JP2005152534A - Magnetic resonance imaging apparatus and rf wave generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the contrast of a magnetic resonance image and to shorten the imaging time by restraining magnetic resonance signals of two or more frequency bands. <P>SOLUTION: MT pulse and CHESS pulse have an excitation frequency band capable of exciting bound water and fat of a subject, respectively. The CHESS pulse is amplitude-modulated so that the center frequency of the excitation frequency band of the MT pulse and the center frequency of the excitation frequency band of the CHESS pulse are shifted from each other by a desired offset frequency, and the MT pulse and the modulated CHESS pulse obtained by amplitude-modulation are synthesized to generate a synthetic pulse. Since the generated synthetic pulse can excite the bound water and fat at one time, the imaging time can be shortened. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus and an RF wave generation method, and more particularly to a magnetic resonance imaging apparatus that applies RF waves that can excite a plurality of frequency bands to a subject, and an RF wave generation method that generates the RF waves. .

Magnetic Resonance Imaging (MRI) applies a gradient magnetic field and RF (Radio Frequency) waves to a subject in a static magnetic field, and the tomogram of the subject is radiated from the subject and received. This is a technique for generating images.
In MRI, for example, a specific frequency band is suppressed, such as MR angiography (hereinafter abbreviated as MRA), in which blood vessels are clearly imaged by suppressing magnetic resonance signals from fat in the subject. There is a case where it is desired to collect the magnetic resonance signal.

As a technique for collecting magnetic resonance signals in which a specific frequency band is suppressed, for example, a CHESS (chemical selective suppression) method and an MT (Magnetization Transfer) imaging method are known.
In the CHESS method, for example, an RF wave having the same frequency as the resonance frequency of fat protons is applied to the test site of the subject to excite only fat, and then a gradient magnetic field that saturates the excited fat is applied. The magnetic resonance signal from fat is suppressed (for example, refer nonpatent literature 1).

  In the MT imaging method, for example, an RF wave having the same frequency as the resonance frequency in the bound water including muscle and brain cell tissue is applied to the test site of the subject to excite only the proton in the bound water, and then excited. A gradient magnetic field that saturates the bound water is applied to suppress magnetic resonance signals from the bound water (see, for example, Non-Patent Document 2).

In addition, a technique for suppressing magnetic resonance signals from both fat and bound water by using both the CHESS method and the MT imaging method is also known (see, for example, Non-Patent Document 3).
In the said nonpatent literature 3, the RF wave application sequence in MT imaging method is performed after performing the RF wave application sequence in CHESS method. By using the CHESS method and MT imaging method together, the magnetic resonance signal from fat and the magnetic resonance signal from muscle, brain cell tissue, etc. can be suppressed, so the contrast between the image of these parts and the image of the blood vessel It is possible to obtain a magnetic resonance image in which blood vessels are drawn in more detail.
Mao J et al., "Fat suppression with an improved selective presaturation pulse.", Journal of Magnetic Resonance Imaging (Journal of Magnetic Resonance Imaging) 1992, 10 (1): p49-53 Hajnal JV et al., “Design and implementation of magnetization transfer pulse sequences for clinical use”, Journal of Computer Assisted Tomography (Journal of Computer Assisted Tomography) 1992, January / February, 16 (1): p7-18 Flaming DP and others, “Magnetization transfer contrast in fat-suppressed steady-state three-dimensional MR images”, Magnetic Resonance in Medicine (Magnetic Resonance in Medicine) July 1992, 26 (1): p122-31

  However, in Non-Patent Document 3 described above, since the RF wave application sequence in the MT imaging method is executed after the RF wave application sequence in the CHESS method, the application time of the RF wave necessary for obtaining the magnetic resonance signal is determined. This is the sum of the execution times of these RF wave application sequences. As a result, there is an inconvenience that the imaging time is long in order to acquire a high-contrast magnetic resonance image.

  Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus and an RF wave generation method that can suppress magnetic resonance signals in a plurality of frequency bands, improve the contrast of the magnetic resonance image, and shorten the imaging time. is there.

  A magnetic resonance imaging apparatus according to the present invention is a magnetic resonance imaging apparatus that generates image data of a magnetic resonance image of a subject based on a magnetic resonance signal from the subject to which an RF wave is applied. RF wave applying means for applying a multi-band excitation RF wave for exciting a frequency band to the subject as the RF wave is provided.

  According to the magnetic resonance imaging apparatus of the present invention, the RF wave applying means applies to the subject as multi-band excitation RF waves for exciting a plurality of different frequency bands as RF waves.

  The RF wave generation method according to the present invention is an RF wave generation method for generating an RF wave to be applied to a subject in order to obtain a magnetic resonance signal used for magnetic resonance image generation, and the frequency bands to be excited are different. The first step of generating the first and second RF waves, and the center frequency of the frequency band excited by the first RF wave and the center frequency of the frequency band excited by the second RF wave are shifted by a predetermined offset frequency. As described above, the second step of amplitude-modulating the second RF wave, and the second RF wave that has been amplitude-modulated and the first RF wave are combined to generate a multi-band excitation RF wave that excites a plurality of different frequency bands. And a third step of generating as an RF wave.

