Technical field

The present invention relates to the field of medical imaging and, particularly, to an apparatus for selecting X-ray energy spectrum.

Background art

In the field of X-ray imaging, it is desirable to further reduce the X-ray dose received by a human body from within the range of the current X-ray dose, so as to further reduce the X- ray radiation on the human body. Since the X-rays emitted by an X-ray tube have a continuous wide energy spectrum, for example, it is distributed in the range of 1-150 keV, in which low- energy X-rays (such as 1-10 keV) do not contribute to the final image result and they will be attenuated off when passing through the tissues and organs of a human body, thus causing the human body to absorb useless X-rays. In order to reduce the low energy X-ray radiation to the human body, the current common practice is to arrange some thin layers (hereafter referred to as attenuation layer) made of attenuation materials in the propagation direction of the X-rays, and these attenuation materials, for example, are metal (such as copper and aluminum) or non-metallic materials (such as graphite and PTFE) so as to attenuate the low-energy X-rays before the X- rays reach the human body. Although the X-ray energy spectrum has changed after having been attenuated by the attenuation layer, it still has a continuous wide energy spectrum, such as 10-150 keV, and as to the current X-ray imaging technology, the wider the X-ray energy spectrum, the worse the signal-to-noise ratio; and at the same time the X-rays with a wide energy spectrum will also affect the further development of X-ray imaging methods .

In the X-ray energy spectrum imaging (such as dual-energy imaging), a subject to be examined is irradiated by a series of X-rays with different energies, so as to acquire histologically different presentations of the subject to be examined, for example, when the voltage of a limiter tube is 70 keV in the X- ray dual-energy imaging, the resulting X-ray energy band is 0- 70 keV; when the voltage of the limiter tube is 140 keV, the resulting X-ray energy band is 0-140 keV; as a result, the X- ray energy bands under the two types of tube voltages have an overlapping part, and in order to reduce this overlapping part, it is required to design an energy spectrum selector so as to carry out the selective shielding of X-rays with different energies when different tube voltages are applied. However, in the prior art, this overlapping part can only be reduced, and the X-rays under two types of tube voltages cannot be separated completely. In addition, the X-rays still have a wide energy spectrum after energy spectrum selection is carried out thereon by an energy spectrum selector, and the signal-to- noise ratio is rather poor, which results in difficulties in expanding the application of energy spectrum imaging.

In addition, as to some emerging X-ray imaging technologies (such as phase difference imaging) , single-energy X-rays are an important foundation, and currently such single-energy X-rays are mainly obtained in the laboratory by synchrotron radiation under specific conditions, and since such single-energy X-rays cannot be obtained under normal conditions, it is very difficult to carry out their commercialization .

Furthermore, the current X-ray imaging technologies focus on the X-ray imaging with extended functions (such as by energy spectrum and photon counting) instead of anatomical X-ray imaging, thus the X-ray imaging mode is changed.

The patent of the grant announcement no. CN100373184C discloses a wide stop band dual-channel bandpass filter, which can restrain sidebands very well at both ends of the transmission peak and at the same time has a high peak value passing rate. However, this filter is for filtering visible light and it cannot perform energy spectrum selection on the X-rays with energy far higher than visible light; and this patent is used in the field of optical detectors and space technology rather than the field of X-ray imaging.

Contents of the invention

In order to overcome the above-mentioned defects in the prior art, the present invention proposes an apparatus for selecting X-ray energy spectrum, so as to carry out energy spectrum selection on the X-rays, i.e. without changing the original X-ray energy spectrum, the X-rays in a predetermined energy band are obtained from the original X-ray spectrum by this apparatus for selecting X-ray energy spectrum, so as to provide a new way of imaging and a new application extension field for X-ray imaging.

In the view of this situation, the present invention provides an apparatus for selecting X-ray energy spectrum, and said apparatus comprises an X-ray tube and further comprises a reflective film filtering mirror, wherein said reflective film filtering mirror is arranged to form an angle between its surface and the propagation direction of an X-ray beam emitted by said X-ray tube, so as to reflect the X-rays in a predetermined energy band.

According to an embodiment of the present invention, said predetermined energy band comprises more than one energy band with the level of magnitude of keV.

