Abstract:
An extracting step (step S 1 ) extracts uninfluenced pixels with a relatively high degree of certainty, while avoiding influences of random quantum noise as much as possible. An approximate fluoroscopic image is obtained based on such uninfluenced pixels (step S 2 ). Thus, accuracy of the approximate fluoroscopic image can be improved over that of the prior art. Therefore, a grid foil shadow image (step S 3 ) and a foil shadow standard image (step S 4 ) calculated successively based on the approximate fluoroscopic image have improved accuracy over the prior art. As a result, while inhibiting influences of random quantum noise, a foil shadow removed image can be obtained which is free from artifacts due to distortion of a synchronous grid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2010-277193, filed Dec. 13, 2010, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to a method of removing foil shadows of a synchronous grid which removes scattered radiation of a radiographic apparatus, and a radiographic apparatus using the same. 
     (2) Description of the Related Art 
     Conventionally, an X-ray apparatus includes an X-ray tube, and an X-ray detector opposed to the X-ray tube. The X-ray detector has a grid disposed adjacent an X-ray incident plane thereof. The grid reduces image quality degradation due to scattered X-rays, but on the other hand, grid foil shadows are superimposed on radiographic images. 
     An FPD (Flat Panel Detector) has come to be used widely as the X-ray detector in recent years. The FPD can improve the spatial resolution and X-ray sensitivity of radiographic images, and for this and other reasons its use is spreading at a rapid rate. However, as improvement is made in the spatial resolution and X-ray sensitivity of the X-ray detector, the grid foil shadows appear clearly on radiographic images, which are obstructive to interpretation of the radiographic images. In order to remove these grid foil shadows from the radiographic images, it is known to remove the shadows by image processing using frequency conversion. See Patent Document 1 (Japanese Unexamined Patent Publication No. 2000-83951 (paragraphs “0033”-“0036”)), for example. 
     Patent Document 1 describes what is called a fixed grid which is fixedly attached to the X-ray detector although changeable to one different in grid intervals. Besides the above fixed grid, there is a moving grid. The moving grid is moved in a direction perpendicular to grid stripes synchronously with X-ray irradiation to prevent a fixed pattern of the grid from appearing on radiographic images. Although the fixed pattern of the grid does not appear on radiographic images, the moving grid has problems of requiring a complicated moving mechanism and lowering detection efficiency. Such grids are constructed of an alternate arrangement of metal foil strips consisting of an X-ray absorbing material such as lead, and interspacers consisting of aluminum or carbon fiber which does not easily absorb X-rays. However, these interspacers absorb a certain quantity of X-rays, which leads to a sensitivity lowering of desired X-ray images. So, a synchronous grid has been proposed as a solution to these problems. See Patent Document 2 (Japanese Unexamined Patent Publication No. 2002-257939 (paragraphs “0018” and “0019”, and  FIG. 1 )). 
     This synchronous grid has grid foil strips arranged so that grid foil shadows fall in middles of detecting pixels of the FPD. More particularly, the grid foil strips are arranged as inclined such that each has flat surfaces thereof aligned to a straight line extending between a focus of the X-ray tube and an X-ray detecting plane of the FPD. 
     Patent Document 3 (Japanese Unexamined Patent Publication No. 2008-232731 (paragraphs “0001”-“0003” and “0007”)), for example, describes a specific method of manufacturing a synchronous grid. This method excludes the interspacers members to provide layers of air, thereby to obtain X-ray images of improved sensitivity. 
     However, the conventional example with such construction has the following problems. 
     For reasons of manufacture of the grid foil strips and construction for aligning the grid foil strips, the synchronous grid has a certain distortion of the linear grid foil strips and a minute shift in their positions of arrangement. Further, since the grid foil strips of the synchronous grid are higher than those of the other type grids, the foil shadows of the synchronous grid are susceptible to influences of the distortion and shifting of the grid foil strips. Distortion and shifting occur also with the grid foil shadows, which are caused by the distortion and shifting of the grid foil strips. As a result, variations in measurements of the foil shadows will occur to different lines of the grid foil shadows, and density variations will occur to the grid foil shadows. There is a drawback that, even if frequency conversion is used for removing the grid foil shadows, the grid foil shadows in a longitudinal pattern cannot fully be removed. The grid foil shadows failing to be removed will become artifacts on radiographic images after the grid foil shadows are removed therefrom. 
     In an X-ray apparatus having a C-arm, the heavy X-ray tube and FPD are mounted at opposite ends of the C-arm. Thus, subtle bending of the C-arm will occur with movement such as rotation of the C-arm, thereby causing a shift between the FPD and the focus of the X-ray tube. This shift is in the order of 2 mm, for example, but the grid foil shadows on the FPD will also move, and hence a problem that the grid foil shadows cannot fully be removed. 
     In order to solve the above problems, Applicant has proposed the following technique (International Application PCT/JP2010/003221). 
     According to this technique, pixels free from influences of grid foil shadows are first extracted from a fluoroscopic image, and an interpolation process is carried out based on detection signal values of these pixels, to obtain an approximate fluoroscopic image without influences of the grid foil shadows. Next, a grid foil shadow image which is an image of only the grid foil shadows is obtained based on a difference between the fluoroscopic image and the approximate fluoroscopic image. Further, the grid foil shadow image is averaged to obtain a foil shadow standard image inhibiting variations in the grid foil shadows due to random errors such as quantum noise. Then, based on the foil shadow standard image and the fluoroscopic image, the grid foil shadows are removed from the fluoroscopic image. This is a technique intended to obtain, through such processes, a fluoroscopic image with no grid foil shadows appearing thereon. 
     It is important for the above proposed technique how the pixels free from influences of the grid foil shadows should be extracted. However, under the influence of random quantum noise occurring with X-rays, pixels influenced by the grid foil shadows can be extracted in error. Then, since the approximate fluoroscopic image has low accuracy, accuracy of the grid foil shadow image also becomes low. There arises a problem that it is impossible to remove the grid foil shadows from the fluoroscopic image ultimately with high accuracy, with artifacts remaining to impart influence. 
     SUMMARY OF THE INVENTION 
     This invention has been made having regard to the state of the art noted above, and its object is to provide a method of removing foil shadows of a synchronous grid and a radiographic apparatus using the same, which are capable of removing artifacts due to distortion of the synchronous grid while inhibiting adverse influences of random quantum noise. 
