Abstract:
A magnetic resonance imaging (MRI) apparatus sequentially transmits a plurality of radio frequency (RF) pulses for refocusing transverse magnetization of spins, and brings the transverse magnetization of the spins to longitudinal magnetization after the refocusing of the transverse magnetization of the spins.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a magnetic resonance imaging apparatus that transmits RF pulses for refocusing transverse magnetization of spins. 
         [0002]    In a magnetic resonance imaging apparatus, unevenness in signal intensity appears in an image due to ununiformity in static magnetic field (B 0  ununiformity) and hence the quality of the image may be deteriorated. There has therefore been proposed a method for reducing image deterioration due to ununiformity in static magnetic field (refer to Japanese Unexamined Patent Publication No. 2007-190362). 
         [0003]    Japanese Unexamined Patent Publication No. 2007-190362 discloses a method of sequentially transmitting four RF pulses having flip angles of 180°, 180°, −180° and −180° between two RF pulses each having a flip angle of 45° to thereby reduce image deterioration due to ununiformity in static magnetic field. However, the method of the patent document 1 also has the problem of occurrence of unevenness in signal intensity due to ununiformity in RF transmission magnetic field (B 1  ununiformity). 
         [0004]    It is desirable that the problem described previously is solved. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    A first aspect of the invention is a magnetic resonance imaging apparatus which sequentially transmits RF pulses α1° θ1° , α2° θ2° , −α1° θ1°  and −α2° θ2°  (where α1°, α2°, −α1° and −α2°; flip angles of the RF pulses, and θ1° and θ2°: the phases of the RF pulses) for refocusing transverse magnetization of spins and brings the transverse magnetization of the spins to longitudinal magnetization after the refocusing of the transverse magnetization of the spins, wherein a combination of the flip angles α1°, α2°, −α1° and −α2° of the RF pulses α1° θ2° , −α2° θ2° , −α1° θ1°  and −α2° θ2°  corresponds to any of the following combinations: 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°−Δ d 1°, 180°+Δ d 2°, −(180°−Δ d 1°), −(180°+Δ d 2°));
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°+Δ d 1°, −(180°−Δ d 2°), −(180°+Δ d 1°), 180°−Δ d 2°);
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°−Δ d 1°), −(180°+Δ d 2°), 180°−Δ d 1°, 180°+Δ d 2°);
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°+Δ d 1°), 180°−Δ d 2°, 180°+Δ d 1°, −(180°−Δ d 2°));
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°+Δ d 1°, 180°−Δ d 2°, −(180°+Δ d 1°), −(180°−Δ d 2°));
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°−Δ d 1°, −(180°+Δ d 2°), −(180°−Δ d 1°), 180°+Δ d 2°);
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°+Δ d 1°), −(180°−Δ d 2°), 180°+Δ d 1°, 180°−Δ d 2°);
 
         [0000]      and 
         [0000]      and (α1°, α2°, −α1°, −α2°)=(−(180°−Δ d 1°), 180°+Δ d 2°, 180°−Δ d 1°, −(180°+Δ d 2°)), where 0 °&lt;Δd 1° and Δ d 2°&lt;180°.
 
         [0006]    A second aspect of the invention is a magnetic resonance imaging apparatus which sequentially transmits RF pulses α1° θ1° , α2° θ2° , α1° θ1°  and α2° θ2°  (where α1° and α2°: flip angles of the RF pulses, and θ 1 ° and θ 2 °: the phases of the RF pulses) for refocusing transverse magnetization of spins and brings the transverse magnetization of the spins to longitudinal magnetization after the refocusing of the transverse magnetization of the spins, wherein a combination of the flip angles α1° and α2° corresponds to any of the following combinations: 
         [0000]      (α1°, α2°)=(180°−Δ d 1°, 180°+Δ d 2°);
 
         [0000]      (α1°, α2°)=(180°+Δ d 1°, −(180°−Δ d 2°));
 
         [0000]      (α1°, α2°)=(−(180°−Δ d 1°), −(180°+Δ d 2°));
 
         [0000]      (α1°, α2°)=(−(180°+Δ d 1°), 180°−Δ d 2°);
 
         [0000]      (α1°, α2°)=(180°+Δ d 1°, 180°−Δ d 2°);
 
         [0000]      (α1°, α2°)=(180°−Δ d 1°, −(180°+Δ d 2°));
 
         [0000]      (α1°, α2°),=(−(180°+Δ d 1°), −(180°−Δ d 2°); and
 
         [0000]      (α1°, α2°)=(−(180°−Δ d 1°), 180°+Δ d 2°), where 0°&lt;Δ d 1° and Δ d 2°&lt;180°.
 
         [0007]    In the invention, RF pulses whose flip angles are shifted by Δd1 or Δd2 from 180° (or −180° are used. Using such RF pulses makes it possible to reduce unevenness in signal intensity due to B 1  ununiformity. 
         [0008]    Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a magnetic resonance imaging apparatus  1  according to a first embodiment of the invention. 
           [0010]      FIG. 2  is a diagram schematically showing a field of view FOV of a subject  9 . 
           [0011]      FIGS. 3A ,  3 B, and  3 C are explanatory diagrams of a pulse sequence for imaging arterial blood AR. 
           [0012]      FIGS. 4A and 4B  are diagrams for describing a concrete configuration of a longitudinal magnetization adjusting sequence  21 . 
           [0013]      FIG. 5  is a diagram for explaining RF pulses 90° x, α1° y, α2° y, −α1° y, −α2° y and −90° x. 
           [0014]      FIGS. 6A ,  6 B, and  6 C are diagrams showing behaviors of spins while the longitudinal magnetization adjusting sequence  21  is being executed. 
           [0015]      FIG. 7  is a graph showing simulation results. 
           [0016]      FIG. 8  is a diagram illustrating one example of the longitudinal magnetization adjusting sequence  21  where a combination of flip angles expressed in an equation (10) is used. 
           [0017]      FIG. 9  is a graph showing simulation results. 
           [0018]      FIG. 10  is diagram for describing eddy currents. 
           [0019]      FIG. 11  is one example of a longitudinal magnetization adjusting sequence  21  employed in a third embodiment. 
           [0020]      FIGS. 12A ,  12 B,  12 C, and  12 D are diagrams for explaining a procedure for generating a crusher gradient pulse Gcrush and a velocity encode gradient pulse Gv combined together on a single axis. 
           [0021]      FIGS. 13A and 13B  are diagrams showing the manner in which the time i of the sequence can be shortened. 
           [0022]      FIG. 14  is a diagram illustrating a sequence when a second gradient pulse g c2  located between RF pulses α1°y and α2°y is brought close to a third gradient pulse g c3 . 
           [0023]      FIG. 15  is a diagram showing a sequence when a plurality of gradient pulses are shifted. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    While modes for carrying out the invention will be explained below, the invention will not be limited to the following modes. 
       (1) First Embodiment 
       [0025]      FIG. 1  is a block diagram of a magnetic resonance imaging apparatus (hereinafter called “MRI apparatus”, where MRI: Magnetic Resonance Imaging)  1  according to a first embodiment of the invention. 
         [0026]    The MRI apparatus  1  has a coil assembly  2 , a table  3 , a heartbeat sensor  4 , a receiving coil  5 , a controller  6 , an input device  7  and a display device  8 . 
         [0027]    The coil assembly  2  has a bore  21  into which a subject  9  is carried. The coil assembly  2  has a superconductive coil  22 , a gradient coil  23  and a transmitting coil  24 . The superconductive coil  22  applies a static magnetic field B 0  to within the bore  21 . The gradient coil  23  applies a gradient pulse to within the bore  21 . The transmitting coil  24  transmits an RF pulse to within the bore  21 . 
         [0028]    The table  3  has a cradle  31 . The cradle  31  is configured so as to move in a z direction and a −z direction. With the movement of the cradle  31  in the z direction, the subject  9  is conveyed to the bore  21 . With the movement of the cradle  31  in the −z direction, the subject  9  conveyed to the bore  21  is carried out of the bore  21 . 
         [0029]    The heartbeat sensor  4  detects the heartbeat of the subject  9  and generates an electrocardiac signal  4   a.    
         [0030]    The receiving coil  5  receives each magnetic resonance signal therein. 
         [0031]    The controller  6  has an electrocardiac signal analysis device  61 , a coil control device  62  and an image reconstruction device  63 . 
         [0032]    The electrocardiac signal analysis device  61  analyzes the electrocardiac signal  4   a.    
         [0033]    The coil control device  62  controls the gradient coil  23  and the transmitting coil  24  in such a manner that a pulse sequence PS (refer to  FIGS. 3  to be described later) is executed, based on the result of analysis by the electrocardiac signal analysis device  61 . 
         [0034]    The image reconstruction device  63  reconstructs an image, based on the magnetic resonance signal received by the receiving coil  5 . 
         [0035]    The input device  7  inputs various instructions to the controller  6  in response to operations of an operator  10 . 
         [0036]    The display device  8  displays various information and images thereon. 
         [0037]      FIG. 2  is a diagram schematically showing an imaging field of view FOV of the subject  9 . 
         [0038]    A heart  14 , arteries  15  and veins  16  are shown in  FIG. 2 . Arterial blood AR flows from an upstream area UP to a downstream area DW via the field of view FOV. Contrary to the arterial blood AR, venous blood VE flows from the downstream area DW to the upstream area UP via the field of view FOV. In the first embodiment, the field of view FOV contains knees K of the subject  9  and its peripheral regions. A description will hereinafter be made of how an MR image of the arterial blood AR flowing through the field of view FOV is acquired. 
         [0039]    Incidentally, the venous blood VE untargeted for imaging is also contained in the field of view FOV in addition to the arterial blood AR targeted for imaging. Since the first embodiment considers that the arterial blood AR is represented, it becomes difficult to visually identify the state of the bloodstream of the arterial blood AR if the venous blood VE is also represented together with the arterial blood AR. Accordingly, there is a need to avoid the representation of the venous blood VE untargeted for imaging as much as possible. Therefore, when the arterial blood AR is imaged, the following pulse sequence is executed in the first embodiment. 
         [0040]      FIGS. 3A-3C  are diagrams for describing a pulse sequence for imaging the arterial blood AR. 
         [0041]      FIG. 3A  is a graph showing an electrocardiac waveform ECG of the subject  9 ,  FIG. 3B  is a pulse sequence PS for imaging the arterial blood AR, and  FIG. 3C  is a diagram showing a longitudinal magnetization recovery curve Car of the arterial blood AR and a longitudinal magnetization recovery curve Cve of the venous blood VE. 
         [0042]    The pulse sequence PS is executed in sync with the electrocardiac waveform ECG. The pulse sequence PS contains a longitudinal magnetization adjusting sequence  21 , a reverse pulse  22  and a data acquiring sequence  23 . 
         [0043]    The longitudinal magnetization adjusting sequence  21  is of a sequence for adjusting a longitudinal magnetization component Mz of the arterial blood AR to Mz=0 (or value near Mz=0) while keeping a longitudinal magnetization component Mz of the venous blood VE at Mz=1 (or value near Mz=1). The reverse pulse  22  is of a pulse for inverting the longitudinal magnetization of the arterial blood AR and the venous blood VE after the longitudinal magnetization adjusting sequence  21  has been executed. The data acquiring sequence  23  is of a sequence for acquiring data for embedding a k space. 
         [0044]    Magnetization behaviors of spins of the venous blood VE and the arterial blood AR at the time that the pulse sequence PS has been executed will next be explained in brief with reference to  FIG. 3C . 
         [0045]    Assume that the longitudinal magnetization components Mz of the venous blood VE and the arterial blood AR are Mz=1 at a time to prior to the start of the longitudinal magnetization adjusting sequence  21 . At a time t 1 , however, the longitudinal magnetization adjusting sequence  21  is executed. The longitudinal magnetization adjusting sequence  21  is of a sequence for setting the longitudinal magnetization component Mz of the venous blood VE to Mz=1 (or a value near Mz=1) and setting the longitudinal magnetization component Mz of the arterial blood AR to Mz=0 (or a value near Mz=0). Thus, when the longitudinal magnetization adjusting sequence  21  is completed, the longitudinal magnetization component Mz of the venous blood VE is Mz=1 (or a value near Mz=1), whereas the longitudinal magnetization component Mz of the arterial blood AR becomes Mz=0 (or a value near Mz=0). Incidentally, a specific configuration of the longitudinal magnetization adjusting sequence  21  will be explained in detail later. 
         [0046]    After the longitudinal magnetization adjusting sequence  21  has been completed, the longitudinal magnetization component Mz of the venous blood VE is inverted from Mz=1 to Mz=−1 by the reverse pulse  22  (time tb). Thereafter, the longitudinal magnetization component is brought to a longitudinal magnetization recovery again and reaches a null point at a time tc. 
         [0047]    On the other hand, during a period from the completion of the longitudinal magnetization adjusting sequence  21  to the transmission of the reverse pulse  22 , the longitudinal magnetization component Mz of the arterial blood AR is recovered from Mz=0 to Mz=m1 and inverted from Mz=m1 to Mz=−m1 by the reverse pulse  22 . Since, however, a waiting time Tw 1  between the longitudinal magnetization adjusting sequence  21  and the reverse pulse  22  is a very short time (a few msec), the value of m1 is a value near zero. Accordingly, the value of −m1 is also a value near zero. The longitudinal magnetization component Mz of the arterial blood AR inverted to Mz=−m1 is brought to a longitudinal magnetization recovery and recovered to Mz=m2 at the time tc. 
         [0048]    The execution of the data acquiring sequence  23  is started at the time tc and ends at a time td. 
         [0049]    At the data acquisition start time tc, the longitudinal magnetization component Mz of the arterial blood AR is 0.5 or so, whereas the longitudinal magnetization component Mz of the venous blood VE is Mz=0. Thus, an MR image in which the arterial blood AR is emphasized and the venous blood VE is suppressed can be obtained by acquiring data at the data acquisition start time tc. 
         [0050]    Incidentally, in the first embodiment, a sequence shown in  FIGS. 4A and 4B  to be described later is used as the longitudinal magnetization adjusting sequence  21 . Using the longitudinal magnetization adjusting sequence  21  shown in  FIGS. 4A and 4B  brings about the effect that it is possible to reduce the unevenness in signal intensity due to variations (B 1  ununiformity) in B 1  intensity within the bore  21  (refer to  FIG. 1 ). The reason why such an effect is obtained will be explained. 
         [0051]      FIGS. 4A and 4B  are diagrams for describing a concrete configuration of the longitudinal magnetization adjusting sequence  21 . 
         [0052]    The longitudinal magnetization adjusting sequence  21  has RF pulses 90° x, α1° y, α2° y, −α1° y, −α2° y and −90° x. The respective RF pulses will be explained below. 
         [0053]      FIG. 5  is a diagram for describing the RF pulses 90° x, α1° y, α2° y, −α1° y, −α2° y and −90° x. 
         [0054]    The RF pulse 90° x is of an RF pulse that coincides with an x axis in phase at a flip angle 90° and is transmitted during a period from times t 1  to t 3 . A central time point of the RF pulse 90° x is t 2 . 
         [0055]    The RF pulse α1° y is of an RF pulse that coincides with a y axis in phase at a flip angle α1° (i.e., it is shifted 90° from the x axis) and is transmitted during a period from times t 4  through t 6 . A central time point of the RF pulse α1° y is t 5 . A time interval Δt between the RF pulses α1° y and 90° x is Δt=Δt 1 . Incidentally, the flip angle α1° is a flip angle smaller than 180° by Δd1°. It is expressed in the following equation (1): 
         [0000]      α1°=180°−Δ d 1°  (1)
 
