Systems and methods for correcting phase errors

Details Systems and methods for correcting phase errors Systems and methods for correcting phase errors

2002-04-04

2004-03-09

Gutierrez, Diego (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

Reexamination Certificate

active

06703834

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a phase correction method and an MRI (Magnetic Resonance Imaging) system, more specifically, to a phase correction method and an MRI system which can correct motion phase errors and simplify the pulse sequence.
FIG. 18
shows a basic example of a pulse sequence of a multishot diffusion enhancement EPI (Echo Planar Imaging) method.
In the pulse sequence, an excitation pulse RF
90
and a slice gradient SG
90
are applied. An MPG (Motion Probing Gradient) pulse MPG is then applied. An inverting RF pulse RF
180
and a slice gradient SG
180
are applied. An MPG pulse MPG is then applied. A phase encode gradient pdn is applied. Data collecting read gradients r
1
. . . rM which are alternately inverted to be positive or negative are applied continuously, phase encode gradients p
2
, . . . , pM are applied at inverting, and they are sampled by being timed to successively focus the first echo el to the Mth echo eM so as to collect data F (n,
1
), . . . , F (n, M) corresponding to the echoes el, . . . , eM. This is repeated for n×
1
, . . . , N while changing the magnitude of the phase encode gradient pdn, to thereby collect data F (
1
,
1
) to F (N, M) for filling the k space. This is called N shot and M echo. A number n given to a shot in the order of execution time is called a shot number. A number given to the echo of the echo train of a certain shot in the order of focusing time is called an echo number.
FIG. 19
is a schematic diagram showing collection trajectories of data F (
1
,
1
) to F (N, M) in a k space KS, where N=
4
and M=
4
.
When the k space KS is divided in the phase encode axis direction from the first line to the N×Mth line (or to the
16
th line in FIG.
19
), phase encodes pdn, p
2
, . . . , pM are applied so as to collect data F (n, m) of the (n+(m−
1
) N) th line by the Mth echo of the nth shot.
As shown in
FIG. 20
, the k space KS can be successively divided into blocks from the first echo block filled by data F (n,
1
) obtained from the first echo of the first to the Nth shot, to the Mth echo block (M=
4
in
FIG. 20
) filled by data F (n, M) obtained from the Mth echo of the shots.
Phase errors which are a problem in the pulse sequence of the multishot diffusion enhancement EPI method include a motion phase error caused by motion (for example, a pulse of a brain) and a magnetic field inhomogeneity error caused by magnetic field inhomogeneity.
The magnitude of the motion phase error periodically fluctuates in synchronization with, e.g., a pulse. In other words, the motion phase error fluctuates by the magnitude which cannot be ignored at relatively long time intervals between the shots. The motion phase error is changed only by the degree which can be ignored within a relatively short time such as an echo train of one shot. The motion phase errors of the same shot number n can thus be regarded as the same magnitude although the echo number m is different.
The magnitude of the magnetic field inhomogeneity phase error is increased in proportion to time from the excitation pulse RF
90
. In other words, the magnetic field inhomogeneity phase error is increased in proportion to the echo number m. Time to the echo number m from the excitation pulse RF
90
is the same or is slightly different (there are the cases that the time is the same and that the time is slightly different) although the shot number n is different. The magnetic field inhomogeneity phase error of the echo number m can be regarded as the same magnitude although the shot number n is different.
FIG. 21
is an explanatory view showing phase errors of data F (n, m) according to a basic example of the pulse sequence of the multishot diffusion enhancement EPI method of FIG.
18
.
After the motion phase error is synthesized with the magnetic field inhomogeneity phase error, the phase errors are stepwise and periodical, causing ghost artifacts.
Like the pulse sequence shown in
FIG. 22
, a navigation phase encode gradient Nr is applied before the phase encode gradient pdn of the pulse sequence (the basic example) of
FIG. 18
so as to focus a navigation echo Ne and to collect correcting data H (n) from the navigation echo Ne.
A phase difference between the correcting data H (n) of the shots represents a difference between the motion phase errors of the shots. Based on the correcting data H (n), imaging data F (n,
1
) to F (n, m) of the same shot are phase corrected to correct the motion phase errors.
FIG. 23
is an explanatory view showing phase errors of data F′ (n, m) after phase correction.
Imaging data F (
1
,
1
) to F (
4
,
4
) are phase corrected (indicated by the black-headed arrows) so that the phase of correcting data H (
1
) of the first shot is matched with the phase of correcting data H (
2
) of the second shot to the phase of correcting data H (
4
) of the fourth shot. This can correct the motion phase errors and can suppress ghost artifacts as compared with the case of FIG.
23
.
Since the magnetic field inhomogeneity phase errors remain, a phase difference is caused between the echo blocks. The ghost artifacts cannot be removed completely.
The pulse sequence of
FIG. 18
, because of the motion phase errors and the magnetic field inhomogeneity phase errors, has a problem of causing ghost artifacts.
The pulse sequence of
FIG. 22
, because of the magnetic field inhomogeneity phase errors, has a problem that ghost artifacts cannot be removed completely. The navigation echo Ne and the imaging echoes from e
1
to em are independent. The pulse sequence is complex and the control is thus complicated.
SUMMARY OF THE INVENTION
Therefore, a first object of the present invention is to provide a phase correction method and an MRI system which can correct motion phase errors and can simplify the pulse sequence.
In addition to the first object, a second object of the present invention is to provide a phase correction method and an MRI system which can correct magnetic field inhomogeneity phase errors.
In a first aspect, the present invention provides a phase correction method including: repeating by N shots a pulse sequence in which when a k space is divided in the phase encode axis direction from the first line to the N=Mth (N and M are a natural number of 2 or more) line, data collecting read gradients are applied while inverting so as to focus M-piece imaging echoes per inverting RF pulse and to focus one or more navigation echoes as an echo train continuous to the M-piece imaging echoes before the M-piece imaging echoes and an MPG pulse is applied before and after the inverting pulse; collecting diffusion enhancement imaging data for filling the k space from the imaging echo; collecting correcting data from the navigation echo; and phase correcting the imaging data based on the correcting data.
In the phase correction method of the first aspect, since the correcting data collected for each of the shots corrects the phase of the imaging data of the same shot, the motion phase errors can be corrected. In addition; the navigation echo is focused so as to be an echo train continuous to the imaging echo. The applying pattern of data collecting read gradients can be simplified and the pulse sequence can be simplified.
In a second aspect, the present invention provides the phase correction method thus constructed, wherein two or more navigation echoes are provided to one shot, and the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data.
In the phase correction method of the second aspect, the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data. Proper correction can thus be performed as compared with the case that the polarities are different. In

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