JP4305706B2 - RF coil and magnetic resonance imaging apparatus - Google Patents

Description

【００６９】
ここで、
ω：閉回路に流れる電流の角周波数
Ｚｉｎ（Ｒｅ）：Ｚｉｎの実部（リアルパート：ｒｅａｌ ｐａｒｔ）
Ｚｉｎ（Ｉｍ）：Ｚｉｎの虚部（イマジナリパート：ｉｍａｇｉｎａｒｙ ｐａｒｔ）
図８に、Ｚｉｎ（Ｉｍ）の変化に対するＺ１の変化を実線で示す。なお、破線は抵抗ＲおよびＺｉｎ（Ｒｅ）が０である理想的な場合のＺ１変化であり、対比のために示す。
【００７０】
現実の回路では抵抗ＲおよびＺｉｎ（Ｒｅ）が０でないために、同図に示すように、Ｚ１は理想的な場合のようにＺｉｎ（Ｉｍ）＝０のときに最大になるのではなく、Ｚｉｎ＝ａのときに最大になる。ａは正の値である。
【００７１】
すなわち、ブロッキング回路のインピーダンスＺ１は、プリアンプ３０６の入力インピーダンスの虚部が正のある値をとるとき、言い換えれば入力インピーダンスの虚部がインダクティブ（ｉｎｄｕｃｔｉｖｅ）であるときに最大になる。
【００７２】
そこで、プリアンプ３０６の入力回路のキャパシタ３６６を調節して、Ｚｉｎ（Ｉｍ）＝ａの条件を満足するようにする。これによってＺ１の最大値が得られ、ブロッキング回路は、回路構成上可能な最高のブロッキング性能を発揮するようになる。
【００７３】
プリアンプ３０６のＺｉｎ（Ｉｍ）をインダクティブにすることにより、ブロッキング回路の共振周波数は、コイルループの共振周波数から低域側にシフト（ｓｈｉｆｔ）したものとなる。したがって、上記のような調節はブロッキング回路の共振周波数をコイルループの共振周波数から低域側にシフトすることに相当する。
【００７４】
これと同等の効果は、Ｚｉｎ（Ｉｍ）の調節に代えて、あるいは、それに加えてインダクタ３０８のインダクタンスを増やすことによって達成するようにしても良い。
【００７５】
また、Ｚｉｎ（Ｉｍ）の調節に代えて、あるいは、それに加えてコイルループのキャパシタ３０２のキャパシタンスを増やすことによって実現するようにしても良い。ただし、その場合は、コイルループの共振周波数の変化を防ぐためにコイルループ中の他のキャパシタを調節する必要がある。さらには、Ｚｉｎ（Ｉｍ）、インダクタ３０８およびキャパシタ３０２の調節を全て行うようにしても良い。
【００７６】
上部コイル８０２，８０４，８０６および下部コイル９０２，９０４，９０６，９０８をいずれもこのような構成にすることにより、相互にカップリングしないものとなり、それぞれ独立に存在するのと等価になる。すなわち、フェーズドアレイコイルとして効果的に動作する受信コイル部を得ることができる。また、ＲＦコイル部１０８とのデカップリングも効果的に行われる。これによって、磁気共鳴信号をＳＮＲ良く受信することができる。
【００７７】
なお、フェーズドアレイコイルは、上記のように同軸的に配置したものに限るものではなく、例えば図９に示すように、平板状の支持体に複数のコイル７０２〜７１０を主感度方向を上に向けて配設したものであっても良い。
【００７８】
これは静磁場の方向が対象３００の体軸に平行となるいわゆる水平磁場を利用する磁気共鳴撮影装置用のサーフェイスコイル（ｓｕｒｆａｃｅ ｃｏｉｌ）として好適である。なお、コイルループの形状は図示のような長方形、前述のような円形、さらには楕円や長円あるいは適宜の多角形であって良い。
【００７９】
また、コイルループはフェーズドアレイを構成せずに単一のコイルとして使用しても良いのはいうまでもない。この場合はＲＦコイル部１０８やその他のコイルとのデカップリングを効果的に行う受信コイルとなる。
【００８０】
【発明の効果】
以上詳細に説明したように、本発明によれば、ブロッキングを効果的に行うＲＦコイルおよびそのようなＲＦコイルを具備する磁気共鳴撮影装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の実施の形態の一例の装置のブロック図である。
【図２】図１に示した装置が実行するパルスシーケンスの一例を示す図である。
【図３】図１に示した装置が実行するパルスシーケンスの一例を示す図である。
【図４】図１に示した装置における受信コイル部のコイル配置を示す模式図である。
【図５】図４に示したコイルの回路図である。
【図６】図５に示したプリアンプの入力回路部分を示す図である。
【図７】ブロッキング回路の等価回路を示す図である。
【図８】ブロッキング回路のインピーダンス特性を示すグラフである。
【図９】フェーズドアレイコイルの模式的構成を示す図である。
【符号の説明】
１００ マグネットシステム
１０２ 主磁場マグネット部
１０６ 勾配コイル部
１０８ ＲＦコイル部
１１０ 受信コイル部
１３０ 勾配駆動部
１４０ ＲＦ駆動部
１５０ データ収集部
１６０ 制御部
１７０ データ処理部
１８０ 表示部
１９０ 操作部
３００ 対象
３０２ キャパシタ
３０４ 導体
３０６ プリアンプ
３０８ インダクタ
３６２ ＦＥＴ
３６６ キャパシタ
３６４ インダクタ
５００ クレードル
８０２，８０４，８０６ 上部コイル
９０２，９０４，９０６，９０８ 下部コイル[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an RF coil (radio frequency coil) and a magnetic resonance imaging apparatus, and more particularly, an RF coil having a blocking circuit that prevents coupling with other coils, and such an RF coil. The present invention relates to a magnetic resonance imaging apparatus.
[0002]
[Prior art]
