Radiographic diagnosis apparatus, radiographic diagnosis method, plate member, and position detecting method

A radiographic diagnosis apparatus includes an X-ray generation device for irradiating an object with X-rays, a two-dimensional array detector which is formed by arraying small detecting elements in a two-dimensional matrix and opposing the X-ray generation device with the object interposed between them and detects X-rays transmitted through the object in units of pixels, a ray conversion plate adhered to the detecting surface of the array detector to change the properties of X-rays in units of pixels, and an image processor for performing image processing based on data from the array detector, thereby generating a radiographic diagnosis image. The ray conversion plate includes a plurality of different types of attenuating elements arrayed in a checkerboard pattern to attenuate radiation at different attenuation ratios. The properties of X-rays are changed via the ray conversion plate. The ray conversion plate changes the properties into two types. The array detector detects two types of property-changed X-ray signals transmitted through the object in units of pixels, and converts the signals into digital signals.

BACKGROUND OF THE INVENTION
 The present invention relates to a radiographic diagnosis apparatus which
 irradiates an object with radiation such as X-rays from a radiation source
 and generates a radiographic diagnosis image by detecting the radiation
 transmitted through the object and, more particularly, to a radiographic
 diagnosis apparatus which uses a two-dimensional array detector, in which
 a plurality of detecting elements are arrayed in a two-dimensional matrix,
 as a radiation detector, irradiates an object with a plurality of
 radiations differing in energy, and reconstructs a desired image by using
 the energy differences.
 This application is based on Japanese Patent Application No. 10-326993,
 filed Nov. 17, 1998, the entire content of which is incorporated herein by
 reference.
 One conventional radiographic diagnosis apparatus using radiation such as
 X-rays is a technique which acquires images by using two different X-ray
 energies (high and low energies) and performs energy subtraction by linear
 arithmetic operations (weighted differential processing) for the images
 (William R. Brody et. al., "A method for selective tissue and bone
 visualization using dual energy scanned projection radiography", Med.
 Phys. 8(3), May/June 1981). The purpose of this technique is to display
 only a soft tissue by erasing information, such as a bone, unnecessary for
 diagnosis by subtraction and to thereby allow easy diagnosis of a soft
 tissue hidden behind a bone.
 Also, a technique by which images are acquired by a plurality of different
 X-ray energies, not by two different energies, and the energy absorption
 characteristic of a substance is visualized by linear arithmetic
 operations for the images is described in Japanese Patent Application No.
 3-334788 assigned to the same assignee as the application concerned. The
 purpose of this technique is to visualize the differences between X-ray
 energy spectrum absorbed by, e.g., bones, soft tissues, and lungs. Since
 the method of display is different from the one that displays X-ray
 attenuation amounts, the method is expected to be applied to tissue
 characterization.
 As a means for acquiring a plurality of different energy images, a method
 of acquiring images by emitting a plurality of different radiations at
 different timings is known. This method sequentially emits a plurality of
 different radiations and acquires a plurality of image data by detecting
 the emitted radiations by a single detector. In this method, the same
 object is irradiated at least twice at different timings with different
 radiations. Therefore, if the object moves, the data acquisition positions
 of the second and subsequent radiations deviate from that of the first
 radiation. This generates an artifact and thereby degrades the image
 quality.
 To prevent this, methods are being developed by which a plurality of
 different radiations are simultaneously emitted and detected by a
 plurality of different detectors. One example is a method which uses a
 detecting device formed by overlapping a plurality of detectors with a
 substance which changes X-ray properties sandwiched between them. This
 method emits one type of radiation, changes the radiation properties by
 passing it through the substance, and acquires a plurality of images at
 the same time by the first detector and the second and subsequent
 detectors. Since a plurality of different radiations are simultaneously
 emitted and a plurality of images are simultaneously acquired, no such
 artifact as caused by the motion of an object as described above is
 produced. However, a detector (first detector) placed nearest to an X-ray
 generator must transmit X-rays to a certain degree to a subsequent
 detector (second detector), i.e., must not absorb X-rays 100%. Also, the
 second detector detects X-rays attenuated by the first detector, so the
 incident dose is reduced compared to that to the first detector. As a
 consequence, the dose detected by each detector reduces, and this
 decreases the ratio of the effective dose to noise, i.e., the S/N ratio of
 an image; the influence of noise increases to degrade the image quality.
 Furthermore, since the detectors overlap each other in the form of a
 sandwich, scattering rays generated by the individual detectors have
 influence on each other. This degrades the image quality acquired by each
 detector.
 An array detector in which small detecting elements are arrayed in a
 two-dimensional matrix has been developed recently as an X-ray detector
 (e.g., U.S. Pat. No. 4,672,454). This detector is characterized in that
 the positions of pixels corresponding to the pixels of a digital image are
 spatially determined. Feasibility of applying the aforementioned diagnosis
 methods such as energy subtraction to a detector having this
 two-dimensional array structure is under consideration.
 This X-ray array detector, however, has a structure in which detecting
 elements are arrayed on the detector. Hence, it is necessary to lay in
 electric wires, for propagating input control signals to the detecting
 elements and output electrical signals from the detecting elements, inside
 the detector. If this detector is constructed into the shape of a
 sandwich, therefore, the shadow of the wiring in the first detecting
 element is projected onto the second detecting element to generate an
 artifact. This makes this detector difficult to put into practice.
 As described above, no conventional means exists which acquires a plurality
 of different energy images without generating any artifact, reducing the
 S/N ratio, and degrading the image quality by scattering rays.
 BRIEF SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide a
 radiographic diagnosis apparatus which uses a two-dimensional array
 detector, acquires an image of an object by using radiations differing in
 energy, and prevents an artifact being caused by the motion of the object,
 or by the shadow of an internal structure of the detector, without
 deteriorating the S/N ratio.
 According to the present invention, there is provided a radiographic
 diagnosis apparatus comprising a radiation source for irradiating an
 object with radiation, a detector comprising a plurality of detecting
 elements for detecting the radiation generated by the radiation source and
 transmitted through the object, and a ray conversion member placed between
 the radiation source and the detector and comprising a plurality of
 different types of attenuating elements for attenuating radiation at
 different attenuation ratios, wherein radiations differing in energy
 attenuated by the plurality of different types of attenuating elements are
 detected by the detecting elements respectively corresponding to the
 attenuating elements, thereby acquiring radiation images differing in
 energy.
 In the present invention, it is possible to irradiate the detector with a
 plurality of different radiations and acquire a plurality of different
 images at once. Accordingly, it is possible to prevent an artifact caused
 by the motion of an object, or by the shadow of an internal structure of a
 detector, without deteriorating the S/N ratio.
 Additional objects and advantages of the present invention will be set
 forth in the description which follows, and in part will be obvious from
 the description, or may be learned by practice of the present invention.
 The objects and advantages of the present invention may be realized and
 obtained by means of the instrumentalities and combinations particularly
 pointed out hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 A preferred embodiment of a radiographic diagnosis apparatus according to
 the present invention will now be described with reference to the
 accompanying drawings.
 First Embodiment
 (Arrangement of Radiographic Diagnosis Apparatus)
 FIG. 1 is a block diagram showing the arrangement of the first embodiment
 of a radiographic diagnosis apparatus. Although this first embodiment will
 be explained by taking X-rays as an example of radiation, some other
 radiation can also be used. An X-ray diagnosis apparatus 10 includes an
 X-ray generation device 1, a two-dimensional array detector 3, a ray
 conversion plate 4, an image processor 5, and a display 9. The X-ray
 generation device 1 irradiates an object 2 with X-rays. The
 two-dimensional array detector 3 is formed by arraying small detecting
 elements in a two-dimensional matrix. This two-dimensional array detector
 3 opposes the X-ray generation device 1 with the object 2 interposed
 between them, and detects X-rays transmitted through the object 2 in units
 of pixels. The ray conversion plate 4 is placed between the array detector
 3 and the X-ray generation device 1, or is adhered to the radiation
 incident surface of the array detector 3, and changes the properties of
 X-rays in units of pixels. The image processor 5 performs image processing
 on the basis of data from the array detector 3 and generates a
 radiographic diagnosis image. The display 9 displays the image
 reconstructed by the image processor 5. A detector described in U.S. Pat.
