now , a detailed description will be given to the preferred embodiments of the present invention with reference to the figures , throughout which like reference signs denote identical or similar component , though illustrated in different figures . for clarity and conciseness , specific description of any known function or structure incorporated here will be omitted otherwise the subject of the present invention may be obscured . subjecting a 2d distribution u ( x , y ) to line integration along a direction θ will result in a 1d function p θ ( t ) which is referred to as the projection of u ( x , y ) at an angle of θ . if the projection p θ ( t ) along respective directions can be obtained , the 2d distribution u ( x , y ) can be calculated accurately based on radon transformation . the procedure of deriving a 2d distribution from its projection is called reconstruction , which acts as the mathematic principle for ct . in practice , after an x - ray and a detector go round an object for one cycle , there measured and obtained the projections of the attenuation coefficient distribution along respective directions for some slice of the object , and the 2d distribution of attenuation coefficients of the object slice can be reconstructed on the basis of the ct principle . the linear attenuation coefficient of a material with respect to x - rays can be represented by a sum of linear attenuation coefficients of three primary effects with the following expression : μ = μ p + μ c + μ e = a 1 f p ( e )+ a 2 f kn ( e )+ a 3 f e ( e ), in which μ represents the linear attenuation coefficient of a material with respect to x - rays , μ p , μ c , μ e represents linear attenuation coefficients corresponding to photoelectric , compton scatter and electron pair effect , respectively . each item of μ p , μ c , μ e can be approximated as a product of two items , a and f ( e ), the coefficient a depending on the atomic number and density of the material , and f ( e ) depending on the energy of x - rays . as shown in theoretical analysis and experiments of material attenuation coefficient , the photoelectric effect makes a contribution much less than the other effects in a high energy situation , and thus the effect can be neglected in calculation . as a result , the above expression is simplified as μ = μ c + μ e = a 2 f kn ( e )+ a 3 f e ( e ). since the linear attenuation coefficient of each material can be uniquely determined by the two coefficients , a 2 and a 3 , two base materials , such as carbon and aluminum , can be selected so as to represent the linear attenuation coefficient of any other material with a linear combination of the linear attenuation coefficients of these base materials , as illustrated in the following expression : μ = μ c + μ e = b 1 μ 1 + b 2 μ 2 ( 1 ), in which μ denotes the linear attenuation coefficient of one arbitrary material , μ 1 , μ 2 are the linear attenuation coefficients of the selected base material , b 1 and b 2 are called base material coefficients . this is exactly a kernel expression in base material decomposition . the above a 2 , a 3 are defined as in which ρ represents the material &# 39 ; s density , z represents the material &# 39 ; s atomic number , and a denotes the material &# 39 ; s atomic mass number . subsequently , a formula for calculating effective atomic number and characteristic density through a base material decomposition scheme can be derived as : the x - ray tube or the accelerator generally produces x - rays with a continuous energy spectrum . during the process of x - ray projection , the ray source energy spectrum and the detector spectrum can be combined into d ( e ) for the purpose of simplifying the calculation process . the combined d ( e ) satisfies the normalization condition as : for a projection equation , it can be rewritten in the form of a continuous spectrum the dual - energy projection can be expressed in the following by substituting a base material decomposition model into the above expression p 1 ( b 1 , b 2 )=− ln ∫ d 1 ( e ) exp [− b 1 μ 1 ( e )− b 2 μ 2 ( e )] de p 2 ( b 1 , b 2 )=− ln ∫ d 2 ( e ) exp [− b 1 μ 1 ( e )− b 2 μ 2 ( e )] de ( 3 ) in which p 1 , p 2 represent projections at dual energy levels , a high - energy projection and a low - energy projection , respectively , d 1 , d 2 represent x - ray system energy spectrums at high and low energy level , and each of b 1 , b 2 is thickness of the base material defined as : b 1 =∫ b 1 ( { right arrow over ( r )} ) dl b 2 =∫ b 2 ( { right arrow over ( r )} ) dl . the projection values of base material coefficients , b 1 and b 2 can be acquired with a lookup table or by solving the above set of equations ( 3 ) directly . then , the base material coefficients b 1 and b 2 can be obtained with a generic filtering inverse projection reconstruction algorithm , in which ( ρ , θ ) represents radial and angular coordinates for the projection values . the atomic number z eff and the characteristic density ρ e can be obtained with the above equation ( 2 ), and the image of linear attenuation coefficient at any energy level can be obtained with the above equation ( 1 ). fig1 is a schematic diagram of a dual - energy ct system according to an embodiment of the present invention . as shown in fig1 , a ray source 100 generates dual - energy x - rays having a continuous energy distribution at predefined timing under the control of a controller 500 . the object 200 is placed on a bearing mechanism 300 , which can rotate uniformly and be lifted up and down under the control of the controller 500 . an array of detectors 400 is arranged at a position opposite to the ray source 100 , and receives the penetrative rays , which have traveled through the object 200 , under the control of the controller 500 so as to obtain detection signals for a first energy level and detection signals for a second energy level . the signals detected by the detector array 400 are converted into digital signals and stored in a computer for subsequent reconstruction processing . ( 1 ) the x - ray source 100 comprises a high - energy dual - energy accelerator ray source , which can produce x - rays at two high voltage levels in a rapid and alternative manner . with respect to large - sized cargoes , the use of an accelerator ray source can produce rays of higher power to ensure a clearer reconstructed image . ( 2 ) the bearing mechanism 300 can be , for example , an object - bearing platform which rotates steadily and stably . ( 3 ) the linear array of detectors 400 is deployed in a horizontal direction while being kept perpendicular to an axis across the center of the x - ray source 100 and the bearing mechanism 300 . ( 4 ) mechanical / electrical control , data transmission and image reconstruction of the overall ct system are fulfilled by a computer work station . the work station performs reconstruction of a tomogram , which is subsequently displayed on a display in 2 or 3 dimensions . ( 5 ) for the purpose of an accurate image reconstruction , the ct system measures or calibrates precisely system parameters including distance d from the x - ray source to a detector , distance r from the x - ray source to the rotation axis of the object - bearing platform , position c at which the x - ray source is mapped onto a detector , pixel size d of a detector , the precise geometric position x i of each of the detectors , and the rotation angle θ of the platform . ( 6 ) the scanning scheme used in the system is a standard fan - beam circle trajectory scanning scheme , in which the ray source and the detectors are fixed at a height while the object moves together with the rotation of the object - bearing platform . the dual - energy ct projection data is collected each rotation the object moves . ( 7 ) with the above image reconstruction process , the atomic number image and the characteristic image of the material can be obtained by a computer from the dual - energy ct projection data of a scanned object . the obtained images provide a basis for subsequent material identification and determination . now , details of the image reconstruction method according to the embodiment of the present invention will be described in connection with fig2 . at step s 11 , dual - energy rays are used in scanning the object to obtain the dual - energy projection values . at step s 12 , the projection values of base material coefficients corresponding to the dual - energy projection values are calculated on the basis of a pre - created lookup table or by analytically solving a set of equations . the lookup table can be created by selecting two different base materials , calculating projection values with dual - energy rays penetrating the two base materials under different combinations of thickness , and creating a lookup table in a form of correspondence between high - and low - energy projection values and the different combinations of thickness . on the other hand , analytically solving a set of equations can be realized by acquiring a corresponding combination of thickness by solving a high - and low - energy projection set of equations under base material decomposition using the high - energy dual - energy projection values actually obtained . the latter analytical solution of an equation set , though providing a higher accuracy , requires a long time period of calculation . considering the slow calculation , a lookup table is thus preferably used in practical applications . at step s 13 , an image of base material coefficient distribution can be obtained from the projection values of the base material . at step s 14 , the atomic number image and characteristic density image of the detected object as well as the attenuation coefficient image of the detected object at an arbitrary energy level can be derived from the image of base material coefficient distribution . this step is implemented in a hardware system . with these steps , the present invention establishes a base material decomposition dual - energy ct reconstruction method with high - energy x - rays , which method can derive the atomic number and characteristic density images of a tomogram from the dual - energy projection in a more accurate manner . the derived atomic number and characteristic density are equivalent physical quantity in the case that the detected object contains a mixture or a compound . fig3 a to 3d show part of experimental results obtained by simulation experiments with a single graphite model . fig3 c and 3d show sectional diagram of the atomic number and characteristic density images in a horizontal direction , respectively , wherein the dotted line denotes reconstructed values , and the solid line denotes theoretical values . as a generic image reconstruction method in the case of high - energy dual - energy condition , the present invention is applicable to any dual - energy ct system using high - energy x - rays as ray source . therefore , the present invention is expected to have a bright prospect in a larger range of applications . the foregoing description is only intended to illustrate the embodiments of the present invention other than limiting the present invention . for those skilled in the art , any change or substitution that can be made readily within the scope of the present invention should be encompassed by the scope of the present invention . therefore , the scope of the present invention should be defined by the claims .