 1   c  shows a two planar piezoelement transducer configuration; 
           [0021]      FIG. 2   a  shows schematically a burst of a “direct” ultrasonic wave generated by a first transducer T 1  or by cylindrical element C, which propagates forward and creates a radiation force F rad ; 
           [0022]      FIG. 2   b  shows schematically the burst in  FIG. 2   a  after reflection from a solid boundary (a second transducer T 2  or C itself) and amplification by the burst emitted by 
           [0023]    T 2  in opposite polarity; 
           [0024]      FIG. 3   a  shows schematically the superposition of direct and reflected bursts at reflection from a cylindrical or a planar piezoelement (without pumping); 
           [0025]      FIG. 3   b  shows schematically the superposition of direct and amplified (double amplitude) reflected bursts in a SP regime; 
           [0026]      FIG. 4   a  shows schematically a cross section of a perspective view of an embodiment of an apparatus for treatments using a combination of SP and RF; 
           [0027]      FIG. 4   b  shows in detail (side and upper view) an RF metal electrode mounted to a plunger in the apparatus of  FIG. 4   a;    
           [0028]      FIG. 5   a  shows schematically a cross section of a perspective view of an apparatus according to another embodiment disclosed herein, having a shear mode receiving piezoelement mounted to a plunger; 
           [0029]      FIG. 5   b  shows details of a shear mode receiving piezoelement mounted to a plunger; 
           [0030]      FIG. 5   c  shows a combination of an RF electrode and a shear mode receiving piezoelement mounted on a plunger head; 
           [0031]      FIG. 6  shows schematically the generation of shear waves by an ultrasonic burst; 
           [0032]      FIG. 7  shows schematically the formation of shear waves from the interaction of the direct and reflected bursts in a SP regime; 
           [0033]      FIG. 8  shows schematically one use of an apparatus with multi-electrode parallel or cylindrical transducers to produce shear waves; 
           [0034]      FIG. 9  shows schematically another use of the apparatus of  FIG. 8  to produce shear waves; 
           [0035]      FIG. 10  shows schematically one use of an apparatus with multi-electrode transducers to produce supersonic shear waves; 
           [0036]      FIG. 11  shows schematically another use of an apparatus with multi-electrode transducers to produce supersonic shear waves; 
           [0037]      FIG. 12  shows schematically a physical mechanism of combinational treatment using SP and RF. 
       
    
    
     DETAILED DESCRIPTION 
     Synchronous Pumping (SP) 
       [0038]    The present inventor has determined that an apparatus similar to apparatus  50  in  FIG. 1   a  can be used for “Synchronous Pumping” (SP) of ultrasonic burst waves. SP is defined hereinbelow. While SP of ultrasonic burst waves is described next in detail with reference to a biological tissue medium, it is to be understood that apparatus and methods disclosed herein may equally be used with other media. For example, apparatus and methods disclosed herein may be used for SP of ultrasonic waves in lossy viscoelastic media (e.g. polymers or resins) or in solids having high attenuation (e.g. composites) and for non-destructive testing (NDT) or diagnostics in such media. 
         [0039]    In an embodiment shown in  FIG. 1   b , a transducer similar to transducer  30  is in the shape of a cylindrical piezoelement C, which includes a front electrode  133  and a back electrode  134  which yield a piezoelement polarization  144 . In another embodiment shown in  FIG. 1   c , transducers similar to transducer  30  are organized as two planar and parallel transducers T 1  and T 2 . T 1  includes a first planar piezoelement  130  and a front electrode  133  and a back electrode  134  which yield a piezoelement polarization  144 . T 2  includes a second planar piezoelement  131  and a front electrode  133 ′ and a back electrode  134 ′ which yield a piezoelement polarization  144 ′. In both embodiments, the transducers are coupled to a power supply power supply  150  which includes a frequency generator (not shown) and a controller (not shown), and which is configured to provide ultrasonic bursts with appropriate frequency, bursts parameters and phase shifts. In particular, the power supply is configured to excite each piezoelement to emit a direct ultrasonic burst in synchronization with the reflection of a previously emitted ultrasonic burst arriving at the same piezoelement, whereby the direct ultrasonic burst and the reflected ultrasonic burst combine to form a resonantly amplified ultrasonic burst wave in a SP regime (see below). 
