JP4129339B2 - MRI equipment - Google Patents

Description

の関係になる。
【００５２】
次いで、画像再構成のときに発生する画像歪みやケミカルシフトアーチファクトの量が決定されるＥＴＬ_ｅｆｆ、最小の繰返し時間ＴＲを規定するＥＴＬ_{ｔｏｔａｌ}について説明する。一例として、位相エンコード方向マトリクスサイズＰＥ＝１２８、ＥＴＬ_{ｃｏｎｔｒａｓｔ}＝４とすると、ＲＦパルスの印加数、すなわちショット数Ｓｈに応じて以下の３態様についてＥＴＬ_ｅｆｆ及びＥＴＬ_{ｔｏｔａｌ}を計算できる。
【００５３】
【数２】

である。
【００５４】
また、エコー間隔ＥＴＳを考慮するために、データ収集時間Ｔ_０の観点からＥＰＩ法の撮影を以下のように分類する。
【００５５】
【数３】

データ収集時間Ｔ_０をこのように規定すると、前述のショット数Ｓｈ＝小（図２（ａ））のときのＥＴＬ_ｅｆｆ＝３２の条件で、上述の（Ｃ）のデータ収集時間Ｔ_０を満足させるＥＴＳは存在しない。このことは、シングルショットＥＰＩ法では脂肪抑制が必須であることの根拠でもある。ＥＴＬ_ｅｆｆ＝３２の条件で、上述の（Ｂ）のデータ収集時間Ｔ_０を満足させるだけでも、ＥＴＳ＜０．８ｍｓになる必要がある。さらに、上述の（Ａ）のデータ収集時間Ｔ_０を満足させるには、ＥＴＳ＜２．４ｍｓが限界である。
【００５６】
これに対して、ショット数Ｓｈ＝大（図２（ｃ））のときのＥＴＬ_ｅｆｆ＝４の条件のときには、上述の（Ｂ）のデータ収集時間Ｔ_０はＥＴＳ＝４．８ｍｓでも満足させ得る。（Ｃ）のデータ収集時間Ｔ_０はＥＴＳ＝２．４ｍｓで十分である。
【００５７】
つまり、位相エンコード方向マトリクスサイズＰＥが与えられたとき、ショット数を変更すれば、ＥＴＬ_ｅｆｆ（すなわち、画像歪みやケミカルシフトアーチファクトの量が決定される、１回のＲＦ励起当たりのエコー数）をコントロールすることができる。
【００５８】
ＥＴＬ_ｅｆｆを短縮させることは、画像歪やケミカルシフトアーチファクトを低減させる上では非常に有効であるものの、ＥＰＩとしての性能、すなわち高速撮影の利点は減弱される。反対に、ＥＴＬ_ｅｆｆを延長させると、ＲＦパルス、位相エンコード量の巻き（ｗｉｎｄ）／巻き戻し（ｒｅｗｉｎｄ）に要する時間、エコー時間シフトなどのオーバーヘッド分をカバーしてデータ収集効率を向上させることができる。したがって、画像歪やケミカルシフトアーチファクトの抑制とデータ収集効率とのバランスを考慮したＥＴＬ_ｅｆｆを設定したいものである。
【００５９】
通常、Ｔ１コントラストを得るには、繰返し時間ＴＲ＜５００ｍｓ程度が望まれる。ＴＲ＞１００ｍｓ以上でフリップ角を７０°程度まで上げてＳＮＲを稼ぐことができる。さらに、ＲＦパルスの空射ちや位相補正データ収集スキャンなど、本撮影用のパルスシーケンスの外側でオーバーヘッドが発生するが、このオーバーヘッド分は、ショット数を増やし且つＴＲを短縮させることで、相対的に減らすことができる。
【００６０】
すなわち、データ収集時間Ｔ_０は、
【数４】

