WO2016194315A1

WO2016194315A1 - Radiation imaging apparatus, control method therefor, and storage medium

Info

Publication number: WO2016194315A1
Application number: PCT/JP2016/002377
Authority: WO
Inventors: Shinichi Takeda; Katsuro Takenaka; Atsushi Iwashita; Kosuke Terui
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-06-01
Filing date: 2016-05-16
Publication date: 2016-12-08
Also published as: JP2016223952A

Abstract

The present invention provides a radiation imaging apparatus comprising a scintillator configured to convert radiation into light, a detector including a plurality of pixels each configured to detect an intensity of light converted by the scintillator and configured to count, for each pixel, the number of times of detection of light at each of a plurality of levels concerning the intensity of light, and a processor configured to perform processing of generating an image based on the number of times of detection at each level obtained for each pixel, wherein the processor corrects the number of times of detection, for each pixel, at each level obtained by using information indicating a relationship between energy of radiation and a probability that the detector will count light at each level, and generates an image based on the corrected numbers of times of detection.

Description

RADIATION IMAGING APPARATUS, CONTROL METHOD THEREFOR, AND STORAGE MEDIUM

The present invention relates to a radiation imaging apparatus, a control method therefor, and a storage medium.

As an imaging apparatus used for medical imaging diagnosis or non-destructive inspection using radiation (X-rays), a radiation imaging apparatus using an FPF (Flat Panel Detector) formed from a semiconductor material has been known. Such a radiation imaging apparatus can be used as a digital imaging apparatus which generates still images, moving images, and the like in, for example, medical imaging diagnosis.

As FPDs, for example, an integration type sensor and a photon counting sensor are available. The integration type sensor measures the total amount of charge generated by the incidence of radiation. In contrast to this, the photon counting sensor identifies the energy (wavelength) of incident radiation and counts the number of times of detection of radiation at each of a plurality of energy levels. That is, the photon counting sensor has an energy resolution and hence can be improved in diagnostic ability as compared with the integration type sensor.

Photon counting sensors are proposed in, for example,

PTLs

1 and 2. PTL 1 has proposed a direct sensor which counts the number of times of detection of radiation by directly detecting the energy of radiation using CdTe. PTL 2 has proposed an indirect sensor which detects the intensity of light generated by a scintillator upon incidence of light and counts the number of times of detection of the light.

A single crystal of CdTe used for a direct sensor can be grown to only several cm squares. For this reason, it is difficult to form a large-area direct sensor, which costs very much. There is also available a method of implementing a large-area direct sensor by depositing amorphous Se. However, a sensor manufactured by this method has drawbacks such as low operation speed and the necessity of temperature management.

In contrast to this, an indirect sensor has advantages of facilitating the formation of a large area and being inexpensive. The indirect sensor, however, sometimes receives incident light with an intensity different from that corresponding to the energy of radiation incident on the scintillator. This makes it impossible to sufficiently identify the energy of radiation with high accuracy. This can lead to an error in the number of times of detection of light counted for each level concerning the intensity of light. As a result, an error can occur in an image generated based on the number of times of detection of light.

Japanese Patent Laid-Open No.2013-501226 Japanese Patent Laid-Open No. 2003-279411

The present invention provides, for example, a technique advantageous in reducing errors occurring in images generated based on the number of times of detection of light.

According to one aspect of the present invention, there is provided a radiation imaging apparatus comprising: a scintillator configured to convert radiation into light; a detector including a plurality of pixels each configured to detect an intensity of light converted by the scintillator and configured to count, for each pixel, the number of times of detection of light at each of a plurality of levels concerning the intensity of light; and a processor configured to perform processing of generating an image based on the number of times of detection at each level obtained for each pixel by the detector, wherein the processor corrects the number of times of detection, for each pixel, at each level obtained by the detector by using information indicating a relationship between energy of radiation and a probability that the detector will count light at each level, and generates an image based on the corrected numbers of times of detection.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1 is a schematic view showing a radiation imaging apparatus according to the first embodiment;

Fig. 2 is a block diagram showing the arrangement of a detector;

Fig. 3 is a block diagram showing the arrangement of each pixel;

Fig. 4 is a timing chart showing irradiation periods and readout periods;

Fig. 5 is a chart showing the operation of each pixel in an irradiation period;

Fig. 6 is a timing chart showing the operation of each pixel in a readout period;

Fig. 7A is a view for explaining the intensity of light detected by a light detector;

Fig. 7B is a view for explaining the intensity of light detected by the light detector;

Fig. 7C is a view for explaining the intensity of light detected by the light detector;

Fig. 8 is a view for explaining the intensity of light detected by the light detector;

Fig. 9 is a view showing the relationship between the energy of radiation and the probability that a detector will count light at each level;

Fig. 10 is a view for explaining the processing of correcting the number of times of detection of light; and

Fig. 11 is an equivalent circuit diagram of each pixel of a detector according to the second embodiment.

Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the same reference numerals denote the same members throughout the drawings, and a repetitive description thereof will not be given. In addition, assume that radiation in the present invention includes a-rays, b-rays, and g-rays which are beams generated by particles (including photons) emitted by radioactive decay, and beams having similar energies, such as X-rays, particle rays, and cosmic rays.

First Embodiment

A radiation imaging apparatus 100 (radiation imaging system) according to the first embodiment of the present invention will be described. Fig. 1 is a schematic view showing the radiation imaging apparatus 100 according to the first embodiment. The radiation imaging apparatus 100 according to the first embodiment can include an irradiation unit 101 which irradiates an object with radiation, a control unit 102 which controls the irradiation unit 101, an imaging unit 104 which images an object irradiated with radiation, and a processor 103. The control unit 102 and the processor 103 each can be formed from a computer including a CPU and a memory. In the first embodiment, the control unit 102 and the processor 103 are separately formed. However, this is not exhaustive. They can be integrally formed. That is, the control unit 102 and the processor103 may be formed from one computer having their functions.

The imaging unit 104 is a photon counting sensor including a scintillator 105 which converts incident radiation into light (for example, visible light) and a detector 106 which detects the intensity of light generated by the scintillator 105. The detector 106 is, for example, a sensor having a two-dimensional array (an array of X rows x Y columns) of a plurality of pixels 201 each configured to detect the intensity of light generated by the scintillator 105. This sensor counts the number of times of detection of light generated by the scintillator 105. The processor 103 performs the processing of generating an image to be displayed on, for example, a display (display unit) based on the number of times of detection of light counted by the detector 106 for each pixel. In addition, the processor 103 can include a correcting unit 130 which corrects the number of times of detection of light counted by the detector 106 for each pixel.

The arrangement of the detector 106 will be described next with reference to Fig. 2. Fig. 2 is a block diagram showing the arrangement of the detector 106. The detector 106 includes a plurality of pixels 201, a vertical shift register 202, a horizontal shift register 203, a vertical data line 204, a vertical selecting line 205, an output data line 206, a horizontal selecting line 207, and a horizontal selecting circuit 208. Each of the plurality of pixels 201 can be configured to count the number of times of detection of light generated by the scintillator 105 at each of a plurality of levels concerning the intensity of light. Each pixel 201 supplies the data of the number of times of detection at one of a plurality of levels which is selected by supplying a signal through the vertical selecting line 205 to the horizontal selecting circuit 208 via the vertical data line 204.

The vertical shift register 202 sequentially changes the vertical selecting lines 205 for supplying signals to output the data of the number of times of detection at a desired level from each pixel 201. Upon reception of a signal via the horizontal selecting line 207, the horizontal selecting circuit 208 outputs the data of the number of times of detection supplied from each pixel 201 to the output data line 206. The horizontal shift register 203 sequentially changes the horizontal selecting lines 207 for supplying signals to cause the plurality of horizontal selecting circuits 208 to sequentially output the data of the numbers of times of detection to the output data line 206. In this case, for the sake of simplicity, Fig. 2 shows the detector 106 having the pixels 201 arrayed in 3 rows x 3 columns. In practice, however, it is possible to use the detector 106 having an array of more pixels 201. For example, the 17-inch detector 106 (FPD) can have pixels 201 arrayed in about 2,800 rows x about 2,800 columns.

The arrangement of each pixel 201 will be described next with reference to Fig. 3. Fig. 3 is a block diagram showing the arrangement of each pixel 201. Each pixel 201 of the detector 106 includes a light detector 301 which detects light generated by the scintillator 105. Each pixel 201 can be configured to compare an output value from the light detector 301 with each of a plurality of reference values and count, as the number of times of detection of light at each of the plurality of levels, the number of times that output values equal to or more than each of the plurality of reference values are obtained. For example, each pixel 201 can be configured to include a differentiating circuit 302, a comparator 303, a counter 304, and a row selecting circuit 305, in addition to the light detector 301.