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, that is, a space in which a static magnetic field is formed, and a gradient magnetic field and a high-frequency magnetic field are applied to apply magnetic field to the object. A resonance signal is generated, and a tomographic image is generated (reconstructed) based on the received signal.
[0003]
An RF coil is used to receive the magnetic resonance signal. When a so-called phased array coil consisting of a plurality of coil loops is used as an RF coil for reception, coupling between the coil loops is prevented by a blocking circuit provided in each coil loop. Thus, a decrease in the SNR (signal-to-noise ratio) of the received signal is prevented.
[0004]
The blocking circuit is configured by connecting an input circuit of an amplifier (preamplifier) having a low input impedance in parallel to a capacitor in a coil loop via an inductor.
[0005]
The resonance frequency of the closed circuit consisting of the preamplifier input circuit, inductor and capacitor is tuned to the magnetic resonance frequency, and the individual coil loops are blocked by utilizing the high impedance due to the parallel resonance, and the decoupling is mutually performed. Realize.
[0006]
[Problems to be solved by the invention]
The closed circuit parallel resonance impedance should theoretically be infinite, but there is a real part of the preamplifier input impedance (real part), copper resistance of the inductor, or equivalent resistance due to loss of the capacitor. By doing so, the parallel resonance impedance becomes much smaller than the theoretical value, and a sufficient blocking effect cannot always be obtained.
[0007]
Therefore, an object of the present invention is to realize an RF coil that effectively performs blocking and a magnetic resonance imaging apparatus including such an RF coil.
[0008]
[Means for Solving the Problems]
(1) An invention according to one aspect for solving the above-described problem includes a closed loop having a series capacitor and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor. An RF coil, characterized in that a resonance frequency of a closed circuit including an input circuit of the low input impedance amplifier, the inductor, and the capacitor is on a lower frequency side than a resonance frequency of the closed loop. .
[0009]
In the invention in this aspect, the resonance frequency of the closed circuit including the input circuit, inductor, and capacitor of the low input impedance amplifier is shifted to a lower frequency side than the resonance frequency of the coil loop. The seen impedance of the closed circuit can be made higher than when the resonant frequency of the closed circuit is matched with the resonant frequency of the coil loop. As a result, decoupling with other coil loops can be further ensured.
[0010]
(2) In another aspect of the invention for solving the above problem, a plurality of closed loops having a series capacitor and low input impedance amplifiers having an input circuit connected in parallel to the capacitor via an inductor An RF coil having a combination, wherein a resonance frequency of a closed circuit including an input circuit of the low input impedance amplifier, the inductor, and the capacitor is lower than a resonance frequency of the closed loop It is.
[0011]
In the invention in this aspect, the resonance frequency of the closed circuit including the input circuit of the low input impedance amplifier, the inductor, and the capacitor is shifted to a lower frequency side than the resonance frequency of the coil loop. The impedance of the closed circuit viewed from the coil loop side at the time of resonance of the loop can be increased as compared with the case where the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop. Thereby, the decoupling between a plurality of coil loops can be made more reliable.
[0012]
(3) According to another aspect of the invention for solving the above-described problem, the imaginary part of the impedance of the input circuit of the low input impedance amplifier under the resonance frequency of the closed loop is positive. The RF coil according to (1) or (2).
[0013]
In this aspect of the invention, since the imaginary part of the impedance of the input circuit of the low input impedance amplifier under the resonance frequency of the coil loop is positive, the impedance of the closed circuit viewed from the coil loop side when the coil loop resonates is set. The resonance frequency of the closed circuit can be made higher than when the resonance frequency of the coil loop is matched.
[0014]
(4) In another aspect of the invention for solving the above-described problem, the inductance of the inductor is larger than a value obtained when matching the resonance frequency of the closed circuit with the resonance frequency of the closed loop. The RF coil according to any one of (1) to (3).
[0015]
In the invention in this aspect, the inductance of the inductor is made larger than the value when the resonance frequency of the closed circuit matches the resonance frequency of the coil loop, so that the impedance of the closed circuit viewed from the coil loop side when the coil loop resonates. Can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop.
[0016]
(5) In another aspect of the invention for solving the above-described problem, the capacitance of the capacitor is larger than a value when the resonance frequency of the closed circuit is matched with the resonance frequency of the closed loop. The RF coil according to any one of (1) to (4).
[0017]
In the invention in this aspect, the capacitance of the capacitor is made larger than the value when the resonance frequency of the closed circuit matches the resonance frequency of the coil loop, so that the impedance of the closed circuit viewed from the coil loop side during the resonance of the coil loop. Can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop.
[0018]
(6) Another aspect of the invention for solving the above-described problem is that magnetic resonance imaging is used to construct an image based on a magnetic resonance signal generated inside a subject using a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field. An RF coil having a closed loop having a series capacitor and a low input impedance amplifier in which an input circuit is connected in parallel to the capacitor via an inductor as an RF coil for receiving the magnetic resonance signal. A magnetic resonance comprising an RF coil in which a resonance frequency of a closed circuit including an input circuit of the low input impedance amplifier, the inductor, and the capacitor is lower than a resonance frequency of the closed loop It is a photographing device.
[0019]
In the invention of this aspect, in the receiving RF coil, the resonance frequency of the closed circuit composed of the input circuit of the low input impedance amplifier, the inductor and the capacitor is shifted to a lower frequency side than the resonance frequency of the coil loop. The impedance of the closed circuit viewed from the coil loop side at the time of resonance can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop. Accordingly, decoupling with other coils can be effectively performed, reception with good SNR can be performed, and high-quality imaging can be performed.
[0020]
(7) Another aspect of the invention for solving the above-described problem is that magnetic resonance imaging is used to construct an image based on a magnetic resonance signal generated inside a target using a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field. An RF device comprising a plurality of sets of a closed loop having a series capacitor as an RF coil for receiving the magnetic resonance signal, and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor A coil having an RF coil in which a resonance frequency of a closed circuit including the input circuit of the low input impedance amplifier, the inductor, and the capacitor is lower than a resonance frequency of the closed loop. This is a magnetic resonance imaging apparatus.
[0021]
In the invention of this aspect, in the receiving RF coil, the resonance frequency of the closed circuit including the input circuit of the low input impedance amplifier, the inductor, and the capacitor is shifted to a lower frequency side than the resonance frequency of the coil loop. For each of the coil loops, the impedance of the closed circuit viewed from the coil loop side when the coil loop resonates can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop. Accordingly, it is possible to effectively perform decoupling between a plurality of coil loops, perform reception with good SNR, and perform photographing with high quality.
[0022]
(8) According to another aspect of the invention for solving the above-described problem, the imaginary part of the impedance of the input circuit of the low input impedance amplifier under the closed-loop resonance frequency is positive. The magnetic resonance imaging apparatus according to (6) or (7).
[0023]
In this aspect of the invention, since the imaginary part of the impedance of the input circuit of the low input impedance amplifier under the resonance frequency of the coil loop is positive, the impedance of the closed circuit viewed from the coil loop side when the coil loop resonates is set. The resonance frequency of the closed circuit can be made higher than when the resonance frequency of the coil loop is matched.
[0024]
(9) In another aspect of the invention for solving the above-mentioned problem, the inductance of the inductor is larger than a value when the resonance frequency of the closed circuit is matched with the resonance frequency of the closed loop. The magnetic resonance imaging apparatus according to any one of (6) to (8).
[0025]
In the invention in this aspect, the inductance of the inductor is made larger than the value when the resonance frequency of the closed circuit matches the resonance frequency of the coil loop, so that the impedance of the closed circuit viewed from the coil loop side when the coil loop resonates. Can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop.
[0026]
(10) In another aspect of the invention for solving the above-mentioned problem, the capacitance of the capacitor is larger than a value when the resonance frequency of the closed circuit is matched with the resonance frequency of the closed loop. The magnetic resonance imaging apparatus according to any one of (6) to (9).
[0027]
In the invention in this aspect, the capacitance of the capacitor is made larger than the value when the resonance frequency of the closed circuit matches the resonance frequency of the coil loop, so that the impedance of the closed circuit viewed from the coil loop side during the resonance of the coil loop. Can be made higher than when the resonance frequency of the closed circuit is matched with the resonance frequency of the coil loop.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment. FIG. 1 shows a block diagram of the magnetic resonance imaging apparatus. This apparatus is an example of an embodiment of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus.
[0029]
As shown in FIG. 1, the apparatus has a magnet system 100. The magnet system 100 includes a main magnetic field magnet unit 102, a gradient coil unit 106, and an RF coil unit 108. Each of the main magnetic field magnet unit 102 and each coil unit is composed of a pair facing each other across a space. Moreover, all have a disk-shaped external shape and are arrange | positioned sharing a central axis. The object 300 is mounted on the cradle 500 and carried into and out of the internal space of the magnet system 100 by a conveying means (not shown).
[0030]
The cradle 500 is provided with a receiving coil unit 110. The receiving coil unit 110 has a substantially cylindrical shape and is installed on the upper surface of the cradle 500. The object 300 is mounted on the cradle 500 in a posture in which the head is placed in the cylinder of the receiving coil unit 110 and is supine.
[0031]
Hereinafter, in the receiving coil unit 110, the top side of the subject 300 is referred to as the upper side, and the neck side is referred to as the lower side. Further, the face side of the object 300 is referred to as the front, and the occipital side is referred to as the rear. Further, the left and right when the object 300 is viewed from below is referred to as the left and right of the receiving coil unit 110.
[0032]
The reception coil unit 110 is an example of an embodiment of the RF coil of the present invention. An example of an embodiment related to the RF coil of the present invention is shown by the configuration of the coil. The receiving coil unit 110 will be described later.
[0033]
The main magnetic field magnet unit 102 forms a static magnetic field in the internal space of the magnet system 100. The direction of the static magnetic field is orthogonal to the body axis direction of the object 300. That is, a so-called vertical magnetic field is formed. The main magnetic field magnet unit 102 is configured using, for example, a permanent magnet. Of course, not only permanent magnets but also superconducting electromagnets or normal conducting electromagnets may be used.
[0034]
The gradient coil unit 106 generates a gradient magnetic field for giving a gradient to the static magnetic field strength. There are three types of gradient magnetic fields that are generated: a slice gradient magnetic field, a read out gradient magnetic field, and a phase encode gradient magnetic field. The gradient coil unit 106 corresponds to these three types of gradient magnetic fields. Has three gradient coils (not shown).
