JP2005000270A - Magnetic resonance imager - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To implement an SSFP pulse sequence in which the vibration of a nuclear spin in a transient state is restrained and the TR is short even in an MRI apparatus whose rated output of an inclined magnetic field power supply is small. <P>SOLUTION: An impressing time interval between a high-frequency pulse of a flip angle α/2 and a high-frequency pulse of a first flip angle α is TR/2+ΔT, an impressing time interval between a high-frequency pulse of a first flip angle α and a high-frequency pulse of a second flip angle α is TR+ΔT and an impressing time interval among high-frequency pulses of a flip angle α continuously applied more than one time after the high-frequency pulse of a second flip angle α is TR, respectively. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

を満たすようにしたものであり、
特に、ｎ＝２のとき、
Ｔ（１）＝ＴＲ／２＋ΔＴ
Ｔ（２）＝ＴＲ＋ΔＴ （ΔＴ＞０）
を満たすことを特徴とする請求項１に記載の磁気共鳴イメージング装置。
【００１３】
【発明の実施の形態】
以下、本発明の実施形態を、図面を参照して説明する。図３は本実施形態で用いられるＭＲＩ装置の概略構成を示すブロック図である。図３において、３０１は被検体、３０２は被検体３０１を含む所定の大きさの空間へ所定の強度でかつ均一な静磁場を発生する静磁場発生用磁石で、公知の永久磁石タイプ、常電導磁石タイプ、あるいは超電導磁石タイプの物が用いられる。３０３は傾斜磁場コイルで、前記被検体３０１を含む空間において前記静磁場へ重畳されほぼリニアな磁場勾配を形成するもので、基本的にはＸ，Ｙ，Ｘの直交する３軸方向へそれぞれが傾斜磁場を発生する３組の傾斜磁場コイルから構成されている。３０４は前記被検体３０１へ高周波（ＲＦ）磁場をパルス状に照射する高周波コイルであり、ＲＦ送信コイルとここでは称する。３０５は前記ＲＦ送信コイル３０４から被検体へＲＦパルスを照射したときに被検体内の組織を構成する原子の原子核、例えばプロトンが核磁気共鳴現象を起こすことにより放出される電磁波を電磁誘導によって検出するＲＦ受信コイルである。なお、ＲＦ送信コイルとＲＦ受信コイルは別個に設けても、両者を兼用する構成としても良い。
【００１４】
３０６は信号検出部で、増幅器と直交検波器とＡ／Ｄ変換器とから成り、前記ＲＦ受信コイルで検出された信号を増幅、直交検波、Ａ／Ｄ変換の順に処理し、画像を形成する信号に変換するもの、３０７は信号処理部で、信号検出部３０６から入力した信号に対して画像再構成演算や各種解析演算等を行うもの、３０８は表示部で、信号処理部３０７で再構成された画像データや解析データをディスプレイの画面へ表示したり、それらのデータを記録保存するもの、３０９は前記傾斜磁場コイル３０３へ電流を供給する傾斜磁場電源である。
