CN101692121B - Optical pumping effect based magnetic resonance signal detection method - Google Patents
Optical pumping effect based magnetic resonance signal detection method Download PDFInfo
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- CN101692121B CN101692121B CN 200910236004 CN200910236004A CN101692121B CN 101692121 B CN101692121 B CN 101692121B CN 200910236004 CN200910236004 CN 200910236004 CN 200910236004 A CN200910236004 A CN 200910236004A CN 101692121 B CN101692121 B CN 101692121B
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Abstract
The invention discloses an optical pumping effect based magnetic resonance signal detection method, which comprises that: a biased magnetic field is supplied to an alkali metal atom absorption bulb placed in the center of an electromagnetic coil and near a sample to be detected by using the principle of optical pumping effect; magnetic fields emitted by the sample to be detected are overlapped on the biased magnetic field to form a total magnetic field; the intensity of transmission light incident to the alkali metal atom absorption bulb is changed along with the change of the total magnetic field, and the change condition of the magnetic field of the sample to be detected can be acquired by recording the change condition of the transmission light intensity of the alkali metal atom absorption bulb; and the transmission light intensity is converted into corresponding electrical signals through a photoelectric detector, and a magnetic resonance image of the sample to be detected can be acquired through two-dimensional Fourier transformation of the electrical signals.
Description
Technical field
The present invention relates to a kind of magnetic resonance signal detection method, particularly a kind of method of carrying out the magnetic resonance signal detection based on optical pumping effect.
Background technology
The conventional MRI imaging device utilizes radio-frequency coil to receive signal.According to Faraday's electromagnetic induction law, the induction electromotive force that the magnetic flux of variation produces in coil is
According to decrease rate, induction electromotive force is write as again in magnetic resonance imaging usually
Wherein,
Be magnetization intensity vector,
Be the detection sensitivity of radio-frequency coil, i.e. the magnetic field, laboratory that produces of unitary current.Signal is mainly derived from the contribution of cross magnetization strength vector, can further it be expressed as
Because ω
0=γ B
0,
As seen the size of induction electromotive force extremely depends on the field intensity of main field, and when the main field field intensity was very low, the signal intensity that radio-frequency coil receives descended greatly.In order to improve signal to noise ratio (S/N ratio), MR imaging apparatus is more and more pursued the magnet of high field intensity, so that the manufacturing maintenance cost of equipment significantly rises, becomes the key factor that the restriction magnetic resonance imaging is popularized.
In order to improve the input ability of MR imaging apparatus, the Measurement Method for Magnetic Field that people are very high with some sensitivity is incorporated in the magnetic resonance imaging to replace radio-frequency coil.For example, in United States Patent (USP) 20070205767A1, the people such as the Shoujun Xu of Lao Lunsi Berkeley National Laboratory and University of California Berkeley utilize optics atomic magnetic force meter to obtain for the first time the magnetic resonance image (MRI) of water.
Optics atomic magnetic force meter is a kind of magnetic-field measurement instrument based on magnetic rotation effect, is mainly used in atom research, space exploration, and mineral investigate, the fields such as medical diagnosis, sensitivity is up to 80fT/Hz
1/2The main body of magnetometer comprises two rubidium Atomic absorption bubbles, and they have formed a First-order Gradient meter to suppress common-mode noise, absorb bubble and are the cube of length of side 2cm, and temperature maintains about 43 ℃ during work.The light that laser instrument sends is corresponding to the D of rubidium
1Transition (794.8nm), this light beam enter into respectively two absorption bubbles after being divided into two bundles by spectroscope, and the polarizer on the light path of every Shu Jiguang and analyzer are 45 ° and put to detect the optically-active angle.B
1Be a guide field, size is about 0.05mT, and this can not act on and absorb bubble.B
bBe the bias-field that a size is about 70nT, this has not only determined the detection direction of magnetometer, and the Laser Modulation frequency when also having determined not add sample is about 1000Hz.
The Larmor precession frequency that absorbs rubidium atom in the bubble is determined by the magnetic field that sample sends, and will observe resonance effect when the modulating frequency of incident laser equals the twice precession frequency.At this moment, from absorbing the plane of polarization that steeps the laser that penetrates certain rotation will occur, indirectly obtain sample magnetic field by the anglec of rotation of measuring plane of polarization, process further obtaining magnetic resonance image (MRI) via follow-up data again.
Experimental subjects is distilled water, by water pump water circulation is flowed during experiment.Sample is divided into first the coded fields that two-way enters into a 3.1mT and carries out the locus coding behind the polarization 15s in the magnet that a size is 0.3T, converge into again at last one the road and enter into magnetometer.
Optics atomic magnetic force meter does not need the Cryo Equipment maintenance work when detecting, it directly measures the magnetic field that sample sends, and need not apply radio-frequency pulse during detection again.Because this magnetometer still can keep high sensitivity under downfield, therefore also can be used for the higher sample of detection of magnetized rate gradient.But the design of optics atomic magnetic force meter and operate all more complicated must be finished in different zones pre-polarization, coding and detection to sample, is only applicable to liquid, and this has just limited its application in magnetic resonance imaging.
Polarization and the coding to sample finished in Chinese patent 200810240408 propositions in same place, at first open the higher pre-polarizing magnetic field of field intensity during imaging imaging object is polarized, and makes the poor increasing of particle number on the different energy levels, and object magnetization degree strengthens.Then, close rapidly pre-polarizing magnetic field, open the coding static magnetic field, make imaging object in the lower coding static magnetic field of this field intensity, finish radio-frequency (RF) excited and locus coding.The Larmor precession frequency of particle depends on the static magnetic field that this is lower, but its magnetization is still determined by higher pre-polarizing magnetic field.After coding was finished, each magnetic field that applies when removing rapidly coding utilized optics atomic magnetic force meter that imaging object is detected.This invention broken through can only the convection cell imaging restriction, sample need not move in the experimentation, but magnetic field measuring device still is optics atomic magnetic force meter.
Summary of the invention
The objective of the invention is to overcome the shortcoming that prior art is brought according to magnetic rotation effect, proposed a kind of magnetic resonance signal detection method based on optical pumping effect.
Detection method of the present invention is based on the optical pumping effect principle, under the magnetic resonance signal and externally-applied magnetic field acting in conjunction that detected sample sends, alkali metal atom absorbs for the specific wavelength incident photon and is affected, the transmitted intensity that passes alkali metal atom absorption bubble changes thereupon, detect the transmitted light light intensity that alkali metal atom absorbs bubble, convert the transmitted light light intensity to electric signal, again these electric signal are carried out the magnetic resonance image (MRI) that two-dimensional Fourier transform obtains detected sample.
The present invention is based on following principle:
Utilize the irradiation alkali metal atom of specific wavelength, when incident light is that left circularly polarized light is σ
+The time, alkali metal atom has absorbed a σ
+Behind the photon, its magnetic quantum number will increase by 1 and transit to the lowest excited attitude; Meanwhile, the atom that is positioned on the lowest excited attitude can be by spontaneous radiation to equate that probability returns each sub-energy level of ground state, and the selection rule of following in this process is Δ m
F=+1,0 ,-1.Because the magnetic quantum number of lowest excited attitude can not be m
F=+3, be positioned at ground state m
FThe transition that can not make progress of particle on=+ 2 sub-energy levels, like this through after the several times circulation, ground state m
FPopulation on=+ 2 sub-energy levels increases greatly.In like manner, be σ if incident light is right-circularly polarized light
-The time, the make progress rule of transition of atom is Δ m
F=-1, the rule that transition is downwards followed still is Δ m
F=+1,0 ,-1, because the magnetic quantum number of lowest excited attitude can not be m
F=-3, be positioned at ground state m
FThe transition that can not make progress of particle on=-2 sub-energy levels, final a large amount of particles can be come ground state m
FOn=-2 sub-energy levels.When particle set was distributed on the sub-energy level of some ground state, alkali metal atom externally showed a Net magnetic moment, the principle of optical pumping that Here it is.
Linearly polarized light can be considered as equivalent σ
+And σ
-Mixing, when incident light was the linearly polarized light of specific wavelength, but alkali metal atom had strong absorption for incident light can externally not show magnetic, did not have optical pumping effect to occur.Elliptically polarized light can be considered inequality σ
+And σ
-Mixing, be the elliptic polarization light time of specific wavelength when incident light, but alkali metal atom absorb equally the incident photon optical pumping effect not as circularly polarized light obvious.
When optical pumping occurs, in fact two physical processes of pumping and relaxation are arranged simultaneously in action.Alkali metal atom can be pumped on the ground state sub-energy level after having absorbed a photon, simultaneously because and the collision of container inner wall and the collision between the atom, alkali metal atom can relaxation be got back to other sub-energy levels of ground state again, and final pumping and relaxation reach mobile equilibrium.When the atom in the container reached mobile equilibrium, absorbed photon number was proportional to the atom number of leaving this sub-energy level of ground state.
Externally-applied magnetic field affects the relaxation of atom.When externally-applied magnetic field was zero, relaxation was the strongest, and a large amount of incident photons are by Atomic absorption, and transmitted intensity reaches minimum value.Along with the increasing of externally-applied magnetic field, the relaxation of alkali metal atom weakens gradually, and transmitted intensity recovers to some extent, and finally tends towards stability.Therefore, transmitted intensity has been subject to the modulation of externally-applied magnetic field.
In order further to determine the relation of transmitted intensity and externally-applied magnetic field, alkali metal atom is absorbed bubble test.A part is offset in the terrestrial magnetic field that utilizes solenoid will absorb place, bubble present position, and then applies another magnetic field to absorbing bubble.Absorb bubble near magnetic field sensor of placement measure the magnetic field size that absorbs bubble, the size of regulating externally-applied magnetic field by the electric current that changes solenoid is recorded in to absorb under each field intensity and soaks the stationary value of penetrating light intensity.
Magnetic resonance signal derives from the magnetic field that the proton magnetization intensity vector of testing sample inside sends when doing Larmor precession, the conventional MRI imaging adopts the radio frequency induction coil indirectly to obtain the situation of change of this magnetization intensity vector just according to Faraday's electromagnetic induction law.Based on the above principle, the magnetic field that testing sample sends will change the physical characteristics that alkali metal atom absorbs bubble, affects alkali metal atom for the absorption of specific wavelength incident photon, and the transmitted intensity that passes alkali metal atom absorption bubble changes thereupon.Arbitrarily during the magnetic fields of field intensity, it is very fast that process is set up in the mobile equilibrium that absorbs alkali metal atom in the bubble, and it is short that transmitted intensity reaches the required time compole of stationary value, sends the cycle in magnetic field much smaller than testing sample.Detect the transmitted intensity that alkali metal atom absorbs bubble, convert light intensity to electric signal, these electric signal just can obtain the magnetic resonance image (MRI) of testing sample again by two-dimensional Fourier transform.
First a part is offset in the terrestrial magnetic field according to method mentioned above before detecting, provide a bias magnetic field for absorbing bubble again.Bias magnetic field is the solenoid magnetic field that produces and the summation that remains geomagnetic field component, utilizes magnetic field sensor can record the field intensity value of this bias magnetic field.The selection of bias magnetic field soaks the relation of penetrating light intensity and externally-applied magnetic field with reference to absorption.Because the light intensity variation was comparatively violent when externally-applied magnetic field was zero, light intensity approached to saturation when externally-applied magnetic field was excessive, so the selection of the field intensity of bias magnetic field should be moderate, made the transmitted intensity that absorbs bubble along with the magnetic field linear change.The magnetic field superposition that testing sample sends forms a total magnetic field on this bias magnetic field, the feature in magnetic field, geomagnetic field component and testing sample magnetic field that solenoid produces has been reflected in this total magnetic field.Transmitted intensity will change along with the variation of total magnetic field, and absorb situation of change that the change situation of steeping light intensity just can obtain testing sample magnetic field by record this moment.
Bias magnetic field can begin to carry out magnetic field detection after determining.In order further to improve detection sensitivity, the present invention takes differential mode to detect light intensity, namely puts a plurality of alkali metal atoms and absorb bubble near testing sample, and light path is not passed from testing sample inside.The magnetization intensity vector of testing sample is with the determined Larmor frequency precession of bias magnetic field, and the total magnetic field that absorbs bubble is magnetic field and the bias magnetic field sum that testing sample sends.
The light of the specific wavelength that light source sends is converted into polarized light after being split multichannel again, polarized light passes and is converted to corresponding electric signal by photodetector after alkali metal atom absorbs bubble, utilize the methods such as Gauss's difference that these electric signal are processed, obtain a final signal.
Because transmitted intensity directly reacts the motion conditions of testing sample magnetization intensity vector, so its Changing Pattern is equal to the induction electromotive force of the used receiving coil of conventional MRI imaging.Image rebuilding method and conventional MRI imaging are identical.After the electric signal that the computer acquisition transmitted light converts to, can obtain again the magnetic resonance image (MRI) of testing sample through two-dimensional Fourier transform.
The magnetic resonance signal detection method that the present invention proposes relies on basic optical pumping magnetic resonance experiment device can detect magnetic field based on optical pumping effect, and apparatus structure is greatly simplified than optics atomic magnetic force meter.Compare with the used radio-frequency coil of conventional MRI imaging, the method that the present invention proposes in the situation that coding static magnetic field is lower still can obtain higher sensitivity, reduced the design maintenance expense of magnet.After adopting Micrometer-Nanometer Processing Technology, the volume of experimental provision also can further dwindle, and becomes a kind of portable magnetic field checkout equipment.
Description of drawings
Fig. 1 is experiment device schematic diagram, and wherein 1 is three groups of mutually orthogonal Helmholtz coilss, and 2 is pre-polarizing coil, and 3 are coding static magnetic field coil, and 4 is gradient coil and radio-frequency coil, and 5 is object under test, and 6 and 7 is magnetic field measuring device;
The transmitted intensity that Fig. 2 steeps for the used absorption of experiment-externally-applied magnetic field graph of a relation;
Fig. 3 is the structural representation of two-way light path, and wherein 8 is light source, and 9 is interference filter, and 10,19,20 is lens, 12 is spectroscope, and 11 is completely reflecting mirror, and 13,14 is polaroid, and 15,16 is quarter-wave plate, 17,18 for absorbing bubble, and 5 is sample, and 21,22 is photodetector.
Embodiment
Further specify the present invention below in conjunction with the drawings and specific embodiments.
Fig. 1,2,3 is embodiments of the invention.The present embodiment take alkali metal atom rubidium atom as example illustrates detection method of the present invention.
Figure 1 shows that the experimental provision of implementing detection method of the present invention, the transmitted intensity that Fig. 2 steeps for the used rubidium Atomic absorption of experiment-externally-applied magnetic field curve, it also is the working curve of rubidium Atomic absorption bubble, the horizontal ordinate of Fig. 2 is the induction level of externally-applied magnetic field, unit is tesla (T), ordinate is the transmitted light light intensity, and size is with the output voltage value representation of photodetector, and unit is volt (V).Fig. 3 is the light channel structure figure that implements the experimental provision of detection method of the present invention.
Magnetic resonance imaging can be divided into pre-polarization, coding and three steps of detection, at first determine to detect the transmitted intensity of used rubidium Atomic absorption bubble and the relation between the externally-applied magnetic field before the imaging, and absorb the required bias magnetic field size of bubble when determining to detect according to the light intensity magnetic field dependence.
As shown in Figure 1, lambda1-wavelength is about 794.8nm, Helmholtz coils 1 is comprised of three groups of mutually orthogonal coils, utilizes wherein the two groups geomagnetic field component shieldings on will the both direction vertical with light path, applies a magnetic field that is parallel to optical path direction with another group coil to absorbing bubble again.The placement magnetic field sensor is big or small to measure absorption bubble magnetic field near the absorption bubble, the transmitted intensity situation of change under the effect of record different magnetic field.The absorption of actual measurement soak penetrate light intensity and externally-applied magnetic field relation as shown in Figure 2, transmitted intensity is the most weak when externally-applied magnetic field is zero, along with the increasing transmitted intensity in magnetic field recovers gradually, and finally reaches a stationary value.
