Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-07-02
2002-05-14
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S300000
Reexamination Certificate
active
06388442
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance signal acquiring method and apparatus, a recording medium and a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance signal acquiring method and apparatus for effecting RF (radio frequency) excitation for preparation on spins within an object, followed by effecting RF excitation again and acquiring a navigator echo, and effecting RF excitation for preparation on the spins within the object, followed by effecting RF excitation again and acquiring an imaging echo, a recording medium recorded with a program for causing a computer to perform such a magnetic resonance signal acquiring function, and a magnetic resonance imaging apparatus comprising such magnetic resonance signal acquiring means.
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, i.e., a space in which a static magnetic field is generated; a gradient magnetic field and a high frequency magnetic field are applied to generate magnetic resonance signals from spins within the object; and a tomographic image is constructed based on the received signals. The magnetic resonance signal for use in constructing the tomographic image is sometimes referred to as an imaging echo.
Since the spin frequency is proportional to the magnetic field strength, the spin frequency varies as the static magnetic field strength varies. If the temporal variation in the spin frequency is considerably large, the current value of the spin frequency is determined before effecting RF excitation for generating an imaging echo, and the RF excitation for generating the imaging echo is effected by the high frequency magnetic field having a frequency equal to the current value of the spin frequency.
In order to evaluate the current value of the spin frequency, the spins are RF-excited separately to measure an FID (free induction decay) signal. The FID is sometimes referred to as a navigator echo. The spin frequency is determined from the navigator echo by a calculation.
Before effecting the RF excitation for the imaging echo, RF excitation for bringing the state of spins within the object to a predetermined state may be effected. Such RF excitation is sometimes referred to as preparation. The RF excitation signal for the preparation is sometimes referred to as a preparation pulse.
The preparation includes, for example, spatial pre-saturation for saturating spins in a desired region within the object, and chemical saturation for saturating spins contained in a particular molecular structure.
Moreover, magnetization transfer for reducing the signal strength from tissues having a high protein concentration, and inversion for inverting spins in order to perform imaging according to an IR (inversion recovery) technique are also included in the category of preparation. Preparation is performed also in acquiring the navigator echo.
FIG. 1
is a time chart of navigator echo collection accompanied by preparation, a calculation on the navigator echo and imaging echo collection accompanied by preparation. The timing of gradient magnetic field application is omitted in the drawing.
As shown, the navigator echo collection accompanied by preparation and the imaging echo collection accompanied by preparation are separately performed during two consecutive periods. The length of each period is TR (repetition time).
In the earlier period, a preparation pulse such as, for example, a spatial pre-saturation pulse is applied at a time t
1
. Next, at a time t
2
, RF excitation is effected for generating a navigator echo. Next, from a time t
3
to a time t
4
, navigator echo collection is performed.
Next, from a time t
5
to a time t
6
, a calculation is performed based on the collected echo data. The spin frequency is determined by the calculation. The result of the calculation is reflected on the frequency of the RF excitation in the later period.
In the later period, a preparation pulse such as, for example, a spatial pre-saturation pulse is applied at a time t
7
. The frequency of the preparation pulse is made equal to the spin frequency determined by the calculation in the earlier period.
Next, at a time t
8
, RF excitation is effected for generating an imaging echo. The frequency of the RF excitation is made equal to the spin frequency determined by the calculation in the earlier period. Next, from a time t
9
to a time t
10
, imaging echo collection is performed.
By repeating the above operation, imaging echoes for a plurality of views are sequentially collected. Each of the imaging echoes is given different phase encoding for each view by the gradient magnetic field, which is omitted in the drawing.
In the above operation, the calculation on the navigator echo should be completed within an interval after the navigator echo collection up to the beginning of the next period. However, this interval is short because it is the remainder of the period TR after subtraction of the time required for the preparation, RF excitation and navigator echo collection, and the time available for performing the calculation is insufficient. Accordingly, the calculation must be sped up to complete the calculation within the interval, or, if this is not possible, the period TR must be extended.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a magnetic resonance signal acquiring method and apparatus in which the time available for a calculation on a navigator echo is large, and a recording medium recorded with a program for causing a computer to perform such a magnetic resonance signal acquiring function, and a magnetic resonance imaging apparatus comprising such magnetic resonance signal acquiring means.
(1) The present invention, in accordance with one aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring method that executes the steps of effecting RF excitation for preparation on spins within an object, followed by effecting RF excitation again and acquiring a navigator echo; effecting RF excitation for preparation on the spins within the object, followed by effecting RF excitation again and acquiring an imaging echo; and performing a calculation on said acquired navigator echo to adjust the frequency of said RF excitation based on the result of the calculation, which method is characterized in comprising: effecting the RF excitation on the spins within the object and acquiring the navigator echo, and thereafter effecting the RF excitation for preparation during one of two consecutive periods; and effecting the RF excitation on the spins within the object and acquiring the imaging echo, and thereafter effecting the RF excitation for preparation during the other of the two consecutive periods.
In this aspect of the invention, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for preparation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for preparation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(2) The present invention, in accordance with another aspect for solving the aforementioned problem, is the magnetic resonance signal acquiring method as described regarding (1), characterized in that said preparation is spatial pre-saturation.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for spatial pre-saturation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for spatial pre-saturation is effected
Kosugi Susumu
Uetake Nozomu
GE Medical Systems Global Technology Company LLC
Kojima Moonray
Lefkowitz Edward
Shrivastav Brij B.
LandOfFree
Magnetic resonance signal acquiring method and apparatus,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic resonance signal acquiring method and apparatus,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic resonance signal acquiring method and apparatus,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2840126