JPH02144039A - Inclined magnetic field applying method of mri apparatus - Google Patents
Inclined magnetic field applying method of mri apparatusInfo
- Publication number
- JPH02144039A JPH02144039A JP63300138A JP30013888A JPH02144039A JP H02144039 A JPH02144039 A JP H02144039A JP 63300138 A JP63300138 A JP 63300138A JP 30013888 A JP30013888 A JP 30013888A JP H02144039 A JPH02144039 A JP H02144039A
- Authority
- JP
- Japan
- Prior art keywords
- magnetic field
- pulse
- eddy current
- gradient magnetic
- gradient
- Prior art date
- Legal status (The legal status 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 status listed.)
- Granted
Links
- 238000000034 method Methods 0.000 title claims abstract description 14
- 230000002123 temporal effect Effects 0.000 claims description 6
- 230000000630 rising effect Effects 0.000 abstract description 12
- 238000001208 nuclear magnetic resonance pulse sequence Methods 0.000 abstract description 11
- 238000005070 sampling Methods 0.000 abstract description 4
- 230000000694 effects Effects 0.000 abstract description 3
- 239000013598 vector Substances 0.000 abstract description 3
- 238000002595 magnetic resonance imaging Methods 0.000 description 5
- 230000005284 excitation Effects 0.000 description 4
- 230000010363 phase shift Effects 0.000 description 4
- 230000005415 magnetization Effects 0.000 description 2
- 238000005481 NMR spectroscopy Methods 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 230000002238 attenuated effect Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000000593 degrading effect Effects 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000013421 nuclear magnetic resonance imaging Methods 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
Landscapes
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
Description
この発明は、ＭＨＩ装置（核磁気共鳴イメージング装置
）において傾斜磁場を印加する方法に関する。The present invention relates to a method of applying a gradient magnetic field in an MHI device (nuclear magnetic resonance imaging device).
一般にＭ ＲＩ装置では、静磁場用の主磁石をとりまい
てクライオスタットシールドやシムコイル、高周波コイ
ル、傾斜磁場用コイルなどが取り付けられているため、
傾斜磁場の時間的変化によって渦電流による磁場が発生
する。この渦電流磁場は核スピンの位相のずれを引き起
こし、取得したデータのＳ／Ｎ比や画像の分解能を劣化
させる。
そこで、従来では、あらかじめ渦電流による磁場を計測
し、それをキャンセルする傾斜磁場を発生させるように
電流を流す電気回路を用いて、渦電流による磁場を補正
するようにしている。Generally, in an MRI device, a cryostat shield, shim coil, high frequency coil, gradient magnetic field coil, etc. are attached to the main magnet for the static magnetic field.
A magnetic field is generated by eddy currents due to temporal changes in the gradient magnetic field. This eddy current magnetic field causes a phase shift in the nuclear spins, degrading the S/N ratio of the acquired data and the resolution of the image. Conventionally, therefore, the magnetic field caused by the eddy current is measured in advance and an electric circuit is used to flow a current to generate a gradient magnetic field that cancels the magnetic field, thereby correcting the magnetic field caused by the eddy current.
しかしながら、パルスシーケンスが異なれば渦電流の発
生の仕方が異なるため、従来のような方法ではさまざま
なパルスシーケンスにおいて渦電流による磁場を最適に
補正することが困難である。
この発明は、どのようなパルスシーケンスでも渦電流磁
場を最適に補正できる、ＭＲＩ装置の傾斜磁場印加方法
を提供することを目的とする。However, since eddy currents are generated in different ways depending on the pulse sequence, it is difficult to optimally correct the magnetic field due to eddy currents in various pulse sequences using conventional methods. An object of the present invention is to provide a gradient magnetic field application method for an MRI apparatus that can optimally correct an eddy current magnetic field in any pulse sequence.
上記目的を達成するため、この発明によるＭＲＩ装置の
傾斜磁場印加方法においては、渦電流磁場を生じさせる
磁場強度の時間的変化を持つ傾斜磁場を印加する際に、
その磁場強度の時間的変化方向とは反対方向に、その強
度が時間的に変化するような別の傾斜磁場を、時間的に
近い時点において与えることが特徴となっている。In order to achieve the above object, in the gradient magnetic field application method for an MRI apparatus according to the present invention, when applying a gradient magnetic field having a temporal change in magnetic field strength that generates an eddy current magnetic field,
It is characterized by applying another gradient magnetic field whose intensity changes over time in the opposite direction to the direction in which the magnetic field intensity changes over time, at a time point that is close to the time.