According to another embodiment of the present invention, said predetermined energy band comprises more than one energy band in the range of 10 keV to 150 keV.

According to another embodiment of the present invention, said energy bands are separated from each other.

Furthermore, said apparatus comprises more than one reflective film filtering mirrors, and these reflective film filtering mirrors are parallel to each other.

According to an embodiment of the present invention, said reflective film filtering mirror comprises a plurality of types of materials.

According to an embodiment of the present invention, said reflective film filtering mirror comprises a plurality of layers of reflective film.

In this case, the layer thickness of each reflective film layer is at the level of magnitude of nm.

The layer thickness of said each reflective film layer is set according to Bragg' s formula as follows: 2i sin 6' = l, wherein d represents the layer thickness of said each reflective film layer, Θ represents said included angle, and λ represents the wavelength of the X-rays in said predetermined energy band.

According to an embodiment of the present invention, said plurality of layers of reflective film comprise a reflective film layer made of a material W and a reflective film layer made of a material B ₄ C, and these two types of reflective film layers are arranged alternately.

According to an embodiment of the present invention, the layer thickness of said reflective film layer made of the material W is 1 nm, and the layer thickness of said reflective film layer made of the material B ₄ C is 1.5 nm.

Preferably, said included angle is in the range of 0.2 to 2 degrees . It can be seen from the above solution that since the present invention provides an apparatus for selecting X-ray energy spectrum, which uses a reflective film filtering mirror as an energy spectrum selector for X-rays, the energy spectrum selection of the original X-rays (i.e. the X-ray beam emitted by an X-ray tube) is achieved by reflection, so as to obtain the X-rays in the predetermined energy band, thus it provides a new way of imaging and a new application extension field for the X-ray imaging. At the same time, as compared to the attenuation layer in the prior art, the X-ray energy band produced by the present invention is narrow, or even a narrow energy band which is close to single energy with the magnitude at keV level, and it further reduces the X-ray dose received by the subject to be examined while improving the signal-to-noise ratio. Moreover, as compared to the currently available X-ray energy spectrum imaging technologies, a plurality of X-ray energy bands produced by the present invention can be separated from each other, without overlapping, which are advantageous for image analysis and improving imaging quality. Furthermore, the apparatus of the present invention can be produced on a large scale, which is advantageous for commercialization. In addition, the present invention further uses a plurality of reflective film filtering mirrors to reflect the incident X- rays more than once and can obtain the X-rays in the predetermined energy band without changing the propagation direction of the incident X-rays or rotating the X-ray tube relative to the original location, so that it can be easily implemented on the basis of the current X-ray imaging equipment .

Description of the accompanying drawings

Fig. 1 is a schematic diagram of the comparison results by using an apparatus for selecting energy spectrum of the present invention and an attenuation layer in the prior art to perform energy spectrum selection to the original X-rays respectively .

Fig. 2 is a schematic diagram of the reflectivities of the X-rays in different bands by the reflective film filtering mirror shown in Fig. 1.

Fig. 3 is a schematic diagram of the results using the reflective film filtering mirror of the present invention to perform energy spectrum selection to the original X-rays to obtain close to single-energy X-rays, which are compared with the X-rays having full width at half maximum 1 keV.

Fig. 4 is a schematic diagram of using the reflective film filtering mirror of the present invention to perform energy spectrum selection to the original X-rays to obtain two separated narrow energy bands X rays .

Fig. 5 is a schematic diagram of a geometrical beam path to carry out energy spectrum selection, in which one reflective film filtering mirror of the present invention is used to reflect the original X-rays.

Fig. 6 is a schematic diagram of a geometrical light path to carry out energy spectrum selection, in which two reflective film filtering mirrors of the present invention are used to reflect the original X-rays.

Particular embodiments

In order to make the object, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail hereinbelow by way of embodiments in conjunction with the accompanying drawings.

The present invention provides an apparatus for energy spectrum selection which is different from the attenuation layer, which apparatus uses a reflective film filtering mirror rather than attenuation materials to carry out the energy spectrum selection of the original X-rays by reflecting the X-rays in a predetermined energy band. The apparatus for selecting X-ray energy spectrum of the present invention comprises an X-ray tube and a reflective film filtering mirror. Such a reflective film filtering mirror comprises a plurality of materials. According to an embodiment of the present invention, the reflective film filtering mirror comprises a plurality of layers of reflective film plated on one surface thereof, such as a reflective film layer made of a material W and a reflective film layer made of a material B ₄ C, wherein the layer thickness of the reflective film layer made of the material W is 1 nm, the layer thickness of the reflective film layer made of the material B ₄ C is 1.5 nm, and these two types of reflective film layers are arranged alternately to form a one-dimensional artificial crystal structure, with a periodicity of 150; as a result, the structure of such a reflective film filtering mirror is: [W(1.0 nm)/B ₄ C(1.5 nm) ] i ₅ o. In said structure, said one-dimensional artificial crystal can form coherent reflection to a specific energy band of the incident X-rays, thus obtaining a high reflectivity, and the specific energy band here is just a predetermined energy band in the present invention. As a result, different reflective film filtering mirrors can be selected according to different X-ray predetermined energy bands; for example, different reflective film filtering mirrors are obtained by selecting the materials which form the reflective film, the layer thickness and periodicity.

The reflective film filtering mirror of the present invention is produced on a scale by specialized equipment which is used for producing reflective films. When preparing this reflective film filtering mirror, a plurality of layers of reflective film are plated on an ultra-smooth base by using the magnetron sputtering coating technology. The surface of the base should be ultra-smooth, so as to effectively reduce scattering caused by the base surface being rough. After having the base washed, the base is placed into a magnetron sputtering coating machine, the air is evacuated until it reaches an ultra-high vacuum. At this moment the sputtering gas is then filled therein, and the W reflective film layer and the B ₄ C reflective film layer are plated alternately. Furthermore, the reflective film layer can also be plated by using other materials. However, the energy spectrum selection effect of the X-rays in the range of X-ray imaging ( 1 0 - 150 keV) by [W( 1 . 0 nm)/B ₄ C( 1 . 5 nm) ] i ₅₀ of the present invention is quite good.

In addition, the reflective film filtering mirror can be designed as a double-peak reflective film filtering mirror which can obtain two energy bands, or a single-peak reflective film filtering mirror with one energy band, or a multi-peak reflective film filtering mirror with more than three energy bands after one energy spectrum selection, and the size of reflectivity in the single-peak energy band, the double-peak energy band and the multi-peak energy band can be achieved by different designs to the reflective film filtering mirror, for example, by selecting the materials of the reflection film, the layer thickness of the reflective film, periodicity, the included angle between the incident X-rays and the reflective film surface, and so on. For example, when the included angle is 1 degree, the reflective film filtering mirror can be designed so that the reflectivity of the X-rays of 4 0 - 60 kev reaches above 90 % , while the reflectivity of the X-rays of other energies is below 2 0 % ; and when the included angle is 1 . 1 degree, the reflectivity of the reflective film filtering mirror to the X-rays of 60 - 80 kev can reach above 90 % , and the reflectivity of the X-rays of other energies is below 2 0 % .

The reflective film filtering mirror of the present invention can achieve the energy selection by reflection according to the principle of Bragg' s equation: 2d sin Θ = A£ = λ , wherein d is the layer thickness of the reflective film, λ is the wavelength of the X-rays reflected by the reflective film (i.e. the wavelength of the X-rays in the predetermined energy band) , and Θ is the included angle between the incident X-rays and the reflective film surface (or the surface of the reflective film filtering mirror) . In the original X-rays incident onto the reflective film filtering mirror, only the X- rays with the wavelength λ can be reflected by each layer of the reflective film, and the X-rays in the predetermined energy band can be obtained after the reflections of a plurality of layers of reflective film being added together. According to Bragg' s equation, if it is required to reflect high-energy X- rays, then it needs to reduce the included angle between the incident X-rays and the reflective film surface, or reduce the layer thickness of the reflective film layer. The smaller the included angle, the higher the accuracy requirements of angle, and in the embodiments of the present invention, according to the selected predetermined energy band, generally the included angle is set in the range of 0.2-2 degrees, which indicates that the X-rays are incident along a small angle which is close to the reflecting surface of the reflective film filtering mirror. The layer thickness of the reflective film can be calculated according to the peak value and the width of the predetermined energy band, and the layer thickness is generally of the level of magnitude of nm, generally less than 5 nm.