[0035]
The RF coil unit 108 transmits an RF excitation signal for exciting spins in the body of the target 300 to the static magnetic field space. The receiving coil unit 110 receives a magnetic resonance signal in which excited spin occurs.
[0036]
A gradient driving unit 130 is connected to the gradient coil unit 106. The gradient driving unit 130 gives a driving signal to the gradient coil unit 106 to generate a gradient magnetic field. The gradient driving unit 130 includes three systems of driving circuits (not shown) corresponding to the three systems of gradient coils in the gradient coil unit 106.
[0037]
An RF drive unit 140 is connected to the RF coil unit 108. The RF drive unit 140 provides a drive signal to the RF coil unit 108 and transmits an RF excitation signal to excite spins in the body of the target 300.
[0038]
A data collection unit 150 is connected to the reception coil unit 110. The data collection unit 150 takes in the reception signal received by the reception coil unit 110 and collects it as digital data.
[0039]
A control unit 160 is connected to the gradient driving unit 130, the RF driving unit 140, and the data collection unit 150. The control unit 160 controls the gradient driving unit 130 or the data collection unit 150, respectively.
[0040]
The data processing unit 170 stores the data acquired from the data collection unit 150 in a memory. A data space is formed in the memory. The data space constitutes a two-dimensional Fourier space. The data processing unit 170 generates (reconstructs) an image of the target 300 by performing two-dimensional inverse Fourier transform on the data in the two-dimensional Fourier space. The two-dimensional Fourier space is also referred to as k-space.
[0041]
The data processing unit 170 is connected to the control unit 160. The data processing unit 170 is above the control unit 160 and controls it. A display unit 180 and an operation unit 190 are connected to the data processing unit 170. The display unit 180 is configured by a graphic display or the like. The operation unit 190 includes a keyboard having a pointing device.
[0042]
The display unit 180 displays the reconstructed image and various information output from the data processing unit 170. The operation unit 190 is operated by an operator and inputs various commands and information to the data processing unit 170. An operator operates the apparatus interactively through the display unit 180 and the operation unit 190.
[0043]
FIG. 2 shows an example of a pulse sequence used for magnetic resonance imaging. This pulse sequence is a pulse sequence of a gradient echo (GRE) method.
[0044]
That is, (1) is a sequence of α ° pulses for RF excitation in the GRE method, and (2), (3), (4) and (5) are slice gradient Gs, readout gradient Gr, It is a sequence of a phase encoding gradient Gp and a gradient echo MR. The α ° pulse is represented by a center signal. The pulse sequence proceeds from left to right along the time axis t.
[0045]
As shown in the figure, the α ° excitation of the spin is performed by the α ° pulse. The flip angle α ° is 90 ° or less. At this time, the slice gradient Gs is applied, and selective excitation for a predetermined slice is performed.
[0046]
After the α ° excitation, spin phase encoding is performed by the phase encoding gradient Gp. Next, the spin is first dephased by the readout gradient Gr, and then the spin is rephased to generate a gradient echo MR. The signal intensity of the gradient echo MR becomes maximum at a time point after the echo time TE after the α ° excitation. The gradient echo MR is collected as view data by the data collection unit 150.
[0047]
Such a pulse sequence is repeated 64 to 512 times with a period TR (repetition time). The phase encoding gradient Gp is changed every time it is repeated, and a different phase encoding is performed each time. Thereby, view data of 64 to 512 views filling the k space is obtained.
[0048]
Another example of a pulse sequence for magnetic resonance imaging is shown in FIG. This pulse sequence is a pulse sequence of a spin echo (SE: Spin Echo) method.
[0049]
That is, (1) is a sequence of 90 ° pulses and 180 ° pulses for RF excitation in the SE method, and (2), (3), (4) and (5) are respectively the slice gradient Gs and the lead. This is a sequence of an out gradient Gr, a phase encode gradient Gp, and a spin echo MR. The 90 ° pulse and the 180 ° pulse are represented by center signals. The pulse sequence proceeds from left to right along the time axis t.
[0050]
As shown in the figure, 90 ° excitation of spin is performed by a 90 ° pulse. At this time, the slice gradient Gs is applied, and selective excitation for a predetermined slice is performed. After a predetermined time from the 90 ° excitation, 180 ° excitation by a 180 ° pulse, that is, spin inversion is performed. At this time, the slice gradient Gs is applied, and selective inversion is performed for the same slice.
[0051]
In the period between 90 ° excitation and spin reversal, a readout gradient Gr and a phase encode gradient Gp are applied. Spin dephase is performed by the lead-out gradient Gr. Spin phase encoding is performed by the phase encoding gradient Gp.
[0052]
After the spin inversion, the spin is rephased at the readout gradient Gr to generate the spin echo MR. The signal intensity of the spin echo MR becomes maximum at the time after TE from 90 ° excitation. The spin echo MR is collected as view data by the data collection unit 150. Such a pulse sequence is repeated 64 to 512 times with a period TR. The phase encoding gradient Gp is changed every time it is repeated, and a different phase encoding is performed each time. Thereby, view data of 64 to 512 views filling the k space is obtained.
[0053]
Note that the pulse sequence used for imaging is not limited to the GRE method or the SE method. For example, the FSE (Fast Spin Echo) method, the fast recovery FSE (Fast Recovery Fast Spin Echo) method, the echo planer imaging (EPI: Echo Planar). Other suitable techniques, such as Imaging).
[0054]
The data processing unit 170 reconstructs a tomographic image of the object 300 by performing two-dimensional inverse Fourier transform on k-space view data. The reconstructed image is stored in the memory and displayed on the display unit 180.
[0055]
As will be described later, since the receiving coil unit 110 is a phased array coil having an effective blocking circuit, the magnetic resonance signal can be received with good SNR, and the quality is good based on such a received signal. A reconstructed image can be obtained.
[0056]
FIG. 4 is a perspective view showing a schematic configuration and arrangement of a coil loop included in the receiving coil unit 110. In the same figure, x, y, and z are the left-right direction, the front-back direction, and the up-down direction of the object 300, respectively.
[0057]
As shown in the figure, the receiving coil unit 110 includes upper coils 802, 804, 806 and lower coils 902, 904, 906, 908. Each of the upper coils 802, 804, 806 and the lower coils 902, 904, 906, 908 is, for example, a one-turn coil loop, and is arranged at a predetermined interval in the z direction. The coil loop is not limited to one turn, and may be an appropriate plural number of turns.
[0058]
The main sensitivity direction of these coils is the z direction. Each of these coils has a blocking circuit, which will be described later, and is a so-called phased array coil having no electromagnetic coupling.
[0059]
The upper coils 802, 804, and 806 are provided at positions that surround the head of the object 300 in the forehead portion. The upper coils 802, 804, and 806 are shown as three coil loops, but the present invention is not limited to this, and an appropriate number of coil loops may be used. The length of the loop of the upper coils 802, 804, 806 is slightly longer than the perimeter of the head of the target 300 at the forehead.
[0060]
The lower coils 902, 904, 906, and 908 are provided at positions that surround the head of the object 300 in a portion below the nose. The lower coils 902, 904, 906, and 908 are shown as four coil loops, but the present invention is not limited to this, and an appropriate number of coil loops may be used. The lower coils 902, 904, 906, and 908 form a loop partially protruding in the y direction corresponding to the bulge of the nose. The length of the loop is slightly longer than the circumference of the head of the object 300 at the nose.
[0061]
FIG. 5 shows an electric circuit of a unit coil constituting the phased array coil. As shown in the figure, the unit coil is constituted by a series connection of a capacitor 302 and a conductor 304. If necessary, one or more other capacitors may be provided in the middle of the conductor 304.
[0062]
An input circuit of a preamplifier 306 that amplifies a magnetic resonance signal received by the unit coil is connected to both ends of the capacitor 302 through an inductor 308. The resonance frequency determined by the capacitance of the capacitor 302 and the inductance of the inductor 308 is tuned to the magnetic resonance frequency. As the preamplifier 306, an amplifier whose input circuit impedance is sufficiently low, that is, a low input impedance amplifier is used.
[0063]
The preamplifier 306 is an example of an embodiment of a low input impedance amplifier in the present invention. The inductor 308 is an example of an inductor according to the present invention. Capacitor 302 is an example of an embodiment of a capacitor in the present invention.
