JPH03297445A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To prevent the occurrence of nonconformity in a uniformity correcting means and to keep the uniformity correction for a static magnetic field from being readjusted by providing a uniformity correcting means comprising a female screw portion formed by a magnet device and an iron-containing screw member screw-engaged therewith. CONSTITUTION:A uniformity correcting means 18 for correcting the uniformity of static magnetic field in a superconductive state comprises a female screw portion 18a formed on a superconductive magnet device 17 and a bolt 18b as an iron-containing screw member screw-engaged with the female screw portion 18a. In the above arrangement, the bolt 18b is screw engaged with the female screw portion 18a formed on the superconductive magnet device 17, whereby the installation can be accomplished more stably than the case of fitting an iron shim with an adhesive agent. Accordingly, the nonconformity of the uniform ity correcting means 18 can be prevented, and the uniformity correction for static magnetic field is not be readjusted.

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention relates to magnetic resonance (MR)
The present invention relates to a magnetic resonance imaging apparatus that obtains an MR image of a subject using the MR (nance) phenomenon.

(Prior art) Magnetic resonance imaging device (hereinafter referred to as MRI device)
The method applies a -like static magnetic field to a desired part of the subject, and uses a transmitter coil to form an RF (radio frequency) magnetic field perpendicular to this static magnetic field, thereby producing an MR phenomenon only in a specific slice part from which a tomographic image is obtained. Furthermore, after the RF magnetic field is removed, the MR multiplied signal generated from the nucleus is detected by a receiving coil, and the inclination is linear with respect to the X' axis direction (coordinate system rotated by θ0 from the X axis). A composite MR multiplied signal is obtained by applying a magnetic field to a static magnetic field, and based on this signal, M
An R image can be formed.

Incidentally, in such an MRI apparatus, the uniformity of the static magnetic field must satisfy, for example, 25 PPM or less within the imaging region. Therefore, this adjustment method involves determining the non-uniformity of the magnetic field using a method called the 7-plane method or the 12-plane method, and determining the location where the uniformity correction means should be installed inside the magnet device corresponding to the non-uniform location.
Attach iron shims as a uniformity correction means with adhesive, etc.
The uniformity of the static magnetic field is corrected by locally changing the magnetic field strength.

(Problem to be Solved by the Invention) However, in the conventional MRI apparatus as described above, the iron shim (uniformity correction means)
In some cases, problems such as peeling of the magnetic field may occur, resulting in the problem that the uniformity correction of the static magnetic field must be readjusted.

Therefore, the present invention has been made in view of the above problems, and
It is an object of the present invention to provide an MRI apparatus that does not cause a malfunction in the uniformity correction means and does not require readjustment of uniformity correction of a static magnetic field.

[Configuration of the Invention (Means for Solving the Problem) The configuration of the present invention for achieving the above object includes a magnet device that forms a static magnetic field, and a method that corrects the uniformity of the static magnetic field formed by the magnet device. A magnetic resonance imaging apparatus comprising a uniformity correction means for applying a high frequency pulse to a subject placed in the static magnetic field and collecting magnetic resonance signals from the subject to obtain a magnetic resonance image. The present invention is characterized in that the uniformity correcting means is constituted by a female screw portion formed in the magnet device and an iron-containing screw member screwed into the female screw portion.

(Function) By screwing into the iron-containing screw member or the female threaded portion formed on the magnet device, more stable installation can be achieved than when conventional iron shims are installed with adhesive or the like.

(Example) Hereinafter, the present invention will be specifically explained with reference to Examples. Note that a case where a superconducting magnet device is used as the magnet device will be exemplified.

FIG. 1 is a block diagram showing an embodiment of the MHI apparatus according to the present invention, and FIGS. 2 and 3 are side views and schematic front views for explaining a magnet device of an embodiment of the MRI apparatus according to the present invention. The sectional views in FIGS. 4(a) to 4(C) are diagrams for explaining the selection of bolts and the positional relationship of the internal threads, respectively, and FIG.
a) and (b) are diagrams each for explaining the screwing depth of a bolt.

In FIG. 1, the high frequency oscillator 6 generates a high frequency (RF) signal under the control of the system controller 5.
The oscillation output of this high frequency oscillator 6 is transmitted to the RF transmitting coil 1 via a gate circuit 7 and a power amplifier 8 arranged at a later stage.
5. RF receiving coil 14
is for detecting the MR multiplied signal from the subject, and the MR multiplied signal preamplifier 1 detected by this RF receiving coil 14
3 to the phase detection circuit 12, where the phase is detected. This phase detection output is then taken into the data processing computer 2 via a waveform memory 11 arranged at a subsequent stage. This data processing computer 2 functionally includes frequency correction control means 3 and image reconstruction means 4.

The frequency correction control means 3 corrects the carrier frequency of the high-frequency pulse according to fluctuations in the static magnetic field strength. For example, the MR multiplied signal of the first subject (patient, etc.) is Fourier-transformed, its peak detected, and the reference The oscillation frequency of the high frequency oscillator 6 under the control of the system controller 5 is adjusted according to the comparison result.

Further, the image reconstruction means 4 reconstructs an MR image from the MR information sent out from the waveform memory 11.
The R image is sent to a display device 1, where it is visualized.

Furthermore, the gradient magnetic field coil 16 generates a gradient magnetic field to be superimposed on the static magnetic field, and has three types: one for generating a slice direction gradient magnetic field G8, one for generating an encoding direction gradient magnetic field G), one for generating a readout direction gradient magnetic field GR. It has a coil. The gradient magnetic field power supply 9 supplies current to the gradient magnetic field coil 16 and is under the control of the system controller 5.

The superconducting magnet device 17 generates a static magnetic field in a superconducting state, and includes a static magnetic field coil 17a formed of a superconductor,
The coil 17a is provided with a cooling means for cooling the coil 17a.

The static magnetic field power supply 10 supplies current to the static magnetic field coil before the permanent mode.

Further, the static magnetic field coil 17a is cooled using liquid helium, liquid nitrogen, or the like.

The uniformity correcting means 18 corrects the uniformity of the static magnetic field in a superconducting state, and includes a female threaded portion 18a formed in the superconducting magnet device 17, and a bolt (as an iron-containing threaded member) screwed into the female threaded portion 18a. (hereinafter simply referred to as bolt)1
8b (Fig. 5(a), (b)
)reference).

Further, the female threaded portion 18a of the uniformity correction means 18 is
For example, as shown in FIG. 3, every 50 holes in the arrow Z-axis direction (the direction of the static magnetic field formed in the imaging hole 20 of the superconducting magnet device 17 formed in a substantially cylindrical shape), as shown in FIG. They are formed on the inner wall of the magnet device 17 at intervals of 10° in the direction of the arrow θ (the outer peripheral direction of the superconducting magnet device 17 formed in a substantially cylindrical shape). Note that 17b in FIGS. 2 and 3 is a vacuum layer provided to maintain a good superconducting state.

Further, among the uniformity correcting means 18, the bolt 18b
For example, the following conditions are formed.

■ Regarding iron content, 0, 5.1.3.5.10.15.20.30. The weight (g) of each is required, but as for the iron content, it is better to have as many types as possible in the widest possible range.

■ The shape of the bolt should be such that the head of the bolt occupies a larger proportion than the shaft.

This is because when working in a magnetic field, a force acts toward the long axis of the magnetic material.

■ Regarding the material and classification of bolts ■ Assuming that the material is iron, the size of the head of the bolt determines the iron content of the entire bolt.

■Assuming that the material is iron, the thickness of the bolt head determines the iron content of the entire bolt.

■Determine the iron content by placing different amounts of iron pieces into a synthetic resin bolt of a fixed size and shape.

■Make bolts from an alloy of non-magnetic materials and iron materials, and determine the iron content by changing the content.

