JPH10201732A - Power supply unit and magnetic resonance imaging system using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high quality MR image, even if there were any error in the reactor for suppressing the circulation current, by adjusting a relative distance between a conductor to run an over-current via a magnetic flux generated near to the reactor and the reactor for adjusting an inductance value of the reactor. SOLUTION: The reactor 61 to be connected to a switching power supply unit is set inside the reactor box 63, in which an up-and-down movable adjusting bar 64 available to be turned around the fixing seat 66 is fitted to a given place n the reactor box 63. Impressing a current into the reactor 61 generates an over-current I the adjusting board 65 via a magnetic flux generated to generate magnetic flux so as to de-excite the magnetic flux generated in the reactor in the vicinity of the adjusting board 65 to reduce an apparent impedance of the reactor. Turning the adjusting bar 64 changes the relative distance between the adjusting board 65 and the reactor 61 to adjust the impedance value to a given value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device suitably used for a magnetic field generating coil such as a magnetic resonance imaging device (hereinafter, referred to as an MRI device). The present invention relates to a power supply device suitable for various power supplies required for generating a high-frequency magnetic field.

[0002]

2. Description of the Related Art An MRI apparatus applies a high-frequency magnetic field in a pulsed manner to a test object placed in a static magnetic field, detects a nuclear magnetic resonance signal generated from the test object, and uses the detected signal to obtain a spectrum or the like. The image is reconstructed, and the MRI apparatus generates a superconducting or normal conducting coil for generating a static magnetic field as a magnetic field generating coil, a gradient magnetic field coil for generating a gradient magnetic field superimposed on the static magnetic field, and a high frequency magnetic field. A high frequency coil for performing the operation. These magnetic field generating coils include a power supply device for controlling the magnitude and timing of an applied current to generate a magnetic field having a predetermined magnetic field strength. In such an MRI apparatus, the magnetic field strength of a static magnetic field, a gradient magnetic field, or a high-frequency magnetic field greatly affects noise on an image finally obtained and an imaging time. A large current power supply is required as a magnetic field power supply for the apparatus, and a power supply apparatus for this purpose has been proposed by the present inventors in Japanese Patent Application Laid-Open No. Hei 8-221139.

FIG. 9 shows a power supply device described in Japanese Patent Application Laid-Open No. Hei 8-21139. In FIG. 9, four switching power supplies 11 to 14 and reactors 20 to 2 for suppressing a sneak current (circulating current) between these switching power supplies.
7 and a control circuit 30 for driving and controlling each switching power supply
And The switching power supplies 11 to 14 are respectively connected in parallel to a DC power supply (not shown), one output terminal of which is connected via reactors 20, 22, 24, 26, respectively, and the other output terminal of which is 21, 23, 25,
27, and is connected to a magnetic field coil 40 as a load. These reactors 20 to 27 supply current from one switching power supply to another switching power supply when the timing for driving the switching power supplies 11 to 14 is slightly deviated, especially when the driving is performed with a phase shifted as described later. To prevent sneaking around.

Further, a current detector 41 for detecting a current flowing through the magnetic field coil 40 is provided on the output side of the power supply device 10. The current command value applied to the magnetic field coil 40 and the current detection value output from the current detector 41 are input to the control circuit 30, and the control circuit 30 controls each switching power supply so that the difference between the two becomes zero. . An example of this known switching power supply is shown in FIG. This switching power supply 10 'includes four switching elements 51 to 54,
Reactor 5 for smoothing output of switching power supply
5 and 56 and capacitors 57 and 58. As the switching element, an insulated gate bipolar transistor (IGBT) is employed.
And 52 (the switching element on the left) and 53 and 54 (the switching element on the right) are connected in series to the DC power supply 50, and the switching elements 51 and 52 and the switching elements 53 and 54 are connected in parallel. I have. Reactor 55 and capacitor 57 are connected in parallel with switching element 52, reactor 56 and capacitor 58 are connected in parallel with switching element 54,
The voltage V on the collector side of the switches 52 and 54, respectively
A smoothing circuit for smoothing L ′ and VR ′ is configured. One output terminal of the switching power supply 10 ′ is connected to a reactor 55.
And the other output terminal is connected to a connection point between the reactor 56 and the capacitor 58.

