JPH0268037A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To relieve the conditions for limiting installation of a shielding room by controlling the temp. of a permanent magnet as well as an irradiation magnetic field and the frequency of a high-frequency signal receiving section in such a manner that the frequency of magnetic resonance is turned to the frequency deviated form the frequency of unnecessary radio waves, etc. CONSTITUTION:This imaging device is so constituted that the set temp. of a temp. controller 8 controlling a magnetic circuit 1 to retain the temp. thereof can be changed to change the temp. of the magnetic circuit. The magnetic circuit is constituted of the permanent magnet. The magnetic field intensity of the permanent magnet in general changes with temp. The temp. coefft. of a neodium/iron/boron (Nd.Fe.B) magnet having powerful PH product to the permanent magnet is about-0.12%/ deg.C. The central frequency changes from 8.514MHz of a standard to 8.504MHz by increasing the temp. of the permanent magnet in 1 deg.C. The device is constituted to change the tuning frequency of the high-frequency signal receiving section and the detecting frequency of a detecting section 12 in this way. The frequency which is not affected by the noises of the electromagnetic waves is set by deviating the temp. of the magnet, the tuning frequency of the signal transmitting and receiving section, the frequency of the detector, etc., so as to avert the frequency of the electromagnetic noises when the noises appear within the image according to the conditions of the installation place.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to magnetic resonance imaging (hereinafter abbreviated as MHI).
The present invention relates to means suitable for avoiding noise caused by external radio waves, etc. in a device.

[Conventional technology]

When an MHI device applies an irradiation pulse that causes magnetic resonance to a subject placed in a static magnetic field, certain atomic nuclei in the subject absorb this energy and become in a resonant state. When the irradiation pulse is cut off at this point, it relaxes to its original state while releasing the absorbed energy. The weak signal at this time is detected and the image is reconstructed. The energy of this emitted electromagnetic wave is as weak as about 10-'eV. For this reason, conventionally, in order to receive this weak magnetic resonance signal, an electromagnetic wave shield room was created to block electromagnetic waves coming from outside, and the MHI device was installed in this room.

[Problem to be solved by the invention]

In the above-mentioned conventional technology, it is necessary to create a chamber of approximately 30 to 100 m' in length surrounded by shielding material for electromagnetic wave shielding. Along with this, air conditioning and lighting in this room.

Medical piping and other incidental equipment is required when installing equipment,
It will be a large-scale construction project, and both the construction period and cost will be problematic.

Normal radio wave shielding work has a shielding ability of 60 dB.
However, when there is a wireless station nearby, this shielding performance becomes a problem, causing frequency noise on the image. In such cases, in order to improve shield performance, the shield should be set to 2.
The installation of a shield room is a major problem, as it requires heavy weight, special materials, and construction methods.

SUMMARY OF THE INVENTION An object of the present invention is to provide a means for alleviating the restrictive conditions for installing a shield room in order to solve the above-mentioned problems.

[Means to solve the problem]

The above object can be achieved by means of selecting the frequency of magnetic resonance while avoiding the frequency band of radio waves from radio stations and the like.

That is, those that use a permanent magnet to generate a static magnetic field utilize the temperature dependence of the magnetic properties of the permanent magnet material, and have means for changing the temperature of the permanent magnet, thereby changing the static magnetic field strength.

Since the frequency of magnetic resonance is proportional to the strength of the static magnetic field, the frequency of magnetic resonance changes as the strength of the static magnetic field changes. On the other hand, the irradiating magnetic field system that causes magnetic resonance and the receiving system that detects signals are also provided with means that can accommodate these changed frequencies of magnetic resonance. By such means υ,
Depending on the installation location of the device, you can select a frequency that causes less radio interference.

[Effect]

The permanent magnet circuit is insulated and kept at a constant temperature by a heat retention control mechanism that combines a heat insulating member, a heater, and the like. This temperature is controlled by controlling the heater current by adjusting the set temperature of the heat retention control mechanism.

It works so that the heat retention temperature can be varied. On the other hand, by varying the oscillation frequency of the frequency source that causes magnetic resonance and the tuning frequency of the tuning circuit that detects the magnetic resonance signal, magnetic resonance can be generated efficiently and the magnetic resonance signal can be detected with maximum sensitivity. The tuning frequency of the receiving system, including the receiving coil, can be selected to enable detection. From the above,
Furthermore, the reference frequency during reception is also variable, so that even if the frequency of the magnetic resonance signal changes, it can always operate at maximum efficiency.

〔Example〕

An embodiment of the present invention will be explained below with reference to FIG. 1st
The figure is a block diagram of an MHI device that is an embodiment of the present invention. The device includes a permanent magnet 1 that generates a strong and uniform magnetic field in a space where a subject 4 exists, a heat retention control mechanism 5 that keeps the temperature of this permanent magnet constant, a temperature control device 8, and x, y, z.
A gradient magnetic field coil 2 that generates a gradient magnetic field to provide three-dimensional spatial information, a gradient magnetic field power supply 6, an irradiation coil 3 that causes a magnetic resonance phenomenon, and a high frequency receiver 7.
A coil 1o for receiving a magnetic resonance signal, a high frequency receiving section 11 for amplifying this signal, a detection section 12, an analog/digital converter 13, and an arithmetic device 14 for reconfiguring me.
, a display 15 for displaying an image, and a reference frequency source 9
The overall control circuit 16 is also well established. below,
The operation of the apparatus shown in FIG. 1 will be explained. Here, an example will be explained in which the magnetic field strength Ho of the permanent magnet is 0.2 Tesla (T) and the gradient magnetic field strength G is 2 mT/m. As is well known, the frequency f of magnetic resonance is (
1) Given by Eq.

