CN100569180C - A kind of device that reduces the magnet for magnetic resonant imaging eddy current - Google Patents
A kind of device that reduces the magnet for magnetic resonant imaging eddy current Download PDFInfo
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- CN100569180C CN100569180C CNB2006101147051A CN200610114705A CN100569180C CN 100569180 C CN100569180 C CN 100569180C CN B2006101147051 A CNB2006101147051 A CN B2006101147051A CN 200610114705 A CN200610114705 A CN 200610114705A CN 100569180 C CN100569180 C CN 100569180C
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Abstract
A kind of device that reduces the magnet for magnetic resonant imaging eddy current, mainly comprise following ingredient: the pole plate head [401] that is used to produce uniform magnetic field, an insulating barrier [402] that is used to isolate eddy current, dredge layer [403] that guide gradient magnetic, main gradient coil [407], a shielded coil [404] and a grading ring [406] that produces main gradient magnetic.Shielded gradient coil [404] is positioned at pole plate head [401] and dredges between the layer [403], is embedded in the insulating barrier [402], and the same main gradient coil of shielded gradient coil [404] [407] is electrically connected.Pole plate head [401] is made by the material that magnetic conduction conducts electricity again, and its one side towards imaging space has the not nonconducting insulating barrier of magnetic conduction [402] of one deck, and insulating barrier [402] has the nonconducting layer [403] of dredging of a floor height magnetic conduction again towards the one side of imaging space.Shielded gradient coil [404] can import to the magnetic flux of the vertical gradient magnetic of the overwhelming majority to be dredged in the layer [403], can reduce the eddy current that vertical gradient magnetic produces greatly on pole plate, thereby improve picture quality greatly.
Description
Technical field
The present invention relates to nuclear magnetic resonance (MRI) system, particularly relate to the device that reduces the magnet for magnetic resonant imaging eddy current.
Background technology
During magnetic resonance imaging system work, human body is placed a strong magnetostatic field, by the atomic nucleus in human body parts zone being excited to human body emission radio-frequency pulse.After radio-frequency field was removed, these nuclear radiations that are excited went out radiofrequency signal, are received by antenna.After adding gradient magnetic in this course, just can obtain the space distribution information of human body, thereby reconstruct the two dimension or the 3-D view of human body by radiofrequency signal.
Gradient magnetic is produced by gradient coil, and gradient coil is installed in the position near pole plate, applies the size pulse current different with width during work.Because the existence of pulse current, it will induce eddy current in the pole plate of conduction in the process that changes.Because size, distribution and the die-away time of eddy current are not controlled by imaging system, but the material of plates of similar polarity is relevant with structure, therefore its existence will have a strong impact on image quality, cause the phase place of signal to produce error, signal deforms, make image produce pseudo-shadow, signal to noise ratio descends, and influences the numerous performance of imaging system.
For reducing the influence of eddy current, a method is to adopt active shielded gradient coil, and when existing this method was used in the magnet of pole plate is arranged, efficient was very low, makes the system structure complexity, and cost rises, the operation and maintenance inconvenience.
Another kind method attaches various permeability magnetic materials on the pole plate of conduction, these materials are cut into different shape, reducing the generation of eddy current, and between pole plate and the permeability magnetic material that attached, one deck sealing coat is arranged, make eddy current in pole plate, not produce to isolate eddy current.
Figure 1 shows that the general structure of magnet for magnetic resonant imaging and gradient coil structures thereof.101 is the ferrum yoke of magnetic conduction, the 102nd, and the magnetic pole head is made by magnetic material.The 103rd, pole plate is used to make the magnetic field of imaging area to become evenly, and the 105th, grading ring is used to enlarge the zone of uniform magnetic field.104 is gradient coil, be used to produce gradient magnetic, 106 is radio-frequency coil, the atomic nucleus of emission radio-frequency electromagnetic wave excitation testee, and receive the electromagnetic wave that testee sends, because the adding of gradient fields can make the electromagnetic frequency spectrum of testee emission that distribution is spatially arranged, thereby reconstruct the image of testee.
For saving the space, gradient coil 104 generally all is close to pole plate 103 and is installed, all adopt the pulsed gradient field during nuclear magnetic resonance at present, this just need pass to pulse current in gradient coil, pulse current can arrive hundreds of amperes greatly, at this moment, if pole plate 103 is conductors, according to the law of electromagnetic induction, it will induce eddy current in pole plate 103.Eddy current produces magnetic field equally, because its distribution non-linear, die-away time uncontrollable will seriously disturb imaging.
Be head it off, class methods are to adopt active shielded gradient coil.Figure 2 shows that the common structure of the active shielded gradient coil of prior art, 201 is the pole plate head, is made by permeability magnetic material, owing to the needs of frame for movement, normally made by pure iron, so it also is a good electric conductor.If gradient coil obviously will produce eddy current near pole plate.For this reason, at main gradient coil 206 arranged outside shielded coils 205.The sense of current of shielded coil 205 is opposite with main gradient coil 206, therefore it produces reverse gradient magnetic, magnetic field cancellation with main gradient coil, thereby in pole plate, there is not gradient magnetic, do not produce eddy current, the Distribution of Magnetic Field that they produce as shown in Figure 3, the magnetic line of force can not enter in the pole plate fully.Obviously, shielded coil 205 has not only been offset the gradient magnetic of main gradient coil in pole plate, also partial offset the gradient magnetic of main gradient coil at imaging space, therefore, the gradient magnetic intensity that is held in the image space does not change, the electric current of coil will increase exponentially, perhaps increase considerably the distance of main gradient coil and shielded gradient coil, the former will increase system power consumption, increase water-cooling system, strengthened the complexity of system, the latter will have to increase the air gap of magnet, thereby increase considerably the manufacturing cost of magnet, increase the weight of magnet.
Another kind of method generally is to attach lamellar or zonal permeability magnetic material on pole plate, such as Chinese patent 01114389.4 disclosed method, the pole plate of its design is divided into three parts, has the nonconducting so-called sealing coat of one deck on the pole plate body of conduction, and then attach various permeability magnetic materials, produce in order to reduce eddy current.Obviously, these methods all have certain effect to reducing eddy current.Yet, because raising to the requirement of imaging level, these methods are powerless to further reducing eddy current, because, the purpose of these class methods is when being desirably in gradient coil work, the high permeability material inside that the magnetic line of force of gradient magnetic all passes through to be attached forms magnetic circuit, thereby does not enter the pole plate head of conduction, can suppress the generation of eddy current.But, because this structure only has good inhibitory effect to the eddy current that the transverse gradients field produces, and when vertically gradient is worked, because pole plate head and the material that is attached have close pcrmeability, inevitable some gradient magnetic line of force enters in the pole plate, and this part magnetic line of force is through the center of imaging area.Again because non-magnetic sealing coat of existence between pole plate head and the material that attached, make the magnetic line of force can't enter the material that is attached very soon, the distance that in the pole plate head, advance and quite grow, therefore still form certain eddy current on pole plate, the existence of these eddy current is enough to some fast imaging sequences are constituted influence.
Figure 4 shows that the vertical gradient magnetic line of force of this type of pole plate without shielded gradient coil the time distributes.301 is pole plate, and 302 is insulating barrier, and 303 is the non-conductive permeability magnetic material that is attached, and 306 is grading ring, and 307 is vertical gradient coil.As seen the most magnetic line of force is arranged through 303 layers among the figure, but, because plate material and the material permeability that is attached are approaching, some magnetic line of force has entered 301 pole plates, and, the non-magnetic insulating barrier of layer arranged owing to pole plate and the material that is attached are middle, make the magnetic line of force will in pole plate, walk very long path, thereby in pole plate, produce eddy current, and therefore this part magnetic line of force can exert an influence still through imaging area to imaging.
By contrast, still adopt active shielded gradient coil than successful method at present, it can prevent that the magnetic field that gradient coil produces from entering cartridge, thereby avoids producing eddy current.Present active shielded gradient coil all adopts double-decker, the internal layer circle of serving as theme, skin is a shielded coil, though this structure can shield eddy current effectively,, cause number of turns a lot of because two-layer coil-span is too near, efficient is very low, also be equipped with cooling system possibly, cause the system structure complexity, buy and the use cost rising.
Summary of the invention
The pole plate that the objective of the invention is to solve magnet for magnetic resonant imaging in the prior art produces the problem than maelstrom in when the axial gradient coil working, further improves the imaging level of magnetic resonance imaging system.For this reason, the present invention proposes a kind of nuclear magnetic resonance active shielded gradient coil and pole plate that adopts new construction, and both combinations can effectively reduce the eddy current that axial gradient magnetic field produces.
The present invention is by the following technical solutions:
Magnet for magnetic resonant imaging mainly comprises the magnetic pole head of being made by magnetic material, yoke and the pole plate that permeability magnetic material is made.Magnet produces required magnetostatic field by pole plate in imaging space.The effect of pole plate is to produce uniform magnetic field at imaging space, and it must be made by the material of high magnetic conduction.
Gradient coil is used for producing gradient magnetic, and it generally is installed on the pole plate, and very close to pole plate.
According to direction sequence arrangement, the present invention includes pole plate head, insulating barrier, shielded gradient coil, dredge layer, main gradient coil from the magnetic pole head to imaging space.Simultaneously, also has a grading ring in one side near imaging space.Wherein, shielded gradient coil is clipped in head and dredges between the layer.The pole plate head is to be made by the material that magnetic conduction conducts electricity again, and insulating barrier is to be made by non-conductive non-magnetic material, and dredging layer is to be made by high magnetic conduction non electrically conductive material.What is called is dredged layer, is to be used for dredging the gradient magnetic magnetic line of force, makes it not enter the pole plate head.
The pole plate head is high magnetic conduction conductive material, its one side towards imaging space has the not nonconducting insulating barrier of magnetic conduction of one deck, insulating barrier has the nonconducting layer of dredging of a floor height magnetic conduction again towards the one side of imaging space, shielded gradient coil is embedded in the middle of the insulating barrier, and is electrically connected with main gradient coil.
Main gradient coil produces the magnetic line of force, and attempt enters head inside along the edge of head, and passes imaging center always, in case enter head, it will produce very big eddy current.Shielded gradient coil of the present invention, because its sense of current is opposite with main coil, therefore, it will produce enough opposing magnetic fields at the edge of head, stop the main gradient magnetic magnetic line of force to enter the head of conduction, dredge layer thereby force it to enter, because dredge between layer and the pole plate head the non-magnetic insulating barrier of one deck is arranged again, the magnetic line of force can't enter in the pole plate head again, and it will form closed magnetic circuit by the air gap of imaging space again along dredging layer.Because it is non-conductive to dredge layer, therefore the axial gradient of this moment will not produce eddy current.Therefore, the present invention can guarantee that the gradient magnetic magnetic line of force does not enter in the pole plate head, thereby does not produce eddy current.
Simultaneously, because among the present invention, the magnetic line of force of shielded gradient coil only is confined to pole plate inside, and the number of turn seldom, therefore main gradient magnetic is not had weakening effect substantially, thereby efficient is very high.
Obviously, the material that the present invention requires to dredge layer has high permeability and high saturation induction density, and low coercivity is arranged.At present more existing materials and method are made such layer of dredging, and such as adopting extremely thin amorphous alloy strip to be wound into such layer of dredging, this amorphous alloy ribbon has very high saturation flux density, big pcrmeability, big resistivity; In addition, also can be cut into the fillet of certain-length and width, stick with glue the outside that is connected on insulating barrier then with thin stalloy.
Shielded coil of the present invention is that to be embedded in insulating barrier intermediary, can finish by integral manufacturing in the process of making pole plate.
Description of drawings
Fig. 1 is the magnetic resonance magnet and the gradient coil structures sketch map thereof of prior art.Among the figure: the ferrum yoke of 101 magnetic conductions, 102 magnetic pole heads, 103 pole plates, 104 gradient coils, 105 grading rings, 106 radio-frequency coils;
Fig. 2 is the active shielded gradient coil and the electrode plate structure schematic diagram of prior art;
Fig. 3 is the scattergram of axial gradient magnetic field magnetic line of the active shielded gradient coil of prior art;
Fig. 4 is the scattergram of axial gradient magnetic field magnetic line of the non-shielded gradient coil of prior art;
Fig. 5 is a basic structure schematic diagram of the present invention.Among the figure: 401 pole plate heads, the 402nd, insulating barrier, 403 dredge layer, 404 shielded gradient coils, 406 grading rings, the 407th, main gradient coil;
Fig. 6 is the scattergram of the gradient magnetic magnetic line of force in pole plate among the present invention;
Fig. 7 is the structural representation of the specific embodiment of the invention.
The specific embodiment
Further specify the present invention below in conjunction with the drawings and specific embodiments.
Figure 5 shows that basic structure principle of the present invention.Arrange by the direction from the magnetic pole head to imaging space, order is followed successively by pole plate head 401, insulating barrier 402, dredges layer 403, shielded gradient coil 404, main gradient coil 407.Main gradient coil 407 is positioned at dredges the outside of layer 403 towards the one side of imaging space.Simultaneously, also has a grading ring 406 in one side near imaging space.Pole plate head 401 usefulness not only magnetic conduction but also the material that conducts electricity are made.For stoping the magnetic line of force to enter pole plate head 401, attaching one deck nonconducting again insulating barrier 402 of magnetic conduction not below pole plate head 401, attach the nonconducting layer 403 of dredging of magnetic conduction of adequate thickness then outside it, its thickness makes it be in undersaturated condition when work.In insulating barrier 402, be inlaid with shielded gradient coil 404.Shielded gradient coil 404 is electrically connected with main gradient coil 407, the current opposite in direction of its sense of current and main gradient coil 407.The effect of shielded gradient coil 404 is that the magnetic line of force with herein main gradient magnetic is urged to and dredges in the layer 403, and does not enter pole plate head 401.
The layer of dredging of the present invention 403 is operated in undersaturated condition, because if dredge layer 403 when saturated, its pcrmeability is also very low, will cause being forced into the magnetic line of force of dredging in the layer 403 and reenter in the pole plate head 401.Dredge layer and 403 be operated in undersaturated condition for making, need design its operating point according to the magnetic characteristic of material.It is relevant with the main field of magnet dredging the magnetic flux density of layer in 403, increases the thickness of dredging layer 403, can reduce the magnetic flux density of dredging in the layer 403, makes material be operated in linearity range in the magnetization curve, and the pcrmeability of material is very high herein, and magnetic resistance is little.406 is grading ring, and its effect is the shimming district that is used for being expanded into picture.
Fig. 6 is the trend and the path of the magnetic line of force of the present invention, the visible magnetic line of force is dredged in the layer 403 through having entered into after the compressing of shielded gradient coil 404 among the figure, because the obstruct of insulating barrier 402 is arranged, its magnetic resistance is very big, and the layer 403 of dredging of this moment is in undersaturated state, and permeability is very high, so the magnetic line of force no longer enters in the pole plate 401, thereby reduced the eddy current in the pole plate greatly, can improve the image quality of fast imaging sequence.
Fig. 7 is one embodiment of the present of invention, and wherein, shielded gradient coil 404 is embedded in the insulating barrier 402, and its number of turn is two circles, and the radius of a circle is 320mm, and the radius of a circle is 380mm, and main gradient coil is 12 circles.Shielded gradient coil 404 and whole pole plate can be made one during manufacturing.
