Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-01-03
2002-05-21
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06392411
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an MR imaging method, a phase shift measuring method and an MR (Magnetic Resonance) imaging system. More specifically, the present invention is concerned with an MR imaging method using a pulse sequence, which is capable of preventing the influence of variations in residual magnetization due to a change in gradient or gradient magnetic field pulse, a phase shift measuring method for measuring phase shifts in subsequent echoes due to the influence of eddy currents and residual magnetization caused by each preceding phase encoding pulse or the like in the pulse sequence, and an MR imaging system for executing these methods.
2. Description of the Related Art
The following prior arts have been disclosed in Japanese Patent Application Laid-Open No. Hei 10-75940.
(1) A phase shift measuring method for executing a pre-scan sequence for transmitting an excitation pulse, transmitting an inversion pulse, applying a phase encoding pulse to a phase axis, applying a read pulse to a read axis, applying a rewind pulse to the phase axis, continuously transmitting an inversion pulse, from echoes while a read pulse is being applied to the phase axis, and measuring a phase shift in each subsequent echo due to the influence of eddy currents and residual magnetization caused by each phase encoding pulse or the like, based on phase data obtained by converting the collected data into one-dimensional Fourier form.
(2) An MR imaging method wherein in a pulse sequence using a high-speed spin echo process for transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while a read pulse is being applied to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, and collecting data of a plurality of echoes by one excitation, a compensating pulse for compensating for the amount of the phase shift measured by the phase shift measuring method described in the paragraph (1) is built or integrated into a phase encoding pulse, added to one or both immediately before or after the phase encoding pulse, built in a rewind pulse or added to one or both immediately before or after the rewind pulse.
The prior arts have been based on the precondition that the amount of the phase shift measured by the phase shift measuring method described in the paragraph (1) and the amount of the phase shift produced when no compensating pulse is provided in the pulse sequence of the high-speed spin echo process described in the paragraph (2), are equal to each other.
In a permanent magnet type MR imaging system, however, the two are not necessarily made equal to each other due to a magnetic hysteresis characteristic of a magnetic shunt plate or the like. Thus, the precondition employed in the prior arts is not established. This will be explained with reference to
FIGS. 1 through 3
.
FIG. 1
is a pulse sequence diagram showing the conventional high-speed spin echo process.
In this FSE (Fast Spin Echo) sequence SQ, an excitation pulse R and a slice gradient or gradient ss are first applied. Next, a first inversion pulse P
1
and a slice gradient ss are applied. Then a phase encoding pulse gy
1
i is applied to a phase axis. Next, an NMR signal is received from a first echo echo
1
while a leas pulse gxw is being applied. Thereafter, a rewind pulse gy
1
ri equal in time integrated value and opposite in polarity to the phase encoding pulse gy
1
i is applied to the phase axis. Incidentally, i indicates repetitive numbers. i=1 through I (e.g., I=128).
Next, a second inversion pulse P
2
and a slice gradient ss are applied, a phase encoding pulse gy
2
i is applied to the phase axis, and an NMR signal is received from a second echo echo
2
while the read pulse gxw is being applied. Afterwards, a rewind pulse gy
2
r equal in time integrated value and opposite in polarity to the phase encoding pulse gy
2
i is applied to the phase axis.
A jth inversion pulse Pj and a slice gradient ss are applied subsequently in the same manner as described above. A phase encoding pulse gyji is applied to the phase axis. An NMR signal is received from a jth echo echoj while the read pulse gxw is being applied. Thereafter, the application of a rewind pulse gyjri identical in time integrated value and opposite in polarity to the phase encoding pulse gyji to the phase axis is repeated for j=3 to J (e.g., J=8).
Assuming that the residual magnetization prior to the application of a phase encoding pulse gy
1
l (i=1) is defined as m
1
, the residual magnetization traces histories a
1
and a
2
so as to reach m
1
again by the application of the phase encoding pulse gy
1
l, as shown in
FIG. 2
due to the magnetic hysteresis characteristic of the magnetic shunt plate or the like in the permanent magnet type MR imaging system. Further, the residual magnetization traces histories a
3
and a
4
so as to reach m
2
by the application of a rewind pulse gy
1
rl.
Since the residual magnetization preceding the application of a phase encoding pulse gy
2
l becomes m
2
, the residual magnetization traces histories a
5
and a
6
so as to reach m
3
by the application of the phase encoding pulse gy
2
l, as shown in FIG.
3
. Further, the residual magnetization traces histories a
7
and a
8
so as to reach m
2
again by the application of a rewind pulse gy
2
rl.
In the permanent magnet type MR imaging system as described above, the residual magnetization varies depending on the histories in which the gradient magnetic field pulses such as the phase encoding pulse and the rewind pulse are varied, due to the magnetic hysteresis characteristic of the magnetic shunt plate or the like.
However, since the pre-scan sequence takes such a form as obtained by cutting down ones up to the collection of the first echo in the FSE sequence, the gradient magnetic field pulses coincide in application history with each other and the residual magnetization makes consistence, with respect to the first echo. Therefore, the aforementioned precondition is established. However, since the gradient magnetic field pulses do not coincide in application history with each other in the second echo or later, the residual magnetization does not make consistence and hence the aforementioned precondition is no longer established.
Therefore, a problem arises in that in the second echo or later, a phase shift in subsequent echo due to the influence of the residual magnetization caused by each gradient magnetic field pulse cannot be sufficiently corrected.
Since a linear relationship is established between the area of each gradient magnetic field pulse and a generated phase error, the above-described problem is considered to be free from occurring in eddy currents.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an MR imaging method capable of preventing the influence of a variation in residual magnetization due to a change in gradient magnetic field pulse.
A second object of the present invention is to provide a phase shift measuring method capable of causing a phase shift in subsequent echo due to the influence of eddy currents and residual magnetization caused by a preceding phase encoding pulse or the like in an FSE pulse sequence to coincide with an actual FSE sequence to thereby measure the phase shift.
Further, a third object of the present invention is to provide an MR imaging system for executing the above-described methods.
In a first aspect, the present invention provides an MR imaging method using a high-speed spin echo process comprising the steps of transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while applying a read pulse to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, collecting data from a plural
Arana Louis
GE Yokogawa Medical Systems Limited
Kojima Moonray
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