γ f=-・Ho (1) 2π Here, γ is called the gyromagnetic ratio and is a value specific to the atomic nucleus.
, for proton IH, r/2π=42.57MH2/T (2). Therefore, in proton imaging with a static magnetic field strength of 0.2 T, the magnetic resonance frequency is f-8,514M.
Hz. Here, the imaging field of view is 40Cr11 (±20
cm), the band Δf to be handled is (1
) ceremony! is given by equation (3).

By substituting the above-mentioned conditions here, Δf=±8.5 KHz is obtained. That is, in imaging, 8.514MH
It is sufficient to handle the frequency range of z±8.5KHz. Here, a description will be given of when a cross-sectional image of the subject (in a direction orthogonal to the body axis) is captured. At this time, first, in order to select the slice position, a gradient magnetic field having a gradient is applied along the body axis of the subject. Here, a high frequency wave having a center frequency fO and a band Δf1 is applied.

At this time, the slice position of the body axis is determined by the center frequency f, and the thickness of the slice surface is determined by the frequency of the band Δfx. Only a certain slice plane selected by high frequency waves causes a magnetic resonance phenomenon (slice selection).

A gradient magnetic field of X and Y is applied to the two-dimensional plane selected here, the signal at this time is detected by the receiving coil 10, and the signal in the frequency band f, ±Δf is amplified by the high frequency receiving section 11. Then, the center frequency f is removed by the phase detection section 12 and reduced to an audible frequency of ±Δf. This low frequency signal is digitized by the A/D converter 13 and sent to the arithmetic unit 14. The arithmetic unit 14 usually consists of a frequency analyzer, which knows the position information according to the frequency of the magnetic resonance signal and rearranges it into a two-dimensional matrix.

The frequency band at this time is determined by the field of view and the magnitude of the gradient magnetic field, as described above, and is ±8.5 KH2 under the above conditions. In other words, the frequency band handled by the receiving coil is 8
．． If there is a band of 514±&5KHz, the desired field of view can be covered, and other frequencies are unnecessary.

Now, depending on the installation location, a radio frequency such as a wireless station may exist at the above frequency. At this time, the vertical position on the image corresponds to that frequency.

Alternatively, bright lines may appear horizontally, causing noise and disturbing the image. MHI deals with weak signals, and if the field strength of such radio waves from an external source is not smaller than 1 normal OdBμV/m, it will appear as noise. The electric field strength due to radio waves from broadcasting stations and radio stations varies depending on the location, but as a guide for installation conditions, it is 60 d depending on the shielding of the equipment or room.
The shielding ability is level B, and the electric field strength at the installation location is 6.
0dBμV/m or less is required. This shielding ability allows it to block noise emitted by its own devices and electromagnetic waves from broadcasts in normal locations.

However, depending on the proximity of a wireless station or the surrounding environmental conditions, this performance may be insufficient and noise may become a problem. At this time, in the present invention, the set temperature of the temperature control device 8 which controls the magnetic circuit 1 to keep it warm can be changed, and the temperature of the magnetic circuit is changed. The magnetic circuit is composed of permanent magnets, and in general, the magnetic field strength of permanent magnets changes depending on the temperature. Strong against Kukaishi - Kaka =
Table 1 Examples of properties of permanent magnet materials Table 1 shows nine temperature coefficients of various magnet materials.

For neodymium-iron-boron (Nd-Fe-33) magnets, which are strong permanent magnets and have a high H product, this temperature coefficient is approximately -0.
, 12%/l:'. Therefore, in this example, we set the temperature to I
By changing C, 8,514 KM zX O,
It can be shifted by 12X 10-" = 10.2 KHz. That is, by increasing the temperature of the permanent magnet IC, the center frequency changes from the standard 8.514 MHz to &504 MHz. Accordingly, High frequency receiving section 11
The configuration is such that the tuning frequency of the detector 12 and the detection frequency of the detector 12 are changed.

With the above configuration, when radio wave noise appears in an image due to the conditions of the installation location, the temperature of the magnet, the tuning frequency of the transmitter/receiver system, the frequency of the detector, etc. are shifted to avoid that frequency, and the electromagnetic wave is removed. You can set a frequency that is not affected by noise. As a result, this effect can be avoided even in areas with particularly strong broadcast signal strength.

A clear image can be obtained.

〔Effect of the invention〕

According to the present invention, electromagnetic waves from radio stations, etc., which vary depending on the installation location, can be dealt with even with a general shield room having a performance of about 60 dB or a gantry shield with a performance inferior to this. Therefore, there is no need for special double shielding or complicated construction, and it is possible to significantly ease installation conditions.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention. 1... Magnetic circuit, 2... Gradient magnetic field coil, 3...
Irradiation coil, 4... Subject, 5... Heat retention circuit, 6.
... Gradient magnetic field power supply, 7 ... High frequency transmission system, 8 ... Temperature control device, 9 ... Reference frequency source, 10 ... Detection coil %11 ... High frequency receiving section, 12 ... Detection Part 1
3...A/D converter. 14... Arithmetic device, 15... Display, 16.
...Control circuit.

Claims (1)

[Claims] 1. In a magnetic resonance imaging device that uses a permanent magnet to generate a static magnetic field, there is a means for keeping the permanent magnet at a constant temperature, a means for changing this temperature, and a frequency of an irradiation magnetic field and a high-frequency receiver that cause magnetic resonance. and controlling the temperature of the permanent magnet and the frequency of the irradiation magnetic field and the high-frequency receiving section so that the frequency of magnetic resonance is tuned to a frequency shifted from the frequency of unnecessary radio waves from the outside. A magnetic resonance imaging device characterized by: