Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-06-20
2002-11-19
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06483305
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an MR (magnetic resonance) imaging method and MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method and MRI apparatus which can reduce residual magnetization caused by gradient pulses.
In prior art, Japanese Patent Application Laid Open No. H10-75940 discloses a technique involving:
(1) a phase shift measurement method comprising the steps of: executing a prescan sequence comprising transmitting an excitation RF pulse, transmitting an inversion RF pulse, applying a phase encoding pulse on a phase gradient axis, applying a read pulse on a read gradient axis and applying a rewinder pulse on the phase gradient axis, and subsequently transmitting an inversion RF pulse, applying a dephaser pulse on the phase gradient axis and collecting data from an echo while applying a read pulse on the phase gradient axis; and measuring a phase shift in subsequent echoes caused by the effect of eddy current or residual magnetization from the phase encoding pulse and so forth based on phase data obtained by performing one-dimensional Fourier transformation on the collected data, and
(2) an MR imaging method employing a fast spin echo pulse sequence that involves repeatedly executing the following steps a plurality of times with a varying phase encoding pulse: after transmitting an excitation RF pulse, transmitting an inversion RF pulse, applying a phase encoding pulse on a phase gradient axis, collecting data from an echo while applying a read pulse on a read gradient axis, and applying a rewinder pulse on the phase gradient axis, thereby collecting data for a plurality of echoes with one excitation RF pulse, wherein a compensation pulse for compensating for a phase shift amount measured by the phase shift measurement method as described regarding (1) is either incorporated in the phase encoding pulse or appended either immediately before or immediately after or immediately before and after the phase encoding pulse, or incorporated in the rewinder pulse or appended either immediately before or immediately after or immediately before and after the rewinder pulse.
The above technique presupposes that the phase shift amount measured by the phase shift measurement method of (1) is equal to the phase shift amount generated in the fast spin echo pulse sequence of (2) before the compensation pulse is added.
However, these amounts are not always equal in the MRI apparatus because of the magnetic hysteresis characteristics of a magnetism conditioning plate or the like, and the presupposition in the above technique does not always hold. This will be explained with reference to
FIGS. 1 and 2
hereinbelow.
FIG. 1
is a pulse sequence chart according to a conventional fast spin echo (FSE) technique.
In an FSE sequence SQ, an excitation RF pulse R and a slice selective pulse ss are first applied. Next, a dephasing pulse gx
1
is applied on a read gradient axis. Next, a first inversion RF pulse P
1
and a slice selective pulse ss are applied. Next, a phase encoding pulse gy
1
i is applied on a phase gradient axis. Then, data is collected from a first echo echo
1
while applying a read pulse gxw. Thereafter, a rewinder pulse gy
1
ri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gy
1
i. Reference symbol i represents a repetition number of the FSE sequence SQ in
FIG. 1
, and i=1—I (for example, I=128).
Next, a second inversion RF pulse P
2
and a slice selective pulse ss are applied, a phase encoding pulse gy
2
i is applied on the phase gradient axis, data is collected from a second echo echo
2
while applying a read pulse gxw, and then, a rewinder pulse gy
2
ri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gy
2
i.
Thereafter, and similarly, a j-th inversion RF pulse Pj and a slice selective pulse ss are applied, a phase encoding pulse gyji is applied on the phase gradient axis, data is collected from a j-th echo echoj while applying a read pulse gxw, and thereafter, a rewinder pulse gyjri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gyji, repeatedly for j=3—J (although J=8 for example,
FIG. 1
shows a case that J=3).
And finally, a killer pulse of a large amplitude is applied on the phase gradient axis.
FIG. 2
is diagram of the magnetic hysteresis characteristics of a ferromagnetic material such as a magnetism conditioning plate in the MRI apparatus.
The magnetization strength B of the ferromagnetic material such as the magnetism conditioning plate varies as indicated by a main loop Ma when the external magnetic field strength H is substantially changed, while it varies as indicated by a minor loop Mi when the change in the external magnetic field strength H is small. A gradient pulse corresponds to the small change in the external magnetic field strength H. Accordingly, application of a gradient pulse causes the magnetic strength B of the ferromagnetic material such as the magnetism conditioning plate to vary as indicated by the minor loop Mi.
Thus, the MRI apparatus has residual magnetization varying depending on a history of applying gradient pulses, owing to the magnetic hysteresis characteristics of a ferromagnetic material such as a magnetism conditioning plate.
However, since the prescan sequence as described regarding (1) does not take care of residual magnetization due to the killer pulse kp, the phase shift amount measured by the phase shift measurement method of (1) is not equal to the phase shift amount generated in the fast spin echo pulse sequence of (2) before the compensation pulse is added. That is, residual magnetization due to a killer pulse in an (i−1)th FSE sequence SQ affects all the echoes in an i-th FSE sequence SQ.
Moreover, the prescan sequence of (1) is in the form of a partially cut-out FSE sequence up to the first echo, and the history of applying gradient pulses of the prescan sequence is not equal to that of the MR imaging scan at and after the second echo. Therefore, the residual magnetization affects the second echo and the following echoes.
Thus, the conventional technique as described above has a problem that the effect of residual magnetization caused by gradient pulses cannot be sufficiently reduced.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an MR imaging method and MRI apparatus which can sufficiently reduce the effect of residual magnetization due to gradient pulses.
In accordance with a first aspect of the present invention, there is provided an MR imaging method comprising the steps of applying a gradient pulse having either a positive or negative polarity on a gradient axis, and thereafter applying a residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the gradient pulse.
In the MR imaging method of the first aspect, a residual magnetization reducing pulse is applied after a gradient pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the gradient pulse, and has an amplitude that can reduce residual magnetization caused by the gradient pulse. Thus, the residual magnetization after the application of the residual magnetization reducing pulse is reduced to a negligible degree. Therefore, the effect of residual magnetization caused by a gradient pulse can be sufficiently reduced.
In accordance with a second aspect of the invention, there is provided an MR imaging method comprising the steps of applying a killer pulse on a gradient axis, and thereafter applying a residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the killer pulse.
In this configuration, the killer pulse is a gradient pulse for eliminating transverse magnetization by forcible dephasing. The killer pulse is also referred to as a spoiler pulse.
In the MR imaging meth
Fetzner Tiffany A.
GE Yokogawa Medical Systems Limited
Kojima Moonray
Lefkowitz Edward
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