第１図に示すように、あるパルス状の傾斜磁場Ａを印加
する際のたとえばその立上りＡ１時における磁場強度の
時間的に急激な変化によって生じる渦電流磁場をキャン
セルしようとする場合、Ａ１の付近、この図では時間的
に前の接近した時点で、その立上りＡ１時の磁場強度の
時間的変イビ方向（図では正側に大きくなる方向に変化
している）とは反対方向に、その強度が時間的に変化す
るような別の傾斜磁場Ｂ１を与えている。つまり、Ｂ１
では負側に大きくなるような方向に磁場強度が時間的に
変化している。
磁場強度の急激な時間的変化を持つ傾斜磁場Ａ１によっ
て渦電流磁場が生じるが、Ａ１とは反対方向に強度の時
間的変化を持つ傾斜磁場Ｂ１を、Ａ１の直前で与えて、
Ａ１によって生じる渦電流磁場と反対方向の渦電流磁場
を生じさせているため、これによりＡ１による渦電流磁
場がキャンセルされる。
渦電流磁場はＫｅ−ｔ′℃の形で時間的に減衰し、大き
さＫは時間勾配とその勾配の持続時間で決まる。そこで
、Ａ１の部分での渦電流磁場は、Ｂ１からＡ１までの時
間に応じてＢ１の時間勾配とその持続時間とを調整する
ことによってキャンセルすることが可能となる。
なお、実際にはこのキャンセル用の傾斜磁場Ｂ１はＢ１
だけという訳にはいかず、０に戻すために反対方向に減
少してくる部分Ｂ２．Ｂ３をも持ち、全体としてＢのよ
うな傾斜磁場波形となる。
Ｂ２の部分は緩やかな時間勾配であるため過電流磁場は
あまり問題とならないが、Ｂ３の部分は急激な時間勾配
を持つのでこの部分の渦電流磁場は無視できない（なお
このようなり３の部分をもたす必要があるのは、なだら
かな部分Ｂ２だけでは時間がかかり、Ｂ１による渦電流
磁場が減衰してし、よってキャンセルできないからであ
る）。そしてこのＢ３によって生じる渦電流磁場はＡ１
と同方向のものであり、Ａ１に加算される。そこで、Ｂ
１の時間勾配と持続時間を決定する際には、この８３部
分での渦電流磁場をもキャンセルするに足りるよう定め
る必要がある。
このように、パルスシーケンス自体に、傾斜磁場を印加
する際に生じる渦電流による磁場をキャンセルするため
の仕組みを加えているため、種々のパルスシーケンスに
おいて、渦電流磁場による核スピンの位相のずれを少な
くすることができる。
その結果、ＭＨＩ装置のさまざまなパルスシーケンスに
おいてＳ／Ｎ比や分解能を向上させることができる。As shown in Fig. 1, when trying to cancel an eddy current magnetic field generated by a sudden change in magnetic field strength over time, for example at the time of rising edge A1 when applying a certain pulsed gradient magnetic field A, in the vicinity of A1 , in this figure, at the previous point in time, the magnetic field strength changes in the opposite direction to the temporal change direction of the magnetic field strength at the time of rising A1 (in the figure, it changes in the direction of becoming larger on the positive side). provides another gradient magnetic field B1 that changes over time. In other words, B1
In this case, the magnetic field strength changes over time in a direction that becomes larger on the negative side. An eddy current magnetic field is generated by a gradient magnetic field A1 having a rapid temporal change in magnetic field strength, but by applying a gradient magnetic field B1 having a temporal change in strength in the opposite direction to A1 immediately before A1,
Since an eddy current magnetic field is generated in the opposite direction to the eddy current magnetic field generated by A1, the eddy current magnetic field due to A1 is canceled. The eddy current magnetic field decays over time in the form Ke-t'°C, and the magnitude K is determined by the time gradient and the duration of that gradient. Therefore, the eddy current magnetic field in the portion A1 can be canceled by adjusting the time gradient of B1 and its duration according to the time from B1 to A1. In addition, in reality, this gradient magnetic field B1 for cancellation is B1
However, the part B2. decreases in the opposite direction to return to 0. It also has B3, and the gradient magnetic field waveform as a whole becomes B. Part B2 has a gentle time gradient, so the overcurrent magnetic field is not much of a problem, but part B3 has a steep time gradient, so the eddy current magnetic field in this part cannot be ignored. This is necessary because it takes time to create only the gentle portion B2, and the eddy current magnetic field due to B1 is attenuated, so it cannot be canceled). And the eddy current magnetic field generated by this B3 is A1
is in the same direction as A1, and is added to A1. Therefore, B
When determining the time gradient and duration of 1, it is necessary to set them so that they are sufficient to cancel the eddy current magnetic field in this 83 portion. In this way, a mechanism is added to the pulse sequence itself to cancel the magnetic field caused by the eddy current that occurs when applying a gradient magnetic field, so in various pulse sequences, the phase shift of nuclear spins due to the eddy current magnetic field can be canceled out. It can be reduced. As a result, the S/N ratio and resolution can be improved in various pulse sequences of the MHI device.
