Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-06-20
2004-03-16
Philogene, Haissa (Department: 2821)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S319000, C324S320000, C324S322000, C324S331000
Reexamination Certificate
active
06707301
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2001-187532 filed Jun. 21, 2001.
BACKGROUND OF THE INVENTION
The present invention relates to an external magnetic field measuring method, a static magnetic field correcting method, an external magnetic field measuring apparatus, and a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to an external magnetic field measuring method for measuring the strength of a magnetic field applied externally to an MRI system. The present invention also relates to a static magnetic field correcting method for correcting a variation in the strength of a static magnetic field caused by a magnetic field applied externally to an MRI system. Moreover, the present invention relates to an external magnetic field measuring apparatus for measuring the strength of a magnetic field applied externally to an MRI system. Furthermore, the present invention relates to an MRI system having the ability to correct a variation in the strength of a static magnetic field caused by a magnetic field applied externally thereto.
In MRI systems, a magnet assembly generates a static magnetic field of a predetermined strength. However, the strength of the static magnetic field is varied with application of an external magnetic field to an MRI system.
In efforts to prevent the variation in the strength of a static magnetic field caused by an external magnetic field, the magnet assembly included in the MRI system may be stored in a shield room or buried under the ground (Japanese Unexamined Patent Publication No. 2000-70245).
However, when only the passive method of storing a magnet assembly of an MRI system in a shield room or burying the magnet assembly under the ground is adopted, a variation in the strength of a static magnetic field caused by an external magnetic field cannot be prevented satisfactorily.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide an external magnetic field measuring method and apparatus for measuring the strength of a magnetic field applied externally to an MRI system.
The second object of the present invention is to provide a static magnetic field correcting method for correcting a variation in the strength of a static magnetic field caused by a magnetic field externally applied to an MRI system, and to provide an MRI system having the ability to correct the variation in the strength of a static magnetic field caused by a magnetic field applied externally thereto.
From the first aspect of the present invention, there is provided an external magnetic field measuring method according to which a magnetization detecting means is placed near a magnet assembly included in an MRI system. A magnetic field generating means is located near the magnetization detecting means. The magnetic field generating means generates a compensation magnetic field so that the compensation magnetic field will cancel a magnetic field generated by the magnet assembly and detected by the magnetization detecting means in the absence of an external magnetic field. The magnetization detecting means measures the external magnetic field with the compensation magnetic field generated.
In order to highly precisely measure an external magnetic field that adversely affects a static magnetic field generated in an MRI system, a magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field is preferably located near a magnet assembly. This is however unfeasible because near the magnet assembly, a static magnetic field generated by the magnet assembly is much stronger than the external magnetic field. Therefore, if the magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field is adopted, the strength of the static magnetic field generated by the magnet assembly exceeds the dynamic range. This results in a failure to measure the external magnetic field. In reality, therefore, the dynamic range of the magnetization detecting means is expanded or the magnetization detecting means is located apart from the magnet assembly. If the former countermeasure is adopted, the external magnetic field cannot be detected high precisely. In contrast, if the latter is adopted, it is uncertain whether an external magnetic field being measured is an external magnetic field that adversely affects a static magnetic field generated in the MRI system.
In contrast, according to the external magnetic field measuring method provided from the first aspect, the magnetic field generating means generates a compensation magnetic field so that the compensation magnetic field will cancel a magnetic field generated by the magnet assembly. The magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field can therefore be located near the magnet assembly. Consequently, an external magnetic field that adversely affects a static magnetic field generated in an MRI system can be measured highly precisely.
From the second aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring method. Herein, when the magnetization detecting means includes a Z-direction magnetic sensor. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction, the Z-direction magnetic sensor detects a magnetization exhibited in the Z direction.
According to the external magnetic field measuring method provided from the second aspect of the present invention, the Z-direction magnetic sensor is included. Therefore, the major component of an external magnetic field that is directed in the same direction as the static magnetic field and that adversely affects the static magnetic field generated in an MRI system can be measured preferably.
From the third aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetization detecting means includes a Z-direction magnetic sensor, a Y-direction magnetic sensor, and an X-direction magnetic sensor. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction and that two mutually orthogonal directions orthogonal to the Z direction are regarded as Y and X directions respectively, the Z-direction magnetic sensor, Y-direction magnetic sensor, and X-direction magnetic sensor detect magnetizations exhibited due to the magnet in the Z, Y, and X directions respectively.
According to the external magnetic field measuring method provided from the third aspect of the present invention, the Z-direction magnetic sensor is included. Therefore, the major component of an external magnetic field that adversely affects the static magnetic field generated in an MRI system and that is directed in the same direction as the static magnetic field can be measured preferably. Moreover, since the Y-direction magnetic sensor and X-direction magnetic sensor are also included, the components of the external magnetic field other than the major component that adversely affect the static magnetic field generated in the MRI system can be measured preferably.
From the fourth aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetic field generating means includes at least a pair of small-size coils and a coil drive circuit. The pair of small-size coils is arranged to sandwich the Z-direction magnetic sensor in the Z direction. The coil drive circuit feeds a current to the pair of small-size coils.
According to the external magnetic field measuring method provided from the fourth aspect of the present invention, a current flows into the pair of small-size coils that is opposed to each other in the Z direction. Consequently, a comp
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
Philogene Haissa
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