Claims (3)
1, a kind of device that reduces the magnet for magnetic resonant imaging eddy current, it constitutes by the direction from the magnetic pole head to imaging space arranges, order for pole plate head [401], insulating barrier [402], shielded gradient coil [404], dredge layer [403], a main gradient coil [407]; Grading ring [406] is positioned at the one side near imaging space, main gradient coil [407] is positioned at dredges the outside of layer [403] towards the one side of imaging space, the same main gradient coil of shielded gradient coil [404] [407] is electrically connected, it is characterized in that, shielded gradient coil [404] is positioned at pole plate head [401] and dredges between the layer [403], is embedded in the insulating barrier [402]; Pole plate head [401] is made by the material that magnetic conduction conducts electricity again, and its one side towards imaging space has the not nonconducting insulating barrier of magnetic conduction [402] of one deck, and insulating barrier [402] has the nonconducting layer [403] of dredging of a floor height magnetic conduction again towards the one side of imaging space.
2. as the said device that reduces the magnet for magnetic resonant imaging eddy current of claim 1, it is characterized in that: the current opposite in direction of main gradient coil [407] and shielded gradient coil [404].
3. as claim 1 or the 2 said devices that reduce the magnet for magnetic resonant imaging eddy current, it is characterized in that: dredge layer [403] and be operated in undersaturated condition, have higher permeability and saturation flux density.
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| Application Number | Priority Date | Filing Date | Title |
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| CNB2006101147051A CN100569180C (en) | 2006-11-22 | 2006-11-22 | A kind of device that reduces the magnet for magnetic resonant imaging eddy current |
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| CNB2006101147051A CN100569180C (en) | 2006-11-22 | 2006-11-22 | A kind of device that reduces the magnet for magnetic resonant imaging eddy current |
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| CN1951322A CN1951322A (en) | 2007-04-25 |
| CN100569180C true CN100569180C (en) | 2009-12-16 |
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Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN102403081B (en) * | 2010-09-17 | 2015-02-11 | 通用电气公司 | Magnet assembly and manufacturing method thereof |
| CN102879754B (en) * | 2012-09-19 | 2015-07-22 | 中国科学院电工研究所 | Magnetic field circulating device for proton/electron double resonance imaging |
| CN104391262A (en) * | 2014-11-27 | 2015-03-04 | 南京磁晨医疗技术有限公司 | MRI (magnetic resonance imaging) gradient coil assembly method |
| CN106908746B (en) * | 2015-12-22 | 2020-01-21 | 通用电气公司 | Head magnetic resonance imaging apparatus and head gradient coil assembly thereof |
| CN106291422B (en) * | 2016-09-29 | 2023-09-01 | 中国科学院苏州生物医学工程技术研究所 | Magnetic resonance imaging system and parameter determination method thereof |
| CN109696645A (en) * | 2018-12-29 | 2019-04-30 | 佛山瑞加图医疗科技有限公司 | A kind of non-planar gradient coil |
| CN111913144B (en) * | 2020-06-01 | 2023-06-23 | 上海联影医疗科技股份有限公司 | Housing structure for a magnetic resonance system, electronic component and magnetic resonance system |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN1399940A (en) * | 2001-07-27 | 2003-03-05 | 武汉宏建电气技术有限公司 | Treatment of eddy current on polar plate in permanent magnet device of magnetic resonance imaging system |
| EP1491138A1 (en) * | 2002-04-01 | 2004-12-29 | Shenyang Neusoft BOPU NMR Tech Co., Ltd. | A permanent magnet for magnet resonance |
| CN1711964A (en) * | 2004-06-22 | 2005-12-28 | 中国科学院电工研究所 | Magnet polar plate of magnetic resonance |
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1399940A (en) * | 2001-07-27 | 2003-03-05 | 武汉宏建电气技术有限公司 | Treatment of eddy current on polar plate in permanent magnet device of magnetic resonance imaging system |
| EP1491138A1 (en) * | 2002-04-01 | 2004-12-29 | Shenyang Neusoft BOPU NMR Tech Co., Ltd. | A permanent magnet for magnet resonance |
| CN1711964A (en) * | 2004-06-22 | 2005-12-28 | 中国科学院电工研究所 | Magnet polar plate of magnetic resonance |
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