Abstract:
The present invention provides an MRI apparatus with selective saturation without affection to the integral value of gradient magnetic field in 1TR. The MRI apparatus comprises a signal acquisition device for applying static field, gradient magnetic field pulses and RF pulses to an object to acquire magnetic resonance signals therefrom, an image reconstruction device for reconstructing an image based on the magnetic resonance signals acquired, and a controller device for controlling both device. The controller device directs the signal acquisition device to apply gradient magnetic field pulses and RF pulses for selective saturation for a number of times prior to applying gradient magnetic field pulses and RF pulses for signal acquisition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Japanese Application No. 2005-277174 filed Sep. 26, 2005. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an MRI (magnetic resonance imaging) apparatus, more specifically to an MRI apparatus which acquires magnetic resonance signals in combination with the spatial selective saturation. 
   In the MRI apparatus, magnetic resonance signals are acquired in combination with the spatial selective saturation when performing blood vessel imaging. More specifically, spatial selective saturation is performed at the upstream of the blood flow outside of the imaging area, prior to the signal collection for the imaging area (patent reference 1, for example). 
   [Patent reference 1] Japanese Unexamined Patent Publication No. Hei 10(1998)-33498 (pp. 4-5, FIGS. 1-3) 
   For the spatial selective saturation, RF pulses for spin excitation and the gradient magnetic field pulses for spatial selection and killer are respectively used. When combining with the pulse sequence for the magnetic resonance signal collection, it is impossible for the integral value of the gradient magnetic field within 1TR (repetition time) to be 0. 
   For this reason, the pulse sequence as in the steady state free precession (SSFP), in which the gradient magnetic field integral value must be 0 within 1TR, may not perform the spatial selective saturation for 1TR. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide an MRI apparatus which provides the spatial selective saturation that does not affect the integral value of the gradient magnetic field within 1TR. 
   A first aspect of the present invention for solving the problem provides an MRI apparatus comprising: a signal acquisition means for applying static field, gradient magnetic field pulses and RF pulses to an object to acquire magnetic resonance signals therefrom; an image reconstruction means for reconstructing an image based on the magnetic resonance signals acquired; and a controller means for controlling both the signal acquisition means and the image reconstruction means, wherein the controller means comprises a signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pluses and RF pulses for a plurality of times for spatial selective saturation prior to directing to apply the gradient field pulse and RF pulse for collecting the magnetic resonance signals. 
   A second aspect of the present invention for solving the problem provides an MRI apparatus comprising: a signal acquisition means for applying static field, gradient magnetic field pulses and RF pulses to an object to acquire magnetic resonance signals therefrom; an image reconstruction means for reconstructing an image based on the magnetic resonance signals acquired; and a controller means for controlling both the signal acquisition means and the image reconstruction means, wherein the controller means comprises a first signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pulses and RF pulses for a plurality of times for spatial selective saturation prior to directing to apply the gradient field pulse and RF pulse for collecting the magnetic resonance signals; a second signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pulse and RF pulse for collecting the magnetic resonance signals, without directing to apply the gradient magnetic field pulses and RF pulses for spatial selective saturation; and an image reconstruction controller unit for directing the image reconstruction means to reconstruct the image based on the magnetic resonance signals collected under the control of the first signal acquisition controller unit and the image based on the magnetic resonance signals collected under the control of the second signal acquisition controller unit, and then to generate a differential image between these images. 
   It is preferable for imaging an artery that the spatial selective saturation is a spatial selective saturation at the upstream of an artery. 
   It is also preferable for imaging a vain that the spatial selective saturation is a spatial selective saturation at the upstream of a vein. 
   It is preferable for properly performing blood vessel imaging that the application of the gradient magnetic field pulses and RF pulses for the spatial selective saturation is repeated at least for two seconds. 
   It is also preferable for properly performing blood vessel imaging that the application of the gradient magnetic field pulses and RF pulses for the spatial selective saturation for a plurality of times is performed so as to gradually change flip angle. 
   It is preferable for properly performing blood vessel imaging that the flip angle gradually changes from 180 degrees to 90 degrees. 
   It is also preferable for properly performing blood vessel imaging that the application of the gradient magnetic field pulses and RF pulses for the spatial selective saturation for a plurality of times is performed so as to gradually change the phase of RF pulses. 
