JPH04371136A - Magnetostatic field generating device for mri device - Google Patents

Abstract

PURPOSE:To enhance the S/N ratio of images by making variable the distance between the pole pieces of a magnetostatic field generating device. CONSTITUTION:In an MRI device having means for generating magnetostatic and gradient magnetic fields, high-frequency applying and receiving coils, and an image reconfiguring means, the magnetostatic field generating means is provided with a hydraulic cylinder 17 which moves a yoke 3b supported to a column 4, a linear bearing 30 which guides the yoke 3b in the direction of its movement, a microswitch 31 which detects the shift position of the yoke, and a striker 32. Further, a shim coil 60 is provided for adjusting magnetic field uniformity. Thereby the distance between pole pieces can be varied with the size of a subject and so optimum magnetic field intensity can be given at each part of the subject while the S/N ratio of images can be enhanced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a static magnetic field generating device for a nuclear magnetic resonance imaging system, and more particularly to a static magnetic field generating device that can vary the magnetic field strength depending on the region to be imaged of a subject.

0002

2. Description of the Related Art A conventional apparatus will be explained with reference to FIGS. 7 and 8. FIGS. 7 and 8 are diagrams showing a static magnetic field generating device that uses permanent magnets to generate a static magnetic field and constitutes a magnetic circuit. A pair of iron yokes 3a, 3b support permanent magnets 2a, 2b and magnetic pole pieces 1a, 1b, respectively, and four columns 4 hold the yokes 3a, 3b facing each other at a predetermined distance apart. has been done. In this static magnetic field generator, the opposing permanent magnets 2a and 2b have different polarities, and the magnetic circuit includes the permanent magnet 2a, the magnetic pole piece 1a, the magnetic pole piece 1b, and the permanent magnet 2.
b, yoke 3b, column 4, yoke 3a, permanent magnet 2a
It is configured through. Among these components, pole piece 1
a, 1b are spaces into which the subject 6 enters, that is, magnetic pole pieces 1a, 1
This is to make the magnetic field uniformity in the center between b. (Homogeneity = amount of change in magnetic field in a certain space ÷ central magnetic field strength x 1
06≒10ppm)

Generally, in order to obtain high uniformity, the distance L between the magnetic pole pieces and the diameter D0 of the magnetic pole pieces 1a, 1b have a relationship of D0≧2×L. Furthermore, the peripheral edges of the opposing magnetic pole pieces 1a and 1b have annular protrusions 7 of the same shape on both the upper and lower sides. This annular protrusion 7 is for suppressing leakage of magnetic flux to the periphery and improving the uniformity of the internal space. (For details, see Japanese Unexamined Patent Publication No. 60-8
8407).

Therefore, the effective gap into which the subject 6 can enter is the distance Lg between the tip ends of the magnetic pole pieces 1a and 1b. In addition to the subject 6, gradient magnetic field coils 8a and 8b necessary for imaging and an RF irradiation coil (
(not shown) and an RF receiving coil (not shown) are arranged.

In the static magnetic field generator as described above, since the magnetic field uniformity required for imaging changes depending on the environment, the upper yoke 3b or the upper yoke 3b and the upper permanent magnet 2b are adjusted at the installation stage. The magnetic field uniformity is adjusted by moving the moving member 9 fitted in the center up and down. However, this adjustment is only made during installation, maintenance and inspection, and during normal use, the distance Lg between the projecting ends remains fixed.

[0006]

[Problems to be Solved by the Invention] The above-mentioned prior art does not consider changing the magnetic field strength of the static magnetic field generating device. I was shooting at a distance. However, when photographing the head, a better image can be obtained if the distance between the tips is narrower than when photographing the abdomen, so if the distance between the tips can be narrowed, the static magnetic field strength can be increased, improving the S/N ratio of the image, Shooting time can be shortened.

The purpose of the present invention is to realize a variable distance between opposing magnetic pole pieces of a static magnetic field generator for the purpose of improving the S/N ratio of images, etc., by providing a variable mechanism and a change in magnetic field uniformity due to variable distance. The purpose is to provide a means of compensation.