【００１５】
３１１は中央演算処理装置（ＣＰＵ）であり、上記各構成要件を所定のタイミングチャート（これをパルスシーケンスと称す。）に則り動作制御するものである。なお、図示を省略されているが、撮像の各種パラメータや解析条件等の他に装置の起動のオン／オフ指令を入力するコンソールがＣＰＵ３１１へ接続されている。
【００１６】
次に、上記の構成になるＭＲＩ装置で動作させられるパルスシーケンスの一実施形態を説明する。図１は本実施形態のパルスシーケンスを示し、図２は図１に示すパルスシーケンスが進行中のオフレゾナンス状態にある核スピンの挙動を示す。図１に示すパルスシーケンスにおいては、先ずスライス選択傾斜磁場２０１の印加と共にフリップ角（−α／２）のプリパルス１０１が照射される。すると、スライス選択傾斜磁場２０１の傾斜とプリパルス１０１の周波数帯域によって決まるスライス内の核スピンは図２（ａ）に示すようにＸ軸上でＸ−Ｙ平面方向へ励起される。プリパルスによる励起が終了後、極性を反転されたスライス方向傾斜磁場２０２が印加され、励起された核スピンのスライス方向における位相合わせが行われる。ＲＦパルス１０１によって励起された核スピンのうちオフレゾナンス状態にある核スピンは位相回転を生じ、図２（ａ）の状態から図２（ｂ）の状態へ時間の経過とともに位相回転をする。オフレゾナンスは静磁場不均一や磁気感受率の相違によって生ずる。
【００１７】
そしてプリパルス１０１の印加から信号計測時のシーケンスの繰返し時間ＴＲの１／２の時間すなわちＴＲ／２と、ΔＴ時間とが経過した時刻（ＴＲ／２＋ΔＴ）にフリップ角αのＲＦパルス１０２を２０１と同様なスライス選択傾斜磁場２０３と共に印加する。これによってスライス内の核スピンの位相は図２（ｂ）及び図２（ｃ）に示すように、ＴＲ／２＋ΔＴの待ち時間にディフェーズされ（図２（ｂ））、ＲＦパルス１０２の印加によって核スピンはその位相がｚ軸周りにαだけ進められる（図２（ｃ））。なお、スライス選択傾斜磁場２０３の後半分とスライス選択傾斜磁場２０３の前半分とは印加量が等しく、核スピンのスライス方向の位相回転をキャンセルするようになっている。
【００１８】
次に、ＲＦパルス１０２とスライス選択傾斜磁場２０３を印加したことによって励起された核スピンのスライス方向の位相ずれを修正するリフェージング用傾斜磁場２０４を印加する。
【００１９】
さらに次のＲＦパルス１０３とスライス方向傾斜磁場２０６の印加に対する核スピンの位相ずれを事前にリフェージングするスライス方向傾斜磁場２０５とを印加し、ＲＦパルス１０２の印加からＴＲ＋ΔＴの経過後にスライス選択傾斜磁場２０６の印加の下にＲＦパルス１０３（フリップ角−α）を印加する。このとき核スピンの位相はＴＲ＋ΔＴの待ち時間によって図２（ｄ）にあった位置からＲＦパルス１０３の印加により図２（ｅ）の位置に移動する。
【００２０】
その後、繰返し時間ＴＲの間に、スライス方向のリフェーズ、ディフェーズのための傾斜磁場２０７，２０８の印加、位相エンコード傾斜磁場３０１（エンコード用），３０２（リフェーズ用）及びリードアウト傾斜磁場４０１（ディフェーズ用）、４０２（エンコード用）、４０３（リフェーズ用）を順次印加し、Ａ／Ｄ変換器のサンプリング期間５０１の間にエコー信号６０１を計測する。このエコー信号計測中に核スピンの位相は図２（ｅ）から図２（ｆ）へ変化する。
【００２１】
以後、同様に位相エンコード量を変化させながら繰返し時間ＴＲで極性が交互に反転するＲＦパルスを印加してエコー信号を計測する。
なお、上記実施形態はｎ＝２のときを説明したものであるが、本発明はｎ≧３にも適用することが可能であり、ｎ＝２，３，４のときのＴ（ｉ）の値を表１に示す。
【００２２】
【表１】

本実施形態では、ＲＦパルス１０２と１０３の間に生ずるエコー信号は計測せず、ＲＦパルス１０３と１０４の間、１０４と１０５の間、…に生ずるエコー信号しか計測しない。したがって、計測時間の延長が懸念されるが、実際の撮影では、最初の１０エコー程度は過度状態の振動の影響を避けるために画像再構成には用いないために撮影時間の延長やデータ取得効率の低下にはほとんど影響はないと考えてよい。
【００２３】
本発明は心電同期計測にも用いることができる。以下、その実施形態を図４を用いて説明する。図４は１心拍中に４枚の画像を得る、すなわち、１心拍中に１から４までの４つの心時相画像を取得する例を示している。図には、第１心周期の第１心時相についてシーケンスの詳細を示しているが、このシーケンスは、プリパレーションパルスＲＦｐとそれに続き図１に示すパルスシーケンスとから成っている。プリパレーションパルスＲＦｐは、例えば脂肪組織の核スピンを選択的に飽和するもので、各時相の始めに印加される。このプリパレーションパルスＲＦｐを印加すると各時相で定常状態が崩れている。そこで、プリパレーションパルスの印加後に図１のパルスシーケンスを適用するようにして、ＴＲの短縮と、所望の画像コントラストを得られるようにした。なお、ここにＴＲとΔＴの具体的な数値を示すと、ＴＲ＝４．０ｍｓ、ΔＴ＝１．５ｍｓである。
【００２４】
また、本発明はα／２ポストパルスにも適用できる。ここで、α／２ポストパルスとは、ＳＳＦＰシーケンスの途中にプリパレーションパルスを挿入したいときなどに印加するもので、ＳＳＦＰ状態にある核スピンの横磁化を縦磁化に戻すＲＦパルスを言い、〔非特許文献２〕に記載されている。本発明をα／２ポストパルスに用いた実施形態を図５に示す。図５は図１に記載したシーケンスを時間経過を逆転したもので、α／２ポストパルス７０３を印加するには、その２つ前のＲＦパルス７０１の印加後にＴＲ＋ΔＴの時間を置いてＲＦパルス７０２を印加し、ＲＦパルス７０２の印加からＴＲ／２＋ΔＴの時間経過後にα／２ポストパルスを印加するようにする。これによりスライス方向のリワインド傾斜磁場８０１の印加時間に余裕を生じさせることができるので、リワインド傾斜磁場８０１の印加強度を下げることができる。
【００２５】
【非特許文献２】
Ｋ．Ｓｃｈｅｆｆｌｅｒ ｅｔ ａｌ．： Ｍａｇｎｅｔｉｚａｔｉｏｎ Ｐｒｅｐａｒａｔｉｏｎ ｄｕｒｉｎｇ ｔｈｅ Ｓｔｅａｄｙ Ｓｔａｔｅ： Ｆａｔ Ｓａｔｕｒａｔｅｄ ３Ｄ Ｔｒｕｅ ＦＩＳＰ， Ｐｒｏｃ．， ＩＳＭＡＭ， ９^ｔｈ Ａｎｎｕａｌ Ｍｅｅｔｉｎｇ， ４４０（２００１）
【００２６】
【発明の効果】
以上述べたように本発明によれば、傾斜磁場電源の定格が小さなＭＲＩ装置でも過度状態での磁化の振動を抑制したＴＲの短いＳＳＦＰシーケンスを実施することが可能となる。
【図面の簡単な説明】
【図１】本発明の一実施形態のパルスシーケンスを示す図。
【図２】図１のパルスシーケンスの進行中におけるオフレゾナンス状態にある核スピンの挙動を示す図。
【図３】本発明が適用されるＭＲＩ装置の概略構成を示すブロック図。
【図４】本発明を心電同期撮影法へ適用した実施形態を示す図。
【図５】本発明をα／２ポストパルスへ適用した実施形態を示す図。
【符号の説明】
１０１…フリップ角α／２のＲＦパルス
１０２〜１０５…フリップ角αのＲＦパルス
２０１〜２１３…スライス選択傾斜磁場
３０１〜３０５…位相エンコード傾斜磁場
４０１〜４０７…リードアウト傾斜磁場
５０１〜５０３…サンプリング期間
６０１〜６０３…エコー信号[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to an MRI apparatus that can use an SSFP sequence with a short repetition time (TR) even in an apparatus with a low gradient magnetic field generation system performance. Is.
[0002]
[Prior art]
An MRI apparatus irradiates a subject placed in a static magnetic field space having a predetermined uniform intensity with a high-frequency magnetic field (RF magnetic field), thereby nucleating magnetic nuclei of atoms constituting the subject's tissue, for example, protons. A resonance phenomenon is caused, and an electromagnetic wave signal emitted from the subject is measured and processed to be imaged. In order to cause the nuclear magnetic resonance phenomenon and to detect the signal, an RF magnetic field, a slice gradient magnetic field, a phase encoding gradient magnetic field, and a readout gradient magnetic field are applied to a subject placed in a uniform static magnetic field with a predetermined pulse sequence. Apply in accordance with.
[0003]
Conventionally, various types of pulse sequences have been developed and put into practical use. However, there are two types that belong to the spin echo system and those that belong to the gradient echo system. Since the gradient echo pulse sequence does not apply an inversion RF pulse for phase inversion of the excited nuclear spin, the echo time TE and the sequence repetition time TR can be made shorter than the spin echo pulse sequence. On the other hand, if TR is shortened, the transverse magnetization of the nuclear spin remains without disappearing until the end of each sequence and the start of the next sequence, and the residual transverse magnetization is carried over into the next sequence.
[0004]
There is a measurement method that positively utilizes the fact that this residual transverse magnetization is brought into the next sequence. In other words, when TR is shortened and the nuclear spin is repeatedly excited, the nuclear spin oscillates for a while from the start of excitation, and then shifts to a steady state after a lapse of time T1 or T2. This steady state is referred to as a state of steady state free precession (hereinafter referred to as SSFP). The process until the SSFP state is reached is referred to as a transient state. During this transient state, the signal measured due to the oscillation of the nuclear spin as described above is not stable. Is considered to be undesirable for image formation.
[0005]
In order to improve this, an excitation angle (1/2) of the RF excitation pulse (excitation angle α) is TR / 2 hours before the RF excitation pulse in the first sequence of the gradient echo sequence executed repeatedly. It has been proposed in [Non-Patent Document 1] to apply an α / 2) RF pulse as a pre-pulse. By applying this pre-pulse, it is said that strong vibrations in a transient state can be suppressed, and signal acquisition can be started earlier than when no pre-pulse is used.
[0006]
[Non-Patent Document 1]
M.M. Deimling and O.D. Heid: Magnetization Prepared True FISP Imaging, Proc. ^{, SMRM, 2 nd Annual Meeting,} p495, 1994
[0007]
Examples of prepulses applied before the execution of a sequence include fat suppression pulses and inversion recovery pulses.However, the effects of the prepulses decrease with time, so signal measurement is performed immediately after the prepulses are applied. It is desirable to do so.
[0008]
When the True FISP sequence described in Non-Patent Document 1 is used clinically, for example, when performing cardiac imaging, TR ≦ 3 ms is preferable for an apparatus having a static magnetic field strength of 1.5 T. The reason is that a good contrast between blood and myocardium is obtained when TR ≦ 3 ms, and when TR becomes long, dark band artifacts due to the effect of static magnetic field inhomogeneity occur. Is needed.
[0009]
[Problems to be solved by the invention]
In order to execute the True FISP sequence described in Non-Patent Document 1, it takes time TR / 2 to apply the pre-pulse with the excitation angle α / 2. However, during TR / 2, it is necessary to apply a pre-pulse, a slice selective gradient magnetic field, and a slice-direction rephasing gradient magnetic field to the slice selective gradient magnetic field, and the more TR is shortened It is necessary to shorten the application time of the magnetic field. Shortening the application time of the gradient magnetic field means that it is necessary to increase the strength of the gradient magnetic field.
[0010]
Therefore, in order to execute True FISP, it is required that the capability of the gradient magnetic field generation system is high. That is, a gradient magnetic field generation system having a large power supply capacity and high-speed switching performance is required.
