Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-01-18
2001-04-17
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06218834
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of phase shift measurement, method of phase shift correction, and magnetic resonance imaging (MRI) apparatus. More particularly, the invention relates to a method of measuring the phase shift of echos caused by the influence of the eddy current and residual magnetization attributable to the preceding encode gradient, etc., a method of correcting the phase shift of echos, and an MRI apparatus which carries out these methods.
2. Description of the Prior Art
The split echo train method is intended to divide multiple echos of an echo train into former-part echos and latter-part echos and produce a first image (proton weighted image) from the former-part echos and a second image (T
2
weighted image) from the latter-part echos.
The pulse sequence of the split echo train method has a large amplitude of the encode gradient on its waveform, with its duration being reduced accordingly, in order to reduce the cycle time.
The same technique is used in the conventional high-speed spin echo (SE) method, i.e., it has a pulse sequence including an encode gradient of a large amplitude and short duration in order to reduce the cycle time.
However, the eddy current emerging in the conductor around the gradient coil increases as the amplitude of gradient pulse increases and its duration decreases. The eddy current effects the increase of the phase shift among echos, and the phase shift produces ghost images in the phase axis direction on the image, i.e., creation of artifacts.
A technique for reducing such ghost images is proposed in publication: Proc. SMR, p. 634, 1995, by R. Scott Hinks et al., in which pre-scan data is sampled without applying the encode gradient at pre-scanning, the sampled data is rendered the first-order Fourier transform along the frequency axis of the K-space thereby to obtain phase data, and the offset phase for the read gradient and inversion pulse P is corrected based on the phase data for the main scanning of imaging.
A method of phase correction process is proposed in publication: Mag. Reso. in Med., pp.632-638, 1995, by Xin Wan, Dennis L. Parker, et al., in which after the main scanning of imaging, correction data is sampled without applying the encode gradient, and the phase correction process is implemented based on the correction data at the calculation of image rearrangement.
MRI apparatus based on the permanent magnet also involve the above-mentioned problem of phase shift caused by the eddy current and, in addition, the problem of phase shift caused by the residual magnetization. Specifically, the residual magnetization emerging in the magnetic rectifying plates, for example, increases and affects the phase shift significantly as the pulse amplitude increases.
However, in the foregoing prior art methods which sample correction data without applying the encode gradient, the correction data does not include the influence of the residual magnetization, and therefore these methods cannot correct the phase shift caused by the influence of the residual magnetization.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a method of phase shift measurement for measuring the phase shift of echos caused by the influence of the eddy current and residual magnetization attributable to the preceding encode gradient, etc.
A second object of the present invention is to provide a method of phase shift correction for correcting the phase shift of echos thereby to prevent the deterioration of image cause by the influence of the eddy current and residual magnetization attributable to the preceding encode gradient, etc.
A third object of the present invention is to provide an MRI apparatus which carries out the above-mentioned phase shift measuring method and phase shift correcting method.
In a first aspect, the present invention resides in a method of phase shift measurement which comprises the steps of emitting an excitation pulse, emitting an inversion pulse, applying an encode gradient to the phase axis, applying a read gradient to the read axis and applying a rewind gradient to the phase axis, with these operations being implemented once or more, and subsequently emitting an inversion pulse, applying a dephaser gradient to the phase axis, sampling data from an echo while applying a readout gradient to the phase axis, and evaluating the phase shift caused by the influence of the encode gradient based on phase data resulting from the first-order Fourier transform for the sampled data.
The phase shift measuring method of the first aspect samples data from an echo while applying the dephaser gradient and readout gradient to the phase axis, but without applying a read gradient. The sampled data align on the trajectory along the phase axis in the K-space, and accordingly the 1st-order phase shift (a phase shift component which exhibits the 1st-order variation of the phase shift value along the phase axis of the K-space) can be known accurately from the phase data resulting from the first-order Fourier transform for the sampled data. Accordingly, by sampling data in this manner following the application of the encode gradient, read gradient and rewind gradient, it is possible to measure accurately the phase shift caused by the influence of the eddy current and residual magnetization attributable to the encode gradient, etc.
A first variant aspect, which is derived from the phase shift measuring method of the first aspect, includes the steps of sampling data by applying a dephaser gradient and readout gradient, with their polarity being inverted, and evaluating the phase shift caused by the influence of the encode gradient based on phase data resulting from the first-order Fourier transform for the sampled data and the phase data obtained prior to the polarity inversion.
The eddy current is also created by the dephaser gradient, and therefore the measurement result can possibly be inconsistent with that of main scanning unless the influence of dephaser gradient is eliminated. The eddy current created by the dephaser gradient acts in the same direction irrespective of the polarity of dephaser gradient in regard to the 1st-order phase shift, whereas the phase shift caused by the preceding encode gradient, etc. acts in the opposite direction when the dephaser gradient has its polarity inverted. Accordingly, by conducting the differential process for the data with and without inversion of the polarity of dephaser gradient, the influence of the eddy current caused by the dephaser gradient can be eliminated and the phase shift attributable to the preceding encode gradient, etc. can be measured accurately.
In a second aspect, the present invention resides in a method of phase shift measurement which comprises the steps of emitting an excitation pulse, emitting an inversion pulse, applying an encode gradient to the phase axis, applying a read gradient to the read axis and applying a rewind gradient to the phase axis, with these operations being repeated twice or more, and subsequently emitting an inversion pulse, applying a dephaser gradient to the phase axis, sampling first data from an echo while applying a readout gradient to the phase axis, applying a rephaser gradient to the phase axis, and subsequently emitting an inversion pulse, applying a dephaser gradient to the phase axis, sampling second data from another echo while applying a readout gradient to the phase axis, and evaluating the phase shift caused by the influence of the encode gradient based on phase data resulting from the first-order Fourier transform for the first data and phase data resulting from the first-order Fourier transform for the second data.
Even if the encode gradient is “0”, the phase shift is not “0” and an offset phase shift component exists. The offset phase shift component has the same direction for the first and second data, whereas the phase shift attributable to the preceding encode gradient, etc. has opposite directions between the first and second data due to the emission of inver
Arana Louis
GE Yokogawa Medical Systems Limited
Kojima Moonray
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