JPH0282942A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To prevent the artifact due to chemical shift and to obtain an image having a high S/N ratio by inversing the polarity of a read inclined magnetic field two or more times with the elapse of time in succession to the excitation of magnetization and the convergence of a spin phase and then adding and averaging a plurality of collected echo signals on a frequency space. CONSTITUTION:The specific region of an examinee is excited in a section I, and a spin is dephased by a read inclined magnetic field Gr and a spin phase is converged by a 180 deg. pulse and a slice inclined magnetic field Gs. In a section II, a spin is rephased by the read inclined magnetic field Gr and the first echo signal S1 is collected. In a section III, the polarity of said magnetic field Gr is inverted to collect the second echo signal S2. Further, the polarity of the magnetic field Gr is inverses to collect the third echo signal S3 and the spin is dephased. Herein, the echo signals S1, S2, S3 are subjected to Fourier transform. Then, by adding and averaging the signals S1, S2, S3, an S/N ratio can be enhanced. In this case, by intensifying inclined magnetic fields including the read inclined magnetic field Gr, the generation of the artifact due to chemical shift can be prevented.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to magnetic resonance (MR)
Regarding a magnetic resonance imaging method that utilizes the 5onance phenomenon to obtain morphological information such as biological slice images and functional information such as spectroscopy, in particular, to prevent artifacts due to chemical shift and obtain images with high S/N. The present invention relates to a magnetic resonance imaging method that makes it possible to

(Prior art) General [%Jl1%SR1 (Magnetic Re
In a 5onanse imaging (magnetic resonance imaging) system, a gradient magnetic field and an excitation radio frequency pulse (RF pulse) are applied to a subject placed in a static magnetic field according to a predetermined excitation/detection procedure. A magnetic resonance phenomenon is generated in a specific part of the examiner, the induced magnetic resonance signals (echo signals and FID signals) are detected, and signal processing is performed using reconstruction techniques such as two-dimensional Fourier transform. By rubbing, anatomical information and qualitative information of the specific region of the subject is imaged.

An example of this type of MRI apparatus will be described below with reference to FIG. In FIG. 5, the cylindrical main body MA includes a superconducting or normal conducting static magnetic field coil I as a static magnetic field generator,
X-axis, Y-axis, and Z-axis gradient magnetic field coils 2 and transmitting/receiving coils 3 are provided.

The subject P is introduced into the diagnosable magnetic field generation area within the main body MA.

Further, the static magnetic field coil I is driven by the static magnetic field control system 4, the transmitting/receiving coil 3 is driven by a transmitter 5 for excitation, by a receiver 6 for detection (collection), and by a receiver 6 for detection (collection). The axis, Y axis, and Z axis gradient magnetic field coils 2 are each
It is driven by an axial gradient magnetic field power source 7, a Y-axis gradient magnetic field power source 8, and a Z-axis gradient magnetic field power source 9.

Y-axis, Y-axis, Z-axis gradient magnetic field power supply 7°8.9 in the above
The transmitter 5 is driven according to a predetermined image data acquisition sequence I by the sequencer 101. For example, as shown in FIG. 6, an SE (Spie echo) method is performed using a slicing gradient magnetic field G5, a read gradient magnetic field Gr1, an encoding gradient magnetic field Ge, and 90 pulses and 180 pulses.

The data group obtained by executing the above sequence is taken into the receiver 6 every time it is collected, converted into a digital signal, and then
This is sent to the computer system 11-. Computer system II uses two-dimensional Fourier transform processing to
A dimensional image (slice image) is created and displayed on the monitor.

(Problems to be Solved by the Invention) In such magnetic resonance imaging, in order to prevent artifacts based on chemical shifts (generally, chemical shifts between water and fat related to protons ('H)), lead slopes are used. The intensity of the magnetic field Gr is applied strongly as shown in the following formula.

r-Gr・Δノ≧δ-r-B.

r: gyromagnetic ratio #: resolution, δ: chemical shift between water and fat F N Boil static magnetic field strength Therefore, the purpose of the present invention is to widen the frequency band Δf within one pixel.
An object of the present invention is to provide a magnetic resonance imaging method that can obtain high S/N images while preventing artifacts due to chemical shifts.

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems and achieve the objects, the present invention is configured to take the following measures.

In other words, in a magnetic resonance imaging method that uses magnetic resonance phenomena to obtain an image that reflects the spin density and relaxation time constant of a specific atomic nucleus, following the excitation of magnetization and the convergence of the spin phase, the polarity of the read gradient magnetic field is changed. It is characterized in that a plurality of echo signals are collected by inverting multiple times over time, and one piece of echo signal data is obtained by averaging the plurality of echo signals in frequency space.

(Function) According to such a configuration, even if the strength of the read gradient magnetic field Gr is increased to prevent chemical shift artifacts, multiple echo signals are collected and averaged within the data collection repetition time. Since the information is generated as one piece of data, the S/N ratio is improved.

(Example) An example of the magnetic resonance imaging method using the F'A4 of the present invention will be described below with reference to FIGS. 1 to 3.

FIG. 1 is a diagram of a pulse sequence showing a first embodiment. In section I shown in FIG. Phase encoding, 180 pulse and slicing gradient vj
The spin phase is converged by G.

In section (3), the first echo signal S is collected by rephasing with the read gradient magnet %Gr.

In section (3), the polarity of Gr is reversed (negative in this case), and an echo signal due to spins after the day phase, that is, a second echo signal S, is collected. This is called a gradient echo signal, gradient, field echo signal, field echo signal, etc.

In section m, the echo signals gathered in the second half of section ■ are day-phased again, and the G is further increased.

(in this case, positive), the echo signals (third echo signal S, ) are collected and dephased.

Here, when SI e "F v Sm is Fourier transformed, Sl becomes Fig. 2 (a), and S becomes Fig. 2 Φ
).

As is clear from Fig. 2(b), the gradient magnetic field G for read
Since r is inverted, S! on the frequency axis! is St
is inverted against.

Therefore, the data in FIG. 2(b) is converted into data shown in FIG. 2(C) which is inverted with respect to the frequency axis, and then averaged in the frequency space, that is, the signals S, , S, , S are obtained.

By calculating (adding and averaging) 8=(E, +E, +E,)/3, it is possible to improve the S/N. In this case, the read gradient magnetic field Gr
By increasing the strength of the gradient magnetic field, it is possible to prevent the occurrence of chemical shift artifacts, and as described above, it is also possible to improve the S/N ratio.

In addition to the above effects, there are also the following effects. That is,
Since 8/N becomes high, thin slices become possible, and as a result, the accuracy of diagnosis improves.

Image resolution is improved because chemical shift artifacts can be prevented.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 3 shows an application to the gradient field echo method (FE method) in which the read gradient magnetic field Gr is inverted instead of 180 pulses in order to converge the spin phase. In this example, the application time and intensity of day phase Gr are the same for positive, negative, and positive. In FIG. 1, the application time and intensity of the day phase Gr are long and weak for the first positive, and short and strong for the next negative and positive.

Next, an embodiment of M3 of the present invention will be described with reference to FIG. In Fig. 4, multi-echo signals are collected, and Sl is obtained by S, , S, , S, and S□.

S, is obtained by S, , 8□, and the application time and intensity of Gr are changed between S, , S, and S.

The present invention is not limited to the first embodiment described above, and can be implemented with various modifications without departing from the gist of the present invention.

〔Effect of the invention〕

As described above, in the present invention, following magnetization excitation and spin phase convergence, the polarity of the read gradient magnetic field is reversed multiple times over time to collect multiple echo signals. , one echo signal data is obtained by averaging in the frequency space, so even if the strength of the read gradient magnetic field Gr is increased to prevent chemical shift artifacts, it is possible to obtain data within the repetition time of data collection. , multiple echo signals are collected and averaged to generate one data, so S
Ha is improved.

Therefore, according to the present invention, it is possible to provide a magnetic resonance imaging method that can obtain high-Si images while preventing artifacts due to chemical shift.

[Brief explanation of the drawing]

1 and 2 are views showing a first embodiment of the magnetic resonance imaging method according to the present invention, and FIGS. 3 and 4 are views showing second and third embodiments of the present invention. FIG. 5 is a diagram showing the configuration of a magnetic resonance imaging apparatus, and FIG. 6 is a diagram showing a conventional example. ! ... Static magnetic field coil, 2... Gradient magnetic field coil, 3.
... Transmission/reception coil, 4... Static magnetic field control system, 5... Transmitter, 6... Receiver, 7, 8.9... Gradient magnetic field power supply, IO... Sequencer, II... computer system.

Claims (1)

[Claims] In a magnetic resonance imaging method that uses magnetic resonance phenomena to obtain images that reflect the spin density and relaxation time constant of a specific atomic nucleus, following magnetization excitation and spin phase convergence,
By reversing the polarity of the read gradient magnetic field multiple times over time, multiple echo signals are collected, and these multiple echo signals are averaged in frequency space.
A magnetic resonance imaging method characterized by obtaining one echo signal data.