For this reason, the True FISP sequence is introduced only in high-end machines, and the MRI system that does not have a very high performance of the gradient magnetic field generation system uses such a short-TR SSFP sequence to suppress the transient spin oscillation. It has not been realized and its development is desired.
[0011]
The present invention has been made in view of the above, and it is possible to execute a short TR SSFP sequence even in an MRI apparatus in which the performance of a gradient magnetic field generation system is not so high, and to suppress nuclear spin oscillation in a transient state and thereby at an early stage. An object of the present invention is to provide an MRI apparatus capable of signal measurement.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, after applying a high-frequency pulse with a flip angle α / 2, a high-frequency pulse with a flip angle α is continuously applied with a short repetition time TR while reversing the polarity between positive and negative. In the magnetic resonance imaging apparatus for obtaining an image by acquiring an echo signal after the magnetization reaches a steady-state free precession from the excessive vibration state, the (i−1) th high-frequency pulse of the flip angle α and the i th When the application time interval with the high-frequency pulse with the flip angle α is T (i) (where the 0th high-frequency pulse is the high-frequency pulse with the flip angle α / 2), the specific n (n ≧ 2) th T (i) is given by the following equation when a high-frequency pulse with a flip angle α of

Is to satisfy
In particular, when n = 2
T (1) = TR / 2 + ΔT
T (2) = TR + ΔT (ΔT> 0)
The magnetic resonance imaging apparatus according to claim 1, wherein:
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 is a block diagram showing a schematic configuration of the MRI apparatus used in the present embodiment. In FIG. 3, reference numeral 301 denotes a subject, 302 denotes a static magnetic field generating magnet that generates a uniform static magnetic field with a predetermined strength in a space of a predetermined size including the subject 301, and is a known permanent magnet type, normal conduction A magnet type or a superconducting magnet type is used. A gradient magnetic field coil 303 is superimposed on the static magnetic field in the space including the subject 301 to form a substantially linear magnetic field gradient. It is composed of three sets of gradient magnetic field coils that generate a gradient magnetic field. Reference numeral 304 denotes a high-frequency coil that irradiates the subject 301 with a high-frequency (RF) magnetic field in a pulse form, and is referred to as an RF transmission coil here. 305 detects by electromagnetic induction the electromagnetic waves emitted by the nuclear magnetic resonance phenomenon of atomic nuclei, eg, protons constituting the tissue in the subject when the subject is irradiated with an RF pulse from the RF transmitter coil 304. RF receiving coil. Note that the RF transmission coil and the RF reception coil may be provided separately or may be configured to use both.
[0014]
A signal detection unit 306 includes an amplifier, a quadrature detector, and an A / D converter. The signal detected by the RF receiving coil is processed in the order of amplification, quadrature detection, and A / D conversion to form an image. What is converted into a signal, 307 is a signal processing unit, which performs image reconstruction calculation and various analysis calculations on the signal input from the signal detection unit 306, 308 is a display unit, and is reconstructed by the signal processing unit 307 309 is a gradient magnetic field power source for supplying current to the gradient magnetic field coil 303, which displays the image data and analysis data on the display screen and records and saves the data.
[0015]
Reference numeral 311 denotes a central processing unit (CPU) that controls the operation of each of the above-described constituent elements in accordance with a predetermined timing chart (referred to as a pulse sequence). Although not shown, a console for inputting an on / off command for starting the apparatus in addition to various imaging parameters and analysis conditions is connected to the CPU 311.