 No. 4,672,454 explained in "BACKGROUND OF THE INVENTION" can be used as
 the two-dimensional array detector 3.
 The properties of X-rays radiated from the X-ray generation device 1 and
 transmitted through the object 2 are changed via the ray conversion plate
 4 on the front surface of the two-dimensional array detector 3. Assume
 that this ray conversion plate 4 changes the properties into two types.
 The array detector 3 detects the property-changed X-ray signal transmitted
 through the object in units of pixels. The array detector 3 converts the
 detected signal into a digital signal and transfers this digital signal to
 the image processor 5. The signal is stored in a memory 6 and subjected to
 image processing by an arithmetic circuit 8. After that, the processed
 signal is transferred to the display 9, such as a display device or a film
 imager, via an I/F 7, and is displayed as an image.
 (Configurations of Array Detector 3 and Ray Conversion Plate 4)
 The configurations of the array detector 3 and the ray conversion plate 4
 will be described in detail below. FIG. 2A is a perspective view of the
 array detector 3 and the ray conversion plate 4. FIG. 2B is an enlarged
 plan view of a portion A indicated by the broken line in FIG. 2A.
 The array detector 3 is formed by arraying, in a two-dimensional matrix
 (lattice), a large number of small detecting elements 3a for detecting
 X-rays generated from the X-ray generation device 1 (and transmitted
 through the object 2). Each detecting element 3a stores an electric charge
 corresponding to the incident X-ray dose and outputs it as an electrical
 signal.
 The ray conversion plate 4 is formed by arraying, into a predetermined
 pattern, a plurality of (in this embodiment, two) different types of X-ray
 attenuating elements A1 and A2, for attenuating X-rays at different
 attenuation ratios, on a support plate 4c which hardly attenuates but
 transmits X-rays nearly 100%. In this embodiment, these attenuating
 elements A1 and A2 are two different types of metal elements having
 different atomic numbers and set to have a thickness by which, e.g., at
 least a 20% portion of 100-keV X-rays is transmitted. Each attenuating
 element A1 or A2 has the same pixel size as the detecting element 3a of
 the array detector 3. As shown in FIG. 2B, these attenuating elements A1
 and A2 alternate in a checkerboard pattern.
 The ray conversion plate 4 is overlapped on the array detector 3 with
 pixels of the two members aligned. For this alignment, projections 4a are
 formed on the four sides of the ray conversion plate 4, and recesses 4b
 are cut in corresponding portions of the array detector 3. In FIG. 2B, the
 support plate 4c of the ray conversion plate 4 and a support member of the
 array detector 3 are not shown. Also, the projection 4a and the recess 4b
 on only one of the four sides are illustrated for explanation. Since the
 recesses 4b and the projections 4a are so processed as to fit with no
 spacing between them, the relative position relationship between the ray
 conversion plate 4 and the array detector 3 does not change. Even when the
 ray conversion plate 4 is detached and attached, the positional
 relationship between the array detector 3 and the ray conversion plate 4
 is always correct.
 When the object 2 is irradiated with X-rays in this state, each pixel of
 the array detector 3 detects X-rays whose X-ray energy intensity is
 attenuated by either the X-ray attenuating element A1 or A2 of the ray
 conversion plate 4. Consequently, the detector 3 alternately outputs A1-
 and A2-attenuated X-ray signals.
 (Operations of Array Detector 3 and Ray Conversion Plate 4)
 The array detector 3 and the ray conversion plate 4 constructed as above
 operate in the following manner. FIG. 3 is a view schematically showing
 the pixel signal distribution on the array detector 3 and processing for
 the distribution.
 On the array detector 3, pixels (hatched portions in FIG. 3) which detect
 X-rays attenuated through the X-ray attenuating elements A1 and pixels
 (blank portions in FIG. 3) which detect X-rays attenuated through the
 attenuating elements A2 are arranged in a checkerboard pattern (this image
 will be referred to as an original image hereinafter). From this original
 image, component images M1 and M2 are formed. The component image M1 is
 composed of only pixels which detect X-rays attenuated through the X-ray
 attenuating elements A1. The component image M2 is composed of only pixels
 which detect X-rays attenuated through the attenuating elements A2.
 Referring to FIG. 3, pixels (1,1), (1,3), . . . , (2,2), . . . , (3,1), .
 . . , are pixels which detect X-rays attenuated through the X-ray
 attenuating elements A1. Pixels (1,2), (1,4), . . . , (2,1), . . . ,
 (3,2), . . . , are pixels which detect X-rays attenuated through the
 attenuating elements A2.
 Each component image contains the number of pixels half that of the
 original image. Therefore, the number of pixels is made equal to that of
 the original image by allocating one pixel of the original image to two
 adjacent pixels. As an example, data of pixels (1,1), (1,1), (1,3), (1,3),
 . . . , (2,2), (2,2), (2,4), (2,4), . . . , on the original image are
 embedded in pixels (1,1), (1,2), (1,3), (1,4), . . . , (2,1), (2,2),
 (2,3), (2,4), . . . , respectively, on the component image M1, and data of
 pixels (1,2), (1,2), (1,4), (1,4), . . . , (2,1), (2,1), (2,3), (2,3), . .
 . , on the original image are embedded in pixels (1,1), (1,2), (1,3),
 (1,4), . . . , (2,1), (2,2), (2,3), (2,4), . . . , respectively, on the
 component image M2. This processing halves the spatial resolution in the
 horizontal direction. However, the component images M1 and M2 are formed
 by picking up only pixels transmitted through the X-ray attenuating
 elements A1 and A2, respectively.
 This data embedding is not restricted to the above example. For instance,
 to make the horizontal and vertical resolutions equal to each other, it is
 possible to embed the average value of pixels (1,1) and (2,2) on the
 original image into four pixels (1,1), (1,2), (2,1), and (2,2) on the
 component image M1, and to embed the average value of pixels (1,2) and
 (2,1) on the original image into four pixels (1,1), (1,2), (2,1), and
 (2,2) on the component image M2.
 After these component images M1 and M2 are formed as above, they are
 multiplied by predetermined coefficients C1 and C2, respectively, and
 subjected to arithmetic processing by a subtracter 11, thereby forming an
 energy-subtraction image or an image representing the energy absorption
 characteristic. That is, the values of individual pixels of the component
 images M1 and M2 are substituted into the equation
 Y=C1.times.M1+C2.times.M2, and a linear arithmetic operation is performed
 to construct an image Y. Note that the coefficients C1 and C2 can be
 determined on the basis of any conventionally known method.
 One suitable example will be described below. In this example, the ray
 conversion plate 4 is formed by using the X-ray attenuating elements A1
 made of copper and having a thickness of 0.3 mm and the attenuating
 elements A2 made of aluminum and having a thickness of 3 mm. Assuming a
 human lung produces X-ray attenuation substantially equivalent to 10 cm of
 water, if a 1-cm thick rib exists in this lung, the array detector 3
 detects the following signal. Note that the values described below are
 relative values when the lung on the component image M1 is 1.0.
 (i) Signals (on the component image M1) obtained by detecting X-rays
 attenuated through the X-ray attenuating element A1
 Lung: 1.00
 Rib: 0.80
 (ii) Signals (on the composition image M2) formed by detecting X-rays
 attenuated through the attenuating element A2
 Lung: 0.78
 Rib: 0.64
 Accordingly, to erase the shadow of the rib by energy subtraction, the
 component image M2 is multiplied by 0.80/0.64(=1.25), and the product is
 subtracted from the component image M1. Consequently, a signal having a
 component of 1-0.78.times.1.25(=0.025) remains in the lung, so the shadow
 of only the lung is displayed on an image from which the rib is
 eliminated. In this example, energy subtraction is described by:
EQU Y=1.times.M1-1.25.times.M2
 That is, the coefficients C1 and C2 are set such that C1=1 and C2=-1.25.
 Since these coefficients are determined by the tube voltage of the X-ray
 generation device 1, they are previously calculated in accordance with the
 tube voltage used. Note that if the X-ray attenuating elements A1 and A2
 absorb X-rays too much, only little X-rays enter the array detector 3, and
 this lowers the S/N ratio. Therefore, the materials of the attenuating
 elements A1 and A2 must be carefully chosen.
 In the first embodiment as described above, radiation generated from a
 radiation source is passed through a plurality of attenuating elements
 constructing a plate-like member. This radiation is attenuated at
 predetermined ratios and converted into a plurality of different types of
 energy radiations. These different types of radiations are detected by
 detecting elements placed in positions corresponding to the attenuating
 elements. Consequently, in a radiographic diagnosis apparatus using a
 two-dimensional array detector, it is possible to prevent an artifact
 resulting from the motion of an object, or by the shadow of an internal
 structure of the detector, without deteriorating the S/N ratio, thereby
 simultaneously obtaining images of the object by using radiations
 differing in energy.
 Other embodiments of the present invention will be described below. In the
 following embodiments, the same reference numerals as in the first
 embodiment denote the corresponding parts, and a detailed description
 thereof will be omitted.
 Second Embodiment
 FIG. 4 shows an X-ray diagnosis apparatus 20 according to the second
 embodiment. In this second embodiment, a ray conversion plate 12 is placed
 between the object 2 and the X-ray generation device 1. Since, therefore,
 X-rays are attenuated before irradiating the object 2, the dose of the
 radiation to the object 2 can be reduced compared to the first embodiment.
 However, the ray conversion plate 12 and the array detector 3 must be
 aligned (so that X-rays passing through attenuating elements correctly
 enter the corresponding detecting elements).
 Additionally, each detecting element of the array detector 3 is so placed
 as to have an area substantially equal to an X-ray irradiation area when
 radiation passing through the corresponding detecting element is applied
 to the array detector 3. More specifically, the ray conversion plate 12 is
 placed in the middle of the distance between the X-ray generation device 1
 and the array detector 3. Also, the length and width (the length of one
 side of a square) of each attenuating element of the ray conversion plate
 12 are one-half of those (the length of one side of a square) of each
 detecting element of the array detector 3. That is, each attenuating
 element of the ray conversion plate 12 is projected onto the corresponding
 detecting element of the array detector 3 after being magnified at four
 times.
 Note that this magnification further increases when the size of each
 attenuating element of the ray conversion plate 12 is decreased compared
 to that of the detecting element of the array detector 3, so the ray
 conversion plate 12 can be placed nearer to the X-ray generation device 1.
 However, the penumbra is produced on the array detector 3 by the thickness
 of the metal of the attenuating element. This significantly deteriorates
 the spatial resolution on the edges of an image. That is, the
 magnification has its limit. Under geometric conditions in actual image
 sensing, four times is presumably the limit of the magnification when the
 thickness of the metal element is 3 mm. Consequently, the attenuating
 element size of the ray conversion plate 12 must be so designed as to meet
 a magnification of four times or less. So, the length of one side of the
 attenuating element is half that of the detecting element or less.
 Third Embodiment
 FIG. 5 schematically shows an X-ray diagnosis apparatus 30 according to the
 third embodiment. In this third embodiment, as in the second embodiment, a
 ray conversion plate 15 is placed between the object 2 and the X-ray
 generation device 1 and achieves the same effect as in the first
 embodiment while reducing the exposure of the object 2 to radiation.
 However, the relative position of the ray conversion plate 15 with respect
 to the array detector 3 need not be precisely fixed, unlike in the second
 embodiment.
 That is, X-ray blocking elements 13 for blocking X-rays at a predetermined
 ratio are formed, instead of attenuating elements, on the edges of the
 four sides of the ray conversion plate 15 according to this embodiment.
 The array detector 3 detects the positions (blocking positions) of these
 X-ray blocking elements 13, thereby detecting the relative position of the
 ray conversion plate 15 with respect to the array detector 3 and
 correcting any positional deviation by signal processing. Although FIG. 5
 shows only blocking elements formed on the upper and left-hand edges,
 blocking elements are also formed in corresponding portions on the lower
 and right-hand edges.
