JP4871457B2 - Magnetic resonance imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging technique, and more particularly to a technique for suppressing artifacts caused by a moving substance such as a blood flow.
[0002]
[Prior art]
A magnetic resonance imaging (MRI) apparatus irradiates a substance constituting a subject with a magnetic resonance phenomenon by irradiating a living body with a high frequency excitation pulse (excitation RF pulse) of a specific frequency in a state where a uniform static magnetic field is applied to a living body. The device is used for medical diagnosis by measuring a magnetic resonance signal (NMR signal) generated from the substance and imaging chemical and physical information of the substance. For example, when a tomographic image of a living body is imaged, an excitation RF pulse is applied while applying a uniaxial slice gradient magnetic field that specifies a tomographic site, and the site is excited. Then, two-dimensional gradient magnetic fields orthogonal to the gradient axis direction of the slice gradient magnetic field are applied, two-dimensional position information is given to the magnetic resonance signal generated by excitation, and image data of the tomographic image is measured. I am doing so. Usually, two gradient magnetic fields that give two-dimensional position information are referred to as a phase encode gradient magnetic field and a frequency encode gradient magnetic field, respectively.
[0003]
In such MRI, if there is a moving substance such as a blood flow in the imaging region, the imaging region is applied after excitation by applying a slice gradient magnetic field and before applying a phase encoding gradient magnetic field or a frequency encoding gradient magnetic field. As a result, the state of the excited substance changes, resulting in artifacts in the obtained tomogram, which hinders diagnosis. For example, when imaging the abdomen with a circular section, that is, when imaging an abdominal organ such as the liver with a transverse section, the abdomen has an aorta and a vena cava that flow in opposite directions. Artifacts can be caused by blood flow, which can interfere with diagnosis.
[0004]
In order to suppress such artifacts due to blood flow, an imaging technique called presaturation is conventionally known (Japanese Patent Laid-Open No. 3-32642). This method irradiates the slice gradient magnetic field pulse and the excitation RF pulse upstream of the blood flow from the slice region of the imaging region before executing the imaging sequence (pulse sequence) of the desired region, This is a method of applying a gradient magnetic field pulse. In other words, before exciting blood flow into the imaging region by exciting at least the blood flow part upstream from the imaging region, applying a gradient magnetic field to the nuclear spin generated thereby, This is a technique for preventing the occurrence of artifacts by saturating and suppressing magnetic resonance signals related to blood flow. That is, if the phases of the nuclear spins are separated and randomized, a signal obtained by synthesizing the nuclear spins becomes a sufficiently small signal. As a result, it is possible to suppress artifacts caused by the blood flow portion flowing into the imaging region.
[0005]
[Problems to be solved by the invention]
However, according to the pre-saturation method described in the above publication, when two blood vessels that flow in opposite directions, such as an aorta or a vena cava, are present in an imaging region, for example, two locations on the head side and the foot side of the imaging region Since the pre-saturation process is performed on this part, there is a problem that the imaging time becomes long. That is, it takes twice as long to perform the pre-saturation process in which the pre-saturation RF pulse is applied and the gradient magnetic field pulse for signal saturation is applied twice before the normal imaging sequence. Accordingly, if the repetition time TR of photographing is constant, the number of slices that can be photographed is reduced. Further, if the number of slices is made constant, there is a problem that an increase in TR leads to a change in image contrast and an increase in shooting time.
[0006]
An object of the present invention is to eliminate the above-described conventional problems and suppress magnetic resonance signals at a plurality of sites by one presaturation process.
[0007]
[Means for Solving the Problems]
The present invention, before performing the shooting sequence, by applying an RF pulse for pre-saturation pulse having at least two fundamental frequency components, to solve the above problems.
[0008]
For example, while applying the same gradient as the slice gradient magnetic field at the time of normal imaging, a pre-saturation with at least two fundamental frequency components of the imaging slice excitation RF pulse base frequency f0 frequency f2 higher and lower frequency f1 than the When two RF pulses are irradiated, two portions X1 and X2 that are determined by the relationship between the intensity of the slice gradient magnetic field and the frequencies f1 and f2 are excited. These parts X1 and X2 are on both sides of the slice part X0 during normal imaging. Then, the phase of the nuclear spins of the excited sites X1 and X2 becomes random by the subsequently applied gradient magnetic field, and the magnetic resonance signal obtained by synthesizing a plurality of nuclear spins becomes a sufficiently small signal. That is, a single presaturation process can suppress magnetic resonance signals at a plurality of sites, and can suppress, for example, artifacts caused by blood flow flowing from both sides of the imaging region. In addition, problems such as a reduction in the number of slices and an increase in shooting time can be avoided.
