JP3400513B2 - MRI equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to NMR (nuclear magnetic resonance).
By utilizing a phenomenon related to MRI (Magnetic Resonance Imaging) apparatus for obtaining a tomographic image of a desired region of the object (human body), in particular,
When generating a high frequency magnetic field waveform to be applied to the subject, about the MRI apparatus can be easily be varied strength of the RF magnetic field along the frequency direction.

[0002]

2. Description of the Related Art An MRI apparatus measures the density distribution of nuclear spins, relaxation time distribution, etc. at a desired inspection site in an object using the NMR phenomenon, and images an arbitrary cross section of the object from the measured data. It is something to display. Then, the gradient magnetic field the conventional MRI apparatus, for generating a static magnetic field generating means for applying a static magnetic field to the subject, a gradient magnetic field along the two directions perpendicular to the static magnetic field direction and thereto which is generated by the static magnetic field generating means and generating means, a gradient magnetic field power supply for driving the gradient magnetic field generating means, a high-frequency probe for detecting echo signals emitted from the analyte sends a high frequency magnetic field to the subject, a high-frequency magnetic field to the high frequency probe includes a transceiver for processing the echo signal detected is supplied, and a computer for controlling the generation and transmission of high-frequency magnetic field of the gradient magnetic field according to a predetermined pulse sequence, image reconstruction using the further echo signals the detected Equipped with a signal processing system (not shown) that performs configuration calculation and creates a two-dimensional image, and repeats measurement of echo signals emitted by nuclear magnetic resonance It was supposed to obtain a tomographic image I.

In such a MRI apparatus irradiates <br/> frequency magnetic field to the subject, as a way to excite magnetic spins by the high-frequency magnetic field, conventionally, by amplitude-modulating a carrier wave with a sinc function, a certain frequency band A method of irradiating with a uniform distribution of high-frequency magnetic field strength is generally known. According to this method, the excitation angles of the magnetic spins in the excited band, that is, the magnitudes of the flip angles become equal, and an excitation signal of constant intensity is obtained, but the image signal intensity obtained by the signal processing system is When the signal intensity of the flow information changes in the slice direction, it changes accordingly, and the image quality of the displayed tomographic image may deteriorate.

On the other hand, a method of changing the strength of the high-frequency magnetic field by combining two kinds of high-frequency magnetic fields orthogonal to each other and linearly changing the magnitude of the flip angle along the frequency direction is David Purdy, Geneva Cadena. , and
Gerhard Laub, "The Design of Variable Tip Angle Slab
Selection (TONE) Pulses for Improved 3-D MR Angio
graphy ": 11th Magn.Reson.Med.Mtg., 882 (1992). According to this method, the excitation signal from the magnetic spin moving in the frequency direction becomes constant, and for example, the signal intensity of blood flow information. Even when is changing in the slice direction, the image signal intensity obtained by the signal processing system can be made substantially constant, and deterioration in image quality of the displayed tomographic image can be suppressed.

[0005]

However, in such a second conventional method, since the high-frequency magnetic field strength is changed by the combination of two types of high-frequency magnetic fields which are orthogonal to each other, two systems of high-frequency magnetic field generating means are required. However, the device configuration was complicated and large. Alternatively, in the case of the one-system high-frequency magnetic field generating means, it is necessary to irradiate the high-frequency magnetic field twice in the directions orthogonal to each other, which increases the irradiation time of the high-frequency magnetic field for measurement. Therefore, in the conventional method, it may not be possible to easily change the strength of the high-frequency magnetic field along the frequency direction.

Therefore, the present invention addresses such a problem, and can change the strength of the high-frequency magnetic field along the frequency direction by one system of high-frequency magnetic field generating means and by one irradiation of the high-frequency magnetic field. It is an object to provide an MRI apparatus that can be easily performed.

[0007]

In order to achieve the above object, an MRI apparatus according to the present invention comprises a static magnetic field generating means for applying a static magnetic field to a subject, a gradient magnetic field generating means for generating a gradient magnetic field, and this gradient. A gradient magnetic field power supply for driving the magnetic field generating means, a high frequency probe for transmitting a high frequency magnetic field to the subject and detecting an echo signal emitted from the subject, and an echo detected by supplying a high frequency magnetic field to the high frequency probe In an MRI apparatus having a transceiver for processing a signal and an electronic computer for controlling the generation of the gradient magnetic field and the transmission of a high frequency magnetic field, the carrier of the central frequency ωc is controlled by the Gaussian function and
A frequency shift component Δω different from the heart frequency ωc
The intensity of the high-frequency magnetic field is changed by irradiating at least two high-frequency magnetic fields, which are subjected to different amplitude modulation using a wave number waveform, and combining the intensity distributions of the high-frequency magnetic fields.