         [0000]    where 0&lt;Δd1°&lt;180° 
         [0056]    The RF pulse α2° y is of an RF pulse that coincides with the y axis in phase at a flip angle α2° (i.e., it is shifted 90° from the x axis) and is transmitted during a period from times t 7  through t 9 . A central time point of the RF pulse α2° y is t 8 . A time interval At between the RF pulses α2° y and α1° y is Δt=Δt 2 . Incidentally, the flip angle α2° is a flip angle larger than 180° by Δd2°. It is expressed in the following equation (2): 
         [0000]      α2°=180°+Δ d 2°  (2)
 
         [0000]    where 0&lt;Δd2°&lt;180° 
         [0057]    The RF pulse −α1° y is of an RF pulse that coincides with the y axis in phase at a flip angle −α1° (i.e., it is shifted 90° from the x axis) and is transmitted between times t 10  and t 12 . A central time point of the RF pulse −α1° y is t 11 . A time interval Δt between the RF pulses −α1° y and α2° y is Δt=Δt 3 . Incidentally, the flip angle −α1° is a flip angle identical in absolute value to the flip angle α1° although opposite in sign thereto. It is expressed in the following equation (3): 
         [0000]      −α1°=−1 (180° −Δd 1°)   (3)
 
         [0000]    where 0&lt;Δd1°&lt;180° 
         [0058]    The RF pulse −α2° y is of an RF pulse that coincides with the y axis in phase at a flip angle −α2° (i.e., it is shifted  90 ° from the x axis) and is transmitted between times t 13  and t 15 . A central time point of the RF pulse −α2° y is t 14 . A time interval Δt between the RF pulses −α2° y and −α1° y is Δt=Δt 4 . Incidentally, the flip angle −α2° is a flip angle identical in absolute value to the flip angle α2° although opposite in sign thereto. It is expressed in the following equation (4): 
         [0000]      −α2°=−1 (180°+Δ d 2°)   (4)
 
