JPS62122645A - Magnetic resonance imaging apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a magnetic resonance imaging apparatus using 2DFT (two-dimensional Fourier transform).

[Technical background of the invention and its problems]

The magnetic resonance imaging system W (hereinafter also referred to as MRI) uses the spin echo method to detect MR and other signals, and the pulse sequence for imaging using the ZDFT method is as shown in Figure 6. . RF shown in the same figure
By repeating the pulse, the slicing gradient magnetic field Gz, the phase coding gradient magnetic field Gy (set to a different strength for each repetition), and the readout gradient magnetic field Gx at the timing shown in the figure, the receiving coil can be used to Signal detected. This MR and other signals are F I D (Fre
The FID signal includes an eInduction Decay signal and an echo signal, but the FID signal is an unnecessary signal in the spin echo method. In this case, if the FID signal is detected superimposed on the echo signal, this will appear as an artifact on the image (see FIG. 7), which will greatly affect the deterioration of diagnostic performance.

In order to prevent the FID signal from being detected as being superimposed on the echo signal, a method of lengthening the period T shown in FIG. 6 can be considered. According to this method, since the echo signal is detected after the FID signal is sufficiently attenuated, the above-mentioned disadvantages can be prevented. However, since the shorter the period TE can contribute to improving the contrast of the image, it is difficult to adopt this method from the viewpoint of image quality.

As an alternative method, there is a prior patent that prevents the occurrence of artifacts by the means described below. That is, if the RF pulse 180°i<rus is P(φ), then this 180° pulse P(φ) is the FID signal signal t)
However, by inverting the phase of P(φ+π), that is, the high frequency, −F (t)
occurs. Using this principle, P(φ),
P(φ+π) is scanned once each, and the data obtained from the two scans are averaged to obtain F.
This is the one from which the ID signal has been removed. The method is as follows.

■ Collect data using P(φ).

The strength of the gradient magnetic field for phase coding is C, the time from the start of data collection in each iteration is t, and the above P(φ)
If the data collected using dI(c, t〉) is d+(c+t)=at(c,t)+F+(C,t), εI(c+t) is Word IF
I (c+t) is the F'ID signal.

■ Collect data using P(φ+π).

At this time, if the collected data is ' t (c, t), then d z (c + t) - g 2 (C1t) +
F z(c+t).

■ Perform 2DFT on the data obtained by adding and averaging both data.

2-dimensional FFT is performed to construct an image.

Here, ε+ (c+ t) #εZ (c+ t)F + (
c+t) -F z (c+t), εI (c+t)#εz(c+D), the FID signal is canceled, and the generation of artifacts can be prevented.

However, since the subject may move between the two scans, ε and 82. F and -F2 do not necessarily match, which adversely affects the reproducibility of the image based on the data after averaging. Furthermore, since the scan must be performed at least twice, not only does the diagnosis time become longer, but the burden on the subject also increases.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and allows image reconstruction without deterioration of image quality due to artifacts based on FID signals even though data is collected by one scan.

It is an object of the present invention to provide a magnetic resonance imaging apparatus whose scanning operation is not more complicated than conventional ones.

[Summary of the invention]

The outline of the present invention is that an RF pulse transmitter that applies an RF pulse inverts the phase of high frequency waves every time data collection is repeated by varying the strength of a gradient magnetic field for phase coding.
It is configured to apply an F pulse, and by performing 2DFT on the collected data, artifacts based on the FED signal can be moved to the edges of the image and displayed.

[Embodiments of the invention]

First, the principle of the present invention will be explained.

Analysis of artifacts based on FID signals revealed the following. That is, an image obtained by performing two-dimensional FFT (fast Fourier transform) on data d(c, t) including an FID signal and an echo signal is as shown in FIG. Here, r
cJ is the strength of the gradient magnetic field ay for phase coding,
t is the time from the start of data collection at each iteration. Then, the two-dimensional FFT is performed by converting C→y, t-4x. As can be seen from the image in Figure 8, artifacts on the image appear along the
It can be seen that the lt number is detected. That is, the FID signal is a very similar signal for each repetition as shown in FIG.
(Co+D#F (CI+t)"'+.

Furthermore, Fig. 9 shows the real part Re (F(c, t)) of the FID signal.
) is shown here, but the imaginary part is also considered to be similar.

Now, in data collection that is performed repeatedly, instead of continuously changing the strength C of the gradient magnetic field GV for phase coding, the gradient magnetic field cy is The strength ck is set to, for example, c,=00k·ΔC, and the strength C of the gradient magnetic field cy is changed stepwise by varying k sequentially from 0 to n-1.

Therefore, using Pφ(1) as 1800 pulses when the strength of the k-th gradient magnetic field Gy is ck, (k+1)
1 when the strength of the first gradient magnetic field Gy is Ck+1
A pulse Pφ+π(1), which is obtained by inverting the phase of Pφ(1), is used as an 80° pulse, and in this way, data is collected using a 180° pulse whose phase is inverted every time data collection is repeated.

By the above operation, the data obtained for the kth time is d
((, ++t), and the data obtained the (k+1)th time is d (CIllI+ t), then both data d (
cm + t) 1d F of (ch++ + t)
As mentioned above, the sign of the ID signal is reversed (Pφ(1)
then F(c, t).

When Pφ+π(1), -F (C1t) is obtained)
. That is, as shown in Fig. 3, FID signals adjacent on the C-axis (Fig. 3 shows only the real part Re (F(c, t)), but the imaginary part is also the same) are of opposite signs. can get.

Here, the FID signal shown in FIG. 3 is set to 1=1. FIG. 4 shows a signal train on the C-axis when cut at (10 is an arbitrary time). As shown in the same figure, it can be seen that the FID signal has a high frequency on the C-axis. On the other hand, the FID signal obtained with the conventional device is as shown in Fig. 9, and similarly, 1 = The signal train on the C-axis obtained by cutting at point 10 is as shown in FIG.

Let FID signal when 1=10 obtained by the present invention be F (c, to), and 1 obtained by the conventional device
The FID signal when =10 is F o (c+ to>
Then, as is clear from FIGS. 4 and 10, the relationship between both signals is π. Note that π/ΔC=ωN (Nyquist frequency).

Next, an image obtained by performing two-dimensional Fourier transform on the data collected as described above will be described.

Let the data be d=ε10F, and apply the two-dimensional inverse FFT to this data.
By doing so, it is assumed that F z (d) = F z (ε) + F 2 (F). Here, Ft(F) is an unnecessary image based on the FID signal, and F2(ε) is a diagnostic image based on the echo signal.

Here, the image based on the 180" pulse signal is A(x, y
) (A(x, y) = F z(F )), and the image based on the FID signal obtained by the conventional device is Ao(x+y) (Ao(x+y) = F 1(Fo)
). However, x and y have a period of 2π.

Image A.

is an artifact as shown in FIG.

Now, if we express the one-dimensional inverse FFT as F(ω→X), then (F
(ω→x) (f(ω) = g(X)), A(
x, y) = F t (F(c, t) ) = F
(t-=x) (F (c-y) CF (c, t))
) = F (t-”x) (F (c-y+yr)
(Fo(c, t)) = A o (X+V+π). Therefore, A(x, y) is an image obtained by shifting Ao(x, y) by π in the y direction, that is, an image in which the artifact is moved to the edge of the image as shown in FIG. Furthermore, Ft(
Regarding ε), there is no difference between the present invention and the conventional device.In the end, the present invention can reproduce the image based on the echo signal as it is,
Furthermore, it is possible to reproduce the movement of artifacts based on the FID signal to the edges of the image, and it is also possible to remove the artifacts by covering the edges of the image with a mask, for example.

Next, an embodiment of the present invention based on the above principle will be described.

The feature of this embodiment device that differs from the conventional device is that the RF pulse transmitter 2 shown in FIG. 2 applies an RF pulse with the high frequency phase inverted for each repetitive operation for data collection. The other configurations are the same as before.

In Fig. 2, P is a subject, 1 is a static magnetic field generator that applies a static magnetic field Ho to the subject P, and 2 is an RF pulse transmitter that provides an RF pulse, which is an excitation pulse, to the subject P (details will be described later). ), 3 is a gradient magnetic field generating section that generates a gradient magnetic field to be superimposed on the static magnetic field Ho. This gradient magnetic field generating section 3
are the gradient magnetic field Gz for slicing, the gradient magnetic field Gy for phase coding, and the gradient magnetic field G for readout as described above.
x respectively.

Reference numeral 4 denotes a signal collecting unit that receives the MR multiplied signal from the subject P. Reference numeral 5 indicates a signal collecting unit that receives the MR multiplied signal from the subject P, and 5 executes two-dimensional Fourier transformation on the MR multiplied signal received by the signal collecting unit 4 to obtain the M of the subject P.
This is an image creation unit that reconstructs an R image. Reference numeral 6 denotes an image display unit that visualizes the MR image reconstructed by the image creation unit 5. 7 is a system controller that controls the operation of the apparatus of this embodiment, and specifically, the static magnetic field generating section 1. Excitation pulse transmitter 2. Gradient magnetic field generator 3. The operation of the signal collecting section 4 is controlled according to a predetermined pulse sea fence.

Next, an example of the RF pulse transmitter 2 will be explained with reference to FIG. The RF pulse transmitter 2 includes a high frequency generation circuit 2A, an envelope generation circuit 2B, and a modulation circuit 2C that modulates the high frequency and the envelope to generate an RF pulse. The high frequency generation circuit 2A generates a first high frequency cos (ω
t→-φ) and a second high-frequency wave cos (ω=1φ+π) obtained by inverting this phase by 1806 degrees. Then, from the modulation circuit 2C, <RF pulse Pφ is generated based on the first high frequency, and <RF pulse Pφ is generated based on the second high frequency.
The pulses Pφ1π are output alternately. still,
The operation timing of each circuit 2A to 2C is controlled by the system controller 7.

In this way, by alternately outputting RF pulses whose phases are inverted each other for each repeated data acquisition operation, artifacts based on the FID signal are moved to the edge of the image and displayed as described in detail above, and the echo signal images with high diagnostic ability can be obtained. Furthermore, since there is no difference from the conventional apparatus except for the control of generation of RF pulses, image reconstruction and scanning operations are not more complicated than in the conventional apparatus.

Furthermore, compared to the prior patent, the present invention can remove artifacts by scanning the subject only once, and the data collection time does not increase, reducing the burden placed on the subject.

Note that the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the gist of the present invention.

Furthermore, the present invention has the potential to be applied to fields other than MRI. In other words, if it is an unnecessary signal that almost reappears each time data collection is repeated, and the sign of the unnecessary signal can be reversed by applying pulses, then the unnecessary signal can be converted into an effective image by the same means. can be removed from the area.

〔Effect of the invention〕

As explained above, according to the present invention, artifacts based on FID signals can be moved to the edge of an image and displayed using a simple configuration in which an RF pulse is applied with the phase reversed every time data collection is repeated. Therefore, an excellent effect can be achieved in that an image based on an echo signal can be displayed as an image with high diagnostic ability.

[Brief explanation of drawings]

Figure 1 is a block diagram of the RF pulse transmitter, Figure 2 is the MH
3 is a schematic block diagram of the I device, FIG. 3 is a characteristic diagram of the FID signal obtained by the present invention, FIG. 4 is a characteristic diagram showing a signal train when the FID signal shown in FIG. 3 is cut at 1=1o, 5
The figure is a schematic diagram showing the effect of 2DFT artifacts moving to the edge of the image, Figure 6 is a timing chart showing a pulse sequence by the spin echo method, Figure 7 is a schematic diagram showing an image containing artifacts, and Figure 8 is a schematic explanatory diagram showing the occurrence position of artifacts due to two-dimensional Fourier transform, FIG. 9 is a characteristic diagram of the FID signal obtained with the conventional device, and FIG. 10 is the FID signal shown in FIG.
FIG. 3 is a characteristic diagram showing a signal train when cut at 1o. 2...RF pulse transmitter. C Figure 8 Fo (Cs'O) C

Claims (1)

[Claims] A subject is placed in a uniform static magnetic field, a gradient magnetic field is superimposed on this uniform static magnetic field, and an RF pulse for setting an excitation rotating magnetic field is applied each time the strength of the gradient magnetic field for phase coding is varied. The RF pulse is applied in a magnetic resonance imaging apparatus that repeatedly performs the operation, detects the magnetic resonance signal induced for each repetitive operation, and images a cross section of the subject by performing two-dimensional Fourier transform on the magnetic resonance signal. A magnetic resonance imaging apparatus, wherein the RF pulse transmitter applies an RF pulse with a high frequency phase inverted for each repetitive operation.