[0008]

Further, said different at least two high-frequency magnetic field amplitude modulated has a first high-frequency magnetic field the carrier sine wave or cosine wave of the center frequency ωc amplitude modulated with a Gaussian function, of the center frequency ωc sine A second high-frequency magnetic field in which a carrier wave of a waveform or a cosine waveform is amplitude-modulated using at least one cosine wave having a frequency shift component Δω different from the Gaussian function and its center frequency ωc, and a cosine of the center frequency ωc. At least two of a third high-frequency magnetic field in which a carrier wave of a waveform or a sine wave is amplitude-modulated by using a Gaussian function and at least one sine wave having a frequency shift component Δω different from the center frequency ωc as a frequency. There is a change in the strength of the high frequency magnetic field of the first high frequency magnetic field having a peak at the center frequency ωc in the frequency domain. And degree distribution, and intensity distribution of the second high-frequency magnetic field having a peak at a position shifted in frequency with respect to the center frequency .omega.c, also the center frequency ω
By combining at least two intensity distributions of the intensity distribution of the third high-frequency magnetic field having a peak at a position shifted in frequency with respect to c, the intensity of the high-frequency magnetic field is changed along the frequency direction.

Further , a high frequency magnetic field corresponding to the case where at least two high frequency magnetic fields are simultaneously irradiated is combined by amplitude modulation and phase modulation, and the strength of the high frequency magnetic field is changed along the frequency direction by one irradiation. It is a thing.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Figure 1 is a block diagram showing the overall configuration of an MRI apparatus according to the present invention. This MRI device is
The R phenomenon is used to obtain a tomographic image of a desired portion of a subject. As shown in FIG. 1, static magnetic field generating means 2 and 2, gradient magnetic field generating means 3 and 3, a gradient magnetic field power source 4, It has a high frequency probe 5, a transceiver 6, and a computer 7.

The static magnetic field generating means 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 is composed of, for example, a static magnetic field coil, and the subject 1 It is located in a spacious space around the. The gradient magnetic field generating means 3 is the static magnetic field generating means 2 described above.
For generating a gradient magnetic field for inclining the magnetic field strength along the direction of the static magnetic field generated in (1) and two directions orthogonal to this, for example, a gradient magnetic field wound in three axial directions of X, Y, Z Composed of coils. Further, the gradient magnetic field power source 4 is
Electric power is supplied to drive the gradient magnetic field generating means 3. Then, the slice plane for the subject 1 can be set by the method of applying the gradient magnetic field by the gradient magnetic field generating means 3.

The high-frequency probe 5 transmits a high-frequency magnetic field to the subject 1 and detects an echo signal emitted from the subject 1. Although not shown in the figure, the atoms constituting the biological tissue of the subject 1 are not shown. A high-frequency coil on the transmitting side that irradiates a high-frequency signal to cause nuclear magnetic resonance in the nuclei of the above, and a high-frequency coil on the receiving side that detects an echo signal emitted by the nuclear magnetic resonance of the biological tissue of the subject 1. Built-in. The transceiver 6 supplies a high-frequency magnetic field to the high-frequency probe 5 and processes the detected echo signal. For example, the transceiver 6 includes a high-frequency oscillator on the transmission side, a modulator, and a high-frequency amplifier, and an amplifier on the receiving side. And a quadrature detector and an A / D converter.

The electronic computer 7 controls the generation of the gradient magnetic field and the transmission of the high frequency magnetic field according to a predetermined pulse sequence, and sends a control signal to the gradient magnetic field power source 4 and the transceiver 6. There is. Although not shown, a signal processing system for creating a two-dimensional image by performing image reconstruction calculation using the echo signals detected by the high frequency probe 5 and the transceiver 6 is emitted by nuclear magnetic resonance. The echo signal is repeatedly measured to obtain a tomographic image of the subject 1.

Next, the MRI apparatus configured as described above
The generation of the high frequency magnetic field waveform will be described. First, the generation of the high frequency magnetic field waveform according to the first embodiment will be described with reference to FIG. FIG. 2 is a graph showing a profile of a high frequency magnetic field in the frequency domain, and here shows an example in which the high frequency magnetic field strength F (ω) linearly changes along the direction of the frequency ω. First, a carrier wave having a sinusoidal waveform is expressed as in Equation 1 when the center frequency is ωc.

[Equation 1] sin (ωct)

## EQU1 ## The following equation 2 is obtained by subjecting the above equation 1 to amplitude modulation using a Gaussian function and Fourier transform.

[Equation 2] Here, a is a constant and j is an imaginary unit. Then, the result of Equation 2 is shown in FIG. Figure 2
In (a), F (ω) represents the strength of the high-frequency magnetic field and corresponds to the magnitude of the flip angle of the magnetic spin during excitation. That is, the magnetic spin of a certain bandwidth is
It means that excitation is performed with the distribution of flip angles shown in FIG. Hereinafter, this distribution will be referred to as a high-frequency magnetic field profile. Then, this profile becomes the first high-frequency magnetic field H ₁ obtained by amplitude-modulating the sine-wave carrier having the center frequency ωc by using the Gaussian function.

Next, the frequency shift component from the center frequency ωc is set to Δω, and the following waveform is Fourier-transformed to obtain Equation 3.

[Equation 3] Furthermore, the frequency shift component from the center frequency ωc is 2Δ
When ω is used and the following waveform is Fourier-transformed, Formula 4 is obtained.

[Equation 4] Therefore, by superimposing the results of Equations 3 and 4 above, a profile as shown in FIG. 2B is obtained. Then, this profile amplitude-modulates a sine-wave carrier having a center frequency ωc by using a Gaussian function and at least one cosine wave having two different frequency shift components Δω with respect to the center frequency ωc. Of the high frequency magnetic field H ₂ .

Next, as in the case of the equation 3, the center frequency ωc
When the frequency shift component from is set to Δω and the waveform shown below is Fourier-transformed, equation 5 is obtained.

[Equation 5] Further, as in the case of the equation 4, the frequency shift component from the center frequency ωc is set to 2Δω, and the following waveform is subjected to the Fourier transform to obtain the equation 6.

[Equation 6] Therefore, by superimposing the results of the above equations 5 and 6, a profile as shown in FIG. 2C is obtained. Then, this profile amplitude-modulates the carrier of the cosine waveform of the center frequency ωc by using a Gaussian function and at least one sine wave whose frequency is two different frequency shift components Δω with respect to the center frequency ωc. Of high frequency magnetic field H ₃ .

In this case, in FIG. 2 (b), the polarities of the profile are the same with the frequency ωc as the center, and FIG.
Shows that the polarity of the profile is inverted around the frequency ωc. Focusing on this, it can be seen that a profile of the high-frequency magnetic field having a gradient can be obtained by multiplying the respective terms of the above equations 2 to 6 by multiplying them by an appropriate coefficient. If the above coefficients are, for example, A ₁ , A ₂ ,
When A ₃ , B ₁ and B ₂ are used and the formulas 2 to 6 are added, the following formula 7 is obtained.

[Equation 7]

[0020] Therefore, in order coefficient of each term the frequency _{direction, - (A 2 -B 2)} <- (A 1 -B 1) <- A 3 <- (A 1
If it is set such that + B ₁ ) <− (A ₂ + B ₂ ), then FIG.
It is possible to realize a profile having a gradient as shown in (d). For that purpose, each coefficient is, for example, A ₁ =
A ₂ = A ₃ <0, B ₂ > B ₁ > 0. Then, the above-mentioned profile irradiates the first high-frequency magnetic field H ₁ , the second high-frequency magnetic field H ₂ and the third high-frequency magnetic field H _3, and has a peak at the center frequency ωc in the frequency region. The intensity distribution of the high-frequency magnetic field H _{1 and} the intensity distribution of the second high-frequency magnetic field H ₂ having a peak at a position shifted in frequency with respect to the center frequency ωc, and a peak at a position similarly shifted in frequency with respect to the center frequency ωc. Third high frequency magnetic field H to have
By combining with the intensity distribution of ₃ , the high-frequency magnetic field H ₄ whose intensity is changed along the frequency direction is obtained. Note that FIG. 2D is a simulated graph, and it is possible to obtain a smooth profile of the high-frequency magnetic field H ₄ by optimizing each parameter.

