JPH03224536A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To suppress the component caused by incompleteness of an RF pulse and to obtain a useful image for a diagnosis being free from an artifact by providing a pulse sequence having the same plural pulses as a first and a second pulses which can select or set arbitrarily an angle. CONSTITUTION:In a section I, macroscopic magnetization is inclined by 30 deg. by a first pulse 22, and in a section II, a phase encoding inclined magnetic field and an inclined magnetic field in the frequency direction are sensed, and also, in a section III, the macroscopic magnetization is inclined by -60 deg. by a second pulse 23. Unless a relaxation phenomenon in the sections I, II and III is taken into consideration, the macroscopic magnetization is inclined by -30 deg. by a first and a second pulses, a signal is formed in a section IV. The sections I, II, III and IV to this signal instrumentation are set as one unit, and while varying successively phase encoding, the instrumentation is executed, and by executing a two-dimensional Fourier-transformation, an image is obtained. In such a way, incompleteness of an RF pulse can be decreased.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to a nuclear magnetic resonance imaging apparatus that utilizes the nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject. The present invention relates to an MHI device that suppresses artifacts caused by uniform components and obtains clear images.

[Conventional technique tl13 Nuclear magnetic resonance imaging equipment uses nuclear magnetic resonance phenomena to measure the density distribution, relaxation time distribution, etc. of atomic nuclear spins (hereinafter simply referred to as spins) at a desired inspection site in a subject. Then, from that measurement data,
It displays an image of a cross section of a subject.

In this apparatus, as shown in FIG. 1, a subject 7 is placed in a static magnetic field generator 4 that generates a static magnetic field of about 0.02 to 2 Tesla. At this time, the spins in the subject perform precession with the direction of the static magnetic field as an axis at a frequency determined by the strength Ho of the static magnetic field.

This frequency is called the Larmor frequency, Larmor frequency 111
'a can be expressed as ν, = γ/2π・HI D). Here, γ is the gyromagnetic ratio, which has a unique value for each type of nucleus, and ω is the angular velocity of Larmor precession. If so.

ω, 2π ya, (2) Because there is a relationship between

ω, = γH0 (3) is given by

Here, the Larmor frequency ν of the atomic nucleus to be measured by the high-frequency irradiation coil 11. When a high frequency magnetic field (electromagnetic wave) with a frequency equal to is applied, the spins are excited and transition to a high energy state.When the high frequency magnetic field is interrupted, the spins return to their original low energy state. The electromagnetic waves emitted at this time are received by the high-frequency receiving coil 14, amplified and waveform-shaped by the amplifier 15, digitized by the A/D converter 17 (hereinafter referred to as ADC), and sent to the central processing unit 1 (hereinafter referred to as ADC). , referred to as CPU)
send to In cpUl, reconstruction calculations are performed based on this data, and the calculated data is displayed on the display 18 as a tomographic image of the subject 7. The above high frequency magnetic field is
It is obtained by amplifying the signal sent out by the sequencer 2 controlled by the CPUI by the high frequency transmitting coil amplifier 10 and sending it to the high frequency transmitting coil 11.

In addition to one or more static magnetic fields 4 and high-frequency magnetic fields, the MRI apparatus is equipped with a group of gradient magnetic field coils 21 to generate gradient magnetic fields for obtaining positional information in space. These gradient magnetic field coils 13 are supplied with current from a gradient magnetic field coil power supply 12 operated by signals from the sequencer 2, and generate gradient magnetic fields.

Here, the imaging principle of the MHI device will be described.As shown in Fig. 6(a), the static magnetic field H1 in two directions is
Viewed from classical physics, the atomic nucleus placed inside behaves like a single bar magnet, precessing around the Z-axis at the Larmor frequency ν mentioned above.

This frequency is given by the above equation (2) and is proportional to the strength of the static magnetic field. γ in equations (1) and (3) is called the gyromagnetic ratio.

Each atomic nucleus has a unique value. Generally, the number of atomic nuclei to be measured is enormous.

