JPH0956691A - Method of regulating magnetic field - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of returning the magnetic field uniformity coefficient of a MRI magnetic circuit disturbed by the application of a gradient magnetic field to the state just after magnetic field regulation with a simple method. SOLUTION: In this method, a pair of disc-like or ring-like permanent magnets are manufactured so that the magnetizing direction is conformed to the disc or ring thickness direction, the permanent magnets are arranged opposite to each other within a relay, magnetic shunt plates 1 are fitted on the respective magnetic pole surfaces of the permanent magnets, so that a magnetic field is generated in the space between the permanent magnets to perform the magnetic field regulation of a MRI magnetic circuit. In this case, after the magnetic shunt plates 1 are demagnetized by use of an attenuated AC magnetic field having a maximum magnetic field larger than the maximum value of the magnetic field used in imaging, the magnetic field regulation is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a permanent magnet type MRI apparatus, and more particularly to a magnetic field adjusting method for a magnetic circuit for permanent magnet type MRI.

[0002]

2. Description of the Related Art A permanent magnet type magnetic circuit for MRI is mainly a magnet facing type using a rare earth magnet. However, in this magnet facing type, in order to generate a uniform magnetic field, a magnetizing plate is provided on the magnet surface. A magnetic pole piece made of a soft magnetic material such as soft iron is provided. In MRI, a gradient magnetic field is generated in order to obtain an image. At that time, a magnetic field is applied to the shunt plate to cause residual magnetization, and the residual magnetization disturbs a uniform magnetic field.
There is a problem that the MRI image is blurred or distorted. The cause of residual magnetization in the magnetic shunt is due to the hysteresis phenomenon of the soft magnetic material used in the magnetic shunt, and is unavoidable as long as a magnetic material is used. The hysteresis phenomenon depends on what kind of magnetic field the magnetic body has received before, that is, the history, but the history becomes quite complicated in the rectifying plate of the magnetic circuit for permanent magnet type MRI.

In the magnetic circuit for permanent magnet type MRI,
Generally, as shown in FIG. 1, the magnetic shunt plate 1 is the last to be incorporated in the magnetic circuit 3 incorporating the magnetized magnet 2. Therefore, the magnetic shunt plate moves in the magnetic circuit when it is assembled, and each part of the magnetic shunt plate receives different magnetic fields. Generally, the magnetic shunt plate has a symmetric shape such as a disc shape, but it is possible that parts at symmetrical positions do not necessarily have the same history. For example, in the method of assembling the magnetic shunt plate in FIG. 1, the end 11 of the magnetic shunt plate 1 passes through the magnetic circuit and receives a complicated history. On the other hand, the end 12 hardly passes through the magnetic circuit.

Further, in MRI, a gradient magnetic field is generated to obtain an image, but the magnetic field is applied to the magnetic shunt plate at that time. This magnetic field causes remanent magnetization. The remanent magnetization also differs depending on the history of each position of the magnetic shunt plate, and the way of generating remanent magnetization also differs depending on each position. By receiving this gradient magnetic field, the history is further accumulated, and the gradient magnetic field is also given with various intensities and angles (x, y, z or compound directions) by the method of imaging. , Its accumulated history becomes very complicated.

[0005]

Generally, a permanent magnet type M is used.
The magnetic field adjustment of the RI magnetic circuit is performed after the magnetic shunt plate is incorporated, but as described above, since the history of each part of the magnetic shunt plate is different, even if a high degree of uniformity is realized in this state,
After that, when the magnetic field is actually used for imaging and a gradient magnetic field is applied, different residual magnetizations are generated. Therefore, the homogeneity is not only disturbed by the residual magnetization, but also the homogeneity is disturbed by having different histories depending on the position of the magnetic shunt plate. This is a serious problem for a magnetic circuit for MRI which requires the highest possible uniformity.

[0006]

The present invention solves the above-mentioned drawbacks and provides a magnetic field adjusting method for a magnetic circuit for a permanent magnet type MRI in which disturbance of the magnetic field homogeneity is less likely to occur due to the difference in the history of each part of the magnetic shunt plate. It is provided.

