Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-06-20
2004-11-16
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06819104
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2001-187956 filed Jun. 21, 2001.
BACKGROUND OD THE INVENTION
The present invention relates to a magnetic resonance (MR) imaging method and a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to an MR imaging method and an MRI system capable of reconstructing good-quality images.
U.S. Pat. No. 2,898,329 has disclosed an MR imaging method according to which:
(1) data acquisition in steady-state free precession (SSFP) is repeated by sequentially changing a phase for phase encoding until data fv(
0
) is acquired from views v constituting a k-space;
(2) data acquisition in SSFP is repeated by sequentially changing a phase for phase encoding and alternately shifting the phase of an RF pulse by 180°, whereby data fv(
1
) is acquired from views v constituting a k-space;
(3) fv(
0
) and fv(
1
) are subjected to addition or subtraction in order to produce data Av that is expressed as follows:
Av
=0.5
×Fv
(
0
)+0.5
×Fv
(
1
) or
Av
=0.5
×Fv
(
0
)−0.5
×Fv
(
1
); and
(4) an image is reconstructed based on the produced data Av.
According to the MR imaging method disclosed in U.S. Pat. No. 2,898,329, good-quality images are produced in some cases. However, only poor-quality images (for example, images having band artifacts caused by an inhomogeneous magnetic field) can be produced in other cases.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an MR imaging method and an MRI system capable of reconstructing good-quality images in cases where any conventional MR imaging method can provide only poor-quality images.
From the first aspect of the present invention, there is provided an MR imaging method having steps described below. That is to say, at the first step (1), data acquisition in steady-state free precession (SSFP) is repeated N times (where N equals the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(
0
) to data fv(N−1) is acquired from views v constituting a k-space. At this time, the phase of a radio-frequency (RF) pulse is varied based on an expression of 360°·v·k/N. At the second step (2), if an operator designates Fourier transform (FT) imaging, the data fv(k) is phase-encoded relative to the phases indicated by the RF pulse and then subjected to a Fourier transform. This results in data Fv(n). In contrast, if the operator does not designate Fourier transform imaging, the data fv(k) is regarded as data Fv(n) as it is. At the third step (3), any of at least either of weighted addition and maximum intensity projection (MIP) processing and root-means-square conversion which is selected by the operator is performed on the data Fv(n) in order to produce data Av. At the fourth step (4), an image is reconstructed based on the produced data Av.
According to the MR imaging method provided from the first aspect of the present invention, an operator can designate whether a Fourier transform (FT) should be performed on the data fv(k) relative to each of the phases indicated by the RF pulse. The Fourier transform makes it possible to designate whichever of the free induction decay (FID) component of data and the spin echo or stimulated echo component thereof should be dominant owing to the principle described below.
For example, when N=4, if
k=0
, the phase of an RF pulse is set to 0 for all times of data acquisition. The polarity of the FID component of data fv(
0
) agrees with a positive Y direction (the positive direction of a Y axis), while the polarity of the spin echo or stimulated echo component thereof agrees with a negative Y direction (the negative direction of the Y axis). If k=1, the phase of an RF pulse is set sequentially to 0, &pgr;/2, &pgr;, 3&pgr;/2, etc. The polarity of the FID component of data fv(
1
) agrees with the positive Y direction, while the polarity of the spin echo or stimulated echo component thereof agrees with a positive X direction (the positive direction of the X axis). If k=2, the phase of an RF pulse is set alternately to 0 and &pgr;. The polarity of the FID component of data fv(
2
) agrees with the positive Y direction, and the polarity of the spin echo or stimulated echo component thereof also agrees with the positive Y direction. If k=3, the phase of an RF pulse is set sequentially to 0, 3&pgr;/2, &pgr;, &pgr;/2, etc. The polarity of the FDI component of data fv(
3
) agrees with the positive Y direction, while the polarity of the spin echo or stimulated echo component thereof agrees with a negative X direction (the negative direction of the X axis).
Since data Fv(
0
)=fv(
0
)+fv(
1
)+fv(
2
)+fv(
3
), the FID components are left intact because the spin echo or stimulated echo components are canceled out due to the above polarities. In reality, a situation disagrees with the ideal. Nevertheless, in the resultant data Fv(
0
), the FID component thereof is dominant. Moreover, since data Fv(
1
)=fv(
0
)−j·fv(
1
)−fv(
2
)+j·fv(
3
), the spin echo or stimulated echo components are left intact because the FID components are canceled out due to the above polarities. Consequently, the spin echo or stimulated echo component of the data Fv(
1
) is dominant. In general, if n in data Fv(n) assumes an odd-numbered value, the FID component is dominant. If n assumes an even-numbered value, the spin echo or stimulated echo component is dominant. Thus, whichever of the FID component and the spin echo or stimulated echo component is dominant can be designated.
Moreover, according to the MR imaging method provided from the first aspect of the present invention, an operator can select processing to be performed on the data Fv(n) from among at least either of weighted addition and MIP and root-mean-square conversion. If weighted addition is performed on the data, whichever of the FID component and the spin echo or stimulated echo component is dominant can be designated. If MIP is performed, a signal-to-noise ratio can be improved. Moreover, If root-mean-square conversion is performed, the signal-to-noise ratio can be improved.
According to the MR imaging method provided from the first aspect of the present invention, processing can be selected from among at least four kinds of processing. In cases where any conventional MR imaging method can produce only poor-quality images, good-quality images may be able to be produced.
Studies made by the present inventor have revealed that: according to the MR imaging method disclosed in the U.S. Pat. No. 2,898,329, if root-mean-square conversion is performed on data fv(
0
) and fv(
1
) that represent view images having band artifacts, an image devoid of the band artifacts may be produced. Moreover, if the number of times of repetition N is increased (for example, 8 or more) and a Fourier transform and root-mean-square conversion are selected, a good-quality image is produced in many cases.
From the second aspect of the present invention, there is provided an MR imaging method comprising the steps described below. Namely, at the first step (1), data acquisition in SSFP is repeated N times (where N equals the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(
0
) to data fv(N−1) is acquired from views v constituting a k-space. At this time, the phase of an RF pulse is varied based on an expression of 360°·v·k/N. At the second step (2), a Fourier transform is performed on the data fv(k) relative to each of the phases indicated by the RF pulse in order to produce data Fv(n). At the third step (3), any of at least either of weighted addition and MIP processing and root-mean-square conversion selected by an operator is performed on the data Fv(n) in order to produce data Av. At the fourth step (4), an image is reconstructed based on the produced data Av.
According to the MR imaging method provided from the second aspect
Adachi Naotaka
Oda Yoshihiro
Yamazaki Aki
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Gutierrez Diego
Horton Esq. Carl B.
Vargas Dixomara
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