[0009]
The magnetic resonance imaging apparatus of the present invention, before performing the shooting sequence, by applying RF pulses for presaturation to the subject, wherein the RF pulse for presaturation is that shall have at least two fundamental frequency components ,realizable.
[0010]
In the present invention, in particular, it is preferable that the at least two fundamental frequency components have a phase shifted relationship with each other. Thereby, the maximum value (crest value) of the amplitude when the synthesized RF pulse is amplitude-modulated with a Sinc function or the like can be suppressed low, and an increase in the capacity of the RF pulse generator can be suppressed.
[0011]
Embodiment
Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a pulse sequence of an embodiment of the method of the present invention, FIG. 2 is an overall configuration diagram of a magnetic resonance imaging apparatus to which the method of the present invention can be applied, FIGS. 3 and 4 are operation explanatory views of the method of the present invention, FIG. FIG. 6 is a waveform diagram of a presaturation RF pulse according to the present invention.
[0012]
As shown in FIG. 2, the magnetic resonance imaging apparatus includes a static magnetic field generator 1, a gradient magnetic field generator 2, a transmission system 3, a reception system 4, a signal processing system 5, a sequencer 6, a central processing unit (CPU) 7, and an operation. It is configured with a part 8 and the like. The static magnetic field generator 1 generates a uniform static magnetic field in the space where the subject 9 is placed. The direction of the static magnetic field is usually the body axis direction of the subject 9 or a direction orthogonal to the body axis. The static magnetic field generator 1 is formed by a magnetic field generating means of a permanent magnet system, a normal conductive magnet system, or a superconductive magnet system. The gradient magnetic field generator 2 includes a gradient magnetic field coil 10 that generates a gradient magnetic field in three orthogonal axes (X, Y, Z), and a gradient magnetic field power supply 11 that supplies a drive current for the gradient magnetic field coil 10. Has been. The gradient magnetic field power supply 11 applies gradient magnetic fields Gs, Gp, Gr in three orthogonal axes (X, Y, Z) to the subject 9 in accordance with instructions from the sequencer 6. The slice plane of the tomographic image can be set by applying this gradient magnetic field, and the position information can be encoded into the measured magnetic resonance signal (echo signal). The sequencer 6 operates under the control of the CPU 7 and sends commands to the gradient magnetic field generator 2, the transmission system 3, the reception system 4, etc. according to an imaging sequence called a pulse sequence, and performs the control necessary for capturing a tomographic image. It is something to execute.
[0013]
The transmission system 3 irradiates an RF pulse (high frequency magnetic field pulse) for causing nuclear magnetic resonance to nuclei constituting the living tissue of the subject 9. The transmission system 3 is a high frequency oscillator 12, a modulator 13, a high frequency amplifier 14, and a high frequency. An irradiation coil 15 is provided. Then, the transmission system 3 modulates the amplitude of the high frequency pulse output from the high frequency oscillator 12 by the modulator 13 and amplifies it by the high frequency amplifier 14 in accordance with an instruction from the sequencer 6, and then supplies the RF pulse to the high frequency irradiation coil 15. To the subject 9.
[0014]
The receiving system 4 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the living tissue of the subject 9, and receives a high-frequency receiving coil 16 on the receiving side, an amplifier 17, a quadrature detector 18, and An A / D converter 19 is included. The NMR signal received by the high-frequency receiving coil 16 is amplified by the amplifier 17, detected by the quadrature detector 18, and then converted into digital signal measurement data by the A / D converter 19. Note that two series of measurement data sampled with the phase shifted by 90 ° by the quadrature detector 18 at the timing controlled by the sequencer 6 is sent to the signal processing system 5.
[0015]
The signal processing system 5 includes a CPU 7, a ROM 20, a RAM 21, a magneto-optical disk 22, a display 23 such as a CRT, and a magnetic disk 24. The CPU 7 performs processing such as Fourier transform, correction coefficient calculation, image reconstruction processing, and control of the sequencer 6 for the input echo signal, and creates an image having a signal intensity distribution of an arbitrary cross section or predetermined processing, It is displayed on the display 23 as a tomographic image. The ROM 20 stores a program for performing image analysis processing and measurement over time, an invariant parameter used for the execution, and the like. The RAM 21 temporarily stores measurement parameters, echo signals detected by the transmission system 4, and necessary images. The magneto-optical disk 22 and the magnetic disk 24 record image data reconstructed by the CPU 7. The display 23 visualizes the image data stored in the magneto-optical disk 22 and the magnetic disk 24 and displays it as a tomographic image. The operation unit 8 is input means for inputting control information of processing executed in the signal processing system 5 and includes, for example, a trackball or a mouse 25 and a keyboard 26.
[0016]
In particular, in order to perform the pre-saturation process according to the features of the present invention, the CPU 7 instructs the sequencer 6 to execute the pre-saturation process before executing normal photographing based on a command input from the operation unit 8. It has become. The sequencer 6 has a function of executing a pulse sequence of presaturation processing in accordance with a command from the CPU 7. Further, the high-frequency oscillator 12 and the modulator 13 of the transmitter 3 generate RF pulses for presaturation.
[0017]
An embodiment of a magnetic resonance imaging method according to the present invention will be described using the magnetic resonance imaging apparatus configured as described above. In FIG. 1, the horizontal axis indicates time, and the vertical axis indicates the intensity of each signal. From the top, the RF pulse, the slice gradient magnetic field Gs, the phase encode gradient magnetic field Gp, the frequency encode gradient magnetic field Gr corresponding to the readout gradient magnetic field, A magnetic resonance signal (echo signal) is shown.
[0018]
First, an RF pulse 101 for presaturation is irradiated while applying a slice gradient magnetic field pulse 111 for selecting a slice part to be presaturated. The slice gradient magnetic field pulse 111 is applied in a predetermined axial direction (in this embodiment, the X-axis direction is the body axis direction of the living body) and intensity. Thereafter, a predetermined gradient magnetic field Gs pulse 112, the same Gp pulse 121, and the same Gr pulse 131 are applied. As a result, the phase of the nuclear spin excited by the RF pulse 101 is diffused so that a magnetic resonance signal is not generated when a pulse sequence 151 at the time of normal imaging described later is executed. The gradient magnetic field Gs pulse 112, the Gp pulse 121, and the Gr pulse 131 are preferably applied to all three axes, but are not necessarily applied to all three axes. In short, it is only necessary that the phase of the nuclear spin excited by the RF pulse 101 is diffused, the phase of the macroscopic magnetic field of the nuclear spin is randomly diffused, and the synthesized component can be reduced. In addition, the intensity of each gradient magnetic field pulse is selected so as to sufficiently diffuse the phase of the nuclear spin.
[0019]
After performing the pre-saturation process in this way, a normal photographing sequence 151 is executed. In the imaging sequence 151 of the present embodiment, an RF pulse 102 is applied to a subject placed in a static magnetic field together with a slice gradient magnetic field Gs pulse 113, followed by a phase rephasing gradient magnetic field Gs pulse 114, and a phase encoding gradient. A magnetic field Gp pulse 122 and a phase diffusion gradient magnetic field Gr pulse 132 in the frequency encoding direction are applied. Thereafter, the readout gradient magnetic field pulse 133 is applied to measure the echo signal 141. The normal imaging sequence is a gradient pulse sequence. However, the present invention is not limited to this, and the effect of presaturation can be obtained even when another known pulse sequence is applied.
[0020]
In this way, by performing the pre-saturation process once, if the echo signal 141 is measured and the image reconstruction process is performed, the image is moved from both sides of the slice portion corresponding to the Gs pulse 113 during normal imaging. Artifacts caused by moving substances such as blood flow flowing into the slice portion can be suppressed, and a clear image can be obtained.
[0021]
This will be described in detail with reference to FIGS. FIG. 3 schematically shows a region imaged by a normal pulse sequence and a region excited by presaturation processing. As shown in the figure, an artery 202 and a vein 203 are present in the trunk 201 of the human body, and they flow in opposite directions in the long axis direction (head-foot direction) of the human body. A tomographic image of the cross section 204 at the position X0 in the X-axis direction is taken on such a trunk 201 of the human body. In this case, in order to prevent the generation of blood flow artifacts from the artery 202, the presaturation process is performed on the region 205 at the position X1 in the X-axis direction, and at the same time, in order to prevent the generation of blood flow artifacts from the veins 203, Presaturation processing is executed on the region 206 at the position X2 in the axial direction.
