Source: http://www.google.com/patents/US6815949?dq=6,163,776
Timestamp: 2016-05-06 10:33:37
Document Index: 756425826

Matched Legal Cases: ['art 119', 'art 119', 'art 119', 'art 120', 'arts 119', 'art 1509', 'art 1508', 'art 1509', 'art 1510', 'art 1509', 'art 1509', 'art 1508', 'art 1508', 'art 1508', 'art 119']

Patent US6815949 - Apparatus for measuring a magnetic field - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsAn apparatus can detect a magnetic field with a high sensitivity using an ordinary-temperature pickup coil even when the pickup coil is arranged outside a cryostat. Specifically, the apparatus for measuring a magnetic field includes a pickup coil for detecting an external magnetic field, a SQUID electrically...http://www.google.com/patents/US6815949?utm_source=gb-gplus-sharePatent US6815949 - Apparatus for measuring a magnetic fieldAdvanced Patent SearchPublication numberUS6815949 B2Publication typeGrantApplication numberUS 10/162,748Publication dateNov 9, 2004Filing dateJun 6, 2002Priority dateJul 19, 2001Fee statusPaidAlso published asUS20030016010Publication number10162748, 162748, US 6815949 B2, US 6815949B2, US-B2-6815949, US6815949 B2, US6815949B2InventorsAkihiko Kandori, Tsuyoshi Miyashita, Keiji Tsukada, Koichi Yokosawa, Daisuke Suzuki, Akira TsukamotoOriginal AssigneeHitachi, Ltd.Export CitationBiBTeX, EndNote, RefManPatent Citations (11), Non-Patent Citations (6), Referenced by (30), Classifications (15), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetApparatus for measuring a magnetic field
US 6815949 B2Abstract
What is claimed is: 1. An apparatus for measuring a magnetic field, comprising:
an oscillator which generates an alternating voltage; a transformer which transfers said alternating voltage to an alternating current; two electrodes which are placed in two positions of a living body, said alternating current being fed to said living body via said two electrodes; a superconducting quantum interference device which is arranged in a cryostat; a pickup coil which detects a magnetic field induced in said living body by feeding said alternating current therethrough, said pickup coil being connected to said superconducting quantum interference device electrically or magnetically, made of a normal conducting material and arranged outside said cryostat; a differential amplifier which amplifies a potential between both ends of a resistance arranged in a line connecting said two electrodes in order to monitor a frequency of said alternating current fed to said living body; a compensation coil which produces a compensation magnetic field with an inverse phase by feeding an alternating current with an inverse phase with respect to said alternating current fed to said living body, said compensation coil being made of a normal conducting material, arranged outside said cryostat and arranged in the vicinity of said pickup coil such that a magnetic flux is transferred to said pick up coil; a control device which controls the amount of the alternating current fed to said compensation coil based on current data obtained from said differential amplifier; a driving circuit which drives said superconducting quantum interference device as a magnetometer and is arranged outside said cryostat; a high-pass filter circuit to which an output of said driving circuit is fed in order to remove a low frequency noise from said output of said driving circuit; a phase-shift detector to which an output of said high-pass filter circuit and an output of said differential amplifier are fed in order to detect a phase shift using the frequency of said alternating current fed to said living body as a reference signal; a band-pass filter circuit to which an output of said phase-shift detector is fed; an amplifier which amplifies an output of said band-pass filter; and a computer which collects an output of said amplifier and displays said output of said amplifier. 2. The apparatus for measuring a magnetic field according to claim 1, wherein the pickup coil and the compensation coil are placed around a bobbin.
3. The apparatus for measuring a magnetic field according to claim 2, wherein
the pickup coil is connected to said superconducting quantum interference device via a first lead line, and the compensation coil is connected to the control device via a second lead line. 4. The apparatus for measuring a magnetic field according to claim 3, wherein the first and second lead lines are twisted.
5. The apparatus for measuring a magnetic field according to claim 3, wherein the first and second lead lines are shielded against external electromagnetic waves.
6. The apparatus for measuring a magnetic field according to claim 3, wherein the first and second lead lines are shielded by a shielding wire which is grounded.
7. The apparatus for measuring a magnetic field according to claim 6, wherein the shielding wire is made of aluminum.
Alternatively, SQUIDs are used to detect a magnetic resonance signal with a high sensitivity [Appl. Phys. Lett.; Vol. 70, No. 8 (1997), pp. 1037-1039 (Reference 5) and Rev. Sci. Instrum.; Vol. 69, No. 3 (1998), pp. 1456-1462 (Reference 6)]. In such an apparatus using SQUIDs, the magnetic resonance signal is detected by a process in which a pickup coil is placed inside a cryostat as in conventional apparatus for measuring a magnetic field in a living body, or by a process in which a sample is placed in the cryostat, and the magnetic resonance signal in the sample is detected at cryogenic temperatures. According to the former process, the pickup coil cannot be sufficiently brought close to the inspected subject and SQUID magnetometer can not be operated because it should be placed in a static magnetic field. According to the latter process, the sample must be cooled to cryogenic temperatures, and the magnetic resonance signal cannot be detected in samples at an ordinary temperature.
