JP3911605B2 - Magnetic resonance imaging system - Google Patents

Description

次に、ＲＦ受信コイルの感度分布と画像について述べる。ｉ番目のＲＦ受信コイルの２次元的な感度分布を、ｃｉ（ｘ，ｙ）とすると、受信した信号ｓｉ（ｘ，ｙ）は、ＲＦ受信コイルの感度分布ｃｉ（ｘ，ｙ）と被検体のプロトン密度分布ｐ（ｘ，ｙ）との積であるから、式（２）で表せる。
【００２９】
【数２】
ｓｉ（ｘ，ｙ）＝ｃｉ（ｘ，ｙ）×ｐ（ｘ，ｙ） （２）
ここで、式（２）を用いると、式（１）は式（３）で表せる。
【００３０】
【数３】

ここで、簡単のため、
【００３１】
【数４】
Ｓｉ＝ｓ’ｉ（ｘ，ｙ’）
Ｃｉｊ＝ｃｉ（ｘ，ｙ’＋（ｊ−１）×Ｙ’） （４）
Ｐｊ＝ｐ（ｘ，ｙ’＋（ｊ−１）×Ｙ’）
とおくと、式（３）は、式（５）となる。
【００３２】
【数５】

式（５）は、Ｎ行２列の行列として、式（６）のように表すことができる。
【００３３】
【数６】

式（６）から、ＲＦ受信コイルの感度分布Ｃｉｊが分かれば、逆行列を計算することで、披検体の測定対象の核、例えば、プロトンの密度分布Ｐｊが求められる。
【００３４】
同様に、Ｎ個のコイルを用いて、Ｍ倍速で撮影を行った場合の一般式は、Ｙ’≡Ｙ／Ｍ、１≦ｙ’≦Ｙ’として、式（７）で表せる。ここで、ｂは定数である。
【００３５】
【数７】

このようにして、ＲＦ受信コイルの感度分布Ｃｉｊを用い、計測データｓ’ｉ（ｘ，ｙ’）について式（７）の行列演算をすることにより、折り返しが除去された画像データとして測定対象内の核スピンの密度分布Ｐｊが求められる。なお、パラレルイメージング法のＲＦ受信コイル数（Ｎ）と倍速数（Ｍ）との関係は、数学的にＮ≧Ｍである。
【００３６】
本実施形態のＭＲＩ装置におけるパラレルイメージング法では、図７に示すように、本計測シーケンスを実行して取得したマルチプルＲＦ受信コイル３０１の各ＲＦ受信コイル４０５１〜４０５３の折り返し画像７０３１〜７０３３の折り返しの除去は、信号処理部４０７において行われる。信号処理部４０７は、送受信兼用コイル４０４の感度画像７０１および各ＲＦ受信コイル４０５１〜４０５３の感度画像７０２１〜７０２３を用いて、感度分布算出処理７０４１〜７０４３を行い、各ＲＦ受信コイル４０５１〜４０５３の感度分布データ７０５１〜７０５３を得る。感度分布算出処理７０４としては、送受信兼用コイル４０４の画像データｓｃ（ｘ，ｙ）を基準に、例えば、式（８）により相対化して、各ＲＦ受信コイル４０５ｉの画像データｓｉ（ｘ，ｙ）により感度分布ｃｉ（ｘ，ｙ）を求める。
【００３７】
【数８】
ｃｉ（ｘ，ｙ）＝ｓｉ（ｘ，ｙ）／ｓｃ（ｘ，ｙ） （８）
このようにして算出した感度分布データ７０５１〜７０５３と、各ＲＦ受信コイル４０５１〜４０５３の折り返し画像７０３１〜７０３３とを用いて、行列作成処理７０６により式（７）の行列式を作成後、逆行列計算処理７０７によって折り返しのない結果画像７０８を得る。
【００３８】
ここで、本実施形態のＭＲＩ装置の制御部におけるデータ取得および信号処理部におけるデータ合成について図１を参照して説明する。本実施形態のＭＲＩ装置の撮影シーケンスは、図１（ａ）に示すように、前計測シーケンス１０１とフルオロスコピーシーケンス１０３とから成り、パラレルイメージング法の折り返し画像７０３を取得する本計測シーケンスとして、フルオロスコピーシーケンス１０３が用いられ、パラレルイメージング法とフルオロスコピー法を組み合わされている。図１（ｂ）の低周波領域データ１０２および間引きデータ１０５、１０６は、図１（ａ）の前計測シーケンスおよびフルオロスコピーシーケンス１０３を実行することにより取得したデータのｋ空間での配置を模式的に示したものであり、図６と同様に、白いラインはデータを取得した部分、黒いラインはデータを取得していない部分を示している。
【００３９】
本実施形態のＭＲＩ装置において、フルオロスコピーシーケンス１０３は、時間間隔１０４１、１０４３、…内で１枚の画像を再構成するのに必要な間引きデータ１０５を取得する第１のシーケンスと、時間間隔１０４２、１０４４、…内で１枚の画像を再構成するのに必要な間引きデータ１０６を取得する第２のシーケンスとにより構成されている。本実施形態のＭＲＩ装置は、第１のシーケンスと第２のシーケンスを時間間隔１０４で交互に繰り返し実行することにより、間引きデータ１０５１、１０６１、１０５２、…を時系列に順次取得する。第１のシーケンスは、例えば、位相エンコード数が６４の場合、１つおきに位相エンコード１、３、５、…、６３のデータを計測する。このため、間引きデータ１０５は、奇数の位相エンコードラインのデータにより構成される。このとき、第２のシーケンスは、１つおきに位相エンコード２、４、６、…、６４のデータを計測する。このため、間引きデータ１０６は、偶数の位相エンコードラインのデータにより構成される。
【００４０】
このように、第１のシーケンスと第２のシーケンスで、それぞれ異なる位相エンコードラインを間引いて間引きデータ１０５、１０６を取得すると、これら間引きデータ１０５、１０６を合成することで、ｋ空間全てを計測データで埋めて補完することができる。そこで、本実施形態では、データ合成処理１０７により、隣り合う２つの間引きデータ１０５と間引きデータ１０６とを合成し、この合成された計測データの中央領域を抜き出すことで、感度分布データ取得用の低周波領域データ１１０２、１１０３、…を生成している。
【００４１】
具体的には、前計測シーケンス１０１が実行されて、各コイル４０４、４０５１〜４０５３の低周波領域データ１０２がそれぞれ取得され、第１のシーケンスが時間間隔１０４１内で実行されて、各コイル４０４、４０５１〜４０５３の間引きデータ１０５１がそれぞれ取得される。前計測シーケンス１０１では、位相エンコードステップの間隔を間引かずに通常の位相エンコードステップと同等にし、位相エンコード数を通常の数分の１にしてシーケンスの繰り返し回数を減らしている。例えば、ｋ空間の位相エンコード数が６４の場合、通常は１〜６４までの位相エンコードの計測データを全て計測するのに対し、前計測シーケンス１０１では、位相エンコードステップを同じにして、例えば、低周波領域の位相エンコード２５〜４０のデータを計測する。この場合、位相エンコード数は、１／４となる。
【００４２】
信号処理部４０７は、各コイル４０４、４０５１〜４０５３の低周波領域データ１０２と各コイル４０４、４０５１〜４０５３の間引きデータ１０５１とをそれぞれデータ合成処理１０７１することにより、各コイル４０４、４０５１〜４０５３の低周波領域データ１１０１を得る。このデータ合成処理１０７１では、低周波領域データ１０２の位相エンコードラインのデータが、低周波領域データ１０２の位相エンコードラインと対応する間引きデータ１０５１の位相エンコードラインのデータと差し換えられ、低周波領域データ１１０１が生成される。つまり、低周波領域データ１０２のうち、奇数の位相エンコードラインのデータは、データ合成処理１０７１により、間引きデータ１０５１の低周波領域における奇数の位相エンコードラインのデータに更新される。信号処理部４０７は、このように更新された各コイル４０４、４０５１〜４０５３の低周波領域データ１１０１をそれぞれフーリエ変換して、時間間隔１０４１に対応する各コイル４０４、４０５１〜４０５３の感度画像７０１、７０２１〜７０２３を生成する。
【００４３】
一方、第１のシーケンスにより、奇数の位相エンコードラインのデータが等間隔に取得されたＲＦ受信コイル４０５１〜４０５３の間引きデータ１０５１は、信号処理部４０７によりそれぞれ領域縮小処理１０８１が行われ、実際に計測した部分のみが取り出される。これにより、ＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９１がそれぞれ作成される。信号処理部４０７は、これらＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９１をそれぞれフーリエ変換して、時間間隔１０４１に対応する折り返し画像７０３１〜７０３３を生成する。このように生成された時間間隔１０４１に対応する折り返し画像７０３１〜７０３３は、時間間隔１０４１に対応する感度画像７０１、７０２１〜７０２３から算出された感度分布データ７０５により折り返しが除去され、時間間隔１０４１に対応する結果画像７０８となり、表示部４０８に表示される。
【００４４】
また、第１のシーケンスが実行された後、第２のシーケンスは、時間間隔１０４１の後の時間間隔１０４２内で実行されて、各コイル４０４、４０５１〜４０５３の間引きデータ１０６１が取得される。信号処理部４０７では、各コイル４０４、４０５１〜４０５３の間引きデータ１０５１と各コイル４０４、４０５１〜４０５３の間引きデータ１０６１とをそれぞれデータ合成処理１０７２することにより、各コイル４０４、４０５１〜４０５３の低周波領域データ１１０２を得る。このデータ合成処理１０７２では、間引きデータ１０５１の奇数の位相エンコードラインのデータと、間引きデータ１０６１の偶数の位相エンコードラインのデータが合成され、中央領域が抜き出されて低周波領域データ１１０２が生成される。この低周波領域１１０２は、信号処理部４０７で低周波領域データ１１０１と同様に処理され、時間間隔１０４２に対応する感度画像７０１、７０２が生成される。
【００４５】
一方、第２のシーケンスにより、偶数の位相エンコードラインのデータが等間隔に取得されたＲＦ受信コイル４０５１〜４０５３の間引きデータ１０６１は、間引きデータ１０５１と同様に、領域縮小処理１０８２がそれぞれ行われ、ＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９２が作成される。これらＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９２は、信号処理部４０７で間引きデータ１０９１と同様に処理され、時間間隔１０４２に対応する折り返し画像７０３が生成される。この折り返し画像７０３は、時間間隔１０４２に対応する感度画像７０１、７０２から算出された感度分布データ７０５により折り返しが除去され、時間間隔１０４２に対応する結果画像７０８となり、表示部４０８に表示される。
【００４６】
また、第２のシーケンスが実行された後、第１のシーケンスは、再び、時間間隔１０４２の後の時間間隔１０４３内で実行されて、各コイル４０４、４０５１〜４０５３の間引きデータ１０５２がそれぞれ取得される。信号処理部４０７では、各コイル４０４、４０５１〜４０５３の間引きデータ１０６１と各コイル４０４、４０５１〜４０５３の間引きデータ１０５２とをデータ合成処理１０７３することにより、各コイル４０４、４０５１〜４０５３の低周波領域データ１１０３を得る。このデータ合成処理１０７３では、間引きデータ１０６１の偶数の位相エンコードラインのデータと、間引きデータ１０５２の奇数の位相エンコードラインのデータとが合成され、中央領域が抜き出されて低周波領域データ１１０３が生成される。以下、低周波領域データ１１０１、１１０２と同様の処理が行われ、時間間隔１０４３に対応する感度画像７０１、７０２が生成される。
【００４７】
一方、再び第１のシーケンスが実行され取得された各ＲＦ受信コイル４０５１〜４０５３の間引きデータ１０５２は、間引きデータ１０５１と同様に処理され、各ＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９３がそれぞれ作成される。これら各ＲＦ受信コイル４０５１〜４０５３の間引きデータ１０９３は、間引きデータ１０９１、１０９２と同様に処理され、時間間隔１０４３に対応する折り返し画像７０３が生成される。この折り返し画像７０３は、時間間隔１０４３に対応する感度画像７０１、７０２から算出された感度分布データ７０５により折り返しが除去され、時間間隔１０４３に対応する結果画像７０８となる。
【００４８】
つまり、本実施形態のＭＲＩ装置は、間引きデータを取得する本計測シーケンスが間引き数に応じた数の位相エンコードのグループに分割され、この分割されたグループ毎に撮影に使用する位相エンコードが選択されるとともに、グループで取得した間引き計測データを組み合せて各ＲＦ受信コイルの感度分布データを作成し、この感度分布データに基づいて画像の折り返しを除去している。また、本計測シーケンスを複数のグループに分割する場合、間引きステップとグループ数を同じとし、時間的に繰り返すグループ番号に分割することができる。また、位相エンコードを選択する場合、グループ番号に応じて位相エンコードの開始量を変えることができる。また、グループで取得した信号を組み合わせて、各ＲＦ受信コイルの感度分布を作成する場合、時間的に隣接するグループで取得した信号を組み合わせることができる。
【００４９】
また、２倍速以上のパラレルイメージングに本発明を適用する場合、フルオロスコピーシーケンスを、パラレルイメージングの倍速数Ｂと等しい数に分割し、例えば、(９)式に従って取得する位相エンコードラインＰ(ｉ,ｊ)をサイクリックに変化させて実行する。
【００５０】
【数９】
Ｐ(ｉ，ｊ)＝Ｂ×ｊ−ｉ％Ｂ （９）
ここで、ｉはフルオロスコピーシーケンスの画像番号(１≦ｉ≦Ｎ、Ｎは画像数)で時間間隔を繰り返す毎に１づつ増加する。ｊはパラレルイメージング法で取得する位相エンコード番号(１≦ｊ≦ＫＹ、ＫＹは位相エンコード数)であり、％は除算の余りを求める演算子である。
【００５１】
従来のＭＲＩ装置は、前計測シーケンスの実行時間がパラレルイメージングの総撮影時間の大きな割合を占める。例えば、フルオロスコピーシーケンスを繰り返し実行する時間間隔の中で、感度分布データ用の低周波領域データと、折り返し画像用の間引きデータとを毎回取得すると、間引きデータから得られる画像の折り返しの除去は、同じ時間間隔内で取得された感度分布データに基づいて行われる。しかし、フルオロスコピーシーケンスを繰り返し実行する時間間隔の中で、感度分布データ取得用の前計測シーケンスを毎回実行しているため、シーケンスの繰り返し時間間隔が長くなり、得られる画像列の時間分解能が低下する。つまり、連続撮影を行い被検体の動態を観察する場合、シーケンスの繰り返し内で感度分布データを合せて取得すると、前計測シーケンスを実行する分、画像１枚の更新時間間隔が長くなり問題がある。
【００５２】
これに対し、本実施形態のＭＲＩ装置は、互いに異なる位相エンコードラインのデータを取得する第1および第２のシーケンスを、時間間隔１０４で交互に繰り返し実行し間引きデータ１０５、１０６を取得して合成し、感度分布データ取得用の低周波領域データ１１０２、１１０３、…を生成している。このため、繰り返し時間間隔の中で前計測シーケンスを毎回実行する場合に比べ、画像１枚あたりのデータ取得時間を短縮し、画像列の時間分解能を向上することができる。
【００５３】
また、従来のＭＲＩ装置の撮影方法として、最初に前計測シーケンスを実行して感度分布データを取得し、次に、本計測のフルオロスコピーシーケンスを所定の時間間隔で繰り返し実行し、この時間間隔内で１枚の画像を再構成するのに必要なデータを取得し、前計測シーケンスで最初に取得した感度分布データを用いて画像の折り返しを除去する方法がある。この場合、最初に取得された感度分布データを用いて、全ての画像の折り返しを除去しているため、被検体が動いたり、インタラクティブスキャンコントロール等を用いて撮影断面を変更したりすると、感度分布が算出されていない領域に折り返しが発生するなどして、画像が劣化したり、正しく画像再構成を行えない場合がある。
【００５４】
これに対し、本実施形態のＭＲＩ装置では、撮影の進行に伴って、感度分布データ取得用の低周波領域データ１１０を順次更新しているので、各時間間隔の折り返し画像に対応した好適な感度分布データを用いてパラレルイメージングの再構成処理を行うことができる。このため、被検体が動いた場合や、撮影断面が変更された場合でも、画質の劣化を低減することができる。
【００５５】
また、本実施形態では、間引きデータ１０５、１０６を合成したデータから低周波領域を抜き出し感度画像を生成しているので、感度画像は強いローパスフィルタがかかった画像となる。このため、データ間の被検体内の変化が結果画像に与える影響を小さくすることができる。
【００５６】
（第２の実施形態）
本発明を適用してなるＭＲＩ装置の第２の実施形態について図８を参照して説明する。図８は、ＭＲＩ装置の第２の実施形態において取得した計測データを計測空間に配置した状態を説明する図である。なお、本実施形態では、第１の実施形態と同一のものには同じ符号を付して説明を省略し、第１の実施形態と相違する構成及び特徴部などについて説明する。
【００５７】
本実施形態が第１の実施形態と相違する点は、４つのＲＦ受信コイルを使用し、高周波領域の位相エンコードステップを４倍に間引き、低周波領域の位相エンコードステップを２倍に間引く点にある。したがって、本実施形態では、図８に示すように、計測データの取得中に間引く位相エンコードステップを変更し、高周波領域１２４１、１２４２において、位相エンコードステップの間隔を４倍、低周波領域１２５において位相エンコードステップの間隔を２倍にして、低周波領域１２５のエンコードステップより高周波領域１２４のエンコードステップを大きく設定している。このとき、データが取得されるｋ空間の位相エンコードラインは、１つまたは３つおきに存在する。言い換えれば、本実施形態では、ｋ空間全域において位相エンコードステップを４倍に間引いてデータを取得し、低周波領域１２５の位相エンコードステップの間隔が２倍になるようにデータを取得したということができる。
【００５８】
図８（ａ）は、取得する計測データのｋ空間を模式的に示したもので、ｋ空間は、網掛けで示した高周波領域１２４１、１２４２と、白色で示した低周波領域１２５に分割されている。図８(ｂ)は、本実施形態のシーケンスを実行して取得したデータをｋ空間に配置した状態を模式的に示したものである。また、図８（ｃ）の低周波領域データ１０２は、前計測シーケンス１０１を実行することにより取得したデータをｋ空間に配置した状態を、間引きデータ１０５、１０６は、高周波領域１２４の位相エンコードステップを４に、低周波領域１２５の位相エンコードステップを２に設定したフルオロスコピーシーケンス１０３の第１および第２のシーケンスを実行することにより取得したデータをｋ空間に配置した状態を模式的に示したものである。
【００５９】
ここで、間引きデータ１０５１の高周波領域１２４の位相エンコードステップは、４倍に間引かれているが、低周波領域１２５の位相エンコードステップの間引き率は、第１の実施形態と同じなので、信号処理部４０７は、低周波領域データ１０２と間引きデータ１０５１を第１の実施形態と同様に処理することにより、時間間隔１０４１に対応する各コイル４０４、４０５１〜４０５３の低周波領域データ１１０１を生成することができる。
【００６０】
また、間引きデータ１０５と間引きデータ１０６をデータ合成処理１０７すると、高周波領域１２４において位相エンコードステップが４倍に間引かれているので、高周波領域１２４は計測データで埋まらないが、低周波領域１２５の位相エンコードステップの間隔は２倍で、かつ、互いに異なる位相エンコードラインが間引かれているので、低周波領域１２５を計測データで埋めることができる。このため、信号処理部４０７は、第１の実施形態と同様に、時間間隔１０４２に対応する各コイル４０４、４０５１〜４０５３の低周波領域１１０２、１１０３、…を生成することができる。
【００６１】
一方、間引きデータ１０５、１０６は、少なくとも３つおきに位相エンコードが計測されているので、領域縮小処理１０８により、位相エンコードステップが４のラインを抜き出して、位相エンコード方向のデータ数を１／４に縮小した間引きデータ１０９をＲＦ受信コイル４０５１〜４０５３毎にそれぞれ作成することができる。ＲＦ受信コイル４０５１〜４０５３毎にそれぞれ作成された間引きデータ１０９は、位相エンコード方向のデータ数が１／４に縮小されているので、その分画像の折り返しも発生するが、ＲＦ受信コイル４０５１〜４０５３が４つのコイルで構成されているため、１／４に縮小された画像の折り返しも除去することができる。
【００６２】
このように、データ取得中に位相エンコードステップを変えて、低周波領域１２５の位相エンコードステップを２倍に、高周波領域１２４の位相エンコードステップを４倍に間引くことにより、位相エンコードステップを全て２倍に間引く場合に比べ、画像１枚あたりのシーケンスの繰り返し回数を減らして画像取得の時間間隔を短くすることができ、更に、画像の時間分解能を向上することができる。
【００６３】
また、本実施形態では、間引きデータを取得する毎に、感度分布取得用の低周波領域データを更新することができるので、各折り返し画像に対応した好適な感度分布データを用いてパラレルイメージングの再構成処理を行うことができる。また、本実施形態では、二次元のフルオロスコピーシーケンスの場合について説明しているが、マルチフェーズ撮影、三次元撮影にも同様に適用することができる。
【００６４】
（第３の実施形態）
本発明を適用してなるＭＲＩ装置の第３の実施形態について図９を参照して説明する。図９は、ＭＲＩ装置の第３の実施形態の信号処理部におけるデータ合成を説明する図である。なお、本実施形態では、第１の実施形態と同一のものには同じ符号を付して説明を省略し、第１の実施形態と相違する構成及び特徴部などについて説明する。
【００６５】
本実施形態が第１の実施形態と相違する点は、位相エンコードステップを２倍に間引くとともに、スライスエンコードステップを２倍に間引くことにより、三次元撮影を行っていることにある。すなわち、本実施形態のＭＲＩ装置において、フルオロスコピーシーケンス１０３は、それぞれ互いに異なる位相エンコードステップおよびスライスエンコードステップがともに２倍に間引かれた第１乃至第４のシーケンスにより構成され、これら第１乃至第４のシーケンスを実行して、それぞれ互いに異なる位相エンコードライン、および、スライスエンコードラインのデータを取得し、間引きデータ１１６、１１７、１１８、１１９、…をそれぞれ取得している。本実施形態のＭＲＩ装置は、第１乃至第４のシーケンスを１セットとし、このセットを繰り返し実行することにより、間引きデータ１１６、１１７、１１８、１１９、…を時系列に順次取得する。これら間引きデータ１１６、１１７、１１８、１１９、…をデータ合成処理１２１にて合成することにより、ｋｙ−ｋｚ空間の中央領域全ての格子をデータで埋めて、感度分布取得用のデータ１２３を生成する。
【００６６】
ここで、図９の間引きデータ１１６〜１１９は、第１乃至第４のシーケンスを実行することにより取得したデータをｋｙ−ｋｚ空間にそれぞれ配置した状態を模式的に示したものであり、位相エンコード方向ｋｙとスライスエンコード方向ｋｚのそれぞれに２倍速のパラレルイメージングを適用した場合を示している。したがって、白い格子はデータを取得した部分、黒い格子はデータを取得していない部分を示している。
【００６７】
一方、第１のシーケンスが実行されて取得された間引きデータ１１６は、領域縮小処理１２０１にて、実際に取得したデータが集められ、位相エンコード方向およびスライスエンコード方向にそれぞれ縮小されて間引きデータ１２２１が作成される。第２乃至第４のシーケンスが実行されて取得された間引きデータ１１７、１１８、１１９も同様に、それぞれ領域縮小処理１２０２、１２０３、１２０４にて縮小され、間引きデータ１２２２、１２２３、１２２４となる。
【００６８】
つまり、本実施形態のＭＲＩ装置は、間引き数に応じた数の位相エンコード、スライスエンコードのグループに分割し、このグループ毎に撮影に使用する位相エンコード、スライスエンコードを選択し、このグループで取得した信号を組み合わせて、各ＲＦ受信コイルの感度分布を作成し、この感度分布により画像の折り返しを除去している。
【００６９】
また、２倍速以上のパラレルイメージングに本発明を適用する場合、位相エンコード方向の倍速数Ｂｙ、スライスエンコード方向の倍速数Ｂｚを用いて、例えば（１０）、（１１）式に従って取得する位相エンコ−ドラインＲｙ（ｉ，ｊ）およびスライスエンコードラインＲｚ（ｉ，ｋ）をサイクリックに変化させて実行する。
【００７０】
【数１０】
Ｒｙ（ｉ，ｊ）＝Ｂｙ×ｊ−ｉ％Ｂｙ （１０）
【００７１】
【数１１】
Ｒｚ（ｉ，ｋ）＝Ｂｚ×ｋ−（（ｉ−１）％（Ｂｙ×Ｂｚ））／Ｂｙ （１１）
ここで、ｉはフルオロスコピーシーケンスの画像番号（１≦ｉ≦Ｎ、Ｎは画像数)、ｊはパラレルイメージング法で取得する位相エンコード番号（１≦ｊ≦ＫＹ、ＫＹは位相エンコード数)、ｋはパラレルイメージングで取得するスライスエンコード番号（１≦ｋ≦ＫＺ、ＫＺはスライスエンコード数)、％は除算の余りを求める演算子である。また、（１１）式の除算では、解の整数部のみを用いる。
【００７２】
また、本実施形態のＭＲＩ装置は、間引き数に応じた数の位相エンコード、スライスエンコードのグループに分割するとき、位相エンコード方向の間引きステップ及びスライス方向の間引きステップからグループ数を算出し、時間的に繰り返すグループ番号に分割することができる。また、位相エンコード、スライスエンコードを選択するとき、グループ番号に応じて、位相エンコード、スライスエンコードの開始量を変えたり、ｋ空間を低周波領域と高周波領域の２つに分割し、低周波領域のエンコードステップと、高周波領域のエンコードステップとを変えたりすることができる。
【００７３】
（第４の実施形態）
本発明を適用してなるＭＲＩ装置の第４の実施形態について図１０および１１を参照して説明する。図１０は、ＭＲＩ装置の第４の実施形態の制御部におけるデータ取得を説明する図である。図１１は、ＭＲＩ装置の第４の実施形態の信号処理部におけるデータ合成を説明する図である。なお、本実施形態では、第１の実施形態と同一のものには同じ符号を付して説明を省略し、第１の実施形態と相違する構成及び特徴部などについて説明する。
【００７４】
本実施形態が第１の実施形態と相違する点は、心電計を被検体に装着し、被検体の心臓の拍動周期を表す信号をＭＲＩ装置に取り込み、拍動周期を複数の区間に分割してなる時相を設定し、時相に対応させて複数のシーケンスを実行するとともに、隣り合う時相で互いに異なる位相エンコードステップを間引いたシーケンスを実行する点にある。また、複数の拍動周期に渡って取得された計測データを各時相にそれぞれ対応するｋ空間に配置し、このｋ空間に配置された計測データに基づいて各時相に対応する画像を生成し、隣り合う時相のｋ空間の計測データを合成して感度分布データを生成し、画像の折り返しを除去する点にある。
【００７５】
本実施形態のＭＲＩ装置は、被検体４０１が周期的に発生する生体信号、例えば、心電信号を検出する心電計（以下、ＥＣＧと称する。）を備えている。ＥＣＧは、被検体４０１に装着され、被検体４０１が発生する心電信号を検出し制御部４１１へ出力する。心臓領域を撮影する場合、ＥＣＧから出力された心電信号に同期して計測を行うことで、血流や心臓自体の動きによるアーチファクトを抑制する。また、本実施形態では、ＥＣＧに限らず、脈波計などの生体信号を検出する様々な計器を適用することができる。
【００７６】
本実施形態のＭＲＩ装置の心電同期撮影では、図１０に示すように、例えば、心電波形８００のＲ波８０１１をトリガーとし、そこから数十〜数百ｍｓ程度のディレイ時間８０３１を設ける。このディレイ時間８０３１の後のデータ取得ウインド８０４１内を複数の区間に分割し、時相８１１１、…、８１５１を設定する。この設定された時相８１１１、…、８１５１毎にシーケンスを実行し、データ８２１１、…、８２５１を取得する。これを、複数の拍動周期に渡って行う。拍動周期は、例えば、Ｒ波８０１１からＲ波８０１２までの１心拍の時間間隔８０２１で周期的に繰り返される。
【００７７】
また、本実施形態のＭＲＩ装置は、図１１に示すように、各時相８１１、…、８１５にそれぞれ対応するｋ空間１１２１、１１３１、１１２２、１１３２、…を備えている。この場合、ｋ空間１１２、１１３は、例えば、信号処理部４０７内のメモリにデータ空間として設けられ、各時相８１１、…、８１５内でそれぞれ取得されたデータ８２１、…、８２５は、対応する各ｋ空間１１２１、１１３１、１１２２、１１３２、…にそれぞれ配置される。
【００７８】
