Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-08-20
2003-12-16
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06664787
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a phase error measuring method and apparatus, phase error correcting method and apparatus, recording medium and magnetic resonance imaging apparatus, and more particularly to a method and apparatus for measuring a phase error in spins due to the effect of residual magnetization etc., a method and apparatus for correcting such a phase error, a recording medium recorded with a program for operating a computer to perform a phase error measuring function, a recording medium recorded with a program for operating a computer to perform a phase error correcting function, a magnetic resonance imaging apparatus comprising phase error measuring means, and a magnetic resonance imaging apparatus comprising phase error correcting means.
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, i.e., a space in which a static magnetic field is generated; gradient magnetic fields and a high frequency magnetic field are applied to generate magnetic resonance signals from spins within the object; and a tomographic image is reconstructed based on the received signals.
The gradient magnetic fields are applied in three mutually perpendicular axis directions. The three mutually perpendicular axes are slice, phase and frequency axes. The gradient magnetic field in the slice axis direction serves to selectively excite a desired slice on the slice axis by an RF (radio frequency) excitation signal, and is sometimes referred to as a slice gradient. The gradient magnetic field in the phase axis direction serves for phase encoding of the spins, and is sometimes referred to as a phase encoding gradient. The gradient magnetic field in the frequency axis direction serves for readout of the magnetic resonance signal, and is sometimes referred to as a readout gradient. The magnetic resonance signal is read out as an echo signal.
One magnetic resonance imaging method is the fast spin echo (FSE) technique. This technique involves exciting spins by 90°, followed by repeating inversion of the spins by a 180° excitation a plurality of times to acquire spin echoes for a plurality views for each 90° excitation.
A pulse sequence of the FSE technique is shown in FIG.
1
. In
FIG. 1
, (1) is a sequence of 90° and 180° excitations; (2), (3) and (4) are sequences of slice gradients Gs, phase encoding gradients Gp and readout gradients Gr, respectively; and (5) is a sequence of spin echoes SE. These sequences proceed along a time axis t.
As shown, a 90° excitation is effected while applying a slice gradient Gs
1
. Next, after a time period U
1
, a first 180° excitation is effected while applying a slice gradient Gs
2
. Next, after a time period U
2
, a second 180° excitation is effected while applying a slice gradient Gs
3
. Thereafter, third, fourth, . . . 180° excitations are effected while applying respective slice gradients Gs
4
, Gs
5
, . . . at every time period U
2
in a similar manner.
During the time period from the 90° excitation to the first 180° excitation, a readout gradient Gr
1
is applied to perform phase dispersion, or dephasing, of the spins. Next, during the time period from the first 180° excitation to the second 180° excitation, a readout gradient Gr
2
is applied to perform phase focusing, or rephasing, of the spins and generate a first spin echo SE
1
. The readout gradient Gr
2
, which generated the spin echo, dephases the spins in its latter half portion.
Prior to the application of the readout gradient Gr
2
, a phase encoding gradient Gp
1
is applied to perform phase encoding, and after the application of the readout gradient Gr
2
is completed, a phase encoding gradient Gp
1
′ is applied in the opposite direction to cancel the phase encoding.
Thereafter, readout gradients Gr
3
, Gr
4
, . . . are applied during every time period between the 180° excitations to generate respective spin echoes SE
2
, SE
3
, . . . in a similar manner. Moreover, phase encoding is achieved by phase encoding gradients Gp
2
, Gp
3
. . . The phase encoding is differentiated every time.
The spin echo is an RF signal having maximum amplitude at the center of the echo. The maximum amplitude, or a peak, of the first spin echo SE
1
occurs after a time period TE (echo time) from the 90° excitation. A peak of the second spin echo SE
2
occurs after the time period TE from the peak of the first spin echo SE
1
. Thereafter, peaks of the spin echoes SE
3
, SE
4
, . . . occur at intervals of time period TE in a similar manner. The generation of a peak is sometimes referred to as focalization (image formation) of a spin echo.
In a magnet system that achieves static magnetic field generation by permanent magnets, residual magnetization may occur owing to, for example, magnetization of pole pieces of the permanent magnets by the gradient magnetic fields. Since the residual magnetization in the frequency axis direction affects the dephasing of the spins during the time period between the 90° and 180° excitations, timing of the spin echo focalization, or timing of the peak generation, experiences an error. A similar phenomenon is produced by eddy currents.
For example, if the first spin echo SE
1
focalizes in a time period TE′ shorter than proper TE, the second spin echo SE
2
focalizes in a time period TE″ longer than proper TE, and the third spin echo SE
3
focalizes at timing TE′ shorter than proper TE, due to the timing error in the focalization. Thereafter, the spin echo SEi (i:
4
,
5
,
6
. . . ) focalizes at timing such that a time period longer than proper TE and a time period shorter than proper TE alternate.
Since such a focalization error is a source of artifact generation in a reconstructed image, an attempt has been made to cancel the effect of the resident magnetization, eddy current etc. in the frequency axis direction by adjusting the readout gradient to correct the focalization error.
When residual magnetization exists also in the phase axis direction, a focalization error occurs due to the effect of the residual magnetization. The focalization error, however, cannot be corrected by adjusting the readout gradient because the axis of the gradient is different. Moreover, in the first place, the effect of the residual magnetization etc. in the phase axis direction on the phase of spins cannot be accurately measured.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide method and apparatus for accurately measuring a phase error in spins in the phase axis direction, method and apparatus for correcting such a phase error, a recording medium recorded with a program for operating a computer to perform a phase error measuring function, a recording medium recorded with a program to operate a computer to perform a phase error correcting function, a magnetic resonance imaging apparatus comprising phase error measuring means, and a magnetic resonance imaging apparatus comprising phase error correcting means.
(1) The present invention, in accordance with one aspect for solving the aforementioned problem, is a phase error measuring method characterized in comprising: effecting a 90° excitation on object spins; effecting a first 180° excitation after a first time period from said 90° excitation; effecting a second 180° excitation after a second time period from said first 180° excitation; effecting a third 180° excitation after said second time period from said second 180° excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180° excitation to said second 180° excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180° excitation to said third 180° excitation to read out a second spin echo signal; and determining a phase error of the spins in the phase axis direction during the time period from said 90° excitation to sa
Miyoshi Mitsuharu
Yamazaki Aki
GE Medical Systems Global Technology Company LLC
Gutierrez Diego
Moonray Kojima
Vargas Dixomara
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