JP2004016657A

JP2004016657A - Magnetic resonance imaging apparatus

Info

Publication number: JP2004016657A
Application number: JP2002178898A
Authority: JP
Inventors: Kenji Sakakibara; 榊原　健二; Hirotaka Takeshima; 竹島　弘隆; Takeshi Yao; 八尾　武
Original assignee: Hitachi Medical Corp
Current assignee: Hitachi Healthcare Manufacturing Ltd
Priority date: 2002-06-19
Filing date: 2002-06-19
Publication date: 2004-01-22
Anticipated expiration: 2022-06-19
Also published as: JP3984874B2

Abstract

<P>PROBLEM TO BE SOLVED: To realize a magnetic resonance imaging apparatus wherein opening property of a magnetic structure is improved by improving the correction of heterogeneity of a static magnetic field generated in a photographing space. <P>SOLUTION: An open magnet 1 is provided with a generation source of the static magnetic field, which comprises magnetic poles 4a and 4b where annular ferromagnetic bodies 5a and 5b are formed. The magnetic poles 4a and 4b generate the static magnetic field in a volume space on the side of surfaces opposing each other. On opposite sides of the opposing surfaces of the magnetic poles 4a and 4b, sheet-like yokes 2a and 2b are magnetically connected via the generation source of the static magnetic field, and the sheet-like yokes 2a and 2b are magnetically connected with each other by a pillar-shaped yoke 6. For the magnetic pole 4a-1 positioned on the side of an opening part and the magnetic pole 4a-2 positioned on the side of the pillar-shaped yoke 6 among the magnetic poles 4a, ferromagnetic materials having different magnetic characteristics are used. For an annular protrusion 5a-1 positioned on the side of the opening part and an annular protrusion 5a-2 positioned on the side of the pillar-shaped yoke 6 among the annular protrusions 5a, ferromagnetic materials having different magnetic characteristics are used. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、均一性の良好な磁場を発生させる磁場発生手段を有する磁気共鳴イメージング装置に関する。
【０００２】
【従来の技術】
磁気共鳴イメージング装置において、開放性を有し、被検体へのアクセス性に優れた開放型ＭＲＩ装置用磁石としては、被検体が挿入される撮影空間の上下、あるいは横方向に静磁場発生源が互いに対向して配置され、被検体に対して、赤道方向あるいは上下周方向が開放された磁石（以下、開放型磁石という）が公知となっている。
【０００３】
例えば、静磁場発生源として、超電導コイルを用いたものが、特開平５−２３４７４６号公報、特開平４−１３２５３９号公報などに開示されている。また、静磁場発生源として永久磁石を用いたものが、特開平６−２５１９３０号公報などに開示されている。
【０００４】
上述した公知例において、開放型磁石とすると、静磁場の均一性が阻害される場合が生じるので、撮影空間に形成する静磁場の磁場均一度を調整するために、静磁場発生源に設けられた磁極の周辺部に、環状の突起部を設け、その内周側の撮影空間に対向する面に適切な凹凸を設けることで、磁極の周辺部における磁界の方向を、対向する磁極に向かう方向に補正することにより撮影空間の磁場均一度を高めている。
【０００５】
体積空間に発生する静磁場の不均一性を補償する方法として、環状の突起部を有する磁極の磁極周辺の形状を変化させる方法は、例えば、特開平５−２３４７４６号公報に記載されている。
【０００６】
ここで、開放性を、さらに向上させた磁石として、対向する円盤状の本体部の中心に撮影空間を有し、この中心より後方に継鉄（鉄柱）を配置した磁石が、特開平８−５０１７０号公報に開示され公知となっている。この場合、磁束は上記継鉄通過することとなり、磁場の不均一性が生じることとなる。
【０００７】
そこで、上記鉄柱による磁場の非対称成分を補正するために、磁極の環状突起部分の高さ、あるいは幅を、周方向に対して変化させた磁石が、特開平８−２４３０８７公報に開示されている。
【０００８】
【発明が解決しようとする課題】
ところで、近年では、磁気共鳴イメージング（ＭＲＩ）装置の分野では、医療の可能性を広げるＩＶＲ（Ｉｎｔｅｒｖｅｎｔｉｏｎａｌ　Ｒａｄｉｏｌｏｇｙ）を可能とするため、被検体へのアクセス性のより高い開放型磁石が要求されている。
【０００９】
しかしながら、磁気共鳴イメージング装置において、被検体についての自由な開放空間を追求すると、撮影空間に対して構造的の対称性の低い磁石構造となる。
【００１０】
したがって、開放性を向上させた構造では、静磁場の非対弥成分が大きくなり、上述した方法だけでは、非対称成分を充分に補正しきれない場合がある。
【００１１】
つまり、従来技術においては、撮影空間に発生する静磁場の不均一性の補正限界から、磁石構造の開放性を向上させることはできなかった。
【００１２】
本発明の目的は、撮影空間に発生する静磁場の不均一性の補正を向上させることにより、磁石構造の開放性を向上させることが可能な磁気共鳴イメージング装置を実現することである。
【００１３】
【課題を解決するための手段】
上記目的を達成するため、本発明は次のように構成される。
（１）静磁場発生手段と、傾斜磁場発生手段と、高周波磁場を発生する照射コイルと、核磁気共鳴信号を受信する受信コイルと、断層画像を作成する画像作成手段とを有する磁気共鳴イメージング装置において、上記静磁場発生手段は、互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起と、上記一対の磁極を支持する柱状の継鉄とを備え、上記磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とは、互いに磁気特性が異なる。
【００１４】
（２）静磁場発生手段と、傾斜磁場発生手段と、高周波磁場を発生する照射コイルと、核磁気共鳴信号を受信する受信コイルと、断層画像を作成する画像作成手段とを有する磁気共鳴イメージング装置において、上記静磁場発生手段は、撮影空間を介して互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起と、上記一対の磁極を支持する柱状の継鉄と、この柱状継鉄の上記撮影空間側の面に配置され、柱状継鉄の面方向に平行な磁化容易軸を有する強磁性体とを備える。
【００１５】
（３）静磁場発生手段と、傾斜磁場発生手段と、高周波磁場を発生する照射コイルと、核磁気共鳴信号を受信する受信コイルと、断層画像を作成する画像作成手段とを有する磁気共鳴イメージング装置において、上記静磁場発生手段は、互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起とを備え、上記一方の磁極に形成される環状突起と、他方の磁極に形成される環状突起は、互いに磁気特性が異なる。
【００１６】
（４）互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起と、上記一対の磁極を支持する柱状の継鉄とを備え、上記一対の磁極の互いに対向する面側の空間に静磁場を発生する磁気共鳴イメージング装置の静磁場発生装置において、上記磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とは、互いに磁気特性が異なる。
【００１７】
（５）互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起と、上記一対の磁極を支持する柱状の継鉄とを備え、上記一対の磁極の互いに対向する面側の空間に静磁場を発生する磁気共鳴イメージング装置の静磁場発生装置において、上記柱状継鉄の上記空間側の面に配置され、柱状継鉄の面方向に平行な磁化容易軸を有する強磁性体を備える。
【００１８】
（６）互いに対向する一対の磁極と、この一対の磁極の互いに対向する面側に形成された環状の強磁性体からなる突起とを備え、上記一対の磁極の互いに対向する面側の空間に静磁場を発生する磁気共鳴イメージング装置の静磁場発生装置において、上記一方の磁極に形成される環状突起と、他方の磁極に形成される環状突起は、互いに磁気特性が異なる。
【００１９】
（７）好ましくは、上記（１）、（４）において、上記磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とで、互いに磁気特性が異なる。
【００２０】
（８）また、好ましくは、上記（１）、（４）において、上記磁極及び環状突起の材料に、飽和磁化の異なる複数の材料を使用して、上記磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とは、磁化方向が互いに異方性を有する材料を使用することを特徴とする静磁場発生装置。
【００２１】
（９）また、好ましくは、上記（３）、（６）において、上記環状突起の材料に、飽和磁化の異なる複数の材料を使用して、上記一方の磁極に形成される環状突起と、他方の磁極に形成される環状突起とが、互いに磁気特性が異なる。
【００２２】
磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とを、互いに磁気特性が異なるように構成し、柱状継鉄の存在による磁場の不均一性が補償される。
【００２３】
飽和磁化の異なる複数の材料を使用して、又は磁化方向が互いに異方性を有する材料を使用することにより、磁極及び環状突起の柱状継鉄に面する側の部分と、上記磁極及び環状突起の柱状継鉄に面する側とは反対側に位置する部分とを、互いに磁気特性が異なるように構成することができる。
【００２４】
【発明の実施の形態】
以下、本発明の実施形態を添付図面を参照して説明する。
図１は、本発明の第１の実施形態である磁気共鳴イメージング装置における開放型磁石１の概略断面図である。図１の一点鎖線で示すＸ軸を水平方向、Ｚ軸を鉛直方向とする。
【００２５】
また、図示していないが、磁気共鳴イメージング装置は、静磁場発生装置の他に、傾斜磁場発生装置と、高周波磁場を発生する照射コイルと、核磁気共鳴信号を受信する受信コイルと、被検体が配置される寝台と、磁場発生装置等の動作を制御する制御手段と、断層画像を作成する画像作成手段と、画像表示手段とを備える。
【００２６】
図１において、開放型磁石１は、静磁場発生源３ａ、３ｂを備え、この静磁場発生源３ａ、３ｂは、環状強磁性体５ａ、５ｂが形成された一対の互いに対向する磁極４ａ、４ｂを有する。そして、磁極４ａ、４ｂが互いに対向する面側の体積空間７に静磁場を発生する。
【００２７】
磁極４ａ、４ｂの対向面の反対側には、静磁場発生源３ａ、３ｂを介して板状継鉄２ａ、２ｂが磁気的に結合されている。さらに、板状継鉄２ａ、２ｂは、柱状継鉄６によって、互いに磁気的に連結され、開放型磁石１は磁気回路を形成している。
【００２８】
図２は、図１に示した磁石１の上側方向から見た磁極４ａ及び環状強磁性体５ａと柱状継鉄６との関係を示す図である。なお、Ｙ軸は、Ｘ軸及びＺ軸に直交する方向の軸である。
【００２９】
図１及び図２に示すように、柱状継鉄６が体積空間７に対し、偏って配置される場合、つまり、体積空間７の周囲に均等に又は点対称に配置されず、４辺形状の一辺に偏って配置される場合には、磁極４ａ、４ｂに発生する磁束が柱状継鉄６に吸引される。
【００３０】
したがって、何ら、対策を施していなければ、柱状継鉄６が配置される側と、その他の部分における体積空間７の磁束密度が異なることとなり、磁場の均一性が悪化する。このため、本発明の第１の実施形態においては、以下に説明するように構成して、磁場の均一性を補償している。
【００３１】
図２において、磁極４ａのうち、開口部側（継鉄６が配置されている側とは反対側）に位置する磁極４ａ−１（斜線以外の領域）と柱状継鉄６側にある磁極４ａ−２（斜線で示す領域）について、磁気特性の異なる強磁性材料を使用している。
【００３２】
また、同様に、環状突起５ａのうち、開口部側に位置する環状突起５ａ−１（斜線以外の領域）と柱状継鉄６側に位置する環状突起５ａ−２（斜線で示す領域）について、磁気特性の異なる強磁性材料を使用する。
【００３３】
磁極４ａ−１及び環状突起５ａ−１の磁気特性と、磁極４ａ−２及び環状突起５ａ−２の磁気特性とを異なるように構成することにより、継鉄６の影響による磁場不均一性を補正する。
【００３４】
なお、磁極４ａ−２及び環状突起５ａ−２の領域は、磁極４ａの中心点又は環状強磁性体５ａの中心点から柱状継鉄６側に向かう拡がり角度が１８０°以下であり、磁極４ａ−１及び環状突起５ａ−１と、磁極４ａ−２及び環状突起５ａ−２と境界線の延長線が継鉄６の幅以上となる領域が好ましい。なお、継鉄６の幅とは、図２のＹ軸方向の長さである。
【００３５】
図３及び図４は、磁極４ａ−１及び環状突起５ａ−１と、磁極４ａ−２及び環状突起５ａ−２とが、飽和磁化特性が異なる強磁性材料を使用した場合を示す図であり、磁石１の上側半分部分を示す図である。
【００３６】
図５は、強磁性体の磁気特性（Ｍ−Ｈ曲線）を示す図であり、互いに異なる磁化特性Ｍｓ１、Ｍｓ２（Ｍｓ１＞Ｍｓ２）の例を示す図である。
【００３７】
柱状継鉄６側に位置する磁極４ａ−２、環状突起５ａ−２の強磁性体には飽和磁化の大きい材料（Ｍｓ１）を用いる。
【００３８】
一方、開口側の磁極４ａ−１、環状突起５ａ−１には、飽和磁化の小さい材料（Ｍｓ２）を用いる。
【００３９】
飽和磁化の異なる材料としては、例えば、純鉄と炭素鋼を使用することができる。このように、飽和磁化の異なる材料を用いる構成により、体積空間７の柱状継鉄６側における、磁場強度が増加し、磁場不均一性を補償することができる。
【００４０】
上述した例は、磁極４ａ−１及び環状突起５ａ−１と、磁極４ａ−２及び環状突起５ａ−２とは、飽和磁化特性が異なる強磁性材料を使用した例であるが、磁極４ａ−１及び環状突起５ａ−１と、磁極４ａ−２及び環状突起５ａ−２とを、磁化容易軸方向が異なる（磁化方向が互いに異方性を有する）強磁性材料を使用しても同様な効果を得ることができる。
【００４１】
柱状継鉄６が存在する場合には、体積空間７の磁束が柱状継鉄６によって吸引されるため、体積空間７の柱状継鉄６側では、柱状継鉄６に向かう水平（Ｘ軸、Ｙ軸方向）成分が大きい。
【００４２】
そこで、柱状継鉄６側に位置する磁極４ａ−２、環状突起５ａ−２に使用する強磁性体には、Ｚ軸方向に磁化容易軸を有する強磁性体、例えば、ケイ素鋼板を使用する。
【００４３】
この構成により、体積空間７の柱状継鉄６側では、Ｚ軸方向に向かう磁束量が増加し、磁場不均一性を補償することができる。
【００４４】
なお、図示した例は、磁石１の上方部分のみ示したが、下方部分も上方部分と同様に構成されている。
【００４５】
以上のように、本発明の第１の実施形態によれば、開口側の磁極４ａ−１及び環状突起５ａ−１と、柱状継鉄６の磁極４ａ−２及び環状突起５ａ−２とを、飽和磁化特性又は磁化容易軸方向が異なる強磁性材料を使用するように構成したので、（飽和磁化特性は開口側より継鉄側が大であり、磁化容易化軸方向は開口側はＸ軸又はＹ軸方向、継鉄側がＺ軸方向）、撮影空間に発生する静磁場の不均一性の補正を向上させることにより、磁石構造の開放性を向上させることが可能な磁気共鳴イメージング装置用の静磁場発生装置及びそれを用いた磁気共鳴イメージング装置を実現することができる。
【００４６】
図６は、本発明の第２の実施形態である磁気共鳴イメージング装置における開放型磁石１の上側方向から見た図である。
【００４７】
この第２の実施形態は、互いに対向して配置される２つの柱状継鉄１６、１６’が配置される場合の例である。
【００４８】
磁極１４ａのうち開口側に位置する磁極１４ａ−１、環状突起１５ａ−１と、柱状継鉄１６、１６’側に位置する磁極１４ａ−２、環状突起１５ａ−２とで磁気特性の異なる材料を使用する。１２ａは板状継鉄である。
【００４９】
なお、磁極１４ａ−２及び環状突起１５ａ−２の領域は、磁極１４ａの中心点又は環状強磁性体１５ａの中心点から柱状継鉄１６、１６’側に向かう拡がり角度が１８０°以下であり、磁極１４ａ−１及び環状突起１５ａ−１と、磁極１４ａ−２及び環状突起１５ａ−２と境界線の延長線が継鉄１６、１６’の幅以上となる領域が好ましい。
本発明の第２の実施形態においても、第１の実施形態と同様な効果を得ることができる。
【００５０】
図７は、本発明の第３の実施形態である磁気共鳴イメージング装置における開放型磁石１の概略断面図である。
【００５１】
この第３の実施形態は、板状継鉄２ａと２ｂとを磁気的に連結する柱状継鉄６を備える開放型磁石１において、柱状継鉄６の体積空間７に面する表面の磁気特性を変化させる例である。
【００５２】
図７において、柱状継鉄６の体積空間７側の面には、この面方向に平行な磁化容易軸を有する、例えばケイ素鋼板などの板状の強磁性体９が配置されている。
【００５３】
柱状継鉄６の面方向に平行な磁化容易軸を有する強磁性体９を、体積空間７側に配置することにより、体積空間７から柱状継鉄６に向かう磁束、つまり、水平成分の磁束が減少し、体積空間７における磁場の不均一性を改善することができる。
【００５４】
なお、上述した例は、板状継鉄２ａと２ｂとを磁気的に連結する柱状継鉄６を備える開放型磁石１に本発明を適用した場合の例であるが、柱状継鉄を使用しない例にも適用可能である。
【００５５】
つまり、柱状継鉄を使用せず、互いに対向する磁石を用いる場合、対向する面積は、互いに異なる場合があり、この場合にも、上下（又は左右）磁極に環状強磁性体が使用され、環状強磁性体の径が、互いに異なることとなる。
【００５６】
その場合には、上下磁極に挾まれる体積空間のうち、中心付近と、周辺付近とで磁場の強度が異なり、磁場が不均一となる。面積が大きな磁極の周辺の磁束は、面な小さな磁極に向かわず、外部に向かうものがあるからである。
【００５７】
そこで、面積が小さな磁極の環状強磁性体の飽和磁化を、面積が大きな磁極の環状強磁性体の飽和磁化より大として、磁場の不均一性を補償することができる。
【００５８】
また、上述した例は、環状突起の飽和磁化特性を調整することにより、磁場の不均一性を補償するようにしたが、コイルに電流を流して不均一性を補償することもできる。
【００５９】
つまり、柱状継鉄側にコイルを配置し、Ｚ軸方向の磁界を発生させて不均一性を補償することができる。
【００６０】
しかしながら、このコイルの径を大とすると、電流と磁界の関係から受ける機械的な力が大となるので、支持強度等が問題となる。
【００６１】
そこで、環状突起と、支持強度等が問題とならない小さな径のコイルとを用いて、環状突起で不均一性をある程度補償し、さらに、微調整としてコイルによる均一性の改善を行ってもよい。
【００６２】
【発明の効果】
本発明によれば、撮影空間に発生する静磁場の不均一性の補正を向上させることにより、磁石構造の開放性を向上させることが可能な磁気共鳴イメージング装置の静磁場発生装置及びそれを用いた磁気共鳴イメージング装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態である磁気共鳴イメージング装置における開放型磁石の概略断面図である。
【図２】図１に示した磁石の上側方向から見た磁極及び環状強磁性体と柱状継鉄との関係を示す図である。
【図３】開口側の磁極と継鉄側の磁極とは飽和磁化特性（磁化容易化軸）が異なる強磁性材料を使用した場合を示す図である。
【図４】開口側の環状突起と継鉄側の環状突起とは飽和磁化特性（磁化容易化軸）が異なる強磁性材料を使用した場合を示す図である。
【図５】強磁性体の磁気特性（Ｍ−Ｈ曲線）を示す図である。
【図６】本発明の第２の実施形態である磁気共鳴イメージング装置における開放型磁石の上側方向から見た磁極及び環状強磁性体と柱状継鉄との関係を示す図である。
【図７】本発明の第３の実施形態である磁気共鳴イメージング装置における開放型磁石の概略断面図である。
【符号の説明】
１　　　　　　　　　開放型磁石
２ａ　　　　　　　　上側板状継鉄
２ｂ　　　　　　　　下側板状継鉄
３ａ、３ｂ　　　　　静磁場発生源
４ａ　　　　　　　　上側磁極
４ｂ　　　　　　　　下側磁極
