Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-04-10
2004-03-02
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06700378
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the Japanese Application No. 2001-140310 filed May 10, 2,001.
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging (MRI) coil structure and a magnetic resonance imaging apparatus employing the coil structure, and particularly to a magnetic resonance imaging coil structure comprising a main magnetic field generating magnet, a gradient magnetic field generating coil, a shield, a magnetic field correcting shim plate and a transmission coil stacked in this order.
In recent years, magnetic resonance imaging apparatuses (MRI apparatuses) have attracted attention for their ability to provide tomographic images of a subject, such as the human body. In the MRI apparatuses, the magnetic property of hydrogen atomic nuclei (protons) within the subject is used and therefore a strong, homogeneous and stable magnetic field is generated.
The MRI apparatuses have employed a superconductive magnet to generate a main magnetic field. However, in using such a superconductive magnet, liquid helium is employed to attain the cryogenic state for realizing the superconductive state. MRI apparatuses that employ a permanent magnet, use no liquid helium, and have excellent openness to mitigate claustrophobic feeling experienced by the subject are coming into widespread use.
The MRI apparatuses employing the permanent magnet are configured to position the subject in a magnetic field space formed between a pair of magnetic resonance imaging coil structures disposed facing each other, and obtain a tomographic image of the subject. The coil structure is constructed by stacking a main magnetic field generating magnet (permanent magnet), a gradient magnetic field generating coil, a shield, a magnetic field correcting shim plate and a transmission coil in this order. Over the transmission coil, it is common to stack a cover made of a material like FRP.
The magnetic resonance imaging coil structure is constructed by sequentially stacking and assembling the main magnetic field generating magnet, gradient magnetic field generating coil, shield, magnetic field correcting shim plate and transmission coil that have been separately formed.
In such a magnetic resonance imaging coil structure, the distance (separation) between the shield and transmission coil must be controlled with a good accuracy. This is conducted because error in the distance causes an increase in the frequency shift and the error significantly affects the image quality of the resulting tomographic image. As an example, if the distance between the shield and transmission coil is generally about 20 mm, a tolerance of the order of 1 mm arises in practice during the aforementioned assembling and the amount of frequency shift due to the tolerance of 1 mm is about 100 kHz.
For this reason, a smaller tolerance of the distance between the shield and transmission coil is preferred. In a trial and error process, a plurality of alternative shim plates configured to be located between the shield and transmission coil are inserted or removed for correcting the magnetic field to adjust spatial homogeneity of the main magnetic field generated by the main magnetic field generating magnet. However, the tolerance of the thickness of the shim plate itself affects the distance between the shield and transmission coil, thus making it difficult to reduce the current tolerance.
It is desirable to reduce the tolerance under the present circumstances where further improvement of the image quality is desired.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic resonance imaging coil structure in which the tolerance of the distance between the shield and transmission coil can be reduced to reduce the amount of frequency shift, and a magnetic resonance imaging apparatus employing such a coil structure.
In its first aspect, the present invention provides a magnetic resonance imaging coil structure including a main magnetic field generating magnet, a gradient magnetic field generating coil, a shield, a magnetic field correcting shim plate and a transmission coil stacked in this order, characterized in that at least the shield and the transmission coil are integrally formed.
The phrase “at least a shield and a transmission coil are integrally formed” as used herein means that in a coil structure in which the shield and the transmission coil are integrally formed, the main magnetic field generating magnet and gradient magnetic field generating coil may be additionally integrally formed.
To be integrally formed, the distance between the shield and the transmission coil contains the tolerance during formation and is not affected by the tolerance of the magnetic field correcting shim plate positioned between the shield and transmission coil. Thus, the tolerance of the distance between the shield and transmission coil can be reduced relative to the tolerance of the distance between the shield and transmission coil in a conventional magnetic resonance imaging coil structure that is assembled by stacking the shield, magnetic field correcting shim plate and transmission coil. Consequently, the image quality of resulting tomographic images can be improved relative to the conventional ones. In addition, the adjustment work for correcting the amount of frequency shift cart be significantly reduced since the amount of frequency shift is reduced. For example, actual results achieved by the present inventors show that when the distance between the shield and transmission coil is 20 mm, the tolerance of the distance is of the order of 0.5 mm, which has been reduced by about half as compared to the conventional tolerance (about 1 mm). The thus-reduced tolerance results in a frequency shift of about 40 kHz. Moreover, the required RF power can also be reduced.
In its second aspect, the present invention provides a magnetic resonance imaging coil structure characterized in that a shim plate space is formed between the shield and transmission coil. The shim plate space is a space into which the magnetic field correcting shim plate can be inserted from the outer peripheral side. The shim plate space is also a space from which the magnetic field correcting shim plate can be removed from the outer peripheral side. The term “outer peripheral side” as used herein refers to the outside of the shield and transmission coil in a plane orthogonal to the stacking direction of the shield and transmission coil.
A trial and error process of inserting/removing the magnetic field correcting shim plate can be made easy since the magnetic field correcting shim plate can be arbitrarily and separately inserted from the outer peripheral side into the shim plate space formed between the shield and transmission coil. Moreover, the magnetic field correcting shim plate can be arbitrarily and separately removed from the outer peripheral side from the shim plate space formed between the shield and transmission coil.
In its third aspect, the present invention provides a magnetic resonance imaging coil structure, characterized in that the magnetic field correcting shim plate is divided into a plurality of generally lath-shaped portions, and the shim plate space is formed as tubular cavities into/from which the magnetic field correcting shim plate can be individually or removed from the outer peripheral side. The magnetic field correcting shim plate is divided into the lath-shaped portions.
The division of the magnetic field correcting shim plate into a plurality of lath-shaped portions reduces the work of inserting into or withdrawing from the shim plate space, an undivided large integral magnetic field correcting shim plate. When inhomogeneity of the main magnetic field is to be corrected, a lath-shaped portion corresponding to the inhomogeneous space may be replaced.
In its fourth aspect, the present invention provides a magnetic resonance imaging coil structure, characterized in that the magnetic field correcting shim plate is divided into a plurality of generally
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Gutierrez Diego
Horton Esq. Carl B.
Vargas Dixonmara
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