JPH0578337B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetic resonance imaging method that applies two-dimensional Fourier transform, and in particular, to a magnetic resonance imaging method that applies two-dimensional Fourier transform, and in particular, to a magnetic resonance imaging method that does not cause ringing in images or decrease in resolution. S/
The present invention relates to a magnetic resonance imaging apparatus capable of improving N.

(Prior art) Magnetic resonance (MR) 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 =γH ₀ where γ is the gyromagnetic ratio specific to the type of atomic nucleus, and H 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 above induced after the above-mentioned resonance absorption are signal-processed to determine the nuclear density and longitudinal relaxation time T.
1. Diagnostic information reflecting information such as transverse relaxation time T2, flow, chemical shift, etc., such as a slice image of a subject, can be 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. from,
The actual device excites a specific region and collects its signals.

FIG. 5 is a diagram showing the overall configuration of an apparatus capable of implementing this type of magnetic resonance imaging method.

As shown in Fig. 5, a magnet assembly capable of accommodating the 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. 6, 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. 6.

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 , _Gz ,
They are applied as a slicing gradient magnetic field (G _s ), an encoding gradient magnetic field (G _e ), and a read gradient magnetic field (G _r ), and the signals from the local excitation site are collected by the receiver coil of the probe 3 and are 1
I'm trying to get line data. and,
In order to generate one screen, data is obtained by repeatedly executing the sequence shown in Fig. 6 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, the spin echo method in the two-dimensional Fourier transform method, which combines a high frequency pulse for spin excitation and a gradient magnetic field pulse for position identification, as described above and shown in FIG. 6, is used. The sequence is well known.

In the sequence shown in FIG. 6, a slice cross section to be imaged is selected using a 90° pulse and a slicing gradient magnetic field pulse _Gs . Then, the position of the magnetization within the selected cross section is identified based on the phase and frequency specific to each position by the gradient magnetic fields G _e and G _r in the encoding direction and the read direction. In this case, the information handled is two-dimensional (phase,
frequency), so encode the gradient magnetic field pulse
G _e is pulsed stepwise and with this encoding gradient magnetic field to obtain the desired resolution in the encoding direction.
Changes are made in each encoding process so that the increase in area of G _e remains the same.

In the above, the resolution of the image reading method Δlr
is uniquely determined by the following equation based on the strength Grt of the gradient magnetic field during the echo signal observation time 2Tac and the echo signal observation time 2Tac.

Δlr=1/(2Tac・Grt) In order to improve the S/N without causing a decrease in resolution, the observation time of the echo signal must be 2Tac.
However, since the echo signal observation time 2Tac is limited by the echo time Te and the length of the excitation high-frequency pulse, it is generally determined as follows.

In other words, the observation time 2Tac of the echo signal is divided into the first half of the observation time Tac ⁺ and Generally, the period is set so that the second half Tac ^- and the second half have the same length.

2Tac=Tac ⁺ +Tac ^- (Tac ⁺ =Tac ^- ) (Problem to be solved by the invention) In this way, in the conventional technology, by setting the observation time of the echo signal for a long time, image quality deterioration such as a decrease in resolution occurs. Although it is possible to improve the S/N ratio without incurring a problem, the observation time of this echo signal is limited by the echo time Te and the length of the excitation high-frequency pulse, so in the end, the S/N ratio is not sufficiently improved. There was a problem that N could not be improved.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus that is capable of improving S/N without causing image quality deterioration such as a decrease in resolution.

[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 generating means for generating a static magnetic field to be applied to a subject; a gradient magnetic field generating means for generating slicing, read and encoding gradient magnetic fields to be applied to the subject; a high-frequency pulse generating means for generating a high-frequency pulse related to the excitation of a specific atomic nucleus, which is applied to a subject; A high-frequency pulse is applied under predetermined conditions, and after selectively exciting the specific atomic nuclei present in a specific region of the subject, the magnetic field is observed for a short time in front of the peak and for a long time in the back. A first lead gradient magnetic field for generating a first echo signal as a resonance signal and a second echo signal that is observed for a short period of time in front of the peak and for a long period of time behind the peak. a second read gradient magnetic field of opposite polarity to the first read gradient magnetic field;
a control means for executing a pulse sequence including a read gradient magnetic field; and a first pulse sequence for applying the first read gradient magnetic field.
and a second echo signal related to the application of the second lead gradient magnetic field; a second echo signal generated from the subject and obtained by the collecting means when the control means is driven; one line data for each encoding direction on a Fourier space plane defined by a read direction and an encoding direction is created based on the first echo signal and the second echo signal, wherein By mutually supplementing the missing data in the read direction of the echo signal and the missing data in the read direction of the second echo signal using the first echo signal and the second echo signal,
a line data creation means for creating one line data for each encoding direction; a means for obtaining image information of a specific region of the subject based on a large number of line data on the Fourier space plane created by the data creation means; A magnetic resonance imaging apparatus comprising:

(Function) Since one line of data in the Fourier space plane obtained in this way is obtained by setting the observation time of the echo signal to be sufficiently long, the image generated thereby has image quality such as a decrease in resolution. It has a high S/N ratio that does not cause deterioration.

(Example) Hereinafter, an example of the magnetic resonance imaging apparatus according to the present invention will be described with reference to the drawings.

FIG. 1 is an image data collection sequence showing the relationship between the application timings of high-frequency pulses and gradient magnetic field pulses according to the first embodiment of the present invention, in which data of line n (n is a natural number) in the Fourier space plane is obtained. The process (AV1, AV2) is shown.

