JPH03109043A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To keep a magnetic circuit at an optional temperature by enabling a temperature control for not only heating but also cooling. CONSTITUTION:Temperature control elements 20 capable of both heating and cooling are stuck to top side of an upper yoke 3 and bottom side of an lower yoke 3 respectively. A magnetic circuit except these temperature control elements 20 is covered with a heat insulation material such as styrene foam or sponge body so as to form a heat insulation part 6. Surfaces of the temperature control elements 20 are not covered with the heat insulation material so as to improve ventilation by external air, and an ornamental cover, which envelops the outside of insulation part, are not tightly closed but holed in the parts covering the temperature control elements 20 for the same purpose. Furthermore, a temperature sensor 10A is fitted to a certain peripheral part of a pole piece 2 and another temperature sensor 10B is set between the heat insulation part 6 and the outer ornamental cover. A temperature in a magnetic field is detected by the temperature sensor 10A and that outside the insulation part 6 is detected by the temperature sensor 10B. It is thus possible to keep the temperature of magnetic circuit constant irrespective of room temperature and to obtain an image without distortion and of improved S/N ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnetic resonance imaging apparatus, and particularly to a nuclear magnetic resonance imaging apparatus in which the temperature of a magnetic circuit is controlled.

[Prior Art] A conventional device controls the temperature of a magnetic circuit as described in Japanese Patent Laid-Open No. 112358/1983.

Magnetic resonance imaging device (hereinafter referred to as MRI device)
A magnetic circuit using permanent magnets has the disadvantage that the magnetic field strength changes with changes in ambient temperature. N to the permanent magnet
The temperature coefficient when using dFe-B is -1000pp
m/]000ppm becomes weaker.

In the MR,I device, a gradient magnetic field is added to a static magnetic field, the position is made to correspond to the magnitude of the magnetic field, and a resonance frequency corresponding to the position is generated. Detecting the NMR signal with this resonance frequency,
Specify the location. The actually observed NMR signal is
This is a signal in which NMR signals from many positions are overlapped. Therefore, this is divided into components for each frequency, the frequency detected at the base position is used as a reference frequency, the deviation frequency from the reference frequency is determined, and the position is specified from the deviation frequency.

However, if the magnetic field fluctuates due to temperature, the frequency will change correspondingly, causing a change in the apparent reference position,
and other positional variations relative to the reference position also occur. Furthermore, the position detection deviation also causes image distortion and blur.

Generally, the limit value for influencing the image by changes in the magnetic field is 5 ppm/hour.

One method for this is to cover the magnetic circuit with a heat insulating material, install a temperature adjustment heater (thermal heater) inside, and control the current to the heater to keep the magnetic circuit temperature at a temperature higher than the indoor temperature. We are proposing a control method to maintain this.

In order to keep the temperature of the magnetic circuit of the MR, I device constant, the heat-retaining heater is, for example, an electric fan heater (a type of heater that uses nichrome wire as the heater wire and releases the generated heat into the air by a fan). be able to.

[Illness that the invention aims to solve] In the above-mentioned conventional technology, the magnetic circuit is kept at a constant temperature using air heated by a heater, so the keeping temperature needs to be set to a temperature higher than room temperature. .

If the indoor temperature is likely to be higher than the magnetic circuit's insulation temperature, conventional devices do not have a cooling function.
Temperature control may become impossible.

Furthermore, if this heat retention temperature is set to a temperature higher than room temperature, it is necessary to constantly turn on electricity to keep warm, so the higher the heat retention temperature, the greater the power consumption. Another problem is that the magnetic field strength also becomes weaker.

Means for Solving the Problem] In order to achieve the above object, temperature control is made possible not only for heating but also for cooling.

This allows the magnetic circuit to be maintained at any set temperature.

[Function] In the present invention, by using a temperature control mechanism capable of temperature control through heating and cooling, the magnetic circuit can be kept warm without being affected by the outside temperature. Therefore, it becomes possible to set the temperature of the magnetic circuit to an arbitrary temperature, and the conditions for setting the magnetic circuit are relaxed.