         [0040]    If short bursts are supplied on piezoelement C (or on opposite piezoelements T 1  and T 2 ) and if the burst duration is shorter than a transit time between elements, then a standing wave will not form. It is well known that an ultrasonic wave is reflected from a rigid boundary (piezoelement) in an opposite phase (180°). Therefore, an ultrasonic burst excited by a cylindrical piezoelement arrives at the (diametrically opposite side of the cylindrical) piezoelement and is reflected by it with a change of 180° in phase. The present inventor has determined that if the cylindrical piezoelement is switched on in opposite polarity with a delay equal to a transit time T of a previous burst each time it receives (reflects) a previous burst (“synchronization”), there will be a resonant growth of the burst amplitude equal to that of a standing wave, but without formation of the nodal structure typical for standing waves. A similar effect of resonant growth of the burst amplitude can be created using the T 1  and T 2  planar configuration. This effect (and the action causing it) are named herein “Synchronous Pumping”. An SP wave has a running wave character. 
         [0041]    Let D be the diameter of cylindrical piezoelement C or the distance between T 1  and T 2 . Let the frequency of the burst be f, its amplitude be A, and the propagation velocity in tissue be V. The transit time “T” of an ultrasonic wave between T 1  and T 2  or between opposite sides of a cylinder is then T=D/V. 
         [0042]    Let C or T 1  be excited by a first electrical burst signal with a burst length in tissue=τ and with a frequency f, and let τ=T/2 (see  FIG. 2   a ). In a general case, to prevent formation of standing waves as a result of the superposition of direct and reflected waves, T must be less then T and preferably less than T/2. The excitation provides a first burst  202  which propagates to the left in  FIG. 2   a . While propagating, the burst produces a radiation force F rad    204 . At time t 1 =T, burst  202  arrives at a diametrically opposite (“second”) side of C or at T 2  and is reflected therefrom. At this moment, C (or T 2 ) is excited by an identical electrical burst signal with a burst length τ=T/2, frequency f and amplitude A, but with opposite polarity, to produce a second burst identical with the first and in phase with the reflected burst (see  FIG. 2   b ). The reflected burst thus acquires an added amplitude A, becoming an “amplified” burst  203  propagating from left to right and having a  2 A amplitude. At time t2=T, amplified burst  203  arrives at the first side of C (or at T 1 ) and is reflected therefrom. At this moment, C (or T 1 ) emits again a (third) burst identical (in phase and polarity) with the first burst. The third burst is in phase with, and added in amplitude to the reflected burst, which now has  3 A amplitude and propagates from right to left (not shown). In terms of timing, C (or T 1 ) generates a burst with length τ at times t=0, 4τ, 8τ etc. (and more generally, at t=4NτT, where N=0, 1, 2, 3 . . . etc.). Then C (or T 2 ) generates a burst with length τ at times t=2τ, 6τ, 10τ etc (and more generally, at t=2+4Nτ, where N=0, 1, 2, 3 . . . etc.). In principle, this process can continue to infinity, resulting in ultrasonic burst resonance amplification (restricted only by ultrasonic wave attenuation). 
         [0043]    Note that while the excitations described in the embodiment above were done in sequence, first on one piezoelement then on the other, in other embodiments they can also be done not in sequence, for example simultaneously. 
         [0044]    In synchronous pumping, upon (during) burst reflection, direct and reflected waves have opposite polarity, as in the case of standing waves. As shown in  FIG. 3   a , in the standing wave case the waves compensate each other at odd λn/4 distances (n=1, 3, 5, 7 . . . etc), with a first compensation “plane” at distance λ/4 marked  205 . In contrast, the compensation process using bursts is transient, and in SP there is compensation only during burst reflection (“transient” compensation). That is, in the standing wave case, if the skin is positioned at the λ/4 plane there is a node of pressure on the skin at all times. In contrast, in burst mode there is zero pressure on the skin at that position only during the superposition of direct and reflected bursts. Further in contrast, at t1 (beginning of the first reflection process) the amplitude of the reflected and amplified burst is 2A, while the direct (received) burst has an amplitude A. Therefore, as shown in  FIG. 3   b , in SP the compensation is not full and instead of zero pressure at λ/4 distance from the boundary (as in standing waves), there is a pressure with amplitude A after each instance of reflection. Note that if one accounts for acoustic attenuation, the burst amplitude grows to and saturates at Q×A, where Q is a mechanical quality factor. 