The light detector 301 is a photoelectric converter which detects light generated by the scintillator 105 upon incidence of light on the scintillator 105, and outputs an electrical signal. The differentiating circuit 302 converts the electrical signal output from the light detector 301 into a voltage pulse signal and outputs it. The comparator 303 compares the voltage value of the pulse signal output from the differentiating circuit 302 with a reference voltage 306 (reference value), and outputs a signal corresponding to the comparison result. If, for example, the voltage value of the pulse signal output from the differentiating circuit 302 is equal to or more than the reference voltage 306, the comparator 303 outputs the digital value "1" as a signal corresponding to the comparison result. In contrast to this, if the voltage value of the pulse signal output from the differentiating circuit 302 is less than the reference voltage 306, the comparator 303 outputs the digital value "0" as a signal corresponding to the comparison result. The reference voltage 306 supplied to the comparator 303 can be set as a value common to all the pixels 201 of the detector 106. The counter 304 counts the digital value "1" output from the comparator 303. That is, the counter 304 counts the number of times of detection of light generated by the scintillator 105 upon incidence of radiation on the scintillator 105. The row selecting circuit 305 supplies the data of the number of times of detection to the horizontal selecting circuit 208 via the vertical data line 204 upon reception of a signal from the vertical shift register 202 via the vertical selecting line 205 connected to the row selecting circuit 305. Upon reception of a signal via the horizontal selecting line 207, the horizontal selecting circuit 208 supplies the data of the number of times of detection to the processor 103.

In the arrangement shown in Fig. 3, each pixel 201 is provided with two comparators 303B and 303R, two counters 304B and 304R, and two row selecting circuits 305B and 305R. Reference voltages 306B and 306R having different values are respectively supplied to the two comparators 303B and 303R. In this manner, each pixel 201 in the detector 106 according to the first embodiment is provided with pluralities of comparators 303, counters 304, and row selecting circuits 305. The reference voltages 306 supplied to the respective comparators 303 are set to different values. This allows each pixel 201 to count the number of times of detection of light at each of a plurality of levels concerning the intensity of light.

In this case, the arrangement shown in Fig. 3 is provided with the two comparators 303, the two counters 304, and the two row selecting circuits 305. However, this is not exhaustive. The arrangement may be provided with three or more each of them. Assume that one pixel of a display for displaying an image generated by the processor 103 includes three sub-pixels of R (Red), G (Green), and B (Blue). In this case, each pixel 201 of the detector 106 may be configured to include three comparators 303, three counters 304, and three row selecting circuits 305.

Fig. 4 is a timing chart showing irradiation periods and readout periods. Referring to Fig. 4, an irradiation period is a period during which the irradiation unit 101 irradiates an object with radiation, and the detector 106 counts the number of times of detection of light, and a readout period is period during which the detector 106 outputs the data of the number of times of detection. As shown in Fig. 4, the detector 106 alternately counts the number of times of detection of light and outputs the data of the number of times of detection.

The operation of the pixel 201 having the arrangement shown in Fig. 3 in an irradiation period will be described next with reference to Fig. 5. Fig. 5 is a chart showing the operation of each pixel 201 in an irradiation period. The waveforms in Fig. 5 respectively represent an output from the differentiating circuit 302, an output from the comparator 303, and an output from the counter 304, with the abscissa representing the time. If the voltage value of a pulse signal output from the differentiating circuit 302 is equal to or more than the reference voltage 306R, the comparator 303R outputs the digital value "1". The counter 304R then counts the digital value "1" output from the comparator 303R, and outputs the count value as the number of times of detection. If the voltage value of a pulse signal output from the differentiating circuit 302 is equal to or more than the reference voltage 306B, the comparator 303B outputs the digital value "1". The counter 304B then counts the digital value "1" output from the comparator 303B, and outputs the count value as the number of times of detection. In this manner, the number of pulse signals output from the differentiating circuit 302 is counted for each of the plurality of reference voltages 306. This allows each pixel 201 to count the number of times of detection of light generated by the scintillator 105 upon incidence of light at each of a plurality of levels concerning the intensity of light.

The operation of the pixel 201 having the arrangement shown in Fig. 3 in a readout period will be described next with reference to Fig. 6. Fig. 6 is a timing chart showing the operation of each pixel 201 in a readout period. The waveforms in Fig. 6 respectively represent the supply of a signal to the vertical selecting line 205, the supply of a signal to the horizontal selecting line 207, and the output of the data of the number of times of detection from the horizontal selecting circuit 208, with the abscissa representing the time. As shown in Fig. 6, signals are sequentially supplied to the plurality of vertical selecting lines 205 and the plurality of horizontal selecting lines 207. For example, when the supply of a signal to a vertical selecting line 205-0R starts, the counter 304R of the pixel 201 connected to the vertical selecting line 205-0R supplies the data of the number of times of detection to the horizontal selecting circuit 208. In a period during which a signal is supplied to the vertical selecting line 205-0R, signals are sequentially supplied to the plurality of horizontal selecting lines 207, and the plurality of horizontal selecting circuits 208 are caused to sequentially output the data of the numbers of times of detection to the output data line 206.

When the supply of a signal to the vertical selecting line 205-0R ends and the supply of a signal to a vertical selecting line 205-0B starts, the counter 304B of the pixel 201 connected to the vertical selecting line 205-0B supplies the data of the number of times of detection to the horizontal selecting circuit 208. In a period during which a signal is supplied to the vertical selecting line 205-0B, signals are sequentially supplied to the plurality of horizontal selecting lines 207, and the plurality of horizontal selecting circuits 208 are caused to sequentially output the data of the numbers of times of detection to the output data line 206. This allows the plurality of pixels 201 on one row to supply the data of the numbers of times of detection at the respective levels to the processor 103 via the output data line 206. Sequentially supplying signals to the plurality of vertical selecting lines 205 and the plurality of horizontal selecting lines 207 in this manner allows the plurality of pixels 201 to supply the data of the numbers of times of detection at the respective levels to the processor 103 via the output data line 206.

In the radiation imaging apparatus 100 having the above arrangement, as shown in Fig. 7A, light 115 generated by the scintillator 105 upon incidence of a radiation quantum 110 is radiated inside the scintillator and incident on the light detector 301 of each pixel 201. At this time, the light 115 generated by the incidence of the radiation quantum 110 may be prevented from influencing the adjacent pixels 201. For this reason, each pixel 201 of the detector 106 may be configured to have a size larger than a range in which the light 115 generated by the scintillator 105 is diffused and incident on the detector 106. The light detector 301 of each pixel 201 may be configured to have a size smaller than that of the pixel 201 to prevent the incidence of light to be detected by the light detector 301 of each adjacent pixel. In addition, as the thickness of the scintillator 105 increases, the light 115 generated by the incidence of the radiation quantum 110 is diffused to enlarge the range in which the light is incident on the detector 106. For this reason, the thickness of the scintillator 105 may be determined in accordance with the size of each pixel 201. For example, this thickness may be smaller than the pitch of the plurality of pixels 201.

When the radiation imaging apparatus 100 has the above arrangement, however, light is sometimes incident on the light detector 301 with an intensity different from that corresponding to the energy of radiation incident on the scintillator 105 in each pixel 201. For example, in each pixel 201, depending on the position in the scintillator at which radiation is incident, the distance between the light-emitting position and the light detector 301 changes, and the intensity of light detected by the light detector 301 can change. In this case, it is impossible to satisfactorily detect the energy of radiation with high accuracy. This can cause an error in the number of times of detection of light counted at each level concerning the intensity of light. This can lead to an error in an image generated based on the numbers of times of detection of light.

Assume that the radiation quantum 110 with the same energy is incident at different positions in the scintillator 105, as shown in Figs. 7B and 7C. In this case, depending on the position in the scintillator at which the radiation quantum 110 is incident, the distance between the light detector 301 and the position of light emission caused by the incidence of the radiation quantum 110 can change. That is, a distance Lb between the light-emitting position and the light detector 301 in Fig. 7B is different from (longer than) a distance Lc between the light-emitting position and the light detector 301 in Fig. 7C. As a result, the intensity of light detected by the light detector 301 in Fig. 7B is different from that in Fig. 7C. That is, even with the radiation quantum 110 having the same energy, the detection of light is counted at different levels. In this case, the numerical values written in the respective regions in the scintillator in Figs. 7B and 7C represent levels concerning the intensity of light detected by the light detector 301 when light generated in the respective regions is incident on the light detector 301, and the broken lines represent boundaries.