で表される。
【００６１】
以上、画質を左右する各種の撮影パラメータについて検討したが、パラメータの重み付けが評価関数により夫々異なるため、従来のように、操作卓上の１次元のスライドバーを用いた撮影条件の設定法の場合、最適な撮影条件を設定するには、熟練操作者でも相当に難しい。
【００６２】
この困難さを著しく軽減するインタフェースが、本ＭＲＩ装置の一実施形態によって以下のように提供される。
【００６３】
このインタフェースは、オペレータが撮影計画を立てる段階でホスト計算機６、表示器１２、及び入力器１３が協働して提供する視覚的且つ対話的なグラフィックインタフェースである。
【００６４】
ところで、本ＭＲＩ装置で扱う撮影計画時のハード的、ソフト的な各種のパラメータは３００個以上にもなるが、実際には装置のホスト計算機６の処理（図示せず）により、５０個程度のパラメータが、ユーザが変更できるパラメータとして、ユーザに開放されている。撮影計画の処理を起動させると、ユーザには見えない装置側パラメータ２５０個程度が所定の手順によりホスト計算機６で自動的に設定される。このため、撮影計画の処理を起動したときにはそのまま本スキャン実行可能な状態になって、ユーザ開放のパラメータを設定するようオペレータに催促がある。オペレータが設定したユーザ開放のパラメータの値に応じて、ホスト計算機５は装置側に任されている２５０個程度のパラメータ値の修正を許容範囲において即座に行う。この修正結果は、必要に応じて、ユーザ開放のパラメータの訂正を迫ることにも反映される。つまり、装置側との対話的な撮影計画の処理が進められる。このため、オペレータがユーザ開放のパラメータをどのような値に設定した場合でも、本スキャンの実行可能な状態は常に維持されている。つまり、パラメータの設定如何によっては、実際に撮影を行ったときに初めて本スキャンが開始されないということが判明するという事態が回避される。
【００６５】
本実施形態の視覚的且つ対話的なグラフィックインタフェースは上述したユーザ開放のパラメータを設定するために提供されるようになっている。ユーザ開放型のパラメータには、撮影対象の種類、シーケンスの種類、繰返し時間ＴＲ、エコー時間ＴＥ、ＥＴＳ、ＥＴＬ、ショット数、フリップ角、位相エンコード方向マトリクスサイズ、セグメント数、スキャン時間、ＦＯＶ、ゲートの有無、ゲートの種類、脂肪抑制の有無など、主要なパラメータが含まれる。
【００６６】
このグラフィックインタフェースの画像として、表示器１２には、図３に示す２次元のグラフが表示される。同図に示すように、変数として横軸にショット数Ｓｈ、縦軸に位相エンコード方向マトリクスサイズＰＥを夫々とった２値とし、評価関数として、ＥＰＩ法の場合には上述した（Ａ）〜（Ｃ）項に示したデータ収集時間Ｔ_０をとる。
【００６７】
２つの変数Ｓｈ，ＰＥ間の関係は、
【数５】
ＰＥ＝ＥＴＬ × Ｓｈ
の直線式で表され（図３の直線Ｓ参照）、この直線Ｓの傾きはＥＴＬである。この直線Ｓが入ることが禁止される禁止領域は、図３において、
【数６】
ＰＥ＞Ｔ_０／ＥＴＳ×Ｓｈ
の領域Ｒで表される（図３の斜線領域参照）。前述したように、Ｔ_０はデータ収集時間であり、ＥＰＩ法の場合には一例として、Ｔ_０＝１００、３０、１０ｍｓの値を選択的にとる（前記条件項（Ａ）〜（Ｃ）参照）。
【００６８】
つまり、前記条件（Ａ）に対応したデータ収集時間Ｔ_０＝１００ｍｓで決まる直線Ｔ０（Ａ）が仕切る領域：ＰＥ＞Ｔ_０／ＥＴＳ×Ｓｈが禁止領域Ｒを形成する。この領域はグラフ上では例えば赤色で描出される。これに対し、前記条件（Ｂ），（Ｃ）に対応したデータ収集時間Ｔ_０＝３０ｍｓ、１０ｍｓで決まる直線Ｔ０（Ｂ）、Ｔ０（Ｃ）が仕切る領域は例えば黄色、青色で夫々描出される。この黄色及び青色の領域は、本実施形態では、ショット数設定の推奨領域として設定される。
【００６９】
この２変数のグラフィックインタフェースにおいて、横軸Ｘ及び縦軸Ｙ上に表した矩形バーＢ０〜Ｂ２は、装置側で自動的に演算してショット数Ｓｈ及びマトリクスサイズＰＥのユーザが設定し得る範囲を示す。ショット数Ｓｈについては、この矩形バーＢ０、Ｂ１により規定される横軸上の範囲Ｒ１が現在とり得る値の範囲である。マトリクスサイズＰＥについては、矩形バーＢ０、Ｂ２により規定される横軸上の範囲Ｒ２が現在とり得る値の範囲である。矩形バーＢ０〜Ｂ２の位置、すなわち変数の取り得る値の範囲Ｒ１，Ｒ２は、オペレータが操作するユーザ開放のパラメータの値に応じて、装置側の演算により、リアルタイムに変わる。
【００７０】
オペレータは、このグラフを見ながら入力器１３を操作してショット数ＳｈのハンドルＸ１、マトリクスサイズＰＥのハンドルＸ２、直線Ｓ：ＰＥ＝ＥＴＬ×ＳｈのハンドルＹ１を操作しながら最適な撮影条件を決めることになる。このハンドル操作の例として以下のような態様が採り得る。
【００７１】
例えば、マトリクスサイズＰＥのハンドルＸ２が固定されている場合、ショット数ＳｈのハンドルＸ１を移動させると（図３中の矢印ａ参照）、直線Ｓ：ＰＥ＝ＥＴＬ × Ｓｈの傾きが変化する（図３中の矢印ａ'参照）。このハンドルＸ１の移動に伴い、データ収集時間Ｔ_０に前記条件（Ａ）の数値を代入して決まる禁止領域Ｒ：ＰＥ＞Ｔ_０／ＥＴＳ×Ｓｈが連動してリアルタイムに変化する（矢印ａ"参照）。このとき、直線Ｓは禁止領域Ｒに入らないようにその傾きが制御される。ハンドルＸ１の移動に伴って、データ収集時間Ｔ_０に前記条件（Ｂ）、（Ｃ）の数値を夫々代入して決まる各領域も連動してリアルタイムに変化する（ａ"参照）。
【００７２】
さらに、別の態様として、脂肪抑制パルス印加などの他の撮影条件の参酌、変更に伴って禁止領域Ｒの範囲を変更するようにしてもよい。
【００７３】
本実施形態のＭＲＩ装置においては、パルスシーケンスが以上の原理にしたがってスキャン計画の一環として設定される。図４にその設定手順の概要を示し、図５にそのように手順を経て設定されたパルスシーケンスの例を示す。
【００７４】
オペレータはスキャン計画を立てる処理の中で、ホスト計算機６のソフトウエア処理によって、表示器１２及び入力器１３の協働を得て実現される対話式のユーザーインタフェースが起動する。
【００７５】
このインタフェース起動によって、最初に、パルスシーケンスの種類及びエコー間隔ＥＴＳが選択入力される（ステップＳ１）。いま、パルスシーケンスとしてマルチショットのＦＥタイプＥＰＩ法が選択されたとする。このＥＴＳ値に応じてエコー時間ＴＥが決定される（ステップＳ２）。この決定により、ｋ空間の位相エンコード方向中心のデータ収集を行うエコーがＥＴＬ_{ｃｏｎｔｒａｓｔ}により内部的に設定される（ステップＳ３）。
【００７６】
次いで、オペレータは撮影目的に応じて、選択したＥＰＩ法に拠るパルスシーケンスに対するデータ収集時間Ｔ_０を評価関数として入力する（ステップＳ４）。これは例えば、前述した条件（Ａ）〜（Ｃ）の如く分類された時間値を入力して行う。
【００７７】
次いで、ホスト計算機６（ユーザーインタフェースとしても機能する）は、グラフィック表示のためのデータをそれまでに受け付けたデータ及び演算したデータから作成し、これを表示器１２上に表示してオペレータ（ユーザ）に提示する（ステップＳ５）。この提示されたグラフィックインタフェースを参照しつつ、オペレータは入力器１３を介してグラフィックインタフェース画面のハンドルを適宜に動かす。これにより、位相エンコード方向マトリクスサイズＰＥを大きくした場合の画質（画像歪やケミカルシフトアーチファクトの抑制）を維持又は所望の状態にするためのショット数などの撮影パラメータを直感的（視覚的）に捕らえつつ、設定できる。
【００７８】
このような対話入力方式のグラフィックユーザーインタフェースを通して確定した撮影パラメータは本スキャンの制御データとして格納される（ステップＳ７）。
【００７９】
この結果、本スキャンは例えば図５に示す如く、マルチショット法に従うＦＥタイプのＥＰＩ法に基づくパルスシーケンスが実行される。図５において、位相エンコード用傾斜磁場Ｇ_Ｓは各エコー信号に対して位相エンコードが施されることのみを示す。この位相エンコードの印加の態様は様々である。
【００８０】
この本スキャンが例えばショット数＝小（図２（ａ）の状態参照）で実行された場合、演算ユニット１０により、ハーフフーリエ法により画像再構成が行われる。反対にショット数＝大（図２（ｃ）の状態参照）で実行された場合、エコートレインの内の初めの方と終わりの方のエコー全部又は一部は演算ユニット１０により破棄され、ｋ空間には配置されずに再構成処理に付される。
【００８１】
Ｔ１強調イメージングにおけるマルチショットのＦＥタイプＥＰＩ法を用いる場合、制御しなければならないパラメータ数が増え、パルスシーケンスのエコートレイン長（ＥＴＬ）やショット数など、画質に影響するパラメータの値を決める上での制約もその分、多くなるが、本実施形態のＭＲＩ装置によれば、撮影目的に応じた最適な撮影条件を満たすパルスシーケンスを迅速に、操作労力少なく、かつ精度良く設定して撮影を行い、高画質のＭＲ画像を提供できる。
【００８２】
とくに、対話入力方式のグラフィックユーザーインタフェースを用いているので、従来の１次元スライドバー方式のインタフェースに比べて、かかるパルスシーケンスを迅速に、操作労力少なく、かつ精度良く設定することができる。加えて、実際の撮影においてパルスシーケンスは常に確実に実行され、高画質の画像を提供することができる。
【００８３】
このため、再撮影が少なくなり、操作上の労力が減って、かつ患者スループットも上がる。これは、患者にとっても体力的、精神的な負担が著しく軽減される。
【００８４】
なお、上述したＥＰＩ法に基づくパルスシーケンスは脂肪抑制パルスを印加しないものとして説明したが、これは、図６に示すように、各ショット前に、脂肪抑制パルスＰ_ｆａｔを印加するように設定してもよい。この脂肪抑制パルスとしては、バイノミアルパルス、シンク関数、またはガウシャン関数などをスライス選択的又はスライス非選択的に用いればよい。
【００８５】
また、上述した第１の実施形態のグラフィックユーザーインタフェースにおいて、禁止領域及びそれ以外の領域の表示法は必ずしもカラーを着色したものに限定されるものではなく、メッシュを掛けるなど、その領域を視覚的に区分けできればよい。
【００８６】
また、禁止領域以外の領域上に、推奨領域を重ねて表示してもよい。また、推奨条件を別途、数値などにより表示してもよい。
【００８７】
さらに、このグラフィックユーザーインタフェースにおいて、前述した直線Ｓ：ＰＥ＝ＥＴＬ×Ｓｈの代わりに、オペレータが任意の傾きの直線を入力し、撮影条件をその直線上でのみ選択可能に構成してもよい。
【００８８】
さらに、第１の実施形態のグラフィックユーザインタフェースでは評価関数としてＥＰＩ法のときのデータ収集時間Ｔ_０を説明したが、パルスシーケンスとしてＦＡＳＥ法を用いる場合には、一例として、
【数７】

