JP5200259B2 - X-ray tube - Google Patents

Description

  The present invention relates to an X-ray tube with reduced out-of-focus X-rays.

Conventionally, X-ray generation sources are usually used in medical X-ray devices that use X-rays to see through a subject and diagnose the subject, and industrial X-ray devices that inspect a subject such as an article with X-rays. X-ray tubes are used.
In this X-ray tube, a cathode and an anode are arranged opposite to each other in a vacuum vessel, and thermoelectrons generated from a filament provided on the cathode side collide with a target provided on the anode side. Is configured to be generated.
That is, a high voltage is applied between the cathode and the anode to generate X-rays from the anode, but the conversion rate of X-rays is about 1% with respect to the applied power, and the remaining power is Heat is emitted from the X-ray tube.
For this reason, a metal such as copper having a high thermal conductivity is used for the anode for the purpose of enhancing the heat dissipation effect, and a metal such as tungsten having a high X-ray conversion rate is used for the target.

On the other hand, some of the thermoelectrons that have collided with the target are scattered as secondary electrons, and all of these secondary electrons re-impact on the outer surface of the anode.
As a result, characteristic X-rays specific to the material are generated from the outer surface of the anode, and these are mixed into the X-rays as out-of-focus X-rays, which causes the image quality of the X-ray image to deteriorate.
In order to improve such a problem, for example, Patent Document 1 proposes an anode target capable of reducing out-of-focus X-rays.

In the anode target described in Patent Document 1, a plurality of focal portions are provided at intervals along the rotation direction on the surface of the rotating anode target, and an annular groove is formed around the focal portions. When the secondary electrons scattered from the focal part re-impact in the groove part and re-emitted as out-of-focus X-rays from inside the groove part, the groove part blocks out-of-focus X-rays. This has the effect of reducing mixing.
JP 2007-73297 A

As in conventional X-ray tube, when the X-ray tube to be used by applying a higher tube voltage 30 kV, also out of focus X-ray generated from the anode is mixed into the X-ray, the influence of the X-ray image Few.
The reason is that the characteristic X-ray energy of the K columns of the copper used in the anode whereas a 8～9KeV, since the energy of X-rays is large enough, the out-of-focus X-ray to X-rays is mixed However, this is because it rarely appears as noise in the X-ray image.
However, in recent years, the use of an X-ray tube at a low tube voltage such as subtraction has attracted attention. However, when a conventional X-ray tube is used at a low tube voltage, the following problems occur.

As shown in FIG. 4, the energy spectrum of white X-rays generally has a Gaussian distribution with a peak energy of 1/2 to 2/3 of the tube voltage (in the figure, A indicates that there is no permeate, and B indicates that An energy spectrum of 0.1 mm iron is permeated, and C is an energy spectrum of 0.3 mm iron permeated).
For example, when an X-ray tube is used at a low tube voltage of 20 kV, the maximum energy spectrum of X-rays is 10 to 15 keV.
For this reason, when copper is used for the anode, the X-ray energy and the out-of-focus X-ray energy, that is, the characteristic X-ray energy of the copper K row are equivalent, so that the out-of-focus X-ray is mixed into the X-ray. In this case, it appears as noise in the X-ray image, which causes the image quality to deteriorate.
Further, since all secondary electrons scattered from the anode collide with the outer surface of the anode and generate out-of-focus X-rays, the dose of out-of-focus X-rays increases and causes the quality of the X-ray image to deteriorate. .
In order to prevent this, it is conceivable to use molybdenum having high thermal conductivity for the anode. However, since the characteristic X-ray energy of the K row of molybdenum is about 20 keV, when it is used at a tube voltage of 50 kV or less. A similar problem occurs.

In the case where a groove portion is provided around the focal portion as in the anode target described in Patent Document 1 and the out-of-focus X-ray is shielded by this groove portion, the thermoelectrons scattered from the focal portion like the rotating anode are Appropriate shielding effect can be expected for those diffused in a relatively narrow range.
However, for those in which thermionic electrons scattered from the target diffuse over a wide area such as the outer surface of the fixed anode, such as a fixed anode, the shielding effect of out-of-focus X-rays can be achieved by simply providing a groove around the focus. Is hardly obtained, and there is a possibility that out-of-focus X-rays cannot be reduced.
The present invention has been made to remedy such problems, and an object of the present invention is to provide an X-ray tube capable of reducing out-of-focus X-rays generated from an anode.

  In the X-ray tube of the present invention, a tube voltage is applied between a cathode and an anode arranged opposite to each other in a vacuum vessel so that thermoelectrons generated from a filament provided on the cathode collide with a target provided on the anode. An X-ray tube that generates X-rays by using an out-of-focus X-ray reduction layer that reduces the dose of out-of-focus X-rays generated when secondary electrons generated when X-rays are generated collide with the anode. Formed on the outer surface of the anode excluding.

  According to the X-ray tube of the present invention, since the dose of out-of-focus X-rays generated when secondary electrons collide with the outer surface of the anode is reduced, The out-of-focus X-rays are less likely to appear as noise in the line image, and even in the case of an X-ray tube used at a low tube voltage, it is possible to prevent deterioration in the image quality of the X-ray image.

Embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a partially cutaway side view of an X-ray tube, FIG. 2 is an explanatory view of the operation of the X-ray tube, and FIG. 3 is an enlarged view in a circle D of FIG.
The X-ray tube shown in FIG. 1 is a reflection type X-ray tube used at a low tube voltage of, for example, 20 kV, and the cathode 2 and the anode 3 are opposed to each other on the same center line in the vacuum vessel 1 with a space therebetween. Has been placed.
The cathode 2 is formed in a round bar shape with a metal having a high thermal conductivity, such as copper, and a filament 2a is provided at substantially the center of the end surface facing the anode 3.
A voltage is applied to the filament 2a from a filament heating circuit (not shown) so that thermoelectrons (primary electrons) are generated from the filament 2a.

The anode 3 disposed opposite to the cathode 2 is also formed in a round bar shape having substantially the same diameter as the cathode 2 from a metal such as copper having high thermal conductivity, and the end surface facing the cathode 2 is the outer surface of the vacuum vessel 1. A target 3b formed of a metal such as tungsten having a high X-ray conversion efficiency is formed at a substantially center of the inclined surface 3a. The target 3b is inclined at the center of the inclined surface 3a. It is provided so as to face.
A low tube voltage of about 20 kV, for example, is applied to the anode 3 from a high voltage generation circuit (not shown) so that the thermoelectrons generated from the filament 2a of the cathode 2 collide with the target 3b of the anode 3. X-rays 4 are generated from the target 3b toward the X-ray irradiation port 1a.

Further, when the thermoelectrons generated from the filament 2a of the cathode 2 collide with the target 3b of the anode 3, some of the thermoelectrons are scattered and secondary electrons 5 are generated as shown in FIG. A characteristic X-ray unique to the material (copper) of the anode 3 is generated when it collides with the outer surface of the anode 3 again, and this becomes an out-of-focus X-ray.
When this out-of-focus X-ray is mixed into the X-ray 4 generated from the target 3b, particularly in the case of an X-ray tube used at a low tube voltage, it appears as noise in the X-ray image, causing a deterioration in the image quality of the X-ray image. It becomes.
In order to prevent this, in this embodiment, the out-of-focus X-ray reduction layer 6 is formed on the entire outer surface of the anode 3 excluding the target 3b portion.

As the out-of-focus X-ray reduction layer 6, irregular irregularities 6 a are formed on the outer surface of the anode 3 as shown in FIG. 3 to diffuse the secondary electrons 5 that collide with the outer surface of the anode 3, thereby By dispersing the energy of the secondary electrons 5, the dose of out-of-focus X-rays is reduced.
As a method of forming the out-of-focus X-ray reduction layer 6 made of unevenness 6a on the outer surface of the anode 3, for example, sand blasting is adopted.
That is, fine sand particles are sprayed on the outer surface of the anode 3 with a high-pressure gas to form irregular irregularities 6a on the outer surface, and the surface roughness of the irregularities 6a is, for example, about Rmax: 200.
Next, the operation of the X-ray tube constructed as described above will be described.

The X-ray tube used at the low tube voltage of the present invention is used for a medical X-ray apparatus or an industrial X-ray apparatus, but can be used for other X-ray apparatuses.
When a low tube voltage of about 20 kV, for example, is applied between the cathode 2 and the anode 3 in a state where the filament 2a of the cathode 2 is heated by the filament heating circuit and is generated in the X-ray apparatus, the cathode 2 generates the filament 2a. The thermoelectrons collide with the target 3b provided on the anode 3 to generate X-rays 4. The X-rays 4 are emitted from the X-ray irradiation port 1a, and then collimated by a collimator, to a subject (both not shown). Irradiated and used for medical examination and examination of the subject.

On the other hand, when the thermoelectrons generated from the filament 2 a collide with the target 3 b, some of them are scattered and secondary electrons 5 are generated separately from the X-rays 4.
The secondary electrons 5 directly collide with the outer surface of the anode 3 or are reflected by the inner surface of the vacuum vessel 1 so that almost all of them re-impact on the outer surface of the anode 3.
However, irregular fine irregularities 6a are formed on the outer surface of the anode 3, and the secondary electrons 5 that have collided as shown in FIG. Distributed.
As a result, the dose of out-of-focus X-rays generated when the secondary electrons 5 collide with the outer surface of the anode 3 is reduced. Therefore, even if out-of-focus X-rays are mixed into the X-rays 4, they are out of focus on the X-ray image. X-rays are less likely to appear as noise, and even in the case of an X-ray tube used at a low tube voltage, it is possible to prevent deterioration in the image quality of the X-ray image.

In the above-described embodiment, the out-of-focus X-ray reduction layer 6 is formed of irregular irregularities 6a. However, a substance whose characteristic X-ray of the material used for the anode 3 is higher than the tube voltage on the outer surface of the anode 3; For example, the out-of-focus X-ray reduction layer 6 may be formed by vacuum-depositing a metal having a large atomic number such as tungsten or tartar on the outer surface of the anode 3 made of copper or molybdenum.
That is, since the secondary electrons 5 scattered when X-rays are generated do not have energy higher than the tube voltage, even if a substance has an absorption edge higher than the tube voltage, it has an energy lower than the absorption edge. Since no excitation occurs at the absorption edge, characteristic X-rays are not generated, and the X-ray image is hardly affected by out-of-focus X-rays.

It is a partially cutaway side view of the X-ray tube which becomes embodiment of this invention. It is operation | movement explanatory drawing of the X-ray tube which becomes embodiment of this invention. FIG. 3 is an enlarged view in a circle D of FIG. 2. It is a diagram which shows the X-ray spectrum distribution of a general X-ray tube.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum container 1a X-ray irradiation port 2 Cathode 2a Filament 3 Anode 3b Target 4 X-ray 5 Secondary electron 6 Out-of-focus X-ray reduction layer

Claims (1)

X-rays are generated by applying a tube voltage between a cathode and an anode disposed opposite to each other in a vacuum vessel and causing thermoelectrons generated from a filament provided on the cathode to collide with a target provided on the anode. An X-ray tube, wherein an out-of-focus X-ray reduction layer that reduces a dose of out-of-focus X-rays generated by secondary collision of secondary electrons generated when X-rays are generated collides with the anode, the anode excluding the target Formed on the outer surface of the
The out-of-focus X-ray reduction layer is formed by coating a material having a characteristic X-ray higher than the tube voltage on the outer surface of the anode excluding the target.

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