  According to the RF wave generation method of the present invention, in the first step, first and second RF waves having different frequency bands to be excited are generated. In the second step, the second RF wave is amplitude-modulated so that the center frequency of the frequency band excited by the first RF wave and the center frequency of the frequency band excited by the second RF wave are shifted by a predetermined offset frequency. In the third step, the amplitude-modulated second RF wave and the first RF wave are combined to generate a multi-band excitation RF wave that excites a plurality of different frequency bands as an RF wave.

  The present invention can provide a magnetic resonance imaging apparatus and an RF wave generation method that can suppress magnetic resonance signals in a plurality of frequency bands, improve the contrast of a magnetic resonance image, and shorten the imaging time.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
First, a configuration example of a magnetic resonance imaging apparatus (MR imaging apparatus) will be described.
Hereinafter, a case where blood vessel imaging of the head is performed as magnetic resonance imaging will be described as an example.

FIG. 1 is a schematic configuration diagram of a main part showing a configuration example of an MR imaging apparatus according to an embodiment of the present invention.
The MR imaging apparatus 1 illustrated in FIG. 1 includes a main body unit 20 and an operator console 280. The main body unit 20 further includes a magnet system 21, an RF (Radio Frequency) coil driving unit 271, a gradient coil driving unit 272, a data collection unit 273, and an image processing unit 275.
The operator console 280 further includes a control unit 274, a signal generation unit 17, a storage unit 18, an operation unit 19, and a display unit 20.
One embodiment of the RF wave applying means in the present invention includes an RF coil unit 214 (to be described later) of the magnet system 21 and an RF coil driving unit 271.

The magnet system 21 shown in FIG. 1 is of a so-called cylindrical type in which the direction Z of the static magnetic field is along the body axis direction from the head of the subject 99 to the legs.
The magnet system 21 includes a static magnetic field generating magnet unit 212, a gradient coil unit 213, and an RF coil unit 214.

The gradient coil part 213 is also arranged in the same cylindrical shape on the inner peripheral side of the cylindrical static magnetic field generating magnet part 212, and the space on the inner peripheral side of the gradient coil part 213 becomes the bore 94.
The subject 99 is placed on the cradle 243 and conveyed into the bore 94. The head of the subject 99 is accommodated in a cylindrical head imaging RF coil unit 214 placed on the cradle 243 when MR imaging of the head is performed.

The static magnetic field generating magnet unit 212 is configured using, for example, a superconducting magnet. In addition to the superconducting magnet, a magnetic field generating magnet such as a permanent magnet or a normal conducting magnet may be used.
A uniform static magnetic field is formed in the bore 94 along the Z direction in the figure by these magnetic field generating magnets configured in a cylindrical shape.

The RF coil unit 214 according to the present embodiment has a form that serves as both an RF wave transmitting coil and a receiving coil.
The RF coil unit 214 for applying an RF signal to the test site of the subject 99 and detecting a magnetic resonance signal from the test site forms a uniform static magnetic field in order to obtain a good magnetic resonance image. In FIG. 1, the head of the subject 99 and the RF coil unit 214 are drawn outside the bore 94 for clarity of illustration.

The RF coil unit 214 is configured to apply an RF wave having a predetermined frequency band for inclining the central axis of proton spin by a coil having a shape designed so that the distribution of the RF wave in the sensitivity region is uniform. Apply to. When the application of the RF wave is stopped, a magnetic resonance signal having a resonance frequency caused by the spin inclination of the test site re-radiated from the test site is detected again by the coil of the RF coil unit 214. The
The frequency range of the RF wave is, for example, from 2.13 MHz to 85 MHz.
The excitation of the central axis of proton spin by application of RF waves is called excitation.

  The RF coil section is not limited to the RF coil section 214 as shown in FIG. 1, but may be arranged in a cylindrical shape further on the inner peripheral side of the gradient coil section 213 depending on the test site. A surface coil that is used in the vicinity of the surface of the part may be used. Also in these other RF coil sections, a coil for transmitting an RF wave and a coil for receiving a magnetic resonance signal may be shared by the same coil, or different dedicated coils may be used.

The gradient coil unit 213 has three systems of gradient magnetic field coils so that the magnetic resonance signal detected by the RF coil unit 214 has three-dimensional position information. The gradient coil unit 213 uses these gradient magnetic field coils to generate a gradient magnetic field that gives gradients in the X, Y, and Z directions to the strength of the static magnetic field formed by the static magnetic field generating magnet unit 212.
Of these three gradient magnetic fields, one is a slice selection gradient magnetic field for selecting a slice at the test site, one is a phase encoding gradient magnetic field, and the other is a reading gradient magnetic field (also referred to as a frequency encoding gradient magnetic field).

An RF coil drive unit 271 is connected to the RF coil unit 214. The RF coil driving unit 271 is further connected to the signal generation unit 17 of the operator console 280.
The RF coil driving unit 271 provides the RF coil unit 214 with an RF wave excitation signal having a signal waveform transmitted from the signal generation unit 17. As a result, an RF wave is applied from the RF coil unit 214 to the subject in the bore 94, and as a result, the inclination of the central axis of the spin at the site of the subject 99 can be changed.

A gradient coil drive unit 272 is connected to the gradient coil unit 213. The gradient coil drive unit 272 is further connected to the signal generation unit 17.
The gradient coil drive unit 272 provides the gradient coil unit 213 with a gradient magnetic field excitation signal having a signal waveform transmitted from the signal generation unit 17. This generates a gradient magnetic field in the bore 94. The gradient coil drive unit 272 includes three drive circuits (not shown) corresponding to the three gradient coils of the gradient coil unit 213.

In the present embodiment, the data collection unit 273 is connected to a detection channel in the RF coil unit 214, takes in the detected magnetic resonance signal, and collects it as original data for magnetic resonance image generation.
For example, after collecting all data for generating one image, the data collection unit 273 transmits the collected data to the image processing unit 275.

  The image processing unit 275 performs predetermined image processing on the original data received from the data collection unit 273 to generate image data of a magnetic resonance image.

The data collection unit 273, the image processing unit 275, the control unit 274 in the operator console 280, the storage unit 18, and the display unit 20 are connected to each other.
The control unit 274 is further connected to the signal generation unit 17 and the operation unit 19.

  The operation unit 19 is realized by an input device such as a keyboard and a mouse, for example. A command signal from an operator who operates the operator console 280 is input to the control unit 274 via the operation unit 19.

The control unit 274 is realized by hardware such as a CPU for calculation and software such as a program for driving the hardware.
The above program is stored in the storage unit 18 realized by, for example, a RAM (Random Access Memory) or a hard disk drive. In addition, the data collected by the data collection unit 273 and the image data generated by the image processing unit 275 can be stored in the storage unit 18.
The control unit 274 integrally controls the signal generation unit 17, the data collection unit 273, the image processing unit 275, and the display unit 20 so as to realize a command from the operator input via the operation unit 19. When there is a restriction such as a hardware restriction of the main body unit 20, the control unit 274 displays on the display unit 20 that the input command cannot be executed.

The image generated by the image processing unit 275 or the image stored in the storage unit 18 can be displayed on the display unit 20 by the display unit 20 executing a display command from the operator.
The display unit 20 is realized by a monitor such as a liquid crystal display panel or a CRT (Cathode-Ray Tube), for example.
The display unit 20 also displays an operation screen for operating the MR imaging apparatus 1.

With the above configuration, a magnetic resonance image of the subject 99 can be generated using the MR imaging apparatus 1. At this time, according to the frequency band of the RF wave applied from the RF coil unit 214 to the subject 99, it is possible to excite and suppress the frequency band corresponding to the frequency band of the RF wave in the magnetic resonance signal obtained from the test site. Become. For example, in MRA, an image in which blood vessels are relatively emphasized can be generated by suppressing magnetic resonance signals from fat and brain cell tissue.
As described above, in magnetic resonance imaging, in order to obtain a desired image, it is important to generate and apply an RF wave having a frequency band.
Hereinafter, the RF wave generation method according to the present embodiment will be described with reference to FIGS.

FIG. 2 is a diagram illustrating an example of a procedure for generating an RF wave according to the present embodiment.
FIG. 3 is a graph showing changes in the waveform signal of the RF wave accompanying the procedure of generating the RF wave according to the present embodiment. In the graphs (a-1), (a-2), (b-1) to (b-3), and (c) shown in FIG. 3, the horizontal axis represents time t (sec) and the vertical axis represents the waveform. Each of the signal strengths is shown. However, the signal strength values in the graphs of FIGS. 3 (a-1), (a-2), (b-1) to (b-3), and (c) do not necessarily match the actual values. Absent.
FIG. 4 is a diagram showing changes in the frequency band of the RF wave accompanying the procedure of generating the RF wave according to the present embodiment. In each figure of (i) to (vi) shown in FIG. 4, the horizontal axis represents frequency (Hz), and the frequency band of the RF wave in each procedure of RF wave generation is drawn on this horizontal axis.

In the procedure of generating an RF wave according to the present embodiment, first, the signal generation unit 17 obtains an imaging condition indicating under what conditions the imaging is performed (step ST1).
The imaging conditions include conditions such as the magnetic field strength of the static magnetic field formed by the static magnetic field generating magnet unit 212 and the maximum magnetic field strength of the magnetic field that can be applied by the gradient coil unit 213 and the RF coil unit 214, for example.

