Patent Application
20090316860
The present invention relates to a multi X-ray generator used for nondestructive X-ray imaging, diagnosis, and the like in the fields of medical equipment and industrial equipment which use X-ray sources.
Conventionally, an X-ray tube uses a thermal electron source as an electron source, and obtains a high-energy electron beam by accelerating the thermal electrons emitted from a filament heated to a high temperature via a Wehnelt electrode, extraction electrode, acceleration electrode, and lens electrode. After shaping the electron beam into a desired shape, the X-ray tube generates X-rays by irradiating an X-ray target portion made of a metal with the beam.
Recently, a cold cathode electron source has been developed as an electron source replacing this thermal electron source, and has been widely studied as an application of a flat panel display (FPD). As a typical cold cathode, a Spindt type electron source is known, which extracts electrons by applying a high electric field to the tip of a needle with a size of several 10 nm. There are also available an electron emitter using a carbon nanotube (CNT) as a material and a surface conduction type electron source which emits electrons by forming a nanometer-order microstructure on the surface of a glass substrate.
Patent references 1 and 2 propose, as an application of these electron sources, a technique of extracting X-rays by forming a single electron beam using a Spindt type electron source or a carbon nanotube type electron source. Patent reference 3 and non-patent reference 1 disclose a technique of generating X-rays by irradiating an X-ray target portion with electron beams from a multi electron source using a plurality of these cold cathode electron sources.
Patent reference 1: Japanese Patent Laid-Open No. 9-180894
Patent reference 2: Japanese Patent Laid-Open No. 2004-329784
Patent reference 3: Japanese Patent Laid-Open No. 8-264139
Non-patent reference 1: Applied Physics Letters 86, 184104 (2005), J. Zhang “Stationary scanning x-ray source based on carbon nanotube field emitters”
In addition, as shown in
It is an object of the present invention to provide a compact multi X-ray generator which can solve the above problems and form multi X-ray beams with few scattered X-rays and excellent uniformity and an X-ray imaging apparatus using the generator.
In order to achieve the above object, a multi X-ray generator according to the present invention is technically characterized by comprising a plurality of electron emission elements, acceleration means for accelerating electron beams emitted from the plurality of electron emission elements, and a target portion which is irradiated with the electron beams, wherein the target portion is provided in correspondence with the electron beams, the target portion comprises X-ray shielding means, and X-rays generated from the target portion are extracted as multi X-ray beams into the atmosphere.
According to a multi X-ray generator according to the present invention, X-ray sources using a plurality of electron emission elements can form multi X-ray beams whose divergence angles are controlled, with few scattered and leakage X-rays. Using the multi X-ray beams can realize a compact X-ray imaging apparatus with excellent uniformity of beams.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
The present invention will be described in detail based on the embodiments shown in
A transmission-type target portion 13 upon which the emitted electron beams e impinge is discretely formed on the anode electrode 20 so as to face the electron beams e. The transmission-type target portion 13 is further provided with an X-ray shielding plate 23 made of a heavy metal. The X-ray shielding plate 23 in this vacuum chamber has X-ray extraction portions 24. A wall portion 25 of the vacuum chamber 11 is provided with X-ray extraction windows 27 having X-ray transmission films 26 at positions in front of the X-ray extraction portions.
The electron beams e emitted from the electron emission elements 15 receive the lens effect of the lens electrode 19, and are accelerated to the final potential level by portions of the transmission-type target portion 13 of the anode electrode 20. X-ray beams x generated by the transmission-type target portion 13 pass through the X-ray extraction portions 24 and are extracted to the atmosphere via the X-ray extraction windows 27. The plurality of X-ray beams x are generated in accordance with the plurality of electron beams e from the plurality of electron emission elements 15. The plurality of X-ray beams x extracted from the X-ray extraction portions 24 form multi X-ray beams.
The electron emission elements 15 are two-dimensionally arrayed on the element array 16, as shown in
When voltages of several 10 to several 100 V are applied to the extraction electrodes 33 and 36 of the Spindt type element and carbon nanotube type element, high electric fields are applied to the tips of the emitters 34 and 35, thereby emitting the electron beams e by the field emission phenomenon.
As electron sources for the generation of multi X-ray beams other than the above electron emission elements, MIM (Metal Insulator Metal) type elements and MIS (Metal Insulator Semiconductor) type elements can be used. In addition, cold cathode type electron sources such as a semiconductor PN junction type electron source and a Schottky junction type electron source can be used.
An X-ray generator using such a cold cathode type electron emission element as an electron source emits electrons by applying a low voltage to the electron emission element at room temperature without heating the cathode. This generator therefore requires no wait time for the generation of X-rays. In addition, since no power is required for heating the cathode, a low-power-consumption X-ray source can be manufactured even by using a multi X-ray source. Since currents from these electron emission elements can be ON/OFF-controlled by high-speed driving operation using driving voltages, a multiarray type X-ray source can be manufactured, which selects an electron emission element to be driven and performs high-speed response operation.