One group of Helmholtz coils that geomagnetic field component shielding on still will the both direction vertical with light path during detection, axis are parallel to optical path direction provides one along the magnetic field of optical path direction for rubidium Atomic absorption bubble.Rubidium Atomic absorption bubble is placed on the middle part of Helmholtz coils, and remaining geomagnetic field component sum is located as bias magnetic field in the magnetic field and the testing sample present position that are parallel to optical path direction.The selection of bias magnetic field field intensity soaks the relation of penetrating light intensity and externally-applied magnetic field according to pre-determined absorption, guarantees light intensity magnetic field linear change.Regulate the Helmholtz coils electric current and can change the magnetic field that coil produces, and then change the size of bias magnetic field.Utilize magnetic field sensor to measure and absorb near the bias magnetic field size of bubble, bias magnetic field after determining removes this magnetic field sensor.
Testing sample polarizes 50-3000ms to obtain a larger net magnetisation vector under the effect of the pre-polarizing magnetic field that pre-polarizing coil shown in Figure 12 produces, then under the acting in conjunction of coding static magnetic field 3, gradient magnetic and radio-frequency (RF) magnetic field 4, finish the locus coding, close at last all above-mentioned magnetic fields, begin to carry out magnetic field detection.
The detection method that the present invention proposes can be summarized as: the magnetic field that testing sample sends and bias magnetic field acting in conjunction are steeped in the rubidium Atomic absorption, cause and absorb rubidium atomic physics characteristic changing in the bubble, the change of rubidium atomic physics characteristic affects it to the absorption of incident photon, pass the transmitted intensity that absorbs bubble and change thereupon, can get the changes of magnetic field situation of testing sample after the transmitted light of multichannel light path gained subtracts each other.
Figure 3 shows that the embodiment of two-way light path.As shown in Figure 3, the light that light source 8 sends wavelength after interference filter 9 filters is controlled near the 794.8nm, spectroscope 12 is divided equally into equal two-way with it, wherein one the road successively becomes left circularly polarized light by polaroid 14 and quarter-wave plate 16, and another road becomes left circularly polarized light by polaroid 13 and quarter-wave plate 15 after completely reflecting mirror 11 reflections.
Optical pumping effect will occur after shining and absorbing bubble in left circularly polarized light, and the rubidium atom will be according to Δ m
F=+1 selection rule is pumped into the lowest excited attitude, and the atom that is positioned on the lowest excited attitude again will be according to Δ m
F=+1,0 ,-1 selection rule turns back to ground state, and final a large amount of rubidium atoms rest on ground state m
FOn=+ 2 sub-energy levels, incident photon is by the rubidium Atomic absorption in this process, and transmitted intensity sharply weakens.When the pumping in rubidium steeps and two physical processes of relaxation reached balance, ground state m was come in pumping
FPopulation and relaxation on=+ 2 sub-energy levels are left m
FThe population of=+ 2 sub-energy levels is equal, and this moment, absorbed photon number was fixed up, and transmitted intensity also reaches the stationary value under this magnetic fields.The magnetic fields that testing sample sends absorbs in two and steeps, so that the relaxation probability of rubidium atom changes, mobile equilibrium is broken, and the rubidium atom can attempt again to set up a new equilibrium state under this magnetic fields.It is very fast that actual equilibrium state is set up process, much larger than the change frequency in the magnetic field that testing sample sends.
After two bundle left circularly polarized lights shone respectively two absorption bubbles, scioptics 19,20 converged on the photodetector 21,22 again, and photodetector is converted to voltage signal with light intensity signal.The voltage signal of two photodetectors output subtracts each other by amplifying circuit and amplifies, and amplifying signal sends back computing machine by data collecting card again, and computing machine carries out can obtaining after the two-dimensional Fourier transform magnetic resonance image (MRI) of testing sample to these data.
