Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
  • A61B5/053—Measuring electrical impedance or conductance of a portion of the body
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
  • A61B2562/18—Shielding or protection of sensors from environmental influences, e.g. protection from mechanical damage
  • A61B2562/182—Electrical shielding, e.g. using a Faraday cage

Definitions

• the present invention relates to a method and apparatus for inductively measuring the bio-impedance of a user's body.
• the inductive measurement of bio-impedance is a known method to determine various vital parameters of a human body in a non-contact way.
• the operating principle is the following: Using an inductor loop, an alternating magnetic field is induced in a part of the human body. This alternating magnetic field causes eddy currents in the tissue of the body. Depending on the type and conductivity of tissue, the eddy currents are stronger or weaker. The eddy currents cause losses in the tissue, which can be measured as a decrease of the quality factor of the inductor loop. They also cause a secondary magnetic field, which can be measured as an induced voltage in a second inductor loop or using compensated sensors e.g. gradiometers.
• a fairly simple setup of an apparatus for measuring the bio-impedance comprises a sensor coil as an inductor that induces the magnetic field in the human body. Measuring of the losses of the sensor coil gives information about the bio-impedance.
• the inductive measurement of the bio -impedance has been shown to allow the non-contact determination of several parameters, e.g. breath action and depth, heart rate and change of the heart volume and blood glucose level, as well as fat or water content of the tissue.
• the alternating voltage, that is applied to the sensor coil leads to capacitive common mode currents through the surrounding. These currents may deviate the measurement results. Therefore, a capacitive shield may be applied above and below the sensor coil.
• the capacity between the sensor coil and the shield is minimized in order to minimize unwanted parasitic currents from the sensor coil to the shield. This is achieved by keeping the shield as far away as possible from the sensor coil.
• an apparatus for inductively measuring the bio-impedance of a user' s body comprising an inductor adapted to induce an alternating magnetic field in the user' s body, and further comprising a resonant circuit adapted to measure electrical losses in the user's body, the resonant circuit comprising the inductor and a resonant capacitor, wherein the resonant capacitor is adapted so serve as a capacitive shield for electrically shielding the inductor.
• the object of the present invention is also achieved by a method of inductively measuring the bio-impedance of a user's body, the method comprising the steps of inducing an alternating magnetic field in the user's body by means of an inductor, and measuring electrical losses in the user's body by means of a resonant circuit, said resonant circuit comprising the inductor and a resonant capacitor, wherein the resonant capacitor serves as a capacitive shield for electrically shielding the inductor.
• a basic idea of the present invention is to use the resonant capacitor as a capacitive shield for electrically shielding the inductor. Therewith the number of structural components is reduced. Thus also the number of possible failures is reduced, leading to a very reliable technique for measuring of bio-impedance.
• the size, the reliability and the sensitivity of a contact-less bio-impedance measuring system can be improved, thus allowing an easy and comfortable diagnosis of vital parameters like heart rate, tissue water content or blood glucose level to supervise a user without the need of applying any kind of devices to the user' s body.
• a single inductor is used for inducing a magnetic field and for measuring. This raises the problem to separate the small measurement signal from the large current needed to induce the magnetic field.
• One way to solve this problem is to distinguish the real and imaginary part of the inductor current.
• the imaginary part is used to induce the magnetic field, while the real part is attributed to losses. Parts of the losses are the losses in the tissue to be detected.
• the imaginary part of the current in the inductor is by far the largest contribution to the total current amplitude.
• Real and imaginary part can be separated in a simple and reliably way by using a parallel resonant circuit.
• the capacitor current operates at the resonance frequency, the capacitor current inherently compensates the inductive currents in the inductor, such that only the real part of the current flows externally of the resonant circuit. This way, the large imaginary part of the current, which does not contribute to the measurements, is eliminated such that it does not appear outside of the resonant circuit.
• the resonant capacitor is preferably located close to the sensor inductor in order to avoid additional losses in the interconnections and influences of additional parasitics.
• Using the capacitive shield as a parallel resonant capacitor allows to integrate the sensor inductor and the resonant capacitor by using the parasitic capacity between the sensor inductor and the shield. This also allows a close placement to the sensor inductor.
• the resonant capacitor is positioned in the immediate vicinity of the sensor inductor.
• the shield is very close to the sensor inductor such that it forms a resonant circuit to allow resonant measuring. This way the unwanted parasitic capacity between the sensor inductor and the shield is turned into a well-defined value used for measuring bio- impedance.