         [0045]      FIG. 4   a  shows schematically a cross section of a perspective view of an apparatus  400  for treatments using a combination of SP and RF according to an embodiment disclosed herein. Apparatus  400  includes, in addition to elements shown in apparatus  50 , a plunger electrode  401  attached to plunger head  64 . Plunger electrode  401  may be a simple metal disk which has exemplarily a cylindrical shape ( FIG. 4   b ), with a typical diameter of a few mm. In general, electrode  401  may be made of any biocompatible, electrically conductive material (e.g. Cr, Au, stainless steel, etc) and can be electrically connected to a power source by wires (not shown). Apparatus  400  further includes a cylindrical piezoelement C with electrodes  133  and  134 . Electrode  401  is commonly connected with back electrode  134  to an AC source and used in conjunction with front electrode  133  to generate an RF electric field indicated by field lines  402  by applying opposite polarities to these two electrodes. Exemplarily, a polarity “+” is applied to electrode  401 , while a polarity “−” is applied to electrode  133 . Alternatively, “−” is applied to electrode  401  and “+” is applied to electrode  133 . In other embodiments, C may be replaced by two parallel plate transducers T 1  and T 2 , each having front and back electrodes. In such embodiments, the electrical connections and operation of the plunger electrode and planar piezoelement electrodes will be similar to those employing a cylindrical transducer. 
       Shear Wave Generation Using SP 
       [0046]      FIG. 5   a  shows schematically a cross section of a perspective view of an apparatus  500  according to another embodiment. Apparatus  500  includes a shear mode receiving (or just “shear mode”) element  501  attached to plunger head  64 . More details of the arrangement are shown in  FIG. 5   b . In  FIG. 5   b , element  501  has a ring shape. Shear mode elements are known in the art and are usually made from piezoceramics with a polarization direction such as a direction  502  parallel to main surfaces covered by electrodes  503  and  504 . A ring shape shear mode element has polarization along the radius of the ring. Its thickness must be equal to half a wavelength for working frequency, but the element can be also very thin (0.5 mm or thinner) and broadband (damped by a damper—not shown) for receiving of any shear wave (non-resonance receiver). Shear mode elements are used to “register” shear waves. The registration is needed to be able to get useful information on medium properties (e.g. to estimate shear wave velocity, attenuation, etc), i.e. for diagnostic purposes.  FIG. 5   c  shows an embodiment in which RF electrode  401  and shear mode receiving element  501  are mounted on plunger head  64  for a type of combinational treatment described below. Note that in general, a shear mode element may be positioned differently within the apparatus and not necessarily attached to a plunger head. 
         [0047]    The generation of shear waves  601  by an ultrasonic burst  202  is shown schematically in  FIG. 6 . Details may be found in PCT application PCT/IB2011/051917 by the present inventor. 
         [0048]      FIG. 7  shows schematically the generation of shear waves from the interaction of direct and reflected waves in a SP regime. Shear force F rad  generates shear deformation and, after the burst passes through a fixed plane, generates a shear wave which propagates in a lateral direction (perpendicular to the burst propagation direction, exemplarily up and down in the figure). For now, acoustic attenuation is ignored. After the first burst which generates a force F rad  with a first polarity is fully reflected at t1, as described above, the reflected burst with amplitude  2 A generates a radiation force  4 F rad  with a second, opposite polarity. The reflected burst also generates shear deformations and shear waves in the lateral direction. Thus, the ultrasonic burst runs forward and backward and generates and amplifies shear waves. For example, in the plane distanced from T 1  (or along the diameter D of cylindrical piezoelement C) by 3T/4, an ultrasonic burst  202  generates a shear wave  701  propagating upwards. After reflection from T 2  and amplification (doubling of amplitude) by a burst with opposite polarity, an upward shear wave  702  which is in phase with the second half period of shear wave  701  is generated, and its amplitude grows resonantly. The same situation appears in a plane distanced from T 1  by T/4 (shear waves propagating down exist symmetrically but are not shown). In intermediate planes between T/4 and 3T/4, the shear waves are out of phase and compensate each other, but because the reflected bursts have doubled amplitude (double radiation force) additional attenuation shear waves are generated along the entire ultrasonic beam, their intensity being maximum in some planes and intermediate in all others. The intensity maximum positions can be changed by simply changing the burst length. 