FIG. 1 is a schematic diagram of an apparatus for measuring a magnetic field as a first embodiment of the present invention;
FIG. 2 is a perspective view illustrating a configuration of the pickup coil part of the apparatus of the first embodiment;
FIG. 3 is a diagram of an equivalent circuit in the apparatus of the first embodiment;
FIG. 4 is a graph showing the relationship between the frequency and the magnetometer sensitivity in the apparatus of the first embodiment as actual measurements and calculations;
FIG. 5 is a graph showing the relationship as actual measurements between the flux noise and the frequency in the apparatus of the first embodiment;
FIG. 6 is a waveform chart showing real-time waveforms as actual measurements of impedance magnetocardiograms in the apparatus of the first embodiment;
FIG. 8 is a schematic diagram of an apparatus for measuring a magnetic field as a second embodiment of the present invention;
FIG. 9 is a schematic diagram of an apparatus for measuring a magnetic field as a third embodiment of the present invention;
FIG. 10 is a schematic diagram of an apparatus for measuring a magnetic field as a fourth embodiment of the present invention;
FIG. 11 is a schematic diagram of an apparatus for measuring a magnetic field as a fifth embodiment of the present invention;
FIG. 12 is a schematic diagram of an apparatus for measuring a magnetic field as a sixth embodiment of the present invention;
FIG. 13 is a schematic diagram of an apparatus for measuring a magnetic field as a seventh embodiment of the present invention;
FIG. 14 is a schematic diagram of an apparatus for measuring a magnetic field as an eighth embodiment of the present invention;
FIG. 15 is a schematic diagram of a high-temperature superconducting SQUID in a ninth embodiment of the present invention;
FIG. 16 is a schematic diagram illustrating, in detail, an apparatus for measuring a magnetic field using the high-temperature superconducting SQUID of FIG. 15 as the ninth embodiment of the present invention; and,
FIG. 17 is a schematic diagram of an apparatus for measuring a magnetic field as a tenth embodiment of the present invention.
FIG. 1 is a schematic diagram of an apparatus for measuring a magnetic field as the first embodiment of the present invention. A SQUID 111 is arranged in a cryostat 110 and is in a superconducting state by liquid helium stored in the cryostat 110. The SQUID 111 used in the present embodiment comprises a SQUID ring made of a member such as niobium, an input coil arranged on the SQUID ring, and a feedback coil arranged outside the input coil. These components are patterned on one chip. The input coil is electrically connected to a lead line part 119 and is thereby electrically connected to a pickup coil 108 via the lead line part 119. The SQUID 111 is connected to an FLL (flux locked loop) circuit 107 arranged outside the cryostat 110 to operate as a magnetometer. The output of the FLL circuit 107 is fed through a high-pass filter 106 having a cutoff frequency of 1 kHz to thereby remove low frequency noise. The output of the high-pass filter 106 is transferred to a phase-shift detector 105. The phase-shift detector 105 detects a phase shift using the frequency of an alternating current (a current of 10 kHz in this embodiment) applied to a subject 121 as a reference signal 104. In the present embodiment, the subject is a living subject. The reference signal 104 is generated by an oscillator 114. A signal generator which can vary its oscillating frequency, such as a function generator, is preferably used herein to control the reference signal at a desired level.
FIG. 2 illustrates the configuration of the magnetic field pickup part of the apparatus. The pickup coil 108 and the compensation coil with an inverse phase 109 are placed around a bobbin 122 made of poly(vinyl chloride) and having a diameter of 30 mm. The pickup coil 108 and the compensation coil with an inverse phase 109 are made of an enamel-coated copper wire (a normal conducting wire). The pickup coil 108 comprises two layers of 75 turns of the copper wire, a total of 150 turns, to thereby have an inductance of 0.7 mH. The lead line part 119 is twisted and is arranged in a direction identical to the direction of the detected magnetic field and opposite to that of the pickup coil 108. Likewise, the lead line part 120 of the compensation coil with an inverse phase 109 is twisted and is arranged in a direction identical to the direction of the detected magnetic field and opposite to that of the pickup coil 108. To avoid high frequency interference, it is preferred that the lead line parts 119 and 120 are made of a cable carrying a shielding means against external electromagnetic waves, such as a shielding wire made of aluminium, as an envelope and the shielding wire is grounded with the ground of the FLL circuit. When the electromagnetic noise is significant, the pickup coil is preferably shielded overall with a shielding material such as aluminium.
