JP3847018B2 - RF coil for MRI - Google Patents

Description

【０００９】
一方、リングコイル１とリングコイル２の間の相互インダクタンスをＭ１とし、図２のように、リングコイル１における電流Ｉの正の向きとリングコイル２における電流Ｉ２の正の向きとを同方向に決め、Ｍ１＞０と規定すると、相互インダクタンスＭ１により図のＡＢ間に誘起される電圧ＶBAは、

【００１０】
上記(１)式で表される電圧ＶBAと，上記(２)式で表される電圧ＶBAが等しくなると、相互干渉の影響により、リングコイル２に誘導電流が流れることが防止されるから、

【００１１】
一方、ループＡＢＤＥにおけるキルヒホッフの法則より、

【００１２】
上記(３)式と上記(４)式よりリングコイル１とリングコイル２間の相互干渉を打ち消す条件は、

【００１３】
次に、図３に示すように、前記スイッチＳ１，Ｓ２を開き、スイッチＳ３，Ｓ４を閉じ、エレメント導体６にＲＦ電圧源Ｖｒ’により共鳴角周波数ω２の電流Ｉを流した場合を想定する。この場合、鞍型コイルＫ１と，鞍型コイルＫ２の間の相互インダクタンスをＭ２とし、図３のように、エレメント導体５における電流Ｉ５の正の向きとエレメント導体６における電流Ｉの正の向きとを同方向に決め、Ｍ２＞０と規定すると、鞍型コイルＫ１と，鞍型コイルＫ２の間の相互干渉を打ち消す条件は、

【００１４】
上記(５)式を上記(６)式に代入して、Ｃａについて整理すると、

【００１５】
上記(５)式より、ＣｂがＣｂ＞０の解を持つための条件は、Ｍ１＞０ゆえ、

【００１６】
［１］ω１²・Ｍ１−ω２²・Ｍ２＝０のとき、

従って、上記(５)式，上記(６)式を同時に満たす物理的に意味のある解が存在する。
【００１７】
［２］ω１²・Ｍ１−ω２²・Ｍ２≠０のとき、

とおくと、

従って、ｆ（Ｃａ）＝０は、

の範囲で解を持ち、物理的に意味のある解が存在する。
【００１８】
上記［１］［２］より、第１，第２コンデンサの容量Ｃａおよび第３，第４コンデンサの容量Ｃｂを、上記（５）式，上記（６）式を満足するように調整することが可能であることが証明された。このとき、共鳴角周波数ω１に対しては、図２に示したように、リングコイル１，２間の相互インダクタンスＭ１を実質的に無視でき、前記リングコイル１，２を相互干渉の無いＲＦコイルとして使用することが出来る。また、共鳴角周波数ω２に対しては、図３に示したように、鞍型コイルＫ１と，鞍型コイルＫ２の間の相互インダクタンスＭ２を実質的に無視でき、鞍型コイルＫ１，Ｋ２を相互干渉の無いＲＦコイルとして使用することが出来る。
【００１９】
図４は、本発明の一実施形態にかかるＭＲＩ用ＲＦコイルを含む送受信回路１０１の構成図である。図中、Ｂｚは、垂直方向の静磁場を示す。
この送受信回路１０１において、ＭＲＩ用ＲＦコイル１００は、リングコイル１と、リングコイル２と、前記リングコイル１と前記リングコイル２の間に介設されたエレメント導体３，４，５，６と、前記エレメント導体３と前記リングコイル１の接続点と前記エレメント導体４と前記リングコイル１の接続点の間に介設された第１コンデンサＣａと、前記エレメント導体３と前記リングコイル２の接続点と前記エレメント導体４と前記リングコイル２の接続点の間に介設された第２コンデンサＣａと、前記エレメント導体３に介設された第３コンデンサＣｂと、前記エレメント導体４に介設された第４コンデンサＣｂとを具備している。ただし、前記第１，第２コンデンサＣａおよび前記第３，第４コンデンサＣｂは、上記(５)式および上記(６)式を満たすように調整されている。
また、前記エレメント導体５と前記リングコイル１の接続点および前記エレメント導体６と前記リングコイル１の接続点の間に介設された給電用コンデンサＣ５および共鳴角周波数ω２に対して並列共振する並列共振回路（コンデンサＣ１とインダクタＬ１の並列回路）と、前記エレメント導体５と前記リングコイル２の接続点および前記エレメント導体６と前記リングコイル２の接続点の間に介設された給電用コンデンサＣ６および共鳴角周波数ω２に対して並列共振する並列共振回路（コンデンサＣ２とインダクタＬ２の並列回路）とを具備している。すなわち、前記並列共振回路を構成するコンデンサの容量Ｃ１〜Ｃ４およびインダクタのインダクタンスＬ１〜Ｌ４は、次式を満たす。