Claims (2)
1. magnetic resonance signal detection method based on optical pumping effect, it is characterized in that described detection method is based on the optical pumping effect principle, at magnetic resonance signal that detected sample sends with add under the bias magnetic field acting in conjunction, alkali metal atom absorbs for the specific wavelength incident photon and is affected, the transmitted intensity that passes alkali metal atom absorption bubble changes thereupon, detect the transmitted light light intensity that alkali metal atom absorbs bubble, convert the transmitted light light intensity to electric signal, again these electric signal are carried out the magnetic resonance image (MRI) that two-dimensional Fourier transform obtains detected sample.
2. magnetic resonance signal detection method according to claim 1, when it is characterized in that detecting, to being placed on the solenoid center, near a plurality of alkali metal atoms the detected sample absorb bubble provides one to add bias magnetic field; The magnetic field superposition that detected sample sends adds on the bias magnetic field at this, forms the total magnetic field that alkali metal atom absorbs bubble; The transmitted intensity that alkali metal atom absorbs bubble will change along with the variation of total magnetic field, and the absorption of record alkali metal atom soaks the change situation of penetrating light intensity and just obtains detected sample changes of magnetic field situation; After the light that light source sends is split multichannel, be converted into polarized light; When polarized light incided alkali metal atom and absorbs bubble, the transmitted light light intensity that alkali metal atom absorbs bubble had reflected that alkali metal atom absorbs alkali metal atom in the bubble in detected sample magnetic field and adds under the bias magnetic field acting in conjunction of absorbing state to(for) described polarized light; The polarized light of respectively restrainting that passes alkali metal atom absorption bubble is converted to corresponding electric signal through photodetector, utilize Gauss's difference approach that these electric signal are processed, obtain a final signal, this signal just can obtain the magnetic resonance image (MRI) of detected sample again by two-dimensional Fourier transform.
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| JP5854735B2 (en) * | 2011-09-30 | 2016-02-09 | キヤノン株式会社 | Nuclear magnetic resonance imaging apparatus and nuclear magnetic resonance imaging method |
| CN102830381B (en) * | 2012-08-15 | 2014-11-12 | 中国科学院武汉物理与数学研究所 | Nuclear magnetic resonance (NMR) device and measurement method based on laser atomic magnetometer |
| CN103018764A (en) * | 2012-12-17 | 2013-04-03 | 江汉大学 | Population inversion quantity evaluation system based on energy level translation and control method of population inversion quantity evaluation system |
| CN103969604B (en) * | 2014-05-30 | 2017-03-15 | 华南师范大学 | Radio frequency atomic magnetometer and its method for measurement NMR signal |
| US9964604B2 (en) * | 2014-11-12 | 2018-05-08 | Seiko Epson Corporation | Magnetic field measurement method and magnetic field measurement device for measuring and offsetting original magnetic field |
| JP6597034B2 (en) * | 2014-12-02 | 2019-10-30 | セイコーエプソン株式会社 | Magnetic field measurement method and magnetic field measurement apparatus |
| CN105182257B (en) * | 2015-09-14 | 2018-03-09 | 北京航天控制仪器研究所 | A kind of magnetic vector measurement apparatus and method based on relevant population trapping effect |
| CN105301541B (en) * | 2015-11-16 | 2019-04-09 | 东南大学 | The measuring device and method of the magnetic coil X of atom magnetometer, Y-axis non-orthogonal angles |
| CN108169803B (en) * | 2017-12-04 | 2019-09-03 | 山东航天电子技术研究所 | A kind of broadband measurement system and method for alternating magnetic field |
| CN110161433B (en) * | 2019-05-14 | 2021-03-02 | 中国科学院上海光学精密机械研究所 | Intermediate layer magnetic field remote measuring device based on gated photon counting |
| CN112485732B (en) * | 2020-11-13 | 2021-07-02 | 山西大学 | Magnetometer calibration method and device based on rubidium atomic magnetic resonance spectrum |
| CN112782623B (en) * | 2020-12-16 | 2023-10-24 | 兰州空间技术物理研究所 | Magnetic moment measuring device and method |
| CN113238175A (en) * | 2021-04-30 | 2021-08-10 | 北京航空航天大学 | Reflected light generation assembly, magnetic measurement system and magnetic measurement method |
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