JPH0263434A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To obtain the image of water or fat with high S/N by one-time photographing by setting the applying timing to an inclined magnetic field in such a way that the phases of water and fat are conformed with a spin echo signal and that the phases of the water and fat are 180 deg. shifted with a field echo signal. CONSTITUTION:Until the time t1-t5, a pulse sequence of two-dimensional Fourier transformation method of an ordinary spin echo method is performed and, although a spin echo signal is generated at the time t4, at this time, no phase difference exists between the signals of water and fat. In a field echo method by GR, the phase difference exists between the signals of water and fat. In this case, since there is a specific relation among a chemical shift quantity delta, a static magnetic field strength B0, the phase difference phi, and t7-t4, by determining the t7-t4 so that, phi=180 deg., a phase difference of 180 deg. is generated between the signals of water and fat. Hence, the signal of only water can be obtained using the spin echo signal SSe and the field echo signals Sfe and by SSe+Sfe, also increasing S/N by 2<1/2> times.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Application Field) The present invention collects echo signals of chemically shifted protozoa in water and fat using an NRI device, and collects the echo signals of water and fat images. The present invention relates to a magnetic resonance imaging method for obtaining at least one of the following.

(Prior art) Magnetic resonance (MR)
e) Development is a phenomenon in which an atomic nucleus with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorbs and emits only electromagnetic waves of a specific frequency, and this atomic nucleus has an angular frequency expressed by the following formula. ω. It resonates at (ωo=2πsi0.si0; Larmor frequency).

ω,)=rB,) Here, r is the gyromagnetic ratio specific to the type of atomic nucleus,
Moreover, Bo is the static magnetic field strength.

A device that performs biological diagnosis using the above principle processes the electromagnetic waves (magnetic resonance signals: echo signals and FID signals) of the same frequency as above that are induced after the above-mentioned resonance absorption, and calculates nuclear density, vertical Relaxation time T no 0. Diagnostic information, such as a slice image of the subject, reflecting information such as the flow of transverse relaxation time T2j and chemical shift, is obtained non-invasively.

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. Therefore, the actual device is designed to excite a specific region and collect 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, the magnet assembly that can be housed inside the subject P′ft includes a static magnetic field coil (a static magnetic field correction shim coil is added) using a normal conduction or superconducting method. There are also.)1
, a gradient magnetic field generating coil 2 for generating a gradient magnetic field (gradient magnetic field pulse) for imparting positional information of the induced site of the magnetic resonance signal, and a rotating high frequency magnetic field (RF pulse).
The induced magnetic resonance signal (
For example, if it is a superconducting method, it includes a refrigerant supply control system, and mainly controls the energization of the static magnetic field power supply. Static magnetic field control system 4, transmitter 5, receiver 6, X axis, Y axis, 2-axis gradient magnetic field power supply? , 8, 9. A sequencer 10 capable of implementing, for example, the image data collection sequence shown in FIG. 3 using a two-dimensional Fourier transform method as an imaging method. It is comprised of a computer system 11 that controls these and also performs signal processing and display of detection signals.

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

The computer system lln, through the -7"-1 path, is connected to a controller that performs overall control, and an image memory such as a magnetic disk device that first introduces data from the interface and is used for subsequent reconstruction processing, etc. , reads the data from this image memory and creates an image such as a two-dimensional image.
and a reconstruction device that reconstructs the image by performing a 7-dimensional transformation process.

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

As a result, the transmitter 5 is driven, and the RF' t4 pulse (90' pulse) is applied from the transmitting coil of the probe 3, and the gradient magnetic field power supplies 7, 8.9 are driven, and the gradient magnetic field generating coil 2 outputs the gradient magnetic field. The magnetic fields GX e Gy r Gz are applied as a gradient magnetic field for slicing (GR), a gradient magnetic field for encoding (GW), and a gradient magnetic field for reading (GR), respectively, and the signals from the local excitation site are collected by the receiving coil of the glove 3. , busy trying to obtain one line of data in the Fourier space plane. Then, in order to generate one screen, the third
Data is obtained by repeatedly executing the sequence shown in the figure a predetermined number of times, and the data obtained each time this sequence is executed is transferred to an image memory and a reconstruction device to generate, for example, a two-dimensional image, and the reconstructed image is Display.

In magnetic resonance imaging as described above, very weak magnetic resonance signals from the human body are handled, so the Ics/N
Obtaining good images is a serious problem.

Generally, methods for imaging magnetic resonance signals include:
High-frequency Gurstsite 30°-18 in the two-dimensional Fourier transform method that combines the high-frequency pulse for spin excitation and the gradient magnetic field A' Lus for position identification described above and shown in FIG.
The spin echo method nopulse sequence of 06 is well known.