さらに、前記リングコイル１には、２つの直流阻止コンデンサＣが介設されている。また、前記リングコイル２には、２つの直流阻止コンデンサＣが介設されている。
【００２０】
次に、この送受信回路１０１の動作を説明する。
［１］第１の核種（例えばプロトン）を励起するため、共鳴角周波数（＝ラーモア角周波数）ω１の励起用ＲＦパルスを送信するとき、励起用ＲＦパルスの電力をパワーアンプ１０ａで増幅し、ハイブリッド（hybrid）１１ａで前記電力を等分割して、送信／受信切換スイッチ１２−１，１２−２に入力する。前記送信／受信切換スイッチ１２−１は、前記励起用ＲＦパルスをバラン１３−１側に出力する切換状態とし、当該バラン１３−１を介して前記励起用ＲＦパルスを給電用コンデンサＣ５の両端に印加する。一方、前記送信／受信切換スイッチ１２−２は、前記励起用ＲＦパルスをバラン１３−２側に出力する切換状態とし、当該バラン１３−２を介して前記励起用ＲＦパルスを給電用コンデンサＣ６の両端に印加する。なお、前記送信／受信切換スイッチ１２−１と前記バラン１３−１とを結ぶ伝送路の長さ、および、前記送信／受信切換スイッチ１２−２と前記バラン１３−２とを結ぶ伝送路の長さは、共鳴角周波数ω１に対応する波長をλ１とするとき、λ１／４である。
このとき、図５に示すように、前記コンデンサＣ３およびインダクタＬ３は並列共振状態（＝インピーダンス無限大）となると共に、前記コンデンサＣ４およびインダクタＬ４は並列共振状態となって、当該並列共振部分が実質的に切り離される。これにより、前記リングコイル１，２は、相互干渉のない１組の送信コイルとして機能し、水平方向の振動磁場ｂｘ（図示の向き）を発生する。なお、コンデンサＣ１およびインダクタＬ１の並列回路，コンデンサＣ２およびインダクタＬ２の並列回路は、共鳴角周波数ω１に対しては、共鳴角周波数ω２との大小に依存して誘導性または容量性を示すので、給電用コンデンサＣ５，Ｃ６の容量を調整することで、前記リングコイル１，２の共振周波数を共鳴角周波数ω１に同調させることが出来る。
［２］共鳴角周波数ω１のＮＭＲ信号を受信するとき、前記送信／受信切換スイッチ１２−１は、給電用コンデンサＣ５の両端からバラン１３−１を介して取り出されたＮＭＲ信号をプリアンプ１４−１側に出力する。前記プリアンプ１４−１は、前記ＮＭＲ信号を増幅し、受信器に送る。一方、前記送信／受信切換スイッチ１２−２は、給電用コンデンサＣ６の両端からバラン１３−２を介して取り出されたＮＭＲ信号をプリアンプ１４−２側に出力する。このとき、送信時と同様に、前記ＭＲＩ用ＲＦコイル１００は、図５に示す状態となり、前記リングコイル１，２は、相互干渉のない１組の受信コイルとして機能し、水平方向の振動磁場ｂｘを検出する。なお、各受信器に送られたＮＭＲ信号を処理することで、前記リングコイル１，２をフェーズドアレイコイル（phased array coil）として使用してもよい。
【００２１】
［３］第２の核種（例えばカーボン）を励起するため、共鳴角周波数ω２の励起用ＲＦパルスを送信するとき、励起用ＲＦパルスの電力をパワーアンプ１０ｂで増幅し、ハイブリッド１１ｂで前記電力を等分割して、送信／受信切換スイッチ１２−３，１２−４に入力する。前記送信／受信切換スイッチ１２−３は、前記励起用ＲＦパルスをバラン１３−３側に出力する切換状態とし、当該バラン１３−３を介して前記励起用ＲＦパルスを給電用コンデンサＣ７の両端に印加する。一方、前記送信／受信切換スイッチ１２−４は、前記励起用ＲＦパルスをバラン１３−４側に出力する切換状態とし、当該バラン１３−４を介して前記励起用ＲＦパルスを給電用コンデンサＣ８の両端に印加する。なお、前記送信／受信切換スイッチ１２−３と前記バラン１３−３とを結ぶ伝送路の長さ、および、前記送信／受信切換スイッチ１２−４と前記バラン１３−４とを結ぶ伝送路の長さは、共鳴角周波数ω２に対応する波長をλ２とするとき、λ２／４である。
このとき、図６に示すように、前記コンデンサＣ１およびインダクタＬ１は並列共振状態となり、前記コンデンサＣ２およびインダクタＬ２は並列共振状態となって、当該並列共振部分が実質的に切り離されるため、鞍型コイルＫ１と，鞍型コイルＫ２が形成される。これにより、前記鞍型コイルＫ１，Ｋ２は、相互干渉のない１組の送信コイルとして機能し、水平方向の振動磁場ｂｙ（図示の向き）を発生する。なお、コンデンサＣ３およびインダクタＬ３の並列回路，コンデンサＣ４およびインダクタＬ４の並列回路は、共鳴角周波数ω２に対しては、共鳴角周波数ω１との大小に依存して誘導性または容量性を示すので、給電用コンデンサＣ７，Ｃ８の容量を調整することで、前記鞍型コイルＫ１，Ｋ２の共振周波数を共鳴角周波数ω２に同調させることが出来る。
［４］共鳴角周波数ω２のＮＭＲ信号を受信するとき、前記送信／受信切換スイッチ１２−３は、給電用コンデンサＣ５の両端からバラン１３−３を介して取り出されたＮＭＲ信号をプリアンプ１４−３側に出力する。一方、前記送信／受信切換スイッチ１２−３は、給電用コンデンサＣ８の両端からバラン１３−４を介して取り出されたＮＭＲ信号をプリアンプ１４−４側に出力する。このとき、送信時と同様に、前記ＭＲＩ用ＲＦコイル１００は、図６に示す状態となり、鞍型コイルＫ１と，鞍型コイルＫ２は、相互干渉のない１組の受信コイルとして機能し、水平方向の振動磁場ｂｙを検出する。なお、各受信器に送られたＮＭＲ信号を処理することで、前記鞍型コイルＫ１，Ｋ２をフェーズドアレイコイルとして使用してもよい。
なお、送信時の励起効率を高める見地から、図４の点ｐ，ｐ’と、ｑ，ｑ’と、ｒ，ｒ’と、ｓ，ｓ’の電位および位相がそれぞれ等しくなるように、前記給電用コンデンサＣ５〜Ｃ８に励起用ＲＦパルスの電力を並列給電することが好ましい。また、受信時の検出効率を高める見地から、図４の点ｐ，ｐ’と、ｑ，ｑ’と、ｒ，ｒ’と、ｓ，ｓ’の電位および位相がそれぞれ等しくなるように、前記ＭＲＩ用ＲＦコイル１００の幾何学的形状の対称性を高めることが好ましい。以上のＭＲＩ用ＲＦコイル１００によれば、共鳴角周波数ω１に対しては、リングコイル１，２を相互干渉の無いＲＦコイルとして使用でき、共鳴角周波数ω２に対しては、エレメント導体３〜６およびリングコイル１，２の一部により形成される鞍型コイルＫ１，Ｋ２を相互干渉の無いＲＦコイルとして使用することが出来る。
【００２２】
−他の実施形態−
上記実施形態ではＭＲＩ用ＲＦコイル１００を単独で用いたが、リングコイル１およびリングコイル２の大きさ（直径）を少し変えた２つのＭＲＩ用ＲＦコイル１００Ａ，１００Ｂを入れ子にし、送信時または受信時にクアドラチャ（quadrature）化してもよい。すなわち、ＭＲＩ用ＲＦコイル１００Ａについては共鳴角周波数ω１に対してリングコイル１，２を送受信コイルとして機能させ、共鳴角周波数ω２に対して鞍型コイルＫ１，Ｋ２を送受信コイルとして機能させると共に、ＭＲＩ用ＲＦコイル１００Ｂについては共鳴角周波数ω１に対して鞍型コイルＫ１，Ｋ２を送受信コイルとして機能させ、共鳴角周波数ω２に対してリングコイル１，２を送受信コイルとして機能させる。この場合、受信時に、前記ＭＲＩ用ＲＦコイル１００Ａから取り出した２つのＮＭＲ信号と、前記ＭＲＩ用ＲＦコイル１００Ｂから取り出した２つのＮＭＲ信号をそれぞれ別々の受信器で受信して処理することで、４チャネルのフェーズドアレイコイルとしても使用できる。
【００２３】
【発明の効果】
本発明によるＭＲＩ用ＲＦコイルによれば、特定の共鳴周波数（共鳴角周波数でもよい）に対しては、１組のリングコイルをＲＦコイルとして機能させ、別の共鳴周波数に対しては、エレメント導体およびリングコイルの一部からなる１組の鞍型コイルをＲＦコイルとして機能させることができるので、共鳴周波数によって感度方向が異なるフェーズドアレイコイルを容易に構成できる。特に、垂直磁場型ＭＲＩ装置に有用である。
【図面の簡単な説明】
【図１】本発明にかかるＭＲＩ用ＲＦコイルの基本回路を示す斜視図である。
【図２】図１のＭＲＩ用ＲＦコイルのリングコイルをＲＦコイルとして機能させるときの状態を示す説明図である。
【図３】図１のＭＲＩ用ＲＦコイルに鞍型コイルを形成するときの状態を示す説明図である。
【図４】本発明の一実施形態にかかるＭＲＩ用ＲＦコイルを含む送受信回路の構成図である。
【図５】本発明の一実施形態にかかるＭＲＩ用ＲＦコイルのリングコイルをＲＦコイルとして機能させるときの状態を示す説明図である。
【図６】本発明の一実施形態にかかるＭＲＩ用ＲＦコイルに鞍型コイルを形成するときの状態を示す説明図である。
【符号の説明】
１００ ＭＲＩ用ＲＦコイル
１，２ リングコイル
３，４，５，６ エレメント導体
Ｃａ 第１，第２コンデンサ
Ｃｂ 第３，第４コンデンサ
Ｃ１〜Ｃ４ コンデンサ
Ｃ５〜Ｃ８ 給電用コンデンサ
Ｋ１，Ｋ２ 鞍型コイル
Ｌ１〜Ｌ４ インダクタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an MRI (Magnetic Resonance Imaging) RF (Radio Frequency) coil, and more particularly to an MRI RF coil whose sensitivity direction varies depending on the resonance frequency.
[0002]
[Prior art]
In NMR spectroscopy (Nuclear Magnetic Resonance spectroscopy), from the standpoint of properly performing shimming to increase the spatial uniformity of the magnetic field and decoupling to cut the coupling between different nuclides. There is known a dual tune coil that can transmit excitation RF pulses of different frequencies and receive NMR signals of different frequencies.
