Abstract:
An applicator supplying RF power for therapeutic diathermic treatment of a patient includes a radiation shielding device for shielding the applicator against misapplication of radiation to objects in the surroundings and unintended areas of the patient&#39;s body, and a coupling device for electrically coupling the radiation shielding device to at least one point of the body of a patient in a low impedance manner that reduces the potential drop from the grounded radiation shield to the body tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional application 61/185,393, entitled “SHIELDED DIATHERMY APPLICATOR WITH AUTOMATIC TUNING AND LOW INCIDENTAL RADIATION,” filed Jun. 9, 2009, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of bioelectromagnetics, specifically, the conversion of radio frequency (RF) energy in human or animal tissue to achieve therapeutic purposes both thermal and athermal. It represents advancements in equipment design that substantially reduce the incidental radiation of energy, while improving the consistency of energy conversion within the desired target tissue. 
     2. Description of Related Art 
     RF coil diathermy systems utilize coils to radiate both electric and magnetic fields. The proximity of the coils to the target tissue results in concentration of the electric and magnetic fields generated by RF excitation of the coils and energy conversion in the tissues near the coils. A problem with these coils is that significant fields can also exist at distances away from the coils, which can cause RF energy conversion within other tissue, exposure to workers nearby, and exposure to others in the general vicinity of the coils. It would, therefore, be desirable to provide an RF coil diathermy system that avoids the foregoing problems. 
     SUMMARY OF THE INVENTION 
     Disclosed is an applicator apparatus for supplying RF power for therapeutic diathermic treatment of a patient. The applicator includes radiation shielding for shielding the applicator against misapplication of radiation to objects in the surroundings and unintended areas of the patient&#39;s body, and a coupling device for electrically coupling the radiation shielding device to at least one point of the body of the patient in a low impedance manner. 
     The radiation shielding can include a conductive grid and at least one conductive pad electrically connected to the conductive grid to provide capacitive coupling to the body of the patient at least at one point. The periphery of the radiation shielding can curve or wrap around the non-applying areas of the applicator to form a curved conductive grid having radial spurs or fingers. The conductive pads can be circular in shape and can be connected at the electrical termination of each radial spur or finger of the curved conductive grid. 
     The conductive grid can include a substrate comprised of printed circuit material. The conductive grid can include an electrically conductive pattern disposed on a flexible, insulative substrate. 
     Also disclosed is a method of constructing a radiation shielded diathermy applicator device. The method includes providing a radio frequency diathermy applicator device including a first flexible coil structure, a second flexible coil structure, and a first non-conductive spacer between the first and second flexible coil structures. The method also includes providing a second non-conductive spacer disposed between the first flexible coil structure and a radiation shield that includes an electrically conductive grid pattern on a flexible, insulative substrate having radial fingers, wherein the electrically conductive grid pattern includes conductive pads on the radial fingers; and wherein said radial fingers can be curved around the first non-conductive spacer, the first flexible coil structure, and the second non-conductive spacer, and coupled to the first non-conductive spacer, wherein said conductive pads are positioned on the first non-conductive spacer on a surface thereof opposite the first flexible coil structure. 
     It should be understood that the following descriptions, while indicating various embodiments of the invention and numerous specific details thereof, are given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an exploded perspective view of a flexible coil structure of a prior art radio frequency diathermy device; 
         FIG. 1B  is a cross-sectional view of the radio frequency diathermy device of  FIG. 1A  in an assembled state; 
         FIG. 2  is a cross-sectional view of a radiation shielded diathermy applicator in accordance with the present invention coupled to the body of a patient or treatment target; 
         FIG. 3  is a radiation shielding device of the applicator of  FIG. 2 ; 
         FIG. 4  is an electrical schematic diagram of an RF diathermy device; 
         FIG. 5  is a lumped series tuned circuit model of the tuning circuit of  FIG. 4 ; 
         FIG. 6A  is a plan view of the adjustable dielectric constant variable capacitor that can be used with the tuned circuit model of  FIG. 5 ; and 
         FIG. 6B  is a cross-sectional view of the adjustable dielectric constant variable capacitor shown in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention and the various features and advantageous details thereof are explained more fully and illustrated by the accompanying drawings and detailed in the following description. 
     Referring to  FIG. 1A , a prior art flexible coil structure  1 , like the one shown in U.S. 2006/0119462, which is incorporated herein by reference, includes a secondary flexible coil structure  5  having a flexible, spiral-like winding which is physically coupled or positioned in spaced relation to a primary flexible coil structure  3  that also has a flexible, spiral-like winding, and a non-conductive spacer  4  disposed between and in contact with both primary flexible coil structure  3  and secondary flexible coil structure  5 . 
     Referring to  FIG. 1B , in an unflexed state, coil structures  3 ,  5  are two-dimensional spirals, each occupying a separate plane. Desirably, these separate planes are parallel to each other with spacer  4  disposed between and coupled to both coil structures  3 ,  5 . Desirably, coil structures  3 ,  5  have a common central axis  7  and are positioned in spaced relation along central axis  7 . In one, non-limiting embodiment, coil structures  3 ,  5  include 18-gauge stranded silver-plated copper wire disposed on a sheet or substrate of insulative polytetrafluoroethylene (PTFE). Other types of wires and insulative sheets would also be acceptable. 
