WO1995017684A1 - Procede et appareil de mesure de champs magnetiques de faible intensite - Google Patents

Procede et appareil de mesure de champs magnetiques de faible intensite Download PDF

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Publication number
WO1995017684A1
WO1995017684A1 PCT/DK1994/000480 DK9400480W WO9517684A1 WO 1995017684 A1 WO1995017684 A1 WO 1995017684A1 DK 9400480 W DK9400480 W DK 9400480W WO 9517684 A1 WO9517684 A1 WO 9517684A1
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WO
WIPO (PCT)
Prior art keywords
magnetic field
field
coil
accordance
frequency
Prior art date
Application number
PCT/DK1994/000480
Other languages
English (en)
Inventor
Jan Henrik ARDENKJÆR-LARSEN
Original Assignee
Ardenkjaer Larsen Jan Henrik
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ardenkjaer Larsen Jan Henrik filed Critical Ardenkjaer Larsen Jan Henrik
Priority to AU13102/95A priority Critical patent/AU1310295A/en
Publication of WO1995017684A1 publication Critical patent/WO1995017684A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/62—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using double resonance
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/24—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/281—Means for the use of in vitro contrast agents

Definitions

  • the invention refers to the measurement of weak magnetic fields by means of Nuclear Magnetic Resonance (NMR).
  • NMR Nuclear Magnetic Resonance
  • Magnetometers instruments used to measure static magnetic fields, are used in many areas, including physics, geology (including oil and mineral exploration), archaeology and military applications. Due to the nature of the application, these are often portable, battery-powered instruments, in which a combination of low current consumption and high sensitivity is of major importance.
  • a known method is to use magnetometers of a type in which a container of liquid with a high hydrogen proton content, such as water or a hydrocarbon, is exposed to a strong magnetic field, or polarising field, which is impressed basically at right angles to the weak magnetic field to be measured.
  • the polarising field is impressed for a short period, such as 2 seconds, and is then interrupted very rapidly, for example within a few hundred microseconds.
  • the container is surrounded by a pick-up coil to record changes in the magnetic moment of the liquid.
  • the magnetisation of the protons in the liquid will be aligned with the impressed magnetic field under the influence of the latter.
  • the magnetisation of the protons will become aligned slowly with the weak magnetic field as it precesses, but at an instantaneously established precession frequency determined solely by the weak field to be measured. Since the polarising field is absent, the precessing magnetic field will induce a damped electrical oscillation in the pick-up coil at a frequency directly proportional to the strength of the field to be measured.
  • the amplitude of the signal is dependent, among other factors, on the strength of the polarising field, the nuclear magnetic properties of the liquid, and the relationship between the directions of the polarising field and the field to be measured. The amplitude of the signal decreases and the signal is absorbed by the inherent noise of the measuring system after a few seconds according as the magnetisation of the protons is aligned with the weak field to be measured.
  • the amplitude of the voltage induced in the pick-up coils should be high which, all things being equal, requires a strong polarising field to align the spinning moments of a large number of protons with it.
  • a strong polarising field is synonymous with an undesirably high power consumption, which increases as the square of the field strength.
  • Another known method is based on the use of a solution of radicals, in which the hydrogen proton nuclear spin of the solution is coupled to the free electron spin of the radicals through the magnetic dipole coupling, and in which the free electron spin of the radicals is also and simultaneously related to a nuclear spin, typically that of a nitrogen atom, within one and the same radical molecule (Fermi coupling).
  • a solution of radicals of this type is exposed to a weak magnetic field, such as the earth's magnetic field, the spinning moment of the protons will align itself parallel to the weak magnetic field, while the electron spin will be similarly aligned, but in the opposite direction, so that weak magnetisation is established.
  • the solution which is surrounded by a pick-up coil, is exposed to high-frequency electromagnetic waves in resonance with the electron spin of the free radicals, producing the phenomenon known as the Overhauser effect. This induces an amplified, continuous oscillation of the pick-up coil at a frequency corresponding to the strength of the weak magnetic field to be measured.
  • the electromagnetic field should preferably be impressed at right angles to the weak magnetic field to be measured.
  • the electromagnetic radiation influences the free electron spin in such manner that it essentially becomes saturated, which is to say that the magnetic moment of the electron spin in the opposite direction to the weak magnetic field falls almost to zero. Due to the dipole connection between the electron spin and the proton nuclear spin, the proton nuclear spin of the solution is now aligned in the original direction of the electron spin; in other words, in the opposite direction to the weak magnetic field to be measured.
  • This Overhauser effect may best be illustrated by considering the quantum mechanics processes which occur in a hydrogen proton-bearing solution containing a radical with a free electron spin. Due to the rotation of the charged proton nuclei, the hydrogen protons possess a magnetically positive dipole moment which, because of the quantum mechanics, may assume two energy states, a basic state and an excited state, either with or without an external magnetic field. A greater or lesser proportion of the protons in the equilibrium state may be excited depending on the strength of the magnetic field. At typical magnetic field strengths, the number of protons in the basic state will almost equal the number in the excited state (weak magnetisation) due to the low excitation energy.
