Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-02-20
2004-05-18
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06737865
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2002-046828 filed Feb. 22, 2002.
BACKGROUND OF THE INVENTION
The present invention relates to a static magnetic field inhomogeneity distribution measuring method, static magnetic field homogenizing method, MR (magnetic resonance) data collecting method, and MRI (magnetic resonance imaging) apparatus, and more particularly to a static magnetic field inhomogeneity measuring method, static magnetic field homogenizing method, MR data collecting method, and MRI apparatus by which the effect of residual magnetization caused by gradient pulses can be thoroughly reduced.
A known static magnetic field inhomogeneity distribution measuring sequence SQJ is shown in FIG.
7
.
The static magnetic field inhomogeneity distribution measuring sequence SQJ applies an RF pulse P1 and a slice selective gradient Ss1; subsequently applies a slice rephasing gradient Sr1′ having an intensity (amplitude) equal to that of the slice selective gradient Ss1; applies a phase encoding gradient Pe1; applies a frequency dephasing gradient Fd1′ having an intensity equal to that of a readout gradient Ro1; subsequently collects first MR data while applying the readout gradient Ro1; then, applies a phase rewinding gradient Pr1′ having an intensity equal to that of the phase encoding gradient Pe1; and further applies a killer gradient Sk1′ having an intensity greater than that of the slice selective gradient. Subsequent to the first gradient echo sequence, a second gradient echo sequence in which the echo time is shifted by &dgr;t collects second MR data. Then, the distribution of static magnetic field inhomogeneity is measured based on the phase difference between the first and second MR data.
Moreover, Japanese Patent Application Laid Open No. 2001-54510 discloses means for suppressing variation of residual magnetization in a magnetization conditioning plate, for example, in an MRI apparatus, which variation depends upon the history of gradient pulse application, in which means:
(1) a residual magnetization reducing pulse is applied immediately after the phase encoding gradient;
(2) a residual magnetization reducing pulse is applied immediately after the phase rewinding gradient;
(3) a residual magnetization reducing pulse is applied immediately after the killer gradient;
(4) the intensity (amplitude) of the slice rephasing gradient is made about half of that of the slice selective gradient; and
(5) the intensity of the frequency dephasing gradient is adjusted.
FIG. 8
is a magnetization characteristic graph for explaining the effect of residual magnetization caused by the gradient pulses in the static magnetic field inhomogeneity distribution measuring sequence SQJ shown in FIG.
7
. It should be noted that the graph is presented merely for explanation of a concept, and does not limit the present invention.
Considering first only the effect of the gradient pulses applied to the slice axis in general, when the magnetization lies at a point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the slice selective gradient Ss1 and slice rephasing gradient Sr1′ by the first gradient echo sequence causes the magnetization to move from the point a, through points b, c and e, to a point f. Then, when the killer gradient Sk1′ is applied, the magnetization moves from the point f through the point b, enters a major loop, and travels through a point B′ to a point c′. In the second gradient echo sequence, the magnetization varies along another minor loop containing the point c′.
Considering next only the effect of the gradient pulses applied to the phase axis in general, when the magnetization lies at the point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the phase encoding gradient Pe1 by the first gradient echo sequence causes the magnetization to move from the point a through the point b to the point c, and then application of the phase rewinding gradient Pr1′ causes the magnetization to move from the point c through the point e to the point f. Next, when the phase encoding gradient Pe2 is applied by the second gradient echo sequence, the magnetization moves from the point f through the point b to the point c, and when the phase rewinding gradient Pr2′ is then applied, the magnetization moves from the point c through the point e to the point f.
Next, considering only the effect of the gradient pulses applied to the frequency axis, when the magnetization lies at the point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the frequency dephasing gradient Fd1′ and readout gradient Ro1 by the first gradient echo sequence causes the magnetization to move from the point a, through the points e, f, and b, to the point c. The magnetization varies in a similar manner in the second gradient echo sequence.
As described above, the conventional static magnetic field inhomogeneity distribution measuring sequence SQJ poses a problem in that the distribution of static magnetic field inhomogeneity cannot be accurately measured because magnetization varies due to gradient pulses.
Consequently, correct shimming cannot be achieved, and the image quality may be significantly degraded especially when conducting an imaging method utilizing the resonance frequency difference between water and fat, such as a CHESS (chemical shift selective imaging) method, in an MRI apparatus with a medium-to-low magnetic field (0.3-0.5 T).
To solve the problem, the inventors of the present invention studied use of means disclosed in Japanese Patent Application Laid Open No. 2001-54510, but the distribution of static magnetic field inhomogeneity cannot be highly accurately measured only by this means. Further investigation was therefore made to find new means for suppressing variation of residual magnetization that depends upon the history of gradient pulse application, and completed the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a static magnetic field inhomogeneity measuring method, static magnetic field homogenizing method, MR data collecting method, and MRI apparatus by which the effect of residual magnetization caused by gradient pulses can be thoroughly reduced.
In accordance with its first aspect, the present invention provides a static magnetic field inhomogeneity distribution measuring method characterized in comprising: collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a killer gradient applied to a slice axis is made equal to that of a slice selective gradient, the intensity of a slice rephasing gradient is made half or about half of that of the slice selective gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, and the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient; collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by &dgr;t relative to said first gradient echo sequence; and measuring the distribution of static magnetic field inhomogeneity based on the phase difference between said MR data.
Conventionally, the intensity of the killer gradient applied to the slice axis is greater than that of the slice selective gradient. Therefore, the variation of residual magnetization due to the killer gradient is large. Thus, Japanese Patent Application Laid Open No. 2001-54510 described above proposes application of a residual magnetization reducing pulse immediately after the killer gradient.
In contrast, the intensity of the killer gradient applied to the slice axis is made equal to that of the slice selective gradient in the static magnetic field inhomogeneity distribution measuring method of the first aspect. Therefore, the variation of residual magnetization by the
Asano Kenji
Kosugi Susumu
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
Shrivastav Brij B.
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