５ａ　　　　　　　　上側環状突起
５ｂ　　　　　　　　下側環状突起
４ａ−１、１４ａ−１　　柱状継鉄側磁極
４ａ−２、１４ａ−２　　開口側磁極
５ａ−１、１５ａ−２　　柱状継鉄側環状突起
５ａ−２、１５ａ−２　　開口部側環状突起
６、１６、１６’　　柱状継鉄
７　　　　　　　　　体積空間
９　　　　　　　　　強磁性体[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus having a magnetic field generating means for generating a magnetic field having good uniformity.
[0002]
[Prior art]
In a magnetic resonance imaging apparatus, a magnet for an open type MRI apparatus having openness and excellent access to a subject includes a static magnetic field generating source in a vertical direction or a horizontal direction of an imaging space into which the subject is inserted. 2. Description of the Related Art There are known magnets that are arranged to face each other and that are open to the subject in the equatorial direction or the upper and lower circumferential directions (hereinafter, referred to as open magnets).
[0003]
For example, those using a superconducting coil as a static magnetic field generating source are disclosed in JP-A-5-234746, JP-A-4-132339 and the like. An apparatus using a permanent magnet as a static magnetic field generation source is disclosed in Japanese Patent Application Laid-Open No. 6-251930.
[0004]
In the above-described known example, if an open magnet is used, the uniformity of the static magnetic field may be hindered. Therefore, in order to adjust the magnetic field uniformity of the static magnetic field formed in the imaging space, the static magnetic field is provided in the static magnetic field generation source. By providing an annular projection on the periphery of the magnetic pole, and providing appropriate irregularities on the surface facing the imaging space on the inner peripheral side, the direction of the magnetic field at the periphery of the magnetic pole can be changed in the direction toward the opposite magnetic pole. To improve the uniformity of the magnetic field in the imaging space.
[0005]
As a method of compensating for the non-uniformity of the static magnetic field generated in the volume space, a method of changing the shape of the magnetic pole having an annular protrusion around the magnetic pole is described in, for example, Japanese Patent Application Laid-Open No. 5-234746.
[0006]
Here, as a magnet with further improved openness, a magnet having a photographing space at the center of the opposed disk-shaped main body and a yoke (iron column) disposed behind the center is disclosed in Japanese Patent Application Laid-Open No. H08-08380. It is disclosed and known in Japanese Patent No. 50170. In this case, the magnetic flux passes through the yoke, causing non-uniformity of the magnetic field.
[0007]
Therefore, in order to correct the asymmetric component of the magnetic field due to the iron column, a magnet in which the height or width of the annular protrusion of the magnetic pole is changed in the circumferential direction is disclosed in Japanese Patent Application Laid-Open No. Hei 8-243807. .
[0008]
[Problems to be solved by the invention]
By the way, in recent years, in the field of magnetic resonance imaging (MRI) apparatuses, open magnets with higher accessibility to a subject are required in order to enable an IVR (Interventional Radiology) that expands medical possibilities. .
[0009]
However, in a magnetic resonance imaging apparatus, if a free open space for the subject is pursued, a magnet structure having a low structural symmetry with respect to the imaging space is obtained.
[0010]
Therefore, in a structure with improved openness, the non-opposite component of the static magnetic field becomes large, and the asymmetric component may not be sufficiently corrected by the above-described method alone.
[0011]
That is, in the related art, the openness of the magnet structure cannot be improved due to the correction limit of the nonuniformity of the static magnetic field generated in the imaging space.
[0012]
An object of the present invention is to realize a magnetic resonance imaging apparatus capable of improving the openness of a magnet structure by improving correction of nonuniformity of a static magnetic field generated in an imaging space.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as follows.
(1) A magnetic resonance imaging apparatus including a static magnetic field generating unit, a gradient magnetic field generating unit, an irradiation coil for generating a high-frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image generating unit for generating a tomographic image In the static magnetic field generating means, a pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on the surface of the pair of magnetic poles facing each other, and a columnar shape supporting the pair of magnetic poles A portion of the magnetic pole and the annular protrusion facing the columnar yoke, and a portion of the magnetic pole and the annular protrusion opposite to the side facing the columnar yoke have magnetic properties mutually. Are different.