具体的には、心電波形８００のＲ波８０１１が検出されると、制御部４１１は、Ｒ波８０１１に基づいて、このＲ波８０１１からディレイ時間８０３１が経過した後に第１および第２のシーケンスを交互に繰り返し実行し、１回目の時間間隔８０２１の各時相８１１１、…、８１５１で、それぞれのデータ８２１１、…、８２５１を取得する。各時相８１１１、…、８１５１で取得されたデータ８２１１、…、８２５１は、それぞれｋ空間１１２１、１１３１、１１２２、１１３２、…に配置される。このとき、データ８２１１はｋ空間１１２１の第１ラインに配置され、データ８２２１はｋ空間１１３１の第２ラインに配置され、データ８２３１はｋ空間１１２２の第１ラインに配置され、データ８２４１はｋ空間１１３２の第２ラインに配置される。そして、２回目のＲ波８０１２が検出されると、１回目と同様に、第１および第２のシーケンスを交互に繰り返し実行し、各時相８１１２、８１２２、…で、それぞれのデータ８２１２、８２２２、…を取得する。取得されたデータ８２１２はｋ空間１１２１の第３ラインに、データ８２２２はｋ空間１１３１の第４ラインに、データ８２３２はｋ空間１１２２の第３ラインに、データ８２４２はｋ空間１１３２の第４ラインにそれぞれ配置される。
【００７９】
以上の操作を、奇数番目の時相８１１、８１３、…、８１５に対応するｋ空間１１２の奇数の位相エンコードライン、および、偶数番目の時相８１２、８１４、…に対応するｋ空間１１３の偶数の位相エンコードラインがそれぞれ取得したデータで全て埋まるまで複数の拍動周期に渡って繰り返す。この結果、互いに異なる位相エンコードステップが間引かれたｋ空間１１２、１１３が取得される。つまり、図１１は、ＥＣＧを用いたマルチフェイズの心臓撮影に、倍速数が２のパラレルイメージング法を適用した場合であり、複数の拍動周期に渡ってデータ取得ウインド８０４内で取得したデータ８２１、…、８２５を時相８１１、…、８１５毎に別々のｋ空間１１２、１１３に配置したものを模式的に示したものである。
【００８０】
また、２倍速以上のパラレルイメージングに本実施形態を適用する場合、データ取得ウインド内で取得する時相を、パラレルイメージングの倍速数Ｂと等しい数のデータ群に分割し、例えば、（１２）式に従って取得する位相エンコードラインＱ（ｋ，ｊ）をサイクリックに変化させて実行する。
【００８１】
【数１２】
Ｑ（ｋ，ｊ）＝Ｂ×ｊ−ｋ％Ｂ （１２）
ここで、ｋはマルチフェーズ計測の時相番号(１≦ｋ≦Ｍ、Ｍはマルチフェーズ数)、ｊはパラレルイメージングで取得する位相エンコード番号（１≦ｊ≦ＫＹ、ＫＹは位相エンコード数）であり、％は除算の余りを求める演算子である。また、時相番号は、各時相８１１、…、８１５に時系列に従って順次付与される。
【００８２】
このような位相エンコードを用いてデータ取得を行うことで、本実施形態のＭＲＩ装置は、隣り合う時相のデータを合わせて、ｋ空間の全てを埋めることが可能である。そこで、第１の実施形態の場合と同様に、ｋ空間１１２、１１３のうち、隣り合う２つの時相、例えば、ｋ空間１１２１とｋ空間１１３１をデータ合成処理１０７１により合成し、抜き出すことで感度分布データ取得用の低周波領域データ１１５１を作成する。同様に、低周波領域データ１１５２は、ｋ空間１１３１とｋ空間１１２２をデータ合成処理１０７２により合成したもの、低周波領域データ１１５３は、ｋ空間１１２２とｋ空間１１３２をデータ合成処理１０７３により合成したものである。
【００８３】
また、各時相８１１、…、８１５にそれぞれ対応するｋ空間１１２、１１３は、領域縮小処理１０８によりそれぞれ位相エンコード方向に縮小され、各時相８１１、…、８１５に対応する間引きデータ１１４がそれぞれ作成され、折り返し画像７０３が得られる。この折り返し画像７０３に基づいて生成される結果画像７０８を時相順に連続に並べてシネ表示することで、心臓の動きを動画として観察することができる。
【００８４】
本実施形態では、間引きデータ１１４と低周波領域データ１１５の取得時間が厳密には対応していないので、パラレルイメージング再構成のときに、間引きデータ１１４の時間的に後ろ側／前側のいずれか一方の低周波領域データ１１５を選択して用いたり、隣り合う２つの低周波領域データ１１５を平均して用いる。例えば、間引きデータ１１４２の時間的に前側の低周波領域データ１１５とは、間引きデータ１１４２の前の時相８１１に対応するｋ空間１１２１とｋ空間１１３１とをデータ合成処理１０７１して生成された低周波領域データ１１５１のことであり、後側の低周波領域データ１１５とは、間引きデータ１１４２の後の時相８１３に対応するｋ空間１１２２とｋ空間１１３１とをデータ合成処理１０７２して生成された低周波領域データ１１５２のことである。つまり、間引きデータ１１４２を再構成する場合、低周波領域データ１１５１または低周波領域データ１１５２のいずれか一方に基づいて生成された感度分布データを用いたり、低周波領域データ１１５１と低周波領域データ１１５２に基づいて生成されたそれぞれの感度分布データを平均して用いたりすることができる。
【００８５】
ところで、従来のパラレルイメージング法では、撮影時間を確保するため撮影を２回に分け、それぞれ呼吸を止めて前計測シーケンスと、本計測シーケンスとを実行する場合がある。しかし、心臓のマルチフェーズ撮影では、数十ms以内の短時間で激しく動く心臓を撮影するため、前計測シーケンスと本計測シーケンス時の被検体位置を一致させないと、画像が劣化してしまうことがある。
【００８６】
これに対し、本実施形態のＭＲＩ装置は、感度分布取得用の付加的なデータを取得しないので、１回の息止め時間で撮影することができ、被検体が呼吸することにより発生する位置ズレを回避して、画像の劣化を防止することができる。更に、動画の動きをスムーズにするには、時相を細分化し、マルチフェーズ数を増やす必要があるが、本実施形態では、画像１枚分のデータ取得時間を少なくできるので、被検体が呼吸を止めている間により多くの画像を取得することができる。これにより、より多くのマルチフェーズ数を設定することができる。
【００８７】
また、本実施形態では、感度分布取得用の低周波領域データ１１５に、異なる時相のｋ空間１１２、１１３のデータが混在するが、時間間隔が数十ｍｓ以下と非常に短いため、また、低周波領域データ１１５は、位相エンコード数が少なく、強いローパスフィルタがかかった画像となるため、結果画像の劣化を低減することができる。
【００８８】
また、本実施形態のＭＲＩ装置は、ディレイ時間８０３と、データ取得ウインド８０４を調節することで、特定の時間範囲、つまり、拍動周期を複数の区間に分割してなる時相のうち、特定時相の心臓画像を取得することができる。
【００８９】
また、本実施形態では、１心拍の各時相で取得するデータは、位相エンコードの１ライン分であるが、各時相の時間間隔などにより、複数ライン分のデータを取得することができる。この場合、撮影に必要な心拍数を減らすことができる。
【００９０】
また、本実施形態では、ｋ空間１１２、１１３の上方からデータを取得しているが、中央、下方など様々なところからデータを取得することができる。要は、隣り合う時相に対応するｋ空間のデータが、互いに異なる位相エンコードラインで取得されていればよい。
【００９１】
本発明は、以上の実施形態で開示された内容にとどまらず、本発明の趣旨を踏まえた上で各種形態を取り得る。パラレルイメージングでは、２つ以上の任意数のマルチプルＲＦ受信コイルを使用可能であり、倍速数は用いたＲＦ受信コイルの数と同数迄の範囲内で選択可能である。例えば、第１乃至第４の実施形態では、４つのＲＦ受信コイルを用いた場合を説明したが、ＲＦ受信コイルの数は４個に限定されない。つまり、８個であっても、それ以外の数であってもよい。
【００９２】
また、第１乃至第４の実施形態のＭＲＩ装置は、１つまたは２つ以上のＲＦ受信コイルを組み合せてＲＦ受信コイル群を形成し、感度画像および折り返し画像を各ＲＦ受信コイル群毎に合成することができる。
【００９３】
また、第１乃至第４の実施形態では、グラディエントエコーシーケンスについて記載したが、パラレルイメージング法では、シーケンスの形状には依存しない。例えば、ＳＥシーケンス、ＦＳＥシーケンス、ＥＰＩシーケンス、スパイラルシーケンス、そして、ＳＳＦＰシーケンスなど様々なシーケンスに適用することができる。また、三次元撮影の場合には、位相エンコード方向だけでなく、スライスエンコード方向にデータを間引いて高速化することもできる。
【００９４】
また、本発明のＭＲＩ装置は、第１乃至第４の実施形態の送受信兼用コイル４０４に代わり、複数のＲＦ受信コイルで取得した感度画像データを合成して感度分布の均一な感度画像を生成し、この合成した感度画像を用いてＲＦ受信コイルの感度分布を求めることができる。
【００９５】
また、本発明のＭＲＩ装置のマルチプルＲＦ受信コイルは、第１乃至第４の実施形態に限らず、水平磁場頭部用マルチプルＲＦコイルとして、Ａｒｒａｙ Ｈｅａｄ Ｃｏｉｌ ｆｏｒ Ｉｍｐｒｏｖｅｄ Ｆｕｎｃｔｉｏｎａｌ ＭＲＩ（Ｃｈｒｉｓｔｏｐｈ Ｌｅｕｓｓｌｅｒ），１９９６ ＩＳＭＲＭ ａｂｓｔｒｕｃｔ ｐ．２４９や、水平磁場頭部用ＱＤマルチプルＲＦコイルとして、Ｈｅｌｍｅｔ ａｎｄ Ｃｙｌｉｎｄｒｉｃａｌ Ｓｈａｐｅｄ ＣＰ ＡｒｒａｙＣｏｉｌｓ ｆｏｒ Ｂｒａｉｎ Ｉｍａｇｉｎｇ:Ａ Ｃｏｍｐａｒｉｓｏｎ ｏｆ Ｓｉｇｎａｌ−ｔｏ−Ｎｏｉｓｅ Ｃｈａｒａｃｔｅｒｉｓｔｉｃｓ（Ｈ．Ａ．Ｓｔａｒｋ，Ｅ．Ｍ．Ｈａａｃｋｅ），１９９６ ＩＳＭＲＭ ａｂｓｔｒａｃｔ Ｐ．１４１２、そして、水平磁場腹部用ＱＤマルチプルＲＦコイルとして、Ｆｏｕｒ Ｃｈａｎｎｅ１ Ｗｒａｐ−Ａｒｏｕｎｄ Ｃｏｉｌ ｗｉｔｈ Ｉｎｄｕｃｔｉｖｅ Ｄｅｃｏｕｐｌｅｒ ｆｏｒ １．５Ｔ Ｂｏｄｙ Ｉｍａｇｉｎｇ（Ｔ．Ｔａｋａｈａｓｈｉ ｅｔ．ａｌ)，１９９５ ＩＳＭＲＭ ａｂｓｔｒｕｃｔ Ｐ．１４１８など様々なマルチプルＲＦ受信コイルを適用することができる。マルチプルＲＦ受信コイルは、相対的に高感度な小型ＲＦ受信コイルを複数個並べて、各コイルで取得した信号を合成することより、ＲＦ受信コイルの高い感度を保ったまま視野を拡大し、高感度化を図るものであり、各ＲＦ受信コイルで再構成処理された結果画像を合成して、視野を拡大し、高感度化を図ることができる。
【００９６】
また、第１乃至第４の実施形態では、静磁場方向（Ｚ方向）が水平である水平磁場のＭＲＩ装置に適用する場合について説明したが、本発明はこれに限らず、静磁場の方向が異なる場合、例えば、垂直磁場であっても同様に適用できる。
【００９７】
【発明の効果】
本発明によれば、ＭＲＩ装置の撮影時間を短縮することができる。
【図面の簡単な説明】
【図１】本発明を適用してなるＭＲＩ装置の第１の実施形態の制御部におけるデータ取得および信号処理部におけるデータ合成を説明する図である。
【図２】本発明を適用してなるＭＲＩ装置の第１の実施形態の概略構成図である。
【図３】本発明を適用してなるＭＲＩ装置の第１の実施形態に係るＲＦ受信コイルを説明する図である。
【図４】本発明を適用してなるＭＲＩ装置の第１の実施形態に係るマルチプルＲＦ受信コイルおよび受信部の概略構成図である。
【図５】本発明を適用してなるＭＲＩ装置のパルスシーケンスを説明する図である。
【図６】本発明を適用してなるＭＲＩ装置におけるパラレルイメージング法の画像の折り返しを説明する図である。
【図７】本発明を適用してなるＭＲＩ装置の第１の実施形態に係る信号処理部の信号処理の手順を示す図である。
【図８】本発明を適用してなるＭＲＩ装置の第２の実施形態において取得した計測データを計測空間に配置した状態を説明する図である。
【図９】本発明を適用してなるＭＲＩ装置の第３の実施形態の信号処理部におけるデータ合成を説明する図である。
【図１０】本発明を適用してなるＭＲＩ装置の第４の実施形態の制御部におけるデータ取得を説明する図である。
【図１１】本発明を適用してなるＭＲＩ装置の第４の実施形態の信号処理部におけるデータ合成を説明する図である。
【符号の説明】
１０１ 前計測シーケンス
１０２、１１０ 低周波領域データ
１０３ フルオロスコピーシーケンス
１０４ 時間間隔
１０５、１０６、１０９ 間引きデータ
１０７ データ合成処理
１０８ 領域縮小処理
４０１ 被検体
４０４ 送受信兼用コイル
４０５ ＲＦ受信コイル
４０７ 信号処理部
４０８ 表示部
４１１ 制御部
７０１、７０２ 感度画像
７０３ 折り返し画像
７０４ 感度分布算出処理
７０５ 感度分布データ
７０６ 行列作成処理
７０７ 逆行列計算処理
７０８ 結果画像[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus that measures and visualizes a magnetic resonance signal generated from hydrogen, phosphorus, or the like in a subject, and more particularly to magnetic resonance suitable for high-speed imaging of a heart or the like having a plurality of RF receiving coils. The present invention relates to an imaging apparatus.
[0002]
[Prior art]
In a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), a pulse sequence (hereinafter referred to as a sequence) is repeatedly executed while changing a phase encoding amount, and an echo signal necessary for reconstruction of one image is acquired. . Therefore, the number of repetitions of the sequence greatly affects the shooting time. When performing high-speed shooting, in general, there are multi-echo type sequences that generate a plurality of echo signals by executing a single sequence, or sequences in which the time interval of sequence repetition is shortened to several to several tens of ms. It is used. However, such a sequence may cause a decrease in image contrast or distortion of the image form.