Fig. 8 is a view showing a state in which the radiation quantum 110 having energy lower than that of radiation quantum 110 incident on the scintillator in Fig. 7B is incident on the scintillator 105. Referring to Fig. 8, the position of light emission caused by the incidence of the radiation quantum 110 is closer to the light detector 301 than in Fig. 7B. In this case, regardless of the fact that the energy of the radiation quantum 110 incident on the scintillator 105 in Fig. 8 is different from that in Fig. 7B, the levels at which the detection of the intensity of light is counted sometimes become the same.

If the intensity of light detected by the light detector 301 differs depending on the position on the scintillator 105 at which radiation is incident in this manner, an error can occur in the number of times of detection of light counted in accordance with the energy of radiation incident on the scintillator 105. As a result, an error can occur in an image generated based on the numbers of times of detection of light. For this reason, the radiation imaging apparatus 100 according to the first embodiment corrects for each pixel the numbers of times of detection of light obtained from the detector 106 (the plurality of pixels 201), based on information indicating the relationship between the energy of radiation and the probability that the detector 106 will count light at each level. A method of correcting the number of times of detection of light obtained from the detector 106 will be described below. The following description is about a case in which each pixel 201 of the detector 106 counts the detection of the intensity of light at three levels. In this case, each pixel 201 of the detector 106 is provided with the three comparators 303, the three counters 304, and the three row selecting circuits 305.

Information indicating the relationship between the energy of radiation and the probability that the detector 106 will count light at each level will be described first. As shown in Fig. 9, this information can include, for example, a table indicating the relationship between the energy of radiation incident on the scintillator 105 and the probability that light generated by the scintillator 105 upon incidence of the radiation will be counted as an intensity at each level. The information (table) shown in Fig. 9 indicates that a plurality of levels concerning the intensity of light include three levels (levels 1 to 3), and also indicates the probability that light will be counted at each level for each energy of radiation incident on the scintillator 105. In addition, referring to Fig.9, the energy of radiation at which the intensity of light should be detected at level 3 is expressed as "high", the energy of radiation at which the intensity of light should be detected at level 2 is expressed as "middle", and the energy of radiation at which the intensity of light should be detected at level 1 is expressed as "low".

When radiation having "high" energy is incident on the scintillator 105, the level at which the intensity of light detected by the light detector 301 is counted should be level 3. As described above, however, depending on the position in the scintillator at which radiation is incident, the level at which the intensity of light is counted sometimes becomes level 1 or level 2. Therefore, according to the information shown in Fig. 9, the probabilities that the intensity of light generated by the incidence of radiation having "high" energy on the scintillator 105 is counted are represented by AH₁ to AH₃. When radiation having "middle" energy is incident on the scintillator 105, the level at which the intensity of light detected by the light detector 301 is counted should be level 2. Likewise, when radiation having "low" energy is incident on the scintillator 105, the level at which the intensity of light detected by the light detector 301 is counted should be level 1. However, depending on the position in the scintillator at which radiation is incident, the level at which the intensity of light is counted sometimes becomes another level. Therefore, according to the information shown in Fig. 9, the probabilities that the intensity of light generated by the incidence of radiation having "middle" energy on the scintillator 105 is counted are represented by AM₁ to AM₃. The probabilities that the intensity of light generated by the incidence of radiation having "low" energy on the scintillator 105 is counted are represented by AL₁ to AL₃.

An example of a method of correcting the number of times of detection of light obtained for each pixel will be described next. Assume that the numbers of times of detection of light at a plurality of levels (levels 1 to 3) obtained from a given pixel are represented by N₁, N₂, and N₃. In this case, the correcting unit 130 of the processor 103 corrects the numbers of times N₁ to N₃ of detection based on the information shown in Fig. 9 to obtain corrected numbers of times XN₁, XN₂, and XN₃ of detection. More specifically, as shown in Fig. 10, the corrected numbers of times XN₁ to XN₃ of detection can be obtained by multiplying the numbers of times N₁ to N₃ of detection by the matrix of the respective probabilities in the table shown in Fig. 9. This processing is performed for each pixel. The processor 103 generates an image based on the corrected numbers of times XN₁ to XN₃ of detection obtained for each pixel. This process can correct the number of times of detection of light at each level and reduce an error occurring in an image generated based on the numbers of times of detection of light.

A method of obtaining information indicating the relationship between the energy of radiation and the probability that the detector 106 will count light at each level will be described below. For example, the processor103 causes radiation with constant energy to be incident on the scintillator 105 without through an object, and causes the detector 106 to count the number of times of detection at each level. The processor 103 then obtains the ratio of the number of times of detection at each level to the total sum of the numbers of times of detection at each level. This ratio can be used as the probability that the detector 106 will count light at each level. It is possible to obtain the information shown in Fig. 9 by performing such processing for each of a plurality of energy levels ("high", "middle", and "low") of radiation to be detected at a plurality levels. In addition, the energy of radiation to be incident on the scintillator 105 can be adjusted by adjusting a tube voltage, tube current, and irradiation time in the irradiation unit 101 and the material and thickness of an additional filter. In addition, the probability that light will be counted at each level can change depending on conditions such as the attenuation coefficient, thickness, and light transmittance of the scintillator 105. For this reason, the processor 103 may have a plurality of pieces of such information and select optimal information in accordance with the arrangement of the scintillator 105.

As described above, the radiation imaging apparatus 100 according to the first embodiment corrects the number of times of detection at each level obtained by the detector 106, for each pixel, based on information indicating the relationship between the energy of radiation and the probability that the detector 106 will count light at each level. This can reduce an error occurring in an image generated based on the numbers of times of detection of light.

Second Embodiment

A radiation imaging apparatus (radiation imaging system) according to the second embodiment of the present invention will be described. The radiation imaging apparatus according to the second embodiment has an arrangement similar to that of the radiation imaging apparatus 100 according to the first embodiment. Each pixel of a detector 106 is formed from only a circuit which detects radiation. The arrangement of each pixel of the detector 106 according to the second embodiment will be described below.

Fig. 11 is an equivalent circuit diagram of a pixel in the detector 106 according to the second embodiment. A pixel 40 of the detector 106 according to the second embodiment can include a photoelectric conversion element 401 and an output circuit unit 402. The photoelectric conversion element 401 can be typically a photodiode. The output circuit unit 402 can include an amplification circuit unit 404, a clamp circuit unit 405, a sample/hold circuit unit 407, and a selection circuit unit 408.

The photoelectric conversion element 401 includes a charge accumulation unit, which is connected to the gate of a MOS transistor 404a of the amplification circuit unit 404. The source of the MOS transistor 404a is connected to a current source 404c via a MOS transistor 404b. The MOS transistor 404a and the current source 404c constitute a source follower circuit. The MOS transistor 404b is an enable switch which is turned on to set the source follower circuit in an operating state when an enable signal EN supplied to the gate of the MOS transistor 404b is set at active level.

In the case shown in Fig. 11, the charge accumulation unit of the photoelectric conversion element 401 and the gate of the MOS transistor 404a form a common node. This node functions as a charge voltage conversion unit which converts the charge accumulated in the charge accumulation unit into a voltage. That is, a voltage V (= Q/C) determined by charge Q accumulated in the charge accumulation unit and a capacitance value C of the charge voltage conversion unit appears in the charge voltage conversion unit. The charge voltage conversion unit is connected to a reset potential Vres via a reset switch 403. When the reset signal PRES is set active level, the reset switch 403 is turned on to reset the potential of the charge voltage conversion unit to the reset potential Vres.

A clamp circuit unit 406 clamps noise output from the amplification circuit unit 404 in accordance with the potential of the reset charge voltage conversion unit by using a clamp capacitor 406a. That is, the clamp circuit unit 406 is a circuit for canceling this noise from the signal output from the source follower circuit in accordance with charge generated by the photoelectric conversion element 401 by photoelectric conversion. The noise can include kTC noise at the time of resetting. After a MOS transistor 406b is turned on by setting a clamp signal PCL at active level, the MOS transistor 406b is turned off by setting the clamp signal PCL at inactive level, thereby performing clamping. The output side of the clamp capacitor 406a is connected to the gate of a MOS transistor 406c. The source of the MOS transistor 406c is connected to a current source 406e via a MOS transistor 406d. The MOS transistor 406c and the current source 406e constitute a source follower circuit. The MOS transistor 406d is an enable switch which is turned on to set the source follower circuit in the operating state when an enable signal EN0 supplied to the gate of the MOS transistor 406d is set at active level.

A signal output from the clamp circuit unit 406 in accordance with charge generated by the photoelectric conversion element 401 by photoelectric conversion is written in a capacitor 407Sb via a switch 407Sa when an optical signal sampling signal TS is set at active level. A signal output from the clamp circuit unit 406 when the MOS transistor 406b is turned on immediately after the potential of the charge voltage conversion unit is reset is a clamp voltage. This noise signal is written in a capacitor 407Nb via a switch 407Na when a noise sampling signal TN is set at active level. This noise signal includes an offset component of the clamp circuit unit 406. The switch 407Sa and the capacitor 407Sb constitute a signal sample/hold circuit 407S. The switch 407Na and the capacitor 407Nb constitute a noise sample/hold circuit 407N. The sample/hold circuit unit 407 includes the signal sample/hold circuit 407S and the noise sample/hold circuit 407N.

When a driving circuit unit 41 drives a row selecting signal VST at active level, a signal (optical signal) held in the capacitor 407Sb is output to a signal line 45S via a MOS transistor 408Sa and a row selecting switch 408Sb. At the same time, a signal (noise) held in the capacitor 407Nb is output to a signal line 45N via a MOS transistor408Na and a row selecting switch 408Nb. The MOS transistor 408Sa and a constant current source (not shown) provided on the signal line 45S constitute a source follower circuit. Likewise, the MOS transistor 408Na and a constant current source (not shown) provided on the signal line 45N constitute a source follower circuit. The MOS transistor 408Sa and the row selecting switch 408Sb constitute a signal selecting circuit unit 408S. The MOS transistor 408Na and the row selecting switch 408Nb constitute a noise selecting circuit unit 408N. The selection circuit unit 408 includes the signal selecting circuit unit 408S and the noise selecting circuit unit 408N.

The pixel 40 may include an addition switch 409S which adds optical signals from the plurality of adjacent pixels 40. In the addition mode, an addition mode signal ADD is set at active level to turn on the addition switch 409S. With this operation, the capacitors 407Sb of the adjacent pixels are connected to each other via the addition switch 409S to average optical signals. Likewise, the pixel 40 may include an addition switch 409N which adds noise from the plurality of adjacent pixels 40. When the addition switch 409N is turned on, the capacitors 407Nb of the adjacent pixels are connected to each other via the addition switch 409N, thereby averaging noise. An addition unit 409 includes the addition switch 409S and the addition switch 409N.

The pixel 40 may include a sensitivity changing unit 405 for changing sensitivity. The pixel 40 can include, for example, a first sensitivity changing switch 405a, a second sensitivity changing switch 405a', and accompanying circuit elements. When a first changing signal WIDE is set at active level, the first sensitivity changing switch 405a is turned on to add the capacitance value of a first additional capacitor 405b to the capacitance value of the charge voltage conversion unit. This decreases the sensitivity of the pixel 40. When a second changing signal WIDE2 is set at active level, the second sensitivity changing switch 405a' is turned on to add the capacitance value of a second additional capacitor 405b' to the capacitance value of the charge voltage conversion unit. This further decreases the sensitivity of the pixel 40. Adding the function of decreasing the sensitivity of the pixel 40 in this manner can receive a larger amount of light and expand the dynamic range. When the first changing signal WIDE is set at active level, a MOS transistor 404a' may be caused to perform a source-follower operation, in place of the MOS transistor 404a, by setting an enable signal ENw at active level.

An output from the above pixel circuit is supplied to a processor 103 upon being converted into a digital value by an A/D converter (not shown). The processor 103 performs, on software, processing corresponding to a differentiating circuit 302, a comparator 303, and a counter 304. The processor 103 is provided with the correcting unit 130 described in the first embodiment.

First of all, as processing corresponding to the differentiating circuit 302, the processor 103 calculates a differential value of an output from the pixel circuit. As processing corresponding to the comparator 303, the processor 103 then compares the calculated differential value with a digital value corresponding to a reference voltage 306. If the differential value is equal to or more than the digital value corresponding to the reference voltage 306, the processor 103 obtains the digital value "1". If the differential value is smaller than the digital value corresponding to the reference voltage 306, the processor 103 obtains the digital value "0". As processing corresponding to the counter 304, the processor 103 then counts the obtained digital value "1". This makes it possible for the processor 103 to obtain the number of times of detection of light at each level.

Subsequently, the processor 103 (correcting unit 130) corrects the number of times of detection of light at each level based on information (see Fig. 9) indicating the relationship between the energy of radiation and the probability that light will be counted at each level. The processor 103 generates an image based on the corrected numbers of times of detection. These processes can be executed by, for example, the CPU of the processor 103. In addition, a storage area for storing the numbers of times of detection is ensured in a memory in the processor 103. There may be a plurality of digital values corresponding to the reference voltages 306 and a plurality of functions for performing processing corresponding to the comparator 303 in accordance with levels concerning the intensity of light.

Third Embodiment

The implementation of the differentiating circuit 302, the comparator 303, and the counter 304 according to the present invention is not limited to the form of being arranged in each pixel of the detector as in the first embodiment or the form of being all implemented on software as in the second embodiment. For example, the differentiating circuit 302 and the comparator 303 may be arranged in each pixel of the detector 106 while processing corresponding to the counter 304 may be performed on software. In addition, processing by the correcting unit 130 may be performed by a circuit provided outside the detector 106 instead of being performed on software. In this case, for example, this circuit may be formed from an FPGA. In addition, it is possible to adopt a form of providing a storage area in the detector 106 to store the information shown in Fig. 9, providing a circuit for correcting the numbers of times of detection based on the information, and supplying the corrected numbers of times of detection from the provided circuit to the processor 103.

Other Embodiments

Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a 'non-transitory computer-readable storage medium') to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)^TM), a flash memory device, a memory card, and the like.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2015-111679 filed on June 1, 2015, which is hereby incorporated by reference herein in its entirety.

Claims

A radiation imaging apparatus comprising:
a scintillator configured to convert radiation into light;
a detector including a plurality of pixels each configured to detect an intensity of light converted by the scintillator and configured to count, for each pixel, the number of times of detection of light at each of a plurality of levels concerning the intensity of light; and
a processor configured to perform processing of generating an image based on the number of times of detection at each level obtained for each pixel by the detector,
wherein the processor corrects the number of times of detection, for each pixel, at each level obtained by the detector by using information indicating a relationship between energy of radiation and a probability that the detector will count light at each level, and generates an image based on the corrected numbers of times of detection.
The apparatus according to claim 1, wherein the processor obtains the information by causing radiation having constant energy to be incident on the scintillator and causing the detector to count the number of times of detection at each level with respect to each of a plurality of energies of radiation to be detected at the plurality of levels.
The apparatus according to claim 1, wherein a size of one pixel of the detector is larger than a range in which light generated by the scintillator is diffused and incident on the detector.
The apparatus according to claim 3, wherein each of the plurality pixels includes a light detector, and
the light detector included in each of the plurality of pixels is configured to have a size smaller than that of the pixel to prevent incidence of light to be detected by the light detector of an adjacent pixel.
The apparatus according to claim 4, wherein each of the plurality of pixels compares an output value from the light detector with each of a plurality of reference values and counts the number of times that the output values not less than each of the plurality of reference values are obtained, as the number of times of detection of light at each of the plurality of levels.
The apparatus according to claim 5, wherein each of the plurality of pixels includes a plurality of comparators configured to compare the output value with each of the plurality of reference values and a plurality of counters configured to respectively count the numbers of times that the output values not less than the reference values are obtained, based on comparison results obtained by the plurality of comparators.
The apparatus according to claim 1, wherein a thickness of the scintillator is smaller than a pitch of the plurality of pixels.
The apparatus according to claim 1, further comprising an irradiation unit configured to irradiate an object with radiation,
wherein the processor generates the image based on the number of times of detection at each level obtained by causing the detector to detect the intensity of light generated by the scintillator upon incidence of radiation transmitted through the object.
A control method for a radiation imaging apparatus including a scintillator configured to convert radiation into light and a detector including a plurality of pixels each configured to detect an intensity of light converted by the scintillator, the method comprising:
counting, for each pixel, the number of times of detection of light detected by the detector at each of a plurality of levels concerning the intensity of light; and
correcting the number of times of detection, for each pixel, at each level obtained in the counting by using information indicating a relationship between energy of radiation and a probability that light will be counted in the counting at each level, and generating an image based on the corrected numbers of times of detection.
A non-transitory computer-readable storage medium storing a program for causing a computer in an information processing apparatus to execute a control method, the method controlling a radiation imaging apparatus including a scintillator configured to convert radiation into light and a detector including a plurality of pixels each configured to detect an intensity of light converted by the scintillator and comprising:
counting, for each pixel, the number of times of detection of light detected by the detector at each of a plurality of levels concerning the intensity of light; and
correcting the number of times of detection, for each pixel, at each level obtained in the counting by using information indicating a relationship between energy of radiation and a probability that light will be counted in the counting at each level, and generating an image based on the corrected numbers of times of detection.