HAN, Se Jin (205Ihyun-dong, Daedeok-gu, Daejeon 306-170, KR)
CHAE, Jang Yool (1-402, Sang-a Apt 1-46,Munhwa-dong, Jung-gu, Daejeon 301-130, KR)
KIM, Chang Kyu (#304, Sinseong Villa 105-14,Sinseong-dong, Yuseong-gu, Daejeon 305-804, KR)
PARK, Young Sei (110-806, Saemteo Maeul AptIrwon-dong, Gangnam-gu, Seoul 135-230, KR)
HAN, Se Jin (205Ihyun-dong, Daedeok-gu, Daejeon 306-170, KR)
CHAE, Jang Yool (1-402, Sang-a Apt 1-46,Munhwa-dong, Jung-gu, Daejeon 301-130, KR)
KIM, Chang Kyu (#304, Sinseong Villa 105-14,Sinseong-dong, Yuseong-gu, Daejeon 305-804, KR)
[CLAIMS] [Claim 1]
<i74> A quasi-monochromatic x-ray optical filter formed by stacking a plurality of reflecting mirrors at a predetermined angle with respect to an incident x-ray one on top of another in a chamber, wherein a length of each of the respective reflecting mirrors is set between a length that allows all x-rays, incident between neighboring reflecting mirrors, to collide with surfaces of the reflecting mirrors, and a length that allows all x-rays, incident between neighboring reflecting mirrors, to be reflected and be prevented from colliding with back surfaces of the neighboring reflecting mirrors. [Claim 2]
<175> The quasi-monochromatic x-ray optical filter as set forth in claim 1, wherein a multi-layer film, the thickness of which gradually increases so that a Bragg condition for reflection of identical specific energy is met, is deposited on each of the reflecting mirrors. [Claim 3]
<i76> The quasi-monochromatic x-ray optical filter as set forth in claim 1, wherein a multi-layer film is deposited on each of the reflecting mirrors so that an inclination obtained by dividing a difference between thicknesses of a film material pair at both ends of the reflecting mirror by the length is uniform. [Claim 4]
<177> The quasi-monochromatic x-ray optical filter as set forth in any one of claims 1 to 3, wherein-'
<i78> when the length of the reflecting mirrors is L, a radius of a concentric circle along which the reflecting mirrors are arranged is r, an interval between neighboring reflecting mirrors is dg, a thickness of the reflecting mirrors is t, an incident angle is θi, a rotation angle of a chamber is θr, and an extended length of the reflecting mirrors is δl,
<i79> setting is performed such that the length L of the reflecting mirrors is set to a minimum length that causes all x-rays, incident between the neighboring mirrors when the incident angle is θi, to collide with the reflecting mirrors, δl /L is in a range of 0 ~ 3, θi is in a range of 0.1 ~ 0.6 degrees, d/(d + t) is in a range of 0.1 - 0.8, r is in a range of 10 ~ 1,000 mm, and θr is in a range of ± 0.5 degrees. [Claim 5]
<i80> The quasi-monochromatic χ-ray optical filter as set forth in claim 4, wherein, in order to adjust reflected energy of the quasi-monochromatic x-ray, the plurality of reflecting mirrors is rotated by an angle θr of rotation of a chamber, or a radius r of a concentric circle along which the reflecting mirrors are arranged is adjusted. [Claim 6]
<i8i> An x-ray imaging system using a quasi-monochromatic χ-ray optical filter, the x-ray imaging system including an χ-ray light source and an χ-ray detection unit, wherein the quasi-monochromatic x-ray optical filter set forth in claim 1 is disposed between the x-ray light source and the x-ray detection unit. [Claim 7]
<i82> The quasi-monochromatic x-ray optical filter as set forth in claim 6, further comprising an optical filter alignment unit for moving the quasi-monochromatic x-ray optical filter based on degrees of freedom along three axes, that is, xyz axes. [Claim 8]
<i83> The quasi-monochromatic χ-ray optical filter as set forth in claim 7, wherein the optical filter alignment unit comprises a mechanism for causing the quasi-monochromatic x-ray optical filter to perform vibration or pendulum movement in a direction perpendicular to an incident beam. [Claim 9]
<i84> A quasi-monochromatic multi-energy x-ray imaging system comprising a light source unit for generating and emitting χ-rays and enabling a focal spot of the x-rays to move in a predetermined direction, an optical filter mounted in the light source unit and installed to emit polychromatic x-rays, emitted from the light source unit, in quasi-monochromatic multi-energy form and radiate the quasi-monochromatic multi-energy onto the subject, and a signal detector unit for detecting the x-rays having passed through the subject and acquiring multi-energy image information,
<i85> wherein the optical filter comprises a plurality of reflecting mirrors that is installed such that the polychromatic x-rays are incident at a substantially identical angle and a shutter that is configured to selectively block the quasi-monochromatic multi energy reflected and emitted from the reflecting mirrors, and
<i86> wherein one or more of the reflecting mirrors on which respective film material pairs having different thicknesses are formed are alternately arranged. [Claim 10]
<187> A quasi-monochromatic multi-energy x-ray imaging system comprising a light source unit for generating and emitting x-rays and enabling a focal spot of the χ-rays to move in a predetermined direction, an optical filter mounted in the light source unit and installed to emit polychromatic x-rays, emitted from the light source unit, in quasi-monochromatic multi-energy form and radiate the quasi-monochromatic multi-energy onto the subject, and a signal detector unit for detecting the x-rays having passed through the subject and acquiring multi-energy image information,
<i88> wherein the optical filter comprises a plurality of reflecting mirrors in which film material pairs have an identical thickness, and a shutter that is configured to selectively block the quasi-monochromatic multi energy reflected and emitted from the reflecting mirrors, and
<i89> wherein one or more of the reflecting mirrors that have different incident angles for the polychromatic x-rays are alternately arranged. [Claim 11]
<i90> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 9 or 10, wherein the optical filter further comprises a back slit, the back slit comprising walls that are formed so as to block polychromatic χ-rays passed between the reflecting mirrors and then emitted, and slots that are formed between the walls so as to pass quasi-monochromatic x-rays, reflected and emitted from the reflecting mirrors, therethrough.
[Claim 12]
<i9i> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 11, wherein the walls and slots of the back slit are arranged along a concentric circle having a substantially same origin as the concentric circle along with the reflecting mirrors are arranged.
[Claim 13]
<i92> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 11, wherein the walls of the back slit are extended such that they can adjust a width of reflected beams reflected from the reflecting mirrors.
[Claim 14]
<i93> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 9 or 10, further comprising a collimator unit that is provided with a collimator having an opening for adjusting a size of the x-ray beam that is emitted from the optical filter and radiated onto the subject.
[Claim 15]
<i94> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 14, wherein the collimator is installed such that it can be moved to adjust a distance to the light source unit.
[Claim 16]
<i95> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 9 or 10, wherein, in order to adjust reflected energy of the quasi-monochromatic x-ray, the reflecting mirrors are installed such that they can be rotated together at a predetermined angle around a selected point or such that a radius r of a concentric circle along which the reflecting mirrors are arranged can be adjusted.
[Claim 17] <i96> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 9 or 10, wherein the reflecting mirrors are installed such that an incident angle of the x-ray is less than 1 degree, and a thickness of a film material pair of each of the reflecting mirrors is set in a range of 1.5 ~ 8 nm, thereby reflecting a quasi-monochromatic x-ray in a range of 10 ~ 100 keV. [Claim 18]
<197> The quasi-monochromatic multi-energy χ-ray imaging system as set forth in claim 9 or 10, wherein a direction of movement of the focal spot of the x-ray is a rectilinear direction. [Claim 19]
<i98> The quasi-monochromatic multi-energy x-ray imaging system as set forth in claim 9 or 10, wherein a direction of movement of the focal spot of the x-ray is a direction of rotation. |
[DESCRIPTION]
[Invention Title]
OPTICAL FILTER FOR A QUASI-MONOCHROMATIC X-RAY AND MULTI-ENERGY X-RAY IMAGING SYSTEM WITH THE QUASI-MONOCHROMATIC X-RAY
[Technical Field]
<i> The present invention relates, in general, to a quasi-monochromatic x- ray optical filter and a quasi-monochromatic multi-energy x-ray imaging system using the optical filter, and, more particularly, to an optical filter that is capable of filtering a polychromatic x-ray to obtain a quasi- monochromatic x-ray having a desired single energy band or multi-energy band and a quasi-monochromatic multi-energy χ-ray imaging system that is capable of acquiring images, in which the spectrum regions of multi-energy are clearly distinctive and which are clear even after a small exposure, through imaging using quasi-monochromatic multi-energy obtained through the filtering of the optical filter.
[Background Art]
<2> Electromagnetic waves in the χ-ray region, which have wavelengths shorter than those of ultraviolet rays, are suitable for the construction of various types of imaging equipment using the information of transmitted components because they have a characteristic in which part thereof is absorbed by an object but the remaining part thereof passes through the object. The representative examples of such equipment include various types of x-ray imaging equipment for medical diagnoses and inspection equipment used at airports, and the utility of such a technology is widely known. <3> In various types of commercial imaging equipment, the material of x-ray emission anodes, output, voltage, current and radiation time are selected to meet the purposes thereof, and then χ-rays are radiated. These x-rays have a wide energy spectrum 101, as shown in Fig. 1. When an x-ray passes through material, the absorption rate is high in a relatively low-energy region, whereas the absorption rate is low and scattering frequently occurs in a relatively high-energy region, due to characteristics intrinsic to the χ-ray.