     The above object is fulfilled, according to this invention, by a grid foil shadow removing method for a radiographic apparatus for obtaining fluoroscopic images and having a synchronous grid with grid foil strips arranged at regular intervals so that grid foil shadows fall on middles of pixels which detect radiation, the method comprising an extracting step including a grouping step for dividing pixels forming a fluoroscopic image into groups each having a predetermined number of pixels within each row in a direction of row, a most influenced pixel selecting step for selecting a pixel most influenced by one of the grid foil shadows in each group as most influenced pixel, a voting step for casting, with the most influenced pixel in each group serving as a reference, a predetermined number of votes for other pixels spaced apart forward and backward in the direction of row, and an electing step for electing a pixel given a maximum number of votes within each group as an uninfluenced pixel which is free from influences of a foil shadow of the grid; an approximate fluoroscopic image calculating step for obtaining an approximate fluoroscopic image by carrying out an interpolation process based on detection signal values of the uninfluenced pixels; a grid foil shadow image calculating step for obtaining a grid foil shadow image based on a difference between the fluoroscopic image and the approximate fluoroscopic image; a foil shadow standard image calculating step for obtaining a foil shadow standard image by averaging the grid foil shadow image in a longitudinal direction of the grid foil shadows; and a foil shadow removing step for removing the grid foil shadows from the fluoroscopic image based on a difference between the fluoroscopic image and the foil shadow standard image, thereby to obtain a foil shadow removed image. 
     According to this invention, the pixels arranged in the direction of row are grouped in the grouping step, and the most influenced pixel within each group is selected in the most influenced pixel selecting step. The most influenced pixel is a pixel most influenced by a grid foil shadow, which can be selected relatively easily and relatively reliably compared with selection of pixels not influenced by the grid foil shadows. Next, in the voting step, votes are cast for other pixels spaced forward and backward in the direction of row from the most influenced pixel in each group, and in the electing step, a pixel given a maximum number of votes within each group is elected as an uninfluenced pixel which is free from influences of a grid foil shadow. By executing the extracting step including the above steps, pixels not influenced by the grid foil shadows can be extracted with a relatively high degree of certainty from among pixels with varied detection signal values due to random quantum noise of the radiation. 
     Then, in the approximate fluoroscopic image calculating step, an interpolating process is carried out based on the detection signal values of the uninfluenced pixels, to calculate an approximate fluoroscopic image with the grid foil shadows substantially removed from the fluoroscopic image. Further, the grid foil shadow image calculating step calculates a grid foil shadow image as an image of only the grid foil shadows based on a difference between the fluoroscopic image and the approximate fluoroscopic image. Since this grid foil shadow image has nonuniformity of the grid foil shadows due to influences of the random errors due to quantum noise and the like, the foil shadow standard image calculating step calculates a grid foil shadow standard image without influences of distortions due to noise, for example, by averaging the grid foil shadow image piecewise by units of several tens of pixels in the longitudinal direction. This averaging also can remove some errors in interpolating process. Next, the foil shadow removing step is executed to obtain a foil shadow removed image excluding the grid foil shadows from the fluoroscopic image based on a difference between the fluoroscopic image and the foil shadow standard image. As described above, the extracting step extracts uninfluenced pixels with a relatively high degree of certainty, while avoiding influences of random quantum noise as much as possible. An approximate fluoroscopic image is obtained based on such uninfluenced pixels. Thus, accuracy of the approximate fluoroscopic image can be improved over that of the prior art. Therefore, the grid foil shadow image and foil shadow standard image calculated successively based on the approximate fluoroscopic image have improved accuracy over the prior art. As a result, while inhibiting influence of random quantum noise, the foil shadow removed image is made free from artifacts due to distortion of the synchronous grid. 
     In this invention, it is preferred that, when the predetermined number of pixels constituting each group is four, and the predetermined number of votes is 1; the voting step casts 1 vote for each of a pixel located next but one forward and a pixel located next but one backward in the direction of row; and the electing step elects a pixel having obtained 2 votes as the uninfluenced pixel; the voting step and the electing step having, interposed therebetween, adjusting steps including a first adjusting step for adjusting the number of votes obtained to 2 for a pixel whose number of votes obtained is 1 when pixels located next but three to such pixel forward and backward in the direction of row have 2 votes, respectively; a second adjusting step for adjusting the number of votes obtained to 0 for a pixel whose number of votes obtained is 1 when a pixel located next to such pixel in the direction of row has 2 votes; a third adjusting step for comparing detection signal values of a pixel whose number of votes obtained is 1 and an adjoining pixel, adjusting the number of votes from 1 to 2 for the pixel having the larger detection signal value, and adjusting the number of votes to 0 for the pixel having the smaller detection signal value; and a fourth adjusting step for adjusting the number of votes obtained from 1 to 0 for a pixel whose number of votes obtained is 1 when one of pixels located next but one to such pixel forward and backward in the direction of row has 2 votes. 
     When the number of pixels constituting each group is four and the number of votes is 1, the voting step first casts 1 vote for each of a pixel located next but one forward and a pixel located next but one backward in the direction of row. Then, the electing step elects a pixel having obtained 2 votes at this point of time as the uninfluenced pixel. Further, at this point of time, there exist pixels with the number of votes obtained=1, for which it is unknown whether they are influenced by the foil shadows or not. Then, the adjusting steps including the first to fourth adjusting steps are executed for all pixels whose number of votes obtained is 1. First, the first adjusting step adjusts the number of votes obtained from 1 to 2 for a pixel when pixels located next but three to this pixel forward and backward in the direction of row have 2 votes, respectively. This is because, when four pixels form each group, a pixel next but three to a pixel having 2 votes has a high probability of not being influenced by a foil shadow. Next, the second adjusting step adjusts the number of votes obtained from 1 to 0 for a pixel when a pixel located next to this pixel in the direction of row has 2 votes. This is because the probability of two adjoining pixels not being influenced by a foil shadow or shadows is low. Next, the third adjusting step compares detection signal values of adjoining pixels, adjusts the number of votes to 2 for the pixel having the larger detection signal value, and adjusts the number of votes to 0 for the pixel having the smaller detection signal value. This is because, where pixels with 1 vote adjoin each other, the pixel with the larger detection signal value is more likely not to be influenced by a foil shadow. Next, the fourth adjusting step adjusts the number of votes obtained to 0 for a pixel when one of pixels located forward and backward next but one to this pixel has 2 votes. This is because, when a pixel not influenced by a foil shadow is present close by, the pixel in question has a high probability of being influenced by the foil shadow. These adjusting steps adjust many pixels with the number of votes 1 to have the number of votes=0 or the number of votes=2, which enables uninfluenced pixels to be extracted within the respective groups. 
     In this invention, it is preferred that the method further comprises a fifth adjusting step executed, when there remains a pixel whose number of votes obtained is 1 after the fourth adjusting step, for adjusting the number of votes obtained by such pixel to 2. 
     For a group located in an end portion, the votes are cast only from the group at one side, and there exists a pixel with the number of votes 1 remaining unchanged. So, this remaining pixel is adjusted to have 2 votes. This enables an uninfluenced pixel to be extracted from the end portion for use in the interpolation process. Therefore, the interpolation process for end portions can also be carried out with high accuracy. 
     In this invention, it is preferred that the method comprises a forcible changing step executed after the extracting step, when a predetermined range includes an uninfluenced pixel skipping four pixels, and an uninfluenced pixel skipping two pixels, for forcibly changing the uninfluenced pixels so that each have three pixels at both sides. 
     Even though uninfluenced pixels are extracted through the adjusting steps, there is a possibility of erroneous extraction since, after all, pixels only with a stochastically high degree of certainty are extracted. So, the forcible changing step assumes a high probability of erroneous extraction when a predetermined range includes an uninfluenced pixel skipping four pixels and an uninfluenced pixel skipping two pixels. Then, the uninfluenced pixels are forcibly changed so that each have three pixels at both sides. This can inhibit lowering of the accuracy of an approximate fluoroscopic image due to the erroneous extraction. 
     In another aspect of the invention, a radiographic apparatus for obtaining radiographs comprises a radiation emitting device for emitting radiation to a patient; a radiation detector having pixels arranged in a two-dimensional array for detecting radiation transmitted through the patient; a synchronous grid with foil strips arranged at regular intervals so that grid foil shadows fall on middles of the pixels of the radiation detector; an extracting unit including a grouping unit for dividing pixels forming a fluoroscopic image into groups each having a predetermined number of pixels within each row in a direction of row, a most influenced pixel selecting unit for selecting a pixel most influenced by one of the grid foil shadows in each group as most influenced pixel, a voting unit for casting, with the most influenced pixel in each group serving as a reference, a predetermined number of votes for other pixels spaced apart forward and backward in the direction of row, and an electing unit for electing a pixel given a maximum number of votes within each group as an uninfluenced pixel which is free from influences of a foil shadow of the grid; an approximate fluoroscopic image calculating unit for obtaining an approximate fluoroscopic image by carrying out an interpolation process based on detection signal values of the uninfluenced pixels; a grid foil shadow image calculating unit for obtaining a grid foil shadow image based on a difference between the fluoroscopic image and the approximate fluoroscopic image; a foil shadow standard image calculating unit for obtaining a foil shadow standard image by averaging the grid foil shadow image in a longitudinal direction of the grid foil shadows; and a foil shadow removing unit for removing the grid foil shadows from the fluoroscopic image based on a difference between the fluoroscopic image and the foil shadow standard image, thereby to obtain a foil shadow removed image. 
     According to this invention, the radiation emitting device emits radiation to a patient, and the radiation detector detects radiation transmitted through the patient. The resulting fluoroscopic image has grid foil shadows of the synchronous grid appearing thereon. 
     So, the grouping unit divides the pixels arranged in the direction of row into groups, and the most influenced pixel selecting unit selects the most influenced pixel within each group. The most influenced pixel is a pixel most influenced by a grid foil shadow, which can be selected relatively easily and relatively reliably compared with selection of pixels not influenced by the grid foil shadows. Next, the voting unit casts votes for other pixels spaced forward and backward in the direction of row from the most influenced pixel in each group, and the electing unit elects a pixel given a maximum number of votes within each group as an uninfluenced pixel which is free from influences of a grid foil shadow. With the extracting unit carrying out such processes, pixels not influenced by the grid foil shadows can be extracted with a relatively high degree of certainty from among pixels with varied detection signal values due to random quantum noise of the radiation. 
     Then, the approximate fluoroscopic image calculating unit carries out an interpolating process based on the detection signal values of the uninfluenced pixels, to calculate an approximate fluoroscopic image with the grid foil shadows substantially removed from the fluoroscopic image. Further, the grid foil shadow image calculating unit calculates a grid foil shadow image as an image of only the grid foil shadows based on a difference between the fluoroscopic image and the approximate fluoroscopic image. Since this grid foil shadow image has nonuniformity of the grid foil shadows due to the random errors due to quantum noise and the like, the foil shadow standard image calculating unit calculates a grid foil shadow standard image without influences of distortions, for example, by averaging the grid foil shadow image piecewise by units of several tens of pixels in the longitudinal direction. Next, the foil shadow removing unit obtains a foil shadow removed image excluding the grid foil shadows from the fluoroscopic image based on a difference between the fluoroscopic image and the foil shadow standard image. As described above, the extracting unit extracts uninfluenced pixels with a relatively high degree of certainty, while avoiding influences of random quantum noise as much as possible. An approximate fluoroscopic image is obtained based on such uninfluenced pixels. Thus, accuracy of the approximate fluoroscopic image can be improved over that of the prior art. Therefore, the grid foil shadow image and foil shadow standard image calculated successively based on the approximate fluoroscopic image have improved accuracy over the prior art. As a result, while inhibiting influence of random quantum noise, the foil shadow removed image is made free from artifacts due to distortion of the synchronous grid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. 
