Correction of MR imaging pulse sequence using prescan having application phase adjusted RF refocusing pulses

There is a method of correcting an imaging pulse sequence magnetically applied by an imaging scan to an object. The method comprises a step of performing a first prescan to the object with a first preliminary pulse sequence including an RF excitation pulse and a plurality of RF refocusing pulses each having an application phase set to a specified phase value (for example, 90.degree.) and a step of performing a second prescan to the object with a second preliminary pulse sequence including an RF excitation pulse and a plurality of RF refocusing pulses of which even-numbered RF refocusing pulse has an application phase set to another specified phase value (for example, 270.degree.) differed by 180.degree. from the specified phase value. The method further comprises a step of correcting the imaging pulse sequence on the basis of first and second echo data groups provided by the first and second prescan, prior to the imaging scan. Each of the first and second preliminary pulse sequences and the imaging pulse sequence is formed by a CPMG pulse train or a phase reversal CP pulse train, such as a GRASE sequence and a fast SE sequence.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to magnetic resonance imaging (MRI), or more 
particularly, to magnetic resonance imaging using a pulse sequence 
composed of a plurality of RF refocusing pulses; such as, a CPMG pulse 
sequence or phase reversal CP pulse sequence (a typical pulse sequence 
includes, for example, RARE (fast SE) and GRASE (hybrid EPI) pulse 
sequences). 
2. Description of the Prior Art 
Magnetic resonance imaging (MRI) utilizing magnetic resonance phenomena of 
nuclear spins is being implemented more and more eagerly in the field of 
medical engineering because of its noninvasiveness and its ability to 
produce images of the insides of patient bodies. Along with the 
advancement and sophistication of technologies including that for image 
processing, the demand for high-quality MR images and the degree of fast 
imaging are increasing. 
In the past, various pulse sequences devised for magnetic resonance imaging 
have been implemented or proposed in an effort to satisfy the foregoing 
conditions. One of the pulse sequences is a pulse sequence referred to as 
a RARE pulse sequence (Refer to, for example, literature 1 "Magnetic 
Resonance in Medicine" (Vol. 3, P. 823-833, 1986), and literature 2 
"Journal of Magnetic Resonance Imaging" (Vol. 78, P.397-407, 1988), or a 
GRASE pulse sequence (Refer to, for example, U.S. Pat. No. 5,270,654). 
FIG. 1 shows a RARE pulse sequence, and FIG. 2 shows a GRASE pulse 
sequence. In these drawings, RFex denotes an RF excitation pulse, and 
RFrel to RFre5 denote RF refocusing pulses. The RF refocusing pulses RFrel 
to RFre5 are applied consecutively succeeding the RF excitation pulse 
RFex. Gss denotes a slice-selective magnetic field gradient. Gpe denotes a 
phase-encoding magnetic field gradient. Gro denotes a readout magnetic 
field gradient. Furthermore, E(1) to E(5) (or E(l1) to E(5,3)) denote NMR 
echoes (waveforms subjected to phase detection) produced by a CPMG pulse 
sequence. As for phases .phi. by which rotation is made with applications 
of the RF pulses RFex and RFrel to RFre5, the phase 4) for the RF 
excitation pulse RFex is set to 00, and that for the RF refocusing pulses 
RFrel to REre5 is set to 90.degree. . As for a flip angle 0 by which the 
magnetization of spins is leveled by an RF pulse, the flip angle 0 by the 
RF excitation pulse RFex is a (generally 90.degree. ), and those 0 by the 
RF refocusing pulses RFrel to RFre5 are 131 to B5 (generally 1800). In 
either FIG. 1 or 2, a CPMG pulse sequence is used to generate a plurality 
of echoes. Phase-encoding of a different quantity is then performed on 
each of the echoes (a phase-encoding gradient pulse (Gpe) of a different 
strength (pulse area) is applied to each of the echoes), and thus echo 
data required to reconstruct a one-frame image is acquired. Thus, compared 
with a known spin echo technique, this technique requires one-fifth or 
-sixth or one-hundredth of the scan time needed for spin echo imaging. 
For a pulse sequence using a plurality of RF refocusing pulses; such as, 
the CPIVIG pulse sequence and phase reversal CP pulse sequence, it is 
certain that each echo acquired between RF refocusing pulses is a total 
sum of echo components depicted to trace a plurality of paths on a phase 
diagram (Refer to, for example, literature 2 "Journal of Magnetic 
Resonance Imaging" (Vol.78, P.397-407, 1988) and literature 3 "Magnetic 
Resonance in Medicine" (Vol.30, P.183-191, 1993)). 
For using these pulse sequences, a phase shift exhibiting a different 
spatial distribution occurs in each echo component because of such major 
causes as eddy currents that are derived from switching of gradient pulses 
and induced in a conductor of an MRI system, an error in a gradient coil 
caused during manufacturing, and imperfect calibration of any other 
facility of the MRI system. This is described particularly in, for 
example, patent publication 1, Japanese Patent Laid-Open No. 6-1 21 777; 
patent publication 2, Japanese Patent Laid-Open No. 6-54827; and patent 
publication 3, U.S. Pat. No. 5,378,985. Consequently, ghost artifacts may 
occur in a direction of phase-encoding in a reconstructed image, a signal 
level for an image may decrease locally, a signal-to-noise ratio for a 
whole image may deteriorate, or any other phenomenon may occur. This 
results in markedly deteriorated image quality. This drawback becomes 
outstanding in, especially, imaging with a high spatial resolution in 
which the strength of a gradient pulse must be varied greatly during a 
short switching time, or imaging in which a great many echoes are 
acquired. 
One of the techniques for resolving the drawback of deteriorated image 
quality (first prior art) is described in the aforesaid patent publication 
3; U.S. Pat. No. 5,378,985. Prior to an actual diagnostic scan 
(hereinafter simply a scan) intended to acquire data of an MR image from a 
patient, a scan referred to as a prescan is carried out. In the prescan, 
scanning is performed with the strength of a phase-encoding gradient pulse 
made nil. Each of the resultant echoes is subjected to a one-dimensional 
Fourier transform. Zero-order and first-order components of a resultant 
phase distribution are calculated as correction data, whereby waveforms of 
gradient pulses to be applied during the scan and phases by which rotation 
is made with applications of RF pulses (hereinafter application phases) 
are corrected. The application phase is, as shown in FIG. 3, expressed as 
a phase .phi. of a magnetization M, which is rotated with application of 
an RF pulse, with respect to an axis of coordinates serving as a reference 
on X.sup.1 and Y.sup.1 planes in a rotary coordinate system X.sup.1, 
Y.sup.1, Z.sup.1. 
Incidentally, it has been found that an imaging technique using a pulse 
sequence composed of a plurality of RF refocusing pulses, such as the CPMG 
pulse sequence or phase reversal CP pulse sequence, has another physical 
nature. That is to say, when a transverse magnetization undergoes a phase 
error between applications of an RF excitation pulse and a first RF 
refocusing pulse or a transverse magnetization undergoes a phase error of 
the same magnitude between applications of adjoining RF refocusing pulses 
succeeding the first RF refocusing pulse, each echo (total sum of echo 
components depicted to trace a plurality of paths) is split into two 
groups of echoes. This nature is described in, for example, literature 4 
"Magnetic Resonance in Medicine" (Vol.30, P.251-255, 1993). 
When the RARE or GRASE pulse sequence or any other pulse sequencers used, 
as described previously, since a gradient pulse having substantially the 
same waveform is applied repeatedly, not only the gradient pulse but also 
an eddy magnetic field that has the nature of being proportional in 
strength to a gradient pulse causes a phase error based on the pulse 
repetition pattern. This presumably results in the split into two groups 
of echo components. Consequently, a pattern of phase shifts (regarded as 
phase error components) of echoes and quality deterioration in a 
reconstructed image can be thought approximatively to attribute to the 
cross-interference between two groups of echo components. 
Assuming that a phase error occurring in the transverse magnetization of 
nuclear spins between applications of a RF excitation pulse and a first RF 
refocusing pulse is .DELTA..phi.1, a phase error occurring uniformly in 
the transverse magnetization between applications of adjoining RF 
refocusing pulses succeeding the first RF refocusing pulse is 
.DELTA..phi.2, one of two groups of echo components observed between 
applications of the n-th and n+1-th RF refocusing pulses, which includes 
echo components depicted to trace paths along which the echo components 
are refocused by all RF refocusing pulses, is regarded as a main echo 
component Emain(n), and the other echo group of echo components is 
regarded as a sub-echo component Esub(n). The phases of both the echo 
components Emain(n) and Esub(n) are expressed as follows: 
EQU arg{Emain(n)}=(-1)n.multidot..DELTA..phi.(1)+{1+(-1) .sup.n-1 
}/2.multidot..DELTA..phi.(2) (1) 
EQU arg{Esub(n)}=(-1).sup.n-1 .multidot..DELTA..phi.(1)+{1+(-1) .sup.n 
}/2.multidot..DELTA..phi.(2) (2) 
The echo components of an echo stemming from each point in a scan area have 
the above relationship of coincidence. The phases of (observed) echoes 
each of which is a total sum of the echo components, or the magnitudes of 
phase variances indicating degrees of phase dispersion have the same 
relationship of coincidence. FIG. 4 is a phase diagram for explaining 
graphically a change in magnitude of a phase variance of each echo 
occurring due to a readout magnetic field gradient between applications of 
RF refocusing pulses. The axis of ordinates of the drawing indicates the 
magnitude (relative value) of a phase variance of each echo caused by a 
readout magnetic field gradient. For better depiction of paths in the 
drawing indicating changes of echo components exhibiting the same phase 
variance, a time integral value of a readout magnetic field gradient to be 
applied between an RIF excitation pulse and a first RF refocusing pulse is 
intentionally increased by AA over a normal value A (which is a half of a 
pulse length B in the drawing). As described in the aforesaid literature 
2, a transverse magnetization component (i.e., echo component) exhibiting 
a certain phase variance is known to be split into three components; a 
transverse magnetization component whose phase variance is reversed, a 
transverse magnetization component whose phase variance is not reversed, 
and a component preserved as a longitudinal magnetization by an RF 
refocusing pulse. An application spacing of succeeding RF refocusing 
pulses and waveforms of applied magnetic field gradients are so regular 
that echo components depicted to trace several paths on the phase diagram 
are superposed on one another because of an equal magnitude of a phase 
variance. As illustrated, a regular pattern of variances is created. In 
the drawing, bold lines indicate a group of echo components that are in 
phase with the main echo component Emain. Thin lines indicate a group of 
echoes that are in phase with the sub-echo component Esub. During an echo 
acquisition period, most of the echo components whose magnitudes of phase 
variances are spreading greatly in a positive or negative direction are in 
a so-called spoiled state in which the echo components cannot be observed 
as an echo because of the phase interference among spins associated with 
echo components of the group to which the echo components belong. 
Consequently, a portion of echo components actually acquired as an echo is 
an area indicated by an arrow in the drawing (components exhibiting a 
phase variance of nil during data acquisition). 
On the other hand, when the phase of an echo is discussed, since the phases 
of echoes and the application phases for RF pulses each have the 
relationship of coincidence, assuming that the conditions of application 
phases for the CPMG pulse sequence or phase reversal CP pulse sequence are 
met and there is not an error between phases of spins deriving from 
imperfect hardware, the phases of the main echo component Emain(n) and 
sub-echo component Esub(n) coincide with each other. An actually-acquired 
echo is a sum of both the echo components and expressed as follows: 
E(n)=Emain(n)+Esub(n) 
The .DELTA.A value in FIG. 4 is sufficiently small even in an unadjusted 
state, the peaks of the two echo components are often seen coincident with 
each other. 
Incidentally, it has been verified that the amplitudes of both the echo 
components Emain(n) and Esub(n) are dependent on the number of RF 
refocusing pulses n, the slice-dependent characteristic of an RF pulse, 
and a flip angle (Refer to, for example, literature 5 and literature 6 
"Society of Magnetic Resonance in Medicine, Pro. of Annual Meeting in 1992 
No. 4508 and in 1991 No. 1025"). 
As long as the ratio R(n) of the amplitude of the echo component Emain(n) 
to that of Esub(n) is unknown, a phase error cannot be detected in an 
acquired echo. 
Under these circumstances, another prior art reference (second prior art) 
for resolving the aforesaid drawback has been disclosed in relation to a 
magnetic resonance imaging method and system in patent publication 2; 
Japanese Patent Laid-Open No. 6-54827. Correction of the disclosure is 
expressed using the same symbols as those used above as follows: 
Esub(1)=0 
When the flip angle caused by a first RF refocusing pulse is 180.degree., 
Esub(2)=almost zero 
Consequently, 
E(1)=Emain(1), E(2)=Emain(2) 
arg{E(2)}-arg{E(1)}=2.multidot..DELTA..phi.(1)-A.phi.(2). 
Using this expression, waveforms of magnetic field gradients and 
application phases for RF pulses, which are set for a scan, are corrected 
on the basis of only the information given by a first echo and second echo 
acquired during a prescan that is performed in the same manner as the 
aforesaid one. An attempt is thus made to achieve correction relatively 
simply. 
The second prior art will be described in conjunction with FIGS. 5 to 7. 
FIG. 5 shows a pulse sequence used for a prescan. FIG. 6 shows a pulse 
sequence used for a scan. FIG. 7 is a flowchart describing correction. 
As described in FIG. 7, first, a prescan is executed according to a pulse 
sequence shown in FIG. 5 (step 101). During the prescan, a phase-encoding 
magnetic field gradient is always nil (Gpe=O). If dephasing occurs for 
some reasons, as illustrated, each echo except a first echo E(1) is split 
into two echo components Emain(n) and Esub(n). In reality, a shift between 
two echo components Emain(n) and Esub(n) is so small that the peak of an 
echo is not halved. A displacement p(n) and phase shift .phi.(n) of an 
echo peak vary alternately echo by echo as if echoes vibrated. According 
to the second prior art, displacements p(1) and p(2) of echo peaks and 
phase shifts .phi.(1) and .phi.(2) of first and second echoes E(1) and 
E(2) are measured (step 102). Assuming that a distance of a selected slice 
plane from a positional center of a magnetic field is xs, a quantity of 
correcting an application phase for RF pulses, .DELTA..phi., (or 
.DELTA.Gss*xs when a quantity .DELTA.Gss of correcting a slice-selective 
gradient pulse Gss is taken into consideration) is proportional to a 
difference between the phase shifts .phi.(1) and .phi.(2). A .DELTA.Gro 
value is proportional to a difference between the displacements p(1) and 
p(2). Based on this relationship, the measured p(1), p(2), .DELTA.(1), and 
.DELTA.(2) values are used to calculate either the quantity .DELTA.Gro of 
correcting an application phase for RF pulses or the quantity .DELTA.Gss 
of correcting a slice-selective gradient pulse Gss, and the quantity 
.DELTA.Gro of correcting a readout gradient pulse Gro (step 103). 
Thereafter, the pulse sequence is corrected by these quantities of 
correction .DELTA..phi.(or .DELTA.Gss) and .DELTA.Gro (See FIG. 6). A scan 
is then executed according to the resultant pulse sequence (step 104). 
The aforesaid first and second prior art references attempt to resolve the 
drawback of artifacts occurring in a phase-encoding direction attributable 
to phase shifts of respective echoes exhibiting different spatial 
distributions. There are still the following problems that must be solved: 
(1) First, there is a problem that if the flip angle caused by an RF 
refocusing pulse is deviated from 180.degree., precision in correcting 
application phases for RF pulses and waveforms of magnetic field gradients 
for successful execution of a scan decreases. 
For example, when Emain(n) equals Esub(n), however large a phase difference 
between the two echo components Emain(n) and Esub(n) is, the phase of an 
echo, that is, a sum of vectors representing the two echo components 
Emain(n) and Esub(n) and that is acquired as the n-th echo, does not vary. 
For this reason, whichever of the techniques of the first and second prior 
art references is implemented, correction cannot be achieved properly. The 
quality of MR images does not improve at all, or the improvement is 
insufficient. 
The situation that a flip angle deviates from or is intentionally deviated 
from 180.degree. is not at all special. For example, as described even in 
the aforesaid literature 5 "Society of Magnetic Resonance in Medicine, 
Pro. of Annual Meeting in 1992 No. 4508," it has been reported that even 
when the existing fast SE (FSE) imaging is employed, the echo components 
Emain(n) and sub-echo component Esub(n) of any of the fourth to eighth 
echoes have substantially the same amplitude. Moreover, literature 3 
"Magnetic Resonance in Medicine" (Vol.30, P. 183-191, 1993) describes that 
slice-dependent characteristics of RF refocusing pulses and flip angles 
caused by them are made different from one another in order to stabilize 
the amplitudes of echoes. Furthermore, an attempt of using this art to 
reduce the decay of each echo in a transverse relaxation time T2 has been 
reported (Refer to "Society of Magnetic Resonance, Pro. of 2nd Annual 
Meeting in 1994 No. 27"). Furthermore, there is a report saying that as 
far as multiple slice imaging is concerned, when the flip angle to be 
caused by an RF refocusing pulse is set to a value of 180.degree. or 
smaller, an SAR (RF exposure) can be diminished and a change in tissue 
contrast deriving from an MTC effect can be reduced (Refer to "Society of 
Magnetic Resonance in Medicine, Pro. of Annual Meeting in 1993 No.1244"). 
As described above, the flip angle caused by an RF refocusing pulse often 
deviates from 180.degree.. Even if correction in accordance with the 
aforesaid first or second prior art references is performed under these 
circumstances, correction is achieved insufficiently in terms of precision 
and stability. 
This problem has been pointed out in the aforesaid patent publication 2; 
Japanese Patent Laid-Open No. 6-54827 (Refer to the 18th to 27th lines in 
page 11). The patent publication provides a method for avoiding the 
problem by dephasing one echo component using a slice-selective magnetic 
field gradient. However, there is a problem that since the waveforms of 
gradient pulses are deformed, the method is prone to the influence of 
imperfect calibration of a system. The method is therefore often 
unsuitable for automation of sequence adjustment. 
(2) The second problem is that any existing method can correct phase shifts 
attributable to only some of a plurality of factors but cannot correct 
phase shifts attributable to all of them. Even in the aforesaid first or 
second prior art references, a phase-encoding gradient pulse Gpe to be 
applied during a prescan is always nil. Echoes given by the prescan do not 
contain a dephased component attributable to a phase-encoding gradient 
pulse to be applied during a scan. In other words, a) a phase shift of a 
phase-encoding gradient pulse itself and b) a phase shift exhibiting 
zero-order or first- or higher-order spatial distribution and deriving 
from an eddy magnetic field induced by a phase-encoding gradient pulse 
cannot be corrected from the viewpoint of principles. The advancement of a 
spatial resolution for a scan and the speedup of the scan are progressing. 
For some kinds of pulse sequences, a dephased component attributable to a 
phase-encoding magnetic field gradient cannot therefore be ignored at 
present. This restriction on correction must be lifted. 
(3) The third problem is that the precision of a correction value varies 
depending on the state of an object of scanning. In the aforesaid first or 
second prior art references, correction information is acquired on the 
basis of echoes stemming from an object of scanning. An accurate phase 
error (i.e., phase shift) cannot be measured depending on the shape or 
state of the object of scanning. This poses a problem in terms of 
correction stability. 
Taking a diagnostic system for medical use for instance, a main signal 
source providing a T2-weighted image of an axial plane of the cervical 
vertebrae is cerebral spinal fluid (CSF) flowing in a direction 
substantially perpendicular to a section. A change occurs in the phases of 
nuclear spins, which have influenced a flow velocity, due to gradient 
pulses employed in a pulse sequence. When an attempt is made to correct 
the phase change according to a technique provided by the first or second 
prior art references, since the phase change emerges substantially in its 
entirety during a prescan, quantities of correction given by the prescan 
are devoid of accuracy and low in reliability. An intended phase change 
may be mistakenly corrected. Thus, the technique is low in correction 
precision and poor in stability. Consequently, artifacts may appear in an 
image of a region in which nuclear spins are still or a signal level for 
the image may decrease. This eventually leads to degradation of a 
signal-to-noise ratio or other drawback. Moreover, when a region producing 
high-level echoes makes a violent motion, image quality may deteriorate. 
This may be unavoidable to some extent for a correction technique 
characterized in that correction information (quantities of correction) 
used to correct a pulse sequence to be employed in a scan is acquired in 
advance from an object of scanning whose shape or state is unknown. 
However, it poses a problem in terms of clinical diagnosis. 
SUMMARY OF THE INVENTION 
The present invention attempts to solve the aforesaid drawbacks and 
problems. 
(1) The first object of the present invention is to realize magnetic 
resonance imaging that preconditions execution of the process for 
calculating quantities of correction needed to correct a pulse sequence to 
be employed in a scan by executing a prescan, and to provide an automatic 
correction method for an MRI pulse sequence and an MRI system which can 
execute a prescan in a manner that is more durable to a variation of flip 
angle caused by an RF refocusing pulse and more stable than a conventional 
manner, can correct a pulse sequence employed in a scan with higher 
precision, and can suppress deterioration in image quality of 
reconstructed MR images. 
Moreover, (2) the second object of the present invention is to realize 
magnetic resonance imaging that preconditions execution of the process for 
calculating quantities of correction needed to correct a pulse sequence to 
be employed in a scan by executing a prescan, wherein a dephased component 
attributable to a phase-encoding gradient pulse can be corrected with high 
precision and thus the image quality of reconstructed MR images can be 
improved. 
Moreover, (3) the third object of the present invention is to realize 
magnetic resonance imaging that preconditions execution of the process for 
calculating quantities of correction needed to correct a pulse sequence to 
be employed in a scan by executing a prescan, wherein even when a major 
signal source is moving at a high speed, the decrease of the precision in 
correcting a pulse sequence to be employed in a scan can be prevented and 
the deterioration in image quality of reconstructed MR images can be 
suppressed. 
Furthermore, (4) the fourth object of the present invention is to provide 
an MR imaging method and MR system which can accomplish the first to 
third objects at a time without the necessity of correcting a pulse 
sequence to be employed in a scan using correction data given by a 
prescan. 
In short, an object of the present invention is to accomplish at least one 
of the foregoing first to fourth objects, and to realize imaging using a 
pulse sequence composed of a plurality of RF refocusing pulses; such as, a 
CPMG pulse sequence, phase reversal CP pulse sequence, or the like, 
wherein acquisition and correction of necessary correction information are 
performed more systematically than they are conventionally in order to 
improve the precision and stability of correction, or the influence of 
phase error distribution is avoided by means of any approach other than 
the correction, and thus MR images that are of high quality in terms of 
spatial resolution, ghost artifacts, and image contrast are produced. 
In order to accomplish the above objects, according to one aspect of the 
present invention, there is provided a method of correcting in a magnetic 
resonance imaging (MRI) process an imaging pulse sequence magnetically 
applied by an imaging scan to an object to be imaged in order to obtain a 
magnetic resonance (MR) image, the method comprising the steps of: 
performing a first prescan applied to the object with a first preliminary 
pulse sequence including a first RIF excitation pulse and a plurality of 
first RF refocusing pulses each of which has an application phase set to a 
specified phase value, thereby the first prescan provides a first echo 
data group consisting of a plurality of echoes responsive to the plurality 
of first RF refocusing pulses; performing a second prescan applied to the 
object with a second preliminary pulse sequence including a second RF 
excitation pulse and a plurality of second RF refocusing pulses of which 
even-numbered second RF refocusing pulses are given with an application 
phase set to another specified phase value differed by 180.degree. from 
the specified phase value, thereby the second prescan providing a second 
echo data group consisting of a plurality of echoes responsive to the 
plurality of second RF refocusing pulses; and correcting the imaging pulse 
sequence on the basis of the first and second echo data groups prior to 
the imaging scan. 
It is preferred that each of the first and second preliminary pulse 
sequences and the imaging pulse sequence is formed on either one of a CPMG 
pulse train and a phase reversal CP pulse train. 
It is preferred that each of the first and second preliminary pulse 
sequences and the imaging pulse sequence is formed in accordance with a 
GRASE sequence and a fast SE sequence. 
It is also preferred that a phase encode gradient is set to zero in each of 
the first and second preliminary pulse sequences. 
In this case, preferably, the correcting step further comprises the steps 
of: separating and extracting from the first and second echo data groups a 
main ecno data family consisting of a main echo component and a sub-echo 
data family consisting of components other than the main echo component by 
at least either one of addition and subtraction executed between two sets 
of echo data each belonging to a specified echo mapping block common to 
the first and second echo data groups; obtaining correction data on the 
basis of the main echo and sub-echo data families; and correcting the 
imaging pulse sequence using the correction data. 
Still preferably, the correcting step further comprises the steps of: 
separating and extracting from the first and second echo data groups a 
main echo data family consisting of a main echo component and a sub-echo 
data family consisting of components other than the main echo component by 
at least either one of addition and subtraction executed between two sets 
of echo data each belonging to a specified echo mapping block common to 
the first and second echo data groups; computing a phase shift and a 
position displacement of echo peaks formed by echo data sets odd-numbered 
and even-numbered in at least either one of the main echo and sub-echo 
data families; obtaining a correction data on the basis of at least either 
one of the phase shift and the position displacement; and correcting the 
imaging pulse sequence using the correction data. 
Still it is preferred that the correcting step further comprises the steps 
of: separating and extracting from the first and second echo data groups a 
main echo data family consisting of a main echo component and a sub-echo 
data family consisting of components other than the main echo component by 
at least either one of addition and subtraction executed between two sets 
of echo data each belonging to a specified echo mapping block common to 
the first and second echo data groups; comparing with each other either 
one of zero-th order and first-order phase distributions of two sets of 
echo data sampled at least partly from each of the main echo and sub-echo 
data families; obtaining a correction data on the basis of compared 
results in the comparing step; and correcting the imaging pulse sequence 
using the correction data. 
Preferably, the object to be imaged is different when performing the first 
and second prescans and in performing the imaging scan, the object being a 
phantom in performing the first and second prescans. 
Further preferably, the correcting step further comprises the steps of: 
first obtaining inherent phase error information for a combination of 
imaging channels and physical channels of gradient coils on the basis of 
the first and second echo data groups; second obtaining imaging condition 
information associated with gradients; and computing a correction data 
used for correcting the imaging pulse sequence using the inherent phase 
error information and the imaging condition information. 
For example, the specified application phase value is 90.degree. and 
another specified application phase value is 270.degree.. 
According to another aspect of the present invention, there is provided a 
method of magnetic resonance imaging by which an MR echo data is emanated 
from an object to be imaged, the method comprising the steps of: first 
performing a first imaging scan to the object with a first imaging pulse 
sequence including an RF excitation pulse, a plurality of RF refocusing 
pulses each having an application phase set to a first specified phase 
value, and a phase encode gradient phase-encoding in a k-space in which 
the echo data are mapped, thereby the first imaging scan provides a first 
echo data group; second performing a second imaging scan to the object 
with a second imaging pulse sequence including an RF excitation pulse, a 
plurality of RF refocusing pulses of which even-numbered RF refocusing 
pulse each has an application phase set to a second specified phase value 
differed by 180.degree. from the first specified phase value, and a phase 
encode gradient phase-encoding in another k-space, thereby the second 
imaging scan provides a second echo data group; separating and extracting 
from the first and second echo data groups a first k-space data being 
mapped over a plurality of mapping blocks of the k-space and consisting of 
a group of main echo components and a second k-space data being mapped 
over a plurality of mapping blocks of the k-space and consisting of a 
group of echo components other than the main echo components by performing 
addition and subtraction between the first and second echo data groups; 
exchanging to each other and block by block echo data positioned at either 
one of an even-numbered mapping block of the first and second k-space data 
and an odd-numbered mapping block of the first and second k-space data; 
reconstructing each of the first and second k-space data which has been 
data-exchanged in the exchanging step, thereby two real-space MR images 
are provided; and producing a single real-space MR image from the two 
real-space MR images. 
For example, the exchanging step is composed of a step exchanging block by 
block echo data residing in either of an even-numbered and an odd-numbered 
echo mapping blocks for mapping an echo signal emanating from an interval 
formed between an adjacent two of the plurality of RF refocusing pulses 
applied by each of the first and second imaging scans. 
As another preferred example, the imaging method further comprises the 
steps of: third performing a first prescan to the object with a first 
preliminary pulse sequence including an RF excitation pulse and a 
plurality of RF refocusing pulses each having an application phase set to 
a third specified phase value, thereby the first prescan provides a third 
echo data group; fourth performing a second prescan to the object with a 
second preliminary pulse sequence including an RF excitation pulse and a 
plurality of RF refocusing pulses of which even-numbered RF refocusing 
pulses each has an application phase set to a fourth specified phase value 
differing by 180.degree. from the third specified phase value, thereby the 
second prescan provides a fourth echo data group; second separating and 
extracting first amplitude information of a first group consisting of a 
main echo component and second amplitude information of a second group 
consisting of an echo component other than the main echo component by 
performing addition and subtraction between the third and fourth echo data 
groups; and correcting amplitudes of each of the first and second k-space 
data using each of the first and second amplitude information, the 
amplitude correcting step being interposed between the k-space data 
reconstructing step and the single real-space MR image producing step. 
Preferably, the single real-space MR image producing step includes a step 
for equalizing phases of each pair of pixel signals residing at the same 
positions common to two sample areas selected from entire areas of the two 
real-space MR images, respectively, with absolute values of the pixel 
signals unchanged. 
As still another aspect of the present invention, provided is a system for 
magnetic resonance imaging (MRI) in which an imaging pulse sequence is 
magnetically applied to an object to be imaged by an imaging scan, the 
system comprising: first means for performing a first prescan to the 
object with a first preliminary pulse sequence including an RIF excitation 
pulse and a plurality of RIF refocusing pulses each having an application 
phase set to a first specified value, thereby the first prescan provides a 
first echo data group; second means for performing a second prescan to the 
object with a second preliminary pulse sequence including an RF excitation 
pulse and a plurality of RIF refocusing pulses of which even-numbered RIF 
refocusing pulses have an application phase set to a second specified 
value differing by 1800 from the first specified value, thereby the second 
prescan provides a second echo data group; and means for correcting the 
imaging pulse sequence on the basis of the first and second echo data 
groups prior to the imaging scan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Various embodiments of the present invention will be described below with 
reference to the drawings. The relationship of correspondence between the 
embodiments to be described in detail below and the first to fourth 
objects of the present invention will be described. 
A first embodiment attempts to accomplish the first object. A second and 
third embodiments attempt to accomplish the second object. A fourth 
embodiment also attempts to accomplish the second object. A fifth and 
sixth embodiments attempt to accomplish the third object. A seventh 
embodiment attempts to accomplish the fourth object. 
(First Embodiment) 
The first embodiment of the present invention will be described in 
conjunction with FIGS. 8 to 15. 
FIG. 8 shows the outline configuration of a magnetic resonance imaging 
(MRI) system in accordance with this embodiment and the other embodiments 
described later. The MRI system comprises a magnet unit for generating a 
static magnetic field, a magnetic field gradient unit for appending 
position information to the static magnetic field, a transmitting and 
receiving unit for receiving selective excitation and MR signals, and a 
control and arithmetic unit responsible for system control and image 
reconstruction. 
The magnet unit includes a magnet 1 that is, for example, of a 
superconducting type, and a static power supply 2 for supplying a current 
to the magnet 1, and generates a static magnetic field H.sub.o in a Z-axis 
direction in a cylindrical diagnostic space into which a patient P or an 
object of scanning is inserted. 
The magnetic field gradient unit includes three sets of gradient coils 3x 
to 3z that are incorporated in the magnet 1 and designed to produce 
magnetic field gradients that change in strength in X-axis, Y-axis, and 
Z-axis directions, a gradient power supply 4 for supplying a current to 
the gradient coils 3x to 3z, and a gradient sequencer 5a for controlling 
the power supply 4. The gradient sequencer 5a includes a computer. When 
receiving a signal commanding an acquisition pulse sequence used to 
execute a prescan and a scan; such as, an FSE or GRASE pulse sequence from 
a controller 5 responsible for control of the entire system (including a 
computer), the gradient sequencer 5a executes processing described in FIG. 
9 in response to the command. The gradient sequencer 5a thus controls the 
applications and strengths of magnetic field gradients that change in 
strength in the X-axis, Y-axis, and Z-axis directions according to the 
commanded pulse sequence, so that the magnetic field gradients can be 
superposed on the static magnetic field H.sub.o. In this embodiment, a 
magnetic field gradient changing in strength in the Z-axis direction of 
three axial directions that are orthogonal to one another is regarded as a 
slice-selective magnetic field gradient G.sub.S. A magnetic field gradient 
changing in the X-axis direction is regarded as a readout magnetic field 
gradient G.sub.R. A magnetic field gradient changing in the Y-axis 
direction is regarded as a phase-encoding magnetic field gradient G.sub.E. 
The transmitting and receiving unit includes an RF coil 7 placed in the 
vicinity of the patient P in the scanning space within the magnet 1, a 
transmitter 8T and receiver 8R connected to the RF coil 7, and an RF 
sequencer 5b (including a computer) for controlling the timing of 
operating the transmitter 8T and receiver 8R. The RF sequencer 5b and 
gradient sequencer 5a constitute a sequencer 5. The RF sequencer 5b can 
command application of an RF pulse in a state synchronous with the 
gradient sequencer 5a. The transmitter 8T and receiver 8R supply a 
pulsating RF current with a Larmor frequency at which nuclear magnetic 
resonance (NMR) can be excited under the control of the RF sequencer 5b. 
Besides, the transmitter 8T and receiver 8R perform various kinds of 
signal processing on an MR signal (RF signal) received by the RF coil 7 to 
produce an echo. 
Furthermore, the control and arithmetic unit includes the controller 6 as 
well as a multiplexer 11 for receiving an echo that is a digital quantity 
produced by the receiver 8R. The multiplexer 11 switches over its output 
path selectively to the controller or reconstruction unit in response to a 
control signal sent from the controller 6. Furthermore, a reconstruction 
unit 12 for reconstructing an image by performing a Fourier transform, a 
memory unit 13 for storing reconstructed image data, a display unit 14 for 
displaying an image, and an input unit 15 are located on one output stage 
of the multiplexer 11. The controller 6 includes, as mentioned above, a 
computer so as to control the contents and timing of operations of the 
entire system. 
Next, the operations of this embodiment will be described. Herein, a FSE 
(fast SE) pulse sequence is adopted. 
The controller 6 executes the processing described in FIG. 9. This 
processing includes two kinds of prescans A and B. First, the controller 6 
switches over the switch path of the multiplexer 11 to the controller 6, 
allows the sequencer 5 to execute prescan A according to a pulse sequence 
shown in FIG. 10, and inputs resultant echoes (step S1 in FIG. 9). 
During prescan A, first, an RF excitation pulse RFex (application phase 
.phi.=0.degree., flip angle .theta.=a (90.degree. in this example)) is 
applied together with a slice-selective gradient pulse Gss. In a time 
.tau./2, a first RF refocusing pulse RFre1 (application phase 
.phi.=90.degree., flip angle .theta.=.beta.1 (180.degree. in this example) 
is applied together with a slice-selective gradient pulse Gss. In a time 
.tau. after the application of the first RF excitation pulse RFex, an echo 
E(1) is read out along with application of a readout gradient pulse Gro. 