         [0049]    As well known, shear waves propagate slowly and attenuate strongly. For a 50 kHz shear wave (τ=10 μsec) the propagation velocity is V shear =3 m/sec and the wavelength is λ shear =0.06 mm. This means that for the time of a burst reflection in plane 3T/4 (t=10 μsec, burst velocity 1500 m/sec) shear waves propagate to a distance of 3 m/sec/50 kHz=0.06 mm from the center line of the transducers (i.e. to one shear wavelength). Taking into account the very high attenuation of shear waves (at 50-100 kHz, shear waves fully attenuate at a distance of 3-4 wavelengths), the method described herein can provide high intensity shear deformations localized in a narrow sub-skin tissue layer. 
       EXAMPLE 
       [0050]    If a burst length τ=10 μsec and the duty cycle is 1/4 (i.e. 10 μsec burst, 30 μsec pause), the generated shear waves have a period Θ=20 μsec, i.e. a frequency equal to 50 kHz=1/Θ (in each plane on the ultrasonic burst path). In the general case, a shear wave will have a period equal to Θ=2τ (if the radiation force creates shear deformation during a time equal to τ−Θ/2, then the radiation force is zero and the medium starts to vibrate from + to − deformation). 
         [0051]    If the transit time T between T 1  and T 2  (or along diameter D of C) is given by T=D/V and if the burst has a length τ=T/2, the shear deformations generated by a reflected burst will be in phase with the shear wave generated by the direct burst in each plane for which n′τ/2=nT/4 where n is odd, i.e. in planes distanced (in time scale) from T 1  (or along diameter D from the perimeter of C) by τ/2=T/4 and 3τ/2=3/4T. 
         [0052]    For τ=T/3, shear deformations generated by the reflected burst will be in phase with the shear wave generated by the direct burst in each plane for which nτ/2=nT/6, where n is odd, i.e. in planes distanced (in time scale) from T1 (or along diameter D from the perimeter of C) by τ/2=T/6, 3τ/2=T/2 and 5τ/2=5T/6. 
         [0053]    For τ=T/4, shear deformations generated by the reflected burst will be in phase with the shear wave generated by the direct burst in each plane for which nτ/2=nT/8, where n is odd, i.e. in planes distanced (in time scale) from T 1  (or along diameter D from the perimeter of C) by τ/2=T/8, 3τ/2=3/8T, 5τ/2=5/8T and 7τ/2=7/8T. Similar expressions can be obtained for τ=T/m, where m&gt;4. 
         [0054]    In a particular numerical example, for a burst length T=10 μsec, T=20 μsec, D=30 mm and a longitudinal ultrasound velocity in tissue 1500 m/sec, one will get shear waves with period Θ=2τ=20 μsec i.e. frequency f=1/T=50 kHz, and positions of “in phase” shear waves will be τ/2=T/4=5 μsec and 3τ/2=3/4τ=15 μsec. 
       Shear Wave Generation in the SP Regime by Multi-Electrode Piezoelements 
       [0055]      FIG. 8  shows schematically one use of an apparatus with multi-electrode transducers used to produce shear waves. The use is shown for parallel transducers T 1  and T 2 . For a cylindrical transducer configuration the operation is the same, except that the multi-electrodes are placed as back electrodes on a cylindrical piezoelement. T 1  (or C) has an array of back electrodes  802   a ,  802   b  and  802   c , and T 2  has an array of back electrodes  804   a ,  804   b  and  802   c . Although only 3 electrodes are shown, there can be more. There is a single front electrode  133 . Electrodes  802   a  on T 1  and  804   a  on T 2  are excited by a burst with τT=T/2 and generate an ultrasonic beam (burst)  806  in the SP regime as described above. This produces shear waves (like a shear wave  801 ) along the entire ultrasonic beam length D, and intensive shear waves in planes T/4 and 3T/4. If electrodes  802   b  and  804   b  are excited simultaneously with  802   a  and  804   a  to generate two parallel ultrasonic beams  806  and  808 , and if the distance  812  between the centers of these beams equals nλ shear /2 where n is even, then there is additional amplification of shear waves generated by the two beams. These two beams can be treated like virtual resonators of shear waves. Virtual resonators are described in detail in PCT application PCT/IB2011/051917. 
         [0056]      FIG. 9  shows schematically another use of the apparatus with multi-electrode parallel transducers of  FIG. 8  to produce shear waves. A shear wave  901  is generated by two ultrasonic beams  906  and  908  emitted simultaneously by  802   a  and  804   b . Two ultrasonic beams may also be emitted simultaneously by  802   b  and  804   a  as well as by other pairs, leading to shear wave generation as above. In this embodiment, a distance  912  between the centers of beams  906  and  908  is fixed at nλ shear /2 where n is odd, because of the opposite direction of the ultrasonic bursts (radiation forces) and, as a result, the opposite shear deformations. The beams are narrow and spaced by fixed distances (odd half-wavelength for opposite directions and even half-wavelength for parallel directions of bursts) for any number of electrodes. 