   It is preferable for improving the imaging quality that the application of the gradient magnetic field pulses and RF pulses for the spatial selective saturation, and/or the application of the gradient magnetic field pulses and RF pulses for the magnetic resonance signal acquisition is/are performed in synchronism with the heart beat. 
   It is preferable for improving the imaging quality that the application of the gradient magnetic field pulses and RF pulses for the spatial selective saturation, and/or the application of the gradient magnetic field pulses and RF pulses for the magnetic resonance signal acquisition is/are performed in synchronism with body move. 
   It is preferable for a short span of the signal acquisition time that the application of the gradient magnetic field pulses and RF pulses for the magnetic resonance signal acquisition is performed in a sequence of steady state free precession. 
   It is preferable for properly performing blood vessel imaging that the signal acquisition control units directs the signal acquisition means to apply RF pulses for T 2  preparation prior to the application of the gradient magnetic field pulses and RF pulses for the magnetic resonance signal collection. 
   It is preferable for properly performing blood vessel imaging that the signal acquisition controller units directs the signal acquisition means to apply RF pulses for fat signal suppression prior to the application of gradient magnetic field pulses and RF pulses for magnetic resonance signal collection. 
   In accordance with the first aspect of the present invention, the MRI apparatus comprises: a signal acquisition means for applying static field, gradient magnetic field pulses and RF pulses to an object to acquire magnetic resonance signals therefrom; an image reconstruction means for reconstructing an image based on the magnetic resonance signals acquired; and a controller means for controlling both the signal acquisition means and the image reconstruction means, wherein the controller means comprises a signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pluses and RF pulses for a plurality of times for spatial selective saturation prior to directing to apply the gradient field pulse and RF pulse for collecting the magnetic resonance signals, so that the spatial selective saturation may be performed without affecting the integral value of the gradient magnetic field within 1TR. 
   In accordance with the second aspect of the present invention, the MRI apparatus comprises: a signal acquisition means for applying static field, gradient magnetic field pulses and RF pulses to an object to acquire magnetic resonance signals therefrom; an image reconstruction means for reconstructing an image based on the magnetic resonance signals acquired; and a controller means for controlling both the signal acquisition means and the image reconstruction means, wherein the controller means comprises: a first signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pulses and RF pulses for a plurality of times for spatial selective saturation prior to directing to apply the gradient field pulse and RF pulse for collecting the magnetic resonance signals; a second signal acquisition controller unit for directing the signal acquisition means to apply the gradient magnetic field pulse and RF pulse for collecting the magnetic resonance signals, without directing to apply the gradient magnetic field pulses and RF pulses for spatial selective saturation; and an image reconstruction controller unit for directing the image reconstruction means to reconstruct the image based on the magnetic resonance signals collected under the control of the first signal acquisition controller unit and the image based on the magnetic resonance signals collected under the control of the second signal acquisition controller unit, and then to generate a differential image between these images, so that the spatial selective saturation may be performed without affecting the integral value of the gradient magnetic field within 1TR, and imaging of only the blood vessel can be preformed. 
   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 
       FIG. 1  is a schematic block diagram of an exemplary MRI apparatus indicating a best mode for carrying out the invention; 
       FIG. 2  is an example of pulse sequence for magnetic resonance signal acquisition; 
       FIG. 3  is an example of pulse sequence for spatial selective saturation; 
       FIG. 4  is an example of time chart for magnetic resonance signal acquisition along with the spatial selective saturation; 
       FIG. 5  is an example of positional relationship between the imaging area and the selective saturation; 
       FIG. 6  is an example of time chart for performing the selective saturation and signal acquisition in synchronism with the heart beat; 
       FIG. 7  is an example of time chart for performing the selective saturation and signal acquisition in synchronism with the heart beat and with the body move; 
       FIG. 8  is a pulse sequence for T 2  preparation; 
       FIG. 9  is a pulse sequence for fat suppression; and 
       FIG. 10  is a time chart for signal acquisition along with the selective saturation, T 2  preparation, and fat suppression. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A best mode for carrying out the invention will be described in greater details with reference to the accompanying drawings. It should be understood that the present invention is not to be considered to be limited to the disclosed best mode for carrying out the invention.  FIG. 1  shows a schematic block diagram of an MRI apparatus. The apparatus is an example of best mode for carrying out the invention. The arrangement of the apparatus indicates an example of the best mode for carrying out the invention with respect to the MRI apparatus. 