         FIG. 1  is an overall view showing an outline of an X-ray fluoroscopic apparatus according to this invention; 
         FIG. 2  is a view in vertical section of a grid; 
         FIG. 3  is a perspective view of grid foil strips; 
         FIG. 4  is a view in vertical section showing a relationship between the grid and an FPD; 
         FIG. 5  is an explanatory view of SIDS; 
         FIG. 6  is a view showing a C-arm having been moved; 
         FIG. 7  is a schematic view illustrating movement of an X-ray focus; 
         FIG. 8  is a schematic view illustrating movement of the X-ray focus; 
         FIG. 9  is a schematic view illustrating movement of a grid foil shadow on pixels of the FPD; 
         FIG. 10  is a block diagram of an image processor; 
         FIG. 11  is a schematic view illustrating a positional relationship between the FPD and grid foil shadows at a time of reference SID; 
         FIG. 12  is a schematic view illustrating a positional relationship between the FPD and grid foil shadows at a time of deviation from the reference SID; 
         FIG. 13  is a schematic view showing a relationship between the grid foil shadows and detection values of the pixels in the absence of a patient; 
         FIG. 14  is a schematic view showing a relationship between the grid foil shadows and detection values of the pixels in the presence of a patient; 
         FIG. 15  shows an example of detection values of the pixels within one row of the FPD, in which  FIG. 15A  shows detection values of the entire row,  FIG. 15B  shows detection values of a middle portion A, and  FIG. 15C  shows detection values of an end portion B; 
         FIG. 16  is a schematic view showing a voting process; 
         FIG. 17  is a flow chart showing operation of the image processor; 
         FIG. 18  is a flow chart showing a process of extracting uninfluenced pixels; 
         FIG. 19  includes schematic views showing uninfluenced pixels after the extracting process, in which  FIG. 19A  shows a case of selecting improper uninfluenced pixels, and  FIG. 19B  shows a state after a forceful changing process; 
         FIG. 20  includes views showing a process according to this invention, in which  FIG. 20A  shows a foil shadow removed image, and  FIG. 20B  shows selected uninfluenced pixels; and 
         FIG. 21  includes views showing a process according to a proposed example, in which  FIG. 21A  shows a foil shadow removed image, and  FIG. 21B  shows selected uninfluenced pixels. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of this invention will be described hereinafter with reference to the drawings. In this embodiment, an X-ray fluoroscopic apparatus will be described as an example of radiographic apparatus.  FIG. 1  is an overall view showing an outline of an X-ray fluoroscopic apparatus according to the embodiment.  FIG. 2  is a view in vertical section of a grid.  FIG. 3  is a perspective view of grid foil strips. 
     An X-ray fluoroscopic apparatus  1  includes an X-ray tube  3 , a synchronous grid  5  and a flat panel detector  7  (hereinafter called FPD). The X-ray tube  3  emits X-rays to a patient M. The synchronous grid  5  is attached to an X-ray incident side of the FPD  7  for removing scattered X-rays. The FPD  7  detects transmission X-rays emitted from the X-ray tube  3 . The X-ray tube  3  and the synchronous grid  5 /FPD  7  are mounted at opposite ends of a C-arm  9  to be opposed to each other. The C-arm  9  is supported by an arm support  11 , and is moved by a C-arm moving mechanism  13 . The C-arm moving mechanism  13  is controlled by a C-arm movement controller  15 . 
     The above X-ray tube  3  corresponds to the “radiation emitting device” in this invention. The FPD  7  corresponds to the “radiation detecting device” in this invention. 
     The C-arm  9  is constructed movable up and down in vertical directions R 1  relative to a top board  17  on which the patient M is placed. The arm support  11  is constructed rotatable about an axis R 2  extending vertically. The C-arm  9  is also rotatable about a horizontal axis R 3  and movable in arcuate rocking directions R 4  relative to the arm support  11 . In order to adjust an SID (Source Image Distance) which is a distance between the X-ray tube  3  and FPD  7 , the synchronous grid  5  and FPD  7  are movable in vertical directions R 5  by the C-arm moving mechanism  13 . 
     The X-ray fluoroscopic apparatus  1  further includes an X-ray tube controller  19 , an analog-to-digital converter  21 , an image processor  23 , a main controller  25 , an input unit  27 , a monitor  29  and a storage unit  31 . 
     The X-ray tube controller  19  controls a tube current and tube voltage outputted to the X-ray tube  3 . The analog-to-digital converter  21  converts X-ray detection signals outputted from the FPD  7 , from analog to digital. The image processor  23  carries out various image processes on the digital X-ray detection signals. The main controller  25  has a CPU and so on for performing overall control of the X-ray tube controller  19  and other components. The input unit  27  has input devices such as a mouse used by the radiographer in making varied settings. The monitor  29  is used to give various displays such as control screens for X-ray diagnosis and X-ray fluoroscopic images picked up. The storage unit  31  is formed of a storage device such as hard disk or semiconductor memory for storing the X-ray fluoroscopic images and various data. 
     The synchronous grid  5  will be described with reference to  FIGS. 2 and 3 . The synchronous grid  5  is disposed to cover an X-ray detecting plane of the FPD  7 . The synchronous grid  5  has grid foil strips  5   a  stretched to extend in a longitudinal (Y) direction. The grid foil strips  5   a  are formed of a material for absorbing X-rays. The grid foil strips  5   a  are arranged as inclined such that each has a flat surface thereof aligned to a straight line extending between a focus F of the X-ray tube  3  and the X-ray detecting plane of the FPD  7 . In other words, the synchronous grid  5  has the grid foil strips  5   a  arranged so that grid foil shadows (hereinafter called simply foil shadows) may fall on middles of X-ray detecting pixels DU of the FPD  7 . 
     The grid foil strips  5   a  will be described with reference to  FIGS. 3 and 4 .  FIG. 4  is a view in vertical section showing a relationship between the grid and FPD. 
     The grid foil strips  5   a  are arranged at predetermined intervals in a transverse (X) direction. The arrangement pitch Gp is 400 μm, for example. This arrangement pitch Gp is designed as appropriate to synchronize with the width W DU  of the X-ray detecting pixels DU of the FPD  7 . That is, the grid foil strips  5   a  are arranged so that, in a C-arm standard position at a reference SID, the foil shadows thereof may fall at predetermined pixel intervals on the X-ray detecting pixels DU. Since the width W DU  of the X-ray detecting pixels DU is 100 μm in this embodiment, for example, the foil shadows will be cast in a ratio of one to four of the X-ray detecting pixels DU in the transverse direction. 
     The above grid foil strips  5   a  are formed of a simple substance such as molybdenum, tungsten, lead or tantalum, or an alloy having one or more of these as main component. These metals, preferably, are materials having large atomic numbers and high X-ray absorptivity. The grid foil strips  5   a  usually have a thickness of 20-50 μm. The grid foil strips  5   a  are manufactured by rolling, cutting and so on, but because of being a heavy metal or an alloy thereof, it is very difficult to secure uniformity in shapes such as in the thickness and width of the grid foil strips  5   a . This shape nonuniformity of the grid foil strips  5   a  is a cause of the foil shadows producing variations in detection values. 
     The FPD  7  has X-ray detecting pixels DU arranged in a two-dimensional array for converting X-rays into charge signals. Specifically, for example, 1440×1440 X-ray detecting pixels DU are arranged. 
     The SID will be described now. The SID is a perpendicular distance between the focus of an X-ray source in the X-ray tube  3  and the FPD  7 . When the SID is shortened, an enlarged fluoroscopic image of the patient M can be obtained. On the other hand, when the SID is elongated, a wide-field fluoroscopic image of the patient M can be obtained. That is, a zoom adjustment of fluoroscopic images can be made by adjusting the SID. It is assumed in this embodiment that the SID at 1000 mm is set as “reference SID”. The grid foil strips  5   a  and FPD  7  are positionally adjusted to have one foil shadow falling on every four X-ray detecting pixels DU in the transverse direction of the FPD  7  when in the C-arm standard position at the reference SID. This is because, in the C-arm standard position, the C-arm  9  is considered free from bending due to its rigidity. The C-arm standard position is a position in which, as shown in  FIG. 1 , the C-arm  9  is in a positional relationship set three-dimensionally relative to the top board  17  or an examination room, and to which the C-arm  9  is initialized for every examination. 
     Reference is now made to  FIG. 5 .  FIG. 5  is an explanatory view of SIDs. 
     When the SID is changed, the foil shadows on the X-ray detecting plane will move. At an elongated SID which is longer than the reference SID, for example, although the foil shadows on a middle portion of the FPD  7  are little influenced, the foil shadows away from the middle portion toward side ends of the FPD  7  move inward of the FPD  7 . Conversely, when the SID is made shorter than the reference SID, the foil shadows move outward of the FPD  7 . 
     The above movements of the foil shadows will occur also when the C-arm  9  is rotated, for example. Here, reference is made to  FIGS. 6 through 9 .  FIG. 6  is a view showing the C-arm having been moved.  FIGS. 7 and 8  are schematic views illustrating movement of the X-ray focus.  FIG. 9  is a schematic view illustrating movement of a foil shadow on pixels of the FPD. 
     When the C-arm  9  is rotated to assume a position as shown in  FIG. 6 , a “bending” will occur to the C-arm  9  due to its rigidity. Then, the X-ray focus in the X-ray tube  3  will also move with this bending, and therefore the foil shadows will move, though minutely, also at the reference SID. This movement is, for example, about 2 mm at most. When the X-ray focus F in the X-ray tube  3  moves minutely as shown in  FIG. 7 , for example, the straight lines extending between the X-ray focus F and the detecting plane of the FPD  7  will become misaligned with the inclination angles of the flat surfaces of the grid foil strips  5   a . Consequently, the foil shadows will move minutely on the X-ray detecting plane. As shown in  FIG. 8 , when the reference SID is 1000 mm and the distance between the synchronous grid  5  and the FPD  7  is 20 mm, the ratio between the distance from the focus F of the X-ray tube  3  to the synchronous grid  5  and the distance from the synchronous grid  5  to the FPD  7  is about 50:1. Therefore, when the focus F of the X-ray tube  3  moves 2 mm, the foil shadows of the grid foil strips  5   a  will move about 40 μm on the detecting plane of the FPD  7 . 
     Assume that the thickness of the grid foil strips  5   a  is 30 μm and the width of the foil shadows also 30 μm, since a setting is made such that the foil shadows are located at the middles of the pixels when at the reference SID, there is an allowance of 35 μm from the foil shadows to adjoining pixels. On the other hand, when the above movement of the focus F of the X-ray tube  3  moves the foil shadows 40 μm, the foil shadows will, as shown in  FIG. 9 , protrude into the adjoining pixels from the pixels arranged beforehand to have the foil shadows cast thereon. 
     When an approximate fluoroscopic image is obtained by fixing pixels not influenced by the grid foil shadows, the accuracy of the approximate fluoroscopic image lowers due to such a phenomenon occurring to the foil shadow. This gives rise to a problem of lowering the accuracy of a fluoroscopic image with no grid foil shadows appearing thereon. It is characteristic of this invention to inhibit such an adverse influence. 
     Next, reference is made to  FIG. 10 .  FIG. 10  is a block diagram of the image processor. 
     The image processor  23  receives the digital X-ray detection signals converted by the analog-to-digital converter  21 . The image processor  23  includes a LOG-transforming unit  41 , an image memory unit  43 , an extracting unit  45 , an approximate fluoroscopic image calculating unit  47 , a foil shadow image calculating unit  49 , a foil shadow standard image calculating unit  51  and a subtracting unit  53 . 
     The LOG-transforming unit  41  has a function to LOG-transform the digital X-ray detection signals. This LOG transformation allows arithmetic operations to be carried out by linear sum, which can lighten the load of subsequent arithmetic operations. The image memory unit  43  stores fluoroscopic images based the LOG-transformed X-ray detection signals, and functions also as a buffer. The extracting unit  45  has a function, details of which will be described hereinafter, to extract pixels not influenced by the foil shadows as uninfluenced pixels, based on a fluoroscopic image stored in the image memory unit  43 . The approximate fluoroscopic image calculating unit  47  carries out an interpolating process based on the uninfluenced pixels extracted by the extracting unit  45 , and calculates an approximate fluoroscopic image having the foil shadows removed from the fluoroscopic image read from the image memory unit  43 . The foil shadow image calculating unit  49  calculating a grid foil shadow image which is an image of the grid foil strips  5   a  by determining a difference between the fluoroscopic image and approximate fluoroscopic image. The foil shadow standard image calculating unit  51  calculates a grid foil shadow standard image by averaging the grid foil shadow image in the longitudinal direction of the grid foil strips  5   a . The subtracting unit  53  calculates a foil shadow removed image having the foil shadows removed from the fluoroscopic image by determining a difference between the fluoroscopic image and the grid foil shadow standard image. 
     The above foil shadow image calculating unit  49  corresponds to the “grid foil shadow image calculating unit” in this invention. The subtracting unit  53  corresponds to the “foil shadow removing unit” in this invention. 
     Reference is now made to  FIGS. 11 and 12 .  FIG. 11  is a schematic view illustrating a positional relationship between the FPD and grid foil shadows at a time of reference SID.  FIG. 12  is a schematic view illustrating a positional relationship between the FPD and grid foil shadows at a time of deviation from the reference SID. 
     According to the design adopted here, at the reference SID the foil shadows fall on the X-ray detecting pixels DU of the FPD  7  as shown in  FIG. 11 , for example. That is, assuming that the X-ray detecting pixels DU of the FPD  7  are set to P 4n+1  (where n is an integer 0 or more) in the direction of row (transverse direction), the foil shadows fall on the pixels indicated by P 4n+1  and arranged at intervals of four pixels (or at intervals of three pixels when the three pixels are seen as being skipped). The shape of the grid foil strips  5   a  is not strictly uniform, and minute shifts will occur with the arrangement of the grid foil strips  3   a  also. These result in variations in the width (in the direction of row) of the foil shadows as seen in a foil shadow  55  and a foil shadow  57 . However, of a group consisting of four pixels (P 4n+1 , P 4n+2 , P 4n+3  and P 4n+4 ), the pixels P 4n+2 , P 4n+3  and P 4n+4  forming a group excluding the pixel P 4n+1  are uninfluenced pixels which are not influenced by the foil shadow  55 . Therefore, the foil shadow image calculating unit  49  may carry out an interpolation process using any one of these pixels. However, random quantum noise exists in X-rays, and when uninfluenced pixels are selected based only on the pixel values (X-ray detection signal values), inappropriate pixels can be selected as the uninfluenced pixels. 
     In the case of a deviation from the reference SID or the C-arm  9  moved as described above, for example, the positions of the foil shadows move from the positions of the foil shadows according to the design value of the reference SID as shown in  FIG. 12 . For example, a foil shadow  59  appears as straddling the pixel P 4n+1  and adjoining pixel P 4n+2  in the group of four pixels (P 4n+1 , P 4n+2 , P 4n+3  and P 4n+4 ). In a different condition, a foil shadow  61  may move completely from pixel P 4(n+1)+1  onto pixel P 4n+4 . In this way, the pixels not influenced by the foil shadows are changeable also with the position of the C-arm  9 , and therefore a contrivance is needed for extracting uninfluenced pixels. 
     Reference is now made to  FIGS. 13 and 14 .  FIG. 13  is a schematic view showing a relationship between the grid foil shadows and detection values of the pixels in the absence of a patient.  FIG. 14  is a schematic view showing a relationship between the grid foil shadows and detection values of the pixels in the presence of a patient. 
     When X-raying is carried out without the patient M placed on the top board  17  as shown in  FIG. 13A , X-ray detection signal values will be as follows. As shown in  FIG. 13B , the pixels P 4n+1  with the foil shadows of the grid foil strips  3   a  falling thereon have X-ray detection signal values (● (black circle) mark) which are reduced about 20% from X-ray detection signal values (Δ (triangle) mark and (□ (square) mark) of the other pixels. 
     Next, when X-raying is carried out with the patient M placed on the top board  17  as shown in  FIG. 14A , X-ray detection signal values will be as follows. As shown in  FIG. 14B , the pixels P 4n+1  with the foil shadows of the grid foil strips  3   a  falling thereon have X-ray detection signal values (● (black circle) mark) which are lower than X-ray detection signal values (Δ (triangle) mark and □ (square) mark) of the other pixels. 
     Reference is now made to  FIG. 15  showing actual measurements in graphs.  FIG. 15  shows an example of detection values of the pixels within one row of the FPD, in which  FIG. 15A  shows detection values of the entire row,  FIG. 15B  shows detection values of a middle portion A, and  FIG. 15C  shows detection values of an end portion B. The FPD  7  used here is 9 inch size with 1440×1440 pixels, the tube voltage is 60 keV, and the elongated SID deviating from the reference SID is 1150 mm. 
     As shown in  FIG. 15A , in pixel numbers  1  to  1440  of the X-ray detecting pixels DU in one row of the FPD  7 , there are four locations of turnover in the magnitude relation of the X-ray detection signal values. These locations represent instances of foil shadows straddling the pixels as described hereinbefore.  FIG. 15B  shows an enlarged graph of the middle portion A of  FIG. 15A , in which the foil shadows fall on every fourth pixels, i.e. at regular intervals skipping three pixels.  FIG. 15  C shows an enlarged graph of the end portion B of  FIG. 15A , which includes an instance of a foil shadow straddling the pixels. It will be seen that the X-ray detection signal values assume a complicated pattern in this graph. Such a complicated pattern formed also indicates a difficulty in extracting uninfluenced pixels. 
     Reference is made to  FIGS. 10 and 16 .  FIG. 16  is a schematic view showing a voting process. In  FIG. 16 , the ● (black circle) mark indicates pixels most influenced by the foil shadows, the ◯ (white circle) mark indicates pixels not influenced by the foil shadows, and hatched ◯ (white circle) mark indicates pixels which are neither of the above two types. 
     The extraction of uninfluenced pixels noted above is carried out by the extracting unit  45 . The extracting unit  45  has a grouping unit  71 , a most influenced pixel selecting unit  73 , a voting unit  75 , an adjusting unit  77 , an electing unit  79  and a forcible change unit  81 . 
     The above adjusting unit  77  corresponds to the “first adjusting unit to the fifth adjusting unit” in this invention. 
     The grouping unit  71  carries out a process of dividing a plurality of pixels i (where i=1 to N) arranged in the direction of row (transverse direction) of the FPD  7  as shown in  FIG. 16A , into groups each consisting of a predetermined number of pixels ( FIG. 16B ). Assume here, for example, that four pixels constitute each group. That is, the grouping unit  71  divides the pixels into a plurality of groups each including four consecutive pixels. In  FIG. 16B , the pixels are divided into a group of pixel i, pixel i+1, pixel i+2 and pixel i+3, a group of pixel i+4, pixel i+5, pixel i+6 and pixel i+7, a group of pixel i+8, pixel i+9, pixel i+10 and pixel i+11, a group of pixel i+12 . . . , and so on. The most influenced pixel selecting unit  73  processes each of the groups formed by the grouping unit  71 . Specifically, one pixel most influenced by a grid foil shadow  5   a  in each group is selected as the “most influenced pixel”. This is done only by selecting what has an extremely low detection signal value, and is easy compared with finding uninfluenced pixels. Specifically, pixels i+2, i+6 and i+10 in the respective groups will be selected as the most influenced pixels. 
     The voting unit  75 , based on the positions of the most influenced pixels i+2, i+6 and i+10 in the respective groups selected by the most influenced pixel selecting unit  73 , casts a predetermined number votes for pixels i, i+4, i+8 and i+12 which are located next but one to the respective most influenced pixels i+2, i+6 and i+10 forward and backward in the direction of row ( FIG. 16C ). Here, the predetermined number of votes is set to “1”. The electing unit  79 , based on the result of voting by the voting unit  75 , elects pixels least influenced by the foil shadows as “uninfluenced pixels”. Since the number of votes is set to one vote, the votes are cast for pixels next but one forward and backward, and the number of pixels in each group is four, the number of votes G(i) obtained by each pixel i at this time is 0, 1 or 2. The pixels with G(i)=0 are those influenced by the foil shadows, while the pixels of G(i)=2 are those least influenced by the foil shadows. Therefore, the electing unit  79  elects the pixels having obtained the number of votes G(i)=2 as uninfluenced pixels. 
     Thus, it is so designed that the foil shadows fall on every four pixels, each group is set to every four pixels, and votes are cast for positions spaced from the most influenced pixel in each group by two pixels which are the half of the number of pixels constituting each group, thereby forming peaks of the number of votes at certain places. Moreover, the peaks correspond with a high degree of certainty to positions of the pixels unlikely to be influenced by the foil shadows, and thus the uninfluenced pixels can be elected with a high degree of certainty. 
     The pixels with G(i)=1 are those for which it is unknown whether they are influenced by the foil shadows or not. So, the adjusting unit  77  carries out the following adjustment for the pixels with G(i)=1. 
     First, when the number of votes G(i)=2 is obtained by each of pixels next but three to a given pixel forward and backward in the direction of row (i.e. G(i−4)=2 or G(i+4)=2), the number of votes G(i) obtained by this given pixel is adjusted from 1 to 2. This is because four pixels form each group, and so a pixel next but three to a pixel having obtained the number of votes G(i)=2 has a high probability of not being influenced by a foil shadow. Next, when the number of votes G(i)=2 is obtained by a pixel next to a given pixel in the direction of row (i.e. G(i−1)=2 or G(i+1)=2), the number of votes G(i) obtained by this given pixel is adjusted from 1 to 0. This is because the probability of two adjoining pixels not being influenced by a foil shadow or shadows is low. Next, the detection signal values of adjoining pixels among the pixels of G(i)=1 are compared. The number of votes of the pixel with the larger detection signal value is adjusted to 2, and the number of votes of the pixel with the smaller value to 0. This is because, where pixels of G(i)=1 adjoin each other, the pixel with the larger detection signal value is more likely not to be influenced by a foil shadow. Next, when one of pixels next but one to a given pixel forward and backward has two votes (i.e. G(i−2)=2 or G(i+2)=2), the number of votes of this given pixel is adjusted to 0. This is because, when a pixel not influenced by a foil shadow is present close by, the given pixel has a high probability of being influenced by the foil shadow. These operations adjust many pixels with the number of votes G(i)=1 to have the number of votes G(i)=0 or the number of votes G(i)=2, which enables uninfluenced pixels to be extracted within the respective groups. 
     When there still remain pixels having the number of votes G(i)=1 after the above process, the adjusting unit  77  changes the number of votes of these pixels to G(i)=2. For a group located in an end portion of the FPD  7 , the votes are cast only from the group at one side, and there exists a pixel with the number of votes G(i)=1 remaining unchanged. So, this remaining pixel is adjusted to have the number of votes G(i)=2, thereby to extract an uninfluenced pixel from the end portion for use in the interpolation process. Therefore, the interpolation process for the end portions of the FPD  7  can also be carried out with high accuracy. 
     After the above adjustments are carried out and the uninfluenced pixels are elected by the electing unit  79 , the forcible change unit  81  checks whether a forcible changing condition is fulfilled or not, and carries out the following forcible change when the condition is fulfilled. 
     Even though uninfluenced pixels are extracted through the adjustment described above, there is a possibility of erroneous extraction since, after all, pixels only with a stochastically high degree of certainty are extracted. Under ideal conditions in which no random noise exists, and when an SID used is longer than the reference SID, most of the uninfluenced pixels occurring within one row skip three pixels each. The uninfluenced pixels, skipping two pixels each, occur in only several locations within one row. The uninfluenced pixels, skipping two pixels each, occur substantially equidistantly. Conversely, when the SID used is shorter than the reference SID, most of the uninfluenced pixels occurring within one row skip three pixels each, the uninfluenced pixels, skipping four pixels each, occur in only several locations within one row, and the uninfluenced pixels, skipping four pixels each, occur substantially equidistantly. That is, with whatever SID, the uninfluenced pixels, skipping two pixels each, and the uninfluenced pixels, skipping four pixels each, never occur at the same time. So, a high probability of erroneous extraction is assumed when the forcible change unit  81  finds fulfillment of a “forcible changing condition” that a predetermined range (e.g. a range of five uninfluenced pixels) includes an uninfluenced pixel skipping four pixels, and an uninfluenced pixel skipping two pixels. Then, the uninfluenced pixels are forcibly changed so that each have three pixels at both sides. This can inhibit lowering of the accuracy of an approximate fluoroscopic image due to the erroneous extraction. 
     Next, a process of X-ray fluoroscopic imaging carried out by the above X-ray apparatus  1  will be described with reference to  FIGS. 17 and 18 .  FIG. 17  is a flow chart showing operation of the image processor.  FIG. 18  is a flow chart showing a process of extracting uninfluenced pixels. 
     First, X-ray fluoroscopic imaging carried out before the processes in the flow chart will be described. The radiographer sets an amount of the SID, an amount of movement of the C-arm  9 , a tube voltage and a tube current to the input unit  27 . The main controller  25  outputs the set amount of the SID and amount of movement of the C-arm  9  to the C-arm movement controller  15 . The C-arm movement controller  15  controls the C-arm moving mechanism  13  to move the C-arm  9 . The main controller  25  also outputs instructions to the X-ray tube controller  19  to control the X-ray tube  3  with the set tube voltage and tube current. Next, when the radiographer instructs a start of X-raying from the input unit  27 , the main controller  25  controls the X-ray tube controller  19  and FPD  7 . The X-ray tube controller  19  applies the tube voltage and tube current to the X-ray tube  3  based on the instructions from the main controller  25 . Then, X-rays are emitted from the X-ray tube  3  to the patient M. X-rays transmitted through the patient M, while scattered X-rays are inhibited by the synchronous grid  5 , fall on the FPD  5  to be detected by the X-ray detecting pixels DU. X-ray detection signals generated by the X-ray detecting pixels DU are outputted to the image processor  23  to be LOG-transformed by the LOG-transforming unit  41 . The LOG-transformed X-ray detection signals are stored as a fluoroscopic image in the image memory unit  43 . 
     Step S 1   
     The extracting unit  45  carries out a process of extracting uninfluenced pixels. Specifically, this process follows the flow chart shown in  FIG. 18 . 
