JP2008099823A - Magnetic resonance imaging apparatus and its pulse sequence method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the higher harmonic noise of the clock frequency of control signals for generating a gradient magnetic field from being superimposed on the reception band of MR signals. <P>SOLUTION: The clock frequency of the respective control signals Sa and Sb for controlling the generation of the gradient magnetic field and a high frequency magnetic field is set higher than the frequency band of the reception band Rb of the MR signals m, is set to 500 kHz higher than 400 kHz of the reception band Rb of the MR signals m for instance. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus and a pulse sequence method thereof that improve a pulse sequence that generates a static magnetic field and controls the generation of a gradient magnetic field and a high-frequency magnetic field to acquire a magnetic resonance image.

  A magnetic resonance imaging apparatus (hereinafter referred to as MRI) includes a superconducting magnet, a gradient magnetic field coil, and a high-frequency coil (hereinafter referred to as an RF coil), and generates a static magnetic field by the superconducting magnet. The gradient magnetic field generated by the gradient magnetic field coil is superimposed, and the subject in the static magnetic field is irradiated with the RF magnetic field generated by the RF coil, and the MR signal generated in the subject by the irradiation of the RF magnetic field is transmitted by the RF coil. The MR signal is received and processed to obtain an MR image.

  Usually, the frequency of the clock used in MRI is an integer MHz order, for example, 32 MHz or 8 MHz. As shown in FIG. 4, MRI with a static magnetic field strength of 1.5 T has a reception band Ra for receiving MR signals of about 63.86 MHz ± 200 kHz. In MRI using clock frequencies of 32 MHz and 8 MHz, high frequency noises such as twice (32 MHz × 2) and 8 times (8 MHz × 8) of these clock frequencies appear at 64.00 MHz.

  On the other hand, the generation of the gradient magnetic field from the gradient magnetic field coil and the reception of the MR signal from the RF coil are performed in synchronization. However, in general, a clock control signal s for generating a gradient magnetic field from the gradient coil and a control signal i for receiving the MR signal from the RF coil and incrementing the data are synchronized with each other. As shown in FIG. 5, it is transmitted at a repetition rate of 4 us in a frequency range of 200 kHz to 250 kHz, for example, a clock frequency of 250 kHz. For this reason, harmonic noise of the control signal s for generating the gradient magnetic field is superimposed on the MR signal at intervals of 250 kHz. FIG. 4 shows that harmonic noise is superimposed on, for example, 63.50 MHz, 63.75 MHz, 64.00 MHz, and 64.25 MHz on the MR signal.

  On the other hand, the MR signal reception band Ra is set to about 200 kHz, for example. Since the reception band Ra of the MR signal is narrower than the 250 kHz interval where the harmonic noise is superimposed, the harmonics can be adjusted by moving the reception band Ra of the MR signal in the direction of arrow A as shown in FIG. It is possible to prevent the noise and the reception band Ra of the MR signal from overlapping.

  However, in the MRI, as shown in FIG. 6, the MR signal reception band Ra is set to a reception band Rb of, for example, 400 kHz, which is double of 200 kHz, by increasing the MR image acquisition speed. Therefore, the MR signal reception band Rb of 400 kHz is wider than the 250 kHz interval in which harmonic noise is superimposed, and even if the MR signal reception band Rb is moved and finely adjusted, the MR signal reception band Rb becomes the same. Harmonic noise has entered and cannot be avoided. FIG. 6 shows that even if the MR signal reception band Rb is finely adjusted, for example, 63.75 MHz and 64.00 MHz harmonic noise is necessarily superimposed in the MR signal reception band Rb.

  MRI is extremely sensitive to noise in the used frequency band. In MRI, for example, when harmonic noise that is an integral multiple of the clock frequency such as a control signal for generating a gradient magnetic field enters, this harmonic noise appears on the MRI image as F1 noise or white spot artifact. . When the white spot artifact appears on the MRI image, it affects the diagnosis of a doctor or the like. For example, a filter is used to remove the harmonic noise so that the harmonic noise is not superimposed on the MR signal.