         [0000]    where 0&lt;Δd2°&lt;180° 
         [0059]    The RF pulse −90° x is of an RF pulse that coincides with the x axis in phase at a flip angle −90° and is transmitted between times t 16  and t 18 . A central time point of the RF pulse −90° x is t 17 . A time interval Δt between the RF pulses −90° x and −α2° y is Δt=Δt 5 . 
         [0060]    The time intervals Δt 1 , Δt 2 , Δt 3 , Δt 4  and Δt 5  among the RF pulses 90° x, α1° y, α2° y, −α1° y, −α2° y and −90° x are set in such a manner that the following relationship is established: 
         [0000]      Δt1:Δt2:Δt3:Δt4:Δt5=1:2:2:2:1
 
         [0061]    The longitudinal magnetization adjusting sequence  21  has a crusher gradient pulse Gcrush and a velocity encode gradient pulse Gv as shown in  FIG. 3C . The crasher gradient pulse Gcrush is applied in right-and-left directions RL, and the velocity encode gradient pulse Gv is applied in up-and-down directions SI (refer to  FIG. 2 ). The crusher gradient pulse Gcrush is applied to cause transverse magnetization of each spin of the arterial blood AR to disappear with respect to the right-and-left directions RL. The velocity encode gradient pulse Gv is applied to shift the phase of each spin of the arterial blood AR and the phase of each spin of the venous blood VE by 180° from each other with respect to the up-and-down directions SI. 
         [0062]    The longitudinal magnetization adjusting sequence  21  is configured in the above-described manner. 
         [0063]    In the first embodiment, the longitudinal magnetization adjusting sequence  21  uses the RF pulses α1° y, α2° y, −α1° y and −α2° y having the flip angles shifted from 180° as shown in  FIG. 4B . Using such a longitudinal magnetization adjusting sequence  21  brings about the effect that it is possible to reduce unevenness in signal intensity due to variations in B 1  intensity (B 1  ununiformity) within the bore  21 . In order to describe the reason why such an effect is obtained, the behaviors of spins at the time that the variations in B 1  intensity have occurred within the bore  21  will be described with reference to  FIGS. 6A-6C . 
         [0064]      FIGS. 6A-6C  are diagrams showing the behaviors of spins while the longitudinal magnetization adjusting sequence  21  is being executed. 
         [0065]      FIG. 6A  is a diagram showing the longitudinal magnetization adjusting sequence  21 , and  FIGS. 6B and 6C  are diagrams showing the behaviors of spins in the venous blood VE lying within the bore  21 . 
         [0066]      FIG. 6A  shows only the RF pulses.  FIG. 6B  shows the behaviors of spins of the venous blood VE at a position where the value of a B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity.  FIG. 6C  shows the behaviors of spins of the venous blood VE at a position where the value of the B 1  intensity is lowered by 20% than the ideal value of the B 1  intensity. 
         [0067]    The behaviors of the spins in  FIGS. 6B and 6C  will be explained below. 
         [0068]    Incidentally, assume that for convenience of explanation below, the flip angles α1°, α2°, −α1° and −α2° of the RF pulses α1° y, α2° y, −α1° y and −α2° y are indicative of the following values: 
         [0000]      α1°=150° (Δ d 1°=30° in the equation   (1))
 
         [0000]      α2°=225° (Δ d 2°=45° in the equation   (2))
 
         [0000]      −α1°=−α1°=−150° (Δ d 1°=30° in the equation   (3)), and
 
         [0000]      −α2°=−α2°=−225° (Δ d 2°=45° in the equation   (4))
 
         [0069]    The behaviors of spins at the position where the value of B 1  intensity becomes larger by 20% than the ideal value of B 1  intensity will now be discussed with reference to  FIG. 6B . 
         [0070]    (i) Time ta 
         [0071]    Since the longitudinal magnetization adjusting sequence  21  is not yet started at the time ta, a magnetization vector M 11  of spins is indicative of magnetization in a Z direction (longitudinal magnetization component) (refer to a spin state B 1 ). 
         [0072]    (ii) Time t 2  (Times t 1  Through t 3 ) 
         [0073]    Since the R pulse 90° x is transmitted between the times t 1  and t 3 , the spins are flipped about the x axis and the magnetization vector M 11  of the spins changes to a magnetization vector M 12  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 2  of the RF pulse 90° x for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to 90° but to 108° (=90°×1.2) larger by 20% than 90° (refer to a spin state B 2 ). 
         [0074]    (ii) Time t 2  Through t 5   
         [0075]    Magnetization M 12  of the spins at the time t 2  is dispersed in the phase of transverse magnetization thereof as time advances and is thereby brought to magnetization M 13  (refer to a spin state B 3 ). 
         [0076]    (iii) Time t 5  (Times t 4  Through t 6 ) 
         [0077]    Since the RF pulse 150° y is transmitted between the times t 4  and t 6 , the spins are flipped about the y axis and a magnetization vector M 13  of the spins changes to a magnetization vector M 14  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 5  of the RF pulse 150° y for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to 150° but to 180° (=150°×1.2) larger by 20% than 150° (refer to the spin state B 3 ). 
         [0078]    (iv) Times t 5  Through t 6 ′ 
         [0079]    At the time t 5 , the spins are being flipped 180°. Accordingly, the dispersion of the phase of the transverse magnetization of the spins that has occurred between the times t 2  and t 5  (time Δt 1 ) is cancelled out between the times t 5  and t 6 ′ (time Δt 1 ). The dispersed transverse magnetization of spins is focused at the time t 6 ′ and is thereby brought to magnetization M 15  (refer to a spin state B 4 ). 
         [0080]    (v) Times t 6 ′ Through t 8   
         [0081]    The magnetization M 15  of the spins, which has been focused at the time t 6 ′, is dispersed in the phase of transverse magnetization thereof as time advances and is thereby brought to magnetization M 16  (refer to a spin state B 5 ). 
         [0082]    (vi) Time t 8  (Times t 7  Through t 9 ) 
         [0083]    Since the RF pulse 225° y is transmitted between the times t 7  and t 9 , the spins are flipped about the y axis and the magnetization M 16  of the spins changes to magnetization M 17  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 8  of the RF pulse  225 ° y for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to 225° but to 270° (=225°×1.2) larger by 20% than 225° (refer to the spin state B 5 ). 
         [0084]    (vii) Times t 8  Through t 11   
         [0085]    The magnetization M 17  of the spins, which has been flipped at the time t 8 , is dispersed in the phase of transverse magnetization thereof as time advances and is thereby brought to magnetization M 18  (refer to a spin state B 6 ). 
         [0086]    (viii) Time t 11  (Times t 10  Through t 12 ) 
         [0087]    Since the RF pulse −150° y is transmitted between the times t 10  and t 12 , the spins are flipped about the y axis and the magnetization M 18  of the spins changes to magnetization M 19  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 11  of the RF pulse −150° y for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to −150° but to −180° (=−150°×1.2) larger by 20% than −150° (refer to the spin state B 6 ). 
         [0088]    (ix) Times t 11  Through t 14   
         [0089]    At the time t 11 , the spins are being flipped −180°. Accordingly, the dispersion of the phase of the transverse magnetization of the spins that has occurred between the times t 8  and t 11  (time Δt 3 =2×Δt 1 ) is cancelled out between the times t 11  and t 14  (time Δt 4 =2×Δt 1 ). The magnetization M 19  of the spins changes to magnetization M 20 . 
         [0090]    (x) Time t 14  (Times t 13  Through t 15 ) 
         [0091]    Since the RF pulse −225° y is transmitted between the times t 13  and t 15 , the spins are flipped about the y axis and the magnetization vector M 20  of the spins changes to a magnetization vector M 21  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 14  of the RF pulse −225° y for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to −225° but to −270° (=−225°×1.2) larger by 20% than −225° (refer to a spin state B 7 ). 
         [0092]    (xi) Times t 14  Through t 17   
         [0093]    The dispersion of the phase of the transverse magnetization of the spins that has occurred between the times t 6 ′ and t 8  (time Δt 1 ) is cancelled out between the times t 14  and t 17  (time Δt 5 =Δt 1 ). Accordingly, the transverse magnetization of the spins is focused at the time t 17  and is thereby brought to magnetization M 22  (refer to a spin state B 8 ). 
         [0094]    (xii) Time t 17  (Times t 16  Through t 18 ) 
         [0095]    Since the RF pulse −90° x is transmitted between the times t 16  and t 18 , the spins are flipped about the x axis and the magnetization vector M 22  of the spins changes to magnetization M 23  (incidentally, the spins are assumed to be flipped momentarily at the central time point t 17  of the RF pulse −90° x for convenience of description). However, at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to −90° but to −108° (=−90°×1.2) larger by 20% than −90°. Thus, since the magnetization M 23  is oriented in the Z-axis direction, the longitudinal magnetization component Mz of each spin is brought to Mz=1 (refer to the spin state B 8 ). 
         [0096]    As has been described in each of (i) through (xii), the RF pulse 150° y (−150° y) flips the spins by 180° (−180°) at the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity. Accordingly, even in the position where the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity, the longitudinal magnetization component Mz of each spin can be brought to Mz=1 (refer to the time t 18  of the longitudinal magnetization recovery curve Cve shown in  FIG. 3 ). 
         [0097]    The behaviors of spins at the position where the value of B 1  intensity is lowered by 20% than the ideal value of B 1  intensity will now be discussed with reference to  FIG. 6C . 
         [0098]    Since the behaviors of the spins in  FIG. 6C  can be described in a manner similar to the behaviors of the spins in  FIG. 6C , they will be explained in brief 
         [0099]    Since the longitudinal magnetization adjusting sequence  21  is not yet started at the time ta, a magnetization vector M 31  of spins is indicative of magnetization (longitudinal magnetization component) in the Z direction (refer to a spin state C 1 ). Thereafter, the RF pulse 90° x is transmitted between the times t 1  and t 3 , and the magnetization vector M 31  of the spins changes to a magnetization vector M 32 . However, at the position where the value of the B 1  intensity is lowered by 20% than the ideal value of the B 1  intensity, the flip angle of the spins is not brought to 90° but to 72° (=90°×0.8) smaller than 90° by 20%, even though the RF pulse 90° x is transmitted (refer to a spin state C 2 ). The magnetization M 32  of the spins at the time t 2  is dispersed in the phase of transverse magnetization thereof as time advances. However, the magnetization of the spins dispersed between the times t 2  and t 8  is focused between the times t 8  and t 12 ′ by the RF pulse 225° y (times t 7  through t 9 ) (refer to a spin state C 6 ). The magnetization of the spins dispersed between the times t 12 ′ and t 14  is focused between the times t 14  and t 17  by the RF pulse −225° y (times t 13  through t 15 ) (refer to a spin state C 8 ). Accordingly, the longitudinal magnetization component Mz of each spin can be brought to Mz=1 even at the position where the value of the B 1  intensity is lowered by 20% than the ideal value of the B 1  intensity (refer to the time t 18  of the longitudinal magnetization recovery curve Cve shown in  FIGS. 3A-3C ). 
         [0100]    It is understood from the explanations of  FIGS. 6B and 6C  that even though the value of the B 1  intensity becomes larger by 20% than the ideal value of the B 1  intensity and the value thereof is lowered by 20% than that in reverse, the longitudinal magnetization component Mz of each spin reaches Mz=1. Thus, the flip angles α1°, α2°, −α1° and −α2° of the longitudinal magnetization RF pulses α1° y, α2° y, −α1° y and −α2° y of the spins are shifted from 180° and −180°, thereby making it possible to reduce unevenness in signal intensity due to B 1  ununiformity. In order to verify the above consideration, simulations were performed on the signal intensities where the longitudinal magnetization sequence  21  was executed. The simulations will be explained below. 
         [0101]      FIG. 7  is a graph showing simulation results. 
         [0102]    The horizontal axis of the graph indicates a transmission magnetic field strength ratio Blr at each pixel within the field of view FOV, and the vertical axis thereof indicates integral values S of signal intensities at the respective pixels within the field of view FOV. 
         [0103]    Incidentally, the transmission magnetic field strength ratio Blr is expressed in the following equation: 
         [0000]        Blr=B 1 x/B 1 a    (5)
 
         [0000]    where B 1   a : ideal value of transmission magnetic field strength where no B 1  ununiformity exists, and B 1   x : transmission magnetic field strength at each pixel within field of view FOV where B 1  ununiformity exists. 
         [0104]    Thus, when a transmission magnetic field strength B 1   x  at a predetermined pixel in the field of view FOV coincides with the ideal value B 1   a  of the transmission magnetic field strength, a transmission magnetic field strength ratio is Blr=1 at the predetermined pixel. When, however, the transmission magnetic field strength B 1   x  at the predetermined pixel in the field of view FOV is shifted from the ideal value B 1   a  of the transmission magnetic field strength, the transmission magnetic field strength ratio is Blr≠1 at the predetermined pixel. Specifically, when the transmission magnetic field strength B 1   x  at the predetermined pixel in the field of view FOV becomes larger than the ideal value B 1   a  of the transmission magnetic field strength, the transmission magnetic field strength ratio is Blr&gt;1 at the predetermined pixel. When, for example, the transmission magnetic field strength B 1   x  at the predetermined pixel in the field of view FOV becomes larger by 20% than the ideal value B 1   a  of the transmission magnetic field strength, the transmission magnetic field strength ratio is Blr=1.2 at the predetermined pixel. On the other hand, when the transmission magnetic field strength B 1   x  at the predetermined pixel in the field of view FOV becomes smaller than the ideal value B 1   a  of the transmission magnetic field strength, the transmission magnetic field strength ratio is Blr&lt;1 at the predetermined pixel. When, for example, the transmission magnetic field strength B 1   x  at the predetermined pixel in the field of view FOV is lowered by 20% than the ideal value B 1   a  of the transmission magnetic field strength, the transmission magnetic field strength ratio is Blr=0.8 at the predetermined pixel. 
         [0105]    Three lines L 1 , L 2  and L 3  each indicative of the relationship between the transmission magnetic field strength ratio Blr and the integral value S of each signal intensity are show in the graph. Under simulation conditions of the three lines L 1 , L 2  and L 3 , the values of the flip angles α1°, α2°, −α1° and −α2° of the RF pulses α1° y, α2° y, −α1° y and −α2° y are different from one another. The values of the flip angles α1°, α2°, −α1° and −α2° with respect to the three lines L 1 , L 2  and L 3  are as follows: 
       (1) Line L 1   
       [0106]      (α1°, α2°, −α1°, −α2°)=(180°, 180°, −180° and −180°)   (6)
 
       (2) Line L 2   
       [0107]      (α1°, α2°, −α1°, −α2°)=(150°, 225°, −150° and −225°)   (7)
 
       (3) Line L 3   
       [0108]      (α1°, α2°, −α1°, −α2°)=(138°, 257°, −138° and −257°)   (8)
 
         [0109]    Ranges ΔR 1 , ΔR 2  and ΔR 3  each taken for the transmission magnetic field strength ratio Blr at which the integral value S of the signal intensity becomes 0.8 or more in the vicinity of the transmission magnetic field strength ratio Blr=1 are shown in  FIG. 7  for every L 1 , L 2 , L 3  of lines. 
         [0110]    Comparing the ranges ΔR 1 , ΔR 2  and ΔR 3  taken for the transmission magnetic field strength ratios Blr at the individual lines L 1 , L 2  and L 3  with one another shows that the ranges ΔR 2  and ΔR 3  for the lines L 2  and L 3  become wider than the range ΔR 1  for the line L 1 . It is thus understood that unevenness in signal intensity due to B 1  ununiformity can be reduced by shifting the flip angles (α1°, α2°, −α1° and −α2°) from 180° and −180°. 
         [0111]    Incidentally, the lines L 2  and L 3  become smaller than the line L 1  in the integral value S of the signal intensity when the transmission magnetic field strength ratio Blr=1. Since, however, the integral value S of the signal intensity at the transmission magnetic field strength ratio Blr=1 is S&gt;0.8 even in the case of the lines L 2  and L 3 , the signal intensity is considered to have sufficient magnitude. Thus, even if the integral value S of the signal intensity is S&lt;1 when the transmission magnetic field strength ratio Blr=1, unevenness in signal intensity due to B 1  ununiformity can be reduced sufficiently. 
         [0112]    Although the venous blood VE has been mentioned in the above description, unevenness in signal intensity due to B 1  ununiformity can sufficiently be reduced in like manner even with respect to the arterial blood AR. In the case of the arterial blood AR that flows at a flow velocity sufficiently faster than that for the venous blood VE, its transverse magnetization is cancelled out by a crusher gradient pulse Gcrush and a velocity gradient pulse Gv. Therefore, the transverse magnetization becomes small sufficiently until the last RF pulse −90° x is transmitted. Thus, even if the RF pulse −90° x is transmitted, the longitudinal magnetization component Mz of the arterial blood AR becomes Mz=0 (or a value near Mz=0) (refer to the time t 18  of the longitudinal magnetization recovery curve Car shown in  FIGS. 3A-3C ). 
         [0113]    Using the longitudinal magnetization adjusting sequence  21  as described above makes it possible to sufficiently reduce the unevenness in signal intensity due to the B 1  ununiformity with respect to both the venous blood VE and the arterial blood AR different in flow velocity and thereby obtain a high quality image. 
       (2) Second Embodiment 
       [0114]    In the first embodiment, the flip angles α1°, α2°, −α1° and −α2° of the RF pulses α1° y, α2° y, −α1° y and −α2° y are expressed in the equations (1) through (4). These equations (1) through (4) are summarized as follows: A combination of the flip angles α1°, α2°, −α1° and −α2° is expressed in the following equation (9): 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°−Δ d 1°, 180°+Δ d 2°, −(180°−Δ d 1°, −(180°+Δ d 2°))   (9)
 
         [0115]    The combination of the flip angles α1°, α2°, −α1° and −α2° may, however, be another combination than the equation (5). The second embodiment will explain a case in which a combination expressed in the following equation (10) as another combination of the flip angles α1°, α2°, −α1° and −α2°. 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°+Δ d 1°, −(180°−Δ d 2°), −(180°+Δ d 1°), 180°−Δ d 2°) (10)
 
         [0000]    where 0°&lt;Δd1° and Δd2°&lt;180° 
         [0116]      FIG. 8  is a diagram showing one example of the longitudinal magnetization adjusting sequence  21  where the combination of the flip angles expressed in the equation (10) is used. 
         [0117]    Values obtained when Δd1°=20° and Δd2°=40° are respectively substituted into Δd1° and Δd2° of the equation (10) are used in  FIG. 8  (refer to the following equation (10′)). 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°+Δ d 1°, −(180°−Δ d 2°), −(180°+Δ d 1°), 180°−Δ d 2°)=(200°, −140°, −200°, 140°)
 
         [0118]    Next, simulations for examining unevenness in signal intensity due to B 1  ununiformity where the flip angles expressed in the equation (10)′ were used were done. The simulations will be explained below. 
         [0119]      FIG. 9  is a graph showing simulation results. 
         [0120]    Two lines L 1  and L 4  each indicative of the relationship between a transmission magnetic field strength ratio Blr and an integral value S of each signal intensity are shown in the graph. 
         [0121]    The line L 1  is identical to the line L 1  shown in  FIG. 7  and indicates a simulation result obtained when flip angles (180°, 180°, −180° and −180°) are used. On the other hand, the line L 4  indicates a simulation result obtained when the flip angles (200°, −140°, −200° and 140°) expressed in the equation (10)′ are used. 
         [0122]    Ranges ΔR 1  and ΔR 4  each taken for the transmission magnetic field strength ratio Blr at which the integral value S of the signal intensity becomes 0.8 or more in the vicinity of the transmission magnetic field strength ratio Blr=1 are shown in  FIG. 9  for every L 1  and L 4  of lines. 
         [0123]    Comparing the ranges ΔR 1  and ΔR 4  taken for the transmission magnetic field strength ratios Blr at the individual lines L 1  and L 4  with each other shows that the range ΔR 4  for the line L 4  becomes wider than the range ΔR 1  for the line L 1 . Thus, even in the second embodiment, unevenness in signal intensity due to B 1  ununiformity can be reduced in a manner similar to the first embodiment. 
         [0124]    Incidentally, when the subject is imaged by a 3 T (Tesla) magnetic resonance imaging apparatus, there is a tendency that the transmission magnetic field strength ratio Blr becomes larger with the approach from the body surface of the subject to the internal center of the subject. It is thus desired that when the subject is imaged by the 3 T (Tesla) magnetic resonance imaging apparatus, the integral value S of the signal intensity has as large a value as possible to obtain a high quality image even in the vicinity of the internal center of the subject even though the value of the transmission magnetic field strength ratio Blr becomes larger. In the line L 1 , however, the integral value S of the signal intensity becomes smaller than 0.8 from the vicinity when the transmission magnetic field strength ratio Blr exceeds Blr=1.3. In the line L 4 , contrary to this, the integral value S of the signal intensity can be set to 0.8 or more up to the vicinity of the transmission magnetic field strength ratio Blr=1.6. Thus, when the subject is imaged by the 3 T (Tesla) magnetic resonance imaging apparatus, unevenness in signal intensity due to B 1  ununiformity can be reduced efficiently even in the neighborhood of the internal center of the subject by using the flip angles (200°, −140°, −200° and 140°). 
         [0125]    In the first embodiment, the combination of the flip angles (α1°, α2°, −α1° and −α2°) expressed in the equation (9) is used. In the second embodiment, the combination of the flip angles (α1°, α2°, −α1° and −α2°) expressed in the equation (10) is used. The combination of the flip angles is, however, not limited to the equations (9) and (10). Combinations of flip angles expressed in the following equations (11) through (16) are possible: 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°−Δ d 1°), −(180°+Δ d 2°), 180°−Δ d 1°, 180°+Δ d 2°)   (11)
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°+Δ d 1°), 180°−Δ d 2°, 180°+Δ d 1°, −(180°−Δ d 2°))   (12)
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°+Δ d 1°, 180°−Δ d 2°, −(180°+Δ d 1°), −(180°−Δ d 2°))   (13)
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(180°−Δ d 1°, −(180°+Δ d 2°), −(180°−Δ d 1°), 180°+Δ d 2°)   (14)
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°+Δ d 1°), −(180°−Δ d 2°), 180°+Δ d 1°, 180°−Δ d 2°))   (15)
 
         [0000]      (α1°, α2°, −α1°, −α2°)=(−(180°−Δ d 1°), 180°+Δ d 2°, 180°−Δ d 1°, −(180°+Δ d 2°))   (16)
 
         [0126]    In the first and second embodiments, (α1°, α2°, −α1°, −α2°) are used as each combination of the flip angles. A combination of (α1°, α2°, α1° and α2°) may, however, be used instead of (α1°, α2°, −α1° and −α2°) 
         [0127]    Further, in the first and second embodiments, the phases θ1° and θ2° of RF pulses α1° θ1° , α2° θ2° , −α1° θ1°  and −α2° θ  and θ1°=θ2°=y (i.e., they are coincident with the y axis shifted 90° with respect to the x axis). There is, however, no need to cause the phases θ1° and θ2° of the RF pulses α1° θ1° , α2° θ2° , −α1° θ1° , and −α2° θ2°  to coincide with the y axis shifted 90° with respect to the x axis. They may be used as a phase shifted θ° (0≦θ°&lt;90°) with respect to the x axis. It is also unnecessary to set the phases as θ1°=θ2°. The phases may be taken as θ1°≠θ2°. 
       (3) Third Embodiment 
       [0128]    Upon explaining a third embodiment, eddy currents generated by a crusher gradient pulse Gcrush and a velocity encode gradient pulse Gv will first be explained. 
         [0129]      FIG. 10  is a diagram for explaining the eddy currents. 
         [0130]    In  FIG. 10 , each of diagonally-shaded areas shown immediately after gradient pulses schematically shows the occurrence of the eddy current. Shading may occur in an image due to the eddy current. Since the eddy current due to the crusher gradient pulse Gcrush and the eddy current due to the velocity encode gradient pulse Gv are generated in  FIG. 10 , there are considered shading caused by the eddy current of the crusher gradient pulse Gcrush and shading caused by the eddy current of the velocity encode gradient pulse Gv. Since, however, the eddy current of the crusher gradient pulse Gcrush influences all of four refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y) that exist between RF pulses 90°x and −90°x, they are considered little to result in the occurrence of shading in the image. On the other hand, each eddy current of the velocity encode gradient pulse Gv influences only the third refocus pulse (RF pulse −α1°y), it can result in the occurrence of shading in the image. In order to reduce the shading caused by the eddy current of the velocity encode gradient pulse Gv, the third refocus pulse (RF pulse −α1°y) may be transmitted in wait for the approach of the eddy current generated by a second pulse g v2  of the velocity encode gradient pulse Gv. Since it is however necessary to wait for the approach of the eddy current generated by the second pulse g v2  of the velocity encode gradient pulse Gv to zero, there is a need to make long a waiting time Δτ 0  between the second pulse gv 2  and the RF pulse −α1°y to some extent. If the waiting time Δτ 0  is made long since time intervals Δt 1 , Δt 2 , Δt 3 , Δt 4  and Δt 5  respectively defined among the RF pulses are set in such a manner that the relationship of 1:2:2:2:1 is established as described above, then the time intervals Δt 1 , Δt 2 , Δt 3 , Δt 4  and Δt 5  need to be lengthened correspondingly (namely, it is necessary to make the time τ of a sequence long). Accordingly, a problem arises in that it is not possible to cope with such a case that it is desired to shorten the time t of the sequence. The third embodiment will therefore explain a longitudinal magnetization adjusting sequence  21  (refer to  FIG. 3 ) capable of reducing shading and further shortening the time τ of the sequence. 
         [0131]      FIG. 11  is one example of a longitudinal magnetization adjusting sequence  21  according to the third embodiment. 
         [0132]    In the third embodiment, a crusher gradient pulse Gcrush and a velocity encode gradient pulse Gv are combined together on a single axis. A procedure for generating the crusher gradient pulse Gcrush and velocity encode gradient pulse Gv combined together on the single axis will be explained below with reference to  FIGS. 12A-12D . 
         [0133]      FIGS. 12A-12D  are diagrams for describing the procedure for generating the crusher gradient pulse Gcrush and the velocity encode gradient pulse Gv combined together on the single axis. 
         [0134]      FIG. 12A  is a diagram showing RF pulses,  FIG. 12B  is a diagram showing a pre-combination crusher gradient pulse Gcrush,  FIG. 12C  is a diagram showing a pre-combination velocity encode gradient pulse Gv, and  FIG. 12D  is a diagram showing the crusher gradient pulse Gcrush and velocity encode gradient pulse Gv combined together on the one axis. 
         [0135]    The pre-combination velocity encode gradient pulse Gv (refer to  FIG. 12C ) has a negative gradient pulse g v1  and a positive gradient pulse g v2 . The negative gradient pulse g v2  is applied at the same time as that for a fourth gradient pulse g c4  of the pre-combination crusher gradient pulse Gcrush. The positive gradient pulse g v2  is however applied previous to a fifth gradient pulse g c5  of the pre-combination crusher gradient pulse Gcrush by a time Δτ 1 . Thus, when the crusher gradient pulse Gcrush shown in  FIG. 12B  and the velocity encode gradient pulse Gv shown in  FIG. 12C  are combined together on the signal axis, the crusher gradient pulse Gcrush and velocity encode gradient pulse Gv (hereinafter referred to as “combined gradient pulse Gcrush+Gv”) combined on the single axis, which are shown in  FIG. 12C , are obtained. 
         [0136]    The combined gradient pulse Gcrush+Gv can be obtained by causing the fourth gradient pulse g c4  of the crusher gradient pulse Gcrush to be moved to the position of the positive gradient pulse g v2  of the velocity encode gradient pulse Gv (to approach the fifth gradient pulse g c5 ). 
         [0137]    Even though the crusher gradient pulse Gcrush and the velocity encode gradient pulse Gv are combined together on the one axis, the effect of the velocity encode gradient pulse Gv can be maintained as it is. It is understood that referring to  FIGS. 12A and 12D , eddy currents due to the combined gradient pulse Gcrush+Gv uniformly exert an influence on all of four refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y). Thus, the four refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y) can be transmitted even without waiting for the achievement of the eddy current to zero, thereby making it possible to shorten the time τ of the sequence (refer to  FIGS. 13A and 13B ). 
         [0138]      FIGS. 13A and 13B  are diagrams showing the manner in which the time t of the sequence can be shortened. 
         [0139]      FIG. 13A  is a diagram showing the longitudinal magnetization adjusting sequence  21  before the time t of the sequence is shortened, and  FIG. 13B  is a diagram showing the longitudinal magnetization adjusting sequence  21  after the time t of the sequence has been shortened. 
         [0140]    The eddy currents due to the combined gradient pulse Gcrush+Gv uniformly influence all of the four refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y). Thus, they are considered little to result in the occurrence of shading in an image even if time intervals Am respectively defined between the refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y) and the immediately preceding gradient pulses are set short. Therefore, the intervals between the respective RF pulses and the intervals between the respective gradient pulses can be narrowed as needed as shown in  FIG. 13B , thereby making it possible to shorten the time τ of the sequence. 
         [0141]    Incidentally, although the third embodiment has explained the four refocus pulses (RF pulses α1°y, α2°y, −α1°y and −α2°y), the number of refocus pulses may be, for example, eight or sixteen and is not limited to the four. 
         [0142]    In the third embodiment, the gradient pulse g c4  located between the RF pulses α2°y and −α1°y is brought close to the fifth gradient pulse g c5  as shown in  FIG. 13 . Another gradient pulse may however be shifted as in the case of the approach of the second gradient pulse g c2  located between the RF pulses α1°y and α2°y to the third gradient pulse g c3  as shown in  FIG. 14 , and the like. As shown in  FIG. 15 , a plurality of gradient pulses may be shifted. 
         [0143]    Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.