Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N1/00—Electrotherapy; Circuits therefor
  • A61N1/16—Screening or neutralising undesirable influences from or using, atmospheric or terrestrial radiation or fields

Definitions

• the present invention relates to, a patient-shielding system for use when a patient is exposed to capacitive currents as a result of immersion into a time-varying magnetic field. More particularly, this invention relates to a system for redirecting potentially harmful currents away from organs such as the heart when a medical procedure includes exposing that organ to a time-varying magnetic field.
• FIG. 1 An exemplary navigation system is shown in FIG. 1.
• the exemplary system of FIG. 1 contains platform 10 in which is embedded coils for generating a time-varying magnetic field. Two such coils are depicted as first coil set 12 and second coil set 14.
• Field line 22 depicts the orientation of a magnetic field amplitude at an instant of time. See also U.S. Patent 5,592,939.
• Present techniques for projecting a time varying magnetic field into a surgical region of interest preferably position the patient proximal to the coils that are generating the necessary fields. This is depicted in FIG. 2.
• Patient 24 is generally kept from direct contact with coil sets 12 and 14 by non-conducting layer 20.
• coil sets 12 and 14 located proximally to the surgical region of interest, may have differing voltage potentials.
• coil set 12 is at positive potential 16
• coil set 14 is at negative potential 18.
• a uniform amplitude field that has its major component lateral to a plane determined by an operating room table is thus generated by two coils at different voltage potentials separated along that lateral dimension.
• Field line 22 in FIG.2 indicates the direction of such an amplitude.
• the surgical region of interest has loop characteristics of what is known as a capacitive current.
• a schematic of such a current is depicted in FIG. 3.
• capacitive current 34 denoted by I, can exceed what is considered desirable.
• capacitive current 34 is of the order
• a patient-shielding and coil system including a coil wire electrically coupled to a source of electrical current, an electrically conductive surface, insulation material situated between the coil wire and the conductive surface, and a drain wire connected to the conductive surface and forming a capacitive current loop with respect to the source.
• the conductive surface has a resistance of substantially 1 ohm per square.
• the electrically conductive surface forms an incomplete enclosure of the coil wire, so as to create an incomplete electrical circuit.
• the conductive surface includes an upper portion and a lower portion.
• the conductive surface includes a polyester foiL apor deposited with aluminum.
• FIG. 1 depicts an exemplary coil system for generating a uniform amplitude magnetic field for a navigational system.
• FIG. 2 depicts an effect the exemplary system of FIG. 1 can have on a patient.
• FIG. 3 is a circuit diagram of a capacitive current loop formed by the configuration of FIG. 2.
• FIG. 4 depicts an exemplary patient-shielding and coil system consistent with the present invention.
• FIG. 5 depicts a cross section of a portion of the exemplary system of FIG. 4.
• FIG. 6 depicts an example of how current flows across a cross section of the exemplary system of FIG. 4.
• FIG. 7 depicts an alternative exemplary patient-shielding and coil system consistent with the present invention.
• FIG. 8 depicts a side view of the exemplary patient-shielding and coil system of FIG. 7.
• the present invention is directed to a system for redirecting potentially harmful currents away from organs such as the heart when a medical procedure includes exposing that organ to a time-varying magnetic field.
• FIG. 4 depicts a patient-shielding and coil system in accordance with a preferred embodiment of the present invention.
• the ends of coil wire 44 are attached to a driving voltage source (not shown). Between the ends of coil wire 44 and the coil assembly 40, coil wire 44 is wrapped about itself as twisted pair 47. Within coil assembly 40, coil wire 44 is looped N times. The current along coil wire 40 is denoted I .
• coil form 54 surrounds that portion of coil wire 44 where coil wire 44 is looped N times.
• Coil form 54 is depicted in FIG. 4 as rectangular in shape, but other shapes such can be used as well, and are consistent with the present invention.
• FIG. 4 Other embodiments of the invention may include a coil wire 44 without a coil form, such that the coil wire is looped without the benefit of any coil form.
• drain wire 42 and shield 52 depicted as the dashed line.
• the shield 52 is preferably electrically conductive, so as to support an electrical current in the presence of a voltage potential.
• the shield 52 may include a non-conductive foundation bonded, or otherwise attached, to a conductive surface.
• Drain wire 42 is attached, or otherwise mounted, to shield 52.
• Shield 52 extends along twisted pair 47 and envelops most of coil form 54, and thus envelopes most of coil wire 44.
• shield 52 does not form a complete enclosure around coil axis 45, so as to prevent a compensating current from forming along the surface of shield 52 that would serve to decrease the magnitude of the magnetic field produced by the coil assembly 40.
• shield 52 ends at gap 46.
• FIG. 5 A more detailed cross section of coil assembly 40 consistent with a preferred embodiment of the present invention is shown in FIG. 5.
• Shield 52 is exterior of coil form 54.
• the lower portion of shield 52 is depicted as "U" shaped, and the upper portion of shield 52 is depicted as a cover.
• the lower and upper portions of shield 52 can preferably be connected by conductive silver ink at location 56, but other techniques of connectivity using any type of conducting material can also be used.
• Shield 52 can be composed of a polyester foil with aluminum vapor-deposited on its surface, but other compositions with the resistance discussed below can also be used.
• the resistance of the vapor-deposited aluminum, a thin film, used in one embodiment of the present invention is of the order 1 ohm per square.
• Drain wire 42 is connected to shield 52 and is connected to ground. Drain wire 42 carries the current 1Q along the length of shield 52. At each point along shield 52 the current 1Q in drain wire 42 is the total of all current induced between that point and gap 46. Because of the ground connection, these are capacitive currents as discussed above with regard to patient 24. However, here the capacitive current loop is closed with respect to a ground rather than through patient 24.
• the current IQ at an instant of time, is associated with positive potential 16 and the capacitance of coil form 54, where the current loop of interest is completed by shield 52 connected to ground via drain wire 42.
• FIG. 5 Also depicted in the cross section shown in FIG. 5 are the N cross sections of coil wire 44 contained within coil form 54. Because of the presence of current 1Q along drain wire 42, the current in coil wire 44 is altered by an amount of the order IQ/(2N). This is depicted in FIG. 6 where drain wire 42 along shield 52 has a current -/( ⁇ /2 and coil wire 44 along one loop has an adjusted current lfof+ 1Q/(2 ⁇ ). The net current including the effect of N loops of coil wire 44 and drain wire 42 along coil assembly 40, however, remains the value as before NIM- The current along drain wire 42 is cancelled. The net result is that patient 24 is shielded from capacitive current 34 by an amount of the order 1Q. Nevertheless, the desired magnetic fields for navigation throughout the surgical region of interest remain the same.
• FIGS. 7 and 8 depict a patient-shielding and coil system in accordance with another preferred embodiment of the present invention.
• shield system 70 is placed over platform 10 containing coil sets 12 and 14.
• Shield system 70 is depicted as containing vapor-deposited conductive film 76 on top of non-conductive plastic sheet 74.
• Conductive film 76 is connected to drain wire 42.
• Coil sets 12 and 14 are connected in series and are driven through twisted pair 47 to produced the desired magnetic fields.
• Positive potential 16 and negative potential 18 are shielded from patient 24 the conductive film 76.
• Vapor-deposited conductive film 76 has a resistance of the order 1 ohm per square. This resistance is sufficient to produce little effect on the magnetic fields, indicated in FIG. 8 by field lines 48. Nevertheless, this resistance is sufficient to protect patient 24 from capacitive current 34.

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• Health & Medical Sciences (AREA)
• Engineering & Computer Science (AREA)
• Biomedical Technology (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Life Sciences & Earth Sciences (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Veterinary Medicine (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)
• Electrotherapy Devices (AREA)

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Citations (5)

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• 2000
  • 2000-10-27 WO PCT/US2000/029730 patent/WO2001030437A1/en active Search and Examination
  • 2000-10-27 EP EP00972380A patent/EP1257317A4/de not_active Withdrawn
  • 2000-10-27 AU AU11049/01A patent/AU1104901A/en not_active Abandoned

Patent Citations (5)

Non-Patent Citations (1)

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