Next, FIGS. 4(a) to 5(C) and 5(a) show adjustment work for uniformity correction of the static magnetic field using the uniformity correcting means 18 having the female threaded portion 18a and bolt 18b configured as described above. ) and (b).

First, the distribution of the strength of the magnetic field is determined, and the steps of determining the location where adjustment is required and the location where the uniformity correcting means 18 is to be installed are carried out in the same manner as conventional methods using the 7-plane method, the 12-plane method, or the like.

Then, instead of fixing the conventional iron shim with double-sided tape or adhesive, the bolt 18b with an appropriate iron content is screwed into the female thread 18a at or near the installation location. In this way, the female thread N 18 a and the bolt 18 b are determined,
If further fine adjustment is required, as shown in Figure 5, adjust the screwing depth of the bolt 18b to d□ (see figure (a)).
, d2 (see figure (b)).

If the bolt is installed between two female threads, proceed as follows. For convenience, they are expressed as female thread part A and female thread part B.

■ When installing the bolt 18b, if the position (the same applies below 2 indicated by
Screw b into both ridge threads A and B (see figure (a)).

■For bolt installation, position X is 1 of female thread part A and female thread part B.
: If the bolt 18b is located at a distance of 2, screw the bolt 18b containing 2/3 of the weight of the iron to be installed into the female thread A, and then screw the bolt 18b containing 1/3 of the weight of the iron to be installed into the female thread A. Screw into female thread part B (see figure (b))
.

■In general, when installing the bolt 18b, if the position A bolt 18b is screwed into the female threaded part A and has a content of m/(m10n) of the weight of the iron to be attached, and is screwed into the female threaded part B (see figure (c)).

As mentioned above, the uniformity correction can be adjusted with a considerable degree of freedom depending on both the iron content and the screwing condition of the bolt.

As detailed above, according to the above embodiment, the bolt 18b
However, by screwing together with the female threaded portion 18a formed on the superconducting magnet device 17, it is possible to attach the iron shim more stably than when attaching with an adhesive or the like in the past, so that a malfunction of the uniformity correcting means 18 may occur. This eliminates the need to readjust the static magnetic field uniformity correction.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment (various modifications and implementations are possible).

For example, although the above embodiment describes a superconducting magnet device, it is also applicable to magnet devices with various other configurations, and the formation position of the female thread portion depends on the performance of various magnet devices,
It can be changed as appropriate depending on various conditions. Furthermore, after the -degree uniformity correction adjustment is completed, the iron-containing screw member can be fixed more firmly by using a synthetic resin adhesive or the like.

In this case, problems such as loosening due to movement can be more reliably prevented.

[Effects of the Invention] As described in detail above, according to the present invention, it is possible to provide an MRr apparatus without causing any malfunction of the uniformity correction means and without readjusting the uniformity correction of the static magnetic field.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the MRI apparatus according to the present invention, and FIGS. 2 and 3 are side views and schematic front views for explaining the magnet device of the embodiment of the MRI apparatus according to the present invention. The sectional views in FIGS. 4(a) to 4(C) are diagrams for explaining the selection of bolts and the positional relationship of the internal threads, respectively, and FIG.
a) and (b) are diagrams each illustrating the screwing depth of a bolt. 17... Superconducting magnet device, 18... Uniformity correction means, 18a... Female threaded portion,
18b... Bolt (iron-containing screw member). (0) Figure

Claims (1)

[Claims] It has a magnet device that forms a static magnetic field and a uniformity correction means that corrects the uniformity of the static magnetic field formed by the magnet device, and applies a high frequency pulse to a subject placed in the static magnetic field. In a magnetic resonance imaging apparatus that collects magnetic resonance signals from a subject to obtain a magnetic resonance image, the uniformity correction means includes a female screw portion formed in the magnet device, and an iron-containing screw member screwed into the female screw portion. A magnetic resonance imaging apparatus comprising: