Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-09-04
2003-05-06
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06559643
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an MR imaging method, residual magnetization amount measuring method and MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method capable of restraining residual magnetization caused by a previous pulse sequence from affecting a current pulse sequence, a residual magnetization amount measuring method for verifying a demagnetizing effect, and an MRI apparatus for implementing such methods.
In the publications of Japanese Patent Application Laid Open Nos. H8-322817 and H10-75940, MR imaging methods are disclosed in which a corrective component is appended to a phase encoder gradient or rewind gradient for restraining residual magnetization caused by a phase encoder gradient from affecting an echo next to an echo that corresponds to the phase encoder gradient.
In MR imaging, a pulse sequence involving transmitting an RF pulse, applying a phase encoder gradient and receiving an NMR signal from a subject is repeated with the phase encoding amount varied to thereby collect data for filling the k-space.
The aforementioned conventional techniques restrain the effect of residual magnetization within one pulse sequence, and do not restrain residual magnetization caused by a previous pulse sequence from affecting the current pulse sequence.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an MR imaging method capable of restraining residual magnetization caused by a previous pulse sequence from affecting the current pulse sequence, a residual magnetization amount measuring method for verifying a demagnetizing effect, and an MRI apparatus for implementing such methods.
In its first aspect, the present invention provides an MR imaging method for transmitting an RF pulse, applying a phase encoder gradient, receiving an NMR signal from a subject and producing an MR image based on said NMR signal, characterized in: prior to transmitting the RF pulse, consecutively applying first through I-th (≧2) gradient pulses having alternately inverting polarity and pulse heights reduced in order.
In accordance with the MR imaging method of the first aspect, since first through I-th gradient pulses having alternately inverting polarity and pulse heights reduced in order are consecutively applied prior to substantially beginning a pulse sequence for acquiring data, residual magnetization of a previous pulse sequence is demagnetized, and thereafter the pulse sequence for acquiring data is substantially begun. Therefore, residual magnetization caused by the previous pulse sequence can be restrained from affecting the current pulse sequence.
In its second aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that said first gradient pulse has a pulse height such as to saturate residual magnetization.
In accordance with the MR imaging method of the second aspect, since a first gradient pulse having a pulse height such as to saturate residual magnetization is applied, even if residual magnetization remains after applying the I-th gradient pulse, the amount of the residual magnetization is always constant. For example, in a pulse sequence according to a spin echo technique, while a varying amount of residual magnetization causes artifacts on an MR image, an always constant amount of residual magnetization does not generate artifacts on an MR image. On the other hand, in a pulse sequence according to a fast spin echo technique, even a constant amount of residual magnetization causes artifacts on an MR image as it stands because an offset in an odd-numbered echo and an offset in an even-numbered echo are in opposite directions; however, the offset amounts are constant and therefore it is possible to correct data. Thus, residual magnetization caused by the previous pulse sequence can be restrained from affecting the current pulse sequence.
In its third aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that I=4.
If I is increased, residual magnetization can be further reduced but this processing is time consuming. On the other hand, if I is decreased, the processing time can be reduced but the effect of decreasing residual magnetization is also reduced.
Therefore, in accordance with the MR imaging method of the third aspect, I=4. Thus, tradeoff between the effect of decreasing residual magnetization and the processing time can be optimized.
In its fourth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the first through I-th gradient pulses have trapezoidal pulse waveforms.
In accordance with the MR imaging method of the fourth aspect, by shaping the pulse waveforms as trapezoids, they can be raised at the maximum slew rate possible with the hardware.
In its fifth aspect, the present invention provides a residual magnetization amount measuring method characterized in: consecutively applying first through I-th (≧2) gradient pulses having alternately inverting polarity and pulse heights reduced in order; applying a 90° RF pulse; applying a 180° RF pulse; observing a first echo while applying a read gradient; transmitting a 180° RF pulse; observing a second echo while applying a read gradient; and measuring a residual magnetization amount from offsets of echo peaks of said first and second echoes.
The effect of residual magnetization after applying the gradient pulses appears as an offset of an echo peak from an echo center. The offset of an echo peak from an echo center is, however, difficult to measure because the echo center is difficult to identify when a single echo is taken.
Therefore, in accordance with the residual magnetization amount measuring method of the fifth aspect, an echo peak of a first echo and that of a second echo are compared. Since an offset of the echo peak from the echo center of the first echo and an offset of the echo peak from the echo center of the second echo are in opposite directions, half of the offsets of the echo peaks of the first and second echoes gives the offset of an echo peak from an echo center. Hence, the residual magnetization amount can be determined from the offset. That is, the effect of applying the gradient pulses can be evaluated.
In its sixth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in adjusting at least one of the pulse height and the pulse width of the second through I-th gradient pulses based on the residual magnetization amount measured by the residual magnetization amount measuring method of the aforementioned configuration.
In accordance with the MR imaging method of the sixth aspect, the pulse width and the pulse height of the gradient pulses can be optimized.
In its seventh aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the pulse height of an i-th gradient pulse is half the pulse height of an (i−1)-th gradient pulse.
In accordance with the MR imaging method of the seventh aspect, since the pulse heights of the gradient pulses are halved in order, the processing can be simplified.
In its eighth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in that the pulse widths of the first through I-th gradient pulses are substantially the same.
In accordance with the MR imaging method of the eighth aspect, since the pulse width is not varied among the gradient pulses, the processing can be simplified.
In its ninth aspect, the present invention provides the MR imaging method of the aforementioned configuration, characterized in applying the first through I-th gradient pulses to a gradient axis to which a phase encoder gradient is applied.
It is the residual magnetization caused by a phase encoder gradient that especially becomes an issue in MR imaging.
Therefore, in accordance with the MR imaging method of the ninth asp
Fetzner Tiffany A.
GE Medical Systems Global Technology Company LLC
Lefkowitz Edward
Moonray Kojima
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