Magnetic resonance imaging system

A magnetic resonance imaging system includes a field generation section, an excitation section, an excitation control section, a resonance data acquisition section, and an image generation section. The field generation section generates a static magnetic field, a gradient magnetic field, and a selective excitation pulse. The excitation section controls the field generation section to apply the static field, the gradient field, and the pulse to an object, at a predetermined timing, to selectively excite a magnetic resonance phenomenon in a specific slice of the object. The excitation control section controls the excitation section to excite a plurality of slices. The excitation control section causes the excitation section to excite a certain slice of the object, and causes the excitation section to excite another slice, separated from the previous slice at least by a thickness of the slice determined by the selective excitation pulse, within an excitation-repeating time. The resonance data acquisition section detects a signal induced by the magnetic resonance, to acquire data relating to the resonance. The image generation section generates an image on the basis of the resonance data.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic resonance imaging (MRI) system 
which can significantly reduce the data acquisition time entailed in 
obtaining magnetic resonance (MR) images of a number of slices. 
In a magnetic resonance imaging system which has already been put into 
practical use, a magnetic resonance phenomenon is excited, as the 
resonance of a magnetic moment of a nuclear spin to an electromagnetic 
wave. An MR signal generated by the MR phenomenon is detected, for the 
purpose of obtaining information about a specific atomic nucleus in the 
tissue of an object being examined. 
A principle of the magnetic resonance imaging system will now be briefly 
described below. 
Each atomic nucleus has its own nuclear spin. An atomic nucleus generates a 
magnetic moment caused by the nuclear spin. The nuclear magnetic moment of 
each atomic nucleus normally faces an arbitrary direction. However, when a 
magnetic field is applied to the nuclear magnetic moments, they are 
oriented in accordance with the direction of lines of magnetic force. For 
example, atomic nuclei having spins of 1/2, such as protons (.sup.1 H) as 
atomic nuclei of hydrogen, have only two types of nuclear magnetic 
moments, i.e., those oriented parallel to the magnetic field and in the 
same direction as that of the lines of magnetic force, and those oriented 
parallel to the magnetic field and in a direction opposite to that of the 
lines of magnetic force. These two types of nuclear magnetic moments in 
the magnetic field have energies of different levels (Zeeman fission of an 
energy level). 
Magnetic behavior of a group of nuclear spins, i.e., nuclear magnetic 
moments as a whole, is known by defining macroscopic magnetization vector 
M. Magnetization vector M represents a vector value obtained by adding all 
the nuclear magnetic moments at a portion of interest of an object being 
examined. When the magnetic field is not applied, magnetization vector M 
is naturally zero. However, when the magnetic field is applied, the 
magnetic moments are oriented in accordance with the direction of the 
magnetic field, and magnetization vector M parallel to the magnetic field 
is generated. In accordance with normal practice in this field of art, the 
coordinate axis of the direction of the magnetic field is defined as the 
Z-axis. 
When magnetization vector M is inclined with respect to the Z-axis, it 
commences precession thereabout. In order to incline magnetization vector 
M, a small magnetic field can be applied, which rotates on an X-Y plane 
perpendicular to a static magnetic field along the Z-axis direction. In 
fact, a high-frequency AC magnetic field, instead of a rotating magnetic 
field, is applied by a coil. At this time, in order to generate the 
magnetic resonance, the frequency of the high-frequency magnetic field 
must be set so as to coincide with the resonance frequency of a specific 
nuclear spin to be examined. 
