Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-04-03
2003-04-22
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06552539
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2001-105528 filed Apr. 4, 2001.
BACKGROUND OF THE INVENTION
The present invention relates to a method of correcting a resonance frequency variation and an MRI (Magnetic Resonance Imaging) apparatus and, more specifically, to a method of correcting a resonance frequency variation, which is capable of handling all frequency drifts caused by fluctuations in magnetic field including a frequency drift whose time change is slow, a frequency drift in a slice direction and a frequency drift whose time change is fast.
As a first prior art, Japanese Laid-open Patent Publication No. 1-141656 discloses a technology for correcting a frequency drift (resonance frequency variation) caused by fluctuations in magnetic field by collecting correction data as well at the time of collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding quantity a plurality of times and adjusting the amount of a current running through a magnetostatic coil based on the collected correction data.
As a second prior art, Japanese Patent No. 2528864 disclose a technology for making phase correction operation on imaging data according to magnetostatic intensity by measuring and storing the magnetostatic intensities of a plurality of points.
In the above first prior art, since correction based on correction data collected at the time of a sequence of imaging pulses of a certain time cannot be in time for the imaging data of this time and is effective for imaging data of the next time, a frequency drift whose time change is fast cannot be handled though a frequency drift whose time change is slow has no problem.
In the above second prior art, since imaging data collected at a certain time can be corrected based on correction data collected at the time of a sequence of imaging pulses of that time, a frequency drift whose time change is fast can be handled. However, since the processing is made after the collection of data, correction is effective for a sliced surface alone and a frequency drift in a slice direction cannot be handled.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of correcting a resonance frequency variation, which is capable of handling all frequency drifts including a frequency drift whose time change is slow, a frequency drift in a slice direction and a frequency drift whose time change is fast as well as an MRI apparatus.
According to a first aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well each time when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time without correcting the frequency variation when the amount of the resonance frequency variation is not smaller than the threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
When the amount of the resonance frequency variation is smaller than the threshold value, as the resonance frequency variation is not a frequency drift whose time change is fast, it will suffice without correcting the imaging data of this time if the correction of the imaging data of the next time is effective. Therefore, only the correction of a frequency variation has to be carried out and the amount of a resonance frequency variation or resonance frequency does not need to be stored. On the other hand, when the amount of the resonance frequency variation is not smaller than the threshold value, as the resonance frequency variation is a frequency drift whose time change is fast, correction must be made on the imaging data of this time as well. Therefore, the amount of the resonance frequency variation or the resonance frequency is stored and correction operation may be made based thereon.
The method of correcting a resonance frequency variation according to the first aspect of the present invention is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later, the imaging data of this time can be corrected. When correction operation is made later, frequency variation correction is not made to simplify control.
According to a second aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well each time when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time when the amount of the resonance frequency variation is not smaller than a threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above second aspect of the present invention is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later, the imaging data of this time can be corrected. Since frequency variation correction is made even when correction operation is made later, a frequency drift in a slice direction can always be handled.
According to a third aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well once out of a plurality of times when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times without correcting the frequency variation when the amount of the resonance frequency variation is not smaller than the threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above third embodiment is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later,
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
Lefkowitz Edward
Shrivastav Brij B.
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