Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-02-08
2002-12-03
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06489764
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an MR (magnetic resonance) imaging method and an MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method and an MRI apparatus preventing deterioration in NMR (nuclear magnetic resonance) signals due to a slice-leaned state.
FIG. 1
is a drawing illustrating an example of pulse sequence SQ′ by a conventional spin echo (SE) method.
As shown in FIG.
1
(
a
), first a 90° RF pulse R for exciting a desired slice is applied to effect excitation, and then a 180° RF pulse P for exciting the same slice is applied to invert the slice. Then, as shown in FIG.
1
(
b
), a spin echo signal E
51
is observed.
As shown in FIG.
1
(
c
), when applying the aforementioned 90° RF pulse R and when applying the aforementioned 180° RF pulse P, a slice gradient Bg
1
where the magnetic field intensity with respect to the position in the slice thickness direction varies at the rate of the inclination angle G
1
is added.
As shown in FIG.
2
(
a
), a profile Ar pertaining to the aforementioned 90° RF pulse R takes on a shape relatively close to the ideal rectangular waveform indicated by dotted lines. On the other hand a profile Ap
51
pertaining to the aforementioned 180° RF pulse P takes on a shape whose two shoulders are rounded unlike the ideal rectangular form indicated by dotted lines. Incidentally, the half power width of each profile is supposed to be the excitation width &tgr;.
Therefore, as shown in FIG.
2
(
b
), the slice profile F′ determined by the product of the aforementioned profile Ar and the aforementioned profile AP
51
takes on a rounded shape with both shoulders gently sloped, generating a so-called “slice-leaned” state.
However, if the aforementioned slice-leaned state arises, there will arise the problem that no NMR signal is obtained in the shadowed area U between the profile and the ideal rectangular form indicated by the dotted lines, resulting in a deterioration in the quality of MR images.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an MR imaging method and an MRI apparatus preventing deterioration in NMR signals due to a slice-leaned state.
In its first aspect, the invention provides an MR imaging method for applying a 180° RF pulse to a specimen after applying a 90° RF pulse to it, characterized in that the excitation width pertaining to the aforementioned 180° RF pulse is made broader than the excitation width pertaining to the aforementioned 90° RF pulse.
According to the MR imaging method in the aforementioned first aspect, since the excitation width pertaining to the 180° RF pulse is made broader than the excitation width pertaining to the 90° RF pulse, the width of the profile pertaining to the 180° RF pulse is expanded in the slice thickness direction.
As a result, it is made possible to restrain the slice profile from falling into a slice-leaned state and thereby to prevent NMR signals from deterioration.
As the slice thickness pertaining to the slice profile is restricted by the profile pertaining to the 90° RF pulse, it will not expand excessively.
In its second aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width is expanded by making the inclination angle of the gradient magnetic field with respect to the position in the slice thickness direction at the time of applying the 180° RF pulse smaller than the inclination angle of the gradient magnetic field at the time of applying the 90° RF pulse.
According to the MR imaging method in the aforementioned second aspect, there is no need to alter the 180° RF pulse because the excitation width is expanded by making the inclination angle of the gradient magnetic field smaller at the time of applying the 180° RF pulse. This is particularly useful where widening of the band of the RF pulse frequency component is restricted.
In its third aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width is expanded by reducing the width of the aforementioned 180° RF pulse in the time axis direction.
According to the MR imaging method in the aforementioned third aspect, the excitation width can be expanded even where the inclination angle of the gradient magnetic field with respect to the position in the slice thickness direction at the time of applying the 180° RF pulse is equal to the inclination angle of the gradient magnetic field at the time of applying the 90° RF pulse, because the frequency component band is made broader by reducing the width of the 180° RF pulse in the time axis direction to expand the excitation width. Where it is to be used in combination with a reduction in the inclination angle of the gradient magnetic field at the time of applying the 180° RF pulse, the difference from the inclination angle of the gradient magnetic field at the time of applying the 90° RF pulse, and the control of the gradient magnetic field can be made easier and more precisely.
In its fourth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that, where multi-slice imaging is to be accomplished by successively applying a plurality of 180° RF pulses, the excitation width pertaining to the aforementioned 180° RF pulse is made broader than the excitation width pertaining to the aforementioned 90° RF pulse by not smaller than a 0.4-fold multiple but not greater than a 0.6-fold multiple of the slice spacing.
According to the MR imaging method in the aforementioned fourth aspect, the NMR signal intensity improving performance due to expansion of the excitation width can be sufficiently achieved because the lower limit of expanding the excitation width pertaining to the 180° RF pulse is made a 0.4-fold multiple of the slice spacing. Furthermore, as the upper limit of expanding the excitation width pertaining to the 180° RF pulse is made a 0.6-fold multiple of the slice spacing, interference between adjoining slice profiles can be reduced.
As a result, even where multi-slice imaging is to be carried out, it is possible to prevent NMR signals from deterioration by restraining the occurrence of a slice-leaned state.
In its fifth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width pertaining to an inversion pulse to be applied first out of a pulse sequence is made broader than the excitation width pertaining to the 90° RF pulse.
According to the MR imaging method in the aforementioned fifth aspect, even where a pulse sequence of an inversion recovery (IR) method, by which an inversion pulse which inverts the direction of the nuclear magnetization vector by 180° is applied at the beginning of the sequence, NMR signals can be prevented from deterioration by restraining the occurrence of a slice-leaned state.
In its sixth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width of the aforementioned 180° RF pulse is expanded so that the whole part excited by the aforementioned 90° RF pulse be excited by the aforementioned 180° RF pulse.
According to the MR imaging method in the aforementioned sixth aspect, the best SNR can be obtained.
In its seventh aspect, the invention provides an MRI apparatus provided with a gradient magnetic field generating unit for generating a gradient magnetic field; an RF pulse transmitting unit for transmitting RF pulses; and an NMR signal receiving unit for receiving NMR signals; and the MRI apparatus being characterized in that it is provided with an excitation width adjusting unit for making the excitation width pertaining to a 180° RF pulse broader than the excitation width pertaining to a 90° RF pulse.
The MRI apparatus in the aforementioned seventh aspect can appropriately implement the MR imaging method according to the aforementioned first aspect.
In its eighth aspect, the invention provides an MRI apparatus of the
Ikezaki Yoshikazu
Yamazaki Aki
GE Medical Systems Global Technology Company LLC
Kojima Moonray
Lefkowitz Edward
Vargas Dixomara
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