Further, the left side of the equation 7 is organized as the following equation 8.

[Equation 8] This shows that a desired profile can be obtained only by one system of high-frequency magnetic field generating means by combining amplitude modulation and phase modulation, which is one of the features of the present invention.

FIG. 4 shows the phase φ derived in the above equation 8.
An example of calculation of (t) is shown. It can be seen from FIG. 4 that the phase change can be approximated by a linear expression, and in that case, the driving of the high frequency magnetic field power supply is sufficient with a control function of changing the phase linearly at a constant frequency. However, if the profile obtained by the linear approximation is not satisfied as the required profile, it is necessary to execute the equation 8 by the control function of changing the phase to an arbitrary value moment by moment.

Further, considering the right side of the above equation 8 as the following equation 9, it can be seen that the synthesis of the high frequency magnetic field waveform can be performed only by the amplitude modulation without performing the phase modulation.

[Equation 9]

Next, the high frequency magnetic field waveform according to the second embodiment
Will be described with reference to FIG. FIG. 3 is a graph showing a profile of a high-frequency magnetic field in the frequency domain. Here, even if the sine wave and the cosine wave of the carrier waveform in the expressions 2 to 6 in the first embodiment are replaced, the expression 7 is obtained. It is shown that results similar to those of Eq. Then, by substituting the sine wave and the cosine wave of the carrier waveform in the equations 2 to 6, the following equations 10 to 14 are obtained, respectively.

[Equation 10]

[Equation 11]

[Equation 12]

[Equation 13]

[Equation 14]

Then, by multiplying the respective terms of the above equations 10 to 14 by an appropriate coefficient and superimposing them, a profile of the high frequency magnetic field having a gradient can be obtained. When the above coefficients are set to C ₁ , C ₂ , C ₃ , D ₁ and D _2, and the equations 10 to 14 are added, the following equation 15 is obtained.

[Equation 15] Further, the right side of the above equation 15 is arranged as the following equation 16.

[Equation 16]

Therefore, the coefficients of the above terms are sequentially arranged in the frequency direction (C ₂ + D ₂ ) <(C ₁ + D ₁ ) <C ₃ <(C ₁ -D ₁ ).
If (C ₂ −D ₂ ) is satisfied, finally, as shown in FIG.
It is possible to realize a profile having a gradient as shown in (d). For that purpose, each coefficient is, for example, C ₁ =
C ₂ = C ₃ > 0 and D ₂ <D ₁ <0. Then, it is understood that the same result as that of the first embodiment can be obtained by comparing the above-mentioned equation 8 with the above-mentioned equation 16. Here, the first high-frequency magnetic field H ₁ ′ as a result of Equation 10 is shown in FIG.
The second high-frequency magnetic field H ₂ ′ as a result of the equations 1 and 12 is shown in FIG. 3 (b), and the third high-frequency magnetic field H ₃ ′ as a result of equations 13 and 14 is shown in FIG. 3 (c). The high-frequency magnetic field H ₄ ′ finally obtained as a result of Equation 16 is shown in FIG.
Shown in.

Here, also with respect to the equation 16, the phase change can be approximated by a linear equation as in the first embodiment .

In the description of the embodiments shown in FIGS. 2 and 3, the waveforms whose frequency shift components are ± Δω and ± 2Δω are superposed, but the present invention is not limited to this, and other than that. A plurality of waveforms having frequency shift components may be superposed.

[0029]

Since the present invention is constructed as described above,
By controlling the computer that controls the generation of the gradient magnetic field and the transmission of the high frequency magnetic field, the carrier of the center frequency ωc is Gaussian
Different frequency shifts for the number and its center frequency ωc
Different amplitude modulation is performed using a waveform whose frequency is the component Δω.
At least irradiating the two high-frequency magnetic field has, it is possible to vary the strength of the RF magnetic field by combining the intensity distribution of their high frequency magnetic field. Therefore, the intensity of the high-frequency magnetic field can be easily changed along the frequency direction with the one-system high-frequency magnetic field generating means, and the device configuration can be simplified and downsized.

Further, a high-frequency magnetic field corresponds to the case of irradiating said at least two high-frequency magnetic field simultaneously, synthesized by the amplitude modulation and phase modulation, varying the intensity of the high frequency magnetic field along a frequency direction in a single irradiation This makes it possible to easily change the intensity of the high-frequency magnetic field along the frequency direction by one-system high-frequency magnetic field generation means and by applying the high-frequency magnetic field once. From this, the irradiation time of the high frequency magnetic field for measurement can be shortened.

[Brief description of drawings]

Is a block diagram showing the overall configuration of an MRI apparatus according to the invention, FIG.

FIG. 2 is a graph showing a profile of a high frequency magnetic field in a frequency domain in the generation of the high frequency magnetic field waveform according to the first embodiment .

FIG. 3 is a graph showing a profile of a high frequency magnetic field in a frequency domain in the generation of the high frequency magnetic field waveform according to the second embodiment .

FIG. 4 is a graph showing a phase change state of phase modulation in the first embodiment .

[Explanation of symbols]

1 ... subject 2 ... static magnetic field generating means 3 ... gradient magnetic field generating means 4 ... gradient magnetic field power source 5 ... RF probe 6 ... transceiver 7 ... computer H _1, H ₁ '... first high-frequency magnetic field H _2, H ₂ ′ ... second high-frequency magnetic field H ₃ , H ₃ ′ ... third high-frequency magnetic field H ₄ , H ₄ ′ ... synthesized high-frequency magnetic field

─────────────────────────────────────────────────── ───Continuation of front page (56) References JP-A-5-269107 (JP, A) JP-A-5-154132 (JP, A) JP-A-7-227387 (JP, A) David Purdy, et. a. , The Design of V aria Tip Angle Slab Selection (TON E) Pulses for Improved 3-D MR Angiography, 11th Magn. Res on. Med. Mtg. , USA, 1992, p. 882 (58) Fields investigated (Int.Cl. ⁷ , DB name) A61B 5/055 JISST file (JOIS)

Claims (3)

(57) [Claims] 1. A static magnetic field generating means for applying a static magnetic field to a subject, a gradient magnetic field generating means for generating a gradient magnetic field, a gradient magnetic field power source for driving the gradient magnetic field generating means, and a high frequency magnetic field for transmitting to the subject. A high-frequency probe that detects an echo signal emitted from the subject, a transceiver that supplies a high-frequency magnetic field to the high-frequency probe and processes the detected echo signal, and generates the gradient magnetic field and transmits the high-frequency magnetic field. In an MRI apparatus having a controlling electronic computer, a carrier wave having a center frequency ωc is controlled by controlling the electronic computer.
For different Gaussian functions and their center frequencies ωc.
Different using a waveform whose frequency is the wave number shift component Δω
Irradiate at least two high-frequency magnetic fields with amplitude modulation ,
An MRI apparatus characterized by changing the intensity of the high-frequency magnetic field by combining the intensity distributions of the high-frequency magnetic fields. 2. The at least two high-frequency magnetic fields having different amplitude modulations have a sine waveform or a cosine wave having a center frequency ωc.
Amplitude-modulated carrier wave of shape using Gaussian function
Frequency magnetic field and sine or cosine waveform with center frequency ωc
The carrier is different from the Gaussian function and its center frequency ωc.
With a frequency shift component Δω of
A second high-frequency magnetic field whose amplitude is modulated using a number of cosine waves,
Gauze the carrier of cosine or sine waveform with center frequency ωc.
Frequency function and its center frequency ωc.
At least one sine wave whose frequency is the ft component Δω
At least of the third high-frequency magnetic field that was amplitude-modulated using
There are also two, and it is the frequency range that changes the strength of those high-frequency magnetic fields.
The first high value with a peak at the center frequency ωc in the range
Frequency magnetic field strength distribution and frequency around its center frequency ωc
Of the second high-frequency magnetic field with a peak at the wavenumber-shifted position
Intensity distribution and frequency shift for center frequency ωc
Intensity distribution of the third high-frequency magnetic field with a peak at the selected position
Combining at least two intensity distributions of
Changes the strength of the high-frequency magnetic field along the frequency direction.
MRI apparatus according to claim 1, characterized in that. 3. The at least two high frequency magnetic fields are simultaneously applied.
Amplitude modulation and phase
Modulation and synthesis, along with the frequency direction with one irradiation
The MRI apparatus according to claim 1 or 2 , wherein the intensity of the high-frequency magnetic field is changed by changing the intensity .