JPH0581136B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus capable of obtaining at least one of a water image and a fat image regarding protons.

(Prior art) Magnetic resonance (MR) development is a phenomenon in which atomic nuclei with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorb and emit only electromagnetic waves of a specific frequency. , this nucleus has an angular frequency ω ₀ (ω ₀ =2πν ₀ ,
It resonates at ν ₀ ; Larmor frequency).

ω ₀ =γB ₀ Here, γ is the gyromagnetic ratio specific to the type of atomic nucleus, and B ₀ is the static magnetic field strength.

The device that performs biological diagnosis using the above principles is
The electromagnetic waves (magnetic resonance signals: echo signals and FID signals) with the same frequency as those above, which are induced after the resonance absorption described above, are processed to determine the nuclear density and longitudinal relaxation time.
It is possible to non-invasively obtain diagnostic information such as a slice image of a subject that reflects information such as T1, lateral saturation time T2, flow, and chemical shift.

Collecting diagnostic information by magnetic resonance can excite all parts of a subject placed in a static magnetic field and collect signals, but there are limitations in the equipment configuration and clinical requirements for imaging images. from,
The actual device excites a specific region and collects its signals.

FIG. 2 is a diagram showing the overall configuration of an apparatus capable of implementing this type of magnetic resonance imaging method.

As shown in Fig. 2, a magnet assembly capable of accommodating a subject P therein includes a static magnetic field coil (a static magnetic field correction shim coil is added) using a normal conduction or superconducting method. ) 1, and a gradient magnetic field generating coil 2 for generating a gradient magnetic field (gradient magnetic field pulse) for providing positional information of the induced site of the magnetic resonance signal.
and a rotating high-frequency magnetic field (high-frequency pulse; RF pulse)
It has a probe 3 consisting of a coil, for example, which is a transmission/reception system for transmitting magnetic resonance signals (echo signals, etc.) and detecting the induced magnetic resonance signals (echo signals, etc.), and if it is a superconducting method, it also includes a refrigerant supply control system. A static magnetic field control system 4, which mainly controls the energization of the static magnetic field power supply, a transmitter 5, a receiver 6, X-axis, Y-axis, and Z-axis gradient magnetic field power supplies 7, 8, 9, and a two-dimensional Fourier transform method as the imaging method. For example, a sequencer 10 capable of implementing the image data acquisition sequence shown in FIG. 3, and a computer system 1 that controls these and performs signal processing of detection signals and display thereof.
1.

Here, the receiver 6 includes a preamplifier that amplifies the signal from the receiving coil of the probe 3 to the extent that it can be applied to subsequent processing, and performs phase detection on the output of this preamplifier using a real part and an imaginary part, respectively. It includes a phase detector, an A/D converter that converts the output of the phase detector into a digital signal, and an interface that introduces the output of the A/D converter into the computer system 11.

The computer system 11 also includes a controller that performs overall control via a data bus;
Introducing the data from the interface first,
It is equipped with an image memory such as a magnetic disk device in preparation for subsequent reconstruction processing, etc., and a reconstruction device that reads data from this image memory and reconstructs an image such as a two-dimensional image by two-dimensional Fourier transform processing. .

With the above configuration, the imaging procedure is to place the subject P in a static magnetic field, operate the sequencer 10, and execute the image data acquisition sequence pulse sequence shown in FIG. 3, for example.

This drives the transmitter 5, applies an RF pulse (90° pulse) from the transmitting coil of the probe 3, drives the gradient magnetic field power supplies 7, 8, and 9, and generates gradient magnetic fields G _X , G from the gradient magnetic field generating coil 2. _Y , G _Z ,
They are applied as a gradient magnetic field for slicing (G _S ), a gradient magnetic field for encoding (G _E ), and a gradient magnetic field for reading (G _R ), and the signals from the local excitation site are collected by the receiver coil of the probe 3, and the signals are collected in the Fourier space plane. 1
I'm trying to get line data. and,
In order to generate one screen, data is obtained by repeating the sequence shown in Fig. 3 a predetermined number of times, and the data obtained each time this sequence is executed is transferred to an image memory via a data bus and converted into a two-dimensional image by a reconstruction device, for example. generate an image and display the reconstructed image.

In magnetic resonance imaging as described above, extremely weak magnetic resonance signals from the human body are handled.
How to obtain an image with good S/N is a serious problem.

Generally, as a method for imaging magnetic resonance signals, a high-frequency pulse of 90° is used in the two-dimensional Fourier transform method that combines a high-frequency pulse for spin excitation and a gradient magnetic field pulse for position identification, as described above and shown in FIG. The pulse sequence of the −180° spin echo method is well known.

Next, we will discuss the clinical applications of images obtained by MRI equipment. In other words, regarding protons as target nuclides, most of them are contained in water and fat, and water images (images of protons contained in water)
It is meaningful to observe the fat images (images of protons contained in fat). Conventionally, these images have been obtained using chemical shift. That is, FIG. 3 shows a water image obtained, and FIG. 4 shows a fat image obtained.

In the sequence shown in Figure 3, the timing of the RF pulse and the gradient magnetic field (gradient magnetic field for reading) is set so that the point where the phases of the water and fat signals match coincides with the spin echo time _TE . In the sequence shown in Figure 4, the 180° pulse is shifted by Δt from that shown in Figure 3, so that the phases of the water and fat signals are shifted by 180°. It is applied at the timing of τ + Δt,
An image of water or fat is obtained by the sum or difference between the echo signal according to the sequence of FIG. 3 and the echo signal according to the sequence of FIG. 4.