       Shear Wave (SSW) Generation in a Supersonic Regime 
       [0057]      FIG. 10  shows schematically a use of an apparatus with multi-electrode transducers to produce shear waves in a supersonic regime (also referred to herein as “supersonic shear waves” (SSWs)) or to produce shear wave resonance (SWR). In  FIG. 10 , the configuration of electrodes is the same as in  FIGS. 8 and 9 , but the electrode couples are now switched on successively instead of simultaneously, i.e.  802   a - 804   a , then  802   b - 804   b  and then  802   c - 804   c . This leads to successive generation of ultrasonic beams  1006 ,  1008  and  1010  and to the step by step move of a beam down (e.g. beam  1006 ) or up (e.g. beam  1010 ). At appropriate distances between beams and switching times (delays), one can get SWR or a SSW regime. Supersonic shear waves are described in detail in PCT/IB2010/052624. A supersonic regime is realized if a beam is moved in the shear wave propagation direction with a velocity higher than the shear wave velocity and when there is some constructive interference (similar to that of a Mach cone from an airplane). 
         [0058]    Let the distances between ultrasonic beams  1006 ,  1008  and  1010  be equal to shear wavelength λ shear  (or to a multiple n of this wavelength, nλ shear ). Let electrodes  802   a - 804   a  be switched on at time t=0 to generate a burst of length τ=T/2=10 μsec and then work in a SP regime. Electrodes  802   b - 804   b  are then switched on at time t1=20 μsec and electrodes  802   c - 804   c  are switched on at time t2=40 μsec. Consequently, the SP ultrasonic beam position is moved from  1006  to  1008  to  1010  or vice versa. As a result, the shear waves generated by the first beam reach the center line of the next beam when the next beam is switched on (in phase) and amplify a shear wave resonantly. This effect is superposed on the synchronous pumping and resonant amplification of shear waves in definite planes described above. For SSW generation, an ultrasonic beam has to move faster than a shear wave generated by the same beam, i.e. switching times t1 and t2 must be less than or equal to t1=20 μsec and t2=40 μsec. 
         [0059]      FIG. 11  shows schematically another use of an apparatus with multi-electrode transducers to produce supersonic shear waves. In this embodiment, electrodes arranged as in  FIG. 9  are used to switch on beams  1106 ,  1108  and  1110  sequentially in the directions shown, and shift the beams up or down. The distances between ultrasonic beams  1006 ,  1008  and  1010  in this case must be equal to half of shear wavelength λ shear /2 (or to a multiple n of half of wavelength nλ shear /2, where n is an odd integer). 
         [0060]    To realize a quasi-continuous movement of ultrasonic beams, the distances between beam centers for both cases shown in  FIGS. 10 and 11  can be very small, on the order of 100 micrometers. The shifting “step” can be of the same order, while a beam width can be 2-3 mm. To realize such fine shifting, each back electrode can be subdivided into a plurality of very thin strips with width and gaps of tens of micrometers, as in a standard linear ultrasonic transducer array. Exemplarily, each back electrode may be divided into 20 such strips. In this case, the ultrasonic beam can be moved up or down with a fine step (“quasi-continuously”) at shear wave velocity (providing shear wave resonance (SWR) in a “wave resonance regime”) or at a higher than shear wave velocity (“SSW regime”) using appropriate switching times. 
         [0061]    An apparatus as in  FIGS. 5   a, b  may be used as a measurement cell for measuring tissue properties before and after treatment, for assessment of shear tissue properties (e.g. shear elastic modulus). Shear waves generated by any of the methods described in  FIGS. 7-11 ) are received by the shear mode element. This enables to measure the shear wave velocity (transit time) and calculate the shear elastic modulus of the tissue (from a known distance from the SP beam to the shear mode element and a measured transit time for shear waves). Such a measurement is more informative than longitudinal elastic modulus measurements. 
       Exemplary Shear Wave Applications 
       [0062]    Shear waves generated by wave resonance can be used in various applications. As shown above, shear deformation created by a burst radiation force can be localized in a narrow sub-skin layer, namely in the dermis and epidermis layers where hair roots are placed. Therefore, shear deformations can act like a shaver. Other applications include cellulite removal, skin renewal and fat reduction by shear waves-assisted therapy (shear waves supplemented by ultrasonic burst treatment, etc.). The biological mechanism of shear wave influence on tissue and cells is described in PCT/IB2011/051917. Note that the shear wave frequency depends on burst length. Attenuation and wavelength of shear waves depends on frequency. Therefore, one can use shear wave frequencies at which a shear wave will attenuate practically on one wavelength, and this wavelength can be a tenth of a micron—close to a cell size (causing effective destruction of cells). 
       Combinational Treatment Using SP, SWR, SSW and RF 
       [0063]    The SP, SWR and SSW effects described above can be applied in a “combinational” treatment of tissue, by adding RF capabilities to each of the apparatuses described above and by ensuring that the burst compensation layers are electrically conductive. In addition, application of vacuum suction on tissue, as described in detail in PCT/IB2010/052624, can be used with any apparatus and any method described herein for various treatments. It is well known that RF frequencies of hundreds of KHz to MHz range are effective for RF thermal treatment of live tissues. Physically, the origin of tissue heating by RF is due to ion movements and vibrations as well as orientation and rotations of polar molecules (dipoles) like in dielectrics and electrolytes. In the low MHz range, RF dielectric losses in living tissue have minor importance. At 1-2 MHz, the electrical resistance of tissue with high electrolyte (water with dissolved salts) content, like blood, muscles and internal organs is equal to 100-200 Ohm·cm. In tissue with low electrolyte content, like fat and bones, the electrical resistance is much higher and equals 2000-5000 Ohm·cm. The thermal influence of RF is in direct relationship to the squared current density, which is usually 0, 01-0, 03 A/cm 2 . 
         [0064]      FIG. 12  shows schematically a physical mechanism of combinational treatment using SP, SWR, SSW and RF. An AC burst voltage (exemplarily at 100 kHz-2 MHz) is supplied to a piezoelement (e.g. in T 1 , T 2  or C) to obtain SP, SWR and SSW regimes as explained above. RF electrode  401  mounted to plunger  64  is AC-electrically excited as described above in opposite polarity with a front electrode  133 . Burst compensation layers  132  are made conductive (e.g. of conductive rubber), so the AC electric voltage synchronously appears in tissue as RF lines  402 , leading to RF heating. The SP, SWR and SSW ultrasonic fields provide periodical vibrations of tissue together with ions (deformations S). The spatial distributions of the SP, SWR and SSW ultrasonic fields and the RF electric field coincide. Synchronous (at the same frequency) excitation of RF fields and ultrasonic waves (in form of SP) and simultaneous excitation of SWR and SSW at low frequency (as discussed above, the shear waves frequency is in low kHz range and as a result displacement amplitudes are very high) will lead to multi-resonant movements of ions (ions will vibrate under AC fields, RF and mechanical deformations S in ultrasonic fields). SP, SWR and SSW will intensify the ion movements, leading to current increase and additional RF heating. 
         [0065]    Note that combinational treatment using SP, SWR, SSW and RF is non-trivial, because only in the transducer configurations disclosed herein do RF and ultrasonic fields (RF, SWR, SSW′ and SP) coincide spatially and are superimposed synchronously to provide new biological and physical effects. 
         [0066]    In summary, in some embodiments, there is provided synchronous pumping of ultrasonic waves without standing wave formation but with all related advantages: safety (minimal ultrasound intensity on the skin), efficiency (resonance grows of ultrasonic burst amplitude) and capability for continuous treatment of a tissue region temporarily fixed by vacuum inside an ultrasound transducer without nodal structure inherent for standing waves. In some embodiments, there is provided shear wave generation in a SP regime. In some embodiments, there is provided shear wave resonance. In some embodiments, there are provided supersonic shear waves. In some embodiments, there are provided combinational treatments using a combination of SP ultrasonic waves +RF, SWR+RF, SP+SWR+RF, SP+SWR+SSW+RF, and each of these combinations plus vacuum massage. 
         [0067]    In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily an exhaustive listing of members, components, elements or parts of the subject or subjects of the verb. 
         [0068]    Various embodiments described herein are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments and embodiments comprising different combinations of features than those noted will occur to persons of the art. The scope of the invention is limited only by the following claims. 
         [0069]    All patents, patent applications and publications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent, patent application or publication was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art.