JPH0531094A - Mri device - Google Patents

Abstract

PURPOSE:To remove an artifact from the outside of the selection region of a slice group profile. CONSTITUTION:The sequence exciting the outside of the selection region of a slice group profile for pre-saturation is provided immediately before the three- dimensional measuring sequence. An artifact from the outside of the selection region of the slice group profile can be removed without increasing the number of waves or increasing the echo time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI (magnetic resonance imaging) apparatus for removing artifacts generated from outside a selected area in a three-dimensional measurement sequence.

[0002]

2. Description of the Related Art There are various three-dimensional measuring methods in an MRI apparatus, and the measuring method by the spin warp method is shown in FIG. As shown in FIG. 4, the spin warp method is roughly 4
It can be divided into two sections a, b, c and d, and the sequence in each section is as follows.

(1) Section a ... 90 ° pulse (high frequency pulse) and slice axis gradient magnetic field Gs (where Gs>
0) is applied. At this time, the phase encode axis gradient magnetic field G
p = 0 and frequency encoding axis gradient magnetic field Gf = 0. (2), section b ... Slice axis gradient magnetic field Gs is applied.
However, Gs is Gs1, Gs2, ..., Gsn, over a plurality of repetition periods (TR) of the number n of slice encoding times.
(However, Gs1 <Gs2 <... <Gsn) is updated. This enables three-dimensional measurement instead of the conventional two-dimensional measurement. At this time, Gp = 0 and Gf = 0. (3) Section c ... A phase encode axis gradient magnetic field Gp is applied. However, Gp is Gp1, GP2, ..., GP over a plurality of repetition periods (TR) of the number m of phase encoding times.
m (however, Gp1 <Gp2 <... <Gpm) is updated.
This can improve the resolution in the phase encode direction. Simultaneously with Gp, a frequency encode axis gradient magnetic field Gf (however, Gf <0) is applied. At this time, Gs = 0
Is. (4), Section d ... The frequency encoding axis gradient magnetic field Gf (where Gf> 0) is applied, and the MR signal (echo signal) generated during this period is detected. (5) Note that Gs and Gp are updated in the following relationship. (A), Gs = Gs1. In this state {a → b → c
(Gp = Gp1) → d → echo detection} → {a → b → c
(Gp = Gp2) → d → echo detection} → ... → {a → b →
The sequence of c (Gp = Gpm) → d → echo detection} is executed. (B), Gs = Gs2. In this state, echo detection is performed by updating Gp = Gp1, Gp2, ..., Gpm as in (a). (C) Then, Gs = Gs3, ..., Gsn are updated one after another, and a similar sequence is executed.

[0004]

As the high frequency pulse in the sequence of FIG. 4, various waveforms such as a sync function and an FM wave are used. Such high frequency pulses, for example sink function F
As

[0005]

## EQU1 ## FIG. 5 shows a slice group profile when F = (sin x) / x is used. FIG. 5A is a diagram showing the relationship between the slice position and the signal strength when the sink function is Fourier transformed. Here, the slice position is the frequency axis. In this profile, the selective excitation region E1 has a rectangular spectral characteristic, and this section is given as a region to be selected. On the other hand, the region outside the region E1 is a non-selected region, but due to the characteristics of the sync function, the two regions E2 and E3 on the left and right of the external region have a signal strength that cannot be ignored. As a result, selective excitation also occurs in the regions E2 and E3, and the selectively excited video signal is generated also in the regions E2 and E3, and this is mixed into the echo signal in the region E1. As a result, as shown in FIG. 5B, the artifacts (also called folding artifacts) caused by the selective excitation of the regions E2 and E3 are shown in FIG.
It appears in the reconstructed image as shown in (c).

The cause of the occurrence of artifacts is E2, E
3 exists, which means that the signal strength in the area E1 is
This is due to the fact that it is not a perfect rectangle although it is rectangular as shown in FIG. Although it is physically impossible to make a perfect rectangle, a method of making it close to a rectangle is also conventionally used. It is a method of increasing the number of waves in the area E1 as much as possible. FIG. 6 shows an example thereof. The horizontal axis represents frequency (position) and the vertical axis represents signal intensity. The dotted line shows an example of 3 waves existing in the area E1, and the solid line shows an example of 5 waves existing in the area E2. Thus, it can be seen that as the wave number increases, it approaches a rectangle and the signal strength outside the region also decreases.

As described above, conventionally, the artifacts have been suppressed by increasing the wave number and improving the slice characteristic, but it is difficult to increase the wave number of the sync function infinitely, and an echo for obtaining a video signal is obtained. In order to make the time a reasonable value, there is a limit to increase the wave number of the sinc function, and it is impossible to completely prevent the artifact.
Although the example of the sink function has been described, there is a similar problem with other high frequency pulses such as FM waves.

An object of the present invention is to provide an MRI apparatus capable of removing an artifact from outside the slice group profile area without increasing the wave number of a sync function or the like and extending the echo time.

[0009]

According to the present invention, a sequence for exciting pre-saturation outside a selected region of a slice group profile is provided immediately before a sequence for three-dimensional measurement (claim 1).

Further, according to the present invention, the outside of this region is divided into two, that is, a frequency larger than the center frequency and a frequency smaller than the center frequency, and a sequence for presaturating each separately is provided (claim 2).

[0011]

According to the present invention, the cause of the artifact generation from outside the selected area is eliminated immediately before the execution of the three-dimensional measurement sequence, and the occurrence of the artifact is eliminated.

[0012]

FIG. 2 is a block diagram showing the overall construction of an MRI apparatus according to the present invention. This MRI apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10 and a central processing unit (CPU) 1.
1, a sequencer 12, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15 and a signal processing system 16.

The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 1 in the body axis direction or in the direction orthogonal to the body axis, and has a certain spread around the subject 1. A permanent magnet type, a normal conducting type, or a superconducting type magnetic field generating means is arranged in the held space.
The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmission system 13, the magnetic field gradient generation system 14, and the reception system 15. The transmission system 13 includes a high frequency amplifier 19 and a high frequency coil 20a on the transmission side.
In accordance with the command of 1., the modulator 18 amplitude-modulates, the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19, and then supplied to the high-frequency coil 20a arranged close to the subject 1, whereby the electromagnetic field wave is The sample 1 is irradiated. The magnetic field gradient generation system 14 includes X,
A gradient magnetic field coil 21 wound in three directions of Y and Z, and a gradient magnetic field power source 22 for driving each coil, and driving the gradient magnetic field power source 22 for each coil according to an instruction from the sequencer 12. X,
Gradient magnetic fields Gx, Gy, and Gz in the triaxial directions of Y and Z are applied to the subject 1. The slice plane for the subject 1 can be set by the method of applying the gradient magnetic field. The receiving system 15 includes a high frequency coil 20 on the receiving side.
b, amplifier 23, quadrature detector 24, and A / D converter 2
5, and the electromagnetic wave (NMR) of the response of the subject 1 due to the electromagnetic wave emitted from the high-frequency coil 20a on the transmission side.
Signal) is a high-frequency coil 2 arranged close to the subject 1.
0b, is input to the A / D converter 25 via the amplifier 23 and the quadrature detector 24, is converted into a digital amount, and is further sampled by the quadrature detector 24 at the timing according to the instruction from the sequencer 12. The data is collected into two series, and the signal is sent to the signal processing system 16. This signal processing system 16 is a CP
U11, a recording device such as a magnetic disk 27 and a magnetic tape 29, and a display 28 such as a CRT. The CPU 11 performs processing such as Fourier transform and correction coefficient calculation image reconstruction to obtain a signal intensity distribution of an arbitrary cross section or a plurality of signals. The distribution obtained by performing an appropriate calculation on the signal is imaged and displayed on the display 28. In FIG. 2, the high frequency coils 20a, 20 on the transmitting side and the receiving side
b and the gradient magnetic field coil 21 are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 1.

Here, in the present embodiment, as the sequence in the sequencer 12, immediately before the normal imaging sequence (measurement sequence) is performed, for the subject 1 outside the area of the slice group profile in which the artifact is generated, It was decided to provide a sequence for performing selective excitation and phase rotation for presaturation.

FIG. 1 is a sequence diagram of this embodiment applied to an imaging sequence by the spin warp method. This sequence is given by the sequencer 12 and the CPU 11. In the figure, a to d are the same visualization sequences as the conventional one by the spin warp method, and in this embodiment, two sequences of e and f were added immediately before the visualization sequence. The normal three-dimensional measurement sequence shown in a to d is shown in FIG.
Since it is the same as, the description thereof will be omitted and the sections e and f will be described.

(1) Section e ... FM waves for selective excitation outside the slice profile group region are first applied as shown in FIG. Along with the application of this FM wave, a slice axis gradient magnetic field Gs is applied (Gs> θ). During this period, Gp =
θ and Gf = θ. Thus, the subject is selectively excited outside the slice group profile region where the artifact is generated. (2) Section f ... Gradient magnetic fields Gs, Gp, and Gf are strongly applied (Gs> θ, Gp> θ, Gf> θ, and the magnetic field is large). Thus, the phase of the spin excited in the section e could be extremely rotated and could be diffused.
By the operations in the sections e and f, from the subject that generates an artifact outside the slice group profile region,
It was possible to reduce the false signal extremely.

When the signal of the portion generating the artifacts from outside the slice group profile region is certainly rotated 90 ° by the FM wave in the section e, any of the gradient magnetic fields Gs, Gp and Gf in the section f is selected. Even if it is not applied, the 90 ° pulse (section a) for imaging rotates the portion that generates the artifact by 180 °, so that the signal does not become a substantial signal and the artifact can be reduced.

FIG. 3 shows another embodiment of the present invention. The feature of this embodiment is that the area outside the slice group profile area is divided into an artifact occurrence area in a frequency area higher than the center frequency (meaning the center frequency in the selected area) and an artifact occurrence area in an area lower than the center frequency. Is selectively excited. In FIG. 3, section e1 is the former example, and section e2 is the latter example. The other sections are the same as in FIG. This enhances the effect of presaturation.

1 and 3 of each of the above-described embodiments, the portion of the imaging sequence has been described as the spin warp method sequence, but the present invention is applicable to the sections e and f of FIG. 1 and the section e of FIG.
The present invention can be applied to all cases by performing the selective excitation sequence of 1, e2, and f immediately before various imaging sequence such as SE method and IR method. Further, although the high frequency pulse at e, e1 and e2 is the FM wave, other waveforms such as a sync function may be used. Further, the cause of the folding artifact is the sync function of [Equation 1], but even if it is a sync function or FM wave other than [Equation 1], such an artifact may occur, and it can be applied to these.

[0020]

According to the present invention, as described above, 3
It is possible to significantly reduce the artifacts generated from outside the slice group profile region in the dimension measurement sequence, and to provide an MRI apparatus having high slice characteristics without artifacts.

[Brief description of drawings]

FIG. 1 is an example diagram of a sequence according to a spin warp method of the present invention.

FIG. 2 is a diagram showing an embodiment of the MRI apparatus of the present invention.

FIG. 3 is a diagram showing another embodiment of the sequence according to the spin warp method of the present invention.

FIG. 4 is a diagram showing a sequence according to a conventional spin warp method.

FIG. 5 is a diagram showing generation of slice group profiles and artifacts when a conventional sync function is a high frequency pulse.

FIG. 6 is an explanatory diagram of conventional artifact removal by increasing the wave number.

[Explanation of symbols]

Gs slice direction gradient magnetic field Gp Phase encoding direction gradient magnetic field Gf Frequency encoding direction gradient magnetic field MR echo signal

Claims (2)

[Claims] 1. An MR having a sequence for performing three-dimensional measurement
In the I apparatus, an MRI apparatus in which a sequence for presaturating the area outside the slice group profile is provided immediately before the sequence. 2. An MR having a sequence for performing three-dimensional measurement
In the I-apparatus, immediately before the sequence, MRI is provided with separate sequences for presaturating the regions outside the slice group profile and below the center frequency and above the center frequency. apparatus.