Inclination of magnetization vector M from a balanced position means that 
it transits from a low- to a high-energy level, and this transition occurs 
only when the frequency of the high-frequency magnetic field corresponds 
to a difference in the magnetic energies of the two predetermined energy 
levels. Resonance frequency (Larmor frequency) .omega..sub.0 is equal to a 
value obtained by multiplying intensity H.sub.0, of the magnetic field to 
be applied, by magnetic rotation ratio .gamma.: 
EQU .omega..sub.0 =.gamma..times.H.sub.0 ( 1) 
Magnetic rotation ratio Y is a constant unique to each type of atomic 
nucleus having a nuclear spin. For example, a resonance frequency of the 
hydrogen atomic nucleus(proton) in a magnetic field of 1 T (tesla) is 
42.57 MHz, and that of phosphorus 31 (.sup.31 P) is 17.24 MHz. When a 
frequency of the high-frequency magnetic field is arbitrarily selected to 
be synchronized with a specific atomic nucleus, the magnetic resonance of 
the atomic nucleus can be separately observed. 
Therefore, according to the MRI system, a predetermined linear gradient 
magnetic field is applied to an object being examined disposed in a static 
magnetic field along the Z-axis direction, and a high-frequency excitation 
pulse is applied so as to incline magnetization vector M (i.e., to excite 
the magnetic resonance). An MR signal received by a receiving coil after 
the high-frequency pulse was applied is Fourier-transformed to reconstruct 
an image of MR information in a space. That is, the MRI system excites 
magnetic resonance at a predetermined portion of an object being examined, 
detects a weak MR signal generated by the magnetic resonance, and 
reconstructs an image according to MR information on the basis of the MR 
signal. 
In such an MRI system, a position and a thickness of a slice are determined 
as described below. 
A high-frequency excitation pulse (selective excitation pulse) consisting 
of a frequency component for selectively exciting only a slice having a 
predetermined thickness is applied to an object being examined while a 
gradient magnetic field in a direction (Z-axis direction) perpendicular to 
a static magnetic field and a slice surface is applied thereto, thereby 
generating an MR phenomenon only in a specific slice (this method is 
called a "selective excitation method"). 
Atomic nuclei at the excited portion start precession at the same resonance 
frequency and in different phases in a direction of thickness by the 
gradient magnetic field applied when a slice position is determined, i.e., 
the selected slice is excited. Such variations in phase decrease an MR 
signal to be detected, and quality of a finally obtained MR image is 
degraded. 
For this reason, in order to eliminate the variations in phase to obtain a 
large MR signal, a gradient magnetic field having a polarity opposite to 
that of the gradient magnetic field applied during excitation (i.e., slice 
determination) is conventionally applied after excitation (i.e., slice 
position determination) and before MR signal acquisition. 
An operation of a magnetization vector relating to the above phase shift 
cancelling will be described below with reference to FIG. 1. Note that a 
coordinate system shown in FIG. 1 is a rotating coordinate system which 
rotates with resonance angular frequency .omega..sub.0 in the above 
equation (1), in which the Z-axis represents a direction of static 
magnetic field H.sub.0, and the X'-axis represents a direction of the 
excitation pulse. By applying a 90.degree. pulse, magnetization vector M 
in the Z-axis direction falls onto the Y'-axis. At this time, by gradient 
magnetic field G.sub.Z applied simultaneously with the 90.degree. pulse, 
magnetization vector M tends to resonate at an angular frequency higher 
than resonance angular frequency .omega..sub.0 in a portion having a 
magnetic field intensity higher than magnetic field intensity H.sub.0 at 
the center of the slice, and hence shifts from the Y'-axis along a 
direction indicated by arrow A.sub.1 as shown in FIG. 1. On the other 
hand, magnetization vector M tends to resonate at an angular frequency 
lower than resonance angular frequency .omega..sub.0 in a portion having a 
magnetic field intensity lower than magnetic field intensity H.sub.0 at 
the center of the slice, and hence shifts from the Y'-axis along a 
direction indicated by arrow A.sub.2. Thereafter, when a gradient magnetic 
field having a polarity opposite to that of gradient magnetic field 
G.sub.Z, i.e., a gradient magnetic field -G.sub.Z in which the portion 
having the magnetic field intensity higher than magnetic field intensity 
H.sub.0 when gradient magnetic field G.sub.Z is applied has a lower 
magnetic field intensity and the portion having the magnetic field 
intensity lower than magnetic field intensity H.sub.0 when gradient 
magnetic field G.sub.Z is applied has a higher magnetic field intensity is 
applied, magnetization vector M shifted from the Y-axis moves along 
-A.sub.1 and -A.sub.2 directions contrary to the above case. If an 
application time of gradient magnetic field -G.sub.Z is arbitrarily 
selected, directions of all magnetization vectors M in the selected slice 
can be aligned with the Y'-axis. 