[0064]
FIG. 6 shows an electrical connection diagram of the input circuit portion of the preamplifier 306. As shown in the figure, an input signal is supplied through an inductor 364 to a gate (FET) of an FET (Field Effect Transistor) 362 that is an amplifying element in the first stage of the preamplifier 306. A connection point between the inductor 364 and the gate is connected to the ground through a variable capacitor 366.
[0065]
Here, since the input impedance of the gate of the FET 362 is extremely high, the input impedance Zin of the preamplifier 306 is given by the series impedance of the LC circuit composed of the inductor 364 and the capacitor 366. Such a series circuit of the input impedance Zin and the inductor 308 is connected in parallel to the capacitor 302 of the coil loop to constitute a blocking circuit.
[0066]
FIG. 7 shows an equivalent circuit of the blocking circuit. As shown in the figure, the equivalent circuit is a closed circuit in which the capacitance C of the capacitor 302, the inductance L and the resistance R of the inductor 308, and the input impedance Zin of the preamplifier 306 are connected in series.
[0067]
When a current having a frequency that matches the magnetic resonance frequency flows in the closed circuit, the impedance of the closed circuit viewed from the coil loop side is expressed by the following equation.
[0068]
[Expression 1]

[0069]
here,
ω: angular frequency of current flowing in the closed circuit Zin (Re): real part of Zin (real part: real part)
Zin (Im): Imaginary part of Zin (imaginary part)
In FIG. 8, the change of Z1 with respect to the change of Zin (Im) is shown by a solid line. The broken line is an ideal Z1 change in which the resistance R and Zin (Re) are 0, and is shown for comparison.
[0070]
Since the resistance R and Zin (Re) are not 0 in an actual circuit, Z1 does not become maximum when Zin (Im) = 0 as in the ideal case, as shown in FIG. It becomes maximum when = a. a is a positive value.
[0071]
That is, the impedance Z1 of the blocking circuit is maximized when the imaginary part of the input impedance of the preamplifier 306 takes a positive value, in other words, when the imaginary part of the input impedance is inductive.
[0072]
Therefore, the capacitor 366 of the input circuit of the preamplifier 306 is adjusted so as to satisfy the condition of Zin (Im) = a. As a result, the maximum value of Z1 is obtained, and the blocking circuit exhibits the highest blocking performance possible in the circuit configuration.
[0073]
By making Zin (Im) of the preamplifier 306 inductive, the resonance frequency of the blocking circuit is shifted from the resonance frequency of the coil loop to the low frequency side. Therefore, the adjustment as described above corresponds to shifting the resonance frequency of the blocking circuit from the resonance frequency of the coil loop to the low frequency side.
[0074]
An effect equivalent to this may be achieved by increasing the inductance of the inductor 308 instead of or in addition to the adjustment of Zin (Im).
[0075]
Further, it may be realized by increasing the capacitance of the capacitor 302 of the coil loop instead of or in addition to the adjustment of Zin (Im). However, in that case, it is necessary to adjust other capacitors in the coil loop in order to prevent a change in the resonance frequency of the coil loop. Further, all adjustments of Zin (Im), inductor 308 and capacitor 302 may be performed.
[0076]
By configuring the upper coils 802, 804, 806 and the lower coils 902, 904, 906, 908 as described above, they are not coupled to each other and are equivalent to existing independently. That is, it is possible to obtain a receiving coil unit that effectively operates as a phased array coil. Further, decoupling with the RF coil unit 108 is also effectively performed. Thereby, the magnetic resonance signal can be received with good SNR.
[0077]
The phased array coil is not limited to the coaxial arrangement as described above. For example, as shown in FIG. 9, a plurality of coils 702 to 710 are placed on a plate-like support with the main sensitivity direction facing upward. You may arrange | position toward.
[0078]
This is suitable as a surface coil for a magnetic resonance imaging apparatus using a so-called horizontal magnetic field in which the direction of the static magnetic field is parallel to the body axis of the object 300. The shape of the coil loop may be a rectangle as shown, a circle as described above, an ellipse, an ellipse, or an appropriate polygon.