Next, in order to generate an RF wave that can excite a desired frequency band, the signal generation unit 17 generates a waveform signal of the original RF wave (step ST2).
One embodiment of the first step in the present invention corresponds to step ST2.
As described in detail below, in the present embodiment, an RF wave that can excite a desired frequency band of a magnetic resonance signal is generated by synthesizing two types of RF wave waveform signals.

The two types of RF waves that are the source are, for example, an RF wave that is applied to excite protons of bound water in the MT (Magnetization Transfer) imaging method, and a fat proton that is excited in the CHESS (chemical selective suppression) method. Therefore, an RF wave to be applied is used.
The bound water refers to water that is restricted in movement by binding to other molecules such as muscles and brain cell tissues. Water combined with a polymer tissue such as brain cells is also called a polymer compound. Since the resonance frequency of the bound water is different from that of freely moving water (free water), if the bound water is excited by applying an RF wave in the frequency band including the resonance frequency of the bound water and then saturated, Thus, the magnetic resonance signal from the free water portion such as a blood vessel can be relatively emphasized.

Hereinafter, for simplicity, an RF wave for exciting protons in bound water is called an MT pulse, and an RF wave for exciting fat protons is called a CHESS pulse. One embodiment of the first RF wave in the present invention corresponds to an MT pulse, and one embodiment of the second RF wave corresponds to a CHESS pulse.
Further, an RF wave that is obtained by synthesizing the MT pulse and the CHESS pulse and is generated as an object in the present embodiment is hereinafter referred to as a synthesized pulse. One embodiment of the multi-band excitation RF wave in the present invention corresponds to this synthesized pulse.

  MT pulses can be used to excite bound water and CHESS pulses can be used to excite fat. The details of the CHESS method and the MT imaging method are described in, for example, Non-Patent Documents 1 and 2 described above, so that the description thereof is omitted.

  The graph of FIG. 3 (a-1) shows an example of the waveform of the MT pulse, and the graph of FIG. 3 (b-1) shows an example of the waveform of the CHESS pulse. However, the intensity values of the MT pulse and the CHESS pulse in FIGS. 3A-1 and 3B-1 are not necessarily the actual values.

In synthesizing the MT pulse and the CHESS pulse, in this embodiment, the CHESS pulse is amplitude-modulated (Amplitude Modulation) with a certain offset frequency. This is because the frequency band of the MT pulse and the frequency band of the CHESS pulse are separated by an offset frequency.
The MT pulse is a pulse for exciting bound water, and the CHESS pulse is a pulse for exciting fat, for example. Therefore, the difference between the resonance frequency of the bound water and the resonance frequency of fat is the offset frequency value. However, since the difference between the resonance frequency of the bound water and the resonance frequency of fat varies depending on the magnitude of the static magnetic field in which the subject 99 is located, the MT pulse frequency band and the CHESS pulse frequency band are combined. In order to locate in the resonance frequency band of water and the resonance frequency band of fat, it is necessary to determine the value of the offset frequency according to the magnitude of the static magnetic field.
Therefore, the signal generation unit 17 determines the offset frequency based on the static magnetic field strength information obtained in step ST1 so that the offset frequency value is the difference between the resonance frequency of the bound water and the resonance frequency of fat. Is automatically determined (step ST3).
The absolute value of the difference between the resonance frequency of bound water and the resonance frequency of fat is, for example, about 220 Hz when the static magnetic field strength is 1.5 Tesla (T).

After determining the offset frequency, the signal generator 17 amplitude-modulates the CHESS pulse with the determined offset frequency (step ST4).
One embodiment of the second step in the present invention corresponds to step ST4.
FIG. 3B-2 shows the waveform of the modulated CHESS pulse obtained as a result of modulating the CHESS pulse shown in the graph of FIG. 3B-1 at 1 kHz, for example.
In the case of amplitude modulation, a CHESS pulse as a carrier wave as shown in FIG. 3 (b-1) is modulated by a signal wave represented by a general form of Ipcos (2πfp). 3 (b-2) shows a waveform in which the amplitude changes with time. Here, the symbol Ip represents the amplitude of the signal wave, and the symbol fp represents the modulation frequency. When modulated at 1 kHz, fp = 1 (kHz).

  Note that the CHESS pulse may not be modulated, and the MT pulse may be modulated instead. However, in order to suppress the load on the main body 20 in consideration of energy efficiency, it is preferable to modulate the CHESS pulse. This is because bound water requires more energy than fat because the water is bound to other tissues, and the energy required to modulate the MT pulse to excite the bound water is increased accordingly. Because.

Here, consider the frequency spectrum of the MT pulse shown in FIG. 3 (a-1) and the frequency spectrum of the modulated CHESS pulse shown in FIG. 3 (b-2).
The MT pulse and the modulated CHESS pulse are pulses each having a specific frequency band including a resonance frequency of combined water and a resonance frequency of fat in order to excite the combined water and fat, respectively. Therefore, these unique frequency bands appear as a spectrum.
In addition, since these specific frequency bands are frequency bands for exciting specific tissues, they may be referred to as excitation frequency bands below.