   As shown in the figure, the apparatus has a magnet system  100 . The magnet system  100  includes a main magnetic field coil unit  102 , a gradient coil unit  106 , and an RF coil unit  108 . These coils have a shape of a cylinder, and are placed coaxially. 
   A cradle  500  of the magnet system  100  carrying thereon an object  1  to be imaged moves in and out of the internal space (bore) in the form of a cylinder by means of a conveyor means not shown in the figure. 
   The main magnetic field coil unit  102  forms a static field within the internal space of the magnet system  100 . The direction of the static field is almost in parallel to the body axis direction of the object  1 . More specifically, this forms a so-called horizontal magnetic field. The main magnetic field coil unit  102  may be arranged by using for example a superconductor coil. The main magnetic field coil unit  102  may equally be formed by a normal conduction coil instead of the superconductor coil. 
   The magnet system itself may also be a vertical field type in which the direction of the static field is perpendicular to the body axis direction of the object  1 , instead of the horizontal field type. In the vertical field type a permanent magnet may be used for generating the static field. 
   The gradient coil unit  106  generates three gradient magnetic fields for providing the inclination of the static field intensity in three axes each perpendicular to the others, more specifically in the slice axis, phase axis, and frequency axis. 
   When giving x, y, z as the coordinate axis each normal to the others in the static field space, any of these axes can be the slice axis. In this case one of the remaining two axes will be the phase axis, and the other will be the frequency axis. The slice axis, phase axis, and frequency axis may be inclined arbitrarily with respect to x-, y-, and z-axis while maintaining the perpendicular relationships between them. In this apparatus the x-axis is defined as the direction of the body width of the object  1 , and y-axis as the direction of the body depth, and z-axis as the direction of body axis. 
   The gradient magnetic field in the direction of slice axis is also referred to as the slice gradient magnetic field. The gradient magnetic field in the direction of phase axis is also referred to as the encode gradient magnetic field. The gradient magnetic field in the direction of frequency axis is also referred to as the read out gradient magnetic field. The read out gradient magnetic field is the same definition as the frequency encode gradient magnetic field. To enable generating these gradient magnetic fields, the gradient coil unit  106  has three gradient coil systems not shown in the figure. The gradient magnetic field will be referred to as simply gradient herein below. 
   The RF coil unit  108  forms an RF magnetic field for exciting the spin within the body of the object  1  in the static magnetic field space. The formation of the RF magnetic field will be referred to as the transmission of RF excitation signals, herein below. The RF excitation signals are also referred to as RF pulses. 
   The electromagnetic waves generated by the excited spin, namely the magnetic resonance signals, are received by the RF coil unit  108 . The magnetic resonance signals thus received will be the sampling signals of the frequency domain, or the Fourier space. 
   With the gradient in the direction of phase axis and in the direction of frequency axis, when encoding the magnetic resonance signals in these two axes, the magnetic resonance signals can be obtained as the sampling signals in two-dimensional Fourier space, and when encoding in three axes by additionally using the slice gradient, the signals can be obtained as the three-dimensional Fourier space signals. Each of gradients determines the position of sampling of the signal in the two- or three-dimensional Fourier space. The Fourier space will be also referred to as k-space herein below. 
   The gradient coil unit  106  is connected to a gradient driver unit  130 . The gradient driver unit  130  generates a gradient magnetic field by providing driving signals to the gradient coil unit  106 . The gradient driver unit  130  includes three systems of driving circuits not shown in the figure, corresponding to three systems of gradient coils in the gradient coil unit  106 . 
   The RF coil unit  108  is connected to an RF driver unit  140 . The RF driver unit  140  provides driving signals to the RF coil unit  108  to transmit RF pulses so as to excite the spin within the body of the object  1 . 
   The RF coil unit  108  is also connected to a data collector unit  150 . The data collector unit  150  gathers the receiving signals received by the RF coil unit  108  as digital data. 