[0003]
[Problems to be solved by the invention]
However, there is no known MRI RF coil whose sensitivity direction varies depending on the resonance frequency.
Accordingly, an object of the present invention is to provide an MRI RF coil whose sensitivity direction varies depending on the resonance frequency.
[0004]
[Means for Solving the Problems]
In a first aspect, the present invention is an MRI RF coil in which a plurality of element conductors are interposed between two ring coils, and the two ring coils are used for an RF signal having a first resonance frequency. An RF coil is formed, and the RF coil is formed in a bowl shape by the element conductor adjacent to the RF signal of the second resonance frequency and the ring coil portion sandwiched between the connection points of the element conductor and the ring coil. An MRI RF coil is provided.
In the MRI RF coil according to the first aspect, two ring coils can be formed as RF coils for the RF signal of the first resonance frequency (generally, an excitation RF pulse or NMR signal), and the second resonance For an RF signal having a frequency, a saddle type coil composed of an element conductor and a part of a ring coil can be formed as an RF coil, and an RF coil whose sensitivity direction differs by 90 ° depending on the resonance frequency can be formed.
[0005]
In a second aspect, the present invention provides a first ring coil, a second ring coil, and first to fourth electrodes interposed between the first ring coil and the second ring coil. An element conductor, a first capacitor interposed between a connection point between the first element conductor and the first ring coil, and a connection point between the second element conductor and the first ring coil; A second capacitor interposed between a connection point between the first element conductor and the second ring coil and a connection point between the second element conductor and the second ring coil; and the first capacitor A third capacitor interposed in the element conductor, a fourth capacitor interposed in the second element conductor, a connection point between the third element conductor and the first ring coil, and the fourth Element guidance A first feeding capacitor interposed between a connection point of the first ring coil and a series circuit of a parallel resonance circuit that resonates in parallel with the second resonance frequency, and the third element conductor And a connection point between the second ring coil and a second feeding capacitor interposed between the connection point between the fourth element conductor and the second ring coil and the second resonance frequency. A series circuit of parallel resonant circuits that resonate in parallel; a third circuit for power feeding provided in the third element conductor; and a series circuit of parallel resonant circuits that resonate in parallel with the first resonance frequency; A fourth power supply capacitor interposed in a fourth element conductor and a series circuit of a parallel resonance circuit that resonates in parallel with the first resonance frequency, and the first to fourth capacitors , Such that the mutual inductance between the first ring coil and the second ring coil is substantially negligible for the first resonance frequency, and for the second resonance frequency, A first saddle coil formed by a ring coil portion sandwiched between connection points of the first and third element conductors and the element conductors and the first and second ring coils; The mutual inductance between the four element conductors and the second saddle coil formed by the ring coil portion sandwiched between the connection points of the element conductors and the first and second ring coils is substantially negligible. The MRI RF coil is characterized in that the capacity is adjusted.
In the above configuration, “power feeding” means one or both of transmission of excitation RF pulses, supply of power for receiving NMR signals, and extraction.
In the MRI RF coil according to the second aspect, with respect to the first resonance frequency, the parallel resonance circuit interposed in the third and fourth element conductors enters a parallel resonance state, and the parallel resonance portion is The first ring coil and the second ring coil are substantially separated and function as an RF coil. Here, since the mutual inductance between the first ring coil and the second ring coil can be substantially ignored due to the capacitance conditions of the first to fourth capacitors, the first ring coil and the second ring The mutual interference between the coils can be substantially eliminated, and the SNR (Signal to Noise Ratio) can be sufficiently increased.