Data collection process AV1 in the first embodiment,
AV2 is a pulse sequence of the spin echo method in the two-dimensional Fourier transform method, and is different from AV1 and AV2 with respect to the signal observation time, that is, the application of the read gradient magnetic field Gr. That is, the observation time of the echo signal is set as the first half Tac ⁺ ′ and the second half Tac ⁻ ′, centering on the peak of the echo signal. However, |Tac ⁺ ′|= |Tac ^- ′|
, |Tac ⁺ ′| (or |Tac ^- ′|)＞|Tac ⁺ |
(or |Tac ^- |). The illustrated Td in the first half is a pseudo observation time, and no data is collected during this pseudo observation time Td. In other words,
The observation time of the echo signal is (｜Tac ⁺ ′｜−Td) +
｜Tac ^- ′｜.

Furthermore, the signs of the read gradient magnetic fields are opposite in AV1 and AV2.

According to the above, the echo signal observed with AV1 is e n1, and the echo signal observed with AV2 is e n1.
If n2, in the Fourier space plane, the second
As shown in the figure, the echo signal e n1 is data read from the middle of the negative part in the read direction to the positive part, and the echo signal e n2 is data read from the middle of the positive part in the read direction to the negative part. This is the data read out. Therefore, in AV1, data e n on line n is formed by observed data e n1 + missing data e n1', and in AV2,
Observed data e n2 + missing data e n2' form data en on line n. In this case, the missing data e in AV1
n1' is in the observation data e n2, and
Since the missing data e n2' in AV2 is within the observed data e n1, by calculation, the missing data e n1' in AV1 is included in the observed data e n
Furthermore, the missing data e n2' in AV2 can be obtained from the observed data e n1. In other words, observation data e n1,
Data e n at line n can be determined by e n2. In this case, the above calculation is
For example, with the configuration shown in FIG. 5, it can be performed in the computer system 11 before or within the magnetic disk device or reconstruction device.

In this way, according to the present embodiment, the observation time of the echo signal is set as one line of data in the Fourier space based on the asymmetric sampling.
It is possible to obtain a value (en) that is set sufficiently longer than the SE method. Therefore, it is possible to image a high S/N image without causing image quality deterioration such as a decrease in resolution.

In this case, one line of data is obtained by collecting data twice, that is, the data collection time is doubled compared to the example in Figure 6, but in the conventional method of executing Figure 6, the reliability of the data is In order to ensure accuracy, it is customary to use the average value of 2, 4, 6, etc. data (averaging), so in this embodiment and the conventional example, in practice, the amount of data required to generate one line of data is The time is the same or shorter in this embodiment.

Next, a second embodiment of the present invention will be described with reference to FIG. The image data acquisition sequence shown in Figure 3 does not use a 180° pulse to focus the spin phase (180° pulses are used in the spin echo (SE) method, etc.), but instead reverses the read gradient magnetic field. This is an application of the field echo (FE) method, which is suitable for high-speed scanning.
Asymmetric sampling in AV2 and normal
The observation time is longer than that of the FE method.

Next, a third embodiment of the present invention will be described with reference to FIG. The image data collection sequence shown in Fig. 4 is such that the first echo signal for both AV1 and AV2 is collected by the FE method, and the second echo signal is collected by the SE method, and AV1 and AV2 are asymmetrically sampled. The observation time is longer than the normal FE and SE methods. In this example, the second
As the echo time TE′=TE, especially the transverse relaxation time T
2 enhanced images can be obtained, which is advantageous for tissue identification.

The present invention is not limited to the above embodiments, but can be implemented with various modifications 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 apparatus that can improve S/N without causing image deterioration such as a decrease in resolution.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the magnetic resonance imaging apparatus according to the present invention, FIG. 2 is a diagram showing the operation of the same embodiment, and FIG. 3 is a diagram showing a second embodiment of the present invention. , FIG. 4 is a diagram showing a third embodiment of the present invention, FIG. 5 is a diagram showing an example of the configuration of a magnetic resonance imaging apparatus that can implement the method of the present invention, and FIG. 6 is a diagram showing a conventional example. It is a diagram showing the SE method. 1... Static magnetic field coil, 2... Gradient magnetic field coil, 3...
Probe, 4... Static magnetic field control system, 5... Transmitter, 6...
Receiver, 7...X-axis gradient magnetic field power supply, 8...Y-axis gradient magnetic field power supply, 9...Z-axis 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 applied to a subject; Gradient magnetic field generating means for generating slicing, read and encoding gradient magnetic fields applied to the subject; a high-frequency pulse generating means that generates a high-frequency pulse related to excitation of a specific atomic nucleus, which is applied to the subject; and a gradient magnetic field for slicing, reading, and encoding for the subject placed in the static magnetic field; The high-frequency pulse is applied under predetermined conditions, and after selectively exciting the specific atomic nuclei present in a specific region of the object, the peak is observed for a short period of time and for a long period of time behind the peak. A first lead gradient magnetic field for generating a first echo signal as a magnetic resonance signal, and a second echo signal that is observed for a short time in front of the peak and for a long time in the rear of the peak. A second read gradient magnetic field of opposite polarity to the first read gradient magnetic field of
a control means for executing a pulse sequence including a read gradient magnetic field; and a first pulse sequence for applying the first read gradient magnetic field.
and a second echo signal related to the application of the second lead gradient magnetic field; a second echo signal generated from the subject and obtained by the collecting means when the control means is driven; one line data for each encoding direction on a Fourier space plane defined by a read direction and an encoding direction is created based on the first echo signal and the second echo signal, wherein By mutually supplementing the missing data in the read direction of the echo signal and the missing data in the read direction of the second echo signal using the first echo signal and the second echo signal,
a line data creation means for creating one line data for each encoding direction; a means for obtaining image information of a specific region of the subject based on a large number of line data on the Fourier space plane created by the data creation means; , A magnetic resonance imaging apparatus comprising: