Magnetic resonance imaging apparatus and method

A magnetic resonance imaging apparatus is provided which generates a plurality of echoes by applying a 90.degree. RF pulse for magnetically exciting spins and subsequently repetitively applying a 180.degree. RF pulse for inverting the phase of the magnetized spin. In this apparatus, a pulse sequence is so performed as to enable any specific interval, out of an interval from the application of a 90.degree. RF pulse until a first echo is obtained and intervals each-between sequential adjacent two echoes, to be made to correspond to a 3 or more odd multiple of any other intervals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic resonance imaging apparatus and 
method for generating a plurality of echoes by applying a 90.degree. RF 
pulse for magnetically exciting spins and sequentially and repetitively 
applying a 180.degree. RF pulse. 
2. Description of the Related Art 
The spin echo imaging (SE) method generates a spin echo on the following 
principle. After the application of a 90.degree. RF pulse, the spin phases 
are diffusively varied after a lapse of time. This is called a transverse 
relaxation phenomenon. When a 180.degree. RF pulse is applied after a 
lapse of time of .tau./2, the phases of respective spins are focused after 
a lapse of time of .tau. from the application of the 90.degree. RF pulse 
and a transient peak emerges at a free induction decay (FID), thus 
generating spin echoes. 
With the fast spin echo imaging method (hereinafter referred to as an FSE 
method), a plurality of spin echoes can be obtained by applying a 
90.degree. RF pulse and repetitively applying a 180.degree. RF pulse. The 
FSE method, compared with the SE method, can reduce the application times 
of the 90.degree. RF pulses drastically. In comparison with the SE method, 
the FSE method can achieve faster imaging. 
The FSE method allows no flexibility upon the designing of a pulse sequence 
under the following rule. That is, according to the rule above, every 
interval of a 180.degree. RF pulse can be uniformalized to .tau., and the 
echo time from the application of the 90.degree. RF pulse until a first 
echo is obtained, as well as every echo interval of those echoes E2, E3, . 
. . , emerging after the appearance of the first echo, is uniformalized to 
.tau.. The rule is based on the following. 
It is not possible, in reality, to make a 180.degree. RF pulse have 
180.degree. flip angle components only. For this reason, the magnetized 
spin is dissolved, upon each reception of a 180.degree. RF pulse, into a 
first component undergoing a phase inversion as expected, a second 
component undergoing a longitudinal magnetization and a third component 
undergoing a direct steady phase spread without being given any influence 
by the 180.degree. RF pulse. The first component emerges as a primary 
echo, the second component as a stimulated echo and the third component as 
an indirect echo, by the stimulated echo it is meant an echo generated, 
through the longitudinal magnetization, after the application of the 
90.degree. RF pulse, for example, an echo which, under a varying phase 
spread of spins along an A-B-E-F path in FIG. 1, is generated when the 
spin phase focuses at a 0 point. By the indirect echo it is meant an echo 
generated upon being unusually subjected by a 180.degree. RF pulse to 
phase inversion, for example, an echo which, under a varying phase 
inversion along an A-B-D-G-K path, is generated when the spin phase 
focuses at a 0 point. These stimulated and indirect echoes, together with 
the primary echo, are effective to composing an image. In order to utilize 
the stimulated and indirect echoes for composing an image, it has been 
necessary to generate stimulated and indirect echoes in the same timing as 
the primary echo and it has, therefore, been necessary to make, constant, 
all RF pulse intervals and their echo intervals as set out above. 
Paradoxically, if the stimulated and indirect echoes are generated in a 
different timing from the primary echo, then this results in their mutual 
interference and in a poor image quality. 
From this situation in which the FSE method, being restricted under the 
above-mentioned rules, will not allow any flexible pulse sequence, the 
following disadvantages are reduced. For example, the FSE method enables a 
plurality of echoes to be obtained in the transverse time T2 and, 
therefore, the time and interval of an initial echo are shortened compared 
to the SE method. For a short echo time, there is no decline of a fat 
signal involved. It is, therefore, not possible, according to the FSE 
method, to obtain an image of a T2 contrast substantially equal to that 
obtained according to the SE method. If the echo time is lengthened so as 
to achieve a decline in the fat signal, subsequent echo intervals are also 
lengthened, thus resulting in a longer data collection time or in a 
decline in the number of echoes involved. In order to achieve such a 
decline in the fat signal without lengthening the echo time and echo 
interval, the techniques, such as the presaturation processing or the 
elimination of the fat signal through a chemical shift, have been 
developed, but there arise the problems with an increase in an SAR (RF 
exposure), decline in an S/N ratio, decline in the number of echoes 
resulting from the addition of prepulses, inhomogeneity of a static 
magnetic field, and so on. These problems must be corrected with high 
accuracy. 
Let it be given that the FSE method and its applied GRASE (gradient and 
spin echo) method are applied to a dual contrast mode by which it is 
possible to obtain two kinds of image of different contrast, a proton 
density weighted image and T2 weighted image, from the first half's echoes 
and latter half's echoes. In this case, the S/N of the latter half's 
echoes is lower than that of the first half's echoes in view of its longer 
relaxation time and the image quality is extremely unbalanced among the 
images. In order to improve the S/N ratio on the latter half's echoes and 
eliminate an unbalance among the image quality, it is necessary to 
lengthen the echo collection time of the latter half's echoes and, by so 
doing, narrow down the echo collection band. However, the above-mentioned 
rule does not allow only the echo collection time of the latter half's 
echoes to be lengthened while the echo collection time of the first half's 
echoes is reserved in an "as shortened" way. 
Let it be given that the first half's echoes and latter half's echoes are 
collected by the FSE method and GRASE method, respectively. When the FSE 
method and GRASE methods are used separately, the 180.degree. RF pulses 
are applied in different time intervals. However, the FSE method and GRASE 
method, being used in combination, are so uniformalized as to allow the 
use of a 180.degree. RF pulse interval. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a magnetic resonance 
imaging apparatus and method which, through the use of the FSE method, can 
obtain a T2 contrast image, substantially equal to that by the SE method, 
while suppressing a fat signal involved. 
A second object of the present invention is to provide a magnetic resonance 
imaging apparatus and method which, in a dual contrast mode for forming 
two kinds of images, one using a first half's echoes and one using a 
half's echoes, can reduce an unbalance in quality between these two 
images. 
A third object of the present invention is to provide a magnetic resonance 
imaging apparatus and method which, in a dual contrast mode for collecting 
first half's echoes by the FSE method and second half's echoes by the 
GRASE method, can optimize their 180.degree. RF pulse interval. 
According to one aspect of the present invention, there is provided a 
magnetic resonance imaging apparatus for generating a plurality of echoes 
by applying a 90.degree. RF pulse for magnetically exciting spins and 
subsequently repetitively applying 180.degree. RF pulses for inverting the 
phase of the magnetized spin, comprising: 
pulse sequence performing means for performing such a pulse sequence that, 
one of an interval from the application of a 90.degree. RF pulse until a 
first echo is obtained and intervals each between sequential adjacent two 
echoes, is made to correspond to a 3 or more odd multiple of another of 
intervals; and 
image forming means for forming an image based on these echoes. 
According to another aspect of the present invention there is provided a 
magnetic resonance imaging method for generating a plurality of echoes by 
applying a 90.degree. RF pulse for magnetically exciting spins and 
subsequently repetitively applying 180.degree. RF pulses for inverting the 
phase of the magnetized spins, comprising the step of performing such a 
pulse sequence as to enable any specified interval, out of an interval 
from the application of a 90.degree. RF pulse until a first echo is 
obtained and intervals each between sequentially adjacent two echoes, to 
be made to correspond to a 3 or more odd multiple of another of intervals. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A magnetic resonance imaging apparatus and method will be explained below 
in conjunction with the embodiments of the present invention, while 
referring to the accompanying drawings. 
(First Embodiment) 
FIG. 2 is a diagram illustrating a magnetic resonance imaging apparatus 
according to a first embodiment of the present invention. A gantry 20 
defines a cylindrical inner space for holding a subject P to be examined 
and is equipped with a static magnetic field magnet unit 1, gradient 
magnetic coil 2 and an RF coil 3. The static magnetic field magnet unit 1 
is comprised of a permanent magnet, (normal-conducting magnet or 
super-conducting magnet) and creates a static magnetic field in the 
cylindrical inner space. Here, for convenience in explanation, the Z axis 
represents the static magnetic field direction parallel to the body axis 
of the subject with the x and Y axes normal to the Z axis. 
The gradient magnetic field coil unit 2 comprises an x axis gradient 
magnetic field coil, a Y axis gradient magnetic field coil and Z axis 
gradient magnetic field coil. The X axis gradient magnetic field coil 
receives electric current from the X axis gradient magnetic field power 
source 7 to generate an X axis gradient magnetic field whose intensity 
varies along the X axis. The Y axis gradient magnetic field coil receives 
electric current from a Y axis gradient magnetic field power source 8 to 
generate a Y axis gradient field whose intensity varies along the Y axis. 
Upon receipt of electric current from the Z axis gradient magnetic field 
power source 9, the Z-axis gradient magnetic field coil generates a Z axis 
gradient magnetic field whose intensity varies along the z axis. For 
convenience in explanation, it is assumed that the X, Y and Z axis 
gradient magnetic fields are used for a read gradient magnetic field GR, 
phase encode gradient magnetic field GE and slice gradient magnetic field 
GS, respectively. It is possible to collect data in a range in which the 
intensities of the X, Y and Z axis magnetic fields are linearly varied. 
The range above is referred to as an image obtainable region. Upon data 
collection, the subject can be inserted into the image obtainable region 
in such a state that the subject is held on a top surface of a bed 13. 
A transmit/receive coil 3 is connected to a transmitter 5 at the time of 
excitation and to a receiver at the time of reception. The 
transmit/receive coil 3 receives electric current from the transmitter 5 
to generate a selective excitation pulse of a predetermined frequency 
component. A plurality of spins are magnetically excited, in a selective 
fashion, in a Z position corresponding to the frequency component above. 
This is called a selective excitation method. A magnetic resonance signal 
(hereinafter referred to as an echo) is generated from the relaxing 
magnetized spins, after excitation has been completed, and sent to the 
transmit/receive coil 3. 
A sequence controller 10 performs a later-described pulse sequence by 
controlling the respective operation timings of the X axis gradient 
magnetic field power source 7, the Y axis gradient magnetic field power 
source 8, the Z axis gradient magnetic field power source 9, the 
transmitter 5 and the receiver 6. A computer system 11 controls the whole 
operation of the system and forms a slice image by picking up echoes from 
the transmit/receive coil 3 through the receiver 6. The slice image is 
displayed on a display unit 12. 
The operation of the present embodiment will be explained below. 
FIG. 3 is a phase diagram representing, as a model, a temporal variation in 
the phase spread of the magnetized spin by a pulse sequence unique to the 
present embodiment. FIG. 3 shows the temporal variation of the phase 
spread on the static magnetic field's inhomogeneity .DELTA.B.sub.O, slice 
direction gradient magnetic field G.sub.S, read direction gradient 
magnetic field G.sub.R and phase encode direction gradient magnetic field 
G.sub.E. What is relevant to the present invention is a temporal variation 
in the phase spread by the inhomogeneity of the static magnetic field. 
FIG. 4 shows, in detail, the temporal variation in the phase spread by 
.DELTA.B.sub.O as well as the pulse sequence of FIG. 3. 
Even in the improved FSE method of the present embodiment, after a 
90.degree. RF pulse has been applied, a 180.degree. RF pulse is applied a 
predetermined N number of times to obtain a plurality of echoes, that is, 
an N number of echoes. This is not different from a conventional FSE 
method. The way of making the interval of any two of sequential echoes E1, 
E2, E3, . . . , EN a uniform time .tau. is also not different from the 
conventional FSE method. 
With the conventional FSE method, the time interval (interval: E1 echo 
time) taken from the application of a 90.degree. RF pulse until a start 
echo E1 is obtained, has been set to be the same as the echo interval 
.tau.. With the present embodiment, the E1 echo time is extended under the 
condition that it is set to be equal to a 3 or more odd multiple of its 
echo time interval .tau.. That is, the E1 echo time TE1 is represented by 
EQU TE1=.tau.. (2m+1) (1) 
noting that m denotes an integer of 1 or more. By extending the E1 echo 
time, the extent to which a fat signal declines is greater than that 
according to the conventional FSE method of setting the E1 echo time equal 
to the echo interval. In consequence, the contrast of an image according 
to the improved FSE method of the present invention is more approximate to 
the contrast of an image by an ordinary SE method than that by the 
conventional FSE method. 
The E1 echo time is preferably made short, taking into consideration the 
need to shorten the imaging time and obtain a large number of echoes. It 
is, therefore, ideal to set the E1 echo time equal to three times the echo 
interval, that is, m=1. The experiments conducted show that, even when the 
E1 echo time is fixed to three times the echo interval, it is possible to 
obtain an image whose contrast is substantially the same as that by the 
ordinary SE method. 
In order to satisfy the condition of Equation (1)for m=3, a 90.degree. RF 
pulse is applied wherein, after a lapse of time of .tau.. (m+1/2), a first 
180.degree. RF pulse is applied, which is followed by the application of a 
second 180.degree. RF pulse after an additional lapse of time of .tau.. 
(m+1). Then a sequence of 180.degree. RF pulses at an interval is 
repeatedly applied a predetermined number of times. Stated in another way, 
the interval between the first 180.degree. RF pulse and the second 
180.degree. RF pulse is set to be a 2 or more integral multiple of the 
third or subsequent 180.degree. RF pulse interval .tau., namely, .tau. . 
(m+1). 
It is required that, in order to make the E1 echo time correspond to a 
three or more odd multiple of the echo interval, the slice gradient 
magnetic field G.sub.S and read gradient magnetic field G.sub.R be 
adjusted as will be set out below. 
The area of the slice gradient magnetic field G.sub.S applied together with 
the initial 180.degree. RF pulse, that is, the intensity of the magnetic 
field X application time (time integral value), is made to correspond to a 
3 or more odd multiple of a time integral value (2 . a) of the slice 
gradient magnetic field G.sub.S applied together with the second and 
subsequent 180.degree. RF pulses which is expressible as 2a . (2m+1). Put 
it in another way, the time integral value of the slice gradient magnetic 
field G.sub.S applied between the first 180.degree. RF pulse and the 
second 180.degree. RF pulse is made to correspond to a 2 or more integral 
multiple of the time integral value of the slice gradient magnetic field 
G.sub.S applied between the second and subsequent adjacent 180.degree. RF 
pulses. In order to obtain the time integral relation as set out above, 
the time of the slice gradient magnetic field G.sub.S applied together 
with the first 180.degree. RF pulse is made to correspond to a 3 or more 
odd multiple of the application time of the slice gradient magnetic field 
G.sub.S applied together with the second or sequential 180.degree. RF 
pulse, provided that, as shown in FIG. 4, the intensity of the magnetic 
field, that is, the gradient of the magnetic field, is made constant. 
However, the intensity of the slice gradient magnetic field G.sub.S 
applied together with the first 180.degree. RF pulse may be made to 
correspond to a 3 or more odd multiple of the slice gradient magnetic 
field G.sub.S applied together with the second or sequential 180.degree. 
RF pulse, provided that the application time is constant. 
An echo is read out in the presence of the read gradient magnetic field 
G.sub.R so as to secure an alignment in the read direction, that is, in 
the X direction. The area of the read gradient magnetic field G.sub.R 
applied at a time of the first echo E1, that is, the magnetic field 
intensity X application time (time integral value), is made to correspond 
to a 2 or more integral multiple of a time integral value 2 . b of the 
read gradient magnetic field G.sub.R applied at a time of reading out the 
second or sequential echoes E2, E3, . . . , which is expressible as 2 . b 
. (m+1). In order to obtain the time integral relation as set out above 
the application time of the read gradient magnetic field G.sub.R applied 
together with the first echo E1 is made to correspond to a 2 or more odd 
multiple of the application time of the read gradient magnetic field 
G.sub.R applied at a time of reading out the second or sequential echoes 
E2, E3, . . . , provided that, as shown in FIG. 4, the magnetic field 
intensity, or the magnetic field gradient, is constant. 
Provided that the interval from the application of the 90.degree. RF pulse 
until the first echo E1 is obtained is made to correspond to a 3 or more 
odd multiple of the echo interval of the second or subsequent echoes E2, 
E3, . . . , the above interval taken is extended and, by so doing, a 
stimulated echo or indirect echo is generated in the same timing as that 
of a primary echo so that it is utilizable to construct an image. 
In reality, it is not possible for the 180.degree. RF pulse to have only 
180.degree. flip angle components. Upon receipt of each 180.degree. RF 
pulse, the magnetized spin is resolved into a first component undergoing a 
phase inversion as predicted, a second component undergoing a longitudinal 
magnetization and a third component undergoing its steady phase spread as 
it is without being affected by the 180.degree. RF pulse. The first 
component is generated as a primary echo, the second component as a 
stimulated echo and the third component as an indirect echo. The 
stimulated echo is one which is generated via the longitudinal 
magnetization after the application of the 90.degree. RF pulse. The 
stimulated echo is generated when, through a varying spin phase spread 
along an A-B-E-H-O path in FIG. 4 for instance, the spin phase passes 
through its focusing point O. The indirect echo is one which occurs when 
it encounters an unusual (a not steady) phase inversion by the 180.degree. 
RF pulse. For example, the indirect echo is generated when, through a 
varying spin phase spread along an A-B-D-I-Q path, the spin phase passes 
through its focusing point O. 
An explanation is given below of the principle on which the stimulated and 
indirect echoes can be generated in the same timing as that of the primary 
echo by extending the interval from the application of the 90.degree. RF 
pulse until the first echo E1 is obtained, provided that the above 
interval is made to correspond to a 3 or more odd multiple of the echo 
interval of the second or sequential echoes E2, E3, . . . . Reference is 
made to the pulse sequences of FIGS. 1 and 4 for comparison. For 
convenience in explanation, let it be assumed that m=1. In the pulse 
sequence of the present embodiment, the application timing of the 
180.degree. RF pulse is equivalent to eliminating the first and third 
180.degree. RF pulses in the pulse sequence shown in FIG. 1. The interval 
of the 180.degree. RF pulse of the pulse sequence in FIG. 1 is so adjusted 
that stimulated and indirect echoes are generated in the same timing as 
the primary echo. The phase spread of the stimulated echo, indirect echo 
and primary echo are made constant irrespective of the 180.degree. RF 
pulse. That is, the phase diagram of FIG. 4 is so considered that, in the 
phase diagram shown in FIG. 1, those magnetized spins by the first and 
third 180.degree. RF pulses are not resolved into three components. 
Therefore, so long as the condition is obeyed that the interval from the 
application of the 90.degree. RF pulse until the start echo E1 is obtained 
is made to correspond to a 3 or more odd multiple of the echo interval of 
the second and sequential echoes E2, E3 . . . . , the stimulated and 
indirect echoes can be generated in the same timing as that of the primary 
echo. 
According to an improved FSE method of the present invention, it is 
possible to obtain an image whose contrast is equal to that of an image 
obtained by the ordinary SE method. 
(Second Embodiment) 
The second embodiment is similar to the first embodiment with respect to 
their arrangements and any detailed explanation of its arrangement is 
omitted for brevity's sake. The second embodiment relates to an improved 
FSE method as applied to a dual contrast mode. As shown in FIG. 8, in a 
dual contrast mode, a proton density image IP is formed at the first 
half's echoes E1 to E3 and a T.sub.2 enhanced image I.sub.T is formed at 
the latter half's echoes E4 to E6 of those echoes E1 to E6 obtained by the 
first 90.degree. RF pulse. 
The condition that the E1 echo time of the first embodiment is extended to 
a 3 or more odd multiple of the echo interval can be exchangeably read as 
the condition that a specific echo interval is extended to a 3 or more odd 
multiple of another echo interval. The second embodiment corresponds to an 
improved FSE method applied to the dual contrast mode under the latter 
condition so read. 
FIG. 5 shows a pulse sequence in which, under the condition above, the echo 
interval at the latter half portion above is extended to a 3 or more odd 
multiple, that is, .tau. . (2m+1), of the echo interval .tau. at the first 
half portion as set out above. This pulse sequence is shown in the phase 
diagram of FIG. 7. 
As shown in FIG. 5, the interval of the 180.degree. RF pulse at the latter 
half portion is extended to a 3 or more odd multiple, that is, .tau. . 
(2m+1), of the 180.degree. RF pulse at the first half portion. This makes 
it necessary for the slice gradient magnetic field G.sub.S and read 
gradient magnetic field G.sub.R to be set as will be set out below. The 
time integral value of the slice gradient magnetic field G.sub.S applied 
together with the 180.degree. RF pulse at the latter half portion is made 
to correspond to a 3 or more odd multiple, that is, 2 . a . (2m+1), of the 
time integral value of the slice gradient magnetic field G.sub.S applied 
together with the 180.degree. RF pulse at the first half portion. Further, 
the time integral value of the read gradient magnetic field G.sub.R 
applied upon the reading of the echoes E4, E5 and E6 at the latter half 
portion is made to correspond to a 3 or more odd multiple, that is, 2 . b 
. (2m+1), of the time integral value of the read gradient magnetic field 
G.sub.R applied upon the reading of the first half's echoes E1 and E2. 
In the second embodiment, the pulse sequence of FIG. 5 can be improved upon 
as shown in FIG. 6. Since the latter half's echo interval can be extended 
to .tau. . (2m+1) with respect to the first half's echo interval .tau., a 
data collection time .DELTA.t' of the latter half's echoes is made longer 
than the data collection time .DELTA.t of the first half's echoes with the 
aforementioned time integral relation reversed and the echo collection 
band is narrowed down so that a S/N difference between the first half's 
echoes and the latter half's echoes is decreased. By so doing, it is 
possible to alleviate an unbalance between a proton density weighted image 
IP at the first half portion and a T.sub.2 weighted image IT at the latter 
half portion. Further, the time length of the latter half's echoes is 
greater than that of the first half's echoes and, without varying a 
sampling frequency for those echoes at the first and latter half portions, 
it is possible to enhance the resolution, relative to a read direction (X 
direction), of the T.sub.2 weighted image IT at the latter half portion. 
(Third Embodiment) 
The third embodiment is similar to the first embodiment with respect to 
their structures and an explanation of these is omitted for brevity in 
explanation with the third embodiment, the first half's echoes are 
collected by the FSE method, and the latter half's echoes by the GRASE 
method, in a dual contrast mode. The GRASE method is an ultra-high speed 
imaging method by which, through the application of a 90.degree. RF pulse 
and then the repetitive application of 180.degree. RF pulses, echoes are 
repetitively generated between the adjacent 180.degree. RF pulses while 
the read gradient magnetic field G.sub.R is reversed. In this connection 
it is to be noted that, under the conventional rule of the 180.degree. RF 
pulse interval being constant, there is no proper time for those echoes to 
be repetitively generated between those adjacent 180.degree. RF pulses 
while reversing the read gradient magnetic field G.sub.R. It has been, 
therefore, very difficult to combine the FSE method with the GRASE method. 
Since, as set out in conjunction with the second embodiment, the interval 
of the latter half's 180.degree. RF pulses can be made to correspond to a 
3 or more odd multiple of the interval of the first half's 180.degree. RF 
pulses, it becomes possible to combine the GRASE method with the FSE 
method. With the GRASE method, a ringing artifact is likely to emerge on 
an image of a shorter echo time owing to the T.sub.2 relaxation of a 
tissue involved and the GRASE method is used in combination at the latter 
half portion where a longer echo time is involved. 
FIG. 9 shows a pulse sequence by the third embodiment. Here, as disclosed 
in FIG. 10, a proton density image IP is formed with the use of first 
half's echoes El, E2 and E3 and a T2 enhanced image IT with the use of 
latter half's echoes E4 . . . E12. As shown in FIG. 10, since the interval 
of the latter half`s 180.degree. RF pulses can be made to correspond to a 
3 or more odd multiple (threefold in FIG. 9) of the interval of the first 
half's 180.degree. RF pulses, the pulse sequence by the GRASE method can 
be incorporated into the latter half portion and three echoes can be 
generated between those adjacent 180.degree. RF pulses at the latter half 
portion while the read gradient magnetic field G.sub.R is reversed twice. 
With the interval of the latter half's 180.degree. RF pulses being a 3 or 
more odd multiple of that of the first half's 180.degree. RF pulses, the 
slice gradient magnetic field G.sub.S and read gradient magnetic field 
G.sub.R need to be adjusted as will be set out below. A time integral 
value of the slice gradient magnetic field GS applied together with the 
latter half's 180.degree. RF pulses is made to correspond to a 3 or more 
odd multiple of a time integral value 2 . a, that is, 2 . a . (2m+1), of 
the slice gradient magnetic field G.sub.S applied together with the first 
half's 180.degree. RF pulses. Further, a time integral value of the read 
gradient magnetic field G.sub.R applied between the latter half's adjacent 
180.degree. RF pulses is made to correspond to a 2 or more integral 
multiple of a time integral value 2 . b, that is, 2 . b . (2m+1), of the 
read gradient magnetic field G.sub.R applied between the first half's 
adjacent 180.degree. RF pulses. 
Thus, it is possible to combine the GRASE method with the FSE method 
according to the third embodiment. As shown in FIGS. 9 and 10, in view of 
the situation in which the number of echoes for the first half's proton 
density image IP provides a large unbalance against the number of echoes 
for the latter half's T2 enhanced image IT, the number of matriceses and 
that of averages are varied between the images so that such an unbalance 
can readily be alleviated. There are sometimes cases where, for the 
enhancement of a S/N ratio, echoes are repetitively collected at phase 
encoding so that they are subjected to weight averaging. That average 
number is called here as an echo number. 
(Fourth Embodiment) 
The fourth embodiment is similar to the first embodiment in their 
structures and an explanation as to the arrangement of the apparatus is 
omitted for brevity's sake. With the fourth embodiment, first half's 
echoes are collected by an interleaved EPI method, and latter half's 
echoes by the FSE method, in the dual contrast mode. FIG. 11 shows a pulse 
sequence of the fourth embodiment and FIG. 12 shows the allocation of 
echoes to two images. In the interleaved EPI method, echo data are so 
collected as to place them in an unsymmetrical relation to an encoding 
direction (Y direction). By so doing, any deficient data is replenished 
through the utilization of a conjugate complexity to compose a first image 
IP. 
With the fourth embodiment, it is thus possible to increase the number of 
echoes for the first image. 
Various changes or modifications of the present invention can be made 
without departing from the spirit and scope of the present invention. For 
example, a 90.degree. RF pulse and 180.degree. RF pulses can change with 
those of another flipangle. Although the present invention has been 
explained as being applied, for example, to the fast spin echo method, it 
may also be applied to a multi-echo imaging method. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, representative devices, and illustrated examples 
shown and described herein. Accordingly, various modifications may be made 
without departing from the spirit or scope of the general inventive 
concept as defined by the appended claims and their equivalents.