Next, we will discuss the clinical applications of images obtained by KMRI equipment. In other words, regarding protons as target nuclides, most of them are contained in water and fat, and a water surface image (an image of protons contained in water), a fat image (an image of protons contained in fat), and t - It is meaningful to observe. Conventionally, these images are obtained by using chemical shift. That is, Fig. 3 is an image of water, and Fig. 4 is an image of water. This is to obtain a fat image.

In the sequence shown in FIG. 3, the R
The timing of the p a4 pulse and the gradient magnetic field (gradient magnetic field for reading) is set, and the sequence shown in Figure 4 is a sequence in which the phases of the water and fat signals are shifted by 180 degrees, and the 180 pulses are shifted by Δt. An image of water or fat is obtained by the sum or difference between the echo signal from the sequence button in FIG. 3 and the echo signal from the sequence in FIG. 4.

(Object to be Solved by the Invention! 4!) According to the above-described method, images of water or fat can be obtained, and although it has clinical advantages, it requires two separate acquisitions and requires a long acquisition time. However, using multi-echo technology, the first echo and
Although it is possible to obtain two echoes whose phases are aligned in the same manner as described above and those whose phase is shifted by 1806 degrees with the second echo in a single photographing process.

In the second echo, there is a drop in S/N, which is a problem.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a magnetic resonance imaging method that allows an image of water or fat to be obtained with a high S/N in a single photographing process.

[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 uses an MRI apparatus to collect echo signals of chemically shifted protons in water and fat, and in a method A to obtain at least one of a water image and a fat image, magnetization is excited with a 90° pulse. and collect a spin echo signal using 180 pulses to converge the spin phase, and then collect a field echo signal by reversing the gradient magnetic field to converge the spin phase,
At this time, the spin knee signal, the field echo signal, and the fat are in phase with each other.
tJ For example, the application timing of each gradient magnetic field is set so that the phases of water and fat are shifted by 180°.

(Function) According to such a configuration, a fat image can be obtained from the sum signal of the spin echo signal and the field echo signal, the water image signal, and the difference signal, and both of these images can be captured in one photograph. The result is K, which is obtained by 5 and also has an improved S/N due to the effect of averaging.

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

As shown in Figure 1, from time t1 to time t5 is a 79 Luss sequence of the two-dimensional Fourier transform method of the normal spin echo method, and a spin echo signal is output at time Mt4. There is no phase difference between the signals.

Next K, the phase is disturbed between times t4 and t5, so in order to rephase this -G! Add L' in the negative direction.

Add K and 03 in the positive direction to the rephase degree, and to align the scattered phases for the day phase. In this case, the following relationship needs to be satisfied.

Furthermore, in the field echo method using GR, there is a phase difference between the water and fat signals. Here, the relationship between the chemical shift amount δ, the static magnetic field strength Bo, and the phase difference φ is as follows: φ=γ・δ−Bo(t7−t4) r :(a rotational ratio.

Therefore, if t7-t4 is determined so that φ=180, a phase difference of 180 will occur between the water and fat signals. Alternatively, φ=nX180 (n: odd number) may be set.

1 so that φ=180 as described above. , 14 is determined, S3°+S, using the spin echo signal SB and the field echo signal Sf. A signal of only the waste water can be obtained, and the S/N is also improved by 4 times.

In addition, a signal of only shiitake fat was obtained from S8°-8fo,
The S/N is improved by a factor of Vi. If image reconstruction is performed using these signals, it is possible to obtain a water image and a fat image in one imaging operation.

It should be noted that only S86 is subjected to the re-revision process, and furthermore, Sf
It is also possible to subject only o to reconstruction processing and obtain a water image and a fat image by performing calculations between the images.

In addition, the present invention may be modified and implemented without departing from the gist of the present invention.

[Effects of the Invention] As described above, in the present invention, in order to excite magnetization and converge the spin phase with 9 Q The field echo signals are collected by reversing the gradient magnetic field in order to converge the spin echo signals, and at this time, the spin echo signals are in phase with water and fat, By setting the application timing of each gradient magnetic field so that the phase of Both images can be obtained by one photographing operation, and the S/H can be improved by the averaging effect.

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 one imaging.

[Brief explanation of the drawing]

Fig. 1 is a /4' las sequence diagram of an embodiment of the magnetic resonance imaging method according to the present invention, Fig. 2 is a diagram showing the configuration of a magnetic resonance imaging apparatus, and Figs. 3 and 4 show a conventional example. FIG. 3 is a pulse sequence diagram. 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. Diagram

Claims (1)

[Claims] In a method of collecting echo signals of chemically shifted protons in water and fat using an MRI apparatus to obtain at least one of a water image and a fat image, in order to excite magnetization with a 90° pulse and converge the spin phase. 1
A spin echo signal is collected using an 80° pulse, and then a field echo signal is collected by reversing the gradient magnetic field to converge the spin phase, and at this time, the spin echo signal is in phase with water and fat. In addition, the field echo signal is such that the phase of water and fat is 1.
A magnetic resonance imaging method characterized in that the application timing of each gradient magnetic field is set so as to be shifted by 80 degrees.