In a currently most popular method adopted in conventional MRI systems, not 
only magnetic resonance is excited, but also an MR echo is generated by a 
predetermined excitation sequences. This MR echo is detected to obtain 
necessary MR information. 
Typical examples of a method of generating the MR echo are a pulse echo 
method in which the MR echo is generated using a so-called 180.degree. 
pulse, and a gradient echo method in which the MR echo is generated by 
inverting a gradient magnetic field. 
The pulse echo method uses a pulse sequence as shown in FIGS. 2A to 2C. 
FIG. 2A shows a waveform of gradient magnetic field G.sub.S for determining 
a slice. FIG. 2A also shows envelope waveforms of excitation pulses 
P.sub.E1 and P.sub.E2 and MR echo E in addition to the waveform of 
gradient magnetic field G.sub.S. FIG. 2B shows a waveform of gradient 
magnetic field G.sub.R for reading MR data, and FIG. 2C shows a waveform 
of gradient magnetic field G.sub.E for phase-encoding the magnetic 
resonance. 
Phase-encoding gradient magnetic field G.sub.E and reading gradient 
magnetic field G.sub.R are used to cause an MR signal (MR echo) to include 
information of displacement on a slice as phase information. 
In the pulse echo method, slice-determining gradient magnetic field G.sub.S 
and 90.degree. excitation pulse P.sub.E1 are applied first. Gradient 
magnetic field G.sub.Z in the Z-axis direction is normally used as 
slice-determining gradient magnetic field G.sub.S. 90.degree. excitation 
pulse P.sub.E1 is, in this case, a selective excitation pulse having an 
envelope of the waveform as shown in FIG. 2A. After 90.degree. excitation 
pulse P.sub.E1 is applied, slice-determining gradient magnetic field 
G.sub.S is inverted so as to cancel phase shift as described above. 
Reading gradient magnetic field G.sub.R and phase-encoding gradient 
magnetic field G.sub.E are applied for a predetermined time interval after 
slice-determining gradient magnetic field G.sub.S is inverted. Gradient 
magnetic fields in directions orthogonal to each other on the X-Y plane, 
e.g., gradient magnetic field G.sub.X in the X-axis direction and gradient 
magnetic field G.sub.Y in the Y-axis direction are normally used as 
reading gradient magnetic field G.sub.R and phase-encoding gradient 
magnetic field G.sub.E. After gradient magnetic fields G.sub.R and G.sub.E 
are applied, slice-determining gradient magnetic field G.sub.S is applied 
again and 180.degree. excitation pulse P.sub.E2 is applied. A selective 
excitation pulse having an envelope of the waveform as shown in FIG. 2A is 
also used as excitation pulse P.sub.E2. Reading gradient magnetic field 
G.sub.R is applied again in a predetermined time interval after 
180.degree. excitation pulse P.sub.E2 is applied, and M.sub.R echo E (an 
envelope waveform of which is shown in FIG. 2A) is detected during 
application of reading gradient magnetic field G.sub.R. A time interval 
from application of excitation pulse P.sub.E1 to the timing at which first 
MR echo E is generated is echo interval T.sub.E. The above sequence is 
repeated a plurality of times by changing phase-encoding gradient magnetic 
field G.sub.E stepwise by a predetermined amount every time the sequence 
is repeated to acquire MR data. As a result, the MR data required for 
obtaining an image of the selected slice are acquired. 
