JP3310751B2 - Asymmetric RF pulse generation method and MRI apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an asymmetric RF pulse generating method and an MRI (Magnetic Resonance Imaging) apparatus, and more particularly to an asymmetric RF pulse capable of suppressing deterioration of a slice shape and occurrence of ringing. The present invention relates to an asymmetric RF pulse generation method and an MRI apparatus that can excite a slice while suppressing a decrease in signal strength at a slice end or a signal mixed outside a slice region.

[0002]

2. Description of the Related Art FIG. 12 is a waveform diagram showing an example of a conventional asymmetric RF pulse. This asymmetric RF pulse G (t)
A sink (sinc) function ｛sin as shown in FIG.
A symmetric RF pulse g (t) based on (πt / τ)｝ / (πt / τ) and f (t) = t (τ) as shown in FIG.
2. An asymmetric window function f (t) where t> τ and f (t) = 0.
Is created by multiplying In a conventional MRI apparatus, a target excitation region (slice) is excited using the asymmetric RF pulse G (t). The resulting excitation characteristics are as shown by the solid line frequency distribution S (f) in FIG. This frequency distribution S (f) is represented by an asymmetric RF pulse G
(T) is obtained by Fourier transform. The dotted line in FIG. 15 indicates ideal excitation characteristics.

[0003]

When the excitation is performed using the asymmetric RF pulse G (t), there are the following two problems as can be seen from FIG. At the end of the slice, the actual excitation characteristics are greatly degraded from the ideal excitation characteristics. That is, the signal strength at the slice end decreases. Ringing occurs outside the slice. That is, signals outside the slice are mixed. Accordingly, a first object of the present invention is to provide a method for generating an asymmetric RF pulse capable of generating an asymmetric RF pulse capable of suppressing deterioration of excitation characteristics at a slice end and / or occurrence of ringing outside a slice. It is in. A second object of the present invention is to provide an MRI apparatus capable of suppressing the signal strength at the slice end and / or suppressing the mixing of signals outside the slice and exciting the MRI.

[0004]

According to a first aspect, the present invention provides a sink function {sin (πt / τ)} / (πt /
τ), a symmetric RF pulse is multiplied by an asymmetric window function to generate an asymmetric RF pulse, wherein at least one of the even function part and the odd function part of the asymmetric window function is a continuous function. A method for producing an asymmetrical RF pulse is provided. In a second aspect, the present invention provides a sink function {sin (πt / τ)}
Excitation means for exciting using an asymmetric RF pulse created by multiplying a symmetric RF pulse based on / (πt / τ) with an asymmetric window function in which at least one of the even function part and the odd function part is a continuous function. MR characterized by comprising:
An I device is provided.

In a third aspect, the present invention provides a symmetric RF pulse based on a sink function {sin (πt / τ)} / (πt / τ) and at least one of an even function part and an odd function part. Function and constant size at t <−τ, −τ <
Provided is an MRI apparatus, comprising: an excitation unit that excites using an asymmetric RF pulse created by multiplying an asymmetric window function having a continuous slope at t <τ and a magnitude of 0 at τ <t. . According to a fourth aspect, the present invention provides a symmetric RF pulse based on a sink function {sin (πt / τ)} / (πt / τ), a function in which the even function part has a constant size and the odd function part. Is constant at t ≦ −τ and t ≧ τ,
Provided is an MRI apparatus comprising excitation means for exciting using an asymmetric RF pulse created by multiplying a continuous function, which is a linear function, by an asymmetric window function when τ <t <τ.

[0006]

As a result of intensive studies, the inventors of the present invention have found that, when the conventional asymmetric RF pulse G (t) is used for excitation, the excitation characteristics at the slice end are degraded and ringing occurs outside the slice. It has been found that the cause is the discontinuity of the even function part and the odd function part of the conventional asymmetric RF pulse G (t). That is, the asymmetric R
The F pulse G (t) is created by multiplying the symmetric RF pulse g (t) by the asymmetric window function f (t). G (t) = g (t) · f (t) is there. The frequency distribution S (f) obtained by Fourier transforming the asymmetric RF pulse G (t) is as follows: S (f) = Fourier transform {G (t)} = Fourier transform {g (t)} * Fourier transform {f (T)｝ (* represents a convolution operation). Here, the even function part of the asymmetric window function f (t) is represented by f
Assuming that e (t) and the odd function part are fo (t), Fourier transform {f (t)} = Fourier transform {fe (t)} + j [Fourier transform {fo (t)}]. (F) = Fourier transform ｛g (t)｝ * Fourier transform ｛fe (t)｝ + j [Fourier transform ｛g (t)｝ * Fourier transform ｛fo (t)｝] (1) Since the symmetric RF pulse g (t) is a real even function, the Fourier transform {g (t)} has only a real part. FIG.
As shown in FIG. 6, the even function part fe (t) of the asymmetric window function f (t) that defines the real part of the frequency distribution S (f) is t = −τ
And t = τ have discontinuities. This affects the Fourier transform {g (t)}, which is a basic component, and causes deterioration of the excitation characteristics at the slice end. Also,
As shown in FIG. 17, the odd function part fo (t) of the asymmetric window function f (t) that defines the imaginary part of the frequency distribution S (f) is represented by t =
There are discontinuities at −τ and t = τ. This affects the Fourier transform {g (t)}, which is a fundamental component, and causes ringing outside the slice.

[0007] In the asymmetric RF pulse generation method according to the first aspect, the asymmetric window function f (t) is converted to an even function part fe (t).
And the odd function part fo (t), and the even function part fe (t)
Alternatively, at least one of the odd function part fo (t) is a continuous function. Thereby, the even function part f of the asymmetric window function f (t) is obtained.
When e (t) is a continuous function, the degradation of the excitation characteristics at the slice end can be suppressed. When the odd function part fo (t) of the asymmetric window function f (t) is a continuous function, the outside of the slice can be suppressed. Can be suppressed, and when both the even function part fe (t) and the odd function part fo (t) of the asymmetric window function f (t) are set as continuous functions, the deterioration of the excitation characteristic at the slice end and the deterioration It is possible to suppress the occurrence of ringing outside the slice.

In the MRI apparatus according to the second aspect, since the asymmetric RF pulse G (t) created by the asymmetric RF pulse creating method according to the first aspect is used, the signal intensity at the slice end is reduced and / or the slice is reduced. Excitation can be performed while suppressing mixing of external signals.

In the MRI apparatus according to the third aspect, the even function part fe (t) of the asymmetric window function f (t) is a flat continuous function, and the odd function part fo of the asymmetric window function f (t).
Since (t) is a continuous function having a smooth shape (that is, a shape having a continuous slope at −τ <t <τ), it is possible to suppress a decrease in signal strength at the slice end and a mixing of a signal outside the slice. To be excited. In the MRI apparatus according to the fourth aspect, in addition to the same operation as the MRI apparatus according to the third aspect, the odd function part fo (t) of the asymmetric window function f (t) is a linear function when −τ <t <τ. Therefore, the calculation can be simplified.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the embodiments shown in the drawings. It should be noted that the present invention is not limited by this. FIG. 1 shows an MR according to an embodiment of the present invention.
FIG. 2 is a block diagram of the I device 100. This MRI apparatus 10
0, the magnet assembly 1 has a space portion (hole) for inserting the subject therein, and a static magnetic field coil for applying a constant static magnetic field to the subject so as to surround the space portion; A gradient magnetic field coil for generating a gradient magnetic field (the gradient coil includes coils for each of X, Y, and Z axes) and a transmission coil for applying an RF pulse for exciting spins of nuclei in the subject. And N from the subject
A receiving coil and the like for detecting an MR (Nuclear Magnetic Resonance) signal are arranged. The static magnetic field coil, the gradient magnetic field coil, the transmitting coil and the receiving coil are connected to a main magnetic field power supply 2, a gradient magnetic field driving circuit 3, an RF power amplifier 4 and a preamplifier 5, respectively.

The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 based on a sequence such as a spin warp method according to a command from the computer 7 to generate a gradient magnetic field from the gradient magnetic field coil of the magnet assembly 1. At the same time, the gate modulation circuit 9 is operated to modulate the high-frequency output signal of the RF oscillation circuit 10 into a pulsed signal having a predetermined timing and a predetermined envelope, which is added to the RF power amplifier 4 as an RF pulse. After the power is amplified, it is applied to the transmission coil of the magnet assembly 1 to selectively excite a target slice.

The preamplifier 5 includes a magnet assembly 1
, And amplifies the NMR signal from the subject detected by the receiving coil, and inputs the amplified signal to the phase detector 12. The phase detector 12
The output of the RF oscillation circuit 10 is used as a reference signal and the preamplifier 5
Is phase-detected and supplied to the A / D converter 11. The A / D converter 11 converts the analog signal after the phase detection into a digital signal and inputs the digital signal to the computer 7.
The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image of a target slice (proton density image). This image is displayed on the display device 6. The computer 7 has a console 13.
Responsible for overall control, such as receiving information input from.

[0013] The method of producing an asymmetric RF pulse according to the present invention comprises:
The asymmetric RF pulse G generated and executed by the computer 7
(T) is passed to the sequence storage circuit 8. FIG.
9 is a flowchart illustrating a procedure of an asymmetric RF pulse creation method executed by the computer 2. In step S1, parameters (τ and smoothing window function) input from the console 13 are used.
And the sink function {sin (πt / τ)} / (πt
/ Τ) to create a symmetric RF pulse g (t).
FIG. 3 illustrates a symmetric RF pulse g (t). In addition, as the smoothing window function, for example, there is a Hamming window. The smoothing window is not essential, but should be applied to smooth the RF pulse waveform.

In step S2, the asymmetric window function f (t)
Create FIG. 4 illustrates an asymmetric window function f (t).
This asymmetric window function f (t) is such that the even function part and the odd function part are continuous functions and have a constant size (= 2) at t <−τ,
The slope is continuous when −τ <t <τ, and the magnitude is 0 when τ <t.
As shown in FIG. 5, the even function part fe (t) of the asymmetric window function f (t) is a function of a constant magnitude (= 1). As shown in FIG. 6, the odd function part fo (t) of the asymmetric window function f (t) has a constant size (= 1, −1), −τ at t ≦ −τ and t ≧ τ. For <t <τ, a linear function (=
−t / τ). In step S3, the symmetric RF pulse g (t) is multiplied by the asymmetric window function f (t) to create an asymmetric RF pulse G (t). In FIG.
An asymmetric RF pulse G (t) is illustrated.

The MRI apparatus 100 is excited by using the asymmetric RF pulse G (t). The resulting excitation characteristics are:
A frequency distribution S (f) shown by a solid line in FIG. 8 is obtained. As can be seen by comparing FIGS. 5 and 16, FIGS. 6 and 17, and FIGS. 8 and 15, the even function part fe of the asymmetric window function f (t) is obtained.
Since both (t) and the odd function part Fo (t) are continuous functions and discontinuous parts are eliminated, deterioration of excitation characteristics at the slice end and occurrence of ringing outside the slice are suppressed. For this reason, it is possible to suppress the signal strength at the slice end and to mix the signal outside the slice, and to excite the slice.

FIG. 9 shows another example of the asymmetric RF pulse f (t) according to the present invention. As shown in FIG. 10, the even function part fe (t) of the asymmetric window function f (t) is a function having a constant magnitude (= 1). On the other hand, as shown in FIG. 11, the odd function part fo (t) of this asymmetric window function f (t)
Is a function of magnitude = 1 at t ≦ −τ, magnitude = 0 at −τ <t <τ, and magnitude = −1 at t ≧ τ. When this asymmetric window function f (t) is used, the asymmetric window function f (t)
Since the even function part fe (t) is made a continuous function and the discontinuous part is eliminated, it is possible to suppress the deterioration of the excitation characteristic at the slice end. However, since the odd function part fo (t) has a discontinuous part, occurrence of ringing outside the slice cannot be suppressed. Therefore, it is preferable to perform filtering in order to suppress occurrence of ringing.

In this filtering, first, a symmetric RF
The pulse g (t) is multiplied by the asymmetric RF pulse f (t) in FIG. 9 to create a temporary asymmetric RF pulse G (t) ′.
Next, the provisional asymmetric RF pulse G (t) ′ is Fourier-transformed to obtain a frequency distribution function S (t) ′. Next, when t <−τ and t> τ of the frequency distribution function S (t) ′, S
A frequency distribution function S (t) with (t) ′ = 0 is created. Next, the frequency distribution function S (t) is inversely Fourier transformed to obtain an asymmetric RF pulse G (t). This asymmetric R
When the excitation is performed using the F pulse G (t), it is possible to suppress the deterioration of the excitation characteristics at the slice end and the occurrence of ringing outside the slice.

[0018]

According to the asymmetric RF pulse generation method of the present invention, it is possible to generate an asymmetric RF pulse in which the deterioration of the excitation characteristics at the slice end and / or the occurrence of ringing outside the slice are suppressed. Further, according to the MRI apparatus of the present invention, it is possible to excite the signal by suppressing a decrease in the signal strength at the slice end and / or mixing of a signal outside the slice.

[Brief description of the drawings]

FIG. 1 is a block diagram of an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a procedure of an asymmetric RF pulse generating method according to the present invention.

FIG. 3 is a waveform diagram of a symmetric RF pulse based on a sync function.

FIG. 4 is a waveform diagram of an asymmetric window function according to the present invention.

FIG. 5 is a waveform diagram of an even function part of the asymmetric window function of FIG. 4;

FIG. 6 is a waveform diagram of an odd function part of the asymmetric window function of FIG. 4;

FIG. 7 is a waveform diagram of an asymmetric RF pulse according to the present invention.

FIG. 8 is a waveform diagram of a frequency distribution function corresponding to the asymmetric RF pulse of FIG. 7;

FIG. 9 is another waveform diagram of the asymmetric window function according to the present invention.

FIG. 10 is a waveform diagram of an even function part of the asymmetric window function of FIG. 9;

11 is a waveform diagram of an odd function part of the asymmetric window function of FIG. 9;

FIG. 12 is a waveform diagram of an example of a conventional asymmetric RF pulse.

FIG. 13 is a waveform diagram of a sync function.

FIG. 14 is a waveform diagram of an example of a conventional asymmetric window function.

FIG. 15 is a waveform diagram of a frequency distribution function corresponding to the asymmetric RF pulse of FIG.

FIG. 16 is a waveform diagram of an even function part of the asymmetric window function of FIG. 14;

17 is a waveform diagram of an odd function part of the asymmetric window function of FIG.

[Explanation of symbols]

 Reference Signs List 1 magnet assembly 4 RF power amplifier 7 computer 8 sequence controller 9 gate modulation circuit 10 RF oscillation circuit 13 console 100 MRI apparatus f (t) asymmetric window function fe (t) even function part fo (t) odd function part g (t ) Symmetric RF pulse G (t) Asymmetric RF pulse S (f) Frequency distribution

Continued on the front page (56) References JP-A-2-13431 (JP, A) JP-A-7-39536 (JP, A) JP-A-6-52803 (JP, U) (58) Fields investigated (Int) .Cl. ⁷ , DB name) A61B 5/055 G01R 33/20-33/64

Claims (4)

(57) [Claims] 1. A sink function {sin (πt / τ)} /
An asymmetric RF pulse that creates an asymmetric RF pulse by multiplying a symmetric RF pulse based on (πt / τ) and an asymmetric window function
In the pulse generation method, at least one of the even function part and the odd function part of the asymmetric window function is a continuous function. 2. A sink function {sin (πt / τ)} /
Excitation means for exciting using an asymmetric RF pulse created by multiplying a symmetric RF pulse based on (πt / τ) and an asymmetric window function in which at least one of the even function part and the odd function part is a continuous function is provided. MRI characterized by having
apparatus. 3. A sink function {sin (πt / τ)} /
The symmetric RF pulse based on (πt / τ) and at least one of the even function part and the odd function part are continuous functions and t
<−τ, constant size, −τ <t <τ, continuous slope, τ <
An MRI apparatus comprising: an excitation unit that excites using an asymmetric RF pulse created by multiplying an asymmetric window function having a magnitude of 0 at t. 4. A sink function {sin (πt / τ)} /
A symmetric RF pulse based on (πt / τ), a function whose even function part is a constant size and an odd function part has a constant size when t ≦ −τ and t ≧ τ, and a constant function when −τ <t <τ An MRI apparatus comprising excitation means for exciting using an asymmetric RF pulse created by multiplying a continuous function, which is a linear function, with an asymmetric window function.