つぎにこの発明をスピンワープ法に適用した一実施例に
ついて図面を参照しながら説明する。スピンワープ法で
は、第２図に示すように励起用高周波パルス１を印加す
るとき同時にスライス厚さ方向くたとえばＺ方向）のス
ライス面選択用傾斜磁場Ｇｓのパルス３を加え、その後
炉斜磁場パルス３を印加したことによる磁化ベクトルの
位相の乱れを補正するための傾斜磁場パルス４を加える
。
そして、位相エンコード方向（たとえばＹ方向）の位相
エンコード用の傾斜磁場Ｇｐに関してパルス５を加え、
さらにその後、リードアウト方向くたとえばＸ方向）の
周波数エンコード用の傾斜磁場Ｇｆに関してデイフェー
ズ用傾斜磁場パルス６を加え、続いてリフェーズ用傾斜
磁場パルス７を印加してＮＭＲ信号を発生させ、信号計
測用のサンプリングパルス８によりデータを採取する。
このスピンワープ法において、傾斜磁場パルス３によっ
て特定のスライス面に関する選択励起を行なう際、この
傾斜磁場パルス３の立上り部分３１の渦電流磁場によっ
て選択励起に悪影響が生じたり、傾斜磁場パルス４によ
って位相補正を行なっても渦電流磁場が完全に打ち消さ
れないため磁化ベクトルの位相が揃わないことがある。
そこで、この実施例では、傾斜磁場パルス３の直前に、
立上り時の渦電流磁場をキャンセルするための傾斜磁場
パルス２を加えている。この傾斜磁場パルス２の立上り
部分２１の時間勾配と持続時間、及び立下り部分２２の
時間勾配と持続時間を調整することによって、立下り部
分２２で生じる渦電流磁場で立上り部分３１で生じる渦
電流磁場をキャンセルし、この立上り部分３１により生
じる渦電流磁場の影響を最小限に抑えるようにしている
。傾斜磁場パルス４の立上り部分４１及び立下り部分４
２についても、それらの時間勾配及び持続時間を調整し
て、傾斜磁場パルス３の立下り部分３２により生じる渦
電流磁場をキャンセルし、渦電流磁場による位相の乱れ
が生じないようにしている。
位相エンコード用の傾斜磁場５は特定の方向（たとえば
Ｙ方向）の位置情報を位相情報にエンコードするための
ものであるが、これについても立上り部分及び立下り部
分で生じる渦電流磁場をキャンセルするための傾斜磁場
パルスを与えることができる。ただ、この実施例ではエ
コー時間が長くなることを避けるためにこのような渦電
流磁場キャンセル用の傾斜磁場パルスは与えていない。
そして、周波数エンコード用傾斜磁場に関しては、デイ
フェーズ用傾斜磁場パルス６の立上り部分６１と立下り
部分６２の時間勾配と持続時間とを調整し、リフェーズ
用傾斜磁場パルス７の立上り部分７１で生じる渦電流磁
場をキャンセルするようにしている。これによって、信
号サンプリング時の渦電流の影響を除去することができ
る。
なお、上記では本発明をスピンワープ法に適用した一実
施例について説明したが、この発明は、他のパルスシー
ケンス、たとえばスピンエコー法、アンギオイメージン
グ法、ケミカルシフトイメージング法などのパルスシー
ケンスにも適用可能であることは勿論である。Next, an embodiment in which the present invention is applied to the spin warp method will be described with reference to the drawings. In the spin warp method, as shown in Fig. 2, when applying a high frequency excitation pulse 1, a pulse 3 of a gradient magnetic field Gs for slice plane selection in the slice thickness direction (for example, the Z direction) is simultaneously applied, and then a furnace gradient magnetic field pulse is applied. A gradient magnetic field pulse 4 is applied to correct the disturbance in the phase of the magnetization vector due to the application of the gradient magnetic field pulse 4. Then, pulse 5 is applied to the gradient magnetic field Gp for phase encoding in the phase encoding direction (for example, the Y direction),
Furthermore, after that, a day phase gradient magnetic field pulse 6 is applied to the frequency encoding gradient magnetic field Gf in the readout direction (for example, the X direction), and then a rephase gradient magnetic field pulse 7 is applied to generate an NMR signal, and the signal is measured. Data is collected using a sampling pulse 8. In this spin warp method, when performing selective excitation on a specific slice plane using the gradient magnetic field pulse 3, the selective excitation may be adversely affected by the eddy current magnetic field in the rising portion 31 of the gradient magnetic field pulse 3, or the gradient magnetic field pulse 4 may cause a phase shift. Even if correction is performed, the eddy current magnetic field is not completely canceled out, so the phases of the magnetization vectors may not be aligned. Therefore, in this embodiment, immediately before gradient magnetic field pulse 3,
Gradient magnetic field pulse 2 is added to cancel the eddy current magnetic field at the time of rise. By adjusting the time gradient and duration of the rising portion 21 and the time gradient and duration of the falling portion 22 of this gradient magnetic field pulse 2, the eddy current generated in the rising portion 31 of the eddy current magnetic field generated in the falling portion 22 can be adjusted. The magnetic field is canceled to minimize the influence of the eddy current magnetic field generated by this rising portion 31. Rising portion 41 and falling portion 4 of gradient magnetic field pulse 4
2, the time gradient and duration thereof are also adjusted to cancel the eddy current magnetic field generated by the falling portion 32 of the gradient magnetic field pulse 3, so as to prevent phase disturbance due to the eddy current magnetic field. The gradient magnetic field 5 for phase encoding is used to encode position information in a specific direction (for example, the Y direction) into phase information, but it is also used to cancel the eddy current magnetic field generated at the rising and falling parts. gradient magnetic field pulses can be given. However, in this embodiment, such a gradient magnetic field pulse for canceling the eddy current magnetic field is not applied in order to avoid an increase in echo time. Regarding the frequency encoding gradient magnetic field, the time gradient and duration of the rising portion 61 and falling portion 62 of the day phase gradient magnetic field pulse 6 are adjusted, and the vortex generated at the rising portion 71 of the rephasing gradient magnetic field pulse 7 is adjusted. It is designed to cancel the current magnetic field. This makes it possible to eliminate the effects of eddy currents during signal sampling. Although an embodiment in which the present invention is applied to the spin warp method has been described above, the present invention can also be applied to other pulse sequences such as spin echo method, angioimaging method, chemical shift imaging method, etc. Of course it is possible.
この発明のＭＲＩ装置の傾斜磁場印加方法によれば、傾
斜磁場パルス波形自体を渦電流磁場をキャンセルするよ
うな波形としているため、どのようなパルスシーケンス
においても傾斜磁場を印加する際に生じる渦電流による
磁場を補正することができる。その結果、ＭＲＩ装置の
さまざまなパルスシーケンスにおいて渦電流磁場による
核スピンの位相のずれを少なくすることができ、Ｓ／Ｎ
比や分解能を向上させることができる。According to the gradient magnetic field application method of the MRI apparatus of the present invention, since the gradient magnetic field pulse waveform itself is a waveform that cancels the eddy current magnetic field, the eddy current generated when applying the gradient magnetic field in any pulse sequence It is possible to correct the magnetic field caused by As a result, it is possible to reduce the phase shift of nuclear spins due to eddy current magnetic fields in various pulse sequences of MRI equipment, and S/N
ratio and resolution can be improved.
第１図はこの発明の詳細な説明するためのタイムチャー
１〜、第２図はこの発明の一実施例にかかるパルスシー
ケンスのタイムチャートである。
１・・励起用高周波パルス、３・・・スライス面選択用
傾斜磁場パルス、５・・・位相エンコード用傾斜磁場パ
ルス、６・・・デイフェーズ用傾斜磁場パルス、７・・
・リフェーズ用傾斜磁場パルス、８・・・信号計測用サ
ンプリングパルス。FIG. 1 is a time chart 1 to 1 for explaining the present invention in detail, and FIG. 2 is a time chart of a pulse sequence according to an embodiment of the present invention. 1... High frequency pulse for excitation, 3... Gradient magnetic field pulse for slice plane selection, 5... Gradient magnetic field pulse for phase encoding, 6... Gradient magnetic field pulse for day phase, 7...
・Gradient magnetic field pulse for rephasing, 8...Sampling pulse for signal measurement.
Claims (1)
持つ傾斜磁場を印加する際に、その磁場強度の時間的変
化方向とは反対方向に、その強度が時間的に変化するよ
うな別の傾斜磁場を、時間的に近い時点において与える
ことを特徴とするＭＲＩ装置の傾斜磁場印加方法。(1) When applying a gradient magnetic field whose magnetic field strength changes over time to generate an eddy current magnetic field, there is a gradient magnetic field whose strength changes over time in the opposite direction to the direction of the temporal change in the magnetic field strength. A method for applying a gradient magnetic field to an MRI apparatus, the method comprising: applying a gradient magnetic field at a time close to each other.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP63300138A JP2580747B2 (en) | 1988-11-28 | 1988-11-28 | Method for applying gradient magnetic field of MRI apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP63300138A JP2580747B2 (en) | 1988-11-28 | 1988-11-28 | Method for applying gradient magnetic field of MRI apparatus |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH02144039A true JPH02144039A (en) | 1990-06-01 |
| JP2580747B2 JP2580747B2 (en) | 1997-02-12 |
Family
ID=17881194
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP63300138A Expired - Fee Related JP2580747B2 (en) | 1988-11-28 | 1988-11-28 | Method for applying gradient magnetic field of MRI apparatus |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP2580747B2 (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0595391A1 (en) * | 1992-10-26 | 1994-05-04 | Koninklijke Philips Electronics N.V. | Eddy current compensation in magnetic resonance imaging |
| US5455512A (en) * | 1992-10-26 | 1995-10-03 | U.S. Philips Corporation | Eddy current compensation in magnetic resonance imaging |
| WO2003037183A1 (en) * | 2001-10-30 | 2003-05-08 | Hitachi Medical Corporation | Magnetic resonance imaging device |
| DE19807306B4 (en) * | 1997-02-22 | 2010-10-14 | General Electric Co. | Method for reducing eddy current effects in a diffusion-weighted echo-planar imaging |
| JP2011182916A (en) * | 2010-03-08 | 2011-09-22 | Toshiba Corp | Magnetic resonance imaging apparatus |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6264351A (en) * | 1985-07-29 | 1987-03-23 | ゼネラル・エレクトリツク・カンパニイ | Method for reducing transitional non-homogenuity of magneticfield |
-
1988
- 1988-11-28 JP JP63300138A patent/JP2580747B2/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6264351A (en) * | 1985-07-29 | 1987-03-23 | ゼネラル・エレクトリツク・カンパニイ | Method for reducing transitional non-homogenuity of magneticfield |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0595391A1 (en) * | 1992-10-26 | 1994-05-04 | Koninklijke Philips Electronics N.V. | Eddy current compensation in magnetic resonance imaging |
| US5455512A (en) * | 1992-10-26 | 1995-10-03 | U.S. Philips Corporation | Eddy current compensation in magnetic resonance imaging |
| DE19807306B4 (en) * | 1997-02-22 | 2010-10-14 | General Electric Co. | Method for reducing eddy current effects in a diffusion-weighted echo-planar imaging |
| WO2003037183A1 (en) * | 2001-10-30 | 2003-05-08 | Hitachi Medical Corporation | Magnetic resonance imaging device |
| JP2011182916A (en) * | 2010-03-08 | 2011-09-22 | Toshiba Corp | Magnetic resonance imaging apparatus |
Also Published As
| Publication number | Publication date |
|---|---|
| JP2580747B2 (en) | 1997-02-12 |
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