For this reason, the X-ray shielding plate 23 in the vacuum chamber and the multi transmission-type target portion 13 are integrated into a single structure. The X-ray extraction portions 24 provided in the X-ray shielding plate 23 are arranged at positions corresponding to the electron beams e so as to extract the X-ray beams x, each having a necessary divergence angle, from the transmission-type target portion 13.
Since the transmission-type target portion 13 formed by a thin metal film generally has low heat dissipation, it is difficult to apply large power. The transmission-type target portion 13 in this embodiment is, however, covered by the thick X-ray shielding plate 23 except for areas from which the X-ray beams x are extracted upon irradiation with the electron beams e, and the transmission-type target portion 13 and the X-ray shielding plate 23 are in mechanical and thermal contact with each other. For this reason, the X-ray shielding plate 23 has a function of dissipating heat generated by the transmission-type target portion 13 by heat conduction.
This makes it possible to form an array of a plurality of transmission-type target portions 13 to which power much larger than that applied to a conventional transmission type target portion can be applied. In addition, using the thick X-ray shielding plate 23 can improve the surface accuracy and hence manufacture a multi X-ray source with uniform X-ray emission characteristics.
As shown in
The X-ray generating layer 131 is made of a heavy metal with a film thickness of about several 10 nm to several μm to reduce the absorption of X-rays when the X-ray beams x are transmitted through the transmission-type target portion 13. The X-ray generation support layer 132 uses a substrate made of a light element to support the thin film layer of the X-ray generating layer 131 and also reduce intensity attenuation by the absorption of the X-ray beams x by improving the cooling efficiency of the X-ray generating layer 131 heated by the application of the electron beams e.
It has been generally thought that for the conventional X-ray generation support layer 132, metal beryllium is effective as a substrate material. In this embodiment, however, an Al, AlN, or SiC film with a thickness of about 0.1 mm to several mm or a combination thereof is used. This is because this material has high thermal conductivity and an excellent X-ray transmission characteristic, effectively absorbs X-ray beams, of the X-ray beams x, which are in a low-energy region and have little contribution to the quality of an X-ray transmission image by 50% or lower, and has a filter function of changing the radiation quality of the X-ray beams x.
Referring to
The following condition is required to prevent X-ray beams from adjacent X-ray sources from leaking to the outside by providing the X-ray shielding plate 23 in the vacuum chamber 11 and the X-ray shielding plate 41 outside the vacuum chamber 11. That is, the X-ray shielding plates 23 and 41 and the X-ray extraction portions 24 need to be set to maintain the relationship of d>2D·tan α where d is the distance between the X-ray beams x, D is the distance between the transmission-type target portion 13 and the X-ray shielding plate 41, and α is the radiation angle of the X-ray beam x exiting the X-ray shielding plate 23.
When the high-energy electron beam e strikes the transmission-type target portion 13, not only reflected electrons but also X-rays are scattered in the reflecting direction. These X-rays and electron beams are regarded as the causes of leakage X-rays from the X-ray sources and fine discharge with a high voltage.
When X-ray beams x are to be formed by irradiating the transmission-type target portion 13 with the high-energy electron beams e, the density of the X-ray beams x is not limited by the packing density of the electron emission elements 15. This density is determined by the X-ray shielding plates 23 and 41 for extracting the separate X-ray beams x from multi X-ray sources generated by the transmission-type target portion 13.
Table 1 shows the shielding effects of heavy metals (Ta, W, and Pb) against X-ray beams with energies of 50 keV, 62 keV, and 82 keV, assuming the energies of the X-ray beams x generated when the transmission-type target portion 13 is irradiated with the 100-kev electron beams e.
As a shielding criterion among the X-ray beams x generated from the transmission-type target portion 13, an attenuation factor of 1/100 is a proper value as an amount which does not influence X-ray images. Obviously, a heavy metal plate having a thickness of about 5 to 10 mm is required as a shielding plate for achieving this attenuation factor.
When this scheme is to be applied to a multi X-ray source body using the electron beams e of about 100 keV, it is appropriate to set thicknesses D1 and D2 of the X-ray/reflected electron beam shielding plate 43 and X-ray shielding plate 23 shown in
In the electron beam generating unit 12′, electron beams e emitted from the electron emission elements 15 pass through a lens electrode and accelerated to high energy. The accelerated electron beams e pass through the electron beam incident holes 42′ of the X-ray/reflected electron beam shielding plate 43′ and are applied to the reflection-type target portion 13′. The X-rays generated by the reflection-type target portion 13′ are extracted as X-ray beams x from the X-ray extraction portions 24′ of the X-ray/reflected electron beam shielding plate 43′. A plurality of X-ray beams x form multi X-ray beams. The X-ray/reflected electron beam shielding plate 43′ can greatly suppress the scattering of reflected electrons which cause high-voltage discharge.
As in the arrangement shown in
The second embodiment has exemplified an application of the present invention to the reflection-type target portion 13′ with a planar structure. However, the present invention can also be applied to a multi X-ray source body in which the electron beam generating unit 12′, the anode electrode 20′, and the reflection-type target portion 13′ are arranged in an arcuated shape. For example, placing the reflection-type target portion 13′ in an arcuated shape centered on an object and providing the X-ray shielding plates 23 and 41 can extremely reduce the region of the leakage X-rays x2 in the prior art shown in
As described above, the second embodiment can extract the independent X-ray beam x which has a high S/N ratio with very few scattered X-rays or leakage X-rays, from the X-rays generated by irradiating the reflection-type target portion 13′ with the electron beams e. Using this X-ray beam x can therefore execute X-ray imaging with high contrast and high image quality.
As in the first embodiment, the multi X-ray source body 10 generates a plurality of X-ray beams x by irradiating a transmission-type target portion 13 with a plurality of electron beams e extracted from an electron beam generating unit 12. The plurality of generated X-ray beams x are extracted as multi X-ray beams toward the multi X-ray intensity measuring unit 52 in the atmosphere via X-ray extraction windows 27 provided in a wall portion 25. The multi X-ray beams (the plurality of X-ray beams x) are impinged upon an object after being transmitted through the transmission type X-ray detector 51 of the multi X-ray intensity measuring unit 52. The multi X-ray beams transmitted through the object are detected by the X-ray detector 53, thus obtaining an X-ray transmission image of the object.
In electron emission elements 15 arrayed on an element array 16, slight variations occur in the current-voltage characteristics between the electron emission elements 15. The variations in emission current lead to variations in the intensity distribution of multi X-ray beams, resulting in contrast irregularity at the time of X-ray imaging. It is therefore necessary to uniform emission currents in the electron emission elements 15.
The transmission type X-ray detector 51 of the multi X-ray intensity measuring unit 52 is a detector using a semiconductor. The transmission type X-ray detector 51 absorbs parts of multi X-ray beams and converts them into electrical signals. The switch control circuit 54 then converts the obtained electrical signals into digital data. The control unit 56 stores the digital data as the intensity data of the plurality of X-ray beams x.
The control unit 56 stores correction data for the electron emission elements 15 which correspond to the voltage-current characteristics of the electron emission elements 15 in
The X-ray intensity correction method using the transmission type X-ray detector 51 can measure an X-ray intensity regardless of an object, and hence can correct the intensities of the X-ray beams x in real time during X-ray imaging.
Independently of the above correction method, it is also possible to correct the intensities of multi X-ray beams by using the X-ray detector 53 for imaging. The X-ray detector 53 uses a two-dimensional type X-ray detector such as a CCD solid-state imaging or an imaging using amorphous silicon, and can measure the intensity distributions of the respective X-ray beams.
In order to correct the intensities of the X-ray beams x by using the X-ray detector 53, it suffices to extract the electron beam e by driving the single electron emission element 15 and synchronously detect the intensity of the generated X-ray beam x by using the X-ray detector 53. In this case, it is possible to efficiently measure the intensity distributions of multi X-ray beams by performing measurement upon synchronizing a generation signal for each X-ray beam of multi X-ray beams with a detection signal from the X-ray detector 53 for imaging. This detection signal is converted into a digital signal by the X-ray detection signal processing unit 55. The signal is then stored in the control unit 56.
This operation is performed for all the electron emission elements 15. The resultant data are then stored as the intensity distribution data of all multi X-ray beams in the control unit 56. At the same time, correction values for driving voltages for the electron emission elements 15 are determined by using part or the integral value of the intensity distributions of multi X-ray beams.
At the time of X-ray imaging of the object, the multi electron emission element driving circuit 57 drives the electron emission elements 15 in accordance with the correction values for driving voltages. Performing this series of operations as periodic apparatus calibration can uniform the intensities of the X-ray beams x.
The above description has exemplified the case in which the electron emission elements 15 are individually driven to measure X-ray intensities. However, it is possible to speed up measurement by simultaneously irradiating with X-ray beams x a plurality of portions on the X-ray detector 53 on which the applied X-ray beams x do not overlap.
In addition, this correction method has the intensity distribution of each X-ray beam x as data, and hence can be used to correct irregularity in the X-ray beams x.
The X-ray imaging apparatus using the multi X-ray source body 10 of this embodiment can implement a planar X-ray source with an object size by arranging the X-ray beams x in the above manner, and hence the apparatus size can be reduced by placing the multi X-ray source body 10 near the X-ray detector 53. In addition, as described above, for the X-ray beams x, X-ray irradiation intensities and irradiation regions can be arbitrarily selected by designating driving conditions for the electron emission element driving circuit 57 and element regions to be driven.
In addition, the multi X-ray imaging apparatus can select the radiation angles of the X-ray beams x by changing the X-ray shielding plate 41 provided outside the vacuum chamber 11 shown in
The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention the following claims are made.
This application claims priority from Japanese Patent Application No. 2006-057846 filed on Mar. 3, 2006, and Japanese Patent Application No. 2007-050942 filed on Mar. 1, 2007, the entire contents of which are hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-057846 | Mar 2006 | JP | national |
| 2007-050942 | Mar 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP07/54090 | 3/2/2007 | WO | 00 | 4/13/2009 |