• the resonant capacitor is preferably positioned between the user's body and the inductor. This arrangement allows the best possible shielding results.
• the resonant capacitor comprises a number of non-conductive areas, said areas being arranged orthogonally to the conductor paths of the inductor, thereby avoiding induced eddy currents.
• the resonant capacitor is structured and the non-conductive areas are slits in the body of the resonant capacitor. To keep the structured parts of the shield on the same electrical potential, preferably all parts of the shield are interconnected.
• the positioning of the resonant capacitor close to the inductor not only tolerates the parasitic currents between the sensor coil and the shield. Additionally a quasi-planar structural shape of the apparatus can be achieved.
• the resonant capacitor preferably exhibits the shape of a layer, enabling a very small installation height.
• the inductor is planar.
• the inductor is integrated into a substrate, preferably made of an insulating material.
• the apparatus can be manufactured using printed circuit board technology.
• the resonant capacitor, preferably in form of a layer is integrated into the substrate of the sensor inductor.
• the installation height of the apparatus can be reduced.
• the inductor and the capacitive shield are made as laminated copper layers in a printed circuit board, similar to multilayer printed circuit boards used for electronic circuits.
• the substrate is preferably made of printed circuit board material due to cost reasons.
• the apparatus comprises at least one additional electronic circuit integrated into or on the substrate. Placing the additional electronic circuit beside the sensor inductor, the same copper layers may be used for the interconnections of the electronic circuits and for the sensor conductor with its shielding. If the electronic circuit is located on top of the sensor inductor, a very small-area apparatus can be obtained. Preferably the copper interconnections are then made from additional copper layers laminated on top of the sensor inductor.
• a substrate material is preferably used which exhibits an enhanced dielectric constant.
• the material C-Lam offered by the material manufacturer Isola in D ⁇ ren, Germany, or an equivalent material is use. It is also possible to use ceramic substrates with significantly enhanced dielectric constant.
• the substrate exhibits a flexible structure.
• the sensor inductor can be integrated in cloths or fabric or in a bed sheet.
• materials e.g. flex foil or a polyamide material can be used.
• the complete apparatus can be integrated in wearables.
• a woven or stitched inductor coil made from thin, insulated wire in the fabric and a further stitched or woven layer on top of it used as capacitive shield.
• the capacitive shielding may not only be provided on one side of the inductor (preferably the side that directs to the user's body).
• a second resonant capacitor is provided, wherein the two resonant capacitors are positioned on both sides of the inductor.
• the second resonant capacitor is used to shield the sensor inductor capacitively from those electronic circuits.
• the inductor is preferably surrounded by a double layer shielding. In that case it may even improve the shielding effect, if the top and the bottom shield are connected to each other at the outside of the sensor inductor.
• the second resonant capacitor which is not positioned between the user's body and the inductor, is made of a softmagnetic material, e.g. Mumetal.
• a softmagnetic material e.g. Mumetal.
• Softmagnetic material effectively shields magnetic fields, so that magnetic and electric fields are shielded from the electronic circuit.
• the softmagnetic capacitor which again may be provided in form of a layer, enhances the inductivity of the sensor inductor and thus increases the magnetic field in the sensing area.
• the softmagnetic shielding layer has to be shaped in a similar way as the non-magnetic layer in order to avoid induced eddy currents in the material.
• the apparatus is adapted such, that a change of the resonance frequency corresponds to the distance to the user' s body.
• the apparatus can be used for distance measuring.
• Fig. 1 is a cross sectional view of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding layers
• Fig. 2 is a schematic picture of a planar spiral winding of a bio- impedance sensor
• Fig. 3 is a schematic picture of a first capacitive shielding layer
• Fig. 4 is a schematic picture of a second capacitive shielding layer
• Fig. 5 is a schematic arrangement of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding
• Fig. 6 is a circuit equivalent to the schematic arrangement of fig. 5
• Fig. 7 is a schematic arrangement of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding, adapted for distance detection,
• Fig. 8 is a circuit equivalent to the schematic arrangement of fig. 7
• Fig. 1 shows a planar bio-impedance sensor 1.
• a sensor 1 is preferably used in a contact less medical diagnostic system that measures inductively the bio-impedance of a user's body.
• the bio-impedance sensor 1 comprises a single inductor coil 2, which is adapted to induce an alternating magnetic field in a user's body 14, see Fig. 5.
• the inductor 2 shows the form of a planar spiral copper winding, see Fig. 2.
• the centre pin 3 of the inductor 2 is connected to ground 12 and the outer pin 4 is connected to an AC voltage source 13, see Fig. 5.
• the bio-impedance sensor 1 further comprises a parallel resonant circuit adapted to measure electrical losses in the user's body 14, the resonant circuit comprising the inductor 2 (for serving as sensor inductor) and a first resonant capacitor 5, wherein the first resonant capacitor 5 is adapted so serve as a capacitive shield for electrically shielding the inductor 2.
• the first resonant capacitor 5 is positioned between the user's body and the inductor 2.
• On the opposite side of the inductor 2 a second resonant capacitor 6 is located.
• the second capacitor is also adapted so serve as a capacitive shield for electrically shielding the inductor 2.
• Both the inductor 2 and the capacitors 5, 6 are provided as laminated copper layers. Both are integrated into a planar substrate 7, made of printed circuit board material, such that a monolithic member is formed.
• the capacitors 5, 6 show a slightly larger diameter as the inductor 2.
• the first and the second capacitor 5, 6 are positioned in the immediate vicinity of the inductor 2 such that it forms a resonant circuit to allow resonant measuring. Both capacitors 5, 6 are connected to each other at the outside of the inductor 2 by means of copper tracks 8.
• Each capacitor 5, 6 comprises a number of slits 9 arranged orthogonally to the conductor paths 10 of the inductor 2.
• the structure of the capacitors 5, 6 comprises radial copper stripes separated by radial slits 9 to avoid eddy currents.
• all parts of the capacitors are interconnected.
• a first embodiment of such capacitors 5 comprising an outside connection design is shown in Fig. 3. Thereby a number of capacitive stripes are interconnected at the outer edge of the capacitor 5 except on one position.
• FIG. 4 A second embodiment of such capacitors 5' comprising an centre connection design is shown in Fig. 4.
• the capacitor 5, 6 is connected to one of the connections of the inductor, as shown in Figs. 5 and 6.
• ground connection 12 of the driving AC voltage source 13 To achieve a good shielding effect to the surrounding one pin 11 of the inductor 2 is connected to ground connection 12 of the driving AC voltage source 13.
• the capacitor 5 is connected to the same pin 11, i.e. the capacitor 5 is connected to the ground pin of the inductor 2.
• the capacitor 5 is located between the user's body 14, which is to be examined, and the inductor 2. There is no potential difference between the capacitor 5 and the ground layer 12 of the voltage source 13 and thus no common mode currents are induced.
• the capacitive area is increased by enlarging the track width of one or more dedicated turns to enhance the capacity of the system.
• the highest effect of capacity increase is reached by enlarging that turn, which is connected to the AC voltage source 13.
• slits are provided in that turn.
• the capacitor 5 is not connected to the ground pin of the inductor 2 but to the AC- voltage pin 15.
• a capacitive path from the capacitor 5 through the user's body 14 to the ground plane 12 of the voltage source 13 is provided, as illustrated in Fig. 7.
• This capacity C BODY depends on the distance of the capacitor 5 to the user's body 14.
• Fig. 8 shows, the capacity is located in parallel to the sensing resonant circuit. A change of this capacity will change the resonance frequency of the resonant circuit. Thus, the resonance frequency will change, if the distance to the user's body changes. The variation of the bio-impedance, however, will only change the quality factor and the amplitude in resonance, but not the resonance frequency.
• a change of the resonance frequency can be attributed to the distance between the sensor and the user's body.
• the losses in the user's body which are measured to determine the user' s bio-impedance, depend on the distance between the sensor and the user's body.

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• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Biomedical Technology (AREA)
• Molecular Biology (AREA)
• Radiology & Medical Imaging (AREA)
• Biophysics (AREA)
• Pathology (AREA)
• Engineering & Computer Science (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Heart & Thoracic Surgery (AREA)
• Medical Informatics (AREA)
• Physics & Mathematics (AREA)
• Surgery (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Veterinary Medicine (AREA)
• Measurement Of Resistance Or Impedance (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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• 2006
  • 2006-04-04 WO PCT/IB2006/051026 patent/WO2006111877A1/en not_active Application Discontinuation
  • 2006-04-04 JP JP2008507211A patent/JP2008536606A/ja not_active Withdrawn
  • 2006-04-04 EP EP06727825A patent/EP1874185A1/de not_active Withdrawn
  • 2006-04-04 CN CNA200680012816XA patent/CN101160093A/zh active Pending

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