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 on spins within an object and acquiring a navigator echo and 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 navigator echo. The spin frequency is determined from the navigator echo by a calculation.
FIG. 1 is a time chart of navigator echo collection, a calculation on the navigator echo and imaging echo collection. The timing of gradient magnetic field application is omitted in the drawing.
As shown, the navigator echo collection and the imaging echo collection are separately performed during two consecutive periods. The length of each period is TR (repetition time).
In the earlier period, RF excitation for generating a navigator echo is effected at a time tl. Next, from a time t2 to a time t3, navigator echo collection is performed.
Next, from a time t3 to a time t5, 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, RF excitation for generating an imaging echo is effected at a time t6. 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 t7 to a time t8, 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 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.
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 on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which method is characterized in comprising: making the sampling frequency for acquiring said navigator echo higher than the sampling frequency for acquiring said imaging echo.
(2) The present invention, in accordance with another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, characterized in that the apparatus comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the sampling frequency for said echo acquiring means to acquire the navigator echo higher than the sampling frequency for said echo acquiring means to acquire the imaging echo.
(3) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium that recorded in a computer-readable manner with a program for causing a computer to execute the functions of effecting RF excitation on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which recording medium is characterized in that the program causes the computer to: make the sampling frequency for acquiring said navigator echo higher than the sampling frequency for acquiring said imaging echo.
(4) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signals, characterized in that means for acquiring said magnetic resonance signals comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the sampling frequency for said echo acquiring means to acquire the navigator echo higher than the sampling frequency for said echo acquiring means to acquire the imaging echo.
According to the invention in these aspects, since the sampling frequency for acquiring the navigator echo is made higher than the sampling frequency for acquiring the imaging echo, the time required for the navigator echo data acquisition can be reduced relative to the time required for the imaging echo data acquisition. Thus, the time available for the calculation on the navigator echo can be increased.
(5) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring method that executes the steps of effecting RF excitation on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which method is characterized in comprising: making the number of sampling points for acquiring said navigator echo fewer than the number of sampling points for acquiring said imaging echo.
(6) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, characterized in that the apparatus comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the number of sampling points for said echo acquiring means to acquire the navigator echo fewer than the number of sampling points for said echo acquiring means to acquire the imaging echo.
(7) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium recorded in a computer-readable manner with a program for causing a computer to execute the functions of effecting RF excitation on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which recording medium is characterized in that the causes the computer to: make the number of sampling points for acquiring said navigator echo fewer than the number of sampling points for acquiring said imaging echo.
(8) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signals, characterized in that means for acquiring said magnetic resonance signals comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the number of sampling points for said echo acquiring means to acquire the navigator echo fewer than the number of sampling points for said echo acquiring means to acquire the imaging echo.
According to the invention in these aspects, since the number of sampling points for acquiring the navigator echo is made fewer than the number of sampling points for acquiring the imaging echo, the time required for the navigator echo data acquisition can be reduced relative to the time required for the imaging echo data acquisition. Thus, the time available for the calculation on the navigator echo can be increased.
(9) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring method that executes the steps of effecting RF excitation on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which method is characterized in comprising: making the sampling frequency for acquiring said navigator echo higher than the sampling frequency for acquiring said imaging echo; and making the number of sampling points for acquiring said navigator echo fewer than the number of sampling points for acquiring said imaging echo.
(10) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, characterized in that the apparatus comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the sampling frequency for said echo acquiring means to acquire the navigator echo higher than the sampling frequency for said echo acquiring means to acquire the imaging echo, and for making the number of sampling points for said echo acquiring means to acquire the navigator echo fewer than the number of sampling points for said echo acquiring means to acquire the imaging echo.
(11) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium recorded in a computer-readable manner with a program for causing a computer to execute the functions of effecting RF excitation on spins within an object and acquiring a navigator echo; performing a calculation on said acquired navigator echo and adjusting the frequency of RF excitation based on the result of the calculation; and effecting RF excitation on the spins within the object and acquiring an imaging echo, which recording medium is characterized in that the program causes the computer to: make the sampling frequency for acquiring said navigator echo higher than the sampling frequency for acquiring said imaging echo; and make the number of sampling points for acquiring said navigator echo fewer than the number of sampling points for acquiring said imaging echo.
(12) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signals, characterized in that means for acquiring said magnetic resonance signals comprises: echo acquiring means for effecting RF excitation on spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of RF excitation based on the result of said calculation; and control means for making the sampling frequency for said echo acquiring means to acquire the navigator echo higher than the sampling frequency for said echo acquiring means to acquire the imaging echo, and for making the number of sampling points for said echo acquiring means to acquire the navigator echo fewer than the number of sampling points for said echo acquiring means to acquire the imaging echo.
According to the invention in these aspects, since the sampling frequency for acquiring the navigator echo is made higher than the sampling frequency for acquiring the imaging echo, and the number of sampling points for acquiring the navigator echo is made fewer than the number of sampling points for acquiring the imaging echo, the time required for the navigator echo data acquisition can be reduced relative to the time required for the imaging echo data acquisition. Thus, the time available for the calculation on the navigator echo can be increased.
Therefore, the present invention can provide a magnetic resonance signal acquiring method and apparatus in which the time available for a calculation on a navigator echo is long, and a recording medium on which a program is recorded 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.