  • Ee is the excitation energy of the electron spin
  • Ep is the excitation energy of the proton spin
  • SAT is the degree of saturation of the electron dipoles. This shows clearly that the amplification may be increased provided that the excitation energy of the electron dipoles can be increased.
  • the ratio Ee/Ep is 658, giving a maximum magnetic amplification factor of 329.
  • the excitation energy of the electron dipole in a low external magnetic field will be considerably higher than in the uncoupled state (known as the 'hyperfine coupling constant') since, in practice, the Fermi coupling acts as a local, polarising magnetic field of, for example, 15 Gauss opposing the electron spin.
  • This high (in relation to the weak external magnetic field) hyperfine coupling constant will produce an extremely high amplification factor in accordance with the above formula. Typical amplification factors of 1500, at a power consumption of about 2 W, have been achieved in practice.
  • the purpose of the present invention is to provide a method, and an apparatus for performing the said method, which will permit such measurements to be carried out without increasing the power consumption.
  • This aim is achieved by means of a method in accordance with the invention, which is characterised by the provisions of the characterising section of patent claim 1 , and using an apparatus for performing the said method, which is characterised by the arrangement specified in the characterising section of patent claim 5.
  • the invention affords significantly improved results by using a particular combination of the two methods described earlier, in that when measuring the weak magnetic field, a container with a special medium, such as a liquid containing a stable radical, is placed in a polarising magnetic field arranged essentially at right angles to the weak magnetic field to be measured. It is further assumed that the polarising field can be interrupted quickly so that the proton spin is aligned with the weak magnetic field. Prior to this interruption, the medium in the container is exposed, in the presence of the polarising field, to a high-frequency electromagnetic signal corresponding to the resonance frequency of the free electrons.
  • a container with a special medium such as a liquid containing a stable radical
  • Fig. 1 is a schematic representation of an apparatus in accordance with the invention.
  • Fig. 2 is a schematic representation of another embodiment of the invention.
  • the electron spin will become essentially saturated. Due to the dipolar coupling between the electron spin and the nuclear spin, the electron spin distribution in the solution will now be altered (Overhauser effect) in such manner that the nuclear magnetisation will be aligned in opposition to the polarising field and at right angles to the weak, external magnetic field to be measured.
  • the excitation energies of the dipoles will be increased in direct proportion to the strength of the polarising field, causing a large number of nuclear dipoles to shift between the energy levels and resulting in strong magnetisation of the solution.
  • both the polarising magnetic field and the high-frequency electromagnetic field are interrupted suddenly and simultaneously, the nuclei will precess into the weak magnetic field at a frequency proportional to the strength of that field and with an amplitude proportional to its projection on the normal to the polarising field. Both the frequency and amplitude of nuclear precession can be recorded with the aid of a pick-up coil surrounding the container.
  • a particularly advantageous configuration can be achieved using a low- viscosity solution with a high hydrogen proton concentration, containing a radical with a free electron spin which is not coupled to a nucleus in the molecule itself (i.e. no Fermi coupling), or a radical with a nuclear magnetic spin, in which the said nuclear magnetic spin is coupled to the free electron spin through a pure magnetic dipole coupling and in which the line width at the high-frequency electron transition is narrow, since this ensures extremely efficient utilisation of the high-frequency electromagnetic radiation.
  • a hyperfine coupling between the electron and a nucleus in the radical causes the resonance frequency of the electron spin to be split into several levels, and the high-frequency electromagnetic field thus contains several resonance frequencies.
  • the medium used is a hydrogen proton-based nuclear magnetic type containing a high concentration of radicals with a free electron without a Fermi coupling
  • an overall amplification factor of 329*100/0.5, or 65,800 when using the invention to measure the earth's magnetic field with a 200 Gauss polarising field.
  • an Overhaus- based amplification factor of 200 resulting in an overall amplification factor of approx. 40,000 at a power consumption of about 1 W, may be expected when using a solution volume of a few millilitres and a radical concentration of a few millimoies.
  • Fig. 1 shows an apparatus in accordance with the invention, which is provided with a device 1 , such as a coil, for transmitting a suitably strong and homogeneous magnetic field, which device is capable of interrupting the impressed field within a fraction of a millisecond.
  • the strong magnetic field shall be aligned essentially at right angles to the weak magnetic field to be measured.
  • the apparatus is, furthermore, provided with an arrangement for mounting a container 2 containing a special medium 3 in the strong, polarising magnetic field.
  • a high-frequency electromagnetic field from a variable frequency generator is impressed at right angles to the polarising magnetic field and, finally, the container 2 is surrounded by a pick-up coil 4 connected to recording equipment.
  • the direction of the field in the coil is essentially parallel to that of the polarising magnetic field.
  • Fig. 2 shows another embodiment of the invention featuring two containers 2, with respectively opposed polarising magnetic fields and pick-up coils 4, which arrangement affords the particular .advantage that any induced electromagnetic interference from the surroundings is equalised by 'common mode rejection'.

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)

Abstract

Afin de mesurer des champs magnétiques de faible intensité, un conteneur renfermant un fluide, par exemple une solution contenant un radical stable, est placé dans un champ magnétique polarisant, principalement à angle droit par rapport au champ à mesurer. Le champ polarisant est coupé rapidement, cette coupure étant précédée par l'impression d'un signal électromagnétique haute fréquence. La fréquence du signal correspond à la fréquence de résonance d'un spin d'électron libre. La fréquence et l'amplitude du noyau de précession, qui sont fonction de l'intensité du champ mesuré, sont enregistrées par une bobine exploratrice.
PCT/DK1994/000480 1993-12-22 1994-12-21 Procede et appareil de mesure de champs magnetiques de faible intensite WO1995017684A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU13102/95A AU1310295A (en) 1993-12-22 1994-12-21 Method and apparatus for measuring weak magnetic fields

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DK1431/93 1993-12-22
DK143193A DK143193A (da) 1993-12-22 1993-12-22 Fremgangsmåde og apparat til måling af svage magnetiske felter

Publications (1)

Publication Number Publication Date
WO1995017684A1 true WO1995017684A1 (fr) 1995-06-29

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Application Number Title Priority Date Filing Date
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Country Status (3)

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AU (1) AU1310295A (fr)
DK (1) DK143193A (fr)
WO (1) WO1995017684A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2515131A1 (fr) * 2011-04-22 2012-10-24 Eidgenössische Technische Hochschule (ETH) Détermination de magnétisation axiale d'un objet dans un champ magnétique
CN113555206A (zh) * 2020-11-11 2021-10-26 华为杰通（北京）科技有限公司 目标空间极弱磁场的建立方法、磁化设备及磁化产品
CN113900056A (zh) * 2021-10-18 2022-01-07 国家纳米科学中心 流速测量方法、装置及存储介质

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117129920B (zh) * 2023-10-27 2024-01-12 中国科学院精密测量科学与技术创新研究院 一种高信噪比宽带激发的弱磁测量装置与方法

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4682106A (en) * 1985-03-21 1987-07-21 General Electric Company Methods of, and apparatus for, proton decoupling in nuclear magnetic resonance spectroscopy
US4891593A (en) * 1987-08-05 1990-01-02 National Research Development Corporation Methods of obtaining images representing the distribution of paramagnetic molecules in solution
EP0409292A2 (fr) * 1987-06-23 1991-01-23 Nycomed Innovation AB Améliorations concernant l'imagerie par résonance magnétique
WO1992004640A1 (fr) * 1990-09-06 1992-03-19 British Technology Group Ltd Procede d'obtention d'images representant la repartition d'une matiere paramagnetique dans une solution
US5189370A (en) * 1991-08-16 1993-02-23 Siemens Aktiengesellschaft Chemical shift imaging

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4682106A (en) * 1985-03-21 1987-07-21 General Electric Company Methods of, and apparatus for, proton decoupling in nuclear magnetic resonance spectroscopy
EP0409292A2 (fr) * 1987-06-23 1991-01-23 Nycomed Innovation AB Améliorations concernant l'imagerie par résonance magnétique
US4891593A (en) * 1987-08-05 1990-01-02 National Research Development Corporation Methods of obtaining images representing the distribution of paramagnetic molecules in solution
WO1992004640A1 (fr) * 1990-09-06 1992-03-19 British Technology Group Ltd Procede d'obtention d'images representant la repartition d'une matiere paramagnetique dans une solution
US5189370A (en) * 1991-08-16 1993-02-23 Siemens Aktiengesellschaft Chemical shift imaging

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2515131A1 (fr) * 2011-04-22 2012-10-24 Eidgenössische Technische Hochschule (ETH) Détermination de magnétisation axiale d'un objet dans un champ magnétique
WO2012143571A1 (fr) * 2011-04-22 2012-10-26 Eidgenössische Technische Hochschule (ETH) Observation d'une aimantation axiale d'un objet dans un champ magnétique
US9733318B2 (en) 2011-04-22 2017-08-15 Eidgenossische Technische Hochschule (Eth) Observation of axial magnetization of an object in a magnetic field
CN113555206A (zh) * 2020-11-11 2021-10-26 华为杰通（北京）科技有限公司 目标空间极弱磁场的建立方法、磁化设备及磁化产品
CN113555206B (zh) * 2020-11-11 2024-02-02 华为杰通（北京）科技有限公司 目标空间极弱磁场的建立方法、磁化设备及磁化产品
CN113900056A (zh) * 2021-10-18 2022-01-07 国家纳米科学中心 流速测量方法、装置及存储介质

Also Published As

Publication number Publication date
DK143193D0 (da) 1993-12-22
AU1310295A (en) 1995-07-10
DK143193A (da) 1995-06-23

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