[0079]
Needless to say, the coil loop may be used as a single coil without forming a phased array. In this case, the receiving coil effectively performs decoupling with the RF coil unit 108 and other coils.
[0080]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to realize an RF coil that effectively performs blocking and a magnetic resonance imaging apparatus including such an RF coil.
[Brief description of the drawings]
FIG. 1 is a block diagram of an exemplary apparatus according to an embodiment of the present invention.
2 is a diagram showing an example of a pulse sequence executed by the apparatus shown in FIG. 1. FIG.
FIG. 3 is a diagram showing an example of a pulse sequence executed by the apparatus shown in FIG.
4 is a schematic diagram showing a coil arrangement of a receiving coil unit in the apparatus shown in FIG. 1. FIG.
FIG. 5 is a circuit diagram of the coil shown in FIG. 4;
6 is a diagram showing an input circuit portion of the preamplifier shown in FIG. 5. FIG.
FIG. 7 is a diagram showing an equivalent circuit of a blocking circuit.
FIG. 8 is a graph showing impedance characteristics of a blocking circuit.
FIG. 9 is a diagram showing a schematic configuration of a phased array coil.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 100 Magnet system 102 Main magnetic field magnet part 106 Gradient coil part 108 RF coil part 110 Reception coil part 130 Gradient drive part 140 RF drive part 150 Data collection part 160 Control part 170 Data processing part 180 Display part 190 Operation part 300 Target 302 Capacitor 304 Conductor 306 Preamplifier 308 Inductor 362 FET
366 Capacitor 364 Inductor 500 Cradle 802, 804, 806 Upper coil 902, 904, 906, 908 Lower coil

Claims (6)

An RF coil having a closed loop having a capacitor in series and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor;
The resonant frequency of the closed circuit consisting of the input circuit of the low input impedance amplifier, the inductor and the capacitor is on a lower frequency side than the resonant frequency of the closed loop,
The inductance of the inductor is greater than the value when matching the resonant frequency of the closed circuit to the resonant frequency of the closed loop,
An RF coil characterized by that. An RF coil having a plurality of sets of a closed loop having a series capacitor and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor,
The resonant frequency of the closed circuit consisting of the input circuit of the low input impedance amplifier, the inductor and the capacitor is on a lower frequency side than the resonant frequency of the closed loop,
The inductance of the inductor is greater than the value when matching the resonant frequency of the closed circuit to the resonant frequency of the closed loop,
An RF coil characterized by that. The imaginary part of the impedance of the input circuit of the low input impedance amplifier under the closed loop resonant frequency is positive,
The RF coil according to claim 1 or 2, wherein A magnetic resonance imaging apparatus that constructs an image based on a magnetic resonance signal generated inside a target using a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field,
An RF coil having a closed loop having a series capacitor as a RF coil for receiving the magnetic resonance signal, and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor,
The resonant frequency of the closed circuit consisting of the input circuit of the low input impedance amplifier, the inductor and the capacitor is on a lower frequency side than the resonant frequency of the closed loop, and the inductance of the inductor determines the resonant frequency of the closed circuit as the closed loop. A magnetic resonance imaging apparatus comprising: an RF coil having a value larger than a value obtained when the resonance frequency is matched with the resonance frequency. A magnetic resonance imaging apparatus that constructs an image based on a magnetic resonance signal generated inside a target using a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field,
An RF coil having a plurality of sets of a closed loop having a series capacitor and a low input impedance amplifier having an input circuit connected in parallel to the capacitor via an inductor as an RF coil for receiving the magnetic resonance signal,
The resonant frequency of the closed circuit consisting of the input circuit of the low input impedance amplifier, the inductor and the capacitor is on a lower frequency side than the resonant frequency of the closed loop, and the inductance of the inductor determines the resonant frequency of the closed circuit as the closed loop. A magnetic resonance imaging apparatus comprising: an RF coil having a value larger than a value obtained when the resonance frequency is matched with the resonance frequency. The imaginary part of the impedance of the input circuit of the low input impedance amplifier under the closed loop resonant frequency is positive,
6. The magnetic resonance imaging apparatus according to claim 4, wherein

Images