in the following , the attached drawings will be referred to while the preferred embodiment of the radiation imaging apparatus of the present invention is described . incidentally , in the embodiment of the present invention , x - rays will be exemplified as radiations to be described . fig1 is a schematic view of an x - ray imaging system of the embodiment of the present invention . x - rays emitted from an x - ray tube 501 irradiates an object 507 . the object 507 is mainly a patient . the x - rays which have penetrated the patient is converted to visible light by an x - ray visible conversion phosphor 502 . the visible light from the phosphor 502 is converted to electric signals by a photoelectric conversion device 503 . consequently , the x - ray penetration image of the subject 507 ( patient ) is converted into the electric signals . the x - ray visible conversion phosphor 502 and the photoelectric conversion device 503 are configured to adhere to each other substantially by being bonded with each other , and both of them are integrally called as an x - ray detector . an x - ray power source 504 supplies a high voltage for accelerating electrons in the x - ray tube 501 , and the x - ray power source 504 is called as an x - ray generation device together with the x - ray tube 501 . an image processor 505 is the so - called computer provided with the function of recording x - ray image information , which is converted electric signals , the function of operating obtained image data , the function of generating control signals for operating the x - ray detector , the function of controlling the x - ray generation device , and the function of making a cathode ray tube ( crt ) display 506 display images . the x - ray imaging system of the present embodiment configured to include the x - ray generation device composed of the x - ray power source 504 and the x - ray tube 501 ; the x - ray imaging apparatus composed of the x - ray detector composed of the x - ray visible conversion phosphor 502 and the photoelectric conversion device 503 , and image processor 505 ; and the crt display 506 as a display device . the x - ray imaging apparatus of the present embodiment makes the x - ray tube 501 generate x - rays in pulses , and obtains a plurality of sheets of image information of the patient continuously in time with the x - ray detector . after that , the x - ray imaging apparatus displays radiographed data on the crt display 506 as a dynamic image with the image processor 505 . the feature of the present invention is that the tube voltage of the x - ray tube 501 for obtaining an image f ( 2m − 1 ) of an odd frame ( 2m − 1 ) and the tube voltage of the x - ray tube 501 for obtaining an image f ( 2m ) of an even frame ( 2m ), where m indicates a natural number ( that is similarly supposed in the following ), are set to be different from each other . that is , odd frame images and even frame images are radiographed by the irradiation of pulsed x - rays with the tube voltages being alternatively switched . the image processor 505 performs the subtraction processes ( energy subtraction processes ) between the odd frame images and the even frame images while displaying dynamic images . incidentally , the subtraction processes are not always simple subtractions . in the following , the processes will be concretely described . the image density of a costa component obtained by the radiography of an odd frame with a tube voltage v 1 is denoted by d 1 ( v 1 ), and the blood vessel density is denoted by d 2 ( v 1 ). similarly , the image density of the costa component obtained by the radiography of an even frame with a tube voltage v 2 is denoted by d 1 ( v 2 ), and the blood vessel density is denoted by d 2 ( v 2 ). in case of a costa density ratio d 1 ( v 2 )/ d 1 ( v 1 )= 1 , costa shadows can be removed by performing the simple subtraction process f ( 2m )− f ( 2m − 1 ) in the subtraction process . however , bone components ( though the following is not limited to the bone components ) absorb different amounts of x - rays when the energy of the x - rays is changed . consequently , a difference is produced between the image densities d 1 ( v 1 ) and d 1 ( v 2 ). that is , the costa density ratio d 1 ( v 2 )/ d 1 ( v 1 ) is not one . accordingly , when the costa density ratio d 1 ( v 2 )/ d 1 ( v 1 ) is set as d 1 ( v 2 )/ d 1 ( v 1 )= k 1 , the costa shadows can be removed by performing a subtraction process f ( 2m )−{ k 1 × f ( 2m − 1 )} in the subtraction process . on the other hand , because blood vessels have tissues ( compositions ) different from those of the costae , the density ratio of a blood vessel d 2 ( v 2 )/ d 2 ( v 1 ) becomes k 2 ≠ k 1 . consequently , even if the subtraction process f ( 2m )−{ k 1 × f ( 2m − 1 )} is performed , the image of the blood vessel does not disappear to be displayed . incidentally , the subtraction process mentioned above is one of subtracting f ( 2m − 1 ) from f ( 2m ) ( k = 1 ). for example , in case of k 1 = 1 . 5 , an image of a treble of f ( 2m − 1 ) may be subtracted from an image of a double of f ( 2m ). that is , even if the subtraction process is performed between an image of operated f ( 2m ) and an image of operated f ( 2m − 1 ), the same result can be obtained . the chest simple x - ray radiography can expose a plurality of tissues , such as an esophagus , a trachea , pulmonary blood vessels , alveoli , a heart , heart blood vessels , a diaphragm , costae and clavicles , by x - ray radiography for a radiograph at one time . the subtraction process may be performed not for removing a certain shadow , but for decreasing the shadows of a plurality of tissues . at that time , as the subtraction process , there is a case where the image of operated f ( 2m − 1 ) is subtracted from the image of operated f ( 2m ). moreover , although the example of the removal of the shadow of a costa has been described in the above example , a subtraction process for removing the shadow of a blood vessel may be preformed , and a subtraction operation is selected according to the tissue and the focus , both being wanted to be observed . in the present invention , when the subtraction process is expressed as f ( 2m )− f ( 2m − 1 ), images are continuously displayed on the crt display 506 in such a manner as “ f ( 2 )− f ( 1 )”→“ f ( 4 )− f ( 3 )”→“ f ( 6 )− f ( 5 )”→ . . . “ f ( 2m )− f ( 2m − 1 )” . . . . a doctor can observe the images as a dynamic image while performing a diagnosis . energy subtraction images have the merit of the removal or the decrease of the shadows of bone portions such as costae and clavicles in case of , for example , chest x - ray radiography . moreover , by observing the movements of a patient ( such as the movements of the diaphragm and the lung field portion owing to breathing , and the movements of the heart ) as a dynamic image , there is the case where foci lurking in the shadows of ( behind ) costae , clavicles , the diaphragm , the heart and the like are revealed at the processes of the movements . consequently , the detection ratio of the foci becomes higher . the energy subtraction images are useful not only in case of the chest x - ray radiography , but also in the detection of , for example , the abnormalities of joints composed of bones and tendons ( muscles ). in addition , the x - ray radiographing element of the present invention obtains not only a sheet ( a time ) of a still image , but also a plurality of images , and enables a doctor to observe the movements as the dynamic images of the obtained images . consequently , the possibility of detecting foci which are difficult to be found from still images in the movements of the body of the patient becomes higher . on the other hand , there is also the case where even the shadow of a normal tissue which is to be detected as a focus in the shadows of a still image can be judged as a normal tissue by the observation of the movements of the tissue with the x - ray apparatus of the present invention , and the precision of a diagnosis is improved . because the present invention performs the subtraction process between odd frames and even frames while displaying in case of the radiographing frame rate of fr 1 ( sheets / second ), the frame rate of display is fr 1 / 2 ( sheets / second ). fig2 is a two - dimensional circuit diagram showing the photoelectric conversion device 503 of the x - ray imaging apparatus of the embodiment of the present invention . for simplifying the descriptions , fig2 shows a photoelectric conversion circuit unit 701 to have pixels of 3 × 3 = 9 . hereupon , s 1 - 1 to s 3 - 3 denote metal insulator semiconductor ( mis ) type photoelectric conversion elements ; t 1 - 1 to t 3 - 3 denote switching elements ( thin - film transistors ( tft &# 39 ; s )); g 1 to g 3 denote gate driving wiring for turning on or off the tft &# 39 ; s t 1 - 1 to t 3 - 3 ; m 1 to m 3 denote matrix signal wiring ; and a vs line denotes bias wiring for supplying an accumulation bias to the photoelectric conversion elements s 1 - 1 to s 3 - 3 . the photoelectric conversion elements s 1 - 1 to s 3 - 3 each include a gate ( g ) electrode , which is one on the side painted out into black , and a drain ( d ) electrode on the opposite side of the g electrode . the d electrodes are commonly used as parts of the bias wiring ( vs line ). from the need of the incidence of light , thin n + layers are used as the d electrodes . the photoelectric conversion elements s 1 - 1 to s 3 - 3 , the tft &# 39 ; s t 1 - 1 to t 3 - 3 , the gate driving wiring g 1 to g 3 , the matrix signal wiring m 1 to m 3 and the bias wiring ( vs line ) are collectively called as the photoelectric conversion circuit unit 701 . the bias wiring ( vs line ) is biased by a bias power source . vs . sr 1 denotes a shift register for supplying pulse voltages for driving to the gate driving wiring g 1 to g 3 . to the driving circuit unit ( the shift register sr 1 ), a voltage vg ( on ) for turning on the tft &# 39 ; s t 1 - 1 to t 3 - 3 and a voltage vg ( off ) for turning off the tft &# 39 ; s t 1 - 1 to t 3 - 3 are supplied from the outside . a reading circuit unit 707 reads parallel signal outputs from the photoelectric circuit unit 701 , and serially converts the read signal outputs to output the converted signals . a 1 to a 3 designates operational amplifier , the inverting terminals (−) of which are connected with the matrix signal wiring m 1 to m 3 , respectively . capacitative elements cf 1 to cf 3 are connected between the inverting terminals (−). and output terminals of the operational amplifiers a 1 to a 3 , respectively . the capacitative elements cf 1 to cf 3 convert the singles of the photoelectric conversion elements s 1 - 1 to s 3 - 3 to voltage quantities by integrating the currents flowing through the capacitative elements cf 1 to cf 3 when the tft &# 39 ; s t 1 - 1 to t 3 - 3 are turned on , respectively . res 1 to res 3 denote switches for resetting the capacitative elements cf 1 to cf 3 to a reset bias v ( reset ). the switches res 1 to res 3 are connected to the capacitative elements cf 1 to cf 3 in parallel , respectively . in fig2 , the reset bias v ( reset ) is expressed as 0 v , or the ground potential gnd . cl 1 to cl 3 denote sample hold capacitors for temporarily storing the signals accumulated in the operational amplifiers a 1 to a 3 and the capacitative elements cf 1 to cf 3 . sn 1 to sn 3 denote switches for performing sample hold . b 1 to b 3 denote buffer amplifiers . sr 1 to sr 3 denote switches for converting parallel signals into serial signals . sr 2 denotes a shift register for supplying pulses for the serial conversions to the switches sr 1 to sr 3 . ab denotes a buffer amplifier for outputting the converted serial signals . sw - res denotes a switch for resetting the non - inverting terminals of the operational amplifiers a 1 to a 3 to a reset bias v ( reset ) ( 0 v in fig2 ), and sw - ref denotes a switch for refreshing the non - inverting terminals of the operational amplifiers a 1 to a 3 to a refresh bias v ( refresh ). these switches are controlled by a signal “ refresh ”. when the signal “ refresh ” is a high level “ hi ”, the switch sw - ref is turned on . when the signal “ refresh ” is a low level “ lo ”, the switch sw - res is turned on . that is , the reading circuit unit 707 is configured in order that the switches sw - res and sw - ref may not be turned on at the same time . fig3 is a time chart showing the operation of the photoelectric conversion device 503 shown in fig2 . fig3 shows the operation of the photoelectric conversion device 503 for two frames . in fig3 , the x - ray pulses are shown as if the x - ray pulses have the same amplitudes in a first photoelectric conversion period and a second photoelectric conversion period for the sake of convenience , but the energy of the x - rays is different in the first photoelectric conversion period and in the second photoelectric conversion period in the present invention . in the radiography of a dynamic image , the operation shown in the time chart of fig3 is continuously repeated according to the number of sheets of radiographs . the tube voltages of the x - ray tube 501 are switched in order that the energy of the x - rays corresponding to an odd frame and an even frame may be different from each other . next , the operation of the photoelectric conversion device 503 shown in fig2 will be described using the time chart of fig3 . first , a photoelectric conversion period will be described . the d electrodes of all of the photoelectric conversion elements s 1 - 1 to s 3 - 3 is in a biased state by the bias power source vs ( to positive electric potential ). all of the signals of the shift register sr 1 are at the low level “ lo ”, and all of the switching tft &# 39 ; s t 1 - 1 to t 3 - 3 are in the off state . when an x - ray source is turned on as a pulse in this state , light irradiates the d electrodes ( n + electrodes ) of the respective photoelectric conversion elements s 1 - 1 to s 3 - 3 , and carriers of electrons and holes are generated in the i - layers of the photoelectric conversion elements s 1 - 1 to s 3 - 3 . hereupon , the generated electrons move to the d electrodes owing to the bias wiring ( vs line ), but the holes are accumulated in interfaces between the i - layers and insulating layers in the photoelectric conversion elements s 1 - 1 to s 3 - 3 to be kept after the turning off of the x - ray . successively , a read period will be described . reading operation is preformed in the order of the photoelectric conversion elements s 1 - 1 to s 1 - 3 at the first row , next the photoelectric conversion elements s 2 - 1 to s 2 - 3 at the second row , and then the photoelectric conversion elements s 3 - 1 to s 3 - 3 at the third row . for reading the photoelectric conversion elements s 1 - 1 to s 1 - 3 at the first row first , the shift register sr 1 supplies a gate pulse to the wiring g 1 for driving the gates of the tft &# 39 ; s t 1 - 1 to t 1 - 3 . at this time , the high level of the gate pulse is the voltage vg ( on ) supplied from the outside . thereby , tft &# 39 ; s t 1 - 1 to t 1 - 3 are turned on , and the signal charges stored in the photoelectric conversion elements s 1 - 1 to s 1 - 3 flows through the tft &# 39 ; s t 1 - 1 to t 1 - 3 as currents . the currents flow into the capacitative elements cf 1 to cf 3 connected to the operational amplifiers a 1 to a 3 , respectively , and the currents are integrated by the capacitative elements cf 1 to cf 3 , respectively . reading capacitors , though they are not particularly shown in fig2 , are added to the matrix signal wiring m 1 to m 3 , and the signal charges are transmitted to the read capacitors on the matrix signal wiring m 1 to m 3 side through the tft &# 39 ; s t 1 - 1 to t 1 - 3 , respectively . however , because the matrix signal wiring m 1 to m 3 is imaginarily grounded by the reset bias gnd of the non - inverting terminals (+) of the operational amplifiers a 1 to a 3 , respectively , no changes of electric potential are caused by the transmission operation , and the electric potential of the matrix signal wiring m 1 to m 3 is in the state of being held at the ground potential level gnd . that is , signal charges are transferred to the capacitative elements cf 1 to cf 3 . the output terminals of the operational amplifiers a 1 to a 3 change as shown in fig3 according to the signal quantities of the photoelectric conversion elements s 1 - 1 to s 1 - 3 , respectively . hereupon , because the tft &# 39 ; s t 1 - 1 to t 1 - 3 are turned on at the same time , the outputs of the operational amplifiers a 1 to a 3 simultaneously change . that is , the outputs are parallel outputs . in the state , the tuning on of a signal “ smpl ” transfers the output signals of the operational amplifiers a 1 to a 3 to the sample hold capacitors cl 1 to cl 3 , respectively . then , the turning off of the signal “ smpl ” temporarily holds the output signals in the sample hold capacitors cl 1 to cl 3 , respectively . successively , the shift register sr 2 applies pulses to the switches sr 1 , sr 2 and sr 3 in the order . thereby the signals held by the sample hold capacitors cl 1 to cl 3 are output from the amplifier ab in the order of the sample hold capacitors cl 1 , cl 2 and cl 3 . as a result , the photoelectric conversion signals of the photoelectric conversion elements s 1 - 1 , s 1 - 2 and s 1 - 3 for one row are sequentially serially converted to be output . in the above , the read operation of the photoelectric conversion elements s 1 - 1 , s 1 - 2 and s 1 - 3 at the first row has been described . the read operation of the photoelectric conversion elements s 2 - 1 to s 2 - 3 at the second row and the read operation of the photoelectric conversion elements s 3 - 1 to s 3 - 3 at the third row are similarly performed . the performance of the sample hold of the signals of the operational amplifiers a 1 to a 3 into the sample hold capacitors cl 1 to cl 3 , respectively , by the signal smpl for the first row outputs the signals of the photoelectric conversion elements s 1 - 1 to s 1 - 3 from the photoelectric conversion circuit unit 701 . consequently , the refresh operation of the photoelectric conversion elements s 1 - 1 to s 1 - 3 in the photoelectric conversion circuit unit 701 and the reset operation of the capacitative elements cf 1 to cf 3 can be performed while the signals held in the sample hold capacities cl 1 to cl 3 are serially converted by the switches sr 1 to sr 3 to be output in the read circuit unit 707 . the refresh operation of the photoelectric conversion elements s 1 - 1 to s 1 - 3 is performed by the turning on of the switch sw - ref by the turning of the signal “ refresh ” to the high level “ hi ”, and by making the switches res 1 to res 3 be in their conductive state with a signal “ rc ”, and further by the application of the voltage vg ( on ) to the gate driving wiring g 1 of the tft &# 39 ; s t 1 - 1 to t 1 - 3 . that is , the refresh operation refreshes the g electrodes of the photoelectric conversion elements s 1 - 1 to s 1 - 3 to the refresh bias v ( refresh ). after that , the photoelectric conversion elements s 1 - 1 to s 1 - 3 transition to reset operation . the reset operation is performed by turning the signal “ refresh ” to the low level “ lo ” in the state in which the voltage vg ( on ) is applied to the gate driving wiring g 1 of the tft &# 39 ; s t 1 - 1 to t 1 - 3 , and in the state in which the switches res 1 to res 3 are in the conducted state . by the operation , the g electrodes of the photoelectric conversion elements s 1 - 1 to s 1 - 3 is reset to the reset bias v ( reset ), which is the ground potential level gnd , and at the same time the signals accumulated in the capacitative elements cf 1 to cf 3 are reset . after the completion of the reset operation , next the gate pulse of the gate driving wiring g 2 can be applied . that is , during the operation of the serial conversion of the signals of the first row by the shift register sr 2 , the photoelectric conversion elements s 1 - 1 to s 1 - 3 are refreshed at the same time , and the capacitative elements cf 1 to cf 3 are reset . then , the signal charges of the second photoelectric elements s 2 - 1 to s 2 - 3 at the second row can be transferred to the matrix signal wiring m 1 to m 3 , respectively , by the shift register sr 1 . by the operation described above , the signal charges of all of the photoelectric conversion elements s 1 - 1 to s 3 - 3 from the first row to the third row can be output . then , the operation of one frame is repeated a plurality of times , and thereby a dynamic image can be obtained . fig4 is the pattern wiring diagram of the photoelectric conversion circuit unit 701 . fig4 shows mis type photoelectric conversion elements and switching elements , both made of an amorphous silicon semiconductor thin film as their material . in addition , fig4 also shows wiring for connecting the photoelectric conversion elements and the switching elements . moreover , fig5 is a sectional view taken along a line 5 - 5 of the photoelectric conversion circuit unit 701 shown in fig4 . in the following descriptions , the mis type photoelectric conversion elements are simply called as photoelectric conversion elements for simplification . the photoelectric conversion elements 101 and the switching elements 102 ( the amorphous silicon tft &# 39 ; s , which will be simply referred to as tft &# 39 ; s ) are formed on the same insulation substrate 103 . the lower part electrodes of the photoelectric conversion elements 101 are configures with first metal thin film layers 104 , which are the same ones of the lower electors ( gate electrodes ) of the tft &# 39 ; s 102 to be used commonly . the upper electrodes of the photoelectric conversion elements 101 are configures with second metal thin film layers 105 , which are the same ones of the upper electors ( source electrodes and drain electrodes ) of the tft &# 39 ; s 102 to be used commonly . moreover , the first and the second metal thin film layers 104 and 105 are commonly used with gate driving wiring 106 and matrix signal wiring 107 in the photoelectric conversion circuit unit 701 . in fig4 , four pixel units ( 2 × 2 ) are shown as pixels . hatched portions in fig4 are light receiving surfaces of the photoelectric conversion elements 101 . a reference numeral 109 denotes a power source line for feeding a bias to the photoelectric conversion elements 101 . moreover , a reference numeral 110 denotes a contact hole portion for the connection of each of the photoelectric conversion elements 101 and each of the tft &# 39 ; s 102 . the use of the configuration , which is shown in fig4 and uses amorphous silicon semiconductor as its main material , makes it possible to produce the photoelectric conversion elements 101 , the switching elements 102 , the gate driving wiring 106 and the matrix signal wiring 107 simultaneously on the same substrate ( the insulation substrate 103 ). consequently , the large sized photoelectric conversion circuit unit 701 can easily and inexpensively be provided . next , the device operation of a unit of the photoelectric conversion elements 101 will be described . fig6 a , 6b and 6 c are energy band diagrams for illustrating the device operation of the photoelectric conversion elements 101 shown in fig4 and 5 . hereupon , fig6 a shows the operation in a refresh mode ; fig6 b shows the operation in a photoelectric conversion mode ; fig6 c shows the operation in a saturated state . the abscissas of fig6 a to 6 c indicate the state in the film thickness direction of each layer shown in fig5 . me 1 denotes the lower electrode ( g electrode ) formed of the first metal thin film layer 104 ( e . g . cr ). an amorphous silicon nitride ( a - sinx ) insulation thin film layer 111 is an insulating layer for obstructing both of electrons and holes . the layer 111 should have a thickness thicker than one at which a tunnel effect is generated , and consequently the thickness thereof is generally set to be 50 nm or more . an amorphous silicon hydride ( a - si : h ) semiconductor thin film layer 112 is a photoelectric conversion semiconductor layer formed of an intrinsic semiconductor layer ( i - layer ) in which no dopants are intentionally doped . an n + layer 113 is an obstruction layer of the injection of simple conductivity type carriers , which layer is formed of non - single crystal semiconductor such as n - type a - si : h layer provided for obstructing the injection of holes into the a - si : h semiconductor thin film layer 112 . moreover , me 2 indicates the upper electrode ( d electrode ) formed of the second metal thin film layer 105 ( e . g . a 1 ). although the d electrodes 105 do not cover the n + layers 113 completely in fig5 , because electron movements are freely performed between the d electrodes 105 and the n + layers 113 , the d electrodes 105 and the n + layers 113 always have the same electric potential . in the following descriptions , the fact is premised . each of the photoelectric conversion elements 101 has two kinds of operation modes of the refresh mode and a photoelectric conversion mode on the methods of the application of voltages onto the d electrodes 105 and the g electrodes . in fig6 a showing the refresh mode , negative electric potential is given to a d electrode 105 to a g electrode . holes denoted by black circles in the i - layer ( a - si : h ) are led to the d electrode by the electric field . at the same time , electrons denoted by white circles are injected into the i - layer ( a - si : h ). at this time , a part of the holes and the electrons are recombined with each other in the n + layer and the i - layer ( a - si : h ) to disappear . if this state continues for a sufficiently long time , the holes in the i - layer ( a - si : h ) are swept out from the i - layer ( a - si : h ). for changing the state of the photoelectric conversion element 101 to the photoelectric conversion mode shown in fig6 b from the aforesaid refresh mode , positive electric potential is given to the d electrode to the g electrode . the application of the positive electric potential instantaneously leads the electrons in the i - layer ( a - si : h ) to the d electrode . however , because the n + layer works as the injection obstruction layer , the holes are not led into the i - layer ( a - si : h ). when light enters into the i - layer ( a - si : h ) in this state , the entered light is absorbed to generate electron - hole pairs . those electrons are leaded to the d electrode by the electric field , and those holes move in the i - layer ( a - si : h ) to reach the interface between the i - layer ( a - si : h ) and the a - sinx insulation thin film layer . however , because the holes cannot move into the inside of the a - sinx insulation thin film layer , the holes stay in the i - layer ( a - si : h ) at this time , because the electrons move to the d electrode and the holes move to the a - sinx insulation thin film interface in the i - layer ( a - si : h ), a current for keeping the electric neutrality in the photoelectric conversion element 101 flows from the g electrode . the current is in proportion to the entered light because the current corresponds to the electron - hole pairs generated by the light . after the state of the photoelectric conversion mode shown in fig6 b has been kept for a certain period , when the photoelectric conversion element 101 again returns to the refresh mode shown in fig6 a , the holes staying in the i - layer ( a - si : h ) are led to the d electrode as described above , and at the same time a current corresponding to the holes flows . the quantity of the holes corresponds to the total quantity of the light entered during the photoelectric conversion period . at this time , a current corresponding to the quantity of the electrons injected into the i - layer ( a - si : h ) also flows , but the quantity of the current is almost a constant and then the total current can be obtained by subtracting the current corresponding to the flow of the quantity of the electrons from the current corresponding to the flow of the holes . that is , the photoelectric conversion element 101 outputs the quantity of entered light at real time , and at the same time the element 101 can detect the total quantity of the light entered for a certain period . however , in the case where the period of the photoelectric conversion mode becomes long owing to some reasons , or in the case where the illuminance of entered light is strong , there is the case where no currents flow even if light enters . the phenomenon is caused by numerous holes staying in the i - layer ( a - si : h ) as shown in fig6 c , which holes make the electric filed in the i - layer ( a - si : h ) small to prevent generated electrons from being led to the d electrode , so that the generated electrons are recombined with the holes in the i - layer ( a - si : h ). the state is called as the saturated state of the photoelectric conversion element 101 . there is a case where a change of the incidence state of the light flows currents unstably . in such a case , a second change of the mode of the photoelectric conversion element 101 to the refresh mode shown in fig6 a sweeps out the holes in the i - layer ( a - si : h ), and enables a second current flow in proportion to the light in the next photoelectric conversion mode shown in fig6 b . moreover , although the all of the holes are ideally swept out in the sweeping of the holes in the i - layer ( a - si : h ) in the refresh mode in the above description , only the sweeping of a part of the holes does not causes any problems because the partial sweeping also brings about an effect to make it possible to obtain the same current as that describe above . that is , it is necessary not to be in the saturated state shown in fig6 c in a detection chance in the next photoelectric conversion mode , and it is necessary to determine the electric potential of the d electrode to the g electrode in the refresh mode , the period of the refresh mode and the characteristic of the injection obstruction layer of the n + layer . moreover , further injection of electrons into the i - layer ( a - si : h ) in the refresh mode is not a necessary condition , and the electric potential of the d electrode to the g electrode is not limited to be negative . the reason is that , when many holes are staying in the i - layer ( a - si : h ), the electric field in the i - layer ( a - si : h ) is produced to be in the direction of leading the holes to the d electrode even if the electric potential of the d electrode to the g electrode is positive potential . moreover , it is also not a necessary condition of the characteristic of the n + layer , which is the injection obstruction layer , to be able to inject electrons into the i - layer ( a - si : h ) similarly . in the embodiment of the radiation imaging apparatus of the present invention , a subtraction process may be performed after a gradation conversion process or an edge enhancement process is performed as the need arises to a ( 2m − 1 ) th odd image or a 2mth even image in advance at the time of the performance of the subtraction process . moreover , a material including gd 2 o 2 s , gd 2 o 3 , csi or the like as the principal component can be used as the material of the x - ray visible conversion phosphor 502 . moreover , the mis type sensors are exemplified as the photoelectric conversion elements , but positive - intrinsic - negative ( pin ) type sensors may be used as the photoelectric conversion elements . moreover , the photoelectric conversion elements may be formed from lead iodide , mercuric iodide , selenium , cadmium telluride , gallium arsenide , gallium phosphide , zinc sulfide , silicon and the like , and the radiations which have penetrated the subject 507 may be directly converted into electric signals without using the x - ray visible conversion phosphor 502 as the x - ray detector . according to the present embodiment , x - rays having different energy from each other are alternately irradiated at the time of the producing of the ( 2m − 1 ) th odd image and at the time of the producing of the 2mth even image while radiographing , and a processed image obtained by performing a subtraction process between the odd image and the even image is displayed on the crt display 506 . consequently , the display of the parts where the degrees of changes are larger can be enhanced to be blacker or whiter against that of the parts where the degrees of changes are not so larger . thereby , for example , a doctor can recognize the parts where the degrees of changes are larger while performing the interpretation of a radiograph when the doctor performs a diagnosis . consequently , the detection rate of abnormal parts such as foci can be improved . moreover , in chest simple x - ray radiography , the shadows of bone portions such as costae and clavicles can be removed or decreased while foci lurking in the shadows of ( behind ) the costae , the clavicles , a diaphragm , a heart and the like , which cannot be obtained only from still images , can be revealed in the movement of the diaphragm and a lung field region owing to the breathing of a patient . consequently , the detection rate of abnormal parts such as foci can be improved , and a misdiagnosis rate can be decreased . moreover , the similar effects can be expected to the parts where a plurality of tissues having different absorption characteristics to x - ray energy such as joints composed of bones and tendons ( muscles ) as well as to the chest portion x - ray radiography . such digitalization in medical scenes makes it possible to heighten the efficiency of diagnoses of a doctor in a hospital and the efficiency of works for the management of the hospital . then , medical environments having higher qualities than those at present can be produced in a future aging society and an information technology ( it ) society . as many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof , it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the claims .
a computer - readable medium is encoded with a computer program for operating a computer to control a radiation detector and a display apparatus . the computer is operated to derive a processed image by subtraction processing of a first image derived by a first energy radiation having a first energy and transmitting through an object and a second image derived successively to the first image by a second energy radiation having a second energy different from the first energy and transmitting through the same object , among a plurality of images of successive frames detected by the radiation detector detecting as an electric signal the radiation transmitted through the object . the display is operated to display on the display apparatus successively a plurality of the processed images derived thereby .