Although the low-energy region having a high variability in the absorption rate and a generally high absorption rate is advantageous for the increase in the contrast of an image from the standpoint of the acquisition of images of the human body, there is a possibility that the human body may be damaged by radioactive rays. In contrast, the high-energy region having a low variability in the absorption rate and a generally low absorption rate has low toxicity to the human body, but it has degraded contrast and low image sharpness due to the scattering of χ-rays.
<4> Accordingly, there are needs for a technology which selects only energy useful for imaging from the wide energy spectrum of a commercial x-ray and radiates the selected energy to the human body, thereby being capable of reducing exposure to toxic radioactive rays and increasing contrast.
<5> This technology refers to a technology for generating a monochromatic χ-ray having a line spectrum 103 corresponding to a wide energy spectrum or a quasi-monochromatic χ-ray having a narrow spectrum 102, and using it. The quasi-monochromatic χ-ray can be produced using a method of causing a polychromatic x-ray to be reflected from a multi-layer film in which films made of material having a high atomic number and films of material having a low atomic number are alternately arranged and selecting monochromatic light having a specific wavelength based on Bragg' s diffraction law. The Bragg' s diffraction law is expressed as follows:
<6> E = (12.4 * n)/(2 * d * sinθ)
<7> where E is diffracted energy in kiloelectron Volts (keVs), n is a constant equal to or greater than 1, d is a diffraction grating distance in As, and θ is an incident angle that is formed by an incident beam and an incident surface.
<8> Monochromatic light can be obtained by causing polychromatic light to be incident on and be reflected from a single crystal to which Bragg's law is applied. However, since monochromatic light is obtained from a very small part of a wide spectrum, the number of photons of a monochromatic x-ray is very low, compared to an incident χ-ray. However, since monochromatic light
produced using an χ-ray having high intensity, which is emitted from a synchrotron, has no difficulty with the acquisition of a number of photons sufficient for the purpose of use, it is very useful for imaging, and various effects obtained by the use of monochromatic light have been reported. <9> A synchrotron is not equipment that can be easily and generally used because it is expensive and large. For this reason, there have been many attempts to develop monochromatic x-ray light sources that use commercial x- ray generation devices as light sources. However, the use of commercial x- ray generation devices as light sources cannot guarantee practical use. Two problems that should be solved to enable the realization of practical imaging equipment and which have been known to us are that the amount of monochromatic light should be sufficiently large and that the two dimensional area of the entire subject should be imaged at a single time.
<io> Since the Signal-to-Noise Ratio (SNR) of an image is low when the amount of monochromatic light is not sufficiently large, and thus the quality of the image is degraded, efforts to use high current so as to increase the input power of a light source, to increase the size of the focal spot of a target anode and to increase imaging time are required. However, there are obstacles difficult to overcome, including the limitation of generator capacity, the blurring of an image attributable to the increase in the size of a focal spot and a vibrational effect attributable to the movement of a subject during imaging covering a long period of time. Accordingly, a method of increasing the amount of light by generating a quasi-monochromatic x-ray, such as a ray 102, that has a wider spectrum width than the monochromatic light of a line spectrum was proposed. In practical applications, it is known that the effects of a quasi-monochromatic χ-ray are sufficient.
<π> The imaging of a two-dimensional area subject at one time is a very important condition. The width of monochromatic light obtained using an x- ray light source, such as a synchrotron, is merely several millimeters, even when the distance to the source is several meters. Accordingly, a subject having a size larger than the width should be imaged using a scanning method,
and a method of obtaining large area emission quasi-monochromatic x-rays so as to image a subject at one time needs to be devised, even when a commercial χ-ray light source is employed.
<i2> Meanwhile, the fact that clearer images can be acquired when quasi- monochromatic x-rays having dual energy are used is well known. That is, in the case of a technology using two types of image signals photographed using dual energy, that is, a technology which belongs to x-ray photography, it was found that the use of a quasi-monochromatic x-ray was superior to the use of a polychromatic χ-ray from the standpoint of the increase in contrast attributable to the elimination of a background and was more effective in distinguishing materials having low absorption contrast from each other because it has similar x-ray absorption coefficients.
<i3> For example, in the case of the dual energy subtraction method of angiography using a contrast medium, χ-rays 104 and 105 having two respective types of energy, which are set in regions respectively lower and higher than that of the K-edge absorption energy 106 of iodine, which is contrast medium material, are radiated, as shown in Fig. 2, and the quality of images is considerably influenced by the shape of an incident beam spectrum and an energy range. Here, K-edge absorption occurs based on an x-ray absorption band attributable to the electron structure of each element. This refers to a phenomenon in which, when light energy greater than the energy gap between the electron orbitals Is and 2s2p of arbitrary material is supplied, light is absorbed and the ionization of a K-shell Is electron and photoemission are generated at the same time.
<i4> The absorption coefficient rapidly increases at the point where energy exceeds an energy gap, and decreases again when the energy continuously increases above the energy gap. Furthermore, since the energy gap is a value inherent in each element, the K-edge energy varies depending on the element. It is known that the K-edge energy of iodine, which is a widely used contrast medium material, is 33.164 keV. That is, when a contrast medium is used, the χ-ray absorption coefficient is increased, and division into a portion having
a high absorption coefficient and a portion having a low absorption coefficient is performed depending on the x-ray energy that is used for imaging.
<i5> In the energy subtraction, the effect of elimination of a background is excellent when two images that are acquired in energy regions closest to the front and rear portions of a region corresponding to the K-edge energy of the contrast medium material are subtracted from each other, and the contrast of a portion in which the contrast medium is used is maximized.
<i6> However, when a polychromatic x-ray is used, the spectrum of an incident beam has a wide width, with the result that it is difficult to prevent two spectra from overlapping each other while causing the energy value to approach the K-edge energy value. Accordingly, the conditions of this are considerably different from the conditions for satisfying the purpose of the dual energy subtraction method. In contrast, when a quasi- monochromatic x-ray is used, the difference in the absorption coefficient can be increased by adjusting two types of x-ray energy before and after the K- edge energy of the contrast medium, with the result that the effects of the dual energy subtraction method can be maximally utilized.
<i7> In one of the research results using a quasi-monochromatic χ-ray having dual energy, G. Baldazzi et al. disclosed the production of two types of high and low quasi-monochromatic x-ray incident beams respectively having peaks at 31.7 keV and 34.7 keV using High Oriented Pyrolytic Graphite (HOPG) in "IEEE NSS MIC 2001 Conference Record, San Diego Ca, November 2001." Here, as the result of the research in which the spectra thereof were made to have narrow widths to prevent them from overlapping each in the K-edge energy of iodine and a dual energy subtraction method was conducted by performing imaging while varying the concentration of an iodine contrast medium and varying the diameter of a tube containing liquid, the excellent characteristics of reducing the amount of exposure and improving contrast and sensitivity were exhibited, thereby presenting a possibility of the employment of a quasi-monochromatic χ-ray. However, this research result
still has problems in that noise level should be reduced and imaging time should be decreased because the amount of reflected quasi-monochromatic light is increased and the field size of x-rays to which a subject is exposed is increased.
<i8> A representative example of radiography using a dual energy subtraction method is mammography. Since the constituent tissues of a mamma have a low χ-ray absorption coefficient, images having high contrast are acquired using energy in the vicinity of 20 keV, which is considerably lower than x-ray energy that are radiated onto other human body portions. In this case, since the quantity of radioactive rays absorbed by the human body is large, a method capable of reducing the dose thereof as much as possible while maintaining contrast has always been the target of research, and a dual energy subtraction method using a quasi-monochromatic x-ray may be a solution thereto. Since there is no abrupt change in the absorption coefficient attributable to K-edge absorption in the imaging of a mamma without using a contrast medium, the subtraction between two dual energy image signals does not produce a considerable effect, and the use of an appropriate subtraction algorithm and the use of an appropriate x-ray energy band are required.
<19> Although the use of a quasi-monochromatic x-ray having a narrow energy spectrum is effective in the dual energy subtraction method compared to the use of a polychromatic x-ray, the technology for putting a method for effectively producing dual energy quasi-monochromatic light, a method for reducing imaging time and a method for achieving a uniform light distribution into practical use is still required. [Disclosure] [Technical Problem]
<20> Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an optical filter capable of producing wide-angle quasi-monochromatic light that can obtain a quasi-monochromatic x-ray in which a spectrum in a desired energy band has a slightly narrow width by
causing a polychromatic x-ray, obtained using a commercial x-ray generation device, to be incident on a highly efficient Bragg reflecting mirror, and that can image each of the subjects having various areas at one time by regularly arranging a plurality of reflecting mirrors between an x-ray light source and the subject optical filter, and an χ-ray imaging system using the optical filter.
<2i> Another object of the present invention is to provide a quasi- monochromatic multi-energy x-ray imaging system that generates a quasi- monochromatic x-ray having multi-energy using the optical filter, so that the spectrum regions of the multi-energy are clearly distinguished, and thus clear images can be acquired after a small exposure.
<22> A further object of the present invention is to provide a quasi- monochromatic multi-energy x-ray imaging system that performs scanning imaging using a plurality of reflecting mirrors, so that imaging time is reduced and the system can be effectively used for the imaging of a large area subject. [Technical Solution]
<23> In order to accomplish the above objects, the present invention provides a quasi-monochromatic χ-ray optical filter formed by stacking a plurality of reflecting mirrors at a predetermined angle with respect to an incident χ-ray one on top of another in a chamber, wherein a length of each of the respective reflecting mirrors is set between a length that allows all x-rays, incident between neighboring reflecting mirrors, to collide with surfaces of the reflecting mirrors, and a length that allows all x-rays, incident between neighboring reflecting mirrors, to be reflected and be prevented from colliding with back surfaces of the neighboring reflecting mirrors.
<24> According to the setting of the length of the reflecting mirrors, a phenomenon in which an incident beam passes through the reflecting mirrors, thereby degrading a quasi-monochromatic property, can be eliminated.
<25> Furthermore, a multi-layer film, the thickness of which gradually
increases so that a Bragg condition for reflection of identical specific energy is met, is deposited on each of the reflecting mirrors of the present invention, and a multi-layer film is deposited on each of the reflecting mirrors of the present invention so that an inclination obtained by dividing a difference between thicknesses of a film material pair at both ends of the reflecting mirror by the length is uniform.
<26> ffhen the multi-layer film is deposited as described above, it is possible to reduce the range of reflected energy deviation in the reflecting mirror.
<27> Additionally, the present invention provides an χ-ray imaging system in which the quasi-monochromatic χ-ray optical filter is disposed between an x- ray light source and an x-ray detection unit.
<28> The quasi-monochromatic x-ray optical filter may further include an optical filter alignment unit for moving the quasi-monochromatic x-ray optical filter based on degrees of freedom along three axes, that is, xyz axes, and the optical filter alignment unit may include a mechanism for causing the quasi-monochromatic χ-ray optical filter to perform vibration or pendulum movement in a direction perpendicular to an incident beam.
<29> Meanwhile, in order to accomplish the above objects, the present invention provides a quasi-monochromatic multi-energy x-ray imaging system including a light source unit for generating and emitting x-rays and enabling a focal spot of the χ-rays to move in a predetermined direction, an optical filter mounted in the light source unit and installed to emit polychromatic χ-rays, emitted from the light source unit, in quasi-monochromatic multi- energy form and radiate the quasi-monochromatic multi-energy onto the subject, and a signal detector unit for detecting the χ-rays having passed through the subject and acquiring multi-energy image information, wherein the optical filter includes a plurality of reflecting mirrors that is installed such that the polychromatic χ-rays are incident at a substantially identical angle and a shutter that is configured to selectively block the quasi- monochromatic multi energy reflected and emitted from the reflecting mirrors,
and wherein one or more of the reflecting mirrors on which respective film material pairs having different thicknesses are formed are alternately arranged.
<30> Additionally, the present invention provides a quasi-monochromatic multi-energy x-ray imaging system including a light source unit for generating and emitting x-rays and enabling a focal spot of the x-rays to move in a predetermined direction, an optical filter mounted in the light source unit and installed to emit polychromatic x-rays, emitted from the light source unit, in quasi-monochromatic multi-energy form and radiate the quasi-monochromatic multi-energy onto the subject, and a signal detector unit for detecting the x-rays having passed through the subject and acquiring multi-energy image information, wherein the optical filter includes a plurality of reflecting mirrors in which film material pairs have an identical thickness, and a shutter that is configured to selectively block the quasi-monochromatic multi energy reflected and emitted from the reflecting mirrors, and wherein one or more of the reflecting mirrors that have different incident angles for the polychromatic χ-rays are alternately arranged.
<3i> The imaging system of the present invention produces quasi- monochromatic light using a commercial x-ray light source. In particular, the imaging system prevents spectra from overlapping each other while causing the energy and width of quasi-monochromatic light to approach the front and end of the K-edge of an x-ray contrast medium, and thus it provides an improved contrast and sensitivity effect in dual energy subtraction angiography using a contrast medium. Meanwhile, in dual energy imaging using no contrast medium, an energy value and an energy width, which produce great effects, can be adjusted.
<32> When the reflecting mirrors are arranged at predetermined angles and a predetermined interval in predetermined directions, imaging time can be reduced and the uniformity of incident intensity can be improved by increasing incident intensity.
<33> Furthermore, the optical filter may further include a back slit, the back slit including walls that are formed so as to block polychromatic x-rays passed between the reflecting mirrors and then emitted, and slots that are formed between the walls so as to pass quasi-monochromatic χ-rays, reflected and emitted from the reflecting mirrors, therethrough. The walls and slots of the back slit may be arranged along a concentric circle having a substantially same origin as the concentric circle along with the reflecting mirrors are arranged. The walls of the back slit may be extended such that they can adjust a width of reflected beams reflected from the reflecting mirrors.
<34> The quasi-monochromatic multi-energy x-ray imaging system may further include a collimator unit that is provided with a collimator having an opening for adjusting a size of the χ-ray beam that is emitted from the optical filter and radiated onto the subject. The collimator may be installed such that it can be moved to adjust a distance to the light source unit .
<35> In order to adjust the reflected energy of the quasi-monochromatic x- ray, the reflecting mirrors may be installed such that they can be rotated together at a predetermined angle around a selected point or such that a radius r of a concentric circle along which the reflecting mirrors are arranged can be adjusted,
<36> Furthermore, the reflecting mirrors may be installed such that an incident angle of the x-ray is less than 1 degree, and a thickness of a film material pair of each of the reflecting mirrors is set in a range of 1.5 - 8 nm, thereby reflecting a quasi-monochromatic χ-ray in a range of 10 ~ 100 keV.
<37> The direction of movement of the focal spot of the χ-ray may be set to a rectilinear direction or a direction of rotation.
<38> According to the imaging system of the present invention, it is possible to perform imaging using quasi-monochromatic dual energy. The imaging system of the present invention enables various imaging methods, such
as a method of dividing reflecting mirrors into reflecting mirrors for high energy and reflecting mirrors for low energy, and performing scanning imaging twice while covering the reflecting mirrors for low energy during high energy imaging and covering the reflecting mirrors for high energy during low energy imaging, a method of alternately mounting two types of reflecting mirrors, performing imaging once, dividing the detection time of a signal detection unit (digital detector) into an appropriate number of frames, accumulating signals, and acquiring images for the respective types of dual energy by separating high-energy and low-energy frames from each other and synthesizing related frames, an energy adjustment method of mounting the same type of reflecting mirrors, performing scanning imaging twice, rotating a reflecting mirror chamber around an axis passing through the thickness of reflecting mirrors after first imaging, changing the energy of reflected quasi- monochromatic light and performing second imaging, a method of performing scanning imaging using a one-row line detector corresponding to one reflecting mirror, and a method of performing dual energy imaging in a single scanning imaging using two reflecting mirrors and a two-row line detector.
[Advantageous Effects]
<40> In accordance with the quasi-monochromatic x-ray optical filter and the χ-ray imaging system using the quasi-monochromatic χ-ray optical filter according to the present invention, a large area subject can be imaged using a quasi-monochromatic x-ray at a single time. Large area emission can be achieved by stacking a plurality of reflecting mirrors one on top of another, rather than using a single reflecting mirror. Furthermore, quasi- monochromatic energy in various regions can be generated using a single type of optical filter, and effective quasi-monochromatic x-ray images can be acquired practically because the optical filter can be installed in a small space that is formed in the structure of an imaging system using a commercial x-ray source which has not been considerably reconstructed.
<4i> In accordance with the quasi-monochromatic multi-energy x-ray imaging system of the present invention, the spectrum regions of multi-energy can be clearly distinguished from each other, so that clear images can be acquired after a small exposure when adjustment is performed to use an optimum energy value.
<42> Furthermore, since scanning imaging is performed using a plurality of reflecting mirrors, imaging time is reduced, thus the system of the present invention is useful for the imaging of a large area subject, and the system of the present invention can be used to eliminate the partial non-uniformity of light intensity. Moreover, since quasi-monochromatic multi-energy can be generated using a single x-ray source, it is possible to implement inexpensive and various scanning imaging technologies. [Description of Drawings]
<43> Fig. 1 is a diagram showing polychromatic, quasi-monochromatic and monochromatic x-rays, which correspond to typical x-ray energy spectra, respectively;
<44> Fig. 2 is a diagram showing the dual energy spectra of quasi- monochromatic light and polychromatic light;
<45> Fig. 3 is sectional views of reflecting mirrors, in which Fig. 3(a) is
a diagram of a reflecting mirror without an inclined coating, and Fig. 3(b) is sectional views of reflecting mirrors with inclined coatings! <46> Fig. 4 is a schematic diagram showing a quasi-monochromatic x-ray optical filter according to the present invention; <47> Fig. 5 is a TEM photo showing the reflecting mirror multi-layer film of embodiment 1; <48> Fig. 6 is a diagram showing the reflected x-ray spectrum, reflectivity and the use of a synchrotron of embodiment 2;
<49> Fig. 7 is a photo showing the reflecting mirror image of embodiment 3; <50> Fig. 8 is a diagram showing the reflecting mirror chamber of embodiment
4; <5i> Fig. 9 is a photo showing a large area image that is captured using the reflecting mirror chamber of embodiment 4; <52> Fig. 10 is a diagram showing the fact that various types of energy can be reflected from the same reflecting mirror through manipulation of the angle of rotation of the reflecting mirror chamber; <53> Fig. 11 is a conceptual diagram showing an entire quasi-monochromatic multi-energy x-ray imaging system according to the present invention; <54> Fig. 12 is a sectional view showing a multi-layer film reflecting mirror that is used in the imaging system of the present invention; <55> Fig. 13 is a diagram showing the scanning direction beam profile of quasi-monochromatic light that is used in the present invention; <56> Figs. 14(a) to 14(c) are diagrams showing the concept of dual energy scanning imaging using the imaging system of the present invention; <57> Fig. 15 is a diagram showing a back slit that constitutes part of the imaging system of the present invention; <58> Figs. 16(a) to 16(c) are diagrams showing the arrangements of reflecting mirrors that are used in the imaging system of the present invention; <59> Fig. 17 is a diagram showing the quasi-monochromatic dual energy spectrum of the present invention; and
<60> Fig. 18 is a diagram showing quasi-monochromatic dual energy x-ray images to which the present invention is applied and an effect image to which a subtraction method is applied. <6i> ^Description of reference numerals of principal elements in the drawings^
<62> 10: light source unit
<63> 12: frame
<64> 20: signal detector unit
<65> 30: collimator unit
<66> 32-' collimator
<67> 40: optical filter
<68> 42: reflecting mirror
<69> 44: reflecting mirror chamber
<70> Sl, S2: focal spot
<7i> 50: subject
<72> 60: shutter
<73> 101: polychromatic χ-ray spectrum
<74> 102: quasi -monochromatic x-ray spectrum
<75> 103: mono-chromatic x-ray
<76> 104: low energy x-ray
<77> 105: high energy x-ray
<78> 106: K-edge absorption energy of iodine
<79> 107: reflecting mirror substrate
<80> 108: multi-layer film
<8i> 109-' multi-layer film of inclined coating
<82> 110: reflecting mirror
<83> 111: virtual light source of reflected quasi -monochromatic χ-ray
<84> 112: reflected quasi -monochromatic x-ray
[Mode for Invention] <85> Fig. 4 is a schematic diagram showing the concept of a large area emission χ-ray optical filter according to the present invention.
<86> Bragg multi-layer substrates (hereinafter referred to as 'reflecting mirrors') are perpendicularly erected on a concentric circle of radius r having the focus spot of a polychromatic x-ray light source as its origin 0, and the end points e21 of the surfaces of the reflecting mirrors 110 are located on the concentric circle having the radius r, the surfaces being located on the sides of the reflecting mirrors closest to the origin 0. Furthermore, each of the reflecting mirrors 110 is arranged to be inclined at the same incident angle θi with respect to a radius line having the origin 0 as its center. The length of the reflecting mirrors 110 is set to L + δl, and the thickness thereof is set to t.
<87> Such an arrangement of the reflecting mirrors 110 allows all reflecting mirrors 110 to have the same incident angle with regard to points where a certain concentric circle having origin 0 as its center is placed, and the respective reflecting mirrors 110 have the same incident angle distribution in the longitudinal direction thereof.
<88> The interval dg between the reflecting mirrors 110 is set such that monochromatic light having a specific wavelength can be picked out based on Bragg 1 s diffraction law, and the value of the interval dg is related to L, t, and θi. That is, when θi is fixed, dg is calculated if L and t are determined. In contrast, when t and dg are fixed, L is calculated. t is calculated in the same manner. Here, dg is the interval between the arranged reflecting mirrors 110.
<89> When the values of L, t and θi are determined, the value of dg is set. The value of dg is set to a value that maximizes the intensity of a reflected beam. The value of dg that maximizes the intensity of a reflected beam may be determined using the following method.
<90> With respect to an arbitrary reflecting mirror, assuming that the end point of the surface thereof, which is farthest from the origin 0, is el2, a line that connects the origin 0 with the point el2 has an angle of θi2 with respect to the surface of the reflecting mirror. Thereafter, a line that connects the origin 0 with the end point el2 of the surface of the reflecting
mirror that is farthest from the origin O, is caused to meet the end point of the back surface of its neighboring reflecting mirror that is closest to the origin 0. The reflecting mirrors are arranged at the same intervals dg by repeating the above-described method.
<9i> When L, t, dg and θi are determined using the above method and when one considers rectilinear lines that connect the end points of the surfaces of the reflecting mirrors which are closest to the origin with the origin, the angle between rectilinear lines for two neighboring reflecting mirrors is a divided angle φ that is formed by dividing the portion of a concentric circle, having an origin 0 as the center thereof, where the reflecting mirrors are located, by the number of reflecting mirrors.
<92> If a polychromatic x-ray incident on a reflecting mirror collides with an end of the reflecting mirror, rather than the surface thereof, that is, the surface of the reflecting mirror corresponding to the thickness of the reflecting mirror thickness, the polychromatic χ-ray is absorbed or dispersed, with the result that the shadow of the reflecting mirror appears in the image of a resulting quasi-monochromatic x-ray that passes through the optical filter. Methods for overcoming the non-uniformity phenomenon include a method of allowing an amplitude of at least a divided angle φ along a concentric circle of radius r and providing angular velocity that enables pendulum motion to be performed two or more times within imaging time, and a method of causing the entire optical filter to perform reciprocating rectilinear motion at an amplitude corresponding to at least dg and at a frequency v equal to or higher than 2 within imaging time in a direction (y- axis direction) perpendicular to a line that connects the central point c of an optical filter (for example, the central point of a portion which belongs to concentric circles around an origin and on which reflecting mirrors are located) with the center of the circle, in the form of the rectilinear oscillation motion of the optical filter.
<93> One of the well-known effects of the quasi-monochromatic x-ray is the absorption effect of a specific energy band x-ray with respect to a specific
portion or a specific material (F. E. Carroll et al.). With regard to a quasi-monochromatic reflecting mirror using Bragg 1 s diffraction theory, it is impossible to change the size of the diffraction grating of a manufactured reflecting mirror, but an incident angle can be changed by adjusting it, thereby changing a quasi-chromatic energy band. However, it is difficult to adjust all incident angles to the same value when a plurality of reflecting mirrors is arranged. In order to solve this problem, the present invention proposes a technology for providing a function capable of adjusting a quasi- monochromatic energy within an energy deviation range allowable in actual use while causing a change in the incident angle of each reflecting mirror.
<94> When the center of the reflecting mirrors is set to c on the concentric circle of radius r and rotation is performed around c by θr with the relative positions of all the reflecting mirrors maintained, all incident angles on the reflecting mirrors are changed uniformly. When the incident angle is changed as described above, the shadow region of the substrate is slightly changed. In contrast, within the range of angles that change θr within 1 degree, preferably 0.5 degrees, the difference is insignificant, and the range of adjustable energy is within about ± 30% or higher, thus resulting in a practical value.
<95> If the reflecting mirrors are rotated in the direction of θr shown in Fig. 4, the incident angle is increased and reflected energy is decreased; if the reflecting mirrors are rotated in the opposite direction, the incident angle is decreased and the reflected energy is increased. The rotation of the reflecting mirrors may be implemented using a method of rotating the reflecting mirrors around an axis located at the center of the arrangement of the reflecting mirrors, indicated by c, in a structure in which the reflecting mirrors are fixedly mounted inside a chamber, such as that shown in Fig. 8. Here, it is not necessary to limit the rotating axis to point c, and it may be located at any point in a portion corresponding to a center in the longitudinal direction of the reflecting mirrors.
<96> In another method, in a region having a relatively small r, energy can
be adjusted only by adjusting the distance between the beam source and the reflecting mirrors. This method for increasing or decreasing the reflected energy based on a principle in which the incident angle is decreased as the distance between the origin and the reflecting mirrors is increased and the incident angle is increased as the distance between them is decreased is effective because an energy value is sensitive to a distance in the direction of the z axis when r is equal to or smaller than 500 mm, more preferably, equal to or smaller than 250 mm.
<97> In the above-described example of the method of designing the arrangement of reflecting mirrors, the length of the reflecting mirrors L is the minimum length that allows a polychromatic x-ray having passed through the interval dg between the reflecting mirrors to collide with all the surfaces of the reflecting mirrors. The present invention reveals that an optical filter effective in actual use can be fabricated only by using reflecting mirrors having a length obtained by adding δl to L for the following reasons.
<98> More specifically, most of an χ-ray incident on a reflecting mirror is reflected through constructive interference based on Bragg' s diffraction equation, or undergoes destructive interference, is converted into thermal energy and then becomes extinct. Since the front and rear ends of reflecting mirrors having a length of L are inclined at an incident angle, the state in which a depth of penetration sufficient to reflect or block an x-ray is not ensured is established. Accordingly, a low percentage of, that is, part of, a polychromatic x-ray passes straight through a multi-layer film and a substrate. With regard to a recti linearIy penetrating x-ray, the low energy of an incident beam having a long wavelength becomes extinct, and the high energy thereof having a short wavelength does not become extinct. Since the penetrating beam that does not become extinct degrades the quasi- monochromatic property or causes non-uniformity in an image, it is preferable to eliminate the penetrating beam. In the present invention, in a reflecting mirror arrangement having a length L, the front portion (that is, the front
end portion) of a reflecting mirror substrate close to the origin 0 is determined to have no great problem because a penetrating x-ray collides with an adjacently disposed reflecting mirror again and experiences the effect of seeming to pass through a thick object until it completely passes through the end of the reflecting mirror, whereas light incident on the back portion (that is, the back end portion) thereof, which seems to pass through a relatively thin object, has a strong possibility of rectilinear penetration. Accordingly, an extended length δl is added as a means for increasing the thickness of an object through which a beam passing through a back portion, that is, the back end portion of a reflecting mirror, passes.
<99> Another reason for requiring δl is to prepare for the case where the incident angle is decreased to a value lower than initial angle θi or the reflecting mirrors become farther from the origin in the z axis direction by rotating the reflecting mirror in the direction of -θr at a specific angle so as to adjust the energy. Since the interval dg between the reflecting mirrors is calculated after θi and L are first determined, an x-ray having passed through the interval dg is emitted without colliding with the reflecting mirrors if θi is decreased by (θi - θr), and an incident χ-ray that passes without colliding with the reflecting mirrors if the length is increased occurs, with the result that the above problems should be overcome by adding a length of δl. The minimum value of δl to be added is determined to be a value that enables all of an incident beam entering at an incident angle (θi - θr) through an interval dg to collide with the reflecting mirrors. A value less than the distance between a subject and the reflecting mirrors is sufficient for the maximum value of δl if φ > θi, and the maximum value of δl may be the distance to a point where a line formed by extending the rear surfaces of neighboring reflecting mirror substrates in the longitudinal direction thereof and a reflected beam reflected from the end points of the reflecting mirrors at an angle of θi meet each other if φ < θ i . <ioo> The optical filter presented in the present invention, as shown in Fig.
8, may include a Bragg multi-layer film reflecting mirror 110 and a reflecting mirror chamber 120 for accurately arranging, securing and protecting the reflecting mirror at angles and intervals devised for each purpose. The optical filter may be used in a system that includes a reflecting mirror chamber alignment mechanism including a unit for aligning the reflecting mirror chamber 120 with an χ-ray light source, a unit for providing oscillation or pendulum motion, and a unit for performing fastening to an external frame.
<ioi> Each reflecting mirror includes a multi-layer film which is formed by alternately depositing at least two types of material, that is, a high electron density element material and a low electron density element material, and a substrate on which the film is deposited. A material and manufacturing method that enables a high reflectance for a quasi- monochromatic x-ray, provide an appropriate reflection energy band, provide excellent durability and incur appropriate manufacturing costs may be selected as the material and manufacturing method of the multi-layer film. In the reflecting mirror arrangement of the present invention, the incident angle θ i at a point close to the origin and the incident angle θi2 at a point far from the origin have the relationship of θi > θi2, and the sequence of the sizes of reflected energy is reversed in the Bragg's equation. In consideration of the ranges of values of parameters proposed in the present invention, quasi-monochromatic energy varies by a minimum of 5% and a maximum of 50% due to incident angle deviation, and the quasi- monochromatic energy varies considerably as the length L of the reflecting mirror increases. As a result, a spatial distribution in which the energy of beams emitted from the reflecting mirrors is increased in the y axis of Fig. 4 is constructed and, if a plurality of reflecting mirrors is used, the shape of a distribution in which an energy curve is present is formed. Furthermore, since the energy intensity of the energy spectrum of an incident polychromatic χ-ray is not uniform, the spatial distribution having an energy curve amplifies the non-uniformity of the spatial intensity distribution of a
reflected x-ray.
<io2> The reduction in the deviation range of energy reflected from a reflecting mirror is implemented using an inclination coating method of sloping a multi-layer film by gradually increasing the thickness of the multi-layer film in a direction toward the reflecting mirrors L. With regard to predetermined reflected energy, the thickness of a high and low electron density film pair applied at each of the start and end points of a reflecting mirror can be calculated by substituting incident angles θi and θi2, which are formed between rectilinear lines that connect the start and end points of the reflecting mirror with the origin 0 and the reflecting mirror, into Bragg's equation. Since a film thickness at either end of each reflecting mirror is obtained by multiplying the thickness of a film material pair by the number of layers, the film thickness is proportional to the thickness of the film material pair. If the thicknesses of film material pairs at both ends of the reflecting mirror are mtl and mt2, respectively, there is the relationship of mt2 > mtl, and the thickness of a film is increased from mtl to mt2 in the longitudinal direction thereof. The thickness of each portion of each reflecting mirror is a thickness that satisfies Bragg's equation, and the extent of thickness (inclination) in the longitudinal direction is increased as the incident angle is close to θi2, rather than θi. However, when θi2 is equal to or greater than 0.15 degrees and the difference between θi and θi2 is small, or when it is necessary to simplify a manufacturing process, inclined coating may be applied such that the inclination obtained by dividing the difference in the thickness of the film material pair of each reflecting mirror by the length of the reflecting mirror {(mt2 - mtl)/L} becomes uniform. In this case, although a reflected energy value region slightly moves to a low value region, the reflecting mirror can still be used as an effective quasi-monochromatic χ-ray reflecting mirror (refer to Fig. 3). A film applied in an inclined manner can be manufactured using a variety of widely used vacuum deposition processes; only a high electron density material is inclined, only a low electron density material is inclined, or a
thickness inclination may be provided to both materials. In Fig. 3, reference numeral 107 denotes a substrate, and reference numerals 108 and 109 denote coated portions.
<iO3> Taking the case where inclined coating is not applied to a multi-layer film as an example, a beam having a high intensity close to the origin is reflected in the y axis direction, and thus energy is decreased because the intensity of the beam and an incident angle is large. In the x axis direction, energy is highest at the center, and energy is decreased on the upper and lower portions. The reason for this is that the same energy contour line is drawn along the contour line of a concentric circle that connects the same distances between reflecting mirrors and an origin. In the y axis direction, the curvature of the concentric circle is increased. The energy non-uniformity in the x axis direction is decreased when the divergence angle of a beam is small, the distance from the origin is great and the incident angle is large. The energy non-uniformity is slight when the convergence angle of a commercial χ-ray is equal to or less than 25 degrees, the value r proposed by the present invention is in a range of 50 mm - 500 mm, more preferably in a range of 150 mm ~ 350 mm, and there are no shutters that block the upper and lower portions of the x axis direction at an incident angle equal to or less than 1 degree, preferably equal to or less than 0.5 degrees, at distance r.
<iO4> Although the substrate may be made of various materials, such as glass, various metals or a silicon single crystal wafer, it is preferred that the surface of the substrate be very flat and not rough. More preferably, the substrate has the root mean square of surface roughness equal to or less than 1.0 nm and a low x-ray transparency. Substrate materials having low x-ray transparencies are heavy metals having high atomic numbers, preferably nickel, iron, molybdenum, tungsten and lead. These substrates help block the above-described x-rays that pass straight through the substrate.
<i05> In the optical filter in which reflecting mirrors are arranged, the efficiency OFE at which an incident beam is emitted in the form of a quasi-
monochromatic ray is expressed as follows:
<i06> OFE = {f(r) x f(s) x N x (1 - t/dg)}/I(p)
<iO7> where f(s)-' the energy reflectance curve of a reflecting mirror
<i08> F(s): energy spectrum curve of incident beam
<i09> Kp) : total number of photons of incident beam
<πo> N: photon number normalization factor
<iu> dg: interval between reflecting mirrors
<ii2> t: thickness of reflecting mirror
<ii3> The efficiency of the optical filter is proportional to the x-ray reflectivity of the reflecting mirror. Since reflectivity is the fraction of specific energy that is reflected, the case where reflectivity is high and the energy resolution of a quasi-monochromatic χ-ray (δE/E) is high is highly effective. Accordingly, this feature should be taken into account in the design and manufacture of the multi-layer film of the reflecting mirror. According to the present invention, Mo, W, Pt, Re and Au are excellent for the high electron density material of the multi-layer film, C, Si, SiC and B4C are excellent as the low electron density material thereof, and Mo-Si, W- C, W-Si, W- B4C, W-SiC and and Pt-C have high efficiency as the material pair.
<ii4> Another principal factor that determines efficiency in the optical filter is the gap d between a reflecting mirror and a neighboring reflecting mirror. As the ratio of the thickness t of the reflecting mirror to gap d or the ratio of the interval between reflecting mirrors (dg = d + t) to gap d decreases, the efficiency of the optical filter increases. Accordingly, in order to reduce the thickness of the substrate used in the reflecting mirror as much as possible and increase the gap, the Bragg film thickness of the reflecting mirror is minimized, so that an incident angle having a large value can be obtained, and the length L of the reflecting mirror is increased as much as possible. Although the thickness of the substrate may be reduced to the limit where the substrate is still not bent and does not lose flatness, problems difficult to deal with, such as the creation of broken
pieces at the time when a glass substrate is fastened to a reflecting mirror chamber, occur and the possibility that an x-ray travels straight and passes through the reflecting mirror. The thickness of the substrate of the reflecting mirror is equal to or less than 1 mm, more preferably 0.5 mm. In order to obtain the best efficiency, a substrate having a thickness of 0.1 mm may be used.
<ii5> Although, in the present invention, reflecting mirrors having short lengths equal to or less than 10 mm, or long lengths equal to or greater than 600 mm may be manufactured and used, a limitation is imposed on the length in terms of practical use. For example, in the case of medical imaging equipment using commercial x-rays having beam divergence angles in the range of about 20 ~ 30 degrees, the distance between an χ-ray light source and a subject generally does not exceed 1 m, and thus the length of the reflecting mirror cannot be equal to or greater than that. The width of the reflecting mirror is related to the distance r to the χ-ray light source and the divergence angle of incident light. A commercial polychromatic x-ray diverges in a conical shape, the divergence angle is determined depending on the inherent characteristics of an x-ray light source, and the divergence width may be adjusted by disposing a width adjusting shutter between the light source and the optical filter. When the divergence angle, r and L are great, the width of the reflecting mirror should be increased in order to enable the entire polychromatic x-ray incident light to collide with the reflecting mirror. In contrast, when the divergence angle, r and L are small, the width is reduced. Although the shape of the reflecting mirror may be designed to conform to the conical beam divergence shape in which the width of a portion close to the origin of light is small and the width of a portion far from the origin is large, it is easy to fabricate the reflecting mirror in a rectangular shape.
<ii6> The outer shape of the reflecting mirror chamber may be formed in a hexahedral shape, a spherical shape or other various shapes having no particular limitation. The reflecting mirror chamber should function to
D
accurately arrange reflecting mirrors in the arrangement shown in Fig. 4, secure them, and protect them from the outside.
<ii7> The chamber alignment mechanism is a device unit that has an important function of aligning an origin with the reflecting mirror arrangement of the reflecting mirror chamber when the x-ray light source is set to the origin shown in Fig. 4. Although no particular limitation is imposed on the alignment method, the alignment mechanism of the present invention must include a mechanism for adjusting rectilinear movement with respect to the x, y and z axes and a mechanism for rotating the reflecting mirror chamber around an axis that intersects, at right angles, an arbitrary circumference of radius r along which the reflecting mirrors are arranged, more preferably around an axis that intersects the circumference at the center of the arrangement of the reflecting mirrors shown in Fig. 4. If a mechanism for adjusting α angular movement in the xz plane and β angular movement in the yz plane is included, the adjustment of the reflecting mirror chamber can be more accurately performed. A micro-positioner and an appropriate motor may be used as a mechanism for performing xyz rectilinear movement. The mechanism for performing xyz rectilinear movement requires a precision equal to or lower than 1/10 mm, and prefers x- and y-axis movement precision to z axis movement precision. The mechanism for rotating the reflecting mirror chamber enables rotation within an angular range of ±1, and must have at least a resolution of 0.005 degrees.
<ii8> In the following embodiment, the results of the manufacture and performance evaluation of the components of the optical filter will be enumerated in brief.
<ii9> Embodiment 1 (manufacture and physical properties of reflecting mirror)
<i2o> Reflecting mirrors were manufactured in various sizes based on a variety of substrates using sputtering vacuum deposition. The following Table 1 lists the manufacturing conditions, numbers of layers, thicknesses, measured using X-Ray Reflection (XRR), and roughnesses of the two material interfaces of the films of several molybdenum/silicon multi-layer film
reflecting mirrors that were manufactured by performing sputtering vacuum deposition on 4-in size silicon single crystals. The reflecting mirrors listed in Table 1 are suitable for reflecting x-rays in a range of 10-100 keV at an incident angle less than 1 degree, or are suitable for reflecting x- rays in a range of 15-60 keV at an incident angle less than 0.5 degrees so as to obtain a higher reflectivity. Fig. 5 presents a transmission electron microscopic photo of reflecting mirror # 1.
<121> [Table 1] <122>
<123>
Embodiment 2 (x-ray reflection spectrum of reflecting mirror)
<124> In order to check the χ-ray reflection performance of specimen # 1, reflectivity was measured using the beam line of Pohang Accelerator Laboratory. When the incident angle was about 0.3 degrees, it was observed that a 22.313 keV χ-ray was reflected. When the reflectivity is calculated in the percentage ratio of a reflected beam energy value denoted by 114 in Fig. 6 to an incident beam energy value denoted by 113, 58.72% is obtained. Furthermore, the spectrum of a beam that was reflected from specimen # 1, after the x-ray of a commercial x-ray generation system having a W positive electrode was caused to enter the specimen # 1, was measured using AMPTEK Co.'s CdTe detector and spectrometer. A slit was disposed between an x-ray generation tube and a reflecting mirror and an incident x-ray was adjusted, with the result that an 18 keV reflected x-ray spectrum was obtained at an incident angle of about 0.4 degrees.
<125> Embodiment 3 (reflecting mirror image)
<126> Fig. 7 compares an image, acquired by, under the conditions in which the commercial x-ray system of Embodiment 2 was used, radiating a reflected x-ray onto a CDMAM Phantom and capturing an image using a digital detector, with a W positive electrode and 40 kVp voltage polychromatic x-ray image. It was observed that, as a result of comparison in contrast with the polychromatic x-ray in the gold disk portion of the CDMAN Phantom, indicated by the dotted lines in the drawing, the quasi-monochromatic reflected beam was higher in contrast by 18%.
<i27> Embodiment _4 (manufacture of reflecting mirror chamber, design parameters, and image)
<i28> An example of the design parameter values of a reflecting mirror chamber, the reflecting mirror chamber manufactured using these values, and an image photographed using the reflecting mirror chamber are illustrated in the following Table 2:
<i29> [Table 2]
Fig. 8 is a drawing of Korean Design Application No. 30-2006-0015261 entitled "Front Filter for X-ray Imaging System." Groove-type slots formed over and under the chamber accurately hold and fasten the arrangement of reflecting mirrors. When an actual reflecting mirror chamber manufactured based on design parameter values of Table 2 is aligned in a commercial x-ray generation system, a large area CDMAM phantom image can be obtained, as shown in Fig. 9.
<i3i> Embodiment 5 (various energy reflections based on changes in rotation angle of a chamber)
<i32> Fig. 10 shows an example of the case where the energy value of a reflected beam varies by mounting a plurality of reflecting mirrors in a chamber and adjusting the rotation angle of the chamber to two cases. In this case, Fig. 10(a) shows a 13.5 keV quasi-monochromatic reflected beam spectrum that is obtained at an arbitrary rotation angle, Fig. 10(b) shows a 17.5 keV quasi-monochromatic reflected beam spectrum that is obtained by manipulating a rotation angle in the same reflecting mirror, and Fig. 10(c) shows a reflecting mirror chamber image under the condition in which the spectrum is measured.
<i33> A system devised to generate a quasi-monochromatic x-ray having dual energy using the quasi-monochromatic x-ray optical filter constructed as described above and be capable of capturing clearer radiographs using the system will be described in detail below.
<i34> Fig. 11 is a conceptual diagram showing a quasi-monochromatic dual energy x-ray imaging system according to the present invention.
<i35> The x-ray imaging system of the present invention includes a light source unit 10, a signal detector unit 20 and a collimator unit 30.
<i36> The light source unit 10 includes an optical filter 40 for filtering a polychromatic χ-ray to obtain a quasi-monochromatic x-ray, and the quasi- monochromatic function of the optical filter 40 is implemented using a multilayer reflecting mirror 42.
<i37> The multi-layer reflecting mirror 42 has a structure in which a glass substrate 43 having a thickness less than 1 mm and a very flat surface is coated with films 42a and 42b having a thickness in a range of several tens to several hundreds of nm, as shown in Fig. 12. Reflected energy is determined based on an incident angle corresponding to the incident angle of Bragg' s diffraction equation and the thickness of a film material pair.
<i38> The reflecting mirror 42 reflects a specific energy portion of a primary beam incident on the reflecting mirror, and the spectrum thereof has
a slight narrow band width. Since the reflected energy and the band width are properties inherent in the reflecting mirror that are determined by adjusting the incident angle, the material, the thickness of the material pair, and the number of layers of the material pair, it is possible to predict ideal values.
<!39> Since an actual commercial x-ray light source causes a beam to diverge at a wide angle of several tens of degrees around a focal spot, an incident angle varies depending on the distance between an inclined reflecting mirror and the focal spot. As the reflecting mirror moves farther from the focal spot in the longitudinal direction identical to a direction in which a beam travels, the incident angle is gradually reduced. For the same distance, the incident angle does not have a single value, but has a width inside a certain range. Accordingly, it is necessary to select a polychromatic x-ray light source suitable for the purpose in question, and manufacture and use the film of the reflecting mirror suitable for that purpose.
<i40> In the present invention, the thickness of the material pair of the multi-layer film is about several nm, the energy of reflected quasi- monochromatic light is set to a value equal to or greater than 10 keV at an incident angle less that 1 degree, and the limitation in which the thickness of the material pair of the film should be constant is not imposed. In more detail, in the case of the use of an iodine contrast medium in angiography, in the case of the inclusion of mammography and in the case of some imaging, using a commercial x-ray light source and an optical filter, a reflected energy value and the width of a spectrum can be determined by adjusting the number of layers between a maximum of 200 and a minimum of 5, with the thickness of a film pair being set between 1.5 and 8 nm and the incident angle being set to a value less than 1 degree. The energy range of a reflected x-ray adjusted under these conditions falls between about 10 keV and 100 keV.
<i4i> The multi-layer reflecting mirrors 42 are arranged to have a preset reflection angle and present intervals and are fastened to the reflecting
mirror chamber 44, and the reflecting mirror chamber 44 is fixedly installed in the frame 12 of the light source unit 10 so that it can be arranged in conjunction with the χ-ray light source. That is, the x-ray light source, the reflecting mirror chamber and the reflecting mirrors are integrated into a single body and form the light source unit 10 of a quasi-monochromatic light source device. The quasi-monochromatic light is configured to photograph a subject in a scanning manner while moving recti linearIy along a preset path, as indicated by the focal spots Sl and S2 in Fig. 11 and to photograph a subject in a scanning manner while rotating around the focal spots Sl and S2, as shown in Fig. 14©.
<i42> A plurality of multi-layer reflecting mirrors 42 is fastened to the reflecting mirror chamber 44 so that the multi-layer reflecting mirrors 42 are arranged along a concentric circle at a predetermined reflection angle and predetermined intervals. Here, the reflection angle is an angle that is formed between a radius line that connects the center of a reflecting mirror with the circumference of a concentric circle along which reflecting mirror are arranged and the reflecting mirror, when multi-layer substrate reflecting mirrors satisfying Bragg's diffraction condition are vertically arranged along the concentric circle of radius r that uses the focus spot (Sl or S2) of a polychromatic χ-ray light source as its origin. In this case, the interval between the reflecting mirrors may be determined depending on the purpose. It is preferable to set the interval between the reflecting mirrors to a minimum value so as to filter most x-rays. Furthermore, the minimum value for the interval between the reflecting mirrors is influenced by the length of the reflecting mirrors and the thickness of the substrates, and it is preferred that the minimum value is set such that as many beams pass through the reflecting mirrors as is possible without them passing between the reflecting mirrors.
<i43> Although the penetration depth to which a polychromatic x-ray incident on a reflecting mirror can penetrate without becoming extinct due to scattering and absorption by the film and the substrate material is not great
at an incident angle less than one degree, a considerable part of a high- energy region equal to or higher than several tens of keV can penetrate into and pass through the substrate in the case where the energy range of a commercial polychromatic x-ray source is a maximum of one hundred and tens of keV. Although an x-ray that travels straight may be blocked by covering the back surface of a substrate with heavy metal material in the case where the interval between arranged reflecting mirrors is large or in the case where a small number, for example, equal to or less than 2 or 3, of reflecting mirrors is used, a separate blocking device is required in the case where reflecting mirrors are densely arranged or the number of reflecting mirrors is large.
<i44> The light source unit 10 of the present invention is configured such that a slit structure is disposed behind reflecting mirrors fastened to the reflecting mirror chamber 44, as shown in Fig. 15, and can block straight beams that pass through the reflecting mirrors without being absorbed and dispersed. Since this structure is disposed along a concentric circle having the same origin as the concentric circle along which the reflecting mirrors 42 are arranged, the structure functions to open or block part of space over the circumference of the concentric circle, and is referred to as a back slit 46.
<i45> The back slit 46 is configured to have slots 47 that are open such that light passes therethrough and walls 48 that are formed between the slots 47 and block the paths of light. The gap between the slots 47 and the gap between the walls 48 may be set to appropriate values depending on the interval between the reflecting mirrors and the thickness of the substrates. That is, the gap between the slots 47 or the gap between the walls 48, that is, the back slit 46, is determined depending on the width of a reflected beam and the interval based on the geometrical structure of the reflecting mirror chamber shown in Fig. 11.
<i46> Meanwhile, the size (width) of the walls 48 corresponding to the closed portions of the back slit 46 is set such that the thickness portions of the
ends of neighboring reflecting mirrors are covered with the respective walls 48, and it may be set by adding an additional length such that the width of reflected beams passing through the slots 47 can be adjusted. Portions 48a added in the direction of the reflection of an x-ray function to block beams reflected from the portions of the reflecting mirrors close to the light source, and portions 48b added in the opposite direction function to block beams reflected from the portions of the reflecting mirror far from the light source. When the widths of the reflected beams passing through the back slit 46 are adjusted using the above-described method, the widths of respective beams reflected from a plurality of reflecting mirrors can be made uniform, so that the beams can function as uniform light during imaging including scanning imaging.
<147> A structure for generating quasi-monochromatic dual energy using an optical filter installed in the imaging system of the present invention and a method of performing imaging using the dual energy will be described below.
<148> Since the energy of an x-ray filtered out by the optical filter 40 is dependent on the thickness of the material pair of reflecting mirrors and the incident angle of the reflecting mirror chamber, dual energy can be implemented in a single optical filter by adjusting these two factors.
<i49> In a configuration, a reflecting mirror chamber is fabricated such that it is suitable for two types of incident angles. That is, the reflecting mirror chamber has a structure in which reflecting mirrors can be installed such that the incident angles of x-rays incident on the reflecting mirrors are two types of incident angles. When reflecting mirrors having the same thickness of material pairs are installed in the reflecting mirror chamber, x-rays in a low energy band are reflected by reflecting mirrors disposed in a region for a large incident angle, while χ-rays in a high energy band are reflected by reflecting mirrors disposed in a region for a small incident angle. The reflecting mirror chamber may have a structure in which reflecting mirrors are arranged to alternately have different incident angles, as shown in Fig. 16(a), a structure in which reflecting mirrors are
arranged such that pairs of reflecting mirrors alternately have different incident angles, as shown in Fig. 16(b) , or a structure in which reflecting mirrors are arranged such that groups of three reflecting mirrors alternately have different incident angles. Alternatively, as shown in Fig. 16(c), the reflecting mirror chamber may have a structure in which reflecting mirrors are arranged on both sides of the center of the reflecting mirror chamber so that different incident angles are assigned to either side. Here, it is not necessary that ratio of the reflection mirrors having different incident angles to each other is 50%, and the ratio may be appropriately determined in consideration of the adjustment of the amount of light and the convenience of a scanning imaging method.
<i50> In another configuration, reflecting mirrors including material pairs having different thicknesses may be arranged in a reflecting mirror chamber manufactured to have the same incident angle. In this case, the two types of reflecting mirrors may be alternately arranged, two types of pairs of reflecting mirrors may be alternately arranged, or two types of halves of reflecting mirrors may be respectively arranged on both sides of the reflecting mirror chamber, and the ratio of two types of reflecting mirrors to each other are not limited, as in the former case.
<i5i> In still another configuration, it is possible to employ a configuration that is constructed by combining the above-described configurations with each other, that is, a configuration in which reflecting mirrors having different thicknesses are used for incident angles.
<i52> Meanwhile, a method using the energy tunable function of an optical filter may be employed as a method of generating dual energy. In the case where all reflecting mirrors are installed in a reflecting mirror chamber and have the same incident angle, if the value of the incident angle is minutely adjusted, it is possible to change the quasi-monochromatic energy band within a specific range. That is, in the case where a plurality of reflecting mirrors is arranged along a concentric circle around the focal spot of an x- ray light source and the radius line of the concentric circle at a uniform
inclination, when the reflecting mirrors are rotated around a point located on the central one of the arranged reflecting mirrors together, the inclined angle of all the reflecting mirrors is changed.
<i53> Here, although the incident angles of all the reflecting mirrors are the same before the reflecting mirrors are rotated together, that is, in the state in which the reflecting mirrors are arranged at preset locations, the reflecting mirrors have slightly different incident angles after the reflecting mirrors are rotated around the selected point together. The difference between the inclined angles is greater between reflecting mirrors arranged on the center portion of the reflecting mirrors and reflecting mirrors arranged on both sides of the reflecting mirrors, and is increased as the angle of the rotation of the reflecting mirrors becomes greater. Accordingly, it is preferable to use this method within a minute rotation angle range. Since the difference between the incident angles of the reflecting mirrors is insignificant at a rotation angle equal to or less than 0.5 degrees, within a distance of several tens of cm from the center of the concentric circle and within a beam divergence angle of 30 degrees and the blocking of reflected beams by the walls of the back slit helps reduce the difference in the incident angle, the method is useful within the ranges of incident angles and quasi-monochromatic energy that are intended to be used in the present invention.
<i54> For example, in the case of angiography, the rotation angle of reflecting mirrors that is used to adjust reflected energy within a range of 30-35 keV, which is less than or greater than the K~edge energy of an iodine contrast medium, is equal to or less than 0.05 degrees when the thickness of the material pairs of the reflecting mirrors is equal to or less than 5 nm. In the case where the distance between the focal spot of a light source and the reflecting mirrors is equal to or less than 35 cm, that is, the distance is very short, preferably 15 cm, a similar effect can be achieved by adjusting the distance between the focal spot of the light source focal spot and the reflecting mirrors.
<i55> Although various methods for generating quasi-monochromatic dual energy have been presented, the number of values is not limited to two, but adjustment to three or more energy values may be performed using the above methods. Alternatively, the above methods may be configured to use a single energy value.
<i56> Since the incident angle of a multi-layer film monochromator for acquiring an x-ray in a range of 10 ~ 150 keV (effective for acquiring typical x-ray images) through filtering is equal to or less than 1 degree (which is very small), the width of a reflected beam is merely several mm even when the wide divergence angle of a light source and the distance to a detector are taken into account. Furthermore, since quasi-monochromatic light acquired through filtering corresponds to part of incident polychromatic light, the amount of light is decreased as long as the power of generation of x-rays of the polychromatic light source is not considerably increased, the degree of which depends on the cases.
<i57> Accordingly, a scheme for performing large area photographing and acquiring images having an appropriate SNR is urgently required. The present invention presents an optical filter using a plurality of reflecting mirrors and scanning imaging based on an integrated light source in which the optical filter is installed as inherent solution methods. The present invention enables a method of acquiring respective dual energy images at one time, in which case the effect of a reduction in the photographing time is achieved.
<i58> The scanning imaging method of the present invention enables the same imaging system to perform various types of imaging when an x-ray light source unit performs rectilinear uniform movement along the χ-axis direction shown in Fig. 11 in the state in which a subject and a detector are fixed, or when an χ-ray light source unit is rotated around a focal spot. A method suitable for the status of a subject and the purpose of imaging may be selected and used.
<i59> In the case where three or more reflecting mirrors are installed in a reflecting mirror chamber, scanning imaging is performed in such a way that a
bundle of quasi-monochromatic light is subjected to uniform rectilinear motion and/or rotating motion in a range from one end 34 of the opening of the collimator 32 of the collimator unit 30 to the opposite end 35, or in such a way that quasi-monochromatic light is subjected to uniform rectilinear motion and/or rotation motion in the state in which the quasi-monochromatic light is radiated onto the entire opening of the collimator 32 first. As a rule, a digital detector having a planar two-dimensional pixel arrangement is used as the signal detector unit 20 in which the x-ray signal detection of scanning imaging is performed. In the case where the number of reflecting mirrors is equal to or less than 2, it is desirable to cause a line-type detector in which pixels are arranged in a single row to correspond to a reflected beam. However, this does not mean that the use of other types of detectors is limited in respective cases. Assuming that the focal spot origin of a polychromatic light source is S2, light diverges at divergence angle θl, and one reflecting mirror constituting part of an optical filter reflects light by angle φ, which is part of the divergence angle. A plurality of reflecting mirrors changes the direction of divergence by an incident angle, with the result that a reflected x-ray is emitted from a second virtual light source having a divergence angle of θ2 and a focal spot origin Sl. Since θ2 varies depending on the number of reflecting mirrors, the distance to an origin and an incident angle and the reflecting mirrors have thicknesses, an angle obtained by adding a number of φ, equal to the number of reflecting mirrors, together always has a value less than θ2. Furthermore, since the thickness portions of the reflecting mirrors block polychromatic incident light, the side intensity shape of quasi-monochromatic light emitted from the optical filter is formed like a wave shown in Fig. 13. In Fig. 13, 'A' indicates the intensity profile of a reflected beam in a scanning direction, which is obtained using a single reflecting mirror, B indicates the profile of a reflected beam in a rectangular form, and C indicates the distance between reflected beams.
<i6i> The opening distance d3 of the collimator 32 enables only a desired target portion of a subject to be exposed to quasi-monochromatic light, and is determined depending on geometrical structures, such as divergence angle θ2, the distance dl between the light source focal spot and the collimator 32, and the distance d2 between the collimator 32 and the subject 50.
<!62> When viewed from the standpoint of image quality, an SNR equal to or higher than a specific level should be obtained, which means that the amount of light having passed through a subject should be equal to or larger than a specific level. As a result, assuming that the same power is supplied, the movement of a plurality of reflecting mirrors, under the same conditions, over the same distance can reduce the time that is required to acquire the same amount of light to a value a plurality of times less than that of the scanning imaging using one reflecting mirror. Although scanning speed may be expressed as the speed of the rectilinear motion of the light source unit, it may be expressed as the division of the opening distance d3 of the collimator adjusted to the size of the subject by the time t that it takes for all the reflected beam bundles of reflecting mirrors to make a complete passage, the division of the left or right distance LO of a subject by t , or the like.
<i63> When a subject is exposed to the same amount of light, the use of an optical filter in which n reflecting mirrors are mounted can increase scanning speed n times and can mitigate limitation to a large area, compared to the use of an optical filter in which one reflecting mirror is mounted.
<164> When a dual energy generation area is divided into right and left regions by appropriately designing the structure of the reflecting mirror chamber of an optical filter or varying the thicknesses of the film material pairs of reflecting mirrors, scanning imaging is performed once or twice in order to obtain a dual energy image.
<i65> In the case where imaging is performed twice, image information is acquired through scanning imaging that is performed by moving a light source while emitting quasi-monochromatic light having one type of energy after completely blocking one region of an optical filter that emits one type of
energy, the light source is returned to a start position after the imaging, the energy emission region is switched to the other region, and an image is acquired through second scanning imaging by using quasi-monochromatic light having another type of energy. A method of blocking either region may be implemented by blocking light incident on a reflecting mirror chamber with a metal plate shutter (a movable lid) through which an χ-ray cannot pass, as shown in Figs. 14(a) and 14(b) , or by configuring the metal plate shutter 60 to have a structure for blocking opposite-side reflected beam emission. Furthermore, in the case where rotation around a focal spot is performed, as shown in Fig. 14(c), the metal plate shutter 60 may be used.
<i66> When a method of changing energy by slightly changing the incident angle of rotating reflecting mirrors around a focal spot is used, imaging is performed once without the use of a shutter, the incident angle of the reflecting mirrors are adjusted to an original state, and then imaging is performed. In the case where imaging is performed twice at different incident angles, a subject may be imaged at different points of a detector. Since this may cause error when two images are subtracted from each other, correction is required.
<i67> Since two types of dual energy emitted from the optical filter are clearly distinguished from each other along an axis that intersects a scanning direction at right angles (see Fig. 13), the type of energy of an x- ray incident on each detector pixel line in a direction that intersects the scanning direction at right angles can be detected, and thus dual energy imaging can be performed through a single imaging operation. However, since a single type of energy is not continuously incident on a predetermined pixel line because the light source moves, signals must be detected by dividing exposure time into time frames.
<i68> Quasi-monochromatic light having passed through a plurality of reflecting mirrors has a saw-toothed beam intensity profile A, such as that shown in Fig. 13, due to the shadows of the reflecting mirrors. Assuming that the profile of a reflected beam has an appropriate shape, preferably the
shape of a single rectangle B corresponding to the width of a half tooth of intensity (FWHM), a time frame is designated as L/v when scanning speed is v and the distance between rectangles is L. Assuming that the width of a pixel line is p, L/p lines are present within L, and all pixel lines are classified into two types of dual energy within a single time frame. Although all pixels are classified into two types of dual energy in a subsequent time frame, pixel locations for the dual energy are changed. Since the locations and widths of pixels corresponding to respective components of dual energy always are constant unless the location of emission of the light source and the distance between a detector and a light source are changed, images of the respective components can be acquired by dividing the images of the frames into images corresponding to the respective components of the dual energy and editing the resulting images again. Assuming that the distance along which scanning imaging should be performed is Ld based on the length of a detector, Ld/L frames are required, and Ld/L is the minimum number of required reflecting mirrors.
<i69> In the case where two types of energy are alternately arranged, the same time frame may be applied, in which case scanning imaging can be performed in the state in which the entire subject is exposed to a quasi- monochromatic χ-ray. Here, the minimum number of time frames may be set to one for the time L/v for which movement has been performed over the distance L, and may be set to two until the end of the subsequent time L/v.
<πo> When limitations on the amount of light and imaging time is not strict, one or two reflecting mirrors may be used, in which case it is preferable to use a rectilinear-type line detector in which the pixel lines of a detector correspond to reflected beams. In this case, a large area subject can be scan-imaged by simultaneously moving the line detector and the light source together.
<i7i> In order to check the effects of the present invention, the following experiment was carried out.
<i72> Iodine contrast medium solutions having different concentrations of
11.6 mgl/ml, 23.1 mgl/ml and 92.5 mgl/ml were put into an acrylic phantom with a square hole having a width of 5 mm and a length of 30 mm, and scanning imaging was performed using quasi-monochromatic dual energy twice under the conditions in which the distance between a light source and reflecting mirrors was 250 mm, an incident angle was 0.25 degrees, the size of a reflecting mirror light source focal spot was 0.3 mm, the length of the reflecting mirrors was 100 mm, the thickness of the reflecting mirrors was 0.5 mm, and the distance between a back slit and the light source was 355 mm. The spectra of dual energy used in the imaging are shown in Fig. 17, and the central energy values thereof are 30.5 keV and 35.7 keV, which are close to and come before and after the K-edge energy of iodine. Fig. 18(a) shows iodine liquid portions having three types of concentrations that were imaged using 30.5 keV quasi-monochromatic light, Fig. 18(b) shows iodine liquid portions having three types of concentrations that were imaged using 35.7 keV quasi-monochromatic light, in which case a clearer image was acquired because the x-ray absorption effect of an iodine contrast medium was great. Fig. 18(c) shows an image after subtraction, which was obtained by performing subtraction between the signals of Figs. 18(a) and 18(b), and is represented in inversed gray scale. The effect in which the background of an acrylic portion, other than a contrast medium portion, was eliminated could be observed.