Note that when 180.degree. excitation pulse P.sub.E2 is used as in the 
above case, phase shift can be cancelled either by inverting 
slice-determining gradient magnetic field G.sub.S after 90.degree. 
excitation pulse P.sub.E1 is applied as described above, or by applying 
non-inverted slice-determining gradient magnetic field G.sub.S immediately 
after 180.degree. excitation pulse P.sub.E2 is applied. 
In the gradient echo method, a pulse sequence as shown in FIGS. 3A to 3C is 
used. 
FIG. 3A shows a waveform of slice-determining gradient magnetic field 
G.sub.S. FIG. 3A shows waveforms of excitation pulse P.sub.E0 and MR echo 
E in addition to the waveform of gradient magnetic field G.sub.S. FIG. 3B 
shows a waveform of gradient magnetic field G.sub.R for reading MR data, 
and FIG. 3C shows a waveform of gradient magnetic field G.sub.E for 
phase-encoding magnetic resonance. 
In the gradient echo method, slice-determining gradient magnetic field 
G.sub.S and 90.degree. excitation pulse P.sub.E0 are applied first. 
Gradient magnetic field G.sub.Z in the Z-axis direction is normally used 
as slice-determining gradient magnetic field G.sub.S. After 90.degree. 
excitation pulse P.sub.E0 is applied, slice-determining gradient magnetic 
field G.sub.S is inverted so as to cancel phase shift as described above. 
Negative reading gradient magnetic field G.sub.R and phase-encoding 
gradient magnetic field G.sub.E are applied for a predetermined time 
interval after slice-determining gradient magnetic field G.sub.S is 
applied. Gradient magnetic fields in directions orthogonal to each other 
on the X-Y plane, e.g., gradient magnetic fields G.sub.X and G.sub.Y in 
the X- and Y-axis directions are normally used as reading gradient 
magnetic field G.sub.R and phase-encoding gradient magnetic field G.sub.E, 
respectively. After gradient magnetic fields G.sub.R and G.sub.E are 
applied, reading gradient magnetic field G.sub.R is inverted, and positive 
reading gradient magnetic field G.sub.R is applied for a predetermined 
time interval, thereby detecting an MR echo E during application of 
reading gradient magnetic field G.sub.R. A time interval from application 
of excitation pulse P.sub.E0 to the timing at which first MR echo E is 
generated is echo interval T.sub.E. The above sequence is repeated a 
plurality of times by changing phase-encoding gradient magnetic field 
G.sub.E stepwise by a predetermined amount within the positive and 
negative ranges every time the sequence is repeated to acquire MR data. As 
a result, MR data required for obtaining an image of the selected slice 
are acquired. 
In general, when a slice once excited is to be excited again so as to 
acquire the MR data, the slice is excited after a resonance state caused 
by the previous excitation is relaxed. 
When the MRI system is utilized, images of a plurality of slices spatially 
adjacent to each other are often required for facilitating diagnosis or 
for obtaining three-dimensional information. For this reason, a technique 
called multi-slice imaging is used in some MRI systems. According to the 
multi-slice imaging, other slices are excited while relaxation of 
resonance of nuclear spins in one excited slice is waited, thereby 
sequentially acquiring MR data. As a result, image data of a number of 
different slice positions can be acquired in a time substantially the same 
as that required for acquiring all the data about one slice. This 
technique effectively utilizes a relaxation waiting time which is 
essential in magnetic resonance imaging. 
For example, the case wherein an image of only a single slice is acquired 
will be described below. FIG. 4 shows an example of an operation sequence 
of a typical pulse echo method in the MRI system. In this example, 
90.degree. and 180.degree. excitation pulses P.sub.E1 and P.sub.E2 are 
applied to excite magnetic resonance to a predetermined slice, and MR data 
at a predetermined encoding phase is acquired. Thereafter, in order to 
excite magnetic resonance in the same slice so as to obtain data at the 
next encoding phase, waiting time T.sub.W for waiting for relaxation of 
resonance of a nuclear spin is required. In FIG. 4, reference symbol 
T.sub.AQ denotes an interval in which MR echo signals are acquired; and 
T.sub.E, a time interval from excitation of magnetic resonance to the 
timing at which the echo signal reaches a maximum level. 