 More specifically, in this embodiment the ray conversion plate 15 is placed
 in a location (in the middle of the distance from the X-ray generation
 device 1 to the array detector 3) where the magnification is four times,
 as in the second embodiment. However, each attenuating element of the ray
 conversion plate 15 has the same size as the detecting element of the
 array detector 3. The number of the attenuating elements of the ray
 conversion plate 15 is one fourth of that of the detecting elements of the
 array detector 3 if the array detector 3 exactly covers the irradiation
 area of the X-ray. However, in the actual product, the number of the
 attenuating elements of the ray conversion plate 15 is more than one
 fourth of that of the detecting elements of the array detector 3 in order
 to sufficiently covers the irradiation area of the X-ray by the array
 detector 3.
 As the X-ray blocking element 13, a material such as lead having very large
 X-ray absorption is used. For example, when lead with a thickness of 2 mm
 is placed, X-rays behind this lead are attenuated to 0.1% or less. So,
 lead is well usable as a blocking material. Note that the position of a
 blocking material can be well estimated if an X-ray attenuation of 95% or
 more is performed. Therefore, a material by which X-ray transmission is 5%
 or less can be suitably used as a blocking material.
 In the X-ray diagnosis apparatus 30 with the above arrangement, any
 positional deviation of the ray conversion plate 15 from the array
 detector 3 is detected by the following operation. Assume that the ray
 conversion plate 15 deviates from the array detector 3 as shown in FIG. 6.
 The detecting elements of the array detector 3 and the attenuating elements
 of the ray conversion plate 15 have the same size, and the magnification
 is four times (two times in the horizontal direction and two times in the
 vertical direction). Therefore, whatever positional deviation takes place,
 a state in which the shadows (black solid portions) of the X-ray blocking
 elements 13 completely cover the detecting elements on the array detector
 3 necessarily exists. FIG. 6 also shows a profile of the (k+1)th line on
 the array detector 3. As shown in FIG. 6, pixels in the (i+1)th column are
 completely covered with the shadow of the X-ray blocking element 13.
 Pixels in the (i+2)th column slightly contain the shadow of the X-ray
 blocking element 13. FIG. 6 also reveals that the shadow of the second
 X-ray blocking element 13 is produced in the (i+5)th column at an interval
 of 3 pixels.
 That is, when the X-ray blocking element 13 is a substance which completely
 blocks X-rays, pixels in the (i+1)th and (i+5)th columns produce pixel
 signals corresponding to scattering ray components generated from the
 object 2. A pixel adjacent to a blocked pixel in the horizontal direction
 always has a larger pixel value than that of the blocked pixel.
 In other words, a pixel completely covered with the X-ray blocking element
 13 gives a minimum value in a local area including the target pixel. The
 local area can be determined beforehand based on the situation of
 radiography. Accordingly, after image acquisition, an image processor 5
 calculates a minimum value of the acquired images in the local area. If a
 value close to this value is present at an interval of four pixels, pixels
 (in the (i+1)th and (i+5)th columns in FIG. 6) which give this minimum
 value are pixels corresponding to the position of the X-ray blocking
 element 13 on the ray conversion plate 15.
