JP3559364B2 - Control method of MRI apparatus and MRI apparatus - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a control method of an MRI (Magnetic Resonance Imaging) apparatus and an MRI apparatus, and more particularly, to a control method of an MRI apparatus and an MRI apparatus capable of adjusting magnetic field uniformity without requiring an operation by a skilled operator. Equipment related.
[0002]
[Prior art]
FIG. 6 is a perspective view of a magnetic material portion of a conventional magnet assembly 1 '.
In this magnet assembly 1 ', TY is an upper yoke, BY is a lower yoke, and LY and RY are side yokes. On the lower surface of the upper yoke TY and the upper surface of the lower yoke BY, permanent magnets M for generating a static magnetic field are attached to face each other. Further, a heater h is attached to each yoke. The positional relationship between the yokes can be adjusted with the adjusting screw b.
[0003]
FIG. 7 is a perspective view of an enclosure and a cover that cover the magnet assembly 1 ′.
ET is an enclosure that covers the upper yoke TY, EB is an enclosure that covers the lower yoke BY, EL is an enclosure that covers the side yoke LY, and ER is an enclosure that covers the side yoke RY. Insulation is filled between the yoke and the enclosure that covers it. The adjustment screw b can be adjusted from outside the enclosure ET. The enclosure ET is protected by a cover c. Although not shown, the enclosures EB, EL, and ER are also protected by covers.
[0004]
FIG. 8 is a flowchart showing a procedure for adjusting the uniformity of the magnetic field.
In step T1, the heater h is energized with the cover c removed.
In step T2, the process waits until the temperature of the yoke of the magnet assembly 1 'is stabilized (for example, 8 hours).
In steps T3 to T5, the non-uniformity of the magnetic field is measured (for example, measured using a mechanical jig), and the positional relationship between the yokes is adjusted with the adjusting screw b so as to reduce the non-uniformity of the magnetic field. Is repeated to keep the magnetic field inhomogeneity within an allowable value.
In Step T6, the cover c is attached.
[0005]
JP-A-63-43649 and JP-A-63-278310 disclose techniques for stabilizing the temperature of a yoke of a magnet assembly.
[0006]
[Problems to be solved by the invention]
In the above-mentioned conventional technology, the yoke of the magnet assembly 1 'is heated by a heater to stabilize the yoke of the magnet assembly 1' at a constant temperature independent of the external temperature so as not to affect the non-uniformity of the magnetic field. The positional relationship between the yokes is adjusted to keep the magnetic field inhomogeneity within an allowable value.
However, skill is required to keep the magnetic field non-uniformity within an allowable value by a mechanical operation of adjusting the adjusting screw b. That is, the conventional MRI apparatus has a problem that a skilled operator is required to adjust the magnetic field uniformity.
Therefore, an object of the present invention is to provide a control method of an MRI apparatus and an MRI apparatus that can adjust the magnetic field uniformity without requiring an operation by a skilled worker.
[0007]
[Means for Solving the Problems]
In a first aspect, the present invention relates to a method for controlling an MRI apparatus including a magnet assembly for forming a magnetic field by using a permanent magnet, wherein the temperature distribution of a magnetic material constituting the magnet assembly is controlled so as to make the magnetic field uniform. And a method for controlling the MRI apparatus, characterized in that the control is performed unevenly.
In the past, in order to prevent the magnetic field from changing due to the temperature change of the magnetic material, passive control was performed to stabilize the temperature of the magnetic material at a constant temperature by making the magnet assembly a so-called constant temperature bath. . On the other hand, in the present invention, the fact that the magnetic field changes due to the temperature change of the magnetic material is positively utilized, and the temperature distribution of the magnetic material is non-uniformly controlled so as to make the magnetic field uniform. According to this, since the magnetic field uniformity can be electrically adjusted, the operation by a skilled operator is not required. It is also possible to adjust the magnetic field uniformity by remote communication.
[0008]
According to a second aspect, the present invention provides an MRI apparatus including a magnet assembly that forms a magnetic field with a permanent magnet, a plurality of heating means for heating different portions of a magnetic material forming the magnet assembly, An MRI apparatus comprising: a control unit that drives the plurality of heating units to control the temperature distribution of the magnetic material to be non-uniform so as to make the magnetic material uniform.
According to this MRI apparatus, different portions of the magnetic material constituting the magnet assembly can be separately heated, so that the control method according to the first aspect can be suitably implemented.
[0009]
According to a third aspect of the present invention, in the MRI apparatus having the above-described configuration, a heat radiating unit for suppressing heat transfer between different portions of the magnetic material forming the magnet assembly is provided. I will provide a.
According to this MRI apparatus, different portions of the magnetic material constituting the magnet assembly can be thermally separated, so that the control method according to the first aspect can be suitably implemented.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described based on an embodiment shown in the drawings. It should be noted that the present invention is not limited by this.
FIG. 1 is a block diagram showing one embodiment of the MRI apparatus of the present invention.
In the MRI apparatus 100, the magnet assembly 1 has a space portion (hole) for inserting a subject therein, and a permanent magnet that applies a constant static magnetic field to the subject so as to surround the space portion. M, a gradient magnetic field coil 1g for generating a gradient magnetic field pulse on the slice selection axis, the readout axis, and the phase encoding axis, a transmission coil 1t for supplying an RF pulse for exciting spins of nuclei in the subject, A receiving coil 1r for detecting an NMR signal from a sample, temperature sensors s1 to s20, and heaters h1 to h16 are arranged. The gradient magnetic field coil 1g, the transmission coil 1t and the reception coil 1r are connected to a gradient magnetic field drive circuit 3, an RF power amplifier 4 and a preamplifier 5, respectively.
[0011]
The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 based on the stored pulse sequence in accordance with a command from the computer 7 to generate a gradient magnetic field pulse from the gradient magnetic field coil 1g of the magnet assembly 1 and to perform gate control. The modulation circuit 9 is operated to modulate the carrier wave output signal of the RF oscillation circuit 10 into a pulse signal having a predetermined timing and a predetermined envelope shape, which is applied to the RF power amplifier 4 as an RF pulse, and the RF power amplifier 4 amplifies the power. After that, the voltage is applied to the transmission coil 1t of the magnet assembly 1 to selectively excite a target slice area.
[0012]
The preamplifier 5 amplifies the NMR signal from the subject detected by the receiving coil 1 r of the magnet assembly 1 and inputs the amplified NMR signal to the phase detector 12. The phase detector 12 uses the carrier output signal of the RF oscillation circuit 10 as a reference signal, performs phase detection on the NMR signal from the preamplifier 5, and supplies the NMR signal to the A / D converter 11. The A / D converter 11 converts the analog signal after the phase detection into a digital signal and inputs the digital signal to the computer 7.
The computer 7 reads the MR data from the A / D converter 11, performs an image reconstruction operation, and generates an image of a target slice area. This image is displayed by the display device 6. Further, the computer 7 is responsible for overall control such as receiving information input from the console 13.
[0013]
Further, the computer 7 measures the distribution of the magnetic field inhomogeneity, and gives the distribution of the magnetic field inhomogeneity to the magnetic field uniformity controller 14.
The temperature distribution calculator 14a of the magnetic field uniformity controller 14 calculates the temperature distribution of each part of the magnet assembly 1 that reduces the magnetic field non-uniformity based on the magnetic field non-uniformity distribution provided from the computer 7. I do. Then, the heater driving unit 14b of the magnetic field uniformity control unit 14 drives the heaters h1 to h16 while monitoring the temperature of each unit of the magnet assembly 1 with the temperature sensors s1 to s20, thereby realizing the temperature distribution.
[0014]
FIG. 2 is a perspective view of a magnetic material portion of the magnet assembly 1.
In the magnet assembly 1, TY is an upper yoke, BY is a lower yoke, and LY and RY are side yokes. On the lower surface of the upper yoke TY and the upper surface of the lower yoke BY, permanent magnets M for generating a static magnetic field are attached to face each other. Each yoke is thermally divided into four parts by a heat dissipation block q made of, for example, aluminum. Heaters h1 to h16 (partially omitted) are attached to each of the four divided parts. Furthermore, temperature sensors s1 to s20 (partially omitted) are attached to the four corners and the center of each yoke.
Although not shown, each yoke is surrounded by an enclosure and is thermally isolated from the outside by a heat insulating material.
[0015]
FIG. 3 is a flowchart showing a procedure for adjusting the uniformity of the magnetic field.
In step V1, the magnetic field inhomogeneity is measured. The measurement of the magnetic field inhomogeneity may be performed using a mechanical jig in the same manner as in the related art. Is preferable.
In step V2, the temperature distribution of each part of the magnet assembly 1 that reduces the magnetic field non-uniformity is calculated based on the magnetic field non-uniformity distribution. This temperature distribution generally makes the temperature near the portion where the magnetic field strength is higher than the reference value relatively high, and makes the temperature near the portion where the magnetic field strength is lower than the reference value relatively low.
In step V3, the heaters h1 to h16 are driven while monitoring the temperature of each part of the magnet assembly 1 with the temperature sensors s1 to s20 so as to approximate the temperature distribution.
By repeating the above process several times, the magnetic field inhomogeneity can be kept within an allowable value.
[0016]
FIG. 4 is a flowchart of the magnetic field inhomogeneity measurement process.
In Step P1, the phantom is photographed by CSI (Chemical Shift Imaging) using the pulse sequence PS shown in FIG. In the pulse sequence PS of FIG. 5, a 90 ° excitation pulse and a slice selection gradient ss are applied to selectively excite a slice at a predetermined position. Next, the phase encode gradients px and py are applied. Next, a 180 ° inversion pulse is applied. Then, the focused echo E is sampled to obtain MRS (Magnetic Resonance Spectroscopy) data.
In Step P2, the frequency spectrum of each voxel is obtained based on the acquired MRS data.
In step P3, the peak frequency of the frequency spectrum of each voxel is determined, and one voxel is set as a reference voxel, and the amount of frequency deviation from the peak frequency of the reference voxel is determined.
In Step P4, a distribution of the magnetic field inhomogeneity is obtained based on the frequency shift amount of each voxel. Assuming that the peak frequency of the reference voxel is f0, the frequency deviation is Δfi, and the static magnetic field strength is H, the magnetic field inhomogeneity ΔHi is
ΔHi = H · Δfi / f0
Required by
By changing the slice position and performing the above-described magnetic field inhomogeneity measurement processing several times, the magnetic inhomogeneity in the required space can be measured.
[0017]
According to the above-described MRI apparatus 100, since the magnetic field uniformity can be electrically adjusted, the operation by a skilled operator is not required. It is also possible to adjust the magnetic field uniformity by remote communication.
[0018]
As another embodiment, a pulse sequence for MRI imaging or a so-called Dixon sequence (excitation RF pulse is applied, then a readout gradient pulse is applied, then an inverted RF pulse is applied, and then the inverted RF pulse is applied. (A pulse sequence in which sampling is performed while applying a readout gradient pulse so as to acquire MR data so that the echo intensity reaches a peak at a time when the time interval of the excitation RF pulse does not coincide with the time interval of the inversion RF pulse). The non-uniform distribution of the magnetic field may be measured.
[0019]
As yet another embodiment, the gradient coil may be energized to generate heat and heat the magnetic material constituting the magnet assembly.
[0020]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the control method of an MRI apparatus and the MRI apparatus of this invention, since the magnetic field uniformity can be adjusted electrically, the operation | work by a skilled worker is unnecessary. It is also possible to adjust the magnetic field uniformity by remote communication.
[Brief description of the drawings]
FIG. 1 is a block diagram showing one embodiment of an MRI apparatus of the present invention.
FIG. 2 is a perspective view of a magnetic material portion of a magnet assembly of the MRI apparatus of FIG.
FIG. 3 is a flowchart showing a procedure for adjusting magnetic field uniformity according to the present invention.
FIG. 4 is a flowchart of a magnetic field inhomogeneity measurement operation.
FIG. 5 is a pulse sequence diagram used for measuring magnetic field inhomogeneity.
FIG. 6 is a perspective view of a magnetic material portion of a conventional magnet assembly.
FIG. 7 is a perspective view of an enclosure and a cover of the magnet assembly of FIG. 6;
FIG. 8 is a flowchart showing a conventional procedure for adjusting magnetic field uniformity.
[Explanation of symbols]
100 MRI apparatus 1 Magnet assembly 14 Magnetic field uniformity controller 14a Temperature distribution calculator 14b Heater drivers h1 to h16 Heaters s1 to s20 Temperature sensor TY Upper yoke BY Lower yoke LY, RY Side yoke M Permanent magnet b Adjustment screw c Cover

Claims (3)

A method for controlling an MRI apparatus including a magnet assembly for forming a magnetic field by a permanent magnet, comprising:
A method for controlling an MRI apparatus, comprising: controlling a temperature distribution of a magnetic material constituting a magnet assembly to be non-uniform so as to make a magnetic field uniform. In an MRI apparatus including a magnet assembly that forms a magnetic field with a permanent magnet,
A plurality of heating means for heating different portions of the magnetic material constituting the magnet assembly, and the plurality of heating means are driven to non-uniformly control the temperature distribution of the magnetic material so as to make the magnetic field uniform. An MRI apparatus comprising control means. 3. The MRI apparatus according to claim 2, further comprising a radiator for suppressing heat transfer between different portions of the magnetic material forming the magnet assembly.

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