For example, Patent Document 1 discloses a technique for removing switching noise generated by a gradient magnetic field power supply. This patent document 1 uses a common operation reference synchronization clock signal between the excitation RF transmitting / receiving means and the gradient magnetic field generation means, and generates a gradient magnetic field using a synchronization clock signal obtained by shifting the operation reference synchronization clock signal into a reconstructed image band. Disclosing operating means.
Japanese Patent Laid-Open No. 10-243934

  An object of the present invention is to provide a magnetic resonance imaging apparatus and its pulse sequence method for preventing harmonic noise of a clock frequency of a control signal for generating a gradient magnetic field from being superimposed on an MR signal reception band. .

  The present invention according to claim 1 generates a static magnetic field, generates a gradient magnetic field and superimposes it on the static magnetic field, irradiates a subject in the static magnetic field with a high-frequency magnetic field, and irradiates the subject by irradiation with the high-frequency magnetic field. In a magnetic resonance imaging apparatus that receives a magnetic resonance signal generated in a specimen and processes a magnetic resonance signal to acquire a magnetic resonance image, the clock frequency of a control signal that controls generation of a gradient magnetic field and a high-frequency magnetic field is set to the magnetic resonance signal. Pulse sequence means for setting higher than the frequency band of the reception band is provided.

  The present invention according to claim 8 generates a static magnetic field, generates a gradient magnetic field and superimposes it on the static magnetic field, irradiates a subject in the static magnetic field with a high-frequency magnetic field, and irradiates the subject by irradiation with the high-frequency magnetic field. In a pulse sequence method of a magnetic resonance imaging apparatus that receives a magnetic resonance signal generated in a specimen and processes the magnetic resonance signal to acquire a magnetic resonance image, a clock frequency of a control signal that controls generation of a gradient magnetic field and a high-frequency magnetic field is set. A magnetic resonance image is acquired by setting it higher than the frequency band of the reception band of the magnetic resonance signal.

  ADVANTAGE OF THE INVENTION According to this invention, the magnetic resonance imaging apparatus and its pulse sequence method which prevent that the harmonic noise of the clock frequency of the control signal for generating a gradient magnetic field is superimposed on the reception band of MR signal can be provided.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration diagram of MRI. A subject 1 such as a patient is placed on the bed 1. The bed 1 is provided so as to be movable in the direction of arrow B.
The superconducting shield body 3 has a structure in which a cavity is provided. The bed 1 is movable in the cavity of the superconducting shield body 3. The superconducting shield body 3 is provided with a superconducting magnet 4, a gradient magnetic field coil 5, and an RF coil 6. A static magnetic field power source 7 is connected to the superconducting magnet 4. The static magnetic field power supply 7 supplies a superconducting current for generating a static magnetic field to the superconducting magnet 4 and generates a static magnetic field having a strength of 1.5 T, for example, from the superconducting magnet 4.

  A gradient magnetic field power supply 8 is connected to the gradient coil 5. The gradient magnetic field power supply 8 supplies a pulse current for generating a gradient magnetic field to the gradient magnetic field coil 5 and generates a gradient magnetic field from the gradient magnetic field coil 5. This gradient magnetic field is superimposed on the static magnetic field generated by the superconducting magnet 4. This gradient magnetic field has an inclination with respect to the directions of the axes x, y, and z orthogonal to each other in the space. For example, the gradient magnetic field has a linear inclination in the axis z direction.

  The RF coil 6 irradiates the subject 2 in the magnetic field in which the gradient magnetic field is superimposed on the static magnetic field, receives the MR signal generated in the subject 2 by the irradiation of the RF magnetic field, and receives the MR signal. Output a signal.

  The pulse sequence control system 9 determines the clock frequencies of the clock signals Ca and Cb for generating the control signals Sa and Sb for controlling the gradient magnetic field generated by the gradient magnetic field coil 5 and the RF magnetic field generated by the RF coil 6, respectively. It is set higher than the frequency band of the reception band of the MR signal m.

  Specifically, the pulse sequence control system 9 includes a sequence control circuit 10, and a clock generation circuit 11 is connected to the sequence control circuit 10. The sequence control circuit 10 is composed of a computer, and executes the sequence program stored in advance to generate the control signals Sa and Sb of the gradient magnetic field and the RF magnetic field to generate the respective clock frequencies of the clock signals Ca and Cb as the MR signal m. It is set higher than the frequency band of the receiving band.

The clock generation circuit 11 generates clock signals Ca and Cb having a clock frequency in the range of 500 kHz to 1000 kHz, for example, a clock frequency of 500 kHz. A gradient magnetic field control circuit 12 and an RF control circuit 13 are connected to the clock generation circuit 11.
The gradient magnetic field control circuit 12 receives a clock signal Ca having a clock frequency of 500 kHz generated by the clock generation circuit 11 and sends a control signal Sa having a frequency synchronized with the clock signal Ca to the gradient magnetic field power supply 8. FIG. 2 shows a clock waveform of the control signal Sa. Thereby, the gradient magnetic field power supply 8 supplies the gradient magnetic field coil 5 with a pulse current for generating a gradient magnetic field synchronized with the control signal Sa.

The RF control circuit 13 receives the clock signal Cb having a clock frequency of 500 kHz generated by the clock generation circuit 11 and sends a control signal Sb having a frequency synchronized with the clock signal Cb to the RF generation circuit 14. FIG. 2 shows a clock waveform of the control signal Sb.
The RF generation circuit 14 receives the control signal Sb sent from the RF control circuit 13 and supplies an RF current to the RF amplifier 15 in synchronization with the clock frequency 500 kHz of the control signal Sb. The RF amplifier 15 amplifies the RF current and supplies it to the RF coil 6.
Note that the RF control circuit 13, the RF generation circuit 14, and the RF amplifier 15 constitute a transmission unit of the pulse sequence control system 9.

  An image processing circuit 19 is connected to the RF coil 6 via a preamplifier 16, a reception band changing circuit 17, and an A / D converter 18. The preamplifier 16, the reception band changing circuit 17, and the A / D converter 18 constitute a receiving unit of the pulse sequence control system 9. The preamplifier 16 amplifies and outputs the MR signal m output from the RF coil 6.

The reception band changing circuit 17 passes the MR signal m amplified and output by the preamplifier 16 through the reception band Rb having a width of 400 kHz, for example, in order to cope with the high speed of MR image acquisition. Send to. The reception band changing circuit 17 enables the reception band Rb to move in the arrow A direction.
The A / D converter 18 performs analog / digital conversion on the MR signal m that has passed through the reception band changing circuit 17 and sends it to the image processing circuit 19.
A frequency divider circuit 20 is connected to the preamplifier 16, the reception band changing circuit 17, and the A / D converter 18. The frequency dividing circuit 20 receives the clock signal Cb having a clock frequency of 500 kHz generated by the clock generating circuit 11, and divides the clock signal Cb into a clock signal Cd having a frequency of 1/2, for example, 1/2. To the preamplifier 16, the reception band changing circuit 17, and the A / D converter 18. As a result, the preamplifier 16, the reception band changing circuit 17, and the A / D converter 18 are configured to increment the MR signal m in synchronization with the clock signal Cd of 250 kHz, for example, as shown in FIG. The MR signal m is amplified at intervals of 250 kHz of the clock signal Cd, passed through the reception band Rb of 400 kHz, and converted from analog to digital.

The image processing circuit 19 processes the MR signal m to obtain an MR image.
A memory device 21, an input device 22 such as a keyboard and a mouse, and a display 23 are connected to the sequence control circuit 10. The sequence control circuit 10 stores the MR image acquired by the image processing circuit 19 in the memory device 21 and displays it on the display 23.

  Here, the pulse sequence control system 9 can acquire an MR image as a sequence for generating a gradient magnetic field and an RF magnetic field by, for example, an imaging method of an echo planar method (EPI). In this echo planar method, for example, when a gradient magnetic field by the gradient magnetic field coil 5 and an RF magnetic field by the RF coil 6 are repeatedly generated in synchronization with the clock frequency of 500 kHz, one generation of the RF magnetic field and gradient magnetic field is generated at each clock frequency of 500 kHz. Sometimes, the gradient magnetic field is continuously reversed at high speed, and the MR signal m from the RF coil 6 is continuously passed through the preamplifier 16, the reception band changing circuit 17 and the A / D converter 18 to the image processing circuit 19. send. Note that the pulse sequence control system 9 is not limited to the echo planar method, and can be applied to, for example, a single shot EPI, a spin echo method (SE), and a field echo method (FE).

Next, an MR image acquisition sequence in the apparatus configured as described above will be described.
The bed 1 is placed with a subject 1 such as a patient, moved in the direction of arrow B, and positioned in the cavity of the superconducting shield body 3.
A superconducting current for generating a static magnetic field is supplied to the superconducting magnet 4 from a static magnetic field power supply 7. Thereby, the superconducting magnet 4 generates a static magnetic field having a strength of 1.5 T, for example.
When a command for MRI operation is issued from the sequence control circuit 10 to the clock generation circuit 11, the clock generation circuit 11 generates clock signals Ca and Cb having a clock frequency of 500 kHz, and supplies the clock signal Ca to the gradient magnetic field control circuit 12. At the same time, the clock signal Cb is sent to the RF control circuit 13.

  The gradient magnetic field control circuit 12 receives a clock signal Ca having a clock frequency of 500 kHz generated by the clock generation circuit 11 and supplies a control signal Sa having a clock waveform as shown in FIG. Send it out. The gradient magnetic field power supply 8 supplies the gradient magnetic field coil 5 with a pulse current for generating a gradient magnetic field synchronized with the control signal Sa. Thereby, the gradient coil 5 generates a gradient magnetic field having a linear gradient in the axis z direction. This gradient magnetic field is superimposed on the static magnetic field generated by the superconducting magnet 4.

  At the same time, the RF control circuit 13 receives a clock signal Cb having a clock frequency of 500 kHz generated by the clock generation circuit 11, and outputs a control signal Sb having a clock waveform frequency in synchronization with the clock signal Cb as shown in FIG. It is sent to the generation circuit 14.

The RF generation circuit 14 receives the control signal Sb sent from the RF control circuit 13 and supplies an RF current to the RF amplifier 15 in synchronization with the clock frequency 500 kHz of the control signal Sb. The RF amplifier 15 amplifies the RF current and supplies it to the RF coil 6. Thereby, the RF coil 6 generates an RF magnetic field in synchronization with the clock frequency of 500 kHz. This RF magnetic field is applied to the subject 2.
Therefore, the subject 2 in the static magnetic field is irradiated with the gradient magnetic field in synchronization with the clock frequency 500 kHz and with the RF magnetic field.

  The RF coil 6 irradiates the subject 2 in the magnetic field in which the gradient magnetic field is superimposed on the static magnetic field, receives the MR signal generated in the subject 2 by the irradiation of the RF magnetic field, and receives the MR signal. Output a signal.

On the other hand, the preamplifier 16, the reception band changing circuit 17 and the A / D converter 18 respectively output the MR signal m in synchronization with the 250 kHz clock signal Cd as shown in FIG. In order to increment the data, the clock signal Cd operates at intervals of 250 kHz. That is, the preamplifier 16 amplifies and outputs the MR signal m output from the RF coil 6. The reception band changing circuit 17 passes the MR signal m amplified and output by the preamplifier 16 through the reception band Rb having a width of 400 kHz, for example, in order to cope with the high-speed MR image acquisition. Send to. The A / D converter 18 performs analog / digital conversion on the MR signal m that has passed through the reception band changing circuit 17 and sends it to the image processing circuit 19. As a result, the image processing circuit 19 processes the MR signal m to obtain an MR image.
The sequence control circuit 10 stores the MR image acquired by the image processing circuit 19 in the memory device 21 and displays it on the display 23.

  Here, the control signal Sa synchronized with the clock signal Ca for generating the gradient magnetic field from the gradient coil 5 and the control signal Sb for receiving the MR signal m from the RF coil 6 and incrementing the data are respectively As shown in FIG. 2, the signals are transmitted at a clock frequency of 500 kHz. For this reason, the harmonic noise of the control signal Sa for generating the gradient magnetic field is superimposed on the MR signal m at intervals of 500 kHz. FIG. 3 shows that harmonic noise is superimposed on the MR signal m at, for example, 63.50 MHz, 64.00 MHz, and 65.00 MHz at intervals of 500 kHz.

On the other hand, the reception band Rb of the MR signal is set to a reception band Rb of 400 kHz, for example, by speeding up MR image acquisition.
Therefore, since the reception band Rb of the MR signal m is narrower than the 500 kHz interval in which the harmonic noise is superimposed as shown in FIG. 3, the MR signal m reception band Rb is moved in the direction of the arrow A, for example, and finely adjusted. Therefore, the harmonic noise and the reception band Rb of the MR signal m do not overlap. The reception band changing circuit 17 can finely adjust the reception band Rb of the MR signal m by moving it in the direction of the arrow A, for example. For example, as shown in FIG. 3, a reception band Rb of 400 kHz can be set between harmonic noises of, for example, 63.50 MHz and 64.00 MHz having an interval of 500 kHz wider than the reception band Rb.

  As described above, according to the above-described embodiment, the clock frequency of each control signal Sa and Sb for controlling the generation of the gradient magnetic field and the high-frequency magnetic field is set higher than the frequency band of the reception band Rb of the MR signal m. Since the receiving band Rb of m is set to 500 kHz which is higher than 400 kHz of the receiving band Rb, the receiving band Rb of 400 kHz set by speeding up of MR image acquisition is, for example, 63.50 MHz having an interval of 500 kHz wider than the receiving band Rb. It can be set between each harmonic noise of 64.00 MHz, and these harmonic noises can be brought to a frequency band outside the reception band Rb. Accordingly, each harmonic noise such as 63.50 MHz or 64.00 MHz is not mixed in the reception band Rb.

  As a result, F1 noise or white spot artifacts due to harmonic noise appearing on the MRI image can be suppressed, and accurate diagnosis can be performed by a doctor or the like. Further, for example, a filter or the like for removing harmonic noise is not necessary.

Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.
For example, in the above embodiment, the clock frequency of each control signal Sa, Sb is set to 500 kHz, which is higher than the frequency band of the reception band Rb of the MR signal m, but any clock within the range of 500 kHz to 1000 kHz. The frequency may be set. In this case, the frequency dividing circuit 20 divides the clock signal Cb having a clock frequency of 500 kHz generated by the clock generating circuit 11 into a clock signal Cd by a division ratio corresponding to an integer, for example, an arbitrary clock frequency. The signal is sent to the preamplifier 16, the reception band changing circuit 17, and the A / D converter 18.

  Further, as shown in FIG. 3, the reception band Rb of 400 kHz is set between the harmonic noises of 63.50 MHz and 64.00 MHz, for example. However, the present invention is not limited to this, for example, 64.00 MHz and 64.00 MHz. You may set between each harmonic noise with 50 MHz.

The block diagram which shows one Embodiment of MRI which concerns on this invention. The figure which shows the timing which carries out data increment of the clock waveform and MR signal of each control signal which controls generation | occurrence | production of the gradient magnetic field and RF magnetic field in the same apparatus. The figure which shows the harmonic noise for every 500 kHz interval superimposed on MR signal in the same apparatus. The figure which shows the timing which carries out data increment of the clock waveform and MR signal of each control signal which control generation | occurrence | production of the gradient magnetic field and RF magnetic field in a conventional apparatus. The figure which shows the timing which carries out data increment of the clock waveform and MR signal of each control signal which control generation | occurrence | production of the gradient magnetic field and RF magnetic field in a conventional apparatus. The figure which shows the receiving band of MR signal according to the acceleration of MR image acquisition in the conventional apparatus.

Explanation of symbols

  1: bed, 2: subject, 3: superconducting shield body, 4: superconducting magnet, 5: gradient coil, 6: RF coil, 7: static magnetic field power supply, 8: gradient magnetic field power supply, 9: pulse sequence control system, 10: Sequence control circuit, 11: Clock generation circuit, 12: Gradient magnetic field control circuit, 13: RF control circuit, 14: RF generation circuit, 15: RF amplifier, 16: Preamplifier, 17: Reception band changing circuit, 18 : A / D converter, 19: Image processing circuit, 20: Frequency dividing circuit, 21: Memory device, 22: Input device, 23: Display.

Claims (11)

A static magnetic field is generated, a gradient magnetic field is generated and superimposed on the static magnetic field, a high-frequency magnetic field is irradiated to the subject in the static magnetic field, and magnetic resonance generated in the subject by the irradiation of the high-frequency magnetic field In a magnetic resonance imaging apparatus for receiving a signal and processing the magnetic resonance signal to obtain a magnetic resonance image,
Pulse sequence means for setting a clock frequency of a control signal for controlling generation of the gradient magnetic field and the high-frequency magnetic field to be higher than a frequency band of a reception band of the magnetic resonance signal;
A magnetic resonance imaging apparatus comprising:   2. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence means sets the clock frequency of the control signal between 500 kHz and 1000 kHz.   The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence unit sets the clock frequency of the control signal to 500 kHz.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence means receives the magnetic resonance signal at a frequency which is a fraction of an integer of the clock frequency. The pulse sequence means includes a transmission unit for transmitting the control signal in which the clock frequency is set to 500 kHz to 1000 kHz,
A receiving unit configured to receive the magnetic resonance signal generated in the subject, wherein the reception band is set to 400 kHz;
The magnetic resonance imaging apparatus according to claim 1, further comprising: The transmitter sends the control signal with the clock frequency set to 500 kHz,
The receiving unit receives the magnetic resonance signal at a frequency of 250 kHz;
The magnetic resonance imaging apparatus according to claim 5.   2. The magnetic resonance imaging according to claim 1, wherein the pulse sequence means acquires the magnetic resonance image of the subject by at least an echo planar imaging technique as a sequence for generating the high-frequency magnetic field and the gradient magnetic field. apparatus. A static magnetic field is generated, a gradient magnetic field is generated and superimposed on the static magnetic field, a high-frequency magnetic field is irradiated to the subject in the static magnetic field, and magnetic resonance generated in the subject by the irradiation of the high-frequency magnetic field In a pulse sequence method of a magnetic resonance imaging apparatus for receiving a signal and processing the magnetic resonance signal to obtain a magnetic resonance image,
A magnetic resonance imaging apparatus for acquiring the magnetic resonance image by setting a clock frequency of a control signal for controlling generation of the gradient magnetic field and the high-frequency magnetic field to be higher than a frequency band of a reception band of the magnetic resonance signal. Pulse sequence method.   9. The pulse sequence method for a magnetic resonance imaging apparatus according to claim 8, wherein the clock frequency of the control signal is set to 500 kHz to 1000 kHz.   9. The pulse sequence method for a magnetic resonance imaging apparatus according to claim 8, wherein the clock frequency of the control signal is set to 500 kHz.   9. The pulse sequence method for a magnetic resonance imaging apparatus according to claim 8, wherein the clock frequency of the control signal is set to 500 kHz and the magnetic resonance signal is received at a frequency of 250 kHz.

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