[0014]
(2) A magnetic resonance imaging apparatus having a static magnetic field generating means, a gradient magnetic field generating means, an irradiation coil for generating a high frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image forming means for forming a tomographic image In the above, the static magnetic field generating means includes a pair of magnetic poles facing each other via an imaging space, a projection made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and the pair of magnetic poles. And a ferromagnetic material disposed on the surface of the column-shaped yoke on the imaging space side and having an easy axis of magnetization parallel to the surface direction of the column-shaped yoke.
[0015]
(3) A magnetic resonance imaging apparatus including a static magnetic field generating unit, a gradient magnetic field generating unit, an irradiation coil for generating a high-frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image generating unit for generating a tomographic image. Wherein the static magnetic field generating means includes a pair of magnetic poles facing each other, and a protrusion made of an annular ferromagnetic material formed on the surface of the pair of magnetic poles facing each other, and is formed on the one magnetic pole. The annular projection formed on the other magnetic pole and the annular projection formed on the other magnetic pole have different magnetic characteristics.
[0016]
(4) a pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on the surface of the pair of magnetic poles facing each other, and a column-shaped yoke supporting the pair of magnetic poles; In the static magnetic field generating device of the magnetic resonance imaging apparatus for generating a static magnetic field in a space on a surface side of the pair of magnetic poles facing each other, a portion of the magnetic pole and the annular protrusion facing the columnar yoke, the magnetic pole and the annular shape The magnetic properties of the protrusion are different from those of the portion located on the side opposite to the side facing the columnar yoke.
[0017]
(5) a pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and a columnar yoke supporting the pair of magnetic poles; In the static magnetic field generator of the magnetic resonance imaging apparatus that generates a static magnetic field in the space on the surface of the pair of magnetic poles facing each other, the static magnetic field generator is disposed on the space-side surface of the columnar yoke, and in the surface direction of the columnar yoke. A ferromagnetic material having a parallel easy axis of magnetization is provided.
[0018]
(6) a pair of magnetic poles opposed to each other, and a projection made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other; In the static magnetic field generator of the magnetic resonance imaging apparatus that generates a static magnetic field, the annular protrusion formed on the one magnetic pole and the annular protrusion formed on the other magnetic pole have different magnetic characteristics.
[0019]
(7) Preferably, in the above (1) and (4), a portion of the magnetic pole and the annular projection facing the columnar yoke and a side opposite to a side of the magnetic pole and the annular projection facing the columnar yoke. Are different in magnetic properties from each other.
[0020]
(8) Preferably, in the above (1) and (4), a plurality of materials having different saturation magnetizations are used for the material of the magnetic pole and the annular protrusion, and the material of the pole and the columnar yoke of the annular protrusion is used. The magnetic field is characterized by using materials whose magnetization directions are anisotropic with respect to each other. Generator.
[0021]
(9) Preferably, in the above (3) and (6), a plurality of materials having different saturation magnetizations are used for the material of the annular projection, and the annular projection formed on the one magnetic pole and the other are formed. And the annular protrusions formed on the magnetic poles have different magnetic properties.
[0022]
A portion of the magnetic pole and the annular projection facing the columnar yoke and a portion of the magnetic pole and the annular projection opposite to the side facing the columnar yoke are configured to have different magnetic properties from each other, The non-uniformity of the magnetic field due to the presence of the columnar yoke is compensated.
[0023]
By using a plurality of materials having different saturation magnetizations or using materials having magnetization directions that are anisotropic with each other, the portion of the magnetic pole and the annular projection facing the columnar yoke and the magnetic pole and the annular projection And a portion located on the side opposite to the side facing the columnar yoke can be configured to have different magnetic properties from each other.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic sectional view of an open magnet 1 in a magnetic resonance imaging apparatus according to a first embodiment of the present invention. The X axis indicated by the dashed line in FIG. 1 is the horizontal direction, and the Z axis is the vertical direction.
[0025]
Although not shown, in addition to the static magnetic field generating device, the magnetic resonance imaging device includes a gradient magnetic field generating device, an irradiation coil for generating a high-frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and a subject. And a control unit for controlling the operation of the magnetic field generator and the like, an image creating unit for creating a tomographic image, and an image display unit.
[0026]
In FIG. 1, an open type magnet 1 includes static magnetic field sources 3a and 3b, and the static magnetic field sources 3a and 3b are formed of a pair of opposed magnetic poles 4a and 4b on which annular ferromagnetic bodies 5a and 5b are formed. Having. Then, the magnetic poles 4a and 4b generate a static magnetic field in the volume space 7 on the side facing each other.
[0027]
Plate yokes 2a, 2b are magnetically coupled to the opposite sides of the opposing surfaces of the magnetic poles 4a, 4b via static magnetic field sources 3a, 3b. Further, the plate-shaped yokes 2a and 2b are magnetically connected to each other by the column-shaped yoke 6, and the open magnet 1 forms a magnetic circuit.
[0028]
FIG. 2 is a diagram showing the relationship between the pole 4a and the annular ferromagnetic material 5a and the columnar yoke 6, as viewed from above the magnet 1 shown in FIG. The Y axis is an axis in a direction orthogonal to the X axis and the Z axis.
[0029]
As shown in FIGS. 1 and 2, when the columnar yoke 6 is arranged to be deviated with respect to the volume space 7, that is, the column-shaped yoke 6 is not arranged uniformly or point-symmetrically around the volume space 7, When the magnetic poles 4a and 4b are arranged on one side, magnetic fluxes generated in the magnetic poles 4a and 4b are attracted to the columnar yoke 6.
[0030]
Therefore, if no countermeasure is taken, the magnetic flux density of the volume space 7 in the side where the columnar yoke 6 is arranged and the other part will be different, and the uniformity of the magnetic field will be deteriorated. Therefore, the first embodiment of the present invention is configured as described below to compensate for the uniformity of the magnetic field.
[0031]
In FIG. 2, of the magnetic poles 4 a, a magnetic pole 4 a-1 (area other than the hatched area) located on the opening side (opposite to the side where the yoke 6 is disposed) and a magnetic pole 4 a on the columnar yoke 6 side For -2 (regions indicated by oblique lines), ferromagnetic materials having different magnetic properties are used.
[0032]
Similarly, of the annular projections 5a, the annular projection 5a-1 (the area other than the hatched area) located on the opening side and the annular projection 5a-2 (the area shown with the hatched area) located on the columnar yoke 6 side Use ferromagnetic materials with different magnetic properties.
[0033]
By configuring the magnetic characteristics of the magnetic pole 4a-1 and the annular protrusion 5a-1 different from the magnetic characteristics of the magnetic pole 4a-2 and the annular protrusion 5a-2, the magnetic field non-uniformity due to the influence of the yoke 6 is corrected. I do.
[0034]
In the region of the magnetic pole 4a-2 and the annular protrusion 5a-2, the divergence angle from the center of the magnetic pole 4a or the center of the annular ferromagnetic body 5a toward the columnar yoke 6 is 180 ° or less. A region where the extension of the boundary line between the magnetic pole 4 and the annular projection 5a-1 and the magnetic pole 4a-2 and the annular projection 5a-2 is equal to or larger than the width of the yoke 6 is preferable. The width of the yoke 6 is the length in the Y-axis direction in FIG.
[0035]
FIGS. 3 and 4 are diagrams showing a case where the magnetic pole 4a-1 and the annular protrusion 5a-1 and the magnetic pole 4a-2 and the annular protrusion 5a-2 use ferromagnetic materials having different saturation magnetization characteristics. FIG. 3 is a diagram illustrating an upper half portion of the magnet 1.
[0036]
FIG. 5 is a diagram illustrating magnetic characteristics (MH curves) of the ferromagnetic material, and is a diagram illustrating examples of different magnetization characteristics Ms1 and Ms2 (Ms1> Ms2).
[0037]
A material (Ms1) having a large saturation magnetization is used for the ferromagnetic material of the magnetic pole 4a-2 and the annular protrusion 5a-2 located on the columnar yoke 6 side.
