Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-09-26
2004-02-24
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06696835
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a second-order static magnetic field correcting method and an MRI (magnetic resonance imaging) apparatus, and more particularly to a second-order static magnetic field correcting method for correcting second-order static magnetic field components to improve homogeneity in an MRI apparatus and an MRI apparatus that can implement the method.
The static magnetic field of an MRI apparatus should be homogeneous. Homogeneity of the static magnetic field is achieved by mechanical shimming or by adding small pieces of magnet, iron or the like.
A metal mass (e.g., an automobile) moving near the MRI apparatus or an environment change (e.g., a change in temperature) varies the static magnetic field, and second-order static magnetic field components are generated.
Pulse sequences that observe gradient echoes, such as one according to GRASS (gradient recalled acquisition in the steady state) or SPGR (spoiled GRASS), are very sensitive to the static magnetic field components, and the generation of the second-order static magnetic field components leads to degradation of image quality.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a second-order static magnetic field correcting method for correcting second-order static magnetic field components to improve homogeneity and an MRI apparatus that can implement the method.
In accordance with its first aspect, the present invention provides a second-order static magnetic field correcting method characterized in: disposing a first circular loop coil and a second circular loop coil at positions spaced in a static magnetic field direction to be symmetrical with respect to a center of an imaging region of an MRI apparatus; disposing a third circular loop coil and a fourth circular loop coil having a larger diameter than that of said first and second circular loop coils at positions spaced in the static magnetic field direction to be symmetrical with respect to the center of said imaging region; generating a first corrective magnetic field and a second corrective magnetic field in the same direction by said first and second circular loop coils; and generating a third corrective magnetic field and a fourth corrective magnetic-field in the same direction and opposite to said first corrective magnetic field by said third and fourth circular loop coils; thereby correcting second-order static magnetic field components in the static magnetic field direction.
In this specification, by a “second order” is meant a quadric function of a position in a static magnetic field direction. By a “zeroth order” is meant independence of the position in the static magnetic field direction. Furthermore, by a “first order” is meant a linear function of the static magnetic field direction.
In the second-order static magnetic field correcting method of the first aspect, since the direction of the first and second corrective magnetic fields generated by the first and second circular loop coils is opposite to the direction of the third and fourth corrective magnetic fields generated by the third and fourth circular loop coils, zeroth-order corrective magnetic field components in the static magnetic field direction can cancel one another, and thus the zeroth-order static magnetic field components are not affected. On the other hand, since the zeroth-order corrective magnetic field components are independent of the second-order corrective magnetic field components, the second-order corrective magnetic field components remain even after the zeroth-order corrective magnetic field components have canceled one another. Therefore, the second-order corrective magnetic field components can cancel the second-order, static magnetic field components, and homogeneity of the static magnetic field can be improved.
Since the first and second corrective magnetic fields are in the same direction, these fields will not generate first corrective magnetic field components. Similarly, since the third and fourth corrective magnetic fields are in the same direction, these fields will not generate first corrective magnetic field components.
In accordance with its second aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that said first and second circular loop coils are disposed substantially coplanar with gradient coils for the static magnetic field direction and outside said gradient coils.
In the second-order static magnetic field correcting method of the second aspect, since the first and second circular loop coils are disposed substantially coplanar with gradient coils for the static magnetic field direction and outside the gradient coils, symmetry with respect to the gradient coils for the static magnetic field direction is conserved; moreover, the first and second corrective magnetic fields are in the same direction, and therefore the gradient magnetic fields are not substantially affected. Furthermore, since circular loop coils, which have no return path, exhibit better linearity than that of gradient coils having return paths, linearity of the gradient magnetic fields is not substantially affected.
In accordance with its third aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that said third and fourth circular loop coils are disposed surrounding magnetism conditioning plates.
In the second-order static magnetic field correcting method of the third aspect, since the third and fourth circular loop coils are disposed surrounding magnetism conditioning plates, the space for installing the circular loop coils can be easily secured. Moreover, there is no need for concern about coupling with the gradient coils.
If the third and fourth circular loop coils are disposed so that the conditions of Helmholtz coils are fulfilled, second-order corrective magnetic field components generated by the third and fourth circular loop coils can be ignored. Therefore, in order to cancel second-order static magnetic field components, only the second-order corrective magnetic field components generated by the first and second circular loop coils need to be adjusted, and thus the adjustment can be done easily.
In accordance with its fourth aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that at least one of the ratio of electric currents and the turns ratio of said first through fourth circular loop coils is determined so that zeroth-order corrective magnetic field components in the static magnetic field direction cancel one another.
In the second-order static magnetic field correcting method of the fourth aspect, since the ratio of electric currents or the turns ratio is adjusted so that the zeroth-order corrective magnetic field components are canceled out, only the electric current values need to be adjusted to cancel the second-order static magnetic field components while maintaining the ratio of corrective electric currents, and thus the adjustment can be done easily.
In accordance with its fifth aspect, the present invention provides a second-order static magnetic field correcting method, characterized in: disposing three RF probes at different positions in a static magnetic field direction of an MRI apparatus, each of which probes has a small phantom capable of emitting an FID (free induction decay) signal and a small coil combined; transmitting RF pulses from said RF probes and receiving FID signals at a time when a magnetic field variation is to be measured; determining frequencies f
1
, f
2
and f
3
from the FID signals; determining a second-order static magnetic field component, &bgr;
2
by solving the following simultaneous equations:
f
1
=&bgr;
0
+&bgr;
1
·r
1
+&bgr;
2
·r
1
2
f
2
=&bgr;
0
+&bgr;
1
·r
2
+&bgr;
2
·r
2
2
,
f
3
=&bgr;
0
+&bgr;
1
·r
3
+&bgr;
2
·r
3
GE Medical Systems Global Technology Company LLC
Kojima Moonray
Shrivastav Brij B.
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