Time interval T.sub.AQ for acquiring echo signals is determined by 
necessary resolution of an image and is generally T.sub.E &gt;T.sub.AQ. Time 
interval T.sub.AQ is also a time interval in which reading gradient 
magnetic field G.sub.R is applied. Excitation pulses P.sub.E1 and P.sub.E2 
(see FIG. 2A) are high-frequency pulses and, in general, P.sub.E1 is a 
90.degree. pulse and P.sub.E2 is a 180.degree. pulse. 
Assuming that a time interval for repeating excitation with respect to one 
slice (i.e., a time interval from the timing at which one slice is excited 
to the timing at which the same slice is excited again) is T.sub.R, 
T.sub.R is obtained by the following equation: 
EQU T.sub.R =T.sub.E +(T.sub.AQ /2)+T.sub.W ( 2) 
where T.sub.E and T.sub.R serve as parameters reflecting relaxation times 
(longitudinal relaxation and transverse relaxation) T.sub.1 and T.sub.2 
and spin density &gt; on an image, and hence can be desirably freely selected 
throughout a wide range. Typical values of T.sub.R and T.sub.E used in 
medical diagnosis are T.sub.R =300 to 3,000 msec and T.sub.E =10 to 120 
msec. 
When MR data are acquired under the condition of T.sub.R &gt;&gt;T.sub.E, T.sub.W 
is increased. In time interval T.sub.W, the system is merely in a waiting 
state and performs no meaningful processing. The multi-slice technique 
utilizes this time interval to excite other slices, thereby acquiring MR 
data. 
Slice excitation by conventional multi-slice imaging will be described 
below with reference to FIGS. 5 and 6. FIG. 5 shows schematic examples of 
a plurality of spatially adjacent slices to be subjected to imaging, i.e., 
slices S.sub. 1 to S.sub.4, and density distribution of nuclear spins in 
which magnetic resonance is excited. FIG. 6 shows excitation sequences of 
first to fourth slices S.sub.1 to S.sub.4. 
As shown in FIG. 6, time T.sub.Rmin (=T.sub.E +(T.sub.AQ /2)) after first 
slice S.sub.1 is excited, second slice S.sub.2 is excited, and after 
another time T.sub.Rmin , third slice S.sub.3 is excited. Thus, a 
plurality of spatially adjacent slices are sequentially excited in the 
order of spatial arrangement, and the MR data are acquired. In this case, 
a frequency component (a high frequency and an envelope) of an excitation 
pulse is set such that nuclear spins in a slice of a predetermined 
thickness are excited to resonate, but the actually excited nuclear spins 
are distributed to a region slightly extending from the slice. Then, when 
a slice is excited during a waiting time of relaxation of resonance of an 
adjacent slice, excitation of resonance in the extending portion 
interferes with resonance of the adjacent slice. 
It is a matter of course that each slice may be excited after a long time 
interval. However, since the number of data acquisitions per slice is 
large, a time interval for data acquisition is increased. Then, an object 
to be examined moves in this time interval for data acquisition, and a 
change in a position and/or a shape is generated in a portion or an organ 
in a region subjected to imaging. If an object to be examined moves in the 
time interval for data acquisition, an artifact is generated in an image 
to be reconstructed. In addition, since it takes a long time for magnetic 
resonance to be attenuated by relaxation, it is impossible to wait for the 
magnetic resonance to be attenuated to have no influence in the 
multi-slice imaging. 
Therefore, when the multi-slice imaging is performed, a lapped portion is 
normally generated between adjacent slices. In this lapped portion, the 
remaining component of magnetic resonance by immediately preceding 
excitation interferes with new magnetic resonance, and adversely affects 
an obtained MR signal, thereby degrading quality of an MR image obtained 
by reconstruction, e.g., decreasing contrast. 