[0003]
On the other hand, when imaging a heart region such as coronary artery imaging, a high-speed imaging method called a parallel imaging method has been proposed because it is necessary to capture an image at a higher speed. The parallel imaging method is a method in which a plurality of RF receiving coils are used to execute a sequence in which phase encoding steps are thinned out at equal intervals, and the number of repetitions of the sequence is reduced to shorten the imaging time. Usually, when the phase encoding step is thinned out at equal intervals and measurement is performed, the image is folded. Therefore, matrix calculation is performed based on the sensitivity distribution of each RF receiving coil calculated in advance to remove the aliasing of the image. In general, in the parallel imaging method, the imaging time can be shortened by the number of RF receiving coils used for imaging. That is, in principle, the number of phase encoding matrices can be thinned by the number of RF receiving coils used for matrix calculation. As a result, for example, if there are N RF receiving coils, the imaging time can be reduced to 1 / N times.
[0004]
[Problems to be solved by the invention]
However, in the parallel imaging method, the sensitivity distribution of each RF receiving coil is necessary to remove the aliasing of the image. In order to obtain the sensitivity distribution of the RF receiver coil, a sequence for obtaining the sensitivity distribution must be executed separately from the sequence in which the phase encoding step is thinned out at equal intervals, and the sequence for obtaining the sensitivity distribution is executed. As a result, there is a problem that the total shooting time becomes longer.
[0005]
An object of the present invention is to shorten the imaging time of the MRI apparatus.
[0006]
[Means for Solving the Problems]
  Main departureAkiraIn order to solve the above problems, a plurality of RF receiving coils that receive magnetic resonance signals generated from a subject,k-spaceThinning out the phase encoding stepAnd the k-space data thinned out for each RF receiving coilgetmeasurementControl means;Magnetic resonance comprising signal processing means for reconstructing one image without aliasing using the thinned k-space data acquired for each RF receiver coil and the sensitivity distribution for each RF receiver coil Intended for imaging devices.
[0007]
  In particular,The measurement control unit obtains two or more k-space data with different thinning-out methods for each RF reception coil, and the signal processing unit reconstructs an image for each k-space data with different thinning-out methods.It is characterized by that. In this case,The signal processing means can generate the sensitivity distribution of the RF receiving coil using at least two of k-space data having two or more different thinning methods. Alternatively, the signal processing means can generate the sensitivity distribution of the RF receiving coil using the data in the low frequency region of the k-space data. Furthermore, the measurement control means can acquire k-space data with two or more different thinning methods by thinning out different phase encoding steps.
[0009]
  Also bookThe MRI apparatus according to the invention includes at least measurement data in a central region of k-space among a plurality of measurement data obtained by executing a plurality of sequences obtained by thinning out different phase encoding steps.make use of, A sensitivity image can be generated by filling the central region of the k space with measurement data, and sensitivity distribution data of the RF receiving coil can be generated based on the sensitivity image. Thus, by generating the sensitivity distribution data, it is possible to reduce the number of repetitions of the pre-measurement sequence before acquiring the sensitivity distribution data, shorten the execution time of the sequence, and shorten the imaging time of the MRI apparatus.
[0010]
Further, when removing the aliasing of the image, the signal processing unit synthesizes a plurality of measurement data obtained by thinning out different phase encoding steps, and complements at least the measurement data in the central region of the measurement space. At least the central region of the measured data is extracted, and sensitivity distribution data of the RF receiving coil is generated based on the extracted measured data to remove the aliasing.
[0011]
In this manner, by synthesizing a plurality of measurement data obtained by thinning out different phase encoding steps, the thinned phase encode line can be complemented and the k-space can be filled with the measurement data. A central region of the k space filled with the measurement data is extracted to generate a sensitivity image of the RF receiving coil and generate sensitivity distribution data of the RF receiving coil. For this reason, it is possible to reduce the number of repetitions of the measurement sequence before acquiring sensitivity distribution data, shorten the execution time of the sequence, and shorten the imaging time of the MRI apparatus.
[0012]
Further, the control means of the MRI apparatus of the present invention takes in a signal representing the pulsation period detected by the means for detecting the pulsation of the heart of the subject and divides the pulsation period into a plurality of sections. Set phase, execute multiple sequences corresponding to this time phase over multiple pulsation cycles, and execute measurement sequence by thinning out different phase encoding steps in adjacent time phases The configuration.
[0013]
At this time, the signal processing means arranges measurement data acquired over a plurality of beat periods in a plurality of measurement spaces respectively corresponding to each time phase, and based on the measurement data arranged in the plurality of measurement spaces. The RF receiver coil sensitivity distribution is generated by generating multiple images corresponding to each time phase and combining the measurement data of at least the central region of the multiple measurement spaces among the multiple measurement data arranged in the multiple measurement spaces. Data is generated and image folding is removed.