[0022]
The conditions for presaturating the region 205 at the position X1 and the region 206 at the position X2 in this way will be described in detail with reference to FIG. 4 (A) shows the relationship between the magnetic field strength by the X-axis direction and the slice gradient magnetic field Gs, the relationship between FIG. 4 (B) resonance frequency and the position of the X-axis direction having a gradient in the figure (A) Show. Here, it is assumed that the magnetic field strengths of the Gs pulse 111 in the pre-saturation process related to slice selection and the Gs pulse 113 at the time of imaging are the same. The application of a Gs pulses 111 when irradiating a RF pulse 101 for pre-saturation can gradient of the X-axis direction as shown in FIG. 4 (A). At this time, the magnetic field strengths of the pre-saturation regions 205 and 206 at the positions X1 and X2 are B1 and B2, respectively. The resonance frequencies are f1 and f2. In FIG. 3, X0 indicates a slice part at the time of imaging, B0 is the magnetic field strength of the part, and f0 is the resonance frequency of the part. From these facts, when the Gs pulse 111 is applied, when the presaturation RF pulse 101 having at least two fundamental frequency components of the frequency f1 lower than the fundamental frequency f0 and the frequency f2 higher than the fundamental frequency f0 of the excitation RF pulse 102 is irradiated. It is possible to excite the nuclear spin of the substance existing in the pre-saturation regions 205 and 206 of the two slice portions X1 and X2. The phase of the nuclear spins of the excited pre-saturation regions 205 and 206 is diffused by the subsequently applied gradient magnetic field pulses 112, 121, and 131. Therefore, the magnetic resonance signal formed by synthesizing a plurality of nuclear spins is The signal is sufficiently small, and artifacts due to blood flow can be suppressed. In the above description, two pre-saturation regions are excited at the same time. However, in the same manner, three or more regions can be pre-saturated.
[0023]
Although the example in which the Gs pulse 111 and the Gs pulse 113 are the same has been described above, they need not have the same intensity. When both have different intensities, the relationship between B0, B1, and B2 and f0, f1, and f2 in the above description changes correspondingly, but the same effect as described above can be obtained.
[0024]
Next, the presaturation RF pulse 101 will be described in detail. Normally, the pre-saturation RF pulse 101 and the excitation RF pulse 102 use a harmonic pulse having a waveform obtained by amplitude-modulating a high-frequency wave having fundamental frequencies f0, f1, and f2 corresponding to the slice position by a sinc function or the like. In general, the phase of the waveform of the fundamental frequency is controlled to be constant for each irradiation at the maximum amplitude position of the sinc function. Here, the maximum amplitude position of the sinc function is when t = 0, where the sinc function is a function of time t.
[0025]
However, the RF pulse whose phase is adjusted in this way has a very large sum of amplitudes composed of a plurality of fundamental frequency components, which may exceed the capacity limit of a transmission system such as a high-frequency oscillator. That is, as shown in FIG. 5, when two waves having a fundamental frequency of 1: 2 are added to RF pulses 401 and 402 that have been subjected to amplitude modulation by a sinc function, respectively, the result is as large as an RF pulse 403. It becomes an amplitude waveform. That is, since the RF pulses 401 and 402 have the same phase at t = 0, the RF pulse 403, which is the sum of both, has a particularly large value at t = 0.
[0026]
In order to solve such a problem, as shown in FIG. 6, the phases of a plurality of fundamental wave components of the RF pulse for presaturation may be shifted. In FIG. 6, an RF pulse 501 having a low frequency as a fundamental frequency is the same as the RF pulse 401 in FIG. 5, but an RF pulse 502 having a high frequency as a fundamental frequency is fundamental to the RF pulse 402 in FIG. 5 . The phase of the wave component is shifted by 90 °. As a result, the RF pulse 503, which is the sum of both, has a smaller amplitude than the RF pulse 403 in FIG. Note that the phase of the sinc function that is the amplitude modulation function of the RF pulses 501 and 502 is the same. The phase to be shifted is not necessarily optimal at 90 °, but it is preferable to have an optimal phase difference according to the fundamental frequency.
[0027]
Here, the thickness (width) of the excitation regions of the pre-saturation regions 205 and 206 will be considered. As described above, when an RF pulse for presaturation is irradiated, an RF pulse obtained by amplitude-modulating a high frequency having a fundamental frequency corresponding to the position of the presaturation regions 205 and 206 by a sinc function is used. This is the basic result of the delta function at the fundamental frequency position that is the Fourier transform result of the fundamental frequency in the frequency space (corresponding to the real space) and the convolution of the rectangular shape that is the Fourier transform result of the sinc function. It means to excite a region having a certain width with respect to the frequency. This width depends on the shape of the sinc function, and more specifically on the frequency of the sinc function. Therefore, if the frequency of the sinc function is increased, the excitation width in the frequency space (real space) becomes wider. From these facts, the harmonics having fundamental frequencies corresponding to the plurality of pre-saturation regions (205, 206) are amplitude-modulated with a plurality of sinc functions having different frequencies, so that the excitation widths of the plurality of pre-saturation regions are different. Can be set to a value.
[0028]
By the way, when the waveform data of the sinc function is held in the form of a table, if a sinc function having an arbitrary frequency is used in order to obtain an arbitrary excitation width, a plurality of tables must be prepared, which is not efficient. . In such a case, the same sinc function is used as the amplitude modulation function of each frequency component, and the excitation widths of the plurality of presaturation regions are made the same, thereby eliminating the need to prepare a plurality of sinc function waveform data tables. Thus, the resources of the computer system can be used effectively.
[0029]
In the above-described embodiment, an example in which a plurality of regions are simultaneously processed by one presaturation processing has been described. However, in this case, the case is limited to a case where a plurality of regions are parallel. However, when it is desired to presaturate a plurality of non-parallel regions, the presaturation process may be performed twice or more by changing the direction of the slice gradient magnetic field.
[0030]
In the above-described embodiment, a gradient pulse sequence is shown as a normal imaging sequence. However, the present invention is not limited to this, and a presaturation effect can be obtained even when another known pulse sequence is applied. be able to. For example, the present invention can be similarly applied to a pulse sequence in which a plurality of echo signals are measured by excitation with one excitation RF pulse, such as an EPI sequence.
[0031]
In the above-described embodiment, the case where the presaturation areas are set on both sides of the imaging slice plane has been shown. However, the present invention is not limited to this, and even if the presaturation area is set at an arbitrary position, the presaturation area is not limited to this. An effect can be obtained.
[0032]
【Effect of the invention】
As described above, according to the present invention, there is an effect that magnetic resonance signals in a plurality of parts distant from the imaging part can be suppressed by one presaturation process.
[Brief description of the drawings]
FIG. 1 is a pulse sequence of an embodiment of the magnetic resonance imaging method of the present invention.
FIG. 2 is an overall configuration diagram of a magnetic resonance imaging apparatus to which the magnetic resonance imaging method of the present invention is applied.
FIG. 3 is an operation explanatory view of the magnetic resonance imaging method of the present invention.
FIG. 4 is an operation explanatory view of the magnetic resonance imaging method of the present invention.
FIG. 5 is a waveform diagram of an example of a presaturation RF pulse according to the present invention.
FIG. 6 is a waveform diagram of another example of the RF pulse for presaturation according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Static magnetic field generator 2 Gradient magnetic field generator 3 Transmission system 4 Reception system 5 Signal processing system 6 Sequencer 7 CPU
8 Operation unit 10 Gradient magnetic field coil 15 High frequency irradiation coil 16 High frequency receiving coil

Claims (8)

An imaging sequence for imaging by irradiating a desired region of the subject with a first RF pulse, and a presaturation sequence for irradiating the other region of the subject with a second RF pulse for presaturation A magnetic resonance imaging apparatus having a control means for controlling imaging of the desired region based on a pulse sequence comprising:
2. The magnetic resonance imaging apparatus according to claim 2, wherein the second RF pulse is formed by amplitude-modulating a plurality of excitation pulses having different fundamental frequencies and phases with a sinc function having the same phase . The magnetic resonance imaging apparatus according to claim 1.
The second RF pulse, a magnetic resonance imaging apparatus, characterized in that the high frequency of each of the fundamental frequency is a composite pulse, each composed by combining the excitation pulse amplitude modulation. The magnetic resonance imaging apparatus according to claim 2 .
The plurality of excitation pulses of the second RF pulse are set to frequencies for exciting different regions, respectively . The magnetic resonance imaging apparatus according to claim 3 .
The magnetic resonance imaging apparatus characterized in that the plurality of excitation pulses of the second RF pulse are set to frequencies for exciting the regions on both sides of the desired region . The magnetic resonance imaging apparatus according to claim 3 or 4 ,
Each of the plurality of excitation pulses of the second RF pulse is amplitude-modulated so that the excitation widths are different from each other . The magnetic resonance imaging apparatus according to claim 3 or 4 ,
Each of the plurality of excitation pulses of the second RF pulse is amplitude-modulated so that the excitation width is the same as each other . The magnetic resonance imaging apparatus according to any one of claims 1 to 6 ,
The magnetic resonance imaging apparatus , wherein the pre-saturation sequence applies the second RF pulse a plurality of times . The magnetic resonance imaging apparatus according to any one of claims 1 to 7 ,
A magnetic resonance imaging apparatus characterized in that a phase of the excitation pulse having a higher frequency of the second RF pulses as the fundamental frequency is shifted by 90 ° .

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