When using different angles, the different reflectivity of the reflective film filtering mirror to the X-rays of 50-80 keV is shown in Table 1 below. The reflective film filtering mirror used in the table includes 200 layers of reflective film, and the layer thickness of each film layer is 1.7 nm.

Table 1

X-ray energy included angle reflectivity

50 keV Θ = 0.423° 76.78%

60 keV Θ = 0.352° 79.39% 80 keV 0.264° 67.75 ^ξ

It can be seen from Table 1 that the smaller the included angle between the incident X-rays and the reflective film surface, the higher the energy of the X-rays reflected by the reflective film filtering mirror. In addition, the reflectivity of the same type of reflective film to the X-rays with different energies is different.

Fig. 1 is a schematic diagram of comparison results by using an apparatus for selecting energy spectrum of the present invention and an attenuation layer in the prior art to perform energy spectrum selection to the original X-rays respectively. The three curves in Fig. 1 are respectively a dotted line representing the original X-rays, a dot-dash line representing the results when the original X-rays are attenuated by using a 2.5 mm aluminum attenuation layer, and a solid line representing the results when the reflective film filtering mirror of the present invention performs energy spectrum selection on the original X-rays. The energy spectrum of the original X-rays is between 10 keV and 110 keV, and this energy spectrum is mainly decided by the anode materials of the X-ray tube and the tube voltage. The energy spectrum after the original X-rays are attenuated by a 2.5 mm aluminum attenuation layer is between 20 keV and 110 keV, and the attenuation layer only attenuates the low-energy X-rays which are below 20 keV, so as to enable the energy spectrum of the X-rays to shift towards the high-energy section, but the energy spectrum selection cannot be carried out on them, and the X- rays after attenuation still retain a wide energy spectrum. However, after the energy spectrum selection is carried out on the original X-rays using the reflective film filtering mirror of the present invention, the X-ray energy band thus obtained is obviously limited in a specified narrow area (e.g. about between 40 keV and 83 keV) , which has a good stop effect on the X-rays below 40 keV and the X-rays above 83 keV at the same time, it can be seen that, as compared to the attenuation layer, the inventive reflective film filtering mirror of the present invention can achieve the energy spectrum selection of the original X-rays very well, and the obtained X-ray energy band is narrow.

Fig. 2 is a schematic diagram of the reflectivity of a reflective film filtering mirror to the X-rays in different energy bands as shown in Fig. 1. It can be seen from Fig. 2 that the reflective film filtering mirror of the present invention has a higher reflectivity to the X-rays of 40-83 keV, which is about between 0.5 and 0.6, and it basically does not reflect the X-rays of less than 40 keV and the X- rays of higher than 83 keV (the reflectivity is about 0.0) . As a result, the reflective film filtering mirror of the present invention achieves a higher reflectivity for the X- rays in the predetermined energy band, while it can also restrain the X-rays which are not in the predetermined energy bands well.

In addition, according to Bragg' s equation, in the case where the reflective film filtering mirror is fixed (i.e. the materials of the reflective film, the layer thickness and the number of layers are invariant) , the original X-rays can be reflected by adjusting the included angle between the incident X-rays and the reflective film surface, so as to obtain the X-rays in the different predetermined energy bands; for example, the X-rays in the energy bands which are quite narrow, such as 20-30 keV, 40-75 keV, 80-100 keV or 90-120 keV and so on. In X-ray imaging technology, the X-ray energy band required is generally 10-150 keV, the X-rays of various narrow energy bands within the range of 10-150 keV can be obtained by setting said included angles differently.

Since each layer of the reflective film of the reflective film filtering mirror of the present invention has a very strong reflecting effect to all the X-rays incident onto this reflective film layer, the X-rays in the predetermined energy band (or energy band window) can be super-strongly reflected, and the X-rays outside the energy band window are shielded off, thus the energy spectrum selection for the X-rays can be achieved by such a reflective film filtering mirror, such as the narrow energy band shown in Fig. 1; or the narrow energy band at the level of magnitude of keV, and they can even be viewed as single-energy X-rays, as shown in Fig. 3; or they have separated dual energy narrow bands, as shown in Fig. 4. However, it is not required to change the X-ray source to generate both of these of quite narrow energy bands and X- rays of such narrow energy bands .

Fig. 3 is a schematic diagram of the results of using the reflective film filtering mirror of the present invention to carry out the energy spectrum selection to the original X- rays to obtain the close to single-energy X-rays and comparing it with the X-rays which is of full width at half maximum 1 keV. The dot-dash line in the figure represents the X-rays of full width at half maximum (FWHM) 1 keV and close to single energy (80 keV) . The reason for using FWHM here is that the curve of spectrum distribution is not a square wave, so there is no width which is commonly referred to, and the magnitude of the curve width can be measured only by FWHM, so as to compare the spreading of different spectral lines; the dotted line represents the original X- rays; and the solid line represents the close to single- energy X-rays (79.5-80.5 keV) which are obtained from the energy spectrum selection by the reflective film filtering mirror of the present invention from the original X-rays represented by the dotted line. It can be seen from the figure that the X-rays obtained by the reflective film filtering mirror of the present invention is very similar to the X-rays of FWHM IkeV. Since such reflective film filtering mirrors of the present invention can be produced on a large scale, as compared to using synchrotron radiation to obtain the single-energy X-rays in the prior art, they are easy for commercialization and for production on a large scale .

Fig. 4 is a schematic diagram of the X-rays of the two separated narrow energy bands (i.e. double-peak energy band) which are obtained using the reflective film light filtering mirror of the present invention to perform energy spectrum selection on the original X-rays. The two narrow energy band X-rays in the figure (about 23-28 keV and 59-61 keV) do not overlap with each other, the reflectivities thereof are about 0.85, and there are also wider stop bands at two ends of the two passing peaks in the figure, which bands effectively stop the X-rays of the energy band of about 28- 57 keV, and both of the two narrow energy band X-rays are of the level of magnitude of keV. In addition to that, the filtering mirror of the present invention can also generate more than three narrow energy bands (i.e. multi-peak energy bands) as shown in Fig. 4, or more than two close to single- energy X-rays as shown in Fig. 3, or more than two narrow energy band X-rays as shown in Fig. 1. As compared to the imaging results of dual-energy X-rays in the prior art, the X- ray energy bands are separated from each other and are not overlapped after the energy spectrum selection has been carried out thereon by the reflective film filtering mirror of the present invention. However, the predetermined energy band after the energy spectrum selection is carried out thereon by the reflective film filtering mirror of the present invention can also be made to overlap, which is particularly determined by a reflecting film which forms the reflective film filtering mirror and the included angle. In addition, different reflectivities in Figs. 3 and 4 are affected by factors such as different predetermined energy bands, included angles and so on. Fig. 5 is a schematic diagram of a geometrical light path of the energy spectrum selection, which is achieved by a reflective film filtering mirror of the present invention for reflecting the original X-rays. An X-ray tube 1 represented by the dotted line in the figure is located at a position 4, here there is no reflective film filtering mirror between the X-ray tube 1 and a detector 3, and the X- ray beam emitted by the X-ray tube 1 and represented by the dotted lines is received by the detector 3. After a reflective film filtering mirror 2 is arranged between the X-ray tube 1 and the detector 3, since it is required to have an included angle (acute angle) between the propagation direction of the X-ray beam 6 emitted by the X-ray tube 1 and the reflective film filtering mirror 2, the X-ray tube 1 is moved from the original position 4 to the current position 5, and the position of the reflective film filtering mirror 2 needs to be adjusted at the same time, so that it is at the position 5, there is an appropriate included angle 12 between the X-ray beam 6 emitted by the X-ray tube 1 and a surface (this surface is plated with a plurality of layers of reflective film) of the reflective film filtering mirror 2, then after the original X-ray beam 6 is reflected by the reflective film filtering mirror 2, the X-rays 7 in the predetermined energy band can be reflected with a very high reflectivity and received by the detector 3, while the X-rays of other energy bands are shielded off by the reflective film filtering mirror 2, here the detector 3 is not moved with the movement of the X-ray tube 1, and its position is not changed. In this case, the included angle 12 can be calculated according to the X-ray wavelength of the predetermined energy band and according to Bragg' s equation. In addition, it needs to calculate the size of the reflective film filtering mirror 2 according to the cone angle of the X-ray beam 6, so that the surface of the reflective film filtering mirror 2 at least can receive all of the X-ray beam 6, but it is not made so large as to waste the materials of the reflective film. However, in Fig. 5, the X-ray tube 1 at the position 5 is not only shifted but also rotated relative to the original position 4, but the position of the detector 3 is not changed at the same time. Therefore, the geometrical arrangement of the X-ray tube and the detector is changed correspondingly. Since both the X-ray tube and the detector are important components of the computed tomography (CT) frame, such changes will affect the design of the whole frame, which not only brings about difficulties in improving the currently available frame design but also increases the costs of the frame design. However, if the X-ray tube 1 is only shifted but not rotated relative to the original position 4, then the frame design would be relatively easy, and significant changes to other components would not be needed.