[0038]
On the other hand, a material (Ms2) having a small saturation magnetization is used for the magnetic pole 4a-1 and the annular protrusion 5a-1 on the opening side.
[0039]
As the materials having different saturation magnetizations, for example, pure iron and carbon steel can be used. As described above, with the configuration using the materials having different saturation magnetizations, the magnetic field intensity on the columnar yoke 6 side of the volume space 7 increases, and the magnetic field inhomogeneity can be compensated.
[0040]
In the example described above, the magnetic pole 4a-1 and the annular projection 5a-1 and the magnetic pole 4a-2 and the annular projection 5a-2 use ferromagnetic materials having different saturation magnetization characteristics. The same effect can be obtained by using a ferromagnetic material in which the direction of the easy axis of magnetization is different (the magnetization directions are mutually anisotropic) for the annular projection 5a-1 and the magnetic pole 4a-2 and the annular projection 5a-2. Obtainable.
[0041]
When the columnar yoke 6 is present, the magnetic flux in the volume space 7 is attracted by the columnar yoke 6, so that the columnar yoke 6 of the volume space 7 is horizontal (X-axis, Y (Axial direction) component is large.
[0042]
Therefore, as the ferromagnetic material used for the magnetic pole 4a-2 and the annular protrusion 5a-2 located on the columnar yoke 6, a ferromagnetic material having an easy axis of magnetization in the Z-axis direction, for example, a silicon steel plate is used.
[0043]
With this configuration, on the column yoke 6 side of the volume space 7, the amount of magnetic flux heading in the Z-axis direction increases, and the non-uniformity of the magnetic field can be compensated.
[0044]
In the illustrated example, only the upper part of the magnet 1 is shown, but the lower part is configured similarly to the upper part.
[0045]
As described above, according to the first embodiment of the present invention, the magnetic pole 4a-1 and the annular protrusion 5a-1 on the opening side and the magnetic pole 4a-2 and the annular protrusion 5a-2 of the columnar yoke 6 are Since the ferromagnetic materials having different saturation magnetization characteristics or different directions of easy magnetization are used, (the saturation magnetization characteristics are larger on the yoke side than on the opening side, and the X-axis or Y-axis on the opening side in the magnetization easy axis direction. The static magnetic field for a magnetic resonance imaging apparatus capable of improving the openness of the magnet structure by improving the correction of the non-uniformity of the static magnetic field generated in the imaging space in the axial direction and the yoke direction on the yoke side). A generator and a magnetic resonance imaging apparatus using the same can be realized.
[0046]
FIG. 6 is a diagram of the open-type magnet 1 in the magnetic resonance imaging apparatus according to the second embodiment of the present invention, as viewed from above.
[0047]
The second embodiment is an example in which two columnar yokes 16, 16 'arranged to face each other are arranged.
[0048]
Among the magnetic poles 14a, materials having different magnetic properties are used for the magnetic pole 14a-1 and the annular projection 15a-1 located on the opening side and the magnetic pole 14a-2 and the annular projection 15a-2 located on the columnar yoke 16, 16 'side. use. 12a is a plate-like yoke.
[0049]
In the region of the magnetic pole 14a-2 and the annular protrusion 15a-2, the divergence angle from the center point of the magnetic pole 14a or the center point of the annular ferromagnetic body 15a toward the columnar yoke 16, 16 ′ is 180 ° or less, A region where the extension of the boundary between the magnetic pole 14a-1 and the annular protrusion 15a-1 and the boundary between the magnetic pole 14a-2 and the annular protrusion 15a-2 is equal to or larger than the width of the yoke 16, 16 'is preferable.
In the second embodiment of the present invention, the same effect as in the first embodiment can be obtained.
[0050]
FIG. 7 is a schematic sectional view of the open magnet 1 in the magnetic resonance imaging apparatus according to the third embodiment of the present invention.
[0051]
In the third embodiment, in the open type magnet 1 including the columnar yoke 6 for magnetically connecting the plate-like yoke 2a and 2b, the magnetic property of the surface of the columnar yoke 6 facing the volume space 7 is determined. This is an example of changing.
[0052]
In FIG. 7, a plate-shaped ferromagnetic material 9 such as a silicon steel plate having an easy axis of magnetization parallel to the surface direction is arranged on the surface of the columnar yoke 6 on the volume space 7 side.