FIG. 4 (i) is a diagram showing the excitation frequency band FMT of the MT pulse, and FIG. 4 (ii) is a diagram showing the excitation frequency band FCH of the modulated CHESS pulse.
Each excitation frequency band FMT, FCH has a certain width. The frequency of the center part of this width is called the center frequency. In FIGS. 4 (i) and (ii), steep excitation frequency bands FMT and FCH are shown in order to clearly show the center frequency. Actually, the peaks of the excitation frequency bands FMT and FCH are more gentle. In many cases.

The center frequency of each excitation frequency band FMT, FCH has a certain value, but the value of the center frequency changes according to the static magnetic field magnetic field strength as described above. May be represented.
In FIG. 4, the center frequency of the MT pulse excitation frequency band FMT is defined as a reference frequency of 0 (Hz).
The center frequency of the modulated CHESS pulse obtained by amplitude modulation at a certain offset frequency appears at a position shifted by the offset frequency from the center frequency of the MT pulse. That is, when the offset frequency is fh (Hz), as shown in FIG. 4 (ii), the excitation frequency band FCH appears at a position separated from the center frequency of 0 (Hz) of the reference MT pulse by the frequency fh. .
At this time, since the modulated CHESS pulse has positive and negative amplitudes as shown in FIG. 3 (b-2), as shown in FIG. 4 (ii), the excitation frequency band FCH whose center frequency is located at the frequency fh, and the frequency Two excitation frequency bands FCH and FCH with an excitation frequency global FCH located at −fh appear.

After generating the MT pulse signal and the modulated CHESS pulse signal, the signal generating unit 17 combines the MT pulse and the modulated CHESS pulse to generate a synthesized pulse (step ST5).
One embodiment of the third step in the present invention corresponds to step ST5.
At this time, since the MT imaging method and the CHESS method have different flip angles in the MT pulse and flip angles in the CHESS pulse, this difference is realized at the time of synthesis.
Specifically, for example, the flip angle in the MT pulse is about 800 ° to 1000 °, and the flip angle in the CHESS pulse is about 90 ° to 95 °. That is, the flip angle of the CHESS pulse is about 1/10 of that of the MT pulse. Therefore, the signal generation unit 17 combines the MT pulse and the CHESS pulse after converting them so as to have a desired flip angle.

  The flip angle means an angle at which the central axis of the spin is inclined by excitation. This flip angle is determined by the intensity and application time of the RF wave applied to the protons in the subject. Therefore, the area of each graph shown in FIG. 3 means a flip angle.

For example, as shown in FIG. 3 (a-2) and FIG. 3 (b-3), the signal generation unit 17 does not change the signal intensity of the MT pulse and reduces the signal intensity of the modulation CHESS pulse to reduce modulation. Generate a new CHESS pulse.
Then, the signal generation unit 17 adds the signal data for each sampling time to synthesize the MT pulse and the reduced modulation CHESS pulse, thereby generating a synthesized pulse as shown in FIG.

In the composite pulse shown in FIG. 3C, the flip angle of the pulse corresponding to the MT pulse is [the area of the composite pulse × the area of the MT pulse / (the area of the MT pulse + the area of the reduced modulation CHESS pulse)].
The flip angle of the pulse corresponding to the reduced modulation CHESS pulse is [the area of the composite pulse × the area of the reduced modulation CHESS pulse / (the area of the MT pulse + the area of the reduced modulation CHESS pulse)].

When the ratio of the flip angle of the MT pulse to the flip angle of the CHESS pulse is about 10: 1, the waveform of the synthesized pulse is reduced and modulated as shown in FIG. The pulse has a shape that fluctuates finely according to fluctuations in the amplitude of the pulse.
The waveform signal of the modulated CHESS pulse shown in FIG. 3 (b-2) and the waveform signal of the reduced modulated CHESS pulse shown in FIG. 3 (b-3) do not necessarily have to be actually generated. The signal generation unit 17 receives an instruction for generating a composite pulse using the waveforms shown in FIGS. 3A-1 and 3B-1 via the operation unit 19 and the control unit 274. In such a case, a signal having the waveform of the composite pulse shown in FIG.

  When considered as a spectrum in the frequency band, the intensity of the excitation frequency band FMT of the MT pulse does not change as shown in FIG. 4 (i) and FIG. 4 (iii). On the other hand, when the signal intensity of the modulated CHESS pulse is reduced to reduce the flip angle, the intensity of the excitation frequency band FCH of the modulated CHESS pulse in FIG. 4 (ii) becomes smaller, and the reduced modulated CHESS pulse shown in FIG. 4 (iv). The excitation frequency band changes to FCR. At this time, the center frequency of the excitation frequency bands FCR and FCR does not change from ± fh.

  As shown in FIG. 4 (v), the spectrum of the frequency of the synthesized pulse obtained by synthesizing the MT pulse and the reduced modulation CHESS pulse is the center of the excitation frequency band FMT and ± fh of the MT pulse having a center frequency of 0 (Hz). The spectrum has three excitation frequency bands, two excitation frequency bands FCR and FCR having frequencies.