     Referring to  FIG. 2 , a cross-sectional view of a radiation shielded diathermy applicator  9  coupled to a patient  12  or treatment target is depicted. The exterior of applicator  9  is a non-conductive, flexible pouch  11  which allows applicator  9  to conform to a patient&#39;s chest, abdomen, back, and/or neck. Desirably, pouch  11  is made from nylon. However, this is not to be construed as limiting the invention. 
     Applicator  9  is in the form of a pad-shaped structure that includes a non-conductive layer  13  that separates pouch  11  from secondary flexible coil structure  5  contained within the pad structure of applicator  9 . Applicator  9  also includes a non-conductive layer  14  that separates a radiation shield  17  (described hereinafter) from pouch  11 . 
     Secondary flexible coil structure  5  is embedded or disposed between layer  13  and spacer  4 . Primary flexible coil structure  3  is embedded or disposed between spacer  4  and a non-conductive radiation shield spacer  15 . Spacer  4  separates secondary flexible coil structure  5  from primary flexible coil structure  3 . Layer  13  provides space between patient  12  and secondary flexible coil structure  5  when applicator  9  is being used by patient  12 . 
     Desirably, layer  13 , layer  14 , spacer  4 , and radiation shield spacer  15 , are each made from closed-cell polyethylene foams with thermoresistance, although other types of flexible, insulative material would also be acceptable. In one embodiment, layer  13 , layer  14  and spacer  4  are made of foam having a thickness of 9.525 mm, and radiation shield spacer  15  is a foam layer having a thickness of 31.75 mm. However, other thicknesses and materials would also be acceptable. 
     With reference to  FIG. 3  and continuing reference to  FIG. 2 , radiation shield  17  covers the non-patient facing side of radiation shield spacer  15  and has radial fingers  19  that curve around radiation shield spacer  15 , primary flexible coil structure  3 , spacer  4 , and connect to the patient-facing side of spacer  4 . Desirably, Velcro®  21  is used to connect each radial finger  19  to spacer  4 . However, this is not to be construed as limiting the invention since it is envisioned that any other suitable and/or desirable means can be utilized to connect each finger to spacer  4 . At the end of each radial finger  19  is a conductive pad  22  that faces the body part of patient  12  under treatment when applicator  9  is worn by a patient. Desirably, each pad  22  provides capacitive coupling to the body of the patient  12 . Radiation shield  17  also includes a conductive pad  24  which is coupled to a ground reference, e.g., a ground  31  sheath of coaxial cable  32  (shown in  FIG. 4 ) in use of radiation shield  17 . 
     Radiation shield  17  includes conducting tracks  23  formed on a flexible printed circuit material made of a flexible, insulative substrate  25 . Conducting tracks  23  are also disposed on substrate  25  and electrically coupled to conductive pads  22  and conductive pad  24 . Non-limiting examples of materials that can be used for this substrate include FR-4, G-10, or Kapton®. Kapton® is a registered trademark of E.I. du Pont de Nemours and Company. Desirably, radiation shield  17  has the grid-like pattern of conducting tracks  23  shown in  FIG. 3 . However, this is not to be construed as limiting the invention as it is envisioned that any suitable and/or desirable pattern having the same effect as the grid-like pattern shown in  FIG. 3  can be used. It should also be noted that other materials could be substituted for substrate  25  provided that any such material has sufficient flexibility and dielectric strength. 
     Desirably, radiation shield  17  adds only a small amount of stray capacitance across secondary flexible coil  5  while allowing electric field lines to terminate on the radiation shield conducting tracks  23 , which are coupled to a ground reference via conducting pad  24  coupled to the ground sheath  31  of coaxial cable  32 . Desirably, radial fingers  19  of radiation shield  17  remain constant in width as the radius of the radial fingers  19  increases radially from the center axis  27  of the radiation shield  17 . The pattern of radial fingers  19  on the periphery allows the fingers to be curved around shield spacer  15 , primary flexible coil structure  3 , and spacer  4 . The conductive pads  22  at the ends of radial fingers  19  define capacitive coupling elements that are positioned in spaced relation to the body tissue of patient  12  when applicator  9  is worn by the patient  12 . Each conductive pad  22  acts as one plate of a capacitor, with the body tissue of patient  12  acting as a second plate of a capacitor, and layer  13  acting as a dielectric between each pad  22  and the body tissue of patient  12 . 
     Conductive pads  22 , along with the body of patient  12  and layer  13 , form a capacitor which capacitively couples conductive tracks  23  to the patient&#39;s body  12 . More specifically, each conductive pad  22  spaced from the body of patient  12  by layer  13  acts as a separate capacitor in parallel with the combination of each of the other pads  22  spaced from the body of patient  12  by layer  13 . When multiple conductive pads  22  come into close proximity (spaced relation) with the body of patient  12  to form multiple parallel capacitors, these parallel capacitors act as a single large capacitor. Radiation shield  17  therefore avoids RF radiation fields from emanating to the surrounding environments by capacitively coupling these fields to the body of patient  12 . 
     Referring to  FIG. 4 , an electrical schematic diagram including supporting circuitry used with primary and secondary flexible coil structures  3 ,  5  in an RF diathermy device  28  is shown. Resistor R p  is a representation of the body of patient  12 . Inductor L 2  is a representation of secondary flexible coil structure  5  and inductor L 1  is a representation of primary flexible coil structure  3 . Capacitor C 1  is a representation of the capacitance that exists by the spacing among patient  12 , primary flexible coil structure  3 , and secondary flexible coil structure  5 . The capacitance of capacitor C 1  may also include the capacitance of conductive pads  22  adjacent the body of patient  12 . Capacitor C 2  is a variable capacitor that can be connected in parallel with R p  and C 1 . Capacitor C 2  enables tuning by matching the impedance of the combination of C 1 , C 2 , L 2 , and R p  to the impedance of supporting circuitry so the same impedance can be realized throughout diathermy device  28  regardless of the patient  12  coupled to the device  28 . This impedance matching allows the resonant frequency of the combination of C 1 , C 2 , L 2 , and R p  to be about the same for each patient  12  that uses the device. An isolation device  29  (e.g. a balun) transforms an unbalanced input signal on the L 3  side of device  29  into a balanced output signal on the L 4  side of device  29 , which output signal is supplied to primary flexible coil structure  3 . Isolation device  29  acts to electrical isolate primary and secondary flexible coil structure  3  and  5  from a ground reference, such as, without limitation, the ground  31  of a 75 ohm coaxial cable  32 , whereupon primary and secondary flexible coil structures  3  and  5  can “float” relative to said ground reference. Coaxial cable  32  connects the L 3  side of device  29  to an RF generator. 
     Referring to  FIG. 5 , a lumped, series tuned model circuit  33  is depicted. The schematic depicted in  FIG. 4  of RF diathermy applicator  28  can be reduced to create model circuit  33 . In model circuit  33 , resistor R p  represents patient  12 ; variable capacitor  30  represents the lumped capacitance of applicator  28 , including variable capacitor C 2 ; variable inductor  29  represents the lumped inductance of applicator  28 , and resistor  35  represents the lumped resistance of applicator  28 . These elements are connected to an RF generator (not shown) via coaxial cable  32 . The tuning range of model circuit  33  may be selected so as to avoid resonance when body tissue is not coupled to applicator  9 . 
     In model circuit  33 , a resistive value R s  of resistor  35  changes with tissue loading. Specifically, resistive value R, is lower when resistor  35  is unloaded and is higher when resistor  35  is “heavily loaded”. Resistive value R s  changes over a range of about 2:1 in practice and the resulting currents and voltages across the tuning circuit elements can then also be expected to vary as much as 2:1 at resonance and even more at detuned conditions. When model circuit  33  is properly matched and resonated, resistor  35  simplifies into a 50 ohm resistor. With 35 watts present, this represents a voltage of about 42 Volts rms and a current of 42/50˜0.84 Amps. These RF currents and voltages are significant values to apply to a tuning circuit, and when model circuit  33  is unloaded, the values increase significantly since the current flowing increases due to the lower load resistance value. 
     Referring to  FIGS. 6A-6B , a non-limiting exemplary embodiment of variable capacitor C 2 , shown in  FIGS. 4 and 5 , is depicted. Variable capacitor C 2  includes a moveable section of two low-loss dielectric materials  37 ,  39  that cause the average dielectric constant between the fixed metal plates (Contact A and Contact B) of capacitor C 2  to vary over a two-to-one range as the moveable section is rotated or moved between Contact A and Contact B. The materials  37 ,  39  selected in this particular embodiment are Teflon® and Noryl®, with approximate dielectric constants of 2 and 4, respectively. Teflon® is a registered trademark of E.I. du Pont de Nemours and Company. Noryl® is a registered trademark of Saudi Basic Industries Corporation (SABIC). It should be noted that other materials could be substituted for either of materials  37 ,  39 , depending on the range of capacitance desired. Use of these materials avoids the need for the metal plates (i.e., Contact A and Contact B) to have a moveable electrical contact, greatly improving reliability and lowering cost. The construction of variable capacitor C 2  in one embodiment is a circular design. It should be noted that other mechanical arrangements (for example a linear array) could be utilized without affecting the intended scope of this invention. 
     Variable capacitor C 2  is used to tune the radio frequency of RF diathermy device  28  to resonance, the value of which depends upon stray capacitances across secondary flexible coil structure  5 . The transformed impedance caused by variable capacitor C 2  varies from inductive to resistive and then to capacitive as the stray capacitances change and as variable capacitor C 2  is adjusted. 
     Numerous characteristics and advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of scope, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.