Abstract:
A one dimensional X-ray image sensor consists of a plurality of unit detecting devices aligned in a row and a filter which covers a portion thereof and is made of a material having an X-ray absorption coefficient equal to or nearly equal to that of a target material to be inspected. An X-ray inspection apparatus includes the one dimensional X-ray image sensor and further includes a data processor for correcting measured values using data measured in regard to the filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a one dimensional X-ray image sensor and an X-ray inspecting apparatus using the same. 
     2. Description of the Related Art 
     Recently, there has been noticed an X-ray inspecting apparatus for measuring a content of a target material using X-ray. This utilizes such a phenomenon that the X-ray absorption coefficient of a target material differs in response to the kind of the target material and the energy of X-ray upon irradiating two kinds of X-rays having different energies to an object. For instance, when high energy and low energy X-rays are irradiated to an object comprised of materials A and B, transmissive intensities I l1  and I h1  of the low energy and high energy X-rays are given as follows. 
     
         I.sub.l1 =I.sub.l0 exp(-μ.sub.la ρ.sub.a T.sub.a -μ.sub.lb ρ.sub.b T.sub.b)                                      (1) 
    
     
         I.sub.h1 =I.sub.h0 exp(-μ.sub.ha ρ.sub.a T.sub.a -μ.sub.hb ρ.sub.b T.sub.b)                                      (2) 
    
     Wherein I l0  and I h0  are intensities of the low energy and high energy X-rays irradiated, μ la  and μ ha  are mass attenuation coefficients of the material A for the low energy and high energy X-rays, respectively, μ lb  and μ hb  are those of the material B, ρ a  and ρ b  are densities of the materials A and B, respectively, and T a  and T b  are thicknesses of the materials A and B, respectively. 
     From these two equations (1) and (2), the following equation is introduced. 
     
         T.sub.b ×ρ.sub.b ={In(I.sub.l0 /I.sub.l1)×μ.sub.ha -In(I.sub.h0 /I.sub.h1)×μ.sub.la }/(μ.sub.ha μ.sub.lb -μ.sub.hb μ.sub.la)                                 (3) 
    
     From the equation, a product of the thickness T b  and the density ρ b  of the material B, i.e., a content of the material is obtained by measuring I l1  and I h1 . For instance, the materials A and B may be soft tissue and bone tissue of the human body in bone salt (mineral) analysis, respectively. In this case, an amount of the bone tissue (T b  ×ρ b ) is obtained according to the equation (3) by measuring the transmissive intensities I l1  and I h1  of the low energy and high energy X-rays. 
     However, so called beam hardening phenomenon is inevitable in such an analysis using X-rays of different energies as mentioned above. This phenomenon is essentially based on an energy distribution of X-ray irradiated from an X-ray source. Absorption of an X-ray having an energy distribution increases as the energy thereof becomes low and the thickness of a target material increases and, accordingly, lower energy components thereof are attenuated much more than higher energy components thereof in transmission through the target material. Due to this, the effective energy of the X-ray after transmission shifts toward higher energy side as compared to that before transmission. 
     This beam hardening phenomenon gives influences to the mass attenuation coefficients μ ha , μ hb , μ la  and μ lb  since they are dependent on energies of X-ray beams passing through. Thus, the content obtained using the equation (3) is not exact. 
     SUMMARY OF THE INVENTION 
     Accordingly, one essential object of the present invention is to provide an X-ray image sensor being free from the beam hardening phenomenon. 
     Another object of the present invention is to provide an X-ray image sensor having a correction filter made of a material having an X-ray absorption coefficient equal to or near to that of a target material to be inspected. 
     A further object of the present invention is to provide an X-ray inspection apparatus capable of measuring an exact content of a target material irrespectively to a thickness of a target. 
     In order to achieve these objects, according to the present invention, there is provided an X-ray image sensor for receiving X-ray beam having been transmitted through an object and outputting electric signals proportional to intensities of the beams received, being characterized by comprising a plurality of unit detecting devices aligned linearly, each unit detecting device outputting an electric signal proportional to an intensity of the beam received and a filter means arranged so as to cover some of said unit detecting devices, said filter means being comprised of a plate-like member having a predetermined thickness which is made of a material having an X-ray absorption coefficient equal to or near to that of a target material to be inspected. 
     Accordingly to the X-ray image sensor of the present invention, two kinds of data, namely data from the unit detecting devices not covered by the filter means and data from the unit detecting covered thereby are obtained always and a correction coefficient for correcting data obtained can be calculated using the data regarding the filter means. 
     Accordingly to another aspect of the present invention, there is provided an X-ray inspection apparatus comprising an X-ray generator for generating at least two kinds of X-rays having different effective energies, a multi-channel X-ray image sensor for outputting electric signals proportional to intensities of X-rays received thereby, a filter means arranged so as to cover a part of said multi-channel X-ray image sensor, said filter means having an X-ray absorption coefficient equal to or near to that of a target material to be inspected, a data processing means for calculating a mass content of said target material using outputs from said multi-channel X-ray image sensor, said data processing means further calculating a correction coefficient based on data obtained regarding said filter and said X-ray absorption coefficient, wherein said mass content obtained is corrected by said correction coefficient. 
     The X-ray inspection apparatus of the present invention is capable of automatically correcting or compensating influences due to so called beam hardening effect of X-ray and fluctuations of the energy of X-ray generated by the X-ray generator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: 
     FIG. 1 is a schematical view showing a composition of an X-ray inspection apparatus according to the present invention; 
     FIG. 2 is a schematical perspective view of the X-ray inspection apparatus installed to a bed; and 
     FIG. 3 is a cross-sectional view of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an X-ray inspection apparatus for analyzing the bone salt regarding a bone tissue 7 such as a backbone of a human body 8 comprised of the bone tissue 7 and a soft tissue 9. 
     Under the human body 8, there is arranged an X-ray image sensor 3 being comprised of a linear array of a plurality of unit detecting devices 3u and a plate-like filter 4 arranged so as to cover a portion of the linear array. 
     Above the human body 8, an X-ray generator 5 such as a cathode ray tube is arranged and irradiates X-ray beams generated, via a K-edge filter 24, to the human body 8. The K-edge filter 24 is made of, for instance, Nd and allows two kinds of X-ray beams splitted to have high and low effective energies to pass therethrough. For instance, when a tube voltage of 80 kV is applied to the X-ray generator 5, X-ray beams having effective energies of 70 keV and 43 keV, respectively, are irradiated to the human body 8 through the K-edge filter 24. 
     Each unit detection device 3u is comprised of a CdTe radiation detector of pulse counting type having a predetermined small detection area. An output therefrom is amplified by a preamplifier 25, is discriminated into signals of high and low levels by a pulse amplitude discriminator 26 and respective numbers of pulses of high and low levels discriminated are counted by a counter circuit 27. Respective count numbers are outputted to a data processor 28 such as a microprocessor and the data processor 28 processes data inputted to obtain the bone salt as will be explained hereinbelow. Results obtained are displayed by a display 29. 
     Intensities I h0  and I l0  of the high energy and low energy X-rays to be irradiated are measured beforehand by the image sensor 3 in absence of the human body. 
     I. Calculation of correction coefficient for correcting influences due to the beam hardening effect 
     Intensities I h1f  and I l1f  of the high energy and low energy X-rays having passed through the soft tissue 9 and the filter 4 are detected by the unit detection devices 3u covered by the filter 4. 
     The filter 4 is an Al plate of a thickness of 1 cm in the present embodiment. The mass attenuation coefficient of Al is 0.567 cm 2  /g for 40 keV X-ray and is 0.203 cm 2  /g for 80 keV X-ray. That of the bone is 0.512 cm 2  /g for 40 keV X-ray and is 0.209 cm 2  /g for 80 keV X-ray. 
     An apparent mass content (T f  ×ρ f ) of the filter 4 is obtained according to the following equation. 
     
         T.sub.f ×ρ.sub.f ={In(I.sub.l0 /I.sub.l1f)×μ.sub.ha -In(I.sub.h0 /I.sub.h1f)×μ.sub.la }/(μ.sub.ha μ.sub.lf -μ.sub.hf μ.sub.la)                                 (5) 
    
     wherein 
     μ la  and μ lf  : mass attenuation coefficients of the soft tissue and the filter for the low energy X-ray, respectively, 
     μ ha  and μ hf  : mass attenuation coefficients of the soft tissue and the filter for the high energy X-ray, respectively, 
     ρ a  and ρ f  : densities of the soft tissue and the filter, respectively, 
     T a  and T f  : thicknesses of the soft tissue and the filter, respectively, 
     The mass content of the filter 4 obtained is shifted from the true value by the beam hardening effect, as stated above. Since the density and the thickness of the filter 4 is known beforehand, the true value of the mass content of the filter 4 is easily obtained from these. 
     Accordingly, the correction coefficient K 1  for compensating the beam hardening effect is obtained as follows 
     
         K.sub.1 =T.sub.f ×ρ.sub.f (true value)/T.sub.f ×ρ.sub.f (measured value)                                          (6) 
    
     II. Calculation of Bone salt 
     Intensities I h1b  and I l1b  of the high energy and the low energy X-rays having passed through the soft tissue 9 and the bone tissue 7 are also detected by the unit detecting devices 3u uncovered by the filter 4. An apparent mass content of the bone 7 i.e. bone salt is obtained according to the equation (3) by equating I l1  and I h1  to I lb  and I h1b , respectively. 
     Namely, 
     
         T.sub.b ×ρ.sub.b ={In(I.sub.10 /I.sub.l1b)×μ.sub.ha -In(I.sub.h0 /I.sub.h1b)×μ.sub.la }/(μ.sub.ha μ.sub.lb -μ.sub.hb μ.sub.la)                                 (3&#39;) 
    
     Since the mass content of the filter 4 is chosen so that it is nearly equal to that of the bone tissue 9, the most probable value of the bone salt is obtained by correcting the measured value with the correction coefficient K 1  calculated beforehand. Namely, it is obtained as follows. 
     
         T.sub.b ×ρ.sub.b (most probable)=K.sub.1 ×T.sub.b ×ρ.sub.b (measured)                             (7) 
    
     III. Sensitivity correction for each unit detecting device 
     Prior to the measurement of the bone salt, the X-ray to be irradiated is detected by all the unit detecting devices of the X-ray image sensor without the human body and in a state that the filter is removed. The sensitivity of each unit detecting device 3u can be corrected from a shift amount of a measured count number thereof from a mean value obtained by averaging all measured count numbers. 
     Respective correction coefficients thus obtained are inputted from a data entry section 30 such as a key board to the data processor 28 beforehand. 
     Count numbers outputted from respective unit detecting devices are corrected using these correction coefficients, respectively, and thereby, the accuracy of measurement is highly improved. 
     Though an Al plate is used for the filter 4 in the above embodiment, materials equivalent to the bone tissue such as calcium carbonate, kalium hydrogenphosphate, potassium iodide and the like are available therefor. 
     Further, the unit detecting device is not limited to the CdTe radiation detector and semiconductor radiation detectors made of Si, Ge, GaAs and the like, scintillation detector, ion chamber detector and the like are available therefor. 
     Also, various known ways to generate X-rays having different energies are available other than the K-edge filtering method. 
     FIG. 2 shows an example of the bone salt analyzing system. 
     A carrier box 31 is arranged under a bed 37 which is movable in a lengthwise direction of the bed 37 and is driven by a driving means 35. Though a concrete composition of the driving means 35 is not shown in FIG. 2, a suitable known mechanism such as a linear motor can be employed to drive the carrier box 31. 
     The carrier box 31 provides an X-ray image sensor 3 on the upper surface thereof which is arranged in a direction of the width of the bed and carries a driving means 33 for moving a filter 4 along the X-ray image sensor 3. 
     Above the bed 37, an X-ray generator 5 is supported by an arm 32 fixed to the carrier box 31 so that the X-ray generator 5 is moved together with the X-ray image sensor 3. The X-ray generator 5 provides with a K-edge filter 24 for generating high and low energy X-rays. 
     Prior to a measurement of the above salt, the filter 4 is moved to a position whereat all unit detecting devices are uncovered thereby to detect the sensitivity of each unit detecting device. By irradiating the high and low energy X-rays to the X-ray image sensor 3 in absence of a human body, each of outputs from the unit detecting devices is read out as a count number. Assuming that I li  and I hi  are count numbers of i-th unit detecting device for the low and high energy X-rays and I l  ave and I h  ave mean values averaged over all count numbers I li  and I hi , respectively, respective correction factors K 2li  and K 2hi  of i-th unit detecting device for the low and high energy X-rays are given as follows. 
     
         K.sub.2li =I.sub.li /I.sub.l ave                           (8) 
    
     
         K.sub.2hi =I.sub.hi /I.sub.h ave                           (9) 
    
     Sensitivities of respective unit detecting devices are corrected using correction factors obtained according to the equations (8) and (9), namely, an output of i-th unit detecting device is corrected using K 2li  and K 2hi  by the data processor 28. Calculation of the mean values and correction factors is done in the data processor 28 and data obtained by the calculation are stored therein to correct outputs from individual unit detecting devices upon measuring the bone salt. 
     Upon starting the measurement, a human body is set on the bed 37 and the filter 4 is moved to cover some of the unit detecting devices lying under a part of the human body including only the soft tissue thereof. Thereafter, the X-ray generator 5 is energized to generate the high and low energy X-rays and the X-ray image sensor 3 receives X-rays passing through the human body 8. 
     The most probable value of the bone salt is calculated by the data processor 28 according to the method as explained regarding FIG. 1. To measure the bone salt along a length of the bone, the X-ray image sensor 3 and the X-ray generator 5 is scanned in the lengthwise direction of the bed 37 by driving the carrier box 31 by the driving means 35 therefor. 
     It is to be noted that a time-dependent fluctuation of the intensity of X-ray is also compensated by the present invention since the correction coefficient K 1  obtained using the filter 4 upon every measurement includes influences due to the time-dependent fluctuation. 
     Thus, the bone salt obtained finally is free from the beam hardening effect, deviations of the sensitivity of each unit detecting device and the time-dependent fluctuation of the intensity of X-ray and, accordingly, represents the real value exactly. 
     In the preferred embodiment, the present invention is applied to the bone salt analysis but is not limited to this and, for instance, is applicable for measuring a content of Cu contained in a plastic material. In this case, the material for the filter is Cu. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.