   The gradient driver unit  130 , the RF driver unit  140 , and the data collector unit  150  are connected to a sequence controller unit  160 . The sequence controller unit  160  controls the gradient driver unit  130  or the data collector unit  150  to perform the acquisition of the magnetic resonance signals. 
   The sequence controller unit  160  is constituted by for example using a computer. The sequence controller unit  160  includes a memory. The memory is served for storing a program and various data used for the sequence controller unit  160 . The function of the sequence controller unit  160  may be achieved by executing the program stored in the memory by the computer. The part comprised of the magnet system  100  and the sequence controller unit  160  is an example of the signal acquisition device in accordance with the present invention. 
   The output of the data collector unit  150  is connected to a data processing unit  170 . The data gathered by the data collector unit  150  will be input into the data processing unit  170 . The data processing unit  170  is constituted by for example a computer. The data processing unit  170  has a memory. The memory stores the program for the data processing unit  170  and a variety of data. 
   The data processing unit  170  is connected to the sequence controller unit  160 . The data processing unit  170  is in the superposition of the sequence controller unit  160  to manage it. The function of the apparatus may be achieved by executing the program stored in the memory by the data processing unit  170 . 
   The data processing unit  170  stores into the memory the data collected by the data collector unit  150 . A data space will be generated in the memory. The data space corresponds to the k-space. The data processing unit  170  reconstructs an image by performing an invert Fourier transform of the data within the k-space. The data processing unit  170  is an example of the image reconstruction means in accordance with the present invention. The data processing unit  170  is also an example of the controller means in accordance with the present invention. 
   A heart beat sensor  112  is attached to the object  1 , through which the heart beat of the object  1  is detected by a heart beat detector unit  110 , and the heat beat detection signals are input into the data processing unit  170 . The data processing unit  170  in turn performs imaging in synchronism with the heart beat based on the heart beat detection signals. 
   Instead of (or in addition to) the heart beat sensor  112  and the heart beat detector unit  110 , a body move sensor and a body move detection unit are provided to detect the body move along with the respiration in order to perform imaging in synchronism with the body move. The body move detection may also be performed based on the movement of diaphragm detected by the magnetic resonance imaging. 
   The data processing unit  170  is connected to a display unit  180  and an operating console unit  190 . The display unit  180  is constituted of a graphic display. The operating console unit  190  is constituted of a keyboard with a pointing device. 
   The display unit  180  displays the reconstructed image output from the data processing unit  170  and various information. The operating console unit  190  is operated by the operator in order to input various instruction and information to the data processing unit  170 . The user is allowed to operate the apparatus interactively through the display unit  180  and the operating console unit  190 . 
     FIG. 2  shows an example of pulse sequence for magnetic resonance signal acquisition. The pulse sequence is a steady state free precession pulse sequence. The steady state free precession will be abbreviated as SSFP herein below. The magnetic resonance signal acquisition may also be performed with any of other techniques than the SSFP. The magnetic resonance signal acquisition will also be abbreviated as signal acquisition herein below. 
   In the figure, ( 1 ) shows a sequence of RF excitation. ( 2 ) to ( 4 ) show a sequence of gradient magnetic field pulses, respectively. ( 5 ) shows a sequence of magnetic resonance signals. Among gradient magnetic field ( 2 ) to ( 4 ), ( 2 ) is a slice gradient, ( 3 ) is a frequency encode gradient, and ( 4 ) is a phase encode gradient. The static magnetic field is always applied at a constant intensity of magnetic field. This condition applies in the following description. 
   RF excitation by 90 degrees pulses is repeated at the distance of 1TR. The 90 degrees excitation is a selective excitation under the slice gradient, slice. Between two 90 degrees excitations, the frequency encode gradient (read), phase encode gradient (warp), and slice encode gradient (slice) are applied in a predetermined sequence, in order to read out the magnetic resonance signals, echo or (echo). 1TR may be 3 msec to 5 msec. 
   Pulses of the slice gradient, slice, the frequency encode gradient, read, and the phase encode gradient, warp have the waveform and amplitude limited so that the integral value in 1TR is made to be 0. 
   The pulse sequence as described above is repeated for a given number of times, and echo is read out each time. The phase encode of the echo is altered in each repetition, and the echo signal acquisition for the entire two-dimensional k-space is performed by the repetition of a given number of times. When phase encoding in the direction of slice, echo signals are acquired for the three-dimensional k-space. 
   By two-dimensional invert Fourier transforming the echo data of two-dimensional k-space, a 2D image is reconstructed. By three-dimensional invert Fourier transforming the echo data of three-dimensional k-space, a 3D image is reconstructed. 
     FIG. 3  shows an example of pulse sequence for spatial selective saturation. In the figure, ( 1 ) designates to the RF excitation, ( 2 ) to a slice gradient, ( 3 ) to a pulse sequence for the killer gradient. The RF excitation by using the 90 degrees pulses is performed as the spatial selective excitation under the slice gradient, slice, then the killer gradient, killer, to be applied later, will disperse the phase of spins. By this, the vertical magnetization and lateral magnetization are both nullified so as not to react with the following RF excitation. The spatial selective saturation will also be referred to as simply selective saturation. 
   The selective saturation as described as above is performed prior to the signal acquisition.  FIG. 4  shows an example of time chart for the signal acquisition along with the selective saturation. As shown in  FIG. 4 , the selective saturation is done at the interval SAT, then the signals are acquired in the interval ACQ. 
   The signal acquisition along with the selective saturation will be performed under the control of the data processing unit  10 . The data processing unit  10  is an example of the signal acquisition controller unit in accordance with the present invention. The data processing unit  10  is also an example of the first signal acquisition controller unit in accordance with the present invention. 
   The selective saturation in the interval SAT is repeated for a plurality of times intermittently. In this example the selective saturation of each repetition will be represented by an RF pulse. The duration of the interval SAT may be for example 2 seconds, during which the selective saturation will be repeated for 40 times, for example. More specifically, the selective saturation will be performed at the interval of 50 msec. 
   The signal acquisition in the interval ACQ will be performed for a plurality of continuous TR. In this example each signal acquisition is represented by a TR. The duration of the interval ACQ may be for example 0.5 second, during which signal acquisition of 128TR will be performed. More specifically, the signal acquisition is done in a very short period of time. 
   As can be seen from the foregoing, the selective saturation is repeated for a plurality of times during the interval SAT, then the signal is acquired for a plurality of continuous TR during the interval ACQ, so that the gradient for the selective saturation will not affect the integral value of the gradient magnetic field within 1TR at the time of signal acquisition. 
   The selective saturation of a plurality of times during the interval SAT may also be performed by gradually changing the flip angle. The flip angle may be changed so as to gradually decrease from 180 degrees to 90 degrees. By this the spin in the earlier selective saturation has a longer recovery time of the vertical magnetization, so that the extension distance of the imageable vessel can be elongated. A plurality of selective saturations during the interval SAT may also be performed by gradually changing the phase of RF excitation. 
     FIG. 5  shows an example of positional relationship between the selective saturation and the imaging area. As shown in  FIG. 5 , when the imaging area FOV is set so as to include the area from the abdominal artery to the femoral artery of both sides, the selective saturation area SST will be set to the upstream of the artery outside the imaging area FOV. The arrow shows the direction of blood stream. If the target vessel is vein, then the selective saturation area SST is set to the upstream of the vein. In other words, the selective saturation area SST may be set to the upstream of the target vessel. 
   The thickness of slab of the selective saturation area SST will be set to for example 10 cm. When set as such, the blood stream of the velocity 100 cm/sec will pass through the selective saturation area SST at 0.1 second. During this time, the selective saturation will be repeated for 50 msec, the blood stream will be saturated twice. By this there will be no gap between saturation points. 
   The relationship between the repetition interval of the selective saturation and the slab thickness of the selective saturation area SST can be appropriately set within the acceptable range that the saturation is not interrupted. In general this is set so as to satisfy the following relationship.
 
Thickness of selective saturation area÷excitation interval&gt;maximum velocity of blood stream
 
   The blood flowing into the selective saturation area SST will undergo the repetitive saturation for two seconds. By this, assuming that the blood stream velocity is 100 cm/sec, the blood stream of at most 200 cm will be saturated, according to the simple calculation. This length is much longer than the length of blood stream in the abdominal artery and femoral arteries. Thus the entire blood stream within the abdominal artery and the femoral arteries within the imaging area FOV can be completely saturated. 
   The time for saturation can be increased or decreased appropriately in accordance with the size of the imaging area and is not limited to 2 seconds. However, the vertical relaxation time T 1  of the blood is at most about 2000 msec, there is no meaning if set much larger than 2 seconds. 
   By reconstructing an image based on the magnetic resonance signals acquired after the selective saturation described as above, an image can be obtained with the abdominal artery and femoral arteries depicted in black. This image includes any other tissues in the abdomen and the femur. 
   To obtain an image depicting solely the vessel with the tissue image removed, the differential image from the tissue image taken aside can be generated. The imaging of tissue image is performed without the selective saturation. The signal acquisition without the selective saturation will be performed under the control of the data processing unit  10 . The data processing unit  10  is an example of the second signal acquisition controller unit in accordance with the present invention. 
   Without selective saturation, a reconstructed image shows the abdominal artery and the femoral arteries not depicted in black. By this, when obtaining the difference from an image with selective saturation, an image can be obtained with the tissue image nullified and the arteries highlighted. 
   The reconstruction of the image with selective saturation, the reconstruction of the image without selective saturation, and the construction of the differential between them will be performed under the control of the data processing unit  10 . The data processing unit  10  is an example of the image construction controller unit in accordance with the present invention. 
     FIG. 6  shows an example of time chart when performing the selective saturation and the signal acquisition in synchronism with the heartbeat. As shown in  FIG. 6 , R wave of ECG will be used as a trigger to perform the selective saturation of 2 seconds for example, then the following R wave is used as the trigger for the signal acquisition. The signal acquisition without the selective saturation will be performed in synchronism with the heartbeat. By this, a vessel image of higher quality without ghost can be obtained. The synchronization with the heart beat may be done for any one of the selective saturation and the signal acquisition. 
   The heart beat synchronization can be combined with the body move synchronization. An example is shown in  FIG. 7 . As shown in  FIG. 7 , during the period of time where the periodic body move accompanied with the respiration is much smaller, both the selective saturation and the signal acquisition in synchronism with the heart beat are performed. The signal acquisition without the selective saturation is also performed in a similar manner. By this the vessel image of further higher quality can be obtained. The synchronization as such may be applied to either one of the selective saturation or the signal acquisition. 
   To completely nullify the muscular tissue image by the differential imaging, T 2  preparation is performed. The T 2  preparation is a process to decrease the muscular signals smaller than the blood signals based on the difference of the lateral relaxation time T 2 . The T 2  preparation will be performed prior to the signal acquisition. 
   RF excitation pulse sequence as shown in  FIG. 8  will be used for the T 2  preparation. As shown in  FIG. 8 , an RF excitation is performed with flip angle of 90 degrees and the phase of 0 degrees, then after the time T, another RF excitation is performed with the flip angle of 180 degrees and the phase of 90 degrees, then after the time 2T, another RF excitation is performed with the flip angle of −180 degrees and the phase of 90 degrees, then after the time 2T, another RF excitation will be performed with the flip angle of −180 degrees and the phase of 90 degrees, and then after the time T another RF excitation will be performed with the flip angle of −90 degrees and the phase of 0 degree. The above RF excitations are non-selective excitations. A killer gradient will be applied after the RF excitations. 
   To completely nullify the fat tissue image by the differential image, a fat suppression will be performed. The fat suppression is a process to decrease the fat signal smaller than the blood signal based on the chemical shift of the magnetic resonance frequency. The fat suppression will be performed prior to the signal acquisition. 
   180 degrees pulses as shown in  FIG. 9  is used for the fat suppression. After 180 degrees excitation a killer gradient will be applied. The frequency of the 180 degrees pulse is tuned to the fat frequency. By this the signal amplitude will be nullified during the process of frequency selection inversion recovery of the fat spin. 
     FIG. 10  shows an example of time chart indicating the selective saturation, T 2  preparation, fat suppression, and signal acquisition. As shown in  FIG. 10 , the selective saturation is repeated for a plurality of times, then T 2  preparation, and fat suppression are performed, followed by the signal acquisition. When the selective saturation is not performed, T 2  preparation and fat suppression are preceded to the signal acquisition. 
   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.