As described above, in the multi-slice imaging, when a sequence from 
excitation at a certain encoding phase to data acquisition is executed 
with respect to one slice, excitation and data acquisition at a certain 
encoding phase are executed with respect to a slice spatially adjacent to 
the above slice, and then, excitation and data acquisition at another 
certain encoding phase are executed with respect to a slice next to the 
above slice, . . . , i.e., excitation and data acquisition are 
sequentially executed with respect to spatially adjacent slices. A 
frequency component of an excitation pulse is set so as to selectively 
excite only nuclear spins in a slice having a predetermined thickness. 
However, a distribution region of actually excited spins slightly extends 
from the slice as shown in FIG. 5. Therefore, when the multi-slice imaging 
is to be efficiently performed in a short time period, adjacent slices 
interfere with each other at a lapped portion therebetween. As a result, 
adverse influences with respect to image quality, such as a decrease in 
contrast of an MR image obtained by reconstruction, cannot be eliminated. 
As described above, in the conventional MRI system, it is impossible to 
execute the multi-slice imaging in a short time period without degrading 
quality of an obtained MR image. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a magnetic 
resonance imaging (MRI) system which is capable of executing multi-slice 
imaging at a high speed to obtain an image of high quality without 
interference between spatially adjacent slices. 
The MRI system according to the present invention comprises a magnetic 
field generation section, an excitation section, an excitation control 
section, a resonance data acquisition section, and an image generation 
section. The magnetic field generation section generates a static magnetic 
field, a gradient magnetic field, and a selective excitation pulse to be 
applied to an object being examined. The excitation section controls the 
magnetic field generation section to apply the static magnetic field, the 
gradient magnetic field, and the excitation pulse to the object at a 
predetermined timing, thereby selectively exciting an MR phenomenon in a 
specific slice of the object. The excitation control section controls the 
excitation section to excite a plurality of slices. The excitation control 
section causes the excitation section to excite a certain slice of the 
object, and then causes the excitation section to excite a slice separated 
from the above slice by at least the thickness of a slice determined by 
the selective excitation pulse within a repeating time of current 
excitation. The resonance data acquisition section detects an MR signal 
induced by magnetic resonance excited by the excitation section and 
acquires data relating to the resonance. The image generation section 
generates an MR image on the basis of resonance data acquired by the data 
acquisition section. 
In this MRI system, after a certain slice is selectively excited, at least 
one slice not spatially adjacent to the above slice, i.e., separated from 
the above slice by at least one slice is sequentially and selectively 
excited within a current time interval for repeating the excitation. 
In this system, a slice located at a position not spatially adjacent is 
selectively excited every time excitation is repeated. Even when slices 
are sequentially changed and excited, spatially adjacent slices do not 
interfere with each other by excitation, and excitation of adjacent slices 
does not adversely affect acquired data. Therefore, even when multi-slice 
imaging is performed efficiently in a short time period, degradation in 
image quality, e.g., decrease in contrast of a reconstructed image is not 
generated. 
Therefore, according to the system of the present invention, multi-slice MR 
data acquisition can be performed at a high speed, and interference 
between spatially adjacent slices can be prevented, thereby obtaining an 
MR image of high quality and good image contrast.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Nuclear spins in which magnetic resonance is excited enter into a 
relaxation/recovery process represented by 
EQU e.sup.(t'/T.sbsp.1.sup.), e.sup.(t/T.sbsp.2.sup.) 
with respect to time t after excitation, immediately after excitation. That 
is, magnetic resonance is relaxed as time t passes, and the amount and 
range (thickness) of a resonance spin distribution are reduced, as shown 
in FIG. 7. Therefore, interference caused by excitation of spatially 
adjacent slices is decreased as an excitation time interval is increased. 
In the MRI system according to the present invention, spatially adjacent 
slices are not excited time-sequentially. On the contrary, a plurality of 
slices are excited so as not to excite spatially adjacent slices 
time-sequentially, and resonance data of the respective slices are 
acquired. Typically, spatially every other slices are sequentially excited 
and their resonance data are acquired. Then, the remaining slices are 
sequentially excited and their resonance data are acquired. That is, 
spatially odd-numbered slices from one end are sequentially excited, and 
then even-numbered slices are sequentially excited. 