[0014]
In this way, using the acquired measurement data to create images of different time phases is called multi-phase measurement. Measurement data acquired over a plurality of pulsation cycles are arranged in a measurement space provided corresponding to each time phase in a state where phase encoding steps are thinned out, and become an image corresponding to each time phase. . The motion of the heart can be observed as a moving image by removing the aliasing of the image corresponding to each time phase based on the sensitivity distribution data and displaying it continuously in time. However, if the subject moves due to breathing or the like during imaging, the acquired image is affected, so the subject must stop breathing during imaging, and the imaging time is limited.
[0015]
In contrast, the MRI apparatus of the present invention generates sensitivity distribution data by synthesizing a plurality of measurement data acquired by thinning out the phase encoding step, so that it is not necessary to perform a sequence for acquiring sensitivity distribution data. Time can be shortened.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Hereinafter, a first embodiment of an MRI apparatus to which the present invention is applied will be described with reference to FIGS. FIG. 1 is a diagram for explaining data acquisition in the control unit and data synthesis in the signal processing unit of the first embodiment of the MRI apparatus to which the present invention is applied. FIG. 2 is a schematic configuration diagram of the first embodiment of the MRI apparatus. FIG. 3 is a diagram illustrating the RF receiving coil according to the first embodiment of the MRI apparatus. FIG. 4 is a schematic configuration diagram of a multiple RF receiving coil and a receiving unit according to the first embodiment of the MRI apparatus. FIG. 5 is a diagram for explaining a pulse sequence of the MRI apparatus. FIG. 6 is a diagram for explaining folding of an image by the parallel imaging method in the MRI apparatus. FIG. 7 is a diagram illustrating a signal processing procedure of the signal processing unit according to the first embodiment of the MRI apparatus.
[0017]
The imaging object of the MRI apparatus that is currently widely used in clinical practice is the main constituent substance of the subject, proton, and by imaging the spatial distribution of proton density and the relaxation phenomenon of the excited state, The shape or function of the head, abdomen, limbs, etc. is depicted two-dimensionally or three-dimensionally.
[0018]
As shown in FIG. 2, the MRI apparatus of this embodiment includes a magnet 402 that generates a static magnetic field in a space including a subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field in this space, and a high-frequency magnetic field in this space. And a transmission / reception coil 404 for detecting a magnetic resonance signal (hereinafter referred to as an MR signal) generated by the subject 401 and an RF probe 405 for detecting the MR signal generated by the subject 401. . The gradient magnetic field coil 403 is composed of a plurality of coils that generate gradient magnetic fields in three directions of X, Y, and Z, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 409. The transmission / reception coil 404 generates a high-frequency magnetic field according to a signal output from the RF transmission unit 410. Signals received by the transmission / reception coil 404 and the RF probe 405 are detected by the signal detection unit 406, processed by the signal processing unit 407, and processed and converted into an image signal. The image is displayed on the display unit 408. The gradient magnetic field power source 409, the RF transmission unit 410, the signal processing unit 407, and the display unit 408 are controlled by the control unit 411. The control time chart is generally called a pulse sequence. The bed 412 is for the subject to lie down.
[0019]
As shown in FIG. 3, the RF probe 405 of the MRI apparatus of the present embodiment is configured to include four 8-shaped RF receiving coils 4051 to 4054. Two of the four RF receiving coils 4051 and 4052 are arranged opposite to each other with a certain distance in the Z direction on a plane substantially parallel to the XY plane, and the other two RF receiving coils 4053 and 4054 are They are arranged facing each other at a fixed distance in the X direction on a plane substantially parallel to the ZY plane. MR signals from the subject 401 placed in the space surrounded by the four RF receiving coils 4051 to 4054 are received.
[0020]
The RF probe 405 uses a technique called “multiple RF receiving coil” or “phased array coil” using four RF receiving coils 4051 to 4054. The multiple RF receiving coil 301 is provided in the RF probe 405, and as shown in FIG. 4, four RF receiving coils 4051 to 4054 are connected to preamplifiers 3021 to 3024, respectively. The reception unit for the measurement signal received by the multiple RF reception coil 301 includes a signal detection unit 406 and a signal processing unit 407. The signal detection unit 406 is provided with four AD conversion / quadrature detection circuits 3031 to 3034 in parallel, and the outputs of the preamplifiers 3021 to 3024 are input to the signal detection unit 406, respectively. The signal processing unit 407 converts each signal received by the RF receiving coils 4051 to 4054 into an MR image by Fourier transform, back projection method, wavelet transform, etc., and a signal synthesis processing unit 304 that synthesizes these image signals. I have. In the figure, elements and processes indicated by 4-digit numbers having the same first three digits indicate the same elements and processes, and the last one-digit number distinguishes a plurality of identical elements and processes. Hereinafter, the same applies in this specification.
[0021]
The imaging method in the MRI apparatus of this embodiment configured as described above will be described in the case where three of the four RF receiving coils 4051 to 4053 are used. For example, when a gradient echo sequence is adopted as an imaging method, as shown in FIG. 5, a high-frequency pulse 6011 and a slice selection gradient magnetic field pulse 6021 are applied, and then a phase encoding gradient magnetic field pulse 6031 and a read gradient magnetic field pulse 6041 are applied. Then, an echo signal 6061 that is measurement data is measured in the sampling window 6051. Such a sequence is repeated at a repetition time 607. The repetition time 6071 is, for example, an interval between the high frequency pulse 6011 and the high frequency pulse 6012. In the MRI apparatus of this embodiment, by changing the amount of the phase encoding gradient magnetic field 603 for each repetition time 607, different phase encoding amounts are given, and echo signals given the respective phase encoding amounts are detected. This is repeated by the number of phase encodings, and a set of echo signals necessary to reconstruct one image at the image acquisition time 608 is obtained.
[0022]
If the repetition time 607 of this sequence is shortened to several ms, the image acquisition time 608 can be reduced to 1 second or less. In the fluoroscopic sequence, a sequence having a short repetition time 607 is continuously executed to obtain a plurality of images having high time resolution, that is, a short image update time interval. The plurality of images are sequentially displayed on the monitor to observe the movement of the subject and devices such as a biopsy needle and a catheter inserted into the subject. In a normal imaging method, the number of echo signals per image is selected from 64, 128, 256, 512, etc. in consideration of the field of view and spatial resolution in the phase encoding direction. Considering the field of view in the out direction and the spatial resolution, it is usually obtained as a time series signal composed of 128, 256, 512, 1024 sampling data. These data are two-dimensionally Fourier transformed to create one MR image.
[0023]
On the other hand, the MRI apparatus according to the present embodiment receives MR signals using the three RF receiving coils 4051 to 4053, and further repeatedly executes this measurement sequence in which the phase encoding step in the k space is thinned to perform thinned data. And a time-series image that is temporally continuous is generated based on the thinned data. Since this time-series image is generated from the thinned data, aliasing occurs. This aliasing is removed using the sensitivity distribution data of the three RF receiving coils 4051 to 4053.
[0024]
In this measurement, the number of repetitions of the sequence is reduced by thinning out the phase encoding step at a constant rate. This thinning rate is generally called a double speed number. For example, if the phase encoding step is thinned out twice, the double speed number becomes 2. When the number of phase encodes is 64, all of the phase encode data from 1 to 64 are usually measured, whereas in the parallel imaging method with a double speed number of 2, for example, every other phase encode 1, 3, 5 ,..., 63 are measured. At this time, the thinning rate becomes 1/2.
[0025]
Here, the folded image in the parallel imaging method will be described with reference to FIG. FIG. 6 (a) shows a case where an image is captured in a normal phase encoding step without thinning out the phase encoding step. By arranging the signals 2021,... The measurement data 201 is not thinned out for one sheet. When this is Fourier transformed, an unfolded image 207 as shown in FIG. 6C can be obtained. On the other hand, in the parallel imaging method, as shown in FIG. 6B, the phase encoding step interval is doubled, for example, for the same field of view as in normal imaging, and thinning is performed by measuring the phase encoding step. Data 203 is acquired. In this case, data 2041,..., 2043 are measured every other line, and data corresponding to the positions of 2051,. As a result, the amount of data to be measured is halved. Therefore, when an image is created by halving the matrix, an image 208 with aliasing is obtained as shown in FIG. In FIG. 6, since the y direction is the phase encoding direction, the image 208 is shown to be folded in the phase encoding direction. That is, as shown in FIG. 6C, the subject image 2061 of the phantom 206 in the upper region 2071 in the image 207 and the subject image 2062 in the lower region 2072 overlap, resulting in FIG. The image 208 in which the folding as shown in FIG.
[0026]
For example, SENSE: Sensitivity Encoding for Fast MRI (Klass P. Prussmant. Al), Magnetic Resonance in Medicine 42: 952-962 (1999), aS ed. It can be removed by a signal processing method such as from Multiple Detectors (JB Ra, CY Rim), Magnetic Resonance in Medicine 30: 142-145 (1993).
[0027]
Here, a calculation method for image aliasing removal will be described. When the image matrices in the x and y directions are X and Y, respectively, the pixel value at the coordinates (x, y) in the image (where 1 ≦ x ≦ X, 1 ≦ y ≦ Y) is represented by si (x, y). And Here, the subscript i is the number of the RF receiving coil, and 2 ≦ i ≦ N. In the case of FIG. 6D, since the phase encoding step is thinned out twice, the matrix in the phase encoding direction of the image after thinning is Y ′ = Y / 2. When the coordinates of the image in FIG. 6D are (x, y ′) (where 1 ≦ y ′ ≦ Y ′), the pixel value si ′ (x, y ′) is 2 of the original image 207. The areas 2071 and 2072 are overlapped and expressed by Expression (1). However, a is a constant.
[0028]
[Expression 1]

Next, the sensitivity distribution and image of the RF receiving coil will be described. If the two-dimensional sensitivity distribution of the i-th RF receiver coil is ci (x, y), the received signal si (x, y) is the RF receiver coil sensitivity distribution ci (x, y) and the subject. Since it is a product of the proton density distribution p (x, y), it can be expressed by equation (2).
[0029]
[Expression 2]
si (x, y) = ci (x, y) × p (x, y) (2)
Here, when Expression (2) is used, Expression (1) can be expressed by Expression (3).
[0030]
[Equation 3]

Here for simplicity,
[0031]
[Expression 4]
Si = s′i (x, y ′)
Cij = ci (x, y ′ + (j−1) × Y ′) (4)
Pj = p (x, y ′ + (j−1) × Y ′)
Then, equation (3) becomes equation (5).
[0032]
[Equation 5]

Expression (5) can be expressed as Expression (6) as an N × 2 matrix.
[0033]
[Formula 6]

If the sensitivity distribution Cij of the RF receiving coil is known from the equation (6), the density distribution Pj of the measurement target nuclei, for example, protons of the specimen is obtained by calculating the inverse matrix.
[0034]
Similarly, a general formula when photographing at M times speed using N coils can be expressed by Formula (7) as Y′≡Y / M, 1 ≦ y ′ ≦ Y ′. Here, b is a constant.
[0035]
[Expression 7]

In this way, by using the sensitivity distribution Cij of the RF receiving coil and performing the matrix calculation of Expression (7) on the measurement data s′i (x, y ′), the image data in which the aliasing is removed is included in the measurement object. The nuclear spin density distribution Pj is obtained. The relationship between the number of RF receiving coils (N) and the number of double speeds (M) in the parallel imaging method is mathematically N ≧ M.
[0036]
In the parallel imaging method in the MRI apparatus of this embodiment, as shown in FIG. 7, the folded images 7031 to 7033 of the RF receiving coils 4051 to 4053 of the multiple RF receiving coils 301 obtained by executing this measurement sequence are folded. The removal is performed in the signal processing unit 407. The signal processing unit 407 performs sensitivity distribution calculation processing 7041 to 7043 using the sensitivity image 701 of the transmission / reception coil 404 and the sensitivity images 7021 to 7023 of the RF reception coils 4051 to 4053, and the RF reception coils 4051 to 4053. Sensitivity distribution data 7051 to 7053 are obtained. As the sensitivity distribution calculation process 704, the image data sc (x, y) of each RF receiving coil 405i is relativized by, for example, Expression (8) with reference to the image data sc (x, y) of the transmission / reception coil 404. Thus, the sensitivity distribution ci (x, y) is obtained.
[0037]
[Equation 8]
ci (x, y) = si (x, y) / sc (x, y) (8)
Using the sensitivity distribution data 7051 to 7053 calculated in this way and the folded images 7031 to 7033 of the RF receiving coils 4051 to 4053, the determinant of Expression (7) is created by the matrix creation process 706, and then the inverse matrix A calculation result 707 obtains an unfolded result image 708.
[0038]
Here, data acquisition in the control unit and data synthesis in the signal processing unit of the MRI apparatus of the present embodiment will be described with reference to FIG. As shown in FIG. 1A, the imaging sequence of the MRI apparatus of the present embodiment is composed of a pre-measurement sequence 101 and a fluoroscopy sequence 103. As the main measurement sequence for acquiring a folded image 703 by the parallel imaging method, The Scopy sequence 103 is used, and the parallel imaging method and the fluoroscopy method are combined. The low-frequency region data 102 and the thinning-out data 105 and 106 in FIG. 1B schematically illustrate the arrangement of the data acquired by executing the pre-measurement sequence and the fluoroscopy sequence 103 in FIG. As in FIG. 6, the white line indicates the portion from which data has been acquired, and the black line indicates the portion from which data has not been acquired.
[0039]
In the MRI apparatus of the present embodiment, the fluoroscopic sequence 103 includes a first sequence for acquiring the thinned data 105 necessary for reconstructing one image within the time intervals 1041, 1043,. , And the second sequence for acquiring the thinning data 106 necessary for reconstructing one image. The MRI apparatus according to the present embodiment sequentially acquires the thinned data 1051, 1061, 1052,... In time series by repeatedly executing the first sequence and the second sequence alternately at the time interval 104. In the first sequence, for example, when the number of phase encodes is 64, data of phase encodes 1, 3, 5,. For this reason, the thinning data 105 is composed of odd phase encode line data. At this time, in the second sequence, data of phase encodes 2, 4, 6,. Therefore, the thinning data 106 is composed of even phase encode line data.
[0040]
As described above, when the thinned data 105 and 106 are obtained by thinning out different phase encode lines in the first sequence and the second sequence, the thinned data 105 and 106 are combined to measure all the k space. It can be filled with and complemented. Therefore, in the present embodiment, two adjacent thinned data 105 and thinned data 106 are combined by the data combining process 107, and the central region of the combined measurement data is extracted, thereby reducing the sensitivity distribution data acquisition low. Frequency domain data 1102, 1103,... Are generated.
[0041]
Specifically, the pre-measurement sequence 101 is executed to acquire the low frequency region data 102 of each of the coils 404 and 4051 to 4053, and the first sequence is executed within the time interval 1041 to Thinned data 1051 of 4051 to 4053 is acquired. In the pre-measurement sequence 101, the interval of the phase encoding step is not thinned out to be the same as that of the normal phase encoding step, the number of phase encoding is reduced to a normal fraction, and the number of repetitions of the sequence is reduced. For example, when the number of phase encodes in k-space is 64, all of the phase encode measurement data from 1 to 64 are usually measured, whereas in the pre-measurement sequence 101, the phase encode step is the same, for example, low Data of phase encoding 25 to 40 in the frequency domain is measured. In this case, the number of phase encodes is 1/4.
[0042]
The signal processing unit 407 performs data synthesis processing 1071 on the low-frequency region data 102 of each coil 404 and 4051 to 4053 and the thinned data 1051 of each coil 404 and 4051 to 4053, so that each coil 404 and 4051 to 4053 Low frequency region data 1101 is obtained. In this data synthesizing process 1071, the data of the phase encode line of the low frequency region data 102 is replaced with the data of the phase encode line of the thinned data 1051 corresponding to the phase encode line of the low frequency region data 102, and the low frequency region data 1101. Is generated. That is, the odd phase encode line data in the low frequency region data 102 is updated to the odd phase encode line data in the low frequency region of the thinned data 1051 by the data synthesis processing 1071. The signal processing unit 407 performs Fourier transform on the low-frequency region data 1101 of each of the coils 404 and 4051 to 4053 updated in this way, and sensitivity images 701 of the coils 404 and 4051 to 4053 corresponding to the time interval 1041. 7021 to 7023 are generated.
[0043]
On the other hand, the thinned data 1051 of the RF receiving coils 4051 to 4053 from which the data of the odd phase encode lines are acquired at equal intervals by the first sequence is subjected to the area reduction processing 1081 by the signal processing unit 407, and actually Only the measured part is taken out. Thereby, the thinning data 1091 of the RF receiving coils 4051 to 4053 are respectively created. The signal processing unit 407 performs Fourier transform on the thinned data 1091 of the RF receiving coils 4051 to 4053, respectively, and generates folded images 7031 to 7033 corresponding to the time interval 1041. The aliased images 7031 to 7033 corresponding to the time interval 1041 generated in this way are eliminated by the sensitivity distribution data 705 calculated from the sensitivity images 701 and 7021 to 7023 corresponding to the time interval 1041, and the time interval 1041. A corresponding result image 708 is displayed on the display unit 408.
[0044]
In addition, after the first sequence is executed, the second sequence is executed within the time interval 1042 after the time interval 1041, and the thinned data 1061 of each coil 404, 4051 to 4053 is acquired. In the signal processing unit 407, the thinned data 1051 of the coils 404 and 4051 to 4053 and the thinned data 1061 of the coils 404 and 4051 to 4053 are subjected to the data synthesis processing 1072, respectively, so that the low frequency of the coils 404 and 4051 to 4053 is obtained. Area data 1102 is obtained. In this data synthesizing process 1072, the odd phase encode line data of the decimation data 1051 and the even phase encode line data of the decimation data 1061 are synthesized, the central area is extracted, and the low frequency area data 1102 is generated. The The low frequency region 1102 is processed in the same manner as the low frequency region data 1101 by the signal processing unit 407, and sensitivity images 701 and 702 corresponding to the time interval 1042 are generated.
[0045]
On the other hand, the thinned data 1061 of the RF receiving coils 4051 to 4053 from which the data of the even-numbered phase encode lines are acquired at equal intervals by the second sequence is subjected to the area reduction processing 1082 in the same manner as the thinned data 1051. Thinned data 1092 of the RF receiving coils 4051 to 4053 is created. The thinned data 1092 of these RF receiving coils 4051 to 4053 is processed in the same manner as the thinned data 1091 by the signal processing unit 407, and a folded image 703 corresponding to the time interval 1042 is generated. The folded image 703 is removed by the sensitivity distribution data 705 calculated from the sensitivity images 701 and 702 corresponding to the time interval 1042, becomes a result image 708 corresponding to the time interval 1042, and is displayed on the display unit 408.
[0046]
In addition, after the second sequence is executed, the first sequence is executed again within the time interval 1043 after the time interval 1042, and the thinned data 1052 of each coil 404, 4051 to 4053 is acquired. The In the signal processing unit 407, the thinned data 1061 of the coils 404 and 4051 to 4053 and the thinned data 1052 of the coils 404 and 4051 to 4053 are subjected to data synthesis processing 1073, whereby the low frequency regions of the coils 404 and 4051 to 4053 are obtained. Data 1103 is obtained. In this data synthesizing process 1073, even phase encode line data of the thinned data 1061 and odd phase encode line data of the thinned data 1052 are synthesized, and the central region is extracted to generate low frequency region data 1103. Is done. Thereafter, processing similar to that of the low-frequency region data 1101 and 1102 is performed, and sensitivity images 701 and 702 corresponding to the time interval 1043 are generated.
[0047]
On the other hand, the thinned-out data 1052 obtained by executing the first sequence again is processed in the same manner as the thinned-out data 1051, and the thinned-out data 1093 of each RF receiving coil 4051-4053 is created. The The thinned data 1093 of each of these RF receiving coils 4051 to 4053 is processed in the same manner as the thinned data 1091 and 1092, and a folded image 703 corresponding to the time interval 1043 is generated. The folded image 703 is removed by the sensitivity distribution data 705 calculated from the sensitivity images 701 and 702 corresponding to the time interval 1043 and becomes a result image 708 corresponding to the time interval 1043.
[0048]
In other words, in the MRI apparatus of this embodiment, the main measurement sequence for acquiring thinned data is divided into a number of phase encoding groups corresponding to the thinning number, and the phase encoding used for imaging is selected for each of the divided groups. At the same time, sensitivity distribution data of each RF receiving coil is created by combining the thinned measurement data acquired in the group, and the aliasing of the image is removed based on the sensitivity distribution data. Moreover, when dividing this measurement sequence into a plurality of groups, it is possible to divide the measurement sequence into group numbers that are repeated in time, with the same number of thinning steps as the number of groups. When phase encoding is selected, the phase encoding start amount can be changed according to the group number. Further, when the sensitivity distribution of each RF receiving coil is created by combining signals acquired in groups, signals acquired in temporally adjacent groups can be combined.
[0049]
When the present invention is applied to parallel imaging at a speed of 2 × or higher, the fluoroscopy sequence is divided into a number equal to the number B of the parallel imaging speed B. For example, the phase encode line P (i, Change j) cyclically and execute.
[0050]
[Equation 9]
P (i, j) = B × j−i% B (9)
Here, i is an image number (1 ≦ i ≦ N, N is the number of images) of the fluoroscopic sequence, and is incremented by 1 each time the time interval is repeated. j is a phase encoding number (1 ≦ j ≦ KY, KY is the number of phase encodings) acquired by the parallel imaging method, and% is an operator for obtaining a remainder of division.
[0051]
In the conventional MRI apparatus, the execution time of the pre-measurement sequence accounts for a large proportion of the total imaging time of parallel imaging. For example, when the low frequency region data for sensitivity distribution data and the thinned data for the folded image are acquired every time in the time interval for repeatedly executing the fluoroscopy sequence, the removal of the folding of the image obtained from the thinned data is: This is performed based on sensitivity distribution data acquired within the same time interval. However, since the pre-measurement sequence for sensitivity distribution data acquisition is executed every time during the time interval for repeatedly executing the fluoroscopy sequence, the sequence repetition time interval becomes longer and the time resolution of the obtained image sequence is reduced. To do. That is, when observing the dynamics of a subject by performing continuous imaging, if the sensitivity distribution data is acquired together within the repetition of the sequence, the update time interval for one image becomes longer due to the execution of the previous measurement sequence. .
[0052]
On the other hand, the MRI apparatus of this embodiment executes the first and second sequences for acquiring the data of the different phase encode lines alternately at the time interval 104 to acquire the thinned data 105 and 106 and synthesize them. In this case, low frequency region data 1102, 1103,... For sensitivity distribution data acquisition is generated. For this reason, it is possible to shorten the data acquisition time per image and improve the time resolution of the image sequence as compared with the case where the previous measurement sequence is executed every time during the repetition time interval.
[0053]
As a conventional MRI imaging method, first, a pre-measurement sequence is executed to acquire sensitivity distribution data, and then the fluoroscopy sequence of this measurement is repeatedly executed at a predetermined time interval. Thus, there is a method of acquiring data necessary to reconstruct one image and removing the aliasing of the image using the sensitivity distribution data first acquired in the previous measurement sequence. In this case, since the aliasing of all images is removed using the sensitivity distribution data acquired first, if the subject moves or the imaging cross section is changed using interactive scan control etc., the sensitivity distribution There is a case where the image is deteriorated or the image cannot be correctly reconstructed due to the occurrence of the aliasing in the area where the value is not calculated.
[0054]
On the other hand, in the MRI apparatus according to the present embodiment, the low frequency region data 110 for acquiring sensitivity distribution data is sequentially updated as the imaging progresses, so that a suitable sensitivity corresponding to the folded image at each time interval is obtained. Parallel imaging reconstruction processing can be performed using the distribution data. For this reason, even when the subject moves or the imaging section is changed, it is possible to reduce the deterioration of the image quality.
[0055]
In this embodiment, since the low frequency region is extracted from the data obtained by combining the thinned data 105 and 106 and the sensitivity image is generated, the sensitivity image is an image on which a strong low-pass filter is applied. For this reason, the influence which the change in the subject between data has on the result image can be reduced.