[0053]
By disposing the ferromagnetic material 9 having an easy axis of magnetization parallel to the plane direction of the columnar yoke 6 on the volume space 7 side, the magnetic flux from the volume space 7 toward the columnar yoke 6, that is, the magnetic flux of the horizontal component is reduced. It is possible to reduce and improve the inhomogeneity of the magnetic field in the volume space 7.
[0054]
The above-described example is an example in which the present invention is applied to the open magnet 1 having the columnar yoke 6 for magnetically connecting the plate-like yoke 2a and 2b, but does not use the columnar yoke. The example is also applicable.
[0055]
In other words, when the magnets facing each other are used without using the columnar yoke, the facing areas may be different from each other. In this case as well, the annular ferromagnetic material is used for the upper and lower (or left and right) magnetic poles, The diameters of the ferromagnetic materials will be different from each other.
[0056]
In this case, in the volume space sandwiched by the upper and lower magnetic poles, the strength of the magnetic field differs between the vicinity of the center and the vicinity of the periphery, and the magnetic field becomes non-uniform. This is because the magnetic flux around the magnetic pole having a large area does not go to the small magnetic pole but goes to the outside.
[0057]
Therefore, the saturation magnetization of the ring-shaped ferromagnetic material having a small-area magnetic pole can be made larger than the saturation magnetization of the ring-shaped ferromagnetic material having a large-area magnetic pole to compensate for the non-uniformity of the magnetic field.
[0058]
Further, in the above-described example, the non-uniformity of the magnetic field is compensated by adjusting the saturation magnetization characteristics of the annular protrusion. However, the non-uniformity can be compensated by supplying a current to the coil.
[0059]
That is, it is possible to arrange the coil on the columnar yoke side and generate a magnetic field in the Z-axis direction to compensate for the non-uniformity.
[0060]
However, when the diameter of the coil is increased, the mechanical force received from the relationship between the current and the magnetic field is increased, so that the supporting strength and the like become a problem.
[0061]
Therefore, the non-uniformity may be compensated to some extent by the annular projection by using the annular projection and a coil having a small diameter that does not cause a problem in supporting strength or the like, and the uniformity may be improved by fine adjustment.
[0062]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the static magnetic field generation apparatus of the magnetic resonance imaging apparatus which can improve the openness of a magnet structure by improving the correction of the nonuniformity of the static magnetic field generated in an imaging space, and using the same Magnetic resonance imaging apparatus can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view of an open magnet in a magnetic resonance imaging apparatus according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a relationship between a magnetic pole and an annular ferromagnetic material and a columnar yoke as viewed from above the magnet shown in FIG. 1;
FIG. 3 is a diagram illustrating a case where ferromagnetic materials having different saturation magnetization characteristics (magnetization facilitating axes) are used for a magnetic pole on the opening side and a magnetic pole on the yoke side;
FIG. 4 is a diagram showing a case where ferromagnetic materials having different saturation magnetization characteristics (magnetization facilitating axes) are used for the annular protrusion on the opening side and the annular protrusion on the yoke side.
FIG. 5 is a diagram showing magnetic characteristics (MH curve) of a ferromagnetic material.
FIG. 6 is a diagram illustrating a relationship between a pole and an annular ferromagnetic material and a columnar yoke as viewed from above an open magnet in a magnetic resonance imaging apparatus according to a second embodiment of the present invention.
FIG. 7 is a schematic sectional view of an open magnet in a magnetic resonance imaging apparatus according to a third embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Open type magnet 2a Upper plate-shaped yoke 2b Lower plate-shaped yoke 3a, 3b Static magnetic field generation source 4a Upper magnetic pole 4b Lower magnetic pole 5a Upper annular protrusion 5b Lower annular protrusion 4a-1, 14a-1 Columnar yoke side Magnetic poles 4a-2, 14a-2 Opening-side magnetic poles 5a-1, 15a-2 Columnar yoke-side annular protrusions 5a-2, 15a-2 Opening-side annular protrusions 6, 16, 16 'Columnar yoke 7 Volumetric space 9 strong Magnetic material

Claims

In a magnetic resonance imaging apparatus having a static magnetic field generating means, a gradient magnetic field generating means, an irradiation coil for generating a high frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image generating means for generating a tomographic image,
The static magnetic field generating means includes a pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on the surface of the pair of magnetic poles facing each other, and a column-shaped yoke supporting the pair of magnetic poles. The portion of the magnetic pole and the annular projection facing the columnar yoke and the portion of the magnetic pole and the annular projection facing the columnar yoke have different magnetic characteristics from each other. A magnetic resonance imaging apparatus characterized by the above-mentioned.

2. The magnetic resonance imaging apparatus according to claim 1, wherein a plurality of materials having different saturation magnetizations are used for the material of the magnetic pole and the annular projection, and a portion of the magnetic pole and the annular projection facing the columnar yoke is provided. A magnetic resonance imaging apparatus, wherein the magnetic pole and the annular projection have different magnetic properties from a portion located on a side opposite to a side facing the columnar yoke.

The magnetic resonance imaging apparatus according to claim 1, wherein a portion of the magnetic pole and the annular protrusion facing the columnar yoke is opposite to a portion of the magnetic pole and the annular projection facing the columnar yoke. A magnetic resonance imaging apparatus, wherein materials having mutually anisotropic magnetization directions are used.

In a magnetic resonance imaging apparatus having a static magnetic field generating means, a gradient magnetic field generating means, an irradiation coil for generating a high frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image generating means for generating a tomographic image,
The static magnetic field generating means supports a pair of magnetic poles facing each other via an imaging space, a protrusion made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and supports the pair of magnetic poles. Magnetic resonance imaging, comprising: a columnar yoke to be formed; and a ferromagnetic body disposed on the surface of the columnar yoke on the imaging space side and having an easy axis of magnetization parallel to the plane direction of the columnar yoke. apparatus.

In a magnetic resonance imaging apparatus having a static magnetic field generating means, a gradient magnetic field generating means, an irradiation coil for generating a high frequency magnetic field, a receiving coil for receiving a nuclear magnetic resonance signal, and an image generating means for generating a tomographic image,
The static magnetic field generating means includes a pair of magnetic poles facing each other, and a protrusion made of a ring-shaped ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and a ring formed on the one magnetic pole. A magnetic resonance imaging apparatus characterized in that the protrusion and the annular protrusion formed on the other magnetic pole have different magnetic properties from each other.

6. The magnetic resonance imaging apparatus according to claim 5, wherein a plurality of materials having different saturation magnetizations are used as the material of the annular projection, and the annular projection formed on the one magnetic pole and the annular projection formed on the other magnetic pole. A magnetic resonance imaging apparatus, wherein the projections and the projections have different magnetic properties.

A pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and a column-shaped yoke supporting the pair of magnetic poles; In a static magnetic field generating device of a magnetic resonance imaging device that generates a static magnetic field in a space on a surface side facing each other of the magnetic poles,
The portion of the magnetic pole and the annular projection facing the columnar yoke and the portion of the magnetic pole and the annular projection opposite to the side facing the columnar yoke have different magnetic characteristics from each other. Magnetic field generator for a magnetic resonance imaging apparatus.

8. The static magnetic field generating device according to claim 7, wherein a plurality of materials having different saturation magnetizations are used for the material of the magnetic pole and the annular projection, and a portion of the magnetic pole and the annular projection facing the columnar yoke is provided. A static magnetic field generator, wherein the magnetic pole and the annular projection have different magnetic properties from a portion located on a side opposite to a side facing the columnar yoke.

8. The static magnetic field generating device according to claim 7, wherein a portion of the magnetic pole and the annular protrusion facing the columnar yoke is opposite to a portion of the magnetic pole and the annular protrusion facing the columnar yoke. A static magnetic field generator using materials having mutually anisotropic magnetization directions.

A pair of magnetic poles facing each other, a projection made of an annular ferromagnetic material formed on a surface of the pair of magnetic poles facing each other, and a column-shaped yoke supporting the pair of magnetic poles; In a static magnetic field generating device of a magnetic resonance imaging device that generates a static magnetic field in a space on a surface side facing each other of the magnetic poles,
A static magnetic field generation device for a magnetic resonance imaging apparatus, comprising: a ferromagnetic material disposed on a surface of the columnar yoke on the space side and having an easy axis of magnetization parallel to a surface direction of the columnar yoke.

A pair of magnetic poles facing each other, and a projection made of an annular ferromagnetic material formed on the surface of the pair of magnetic poles facing each other, and a static magnetic field is applied to a space on the surface of the pair of magnetic poles facing each other. In the generated static magnetic field generator of the magnetic resonance imaging apparatus,
An annular projection formed on one of the magnetic poles and an annular projection formed on the other magnetic pole have different magnetic characteristics from each other.

12. The static magnetic field generator according to claim 11, wherein a plurality of materials having different saturation magnetizations are used for the material of the annular protrusion, and the annular protrusion formed on the one magnetic pole and the annular protrusion formed on the other magnetic pole. A static magnetic field generator, wherein the protrusions have different magnetic properties from each other.