[0008]

[Means for Solving the Problems] Magnetic field generating means for a static magnetic field and a gradient magnetic field, a high-frequency irradiation coil that irradiates an electromagnetic wave to an object to be examined, a high-frequency receiver coil that detects a magnetic resonance signal from the object to be examined, and the detection In a static magnetic field generator for an MRI apparatus, the static magnetic field generating device includes an image reconstruction means for obtaining an image representing the physical properties of a target object using signals, and the magnetic field generating device is provided with a static magnetic field generating device for an MRI apparatus, which is opposite to each other across a gap capable of accommodating a subject, and is arranged on the side of the gap. a pair of permanent magnet devices, each of which is provided with a magnetic pole piece for ensuring uniformity of a static magnetic field on a surface of the permanent magnet device; permanent magnet arrangement means for magnetically coupling the
A magnetic field intensity variable setting means for variably setting the facing distance between the pair of permanent magnet devices, and a magnetic field uniformity correction means for correcting the uniformity of the static magnetic field that occurs when the facing distance between the pair of permanent magnet devices is changed. .

[0009]

[Operation] A hydraulic cylinder is connected between a pair of yokes, and a guide is provided in the direction of movement of the column and the upper yoke to uniformly move the upper yoke to change the magnetic field strength. Further, the detection mechanism that detects the movement position determines the amount of current to be passed through a shimming coil (hereinafter referred to as shim coil) for adjusting magnetic field non-uniformity of each term due to movement. Furthermore, a shim coil is installed in the static magnetic field generator to correct changes in magnetic field uniformity due to movement.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described in detail below with reference to FIGS. 1 to 6. Figure 1 shows a magnetic resonance image according to the present invention.
1 is a block diagram illustrating the overall configuration of a processing device; FIG. In FIG. 1, this magnetic resonance imaging apparatus is a magnetic resonance (N
MR) phenomenon is used to obtain a tomographic image of the subject 6, and for this purpose, a static magnetic field generating magnet 110 having a necessary sufficiently large bore diameter, a central processing unit (hereinafter referred to as CPU) 111, A sequencer 112, a transmission system 113,
Gradient magnetic field system 114, receiving system 115, and signal processing system 116
It consists of. The static magnetic field generating magnet 110 generates a uniform magnetic flux around the subject 6 in the body axis direction or in a direction perpendicular to the body axis. A type of magnetic field generating means is arranged.

The sequencer 112 operates under the control of the CPU 111 and sends various commands necessary for data collection of tomographic images of the subject 6 to the transmission system 113, gradient magnetic field system 14, and reception system 15. It is. The transmission system 113 includes a high frequency oscillator 117, a modulator 118, and a high frequency amplifier 1.
19 and a transmission-side high-frequency coil 120a, which receives high-frequency pulses output from the high-frequency oscillator 117.
The modulator 118 modulates the amplitude according to the command from the controller 112, and the amplitude-modulated high-frequency pulse is sent to the high-frequency amplifier 11.
After being amplified in step 9, the signal is supplied to a high frequency coil 120a placed close to the subject 6. Thereby, the subject 6 is irradiated with electromagnetic waves.

The gradient magnetic field system 14 consists of gradient magnetic field coils 8a, 8b wound in three directions of X, Y, and Z, and a gradient magnetic field power supply 122 for driving each coil. By driving the gradient magnetic field power supply 122 of each coil according to the command,
Gradient magnetic fields Gx, Gy, and Gz in three directions are applied to the subject 6. By changing the way this gradient magnetic field is applied, a slice plane for the subject 6 can be set. The receiving system 115 includes a receiving side high frequency coil 120b, an amplifier 23, a quadrature phase detector 124,
The electromagnetic wave (NMR signal) in response to the electromagnetic wave irradiated from the transmitting side high-frequency coil 120a of the subject is detected by the high-frequency coil 120b placed close to the subject 1. Ru.

Furthermore, an amplifier 123 and a quadrature phase detector 1
24 to the A/D converter 125, where it is converted into a digital quantity, and is sampled by the quadrature phase detector 124 at the timing according to the command from the sequencer 112, resulting in two series of collected data. Signal processing system 11
It is set to be sent to 6. This signal processing system 116
The CPU 111, the magnetic disk 126 and the magnetic tape
A recording device such as a printer 127 and a display 12 such as a CRT.
The CPU 111 performs processing such as Fourier transform and correction coefficient calculation value image reconstruction, and images the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculations on a plurality of signals. is displayed on the display 128. Further, the distance between the magnetic pole pieces of the static magnetic field generating magnet 110 is detected by the position detector 132, the signal is taken into the control circuit 131, and the shim coil power supply 130 outputs a plurality of shim coils 60 (magnetic field uniformity) according to the data. In order to correct each term, the current value to be passed through as many coils as the number of correction terms is determined. In FIG. 1, high frequency coils 120a and 120b on the transmitting side and receiving side, and a gradient magnetic field coil 8
a and 8b are arranged in the magnetic field space of the static magnetic field generating magnet 110 arranged in the space around the subject 6.

FIG. 2 shows a perspective view of an embodiment of the present invention. 3 is a sectional view taken along the line B-B in FIG. 2, and shows the shim coil 60.
is attached. A U-shaped metal fitting 18 is fixed to the lower yoke 10a, a pin 20 is rotatably fitted into a hole at the lower end of the hydraulic cylinder 17, and a bearing has holes that fit into both ends of the pin 20. 19 to the metal fitting 18. A pin 22 is rotatably fitted into a hole at the upper end of the hydraulic cylinder 17, and bearings 21 having fitting holes at both ends are attached to the upper yoke 10b.

FIG. 4 is a diagram showing the structure of the hydraulic cylinder 17. Hydraulic oil 23 is drawn into the interior of the hydraulic cylinder 17, and the rod 17a is moved by this hydraulic oil 23. The pressure generated by the hydraulic pump is 25 to 35 kg/mm
It is 2 or more. In addition, the upper yoke 10b and the upper permanent magnet 2b
Since the total weight of the upper magnetic pole piece 1b is approximately 4 tons, the inner diameter d of each cylinder to support this weight with the hydraulic cylinder 17 is d = 4000 kg/4 pieces/25 kg/m
m2=40mm2 or more is required.

Next, the circuit of the hydraulic cylinder 17 will be explained with reference to FIG. The hydraulic system includes four hydraulic cylinders 17, four electromagnetic valves 41, two throttle valves 42, a check valve 43, a relief valve 44, a filter 45, a motor pump 46, and oil. It consists of 47 tanks. In the operation of the hydraulic system when increasing the distance between the magnetic pole pieces, the oil pumped up from the oil tank 47 by the motor pump 46 flows as shown by the arrow 48. In other words, the oil is filtered of dirt by the filter 45, and the check valve 43 is used to prevent backflow when the hydraulic cylinder 17 is stopped.
and passes through a solenoid valve 41 that electrically opens and closes the valve. When the hydraulic cylinder 17 is extended, the solenoid valve C is closed. Thereafter, it enters the hydraulic cylinder 17, and the rod 17a extends to raise the yoke 10b. The oil in the upper part of the hydraulic cylinder 17 passes through the solenoid valve 41 and returns to the oil tank 47. When the distance between the magnetic pole pieces is narrowed, the flow of oil is reversed. Further, the relief valve 44 serves to return oil to the oil tank 47 so that the pressure does not exceed a certain level when an abnormality occurs in the movement of the upper yoke 10b (an abnormality in the bearing).

Next, the guide mechanism and position detection will be explained with reference to FIG. Basically, the fitting between the column 4 and the linear bearing 30 fixed to the upper yoke 10b serves as a guide. The upper end of the column 4, whose lower end is fixed to the lower yoke 10a, is finished to a dimension that fits the inner diameter of the linear bearing 30 fixed to the upper yoke. - The magnet 10b (upper permanent magnet 2b, upper magnetic pole piece 1b) is movable. Further, the microswitches 31a, 31b attached to the center of the column 4 and the striker 32 attached to the upper yoke 10b constitute a position detection section, and the striker 32 detects which microswitch 31a, 31b The shim coil current is determined depending on whether it is pressed. In addition, the micro switch 31a
, 31b indicate the drive stop position of the hydraulic cylinder 17.

The shim coil 60 will be explained. The shim coil 60 has a pattern for correcting each magnetic field inhomogeneity term,
This pattern is produced by etching the pattern proposed in Japanese Patent Publication No. 40-26368, for example, onto a printed circuit board, and the same pattern is placed one above the other to obtain the desired correction. As the hydraulic cylinder 17 expands and contracts, the upper yoke 10b moves, and the distance between the magnetic pole pieces 1a and 1b changes to achieve magnetic field uniformity, but the amount of change is corrected by the shim coil 60. , 31b are turned on and off to detect the position of the upper yoke 10b, and the value of the current to be passed through the shim coil 60 is determined accordingly.

Next, the cover will be explained with reference to FIG. If the distance between the magnetic pole pieces is changed, a problem arises in covering the entire static magnetic field generator, which will be explained with reference to FIG. Cover - front surface, rear surface (not shown), side surface 52, top surface 53
A frame is taken out from the top of the column 4, and the cover is fixed to the frame so that there is no effect even if the spacing changes. Next, the opening cover 54 will be explained. The opening cover 54 is divided into upper and lower halves, and the upper cover height and width are larger than the lower side.
Both the lower covers are fixed to the magnetic pole pieces 1a and 1b. A restraint 55 is provided in the lateral direction to prevent the cover from opening. With this, even if the distance between the magnetic pole pieces changes, the aperture changes accordingly.

[0020]

According to the present invention, since the distance between the magnetic pole pieces can be changed according to the size of the subject, the distance is narrow when photographing the head, and the magnetic field strength can be increased, so the S/N of the image can be improved.
This has the effect of improving the ratio and shortening the shooting time.

[Brief explanation of drawings]

[Fig. 1] Overall configuration diagram of the magnetic resonance imaging apparatus of the present invention

[Fig. 2] A perspective view of the static magnetic field generator of the present invention.

[Fig. 3] Cross-sectional view of the static magnetic field generator of the present invention

[Figure 4] Cross-sectional view of hydraulic cylinder

[Figure 5] Hydraulic circuit diagram

[Fig. 6] Cover according to the present invention - sectional view

[Figure 7] Plan view of conventional device

[Figure 8] Three-dimensional view of conventional device

[Explanation of symbols]

1a Magnetic pole piece 1b Magnetic pole piece 2a Permanent magnet 2b Permanent magnet 3 York 4 Column 5 Voids 6 Subject 7 Annular protrusion 8a Gradient magnetic field coil 8b Gradient magnetic field coil 9 Moving parts 10a York 10b York 17 Hydraulic cylinder 17a Rod 30 Linear bearing 31a Micro switch 31b Micro switch 32 Striker 60 Shim coil

Claims (1)

[Claims] 1. Magnetic field generating means for a static magnetic field and a gradient magnetic field, a high-frequency irradiation coil that irradiates an electromagnetic wave to an object to be examined, and a high-frequency receiver coil that detects a magnetic resonance signal from the object to be examined;
A static magnetic field generator for an MRI apparatus, comprising: an image reconstruction means for obtaining an image representing the physical properties of the target object using the detection signal; A pair of permanent magnet devices each having a magnetic pole piece for ensuring uniformity of a static magnetic field on a surface on the air gap side, and a pair of permanent magnet devices that are positioned facing each other and that are arranged in a spatial region excluding the air gap. permanent magnet arrangement means for magnetically coupling the magnet devices; magnetic field strength variable setting means for variably setting the facing distance between the pair of permanent magnet devices; and a static magnetic field generated when changing the facing distance between the pair of permanent magnet devices. 1. A static magnetic field generator for an MRI apparatus, comprising a magnetic field uniformity correction means for correcting the uniformity of the magnetic field.