JP4413304B2 - MRI equipment - Google Patents

Description

【００７８】
同式中、第１項目のＴ1(DD) は「internuclear dipole-dipole 相互作用」を表す。この相互作用は、図４に模式的に示す如く、ＲＦ励起に拠ってそのエネルギを格子（lattice ）に移動させることで、カップリングしている分子のスピンＡ，Ｂ間をデカップリング（decoupling）し、信号値を増加させる。
【００７９】
また、第２項目のＴ1(SR) は「スピンrotational」を表す。結合している分子は回転運動をする。分子が回転運動をすると、その運動の大きさにも拠るが、部分的な「local magnetic field」を発生し、Ｔ1 短縮に寄与する。
【００８０】
第３項目のＴ1(SC) は「scaler coupling 」を表す。このカップリングは、カップリングしている原子の片方がQuadrupole（四極子、スピン量子番号Ｉ≧１の原子、例えばＯ¹⁷＝５／２，ｍ_ｎ ⁵⁵＝５／２、参考として挙げるとＨ¹ ＝１／２）と結合していると、四極子はQuadrupole relation という特有の短い緩和時間を有し、四極子とカップリングしているスピンの緩和時間をも短縮させる。
【００８１】
第４項目のＴ1(CSA)は「chemical shift anizotoropy」の項であり、電子遮蔽効果の変化に拠るローカルfield の変化を表す。この変化もＴ1 に影響する。
【００８２】
このようにＴ1 時間は様々な要素で変化する。上述したファクタ以外にもＴ1 に影響する要素はあるが、その影響が大きいものは上述したファクタである。
【００８３】
静止している血液においても、oxyhemogrobin とdeoxyhemogrobin （酸化と無酸化ヘモグロビン）も、paramagnetic ion（主に、鉄Ｉと鉄II）を持つため、local magnetic fieldを作り、Ｔ1 時間を下げる。また、Ｏ_２の有無によっても、spin-spin relaxation（Ｔ2 ）や静脈（w 酸素、Ｔ2 ＝１２０ｍｓｅｃ）、動脈（less酸素、Ｔ2 ＝２２０ｍｓｅｃ）が異なる。
【００８４】
図５に、effective correlation ：Ｔc に対するＴ1 ，Ｔ2 ，およびＴ1Pの依存性を示す。同図中、Ａは堅い固体、Ｂは柔らかい固体、Ｃは粘土の高い液体、Ｄは通常の液体を示している（T.C.Farrar and E.D.Becker, Pulse andFourier Transform NMR, P.98, Academic Press(1971)参照）。
【００８５】
effective correlation ：Ｔc は分子の動きを表すファクタで、動きの速い分子のタンブリング（回転、振動）度を示している。同図の横軸上を右側に進むほど、固体の度合いが高く、また分子の動きがスローになる。このような関係から、流れの速い血液と殆ど静止している血液とでは、Ｔ1 時間値は異なることが分かる。
【００８６】
さらに、ＭＴパルスを分割、複数化して印加することで、流れている血液の見掛けの縦緩和時間Ｔ1 が短縮される。
【００８７】
ＭＴ（magnetization transfer）効果とは、前述したように、dipole-dipole interaction 関係にある高分子Ｈｒ近傍の自由水Ｈｆの平衡状態を、自由水からoff-resonance の周波数をＲＦパルスでＲＦ励起することにより、高分子のプロトンを励起し、これにより、そのプロトンと相互作用している自由水の信号強度に影響が与えられる現象である。すなわち、自由水の磁化をＨｆ、高分子の磁化をＨｒ、反応時間の定数をｋ_１およびｋ_−１とすると、
【数２】

と表される。［ ］はコンセントラーション、Ｋは反応定数である。
【００８８】
そこで、自由水のＴ1 時間をＴ1f、高分子のＴ1 時間をＴ1rとすると、
【数３】

と置くことができ、Ｍ_f(t)：時刻＝ｔ時の自由水の磁化、Ｍ_f(0)：時刻＝０時の自由水の磁化、Ｍ_r(t)：時刻＝ｔ時の高分子の磁化、およびＭ_r(0)：時刻＝０時の高分子の磁化とすると、
【数４】

となる。
【００８９】
ＭＴパルスを印加したときの磁化Ｍ_fSATは
【数５】

となり、そのときの縦緩和時間Ｔ_1SATは、
【数６】

となる。したがって、反応定数ｋ_１は
【数７】

となる。ここで、Ｔ_1SATは見掛けのＴ1 である（「Balaban, Magn. Reson. Quarterly Vol.8, No.2, 1992 」参照）。
【００９０】
そこで、移動している血液中の自由水のプロトンの磁化ＨfA、血液中の高分子のプロトンの磁化をＨrA、静止している実質部の自由水のプロトンの磁化をＨfB、および、静止している実質部の高分子のプロトンの磁化をＨrBとして、このＭＴ印加に伴う本発明の磁化の挙動を模式的に示すと図６のようになる。
【００９１】
移動中の血液において平衡状態にある自由水プロトンの磁化ＨfAと高分子プロトン（同図（ａ）参照）に対して最初の分割化ＭＴパルスＰ₁ （フリップ角は例えば１００°）を印加すると、高分子の磁化ＨrA（原子核プール）から自由水の磁化ＨfA（原子核プール）に磁化スピンが移動し（magnetization transfer）、その反対に、自由水の磁化ＨfAから高分子の磁化ＨrAにエネルギが移動する（同図（ｂ）参照）。分割されたＭＴパルスであるので、それらの移動量は共に小さい。すなわち、ＭＴ効果が小さい。
【００９２】
縦緩和によって高分子の磁化ＨrAが初期の平衡状態に戻ってくるが、このとき、前述したように、dipole-dipole interaction 、常磁性効果などのファクタによって見掛け上、Ｔ1 時間が短縮され、磁化スピンＨrAの戻り速度が速くなる。
【００９３】
このようにして分割化されたＭＴパルスＰ_２，Ｐ_３，…，Ｐ_ｎが順次印加されるが、そのＭＴ効果のトータルは、見掛け上、短縮されたＴ1 時間に拠って小さくなる（図６（ｃ）（ｄ）参照）。したがって、流れている（移動している）血液に対するＭＴ効果は、静止または殆ど動いていない血液や実質部に比べて、ＭＴ効果は小さく、撮像面Ｓ_ｉｍａから収集されるエコー信号の強度が高い。
【００９４】
これに対して、静止していたり、動きの少ない実質部の自由水のプロトン磁化ＨfB及び高分子のプロトン磁化ＨrBの間に生じるＭＴ効果は、前述した図７の場合と同様になる。つまり、この場合、個々の分割化ＭＴパルスの和として効いてくる。このため、このような撮像面Ｓ_ｉｍａ の実質部には従来と同等のＭＴ効果の効きが発揮され、収集されるエコー信号も従来と同様に下がる。
【００９５】
したがって、本実施形態のＭＲＩ装置により、スライス選択的なＭＴ効果を利用して、静止している又は殆ど動いていない対象と動いている対象とを差別化して画像化できる。そして、得られるＭＲＡ像は、長い印加時間で且つ大きなフリップ角のＭＴパルスを１回印加する従来法に比べて、撮像面の血液（血流）／実質部のコントラストを著しく向上させることができる。それにより、撮像面の血流の描出能の高い、より高品質なＭＲＡ像を提供できる。
【００９６】
さらに、この実施形態によるＭＲＡ像はＭＲ造影剤を使用するものでないから、通常のＭＲイメージングの非侵襲性の特性を生かしたものとなる。このため、造影剤を使用したＭＲＡ像の撮影に比べて、患者の精神的、体力的負担が著しく少なくて済む。
【００９７】
なお、前記図２に示したパルスシーケンスの例において、複数個の分割化ＭＴパルスの印加時にスライス用傾斜磁場Ｇ_Ｓのパルスも複数個印加する構成を示したが、この構成に代えて、図７に示すように、複数個の分割化ＭＴパルスを印加している間中、連続してスライス用傾斜磁場Ｇ_Ｓ のパルスを１個印加するように設定することもできる。これにより、ＭＴパルス列Ｐ_ＭＴの印加に必要な時間が短かくて済み、全体の撮像時間も短縮させることができる。
【００９８】
また、複数個の分割化ＭＴパルスを印加する別の例として、図８に示す手法がある。この手法は、スライス方向、読出し方向、および位相エンコード方向のいずれにも傾斜磁場パルスを印加することなく、分割化ＭＴパルスを単独で印加し、その後で、スライス方向、読出し方向、および位相エンコード方向の内の任意の１以上の方向に傾斜磁場スポイラーパルスを印加するものである。つまり、分割化ＭＴパルスはスライス非選択的に印加される。これにより、分割化ＭＴパルスは広い範囲の領域に有効に掛けられ、スライスやスラブに限定されない。なお、上述した図７および図８では、読出し方向および位相エンコード方向の傾斜磁場（スポイラーパルスを含む）の図示を省略している。
【００９９】
またなお、本実施形態のＭＲＩ装置で使用できるデータ収集シーケンスは、上述したように通常ＦＳＥ法に限定されることなく、ＦＥ法、ＳＥ法、ＥＰＩ法、ＦＬＡＩＲ法、ＦＡＳＥ法などのイメージング法を採用することもできる。
【０１００】
［第２の実施形態］
第２の実施形態を図９〜図１６に基づき説明する。
【０１０１】
この実施形態は、上述で基本的な原理を説明した、分割化した複数のＭＴパルスを用いて肺野の実質部を画像化できるＭＲＩ装置に関する。
【０１０２】
ＭＲＩの分野において、肺野については、その主な撮像法として３つの方法が提案されている。それらは、hyper-polarized （キセノンやヘリウム）ガスを使用する方法、造影剤Ｇｄ−ＤＴＰＡを使用したパフュージョン法（例えば文献「Hatabu H, et al., MRM 36:503-508, 1996）、さらには酸素分子を使用した酸素吸入による方法（例えば文献「Edelman RR, et al., Nature Medicine, 2,11,1236-1239, 1996 ）である。
【０１０３】
この内、１番目のhyper-polarized （キセノンやヘリウム）ガスを使用する方法は、肺野にガスを吸入させて、例えばキセノン（Ｘｅ）ガスのＭＲ周波数で画像化する方法である。また２番目の造影剤Ｇｄ−ＤＴＰＡを使ったパフュージョンは、血中のＧｄ−ＤＴＰＡのパフューズしている状態を観察する手法である。第３番目の酸素分子を吸入させる方法は、酸素分子は弱いパラマグネティックであるが、肺胞の表面で、ＭＲＩで観察できる水信号に十分な信号変化を及ぼすとの報告に基づくものである。
【０１０４】
しかしながら、上述した撮像法には以下のような問題がある。第１番目のhyper-polarized （キセノンやヘリウム）ガスを使用する方法の場合、通常のキセノンガスをhyper-polarizeする必要があるので、コストが高いという問題がある。また、第２番目の造影剤Ｇｄ−ＤＴＰＡを使ったパフュージョンは、造影剤を投与する必要があるので、侵襲的な処置が必要で、何よりもまず、患者の精神的、体力的な負担が大きい。また、検査コストも高い。さらに、患者の体質などによっては造影剤を投与できない場合もある。さらに、第３番目の酸素分子を吸入させる方法によっても、信号変化は画像上で十分ではなく、臨床や研究に使用できるほどの満足な画像な画像はおろか、その形態すら撮像できていない。
【０１０５】
肺野の構造は、スポンジ状の肺胞、気管支、肺動脈、および肺静脈がその殆どの表面積を占め、その周りに空気が存在している。スポンジ状の肺胞の表面積は膨大な値になるが、ほかの臓器（肝臓、腎臓など）のように細胞内外に自由水を多く持っている訳ではない。したがって、肺野の場合、ＭＲＩでの検出対象である水信号は、血管系から検出されるのみであり、肺野実質部としての水信号値は不足する。肺胞周辺は、その表面積に比較して水分子が少ないため、ＭＲ信号として反映されないものと推定される。しがって、肺野のＴ２値＝８０ｍｓ（文献「ＪＭＲＩ，２（Ｓ）：13-17,1992」参照）自体は他の臓器と比較しても遜色ないにも関わらず、従来のＭＲイメージングでは、造影剤などを使用しない限り、肺野実質部を描出できないものと推定される。
【０１０６】
そこで、本実施形態では、本発明に係る、分割化した複数のＭＴパルスの優位性を余すところ無く発揮させ、従来困難とされていた肺野実質部を、造影剤などを使用しないで撮像できるようにする。
【０１０７】
この実施形態に係るＭＲＩ装置は、第１の実施形態のものと同等のハード構成を有するが、スキャンの手順に以下のように相違している。
【０１０８】
ホスト計算機６は、位置決め用スキャン（図示しない）や撮像条件の入力などの準備作業に引き続いて、図９に示す如く、第１回目および第２回目の最低２回の撮像を実施する。この２回の撮像はそれぞれ、画像再構成に必要なエコーデータの組を収集する２次元または３次元のＭＲスキャンである。この各撮像は、患者が意識的に一時、息を止めた状態で撮像する息止め法、および、ＥＣＧ信号に依るＥＣＧゲート法を併用して行うことが望ましい。
【０１０９】
この撮像に使用可能なパルスシーケンスとしては、フーリエ変換法を適用したものであれば、２次元（２Ｄ）スキャンまたは３次元（３Ｄ）スキャンのものであってよいし、またそのパルス列の形態としては、ＳＥ法、ＦＳＥ（高速ＳＥ）法、ＦＡＳＥ（高速 Asymmetric ＳＥ）法（すなわち、高速ＳＥ法にハーフフーリエ法を組み合わせたイメージング法）、ＦＥ法、高速ＦＥ法、セグメンテド高速ＦＥ法、ＥＰＩ（エコープラナーイメージング）法、などを使用できる。
【０１１０】
また、演算ユニット１０は、受信器８Ｒが出力したデジタルデータ（原データ）を、シーケンサ５を通して入力し、その内部メモリ上の画像のフーリエ空間（ｋ空間または周波数空間とも呼ばれる）に原データ（生データとも呼ばれる）を配置し、この原データを各組毎に２次元または３次元のフーリエ変換に付して実空間の画像データに再構成する。また演算ユニット１０は、画像に関するデータの合成処理や差分演算処理を行うことが可能にもなっている。
【０１１１】
この合成処理には、複数フレームの画像データを対応画素毎に加算する処理、複数フレームの画像データ間で対応ピクセル毎に最大値を選択する最大値投影（ＭＩＰ）処理などが含まれる。また、上記合成処理の別の例として、フーリエ空間上で複数フレームの軸の整合をとって原データのまま１フレームの原データに合成するようにしてもよい。なお、加算処理には、単純加算処理、加算平均処理、重み付け加算処理などが含まれる。
【０１１２】
記憶ユニット１１は、再構成された画像データのみならず、上述の合成処理や差分処理が施された画像データを保管することができる。表示器１２は画像を表示する。また入力器１３を介して、術者が希望する撮像条件、パルスシーケンス、画像合成や差分演算に関する情報をホスト計算機６に入力できる。
【０１１３】
また、息止め指令部の一要素として音声発生器１６を備える。この音声発生器１６は、ホスト計算機６から指令があったときに、息止め開始および息止め終了のメッセージを音声として発することができる。
【０１１４】
さらに、心電計測部は、被検体の体表に付着させてＥＣＧ信号を電気信号として検出するＥＣＧセンサ１７と、このセンサ信号にデジタル化処理を含む各種の処理を施してホスト計算機６およびシーケンサ５に出力するＥＣＧユニット１８とを備える。この心電計測部による計測信号は、撮像を実行するときにシーケンサ５により用いられる。これにより、ＥＣＧゲート法（心電同期法）による同期タイミングを適切に設定でき、この同期タイミングに基づくＥＣＧゲート法の撮像を行ってデータ収集できるようになっている。
【０１１５】
次に、この実施形態のＭＲＩ装置による撮像の動作を図９〜図１６を参照して説明する。
【０１１６】
ホスト計算機６は、図示しない所定のメインプログラムを実行することにより、図９に示す如く、２回の撮像を、一例としての３次元のＦＡＳＥ（高速 Asymmetric ＳＥ）法で実行する。第１回目の撮像は息止め法およびＥＣＧゲート法を併用し、かつ、後述するようにＭＴパルスは印加しない（ｏｆｆ）の状態で実行される。この第１回目の撮像が終了した後の適宜なタイミングで、第２回目の撮像が第１回目と同様に息止め法およびＥＣＧゲート法を併用して実行される。ただし、この第２回目の撮像は、ＭＴパルスを印加して（ｏｎ）実行される。
【０１１７】
この第１回目および第２回目の撮像のパルスシーケンス例を共に図１２に示す。このパルスシーケンスは、３次元スキャンの高速 Asymmetric ＳＥ法（高速ＳＥ法にハーフフーリエ法を組み合わせたスキャン法）のパルスシーケンスに基づいている。なお、図１２では、位相エンコード方向の傾斜磁場（スポイラーパルスを含む）の図示は省略されている。
【０１１８】
第１回目の撮像の場合、図１２におけるデータ収集シーケンスＳＱ_ａｃｑのみであり、事前シーケンスＳＱ_ｐｒｅは使用されない。つまり、事前シーケンスＳＱ_ｐｒｅを成すＭＴパルス列Ｐ_ＭＴおよび傾斜磁場スポイラーパルスＳＰ_Ｓ，ＳＰ_Ｒ，およびＳＰ_Ｅは第１回目のときには印加されない。このため、第１回目はＭＴパルス＝ｏｆｆの状態でスキャンされる。このＭＴパルス列は前述した第１の実施形態と同様に、分割化されたＭＴパルスで構成されている。
【０１１９】
これに対し、第２回目の撮像の場合、図１２におけるデータ収集シーケンスＳＱ_ａｃｑに先立ち、事前シーケンスＳＱ_ｐｒｅが印加される。つまり、ＭＴパルス＝ｏｎの状態でスキャンされる。
【０１２０】
なお、図１２には示していないが、この第２回目の撮像に用いるパルスシーケンスにおいて、データ収集シーケンスＳＱ_ａｃｑ（本スキャン）のＲＦパルスのスライス選択は不変とした状態で、分割化ＭＴパルスを印加する選択領域をスライス方向のみならず、位相エンコード方向または読出し方向に設定するように傾斜磁場を印加する構成にしてもよい。
【０１２１】
一例として、この図１２に示す３次元ＦＡＳＥ法のパルスシーケンスでは、実効エコー時間ＴＥeff＝１００ｍｓおよびエコー間隔ＥＴＳ＝５ｍｓに設定される。また、ＭＴパルスは、周波数オフセット＝１３００Ｈｚ，分割ＭＴパルス数＝５個（５個全部のＭＴパルスによるトータルのフリップ角＝８００°）に設定される。さらに、繰り返し時間ＴＲ＝３２４７ｍｓ、フリップパルス／フロップパルスのフリップ角度＝９０°／１４０°、マトリクスサイズ＝２５６×２５６、ＦＯＶ＝３７×３７ｃｍに設定される。さらに、肺野内の血流の縦横無尽の走行方向を考慮したとき、各回の撮像の画像データとして、位相エンコード方向を変えて複数回スキャンしたデータの平均値（例えば位相エンコード方向を９０°変えてスキャンして２回の画像データの平均加算値）を採用する手法を用いることが好適である。この手法は、例えば、“J. of Magn. Reson. Imaging (JMRI) 8: 503-507, 1998"で提案されている。
【０１２２】
まず、第１回目の撮像が以下のように実行される。ホスト計算機６は、図示しない所定のメインプログラムを実行する中で、入力器１３からの操作情報に応答して図１０に示す処理を実行する。
【０１２３】
これを詳述すると、ホスト計算機６は、最初に、適宜に決めたＥＣＧゲート法用の遅延時間Ｔ_DLを例えば入力器１３を介して入力する（ステップＳ２０）。次いで、ホスト計算機６は操作者が入力器１３から指定したスキャン条件（位相エンコードの方向、画像サイズ、スキャン回数、パルスシーケンス、ＥＣＧゲート法の遅延時間など）および画像処理法の情報（加算処理か最大値投影（ＭＩＰ）処理かなど）を入力し、それらの情報を制御データに処理し、その制御データをシーケンサ５および演算ユニット１０に出力する（ステップＳ２１）。
【０１２４】
次いで、スキャン前の準備完了の通知があったと判断できると（ステップＳ２２）、ステップＳ２３で息止め開始の指令を音声発生器１４に出力する（ステップＳ２３）。これにより、音声発生器１４は「息を十分に吸ってから息を止めて下さい」といった内容の音声メッセージを発するから、これを聞いた患者は息を十分に吸った状態で息を止めることになる。
【０１２５】
この後、ホスト計算機６はシーケンサ５に撮像開始を指令する（ステップＳ２４および図１１参照）。
【０１２６】
シーケンサ５は、図１１に示す如く、撮像開始の指令を受けると（ステップＳ２４−１）、ＥＣＧ信号の読み込みを開始し（ステップＳ２４−２）、ＥＣＧ信号におけるＲ波（参照波形）のピーク値の所定ｎ回目の出現を、そのピーク値に同期させたＥＣＧトリガ信号から判断する（ステップＳ２４−３）。ここで、Ｒ波の出現をｎ回（例えば２回）待つのは、確実に息止め状態に移行した時期を見計らうためである。これにより、ｎ個目のＲ波の出現を待つ調整時間Ｔ_ｓｐが設定される（図１２参照）。
【０１２７】
所定ｎ回目のＲ波が出現すると、予め適宜に設定した遅延時間Ｔ_DLだけ待機する処理を行う（ステップＳ２４−４）。この遅延時間Ｔ_DLは、前述したように、対象とする肺野組織を撮像する上で最もエコー信号の強度が高くなり、そのエンティティの描出能に優れた値に最適化される。
【０１２８】
この最適な遅延時間Ｔ_DLが経過した時点が最適な心電同期タイミングであるとして、シーケンサ５は撮像を実行する（ステップＳ２４−５）。具体的には、既に記憶していたパルスシーケンス情報に応じて送信器８Ｔおよび傾斜磁場電源４を駆動し、３次元ＦＡＳＥ法のパルスシーケンスに基づく１回目のスキャンが図１２に示す如くＥＣＧゲート法（心電同期法）で実行される。ただし、事前シーケンスＳＱ_ｐｒｅに係るＭＴパルス列Ｐ_ＭＴは印加されない（ＭＴパルス＝ｏｆｆ）。これにより、最初のスライスエンコード量ＳＥ１の元で、約６００ｍｓ程度のスキャン時間で、図１３（ａ）に示す如く、肺野を含んで設定した３次元撮像領域Ｒ_ｉｍａからエコー信号が収集される。
【０１２９】
この１スライスエンコード量ＳＥ１に拠る１回目のスキャンが終了すると、シーケンサ５は、最終のスライスエンコード量ＳＥｎに拠るスキャンが完了したかどうかを判断し（ステップＳ４−６）、この判断がＮＯ（最終スキャンが済んでいない）の場合、ＥＣＧ信号を監視しながら、例えば撮像に使用したＲ波から例えば２心拍（２Ｒ−Ｒ）と、短めに設定した期間が経過するまで待機し、静止している実質部のスピンの縦磁化の回復を積極的に抑制する（ステップＳ２４−７）。つまり、この待機期間が繰返し時間ＴＲとなる。
【０１３０】
このように例えば２Ｒ−Ｒ分に相当する期間待って、例えば３個目のＲ波が出現すると（ステップＳ２４−７，ＹＥＳ）、シーケンサ５は前述したステップＳ２４−４にその処理を戻す。これにより、その３個目のＲ波ピーク値に同期したＥＣＧトリガ信号から指定遅延時間Ｔ_DLが経過した時点で２回目のスライスエンコード量ＳＥ２に拠るスキャンが前述と同様に実行され、３次元撮像領域Ｒ_ｉｍａからエコー信号が収集される（ステップＳ２４−４，５）。以下同様に、最終のスライスエンコード量ＳＥｎ（例えばｎ＝８）までスキャンが繰り返されてエコー信号が収集される。
【０１３１】
スライスエンコード量ＳＥｎに拠る最終回のスキャンが終わると、ステップＳ２４−６における判断がＹＥＳとなり、シーケンサ５からホスト計算機６に撮像の完了通知が出力される（ステップＳ２４−８）。これにより、処理がホスト計算機６に戻される。
【０１３２】
ホスト計算機６は、シーケンサ５からのスキャン完了通知を受けると（図１０、ステップＳ２５）、息止め解除の指令を音声発生器１６に出力する（ステップＳ２６）。このため、音声発生器１６は、例えば「息をして結構です」といった音声メッセージを患者に向けて発し、息止め期間が終わる。
【０１３３】
したがって、図１２のシーケンスで説明する如く、２Ｒ−Ｒ毎にＥＣＧゲート法に基づき、スライスエンコード量毎のスキャンがｎ回（例えば８回）実行される。このｎ回のスキャンに要する時間、すなわち患者に息止めを継続してもらう時間は撮像条件に拠って異なるが、一例として、２０〜２５秒程度である。
【０１３４】
患者Ｐから発生されたエコー信号は、各回のスキャン毎に、ＲＦコイル７で受信され、受信器８Ｒに送られる。受信器８Ｒはエコー信号に各種の前処理を施し、デジタル量に変換する。このデジタル量のエコーデータはシーケンサ５を通して演算ユニット１０に送られ、メモリで形成される画像の３次元ｋ空間に配置される。ハーフフーリエ法を採用していることから、収集しなかったｋ空間のデータは演算により求められ、埋められる。これによりｋ空間全部にエコーデータが配置される。
【０１３５】
この後、所定の待機時間が経過すると、第２回目の撮像が図１０および図１１で説明したと同様に実行される。ただし、図１０のステップＳ２１において、事前シーケンスＳＱ_ｐｒｅに係るＭＴパルス列Ｐ_ＭＴを印加する（ＭＴパルス＝ｏｎ）ための情報がホスト計算機６に与えられ、これを含むスキャン条件、画像処理法などの制御データがシーケンサ５に与えられる。これ以外のスキャン状態は、第１回目のそれと同じである。このため、図１１のステップＳ２４−５で実行される３次元ＦＡＳＥ法に拠るパルスシーケンスには、図１２に示す如く、事前シーケンスＳＱ_ｐｒｅとデータ収集シーケンスＳＱ_ａｃｑが含まれる。つまり、分割化した複数個のＭＴパルスが第１回目の撮像における各回のスキャン先頭に印加された状態で第２回目の撮像が実行される。
【０１３６】
この結果、第２回目の撮像により収集されたエコーデータ（原データ）も第１回目と同様に画像のｋ空間に配置される。
【０１３７】
このように２回の撮像によるエコーデータの収集が終わると、ホスト計算機６は演算ユニット１０に画像データの処理および表示を指令する。この一連の処理を図１４に示す。
【０１３８】
まず、演算ユニット１０は、第１回目の撮像により収集・配置されたｋ空間上のエコーデータに３次元フーリエ変換を施して、実空間の絶対値画像データＩＭ１に再構成する（図１４、ステップ３１）。同様に、第２回目のイメージングスキャンによるそれについても、同様の再構成を実行して実空間の絶対値画像データＩＭ２に再構成する（ステップＳ３２）。
【０１３９】
このように生成された肺野アキシャル像の画像データの信号値レベルを図１５（ａ），（ｂ）に模式的に示す（両者とも便宜的に２次元画像で示す）。同図（ａ）はＭＴパルスｏｆｆ時の画像ＩＭ１を示し、同図（ｂ）はＭＴパルスｏｎ時の画像ＩＭ２を示す。同図（ｂ）では、分割化したＭＴパルスの印加に伴うＭＴ効果によって信号値が低下しているが、その低下を実質部と血流とで比較した場合、実質部の低下の割合が血流のそれよりも大きい。そこで、同図（ｂ）では、低下割合が大きい実質部の部分にのみハッチングを付して模式的に表している。
【０１４０】
この信号値レベルの低下割合の比較を図１６（ａ），（ｂ）によりさらに説明する。第１の画像データＩＭ１はＭＴパルス＝ｏｆｆであるので、肺野ＬＧの実質部および血流の信号値レベルは、同図（ａ）に模式的に示す如く、撮像状態で決まるレベルになる。これに対し、第２の画像データＩＭ２は、分割化した複数のＭＴパルスを印加したデータである。このため、肺野ＬＧの実質部は、血流よりもＭＴ効果を大きく受け、図１６（ｂ）に模式的に示す如く、その信号値の低下割合ΔＴは大きく、血流の低下割合ΔＢよりも大きくなる。この低下割合の差＝「ΔＴ−ΔＢ」が肺野ＬＧの実質部の画像データになる。
【０１４１】
そこで、演算ユニット１０は、適宜な係数α（０＜α≦１）を用い、両方の画像データＩＭ１，ＩＭ２（絶対値データ）について画素毎に、
【数８】
ＩＭ１−α・ＩＭ２
の差分処理を実施する（図１４、ステップＳ３３）。この結果、図１５（ｃ）の画像データＩＭ３に模式的に示す如く（便宜的に２次元画像で示す）、肺野ＬＧの血流の信号値が差分によりほぼ相殺され、実質部が残る。
【０１４２】
このように生成された３次元の実空間の画像データＩＭ３に対して最大値投影（ＭＩＰ）処理を実行し、２次元の画像データを作成する（図１４、ステップＳ３４）。この画像データは表示器１２に画像として表示される一方で、記憶ユニット１１に格納される。また３次元の画像データＩＭ３も同様に格納される（ステップＳ３５）。
【０１４３】
以上のように、本実施形態のＭＲＩ装置によれば、一方の撮像時に、分割化した複数のＭＴパルスを用いたので、静止または殆ど静止している肺野実質部に分割ＭＴパルスの和として効く、大きなＭＴ効果を与えることができる。このＭＴ効果による信号値の低下割合は、肺野血流のそれよりも確実に大きくなる。したがって、ＭＴパルスのオフ時の撮像画像との差分処理（単なる差分処理でも、重み付け差分処理でも可能）を行うことによって、肺野実質部を画像化できる。つまり、従来、ガス、造影剤、酸素などを使用しないで撮像することは不可能とされていた肺野の実質部を良好に画像化することができる。
【０１４４】
このため、造影剤などを投与しなくても済むので、非侵襲に撮像でき、患者の精神的、体力的な負担が著しく軽くなる。同時に、造影効果のタイミングを計る必要があるなど、造影法固有の煩わしさからも解放されるとともに、造影剤法と違って、必要に応じて容易に繰返し撮像が可能になる。また、造影剤やガスを使用しないため、イメージングに要する検査コストの点でも有利である。
【０１４５】
また、本実施形態では、傾斜磁場スポイラーパルスを、分割した複数のＭＴパルスの最終段に1回だけ印加し、その前に複数個のＭＴパルスのみを順次印加するようにしている。このため、ＭＴパルス間の待機時間を少なく（または殆ど零）できるので、ＭＴパルス相互間における磁化スピンの縦緩和の回復量も小さくなる。これにより、より大きな信号値を得ることができる。
【０１４６】
さらに、繰返し時間ＴＲおよびエコー間隔を短く設定し、また、スライス方向を例えば患者の前後（前から背中に抜ける）方向にとることができる。これにより、全体のスキャン時間が短くなること、および、スライス方向の撮像長さが短くなってスライスエンコード回数が少なくて済むので、全体の撮像時間が従来のＴＯＦ法や位相エンコード法に比べて大幅に短縮される。これにより、患者の負担も少なく、患者スループットも上がる。
【０１４７】
これに付随して、２回の撮像それぞれのスキャン（目的としたエコーデータ群を収集するための撮像）が１回の息止め可能期間内に終わることができるから、患者の負担も著しく少なくなる。
【０１４８】
さらに、上述した実施形態の場合、各回の息止め期間に、目的としたエコーデータ群を収集するための撮像全体を終える。このため、肺などの周期的運動による体動アーチファクトの発生を抑制できるとともに、複数回にわたって息止め撮像をするときの患者の体自体の位置ずれに因る体動アーチファクトの発生も合わせて低減できる。これにより、アーチファクトのより少ない高品質の画像を提供できる。
【０１４９】
さらに、ＥＣＧゲート法を併用しているので、心臓の動きに因る体動アーチファクトを殆ど排除した画像データを得ることもできる。
【０１５０】
この実施形態に係る心電同期法はＲ派から遅延時間Ｔ_DLだけ遅らせた時相でスキャン開始する構成としたが、このスキャン開始の時相は個々の臨床上の要求に応じて、これ以外のタイミングに設定してもよい。
【０１５１】
なお、上述した実施形態に記載の内容は、請求項記載の発明を実施するときの例示的な態様に過ぎず、当業者であれば本発明の趣旨を逸脱しない範囲で種々の態様に変更、変形して実施できることは勿論である。
【０１５２】
【発明の効果】
以上説明したように、本発明のＭＲＩ装置の１つの態様によれば、従来使用していたスライス選択的で印加時間の長く且つ大きなフリップ角に設定したＭＴパルスに代えて、印加時間が短く且つ小さなフリップ角に設定した、複数個のＭＴパルスを使用するので、この複数個のＭＴパルスを印加している間に流れている血流（血液）に対しては、見掛け上のＴ1時間が短縮され、その分、ＭＴ効果が緩和されて信号値が高くなり、その一方、静止または殆ど静止している血流や実質部に対しては、個々のＭＴ効果の和としてのＭＴ効果が効くので、静止または殆ど静止している対象と動いている対象の差別化が顕著になり、従来法に比べて、撮像面に流入する血流（血液）と撮像面の実質部とのコトントラストが格段に向上し、血流描出能の高いＭＲＡ像を提供することができる。
【０１５３】
また、本発明のＭＲＩ装置の別の態様によれば、ＭＴパルスおよび傾斜磁場スポイラーパルスを印加させない状態で撮像したＭＲ信号による画像データと、ＭＴパルスおよび傾斜磁場スポイラーパルスをそれぞれ印加して撮像したＭＲ信による画像データとから、例えばデータ間相互の画素毎の差分演算などにより、ＭＲ像データが生成される。このため、分割化した複数個のＭＴパルスによるＭＴ効果が血流よりも実質部に大きく効くという現象を存分に発揮させ、これにより、例えば、従来、造影剤などを投与しなければ不可能とされていた肺野実質部のＭＲ画像を、造影剤などの侵襲性の高い処置を行わずに、提供することができる。つまり、非侵襲で簡単に撮像することができる。
【０１５４】
また、この撮像に息止め法や心電同期法を併用することで、体動アーチファクトなどの少ない高画質のＭＲ像を提供できる。
【図面の簡単な説明】
【図１】本発明の実施形態に係るＭＲＩ装置の構成例を示す機能ブロック図。
【図２】第1の実施形態のパルスシーケンスの一例を示すタイミングチャート。
【図３】（ａ），（ｂ）は診断部位別の撮像面と事前励起面との位置関係を示す図。
【図４】スピン間のデカップリングを説明する図。
【図５】Ｔ1 緩和時間などのＴc 依存性を説明する図。
【図６】動きのある対象の自由水、高分子のスピンに対する分割化ＭＴパルスに拠るＭＴ効果を説明する図。
【図７】分割化ＭＴパルスの印加に係る別の例を示す部分的なパルスシーケンス。
【図８】分割化ＭＴパルスの印加に係る更に別の例を示す部分的なパルスシーケンス。
【図９】第２の実施形態に係る２回の撮像の時間的前後関係と各撮像に並行して実施される手法とを説明する図。
【図１０】ホスト計算機が実行する第１回目の撮像の手順を例示する概略フローチャート。
【図１１】ＥＣＧゲート法を併用した撮像の流れを示すタイミングチャート。
【図１２】ＥＣＧゲート法に基づく２回の撮像のパルスシーケンスを示す概略図。
【図１３】３次元撮像領域と論理軸方向との関係を説明する図。
【図１４】エコー収集後の画像処理の一例を示す概略フローチャート。
【図１５】画像処理の一過程である重み付け差分処理を説明する図。
【図１６】実質部と血流とに対するＭＴ効果の効き方の差異を説明する模式図。
【図１７】従来のＭＴパルスに拠るＭＴ効果を説明する図。
【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
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
１３ 入力器
１６ 音声発生器
１７ ＥＣＧセンサ
１８ ＥＣＧユニット[0001]
BACKGROUND OF THE INVENTION
  The present invention obtains a blood flow image and a substantial part image inside the subject based on the magnetic resonance phenomenon of the nuclear spin of the subject.Regarding MRI equipment. In particular, an image with improved contrast between blood (or blood flow) and the substantial part is obtained using MT (magnetization transfer) pulses.Regarding MRI equipment.
[0002]
As used herein, “blood (or blood flow)” is used to mean “fluid” representing cerebral spinal fluid, blood (blood flow), and the like flowing in the subject.
[0003]
[Prior art]
Magnetic resonance imaging (MRI) is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with a high-frequency signal having a Larmor frequency, and an image is obtained based on an MR signal generated by the excitation. is there.
[0004]
  One field of this magnetic resonance imaging is MR angiography (MR angiography). As one of the conventional methods of MR angiography, a phase method using a pulse called a flow encode pulse is known. As another approach to this MR angiography, the MT effect (sometimes called the MTC (magnetization transfer contrast) effect) is used to provide a blood flow with a contrast between the blood (blood flow) and the substantial part. In recent years, techniques for obtaining images have been actively used. One example isUS Pat. No. 5,050,609(Magnetization Transfer Contrast and Proton Relaxation and Use thereof in Magnetic Resonance Imaging).
[0005]
The study of MT effect is based on ST (saturation transfer) method (Forsen et al., Journal of Chemical Physics, Vol.39 (11), pp.2892-2901 (1963)) based on 'Forsen & Hoffman'. And based on, for example, chemical exchange and / or cross relaxation between protons between free water and macromolecules as multiple nuclear pools.
[0006]
Several techniques have been proposed for conventional MR angiography using the MT effect as follows.
[0007]
17A and 17B show the frequency spectrum of free water and polymer on the left side and the exchange / relaxation relationship of magnetization H on the right side. Looking at the free water and polymer proton spectra, as shown in the figure, free water with a long T2 relaxation (lateral relaxation) time (T2 is about 100 msec) and a polymer with a short T2 relaxation time (T2 is about 0.2). There is a region where 1 to 0.2 msec) resonates at the same frequency. Since the T2 relaxation time of the free water signal value is long, the signal value after the Fourier transform shows a peak with a narrow half-value width as shown in the figure. On the other hand, the proton signal value, such as protein, that is restricted between macromolecules, has a short T2 relaxation time, so the signal value after Fourier transformation has a wide half-value width and peaks on the spectrum. It hardly appears as a value.
[0008]
Therefore, free water resonance peak frequency f₀Is the center frequency, as shown in the left column of FIG.₀For example, a frequency band shifted by 500 Hz is excited (off-resonance excitation). Thus, as shown in the right column, the free water magnetization between the free water magnetization Hf and the polymer magnetization Hr in an equilibrium state as shown in the right column of FIG. Hf moves to the magnetization Hr of the polymer. As a result, the MR signal value of free water protons decreases as shown in the left column of FIG. Therefore, there is a difference in the signal value between the site where chemical exchange and / or cross relaxation between free water and macromolecules is reflected and the site where it is not. A contrast difference can be made between the blood flow and the blood flow image.
[0009]
The MR angiography method based on the MT effect is currently roughly classified into a spatially non-selective imaging method and a slice-selective imaging method. be able to.
[0010]
As the former, for example, as proposed in "GBPike, MRM 25, 327-379, 1992", a frequency-selective binomial pulse is applied spatially non-selectively as the MT pulse, A contrast between the substantial part and the blood flow is obtained based on “MT effect of the substantial part> MT effect of the blood flow”.
[0011]
As an example of the latter, the method proposed in "M.Miyazaki, MRM 32, 52-59, 1994" is known. A slice-selective MT pulse is formed by an RF excitation pulse having a long application time and a gradient magnetic field spoiler pulse. By applying this pulse, a signal from a substantial part (stationary part) of the imaging surface is more than the blood flow by the MT effect. In this method, the MT effect of the blood flow flowing into the imaging surface is greatly reduced (the decrease in the signal from the blood flow is less than that of the substantial part), and the contrast between the blood flow and the substantial part is derived.
[0012]
[Problems to be solved by the invention]
However, in the case of the MR angiography using the slice-selective MT pulse described above, the state of “substantial MT effect> MT effect of blood flow” is maintained, but the spin flip angle associated with the MT pulse application is Since it is set to be a large value (for example, 500 ° to 1000 °), the MT effect on the blood flow flowing into the imaging surface is also considerably large. As a result, the MR signal value from the blood flow on the imaging surface is considerably reduced, so that the blood flow / substantial contrast difference is sufficiently satisfied to meet the recent needs for higher resolution. I couldn't say it.
[0013]
The present invention was made to overcome the above-described current situation of MR imaging using slice-selective MT pulses, and differentiated between a moving blood flow and a stationary blood flow or substantial part. MT pulse is used to reduce the effect of MT effect on blood flow, and the contrast between blood flow / substantial part is greatly improved compared to the conventional method, providing a blood flow image and a real part image with high rendering ability. The purpose is to do.
[0014]
[Means for Solving the Problems]
  In order to achieve the above object, an MRI apparatus of the present invention is an MRI apparatus for obtaining an MR image of an imaging region of a subject, and a plurality of MT pulses that are respectively excited at a frequency different from a frequency defining the imaging region. MT pulse applying means for applying, and spoiler applying means for applying a gradient magnetic field spoiler pulse after applying the MT pulseEach of the plurality of MT pulses is an RF pulse that excites spins in a slice region determined by the frequency of the MT pulse, and is applied in parallel with the RF pulse and is a gradient magnetic field for selecting the slice region. Gradient magnetic field applying means for applying a pulse and correspondingly rising and falling of each of the plurality of MT pulses is provided.It is characterized by that.The MRI apparatus of the present invention is an MRI apparatus that obtains an MR image of an imaging region of a subject, and applies an MT pulse that applies a plurality of MT pulses that are excited at a frequency different from a frequency that defines the imaging region. And a spoiler applying means for applying a gradient magnetic field spoiler pulse after applying the MT pulse, each of the plurality of MT pulses being an RF pulse for exciting a spin in a slice region determined by the frequency of the MT pulse. Applying a gradient magnetic field pulse applied in parallel with the RF pulse and selecting the slice region, and corresponding to the first rise and fall of the plurality of MT pulses, the gradient magnetic field Gradient magnetic field applying means for raising and lowering the pulse is provided.
[0015]
As an example, it is desirable to include an imaging unit that collects MR signals from the imaging region by executing a scan based on a data acquisition sequence after applying the gradient magnetic field spoiler pulse.
[0017]
  Furthermore, preferably,The application time of each of the plurality of MT pulses is about 1300 μsec, and the flip angle of the spin excited by each of the plurality of MT pulses is set to a value of about 90 ° to 100 °.
[0018]
Also preferably, the gradient magnetic field spoiler is configured to apply in any one direction, any two directions, or all three directions among the slice direction, the readout direction, and the phase encoding direction.
[0022]
As described above, a plurality of divided MT pulses are applied to a slice region different from the imaging region, with each application time being short and the flip angle being set to a small value, and the imaging region is off-resonance. Excited by. This divided MT pulse shortens the apparent T1 time of the flowing blood flow. That is, each divided MT pulse is turned on and off to promote the longitudinal relaxation (T1) of the moving blood flow spin and accelerate the return to the steady state. In other words, the MT effect on the free blood flow until the final divided MT pulse application is reduced, and the total MT effect is reduced. Therefore, the signal value of the blood flow flowing into the imaging region from the slice region to which the MT pulse is applied is higher than that in the conventional MT pulse application.
[0023]
On the other hand, the substantial part is usually not moving, or even if it is moving, the amount of movement is small. Therefore, the divided MT pulses act as the sum of them to obtain the desired MT effect. Therefore, the signal value from the substantial part is lowered.
[0024]
As a result, the contrast of the blood flow / substantial portion of the MRA image in the imaging region is improved by the amount corresponding to the effect of shortening the T1 time for the moving blood flow as compared with the conventional slice selective MT pulse application method. The blood flow visualization ability will be excellent.
[0025]
One aspect of the MRI apparatus that performs imaging based on the above principle is as follows.
[0026]
  That is, the MRI apparatus performs scanning based on the first pulse sequence in a state where the MT pulse and the gradient magnetic field spoiler pulse are not applied in addition to the MT pulse applying unit and the spoiler pulse applying unit. Collecting the first MR signal from the region;1An imaging means, and a second imaging means for acquiring a second MR signal from the imaging region by executing a scan based on a second pulse sequence after applying the MT pulse and the gradient magnetic field spoiler pulse, respectively. And image generating means for generating the MR image from the first and second echo signals.
[0028]
  For example, the plurality of MT pulses are arranged adjacent to each other on the time axis..
[0030]
More preferably, the image generation means is means for performing a difference calculation between image data based on the first and second echo signals.
[0031]
Still another preferred configuration is provided with breath holding command means for commanding the subject to hold his / her breath during imaging by the first and second imaging means. On the other hand, the first and second imaging means include a detection means for detecting a signal representing a cardiac time phase of the subject, and a synchronization means for executing the first and second pulse sequences in synchronization with the signal. You may have.
[0032]
More preferably, the first and second pulse sequences are the same two-dimensional scan or three-dimensional scan pulse sequence, and the pulse sequences are SE method, FSE method, FASE method, FE method, FFE method, This is a configuration that is one of the segmented FFE method and the EPI method.
[0033]
Furthermore, for example, the imaging region of the subject including the imaging region is a lung field.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described below with reference to the accompanying drawings.
[0035]
[First Embodiment]
A first embodiment will be described with reference to FIGS.
[0036]
A schematic configuration of an MRI (magnetic resonance imaging) apparatus according to this embodiment is shown in FIG.
[0037]
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 / calculation unit responsible for overall system control and image reconstruction, an electrocardiogram measurement unit that measures an ECG signal as a signal representing the cardiac phase of the subject P, and diagnosis of breathing in the subject (patient) And a breath-holding designation unit for instructing the person to temporarily stop.
[0038]
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 direction₀Is 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 host computer to 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.
[0039]
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 that are orthogonal to each other and generate gradient magnetic fields in the X, Y, and Z axis directions as gantry physical axes. . 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.
[0040]
By controlling the pulse current supplied from the gradient magnetic field power source 4 to the x, y, z coils 3x to 3z, the gradient magnetic fields in the X, Y, and Z directions, which are physical axes, are synthesized and the 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_RCan be set and changed arbitrarily. The slice direction, the phase encoding direction, and the reading direction are logical axis directions orthogonal to each other, and the gradient magnetic field in each direction is a static magnetic field H.₀Is superimposed on.
[0041]
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 operate under the control of the sequencer 5 described later. The transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for causing magnetic resonance (MR). The receiver 8R takes in the MR signal (high frequency signal) received by the RF coil 7, and performs various signal processing on the MR signal (high frequency signal) to generate digital data (original data).
[0042]
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, an input device 13, and a sound generator 16.
[0043]
Among these, the host computer 6 has a function of commanding the pulse sequence information to the sequencer 5 according to the stored software procedure and overseeing the operation of the entire apparatus.
[0044]
The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, controls the operations of the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to this information, The decibel data of the MR signal output from the receiver 8R is temporarily received and transferred to the arithmetic unit 10.
[0045]
Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R 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.
[0046]
The pulse sequence may be a two-dimensional scan or a three-dimensional scan, and the pulse train may have a SE (spin echo) method, an FE (field gradient echo) method, or an FSE (fast). Any pulse train such as SE) method, FASE (Fast Asymmetric SE) method or the like may be used.
[0047]
In this MRI apparatus, as will be described later, under the control of the host computer 6, the sequencer 5 drives the RF pulse as the MT pulse and the logical axis direction (any of the slice direction, phase encoding direction, and reading direction). A pre-sequence including a gradient magnetic field spoiler to be applied is applied prior to the data acquisition sequence (also referred to as this sequence).
[0048]
This MT pulse is set to be applied by slice selection. That is, the MT pulse is set as a plurality of RF pulses for excitation formed by, for example, a sinc function as shown in FIG. 2, and in parallel with the application of each MT pulse, the slice gradient magnetic field G_S Is applied. The number of applied MT pulses is a plurality of n (for example, 10), and the flip angle is a smaller divided value (for example, about 90 ° to 100 °) than a large value (500 ° to 1000 °) by the conventional MT pulse. Set to This MT pulse is formed, for example, by modulating an RF signal having a desired frequency offset value with a sinc function. By applying this MT pulse, the substantial part of the imaging region and the moisture are subjected to the MT effect, and the signal value of the substantial part is significantly lower than the moisture, and the contrast between the two is increased.
[0049]
This MT pulse and slice gradient magnetic field G_SAre sequentially applied, and then a gradient magnetic field spoiler pulse for spin phase dephasing is simultaneously applied in the slice direction, the phase encoding direction, and the reading direction.
[0050]
The arithmetic unit 10 receives the digital data of the MR signal from the receiver 8R and inputs the original data (also referred to as raw data) into the Fourier space (also referred to as k space or frequency space) of the image formed by the built-in memory. And two-dimensional or three-dimensional Fourier transform processing for reconstructing the original data into a real space image.
[0051]
The storage unit 11 can store the original data and the image data subjected to the reconstructed image data. The display device 12 displays an image. Information such as scan conditions and pulse sequences can be input to the host computer 6 via the input unit 13.
[0052]
The voice generator 14 is an element that functions as a part of the breath-hold specifying unit, and can issue a breath-hold start and breath-hold end message as a voice when instructed by the host computer 6.
[0053]
Further, the electrocardiogram measurement unit attaches to the body surface of the subject and detects an ECG signal as an electrical signal, and performs various processing including digitization processing on the sensor signal to perform the host computer 6 and the sequencer. And an ECG unit 18 for outputting to the PC. The measurement signal from the electrocardiogram measurement unit is used by the host computer 6 as a timing signal when executing the scan sequence. As a result, the synchronization timing for electrocardiogram synchronization can be set appropriately, and the MR original (raw) data can be collected by performing an electrocardiogram synchronization scan based on the set synchronization timing.
[0054]
Subsequently, the operation of the MRI apparatus of the present embodiment will be described with reference to FIGS.
[0055]
In the MRI apparatus of this embodiment, the MR angiography pulse sequence shown in FIG. 2 is executed based on a command from the sequencer 5.
[0056]
This pulse sequence is a pre-sequence SQ that is executed first in each RF excitation as shown in FIG._preAnd the subsequent data collection sequence SQ to be executed_acqIt consists of.
[0057]
Pre-sequence SQ_preIs the MT pulse train P causing the MT effect_MTAnd this MT pulse train P_MTGradient magnetic field spoiler pulse SP applied after application_S, SP_R, SP_EIncluding. MT pulse train P_MTAre a plurality of excitation RF pulses P sequentially applied as MT pulses.₁ , P₂ , P_Three , ..., P_nAnd a gradient magnetic field pulse G for slicing applied in parallel with these MT pulses._SIt consists of.
[0058]
Gradient magnetic field pulse G for slicing_S Applied intensity = G_S1As shown in FIG. 3 (a) or (b), the slice selection surface due to this magnetic field is the imaging surface S._imaIt is set to be in a position with a gapless or a gap different from.
[0059]
Each MT pulse P₁ (P₂ , P_Three , ..., P_n) Is formed by a SINC function as an example, and the intensity is set so that the spin flip angle FA accompanying this pulse application is, for example, 90 °. MT pulse P₁ , P₂ , P_Three , ..., P_nThe total number is set to 10 as an example.
[0060]
That is, in this embodiment, instead of the conventional configuration in which one MT pulse having a large flip angle FA (for example, 500 ° to 1000 °) is applied by slice selection, this MT pulse is divided into a plurality of pieces, and sequentially. The configuration of the MT pulse train to be applied is adopted.
[0061]
Each MT pulse P₁ (P₂ , P_Three , ..., P_n) Is a value (a preferable example is 90 ° to 100 °) divided so as to cause a desired MT effect in the entire MT pulse train, and the number thereof is also the MT effect of the entire MT pulse train. In addition, the number is appropriately determined (5 to 10) depending on the balance with the imaging time. The application time of each divided MT pulse is about 1300 μsec, which is shorter than the conventional slice selection MT pulse by the divided amount.
[0062]
Further, the time interval Δt between the divided MT pulses in the MT pulse train is set to a value that can optimize the MT effect of water / fat on the substantial part of the MT pulse application surface (see FIG. 3). This time interval Δt varies depending on the measurement site, and in some cases, Δt = 0 can be set.
[0063]
On the other hand, the gradient magnetic field spoiler pulse SP placed in three directions of the slice direction, readout direction, and phase encode direction_S, SP_R, SP_EIs the pre-sequence SQ_preUsed as an end spoiler. For this reason, gradient magnetic field spoiler pulse SP_S, SP_R, SP_EIn each of the above, after applying a plurality of divided MT pulses, the spin phase is dispersed in each direction, so that interference of the spin phase between the pre-sequence and the data acquisition sequence is eliminated, and generation of pseudo echoes is prevented. I have to. The spoiler pulse may be applied only in one arbitrary direction or two directions.
[0064]
Data collection sequence SQ_acqIs the gradient magnetic field G for slicing in each logical axis direction_S, Gradient magnetic field for reading G_R, And gradient magnetic field G for phase encoding_EFor example, the FSE method is adopted.
[0065]
The host computer 6 executes a predetermined main program and applies each pulse of the pulse sequence shown in FIG. This pulse application is performed via the x, y, z coils 3x to 3z and the RF coil 7 under the control of the sequencer 5.
[0066]
First, the pre-sequence SQ_pre, N (n ≧ 2: for example, n = 10) MT pulses P with a flip angle FA = α ° (for example, 90 °).₁~ P_n Gradient magnetic field G for slicing_S(= Strength G_S1) And sequentially applied. This MT pulse P₁~ P_nEach is formed by a sink function, for example.
[0067]
For example, if the diagnosis site is the lower limb as shown in FIG._S= G_S1For example, by setting the intensity appropriately, a desired imaging surface S_ima Parallel pre-excitation surface S of a predetermined thickness substantially adjacent to the inflow side of the artery_mtIs set. As a result, the pre-excitation surface S_mtThe MT pulses divided into n are sequentially applied every predetermined time Δt.
[0068]
Gradient magnetic field for slice G_SFor example, by adjusting the intensity of the pre-excitation surface S_mtShooting surface S_imaIt can be set to the arterial outflow side, ie, the venous inflow side. In addition, the shooting surface S_imaAnd pre-excitation surface S_mtA gap may be provided between the two as necessary, or a gapless state may be set.
[0069]
Furthermore, the pre-sequence SQ_preImmediately after applying the divided MT pulse, the gradient magnetic field spoiler pulse SP is applied in each of the slice direction, readout direction, and phase encoding direction._S , SP_R , And SP_EAre applied respectively.
[0070]
In this way, first, the pre-excitation surface S_mtAre selectively excited in a plurality of times by a plurality of divided MT pulses, that is, MT pulses having a smaller flip angle and shorter application time than MT pulses according to the conventional method. As a result, the pre-excitation surface S_mtIn the imaging surface S._imaSince it is off-resonance, the MT effect peculiar to the present invention as described later is applied to the imaging surface S._imaTo bring. The spin remaining in the transverse magnetization by the excitation of the plurality of divided MT pulses is converted into the subsequent spoiler pulse SP._S , SP_R, And SP_EFully dispersed by.
[0071]
After that, the data collection sequence SQ_acqAs an example, scanning based on the FSE method is performed on the imaging surface S._imaIs issued from the sequencer 5. By this sequence, the gradient magnetic field for slice G_S= G_S2(≠ G_S1) Is set, the imaging surface S_ima Is set to the desired slice position. Imaging surface S_imaA plurality of echo signals in response to a plurality of refocus RF pulses are collected via the RF coil 7 and sent to the receiver 8R. This series of imaging processes is executed for each excitation.
[0072]
Echo data collected from the subject P is processed into digital data by the receiver 8 </ b> R and sequentially stored in the arithmetic unit 10. In response to the reconstruction command from the host computer 6, the arithmetic unit 10 performs a two-dimensional Fourier transform on a set of echo data arranged in the two-dimensional Fourier space to obtain an imaging surface S._imaMRA images are reconstructed.
[0073]
This MRA image has few artifacts, and the image contrast between its inflow blood flow / substantial part is much improved as compared with the case where the conventional MT pulse is used. This is because the imaging surface S is obtained by using a plurality of divided MT pulses according to the present invention._imaThe echo signal from the substantial part (stationary part) of the image is reduced by the MT effect, and the imaging surface S_imaThis is due to the mitigation (reduction) of the MT effect occurring in the blood flow (arteries and / or veins) flowing into the bloodstream. That is, due to the short MT pulse divided into a plurality of times, the apparent longitudinal relaxation T1 time of the flowing or tumbling blood flow is shortened, and the effect of the MT effect is reduced. Since the substantial part (stationary part) has a signal value reduction effect corresponding to the sum of a plurality of divided MT pulses, the imaging surface S_ima The image contrast between the blood flow (blood) flowing into the body and the substantial part is significantly improved as compared with the conventional MT effect using one MT pulse.
[0074]
This feature will be described in detail below in terms of its principle.
[0075]
First, as an MT pulse, a blood that is flowing or tumbling when an RF pulse with a long application time is applied once and when an RF pulse with a short application time is applied multiple times continuously. Consider the MT effect on the flow.
[0076]
First, general factors that contribute to the T1 time will be described. While the longitudinal relaxation time T1 varies depending on the temperature, paramagnetic component, molecular size, its environment, viscosity, etc., it is expressed by the sum of the following factors between the components. It is known that
[0077]
[Expression 1]

[0078]
In the formula, T1 (DD) of the first item represents “internuclear dipole-dipole interaction”. This interaction is decoupled between the spins A and B of the coupled molecules by transferring the energy to the lattice by RF excitation, as schematically shown in FIG. And increase the signal value.
[0079]
The second item T1 (SR) represents "spin rotation". Bound molecules have a rotational movement. When a molecule rotates, depending on the magnitude of the movement, a partial “local magnetic field” is generated, which contributes to T1 shortening.
[0080]
The third item T1 (SC) represents "scaler coupling". In this coupling, one of the atoms to be coupled is Quadrupole (a quadrupole, an atom having a spin quantum number I ≧ 1, for example, O¹⁷= 5/2, m_n ⁵⁵= 5/2, H for reference¹= 1/2), the quadrupole has a unique short relaxation time called Quadrupole relation, and shortens the relaxation time of the spin coupled with the quadrupole.
[0081]
The fourth item T1 (CSA) is a term of “chemical shift anizotoropy”, and represents a change in local field due to a change in electron shielding effect. This change also affects T1.
[0082]
Thus, T1 time varies depending on various factors. There are other factors that affect T1 besides the factors described above, but the factors that have a large effect are the factors described above.
[0083]
Even in stationary blood, oxyhemogrobin and deoxyhemogrobin (oxidized and non-oxygenated hemoglobin) have paramagnetic ions (mainly iron I and iron II), so create a local magnetic field and lower T1 time. O₂The spin-spin relaxation (T2), veins (w oxygen, T2 = 120 msec), and arteries (less oxygen, T2 = 220 msec) also differ depending on whether or not there is.
[0084]
FIG. 5 shows the dependence of T1, T2 and T1P on effective correlation: Tc. In the figure, A is a hard solid, B is a soft solid, C is a liquid with high clay, and D is a normal liquid (TCFarrar and EDBecker, Pulse and Fourier Transform NMR, P.98, Academic Press (1971). )reference).
[0085]
Effective correlation: Tc is a factor representing the movement of the molecule, and indicates the degree of tumbling (rotation, vibration) of the fast-moving molecule. The further to the right on the horizontal axis in the figure, the higher the degree of solids and the slower the movement of molecules. From this relationship, it can be seen that the T1 time value differs between fast-flowing blood and almost stationary blood.
[0086]
Furthermore, the apparent longitudinal relaxation time T1 of the flowing blood is shortened by dividing and applying the MT pulse.
[0087]
As described above, the MT (magnetization transfer) effect means that the equilibrium state of the free water Hf in the vicinity of the polymer Hr in the dipole-dipole interaction relationship is RF-excited with an RF pulse at a frequency of off-resonance from the free water. Thus, the proton of the polymer is excited, thereby affecting the signal strength of free water interacting with the proton. That is, the free water magnetization is Hf, the polymer magnetization is Hr, and the reaction time constant is k.₁And k_-1Then,
[Expression 2]

It is expressed. [] Is concentration, K is a reaction constant.
[0088]
Therefore, if T1 time of free water is T1f and T1 time of polymer is T1r,
[Equation 3]

And M_{f (t)}: Time = magnetization of free water at time t, M_{f (0)}: Time of free water magnetization at 0:00, M_{r (t)}: Time = t-magnetization of polymer and M_{r (0)}: When the time = 0 is the magnetization of the polymer,
[Expression 4]

It becomes.
[0089]
Magnetization M when MT pulse is applied_fSATIs
[Equation 5]

And the longitudinal relaxation time T at that time_1SATIs
[Formula 6]

It becomes. Therefore, the reaction constant k₁Is
[Expression 7]

It becomes. Where T_1SATIs the apparent T1 (see "Balaban, Magn. Reson. Quarterly Vol. 8, No. 2, 1992").
[0090]
Therefore, the magnetization HfA of free water protons in moving blood, HrA the magnetization of protons of macromolecules in blood, HfB of the magnetization of free water protons in a stationary substantial part, and FIG. 6 schematically shows the magnetization behavior of the present invention accompanying the application of MT, where the magnetization of the protons in the substantial part of the polymer is HrB.
[0091]
First split MT pulse P for magnetization HfA of free water protons and polymer protons (see (a)) in equilibrium with the moving blood₁(Flip angle is 100 °, for example), the magnetization spin moves from the magnetization HrA (nuclear pool) of the polymer to the magnetization HfA (nuclear pool) of free water, and conversely, the magnetization of free water Energy is transferred from HfA to the magnetization HrA of the polymer (see FIG. 5B). Since these are divided MT pulses, their movement amounts are both small. That is, the MT effect is small.
[0092]
Due to longitudinal relaxation, the magnetization HrA of the polymer returns to the initial equilibrium state. At this time, as described above, the T1 time is apparently shortened by factors such as the dipole-dipole interaction and the paramagnetic effect, and the magnetization spin The return speed of HrA increases.
[0093]
MT pulse P divided in this way₂, P₃, ..., P_nAre sequentially applied, but the total MT effect is apparently reduced by the shortened T1 time (see FIGS. 6C and 6D). Therefore, the MT effect on the flowing (moving) blood is smaller than that of the stationary or hardly moving blood or the substantial part, and the imaging surface S_imaThe intensity of the echo signal collected from is high.
[0094]
On the other hand, the MT effect generated between the proton magnetization HfB of the free water in the substantial part that is stationary or moves little is similar to the case of FIG. 7 described above. That is, in this case, it works as the sum of the individual divided MT pulses. For this reason, such an imaging surface S_ima In the substantial part, the effect of the MT effect equivalent to the conventional one is exhibited, and the collected echo signal is lowered similarly to the conventional one.
[0095]
Therefore, the MRI apparatus of this embodiment can differentiate and image a stationary or hardly moving object and a moving object using the slice-selective MT effect. The obtained MRA image can remarkably improve the contrast of blood (blood flow) / substantial portion on the imaging surface as compared with the conventional method in which an MT pulse having a long application time and a large flip angle is applied once. . As a result, it is possible to provide a higher-quality MRA image having a high ability to depict blood flow on the imaging surface.
[0096]
Furthermore, since the MRA image according to this embodiment does not use an MR contrast agent, it takes advantage of the non-invasive characteristics of normal MR imaging. For this reason, the patient's mental and physical burdens can be remarkably reduced as compared with MRA imaging using a contrast agent.
[0097]
In the example of the pulse sequence shown in FIG. 2, the slice gradient magnetic field G is applied when a plurality of divided MT pulses are applied._SAlthough a configuration in which a plurality of pulses are also applied is shown, instead of this configuration, as shown in FIG. 7, the slice gradient magnetic field G is continuously applied while a plurality of divided MT pulses are being applied._S It is also possible to set so that one pulse is applied. As a result, the MT pulse train P_MTThe time required for the application of is sufficient, and the entire imaging time can be shortened.
[0098]
Further, as another example of applying a plurality of divided MT pulses, there is a method shown in FIG. This technique applies a split MT pulse alone without applying gradient magnetic field pulses in any of the slice direction, readout direction, and phase encode direction, and then slice, read, and phase encode directions. The gradient magnetic field spoiler pulse is applied in any one or more directions. That is, the divided MT pulse is applied in a non-slice manner. Thereby, the divided MT pulse is effectively applied to a wide area, and is not limited to a slice or a slab. 7 and 8 described above, illustration of gradient magnetic fields (including spoiler pulses) in the reading direction and the phase encoding direction is omitted.
[0099]
In addition, the data acquisition sequence that can be used in the MRI apparatus of the present embodiment is not limited to the normal FSE method as described above, and imaging methods such as the FE method, the SE method, the EPI method, the FLAIR method, and the FASE method are used. It can also be adopted.
[0100]
[Second Embodiment]
A second embodiment will be described with reference to FIGS.
[0101]
This embodiment relates to an MRI apparatus capable of imaging a substantial part of a lung field using a plurality of divided MT pulses, the basic principle of which has been described above.
[0102]
In the field of MRI, three methods have been proposed as the main imaging method for lung fields. They include a method using hyper-polarized (xenon or helium) gas, a perfusion method using a contrast agent Gd-DTPA (eg, “Hatabu H, et al., MRM 36: 503-508, 1996), Is a method by oxygen inhalation using oxygen molecules (for example, the document “Edelman RR, et al., Nature Medicine, 2,11, 1236-1239, 1996).
[0103]
Of these, the first method using hyper-polarized (xenon or helium) gas is a method in which gas is inhaled into the lung field and imaged, for example, at the MR frequency of xenon (Xe) gas. In addition, perfusion using the second contrast agent Gd-DTPA is a technique for observing the perfused state of Gd-DTPA in the blood. The third method of inhaling oxygen molecules is based on a report that oxygen molecules are weakly paramagnetic but have sufficient signal changes on the alveolar surface to the water signal observable by MRI.
[0104]
However, the imaging method described above has the following problems. In the case of the first method using hyper-polarized (xenon or helium) gas, since it is necessary to hyper-polarize normal xenon gas, there is a problem that the cost is high. In addition, perfusion using the second contrast agent Gd-DTPA requires administration of a contrast agent, and therefore requires invasive treatment. First and foremost, there is a burden on the patient's mental and physical strength. large. Also, the inspection cost is high. Furthermore, the contrast agent may not be administered depending on the patient's constitution. Further, even with the third method of inhaling oxygen molecules, the signal change is not sufficient on the image, and not only a satisfactory image enough to be used for clinical and research, but even its form cannot be imaged.
[0105]
In the structure of the lung field, sponge-like alveoli, bronchi, pulmonary arteries, and pulmonary veins occupy most of the surface area, and air is present around them. The surface area of sponge-like alveoli is enormous, but it does not have a lot of free water inside and outside the cell like other organs (liver, kidney, etc.). Therefore, in the case of the lung field, the water signal that is the detection target in the MRI is only detected from the vascular system, and the water signal value as the lung field parenchyma is insufficient. The area around the alveoli is estimated not to be reflected as an MR signal because there are fewer water molecules than the surface area. Therefore, although the T2 value of the lung field = 80 ms (refer to the document “JMRI, 2 (S): 13-17,1992”) itself is comparable to other organs, the conventional MR In imaging, it is presumed that the lung parenchyma cannot be depicted unless a contrast agent or the like is used.
[0106]
Therefore, in this embodiment, the advantages of the plurality of divided MT pulses according to the present invention can be fully exhibited, and a lung field parenchyma which has been considered difficult in the past can be imaged without using a contrast agent or the like. Like that.
[0107]
The MRI apparatus according to this embodiment has a hardware configuration equivalent to that of the first embodiment, but differs in the scanning procedure as follows.
[0108]
Following the preparatory work such as positioning scanning (not shown) and inputting imaging conditions, the host computer 6 performs the first and second imaging at least twice as shown in FIG. Each of the two imaging operations is a two-dimensional or three-dimensional MR scan that collects a set of echo data necessary for image reconstruction. Each imaging is preferably performed by using a breath holding method in which the patient consciously holds his / her breath temporarily and an ECG gate method using an ECG signal.
[0109]
The pulse sequence that can be used for this imaging may be a two-dimensional (2D) scan or a three-dimensional (3D) scan as long as the Fourier transform method is applied. , SE method, FSE (fast SE) method, FASE (fast Asymmetric SE) method (that is, imaging method combining the fast SE method with the half Fourier method), FE method, fast FE method, segmented fast FE method, EPI (echo) Planar imaging) can be used.
[0110]
The arithmetic unit 10 inputs the digital data (original data) output from the receiver 8R through the sequencer 5, and stores the original data (raw data) in the Fourier space (also referred to as k space or frequency space) of the image on the internal memory. This original data is subjected to two-dimensional or three-dimensional Fourier transform for each set, and reconstructed into real space image data. The arithmetic unit 10 is also capable of performing data composition processing and difference calculation processing on images.
[0111]
This composition processing includes processing for adding image data of a plurality of frames for each corresponding pixel, maximum value projection (MIP) processing for selecting a maximum value for each corresponding pixel between the image data of a plurality of frames, and the like. As another example of the above synthesis process, the axes of a plurality of frames may be aligned in Fourier space, and the original data may be synthesized into one frame of original data. The addition processing includes simple addition processing, addition averaging processing, weighted addition processing, and the like.
[0112]
The storage unit 11 can store not only the reconstructed image data but also the image data that has been subjected to the above-described combining process and difference process. The display device 12 displays an image. In addition, information regarding imaging conditions, pulse sequences, image composition, and difference calculation desired by the operator can be input to the host computer 6 via the input unit 13.
[0113]
Moreover, the sound generator 16 is provided as an element of the breath-hold command unit. This voice generator 16 can emit a breath holding start message and a breath holding end message as voice when instructed by the host computer 6.
[0114]
Further, the electrocardiogram measurement unit attaches to the body surface of the subject and detects an ECG signal as an electrical signal, and performs various processing including digitization processing on the sensor signal to perform the host computer 6 and the sequencer. And an ECG unit 18 for outputting to the PC. The measurement signal from the electrocardiogram measurement unit is used by the sequencer 5 when performing imaging. Thereby, the synchronization timing by the ECG gate method (electrocardiogram synchronization method) can be set appropriately, and data can be collected by performing imaging of the ECG gate method based on this synchronization timing.
[0115]
Next, imaging operation by the MRI apparatus of this embodiment will be described with reference to FIGS.
[0116]
By executing a predetermined main program (not shown), the host computer 6 performs two imaging operations using a three-dimensional FASE (high-speed Asymmetric SE) method as an example, as shown in FIG. The first imaging is performed using both the breath holding method and the ECG gate method, and the MT pulse is not applied (off) as will be described later. At an appropriate timing after the completion of the first imaging, the second imaging is executed using both the breath holding method and the ECG gate method in the same manner as the first imaging. However, the second imaging is performed by applying an MT pulse (on).
[0117]
FIG. 12 shows both pulse sequence examples of the first and second imaging. This pulse sequence is based on a pulse sequence of a three-dimensional high-speed Asymmetric SE method (a scanning method combining a high-speed SE method and a half Fourier method). In FIG. 12, the illustration of the gradient magnetic field (including the spoiler pulse) in the phase encoding direction is omitted.
[0118]
In the case of the first imaging, the data collection sequence SQ in FIG._acqOnly, pre-sequence SQ_preIs not used. That is, the pre-sequence SQ_preMT pulse train P_MTAnd gradient magnetic field spoiler pulse SP_S, SP_R, And SP_EIs not applied at the first time. For this reason, the first scan is performed with MT pulse = off. This MT pulse train is composed of divided MT pulses as in the first embodiment.
[0119]
On the other hand, in the case of the second imaging, the data collection sequence SQ in FIG._acqPrior to the previous sequence SQ_preIs applied. That is, scanning is performed with MT pulse = on.
[0120]
Although not shown in FIG. 12, in the pulse sequence used for the second imaging, the data acquisition sequence SQ_acqA configuration in which a gradient magnetic field is applied so that a selection region to which a divided MT pulse is applied is set not only in a slice direction but also in a phase encoding direction or a reading direction in a state where selection of an RF pulse in (main scan) is unchanged. It may be.
[0121]
As an example, in the pulse sequence of the three-dimensional FASE method shown in FIG. 12, the effective echo time TEeff = 100 ms and the echo interval ETS = 5 ms are set. Further, the MT pulse is set such that the frequency offset = 1300 Hz and the number of divided MT pulses = 5 (total flip angle of all five MT pulses = 800 °). Further, the repetition time TR = 3247 ms, flip pulse / flop pulse flip angle = 90 ° / 140 °, matrix size = 256 × 256, and FOV = 37 × 37 cm are set. Furthermore, when taking into account the endless and transverse running direction of blood flow in the lung field, the average value of the data scanned a plurality of times by changing the phase encoding direction (for example, changing the phase encoding direction by 90 °) as image data for each imaging. It is preferable to use a technique that employs an average addition value of two times of image data after scanning. This technique is proposed in, for example, “J. of Magn. Reson. Imaging (JMRI) 8: 503-507, 1998”.
[0122]
First, the first imaging is performed as follows. The host computer 6 executes the processing shown in FIG. 10 in response to operation information from the input device 13 while executing a predetermined main program (not shown).
[0123]
In detail, the host computer 6 firstly determines the delay time T for the ECG gate method determined appropriately._DLIs input via the input device 13, for example (step S20). Next, the host computer 6 scans the scanning conditions (phase encoding direction, image size, number of scans, pulse sequence, ECG gate method delay time, etc.) specified by the operator from the input device 13 and image processing method information (addition processing or not). The maximum value projection (MIP) process, etc.) is input, the information is processed into control data, and the control data is output to the sequencer 5 and the arithmetic unit 10 (step S21).
[0124]
Next, when it is determined that there is a notice of completion of preparation before scanning (step S22), a command to start breath holding is output to the sound generator 14 in step S23 (step S23). As a result, the voice generator 14 issues a voice message such as “Please hold your breath after you breathe in,” so that the patient who hears this will hold his breath in a state where he has breathed in enough. Become.
[0125]
Thereafter, the host computer 6 instructs the sequencer 5 to start imaging (see step S24 and FIG. 11).
[0126]
As shown in FIG. 11, when the sequencer 5 receives an imaging start command (step S24-1), the sequencer 5 starts reading the ECG signal (step S24-2), and the peak value of the R wave (reference waveform) in the ECG signal. Is determined from the ECG trigger signal synchronized with the peak value (step S24-3). Here, the reason for waiting for the appearance of the R wave n times (for example, twice) is to estimate the time when the state is surely shifted to the breath holding state. Thereby, the adjustment time T for waiting for the appearance of the nth R wave_spIs set (see FIG. 12).
[0127]
When a predetermined n-th R wave appears, a delay time T set appropriately in advance_DLOnly waiting processing is performed (step S24-4). This delay time T_DLAs described above, the intensity of the echo signal is highest when imaging the target lung field tissue, and is optimized to a value excellent in the rendering ability of the entity.
[0128]
This optimum delay time T_DLThe sequencer 5 executes imaging (step S24-5), assuming that the time point after the elapse is the optimal ECG synchronization timing. Specifically, the transmitter 8T and the gradient magnetic field power source 4 are driven according to the pulse sequence information that has already been stored, and the first scan based on the pulse sequence of the three-dimensional FASE method performs the ECG gate method as shown in FIG. (Electrocardiogram synchronization method). However, prior sequence SQ_preMT pulse train P related to_MTIs not applied (MT pulse = off). As a result, the three-dimensional imaging region R set including the lung field as shown in FIG. 13A with a scan time of about 600 ms under the initial slice encoding amount SE1._imaEcho signals are collected from.
[0129]
When the first scan based on the one-slice encode amount SE1 is completed, the sequencer 5 determines whether the scan based on the final slice encode amount SEn is completed (step S4-6), and this determination is NO (final In the case of scanning not completed), while monitoring the ECG signal, for example, from the R wave used for imaging, for example, 2 heartbeats (2R-R) and waits until a short period has elapsed, and is stationary The recovery of the longitudinal magnetization of the substantial part of the spin is positively suppressed (step S24-7). That is, this standby period becomes the repetition time TR.
[0130]
Thus, for example, after waiting for a period corresponding to 2R-R, for example, when the third R wave appears (step S24-7, YES), the sequencer 5 returns the process to step S24-4 described above. As a result, the specified delay time T from the ECG trigger signal synchronized with the third R wave peak value is obtained._DLAt the time when elapses, the second scan based on the slice encoding amount SE2 is executed in the same manner as described above, and the three-dimensional imaging region R_imaEcho signals are collected from (step S24-4, 5). Similarly, the scan is repeated until the final slice encoding amount SEn (for example, n = 8), and echo signals are collected.
[0131]
When the last scan based on the slice encoding amount SEn is completed, the determination in step S24-6 is YES, and an imaging completion notification is output from the sequencer 5 to the host computer 6 (step S24-8). As a result, the processing is returned to the host computer 6.
[0132]
When the host computer 6 receives the scan completion notification from the sequencer 5 (FIG. 10, step S25), it outputs a breath holding release command to the voice generator 16 (step S26). For this reason, the voice generator 16 issues a voice message such as “You can breathe” to the patient, and the breath holding period ends.
[0133]
Therefore, as described in the sequence of FIG. 12, the scan for each slice encoding amount is executed n times (for example, 8 times) based on the ECG gate method every 2R-R. The time required for the n scans, that is, the time for the patient to keep holding his / her breath varies depending on the imaging conditions, but is about 20 to 25 seconds as an example.
[0134]
  Generated from patient PEcho signalIs received by the RF coil 7 for each scan and sent to the receiver 8R. The receiver 8R performs various preprocessing on the echo signal and converts it into a digital quantity. The digital amount of echo data is sent to the arithmetic unit 10 through the sequencer 5 and is arranged in the three-dimensional k-space of the image formed by the memory. Since the half Fourier method is adopted, k-space data not collected is obtained by calculation and filled. As a result, the echo data is arranged in the entire k space.
[0135]
Thereafter, when a predetermined waiting time has elapsed, the second imaging is performed in the same manner as described with reference to FIGS. However, in step S21 of FIG._preMT pulse train P related to_MT(MT pulse = on) is applied to the host computer 6, and control data including the scan conditions and image processing method including the information is applied to the sequencer 5. The other scan states are the same as those in the first scan. Therefore, the pulse sequence based on the three-dimensional FASE method executed in step S24-5 in FIG._preAnd data collection sequence SQ_acqIs included. That is, the second imaging is executed in a state where a plurality of divided MT pulses are applied to the head of each scan in the first imaging.
[0136]
As a result, the echo data (original data) collected by the second imaging is also arranged in the k-space of the image as in the first imaging.
[0137]
When the collection of echo data by two imaging operations is completed in this way, the host computer 6 instructs the arithmetic unit 10 to process and display image data. This series of processing is shown in FIG.
[0138]
First, the arithmetic unit 10 performs three-dimensional Fourier transform on the echo data in the k space collected and arranged by the first imaging, and reconstructs the absolute value image data IM1 in the real space (FIG. 14, step). 31). Similarly, for the second imaging scan, the same reconstruction is executed to reconstruct the absolute value image data IM2 in real space (step S32).
[0139]
The signal value levels of the image data of the lung field axial image generated in this way are schematically shown in FIGS. 15A and 15B (both are shown as a two-dimensional image for convenience). FIG. 4A shows an image IM1 when the MT pulse is turned off, and FIG. 4B shows an image IM2 when the MT pulse is on. In FIG. 5B, the signal value is reduced due to the MT effect associated with the application of the divided MT pulse. However, when the reduction is compared between the real part and the blood flow, the rate of decrease in the real part is blood. Greater than that of the flow. Therefore, in FIG. 4B, only the substantial part where the rate of decrease is large is hatched and schematically shown.
[0140]
The comparison of the signal value level reduction ratio will be further described with reference to FIGS. 16 (a) and 16 (b). Since the first image data IM1 is MT pulse = off, the signal value level of the substantial part of the lung field LG and the blood flow is a level determined by the imaging state as schematically shown in FIG. On the other hand, the second image data IM2 is data obtained by applying a plurality of divided MT pulses. For this reason, the substantial part of the lung field LG receives the MT effect more than the blood flow, and as shown schematically in FIG. 16B, the decrease rate ΔT of the signal value is larger than the decrease rate ΔB of the blood flow. Also grows. The difference of the reduction ratio = “ΔT−ΔB” is the image data of the substantial part of the lung field LG.
[0141]
Therefore, the arithmetic unit 10 uses an appropriate coefficient α (0 <α ≦ 1), and for each image data IM1, IM2 (absolute value data) for each pixel,
[Equation 8]
IM1-α ・ IM2
Is executed (FIG. 14, step S33). As a result, as schematically shown in the image data IM3 in FIG. 15C (shown as a two-dimensional image for convenience), the blood flow signal value of the lung field LG is almost offset by the difference, and the substantial part remains.
[0142]
Maximum value projection (MIP) processing is executed on the three-dimensional real space image data IM3 generated in this way to generate two-dimensional image data (step S34 in FIG. 14). The image data is displayed as an image on the display 12 while being stored in the storage unit 11. Similarly, the three-dimensional image data IM3 is stored (step S35).
[0143]
As described above, according to the MRI apparatus of the present embodiment, since a plurality of divided MT pulses are used during one imaging, the sum of the divided MT pulses is applied to a stationary or almost stationary lung field parenchyma. It is effective and can provide a large MT effect. The decrease rate of the signal value due to the MT effect is surely larger than that of the pulmonary blood flow. Therefore, the lung field parenchyma can be imaged by performing a difference process with a captured image when the MT pulse is off (either a simple difference process or a weighted difference process is possible). That is, the substantial part of the lung field, which has conventionally been impossible to image without using gas, contrast agent, oxygen, etc., can be imaged well.
[0144]
For this reason, since it is not necessary to administer a contrast medium or the like, non-invasive imaging can be performed, and the mental and physical burden on the patient is remarkably reduced. At the same time, it is freed from the troublesomeness inherent in contrast methods, such as the need to measure the timing of the contrast effect, and unlike the contrast agent method, repeated imaging can be easily performed as needed. Further, since no contrast agent or gas is used, it is advantageous in terms of inspection cost required for imaging.
[0145]
In the present embodiment, the gradient magnetic field spoiler pulse is applied only once to the final stage of the plurality of divided MT pulses, and only the plurality of MT pulses are sequentially applied before that. For this reason, since the waiting time between MT pulses can be reduced (or almost zero), the recovery amount of longitudinal relaxation of magnetization spins between MT pulses is also reduced. Thereby, a larger signal value can be obtained.
[0146]
Further, the repetition time TR and the echo interval can be set short, and the slice direction can be taken, for example, in the front-rear direction of the patient (from the front to the back). As a result, the overall scanning time is shortened, and the imaging length in the slice direction is shortened, so that the number of slice encodings can be reduced. Therefore, the overall imaging time is significantly larger than that of the conventional TOF method or phase encoding method. Shortened to This reduces the burden on the patient and increases patient throughput.
[0147]
Accompanying this, since each of the two imaging scans (imaging for collecting the target echo data group) can be completed within one breath-holding period, the burden on the patient is significantly reduced. .
[0148]
Furthermore, in the case of the above-described embodiment, the entire imaging for collecting the target echo data group is completed in each breath holding period. For this reason, it is possible to suppress the occurrence of body movement artifacts due to periodic movements of the lungs and the like, and to reduce the generation of body movement artifacts due to the positional deviation of the patient's body itself when performing breath-hold imaging multiple times. . As a result, a high-quality image with less artifacts can be provided.
[0149]
Furthermore, since the ECG gate method is used together, it is possible to obtain image data in which body motion artifacts due to the motion of the heart are almost eliminated.
[0150]
The electrocardiographic synchronization method according to this embodiment uses a delay time T from the R group._DLAlthough the scan is started at a time phase delayed by a certain amount, the time phase at the start of scan may be set at a timing other than this according to individual clinical requirements.
[0151]
It should be noted that the contents described in the above-described embodiments are merely exemplary aspects when carrying out the invention described in the claims, and those skilled in the art can change to various aspects without departing from the spirit of the present invention. Of course, it can be modified and implemented.
[0152]
【The invention's effect】
  As described above, according to one aspect of the MRI apparatus of the present invention, instead of the MT pulse that has been used in the past and is slice-selective and has a long application time and a large flip angle, the application time is short and Set a small flip angle,MultipleThis is because MT pulses are used.MultipleFor blood flow (blood) flowing during the application of the MT pulse, the apparent T1 time is shortened, and the MT effect is relaxed accordingly, and the signal value is increased. Or for almost static blood flow or parenchyma,IndividualSince the MT effect as a sum of the MT effects is effective, the distinction between a stationary or almost stationary object and a moving object becomes remarkable, and blood flow (blood) flowing into the imaging surface is compared with the conventional method. The dot trust with the substantial part of the imaging surface is remarkably improved, and an MRA image with high blood flow rendering ability can be provided.
[0153]
Further, according to another aspect of the MRI apparatus of the present invention, imaging is performed by applying the MT signal and the gradient magnetic field spoiler pulse respectively, and the image data based on the MR signal captured without applying the MT pulse and the gradient magnetic field spoiler pulse. The MR image data is generated from the image data based on the MR signal, for example, by calculating the difference between the data for each pixel. For this reason, the phenomenon that the MT effect by a plurality of divided MT pulses is more effective in the substantial part than the blood flow is fully exhibited, and this is impossible unless, for example, a contrast medium is conventionally administered. The MR image of the pulmonary parenchyma, which has been assumed, can be provided without performing a highly invasive treatment such as a contrast medium. That is, imaging can be performed easily and non-invasively.
[0154]
Further, by using the breath holding method and the electrocardiogram synchronization method together with this imaging, it is possible to provide a high-quality MR image with less body motion artifacts.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration example of an MRI apparatus according to an embodiment of the present invention.
FIG. 2 is a timing chart showing an example of a pulse sequence according to the first embodiment.
FIGS. 3A and 3B are diagrams showing a positional relationship between an imaging surface and a pre-excitation surface for each diagnostic region. FIGS.
FIG. 4 is a diagram for explaining decoupling between spins.
FIG. 5 is a diagram for explaining Tc dependency such as T1 relaxation time.
FIG. 6 is a diagram for explaining an MT effect due to a divided MT pulse with respect to free water and polymer spin of a moving object.
FIG. 7 is a partial pulse sequence showing another example related to application of a divided MT pulse.
FIG. 8 is a partial pulse sequence showing still another example relating to application of a divided MT pulse.
FIG. 9 is a diagram for explaining a temporal context of two imaging operations according to the second embodiment and a technique implemented in parallel with each imaging operation.
FIG. 10 is a schematic flowchart illustrating a first imaging procedure executed by the host computer.
FIG. 11 is a timing chart showing a flow of imaging using the ECG gate method together.
FIG. 12 is a schematic diagram showing a pulse sequence of two imaging operations based on an ECG gate method.
FIG. 13 is a diagram for explaining a relationship between a three-dimensional imaging region and a logical axis direction.
FIG. 14 is a schematic flowchart showing an example of image processing after echo collection.
FIG. 15 is a diagram illustrating weighted difference processing that is a process of image processing.
FIG. 16 is a schematic diagram for explaining the difference in the effect of the MT effect on the substantial part and blood flow.
FIG. 17 is a diagram for explaining an MT effect based on a conventional MT pulse.
[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
7 RF coil
8T transmitter
8R receiver
10 Arithmetic unit
11 Storage unit
12 Display
13 Input device
16 Sound generator
17 ECG sensor
18 ECG units

Claims (16)

In an MRI apparatus for obtaining an MR image of an imaging region of a subject,
MT pulse applying means for applying a plurality of MT pulses respectively excited at a frequency different from the frequency defining the imaging region;
A spoiler application means for applying a gradient magnetic field spoiler pulse after applying the MT pulse ,
Each of the plurality of MT pulses is an RF pulse that excites spins in a slice region determined by the frequency of the MT pulse, and is applied in parallel with the RF pulse and a gradient magnetic field pulse for selecting the slice region. And a gradient magnetic field applying means for raising and lowering the gradient magnetic field pulses one by one in response to rising and falling of each of the plurality of MT pulses . In an MRI apparatus for obtaining an MR image of an imaging region of a subject,
MT pulse applying means for applying a plurality of MT pulses respectively excited at a frequency different from the frequency defining the imaging region;
A spoiler application means for applying a gradient magnetic field spoiler pulse after applying the MT pulse,
Each of the plurality of MT pulses is an RF pulse that excites spins in a slice region determined by the frequency of the MT pulse, and is applied in parallel with the RF pulse and a gradient magnetic field pulse for selecting the slice region. And a gradient magnetic field applying means for raising and lowering the gradient magnetic field pulse corresponding to the first rise and fall of the plurality of MT pulses as a whole . 3. The MRI according to claim 1 , further comprising an imaging unit that collects MR signals from the imaging region by executing a scan based on a data acquisition sequence after applying the gradient magnetic field spoiler pulse. apparatus. 3. The invention according to claim 1 , wherein the application time of each of the plurality of MT pulses is about 1300 μsec, and the flip angle of the spin excited by each of the plurality of MT pulses is about 90 ° to 100 °. An MRI apparatus with a configuration set to 3. The MRI according to claim 1 or 2 , wherein the gradient magnetic field spoiler is applied in any one of the slice direction, the readout direction, and the phase encoding direction, in any two directions, or in all three directions. apparatus. 3. The method according to claim 1 , wherein a first MR signal is acquired from the imaging region by executing a scan based on a first pulse sequence in a state where the MT pulse and the gradient magnetic field spoiler pulse are not applied. And a second imaging unit that collects a second MR signal from the imaging region by executing a scan based on a second pulse sequence after applying the MT pulse and the gradient magnetic field spoiler pulse, respectively. And an image generating means for generating the MR image from the first and second echo signals. 3. The MRI apparatus according to claim 1 , wherein the plurality of MT pulses are arranged adjacent to each other on a time axis.   7. The MRI apparatus according to claim 6, wherein the image generation means is a means for performing a difference calculation between image data based on the first and second echo signals.   7. The MRI apparatus according to claim 6, further comprising a breath holding command unit that commands the subject to hold a breath during imaging by the first and second imaging units.   7. The invention according to claim 6, wherein the first and second imaging means detect a signal representing a cardiac time phase of the subject, and the first and second pulses in synchronization with the signal. An MRI apparatus having synchronization means for executing a sequence.   The invention according to claim 6, wherein the first and second pulse sequences are the same two-dimensional scan or three-dimensional scan pulse sequences, and the pulse sequences are SE method, FSE method, FASE method, FE method, An MRI apparatus that is one of an FFE method, a segmented FFE method, and an EPI method. 4. The MRI apparatus according to claim 3 , wherein the imaging unit executes an imaging scan with application of RF pulses for each of a plurality of heartbeats in synchronization with a signal representing the cardiac time phase of the subject detected by the time phase detection unit. An MRI apparatus configured to collect the MR signals. The MRI apparatus according to claim 3 , wherein the imaging unit performs a three-dimensional scan in which an operation of collecting echo signals for a predetermined slice encoding amount in synchronization with a signal representing a cardiac time phase of the subject is repeated for each plurality of heartbeats. An MRI apparatus configured to execute as the scan. 14. The MRI apparatus according to claim 13 , wherein the imaging unit is configured to collect the MR signal by executing an imaging scan in a coronal direction, and further includes an image generation unit that generates a coronal image from the MR signal. MRI equipment. 3. The MRI apparatus according to claim 1 , wherein the MT pulse application unit is configured to apply the plurality of MT pulses to a region different from the imaging region. 3. The MRI apparatus according to claim 1 , wherein the MT pulse applying unit is configured to set a region to which the plurality of MT pulses are applied on an artery inflow side or an artery outflow side of the imaging region. .

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