JPH07227387A - Method for magnetic resonance imaging - Google Patents

Abstract

PURPOSE:To obtain a slice characteristic having an arbitrary shape by a series of high-frequency pulse apparatus by a method wherein a plurality of high frequency pulses with different frequencies are applied and selectively excited and the strength is arbitrarily changed. CONSTITUTION:An MR1 apparatus gives a tomographic image of a testee body by utilizing magnetic resonance phenomenon, and a static magnetic field generating magnet 2, a magnetic slope generating system 3, a transmitting system 4, a receiving system 5, a signal processing system 6 and a sequencer 7 are provided. In this case, in the transmitting system, a plurality of high-frequency pulses with different frequencies are applied to a high-frequency coil 14a through the sequencer by means of a CPU 8 and are selectively excited and the strength is arbitrarily changed. In addition, in the signal processing system 6, such processings as Fourier transformation and reconstitution of a correcting coefficient calculating image are performed and either the signal strength distribution of an arbitrary cross-section or a distribution obtd. by performing appropriate operations on a plurality of signals is made into a image and is displayed on a display 20 as a tomographic image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to magnetic resonance (hereinafter referred to as NM
The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus that obtains a tomographic image of a desired portion of a subject by utilizing a phenomenon (abbreviated as R).

[0002]

2. Description of the Related Art An MRI apparatus measures the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as "spins") at a desired inspection site in an object by using the NMR phenomenon, An arbitrary cross section is displayed on the subject. Then, in the conventional MRI apparatus, a high-frequency pulse that causes magnetic resonance in atomic nuclei of atoms forming a biological tissue is repeatedly applied to a subject 1 placed in a static magnetic field in a predetermined pulse sequence, The echo signal emitted by resonance is measured. An image reconstruction calculation is performed by using the echo signals repeatedly measured to obtain a tomographic image.

In this apparatus, a subject is placed in a static magnetic field generating magnet that generates a static magnetic field of about 0.02 to 2 Tesla. At this time, the spins in the subject perform a precession motion with the direction of the static magnetic field as an axis at a frequency determined by the strength H ₀ of the static magnetic field. This frequency is called the Larmor frequency, and the Larmor frequency ν ₀ has a unique value for each type of nucleus represented by ν ₀ = γ / 2πH ₀ (1). Here, H ₀ is the static magnetic field strength, and γ is the gyromagnetic ratio.

When the angular velocity of the Larmor precession is ω ₀ , ω ₀ = 2πν ₀ , and therefore ω ₀ = γ · H ₀ (2)

Measure with the high frequency irradiation coil
Larmor frequency ν of the intended nucleus ₀Frequency f equal to₀
When a high frequency pulse (electromagnetic wave) of
Transition to a higher energy state. This high frequency pulse
When you break it off, the spin has a time constant corresponding to each state.
Return to the original low energy state. Released at this time
The electromagnetic waves are received by the high frequency receiving coil.

At this time, in order to obtain position information in space for each spin, a gradient magnetic field is applied in addition to the static magnetic field and the high frequency pulse. The gradient magnetic field coil for giving a normal gradient magnetic field is composed of gradient magnetic field coils in three axial directions orthogonal to each other,
A high-frequency pulse and these gradient magnetic fields are applied to the subject in a pulsed manner in a predetermined pulse sequence controlled by the sequencer 7 to obtain an NMR signal used for image reconstruction.

Two-dimensional measurement method and three-dimensional measurement method are generally used as an imaging method in an MRI apparatus. In these measurement methods, the tissue of a specific slice plane (region) of a subject is usually measured. To excite the nuclei, a slice gradient magnetic field is applied at the same time as the high frequency pulse is applied. In this case, the width of the slice plane to be excited is determined by the characteristics of the applied radio frequency pulse RF and the magnitude of the gradient of the gradient magnetic field Gz.

That is, as shown in FIG. 5, the gradient magnetic field Gz is applied at the same time as the irradiation of the high frequency pulse having the frequency ω, so that the frequency band (ω
Only spins in the region of ± Δω) are selectively excited. Conventionally, in the case of performing such selective excitation, in order to make the slice characteristic, that is, the signal intensity distribution (frequency distribution) in the slice direction of the selective excitation region rectangular, the carrier wave is shown in FIG. 5 as a high frequency pulse. Like SINC
A function or an even function such as Gaussian is used.

[0009]

On the other hand, in the case of blood flow (angio) measurement, the signal intensity changes in the slice direction in order to suppress saturation of spins in the blood flow.
That is, the slice characteristic having a slope is effective. The slice characteristic having such a slope can be obtained by applying two high-frequency pulses with a phase shift,
Conventionally, such slice characteristics have been obtained by irradiating two types of high-frequency pulses that are orthogonal to each other (David Purdy, Geneva Cadena, and Gerhard Laud, The).
Design of Valiavle Tip Angle Slab Selection (TONE)
Pulse for improved 3-D MR Angiography: 11th Magn.
Reson.Med.Mtg., 882 (1992)).

However, this excitation method requires two generation facilities as a high-frequency pulse generation system, and the apparatus configuration becomes complicated. It is an object of the present invention to provide an MRI method capable of performing selective excitation having a slice characteristic of an arbitrary shape in a single-system high-frequency pulse generating facility having a simple device configuration.

[0011]

According to the MRI method of the present invention which achieves such an object, a high frequency pulse and a gradient magnetic field are applied in a predetermined pulse sequence to a subject placed in a static magnetic field, and a predetermined slice is applied. In the MRI method of selectively resonating the atomic nuclei of the atoms constituting the biological tissue of the subject on the surface, measuring the echo signal emitted by the magnetic resonance of the atomic nuclei, and reconstructing an image, a high-frequency pulse for generating a high-frequency pulse A high-frequency pulse generator of one system is used as a pulse generator, and a plurality of high-frequency pulses having the same or different frequencies are continuously applied by this high-frequency pulse generator to selectively excite a predetermined slice plane, and this selective excitation is performed. The shape of the signal intensity distribution in the slice direction of the different region is adjusted arbitrarily.

In order to obtain a particularly inclined slice characteristic, the application intensity of the high frequency pulse applied continuously is continuously increased or decreased.

[0013]

By applying a plurality of high-frequency pulses having different frequencies to selectively excite them and changing their intensities arbitrarily, it is possible to obtain slice characteristics of an arbitrary shape with a single-system high-frequency pulse device. .

[0014]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 4 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. This MRI apparatus obtains a tomographic image of a subject by utilizing a magnetic resonance (NMR) phenomenon. As shown in FIG.
, Magnetic field gradient generation system 3, transmission system 4, reception system 5, signal processing system 6, sequencer 7, and central processing unit 8 (CP
U) and.

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the body axis direction or in a direction orthogonal to the body axis, and has a certain spread around the subject 1. A permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged in the space. The magnetic field gradient generation system 3 is composed of a gradient magnetic field coil 9 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power supply 10 that drives each of the gradient magnetic field coils, and follows a command from a sequencer 7 described later. By driving the gradient magnetic field power supply 10 of each coil, a gradient magnetic field Gx in the triaxial directions of X, Y, and Z,
Gy and Gz are applied to the subject 1. A slice plane to be inspected 1 can be set by the application method of the gradient magnetic field.

The transmission system 4 irradiates a high frequency pulse in order to cause magnetic resonance in atomic nuclei of atoms constituting the biological tissue of the subject 1, and includes a high frequency oscillator 11 and a modulator 12.
And a high-frequency amplifier 13 and a high-frequency coil 14a on the transmission side. The high-frequency pulse output from the high-frequency generator 11 is amplitude-modulated by the modulator 12 according to the instruction of the sequencer 7, and the modulated high-frequency pulse is generated by the high-frequency amplifier 13. After the amplification, the electromagnetic waves are radiated to the subject 1 by being supplied to the high-frequency coil 14a arranged close to the subject 1. The frequency of the high frequency pulse emitted by the high frequency coil 14a can be adjusted to an arbitrary frequency by a command from the CPU 8.
That is, the reference signal generated from the oscillator of the high frequency generator 11 is a sine wave stored in advance in the RAM in the CPU,
By performing / D conversion and synthesizing, frequency synthesis can be performed with arbitrary resolution. Further, the modulator 12 may be frequency-modulated. In the present invention, high frequency pulses having different frequencies are thus used.

The receiving system 5 is an echo signal (NMR signal) emitted by magnetic resonance of atomic nuclei of the living tissue of the subject 1.
Which includes a high-frequency coil 14b on the reception side, an amplifier 15, a quadrature detector 16, and an A / D converter 17, and the response of the subject 1 to the electromagnetic wave emitted from the high-frequency coil 14a on the transmission side. Of the electromagnetic wave (NMR signal) is detected by the high-frequency coil 14b arranged close to the subject 1, and is detected via the amplifier 15 and the quadrature phase detector 16 as A /
The data is input to the D converter 17, converted into a digital amount, and further converted into two series of collected data sampled by the quadrature phase detector 16 at the timing according to the instruction from the sequencer 7, and the signal is sent to the signal processing system 6. It is like this.

The signal processing system 6 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a CRT.
The CPU 8 performs processing such as Fourier transform, correction coefficient calculation image reconstruction, etc., and performs image processing on the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculation on a plurality of signals. A tomographic image is displayed on the display 20.

The sequencer 7 operates under the control of the CPU 8 and sends various commands necessary for collecting the tomographic image data of the subject 1 to the transmission system 4, the magnetic field gradient generation system 3 and the reception system 5. In FIG. 1, the high frequency coils 14a and 14b on the transmitting side and the high frequency coil on the receiving side are shown separately, but the same ones may be used. Next, an embodiment in which the MRI method of the present invention having such a configuration is applied to the three-dimensional measurement of the Grand Refuge system will be described. FIG. 1 is an explanatory diagram showing a sequence of three-dimensional measurement of a Grand Refuge system suitable for blood flow measurement. This sequence is stored in the sequencer 7, and the sequencer 7 generates a high frequency pulse and a gradient magnetic field according to this sequence. In the figure, (a) shows an irradiation timing of a high-frequency pulse signal and an envelope for selectively exciting the slice position of the subject. In the present embodiment, three high-frequency pulses are used to excite three consecutive slice planes. High frequency pulses RF ₁ , RF ₂ , RF _{3 of three} different frequencies ω ₁ , ω ₂ , ω ₃ selected for are used.
(B) is the application timing of the gradient magnetic field Gz in the slice direction, (c) is the application timing of the phase encoding direction gradient magnetic field Gy, and (d) is the frequency encoding direction gradient magnetic field G.
The application timing of x is shown respectively. Also,
(E) shows the measured echo signal (NMR signal). (F) is a section of measurement.

In this sequence, after the waiting time in the section I, in the section II, the radio frequency pulse RF ₁ for selective excitation is irradiated and the slice-direction gradient magnetic field Gz ₁ is applied. As a result, the region a shown in FIG. 2 is selectively excited. Next, in the section III, the rephasing gradient magnetic field Gz is applied in the slice direction in order to return the spin in the slice direction.
Apply ₂ . Similarly, in the sections IV to V, the high frequency pulse RF ₂ , the slice direction gradient magnetic field Gz ₃ and the rephasing gradient magnetic field Gz ₄ are used, and in the sections VI to VI, the high frequency pulse RF ₃ and the slice direction gradient magnetic field Gz ₅ and the rephasing gradient. By applying the magnetic field Gz ₆ , respectively, the regions b and c shown in FIG. 2 are excited.

These high frequency pulses RF ₁ , RF ₂ , RF ₃
The interval at which is applied is, for example, about several ms, and regions a, b, and c are selectively excited by such continuous application. The frequencies of the high frequency pulses RF ₁ , RF ₂ and RF ₃ at this time are frequencies ω ₁ , ω ₂ and ω ₃ corresponding to the positions of the regions a, b and c, and their intensities are as shown in FIG. It is adjusted so as to increase or decrease in sequence as shown in (b). As a result, the characteristic signal intensity distribution of the selectively excited slice can be inclined. When the intensity of the high frequency pulse is not changed as shown in FIG. 2C, rectangular slice characteristics can be obtained. Thus, by arbitrarily changing the ratio of the intensities of the three high-frequency pulses applied continuously, it is possible to obtain an arbitrary slice characteristic.

After the predetermined slice is selectively excited by the continuous application of the high frequency pulse as described above, in the next section VIII, a slice encode pulse Gz ₇ for obtaining spatial information in the slice direction is applied and a positive pulse in the frequency encode direction is applied. The pulse Gx ₁ is applied, and further the phase encode pulse Gy ₁ is applied in the phase encode direction. Then section I
At X, the slice encode pulse Gz ₇ and the phase encode pulse Gy ₁ are continuously applied, and the negative flow encode pulse Gx ₂ is applied in the frequency encode direction. This returns the spin in the frequency encode direction (blood flow direction).

Next, in the section X, the positive pulse Gx ₃ is applied in the frequency encoding direction and the echo signal S is measured. Section XI is the waiting time. Here, as the echo signal, a signal enhanced by the inflow effect of the blood flow can be obtained, but in this case, since the spin is excited by the high frequency pulse having the gradient in the signal intensity distribution in the slice direction,
A high signal can be obtained without weakening the signal strength due to saturation due to multiple excitation.

Using the pulse sequence described above as a basic unit, the intensity of the phase encode direction gradient magnetic field Gy and the slice encode gradient magnetic field Gz is changed every time, and a predetermined number of times (slice encode x
Phase encoding) Repeat. The spatial distribution of macroscopic magnetization can be obtained by three-dimensional Fourier transforming the measurement signal thus obtained.

FIG. 3 is an explanatory view showing a measurement sequence showing another embodiment of the present invention. In this sequence, the gradient magnetic field Gz in the slice direction, which is applied together with the high frequency pulse at the time of selective excitation, is alternately applied. In this sequence,
Back spin of the high frequency pulse RF ₁ gradient Gz ₁ during a slice selection performed in, is carried gradient Gz ₂ during slice selection performed in the next RF pulse RF _2, spin for slice selection to perform a high-frequency pulse RF ₂ Is returned by the next slice selection gradient magnetic field Gz ₃ .
Also in this case, similarly to the embodiment of FIG. 1, by selectively changing the intensity of irradiation of the high frequency pulse, selective excitation can be performed with arbitrary slice characteristics. After that, the echo signal is measured in the same manner as the sequence of FIG. still,
Since this section does not require a section for spin-back, the sequence repetition time can be shortened and the measurement time can be shortened.

In the above embodiments, the example in which the present invention is applied to the pulse sequence of the grand referation system has been described, but the present invention is not limited to these, and flight such as PS measurement (Phase Contrast) is performed. It can also be applied to other sequences of the time-of-flight method.

[0027]

As is apparent from the above description, according to the MRI method of the present invention, a plurality of high frequency pulses are continuously generated by using one system of high frequency pulse generator, and the frequency and intensity thereof are changed. By doing so, slice characteristics of an arbitrary shape can be obtained without complicating the apparatus. Therefore, particularly in blood flow measurement by the time-of-flight method, it is possible to suppress saturation of blood flow and improve blood flow depiction ability by obtaining slice characteristics having an inclination in the slice direction.

[Brief description of drawings]

FIG. 1 is a timing diagram showing an embodiment in which an MRI method of the present invention is applied to a pulse sequence of a grand rephasing method.

FIG. 2 is a diagram illustrating slice characteristics in the MRI method of FIG.

FIG. 3 is a timing diagram showing another embodiment applied to the pulse sequence of the MRI method grand rephasing method of the present invention.

FIG. 4 is a block diagram showing the overall configuration of an apparatus to which the MRI method of the present invention is applied.

FIG. 5 is an explanatory diagram showing slice plane selective excitation by an MRI method.

[Explanation of Codes] 1 ... ・ Subject RF ₁ , RF ₂ , RF ₃・ ・ High-frequency pulse Gz ・ ・ Slice gradient magnetic field S ・ ・ ・ ・ Echo signal

Claims (2)

[Claims] 1. A high frequency pulse and a gradient magnetic field are applied to a subject placed in a static magnetic field in a predetermined pulse sequence to selectively excite atomic nuclei of atoms constituting a living tissue of the subject on a predetermined slice plane. Then, in a magnetic resonance imaging method of measuring an echo signal emitted by the magnetic resonance of the atomic nuclei and reconstructing an image, a high frequency pulse generator of one system is used as a high frequency pulse generator for generating the high frequency pulse. , A plurality of high-frequency pulses having different frequencies are continuously applied by this high-frequency pulse generator, a predetermined slice plane is selectively excited, and the shape of the signal intensity distribution in the slice direction of the selectively excited region is arbitrarily adjusted. A magnetic resonance imaging method comprising: 2. The magnetic resonance imaging method according to claim 1, wherein the applied intensity of the high-frequency pulse is continuously increased or decreased.