に設定して用いればよい。
【００８９】
（第２の実施形態）
本発明の第２の実施形態を図７〜図８に基づき説明する。なお、この実施形態のＭＲＩ装置のハード的構成は第１の実施形態のものと同一又は同等である。
【００９０】
本実施形態では、本発明に係るグラフィックユーザインタフェースを用いたスキャン計画及び本スキャンの実行を、心臓などの同期撮影法で撮影する例に適用している。図７に、マルチショットセグメントＥＰＩ法（ＦＥタイプ）のシーケンス構造の模式図を示す。
【００９１】
この同期撮影法を実施する場合、上述した第１の実施形態に加え、ＥＣＧ信号のＲ−Ｒ間隔などの制約が更に加わり、撮影条件（パラメータ）の設定は更に複雑になる。すなわち、ＥＰＩ法を用いる場合には、前述したデータ収集時間Ｔ_０の観点からの制約条件（Ａ）〜（Ｃ）に、例えば、以下の条件のいずれかが加わる。
【００９２】
（Ｄ）１心拍撮影：
不整脈、パフュージョンなどの経時変化を観察する撮影法で、心収縮器は除外して２００〜３００ｍｓの時間幅で収集する。セグメント数Ｓｅ＝１であるから、画像歪及びケミカルシフトの発生量の範囲内で、できるだけ繰返し時間ＴＲを長くしてデータ収集の効率を稼ぐ方法となる。
【００９３】
（Ｅ）動態観察（Ｉ）：
患者にとって負担の少ない５〜８秒程度の息止めをさせると同時に１００ｍｓ程度の心時相の分解能で撮影する。
【００９４】
（Ｆ）動態観察（ＩＩ）：
心臓弁などの動きを精密に観察するため、３０秒程度の息止めで、又は、５〜８秒程度の息止めを間欠的に行う撮影であり、データ収集は１０〜３０ｍｓ程度に止めて撮影する。この場合、ショット数Ｓｈ＝１になることもある。
【００９５】
心臓の撮影では、ＥＣＧ信号のＲ−Ｒ間隔に同期して撮影繰り返される。したがって、撮影条件の設定は更に複雑化するので、これを解消するため、対話入力方式のグラフィックユーザインタフェースが提供されている。
【００９６】
図８に、このインタフェースのグラフ表示の状態を示す。変数として、ショット数Ｓｈ、位相エンコード数ＰＥ（＝位相エンコード方向マトリクスサイズ）、セグメント数Ｓｅの３値をとる。縦軸に位相エンコード数ＰＥを、第１象限側の横軸にショット数Ｓｈを、第２象限側の横軸にセグメント数Ｓｅをそれぞれとった２次元グラフを形成している。
【００９７】
この３変数のグラフィックインタフェースにおいて、横軸Ｘ及び縦軸Ｙ上に表した矩形バーＢ０〜Ｂ３は、装置側で自動的に演算してショット数Ｓｈ、マトリクスサイズＰＥ、セグメント数Ｓｅのユーザが設定し得る範囲を示す。ショット数Ｓｈについては、この矩形バーＢ０、Ｂ１により規定される横軸上の範囲が現在とり得る値の範囲である。マトリクスサイズＰＥについては、矩形バーＢ０、Ｂ２により規定される横軸上の範囲が現在とり得る値の範囲である。セグメント数Ｓｅについては、矩形バーＢ０、Ｂ３により規定される横軸上の範囲が現在とり得る値の範囲である。矩形バーＢ０〜Ｂ３の位置、すなわち変数の取り得る値の範囲は、オペレータが操作するユーザ開放のパラメータの値に応じて、装置側の演算により、リアルタイムに変わる。
【００９８】
各変数間の関係は
【数８】