[Example] An example of the MRI apparatus of the present invention is shown in FIG. 1, in which a magnetic pole piece 2 for forming a uniform magnetic field is laminated on a flat permanent magnet 1. Two laminates each consisting of the permanent magnet 1 and the magnetic pole piece 2 are prepared and placed vertically facing each other with a measurement space into which the subject is inserted. As a result, the space between the upper and lower pole pieces forms a uniform magnetic field.

This uniform magnetic field space accommodates a gradient magnetic field generation coil, an irradiation coil for applying electromagnetic waves, and a reception coil for receiving NMR signals. As for the arrangement order, a laminate consisting of a permanent magnet and a magnetic pole piece is provided at a portion corresponding to the outermost circumferential position, and then a gradient magnetic field generating coil 3 is placed inwardly.
1. The irradiation coil 32 for applying electromagnetic waves and the receiving coil 33 are housed in the uniform magnetic field space in this order. The receiving coil 33 located at the innermost circumferential position is a cylindrical solenoid coil, and the space inside this cylinder constitutes a true measurement space, in which the subject is accommodated and measurements are performed.

Further, one side of each of the upper and lower permanent magnets is closely fixed to the yoke plate 3. The yoke plate 3 is rectangular and has a width that covers at least one entire surface of the permanent magnet. This yoke plate 3 has the role of partitioning a space such that the subject can fully enter the measurement space. The upper and lower yoke plates are magnetically connected by the yoke frame 4.
In addition, they are mechanically connected.

The yoke frame 4 magnetically and mechanically connects the rectangular yoke plates at four corners thereof.

Furthermore, a portion of the yoke frame 4 penetrates the lower yoke plate and protrudes outward to form a leg portion 14 of the entire magnetic circuit.

With the above configuration, in the upper and lower stacked bodies, a uniform magnetic field is formed across the measurement space, and the surface opposite to the permanent magnet is connected to the permanent magnet - the yoke plate, the yoke plate, and the other yoke plate. - Another permanent magnet magnetic circuit could be formed. In this way, one magnetic circuit could be formed as a whole.

Temperature control elements 20 capable of heating and cooling are spread and attached to the upper side of the upper yoke plate 3 and the lower side of the lower yoke plate 3. Then, a portion of the magnetic circuit excluding the temperature control element is covered with a heat insulating material 60 made of styrene foam, sponge, or the like to form a heat insulating section 6. The parts covered with the heat insulating material are the entire yoke frame 4, a part of the yoke plate, and the entire laminated pair. However, the laminate and the yoke plate were fixed in close contact with each other, and both were covered with a heat insulating material.

Further, the surface of the temperature control element 20 (the surface not in contact with the yoke plate) is not covered with a heat insulating material, but is made to have good ventilation with the outside air. Even in the decorative cover that covers the outside of the insulation part,
The temperature control element should not be sealed, but should be made with holes to improve ventilation with the outside air.

In FIG. 1, only a portion of the heat insulating section is shown for clarity of drawing. The shaded portion 60 is its cross section.

Although not shown, a gradient magnetic field coil, an electromagnetic wave transmitting coil, and a receiving coil are provided in the space 9 partitioned off by the heat insulating material 60, as in the conventional example.

Furthermore, a temperature sensor 1. OA
Then, install the IOB between the insulation part and the outer decorative cover. The temperature sensor LOA detects the temperature in the magnetic field, and the IOB detects the temperature outside the heat insulating section.

Conventionally, a heat retention control mechanism for a magnetic circuit does not have a cooling function, and therefore, its set temperature needs to be set to a temperature higher than the maximum temperature expected in the measurement chamber where the magnetic circuit is set.

In the present invention, by using a temperature control mechanism capable of cooling and heating, the magnetic circuit can be kept at an arbitrary temperature regardless of the room temperature. Furthermore, by lowering the set temperature, it is possible to increase the magnetic field strength. Furthermore, by setting the set temperature to a temperature close to room temperature, the power consumption required for keeping warm can be reduced.

Here, as an example of a temperature control mechanism capable of heating and cooling, an element using the Beltier effect will be described using FIG. 3.

When two different conductors or semiconductors are connected and a direct current is passed through them, the phenomenon in which heat other than Joule heat is absorbed or generated at each junction is called the Bertier effect.

Utilizing this effect, it is possible to freely control cooling and heating temperatures using direct current. Since this temperature control element performs electrical temperature control, the temperature response is very fast and highly accurate temperature control is possible.

In FIG. 3, when a P-type semiconductor 43 and an N-type semiconductor 44 are bonded with metal and a current is passed as shown in the figure, the temperature of the bonded portion 41 on the left becomes low, and heat absorption occurs at this bonded portion. At the same time, the right joints 42A and 42B become hot, and heat is generated in these joints. In other words, it acts like a heat pump by transferring heat from a low-temperature area to a high-temperature area. This effect is reversible; if the direction of the current is reversed, the high-temperature and low-temperature regions will be reversed, and heat absorption and heat generation will also occur in the opposite direction. This temperature control element can also have a flat plate shape and be fixed to the object to be temperature controlled with screws.

In order to control the magnetic circuit to a constant temperature using this temperature control element, it is necessary to detect and control the temperature difference between the temperature-controlled side and the opposite side.

A specific control method will be explained with reference to a schematic circuit diagram for temperature control shown in FIG. In FIG. 4, 45 is a DC power supply, 46 is a temperature control circuit, 20 is a temperature control element, 1
0A and IOB are temperature sensors (such as thermistors or thermocouples). First, measure the temperature of the magnetic circuit with sensor 1. OA
Then, the temperature outside the heat insulating part is detected by the temperature sensor IOB.

Based on the difference between the temperature of the magnetic circuit and the temperature outside the heat insulating section, the temperature control circuit 46 determines whether the temperature control element absorbs heat or generates heat, and controls the current value and direction of the current to keep the temperature of the magnetic circuit constant. Keep it.

The gist of the present invention is that the number and location of temperature control elements are not limited. Further, the temperature control element is not limited to an element that utilizes the Beltier effect, as long as it is capable of heating and cooling.

[Effects of the Invention] According to the present invention, the temperature of the magnetic circuit can be kept constant regardless of the room temperature, and an image with no image distortion and improved S/N can be obtained.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a longitudinal sectional view of FIG. 1, FIG. 3 is an explanatory diagram of the Bertier effect, and FIG. 4 is a temperature control circuit diagram. 1... Permanent magnet, 2... Magnetic pole piece, 3... Yoke plate,
4...Yoke frame, 6...Insulating section, 10...Temperature sensor, 20...Temperature control element, 46.Temperature control circuit. Kaya / Ro 2nd mouth / 0 0 Gi O

Claims (1)

[Claims] 1. A static magnetic field magnetic circuit consisting of a magnetic pole piece and a permanent magnet for generating a uniform magnetic field, facing each other across a measurement space, and generating a gradient magnetic field to be added to the uniform magnetic field. It includes a gradient magnetic field coil, an irradiation coil that applies an electromagnetic field with a frequency that causes nuclear magnetic resonance to a subject in the measurement space, and a receiving coil that receives a nuclear magnetic resonance signal from the subject in the measurement space. A temperature control device for a magnetic resonance imaging apparatus, characterized in that the temperature of a static magnetic field magnetic circuit is maintained at a set temperature by providing a mechanism capable of heating and cooling. 2. The temperature adjustment mechanism is provided with a temperature control element that utilizes the Peltier effect, and a control means is provided that controls the current to perform heating and cooling to maintain the temperature of the static magnetic field magnetic circuit at a set temperature. A temperature control device in a nuclear magnetic resonance imaging apparatus according to claim 1.