[0056]
(Second Embodiment)
A second embodiment of the MRI apparatus to which the present invention is applied will be described with reference to FIG. FIG. 8 is a diagram for explaining a state in which measurement data acquired in the second embodiment of the MRI apparatus is arranged in the measurement space. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof is omitted, and configurations and features that are different from those in the first embodiment will be described.
[0057]
The difference between this embodiment and the first embodiment is that four RF receiving coils are used, the phase encoding step in the high frequency region is thinned out by 4 times, and the phase encoding step in the low frequency region is thinned out by 2 times. is there. Therefore, in this embodiment, as shown in FIG. 8, the phase encoding step to be thinned out during measurement data acquisition is changed, and the phase encoding step interval is quadrupled in the high frequency regions 1241 and 1242 and the phase in the low frequency region 125. The encode step interval is doubled, and the encode step in the high frequency region 124 is set larger than the encode step in the low frequency region 125. At this time, there are one or three k-space phase encode lines from which data is acquired. In other words, in the present embodiment, the data is acquired by thinning out the phase encoding step four times in the entire k space, and the data is acquired so that the interval of the phase encoding steps in the low frequency region 125 is doubled. it can.
[0058]
FIG. 8A schematically shows a k-space of measurement data to be acquired. The k-space is divided into high-frequency areas 1241 and 1242 indicated by shading and a low-frequency area 125 indicated by white. ing. FIG. 8B schematically shows a state in which data acquired by executing the sequence of this embodiment is arranged in the k space. 8C shows a state where the data acquired by executing the pre-measurement sequence 101 is arranged in the k space, and the thinned data 105 and 106 indicate the phase encoding step of the high frequency region 124. 4 schematically shows a state in which the data acquired by executing the first and second sequences of the fluoroscopy sequence 103 in which the phase encoding step of the low frequency region 125 is set to 2 is arranged in the k space. Is.
[0059]
Here, although the phase encoding step of the high frequency region 124 of the thinned data 1051 is thinned by four times, the thinning rate of the phase encoding step of the low frequency region 125 is the same as that of the first embodiment. The unit 407 generates the low frequency region data 1101 of the coils 404 and 4051 to 4053 corresponding to the time interval 1041 by processing the low frequency region data 102 and the thinned data 1051 in the same manner as in the first embodiment. Can do.
[0060]
In addition, when the thinned data 105 and the thinned data 106 are subjected to the data synthesizing process 107, since the phase encoding step is thinned by four times in the high frequency region 124, the high frequency region 124 is not filled with the measurement data. Since the interval between the phase encoding steps is twice and different phase encoding lines are thinned out, the low frequency region 125 can be filled with the measurement data. Therefore, the signal processing unit 407 can generate the low frequency regions 1102, 1103,... Of the coils 404, 4051 to 4053 corresponding to the time interval 1042, as in the first embodiment.
[0061]
On the other hand, since the phase encoding is measured at least every three of the thinned data 105 and 106, the line with the phase encoding step of 4 is extracted by the area reduction processing 108, and the number of data in the phase encoding direction is reduced to 1/4. It is possible to create the thinning-out data 109 reduced to each of the RF receiving coils 4051 to 4053, respectively. The thinned data 109 created for each of the RF receiving coils 4051 to 4053 is reduced in the number of data in the phase encoding direction to ¼, so that the image may be folded back, but the RF receiving coils 4051 to 4053 are generated accordingly. Since it is composed of four coils, the aliasing of the image reduced to ¼ can be removed.
[0062]
In this way, by changing the phase encoding step during data acquisition, the phase encoding step in the low frequency region 125 is doubled, and the phase encoding step in the high frequency region 124 is thinned out by four times, so that all the phase encoding steps are doubled. Compared with the case of thinning out, the number of repetitions of the sequence per image can be reduced, the time interval of image acquisition can be shortened, and the time resolution of the image can be improved.
[0063]
In the present embodiment, the low frequency region data for sensitivity distribution acquisition can be updated every time thinning data is acquired. Therefore, parallel imaging can be re-executed using suitable sensitivity distribution data corresponding to each folded image. Configuration processing can be performed. In the present embodiment, the case of a two-dimensional fluoroscopy sequence has been described. However, the present embodiment can be similarly applied to multiphase imaging and three-dimensional imaging.
[0064]
(Third embodiment)
A third embodiment of an MRI apparatus to which the present invention is applied will be described with reference to FIG. FIG. 9 is a diagram for explaining data synthesis in the signal processing unit of the third embodiment of the MRI apparatus. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof is omitted, and configurations and features that are different from those in the first embodiment will be described.
[0065]
This embodiment is different from the first embodiment in that three-dimensional imaging is performed by thinning out the phase encode step twice and thinning out the slice encode step twice. That is, in the MRI apparatus of the present embodiment, the fluoroscopy sequence 103 is composed of first to fourth sequences in which phase encoding steps and slice encoding steps which are different from each other are thinned out twice. The fourth sequence is executed to acquire data of phase encode lines and slice encode lines that are different from each other, and acquire thinned data 116, 117, 118, 119,. The MRI apparatus of the present embodiment takes the first to fourth sequences as one set, and by repeatedly executing this set, the thinned data 116, 117, 118, 119,... Are sequentially acquired in time series. By synthesizing the thinned data 116, 117, 118, 119,... In the data synthesis process 121, the data in the central area of the ky-kz space is filled with data, and data 123 for obtaining sensitivity distribution is generated. .
[0066]
Here, the thinned-out data 116 to 119 in FIG. 9 schematically show a state where the data acquired by executing the first to fourth sequences are arranged in the ky-kz space, respectively. A case where double-speed parallel imaging is applied to each of the direction ky and the slice encoding direction kz is shown. Therefore, the white grid indicates a portion from which data has been acquired, and the black grid indicates a portion from which data has not been acquired.
[0067]
On the other hand, the thinned data 116 acquired by executing the first sequence is obtained by collecting the actually acquired data in the area reduction processing 1201 and reducing the thinned data 1221 in the phase encoding direction and the slice encoding direction. Created. Similarly, the thinned data 117, 118, and 119 acquired by executing the second to fourth sequences are also reduced by the area reduction processing 1202, 1203, and 1204, respectively, and become thinned data 1222, 1223, and 1224.
[0068]
That is, the MRI apparatus according to the present embodiment divides the number of phase encoding and slice encoding according to the number of thinning out, selects the phase encoding and slice encoding used for imaging for each group, and acquires the group. The sensitivity distribution of each RF receiving coil is created by combining the signals, and the aliasing of the image is removed by this sensitivity distribution.
[0069]
In addition, when the present invention is applied to parallel imaging at a speed of 2 × or higher, the phase encoding acquired according to the equations (10) and (11), for example, using the speed double By in the phase encoding direction and the speed multiple Bz in the slice encoding direction. The process is executed by cyclically changing the drain line Ry (i, j) and the slice encode line Rz (i, k).
[0070]
[Expression 10]
Ry (i, j) = By × j−i% By (10)
[0071]
## EQU11 ##
Rz (i, k) = Bz × k − ((i−1)% (By × Bz)) / By (11)
Here, i is the image number of the fluoroscopic sequence (1 ≦ i ≦ N, N is the number of images), j is the phase encoding number acquired by the parallel imaging method (1 ≦ j ≦ KY, KY is the number of phase encodings), k Is a slice encoding number (1 ≦ k ≦ KZ, where KZ is the number of slice encodings) acquired by parallel imaging, and% is an operator for calculating the remainder of division. Further, in the division of equation (11), only the integer part of the solution is used.
[0072]
The MRI apparatus according to the present embodiment calculates the number of groups from the thinning step in the phase encoding direction and the thinning step in the slice direction when dividing into a number of phase encoding and slice encoding groups corresponding to the thinning number. Can be divided into group numbers that repeat. Also, when selecting phase encoding and slice encoding, the start amount of phase encoding and slice encoding is changed according to the group number, or the k-space is divided into two regions, a low frequency region and a high frequency region. The encoding step and the encoding step in the high frequency region can be changed.
[0073]
(Fourth embodiment)
A fourth embodiment of an MRI apparatus to which the present invention is applied will be described with reference to FIGS. FIG. 10 is a diagram for explaining data acquisition in the control unit of the fourth embodiment of the MRI apparatus. FIG. 11 is a diagram for explaining data synthesis in the signal processing unit of the fourth embodiment of the MRI apparatus. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof is omitted, and configurations and features that are different from those in the first embodiment will be described.
[0074]
This embodiment is different from the first embodiment in that an electrocardiograph is attached to the subject, a signal representing the heart beat cycle of the subject is taken into the MRI apparatus, and the beat cycle is divided into a plurality of sections. A time phase formed by division is set, a plurality of sequences are executed in correspondence with the time phases, and a sequence in which different phase encoding steps are thinned out in adjacent time phases is executed. In addition, measurement data acquired over a plurality of pulsation periods are arranged in k space corresponding to each time phase, and an image corresponding to each time phase is generated based on the measurement data arranged in this k space. However, the sensitivity distribution data is generated by synthesizing the measurement data in the k space of the adjacent time phases, and the aliasing of the image is removed.
[0075]
The MRI apparatus of the present embodiment includes an electrocardiograph (hereinafter referred to as ECG) that detects a biological signal periodically generated by the subject 401, for example, an electrocardiogram signal. The ECG is mounted on the subject 401, detects an electrocardiogram signal generated by the subject 401, and outputs it to the control unit 411. When imaging a heart region, measurement is performed in synchronization with an electrocardiogram signal output from the ECG, thereby suppressing artifacts due to blood flow and movement of the heart itself. Moreover, in this embodiment, not only ECG but various instruments which detect biological signals, such as a pulse wave meter, can be applied.
[0076]
In the electrocardiogram synchronous imaging of the MRI apparatus of the present embodiment, as shown in FIG. 10, for example, an R wave 8011 of the electrocardiogram waveform 800 is used as a trigger, and a delay time 8031 of about several tens to several hundreds ms is provided therefrom. The data acquisition window 8041 after the delay time 8031 is divided into a plurality of sections, and time phases 8111,. A sequence is executed for each set time phase 8111,..., 8151, and data 8211,. This is done over a plurality of beat cycles. The pulsation cycle is periodically repeated at a time interval 8021 of one heartbeat from R wave 8011 to R wave 8012, for example.
[0077]
Further, as shown in FIG. 11, the MRI apparatus of the present embodiment includes k spaces 1121, 1131, 1122, 1132,... Corresponding to the respective time phases 811,. In this case, for example, the k spaces 112 and 113 are provided as data spaces in the memory in the signal processing unit 407, and the data 821,..., 825 respectively acquired in the respective time phases 811,. Are arranged in the k spaces 1121, 1131, 1122, 1132,.
[0078]
Specifically, when the R wave 8011 of the electrocardiogram waveform 800 is detected, the control unit 411 performs the first and second sequences based on the R wave 8011 after the delay time 8031 has elapsed from the R wave 8011. Are alternately executed, and the respective data 8211,..., 8251 are acquired at the respective time phases 8111,. The data 8211,..., 8251 acquired in each time phase 8111,..., 8151 are respectively arranged in the k spaces 1121, 1131, 1122, 1132,. At this time, the data 8211 is arranged in the first line of the k space 1121, the data 8221 is arranged in the second line of the k space 1131, the data 8231 is arranged in the first line of the k space 1122, and the data 8241 is k space. 1132 is arranged in the second line. Then, when the second R wave 8012 is detected, the first and second sequences are alternately executed in the same manner as in the first time, and the respective data 8212, 8222 in the respective time phases 8112, 8122,. Get ... The acquired data 8212 is in the third line of the k space 1121, the data 8222 is in the fourth line of the k space 1131, the data 8232 is in the third line of the k space 1122, and the data 8242 is in the fourth line of the k space 1132. Each is arranged.
[0079]
.., 815 corresponding to odd-numbered time phases 811, 813,..., 815, and even-numbered k-space 113 corresponding to even-numbered time phases 812, 814,. It repeats over a plurality of pulsation cycles until all the phase encode lines are filled with the acquired data. As a result, k-spaces 112 and 113 in which different phase encoding steps are thinned out are acquired. That is, FIG. 11 shows a case where the parallel imaging method with a double speed number of 2 is applied to multi-phase cardiac imaging using ECG, and data 821 acquired in the data acquisition window 804 over a plurality of beat cycles. ,... 825 are schematically shown in which time phases 811,.
[0080]
When this embodiment is applied to parallel imaging at a speed of 2 × or higher, the time phase acquired in the data acquisition window is divided into a number of data groups equal to the number B of the parallel imaging speed, for example, Expression (12) The phase encoding line Q (k, j) acquired according to is cyclically changed and executed.
[0081]
[Expression 12]
Q (k, j) = B × j−k% B (12)
Here, k is a time phase number of multiphase measurement (1 ≦ k ≦ M, M is the number of multiphases), j is a phase encoding number acquired by parallel imaging (1 ≦ j ≦ KY, KY is the number of phase encodings). Yes,% is an operator for calculating the remainder of division. In addition, the time phase number is sequentially assigned to each time phase 811,.
[0082]
By performing data acquisition using such phase encoding, the MRI apparatus of the present embodiment can fill the entire k space by combining data of adjacent time phases. Therefore, as in the case of the first embodiment, the two adjacent time phases of the k spaces 112 and 113, for example, the k space 1121 and the k space 1131 are combined by the data combining processing 1071 and extracted to extract the sensitivity. Low frequency region data 1151 for obtaining distribution data is created. Similarly, the low frequency region data 1152 is obtained by combining the k space 1131 and the k space 1122 by the data composition processing 1072, and the low frequency region data 1153 is obtained by combining the k space 1122 and the k space 1132 by the data composition processing 1073. It is.
[0083]
In addition, the k spaces 112 and 113 corresponding to the respective time phases 811 to 815 are reduced in the phase encoding direction by the area reduction processing 108, and the thinned data 114 corresponding to the respective time phases 811 to 815 are respectively obtained. A folded image 703 is obtained. By displaying the result images 708 generated based on the folded image 703 in chronological order and displaying them in cine, the motion of the heart can be observed as a moving image.
[0084]
In the present embodiment, since the acquisition time of the thinned data 114 and the low frequency region data 115 does not correspond exactly, either the back side or the front side of the thinned data 114 in terms of time during parallel imaging reconstruction. The low frequency region data 115 is selected and used, or two adjacent low frequency region data 115 are averaged and used. For example, the low-frequency region data 115 on the temporal side of the thinned data 1142 is the low frequency region data 1151 generated by performing the data synthesis processing 1071 on the k space 1121 and the k space 1131 corresponding to the time phase 811 before the thinned data 1142. This is the frequency domain data 1151 and is generated by performing the data synthesis process 1072 on the k space 1122 and the k space 1131 corresponding to the time phase 813 after the thinned data 1142. This is low frequency region data 1152. That is, when the thinning data 1142 is reconstructed, sensitivity distribution data generated based on either the low frequency region data 1151 or the low frequency region data 1152 is used, or the low frequency region data 1151 and the low frequency region data 1152 are used. Each sensitivity distribution data generated based on the above can be averaged and used.
[0085]
By the way, in the conventional parallel imaging method, in order to ensure imaging time, imaging | photography may be divided into 2 times, a breathing may be stopped and a pre-measurement sequence and this measurement sequence may be performed. However, in multi-phase imaging of the heart, the heart moves violently within a short time of several tens of ms, so if the subject position in the previous measurement sequence and the main measurement sequence are not matched, the image may deteriorate. is there.
[0086]
On the other hand, since the MRI apparatus of this embodiment does not acquire additional data for sensitivity distribution acquisition, the MRI apparatus can take an image in one breath holding time, and a position shift that occurs when the subject breathes. Can be avoided and image deterioration can be prevented. Furthermore, in order to smooth the motion of the moving image, it is necessary to subdivide the time phase and increase the number of multiphases. However, in this embodiment, the data acquisition time for one image can be reduced, so that the subject breathes. More images can be acquired while stopping. As a result, a larger number of multiphases can be set.
[0087]
Further, in the present embodiment, the data of the k-spaces 112 and 113 having different time phases are mixed in the low-frequency region data 115 for obtaining the sensitivity distribution, but the time interval is as short as several tens of ms or less. Since the low frequency region data 115 is an image with a small number of phase encodes and a strong low-pass filter, it is possible to reduce degradation of the resulting image.
[0088]
In addition, the MRI apparatus of the present embodiment adjusts the delay time 803 and the data acquisition window 804 to specify a specific time range, that is, a time phase obtained by dividing the pulsation period into a plurality of sections. A temporal cardiac image can be acquired.
[0089]
In this embodiment, the data acquired at each time phase of one heartbeat is for one line of phase encoding, but data for a plurality of lines can be acquired by the time interval of each time phase. In this case, the heart rate required for photographing can be reduced.
[0090]
In this embodiment, data is acquired from above the k-spaces 112 and 113, but data can be acquired from various places such as the center and below. In short, it is only necessary that k-space data corresponding to adjacent time phases are acquired by different phase encode lines.
[0091]
The present invention is not limited to the contents disclosed in the above embodiments, and can take various forms based on the gist of the present invention. In parallel imaging, two or more arbitrary numbers of multiple RF receiving coils can be used, and the double speed number can be selected within a range up to the same number as the number of RF receiving coils used. For example, in the first to fourth embodiments, the case where four RF receiving coils are used has been described, but the number of RF receiving coils is not limited to four. That is, the number may be eight or any other number.
[0092]
The MRI apparatus of the first to fourth embodiments forms an RF receiving coil group by combining one or more RF receiving coils, and synthesizes a sensitivity image and a folded image for each RF receiving coil group. can do.
[0093]
In the first to fourth embodiments, the gradient echo sequence is described. However, the parallel imaging method does not depend on the shape of the sequence. For example, the present invention can be applied to various sequences such as an SE sequence, an FSE sequence, an EPI sequence, a spiral sequence, and an SSFP sequence. In the case of three-dimensional imaging, it is possible to increase the speed by thinning out data not only in the phase encoding direction but also in the slice encoding direction.
[0094]
The MRI apparatus of the present invention generates sensitivity images with uniform sensitivity distribution by synthesizing sensitivity image data acquired by a plurality of RF receiving coils, instead of the transmitting / receiving coil 404 of the first to fourth embodiments. The sensitivity distribution of the RF receiving coil can be obtained using this synthesized sensitivity image.
[0095]
Further, the multiple RF receiving coil of the MRI apparatus of the present invention is not limited to the first to fourth embodiments, but as a multiple RF coil for a horizontal magnetic field head, an array head coil for improved functional MRI (Christophe Leusler), 1996 ISMRM. abstract p. As a QD multiple RF coil for the horizontal magnetic field head, Helmet and Cylindrical Shaped CP Array Coils for Brain Imaging: A Comparison of Signal-to-Noise Characteristics (H.A.M.St. P. 1412, and as a QD multiple RF coil for horizontal magnetic field abdomen, Four Channel 1 Wrap-Around Coil with Inductive Decoder for 1.5T Body Imaging (T. Takahashi et.al), 1995 IMR. Various multiple RF receiver coils such as 1418 can be applied. Multiple RF receiver coils are arranged with multiple relatively high-sensitivity small RF receiver coils, and by synthesizing the signals acquired by each coil, the field of view is expanded while maintaining high sensitivity of the RF receiver coil, and high sensitivity The result image reconstructed by each RF receiving coil is combined to expand the field of view and achieve high sensitivity.
[0096]
In the first to fourth embodiments, the case of applying to a horizontal magnetic field MRI apparatus in which the static magnetic field direction (Z direction) is horizontal has been described. However, the present invention is not limited to this, and the direction of the static magnetic field is When different, for example, a vertical magnetic field can be similarly applied.
[0097]
【The invention's effect】
According to the present invention, the imaging time of the MRI apparatus can be shortened.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining data acquisition in a control unit and data synthesis in a signal processing unit of a first embodiment of an MRI apparatus to which the present invention is applied.
FIG. 2 is a schematic configuration diagram of a first embodiment of an MRI apparatus to which the present invention is applied.
FIG. 3 is a diagram for explaining an RF receiving coil according to a first embodiment of an MRI apparatus to which the present invention is applied.
FIG. 4 is a schematic configuration diagram of a multiple RF receiving coil and a receiving unit according to the first embodiment of the MRI apparatus to which the present invention is applied.
FIG. 5 is a diagram for explaining a pulse sequence of an MRI apparatus to which the present invention is applied.
FIG. 6 is a diagram for explaining folding of an image by a parallel imaging method in an MRI apparatus to which the present invention is applied.
FIG. 7 is a diagram showing a signal processing procedure of the signal processing unit according to the first embodiment of the MRI apparatus to which the present invention is applied.
FIG. 8 is a diagram illustrating a state in which measurement data acquired in a second embodiment of an MRI apparatus to which the present invention is applied is arranged in a measurement space.
FIG. 9 is a diagram for explaining data synthesis in the signal processing unit of the third embodiment of the MRI apparatus to which the present invention is applied;
FIG. 10 is a diagram illustrating data acquisition in the control unit of the fourth embodiment of the MRI apparatus to which the present invention is applied.
FIG. 11 is a diagram for explaining data synthesis in the signal processing unit of the fourth embodiment of the MRI apparatus to which the present invention is applied.
[Explanation of symbols]
101 Pre-measurement sequence
102, 110 Low frequency data
103 Fluoroscopy sequence
104 hour interval
105, 106, 109 Thinned out data
107 Data composition processing
108 Area reduction processing
401 Subject
404 Transmitting and receiving coil
405 RF receiver coil
407 Signal processor
408 display unit
411 control unit
701, 702 Sensitivity image
703 Folded image
704 Sensitivity distribution calculation processing
705 Sensitivity distribution data
706 Matrix creation process
707 Inverse matrix calculation processing
708 result image

Claims (9)

A plurality of RF receiving coils for receiving magnetic resonance signals generated from the subject;
and had thinning phase encoding steps of k-space, the measurement control means for acquiring k-space data thinned every said RF receiver coil,
Magnetic resonance comprising signal processing means for reconstructing one image without aliasing using the thinned k-space data acquired for each RF receiver coil and the sensitivity distribution for each RF receiver coil In the imaging device,
The measurement control means acquires two or more pieces of k-space data in which the phase encoding step is thinned out for each RF receiving coil,
The magnetic resonance imaging apparatus , wherein the signal processing unit synthesizes the image for each k-space data having different thinning methods . The magnetic resonance imaging apparatus according to claim 1.
The signal processing means generates a sensitivity distribution of the RF receiving coil by using at least two of the two or more k-space data of different thinning methods . The magnetic resonance imaging apparatus according to claim 2.
The magnetic resonance imaging apparatus characterized in that the signal processing means generates a sensitivity distribution of the RF receiving coil using data in a low frequency region of the k-space data . The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus, wherein the measurement control unit acquires k-space data having two or more different thinning methods by thinning out different phase encoding steps. The magnetic resonance imaging apparatus according to claim 4.
The measurement control means obtains k-space data obtained by thinning out odd-numbered phase encoding steps and k-space data obtained by thinning out even-numbered phase encoding steps. The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The measurement control means obtains k-space data having different two or more thinning methods by setting a phase encoding step interval in a high frequency region to be larger than a phase encoding step interval in a low frequency region. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The measurement control means alternately and repeatedly acquires k-space data having different thinning methods to the two or more
  The magnetic resonance imaging apparatus, wherein the signal processing unit reconstructs the image every time the k-space data having the different thinning-out methods is acquired. The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
Means for detecting heart beat of the subject;
The measurement control unit divides one cycle of the pulsation into a plurality of time phases, and acquires k-space data having different thinning methods for each time phase. A plurality of RF receiving coils for receiving magnetic resonance signals generated from the subject;
measurement control means for thinning out the k-space phase encoding step and acquiring k-space data thinned out for each RF receiving coil;
While generating a folded image using the k-space data acquired for each RF receiving coil, and removing the folding using a sensitivity distribution for each RF receiving coil. In a magnetic resonance imaging apparatus comprising a signal processing means for synthesizing a plurality of images,
The measurement control means acquires two or more pieces of k-space data in which the phase encoding step is thinned out for each RF receiving coil,
The signal processing means includes the 2 At least of the above k-space data with different thinning methods 2 A magnetic resonance imaging apparatus characterized in that, using the two, a sensitivity distribution for each RF receiving coil is generated and the image is synthesized for each k-space data having a different thinning method.

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