On the other hand, for the second resonance frequency, the parallel resonance circuit interposed between the connection points of the third and fourth element conductors and the first ring coil, and the third and fourth element conductors, The parallel resonance circuit interposed between the connection points with the second ring coil enters a parallel resonance state, and the parallel resonance portion is substantially cut off, and the first and third element conductors and the first and second elements are separated. A first saddle coil formed by a part of the ring coil and a second saddle coil formed by a part of the second and fourth element conductors and the first and second ring coils. Functions as an RF coil. Here, since the mutual inductance between the first saddle coil and the second saddle coil can be substantially ignored due to the capacity conditions of the first to fourth capacitors, the first saddle coil and The mutual interference between the second saddle coils can be substantially eliminated, and the SNR can be sufficiently increased.
From the above, the first resonance frequency and the second resonance frequency function as an RF coil whose sensitivity direction is different by 90 °.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
[0007]
Before describing the embodiment of the present invention, the basic principle of the MRI RF coil according to the present invention will be described.
FIG. 1 is a perspective view showing a basic circuit of an MRI RF coil 100 ′ according to the present invention.
The MRI RF coil 100 ′ includes a ring coil 1, a ring coil 2, element conductors 3, 4, 5, 6 interposed between the ring coil 1 and the ring coil 2, and the element conductor 3. And a connection point between the ring coil 1, a first capacitor Ca interposed between the connection point between the element conductor 4 and the ring coil 1, a connection point between the element conductor 3 and the ring coil 2, and the element conductor. 4 and a second capacitor Ca interposed between the ring coil 2, a third capacitor Cb interposed in the element conductor 3, and a fourth capacitor Cb interposed in the element conductor 4. It comprises. A switch S1 is interposed between the connection point between the element conductor 5 and the ring coil 1 and between the connection point between the element conductor 6 and the ring coil 1, and the connection point between the element conductor 5 and the ring coil 2 and the element. A switch S2 is interposed between the connection points of the conductor 6 and the ring coil 2, a switch S3 is interposed in the element conductor 5, and a switch S4 is interposed in the element conductor 6.
The first and second capacitors Ca and the third and fourth capacitors Cb substantially have a mutual inductance (M1 in FIG. 2) between the ring coils 1 and 2 with respect to the first resonance angular frequency ω1. And with respect to the second resonance angular frequency ω2, the element conductors 3, 5 and the ring coil portion sandwiched between the connection points of the element conductors 3, 5 and the ring coils 1, 2 are formed. The saddle coil (K1 in FIG. 3), the element conductors 4 and 6 and the ring coil portion formed between the element conductors 4 and 6 and the connection points of the ring coils 1 and 2 (FIG. 3). The capacitance is adjusted so that the mutual inductance (M2 in FIG. 3) between K2) is substantially negligible. Next, the principle of determining the capacity will be described in detail.
[0008]
First, as shown in FIG. 2, the switches S1 and S2 are closed, the switches S3 and S4 are opened, an RF voltage source Vr is applied to the ring coil 1 (this is an applied voltage of an excitation RF pulse or an induced voltage by an NMR signal). The case where the current I having the resonance angular frequency ω1 is passed is assumed. In the figure, if the current flowing through the loop ABDE (the path of the third capacitor Ca → the second capacitor Ca → the fourth capacitor Cb) is Im, the voltage VBA appearing between AB in the figure by this current Im is

[0009]
On the other hand, the mutual inductance between the ring coil 1 and the ring coil 2 is M1, and the positive direction of the current I in the ring coil 1 and the positive direction of the current I2 in the ring coil 2 are the same direction as shown in FIG. When M1> 0 is defined, the voltage VBA induced between AB in the figure by the mutual inductance M1 is

[0010]
When the voltage VBA expressed by the above equation (1) is equal to the voltage VBA expressed by the above equation (2), the induced current is prevented from flowing through the ring coil 2 due to the influence of mutual interference.

[0011]
On the other hand, from Kirchhoff's law in loop ABDE,

[0012]
From the above equations (3) and (4), the condition for canceling the mutual interference between the ring coil 1 and the ring coil 2 is:

[0013]
Next, as shown in FIG. 3, it is assumed that the switches S1 and S2 are opened, the switches S3 and S4 are closed, and a current I having a resonance angular frequency ω2 is supplied to the element conductor 6 by the RF voltage source Vr ′. In this case, the mutual inductance between the saddle type coil K1 and the saddle type coil K2 is M2, and the positive direction of the current I5 in the element conductor 5 and the positive direction of the current I in the element conductor 6 as shown in FIG. Are defined in the same direction and M2> 0, the condition for canceling the mutual interference between the saddle coil K1 and the saddle coil K2 is:

[0014]
Substituting the above equation (5) into the above equation (6) and arranging for Ca,

[0015]
From the above equation (5), the condition for Cb to have a solution of Cb> 0 is M1> 0.

[0016]
[1] When ω1 ² · M1-ω2 ² · M2 = 0,

Therefore, there is a physically meaningful solution that satisfies the above equations (5) and (6) at the same time.
[0017]
[2] When ω1 ² · M1−ω2 ² · M2 ≠ 0,

After all,

Therefore, f (Ca) = 0 is

There are solutions that are physically meaningful.
[0018]
From [1] and [2] above, the capacitance Ca of the first and second capacitors and the capacitance Cb of the third and fourth capacitors can be adjusted so as to satisfy the expressions (5) and (6). Proven to be possible. At this time, with respect to the resonance angular frequency ω1, as shown in FIG. 2, the mutual inductance M1 between the ring coils 1 and 2 can be substantially ignored, and the ring coils 1 and 2 are made to be RF coils having no mutual interference. Can be used as For the resonance angular frequency ω2, as shown in FIG. 3, the mutual inductance M2 between the saddle type coil K1 and the saddle type coil K2 can be substantially ignored, and the saddle type coils K1 and K2 are mutually connected. It can be used as an RF coil without interference.
[0019]
FIG. 4 is a configuration diagram of a transmission / reception circuit 101 including an MRI RF coil according to an embodiment of the present invention. In the figure, Bz represents a static magnetic field in the vertical direction.
In this transmission / reception circuit 101, the MRI RF coil 100 includes a ring coil 1, a ring coil 2, element conductors 3, 4, 5, 6 interposed between the ring coil 1 and the ring coil 2, A connection point between the element conductor 3 and the ring coil 1, a first capacitor Ca interposed between the connection point between the element conductor 4 and the ring coil 1, and a connection point between the element conductor 3 and the ring coil 2. And a second capacitor Ca interposed between the connection points of the element conductor 4 and the ring coil 2, a third capacitor Cb interposed between the element conductors 3, and an element conductor 4. And a fourth capacitor Cb. However, the first and second capacitors Ca and the third and fourth capacitors Cb are adjusted so as to satisfy the above expressions (5) and (6).
Further, a parallel resonance occurs in parallel with the feeding capacitor C5 and the resonance angular frequency ω2 interposed between the connection point of the element conductor 5 and the ring coil 1 and the connection point of the element conductor 6 and the ring coil 1. A resonance circuit (a parallel circuit of a capacitor C1 and an inductor L1), a connection point between the element conductor 5 and the ring coil 2, and a power supply capacitor C6 interposed between the connection point between the element conductor 6 and the ring coil 2 And a parallel resonance circuit (a parallel circuit of a capacitor C2 and an inductor L2) that resonates in parallel with the resonance angular frequency ω2. That is, the capacitances C1 to C4 of the capacitors and the inductances L1 to L4 of the inductors constituting the parallel resonant circuit satisfy the following expression.

Further, two DC blocking capacitors C are interposed in the ring coil 1. The ring coil 2 is provided with two DC blocking capacitors C.
[0020]
Next, the operation of the transmission / reception circuit 101 will be described.
[1] When an excitation RF pulse having a resonance angular frequency (= Larmor angular frequency) ω1 is transmitted to excite the first nuclide (for example, proton), the power of the excitation RF pulse is amplified by the power amplifier 10a. The power is equally divided by the hybrid 11a and input to the transmission / reception change-over switches 12-1 and 12-2. The transmission / reception changeover switch 12-1 is in a switching state in which the excitation RF pulse is output to the balun 13-1, and the excitation RF pulse is applied to both ends of the power supply capacitor C5 via the balun 13-1. Apply. On the other hand, the transmission / reception change-over switch 12-2 is in a switching state in which the excitation RF pulse is output to the balun 13-2 side, and the excitation RF pulse is supplied to the feeding capacitor C6 via the balun 13-2. Apply to both ends. The length of the transmission line connecting the transmission / reception changeover switch 12-1 and the balun 13-1, and the length of the transmission line connecting the transmission / reception changeover switch 12-2 and the balun 13-2. When the wavelength corresponding to the resonance angular frequency ω1 is λ1, it is λ1 / 4.
At this time, as shown in FIG. 5, the capacitor C3 and the inductor L3 are in a parallel resonance state (= impedance infinite), and the capacitor C4 and the inductor L4 are in a parallel resonance state, so that the parallel resonance portion is substantially Separated. As a result, the ring coils 1 and 2 function as a set of transmission coils having no mutual interference, and generate a horizontal oscillating magnetic field bx (direction shown). Note that the parallel circuit of the capacitor C1 and the inductor L1 and the parallel circuit of the capacitor C2 and the inductor L2 exhibit inductivity or capacitance depending on the magnitude of the resonance angular frequency ω2 with respect to the resonance angular frequency ω1. By adjusting the capacitances of the power feeding capacitors C5 and C6, the resonance frequency of the ring coils 1 and 2 can be tuned to the resonance angular frequency ω1.
[2] When receiving the NMR signal of the resonance angular frequency ω1, the transmission / reception change-over switch 12-1 receives the NMR signal extracted from both ends of the power supply capacitor C5 via the balun 13-1, and the preamplifier 14-1. Output to the side. The preamplifier 14-1 amplifies the NMR signal and sends it to a receiver. On the other hand, the transmission / reception selector switch 12-2 outputs the NMR signal extracted from both ends of the power supply capacitor C6 via the balun 13-2 to the preamplifier 14-2 side. At this time, as in the transmission, the MRI RF coil 100 is in the state shown in FIG. 5, and the ring coils 1 and 2 function as a set of receiving coils without mutual interference, and a horizontal oscillating magnetic field. bx is detected. In addition, you may use the said ring coils 1 and 2 as a phased array coil (phased array coil) by processing the NMR signal sent to each receiver.
[0021]
[3] When transmitting an excitation RF pulse having a resonance angular frequency ω2 in order to excite the second nuclide (for example, carbon), the power of the excitation RF pulse is amplified by the power amplifier 10b, and the electric power is amplified by the hybrid 11b. Divide into equal parts and input to transmission / reception change-over switches 12-3 and 12-4. The transmission / reception selector switch 12-3 is in a switching state in which the excitation RF pulse is output to the balun 13-3 side, and the excitation RF pulse is applied to both ends of the power supply capacitor C7 via the balun 13-3. Apply. On the other hand, the transmission / reception selector switch 12-4 is in a switching state in which the excitation RF pulse is output to the balun 13-4 side, and the excitation RF pulse is supplied to the feeding capacitor C8 via the balun 13-4. Apply to both ends. The length of the transmission line connecting the transmission / reception changeover switch 12-3 and the balun 13-3, and the length of the transmission line connecting the transmission / reception changeover switch 12-4 and the balun 13-4. When the wavelength corresponding to the resonance angular frequency ω2 is λ2, it is λ2 / 4.
At this time, as shown in FIG. 6, the capacitor C1 and the inductor L1 are in a parallel resonance state, and the capacitor C2 and the inductor L2 are in a parallel resonance state, so that the parallel resonance portion is substantially separated. Coil K1 and saddle type coil K2 are formed. As a result, the saddle coils K1 and K2 function as a set of transmission coils having no mutual interference, and generate a horizontal oscillating magnetic field by (direction shown). The parallel circuit of the capacitor C3 and the inductor L3 and the parallel circuit of the capacitor C4 and the inductor L4 exhibit inductivity or capacitance with respect to the resonance angular frequency ω2, depending on the magnitude of the resonance angular frequency ω1. By adjusting the capacitances of the power feeding capacitors C7 and C8, the resonance frequency of the saddle coils K1 and K2 can be tuned to the resonance angular frequency ω2.
[4] When receiving the NMR signal of the resonance angular frequency ω2, the transmission / reception changeover switch 12-3 receives the NMR signal extracted from both ends of the power supply capacitor C5 via the balun 13-3, and the preamplifier 14-3. Output to the side. On the other hand, the transmission / reception selector switch 12-3 outputs the NMR signal extracted from both ends of the power supply capacitor C8 via the balun 13-4 to the preamplifier 14-4 side. At this time, similarly to the transmission, the MRI RF coil 100 is in the state shown in FIG. 6, and the saddle-shaped coil K1 and saddle-shaped coil K2 function as a set of receiving coils without mutual interference, and are horizontal. An oscillating magnetic field by is detected. The saddle coils K1 and K2 may be used as phased array coils by processing the NMR signals sent to each receiver.
From the viewpoint of increasing the excitation efficiency at the time of transmission, the potentials and phases of the points p, p ′, q, q ′, r, r ′, s, s ′ in FIG. It is preferable to feed the power of the excitation RF pulse to the power supply capacitors C5 to C8 in parallel. Further, from the viewpoint of increasing the detection efficiency at the time of reception, the points p and p ′, q and q ′, r and r ′, and s and s ′ in FIG. The symmetry of the geometric shape of the MRI RF coil 100 is preferably increased. According to the MRI RF coil 100 described above, the ring coils 1 and 2 can be used as RF coils without mutual interference for the resonance angular frequency ω1, and the element conductors 3 to 6 for the resonance angular frequency ω2. And the saddle type coils K1, K2 formed by a part of the ring coils 1, 2 can be used as RF coils without mutual interference.
[0022]
-Other embodiments-
In the above-described embodiment, the MRI RF coil 100 is used alone, but two MRI RF coils 100A and 100B, in which the sizes (diameters) of the ring coil 1 and the ring coil 2 are slightly changed, are nested and transmitted or received. Sometimes it may be quadrature. That is, with respect to the MRI RF coil 100A, the ring coils 1 and 2 function as transmission / reception coils for the resonance angular frequency ω1, the saddle coils K1 and K2 function as transmission / reception coils for the resonance angular frequency ω2, and the MRI. For the RF coil 100B, the saddle coils K1, K2 function as transmission / reception coils for the resonance angular frequency ω1, and the ring coils 1, 2 function as transmission / reception coils for the resonance angular frequency ω2. In this case, at the time of reception, the two NMR signals extracted from the MRI RF coil 100A and the two NMR signals extracted from the MRI RF coil 100B are received and processed by separate receivers, respectively. It can also be used as a channel phased array coil.
[0023]
【The invention's effect】
According to the MRI RF coil of the present invention, one ring coil functions as an RF coil for a specific resonance frequency (may be a resonance angular frequency), and an element conductor for another resonance frequency. In addition, since one set of saddle-shaped coils formed of a part of the ring coil can function as an RF coil, a phased array coil having a different sensitivity direction depending on the resonance frequency can be easily configured. Particularly, it is useful for a vertical magnetic field type MRI apparatus.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a basic circuit of an MRI RF coil according to the present invention.
2 is an explanatory diagram showing a state when the ring coil of the MRI RF coil of FIG. 1 is made to function as an RF coil. FIG.
3 is an explanatory view showing a state when a saddle coil is formed on the MRI RF coil of FIG. 1; FIG.
FIG. 4 is a configuration diagram of a transmission / reception circuit including an MRI RF coil according to an embodiment of the present invention.
FIG. 5 is an explanatory diagram showing a state when the ring coil of the MRI RF coil according to the embodiment of the present invention is caused to function as an RF coil.
FIG. 6 is an explanatory diagram showing a state when a saddle coil is formed on the MRI RF coil according to the embodiment of the present invention.
[Explanation of symbols]
100 MRI RF coil 1, 2 Ring coil 3, 4, 5, 6 Element conductor Ca First and second capacitors Cb Third and fourth capacitors C1 to C4 Capacitors C5 to C8 Feeding capacitors K1 and K2 Saddle coil L1 ~ L4 inductor