In the MRI system according to a first embodiment of the present invention, 
when the number of slices is five, slices S.sub.1, S.sub.2, S.sub.3, 
S.sub.4, and S.sub.5 are excited not in the order of 
S1.fwdarw.S2.fwdarw.S3.fwdarw.S.sub.4 .fwdarw.S.sub.5 from one end but in 
the order of, e.g, S.sub.1 .fwdarw.S.sub.3 .fwdarw.S.sub.5 .fwdarw.S.sub.2 
.fwdarw.S.sub.4 .fwdarw.S.sub.1, i.e., jumping over at least one slice, as 
shown in FIG. 8. 
FIG. 9 shows an excitation sequence in this case. 90.degree. pulse P.sub.E1 
is generated for first data acquisition of slice S.sub.1, and then 
180.degree. pulse P.sub.E2 is generated to obtain MR echo signal E. After 
a predetermined time interval T.sub.EX has passed from generation of pulse 
P.sub.E1 (i.e., a time interval between excitation of a certain slice and 
excitation of the next slice), 90.degree. pulse P.sub.E1 is generated to 
acquire data of slice S.sub.3, and then 180.degree. pulse P.sub.E2 is 
generated to obtain MR echo signal E. After another excitation time 
interval T.sub.EX has passed, in order to acquire data of slice S.sub.5, 
90.degree. pulse P.sub.E1 is generated and then 180.degree. pulse P.sub.E2 
is generated, thereby obtaining MR echo signal E. After still another 
excitation time interval T.sub.EX has passed, in order to acquire data of 
slice S.sub.4, 90.degree. pulse P.sub.E1 is generated and then 180.degree. 
pulse is generated, thereby obtaining MR echo signal E. The above sequence 
is executed in first excitation time interval T.sub.R. Then, after 
excitation time interval T.sub.EX has passed (after first excitation time 
interval T.sub.R has passed), 90.degree. pulse P.sub.E1 is generated and 
then 180.degree. pulse P.sub.E2 is generated, thereby obtaining MR echo 
signal E for second data acquisition of slice S.sub.1. Thus, every other 
slices are excited so that slices not adjacent to each other are 
sequentially and selectively excited to acquire data. 
If excitation time interval T.sub.EX is determined by the following 
equation (3), interference between slices can be effectively prevented: 
EQU T.sub.EX =T.sub.R /Number of Multi-Slices (3) 
where T.sub.EX &gt;T.sub.Rmin. 
FIG. 10 shows an arrangement of the MRI system according to the first 
embodiment of the present invention. The system includes static coils 1A 
and 1B, first and second gradient coils 2 and 3, RF (radio frequency) coil 
4, transmitter 5, receiver 6, A/D (analog-to-digital) converter 7, data 
acquisition section 8, Fourier-transformation section 9, image processor 
10, display 11, sequence controller 12, and power supply 13. 
A pair of static coils 1A and 1B are driven by power supply 13 and generate 
a uniform static magnetic field to be applied to object to be detected 
(patient) P. First gradient coil 2 generated gradient magnetic field 
G.sub.S in the Z-axis direction (normally along a body axis direction of 
patient P) to be applied to patient P so as to determine a position of 
imaging slice S. Second gradient coil 3 generates gradient magnetic fields 
to be applied to patient P and in a predetermined direction on the X-Y 
plane, i.e., reading gradient magnetic field G.sub.R and phase-encoding 
gradient magnetic field G.sub.E. RF coil 4 is driven by transmitter 5 to 
apply a high-frequency magnetic field to patient P, and detects a signal 
caused by magnetic resonance generated in patient P, e.g., an MR echo 
(spin echo) and supplies it to receiver 6. Receiver 6 causes a phase 
detector such as an orthogonal detector to detect the MR signal detected 
through RF coil 4. A/D converter 7 converts the MR signal detected and 
extracted by receiver 6 into digital data and supplies it to data 
acquisition section 8. Data acquisition section 8 acquires and stores the 
MR data supplied through A/D converter 7. In order to improve the S/N 
(signal-to-noise) ratio, data acquisition section 8 accumulates the MR 
data sampled a plurality of times under the same condition as needed. 
Fourier-transformation section 9 Fourier-transforms the MR data acquired 
by data acquisition section 8. Image processor 10 performs predetermined 
processing of the data obtained by Fourier-transformation section 9 to 
generate an MR image. Display 11 displays the MR image generated by image 
processor 10. Sequence controller 12 controls gradient coils 2 and 3, 
transmitter 5, A/D converter 7, and power supply 13 so that excitation of 
magnetic resonance and acquisition of resonance data are performed in 
accordance with a sequence as shown in FIG. 9. 
As described above, in the MRI system for exciting magnetic resonance of 
spins of specific atomic nuclei at specific slice portion S of an object 
being examined, receiving an MR signal of the magnetic resonance to 
acquire MR data, and performing predetermined processing including Fourier 
transformation, thereby obtaining an MR image of the specific atomic 
nuclei in this slice portion S, excitation of the magnetic resonance and 
acquisition of the resonance data are sequentially repeated at a 
predetermined timing, and during this repetition, control is performed 
such that spatially adjacent slices are not time-sequentially excited. 
In general, even if a system is set so as to excite magnetic resonance only 
of nuclear spins in a slice portion, density distribution of nuclear spins 
in which magnetic resonance is excited actually extends to a region 
slightly extending from the slice portion. However, according to the above 
system, magnetic resonance is excited such that spatially adjacent slices 
are not time-sequentially selected. For this reason, even when a plurality 
of spatially adjacent slices are sequentially excited while selected 
slices are changed as needed, interference is not generated between 
acquired data of spatially adjacent slices. Therefore, degradation in 
image quality such as a decrease in contrast of a finally obtained MR 
image is prevented. 
In order to perform multi-slice imaging efficiently in a short time period, 
excitation and data acquisition of a slice are preferably performed 
immediately after data acquisition of the previous slice is performed 
once, but conventional systems cannot perform multi-slice imaging in a 
short time period while maintaining good image quality. However, in the 
above system, interference of magnetic resonance is not generated between 
spatially adjacent slices. Therefore, according to the above system, even 
if multi-slice imaging is performed efficiently in a short time period, 
adverse influence on image quality, such as a decrease in contrast of an 
MR image, is not generated, and a number of MR images of high quality can 
be obtained in a short time period. 
Note that the present invention is not limited to the above first 
embodiment described with reference to the drawings but can be arbitrarily 
modified and practiced without departing from the spirit and scope of the 
invention. 
For example, in a second embodiment of the present invention, when imaging 
is to be performed for spatially adjacent four slices S.sub.1, S.sub.2, 
S.sub.3 , and S.sub.4, slices are 
the order of S.sub.1 .fwdarw.S.sub.2 .fwdarw.S.sub.3 .fwdarw.S.sub.4. In 
this case, slices S.sub.2 and S.sub.3 are time-sequentially excited 
despite that they are spatially adjacent to each other. For this reason, 
interference may occur between slices S.sub.2 and S.sub.3. However, 
interference does not occur between other slices, so that image quality 
degradation is minimal as a whole. In addition, when only an excitation 
time interval between slices S.sub.2 and S.sub.3 is made longer than other 
excitation time intervals, interference between slices S.sub.2 and S.sub.3 
can be reduced. 
Furthermore, the order of excitation is not limited to that in which 
odd-numbered slices are sequentially excited and then even-numbered slices 
are sequentially excited. For example, slices may be excited in the order 
opposite to the above order or every third or fourth slices may be 
excited. That is, the order can be arbitrarily set as long as spatially 
adjacent slices are not time-sequentially excited.