(Problems to be Solved by the Invention) According to the above-described method, images of water or fat can be obtained, and although it has clinical advantages, it requires two separate imaging times and requires a long imaging time. This is a problem. On the other hand, by using multi-echo technology, it is possible to obtain in one shot the first and second echoes whose phases are aligned as described above, and whose phases are shifted by 180°. With 2 echoes, there is a drop in S/N, which is still a problem.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus that can obtain an image of water or fat with a high S/N in a single imaging operation.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a structure in which the following means are taken to solve the above problems and achieve the objects.
That is, the present invention provides: a static magnetic field generation means for generating a static magnetic field applied to a subject; a gradient magnetic field generating means for generating a gradient magnetic field applied to the subject; and a proton magnetic field generation means applied to the subject. a high-frequency pulse generating means for generating a high-frequency pulse related to the excitation of the subject; A first gradient magnetic field whose application timing is set to generate a spin echo signal under the condition that the phases of protons in water and fat align at their peaks after selectively exciting protons present in a specific region, and water and fat. The phase of the proton at the peak is 180°
A pulse including a second gradient magnetic field of opposite polarity to the first gradient magnetic field and a third gradient magnetic field of the same polarity as the first gradient magnetic field, the application timing of which is set to generate a field echo signal under shifted conditions. a control means for executing a sequence; a collection means for collecting a spin echo signal and a field echo signal related to the excitation of the protons; and a collection means for collecting a spin echo signal and a field echo signal related to the excitation of the protons; and a signal processing means for processing the spin echo signal and the field echo signal to obtain information on a specific region of the subject.

(Function) According to such a configuration, a water image can be obtained using the sum signal of the spin echo signal and the field echo signal, and a fat image can be obtained using the difference signal.
Both of these images are obtained by one photographing operation, and the S/N ratio is also improved due to the effect of averaging.

(Example) An example of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to FIG.

As shown in FIG. 1, from time t ₁ to time t ₅ ,
This is a pulse sequence of the two-dimensional Fourier transform method of the normal spin echo method, and a spin echo signal is output at time _t4 , but at this time, there is no phase difference between the water and fat signals.

Next, since the phase is disturbed between times t ₄ and _{t 5} , −G _R ′ is added in the negative direction in order to refase this. By adding -G _R ′ for rephase and even dayphase, the phase is dispersed. In order to align these scattered phases,
Add G _R. In this case, the relationship ∫ ^t5 _t4 G _R d _t +∫ ^t7 _t6 G _R d _t =∫ ^t6 _t5 G _R ′d _t needs to be satisfied.

Furthermore, in the field echo method using _GR , a phase difference occurs between the water and fat signals. Here, chemical shift amount δ, static magnetic field strength
There is a relationship between B ₀ and the phase difference φ: φ=γ・δ・B ₀ (t ₇ −t ₄ ) γ: gyromagnetic ratio.

Therefore, if t ₇ −t ₄ is determined so that φ=180°, a phase difference of 180° will be generated between the water and fat signals. Also, φ=n×180° (n: odd number)
You may make it so that In Figure 1, an echo signal is generated at t3-t4, and at t4-t5 and
Echo signals are also generated at t6−t7, but here the spin echo signals Sse and t6− are collected at t3−t4.
Only the field echo signal Sfe at t7 is shown.

When t7 and t4 are determined so that φ = 180° as described above, a water-only signal is obtained by S _se + S _fe using the spin echo signal S _se and field echo signal S _fe , and the S/N is will also improve by √2 times.

Moreover, a signal of only fat is obtained by S _Se −S _fe , and the S/N is improved by √2 times. If image reconstruction is performed using these signals, it is possible to obtain a water image and a fat image in one imaging operation.

Note that only S _Se is subjected to reconstruction processing, and
A water image and a fat image can also be obtained by subjecting only S _fe to reconstruction processing and performing calculations between images.

In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] Therefore, according to the present invention, it is possible to provide a magnetic resonance imaging method that can obtain an image of water or fat with a high S/N in a single photographing process.

[Brief explanation of the drawing]

Fig. 1 is a pulse sequence diagram in a magnetic resonance imaging apparatus according to an embodiment of the present invention, Fig. 2 is a diagram showing the configuration of the magnetic resonance imaging apparatus, and Fig. 3 is a diagram showing the configuration of the magnetic resonance imaging apparatus.
4 and 4 are pulse sequence diagrams showing a conventional example. 1... Static magnetic field coil, 2... Gradient magnetic field generation coil, 3... Probe, 4... Static magnetic field control system, 5...
...Transmitter, 6...Receiver, 7, 8, 9...Gradient magnetic field power supply, 10...Sequencer, 11...Computer system.

Claims (1)

[Scope of Claims] 1. Static magnetic field generating means for generating a static magnetic field to be applied to a subject; gradient magnetic field generating means for generating a gradient magnetic field to be applied to the subject; applied to the subject; a high-frequency pulse generating means for generating a high-frequency pulse related to excitation of protons, and applying the gradient magnetic field and the high-frequency pulse under predetermined conditions to the subject placed in the static magnetic field, the subject being After selectively exciting protons existing in a specific region of the water and fat, a first gradient magnetic field whose application timing is set to generate a spin echo signal under the condition that the phases of the protons in water and fat align at their peaks. The phase of protons in fat is 180° at its peak.
A pulse including a second gradient magnetic field of opposite polarity to the first gradient magnetic field and a third gradient magnetic field of the same polarity as the first gradient magnetic field, the application timing of which is set to generate a field echo signal under shifted conditions. a control means for executing a sequence; a collection means for collecting a spin echo signal and a field echo signal related to the excitation of the protons; and a collection means for collecting a spin echo signal and a field echo signal related to the excitation of the protons; A magnetic resonance imaging apparatus comprising: signal processing means for processing the spin echo signal and the field echo signal to obtain information on a specific region of the subject.