     Step T 1   
     The grouping unit  71  divides all the pixels in one row into groups as described above. This grouping is carried out for all the rows of the FPD  7 . 
     Step T 2   
     The most influenced pixel selecting unit  73  selects a pixel most influenced by the foil shadows in each group as described above. 
     Step T 3   
     The voting unit  75  casts votes in the procedure described above for fore and aft pixels spaced from the most influenced pixels. 
     Step T 4   
     The adjusting unit  77  adjusts the number of votes obtained by the pixels whose vote is one, as described above. 
     Step T 5   
     Based on the result of voting for each pixel, the electing unit  79  elects uninfluenced pixels as described above. 
     Steps T 6  and T 7   
     The forcible change unit  81  checks whether the forcible changing condition which may take place rarely is fulfilled, and forcibly changes the numbers of votes according to a result. Reference is made to  FIG. 19  for a specific example.  FIG. 19  includes schematic views showing uninfluenced pixels after the extracting process, in which  FIG. 19A  shows a case of selecting inappropriate uninfluenced pixels, and  FIG. 19B  shows a state after a forceful changing process. In these figures, the white lines represent the uninfluenced pixels elected in step T 5 , and the black lines represent pixels other than the uninfluenced pixels and including the most influenced pixels. 
       FIG. 19A  shows a case of the above forcible changing condition being fulfilled. Specifically, in two locations near the right end as indicated by an arrow in  FIG. 19A , an uninfluenced pixel is elected skipping four pixels and an uninfluenced pixel is elected skipping two pixels. When such elections are made, a forcible change is carried out in steps T 6  and T 7 . The result is shown in  FIG. 19B . In this figure, the forcible change has been carried out to show that pixels skipping three pixels are elected as uninfluenced pixels. 
     Step T 1  described above corresponds to the “grouping step” in this invention. Step T 2  corresponds to the “most influenced pixel selecting step”. Step T 3  corresponds to the “voting step”. Step T 4  corresponds to the “adjusting step”. Step T 5  corresponds to the “electing step”. Step T 4  corresponds also to the “first to fifth adjusting steps”. Steps T 6  and T 7  correspond to the “forcible changing step”. 
     Reference is made to the flow chart of  FIG. 17  again. 
     Step S 2   
     The approximate fluoroscopic image calculating unit  47  calculates, by interpolation process, detection signal values corresponding to positions of the most influenced pixels, based on the uninfluenced pixels outputted from the extracting unit  45 . Then, an approximate fluoroscopic image is calculated based on the fluoroscopic image from the image memory unit  4  and results of the interpolation. The interpolation process may employ cubic interpolation such as cubic spline method, for example. 
     Step S 3   
     The foil shadow image calculating unit  49  calculates a grid foil shadow image showing only the foil shadows, by determining a difference between the fluoroscopic image from the image memory unit  43  and the approximate fluoroscopic image from the approximate fluoroscopic image calculating unit  47 . 
     Step S 4   
     The foil shadow standard image calculating unit  51  calculates a grid foil shadow standard image by averaging the grid foil shadow image from the foil shadow image calculating unit  49 , piecewise by units of several tens of pixels in the longitudinal direction corresponding to the direction of length of the grid foil strips  5   a . That is, correction is made by averaging variations in the foil shadows due to random errors such as quantum noise and the like as shown in  FIGS. 11 and 12 . The entire length of each grid foil strip  5   a  corresponds to 1000 to 2000 pixels. By averaging these piecewise, interpolation errors included in the grid foil shadow image are removed therefrom, while leaving distortions of the foil itself in the image. 
     Step S 5   
     The subtracting unit  53  calculates a foil shadow removed fluoroscopic image by determining a difference between the fluoroscopic image from the image memory unit  43  and the grid foil shadow standard image from the foil shadow standard image calculating unit  51 . By removing the standardized foil shadows from the fluoroscopic image, an X-ray fluoroscopic image of the patient from which the interpolation errors have been removed can be obtained. 
     The X-ray fluoroscopic image of the patient obtained in this way is displayed on the monitor  29  or stored in the storage unit  31  through the main controller  25 . 
     Step S 1  described above corresponds to the “extracting step” in this invention. Step S 2  corresponds to the “approximate fluoroscopic image calculating step”. Step S 3  corresponds to the “grid foil shadow image calculating step”. Step S 4  corresponds to the “foil shadow standard image calculating step”. Step S 5  corresponds to the “foil shadow removing step”. 
     Next, reference is made to  FIGS. 20 and 21 .  FIG. 20  includes views showing a process according to this invention, in which  FIG. 20A  shows a foil shadow removed image, and  FIG. 20B  shows selected uninfluenced pixels.  FIG. 21  includes views showing a process according to a proposed example, in which  FIG. 21A  shows a foil shadow removed image, and  FIG. 21B  shows selected uninfluenced pixels. 
     In this invention, as shown in  FIG. 20B , uninfluenced pixels are extracted at substantially equal intervals. As a result, as shown in  FIG. 20A , an X-ray fluoroscopic image which is a foil shadow removed image is free from artifacts. 
     On the other hand, in the proposed example, as shown in  FIG. 21B , uninfluenced pixels are extracted at irregular intervals. As a result, as shown in  FIG. 21A , an X-ray fluoroscopic image obtained has artifacts remaining thereon (encircled area in the figure) under the influence of foil shadows. 
     This invention is not limited to the foregoing embodiment, but may be modified as follows: 
     (1) In the foregoing embodiment, the construction provides one grid foil strip  5   a  for every four pixels, but this invention is not limited to this. For example, one grid foil strip  5   a  may be provided for every eight pixels. In this case, the grouping described hereinbefore may be carried out for every eight pixels, with votes cast for fourth pixels forward and backward. 
     (2) In the foregoing embodiment, for the pixels given one vote and remaining to the last, the number of votes obtained is changed to 2. Such process may be omitted when peripheral portions of the X-ray fluoroscopic image are not processed. This can lighten the load on the process. 
     (3) In the foregoing embodiment, checking is made whether the forcible changing condition is fulfilled. When the frequency of occurrence is low, such checking process may be omitted. This can lighten processing load, and increase processing speed. 
     (4) In the foregoing embodiment, the X-ray detection signals are LOG-transformed by the LOG-transforming unit  41 . It is not necessary to provide the LOG-transforming unit  41  where the arithmetic capability has leeway. This can simplify the construction, and reduce apparatus cost. 
     This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.