Since each of them rotates with an arbitrary phase, when viewed as a whole, the components in the XY plane cancel each other out, leaving only the macroscopic magnetization in the Z direction component. In this state, when a high frequency magnetic field H1 having a frequency equal to the Larmor frequency ν is applied in the X direction (FIG. 6(b)), the macroscopic 1-hi begins to fall in the Y direction. This angle of collapse is approximately proportional to the product of the amplitude of Hl and the application time, and H4 when it collapses 90' with respect to the pulse application time is 90
H8 when tilting 180° is called a 180° pulse.

However, when viewed microscopically, due to the 901 pulse, there are magnetizations that are tilted by 0 degrees and magnetizations that are tilted by 40@50 degrees (No. 611(c)).

Furthermore, variations in magnetization occur due to nonuniformity of the amplitude of the RF pulse in the frequency direction, that is, deterioration of the slice profile. When considered in an orthogonal coordinate system, for example, magnetization tilted by 40' can be decomposed into 90' and 06 components (Fig. 6(d)).

When considering each magnetization, 0°, 90"180',
It can be analyzed by decomposing it into -90' components. In particular, in the SE method described below, the 90'' component of the 180' pulse is superimposed during signal measurement, which often causes image deterioration. This is called pulse imperfection.

Now, a two-dimensional Fourier imaging method is currently commonly used for imaging using an MHI device. A typical pulse sequence of the spin echo method, which is a typical method among these methods, is shown in FIG. After a time of 1800 pulses 28 are applied. After applying 90" pulses 27 (Fig. 8), each spin begins to rotate in the x-Y plane at its own speed, so a phase difference occurs between each spin over the course of 1 hour (Fig. 8511).
11) When the 180° pulse 28 is applied here, each spin is reversed symmetrically around the x I axis as shown in FIG. 8, and in order to continue rotating at the same speed, the time To
Then, the spins are focused again (Fig. 8 (■)) and form an echo signal.

The signals are measured as described above, but in order to construct a tomographic image, the spatial distribution of the signals must be determined, and a linear gradient magnetic field is used for this purpose. A spatial magnetic field gradient is created by superimposing a gradient magnetic field on a uniform static magnetic field. As mentioned earlier, the rotational frequency of the spins is proportional to the magnetic field strength, so when a gradient magnetic field is applied, the rotational frequency of each spin differs spatially. Therefore, by examining this frequency, the position of each spin can be determined. A one-phase encoding gradient and a one-frequency encoding gradient are used for this purpose.

The pulse sequence described above is used as a basic unit and is repeated a predetermined number of times (for example, 256 times) at a constant repetition time (TR) while changing the strength of the one-phase encoding gradient magnetic field each time.The measurement signal obtained in this way is a two-dimensional inverse Fourier In the explanation of 0 or more where the spatial distribution of macroscopic magnetization is obtained by transformation, the three types of gradient magnetic fields can be any of X, Y, and Z, as long as they do not overlap with each other. Regarding the basic principles of MHI of 0 or more, which may be a combination of I am familiar with

The above principle makes the RF pulse ideal.

In other words, the explanation is based on the assumption that the magnetization selectively excited by the 90m pulse is all knocked down by 90''. Here, R
An example of generation of false images due to imperfection of the F pulse will be explained with reference to FIG. First, by 90° pulse 27, O°
After that, the component 29 which fell 90'' due to the 180'' pulse 28.

If a signal is measured before the component 29, which is diffused and decreased due to non-uniform static magnetic fields, gradient magnetic fields, etc., is completely decreased, this component will be superimposed on the signal that should actually be measured.

It becomes a false image. Furthermore, in multi-echo measurement in which a 180'' pulse is added and the echo signal is measured multiple times, many artifacts must be taken into consideration.Normally, in order to remove these artifacts, the RF pulse is optimized. , there is an additional approach of gradient magnetic fields to diffuse the artifact components.

By the way, information such as proton density and TL waiting time T1 time can be added to images captured by MRI. T,, T, is a value unique to each organization, and lesions such as tumors can also be identified, so imaging this two-dimensional distribution can contribute to improving diagnostic performance. An 18-weighted image and a 12-weighted image are used for this purpose, and the repetition time is TR.

Echo time TE, inversion recovery (InversionRec)
recovery time T1 in the over+IR) method. For example, 0 can be obtained by appropriately selecting the flip angle in the gradient echo method (hereinafter referred to as GE method).

In order to obtain T1 image adjustment using the SE method, TR〉〉T,
TE needs to be set appropriately long. The SE method with long TR and TE is particularly called long-5E.

[Problems to be Solved by the Invention] As mentioned above, T3-weighted imaging has been conventionally performed with long-5E, but in order to remove the influence of Ts, TRIt
It was necessary to set it longer than ,T. In this case, imaging takes a long time and the subject, ie, the patient, is restrained for a long time, which not only causes lumbar pain but also causes deterioration in image quality such as artifacts due to body movements. Moreover, this was an obstacle to improving throughput.

Furthermore, as described above, the incompleteness of the RF pulse has caused deterioration of the captured image and reduced diagnostic performance.

An object of the present invention is to solve these problems and provide a pulse sequence that can obtain a T2-weighted image with a shorter TR than before and further reduces imperfections in the RF pulse.

[Means for Solving the Problems] In order to achieve the above object, a first pulse whose angle can be arbitrarily selected or set and a second pulse whose angle can be arbitrarily selected or set, or further. RF
The respective angles are set so that the component of pulse imperfection is not superimposed on the signal during signal measurement. Furthermore, by lowering the excitation angle, the longitudinal magnetization component during the first-order excitation is made independent of 11, so that T1 tuning can be obtained with a short TR.

C action] According to the present invention, there is a first pulse whose angle can be arbitrarily selected or set, and a second pulse whose angle can be arbitrarily selected or set, or an arbitrary pulse which can further select or set an arbitrary angle. A pulse sequence having a plurality of pulses is provided, and the angles of each pulse are set so that the imperfection component of the RF pulse is not superimposed on the signal during signal measurement. moreover.

By lowering the excitation angle, the longitudinal magnetization component at the time of fifth-order excitation is not dependent on T, so that a 12-weighted image can be obtained with a short TR.

For these reasons, a T*'A-tone image can be obtained in a short imaging time, which has the effect of providing clinically useful information in a short time. Additionally, image degradation due to RF pulse imperfections can be reduced.

[Embodiment] An embodiment of the present invention will be described below with reference to FIGS. 1, 2, and 3. FIG. 1 is a block explanatory diagram of the overall configuration of a nuclear magnetic resonance imaging apparatus to which the present invention is applied, FIG. 2 is a schematic explanatory diagram of a pulse sequence according to an embodiment of the present invention, and FIG. It is an explanatory view showing behavior of macroscopic magnetization of an example.

A nuclear magnetic resonance imaging apparatus to which the present invention is applied will be explained with reference to FIG. This nuclear magnetic resonance imaging system can be roughly divided into central processing units (CPU
) l, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a reception system 5, and a signal processing system 6.

The central processing unit t (CPtJ) 1 operates a sequencer 2, a transmission system 3 . Receiving system 5. It controls each of the signal processing systems 6. Sequencer 2
operates based on control instructions from the central processing unit 1,
A transmission system 3. transmits various commands necessary for data collection of tomographic images of the subject 7. 21 to the gradient magnetic field generation 1 of the static magnetic field generation magnet 4
, and is sent to the receiving system 5.

The transmission system 3 includes a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11 as a high-frequency coil.The modulator 9 modulates the amplitude of the high-frequency pulse from the high-frequency oscillator 8 according to a command from the sequencer 2. By amplifying the generated high-frequency pulse via a high-frequency amplifier 10 and supplying it to the irradiation coil 11 , the subject 7 is irradiated with a predetermined pulsed electromagnetic wave.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field in any direction around the subject 7.
Inside this static magnetic field generating magnet, there is an irradiation coil 1.
1, a gradient magnetic field coil 13 for generating a gradient magnetic field, and a receiving coil 14 of the receiving system 5 are installed. The gradient magnetic field generation system 21 includes a gradient magnetic field coil 13 having a configuration capable of independently applying a gradient magnetic field in the mutually orthogonal Cartesian shelf axis directions, a gradient magnetic field power source 12 that supplies current to the gradient magnetic field coils, and a gradient magnetic field power source 12. It is composed of a sequencer 2 that controls the

The receiving system 5 includes a receiving coil 14 as a high-frequency coil and an amplifier 1 connected to the receiving coil 14.
15 and a quadrature phase detection [116] and an A/D converter 17, and receives the NMR signal from the subject 7 into the receiving coil 14.
When detected, the signal is sent to the intensifier IIi 15 and the quadrature phase detector II 16. The data is converted into a digital quantity via the A/D converter 17, and is also converted into two bases of collected data sampled by the quadrature phase detector 16 at the timing according to the command from the sequencer 2, and then sent to the central processing unit 1.
I am trying to send it to.

The signal processing system 6 includes a magnetic disk 20 and an optical disk 19.
A display 1 consisting of an external storage device such as, and a CRT etc.
8, and when data from the receiving system 5 is input to the central processing unit 1, the central processing unit fl executes processing such as signal processing 1 image reconstruction.

The resulting desired cross-sectional image of the subject 7 is displayed on the display 18.
It is also displayed on the magnetic disk 20 of an external storage device or the like.

Here, a pulse sequence according to an embodiment of the present invention will be explained using FIGS. 2 and 3. In FIG. 2, for the sake of simplicity, the first (7) RF pulse 22
is 30'' and the second pulse 23 is -60°,
explain.

This angle may be any angle from 90' for the first pulse 22 to 90' for the second pulse 23 and from 0" to -iso'pulse; furthermore, these angles may be user selectable. First, in section I, the macroscopic magnetization is inverted by 300 by the first pulse 22. (In the third plotting section ■, the phase encoding gradient magnetic field and feel the gradient magnetic field in the frequency direction.

Further, in one section m, the macroscopic magnetization is inverted by -60' by the second pulse 23. (Figure 3 m) Section 1[11[
If the relaxation phenomenon at 1 is not taken into account, a signal is formed in the 0 interval ■ where the macroscopic magnetization is overthrown by -30' by the 1.2 pulse. The interval t until this signal measurement
An image is obtained by performing measurements while sequentially changing one phase encode in units of nm and two-dimensional Fourier transform.

In the conventional SE method, the second pulse is 180°, but
In the present invention, by using a small negative pulse, 9o°
Alternatively, the -90'' component becomes small, and deterioration in image quality due to imperfections in the RF pulse is suppressed.

Furthermore, the macroscopic magnetization ultimately becomes -301, so 5
Before the next excitation, the longitudinal magnetization easily returns to a thermal equilibrium state and is less susceptible to the T1 relaxation phenomenon.

Pulse sequence of another embodiment of the present invention The process will be explained using Figures 4 and 5.

FIG. 4 is a schematic diagram of a pulse sequence according to another embodiment of the present invention. In FIG. 4, for ease of explanation, the first RF pulse 22 is set at 30 degrees, the second pulse 23 is set at -60', and the third pulse 24 is set at 60'.

These angles are: 906 or less for the first pulse 22, between 0'' and -180' for the second pulse 23, and between 0'' and -180' for the second pulse 23;
The pulses 24 may be pulses at any angle between 01 and 1809.Furthermore, these angles may be selectable by the user or may be fixed and not selectable by the user. First, by the first pulse 22 in the interval chamber, the macroscopic magnetization is overturned by 301. (Fifth Great Seal) Section ■
Feel the phase encode gradient magnetic field and the frequency direction gradient magnetic field. moreover.

With the second pulse in interval m, the macroscopic magnetization is -60°
be defeated. (Fig. 5 m) If the relaxation phenomenon in the section ■■m is not taken into account, the macroscopic magnetization due to the 1.2 pulses is as follows.

-306 A signal is formed in one section ■ that will be defeated. Furthermore, in interval V, with the third pulse 24, the macroscopic magnetization becomes .

608, (Fig. 5 V) If we do not take into account the relaxation phenomenon in the section 1nIII■■, the macroscopic magnetization will be 30
@Measure the second echo signal in 6 sections m that will be knocked down. While sequentially changing one phase encode using the interval 1f1mlVVVI as one unit until this signal measurement,
An image is obtained by measuring and performing two-dimensional Fourier transformation. In the conventional SE method, the second and third pulses are
In the present invention, a small negative pulse,
By making the pulse small positive, 90 degrees or more.

The −90” component becomes small, and deterioration in image quality due to imperfections in the RF pulse is suppressed.

Furthermore, since the macroscopic magnetization ultimately becomes 30', the longitudinal magnetization easily returns to the thermal equilibrium state before the first-order excitation, and is less susceptible to the T1 relaxation phenomenon.
Although the relaxation phenomenon was not considered, this is only for the purpose of explanation; even if the relaxation phenomenon is taken into consideration, the present invention can be applied by appropriately selecting the angles of the RF pulses.

[Effects of the Invention] According to the present invention, components due to imperfections in RF pulses can be suppressed, so that it is possible to obtain diagnostically useful images without artifacts.

In addition, after signal measurement, there is a large number of longitudinal magnetization components, so T is less susceptible to relaxation phenomena, so a short TR
That is, since a T1-weighted image can be obtained in a short imaging time, the restraint time of the subject can be shortened, which has the effect of improving throughput, and also has the effect of suppressing artifacts due to body movement.

4,

[Brief explanation of drawings]

FIG. 1 is a block diagram illustrating the overall configuration of a nuclear magnetic resonance imaging apparatus to which the present invention is applied. 5 21i1 is a schematic explanatory diagram of a pulse sequence of an embodiment of the present invention, and FIG. 3 is an embodiment of the present invention. FIG. 4 is a schematic explanatory diagram of a pulse sequence in another embodiment of the present invention, and FIG. 5 is an explanatory diagram showing the behavior of macroscopic magnetization in another embodiment of the present invention. FIG. 6 is a schematic illustration of macroscopic magnetization, FIG. 7 is a schematic illustration of the pulse sequence of the SE method, and FIG. 8 is an illustration of the behavior of magnetization in the SE method. Explanation of symbols 2... Sequencer, 7... Subject, 8... High frequency oscillator, 11... Irradiation coil, 22... First high frequency pulse, 23... Second high frequency pulse , 24...
Third high frequency pulse, 29...False image signal 2' due to imperfection of high frequency pulse 1st Kayo

Claims (1)

[Scope of Claims] 1. Means for applying a static magnetic field to a subject; a means for applying a slicing direction gradient magnetic field, a frequency encoding gradient magnetic field, and a phase encoding gradient magnetic field to the subject; means for repeatedly applying a high frequency pulse that causes magnetic resonance in a certain predetermined pulse sequence;
In the nuclear magnetic resonance imaging apparatus equipped with a means for detecting a nuclear magnetic resonance signal, the sequencer for controlling the pulse sequence is composed of two or more excitation pulses and a gradient magnetic field necessary for signal measurement, The first pulse is at any angle of 90° or less, the second excitation pulse is at any angle between 0 and -180°, and the third pulse is at any angle between 0 and 180°. , hereinafter, even-numbered pulses have an arbitrary angle between 0 and -180°, and odd-numbered pulses have an arbitrary angle between 0 and 180°. Imaging equipment. 2. Above, especially when the first pulse is at an arbitrary angle α° of 90° or less, the second pulse is −2α°, the third pulse is +2α°, and thereafter, even-numbered pulses are −2α°. 2. The magnetic resonance imaging apparatus of claim 1, wherein the odd-numbered pulses have an angle of +2[alpha][deg.].