Thereafter, second and subsequent RF refocusing pulses RFre2, RFre3, etc. 
are applied together with a slice-selective gradient pulse Gss at 
intervals of a time .tau. after the application of the first RF refocusing 
pulse RFrel. Echoes E(2), E(3), etc. are read out in the same manner. In 
the prescan A, as illustrated, a phase-encoding gradient pulse Gpe is 
always nil. 
In prescan A, if nuclear spins are dephased for some reasons, as mentioned 
above, each of the echoes E(2), E(3), etc. except the first echo E(1) is 
split into two echo components; that is, a main echo component Emain(2) 
(Emain(3), etc.) and a sub-echo component Esub(2) (Esub(3), etc.) (See 
FIG. 10). When there is no cause of dephasing, the phase arg{Emain(n)} of 
the main echo component Emain(n) and the phase arg{Esub(n)} of the 
sub-echo component Esub(n) are coincident with each other (n=2, 3, etc.). 
The controller 6 then allows the sequencer 5 to execute the second prescan 
B shown in FIG. 11 and inputs resultant echoes (step S2 in FIG. 9). 
Even during prescan B, first, an RF excitation pulse RFex (application 
phase .phi.=0.degree., flip angle .theta.=a (90.degree. in this example) 
is applied together with a slice-selective gradient pulse Gss. In a time 
.tau./2 an RF refocusing pulse RFrel (application phase .phi.=90.degree., 
flip angle .theta.=a (180.degree. in this example) is applied together 
with a slice-selective gradient pulse Gss. In a time .tau. after the 
application of the first RF excitation pulse RFex, an echo E(1) is read 
out along with application of a readout gradient pulse Gro. Thereafter, 
second and subsequent RF refocusing pulses RFre2, RFre3, etc. are applied 
together with a slice-selective gradient pulse Gss at intervals of a time 
.tau. after the application of the first RF refocusing pulse RFrel. Echoes 
E(2), E(3), etc. are then read out in the same manner. 
In the prescan B, the application phase .phi. for even-numbered RF 
refocusing pulses RFre2, RFre4, etc. are set to a value 
(.theta.=270.degree.) that is advanced by an extra 180.degree. from the 
application phase for odd-numbered RF refocusing pulses RFre3, RFre5, etc. 
Incidentally, a phase-encoding gradient pulse Gpe is always nil in the 
same manner as that in the prescan A. 
If nuclear spins are dephased for some reasons, each of the echoes E(2), 
E(3), etc. except the first echo E(1) is split into two echo components; 
that is, a main echo component Emain(2) (Emain(3), etc.) and a sub-echo 
component Esub(2) (Esub(3), etc.) in the same manner as mentioned above 
(See FIG. 10). Moreover, since the application phase for the even-numbered 
RF refocusing pulses RFre2, RFre4, etc. are advanced by extra 180.degree., 
the phases arg{Esub(n)} of only the sub-echo components Esub(2), Esub(3), 
etc. are advanced by 180.degree. from the corresponding phases attained in 
prescan A. 
This reason will be described in conjunction with FIG. 15. As illustrated, 
assuming that the phase of the n-th echo is .theta..sub.n, and the phases 
of the n+1-th and n+2-th RF refocusing phases are .phi..sub.n+1 and 
.phi..sub.n+2 respectively, the phase .theta..sub.n+2,SE and phase 
.theta..sub.n+2,STE of a spin echo (SE) of stimulated echo (STE), which 
are induced by the n+2-th RF refocusing pulse, are expressed as follows: 
EQU .theta..sub.n+2, SE =2.phi..sub.n+2 -.phi..sub.n+1 +.theta..sub.n 
EQU .theta..sub.n+2,STE =.phi..sub.n+2 +.phi..sub.n+1 -.theta..sub.n (*) 
In the example of FIG. 10, .phi..sub.n equals 90.degree., .theta..sub.n 
equals 90.degree., .theta..sub.n+2,SE equals 90.degree., and 
.theta..sub.n+2,STE equals 90.degree., where n denotes an integer of 1 or 
larger. The phases of the components Emain(n) and Esub(n) each of which is 
regarded as a synthesis of a plurality of echo paths on a phase diagram 
are therefore equal to each other and set to 90.degree.. 
In the example of FIG. 11, n is thought to assume two different values. 
Assume that n equals 2m (where m denotes an integer of 1 or larger). 
.phi..sub.n+2 =.phi..sub.EVEN =270.degree., .phi..sub.n+1 =.phi..sub.ODD 
=90.degree. 
.phi..sub.n+2,SE =.theta..sub.EVEN,SE =90.degree. 
.theta..sub.n+2,STE =.theta..sub.EVEN,STE =270.degree. 
Assume that n equals to 2m+1 (where m denotes an integer of 1 or larger). 
.phi..sub.n+2 =.phi..sub.ODD =90.degree., .phi..sub.n+1 =.phi..sub.EVEN 
=270.degree. 
.theta..sub.n+2,SE =.theta..sub.ODD,SE =90.degree. 
.theta..sub.n+2,STE =.theta..sub.ODD,STE =270.degree. 
The stimulated echo component stemming from the n-th main echo component 
with the phases .phi..sub.n+1 and .phi..sub.n+2 set is expressed as 
Esub(n+2). As mentioned above, the phase of a Sub-echo component Esub(n) 
shown in FIG. 11 is 270.degree., where n denotes any integer of 2 or 
larger. That is to say, the sub-echo component is reversed 180.degree. 
from the corresponding one shown in FIG. 10. 
FIG. 15 shows only the relationship of coincidence between each pair of 
phases of echo components of echoes each occurring between applications of 
adjoining RF refocusing pulses which are depicted to trace a limited 
number of paths on a phase diagram. Since the relationship of coincidence 
between phases of echo components of an echo has the regularity shown in 
FIG. 4, the phases of sub-echo components produced during prescan B are 
advanced by 180.degree. from those of corresponding ones produced during 
prescan A. 
In this embodiment, a combination of application phases set in the CPMG 
pulse sequence is used as a reference for correction. The present 
invention is not limited to this combination. As long as the foregoing 
expression (*) is satisfied and either the main echo component Emain(n) or 
sub-echo component Esub(n) has a 180.degree. phase difference between two 
prescans, any combination of application phases will do. 
Returning to FIG. 2, description will proceed. Assuming that an echo given 
by prescan A is Ea(n) and an echo given by prescan B is Eb(n) (n=2, 3, 
etc.), the following relationships are established: 
EQU Ea(n)=Emain(n)+Esub(n) (3) 
EQU Eb(n)=Emain(n)-Esub(n) (4) 
The controller 6 executes an averaging of echoes given by prescans A and B 
(step S3 in FIG. 9). That is to say, the following calculation is 
performed: 
{Ea(n)+Eb(n)}/ 2 
where n equals 1, 2, etc. The concept of the averaging is shown graphically 
in FIGS. 12A and 12B and 13. As shown in these drawings, each pair of 
sub-echo components Esub(2), Esub(3), etc. of the second and subsequent 
echoes E(2), E(3), etc. given by two prescans A and B are 180.degree. out 
of phase mutually and therefore canceled out. Only main echo components 
Emain(n) (n=1, 2, etc.) are sampled. 
FIG. 13(b) graphically shows sampling of only sub-echo components Esub(n) 
(n=2, 3, etc.) as a result of the following calculation: 
{Ea(n)-Eb(n)}/ 2 
The controller 6 computes authentic phase shifts (.phi.main(1) and 
.phi.main(2) and displacements pmain(1) and pmain(2) of peaks of main echo 
components of first and second echoes (step S4 in FIG. 9). 
Next, similar to the prior art, a quantity .DELTA..phi. for correcting an 
application phase .phi. for RF pulses RF, or a quantity .DELTA.Gss for 
correcting a slice-selective gradient pulse Gss, and a quantity .DELTA.Gro 
for correcting a readout gradient pulse Gro, which are needed for 
execution of a scan, are computed on the basis of values given by the 
following expressions (step S5 in FIG. 9): 
.phi.main(1)-.phi.main(2), and 
pmain(1) pmain(2) 
Finally, the controller 6 switches over the switch path of the multiplexer 
11 to the reconstruction unit 12 and allows the sequencer 5 to execute a 
scan according to a pulse sequence shown in FIG. 14 (step S6 in FIG. 9). 
For the scan, as illustrated, a pulse sequence in which the results of the 
above computation are reflected is employed. Specifically, the application 
phase for an RF excitation pulse RFex to be applied together with a 
slice-selective gradient pulse Gss is equal to .DELTA..phi., or the 
strength of a slice-selective gradient pulse Gss, which is applied 
succeedingly to the slice-selective gradient pulse Gss, reversed in 
polarity, and used to prevent dephasing, is corrected by .DELTA.Gss. At 
the same time, the strength of a readout gradient pulse Gro to be applied 
in parallel with the slice-selective gradient pulse Gss used to prevent 
dephasing is corrected by .DELTA.Gro. 
In a time .tau./2 after the first excitation, a first RF refocusing pulse 
RFrel (application phase .phi.=90.degree., flip angle .phi.=B1 
(180.degree. in this example) is applied together with a slice-selective 
gradient pulse Gss. This results in a refocusing of spins. A 
phase-encoding gradient pulse Gpe whose strength (pulse area) is adjusted 
at every implementation of this pulse sequence is then applied. A readout 
gradient pulse Gro is then applied with the passage of a time .tau. after 
a first selective excitation. A first echo E(1) is acquired in the 
meantime. 
Thereafter, RF refocusing pulses RFre2, RFre3, etc. (application phase 
.phi.=90.degree., flip angle .phi.=B2, B3, etc. (180.degree. in this 
example)) are applied consecutively together with a slice-selective 
gradient pulse Gss. In the meantime, echoes E(2), E(3), etc. are acquired 
consecutively as a result of refocusing of spins. In FIG. 14, after 
acquisition of each echo, a phase-encoding gradient pulse Gpe that is 
reversed in polarity is applied as a rewinding pulse. With this 
application, occurrence of stimulated echoes can be prevented during a 
scan. 
Echoes E(n) acquired by executing the scan are received consecutively by 
the receiver 8R, and then converted into echo data. This processing 
includes orthogonal detection and A/D conversion which are performed on 
the echoes E(n). The echo data are arranged in a memory defining a k-space 
(Fourier space) in order of quantity of encoding by means of the 
reconstruction unit 12. The resultant echo data is subjected to a Fourier 
transform and thus reconstructed into an MR image. 
As mentioned above, in this embodiment, sub-echo components of second and 
subsequent echoes (as far as the second echo is concerned, stimulated echo 
component), which cannot be removed by any existing technique, can be 
removed almost perfectly. Main echo components alone can be sampled 
properly. This brings about the advantages that the precision in 
correcting a pulse sequence employed in a scan improves markedly, and that 
the scan can be executed in a stable manner. 
A procedure for calculating the quantities of correction .DELTA..phi., 
.DELTA.Gss, and .DELTA.Gro in this embodiment is not limited to the 
foregoing one. Alternatively, as apparent from the expression (1), the 
phase differences between even-numbered main echo components and 
odd-numbered main echo components of the third and subsequent echoes 
rather than the first and second echoes, and displacements of echo peaks 
may be used for calculation of the quantities of correction .DELTA..phi., 
.DELTA.Gss, and .DELTA.Gro. Moreover, a one-dimensional Fourier transform 
may be performed in a kr direction on echo data given by prescans in order 
to depict a curve showing distribution of phases in a readout direction. 
In this case, the slope and intercept of the curve; that is, first-order 
and zero-order components of a phase indicated by the phase distribution 
curve in the readout direction, are calculated; and then a displacement 
and phase shift of an echo peak is calculated on the basis of the 
first-order and zero-order components. 
(Second Embodiment) 
The second embodiment of the present invention will be described in 
conjunction with FIGS. 16 to 20. In this embodiment and subsequent 
embodiments, component elements identical to or equivalent to those in the 
first embodiment will be assigned the same reference numerals. The 
description of the component elements will be omitted or briefed. 
An MRI system in accordance with this embodiment is identical to the 
aforesaid one. The controller 6 executes a series of operations described 
in FIG. 16. 
First, the controller 6 switches over the switch path of the multiplexer 11 
to the controller 6, commands the sequencer 5 to successively execute two 
kinds of prescans C and D according to FSE imaging, and inputs resultant 
echoes (steps S11 and S12 in FIG. 16). A pulse sequence used for the first 
prescan C is shown in FIG. 17, and a pulse sequence used f or the 
succeeding prescan D is shown in FIG. 18. 
During the first prescan C, a plurality of RF refocusing pulses RFre1, 
RFre2, etc, (application phase .phi.=90.degree., flip angle .phi.=B1, B2, 
etc. (180.degree. in this example)) are applied consecutively together 
with a slice-selective gradient pulse Gss succeedingly to one RF 
excitation pulse RFex (application phase .phi.=0.degree., flip angle 
.phi.=a(90.degree. in this example)). Echoes Ec(n) are then acquired along 
with application of a readout gradient pulse Gro. A phase-encoding 
gradient pulse Gpe is set to exhibit the same strength change as that for 
a shot intended to acquire data corresponding to a data line in a k-space 
containing the center of the k-space during a scan. As a result. RF pulses 
and gradient pulses employed in prescan C are set to have the same 
waveforms as those for a shot intended to acquire the data corresponding 
to a data line containing the center of k-space during the scan (this 
pulse sequence is different in this point from the pulse sequence employed 
in a prescan in accordance with the prior art and the first embodiment 
alike). 
If nuclear spins are dephased for some reason, each of echoes E(2), E(3), 
etc. except the first echo E(1) is split into two echo components; that 
is, a main echo component Emain(2) (Emain(3), etc.) and a sub-echo 
component Esub(2) (Esub(3), etc.) in the same manner as the aforesaid one 
(See FIG. 17). 
During the succeeding prescan D, a plurality of RF refocusing pulses RFre1, 
REre2, etc. (application phase .phi.=90.degree. or 270.degree., flip angle 
.phi.=B1, B2, etc. (180.degree. in this example) are applied consecutively 
together with a slice-selective gradient pulse Gss succeedingly to one RF 
excitation pulse RFex (application phase 0.degree., flip angle 
.phi.=a(90.degree. in this example)). Echoes Ed(n) are then acquired along 
with the application of a readout gradient pulse Gro. A phase-encoding 
gradient pulse Gpe is set to exhibit the same strength change as that for 
a shot intended to acquire data corresponding to a data line in a k-space 
containing the center of the k-space during a scan. As a result, gradient 
pulses employed in prescan D are set to have the same waveforms as those 
for the shot intended to acquire data corresponding to the data line 
containing the center of k-space during a scan (this pulse sequence is 
different in this point from the one employed in a prescan in accordance 
with the prior art and the first embodiment). However, as for RF pulses, 
similarly to prescan B in the aforesaid first embodiment, an application 
phase for even-numbered RF refocusing pulses RFre2, RFre4, etc. is further 
advanced by 180.degree. and set to 270.degree.. 
If nuclear spins are dephased for some reason, each of echoes E(2), E(3), 
etc. except the first echo E(1) is split into two echo components; that 
is, a main echo component Emain(2) (Emain(3), etc.) and a sub-echo 
component Esub(2) (Esub(3), etc.) (See FIG. 18). Besides, the application 
phase .phi. for even-numbered RF refocusing pulses RFre2, RFre4, etc. is 
advanced by an extra 180.degree.. The phases arg{Esub(n)j of the 
even-numbered sub-echo components Esub(2), Esub(4), etc. are therefore 
advanced by 180.degree. from the corresponding ones attained in prescan C. 
Assume that an echo given by prescan C is Ec(n) and an echo given by 
prescan D is Ed(n). Phase encoding is performed on echoes except an echo 
produced in an effective echo time during each of the prescans; that is, 
an n.sub.eff -th echo E(n.sub.eff) (in the example of FIG. 17 or 18, the 
third echo E(3)). Echoes usable for calculating quantities of correction 
needed for execution of a scan are only the echoes produced in the 
effective echo time; that is, the third echoes Ec(3) and Ed(3) 
The controller 6 then computes a main echo component Emain(3) and a 
sub-echo component Esub(3) of the third echo E(3) corresponding to the 
n.sub.eff -th echo E(n.sub.eff) by calculating the following expressions 
(step S13 in FIG. 16): 
EQU Emain (n.sub.eff)={Ec(n.sub.eff)+Ed(.sub.neff)}/2 (5) 
EQU Esub (n.sub.eff){Ec(n.sub.eff)-Ed(n.sub.eff)}/2 (6) 
that is, 
EQU Emain (3)={Ec(3)+Ed(3)}/2 (5)' 
EQU Esub (3)={Ec(3)-Ed(3)}/2 (6)' 
The main echo component Emain(3) and sub-echo component Esub(3) given by 
the computation are shown in FIG. 19. In this embodiment, the phase of the 
sub-echo component Esub(3) of the third echo E(3) corresponding to the 
n.sub.eff -th echo (E(n.sub.eff) is different by 180.degree. between two 
prescans. Therefore, as mentioned above, the main echo component Emain(3) 
is computed by performing additive averaging, that is, division of a sum 
by 2, and the sub-echo component Esub(3) is computed by performing 
subtractive averaging, that is, division of a difference by 2. 
Furthermore, the controller 6 computes a phase shift .phi.main(3) and 
displacement pmain(3) of the peak of the main echo component of the third 
echo E(3) and a phase shift .phi.sub(3) and displacement psub(3) of the 
peak of the sub-echo component thereof on the basis of the above echo data 
(step S14 in FIG. 16). 
Next, similarly to the known techniques, based on a phase difference 
between the main echo component and sub-echo component of the n-th echo, 
which is provided as follows: 
.phi.main(n.sub.eff)-psub(n.sub.eff) and a difference between peak 
positions which is provided as follows: 
pmain(n.sub.eff)-psub(n.sub.eff) that is: 
.phi.main(3)-.phi.sub(3) 
pmain(3)-psub(3), 
a quantity .DELTA..phi. for correcting an application phase .phi. for RF 
pulses RF or a quantity .DELTA.Gss for correcting a slice-selective 
gradient pulse Gss and a quantity .DELTA.Gro for correcting a readout 
gradient pulse Gro are computed (step S15 in FIG. 16). Since the polarity 
of a result of the corrective computation is reversed depending on whether 
the n.sub.eff -th numeral is odd or even, the order of subtraction should 
be made uniform in order to attain the same polarity all the time. 
Finally, the controller 6 switches over the switch path of the multiplexer 
11 to the reconstruction unit 12 and allows the sequencer 5 to execute a 
scan according to a pulse sequence shown in FIG. 19 (step S16 in FIG. 16). 
For the scan, as illustrated, a pulse sequence in which the results of the 
computation are reflected is employed. Specifically, the application phase 
.phi. for an RF excitation pulse RFex (flip angle .phi.=a(90.degree. in 
this example) to be applied first together with a slice-selective gradient 
pulse Gss equal to .DELTA..phi., or the strength of a slice-selective 
gradient pulse Gss to be applied succeedingly to the slice-selective 
gradient pulse Gss, reversed in phase, and used to prevent dephasing is a 
value corrected by .DELTA.Gss. At the same time, the strength of a readout 
gradient pulse Gro to be applied in parallel with the slice-selective 
gradient pulse Gss used to prevent dephasing is a value corrected by 
.DELTA.Gro. 
Thereafter, similarly to the aforesaid pulse sequence in FIG. 14, a 
plurality of RF refocusing pulses RFrel, RFre2, etc. are used to 
consecutively acquire echoes E(1), E(2), etc. produced as a result of 
refocusing of nuclear spins. The echoes (n) acquired by executing the scan 
are consecutively input as echo data to the reconstruction unit 12 via the 
receiver 8R, and reconstructed into an MR image through a Fourier 
transform. 
As mentioned above, according to this embodiment, in addition to the 
advantages of the aforesaid first embodiment, an advantage that a 
spatially-uniform dephased component of dephased components caused by a 
phase-encoding gradient pulse has is that a zero-order component can be 
corrected preferably, and that a precision in correcting a pulse sequence 
used for a scan improves further. 
A procedure for calculating the quantities of correction .DELTA..phi., 
.DELTA.Gss, and .DELTA.Gro in this embodiment is not limited to the 
aforesaid one. Alternatively, a one-dimensional Fourier transform may be 
performed in a kr direction on echo data given by prescans in order to 
depict a curve showing distribution of phases in a readout direction. In 
this case, the slope and intercept of the curve; that is, first-order and 
zero-order components of a phase indicated by the phase distribution curve 
in the readout direction, are calculated. A displacement and phase shift 
of an echo peak are then calculated on the basis of the first-order and 
zero-order components. 
(Third Embodiment) 
The third embodiment will be described in conjunction with FIGS. 21 to 25. 
In this variant, the GRASE imaging is employed on behalf of the FSE 
imaging. The sequencer 5 can command a GRASE pulse sequence for either a 
prescan and a scan in response to a command sent from the controller 6. 
The controller 6 executes a series of operations described in FIG. 21. 
First, the controller 6 switches over the switch path of the multiplexer 11 
to the controller 6, commands the sequencer 5 to successively execute two 
kinds of prescans E and F according to the GRASE imaging, and inputs 
acquired echoes (steps S21 and S22 in FIG. 21). A pulse sequence employed 
in the first prescan E is shown in FIG. 22, and a pulse sequence employed 
in the succeeding prescan F is shown in FIG. 23. 
The pulse sequences employed in two kinds of prescans E and F use one RF 
excitation pulse RFex and a plurality of RF refocusing pulses RFrel, 
RFre2, etc. which are identical to those shown in FIGS. 17 and 18 
concerning the second embodiment. A readout gradient pulse Gro is applied 
with its polarity reversed three times during interpulse times of RF 
refocusing pulses, whereby echoes Ee(m.n) and Ef(m,n) are acquired. At 
this time, each phase-encoding gradient pulse Gpe is set to exhibit the 
same strength change as that for a shot intended to acquire data 
corresponding to a data line containing the center of the k-space during a 
scan. 
In both prescans E and F, if nuclear spins are dephased, each of echoes 
E(2,1), E(2,2), E(2,3), E(3,1), etc. except the first echoes E(1,1) to 
E(1,3) is split into two echo components; that is, a main echo component 
Emain(2,1) (Emain(2,2), Emain(2,3), Emain(3,1), etc.) and a sub-echo 
component Esub(2,1) (Esub(2,2), Esub(2,3), Esub(3,1), etc.) (an example is 
shown in FIGS. 22 to 24 by taking an echo E(3,2)). Besides, during the 
succeeding prescan F, the application phase (@ for even-numbered RF 
refocusing pulses RFre2, REre4, etc. is advanced by an extra 180.degree. 
and thus set to 270.degree.. The phases of sub-echo components alone of 
even-numbered echoes are therefore advanced by 180.degree. from the 
associated ones attained during prescan E. 
Assume that an echo given by prescan E is Ee(n,m) and an echo given by 
prescan F is Ef(n,m). During both the prescans, phase encoding is 
performed on echoes except an echo produced in an effective.echo time; 
that is, an n.sub.eff -th echo E(n.sub.eff, m') (an echo E(3,2) of the 
third echoes in the example shown in FIGS. 22 and 23). An echo usable for 
computing quantities of correction needed for execution of a main scan is 
only the echo produced in the effective echo time; that is, the echo 
E(3,2) of the third echoes. 
Next, the controller 6 calculates a main echo component Emain(3,2) and a 
sub-echo component Esub(3,2) of the echo E(3,2) by performing the 
computation expressed as follows (step S23 in FIG. 21): 
Emain(3,2)={Ee(3,2)+Ef(3,2)}/2 
Esub(3,2)={Ee(3,2)-Ef(3,2)}/2 
The main echo component Emain(3,2) and sub-echo component Esub(3,2) 
calculated by performing the computation are shown in FIG. 24. 
Furthermore, the controller 6 computes a phase shift .phi.main(3,2) and 
displacement pmain(3,2) of the peak of the main echo component of the echo 
E(3,2) and a phase shift .phi.sub(3,2) and displacement psub(3,2) of the 
peak of the sub-echo component thereof (step S24 in FIG. 21). 
Next, similarly to known techniques, based on a phase difference between 
the main echo component and sub-echo component of the n-th echo; a value 
is given by the following expression: 
.phi.main(3,2)-.phi.sub(3,2) 
pmain(3,2)-psub(3,2), 
and a quantity .DELTA..phi. for correcting the application phase .phi. for 
RF pulses RF or a quantity .DELTA.Gss for correcting a slice-selective 
gradient pulse Gss and a quantity .DELTA.Gro for correcting a readout 
gradient pulse Gro, which are needed for execution of a scan, are computed 
(step S25 in FIG. 21). 
Finally, the controller 6 switches over the switch path of the multiplexer 
11 to the reconstruction unit 12, and allows the sequencer 5 to execute a 
scan according to a pulse sequence shown in FIG. 25 (step S26 in FIG. 21). 
For the scan, as illustrated, the application phase .phi. for an RF 
excitation pulse RFex to be applied first together with a slice-selective 
gradient pulse Gss is equal to .DELTA..phi. or the strength of a 
slice-selective gradient pulse Gss to be applied succeedingly to the 
slice-selective gradient pulse Gss, reversed in polarity, and used to 
prevent dephasing is a value corrected by .DELTA.Gss. At the same time, 
the strength of a readout gradient pulse Gro to be applied in parallel 
with the slice-selective gradient pulse Gss used to prevent dephasing is a 
value corrected by .DELTA.Gro. 
As mentioned above, according to this embodiment, the advantages of the 
first and second embodiments are available. In addition, unlike the FSE 
imaging, the GRASE imaging does not vary the waveform of a phase-encoding 
gradient pulse very much among RF refocusing pulses. The pattern of 
occurrence of a phase error deriving from phase encoding becomes regular. 
Consequently, corrections by .DELTA.Gss and .DELTA.Gro which are shown as 
hatched areas in FIG. 18 make it possible to minimize artifacts 
effectively. 
(Fourth Embodiment) 
An MRI system in accordance with the fourth embodiment will be described 
with reference to FIGS. 26 to 30. Even in this embodiment, two kinds of 
prescans G and Hare used. However, phase encoding is performed on each 
echo during the prescans in the same manner as it is during a scan. 
Herein, description will proceed on the assumption that a GRASE pulse 
sequence is employed. 
The MRI system has the same configuration as those of the aforesaid 
embodiments. The controller 6 executes processing described in FIG. 26. 
The controller 6 switches over the switch path of the multiplexer 11 to the 
controller 6, commands the sequencer 5 to successively execute two kinds 
of prescans G and H according to the GRASE imaging, and inputs acquired 
echoes (step S31 in FIG. 26). 
Unlike the aforesaid prescans, the first prescan G shown in FIG. 27 and the 
succeeding prescan H shown in FIG. 28 vary the strength of a 
phase-encoding gradient pulse Gpe or a quantity of phase encoding, which 
is performed on each echo Eg(n,m) or Eh(n, m) in the same manner as it is 
during a scan, at every shot. The addition of phase encoding is intended 
to acquire two-dimensional unadjusted echo data covering the whole or part 
of a k-space. The application phases for RF pulses RFex, and RFre1, REre2, 
etc. are varied in the same manner as those set for the two kinds of 
prescans A and B in the first embodiment. Two two-dimensional echo data 
sets; Eg(n,m) and Eh(n,m), are therefore acquired by changing the 
combination of application phases for RF pulses. 
For the prescans G and H, a GRASE pulse sequence in which the number of RF 
refocusing pulses is 5, the number of gradient echoes to be acquired 
between adjoining RF refocusing pulses is 3, and the total number of 
echoes per shot is 15, is taken for instance. Shown in the drawings are, 
for the sake of representation, waveforms of magnetic field gradients 
attained when the number of shots is two and the number of matrix elements 
in a phase-encoding direction in an image is 30. The number of shots is 
preferably set to a value permitting stable measurement of a first-order 
phase change in a phase-encoding direction in a screen by analyzing only 
the n.sub.eff -th echo corresponding to an echo produced in an effective 
echo time, for example, 8 or more(in terms of data in a k-space, this is 
comparable to 8 or more adjoining echo data lines with a data line 
containing the center of the k-space as a center data line). 
If nuclear spins are dephased, each of echoes E(2,m), E(3,m), etc. except 
the first echo E(1,m) is split into two echo components; that is, a main 
echo component Emain(2,m) (Emain(3,m), etc.) and a sub-echo component 
Esub(2,m) (Esub(3,m). etc.) (See, for example, echoes Eg(3,2) and Eh(3,2) 
in FIGS. 27 and 28). During the succeeding prescan H, the application 
phase for even-numbered RF refocusing pulses RFre2, RFre4, etc. is 
advanced by an extra 180.degree.. The phases of even-numbered sub-echo 
components Esub(2,m), Esub(4,m), etc. alone are therefore advanced by 
180.degree. from the corresponding ones attained during prescan G. 
Similarly to the aforesaid embodiments, as shown in FIG. 29, assuming that 
a two-dimensional echo given by the first prescan G is Eg(n,m) and an echo 
given by the succeeding prescan H is Eh(n,m), a two-dimensional main echo 
component Emain(n,m) developed in a k-space and a two-dimensional sub-echo 
component Esub(n,m) developed therein are provided as follows: 
EQU Emain (n,m)={Eg(n,m)+Eh(n,m)}/2 (7) 
EQU Esub (n,m)={Eg(n,m)-Eh(n,m)}/2 (8) 
Assuming that an n.sub.eff -th echo E(n,m) corresponding to an echo 
produced in an effective echo time during either of the prescans is, for 
example, E(3,2), a main echo component Emain(3,2) and a sub-echo-component 
Esub(3,2) are provided according to the following expressions (step S33 in 
FIG. 26): 
Emain(3,2)={Eg(3,2)+Eh(3,2)} / 2 
Esub(3,2)={Eg(3,2)-Eh(3,2)} / 2 
The provided echo components Emain(3,2) and Esub(3,2) are shown in FIGS. 
29(c) and 29(d). Among two pairs of echo components of a two-dimensional 
echo E(3,2) corresponding to the n.sub.eff -th echo, a pair of pub-echo 
components Esub(3,2) are 180.degree. out of phase mutually (one of the 
components is shown with dashed lines in FIGS. 28 and 29). As mentioned 
above, therefore, the main echo component Emain(3,2) is provided by 
performing additive averaging, that is, division of a sum by 2, and the 
sub-echo component Esub(3,2) is provided by performing subtractive 
averaging, that is, division of a difference by 2. 
Furthermore, the controller 6 computes a phase shift .phi.main(3,2) and 
displacement pmain(3,2) of the peak of the main echo component of the 
(3,2)-th echo E(3,2) and a phase shift .phi.sub(3,2) and displacement 
psub(3,2) of the peak of the sub-echo component thereof on the basis of 
the above two-dimensional echo data Emain(3,2) and Esub(3,2) developed in 
the k-space (step S34 in FIG. 26). Incidentally, the displacements 
pmain(3,2) and psub(3,2) are two-dimensional vectors. 
Next, a phase difference between the main echo component and sub-echo 
component of the (3,2)-th echo and a difference between the peak position 
vectors of the components are computed according to the expressions (1) 
and (2); that is, by calculating the following expressions: 
.phi.main(3,2)=.phi.sub(3,2) and 
pmain(3,2)=.rho.sub(3,2) Based on the resultant values, a quantity 
.DELTA..phi. of correcting an application phase .phi. for RF pulses RF, a 
quantity .DELTA.Gss of correcting a slice-selective gradient pulse Gss, a 
quantity .DELTA.Gro of correcting a readout gradient pulse Gro, and a 
quantity .DELTA.Gpe of correcting a phase-encoding gradient pulse Gpe are 
computed (step S35 in FIG. 26). Since each result of the corrective 
computation is reversed in polarity depending on whether the n.sub.eff -th 
numeral is odd or even, the order of subtraction should be made uniform in 
order to attain the same polarity all the time. 
Finally, the controller 6 switches over the switch path of the multiplexer 
11 to the reconstruction unit 12, and allows the sequencer 5 to execute a 
scan according to a pulse sequence shown in FIG. 30 (step S36 in FIG. 26). 
For the scan, as illustrated, a pulse sequence in which the results of the 
corrective computation are reflected is employed. Specifically, the 
application phase .phi. for an RF excitation pulse RFex (flip angle 
.phi.=.alpha.(90.degree. in this example)) to be applied first together 
with a slice-selective gradient pulse Gss is equal to .DELTA..phi., or the 
strength of a slice-selective gradient pulse Gss to be applied 
succeedingly to the slice-selective gradient pulse Gss, reversed in 
polarity, and used to prevent dephasing is a value corrected by 
.DELTA.Gss. At the same time, the strength of a readout gradient pulse Gro 
to be applied in parallel with the slice-selective gradient pulse Gss used 
to prevent dephasing is a value corrected by .DELTA.Gro, and the strength 
of a phase-encoding gradient pulse Gpe is a value corrected by .DELTA.Gpe. 
Thereafter, similarly to the aforesaid embodiments, a plurality of RF 
refocusing pulses RFre1, RFre2, etc. are used to consecutively acquire 
echoes E(n,m) produced as a result of refocusing of nuclear spins, and 
then reconstructed into an MR image. 
As mentioned above, according to this embodiment, in addition to the 
advantages provided by the first to third embodiments, an advantage that 
even a dephased component of dephased components caused by a 
phase-encoding gradient pulse, which exhibits a first-order spatial 
distribution in a phase-encoding direction, can be corrected preferably, 
that the precision in correcting a pulse sequence used for a scan is 
improved markedly, and that high-quality MR images can be produced can be 
made available. 
Incidentally, a procedure of calculating the quantities of correction 
.DELTA..phi., .DELTA.G (.DELTA.Gss, .DELTA.Gro, and .DELTA.Gpe) in this 
embodiment is not limited to the aforesaid one. Alternatively, 
two-dimensional images may be reconstructed using the two-dimensional echo 
data given by prescans. In this case, the slopes and intercepts of curves 
depicted to show distribution of phases in a readout direction and 
phase-encoding direction respectively; that is, first-order components and 
zero-order components of phases indicated by the curves in the readout 
direction and phase-encoding direction, are then calculated. Displacements 
and phase shifts of echo peaks are then calculated on the basis of the 
first-order components and zero-order components. Finally, quantities of 
correction .DELTA..phi. and .DELTA.G used to correct a pulse sequence to 
be employed in a scan are calculated. 
The aforesaid embodiments have been described by taking two-dimensional 
scanning for instance. Alternatively, correction can be made for 
three-dimensional scanning or larger-dimensional scanning by increasing 
the dimension of a vector. 
(Fifth embodiment) 
The fifth embodiment of the present invention will be explained in 
conjunction with FIGS. 31 and 32. In the embodiment, although two kinds of 
prescan are performed in the same manner as described above, the prescans 
use a particular object being prescanned in consideration of the nature of 
the prescan. 
The MRI system used in this embodiment is the same in hardware construction 
as that in the first embodiment, but different in that the controller 6 
executes two sets of processes shown in FIGS. 31 and 32. A set of 
processes in FIG. 31 provides a prescan procedure and a set of processes 
in FIG. 32 provides a scan procedure. 
Performing the prescan is to measure phase errors of the main and sub-echo 
components resulting from incompleteness of used hardware such as the 
gradient coils. Accordingly, it is not always necessary that an object to 
be prescanned be the same as that scanned in the scan obtaining diagnostic 
MR images. In the present embodiment, a patient as the object is replaced 
by a phantom. To attain higher measurement accuracy, favorable phantoms 
should be as uniform as possible in signal spatial distribution. For 
example, such phantoms include an acrylic-resin-made container filled with 
copper sulfate aqueous solution. 
Such an objective phantom is placed in the diagnostic space of the magnet 
1. When a start command is given to the controller 6, the controller 6 
initiates the prescanning processes shown in FIG. 31. 
First, at Step S41 in FIG. 31, the controller 6 determines resolution and 
slice thickness which are imaging conditions for two kinds of prescan G 
and H. The prescans G and Hare the same as those described in the fourth 
embodiment, and uses pulse sequences in compliance with the GRASE 
technique. At Step S42, the controller 6 performs in turn the two kinds of 
prescan G and H with the determined imaging conditions. Processing is then 
moved to Step S43, where computation is carried out in the same manner as 
in the fourth embodiment, thereby main echo components Emain(n,m) and 
sub-echo components Esub(n, m) of an echo signal E(n,m) at the n.sub.eff 
-th (i.e., the(n,m)-th) are extracted based on the following formulas: 
Emain(n,m)={Eg(n,m)+Eh(n,m)}/2 
Esub(n,m)={Eg(n,m)-Eh(n,m)}/2 
At Step S44, computation is carried out for obtaining the 0-th and first 
order components of the spatial distribution in which phase errors of each 
of the main echo component Emain(n,m) and sub-echo component Esub(n,m); 
.phi.main(n,m), Pmain(n,m), .phi.sub(n,m), and psub(n,m). The controller 6 
executes in turn processes of Steps S45 and S46. At Step S45, computed are 
differences in phases of the main echo and sub-echo components and 
differences in peak position vectors of the main echo and sub-echo 
components, based on the foregoing formulas (1) and (2). At Step S46, 
information with respect to those differences, together with data of the 
spatial resolution and slice thickness, as correcting data, are stored 
into the memory unit 13. 
When the scan is instructed to start, the controller 6 begins processes in 
FIG. 32. Firstly, at Step S51, resolution and slice thickness which are 
imaging conditions are determined by, for example, making reference to 
data given from an operator. At Step S52, correcting data are read out 
which have been obtained and memorized with the prescans. The correcting 
data are of the imaging conditions (resolution, slice thickness, etc.) and 
the phase difference and peak positional vector difference for the main 
echo and sub-echo components. 
Further, at Step S53, correcting quantities .DELTA..phi. and .DELTA.G are 
computed in the same manner as that described before on the basis of the 
read-out correcting data. In this computation, the imaging conditions and 
spatial resolution in each direction of an MR image for the scan are 
compared with those of the prescan in order to obtain the ratios between 
them. The correcting quantities .DELTA..phi. and .DELTA.G are computed 
correspondingly with the obtained ratios. At next Step S54, the controller 
6 instructs the sequencer 5 to execute in the diagnostic a pulse sequence 
which reflects the computed correcting quantities .DELTA..phi. and 
.DELTA.G. 
As described above, executing the prescan using the phantom having an ideal 
signal distribution makes it possible to have more accurate correcting 
quantities. Specifically, there is no possibility of moving an object 
(because, it is a phantom) in the prescan, which avoids the accuracy of 
correction of phase errors from being lowered and signals emanated from 
the object from being changed. Accordingly, all such merits lead to 
acquisition of more stable and accurate correcting quantities and make it 
possible to execute the diagnostic scan in a more stable and accurate 
manner in acquiring MR data. The prescan for acquiring correcting data of 
phase errors is not necessary for patients, and a diagnostic scan is 
solely performed. Hence, there is a decrease in examining time for 
patients and it makes easier it to provide more medical examinations. 
Where the signal error distribution in the phase encode direction is as 
small as negligible, it may be possible that the prescan is performed in 
which the phase encode amount is zero, such as the prescans A and B in the 
foregoing first embodiment, phase error distributions in directions other 
than the phase encode direction are solely measured, and correction is 
performed on the basis of information of the phase error distributions. It 
may also be possible that, as seen in the prescans C and D in the 
foregoing second embodiment, executed is a prescan which can obtain a 
phase error distribution including the zero-th phase error component 
caused by the phase encode gradient. 
(Sixth embodiment) 
The sixth embodiment of the present invention, which is modified from the 
fifth embodiment, will be explained in conjunction with FIGS. 33 and 34. 
In an MRI system according to the sixth embodiment, phase error 
characteristics of physical channels of gradients are measured with two 
kinds of prescans G and H which are identical to the fifth embodiment, 
prior to a diagnostic scan. When the scan is performed, imaging conditions 
associated with gradients, such as an imaging cross-sectional direction, 
resolution, and slice thickness, are used to compute the correcting 
quantities .DELTA..phi. and .DELTA.G for a pulse sequence of the scan. 
This eliminates the necessity of executing the prescan which should be 
directed in the same direction as the scan, and makes it possible to 
compute the correcting quantities .DELTA..phi. and .DELTA.G to the scan 
directed in an arbitrary cross-sectional direction. 
The controller 6 executes not only the procedures shown in FIGS. 33 and 34 
replaced by FIG. 31 but also the procedures shown in FIG. 32. 
The procedures represented in FIG. 33 include prescans and are used for 
computing the elements (referred to as an "array .alpha.") of correcting 
gradient coefficient matrices Kp,ro, Kp,pe, Kp,ss, while those represented 
in FIG. 34 are for computing the elements (referred to as an "array B") of 
correcting RF phase coefficient vector K.phi.,ro, K.phi.,.rho.e, 
K.phi.,ss. 
The relation between each of the correcting gradient coefficient matrices 
Kp,ro, Kp,pe, Kp,ss and the array a is expressed element by element by the 
following formula (9). 
##EQU1## 
The relation between each of the correcting RF phase coefficient vectors 
K.phi.ro, K.phi.,pe, K.phi.,ss and the array B is shown element by element 
below by the following formula (10). 
##EQU2## 
The correcting gradient coefficient matrices Kp,ro, Kp,pe, Kp,ss relate to 
the correcting gradient .DELTA.G through the following formula (11). 
##EQU3## 
Further, the correcting RF phase coefficient vectors K.phi.jo, K.phi.,pe, 
K.phi.,ss are coupled with the correcting quantity .DELTA..phi. of the 
application phase of the RIF pulse by the following formula (12). 
##EQU4## 
Still further, a matrix R representing rotation of an imaging sectional 
direction can be defined as the following formula (13). 
##EQU5## 
and the correcting gradient .DELTA.Gcan be expressed by elements shown as 
the following formula (14). 
##EQU6## 
When it is supposed that M,imgCh are magnitudes of spatial frequencies in 
the directions of imaging channels imgCh (i.e., the directions of RO, PE, 
SS related to a pulse sequence) and M,phCh are magnitudes of spatial 
frequencies in the directions of physical channels phCh (i.e., the 
directions of physical channels related to the X-, Y-, and Z-gradient 
coils), the formula (13) is supposed to connect those directions with the 
rotation matrix R. 
As shown in the formulas (9) and (11), when imgCh is made to correspond to 
phCh1, 
.alpha.imgCh!phCh1!phCh2! 
are coefficients representing how much the first-order phase distribution 
in the phCh2 direction should be corrected. T appears in the formula (11) 
as an interval of time for the correcting gradient. F,imgCh appeared in 
the formula (11) as ratios of gradient magnitudes required in an actual 
scan, the ratios being computed based on gradient magnitudes required to 
satisfy specified imaging conditions for imaging channels given by each 
suffix. Mo,imgCh seen in the formula (11) are magnitudes of spatial 
frequencies specified as a reference in imgCh directions. 
Also, as shown in the formulas (10) and (12), when imaging channels imgCh 
of RO, PE and SS concerning a pulse sequence are made to correspond to 
physical channels phCh, 
BimgCh!phCh! 
are coefficients representing how much the zero-th order phase distribution 
should be corrected. F,imgCh shown in formula (12) are the same parameters 
as those in formula (11). 
Computation of the elements of each correcting gradient coefficient matrix 
shown in FIG. 33, which is carried out by the controller 6, will now be 
explained. First, an element in a matrix is selected (Step S61). Next, an 
imaging sectional direction is selected in relation to the determined 
element (Step S62). In order to obtain elements of the matrix, imaging 
conditions should be changed in several ways, and in each imaging 
condition, a phase shift distribution of spins should be obtained and 
compared with each other. 
In order to obtain the elements, first, a variable i for counting up the 
number collected is initialized by i=o (Step S63). Then, imaging 
conditions, such as spatial resolution and slice thickness, are determined 
such that a designated element is obtained (Step S64). Thanks to 
characteristics of gradient coils or a used pulse sequence, the number 
collected can be lowered when any element is regarded clearly as zero. 
Further, at Step S65 in FIG. 33, the prescans G and H (i.e., a series of 
processes including Steps S42 to S44 in FIG. 31) are executed. Processing 
is moved to Step S66, where, on the basis of the two-dimensional vectors 
pmain and psub showing positional displacements of the main echo and 
sub-echo components and being obtained with the prescans, differences in 
those peak position vectors are computed for every variable i by the 
formula: 
.DELTA.Pi=pmain-psub 
Processing is moved to Step S67 to determine whether there still remains an 
unselected imaging condition. When there is an imaging condition which has 
not been selected so far, the variable i is incremented (i=i+1) at Step 
S68, and the processing is returned to Step S64. Thus, a series of steps 
S64 to S66 are repeated one time or a plurality of times. Differences APi 
in the peak position vectors are obtained on different imaging conditions 
or one imaging condition. 
When the determination at Step S67 produces No and it is determined that 
all the prescans have ended for the predetermined imaging conditions, the 
controller 6 executes a process at Step S69, where correcting quantities 
of corresponding elements 
.DELTA..alpha.imgCh!phCh1!phCh2! 
are computed based on information of the imaging conditions and the 
difference .DELTA.Pi in the peak position vectors. Then, at Step S70, the 
correcting quantities 
.DELTA..alpha.imgCh!phCh1!phCh2! are used to compute corresponding 
matrix elements: 
##EQU7## 
The foregoing processing is carried out for all the elements selected at 
Step S61 (Step S71). 
The elements of each correcting RF phase coefficient vector are computed in 
the same manner as in FIG. 33. This computation is illustrated in Steps 
S81 to S91 in FIG. 34. At Step S81, a desired element 
BimgCh!phCh! 
of correcting RF phase coefficient vectors is preselectea. As Step S86, 
phase differences between the main echo and sub-echo components 
.DELTA..phi.i=.phi.main-.phi.sub 
are computed for each imaging condition on the basis of the prescans G and 
Hand the postcalculation associated with the prescans. Furthermore, at 
Step S89, information of the imaging conditions and the phase differences 
.DELTA..phi.i are used to compute correcting quantities of corresponding 
vector elements: 
.DELTA.BimgCh!phCh!. 
At Step S90, corresponding vector elements are computed by the following: 
##EQU8## 
The correcting gradient coefficient matrices and correcting RF phase 
coefficient vectors are thus-obtained, and stored in the memory unit 13 or 
other internal memories (not shown) for use with a diagnostic scan which 
is to be carried out later on. The diagnostic scan is based on the manner 
described in FIG. 32. In other words, imaging conditions of the diagnostic 
scan are determined (Step S51), the correcting coefficient matrices and 
coefficient vectors which have been stored so far are read out (Step S52), 
and the correcting quantity .DELTA..phi. of the application phase and the 
correcting quantity .DELTA.G of the gradients are computed using the 
formulas (11) and (12). Finally, a pulse sequence reflecting those 
correcting quantities Ad) and .DELTA.G is executed during the diagnostic 
scan (Step S54). 
The above processing eliminates the necessity of executing the prescans 
performed in the same imaging sectional direction as the scan. Therefore, 
without the prescans, the scan can be done for an imaging section oriented 
in an arbitrary direction. 
The characteristics of imaging described in the above sixth embodiment can 
be compared with the prior art references as follows. In U.S. Pat. No. 
5,378,985 and Japanese Patent Laid-Open No. 6-54827 both described above, 
it is considered that all the elements corrected in each correspond to 
Kp,ro and Kp,ss among the correcting gradient coefficient matrices and 
K.phi.,ro and K.phi.,ss among the correcting RF phase coefficient vectors. 
In techniques of those references, since each element in not clearly 
separated, it is required that the prescan should be performed directly 
before the imaging scan with the same section direction, spatial 
resolution, and slice thickness as in the imaging scan. 
In contrast, the sixth embodiment discloses a correction technique whereby 
not only diagonal elements of the correcting gradient coefficient matrix 
Kp,pe but also the correcting RF phase coefficient vector K.phi.,pe, which 
are impossible to be corrected by the above-mentioned prior techniques, 
are additionally corrected, thereby increasing the number of corrected 
elements. In addition, the non-diagonal elements of all the correcting 
gradient coefficient matrices can be independently measured and corrected 
in the sixth embodiment, whereby correcting quantities can be computed 
even if the direction of an imaging section differs from each other 
between the prescan and the imaging scan. Further, correcting the phase of 
the RF pulse and the quantity of the gradients leads to correction of 
almost all the elements in the matrices and vectors. Accordingly, even 
when the MRI system according to the sixth embodiment employs the same or 
identical gradient coils in construction as conventional ones, the MRI 
system of this embodiment is able to provide more stable and high-quality 
MR images than conventional ones. 
The computation models accomplishing the foregoing formulas (11), (12), 
etc. can be modified in compliance with the characteristics of gradient 
systems. 
(Seventh embodiment) 
The MRI system according to the seventh embodiment of the present invention 
will be described in conjunction with FIGS. 35 to 40. Apart from the 
foregoing embodiments, the MRI system in this embodiment is designed such 
that phase errors including phase error distributions of the second or 
higher orders are properly corrected without correction of waveforms of 
gradients and an application phase of the RF pulse included in an imaging 
pulse sequence of the scan. 
The hardware of the MRI system is constructed in the same way as ones 
described before. The controller 6 is responsible for executing the 
procedure shown in FIG. 35 into which two kinds of prescans and one scan 
are combined. For the prescans and scan, an FSE method is used. 
First of all, in a state that the multiplexer 11 has switched over its 
output to the controller side, the controller 6 sends the sequencer 5 a 
command to execute two kinds of prescans A and B in sequence (Step S 101 
in FIG. 35). These prescans A and B are the same in manner as those in the 
first embodiment. 
In the same way as above, the main echo components Emain(n) and sub-echo 
components Esub(n) are extracted (computed) by 
Emain(n)={Ea(n)+Eb(n)}/2 
Esub(n)={Ea(n)-Eb(n)}/2 
using echo signals Ea(n) and Eb(n) obtained in prescans A and B, their 
absolute values .vertline.{Ea(n)+Eb(n)J/2.vertline. and 
.vertline.{Ea(n)-Eb(n)1/2.vertline. are computed, and the computed 
absolute values are set as amplitude information EPmain and EPsub of the 
main echo and sub-echo components Emain(n) and Esub(n), respectively (Step 
S102; refer to FIG. 39). The controller 6 then instructs not only the 
multiplexer 11 to switch over to the reconstruction unit side but also the 
sequencer 5 to execute in turn two kinds of scans I and J (Step S103). 
Pulse sequences used in these diagnostic scans I and J are each shown in 
FIGS. 36 and 37. As understood from the figures, except the application 
phase patterns of RF pulses, the pulse sequences are the same as a 
normally-used FSE method; namely, no correction is made to the application 
phases of RF pulses and the waveforms of gradients. The application phase 
pattern of RF refocusing pulses in one diagnostic scan I is not changed 
from a normally-used FSE method, while that in the other diagnostic scan J 
is changed in that the application phases .phi. of the even-numbered RF 
refocusing pulses are set to 270.degree. additionally rotated by 
180.degree.. This application phase pattern is, for example, the same as 
one in the foregoing prescan B. 
Then, the controller 6 sends the reconstruction unit 11 a command to 
execute in turn procedures shown at Step S94 and its subsequent steps. 
Responsively to the command, the reconstruction unit 11 extracts and 
separates the two-dimensionally distributed main echo and sub-echo 
components Emain and Esub in the k-space using the data Ei(n) and Ej(n) 
acquired by the diagnostic scans I and J (Step S104). This process is 
carried out by 
Emain={Ei(n)+Ej(n)}/2 
Esub={Ei(n)-Ej(n)}/2 
which are the same formulas described before. The separated and extracted 
main echo and sub-echo components are pictorially shown in FIGS. 38 and 
39. The main echo components Emain are illustrated by solid lines, and the 
sub-echo component by dashed lines. 
As clearly understood from foregoing formulas (1) and (2), for each of the 
main echo components Emain and sub-echo components Esub, the phase 
alternately repeats two values .phi.1 and .phi.2 in a manner such that it 
takes .phi.1 at odd-numbered echo signals and it takes .phi.2 at 
even-numbered echo signals. Additionally, taking account of regularity of 
the gradient fields, the phases of odd-numbered echo signals of the main 
echo components Emain and the phases of even-numbered echo signals of the 
sub-echo components Esub are equal to each other in most cases. 
In accordance with this principle, as shown in FIG. 39, the reconstruction 
unit 11 produces new main echo components Emain' and sub-echo components 
Esub' by exchanging echo data belonging to even- or odd-numbered echo 
blocks, every echo block, in their k-spaces between corresponding echo 
blocks in the main echo and subecho components Emain and Esub (Step S105). 
In detail, the exchange is carried out such that when an even-numbered 
echo signal is placed at each of the centers of the k-spaces, all the 
odd-numbered echo data are exchanged with each other, whereas when an 
odd-numbered echo signal is placed at each of the centers of the k-spaces, 
all the even-numbered echo data are exchanged with each other. As a 
result, in the newly-produced main echo and sub-echo components Emain' and 
Esub' mapped in the respective k-spaces, the phase shifts between echo 
blocks will almost disappear, whereby all the echo data are positionally 
smooth-connected with each other, as pictorially shown in FIG. 39. 
Each of the main echo and sub-echo components Emain' and Esub' at this 
stage are merely arranged alternately at every echo block in the k-space. 
In short, there is still dispersion of the echo signal amplitudes between 
echo blocks, as shown by the graphs Amain and Asub each representing 
changes in the amplitude in the ke-direction. 
Therefore, the reconstruction unit 11 receives from the controller 6 the 
amplitude information EPmain and EPsub made using the prescans A and B, 
designates the information ElPmain and EPsub as amplitude correction data, 
and puts amplitude correction into execution, for every echo block, to 
give a uniform amplitude distribution over the respective entire k-space 
to each of the main echo and subecho components Emain' and Esub' (Step 
S106). This amplitude correction further produces two new sets of main 
echo and sub-echo components Emain" and Esub" each mapped in the k-spaces 
(refer to FIG. 39). 
The reconstruction unit 11 subsequently reconstructs (two-dimensional 
Fourier transformation) each of the two new sets of main echo and sub-echo 
components Emain" and Esub", thereby producing two sets of MR image data 
MRmain and MRsub, respectively (Step S107). Since both the echo data 
Emain" and Esub" mapped in their respective k-space have almost no phase 
shift between echo data, even if adjustment is poor, it is possible to 
exclude most ghost artifacts from the reconstructed MR images MRmain and 
MRsub. 
A further process is executed by the reconstruction unit 11 at Step S108, 
in which the phases of two pixel signals at the same two pixel positions 
common to the two MR images are arranged to be equal to each other (Step 
S108). This phase arrangement is, for example, carried out as follows. 
When it is supposed that pixel numbers (complex data) at the same 
positions common to the two MR images MRmain and MRsub are I.sub.A (r) and 
I.sub.B (r) (refer to FIG. 40), the phase difference .DELTA..GAMMA.(r) is 
computed by 
.DELTA..theta.(r)=arg{I.sub.B (r)}-arg{I.sub.A (r)} 
and using a relation of 
EQU I.sub.A '(r)=I.sub.B (r)-exp{-j.DELTA..theta.(r)} 
a final MR image I(r) is computed at every pixel by the following formula 
(Step S109): 
EQU I(r)={I.sub.A (r)+I.sub.B '(r)}/2 
Alternatively, IB'(r) can be: 
EQU I.sub.B '(r)={(I.sub.B (r)/I.sub.A (r))+(I.sub.B (r)/l.sub.B (r)*)}I.sub.A 
(r) 
where (I.sub.B (r)/I.sub.A (r))* is the complex conjugate of (I.sub.B 
(r)/I.sub.A (r)*)}I.sub.A (r), and data of the final MR image I(r) can be 
computed pixel by pixel using the approximation of IB'(r) (Step S109). 
Still alternatively, data of the final MR image I(r) can be obtained pixel 
by pixel as follows (Step S109). 
EQU I(r)={.vertline.I.sub.A (r).vertline.+.vertline.I.sub.B (r)}/2 
As described above, the MRI system of this embodiment requires at least two 
diagnostic scans. However, even when there exists a phase error 
distribution of the second or higher order which is theoretically 
impossible to be corrected by adjusting the application phase of the RF 
excitation pulse and the waveforms of gradients, the MRI system can 
steadily exclude the phase error distribution including such higher-order 
error components. Thus, it is possible to provide MR images of stable and 
higher-quality, without deterioration of image quality. In addition, since 
it is not necessary to utilize phase information acquired from the 
prescans, an MR imaging technique having resistance against an object 
motion etc. is provided. 
In the foregoing seventh embodiment, when the amplitude correction is 
unnecessary in some cases, the prescans A and B can be excluded. 
Further, lowering the flip angles due to the RF refocusing pulses can cause 
changes in amplitude ratios between the main echo and sub-echo components, 
and can suppress ups and downs in amplitude in the respective k-spaces. 
When such manner of lowering the flip angles is employed, diagnostic scans 
can be performed prior to prescans, because the pulse sequence itself used 
in the diagnostic scan has no relation with information acquired by the 
prescans. 
Still further, the imaging techniques taught by the seventh and its 
modified embodiments can be applied to those including a plurality of RF 
refocusing pulses, such as a CPMG pulse train and phase reversal CP pulse 
train. 
However, when a GRASE method is employed as one of such imaging techniques, 
attention must be paid to the foregoing echo data exchange, for there are 
some cases in the GRASE method that do not hold the above-described 
situation in which even-numbered or odd-numbered echo data belonging to a 
plurality of echo blocks arranged in the phase encode direction of the 
k-space can simply be exchanged, block by block, between the two echo 
components Emain and Esub. One such example is shown in FIG. 41. Two sets 
of k-space data Emain and Esub illustrated therein are derived from the 
GRASE method. In detail, echo data E(N.sub.RF, N.sub.GE are acquired in a 
manner that the RF refocusing pulses are odd in number (=N.sub.RF =3), the 
gradient echoes generated after each RF refocusing pulse are three in 
number (NGE=3). When the number of RF refocusing pulses is odd as 
exemplified above, two new sets of k-space data Emain' and Esub', which 
are almost equal in phase in the respective k-space data, can be produced 
by converting echo data E(NRF, NGO in an odd-numbered (or even-numbered) 
echo block for each RF refocusing pulse at the process corresponding to 
Step S25 shown in the foregoing FIG. 35. Utilizing regularity associated 
with the phase shifts in this way makes it possible to effectively 
suppress generation of artifacts for imaging methods that do not change 
largely the waveform of phase encode gradients applied subsequently after 
each RF refocusing pulse.