Patent Publication Number: US-7589774-B2

Title: Image pickup apparatus having non-destructively readable image pickup means for picking up an object image

Description:
This application is a divisional of application Ser. No. 09/916,264, filed July 30, 2001, now allowed, the contents of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image sensing apparatus and more particularly to an image sensing apparatus for sensing a moving image. 
     2. Related Background Art 
       FIG. 6  is a block diagram showing the structure of a conventional X-ray image sensing apparatus for sensing a moving image by utilizing X-rays. In  FIG. 6 , reference numeral  101  represents an X-ray source for radiating X-rays, and reference numeral  102  represents a X-ray image sensing panel. The X-ray image sensing panel  102  has a plurality of photoelectric conversion elements disposed two-dimensionally and a driver circuit for driving these elements. An X-ray radiated from the X-ray source  101  passes through a subject  103  and becomes incident upon the X-ray image sensing panel  102  which detects the image of the subject. The X-ray passed through the subject  103  is converted into visible light by an phosphor (not shown) and then becomes incident upon the X-ray image sensing panel  102 . 
     Reference numeral  104  represents an A/D converter for A/D converting a signal supplied from the X-ray image sensing panel  102 , reference numeral  105  represents an FPN (fixed pattern noises) memory for storing FPN corrected values, and reference numeral  106  represents an FPN obtaining timing generation circuit for generating a timing signal for obtaining an FPN corrected value. When the FPN obtaining timing generating circuit  106  generates a timing signal, a switch  107  is turned on so that FPN is read from the A/D converter  104  into the FPN memory  105 . Reference numeral  108  represents a subtractor for subtracting a corrected value read from the FPN memory  106  from an output of the A/D converter  104 , reference numeral  109  represents a monitor for displaying a sensed image, and reference numeral  110  represents a recording medium for recording image data. 
       FIGS. 7A to 7C  are timing charts illustrating an operation of the X-ray image sensing apparatus shown in  FIG. 6 .  FIG. 7A  shows the FPN obtaining timing signal supplied from the FPN obtaining timing generating circuit  106 , and  FIG. 7B  shows an output from the A/D converter  104 . At the start of the image sensing, the FPN obtaining timing signal shown in  FIG. 7A  is supplied to the switch  107  to turn it on, so that the FPN corrected value is read from the A/D converter  104  into the FPN memory  105 . Thereafter, during the image sensing, the subtractor  108  subtracts the corrected value stored in the FPN memory  106  from an output of the A/D converter  104  as shown in  FIG. 7C , so that the corrected image data with FPN being removed is supplied to the monitor  109  and recording medium  110 . 
     With the conventional moving image sensing apparatus, since the FPN corrected value is stored in the FPN memory at the start of the image sensing, the FPN corrected value is fixed. Namely, the FPN corrected data cannot be obtained always during the moving image sensing. For this reason, for example, if the offset of an output amplifier (an output stage amplifier in the X-ray image sensing panel) changes with a power supply fluctuation, a temperature change or the like, a change in the offset appears directly in an output as shown in  FIG. 7B . Therefore, as shown in  FIG. 7C , a change in the offset appears directly in the corrected output and the image quality is degraded. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an image sensing apparatus capable of reliably performing FPN correction without being influenced by a change in an offset. 
     In order to achieve the above object, according to aspect of the present invention, there is provided an image sensing apparatus comprising: an image sensing unit having a non-destructive read function, adapted to sense an object image; and a subtractor circuit adapted to sequentially output a plurality of corrected values, wherein each of the plurality of corrected values is a difference between a first frame included in a plurality of frames sequentially read out non-destructively from the image sensing unit and a second frame included in the plurality of frames, read out before the frist frame. 
     Other objects and features of the present invention will become apparent from the following detailed description of embodiments when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the structure of an image sensing apparatus according to an embodiment of the invention. 
         FIG. 2  is a circuit diagram showing part of an X-ray image sensing panel of the embodiment shown in  FIG. 1 . 
         FIGS. 3A ,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G,  3 H,  3 I and  3 J are timing charts illustrating a normal read operation. 
         FIGS. 4A ,  4 B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H,  4 I and  4 J are timing charts illustrating a non-destructive read operation. 
         FIGS. 5A ,  5 B,  5 C,  5 D and  5 E are timing charts illustrating the operation of the embodiment shown in  FIG. 1 . 
         FIG. 6  is a block diagram showing the structure of a conventional X-ray image sensing apparatus. 
         FIGS. 7A ,  7 B and  7 C are timing charts illustrating the operation of the conventional apparatus shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is a block diagram showing the structure of an image sensing apparatus according to the embodiment of the invention. In  FIG. 1 , like elements to those of the conventional apparatus shown in  FIG. 6  are represented by using identical reference numerals. Referring to  FIG. 1 , an X-ray image sensing panel  111  is an image sensing panel capable of performing a normal read and a non-destructive read. The X-ray image sensing panel  111  has a plurality of photoelectric conversion elements arranged two-dimensionally and a driver circuit for driving the elements. The circuit structure and operation of the X-ray image sensing panel  111  will be later detailed. 
     A comparator  112  is a circuit for comparing a reference value which is set when reading the X-ray image sensing panel  111 , with an output value of an A/D converter  104 . In accordance with the comparison result between the output value of the A/D converter  104  and the reference value, the comparator  112  outputs a mode switching signal for switching a read mode of the X-ray image sensing panel  111 . This mode switching signal is delayed by a delay  113  by a predetermined time. 
     During the image sensing, a signal is read non-destructively from the X-ray image sensing panel  111 . During the non-destructive read, as will be described later, signal charges are accumulated without reset of a pixel so that an output of the A/D converter  104  gradually increases. In this embodiment, therefore, the reference value of the comparator  112  is set to a value slightly lower than the output of the A/D converter at a time when each pixel becomes saturated. When the output of the A/D converter  104  takes the reference value or higher, the read mode is changed to the normal read mode and the pixels of the X-ray image sensing panel  111  are reset. 
     A switch  114  is connected between the output of the A/D converter  104  and a positive terminal of the comparator  112 . This switch  114  is driven by a signal supplied from a mask table  115 . The mask table  115  stores beforehand the position information of each defective pixel of the X-ray image sensing panel  111 . When an output of a defective pixel is to be output, the switch  114  is turned off so that the read mode is prevented from being switched by the output of the defective pixel. The mask table  115  also stores the position information of an invalid area other than the image sensing area. The switch  114  is turned off when an output of the invalid area other than the image sensing area is to be output, so that it is possible to prevent the read mode from being wastefully changed by the invalid area. 
     A one-frame delay memory  116  is a memory for storing output values of one frame from the A/D converter  104 . A subtractor  117  is a circuit for subtracting an output value of each pixel of the previous frame stored in the one-frame delay memory  116  from an output value of each pixel currently output from the A/D converter  104 . By reading a signal in the non-destructive mode from the X-ray image sensing panel  111  and subtracting the output value of the previous frame from the current output value, FPN is corrected. 
     A one-frame delay memory  118  is a memory for storing outputs values of one frame from the subtractor  117 . A switch  119  selects either an output from the subtractor  117  or an output from the one-frame delay memory  118 , in accordance with a signal supplied from a timing generation circuit  120 . Although the details will be later given, the corrected value for the frame following the normal read cannot be used. In this embodiment, the switch  119  is switched to the one-frame delay memory  118  to output the corrected value for the previous frame stored in the one-frame delay memory  118 . An X-ray source  101 , a subject  103 , a monitor  109  and a recording medium  110  are similar to those shown in  FIG. 6 . Similar to  FIG. 1 , a phosphor for converting an X-ray into visual light is omitted. 
       FIG. 2  is a circuit diagram of the X-ray image sensing panel  111 .  FIG. 2  shows part of the X-ray image sensing apparatus. In  FIG. 2 , reference numeral  130  represents a vertical shift register, reference numeral  131  represents a horizontal shift register, reference numeral  132  represents an AND gate, and reference numeral  133  represents a pixel portion. Reference numeral  134  represents a constant current source, and reference numeral  135  represents a horizontal output switching MOS transistor. A mode switching signal is input to the AND gate  132  from the comparator  112  via the delay  113  shown in  FIG. 1 . Reference numeral  141  represents a vertical read line, and reference numeral  142  represents a horizontal read line. 
     The pixel portion  133  is constituted of a reset MOS transistor  136 , a vertical output switching MOS transistor  137 , a read MOS transistor  138 , a photoelectric conversion element  139  and a capacitor  140 . The pixel portion  133  and constant current source  134  constitute an amplifier having a voltage gain of 1, and can perform a read operation independently from the reset operation, without moving charges of the photoelectric conversion element  139 . 
     More specifically, the photoelectric conversion element  139  and capacitor  140  are connected to the gate terminal of the read MOS transistor  138  of the pixel portion  133 . This circuit connection and the constant current source  134  constitute a source follower circuit. Therefore, current will not flow into the gate terminal of the read MOS transistor  138 , signal charges of the photoelectric conversion element  139  can be read to the vertical read line  141 , and the signal charges of the photoelectric conversion element  139  will not move. It is therefore possible to perform the non-destructive read. In place of the constant current source  134 , a resistor may be used. However, in order to improve a precision, it is desired to use the constant current source  134 . In this embodiment, although the photoelectric conversion element is connected to the gate terminal of the read MOS transistor, the invention is not limited only thereto. For example, the photoelectric conversion element may be connected to an amplifier circuit or its control terminal to realize the non-destructive read. This is because current will not flow into the control terminal and charges will not move. Even if current flows, only charges much smaller than those necessary for output charges flow because of amplification by the circuit. Such small current is negligible. For example, the photoelectric conversion element may be connected to the base terminal of a bipolar transistor for reading to realize the non-destructive read. 
       FIGS. 3A to 3J  are timing charts illustrating the normal read operation with reset, and  FIGS. 4A to 4J  are timing charts illustrating the non-destructive read operation without reset. First, with reference to  FIGS. 3A to 3J , the normal read operation will be described. As shown in  FIG. 3A , during the normal read operation, a high level mode switching signal is supplied from the comparator  112  to the AND gate  132 . In this state, as shown in  FIG. 3B , when a signal φO n  (high level) is output from the vertical shift register  130 , the vertical output switching MOS transistor  137  is turned on. 
     At this time, since the circuit including the MOS transistor  138  for reading constitutes a source follower which is an amplification circuit having a voltage gain of about 1, signal charges of the photoelectric conversion element  139  are directly read out to the vertical read line  141 . Although omitted in  FIGS. 3A to 3J , a plurality of pixel portions are arranged in the row direction. Signal charges of the pixel portions of one line along the row direction are read out to the vertical read lines  141 . In  FIGS. 3A to 3J , a plurality of pixel portions are arranged also in the column direction. A plurality of pixel portions  133  are therefore arranged in a matrix shape along the row and column directions. 
     Next, as shown in  FIG. 3F , when a signal φH 0  is output from the horizontal shift register  131 , the horizontal output switching MOS transistor  135  turns on. Therefore, as shown in  FIG. 3J , outputs on the vertical read lines  141  are read out to the horizontal output line  142 . Thereafter, as shown in  FIGS. 3G to 3I , signals φH 1 , φH 2 , . . . , φH n  are sequentially output from the horizontal shift register  131 , and as shown in  FIG. 3J , signal charges of pixels of one line along the row direction are sequentially read out to the horizontal read line  142 . With the above operations, reading the first line along the row direction is completed. 
     Next, as shown in  FIG. 3C , when a signal φC n  is output from the vertical shift register  130  to the AND gate  132 , the reset MOS transistor  136  turns on so that signal charges of the photoelectric conversion element  139  are initialized (reset). Similarly, signal charges of other pixel portions of one line along the row direction are reset so that during the next accumulation period, charges are newly accumulated in the photoelectric conversion elements. 
     Next, as shown in  FIG. 3D , for the pixels of the second row (not shown), a signal φO n+1  is output from the vertical shift register  130  and the vertical output switching MOS transistor  137  turns on. Signal charges of the photoelectric conversion elements  139  of the pixel portions of the second row are therefore read out to the vertical read lines  141 . As shown in  FIGS. 3F to 3I , signals φH 0  to φH m  are sequentially output from the horizontal shift register  131 , and as shown in  FIG. 3J , signal charges of the vertical read lines  141  are sequentially read out to the horizontal output line  142 . 
     Thereafter, as shown in  FIG. 3E , a signal φC n+1  is output from the vertical shift register  130  to the AND gate  132  to reset the photoelectric conversion elements  139  of the pixel portions of the second row. Similarly thereafter, signal charges of the pixel portions of the third row, fourth row, . . . are read out. When the signal charges of the last n-th line are read out, reading all the pixel portions of the X-ray image sensing panel  111  is completed. 
     Next, with reference to  FIGS. 4A to 4J , the non-destructive read operation will be described. During the normal read operation, after signal charges are read out, they are reset, whereas during the non-destructive operation, after signal charges are read out, they are not reset. This is a different point between the two read operations. As shown in  FIG. 4A , therefore, a low level mode switching signal is supplied from the comparator  112  to maintain the AND gate  132  to be closed. 
     In this state, as shown in  FIG. 4B , when a signal φO n  is output from the vertical shift register  130 , the vertical output switching MOS transistor  137  is turned on. Signal charges of the photoelectric conversion elements  139  are therefore read out to the vertical read lines  141  via the read MOS transistors  138 . Next, as shown in  FIG. 4F , when a signal φH 0  is output from the horizontal shift register  131 , the horizontal output switching MOS transistor  135  turns on. Therefore, as shown in  FIG. 4J , outputs on the vertical read lines  141  are read out to the horizontal output line  142 . Thereafter, as shown in  FIGS. 4G to 4I , signals φH 1 , φH 2 , . . . , φH n  are sequentially output from the horizontal shift register  131 , and as shown in  FIG. 4J , signal charges of pixels of one line along the row direction are sequentially read out to the horizontal read line  142 . With the above operations, reading the first line along the row direction is completed. 
     Next, as shown in  FIG. 4D , for the pixels of the second row (not shown), a signal φO  n+1  is output from the vertical shift register  130  and the vertical output switching MOS transistor  137  turns on. Signal charges of the photoelectric conversion elements  139  of the pixel portions of the second row are therefore read out to the vertical read lines  141 . As shown in  FIGS. 4F to 4I , signals φH 0  to φH m  are sequentially output from the horizontal shift register  131 , and as shown in  FIG. 4J , signal charges of the vertical read lines  141  are sequentially read out to the horizontal output line  142 . 
     Similarly thereafter, signal charges of the pixel portions of the third row, fourth row, . . . are read out. When the signal charges of the last n-th line are read out, reading all the pixel portions of the X-ray image sensing panel  111  is completed. In the non-destructive read mode, therefore, after signal charges of the pixel portions are read out, signal charges of the photoelectric conversion elements are not reset but the next accumulation starts. Namely, the charge amount does not change before and after reading the pixel portion, and the read operation does not influence the photoelectric conversion element. In this embodiment, the non-destructive reading of the X-ray image sensing panel  111  is used to correct FPN. 
       FIGS. 5A to 5E  are timing charts illustrating the operation of this embodiment. With reference to  FIG. 1  and  FIGS. 5A to 5E , a specific operation of the embodiment will be described. In sensing a moving image, an X-ray is radiated from the X-ray source  101  to the subject  103 . The X-ray passed through the subject  103  becomes incident upon the X-ray image sensing panel  111  which detects an image.  FIG. 5A  illustrates a read mode of sensing a moving image. A represents the normal read mode, and B represents the non-destructive read mode. 
     During the moving image sensing, as shown in  FIG. 5A , the non-destructive read B is used which is switched after each pixel of the X-ray image sensing panel  111  is reset during the normal read A.  FIG. 5B  shows an output of the A/D converter  104 . When the read mode is switched to the non-destructive read B, a signal is read out from the X-ray image sensing panel  111  by the operation described with reference to  FIGS. 4A to 4J . As shown in  FIG. 5B , the non-destructive reading B 0 , B 1 , B 2 , B 3 , . . . are performed for each frame by the A/D converter  104 . The output values of pixels of each frame during the non-destructive reading are sequentially stored in the one-frame memory  116 . 
     The subtractor  117  subtracts an output value of each pixel of the previous frame stored in the one-frame delay memory  116  from a current output value of the corresponding pixel output from the A/D converter  104 , and outputs the corrected values as shown in  FIG. 5E . Namely, the subtractor  117  performs a process of subtracting an output value of each pixel of the previous frame from a current output value of the corresponding pixel, and outputs the corrected values (B 1 −B 0 ), (B 2 −B 1 ), (B 3 −B 2 ), . . . each corresponding to a difference between an output value of each frame and the output value of the previous frame stored in the one-frame delay memory  116 . 
     The corrected values output from the subtractor  117  are supplied via the switch  119  to the monitor  109  and recording medium  110 . The sensed image is displayed on the monitor  109  and stored in the recording medium  110  as image data. During the non-destructive read, the switch  119  is turned to the side a, and the corrected values output from the subtractor  117  are supplied via the switch  119  to the monitor  109  and recording medium  110 . As will be described later, the switch  119  is turned to the side b after the normal read, in response to the signal from the timing conversion circuit  120 . 
     Since the output value of the A/D converter  104  is provided by non-destructive read and thus the charge of each pixel of the X-ray image sensing panel  111  is sequentially accumulated during the non-destructive read without resetting. As shown in  FIG. 5B , the output value of the A/D converter increases gradually. The comparator  112  monitors the output value of the A/D converter  104 . As shown in  FIG. 5B , when the output value of the A/D converter  104  becomes larger than the reference value in the non-destructive read B 7 , the high level mode switching signal is output via the delay  113  to the X-ray image sensing panel  111 . This mode switching signal is delayed by the delay  113  by a predetermined time, and supplied to the AND gate  132  of the X-ray image sensing panel  111  as described with reference to  FIG. 2 . 
     The delay time of the delay  113  is the time from the frame at which the output value of the A/D converter  104  exceeded the reference value to the frame following that frame, as shown in  FIG. 5C . This delay time prevents the read mode from being changed at the midst of the frame. During the ordinary case, the switch  114  is maintained on, whereas it is maintained off for the defective pixel and invalid area as described earlier. The reference value of the comparator  112  is set to a value slightly lower than the output value of the A/D converter  104  at a time when the pixel of the X-ray image sensing panel  111  is saturated. 
     When the read mode is changed to the normal read mode, signals are read from the X-ray image sensing panel  111  by the operation described with  FIGS. 3A to 3J . As shown in  FIG. 5B , one frame is read by the ordinary read A 1 . In this case, as described with  FIGS. 3A to 3J , after signals are read from the respective pixels, the corresponding photoelectric conversion elements are reset. After the normal read A 1 , the output value of the A/D converter  104  lowers so that the mode switching signal from the comparator  104  is inverted to the low level as shown in  FIG. 5C , and the mode is switched to the non-destructive read as shown in  FIG. 5B . Signals are therefore sequentially read out by the non-destructive operations B 8 , B 9 , B 10 , . . . 
     An output signal of the comparator  112  is also supplied via the delay  113  to the timing conversion circuit  120 . The timing conversion circuit  120  is constituted of a delay circuit for delaying the timing by one frame. As shown in  FIG. 5D , a signal output from the timing conversion circuit  120  is delayed by one frame from the mode switching signal ( FIG. 5C ). The signal output from the timing conversion circuit  120  is supplied to the switch  119  so that the switch  119  is turned to the side b. Namely, as shown in  FIG. 5E , the corrected value obtained by subtracting the output value of the previous frame from the current output value is not provided as a normal value for the frame following the normal read. Namely, the corrected value for the frame following the normal read is (B 8 −A 1 ), but this is not provided as a normal value. 
     In this embodiment, therefore, the switch  119  is turned to the side b so that the corrected value for the previous frame stored ih.the one-frame delay memory  118  is output as shown in  FIG. 5D . Since the previous corrected value (A 1 −B 7 ) is stored in the one-frame delay memory  118 , the previous corrected value (A 1 −B 7 ) is output for the frame following the normal read as shown in  FIG. 5E . In this case, therefore, although the corrected value (A 1 −B 7 ) is output twice, the quality of the whole image is not considerably influenced. 
     Similarly thereafter, a signal is read out from the X-ray image sensing panel  111  during the non-destructive read, and the output value of the previous frame is subtracted from the current output value to output the corrected value. This operation is repeated. When the output value of the A/D converter  104  exceeds the reference value, the read mode is changed to the ordinary read mode, and the corrected value of the previous frame is output for the frame following the normal read. This operation is repeated to continuously sense a moving image. 
     As described above, in this embodiment, signals are sequentially read out in the non-destructive read, and the image signal is corrected by subtracting the output value of the previous frame from the current output value. It is therefore possible to correct FPN. In addition, even if an offset of an output amplifier of the X-ray image sensing panel changes with a power supply fluctuation, a temperature change or the like, a change in the offset hardly affects the corrected value. More specifically, a change in the offset is slower than the frame rate and is negligible. Therefore, as shown in  FIG. 5B , even if the output values such as those during the non-destructive reading B 8 , B 9 , B 10 , . . . are changed by the influence of the offset, the corrected output values are hardly influenced by the change in the offset. An image sensing apparatus resistant to offset or other fluctuation can be realized. 
     Furthermore, since the signals read out by the non-destructive read without reset are used, there is no influence of KTC noises and a high S/N ratio can be realized. For the corrected value for the frame following the normal read, the corrected value for the previous frame is used. Therefore, corrected values can be obtained continuously and the moving image can be sensed continuously. 
     Although not shown in  FIG. 1 , a counter may be connected to the output of the comparator  112  to count the number of times when the output of the A/D converter exceeds the reference value. When the count of this counter exceeds a reference value, the read mode may be changed from the non-destructive read mode to the normal read mode. In this case, it is possible to prevent the read mode from being more frequently switched. The previous corrected value is output for the frame following the normal read because the corrected value is not correct. However, in the above case, the read mode is not switched frequently so that the image quality can be improved. 
     In the embodiment described above, phosphor is used for converting an X-ray into visual light. Instead of phosphor, a general scintillator, i.e., a wave conversion body may be used. A photoelectric conversion element itself without phosphor may also be used if it can directly detect radiation waves and generate charges. 
     In the embodiment, although an X-ray is used, other radiation waves such as α, β and γ rays may also be used. 
     As described so far, according to the embodiment, signals are sequentially read out in the non-destructive read, and a difference between the output value of the previous frame and the current output value is output as the corrected value. It is therefore possible to correct FPN. In addition, even if an offset of an output amplifier of the X-ray image sensing panel changes with a power supply fluctuation, a temperature change or the like, a change in the offset hardly affects the corrected value and a good image quality can be realized. Furthermore, since the signals read by the non-destructive read without reset are used, there is no influence of KTC noises and a high S/N ratio can be realized. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.