JPH0432525B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a power supply for a self-shielded normal conducting magnet for a static magnetic field in a nuclear magnetic resonance tomography imaging apparatus.

(Prior art) Nuclear magnetic resonance tomography (hereinafter referred to as NMR-CT) applies a gradient magnetic field to a strong static magnetic field, and generates a magnetic field that is proportional to the strength of the magnetic field generated by the gradient magnetic field along an axis perpendicular to the static magnetic field. This device generates magnetic resonance by applying a high-frequency rotating magnetic field at the Larmor frequency, receives echo signals from atomic nuclei that resonate at that frequency, processes them, and diagnoses them. As mentioned above, NMR-CT requires a strong static magnetic field, and the stability of this static magnetic field is extremely important, and it is no exaggeration to say that it determines the quality of the image obtained.

The main magnetic field magnets for static magnetic fields include normal conducting magnets, permanent magnets, and superconducting magnets. Considering conditions such as weight, maintenance costs, and installation location, we aim to generate a static magnetic field with a wide uniform area with as little power as possible. Excellent normal conducting magnets are often used.

(Problem to be Solved by the Invention) In a normally conducting magnet, a magnetic shield is provided around the magnetic field generator for reasons such as reducing leakage magnetic fields and reducing power consumption by enhancing the magnetic field with ferromagnetic material. ing. In this self-shielded electromagnet, when the temperature of the magnetic shield changes, the magnetic resistance of the shielding material changes, causing the magnetic field strength to fluctuate.

Self-shielded electromagnets use iron shielding material at saturation magnetic flux density Bs. Generally, Bs (T) at temperature T has the following relationship, assuming that the saturation magnetic flux density Bs (O) at absolute temperature T=0 is Bs max.

Bs(T)=Bs max {1-(T/Tc)}3/2 Here, Tc...Curie temperature (〓) Room temperature T
=298〓(25℃), the saturation magnetic flux density Bs changes by 0.09% with a temperature change of 1℃. Normal NMR-CT
With the magnet dimensions for this purpose, a change in Bs of 0.09%/°C results in a drift of -100 ppm/°C. The temperature of the magnetic shielding material changes due to changes in room temperature or heat generation due to current flowing through the coil. As a result,
As mentioned above, Bs drift occurs and the magnetic field strength changes. This state is shown in FIG. The figure is a curve showing the change in magnetic field strength with respect to the temperature change of the magnetic shielding material when the coil current is kept constant. When the magnetic field strength changes in this way,
Fluctuations occur in the output signal of NMR-CT, resulting in errors.

The present invention has been made in view of the above points, and its purpose is to realize a power source for a normally conducting magnet whose magnetic field does not change even if the temperature changes.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, provides a DC power supply for supplying current to a coil in a power supply for a self-shielded normally conducting magnet for a static magnetic field of a nuclear magnetic resonance tomography apparatus. a coil current supply means comprising a current detection means and a current adjustment means inserted in series in the DC power supply circuit; a means for detecting the temperature of the magnetic shield; and a temperature compensation means for outputting a compensation voltage based on the detected temperature. An adjusting means, an adding means for adding the compensation voltage and the output voltage of the standard voltage power supply, and a control means for controlling the current adjusting means based on the difference between the output of the adding means and the output of the current detecting means. It is characterized by:

(Function) The amount of current flowing through the current adjustment means is adjusted by the difference voltage between the standard voltage containing a compensation signal based on the magnetic shield temperature and the output of the current detection means, and magnetic field drift caused by changes in magnetic resistance due to temperature changes in the magnetic shield is controlled. Compensate for.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram of an embodiment of the present invention. 1 is a normally conducting magnet coil that provides a static magnetic field to the NMR-CT, and is surrounded by a magnetic shield 2 to enhance the magnetic field. 3 is coil 1
A DC power source 4 is connected to the DC power source 3 at its collector and connected to the coil 1 via a current detection resistor 5 to adjust the current flowing through the coil 1. The current detection resistor 5, which is a transistor, is a precision resistor with a small temperature coefficient of resistance r, which is inserted in series with the power supply circuit that supplies current to the coil 1, and its both ends are connected to the input terminals of the amplifier 6, respectively. ing. The above DC power supply 3, transistor 4, and current detection resistor 5 constitute a current supply means. An amplifier 6 amplifies the voltage across the current detection resistor 5. 7 is a temperature sensor that detects the temperature of the magnetic shield 2; 8 is a temperature sensor that detects the temperature of the magnetic shield 2 based on the temperature data of the temperature sensor 7;
9 is a standard voltage power supply that outputs a standard voltage Vr, and 9 is a standard voltage power supply that outputs a standard voltage Vr. It is input to the adder 10 together with the output voltage Vth of the device 8. The adder 10 adds the two inputs and outputs a voltage "Vr+Vth". 11 is the output voltage "Vr+Vth" of the adder 10 and the output voltage I of the amplifier 6.
A control signal based on the difference voltage is input to the base of transistor 4, and
This is a controller that changes the internal resistance between the emitter and collector of the

Next, the operation of the circuit configured as described above will be explained. At the start of operation, a current is supplied from the DC power supply 3 to the coil 1 via the internal resistance between the collector and emitter of the transistor 4 and the current detection resistor 5 to provide a desired static magnetic field to the NMR-CT. Let I be the current in the coil 1 at this time. The temperature of the magnetic shield 2, which changes over time, is detected by the temperature sensor 7, inputted to the adder 10 as a compensation voltage Vth in the temperature compensation controller 8, and added to the output voltage Vr of the standard voltage power supply 9. be done. This addition output is input to the controller 11. Controller 11
The voltage I·r across the current detection resistor 5 generated by the current I flowing through the coil 1 is inputted via the amplifier 6, and the output of the adder 10 is “Vr+Vth”.
A control signal is output based on the voltage difference "(Vr+Vth)-I.r". At the start of operation, the current in the coil 1 is adjusted so as to satisfy the following equation.

(Vr+Vth)-I・r=0 As the temperature of the magnetic shield 2 increases with the passage of time, the output of the temperature compensation controller 8 becomes “Vth+△
Vth”, and the adder 9 outputs a voltage “Vr+Vth+△
Vth" is output. This input voltage causes the controller 1 to
The output voltage of 0 is given by the following equation.

(Vr+Vth+△Vth)-I・r>0 Therefore, the base voltage of the transistor 4 increases, the collector-base resistance decreases, and the DC power supply 3
As the load increases, the current flowing through the coil 1 increases. Therefore, the magnetic flux generated by the coil 1 increases, the strength of the magnetic field increases, and the increase in magnetic resistance due to the temperature rise of the magnetic shield 2 is compensated for.

As the temperature of the magnetic shield rises, the current flowing through the coil 1 increases, increasing the magnetic flux to compensate for the increase in magnetic resistance accompanying the rise in temperature of the magnetic shield.

FIG. 3 shows the state of compensation for the temperature rise of the magnetic shield according to this embodiment. In the figure, figure A shows the situation where the coil current I increases due to the rise in temperature of the magnetic shield, and figure B shows the increase in coil current I shown in figure A, which compensates for the decrease in the magnetic field strength shown in figure 2. This is a curve showing the situation where a constant magnetic field strength is obtained. As is clear from the figure, the coil current increases as the temperature of the magnetic shield increases, so the magnetic field strength remains constant regardless of the temperature of the magnetic shield.

Note that the present invention is not limited to the above embodiments. For example, the current detection resistor may be a DC current transformer.

(Effects of the Invention) As explained in detail above, according to the present invention, even if the temperature of the magnetic shield changes due to changes in the ambient temperature, etc., the NMR- It is possible to obtain the static magnetic field of CT, which has a great practical effect.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, and FIG.
The figure is a diagram showing changes in magnetic field strength due to temperature changes in the magnetic shield, and FIG. 3 is a diagram explaining the situation of temperature compensation according to this embodiment. DESCRIPTION OF SYMBOLS 1... Coil, 2... Magnetic shield, 3... DC power supply, 4... Transistor, 5... Current detection resistor, 6... Amplifier, 7... Temperature sensor, 8... Temperature compensation controller, 9... ...Standard voltage power supply, 10...Adder, 11...Controller.

Claims (1)

[Claims] 1. A power source for a self-shielded normal conductive magnet for static magnetic fields in a nuclear magnetic resonance tomography imaging device, consisting of a DC power supply that supplies current to a coil, and a current detection means and a current adjustment means inserted in series in the DC power supply circuit. a coil current supply means, a means for detecting the temperature of the magnetic shield, a temperature compensation adjustment means for outputting a compensation voltage based on the detected temperature, and an addition means for adding the compensation voltage and the output voltage of the standard voltage power supply. A power supply for a normally conducting magnet, comprising: a control means for controlling the current adjusting means based on the difference between the output of the adding means and the output of the current detecting means.