であり、直線Ｓ１の傾きは「ＥＴＬ × Ｓｅ」、直線Ｓ２の傾きは「ＥＴＬ × Ｓｈ」である。直線Ｓ１、Ｓ２の禁止領域Ｒ１、Ｒ２は夫々、
【数９】

である。
【００９９】
また、このインタフェースのグラフには、第１の実施形態のときと同様に、前記条件（Ａ）〜（Ｃ）で表わされるデータ収集時間Ｔ_０が評価関数として用いられ、直線Ｔ０（Ａ）、Ｔ０（Ｂ）及びＴ０（Ｃ）：ＰＥ＞Ｔ_０／ＥＴＳ×Ｓｅ、又は、ＰＥ＞Ｔ_０／ＥＴＳ×Ｓｈで区分けされる領域が象限毎に、一例として色分け表示されている。この色分け領域は変数の変動と伴に画像上でダイナミックに変動する。
【０１００】
このグラフィックユーザーインタフェースの画面を見ながら、オペレータは例えば以下のようにハンドルを操作する。
【０１０１】
例えば、位相エンコード数ＰＥのハンドルＸ２が固定されている場合、ショット数ＳｈのハンドルＸ１を移動させると、直線Ｓ１：ＰＥ＝ＥＴＬ×Ｓｅ×Ｓｈの傾きが変化する。直線Ｓ１傾きはセグメント数Ｓｅの関数となっているので、セグメント数ＳｅのハンドＸ３もリアルタイムに連動する。このハンドルＸ１、Ｘ３の移動に伴い、禁止領域Ｒ１、Ｒ２もリアルタイムに連動して変化する。このとき、直線Ｓ１、Ｓ２は禁止領域Ｒ１、Ｒ２に入らないようにその傾きが制御される。反対に、セグメント数Ｓｅ側のハンドルＸ３を移動させる場合も同様である。
【０１０２】
このインタフェースを対話形式で用いることで、位相エンコード方向マトリクスサイズＰＥを大きくした場合の画質を維持するためのショット数、セグメント数などを撮影条件を視覚的（直感的）に把握しながら設定することができる。したがって、ＥＣＧゲートに拠る心臓撮影であっても、第１の実施形態のときと同等の効果を得ることができる。
【０１０３】
なお、上記第１、第２の実施形態の撮影は２次元撮影法であるとして説明してきたが、全く同様のインタフェース入力を３次元撮影法についても適用できる。
【０１０４】
また、パルスシーケンスはＥＰＩ法に限られず、ＦＳＥ法（ＲＡＲＥ法）であってもよく、このＦＳＥ法の場合には、Ｔ２^＊に因る画像歪の代わりに、Ｔ２緩和に因る画像ボケを考慮することで同様のインタフェースを適用できる。
【０１０５】
さらに、本発明に係るグラフィックユーザインタフェースにおいてユーザがグラフ画面を見ながら調整する変数は前述した２変数または３変数のみに限定されるものではなく、画質に影響する撮影パラメータとしてユーザに開放されたパラメータであれば採用することができる。例えば、図８のグラフ画面において、縦軸を成す位相エンコード数ＰＥの代わりに、別の変数としてＦＯＶを設定してもよい。一方、変数の数自体も４変数以上であってもよい。例えば、図８のグラフ画面において、横軸に図示の如くショット数Ｓｈ及びセグメント数Ｓｅを採り、縦軸の上下に位相エンコード数ＰＥ又はＦＯＶ及び息止めスキャン時間を採った４象限としてもよい。
【０１０６】
したがって、本発明を適用したグラフィックユーザインタフェースの特徴を総括すると以下のようになる。第１に、複数の撮影パラメータを同時に設定することができる。第２に、その複数の撮影パラメータの採り得る値を同時に視覚的に表現して、オペレータに設定範囲の直感的イメージを与えることができ、撮影パラメータの設定を容易にする。第３に、複数のパラメータの組み合わせを適宜変更できる。第４に、撮影パラメータのハンドル位置を変えると禁止領域もダイナミックに連動して、オペレータに常に的確な設定範囲の直感的イメージを与える。
【０１０７】
また、前記ユーザーインターフェースは、操作者が任意の傾きの直線を前記グラフ上に入力する手段と、撮影パラメータをその直線上でのみ選択可能とする手段とを有していてもよい。
【０１０８】
なお、本発明は、代表的に例示した上述の実施の形態に限定されるものではなく、当業者であれば、特許請求の範囲の記載内容に基づき、その要旨を逸脱しない範囲内で種々の態様に変形、変更することができ、それらも本発明の権利範囲に属するものである。
【０１０９】
【発明の効果】
以上説明したように、本発明に係るＭＲＩ装置によれば、例えば、ＥＰＩ法のエコー時間ＴＥ及び繰返し時間ＴＲを制約してマルチショット化する場合、その撮影準備において使用するグラフィック化したユーザインタフェースによって、撮影対象毎に最適な撮影パラメータを視覚的に理解しながら容易に設定することができ、また位相エンコード方向のマトリクスサイズなどを変更しても画質が大きくは変化しない撮影パラメータを確実に設定でき、この撮影パラメータに基づいた安定したＭＲイメージングを実行することができる。したがって、患者やオペレータの負担の少ない高精度・高品質なＭＲ画像を確実に提供できる。
【図面の簡単な説明】
【図１】本発明の実施形態に係るＭＲＩ装置の概略構成を示すブロック図。
【図２】ショット数の大小、ＥＴＬ、及びデータ配置の関係を模式的に説明する図。
【図３】第１の実施形態におけるショット数の制約を説明する、グラフィックユーザインタフェースの画面図。
【図４】グラフィックユーザインタフェースに対するオペレータの操作の概略を示すフローチャート。
【図５】第１の実施形態で用いたマルチショットＥＰＩ法（ＦＥタイプ）のパルスシーケンスを例示するタイミングチャート。
【図６】変形例としての、脂肪抑制パルスを付加したマルチショットＥＰＩ法（ＦＥタイプ）のパルスシーケンスを例示するタイミングチャート。
【図７】本発明の第２の実施形態に係るマルチショットセグメントＥＰＩ法（ＦＥタイプ）のパルスシーケンスを例示するタイミングチャート。
【図８】第２の実施形態におけるショット数及びセグメント数の制約を説明する、グラフィックユーザインタフェースの画面図。
【符号の説明】
１ 磁石
２ 静磁場電源
３ 傾斜磁場コイルユニット
４ 傾斜磁場電源
５ シーケンサ
６ ホスト計算機（ユーザインタフェース）
７ ＲＦコイル
８Ｔ 送信器
８Ｒ 受信器
１０ 演算ユニット
１１ 記憶ユニット
１２ 表示器（ユーザインタフェース）
１３ 入力器（ユーザインタフェース）
１７ ＥＣＧセンサ
１８ ＥＣＧユニット[0001]
BACKGROUND OF THE INVENTION
The present invention obtains an image in a subject by utilizing a magnetic resonance phenomenon of a nuclear spin existing in the subject. MRI (magnetic resonance imaging) equipment Concerning. In particular, an EPI (Echo Planar Imaging) method, FSE (Fast Spin), which can generate data in a short time by generating a plurality of echo signals for one excitation pulse applied to generate a magnetic resonance phenomenon. Echo) method, or magnetic resonance imaging that performs pulse sequences based on them.
[0002]
[Prior art]
In magnetic resonance imaging, a nuclear spin of a subject placed in a static magnetic field is magnetically excited with a high-frequency signal of Larmor frequency, and the subject is detected from an FID (free induction decay) signal or echo signal generated by this excitation. It is a technique to obtain the image inside.
[0003]
This magnetic resonance imaging is particularly effective as a technique for non-invasively obtaining an anatomical tomographic image of a subject. For this reason, for example, it is also used for diagnosis of the central nervous system of the brain covered with bone.
[0004]
In such a field of magnetic resonance imaging, when performing T2 weighted imaging with high clinical effectiveness, it is necessary to use a pulse sequence in which the echo time TE and the repetition time TR are set long. For this reason, the entire imaging time is as long as about 10 minutes, and the burden on the patient is large.
[0005]
Therefore, in order to shorten the imaging time, an EPI method and an FSE method (also called RARE method) for collecting data a plurality of times for a single excitation pulse have been proposed and used.
[0006]
The EPI method is a method proposed by “Mansfield” and uses a process of continuously generating field echoes while switching the polarity of the gradient magnetic field in the readout direction (“Mansfield, P.“ NMR Imaging in Biomedicine ”, Advances in Magnetic Resonance, Academic Press, New York, 1982 "). By using this MRI method, single shot imaging can be performed. However, in the case of single shot photography, however, the influence of magnetic field inhomogeneity and image blurring due to T2 relaxation become obvious. In order to suppress this, in practice, imaging using a multi-shot type EPI method is frequently used.
[0007]
On the other hand, the FSE method is a method proposed by “Hennig” and adopts a process of generating a multi-echo signal by continuously applying a refocus pulse to a single excitation pulse (“Hennig, J , "RARE Imaging: Fast Imaging Method Clinical MR", Magn. Reson. Med 3, 823-33 (1986) "). The FSE method has an advantage that the imaging time is longer than that of the EPI method, but it is not affected by magnetic field inhomogeneity. Also in the case of this FSE method, the use of multiple shots increases the clinical effectiveness due to shortening of the effective echo time, reduction of image blur, etc., so that it is widely used.
[0008]
As described above, the imaging of the T2-weighted image (T2 contrast image) is accelerated by the EPI method or the FSE method, and the degree of acceleration increases as the number of echo trains corresponding to one excitation pulse application increases. The data collection time for one excitation in these high-speed imaging is about 100 to 150 ms in the case of the EPI method and about 200 to 400 ms in the case of the FSE method.
[0009]
On the other hand, regarding the imaging of the T1-weighted image compared with the T2-weighted image, when a multi-echo signal is obtained using the multi-shot type FSE method, the MT (magnetization transfer) contrast associated with the application of the RF refocusing pulse. Since this change occurs in the image, it is not suitable for use as a T1-weighted image. Further, the EPI method cannot be used due to image distortion, and it has been difficult to obtain the effect of shortening the photographing time by echo train.
[0010]
For this reason, with respect to the T1-weighted image, a technique such as using the SE method or the FE method with the repetition time TR shortened is used to shorten the photographing time. However, in this case as well, the problem that the image quality deteriorates due to factors such as the MT effect is unavoidable as the RF pulse application density increases for the purpose of speeding up, as in the FSE method. Further, even if it is attempted to shorten the repetition time TR with the conventional FE method, it is difficult to obtain a desired T1 contrast, and almost only a proton contrast can be obtained.
[0011]
To overcome this situation, an imaging method using a multi-shot FE type EPI method has been proposed (for example, “Slavin S. and Riederer SJ., Magn. Reson. Med. 38 368-377; 1997”, “Epstein FH., Et al., 6th Annual meeting ISMRM 801; 1998”, and “Slavin S. et al., 6th Annual meeting ISMRM 320; 1998”). The sequence based on this imaging method was developed mainly for the purpose of imaging the heart, and the feature is to shorten the ETL (Echo Train Length) of the sequence and reduce the influence of magnetic field inhomogeneity. It is. This makes it possible to take an oblique image of the heart or the like, which was difficult to take with the conventional single shot type EPI method.
[0012]
[Problems to be solved by the invention]
However, when the multi-shot FE type EPI method in the T1-weighted imaging described above is used, the number of parameters that need to be controlled increases, so parameters such as the echo train length (ETL) of the pulse sequence and the number of shots that affect image quality There are also many restrictions on determining the value.
[0013]
For example, if the pulse sequence is set so that the echo train length becomes long, the imaging itself can be speeded up. On the other hand, the data acquisition time becomes long as particularly noticeable in the EPI method. Therefore, image distortion due to the generation amount of chemical shift artifacts and magnetic field inhomogeneity increases. As described above, since the restrictions on the parameters affecting the image quality are many and complicated, a method for setting shooting conditions by operating a one-dimensional slide bar of each parameter installed on a conventional operation panel while calculating with a calculator. In this case, it is extremely difficult to set the shooting conditions optimally.
[0014]
In particular, since the weighting of each parameter varies depending on the evaluation function, it is inevitable that even when a skilled operator operates a one-dimensional slide bar, it is extremely difficult to set an optimum shooting condition. is there.
[0015]
If the shooting condition setting accuracy is low, the pulse sequence is not executed in actual shooting, or only an image with inferior image quality is obtained. For this reason, there are frequent problems such as being forced to re-photograph, so that there is a problem in that operational effort increases and patient throughput decreases. In addition, such a situation is a significant physical and mental burden for the patient.
[0016]
The present invention has been made in view of the above-described problems of the prior art, and a first object of the present invention is to provide a plurality of echo signals (echo trains) for each shot, such as a multi-shot type FSE method and an EPI method. When performing magnetic resonance imaging, it is possible to provide a high-quality MR image by quickly and accurately setting a pulse sequence that satisfies the optimal imaging conditions according to the imaging purpose, with little effort and manipulation. Is to do.
[0017]
The second object of the present invention is to meet the purpose of imaging when performing magnetic resonance imaging such as multi-shot type FSE method, EPI method, etc. to obtain a plurality of echo signals (echo train) for each shot. It is another object of the present invention to provide an interface capable of setting a pulse sequence that satisfies the optimum imaging conditions quickly, with little operation effort, and with high accuracy.
[0018]
[Means for Solving the Problems]
In the present invention, for example, the restriction of the echo train length is effective to improve the speed in the EPI method described above, that is, to improve the relationship between the extension of the data acquisition time and the increase in image distortion due to chemical shift artifacts and magnetic field nonuniformity That's what we focused on. Further, when fat suppression is added, since it is not necessary to consider the occurrence of chemical shift artifacts, the data collection time can be extended to the extent that image distortion due to the effect of magnetic field inhomogeneity does not occur. Further, if the echo train is lengthened, the data collection efficiency does not decrease even if the repetition time TR is extended, so that the SNR per unit time can be improved.
[0019]
In order to achieve the above object, the following configuration is adopted.
[0020]
According to the first aspect of the present invention, in the MRI apparatus for applying the pulse sequence to the subject placed in a uniform static magnetic field to collect echo signals and generating an image inside the subject, the pulse A pulse sequence for generating a plurality of echo signals per application of one RF excitation pulse is set as a sequence, and a data collection time for collecting the plurality of echo signals is set to a desired value, and this desired Pulse sequence setting means for determining the number of echoes of the plurality of echo signals and the number of shots of the RF excitation pulse from time, and executing the pulse sequence for collecting the echo signals by executing the pulse sequence set by the pulse sequence setting means Means.
[0021]
Preferably, the type of the pulse sequence set by the pulse sequence setting means is the polarity of a pulse train or readout direction gradient magnetic field pulse to which a plurality of refocus RF pulses are applied in order to generate the plurality of echo signals. Includes a pulse train that is inverted multiple times. For example, the pulse sequence is a pulse sequence for obtaining a T1-weighted image, and the repetition time TR is a value of 600 ms or less.
[0022]
Further, for example, the pulse sequence is a pulse sequence that includes a pulse train that inverts the polarity of the read direction gradient magnetic field pulse a plurality of times, and applies a fat suppression pulse before the application of the pulse train. Preferably, the data collection time for collecting the plurality of echo signals is set to a value of 30 ms or less.
[0023]
As an example, the data collection time for collecting the plurality of echo signals can be set to a value of 10 ms or less. The pulse sequence may be a pulse sequence that does not use fat suppression pulses.
[0024]
On the other hand, the pulse sequence setting means preferably includes an interactive user interface of a plane graph display for constraining the data collection time of the pulse sequence to an optimum value.
[0025]
This user interface is, for example, an interactive user interface for displaying a plane graph using two variables. These two variables are the number of shots and the number of phase encodings of the RF pulse. For example, the user interface is an interface that designates a prohibited area for a combination of an echo train length (ETL) and the number of shots on the two-dimensional graph and performs parameter selection within a range excluding the prohibited area. For example, the user interface is an interface that displays a recommended range for a combination of an echo train length (ETL) and the number of shots on the two-dimensional graph on the two-dimensional graph. As an example, this user interface is displayed by dividing at least one of the invalid range and the recommended range by a pattern or color.
[0026]
Further, the user interface may include means for allowing an operator to input a straight line having an arbitrary inclination on the graph, and means for allowing a photographing parameter to be selected only on the straight line.
[0027]
Further, the user interface may be an interactive user interface for displaying a two-dimensional graph using three variables. These three variables are, for example, the number of shots of the RF pulse, the number of phase encodings, and the number of segments.
[0028]
Further, as another aspect of the first invention, reception processing means for receiving the plurality of echo signals every time the RF excitation pulse is applied and processing the echo signals into digital echo data, and the echo data Is arranged in a frequency space of a predetermined matrix size, and reconstructing means for performing image reconstruction processing on echo data in the frequency space, and this reconstructing means includes applying the RF excitation pulse to the arrangement of the frequency space. When the echo data exceeding the required number of data is generated, a part of the echo data may be discarded and the remaining echo data may be arranged in the frequency space.
[0029]
Preferably, when the application of the RF excitation pulse generates the echo data exceeding the number of data necessary for the arrangement of the frequency space, the reconstruction means is arranged in the first row of the echo data. A part of the generated data and a part of the data generated at the end are discarded and the remaining echo data is arranged in the frequency space.
[0030]
With this configuration, when the T1 contrast image, which has conventionally been difficult to shorten the photographing time, is accelerated using a pulse sequence such as the EPI method, the variable functions of the ETL and the parameter input thereof are problems. for Graphic interface Thus, the optimum shooting parameter can be determined. The shooting parameters can be set while intuitively understanding the values that can be taken by the shooting parameters according to the desired image quality and the current setting status in relation to other parameters.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0033]
(First embodiment)
A first embodiment will be described with reference to FIGS.
[0034]
The MRI (magnetic resonance imaging) apparatus according to this embodiment generates a plurality of echo signals per application of one excitation pulse, and can collect data in a short time, EPI (echo planar imaging) method, FSE (fast spin echo) ) Method, or an apparatus for executing a pulse sequence based on them, and is characterized by limiting the data collection time by such a pulse sequence. This restriction is performed interactively with an operator using a graph display interface. This restriction can eliminate or suppress image deterioration factors such as chemical shift artifacts, image distortion due to magnetic field inhomogeneity, and image blur due to T2 attenuation, thereby improving image quality.
[0035]
A schematic configuration of this MRI apparatus is shown in FIG. The MRI apparatus includes a bed unit on which the subject P is placed, a static magnetic field generation unit that generates a static magnetic field, a gradient magnetic field generation unit for adding position information to the static magnetic field, a transmission / reception unit that transmits and receives high-frequency signals, A control / arithmetic unit for controlling the entire system and for image reconstruction is provided.
[0036]
The static magnetic field generation unit includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 that supplies current to the magnet 1, and an axial direction of a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. Static magnetic field H in the Z-axis direction ₀ Is generated. In addition, the shim coil 14 is provided in this magnet part. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a controller which will be described later. The couch portion can removably insert the top plate on which the subject P is placed into the opening of the magnet 1.
[0037]
The gradient magnetic field generator includes a gradient magnetic field coil unit 3 incorporated in the magnet 1. The gradient coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in the X, Y, and Z axis directions orthogonal to each other. The gradient magnetic field unit further includes a gradient magnetic field power supply 4 that supplies current to the x, y, z coils 3x to 3z. The gradient magnetic field power supply 4 supplies a pulse current for generating a gradient magnetic field to the x, y, z coils 3x to 3z under the control of a sequencer 5 described later.
[0038]
By controlling the pulse current supplied from the gradient magnetic field power supply 4 to the x, y, z coils 3x to 3z, the gradient magnetic fields in the X, Y, and Z directions, which are the three axes as physical axes, are synthesized, and the logical axis As a slice direction gradient magnetic field G _S , Phase encoding direction gradient magnetic field G _E , And readout direction (frequency encoding direction) gradient magnetic field G _R Each direction can be set and changed arbitrarily. Each gradient magnetic field in the slice direction, the phase encoding direction, and the readout direction is a static magnetic field H. ₀ Is superimposed on.
[0039]
The transmission / reception unit includes an RF coil 7 disposed in the vicinity of the subject P in the imaging space in the magnet 1, and a transmitter 8T and a receiver 8R connected to the coil 7. The transmitter 8T and the receiver 8R supply an RF current pulse having a Larmor frequency for causing a magnetic resonance (MR) phenomenon to the RF coil 7 under the control of the sequencer 5 described later. The high-frequency MR signal received is received and subjected to various signal processing to form a corresponding digital signal.
[0040]
The control / arithmetic unit further includes a sequencer (also referred to as a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, a display device 12, and an input device 13.
[0041]
Among them, the host computer 6 has a function of supervising the entire operation of the apparatus including the sequencer 5, the arithmetic unit 10, the storage unit 11, and the display 12, and also functions as a user interface at the time of scan planning. . That is, the host computer 6 accepts information instructed by the operator based on the stored software procedure, and provides an interactive user interface for instructing the sequencer 5 with scan sequence information based on this information. It comes to offer with. Through this user interface, the user can arbitrarily specify the number of slices and the scan time that can be taken during the repetition time TR corresponding to the variable length echo train according to the matrix size of the image to be taken and the number of shots (excitation number). It can be set to.
[0042]
The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, and controls a series of operations of the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to this information. . The sequencer 5 once inputs the digital data of the MR signal from the receiver 8R and transfers the data to the arithmetic unit 10 that performs the reconstruction process.
[0043]
Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power source 4, the transmitter 8R, and the receiver 8T according to a series of pulse sequences, for example, x, y, z coils 3x to 3z. Includes information on the intensity, application time, application timing, and the like of the pulse current applied to.
[0044]
Pulse sequences that can be employed in the present embodiment include the FE type EPI method, the SE type EPI method, the FSE method, and the FASE (Fast Asymmetric SE) method.
[0045]
The arithmetic unit 10 reads the input raw data, places the raw data in the Fourier space (also referred to as k-space or frequency space) of the image, averages the data, and reconstructs the raw data into real space data. Processing (for example, two-dimensional or three-dimensional Fourier transform processing) In order to generate two-dimensional image data from three-dimensional image data, MIP (maximum value projection) processing and the like are performed in an appropriate order.
[0046]
In particular, in this embodiment, the effective echo time TE _eff In order to improve the degree of freedom of setting, the data arrangement is made possible by shifting the center of the phase encoding direction of the Fourier space, so that the arithmetic unit 10 performs a reconstruction process using the half Fourier method as the reconstruction process described above. Can be done.
[0047]
The storage unit 11 can temporarily store not only raw data and reconstructed image data but also various data generated in the course of arithmetic processing. The display device 12 displays an image. Further, the surgeon can input information necessary for designating imaging conditions such as desired scanning conditions, scanning sequences, and image processing methods to the host computer 6 via the input device 13.
[0048]
As elements of the control / arithmetic unit, a sound generator 16, an ECG sensor 17, and an ECG unit 18 are provided. The voice generator 16 generates a voice message for breath holding to the patient (subject) in response to an instruction from the sequencer 5 or the host computer 6. Further, the ECG sensor 17 and the ECG unit 18 detect a patient's electrocardiogram signal and output it to the sequencer 5, thereby enabling electrocardiographic synchronization imaging.
[0049]
The MRI apparatus of this embodiment is characterized in that the optimum imaging conditions can be set easily. The core of the setting is easy selection and setting of a pulse sequence used for imaging. Here, the relationship between various parameters of the pulse sequence that affects the image quality will be examined in association with the selection method of the pulse sequence.
[0050]
Now, a description will be given assuming that a pulse sequence of the EPI method is used. It is the echo interval (ETS: Echo Train Spacing) that determines the capability of the gradient magnetic field, that is, the resolution and the sampling pitch (bandwidth), so this echo interval is the imaging condition (parameter) that the user first inputs. And According to one of the existing MRI apparatuses, there are seven types of echo intervals such as 0.6 ms, 0.8 ms, 1.2 ms, 1.6 ms, 2.4 ms, 3.2 ms, 4.8 ms, etc. The value of the degree can be selected.
[0051]
Next, the echo time TE is determined. In the case of a pulse sequence based on the FE-type EPI method, In Phase and Out Phase with fat nuclear spins are important in determining image contrast when a fat suppression technique is not used together. Deciding on which echo signal to collect the data of echo time TE, that is, ETL _contrast When (Echo Train Length) is determined, the echo signal responsible for data collection at the center of k-space is the ETL. _contrast Is set internally. Actually ETL _contrast Since it is determined that> 1, the echo time TE is constrained by the echo interval ETS. That is,
[Expression 1]

It becomes a relationship.
[0052]
Next, the amount of image distortion and chemical shift artifacts that occur during image reconstruction is determined. _eff , ETL which defines the minimum repetition time TR _total Will be described. As an example, phase encoding direction matrix size PE = 128, ETL _contrast = 4, ETL is applied to the following three modes according to the number of RF pulses applied, that is, the number of shots Sh. _eff And ETL _total Can be calculated.
[0053]
[Expression 2]

It is.
[0054]
In order to consider the echo interval ETS, the data collection time T ₀ From the viewpoint of the above, EPI method photography is classified as follows.
[0055]
[Equation 3]

Data collection time T ₀ Is defined in this way, the ETL when the number of shots Sh described above is small (FIG. 2A). _eff = 32, the data collection time T in (C) above ₀ There is no ETS that satisfies. This is also the ground that fat suppression is essential in the single shot EPI method. ETL _eff = Data collection time T in (B) above under the condition of 32 ₀ It is necessary to satisfy ETS <0.8 ms simply by satisfying the above. Further, the data collection time T in (A) above. ₀ In order to satisfy the above, ETS <2.4 ms is the limit.
[0056]
On the other hand, ETL when the number of shots Sh = large (FIG. 2C). _eff = 4 When the condition is 4, the data collection time T of (B) described above ₀ Can be satisfied even with ETS = 4.8 ms. (C) Data collection time T ₀ ETS = 2.4 ms is sufficient.
[0057]
In other words, when the phase encoding direction matrix size PE is given, if the number of shots is changed, the ETL _eff (I.e., the number of echoes per RF excitation in which the amount of image distortion and chemical shift artifacts is determined) can be controlled.
[0058]
ETL _eff Is very effective in reducing image distortion and chemical shift artifacts, but the performance as an EPI, that is, the advantage of high-speed imaging is reduced. On the other hand, ETL _eff By extending, it is possible to improve the data collection efficiency by covering the RF pulse, the time required for winding / rewinding the phase encoding amount, and overhead such as echo time shift. Therefore, ETL considering the balance between image distortion and chemical shift artifact suppression and data collection efficiency _eff Is what you want to set.
[0059]
Usually, in order to obtain T1 contrast, a repetition time TR <500 ms is desired. When TR> 100 ms or more, the SNR can be increased by raising the flip angle to about 70 °. In addition, overhead occurs outside the pulse sequence for main imaging, such as RF pulse shooting and phase correction data collection scan, but this overhead is increased by increasing the number of shots and shortening TR. Can be reduced.
[0060]
That is, the data collection time T ₀ Is
[Expression 4]

It is represented by
[0061]
As described above, various shooting parameters that affect image quality have been examined. Since the weighting of the parameters varies depending on the evaluation function, in the case of the shooting condition setting method using a one-dimensional slide bar on the console as in the past, Setting the optimum shooting conditions is considerably difficult even for a skilled operator.
[0062]
An interface that significantly reduces this difficulty is provided by one embodiment of the present MRI apparatus as follows.
[0063]
This interface is a visual and interactive graphic interface provided by the host computer 6, the display device 12, and the input device 13 in cooperation when an operator makes a shooting plan.
[0064]
By the way, although various parameters such as hardware and software at the time of imaging planning handled by the MRI apparatus are 300 or more, in practice, about 50 parameters are processed by the processing (not shown) of the host computer 6 of the apparatus. The parameter is open to the user as a parameter that can be changed by the user. When the photographing plan processing is started, about 250 device-side parameters that are not visible to the user are automatically set by the host computer 6 according to a predetermined procedure. For this reason, when the imaging plan process is started, the main scan is ready to be executed, and the operator is prompted to set the user release parameter. The host computer 5 immediately corrects about 250 parameter values, which are left to the apparatus side, within an allowable range in accordance with the user-open parameter values set by the operator. This correction result is also reflected in pressing the correction of the user release parameter as necessary. That is, the processing of the interactive shooting plan with the apparatus side is advanced. Therefore, regardless of the value set by the operator for the user release parameter, the state in which the main scan can be executed is always maintained. That is, depending on the setting of the parameter, it is possible to avoid a situation where it is found that the main scan is not started for the first time when actual shooting is performed.
[0065]
The visual and interactive graphic interface of the present embodiment is provided for setting the above-mentioned user opening parameters. The user-open parameters include the type of imaging target, the type of sequence, the repetition time TR, the echo time TE, the ETS, the ETL, the number of shots, the flip angle, the phase encoding direction matrix size, the number of segments, the scanning time, the FOV, and the gate. Main parameters such as presence / absence, type of gate, presence / absence of fat suppression are included.
[0066]
As the graphic interface image, a two-dimensional graph shown in FIG. As shown in the figure, the variable is a binary value in which the horizontal axis represents the number of shots Sh and the vertical axis represents the phase encoding direction matrix size PE, and the evaluation function (A) to (A) to ( Data collection time T shown in C) ₀ Take.
[0067]
The relationship between the two variables Sh and PE is
[Equation 5]
PE = ETL x Sh
(See the straight line S in FIG. 3), and the slope of the straight line S is ETL. The prohibited area where the straight line S is prohibited is shown in FIG.
[Formula 6]
PE> T ₀ / ETS x Sh
(Refer to the shaded area in FIG. 3). As mentioned above, T ₀ Is the data collection time. In the case of the EPI method, as an example, T ₀ = 100, 30, and 10 ms are selectively taken (see the condition items (A) to (C)).
[0068]
That is, the data collection time T corresponding to the condition (A) ₀ = Area partitioned by straight line T0 (A) determined by 100 ms: PE> T ₀ / ETS × Sh forms the prohibited region R. This area is depicted in red on the graph, for example. On the other hand, the data collection time T corresponding to the conditions (B) and (C) ₀ = 30 ms, the areas defined by the straight lines T0 (B) and T0 (C) determined by 10 ms are depicted in yellow and blue, for example. In this embodiment, the yellow and blue areas are set as recommended areas for setting the number of shots.
[0069]
In this two-variable graphic interface, the rectangular bars B0 to B2 shown on the horizontal axis X and the vertical axis Y are automatically calculated on the apparatus side, and the range that the user of the shot number Sh and matrix size PE can set is set. Show. Regarding the number of shots Sh, a range R1 on the horizontal axis defined by the rectangular bars B0 and B1 is a range of values that can be currently taken. Regarding the matrix size PE, a range R2 on the horizontal axis defined by the rectangular bars B0 and B2 is a range of values that can be currently taken. The positions of the rectangular bars B0 to B2, that is, the ranges of values R1 and R2 that can be taken by the variable, change in real time according to the calculation on the apparatus side according to the value of the user opening parameter operated by the operator.
[0070]
The operator operates the input unit 13 while viewing this graph, and determines the optimum shooting conditions while operating the handle X1 for the number of shots Sh, the handle X2 for the matrix size PE, and the handle Y1 for the straight line S: PE = ETL × Sh. It will be. The following aspects can be taken as an example of this handle operation.
[0071]
For example, when the handle X2 of the matrix size PE is fixed, if the handle X1 of the number of shots Sh is moved (see the arrow a in FIG. 3), the slope of the straight line S: PE = ETL × Sh changes (see FIG. (See arrow a ′ in FIG. 3). As the handle X1 moves, the data collection time T ₀ Forbidden region R: PE> T determined by substituting the numerical value of the condition (A) for ₀ / ETS × Sh changes in real time in conjunction with each other (see arrow a ″). At this time, the inclination of the straight line S is controlled so as not to enter the prohibited area R. Data collection is performed as the handle X1 moves. Time T ₀ Each region determined by substituting the numerical values of the above conditions (B) and (C) also changes in real time in conjunction with each other (see a ").
[0072]
Furthermore, as another aspect, the range of the prohibited region R may be changed in accordance with the consideration or change of other imaging conditions such as application of fat suppression pulses.
[0073]
In the MRI apparatus of this embodiment, a pulse sequence is set as part of a scan plan according to the above principle. FIG. 4 shows an outline of the setting procedure, and FIG. 5 shows an example of a pulse sequence set through the procedure.
[0074]
In the process of making a scan plan, the operator starts an interactive user interface realized by the cooperation of the display unit 12 and the input unit 13 by software processing of the host computer 6.
[0075]
By starting this interface, first, the type of pulse sequence and the echo interval ETS are selected and input (step S1). Assume that the multi-shot FE type EPI method is selected as the pulse sequence. The echo time TE is determined according to the ETS value (step S2). By this determination, the echo that collects the data at the center of the phase encoding direction of k-space becomes ETL. _contrast Is set internally (step S3).
[0076]
Next, the operator collects the data collection time T for the pulse sequence according to the selected EPI method according to the imaging purpose. ₀ Is input as an evaluation function (step S4). This is performed, for example, by inputting time values classified as in the above conditions (A) to (C).
[0077]
Next, the host computer 6 (which also functions as a user interface) creates data for graphic display from the data received so far and the calculated data, and displays this on the display unit 12 to display it on the operator (user). (Step S5). While referring to the presented graphic interface, the operator appropriately moves the handle of the graphic interface screen via the input device 13. This makes it possible to intuitively (visually) capture shooting parameters such as the number of shots to maintain image quality (suppression of image distortion and chemical shift artifacts) when the phase encoding direction matrix size PE is increased or to obtain a desired state. It can be set.
[0078]
The photographing parameters determined through the interactive input type graphic user interface are stored as control data for the main scan (step S7).
[0079]
As a result, as shown in FIG. 5, for example, the main scan executes a pulse sequence based on the FE type EPI method according to the multi-shot method. In FIG. 5, the phase encoding gradient magnetic field G _S Indicates only that phase encoding is applied to each echo signal. There are various modes of applying this phase encoding.
[0080]
When this main scan is executed, for example, when the number of shots is small (see the state of FIG. 2A), the image reconstruction is performed by the arithmetic unit 10 by the half Fourier method. On the other hand, when executed with the number of shots = large (see the state of FIG. 2C), all or part of the echoes at the beginning and end of the echo train are discarded by the arithmetic unit 10 and k-space. Is attached to the reconstruction process without being arranged.
[0081]
When using the multi-shot FE type EPI method in T1-weighted imaging, the number of parameters that need to be controlled increases, and in determining the values of parameters that affect image quality, such as the echo train length (ETL) of the pulse sequence and the number of shots. However, according to the MRI apparatus of the present embodiment, the pulse sequence satisfying the optimum imaging condition according to the imaging purpose is set quickly and with little operation effort and with high accuracy. High-quality MR images can be provided.
[0082]
In particular, since an interactive input type graphic user interface is used, it is possible to set such a pulse sequence quickly, with less operation effort, and with higher accuracy than a conventional one-dimensional slide bar type interface. In addition, the pulse sequence is always reliably executed in actual photographing, and a high-quality image can be provided.
[0083]
This reduces re-imaging, reduces operational effort and increases patient throughput. This significantly reduces the physical and mental burden on the patient.
[0084]
The pulse sequence based on the above EPI method has been described as not applying a fat suppression pulse. However, as shown in FIG. _fat May be set to be applied. As this fat suppression pulse, a binomial pulse, a sync function, a Gaussian function, or the like may be used in a slice-selective or non-slice-selective manner.
[0085]
Further, in the graphic user interface of the first embodiment described above, the display method of the prohibited area and the other areas is not necessarily limited to the colored one, but the area is visually displayed such as by applying a mesh. It can be divided into
[0086]
Further, the recommended area may be displayed so as to overlap the area other than the prohibited area. Further, the recommended conditions may be separately displayed by numerical values.
[0087]
Further, in this graphic user interface, instead of the above-described straight line S: PE = ETL × Sh, the operator may input a straight line having an arbitrary inclination, and the photographing condition may be selected only on the straight line.
[0088]
Furthermore, in the graphic user interface of the first embodiment, the data collection time T in the EPI method is used as the evaluation function. ₀ However, when the FASE method is used as the pulse sequence, as an example,
[Expression 7]