FIG. 3 is a schematic diagram of an equivalent circuit when the pickup coil is made of the normal conducting member in the present embodiment. A voltage induced by the normal conducting coil is defined as jωΦp (Equation (3)). The relationship between a flux Φp fed to the pickup coil and a flux Φsq transferred to the SQUID ring is calculated according to the following equations:
From the equations (1), (2) and (3), the relationship between Φsq and Φp can be expressed by the following equation:
fc 1=R/(2π(Lp+Li)) (8)
Next, the flux noise generated from the resistance Ri of the pickup coil is calculated. The voltage noise Vn generated by the resistance Ri is expressed by the equation: Vn={square root over ( )}(4*k*T*Ri) wherein k is the Boltzmann constant (1.37�10−23); and T is the temperature. Vn in the present embodiment is 3.6�10−10 V/{square root over ( )} Hz provided that T is 300 K and Ri is 9 Ω. The flux noise Φn detected by the SQUID ring is expressed by the following equation:
When ω is sufficiently low (ω=0), Φsq is 1.3�10−4 �0/{square root over ( )} Hz. The value Φsq satisfactorily coincides with a flux noise level of 1 kHz or less in FIG. 5. The cutoff frequency fc2 in the equation (9) can be expressed by the following equation:
fc 2=(R/(2π(Lp+Li)) (10)
FIG. 4 shows the relationship between the sensitivity of the magnetometer and the frequency as actual measurements and calculation results according to the equation (7). The actual measurements are found to be in good agreement with the calculation results, indicating that the sensitivity increases with an increasing frequency. The sensitivity as the actual measurements decreases at frequencies of 50 kHz or more as compared with the calculation results. This is because the dumping capacitor (0.47 μF) connected in parallel with the input coil serves as a low-pass filter with the cutoff frequency fc2=1/(2πRiC)=38 kHz.
FIG. 5 shows actual measurements of the flux noise. In FIG. 5, values obtained by converting the flux noise to an output voltage are plotted on the right ordinate. FIG. 5 shows that the noise level is as high as Ri noise of 1.3�10−4 Φ0/{square root over ( )} Hz at frequencies of 1 kHz or less as calculated according to the equation (9), and that the cutoff frequency as calculated according to the equation (10) substantially coincides with the actual measurement.
The magnetic field resolution of the overall magnetometer can be calculated by multiplying the sensitivity shown in FIG. 4 by the output voltage shown in FIG. 5. The magnetic field resolution is, for example, 90 fT/{square root over ( )} Hz at 10 kHz. The magnetic field resolution attains the minimum at a frequency of about 10 kHz.
FIG. 6 shows impedance magnetocardiogram waveforms as measured at two positions on the thoracic wall of a healthy male subject (34 years old). A current of 7 mA peak-to-peak was fed during measurement. To avoid the influence of breathing, the waveforms were measured during non-breathing for 15 seconds after inhalation. An impedance magnetocardiogram waveform which is considered as significantly clearly corresponds to the heartbeat was observed at the position 1 near to the heart. A raw waveform of the impedance magnetocardiogram was observed at the position 2, although it was somewhat weak.
The fifth embodiment of the present invention will be illustrated with reference to FIG. 11. The FLL circuit, detecting process and circuitry of the apparatus are the same as in Second Embodiment shown in FIG. 8, and explanations thereof are omitted. The apparatus according to the present embodiment comprises plural units of the configuration shown in FIG. 1. This apparatus includes demodulation circuits 1102. The pickup coils 108 are ordinary-temperature coils, are arranged outside the cryostat 110 and can therefore be arranged in intimate contact with the head of a subject. The apparatus according to the present embodiment includes the pickup coils 108-1 . . . 108-n fixed on a cap 1101 and can thereby detect a magnetic field of the subject only by placing the cap 1101 on the head of the subject. In the apparatus, the accurate positional relationship among the pickup coils can be obtained, and the apparatus enables impedance CT (computed tomography) using a magnetic field.
FIG. 15 shows a device structure of a high-temperature superconducting SQUID as the ninth embodiment of the present invention. A pattern 1500 in the form of the symbol infinity (∞) is made of a high-temperature superconducting member on a print circuit board 1518. By forming the pattern 1500 in the form of the symbol infinity (∞), induced currents I1 and I2 are generated in the right and left portions of the pattern, respectively, by action of a flux fed to the pattern 1500, and the difference between the induced currents I1 and I2 flows as a current I3 through a ring including Josephson junctions 1502 and 1503. The high-temperature superconducting SQUID detects a flux by action of the current I3 and converts the same into a voltage. By forming the pattern in the form of the symbol infinity (∞), the resulting device becomes resistant to external flux noise.
The device includes a feedback coil part 1509 in one of the right and left portions of the pattern 1500 in the form of the symbol infinity (∞) and an input coil part 1508 in the other. In addition, the print circuit board 1518 includes line connection pads 1514, 1515, 1516, and 1517. A pad 1504 is wired patternwise with the line connection pad 1514 and is electrically connected to one end of the feedback coil part 1509 via a bonding part 1510. The pad 1504, line connection pad 1514 and bonding 1510 may be connected with one another by bonding with a metal material such as aluminium. Likewise, a pad 1505 is wired patternwise with the line connection pad 1515 and is electrically connected to the other end of the feedback coil part 1509 by bonding 1511. The feedback coil part 1509 corresponds to the feedback coil 88 shown in FIG. 8, and the line connection pads 1514 and 1515 are electrically connected to the feedback resistance 87 arranged outside the cryostat 110. On the pickup coil side, pads 1506 and 1507 are wired patternwise with the line connection pads 1516 and 1517, respectively, and are electrically connected to the input coil part 1508 via bondings 1512 and 1513. The input coil part 1508 corresponds to an input coil which transfers a flux from the pickup coil 108 to the SQUID 111 shown in FIG. 8. The input coil part 1508 is electrically connected to an ordinary-temperature pickup coil arranged outside the cryostat. The print circuit board 1518 further comprises pads 1519, 1520, 1521, and 1522 and line connection pads 1523, 1524, 1525, and 1526 that are bonded to bonding parts C and D to thereby detect an input current bias and an output voltage. In this connection, FIG. 15 also shows a bicrystal line 1501. As thus described, by forming the pattern 1500 in the form of the symbol infinity (∞), the resulting device becomes resistant to external noise magnetic fields. In addition, by forming the input coil in one of the right and left portions of the pattern and the feedback coil in the other, the high-temperature superconducting SQUID can detect a magnetic field with a high sensitivity.
FIG. 16 shows a configuration of an apparatus for measuring a magnetic field using the high-temperature superconducting SQUID shown in FIG. 15. However, such an apparatus can also be formed by using a niobium SQUID. The apparatus according to the present embodiment corresponds to a detailed configuration of the cryostat in Fourth Embodiment shown in FIG. 10. In the apparatus according to Ninth Embodiment, the SQUID 111 is arranged inside the cryostat 110, and the lead line part 119 from the SQUID 111 penetrates the vacuum layer at the bottom of the cryostat 110 and is electrically connected to the pickup coil 108. The detecting probe 1001 is fixed at the bottom of the cryostat 110. By fixing the detecting probe 1001 with the cryostat 110, the resulting apparatus can easily be handled.
FIG. 17 illustrates in detail an apparatus according to the tenth embodiment of the present invention. A sample is labeled with a magnetic marker as a result of antigen-antibody immunoreaction and is placed on a rotator 1713. The sample is marked in the following manner. Specifically, as shown at the bottom of FIG. 17, an antibody for holding 1705 is fixed on a substrate 1706 and is allowed to react with an antigen 1704, and an antibody for detection 1703 labeled with a polymer 1701 including a magnetic particle 1702 as a marker is allowed to react with the antigen 1704 to thereby constitute the labeled sample. The apparatus also comprises a magnet 1711 for magnetizing the magnetic particle 1702 upon rotation of the sample on the rotator 1713. The sample with the marker magnetic particle 1702 passes in the vicinity of the magnet 1711 on every rotation, and the magnetic field can be detected with a high sensitivity. A rotation controller 1709 controls the rotation of the rotator 1713 by controlling a motor 1708 to rotate with an axis of a rotation axis 1712 under a command of the computer 101. The rotation controller 1709 outputs a trigger signal upon every rotation, and the trigger signal is input into the computer 101 for averaging. The speed of rotation is preferably set at such a speed corresponding to the frequency to be measured, such as 10 kHz. When the frequency to be measured is 10 kHz, the rotation speed is preferably equal to or more than 10000 per second (600000 rpm). However, it is difficult in actuality to rotate the rotator at such a high speed, and the S/N ratio is improved by increasing the number of averaging. According to the apparatus of the present embodiment, the pickup coil 108 is arranged outside the cryostat 110, can thereby be brought close to the inspected subject and can detect a magnetic field with a higher sensitivity. The aforementioned configuration of the present embodiment can also be applied to conventional apparatus in which the pickup coil is arranged inside the cryostat.
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