[0016]
Next, an embodiment of a pulse sequence operated by the MRI apparatus configured as described above will be described. FIG. 1 shows the pulse sequence of this embodiment, and FIG. 2 shows the behavior of nuclear spins in the off-resonance state in which the pulse sequence shown in FIG. 1 is in progress. In the pulse sequence shown in FIG. 1, first, a pre-pulse 101 having a flip angle (−α / 2) is irradiated with application of the slice selective gradient magnetic field 201. Then, the nuclear spin in the slice determined by the gradient of the slice selective gradient magnetic field 201 and the frequency band of the prepulse 101 is excited in the XY plane direction on the X axis as shown in FIG. After the excitation by the pre-pulse is completed, a slice direction gradient magnetic field 202 having a reversed polarity is applied, and phase alignment of the excited nuclear spin in the slice direction is performed. Of the nuclear spins excited by the RF pulse 101, the nuclear spins in the off-resonance state undergo phase rotation, and phase rotation occurs from the state of FIG. 2A to the state of FIG. 2B over time. The off-resonance is caused by a non-uniform static magnetic field and a difference in magnetic susceptibility.
[0017]
Then, the RF pulse 102 with the flip angle α is set to 201 at the time (TR / 2 + ΔT) when 1/2 of the repetition time TR of the sequence at the time of signal measurement from the application of the pre-pulse 101, that is, TR / 2, and ΔT time have elapsed. The same slice selective gradient magnetic field 203 is applied. As a result, the phase of the nuclear spin in the slice is dephased with a waiting time of TR / 2 + ΔT (FIG. 2 (b)) as shown in FIGS. 2 (b) and 2 (c). The phase of the nuclear spin is advanced by α around the z-axis (FIG. 2C). Note that the application amount of the rear half of the slice selective gradient magnetic field 203 is equal to that of the front half of the slice selective gradient magnetic field 203 so that the phase rotation of the nuclear spin in the slice direction is canceled.
[0018]
Next, a rephasing gradient magnetic field 204 for correcting a phase shift in the slice direction of the nuclear spin excited by applying the RF pulse 102 and the slice selective gradient magnetic field 203 is applied.
[0019]
Further, a slice direction gradient magnetic field 205 for rephasing the phase shift of the nuclear spin with respect to the application of the next RF pulse 103 and the slice direction gradient magnetic field 206 is applied in advance, and the slice selection gradient magnetic field is applied after TR + ΔT has elapsed since the application of the RF pulse 102. Under the application of 206, the RF pulse 103 (flip angle -α) is applied. At this time, the phase of the nuclear spin moves from the position shown in FIG. 2D by the waiting time of TR + ΔT to the position shown in FIG.
[0020]
Thereafter, during the repetitive time TR, application of gradient magnetic fields 207 and 208 for rephasing and dephasing in the slice direction, phase encoding gradient magnetic fields 301 (for encoding), 302 (for rephasing), and readout gradient magnetic field 401 (dephasing) Phase (for phase), 402 (for encoding), and 403 (for rephase) are sequentially applied, and the echo signal 601 is measured during the sampling period 501 of the A / D converter. During this echo signal measurement, the phase of the nuclear spin changes from FIG. 2 (e) to FIG. 2 (f).
[0021]
Thereafter, the echo signal is measured by applying RF pulses whose polarities are alternately inverted at the repetition time TR while changing the phase encoding amount.
Although the above embodiment has been described when n = 2, the present invention can also be applied to n ≧ 3, and T (i) when n = 2, 3, 4 Values are shown in Table 1.
[0022]
[Table 1]

In this embodiment, an echo signal generated between the RF pulses 102 and 103 is not measured, and only an echo signal generated between the RF pulses 103 and 104, between 104 and 105, and so on is measured. Therefore, although there is a concern about extending the measurement time, in actual shooting, the first 10 echoes are not used for image reconstruction in order to avoid the influence of excessive vibration, so that the shooting time is extended and the data acquisition efficiency is increased. It can be considered that there is almost no effect on the decline in
[0023]
The present invention can also be used for electrocardiographic synchronization measurement. The embodiment will be described below with reference to FIG. FIG. 4 shows an example in which four images are acquired during one heartbeat, that is, four cardiac phase images from 1 to 4 are acquired during one heartbeat. The figure shows the details of the sequence for the first cardiac time phase of the first cardiac cycle, and this sequence consists of the preparation pulse RFp followed by the pulse sequence shown in FIG. The preparation pulse RFp selectively saturates the nuclear spin of adipose tissue, for example, and is applied at the beginning of each time phase. When this preparation pulse RFp is applied, the steady state is broken in each time phase. Accordingly, the pulse sequence shown in FIG. 1 is applied after the preparation pulse is applied so that TR can be shortened and a desired image contrast can be obtained. Here, specific values of TR and ΔT are shown as TR = 4.0 ms and ΔT = 1.5 ms.
[0024]
The present invention can also be applied to α / 2 post pulses. Here, the α / 2 post pulse is applied when a preparation pulse is to be inserted in the middle of the SSFP sequence. The α / 2 post pulse refers to an RF pulse that returns the transverse magnetization of the nuclear spin in the SSFP state to the longitudinal magnetization. Patent Document 2]. FIG. 5 shows an embodiment in which the present invention is used for an α / 2 post pulse. FIG. 5 is a sequence obtained by reversing the time lapse of the sequence shown in FIG. 1. In order to apply the α / 2 post pulse 703, a time TR + ΔT is applied after the application of the RF pulse 701 two times before the RF pulse 702. And an α / 2 post pulse is applied after the time TR / 2 + ΔT has elapsed from the application of the RF pulse 702. Accordingly, a margin can be generated in the application time of the rewind gradient magnetic field 801 in the slice direction, so that the application intensity of the rewind gradient magnetic field 801 can be lowered.
[0025]
[Non-Patent Document 2]
K. Scheffler et al. : Magnetization Preparation the Steady State: Fat Saturated 3D True FISP, Proc. ^{, ISMAM, 9 th Annual Meeting,} 440 (2001)
[0026]
【The invention's effect】
As described above, according to the present invention, it is possible to implement an SSFP sequence with a short TR that suppresses vibration of magnetization in an excessive state even with an MRI apparatus having a small gradient magnetic field power supply rating.
[Brief description of the drawings]
FIG. 1 is a diagram showing a pulse sequence according to an embodiment of the present invention.
FIG. 2 is a diagram showing the behavior of nuclear spins in an off-resonance state during the progress of the pulse sequence of FIG.
FIG. 3 is a block diagram showing a schematic configuration of an MRI apparatus to which the present invention is applied.
FIG. 4 is a diagram showing an embodiment in which the present invention is applied to an electrocardiogram synchronous imaging method.
FIG. 5 is a diagram showing an embodiment in which the present invention is applied to an α / 2 post pulse.
[Explanation of symbols]
101 ... RF pulses 102 to 105 with a flip angle α / 2 ... RF pulses 201 to 213 with a flip angle α ... Slice selection gradient magnetic fields 301 to 305 ... Phase encoding gradient magnetic fields 401 to 407 ... Lead-out gradient magnetic fields 501 to 503 ... Sampling period 601-603 ... Echo signal

Claims (2)

After applying a high-frequency pulse with a flip angle α / 2, a plurality of high-frequency pulses with a flip angle α are continuously applied with a short repetition time TR while reversing the polarity between positive and negative, so that the magnetization is in a state of excessive vibration. In a magnetic resonance imaging apparatus that obtains an image by acquiring an echo signal after reaching steady-state free precession,
The application time interval between the (i-1) th high-frequency pulse with the flip angle α and the i-th high-frequency pulse with the flip angle α is T (i) (where the 0th high-frequency pulse has a high frequency with the flip angle α / 2) Pulse)
, T (i) is given by applying a specific n (n ≧ 2) th high-frequency pulse of flip angle α.

The magnetic resonance imaging apparatus characterized by satisfy | filling. When n = 2
T (1) = TR / 2 + ΔT
T (2) = TR + ΔT (ΔT> 0)
The magnetic resonance imaging apparatus according to claim 1, wherein:

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