It may be used by setting to.
[0089]
(Second Embodiment)
A second embodiment of the present invention will be described with reference to FIGS. The hardware configuration of the MRI apparatus of this embodiment is the same as or equivalent to that of the first embodiment.
[0090]
In the present embodiment, the scan plan using the graphic user interface according to the present invention and the execution of the main scan are applied to an example in which imaging is performed by a synchronous imaging method such as a heart. FIG. 7 shows a schematic diagram of a sequence structure of the multi-shot segment EPI method (FE type).
[0091]
When this synchronous imaging method is performed, in addition to the first embodiment described above, restrictions such as the RR interval of the ECG signal are further added, and the setting of imaging conditions (parameters) becomes more complicated. That is, when the EPI method is used, the above-described data collection time T ₀ For example, one of the following conditions is added to the constraint conditions (A) to (C) from the above viewpoint.
[0092]
(D) One heartbeat shooting:
This is an imaging method that observes changes over time such as arrhythmia and perfusion, and is collected in a time width of 200 to 300 ms, excluding cardiac contractors. Since the number of segments Se = 1, this is a method of increasing the data collection efficiency by increasing the repetition time TR as much as possible within the range of the generation amount of image distortion and chemical shift.
[0093]
(E) Dynamic observation (I):
The patient is held for about 5-8 seconds, which is less burdensome for the patient, and at the same time, the image is taken with a resolution of about 100 ms.
[0094]
(F) Dynamic observation (II):
In order to precisely observe the movement of the heart valve, etc., it is an image that is taken for about 30 seconds or intermittently for about 5 to 8 seconds. To do. In this case, the number of shots Sh = 1 may be obtained.
[0095]
In the imaging of the heart, imaging is repeated in synchronization with the RR interval of the ECG signal. Therefore, since the setting of shooting conditions is further complicated, an interactive input type graphic user interface is provided to solve this problem.
[0096]
FIG. 8 shows a graph display state of this interface. As variables, there are three values: shot number Sh, phase encode number PE (= phase encode direction matrix size), and segment number Se. A two-dimensional graph is formed with the phase encoding number PE on the vertical axis, the shot number Sh on the horizontal axis on the first quadrant side, and the segment number Se on the horizontal axis on the second quadrant side.
[0097]
In this three-variable graphic interface, the rectangular bars B0 to B3 represented on the horizontal axis X and the vertical axis Y are automatically calculated on the apparatus side and set by the user of the shot number Sh, the matrix size PE, and the segment number Se. The possible range is shown. Regarding the number of shots Sh, the range on the horizontal axis defined by the rectangular bars B0 and B1 is a range of values that can be currently taken. For the matrix size PE, the range on the horizontal axis defined by the rectangular bars B0 and B2 is a range of values that can be currently taken. Regarding the number of segments Se, the range on the horizontal axis defined by the rectangular bars B0 and B3 is a range of values that can be currently taken. The positions of the rectangular bars B0 to B3, that is, the range of values that can be taken by the variable, change in real time according to the calculation on the apparatus side according to the value of the user opening parameter operated by the operator.
[0098]
The relationship between each variable is
[Equation 8]

The slope of the straight line S1 is “ETL × Se”, and the slope of the straight line S2 is “ETL × Sh”. The forbidden areas R1 and R2 of the straight lines S1 and S2 are respectively
[Equation 9]

It is.
[0099]
Also, the graph of this interface shows the data collection time T represented by the conditions (A) to (C), as in the first embodiment. ₀ Are used as evaluation functions, and straight lines T0 (A), T0 (B) and T0 (C): PE> T ₀ / ETS × Se or PE> T ₀ The area divided by / ETS × Sh is displayed in different colors as an example for each quadrant. This color-coded area changes dynamically on the image as the variable changes.
[0100]
While looking at the screen of this graphic user interface, the operator operates the handle as follows, for example.
[0101]
For example, when the handle X2 of the phase encoding number PE is fixed, if the handle X1 of the shot number Sh is moved, the slope of the straight line S1: PE = ETL × Se × Sh changes. Since the slope of the straight line S1 is a function of the number of segments Se, the hand X3 with the number of segments Se is also linked in real time. As the handles X1 and X3 move, the prohibited areas R1 and R2 also change in real time. At this time, the slopes of the straight lines S1 and S2 are controlled so as not to enter the prohibited areas R1 and R2. Conversely, the same applies when the handle X3 on the segment number Se side is moved.
[0102]
Use this interface interactively to maintain image quality when the phase encoding direction matrix size PE is increased For The number of shots, the number of segments, etc. can be set while grasping the shooting conditions visually (intuitively). Therefore, even in cardiac imaging based on the ECG gate, the same effect as in the first embodiment can be obtained.
[0103]
Note that although the imaging in the first and second embodiments has been described as being a two-dimensional imaging method, the same interface input can also be applied to a three-dimensional imaging method.
[0104]
Further, the pulse sequence is not limited to the EPI method, and may be an FSE method (RARE method). In the case of this FSE method, T2 ^* A similar interface can be applied by considering image blur due to T2 relaxation instead of image distortion due to T2.
[0105]
Furthermore, the variable that the user adjusts while viewing the graph screen in the graphic user interface according to the present invention is not limited to the above-described two or three variables, but is a parameter that is open to the user as a shooting parameter that affects image quality. If it can be adopted. For example, in the graph screen of FIG. 8, FOV may be set as another variable instead of the phase encoding number PE forming the vertical axis. On the other hand, the number of variables may be four or more. For example, in the graph screen of FIG. 8, the horizontal axis may be the number of shots Sh and the number of segments Se as shown in the figure, and the vertical axis may be four quadrants with the phase encoding number PE or FOV and the breath holding scan time.
[0106]
Therefore, the characteristics of the graphic user interface to which the present invention is applied are summarized as follows. First, a plurality of shooting parameters can be set simultaneously. Secondly, the values that can be taken by the plurality of shooting parameters can be visually expressed simultaneously to give the operator an intuitive image of the setting range, thereby facilitating the setting of the shooting parameters. Third, the combination of a plurality of parameters can be changed as appropriate. Fourthly, when the shooting parameter handle position is changed, the prohibited area is also dynamically linked to always give the operator an intuitive image of the accurate setting range.
[0107]
Further, the user interface may include means for allowing an operator to input a straight line having an arbitrary inclination on the graph, and means for allowing a photographing parameter to be selected only on the straight line.
[0108]
It should be noted that the present invention is not limited to the above-described exemplary embodiments, and those skilled in the art will recognize that various modifications can be made without departing from the spirit of the invention based on the description of the claims. It is possible to modify and change the embodiment, and these also belong to the scope of rights of the present invention.
[0109]
【The invention's effect】
As described above, according to the present invention MRI equipment For example, when the EPI method echo time TE and repetition time TR are constrained to make a multi-shot, an optimal shooting parameter for each shooting target is visually determined by a graphic user interface used in the shooting preparation. It is possible to easily set the imaging parameters without changing the matrix size in the phase encoding direction, etc., and to reliably set the imaging parameters in which the image quality does not change greatly, and to realize stable MR imaging based on these imaging parameters. Can be executed. Therefore, it is possible to reliably provide high-precision and high-quality MR images with less burden on patients and operators.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an MRI apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram schematically illustrating the relationship between the number of shots, ETL, and data arrangement.
FIG. 3 is a screen view of a graphic user interface for explaining restrictions on the number of shots in the first embodiment.
FIG. 4 is a flowchart showing an outline of an operation of an operator for a graphic user interface.
FIG. 5 is a timing chart illustrating a pulse sequence of a multi-shot EPI method (FE type) used in the first embodiment.
FIG. 6 is a timing chart illustrating a pulse sequence of a multi-shot EPI method (FE type) to which a fat suppression pulse is added as a modified example.
FIG. 7 is a timing chart illustrating a pulse sequence of a multi-shot segment EPI method (FE type) according to the second embodiment of the invention.
FIG. 8 is a screen view of a graphic user interface for explaining restrictions on the number of shots and the number of segments in the second embodiment.
[Explanation of symbols]
1 Magnet
2 Static magnetic field power supply
3 Gradient magnetic field coil unit
4 Gradient magnetic field power supply
5 Sequencer
6 Host computer (user interface)
7 RF coil
8T transmitter
8R receiver
10 Arithmetic unit
11 Storage unit
12 Display (user interface)
13 Input device (user interface)
17 ECG sensor
18 ECG units

Claims (18)

In an MRI apparatus that applies a pulse sequence to a subject placed in a uniform static magnetic field, collects echo signals, and generates an image inside the subject.
A pulse sequence for generating a plurality of echo signals per application of one RF excitation pulse is set as the pulse sequence, a data collection time for collecting the plurality of echo signals is set to a desired value, and Pulse sequence setting means for determining the number of echoes of the plurality of echo signals and the number of shots of the RF excitation pulse from the desired time, and a pulse for collecting the echo signals by executing the pulse sequence set by the pulse sequence setting means An MRI apparatus comprising a sequence execution unit. The MRI apparatus according to claim 1,
The type of pulse sequence set by the pulse sequence setting means is to invert the polarity of a pulse train or readout direction gradient magnetic field pulse to which a plurality of refocus RF pulses are applied in order to generate the plurality of echo signals a plurality of times. An MRI apparatus that is a pulse sequence including a pulse train. The MRI apparatus according to claim 2,
The pulse sequence is a pulse sequence for obtaining a T1-weighted image, and the repetition time TR is a value of 600 ms or less. The MRI apparatus according to claim 2 or 3,
The MRI apparatus is a pulse sequence that includes a pulse train that inverts the polarity of a read direction gradient magnetic field pulse a plurality of times and that applies a fat suppression pulse before application of the pulse train. The MRI apparatus according to claim 4, wherein
An MRI apparatus in which the data collection time for collecting the plurality of echo signals is set to a value of 30 ms or less. The MRI apparatus according to claim 3, wherein
An MRI apparatus in which the data collection time for collecting the plurality of echo signals is set to a value of 10 ms or less. The MRI apparatus according to claim 6, wherein
The MRI apparatus, wherein the pulse sequence is a pulse sequence that does not use fat suppression pulses. The MRI apparatus according to claim 1,
The pulse sequence setting means is an MRI apparatus comprising an interactive user interface of a plane graph display for constraining the data collection time of the pulse sequence to an optimum value. The MRI apparatus according to claim 8, wherein
The user interface is an MRI apparatus which is an interactive user interface for displaying a planar graph using two variables. The MRI apparatus according to claim 9, wherein
The MRI apparatus, wherein the two variables are the number of shots and the number of phase encodings of the RF pulse. The MRI apparatus according to claim 10, wherein
The MRI apparatus is an interface in which the user interface designates a prohibited area for a combination of an echo train length (ETL) and the number of shots on the two-dimensional graph and performs parameter selection in a range excluding the prohibited area. The MRI apparatus according to claim 11,
The MRI apparatus is an interface that displays a recommended range for a combination of an echo train length (ETL) and the number of shots on the two-dimensional graph on the two-dimensional graph. The MRI apparatus according to claim 12, wherein
The MRI apparatus, wherein the user interface is an interface that displays at least one of the invalid range and the recommended range with a pattern or color. The MRI apparatus according to claim 8, wherein
The MRI apparatus, wherein the user interface includes means for allowing an operator to input a straight line having an arbitrary inclination on the graph, and means for allowing an imaging parameter to be selected only on the straight line. The MRI apparatus according to claim 8, wherein
The user interface is an MRI apparatus which is an interactive user interface for displaying a two-dimensional graph using three variables. The MRI apparatus according to claim 15,
The MRI apparatus, wherein the three variables are the number of shots, the number of phase encodings, and the number of segments of the RF pulse. The MRI apparatus according to claim 1,
Receiving processing means for receiving the plurality of echo signals every time the RF excitation pulse is applied and processing the echo signals into digital echo data, and arranging the echo data in a frequency space of a predetermined matrix size Reconstructing means for performing image reconstruction processing on echo data in the frequency space, and the reconstructing means generates the echo data in which the application of the RF excitation pulse exceeds the number of data necessary for the arrangement in the frequency space. An MRI apparatus characterized in that the echo data is partly discarded and the remaining echo data is arranged in the frequency space. The MRI apparatus according to claim 17,
When the RF excitation pulse is applied to generate the echo data exceeding the number of data necessary for the arrangement of the frequency space, the reconstruction means generates the first of the data generated in the echo data sequence. An MRI apparatus characterized in that a part of data generated in part and the last part is discarded and the remaining echo data is arranged in the frequency space.

Images