That is, according to the present invention, a pair of disc-shaped or ring-shaped permanent magnets are manufactured so that the magnetization direction is the thickness direction of the discs or rings, and the pair of permanent magnets are opposed to each other inside the yoke. In the magnetic field adjusting method of the magnetic circuit for MRI in which a magnetic shunt plate is provided on the magnetic pole surface of each of the pair of permanent magnets to generate a magnetic field in the space between the pair of permanent magnets, the maximum value of the magnetic field used for imaging is provided. The gist of a magnetic field adjusting method is characterized in that the magnetic field is adjusted after the degaussing plate is demagnetized by using the attenuated alternating magnetic field having the above-mentioned maximum magnetic field. This will be described in more detail below.

In order to eliminate the disturbance of the magnetic field homogeneity due to the difference in the residual magnetization generation in each part due to the different history of each part of the magnetic shunt plate, the following method can be considered. First, the history of each part of the magnetic shunt is made the same, but it is impossible to actually perform it because the size of the magnetic shunt generally exceeds φ1000 mm. As a next conceivable method, even if residual magnetization occurs due to the application of a gradient magnetic field, it is eliminated to restore the high homogeneity immediately after the magnetic field adjustment. The present inventors have paid attention to this point, and have invented the magnetic field adjusting method of the present invention that restores the disturbance of the homogeneity due to the residual magnetization of the permanent magnet type MRI.

At each position in the magnetic shunt plate, a residual magnetic field is generated according to the history of the gradient magnetic field. The concept will be described below with reference to FIG. In the process of assembling the shunt plate according to FIG. 1, from the magnetic field 0 to the end 11, the magnetic field is 0 to H1 to H2 to H1 to H3,
The magnetization changes from 0 to A to B to C to E. Also the end 1
2, the magnetic field changes from 0 to H1 to H3, and the magnetization changes from 0 to A to D. Here, when the magnetic field is changed between H1 and H3, the values of the remanent magnetization differ even though the positions of the end portions 11 and 12 are symmetrical positions as the magnetic shunt plate.

The value of the remanent magnetization thus generated depends on the magnetic field strength received at each position of the magnetic shunt plate, and its value is shown in FIG.
As shown in (a), when a gradient magnetic field is applied,
It changes within the range of the maximum value of the magnetic field strength received by the gradient magnetic field and forms a smaller minor loop in the minor loop of the maximum value.

On the other hand, as a degaussing method for a soft magnetic material such as soft iron used for a magnetic shunt plate, there is a method called an AC degaussing method. This is that the soft magnetic material can be demagnetized by placing it in an attenuating AC magnetic field. Of course, it is impossible to place the permanent magnet type magnetic circuit for MRI in an alternating magnetic field. However, the residual magnetization that demagnetizes the non-uniformity is
As described above, it depends on the applied gradient magnetic field and changes within the range of the maximum value or less. As a result, if an AC magnetic field that attenuates is applied using the same method as that for applying a gradient magnetic field, the magnetization in the shunt plate will settle at a point with a minor loop in the hysteresis loop of FIG. 3B. Become. That is, if the maximum strength of the damping AC magnetic field is set to a value equal to or larger than the maximum value of the gradient magnetic field used, even if residual magnetization occurs due to the gradient magnetic field, the damping AC magnetic field should be given (hereinafter,
It is possible to return the distribution of magnetization in the magnetic shunt plate to a certain constant value by the demagnetization operation).

However, even if the magnetization distribution of a certain magnetic shunt plate can be returned to a constant value, the magnetic field homogeneity may be disturbed. That is, this is a case where the constant value of the magnetization distribution obtained by the demagnetization operation is different from the magnetization distribution of the shunt plate when the magnetic field adjustment is performed. Therefore, by performing the demagnetization operation before adjusting the magnetic field, the magnetization distribution in the shunt plate is initialized and can be repaired. By the demagnetization operation, it is possible to restore the homogeneity immediately after the magnetic field adjustment.

The frequency of the damping AC magnetic field is 100
Hz or less is desirable. This is because due to the skin effect, the lower the frequency, the higher the demagnetization effect because the attenuated AC magnetic field penetrates deeper into the magnetic shunt plate. Further, although the damping method is not specified, it is preferable that there be a sufficient period of about 10 cycles until the magnetic field strength is reduced to 0.

As described above, according to the present invention, even if the residual magnetization is generated by applying the gradient magnetic field, the disturbance of the magnetic field homogeneity is eliminated by the demagnetization operation, and the high magnetic field homogeneity immediately after the magnetic field adjustment is performed. Can be returned to.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described with reference to Examples and Comparative Examples. Example A magnetic circuit for permanent magnet type MRI was assembled, and after demagnetization operation, magnetic field uniformity was measured immediately after magnetic field adjustment. Next, a current of 90 A is applied to the magnetic field coil in this magnetic circuit to apply a magnetic field gradient, and then the magnetic field homogeneity is measured again. It was measured. The magnetic circuit used here has a magnetic field strength of 0.1 T and a uniform space of φ350.
× 250 It is an ellipsoidal sphere. Table 1 shows the measurement results of the magnetic field homogeneity.

[0016]

[Table 1]

The current waveform used for the demagnetization operation is shown in FIG. As a specification of this MRI apparatus, a maximum current of 90 A is to be passed through the gradient magnetic field coil. Therefore, a damping AC magnetic field having a maximum current value of 90 A was applied to perform demagnetization. The expression of the attenuated AC current is expressed by the following expression (1). However, t is 10 msec in time unit. I (t) = 9 (11-t / 60) sin {2π (t / 60-10)} ‥‥‥ (1)

Comparative Example For comparison, the same conditions as in Example 1 were used except that the first demagnetization operation was not performed. The results are shown in Table 1.

[0019]

According to the present invention, in the permanent magnet type MRI apparatus, even if residual magnetism is generated in the magnetic shunt plate and uniformity is deteriorated,
By performing the demagnetization operation without adjusting the magnetic field using the magnetic material again, the history given within the magnetic field strength range of the gradient magnetic field can be erased and restored, and the homogeneity can be returned to the state immediately after the magnetic field adjustment. It has become possible. This demagnetization operation has the same effect as the magnetic field adjustment, and the magnetic field adjustment that was conventionally carried out by dispatching service personnel can be performed only by the device built into the system and by the user's hand, thus improving serviceability and cost reduction. It is connected and its utility value is extremely high.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a method of incorporating a magnetic shunt plate in a magnetic circuit.

FIG. 2 is a conceptual diagram of a difference in residual magnetization due to a difference in history.

FIG. 3 is a minor loop when a gradient magnetic field and an alternating magnetic field are applied. (A) When a gradient magnetic field is applied. (B) When a damped AC magnetic field is applied.

FIG. 4 is a waveform of a damped AC magnetic field used in an example of the present invention.

[Explanation of symbols]

 1 ・ ・ ・ Magnifier plate 11 ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥

Claims (2)

[Claims] 1. A pair of disc-shaped or ring-shaped permanent magnets are manufactured so that the magnetization direction is the thickness direction of the discs or rings, and the pair of permanent magnets are provided inside the yoke so as to face each other. In the magnetic field adjusting method of the magnetic circuit for MRI in which a magnetic compensating plate is provided on each magnetic pole surface of the pair of permanent magnets to generate a magnetic field in the space between the pair of permanent magnets, the maximum value of the magnetic field used for imaging is not less than the maximum value. A magnetic field adjusting method characterized by performing magnetic field adjustment after demagnetizing a magnetic shunt plate using an attenuated alternating magnetic field having a magnetic field. 2. The magnetic field adjusting method according to claim 1, wherein the damping AC magnetic field is generated by a gradient magnetic field coil.