Based on this situation, in the present invention preferably two reflective film filtering mirrors which are parallel to each other are used and of course more than three reflective film filtering mirrors which are parallel to each other can also be used, the object that only the X-ray tube 1 needs to be shifted relative to the original position 4 to enable the X-rays 7 in the predetermined energy band to be received by the detector 3 can be achieved by having more than two or three reflections to the incident X-rays, and at the same time it also maintains the propagation direction without changing the original X-rays, as shown in Fig. 6.

Fig. 6 is a schematic diagram of a geometrical relationship of an energy spectrum selection, which is achieved by using two reflective film filtering mirrors of the present invention to reflect the original X-rays. Here, the two reflective film filtering mirrors are respectively named as a first reflective film filtering mirror 21 and a second reflective film filtering mirror 22. Three X-ray tubes 1 are illustrated in Fig. 6, in which the X-ray tubes 1 at the position 4 and at the position 5 are both represented by dashed lines. When there is no reflective film filtering mirror 21 and 22 located between the position 4 and the detector 3, the X-ray beam represented by the dashed line and emitted by the X-ray tube 1 at the original position 4 is received by the detector 3. When there is only one reflective film filtering mirror 22 between the position 5 and the detector 3, the X- ray tube 1 needs to be shifted and rotated to reach the position 5 from the original position 4, so that the X-rays (i.e. the reverse extended line of the X-ray 10) emitted by the X-ray tube 1 at the position 5 are received by the detector 3 after having been reflected by the reflective film filtering mirror 22. However, after the two reflective film filtering mirrors 21 and 22 are arranged between the X-ray tube 1 and the detector 3, the X-ray tube 1 only needs to be shifted to the current position 8 from the original position 4, at the same time the positions of the two reflective film filtering mirrors are adjusted such that the first reflective film filtering mirror 21 and the second reflective film filtering mirror 22 are parallel to each other, thus the X-ray beam 6 emitted by the X-ray tube 1 is first reflected by the first reflective film filtering mirror 21 to obtain the X-rays 10 in the predetermined energy band, then the X-rays 10 in this predetermined energy band are reflected by the second reflective film filtering mirror 22 to obtain the X-rays 7 in the predetermined energy band and received by the detector 3, here the energy bands of the X-rays 10 and the X-rays 7 are the same, and the detector 3 is not shifted when the X-ray tube 1 is shifted. As a result, after being reflected by the two reflective film filtering mirrors 21 and 22, the X-ray beam 6 emitted by the X-ray tube 1 at the position 8 can be received by the detector 3, and the propagation direction of the X-ray beam 6 and that of the X-rays 7 in the predetermined energy band are parallel to each other, thus achieving the object of keeping the propagation direction of the original X-rays unchanged . According to Bragg' s equation, the object of reflecting the X-rays of different energy bands can be achieved by changing the included angle between the original X-rays and the reflective film filtering mirror, and changing the included angle can be achieved by adjusting the position of the X-ray tube, as shown in Fig. 6, with the X-ray tube 1 being moved to the position 8 from the original position 4; and it can be seen from the figure that the X-ray tube 1 at the position 8 is shifted upward and rightward relative to the original position 4, and such shift can be achieved by the method as follows: as the upward shift distance is larger than the rightward shift distance, the upward shift can be achieved by shifting the X- ray tube 1, so that in the frame design, the X-ray tube 1 should be made to be able to carry out such a shift of distance of the order of centimeters; and in the horizontal direction, it can be achieved by shifting the focus of the X-ray tube 1 which has been shifted upwards, such a shift generally being of the order of millimeters, since the target surface of the X-ray tube is limited, of course this can also be achieved by shifting the X-ray tube rightward.

In addition, the position 4 and position 5 of the X-ray tube are centrosymmetric about the position 8, and at the same time the position 5 and position 8 are centrosymmetric about a horizontal curve 13 of the reflective film filtering mirror 21, the reason is that the included angle between the horizontal curve 13 and the reverse extended line of the X-rays 10, the included angle between the X-ray beam 6 and a horizontal curve 14 of the second reflective film filtering mirror 22, the included angle between the X-rays 10 and the horizontal curve 14, and the included angle between the horizontal curve 14 and the reverse extended line of the X-rays 7 are all equal to the size of the angle 11. Some parameters of these two reflective film filtering mirrors 21 and 22, such as size and position, can be calculated according to the focus of the X-ray tube, the distance and the included angle between the focus and the second reflective film filtering mirror by using the triangle similarity formula in the case that the focus of the X-ray tube is shifted; in the case that only the X-ray tube is shifted, they can be calculated according to the position of the X-ray tube, the distance and the included angle 11 between the position and the second reflective film filtering mirror, nevertheless at this moment the focus of the X-ray tube is shifted as the X-ray tube is shifted, therefore generally speaking, the position can be calculated according to the focus of the X-ray tube, the position and the arrangement mode of the reflective film filtering mirrors 21 and 22 are calculated according to the focus of the X-ray tube, the distance and the included angle 11 between the focus and the second reflective film filtering mirror. For example, the position 4, the position 8 and the included angle 11 of the X-ray tube are known, and the propagation direction of the X-ray beam emitted by the X-ray tube at the position 8 is paralleled to the emergent direction of the X-rays reflected by the second reflective film filtering mirror, since the position 4 and position 5 are centrosymmetric about the position 8, then in this way the position 5 can be obtained, and the position 5 is located on the reverse extended line of the incident X-rays incident onto the second reflective film filtering mirror 22. The distance between the first reflective film filtering mirror 21 and the position 8 is obtained according to the distance and the size of the included angle 11 between the position 8 and the second reflective film filtering mirror 22. According to the triangle similarity theorem, the distance between the second reflective film filtering mirror 22 and the position 4 is twice the distance between the first reflective film filtering mirror 21 and the position 8, thus the specific position of the second reflective film filtering mirror 22 is obtained. Further, on the basis that the included angle between the X-ray beam emitted by the X-ray tube at the position 8 and the first reflective film filtering mirror is the angle degree 11, thus the arrangement mode of the first reflective film filtering mirror 21 is obtained.

It is worth noting that since the energy band of the X- rays 10 is the same as that of the X-rays 7 in Fig. 6, both of which are the X-rays in the predetermined energy band, then it only needs the second reflective film filtering mirror 22 to reflect the incident X-rays 10, and it does not need to carry out energy spectrum selection on the incident X-rays by reflection, therefore in order to save costs, it is also possible for the second reflective film filtering mirror 22 here not to use the materials and the structure of the reflective film filtering mirror of the present invention, as long as it can reflect all or most of the incident X- rays . In addition, the reflective film filtering mirror of the present invention can also be used as a pre-fil tering filter or a shaping (such as wedge-shape) filter, which is used in X-ray imaging equipment (such as CT, X-ray projector equipment, C-type arm equipment, and micro CT and so on) which needs to adjust or select an X-ray energy band.

What are described above are merely preferred embodiments of the present invention and do not limit the present invention, and any modifications, equivalents, improvements, etc. made within the spirit and principles of the present invention should be covered in the protection scope of the present invention.