The switching power supply 10 'is alternately driven at a constant period so that the switches 52 and 53 are off when the switches 51 and 54 are on, and the switches 52 and 53 are on when the switches 51 and 54 are off. Is done. At this time, on the other hand, for example, the time during which the switches 51 and 54 are turned on is increased, and
, The voltages VL ′ and VR ′ on the collector side of the switches 52 and 54 viewed from the neutral point (not shown) of the DC power supply 50 have waveforms as shown in FIG. To the reactor 55 and the capacitor 5
The voltage VLA ', VRA' at the output terminal becomes a DC voltage by smoothing the output terminal 7 with the reactor 56 and the capacitor 58. The output terminal of such a switching power supply 10 'is connected to a magnetic field coil 40 via reactors 20, 21 as shown in FIG. Although only the switching power supply 11 is shown in FIG.
No. 4 has exactly the same configuration. Each of the switching power supplies 11 to 14 configured as described above is connected to a magnetic field coil via a reactor, and these switching power supplies 11 to 14 are driven by the control circuit 30 by shifting the phases of the switches. In this example,
As shown in FIG. 12, the phases are shifted by 90 degrees corresponding to the provision of four switching power supplies. Note that the solid line waveforms VL1 to VL4 in the figure indicate the voltage on the collector side of the switching element on the left side of each switching power supply, and the chain line waveform indicates the voltage at the output terminal of each switching power supply. Control circuit 3 to shift the phase of the switch
0 shifts the timing of applying the gate voltage of the switching element of each of the switching power supplies 11 to 14 and compares the current command value to be applied to the magnetic field coil 40 with the current detection value detected by the current detector 41. The voltage applied to the switching element of each switching power supply is controlled so that the difference between them becomes zero. As a result, the output current IL having a predetermined current command value is applied to the magnetic field coil even when the switching power supplies are driven with their phases shifted. Here, the waveform of the voltage at the output terminal of each switching power supply is a DC waveform including a ripple having the same frequency as the switching frequency as shown in FIG. 12, but these phases are shifted by 90 degrees. 9, the voltage of the output terminal of the entire power supply device is increased in frequency, and the ripple of the output current IL flowing through the magnetic field coil 40 can be reduced. Therefore, even if the highest frequency for safely operating a high-withstand-voltage, high-current element such as an IGBT, for example, 20 kHz, is set, the ripple frequency of the actual output is increased to 80 kHz to reduce the ripple. In the above description, even when the phases of the switches of a plurality of switching power supplies are not shifted, it is possible to increase the current capacity of the power supply device by providing a plurality of switching power supplies in parallel. However, by shifting the phase, the output current ripple of the power supply device can be increased in frequency, so that a high-withstand-voltage, large-current element such as an IGBT driven at a relatively low operating frequency is used as a switching element. It is possible to increase the capacity as a result.

[0006]

SUMMARY OF THE INVENTION As already mentioned,
A plurality of switching power supplies are connected in parallel in order to flow a large current with a small ripple through the magnetic field coil 40, and a reactor is connected to the output side of each of the switching power supplies so that the sneak caused by imbalance of the output voltage between the switching power supplies. The method of suppressing the current (circulating current) is effective, but if there is a difference in the inductance value of each reactor, a difference occurs in the magnitude of the current flowing through each reactor, and as a result, the combination of the currents flowing through these reactors A large ripple current as shown in FIG. 13 flows through the magnetic field coil current IL which is a value. The structure of these reactors is a cylinder (bobbin) having a certain diameter as shown in FIG.
Is generally formed by winding several turns of the winding, and the inductance value of the reactor thus configured is proportional to the square of the diameter of the bobbin for winding the winding. For example, if there is an error of 1% in the diameter, the error of the inductance of the reactor is about 2
%. This error is inevitable in manufacturing, and it is difficult to secure a higher accuracy. If there is a 2% error in the inductance value of these reactors,
The magnetic field current, which is the output current of the power supply device, generates a ripple current of an effective value of several hundred mA. Since this current ripple causes image noise in the MRI apparatus, it is necessary to set the effective value to several mA or less, for example. For this reason, in order to reduce the noise of the MRI image to an allowable value or less, the variation in the inductance of the reactor is set to 0.1.
Must be around 1%.

An object of the present invention is to provide a power supply device capable of obtaining a high-quality MRI image by preventing the output current ripple from being affected even if there is an error of several percent in the circulating current suppression reactor. And a magnetic resonance imaging apparatus using the same.

[0008]

According to the present invention, there is provided a power supply device for outputting a current having an arbitrary waveform, comprising a plurality of switching power supplies each having a switching element and a load. In a power supply device having reactors connected in parallel and individually connected to the output sides of the plurality of switching power supplies for suppressing a sneak current (circulating current) between the plurality of switching power supplies, the value of the inductance of the reactor is A means for adjusting is provided. The adjustment of the inductance value of the reactor is achieved by adjusting a conductor through which an eddy current flows by a magnetic flux generated in the vicinity of the reactor, and a relative distance between the conductor and the reactor.

The MRI apparatus of the present invention includes the above-described power supply as a power supply for a magnetic field generating coil.

[0010] The power supply device of the present invention configured as described above is individually provided on the output sides of the plurality of switching power supplies for suppressing the sneak current (circulating current) between the plurality of switching power supplies connected in parallel. Even if there is a manufacturing error of several percent in the connected reactor, the value of the inductance of this reactor is adjusted to make the output currents of the switching power supplies substantially equal, and the output currents of the plurality of switching power supplies connected in parallel are combined. The ripple of the magnetic field coil current, which is a value, is set to a very small value. Thus, a power supply device capable of obtaining a high-quality MRI image and a magnetic resonance imaging apparatus using the same can be provided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A power supply according to the present invention is
An embodiment applied to a gradient coil of the apparatus will be described. FIG. 1 is a circuit configuration diagram of a power supply device in which a reactor connected to the output side of a switching power supply connected in parallel is provided with an inductance adjusting function.

The DC power supply 50 supplies a DC voltage to switching power supplies 31 and 32, which are output current amplifiers, and these switching power supplies 31 and 32 output a current having an arbitrary waveform.

The switching power supplies 31 and 32 have reactors 61 and 6 whose inductance values can be adjusted.
2, 71 and 72 are connected. Considering reduction of output current ripple, switching power supplies 31 and 32 have 18
It is operated with a phase difference of 0 degrees. This makes it possible to supply a current without a ripple as shown in FIG. 2 to the magnetic field coil 40. This power supply device is the same as the conventional power supply device shown in FIG. 9 except that only two power supply devices are shown for the sake of simplicity of description, and the function of adjusting the inductance of the circulating current suppression reactor.

When a large current is supplied to the magnetic field coil 40, it is preferable to provide two or more switching power supplies. FIG. 10 shows an example of each of the switching power supplies 31 and 32.
What was explained in can be applied. The reactor 61, 6
2, 71 and 72 have a structure as shown in FIG.
The reactor 61 will be described in detail as an example. The reactor 61 is provided in a reactor box 63, and an adjustment bar 64 is attached to the reactor box 63, which is rotatable with respect to a fixed seat 66 which is fixed to the reactor box 63 and has an internal thread so as to be movable in a vertical direction. An adjusting plate 65 made of a conductor, such as copper, is attached to the tip of the adjusting rod 64. Now, when a current is applied to the reactor 61, a magnetic flux is generated upward (in the direction of the arrow) of the reactor. Since the adjusting plate 65 is near the magnetic flux generated by the current flowing through the reactor, an eddy current is generated in the adjusting plate 65, and a clockwise eddy current flows in the adjusting plate 65 as shown in FIG. Further, due to the eddy current, a magnetic flux is generated (downward) in the vicinity of the adjustment plate 65 so as to cancel the magnetic flux generated by the reactor, thereby reducing the apparent inductance of the reactor.

The relative distance h between the adjustment plate 65 and the reactor 61
And the inductance of the reactor 61 have the relationship shown in FIG. Assuming that the inductance of the reactor 61 is L61, the adjustment rod 64 is rotated, and the adjustment plate 65 and the reactor 6 are rotated.
By changing the relative distance from 1, the value of the inductance can be adjusted to the predetermined value L0.

Similarly, reactors 62, 71, 72
Is also adjusted to the predetermined value L0 so that the switching power supply 3
The output current values of 1, 32 become equal, and the ripple of the current flowing through the magnetic field coil obtained by combining them can be made very small. FIG. 6 shows reactors 61, 62, 7
Another embodiment for adjusting the inductance values of 1, 72 will be described in detail with the reactor 61 as an example.

The reactor 61 is in a reactor box 63, and an adjustment rod 68 which can be moved in the vertical direction is attached by rotating with respect to a fixed seat 66 fixed to the reactor box 63 and having an internal thread. It is. An adjusting cylinder 67 made of a conductor, such as copper, is attached to the tip of the adjusting rod 68. When a current flows through the reactor 61 as in the embodiment of FIG. 3, a magnetic flux is generated upward (in the direction of the arrow) of the reactor. Since the adjusting cylinder 67 is near the magnetic flux generated by the reactor current, an eddy current flows through the adjusting cylinder 67 as shown in FIG.

Due to the eddy current, a magnetic flux is generated (downward) in the vicinity of the adjusting cylinder 67 so as to cancel the magnetic flux generated by the reactor, and the apparent inductance of the reactor decreases. The adjusting cylinder 67 and the reactor 6
The relationship shown in FIG. 8 exists between the relative distance h with respect to 1 and the inductance of the reactor 61. Assuming that the inductance of the reactor 61 is L61 ', the inductance can be adjusted to a predetermined value L0 by turning the adjustment rod 68 and changing the relative distance between the adjustment cylinder 67 and the reactor 61. Similarly, reactors 62, 71,
By adjusting 72 also to the predetermined value L0, the output current values of the switching power supplies 31 and 32 become equal, and the ripple of the magnetic field current obtained by combining them can be extremely reduced.

In the embodiment shown in FIG. 6, the change in the inductance with respect to the change in the relative distance h between the adjusting cylinder 67 and the reactor 61 is more gradual than in the embodiment shown in FIG.
It is more effective for fine adjustment of the inductance.

In this way, by adjusting the variation in inductance of several% that occurs in the production of the reactor,
The current ripple can be made extremely small, and the problem of blurring or artifacts in the image of the MRI apparatus can be solved even in high-speed imaging, and a high-quality MRI image effective for diagnosis can be obtained. In the present embodiment, the relative distance between the reactor and the conductor for adjustment is changed. However, the position of the reactor and the conductor may be changed relatively, and is not affected by the mechanism or the moving direction.

[0021]

According to the present invention as described above,
Even if the reactor connected to the output side of the plurality of switching power supplies for suppressing the sneak current (circulating current) between the plurality of switching power supplies connected in parallel has a manufacturing error of several%, the inductance of these reactors is reduced. The present invention provides a power supply device capable of obtaining a high-quality MRI image because the output current of the switching power supply is adjusted to make the output current of the switching power supply uniform and the ripple of the magnetic field coil current to a very small value, and a magnetic resonance imaging apparatus using the same. be able to.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram in which a power supply device of the present invention is applied to a gradient coil of an MRI apparatus.

FIG. 2 is a waveform diagram of a current flowing through a reactor and a magnetic field coil of the circuit of FIG. 1;

FIG. 3 is a diagram showing an embodiment of adjusting the inductance value of the reactor according to the present invention.

FIG. 4 is a diagram illustrating an eddy current generated in the reactor of FIG. 3;

FIG. 5 is a diagram showing a relationship between a relative distance between a reactor and an adjustment plate and an inductance in FIG. 3;

FIG. 6 is a diagram showing another embodiment for adjusting the value of the inductance of the reactor according to the present invention.

FIG. 7 is a diagram illustrating an eddy current generated in the reactor of FIG. 6;

FIG. 8 is a diagram showing a relationship between a relative distance between a reactor and an adjustment plate and an inductance in FIG. 6;

FIG. 9 is a circuit configuration diagram in which a conventional power supply device is applied to a gradient coil of an MRI apparatus.

FIG. 10 is a circuit diagram illustrating an example of a configuration of a switching power supply.

11 is a waveform diagram of each part of the circuit of FIG. 10 (when the phase of the switch is not shifted).

12 is a waveform diagram of each part of the circuit of FIG. 9 (when the phase of a switch is shifted).

13 is a current waveform of each part when the inductance value of the circulating current suppressing reactor of the conventional power supply device of FIG. 9 varies.

FIG. 14 is a structural diagram of a general reactor.

[Explanation of symbols]

Reference Signs List 1 bobbin 2 winding 31, 32 switching power supply 40 gradient magnetic field coil 50 DC power supply 61, 62, 71, 72 reactor with adjustable inductance

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroyuki Takeuchi 1-1-1 Uchikanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Corporation

Claims (3)

[Claims] 1. A power supply device for outputting a current having an arbitrary waveform, comprising a plurality of switching power supplies having switching elements connected in parallel with a load, and a sneak current (circulating current) between the plurality of switching power supplies. A power supply device comprising: a reactor connected to an output side of each of the plurality of switching power supplies; and a means for adjusting an inductance value of the reactor. 2. The power supply device according to claim 1, wherein the reactor adjusting means includes a conductor arranged near the reactor, and a mechanism for adjusting a relative distance between the conductor and the reactor. 3. A magnetic resonance imaging apparatus comprising the power supply device according to claim 1 as a power supply device for a magnetic field generating coil.