JP3074757B2 - Rotating cathode X-ray tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rotating cathode X-ray tube for irradiating X-rays from all directions around a subject.

[0002]

2. Description of the Related Art A partial cross section of a rotary cathode X-ray tube according to a conventional example is shown in FIG. A ring-shaped anode target 3 is installed and fixed inside a ring-shaped vacuum vessel 2, and a rotating cathode 4 facing the anode target 3 is levitated and supported by a magnetic bearing 5. Rotating cathode 4
A filament 6 for emitting thermoelectrons is attached to the surface facing the anode target 3.

[0003] The filament 6 is connected to a ring-shaped slip ring 7 arranged on the inner peripheral surface of the rotating cathode 4, and a power supply brush 8 is in contact with the slip ring 7. The power supply brush 8 is connected to an AC power supply 9 outside the vacuum vessel 2, and the filament 6 is heated by the output of the AC power supply 9 to emit thermoelectrons. Note that reference numeral 10
Is a DC high-voltage power supply for applying an electric field for accelerating thermionic electrons between the filament 6 and the anode target 3, 11 is a stator for driving the rotating cathode 4 for rotation, and 12 is for detecting a gap between the rotating cathode 4 and the vacuum vessel 2. It is a sensor.

Various types of power supply brushes 8 are used. For example, as shown in FIG. 8, a carbon contactor 14 is inserted into a holder 13 installed on the inner peripheral surface of the vacuum vessel 2. In some cases, a contact 15 is pressed against the slip ring 7 by a spring 15 provided in the holder 13 to maintain the contact state. In the power supply brush 8, the contact 14 is prevented from separating from the rotating cathode 4 by the pressing force of the spring 15, following the depletion of the contact 14 and the vibration of the rotating cathode 4. The AC power supplied to the filament 6 using such a power supply brush 8 is generally a relatively small voltage and large current having a voltage of 15 V and a current of 5 A.

[0005]

However, it is extremely difficult to make the surface of the slip ring 7 that is in contact with the power supply brush 8 flat completely, and even though the contact surface is slightly uneven. Existing. Therefore, no matter how much the power supply brush 8 presses the contact 14 against the rotating cathode 4 by the spring 15, the power supply brush 8 follows the slight uneven portion of the slip ring 7 rotating at high speed and completes it. It is almost impossible to maintain contact. Therefore, the contact area of the two parts changes in some places, and the contact resistance slightly changes. As described above, the power supply brush 8
Since the current flowing through the filament 6 via the slip ring 7 is a relatively large current, the value of the current that changes with the change in the contact resistance also becomes large.

When the current flowing through the filament 6 fluctuates, the amount of thermoelectrons emitted from the filament 6 changes, and accordingly, the X-ray tube current measured between the filament 6 and the anode target 3 changes. Since the amount of generated X-rays depends on the X-ray tube current, there is a problem that a change in the X-ray dose causes deterioration of the image quality of the tomographic image.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a rotating cathode X-ray tube capable of minimizing a current fluctuation to a filament.

[0008]

The present invention has the following configuration to achieve the above object. That is, a ring-shaped anode target installed and fixed in a ring-shaped vacuum vessel, a rotating cathode equipped with an electron emitting portion for emitting an electron beam toward the anode target, and the electron emission contacting the rotating cathode. In a rotary cathode X-ray tube having a power supply electrode for supplying power to the unit, a transformer for increasing an alternating current is provided between the power supply electrode and the electron emission unit.

[0009]

According to the present invention, since a transformer for increasing an alternating current is provided between the power supply electrode and the electron emission portion, a relatively small current may be supplied to the power supply electrode. Changes in the current are kept in a minute range.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a partial sectional view of a rotating cathode X-ray tube. As in the conventional case, a ring-shaped anode target 3 and a rotating cathode 4 which is supported to be levitated and opposed to the ring-shaped anode target 3 are provided inside a vacuum vessel 2 formed in a ring shape. A filament 6 for emitting thermoelectrons is attached to the surface facing 3.

A transformer 31 for converting large-voltage, small-current power into small-voltage, large-current power is fixedly installed inside the rotating cathode 4 via an insulating spacer 30. The filament 6 is connected to the secondary coil 32 of the transformer 31, and the power supply brush 8 is connected to the primary coil 33 via the slip ring 7. The power supply brush 8 is connected to an AC power supply 34 for heating the filament, which is provided outside the vacuum vessel 2. Other components are the same as those of the conventional example shown in FIG.

FIG. 2 shows a circuit diagram from the AC power supply 34 to the filament 6. As described above, the filament 6
The power supplied to the power supply is, for example, power of a relatively small voltage and a large current of 15 V and 5 A. When such a large current flows through the power supply brush 8, the current value also fluctuates greatly due to fluctuations in the contact resistance with the slip ring 7. Therefore, the AC power supply of this embodiment
Numeral 34 supplies a relatively large voltage and small current power of, for example, 150 V, 0.5 A, and converts it into a required 15 V, 5 A by a transformer 31 inside the rotating cathode 4 and supplies it to the filament 6. Have been. That is, the primary coil 33 of the transformer 31
And the winding ratio of the secondary coil 32 is set to 10: 1.

Here, assuming that the contact resistance has fluctuated by "+0.1 Ω", the difference in the fluctuation of the filament supply current between the case where there is no transformer 31 and the case where there is a transformer 31 is shown below. First, when the transformer 31 is not provided, the required power of "15 V, 5 A" is supplied to the power supply brush 8 from the AC power supply 9 of FIG. Now, if the contact resistance between the power supply brush 8 and the slip ring 7 is replaced by a simple resistance element R, this resistance R
Becomes “3Ω”, which fluctuates by “+0.1 Ω”, and becomes “3.1 Ω”. The current value at this time is “4.84
A ”, and the current flowing in the circuit after the resistor R, that is, the current flowing from the power supply brush 8 to the filament 6 has changed by“ −0.16 A ”. Generally, when the heating current of the filament changes by "0.1 A", the X-ray tube current changes by several tens mA or more according to the change in the amount of emitted thermoelectrons.

Next, when there is the transformer 31, electric power of "150 V, 0.5 A" is supplied to the power supply brush 8 from the AC power supply 34 shown in FIG. The contact resistance is changed to the resistance R
If the value of the resistor R is “300 Ω”, “+0.1
Now it changes to “300.1 Ω”. The current value at this time is "0.499 A", which is supplied to the circuit after the resistor R, that is, the primary coil 33 of the transformer 31.
Since the transformer 31 transmits this current to the filament 6 by increasing the current by 10 times and the voltage by 1/10, the current supplied to the filament 6 is about “4.99 A”, which is a fluctuation of “−0.01 A”. . Therefore, in the case where the transformer 31 of this example is provided, the fluctuation of the filament supply current is reduced theoretically to about 1/16, and the fluctuation of the X-ray tube current is conventionally reduced to about 1/16 as described above. Although it fluctuated by several tens of mA, it changed to about several mA
It can be suppressed to a small degree.

An example of mounting the transformer 31 other than the above will be described below. (1) As shown in the partial cross-sectional view of FIG. 3, the transformer 31 is supported by an insulating support member 40 attached to the outer peripheral surface of the rotating cathode 4. (2) As shown in the partial sectional view of FIG.
The transformer 31 is put in the inside, and the container 41 is attached to the outer peripheral surface of the rotating cathode 4. (3) As shown in the partial cross-sectional view of FIG. 5, the transformer 31 is molded with a low vapor pressure material such as Teflon and attached to the outer peripheral surface of the rotating cathode 4. As described in (2) and (3) above, by closing the transformer 31, the temperature of the transformer 31 rises, and the material constituting the coil and the like is vaporized. Deterioration of the degree of vacuum can be prevented. The same applies to the case where the transformer 31 is built in the rotating cathode 4 as in the above embodiment.

FIGS. 3 to 5 show an example in which the transformer 31 is installed on the side opposite to the surface on which the filament 6 is attached. However, the place where the transformer 31 is installed is not limited to this, and as shown in FIG. May be mounted on the surface on which the filament 6 is mounted. In this case, it is preferable to surround the transformer 31 with a magnetic shield material 42 so that the orbit of the thermoelectrons emitted from the filament 6 is not distorted by the leakage magnetic field of the transformer 31.

[0017]

As is apparent from the above description, the rotating cathode X-ray tube according to the present invention has a transformer for increasing the current between the power supply electrode and the electron-emitting portion, so that the current flows to the power supply electrode. The current can be relatively small. Therefore, even if the resistance value at the contact surface between the power supply electrode and the rotating cathode fluctuates, the fluctuation of the current becomes a small value, and X
Fluctuations in the tube current and accompanying fluctuations in the generated X-ray dose are also suppressed to a very small range, and a tomographic image with stable image quality can be obtained without causing a decrease in contrast of the tomographic image.

[Brief description of the drawings]

FIG. 1 is a partial sectional view showing a rotary cathode X-ray tube according to an embodiment of the present invention.

FIG. 2 is a circuit diagram from an AC power supply to a filament.

FIG. 3 is a cross-sectional view showing another example of mounting the transformer.

FIG. 4 is a sectional view showing another example of mounting the transformer.

FIG. 5 is a sectional view showing another example of mounting the transformer.

FIG. 6 is a cross-sectional view showing another example of mounting the transformer.

FIG. 7 is a partial sectional view showing an example of a conventional rotating cathode X-ray tube.

FIG. 8 is a cross-sectional view illustrating a structure of a power supply electrode.

[Explanation of symbols]

 6 Filament 8 Power supply brush 31 Transformer 41 Closed container

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01J 35/26 H01J 35/06

Claims (1)

(57) [Claims] 1. A ring-shaped anode target installed and fixed in a ring-shaped vacuum vessel, a rotating cathode provided with an electron-emitting portion for emitting an electron beam toward the anode target,
In a rotating cathode X-ray tube including a power supply electrode that contacts the rotating cathode and supplies power to the electron emission unit, a transformer that increases an alternating current is provided between the power supply electrode and the electron emission unit. A rotating cathode X-ray tube, characterized by being provided.