Abstract:
An ultrasound therapy system and method for providing therapeutic treatment to tissues using multiple, user-selectable frequencies generated from a single applicator sound head. The multi-frequency ultrasound therapy system and method includes a generator/control unit having a microprocessor for controlling and monitoring the generation and output of ultrasonic energy. A precision oscillator is connected to the microprocessor, and wave shaping circuitry is also provided to generate a wave-form for amplification by an output amplifier. The amplified high frequency wave-form is supplied to and drives an applicator sound head at an appropriate frequency for producing the desired therapeutic ultrasonic waves for treatment. Bonded to a cup member and positioned within the applicator sound head is a transducer crystal which converts high frequency wave-forms into mechanical (acoustic) vibrations, thus producing thermal energy in the form of heat for penetration into tissues under treatment. Both the transducer crystal and the applicator cup are designed with the capability of providing at least three different output frequencies from a single applicator sound head, while supplying three different penetration depths for treating a patient&#39;s tissues.

Description:
BACKGROUND 
     1. The Field of the Invention 
     This invention relates to ultrasound therapy devices and, more particularly, to novel systems and methods for providing ultrasound therapy at multiple, user-selectable frequencies using a single applicator sound head. 
     2. The Background Art 
     Heat has long been known to have many beneficial and necessary effects in the rehabilitation process. Heat increases the extensibility of collagen tissue, decreases joint stiffness, produces pain relief, relieves muscle spasm, increases blood flow, increases local metabolism, increases nerve conduction velocities, and assists in the resolution of inflammatory infiltrates, edema, and exudates. Heat has also been used as part of cancer therapy. 
     Investigations have shown that subjective complaints of stiffness on the part of a patient with rheumatoid arthritis coincide with changes in the measurements of the viscoelastic properties of joints. The joint stiffness, assessed both subjectively and by objective measurement, can be decreased by the application of heat, thereby decreasing the patient&#39;s discomfort. 
     After hot packs, ultrasound is probably the most frequently used physical agent in treating musculoskeletal pain and soft tissue injuries. Millions of ultrasound treatments are performed each year in the United States and Canada. Ultrasound produces the desirable therapeutic effects of any deep-heat modality. The effect of ultrasound that may be the most distinguishable, however, is its ability to selectively increase the temperature in local, well-circumscribed areas. 
     Ultrasound is a form of acoustic vibration occurring at frequencies too high to be perceived by the human ear. Thus, frequencies under 17,000 Hz are usually called sound, whereas those above this level are designated ultrasound. With the exception of the differences in frequencies, the physics of ultrasound is in no way different from that of audible sound. Ultrasonic frequencies typically used for therapeutic purposes range between 0.8 and 3 MHz. 
     The temperature distribution produced by ultrasound is unique among deep-heating modalities. Ultrasound causes comparatively little temperature elevation in the superficial tissues, but has a depth of penetration in the musculature and other soft tissues. In normal biological applications, for example, about 50% of the ultrasound energy is transmitted to a depth of 5 cm (1 inch) or greater, and this depth of penetration can be effectively employed in reaching deep tissues, such as joint capsules and deep muscles. For this reason, ultrasound is generally the treatment of choice when it is desirable to provide deep heat. 
     The therapeutic ultrasound machine consists of a generator that produces a high-frequency alternating current. The high frequency electric current is then converted by a transducer into mechanical (acoustic) vibrations. The transducer consists basically of a crystal inserted between two electrodes. As an alternating electrical charge is applied to the surfaces of the crystal, the crystal is made to vibrate. As the crystal vibrates rapidly, sound waves are produced. 
     Ultrasound waves are transmitted more effectively through water, oil, or transmission gel than air. Consequently, a coupling agent is used in clinical applications to &#34;couple&#34; the applicator sound head to the patient&#39;s body in order to ensure that the ultrasonic waves are properly transmitted to the desired treatment site. Such a coupling agent may, for example, be in the form of a gel or lotion which is applied to the skin of the patient over the area to be treated. Water may also be used as a coupling agent in appropriate cases by submerging a portion of the patient&#39;s body in a water bath. The sound head is then positioned in the coupling agent over the patient&#39;s skin, and the generator is activated. Ultrasonic waves produced at the sound head are transmitted through the coupling agent into the patient to provide the desired therapeutic treatment. 
     The treatment properties of the sound applicator depend upon its diameter and frequency. For example, a small diameter applicator produces a small diameter ultrasound beam. The angle of divergence of the beam is also generally greater than if a larger diameter applicator is used. For this reason, it may be difficult to treat the deep tissues in an area using a small diameter applicator. On the other hand, if the radiating surface of the applicator is too large, it may be difficult to maintain contact with the surface of the body at all times. Consequently, applicators of different sizes are generally provided with ultrasound equipment, and the size used will depend upon the nature of the treatment being performed. 
     Similarly, ultrasound frequencies of about 1 MHz are typically used to treat deep tissue, while higher frequencies do not penetrate the patient as deeply. A practitioner will thus decide which frequency to use depending upon the patient&#39;s condition. Unfortunately, however, prior art ultrasound devices typically have sound heads which are capable of operating effectively at only a single frequency. Consequently, the user will generally be required to purchase several different sound heads of each size in order to provide the varying penetration depths needed for different therapeutic treatments. 
     The need to use several different sound heads in order to provide ultrasonic therapy is, of course, a significant expense. For example, if a user wishes to have three different frequencies available on three different sizes of sound head, nine separate sound heads would have to be supplied. 
     Moreover, the sound heads would need to be changed in order to vary the ultrasound frequency, as well as to vary the head size. Changing sound heads can be both time-consuming and cumbersome since the operational parameters of the ultrasound equipment must be readjusted each time the head is changed. 
     BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
     In view of the foregoing, it is a primary object of the present invention to provide an ultrasound therapy system and method having a single sound applicator head which is capable of operating at three different frequencies, thereby eliminating the need to have a separate head for each frequency. 
     It is also an object of the present invention to provide an ultrasound therapy system and method which uses a microprocessor to detect the size of applicator head being used and to thereafter control and monitor all functional aspects of the generation and output of ultrasound waves. 
     Further, it is an object of the present invention to provide an ultrasound therapy system and method which simplifies use and decreases manufacturing costs. 
     Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an ultrasound therapy system is disclosed in one embodiment of the present invention as including a generator/control unit having a microprocessor for controlling and monitoring the generation and output of ultrasonic energy. Connected to the microprocessor is a precision oscillator providing wave shaping circuitry to generate a wave-form which is further amplified by an output amplifier. 
     The amplified high frequency wave-form is supplied to and drives an applicator sound head at the appropriate output frequency, whereby producing the desired ultrasonic waves for therapeutic treatment. Positioned within the applicator sound head is a transducer crystal which converts high frequency electrical signals into mechanical (acoustic) vibrations, thus producing thermal energy in the form of heat for deep penetration into those tissues being treated. Both the transducer crystal and the applicator cup are designed with the capability of providing at least three different output frequencies from a single applicator sound head, while supplying three different penetration depths for treating a patient&#39;s tissues. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
     FIG. 1 is a perspective view of a multi-frequency ultrasound therapy system in accordance with one presently preferred embodiment of the present invention; 
     FIG. 2 is a functional block diagram illustrating one presently preferred embodiment of the multi-frequency ultrasound therapy system and method of the present invention; 
     FIG. 3 is a cross-sectional view of one presently preferred embodiment of the transducer and cup of one presently preferred embodiment of the applicator head of the ultrasound therapy system of the present invention; and 
     FIGS. 4A through 4E (hereinafter referred to collectively as FIG. 4), are an electrical schematic diagram illustrating one presently preferred configuration of an electrical circuit for use in connection with one presently preferred embodiment of the multi-frequency ultrasound therapy system and method of the present invention. (It should be noted that FIGS. 4A through 4E comprise a single schematic diagram, the labeled tabs illustrating the electrical interconnection between the Figures.) 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in FIGS. 1 through 4, is not intended to limit the scope of the invention, as claimed, but it is merely representative of the presently preferred embodiments of the invention. 
     The presently preferred embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
     One presently preferred embodiment of the multi-frequency ultrasound therapy system of the present invention, designated generally at 10, is illustrated in FIG. 1. As shown, ultrasound system 10 comprises an ultrasound generator/control unit 12 and an applicator sound head 20. Applicator head 20 is removably connected to generator/control unit 12 by means of an electrical cord 14. 
     Referring now to the functional block diagram of FIG. 2, ultrasound system 10 includes a microprocessor 30 which controls and monitors the ultrasound generation and output. Microprocessor 30 is connected to and controls a precision oscillator 32. The output of precision oscillator 32 is provided to wave shaping circuitry 34, and the generated wave-form is amplified by means of output amplifiers 36. The amplified wave-form is supplied to and drives the ultrasound applicator head 20 at the appropriate frequency, thereby producing the desired ultrasonic waves for use in treatment. 
     The central component of applicator head 20 is a transducer crystal for converting electrical signals into ultrasonic waves. This transducer crystal is provided with electrodes in conventional fashion and is electrically connected to generator/control unit 12 via cable 14 (see FIG. 1). Apart from the novel transducer configuration described below, the overall structure of applicator head 20 is substantially the same as that of other applicator heads used for ultrasound therapy and is believed to be well known in the art to which this invention pertains. The overall structure and configuration of applicator head 20 will not, therefore, be described in further detail herein. 
     As depicted in cross-section in FIG. 3, the transducer crystal of applicator head 20 comprises a ceramic crystal 40 which is bonded to an aluminum alloy applicator cup 42. Significantly, crystal 40 and cup 42 are capable of producing three different frequencies of ultrasonic waves. In one presently preferred embodiment, for example, crystal 40 and cup 42 positioned within the applicator head 20 can produce nominal frequencies of 1 Mhz, 2 Mhz and 3 Mhz. Advantageously, therefore, a single applicator head 20 can be used to provide ultrasound therapy at three different frequencies, and, as a result, at three different tissue depths within a patient. 
     Crystal 40 is formed of a suitable poled ceramic material, such as, for example, a high Q ceramic material such as EC97 from EDO Western Corp. The ceramic is formed and prepared in a manner well known in the art to have a natural resonance at 2 Mhz (that is, the ceramic is formed so as to be one-half wavelength thick at 2 Mhz). Thus, for example, the ceramic may be 0.0431 inch (1.0947 mm) thick. 
     Applicator cup 42, on the other hand, is formed of an aluminum alloy. This alloy is preferably hard enough to resist excessive abrasion during use and may, for example, be alloy 6262T8. Applicator cup 42 is likewise formed and prepared to have a natural resonant frequency of 2 Mhz. 
     In order to achieve multiple resonances reliably, both crystal 40 and cup 42 are fabricated to thickness and flatness tolerances of 0.0005 inch (0.01270 mm). In addition, the thickness of the glue bond between crystal 40 and cup 42 is maintained at very nearly zero. 
     The thin bonding of crystal 40 to applicator cup 42 can be done reliably by the following procedure to yield epoxy bonds on the order of 1 to 2 μm thick. The bonds will have excellent adhesion, and it is not necessary to heat the parts being bonded. Room temperature bonding is especially important when the pieces being bonded have different coefficients of thermal expansion, as do ceramic and aluminum. When such pieces are bonded together at elevated temperatures, they may crack or distort when they cool down. 
     The ceramic slab and the aluminum cup are first cleaned and then bonded to each other using a very thin (low viscosity) epoxy adhesive. For example, Dow epoxy resin (DER) 332 may be melted in a disposable beaker of known weight. After the resin is in a liquid state at room temperature, 11 PHR of Dow epoxy hardener (DEH) 20 is added using an eye dropper. Both DER 332 and DEH 20 are manufactured by Dow Chemical Company and are available from a number of commercial vendors. One should exercise caution when using these substances, since DEH 20 is a diethyl triamine and is therefore volatile and fumes when opened; and it is also an irritant with a SPI classification 5. Even the epoxy will fume as it cures. Despite these hazards, however, this adhesive has been found to give very excellent results at room temperature. DEH 20 yields bonds with excellent adhesion and also seems to lower the viscosity of the resin significantly, which is another desirable property for making thin bonds. 
     It is convenient to use a hypodermic needle to deliver the epoxy to the pieces being bonded. Thus, for example, after mixing the hardener with the resin, one or two ml of the epoxy can be poured into a 10 cc Luer-LOK disposable syringe (B-D #5604 from Becton, Dickinson, and Co., Rutherford, N.J.). Next a disposable micropore filter is attached to the syringe, along with a 23 gauge, 1 inch (2.54 cm) needle. The first few drops delivered by the needle are preferably discarded to help insure that the epoxy used in making the bond is not contaminated. A bead of filtered epoxy is placed over the center of one of the parts being bonded, and the other piece is laid down on top of the glue using clean tweezers. Air bubbles in the glue bead must be dealt with either by pushing them off one end of the glue bead, popping them, or by sucking them up with the syringe. 
     The next step in thin bonding is to press out almost all of the epoxy between the pieces being bonded. To do this we employ the nonuniform pressure bonding technique of E. P. Papadakis (Journal of Adhesion, &#34;Nonuniform Pressure Device for Bonding Thin Slabs to Substrates,&#34; 1971, Vol. 3, pp. 181-94). The idea behind this technique is to apply more pressure to the center of the parts being bonded than one applies to the edges, thereby avoiding the problem of having epoxy trapped between. This is accomplished by using a jig and a spherical piece of rubber to press the ceramic slab against the aluminum, thereby exerting more force at the center of the ceramic than we do at the edges. After two days, the bonded parts are removed from the bonding jig. Excess epoxy can be removed with an epoxy stripper or Miller Stevenson Product #MS111. 
     As mentioned above, crystal 40 is provided with electrodes (not shown) in a manner which is well known in the art (such as by vapor deposition). An electrical lead 44 is soldered to the electrode of crystal 40 to provide the needed electrical connection. A thermistor 46 is also provided in order to sense the temperature of crystal 40, for reasons which will become readily apparent from the discussion which follows. 
     The combination of ceramic and aluminum layers, as described above, results in a crystal having a natural resonance at 2 MHz, with additional, useable resonances at 1 and 3 MHz. In addition, the use of Aluminum in combination with the ceramic provides a transition between the acoustic impedance of the ceramic and that of the human body. It has also been observed that the transducer bandwidth is increased, and stressing of the ceramic is reduced. 
     Referring again to FIG. 2, when ultrasound system 10 is first activated, applicator head 20 is scanned at a first known temperature through each of the three output frequency levels to determine the specific driving frequency of maximum efficiency at each level. The scanning of applicator head 20 is then repeated at a second, higher temperature. Finally, a known output (as measured by an ultrasonic power meter) is generated, and the impedance of applicator head 20 is calculated from the detected output voltage and current. For each of the three output frequencies, microprocessor 30 then stores the following three parameters for later use: (1) frequency of output, (2) head impedance, and (3) frequency change over the operating temperature. 
     During operation, the temperature and output current for each of the three output frequencies of applicator head 20 is continuously monitored by microprocessor 30. The driving frequency generated by oscillator 32 is adjusted, as needed, to maintain optimum ultrasonic output over the operating temperature range. If the temperature of applicator head 20 exceeds a pre-set safe level, microprocessor 30 actuates a warning signal (such as a bell and/or display screen), and operation is disabled until the temperature of applicator head 30 returns to a safe level. Applicator head 30 is thus protected from damage in situations where poor coupling results in self-heating. 
     Reference is next made to FIG. 4, which illustrates in more detail one preferred embodiment of a schematic diagram derived from the functional block diagram of FIG. 2. Those of ordinary skill in the art will, of course, appreciate that various modifications to the detailed schematic diagram of FIG. 4 may easily be made without departing from the essential characteristics of the invention, as described in connection with the block diagram of FIG. 2 above. Thus, the detailed schematic diagram of FIG. 4 is intended only as an example, and it simply illustrates one presently preferred embodiment of a schematic diagram that is consistent with the foregoing description of FIG. 2 and the invention as claimed herein. 
     In FIG. 4, the designations &#34;RFI&#34; and &#34;RFC&#34; refer to electrical contacts at opposite ends of a ribbon cable which interconnects portions of the circuitry. RFI 1 thus connects to RFC 1, and so forth. Similarly, the designations &#34;RFP&#34; and &#34;RFPC&#34; refer to electrical contacts at opposite ends of a power cable, with RFP 1 connecting to RFPC 1, etc. 
     Table 1 below identifies the specific electrical components illustrated in FIG. 4. Unless other wise noted, capacitors have 0.2 inch (5.08 mm) spacing and are 25 volt, and resistors are 0.25 watt. 
     
                                           TABLE 1__________________________________________________________________________No.      Description  Manufacturer                         Part Number__________________________________________________________________________C101, 102    Cap 20 pf 25 Volt    CeramicC205, 304    Cap 100 pf 25 Volt    CeramicC103-107 Cap .001 μf 25 Volt    CeramicC108, 109,    Cap .1 μf 50 Volt110, 213,    Ceramic214, 301,302, 305,306, 308,315, 316C900-912 Cap .1 Ufd 50    Ceramic, 0.3&#34; lead    spacingC203     Cap .1 Ufd 100 Volt    CeramicC215     Cap 22 μf 16 Volt    Tant. RadialC202     Cap 100 μf 63 Volt    Elec. RadialC201     Cap 470 μf 15 Volt    Elec. RadialC210     Cap 1000 μf 100 volt    al. elec. axialC211, 212    Cap 1000 μf 50 volt    al. elec. axialC511, 513    39 pfC405     200 Pf 100 Volt low    ESRC516     200 pfC509, 507    .022 CeramicC402, 403,    .01 Ufd Ceramic503, 510,601C401, 411,    0.1 Ufd Ceramic413, 414,415, 416,417, 418,419, 420,502, 504,505, 512,515C406, 410,    .1 Ufd, 65-100 volt412      CeramicC506, 514    .33 Ufd Mono cerC404, 408,    1 Ufd 50 Volt                 Kemet   C330C105K5R5CA409      Ceramic, low ESR     C340C105M105CAC508     4.7 Ufd Tank, 10    voltsC407     100 Ufd 63 Volt Al    ElectC501     220 Ufd Tant 10 Volts    or higherR217     2 Ohm 2 WattR201     3.9 OhmR102     82 OhmR101, 319    100 OhmR209     470 OhmR202, 203,    1K205, 207,216, 220,318R305     2.2KR215     3KR208     3.9KR212, 214,    4.7K219, 301,302R204, 206,    10K213, 218,304, 317,322, 323,324R221     10K 2 WattR210     22KR303     27KR211     56KR315     100KR320     180KR310, 311    1 Meg 1%R312, 313    2.2 Meg 1%RS101, 102    4.7K Ohm Sip 9 Pin                 Bourns  10X-1-472    BussedR403     15 OhmR411     22 OhmR604     39 OhmR501     180 OhmR408     560 OhmR402     1KR606     2.2KR503     3KR504     5.1KR401, 414,    10K602, 603R502     33KR415, 505,    100K507R605     390KR601     680KR422     50 Ohm 2 Watt Non-    inductiveRV201    Pot 10K 17 Turn Vert                 Bourns  3296W-1-103    Adj.RV304, 305    Pot 100K 17 Turn Vert                 Bourns  3296W-1-104    Adj.         Spectrol                         68WR104RV306    Pot 10K 1 Turn                 Bourns  3386T-1-103D206, 301    Diode 1N4148 Si Gen  1N4148    PurposeD201, 202    Diode 1N4004         1N4004D203     Diode 1N5822         1N5822BR201    Bridge Rectifier 4                 GI      KBU4A    AMP 100 voltD204     Diode SF32 Schottky                 SF32    300 Volt (Diodes    Inc.)Z303     Zener Diode 10 Volt  1N758A, 1N4740    1/2 WattQ 2 0 2  Transistor MPSA06    MPSA06301 (Q3),302, 303Q201     Transistor TIP 107   TIP107D407     Schottky diode, 1A @                 Mot     1N5817    40 Volts     IR      11DQ03D408     Gen pur Schottky                 HP      1N5711    diode, 1 pf or lessD401, 402,    Si gen pur diode     1N4148403, 404,601LED602   IR transmitter                 Mot     MLED81LED601   IR Detector  Siemens SFH2030FQ401, 402    Power MOSFET         IRF-510U110                          xx00U103, 109                     xx138U106, 111                     xx244U107, 108                     xx574U306     7574 8 bit ADC       7574U304     Quad Op Amp          LM324U202     Quad Op Amp          LF444U305     8PST CMOS Switch                 MAXIM   DG508U201     H64180 Microprocessor                 Hitachi H64180    6 Mhz        Zilog   Z180U101     Power Supervisor                 MAXIM   MAX690U201     12 bit serial DAC                 MAXIM   MAX543U203     precision reference                 MAXIM   REF02U105     EPROM (200 ns)       27C512U104     RAM 8K               HM6264U301     4 Non-volatile RAM                 CATALYST                         35C104VR202    +12 Volt regulator   7812T    (TO-220)VR203    -12 Volt regulator   7912T    (TO-220)VR201, 204    IC LM2575T-5.0                 NAT     LM2575T-5.0    Voltage Regulator    (LM1575T-5.0 also OK)U401                          xx74U402     MOSFET Driver                 Natl    DS0026                 Teledyne                         4423                 Harris  7667U501     Frequency Synthesizer                 Signetics                         SAA1057U502     Phase Locked Loop                 Mot     &#39;HC4046U601     JFET Op Amp          LF442VR501    7805 Regulator (TO-  7805    220 case)X101     Crystal, HC-18/U                 MTRON   MP-1-12.000    holder, 12 MHZ                 FOXL201     Inductor 100 μH 1 Amp                 Caddell-Burns                         CB7070-25L202     Inductor 150 μH 5 Amp                 Miller  JWM5506                 ACC     FC5-151                 DALE    IH-3-150U102     68 pin PLCC socketU207     Molex carrier socket                 Molex   78805-518BL201, 202    Header 2 pinCHIM     Header 2 Pin (Molex    Latch Type)BL202    Back Light Inverter                 ERG     JLA 05-2-3PSEN      Header 4 Pin latching                 Molex   22-29-2041RFI, LCS, LCD    Header 20 Pin Dual    InlineKB       Header 12 Pin Single    RowSER, CAL 4 pin bergstikVR202    Termalloy Heat Sink                 Termalloy                         6298-2-SF2-P2    w/mtng studDISPLAY  Display Ribbon Cable    20 Pin Double RowRFI      20 Pin dual row    headerFB401, 501,    1 bead: 501, 2; 2                 JW Miller502      Beads: 401RLY401   Omron relay  Omron   G5V-2H-12VDCX401     coupling transformer                 Valor   TT1501OUT      mini-din Jack (NO                 Switchcraft                         SMD8FRA121    sub)         Singatron                         MDJ-106-8PSC-G30SENS     4 pin plug (key: 89-                 Molex   10-11-2043    00-0422)X501     4 Mhz Crystal, HC-18/U                 M-Tron  MP-1-4.000    Holder       FoxRFPC     6 circuit mini-fit                 Molex   39-29-5063    Jr. (Also -4069)Q401, 204    Heat Sink    Termalloy                         6298-2-SF2-P2VR501    Heat Sink    Thermally                         6038B-TTFAN      1.5&#34; 12 VDC Fan, 7 CFM                 Delta   DFB0412MFAN      2 pin locking header                 MolexFAN      Matching header                 Molex    w/terms__________________________________________________________________________ 
    
     The output transformer shown in FIG. 4 (OXFMR) has a Magnetics #A-42616-UG core, a Magnetics #PC-B2616-11 (single section) bobbin, and a Magnetics C2616-14 (with #4-40 mounting tabs) clip. The primary winding has 12 turns CT, and the secondary winding, 6 turns CT. Windings are 1 single layer, using Litz wire, and primary and secondary wires are twisted together prior to winding with 2-5 twists per inch. Two and one-half turns of the primary are wound before starting the secondary to that the secondary is physically in the center of the bobbin. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.