After generating the synthesized pulse signal, the signal generating unit 17 transmits the synthesized pulse waveform signal to the RF coil driving unit 271 and applies the RF wave according to the synthesized pulse waveform from the RF coil unit 214 to the subject 99. Let
At this time, for example, the center frequency fh of the excitation frequency band FCR for exciting fat does not necessarily match the actual resonance frequency of fat at the test site of the subject 99 in the static magnetic field. Therefore, the signal generation unit 17 shifts the transmission frequency of the synthetic pulse by the transmission offset frequency ft and applies the command signal to be applied to the test site in order to match the center frequency of the excitation frequency band FCR with the actual resonance frequency of fat. It transmits to the RF coil drive unit 271.

By shifting the transmission frequency of the composite pulse by the transmission offset frequency ft, the excitation frequency band FMT and FCR, FCR possessed by the composite pulse are shifted by the transmission offset frequency ft as shown in FIG. Therefore, finally, the excitation frequency band FMT of the synthesized pulse excites the tissue of the resonance frequency ft, and the excitation frequency bands FCR and FCR excite the tissue of the resonance frequency (± fh + ft).
For example, if the transmission offset frequency ft is determined so that the frequency (fh + ft) matches the resonance frequency of fat, fat can be excited by the excitation frequency band FCR.
The offset frequency fh used to modulate the CHESS pulse is defined as the difference between the resonance frequencies of fat and bound water, so when the frequency (fh + ft) matches the resonance frequency of fat, the frequency fh It is possible to excite the bound water by the excitation frequency band FMT having the center frequency ft.
For example, when the resonance frequency of fat is ff, the transmission offset frequency ft can be obtained by calculation of ft = ff−fh.

  The composite pulse having a plurality of excitation frequency bands FMT and FCR capable of exciting fat and bound water as described above is applied from the RF coil unit 214 to the test site.

Hereinafter, an example of a pulse sequence for magnetic resonance imaging in the MR imaging apparatus 1 according to the present embodiment will be described with reference to FIG.
In FIG. 5, the horizontal axis represents the elapsed time t, and each graph shows an RF wave application pulse sequence RF, a slice selection gradient magnetic field application pulse sequence G_slice, a phase encoding gradient magnetic field application pulse sequence G_phase, from the top of FIG. A read gradient magnetic field application pulse sequence G_read and a magnetic resonance signal generation sequence Signal are shown.
A pulse sequence is a pulse waveform of an RF wave, gradient magnetic field, and magnetic resonance signal that is shown along the elapsed time, and an RF wave excitation signal in which each pulse waveform has a shape defined by the pulse sequence. And the gradient magnetic field excitation signal are input from the RF coil driving unit 271 and the gradient coil driving unit 272 to the RF coil unit 214 and the gradient coil unit 213, respectively.

The sequence RF indicates the waveform of the RF wave applied from the RF coil unit 214 to the subject 99.
The sequence G_slice represents a waveform of a slice selection gradient magnetic field pulse that the gradient coil unit 213 applies to the test site in order to select an imaging slice of the test site.
The sequence G_phase represents a waveform of a phase encoding gradient magnetic field pulse applied by the gradient coil unit 213 to the test site for use in encoding position information in the phase direction of the subject.
A sequence G_read represents a waveform of a read gradient magnetic field pulse applied to the test site by the gradient coil unit 213 in order to emit a magnetic resonance signal from the test site to which the RF wave is applied by the RF coil unit 214.
The sequence Signal represents the magnetic resonance signal 54 emitted from the test site.

The step of applying the RF wave and performing phase encoding with the phase encoding gradient magnetic field is repeated a predetermined number of times while changing the magnitude of the phase encoding gradient magnetic field according to the pixel size of the target image. This operation is expressed by a plurality of phase encoding gradient magnetic field pulses 52 in the sequence G_phase of FIG.
In addition, for example, in order to form a three-dimensional image of a blood vessel by MRA, a magnetic resonance signal is obtained for a slab that is thick to a certain extent with a plurality of slices superimposed. For this purpose, encoding is also performed in the slice direction. Is required. A plurality of slice selective gradient magnetic field pulses 51d in FIG. 5 represent the encoding operation in the slice direction.

  As shown in FIG. 5, the pulse sequences for magnetic resonance imaging in the present embodiment are roughly divided into three parts: a first partial pulse sequence PS1, a second partial pulse sequence PS2, and a third partial pulse sequence PS3. It has a partial pulse sequence.

In the first partial pulse sequence PS1, the synthesized pulse 50a generated by the method described so far is applied to the test site of the subject 99.
As described above, the synthesized pulse 50a is a multi-band excitation RF wave that can excite a plurality of different frequency bands in a magnetic resonance signal such as a resonance frequency band of bound water and a resonance frequency band of fat. By applying the synthetic pulse 50a, a plurality of tissues such as combined water and fat can be excited at a time.

After the execution of the partial pulse sequence PS1, for example, a gradient magnetic field that attenuates the coherence of the excited tissue signal and makes it insensitive to the subsequent excitation by a new RF wave, as shown by gradient magnetic field pulses 51b and 53a in FIG. A partial pulse sequence PS2 for applying a pulse to a region to be examined is executed.
It is said that the proton is saturated when the coherence of the excited tissue signal is attenuated and it becomes insensitive to excitation by RF waves until longitudinal magnetization is restored.
A gradient magnetic field that saturates protons, such as gradient magnetic field pulses 51b and 53a, is referred to as a crusher gradient magnetic field.
By applying a crusher gradient magnetic field, only the excited bound water and fat protons are saturated.

In partial pulse sequence PS3 after execution of partial pulse sequence PS2, MRA pulse sequence specialized for angiography, spin echo method, gradient echo method, echo planar imaging (echo planar imaging) A pulse sequence for generating a magnetic resonance signal as an echo from a region to be examined, such as a method, can be appropriately applied.
FIG. 5 shows, as an example, a partial pulse sequence for obtaining a magnetic resonance signal from a subject by a gradient echo method. As shown in FIG. 5, an RF wave 50b for generating a magnetic resonance signal is applied to a subject 99 in a state where a slice or slab is selected by applying a slice selective gradient magnetic field pulse 51c.

After applying the RF wave 50b, as shown in FIG. 5, the slice selective gradient magnetic field pulse 51d and the phase encode gradient magnetic field pulse 52 are applied to perform the encoding in the slice direction and the phase encode direction, respectively, and the read gradient magnetic field pulse 53b is applied. Apply to the test site. By applying the readout gradient magnetic field pulse 53b, the magnetic resonance signal 54 as an echo from the slab of the region to be examined is detected by the RF coil unit 214.
At the time of applying the RF wave 50b, since only the combined water and fat protons excited by the synthetic pulse 50a are saturated, they are difficult to be excited by the RF wave 50b. Therefore, in the magnetic resonance signal 54, the frequency components of the resonance frequency of the combined water and fat are relatively suppressed.

The gradient magnetic field pulse 51e shown in FIG. 5 is a pulse as a crusher for attenuating excess coherence after the magnetic resonance signal 54 is acquired.
The time from the center of the RF wave 50b applied to acquire the magnetic resonance signal 54 to the center of the magnetic resonance signal 54 is referred to as an echo time TE.
The time from the partial pulse sequence PS1 to PS3 is referred to as a repetition time TR. The shooting of one slab includes a repetition time TR a predetermined number of times.

As described above, in the present embodiment, a plurality of RF wave signals having different excitation frequency bands are combined by modulating other signals with one signal as a reference to generate a combined pulse 50a. By this synthesis using modulation, the synthesized pulse 50a has all the excitation frequency bands that the original RF wave used for synthesis had. By applying the synthetic pulse 50a to the subject 99, it is possible to excite a plurality of tissues whose central frequencies are included in the excitation frequency band at the subject site of the subject 99 at a time. Therefore, when a plurality of tissues are excited, it is not necessary to individually execute a pulse sequence for exciting each tissue as in the prior art, and the time of the pulse sequence in the previous stage of magnetic resonance signal acquisition can be shortened. it can. As a result, the repetition time TR is shortened and the photographing time is shortened. For example, in the present embodiment, the time of the partial pulse sequence PS1 for applying the composite pulse 50a is about 8 msec and the time of the partial pulse sequence PS2 for applying the crusher is about 2 msec. The time is about 10 msec.
Further, by executing the two partial pulse sequences PS1 and PS2, in the magnetic resonance signal 54, the frequency components of the resonance frequency of the combined water and fat are relatively suppressed. For this reason, in the magnetic resonance image, the tissue portion of the bound water existing in the background of the blood vessel is darkened, the contrast between the blood vessel and the background is improved, and fat unnecessary for image interpretation is not drawn. A magnetic resonance image with good image quality can be obtained.

[Comparative example]
For comparison, FIG. 6 shows a conventional pulse sequence for individually executing a pulse sequence for exciting each tissue when a plurality of tissues are excited. In FIG. 6, parts having the same meaning as in FIG. 5 are given the same reference numerals, and detailed descriptions thereof are omitted.

  As shown in FIG. 6, conventionally, after executing the partial pulse sequence PS4 for applying the CHESS pulse 50c for exciting fat to the subject 99, the partial pulse sequence PS2 for applying the crusher is executed once. Thereafter, a partial pulse sequence PS5 for applying an MT pulse 50d for exciting the bound water to the subject 99 is executed, and then a partial pulse sequence PS2 for applying a crusher is executed. Note that the flip angle of the CHESS pulse 50c and the flip angle of the MT pulse 50d are not necessarily the same as in practice.

Conventionally, the partial pulse sequence PS3 for acquiring the magnetic resonance signal has been executed after individually executing the pulse sequence for exciting each of the plurality of tissues as described above.
As a result, the time of the pulse sequence in the previous stage of the partial pulse sequence PS3 becomes longer, and as a result, the repetition time TR and the imaging time tend to become longer.
For example, the time of the two partial pulse sequences PS4 and PS5 is about 16 msec and 8 msec, respectively, and the time of the partial pulse sequence PS2 is about 2 msec, so that the time of the previous pulse sequence needs to be about 28 msec in total. Since the time of the previous pulse sequence in the above embodiment is 10 msec, it can be seen that the time is greatly shortened in the case of the above embodiment.

In addition, this invention is not limited to the content of the said embodiment, A various change is possible within a claim. For example, not only the MT pulse and the CHESS pulse but also three or more types of RF waves can be combined and applied as long as the resonance frequency band of the target tissue and the flip angle at the time of application are shifted. .
In addition, by appropriately selecting the excitation frequency band in the composite pulse, not only the suppression of bound water and fat in the magnetic resonance image, but also excitation of free water to obtain a magnetic resonance image in which blood vessels are emphasized, The present invention can be applied to the generation of various images such as obtaining images in which the image is suppressed.

  The present invention can be suitably used in the field of magnetic resonance image generation.

It is a schematic block diagram of the principal part which shows the structural example of MR imaging apparatus which concerns on one embodiment of this invention. It is a figure which shows an example of the procedure of the RF wave generation which concerns on one embodiment of this invention. It is the figure which showed the change of the waveform signal of RF wave accompanying the procedure of RF wave generation which concerns on one embodiment of this invention with the graph. It is a figure which shows the change of the frequency band of RF wave accompanying the procedure of RF wave generation which concerns on one embodiment of this invention. It is an example of the pulse sequence performed in the magnetic resonance imaging using the MR imaging apparatus shown in FIG. It is an example of the pulse sequence performed in the conventional magnetic resonance imaging.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... MR imaging device 17 ... Signal generation part 50a ... Synthetic | combination pulse (multiband excitation RF wave)
99 ... Subject 212 ... Magnet part 213 ... Gradient coil part 214 ... RF coil part 271 ... RF coil driving part 272 ... Gradient coil driving part 274 ... Control part

Claims (9)

A magnetic resonance imaging apparatus that generates image data of a magnetic resonance image of the subject based on a magnetic resonance signal from the subject to which an RF wave is applied,
A magnetic resonance imaging apparatus comprising: an RF wave applying unit configured to apply a plurality of band excitation RF waves for exciting a plurality of different frequency bands to the subject as the RF waves. In the first and second RF waves having different frequency bands to be excited, the RF wave applying means includes a center frequency of a frequency band excited by the first RF wave and a center frequency of a frequency band excited by the second RF wave. The second RF wave is amplitude-modulated so as to be shifted by a predetermined offset frequency, and the multi-band excitation RF wave generated by combining the amplitude-modulated second RF wave and the first RF wave is applied. The magnetic resonance imaging apparatus according to 1. 3. The magnetism according to claim 2, wherein the RF wave applying unit applies the multi-band excitation RF wave synthesized after converting the first RF wave and the second RF wave so as to be excited at a desired flip angle. 4. Resonance imaging device. 4. The magnetic resonance according to claim 2, wherein the first RF wave is an RF wave for exciting protons in bound water, and the second RF wave is an RF wave for exciting fat protons. 5. Shooting device. The offset frequency is a magnetic field of a static magnetic field applied to the subject so as to be a difference between a center frequency of a frequency band excited by the first RF wave and a center frequency of a frequency band excited by the second RF wave. The magnetic resonance imaging apparatus according to claim 2, wherein the magnetic resonance imaging apparatus is defined based on intensity. An RF wave generation method for generating an RF wave applied to a subject to obtain a magnetic resonance signal used for magnetic resonance image generation,
A first step of generating first and second RF waves having different frequency bands to be excited;
A second step of amplitude-modulating the second RF wave so that a center frequency of a frequency band excited by the first RF wave and a center frequency of a frequency band excited by the second RF wave are shifted by a predetermined offset frequency;
A third step of synthesizing the amplitude-modulated second RF wave and the first RF wave to generate a multi-band excitation RF wave for exciting a plurality of different frequency bands as the RF wave. . The RF wave generating method according to claim 6, wherein in the third step, the first RF wave and the second RF wave are combined after being converted so as to be excited at a desired flip angle. The RF wave generation according to claim 6 or 7, wherein, in the first step, the first RF wave having a frequency band for exciting bound water and the second RF wave having a frequency band for exciting fat are generated. Method. In the second step, the static voltage applied to the subject is set so as to have a difference between the center frequency of the frequency band excited by the first RF wave and the center frequency of the frequency band excited by the second RF wave. The RF wave generation method according to claim 6, wherein the offset frequency defined based on the magnetic field strength of the magnetic field is used.

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