 As shown in FIG. 6, the (i+1)th column ((i+5)th column) is completely
 covered with the shadow of the X-ray blocking element 13. Therefore, the
 pixel value is a scattering ray component S coming from the perimeter. In
 contrast, pixels in the adjacent i-th column ((i+4)th column) and (i+2)th
 column ((i+6)th column) are partially covered with the shadow of the X-ray
 blocking element 13. Hence, letting a and (1-a) be the ratios of regions
 covered with the shadow of the X-ray blocking element 13 to the whole area
 of pixels in the i-th column ((i+4)th column) and the (i+2)th column
 ((i+6)th column), and letting E be the value of pixels which are not at
 all covered with the X-ray blocking element 13, a pixel value E1 of the
 i-th column ((i+4)th column) and a pixel value E2 of the (i+2)th column
 ((i+6)th column) are represented by:
EQU E1 (the pixel value of the i-th column)=E.times.a+S
EQU E2 (the pixel value of the (i+2)th column)=E.times.(1-a)+S
 From the above equations, the pixel value E when a pixel is not at all
 covered with the X-ray blocking element 13 is given by:
EQU E=(E1+E2)-2.times.S
 That is, a twofold value of the pixel value S of the (i+1)th column is
 subtracted from the sum of the pixel values E1 and E2 of the i-th and
 (i+2)th columns. Accordingly, a is given by:
EQU a=(E1-S)/E
 The above example is the detection of positional deviation based on the
 blocking elements 13 formed on the upper edge. This positional deviation
 detection is also similarly performed on the basis of the blocking
 elements 13 formed on the lower edge. That is, the positional deviations
 of the uppermost line and the lowest line are obtained by these detection.
 The positional deviations of intermediate lines are obtained by
 interpolating the results of the two detections.
 By the above operation, the ratio a of the region covered with the X-ray
 blocking element 13 to the whole area of the pixels is calculated. The
 horizontal positional deviation between the ray conversion plate 15 and
 the array detector 3 can be estimated by calculations to be a times the
 width of one pixel of the array detector 3.
 When the positional deviation of the ray conversion plate 15 is detected,
 the horizontal positional deviation of each horizontal line can be known
 by a linear arithmetic operation. Therefore, the ratios a and (1-a) at
 which pixels are covered with X-ray attenuating elements A1 and
 attenuating elements A2 can be obtained for all pixels.
 A component image M1 can be estimated from an original image by using a.
 Let P be the pixel value of a pixel of the array detector 3 when the ray
 conversion plate 15 is absent. For the sake of convenience of explanation,
 assume that the attenuating elements A1 and A2 of the ray conversion plate
 15 are alternately arrayed into the form of stripes, not a checkerboard
 pattern. As shown in FIG. 7, letting be the pixel value of pixels
 completely covered with the X-ray attenuating elements A1 and be the
 pixel value of pixels completely covered with the attenuating elements A2,
 we have:
EQU P=.times.a+.times.(1-a)
 of a pixel can be estimated from observed values of pixels surrounding
 the pixel of interest and completely covered with the attenuating elements
 A2. Hence, if a pixel whose P is observed is completely covered with the
 X-ray attenuating element A1, the estimated value is &lt;&gt; given by:
EQU &lt;&gt;=(P-.times.(1-a))/a
 The component image M1 can be estimated from and &lt;&gt;.
 Analogously, the component image M2 can be estimated from and
 &lt;&gt;, the latter of which is given by:
EQU &lt;&gt;=(P-.times.a)/(1-a)
 In this manner, the component images M1 and M2 that compensate for any
 positional deviation of the ray conversion plate 15 from the array
 detector 3 are obtained. By using these component images M1 and M2, a
 method such as energy subtraction can be applied in the same way as in the
 first embodiment.
 Although horizontal positional deviation has been described above, vertical
 positional deviation can also be obtained in a similar manner.
 Also, even when the metal elements of the ray conversion plate are arrayed
 in a checkerboard pattern as shown in FIG. 2B, the positions of pixels on
 the array detector which are completely covered with the X-ray attenuating
 elements A1 and A2 can be known by accurately detecting the positions of
 the X-ray blocking elements on the upper, lower, right-hand, and left-hand
 sides described above. Additionally, the component images M1 and M2 can be
 formed by obtaining for each pixel the ratio a at which the pixel is
 covered with the X-ray attenuating element A1.
 Fourth Embodiment
 FIG. 8 is a perspective view showing a ray conversion plate 16 and the
 array detector 3 as main parts of an X-ray diagnosis apparatus 40
 according to the fourth embodiment. Referring to FIG. 8, the ray
 conversion plate 16 according to this embodiment is an example of the
 simplest ray conversion plate. Although FIG. 8 shows only a part of the
 ray conversion plate 16, X-ray attenuating elements A1 and A2 are
 alternately arrayed in every other column, and X-ray blocking elements 19
 are formed in four portions at the ends of two attenuating elements A1.
 This ray conversion plate 16 can be interposed between an X-ray generation
 device and an object as in the second and third embodiments, or attached
 to the surface of the array detector as in the first embodiment. The width
 of the columns of the X-ray attenuating elements A1 and A2 is the same as
 that of the array detector 3 if the ray conversion plate 16 is placed
 between the X-ray generation device 1 and the object 2 as in the third
 embodiment. The width of the columns of the X-ray attenuating elements A1
 and A2 is twice as that of the array detector 3 if the ray conversion
 plate 16 is placed on the array detector 3 as in the first embodiment.
 The image processor 5 detects positions U1, U2, L1, and L2 of pixels on the
 array detector 3 which are completely covered with the shadows of the
 X-ray blocking elements 19. After that, the image processor 5 determines
 that all pixels positioned on a line connecting U1 and L1 are completely
 covered with the X-ray attenuating element A1. Likewise, the image
 processor 5 determines that all pixels positioned on a line connecting U2
 and L2 are completely covered with the X-ray attenuating element A1. The
 image processor 5 also determines that pixels on a line intermediate
 between these two lines are covered with the X-ray attenuating element A2.
 When the positions of pixels on the array detector 3 that are completely
 covered with the X-ray attenuating elements A1 and A2 are thus determined,
 component images M1 and M2 can be formed by using these pixels, since it
 is assumed that the pitch of the X-ray attenuating elements A1 (or A2) is
 four times the pitch of the detecting element columns of the array
 detector and pixels corresponding to A1 and A2 are determined by the
 period of four pixels in detecting element columns which correspond to
 attenuating element lines.
 Fifth Embodiment
 FIG. 9 is a partially exploded perspective view showing a ray conversion
 plate 17 according to this embodiment. This ray inversion plate 17 is
 characterized by having a function of changing the properties of X-rays,
 when the image data is acquired with the ray inversion plate 17 attached
 to the surface of the array detector 3, and also having a function of a
 grid for removing rays scattered by the object 2. That is, in the ray
 conversion plate 17 attenuating elements constructing it are separated by
 separators 18 which attenuate X-rays at a predetermined ratio.
 More specifically, the separators 18 that are thin plate members made of a
 material such as lead having large X-ray absorption are present between
 the metal elements (attenuating elements) of the ray conversion plate 17.
 These separators 18 absorb scattering ray components incident on the ray
 conversion plate 17 and thereby reduce scattering to the ambient
 environment. In FIG. 9, the attenuating elements form lines in the
 vertical direction, and the separators 18 extend in the vertical
 direction. However, it is also possible to enhance the scattering ray
 reducing function by forming separators in both the vertical and
 horizontal directions in a lattice-like pattern.
 The positions of the detecting elements of the array detector 3 and the
 positions of the attenuating elements of the ray conversion plate 17 can
 be aligned by the method as described in the first embodiment. So, the
 scattering ray preventing separators can be formed above the boundaries of
 the detecting elements on the array detector 3. This minimizes reduction
 in the direct X-ray component attributed to the separators. Note that the
 separators cannot well reduce scattering rays unless the thickness and the
 material are such that at least 80% of 100-keV X-rays are transmitted.
 Therefore, the thickness must be about 20 .mu.m or more when lead is used.
 The present invention is not limited to the above embodiments and can be
 practiced in the form of various modifications. That is, practical
 examples such as the arrangement of the ray conversion plate in each
 embodiment, the relative size with respect to the array detector, and the
 image processing by the image processor are merely examples. So, the
 present invention is not restricted to these examples.
 According to the present invention as has been described above, in a
 radiographic diagnosis apparatus which acquires image data based on
 radiations differing in energy by using a two-dimensional array detector,
 it is possible to prevent an artifact caused by the motion of an object,
 or by the shadow of an internal structure of the detector, without
 deteriorating the S/N ratio. That is, it is possible to provide an image
 representing energy subtraction or an X-ray absorption characteristic by
 using a ray conversion plate, and to enhance the X-ray diagnosis
 capability using an array detector.