Claims (2)

An MRI RF coil having a plurality of element conductors interposed between two ring coils,
An RF coil composed of the two ring coils is formed for the RF signal having the first resonance frequency, and the element conductor adjacent to the RF signal having the second resonance frequency and the connection between the element conductor and the ring coil are connected to each other. An RF coil for MRI, wherein an RF coil is formed in a bowl shape by a ring coil portion sandwiched between points. A first ring coil; a second ring coil; first to fourth element conductors interposed between the first ring coil and the second ring coil; and the first element conductors. And a first capacitor interposed between the connection point of the first ring coil and the connection point of the second element conductor and the first ring coil, the first element conductor and the second A ring capacitor connecting point, a second capacitor interposed between the second element conductor and the second ring coil connecting point, and a third capacitor interposed between the first element conductor A capacitor, a fourth capacitor interposed in the second element conductor, a connection point between the third element conductor and the first ring coil, and the fourth element conductor and the first ring coil. A first feeding capacitor interposed between connection points, a series circuit of a parallel resonance circuit that resonates in parallel with the second resonance frequency, a third element conductor, and a second ring coil. A series of a connection point, a second power feeding capacitor interposed between the connection point of the fourth element conductor and the second ring coil, and a parallel resonance circuit that resonates in parallel with the second resonance frequency A circuit, a third power supply capacitor interposed in the third element conductor, a series circuit of a parallel resonance circuit that resonates in parallel with the first resonance frequency, and an intervening element in the fourth element conductor And a series circuit of a parallel resonance circuit that resonates in parallel with the first resonance frequency,
The first to fourth capacitors are configured such that a mutual inductance between the first ring coil and the second ring coil is substantially negligible with respect to the first resonance frequency, and With respect to the second resonance frequency, a first coil formed by the first and third element conductors and a ring coil portion sandwiched between connection points of the element conductors and the first and second ring coils. And a second saddle coil formed by a ring coil portion sandwiched between the second and fourth element conductors and a connection point between the element conductors and the first and second ring coils. An MRI RF coil, wherein the capacitance is adjusted so that the mutual inductance can be substantially ignored.

Images