Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/20—Measuring radiation intensity with scintillation detectors
  • G01T1/2006—Measuring radiation intensity with scintillation detectors using a combination of a scintillator and photodetector which measures the means radiation intensity
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
  • H04N25/30—Circuitry of solid-state image sensors [SSIS]; Control thereof for transforming X-rays into image signals
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
  • H10F39/10—Integrated devices
  • H10F39/12—Image sensors
  • H10F39/191—Photoconductor image sensors
  • H10F39/195—X-ray, gamma-ray or corpuscular radiation imagers

Definitions

• the present invention relates to a radiation imaging apparatus to be preferably used for diagnosis for medical care and industrial non-destructive inspection and its control method.
• electromagnetic waves such as X ray and Y ray and ⁇ ray and ⁇ ray are included in radiation.
• an X-ray digital imaging apparatus for performing imaging by using a converter such as a photoelectric transducer for converting light into an electrical signal and thereby using radiation such as X ray is practically used and spread in accordance with the advancement of semiconductor technology.
• the present invention it is possible to restrain the sensitivity change of a conversion element. Therefore, it is possible to restrain the fluctuation or image uncomfortable feeling of image luminance and obtain an image having a high image quality.
• Fig. 1 is a sectional view showing the structure of a pixel using an MIS-type photoelectric conversion element
• Fig. 2A is an energy band diagram for explaining the photoelectric conversion mode of an MIS-type photoelectric conversion element
• Fig. 2B is an energy band diagram for explaining the saturation state of an MIS-type photoelectric conversion element
• Fig. 2C is an energy band diagram for explaining the refresh mode of an MIS-type photoelectric conversion element
• Fig. 3 is a circuit diagram showing the configuration of one pixel of a two-dimensional sensor of an X-ray imaging apparatus
• Fig. 4 is a timing chart showing changes of potentials of points A and B in the pixel shown in Fig. 3;
• Fig. 5 is a circuit diagram showing the configuration of one pixel of a two-dimensional sensor of the radiation imaging apparatus (X-ray imaging apparatus) of first embodiment of the present invention
• Fig. 6 is a timing chart showing changes of potentials of points C and D in the pixel shown in Fig. 5;
• Fig. 7 is a circuit diagram showing the configuration of two-dimensional sensor in which nine pixels are arranged like a matrix
• Fig. 9 is a timing chart showing operations of the two-dimensional sensor 801 of the first embodiment.
• Fig. 10 is a flowchart showing the driving method of the two-dimensional sensor 801 of the first embodiment
• Fig. 11 is a circuit diagram showing the configuration of one pixel of a two-dimensional sensor of the radiation imaging apparatus (X-ray imaging apparatus) of second embodiment of the present invention.
• Fig. 12 is a timing chart showing changes of potentials of points E and F in the pixel shown in Fig. 11;
• Fig. 13 is a circuit diagram showing the configuration of a two-dimensional sensor in which nine pixels shown in Fig. 11 are arranged like a matrix and its peripheral circuits;
• Fig. 14 is a timing chart showing operations of a two-dimensional sensor 1002 of the second embodiment
• Fig. 15A is a timing chart showing an example of the relation between driving and output of a two- dimensional sensor when light is irradiated;
• Fig. 15B is a timing chart showing another example of the relation between driving and output of a two-dimensional sensor when light is irradiated;
• Fig. 15C is a timing chart showing another example of the relation between driving and output of a two-dimensional sensor when light is irradiated;
• Fig. 15D is a timing chart showing still another example of the relation between driving and output of a two-dimensional sensor when light is irradiated;
• a TFT 216 is formed on a glass substrate 201. Moreover, a gate line 220, gate insulating film 202, channel layer 203, N + amorphous silicon layer 204, drain electrode 205, and source electrode 206 are formed on the TFT 216. Furthermore, a signal line 219 for transferring an electrical signal output from the TFT 216 to a signal amplifying circuit is connected to the source electrode 206.
• MIS-type conversion element 217 serving as an MIS-type conversion element is also formed on a glass substrate. Moreover, a sensor bottom electrode layer 207, insulting layer 208, photoelectric conversion layer 209, N + amorphous silicon layer 210, transparent electrode 211, and sensor bias line 218 are formed on the MIS-type photoelectric conversion element 217. A voltage is supplied from the transparent electrode 211 and sensor bias line 218 to the photoelectric conversion layer 209.
• the N + amorphous silicon layer 210 has an ohmic contact with the photoelectric conversion layer 209 and transparent electrode 211, which is a layer for blocking implantation of positive holes from the sensor bias line 218.
• the phosphor protective layer 215 protects the phosphor 214 from humidity or the like. Then, the operational principle of the MIS-type photoelectric conversion element is described by referring to the energy band diagram of the MIS-type photoelectric conversion element shown in Figs. 2A to 2C.
• a positive voltage is applied to the sensor bias line 218 of the MIS-type photoelectric conversion element 217 and positive holes are accumulated.
• the photoelectric conversion mode when light 301 is irradiated to the photoelectric conversion layer 209, positive hole 303 and electron 302 are generated by the photoelectric effect in the photoelectric conversion layer 209. Then, the positive hole 303 is moved to the interface between the insulating layer 208 and photoelectric conversion layer 209 by an electric field and the electron 302 is moved to the N+ amorphous silicon layer 210 side.
• the positive hole 303 cannot pass through the insulting layer 208, it is accumulated on the interface between the photoelectric conversion layer 209 and the insulating layer 208.
• a voltage proportional to the dose of the light 301 or time is generated in the MIS-type photoelectric conversion element 217 and the potential of the bottom electrode layer 207 is lowered.
• the positive hole 303 generated in the photoelectric conversion layer 209 cannot move up to the interface between the photoelectric conversion layer 209 and the insulating layer 208 and the positive hole 303 is recombined with the electron 302 and disappears. Therefore, a voltage proportional to the dose of the light 301 or time is not generated. Then, because a voltage proportional to the dose of the light 301 or time is not generated in the saturated MIS-type photoelectric conversion element 217, the sensitivity is lowered and a normal X-ray image cannot be obtained.
• the light sensitivity of the MIS-type photoelectric conversion element 217 depends on the voltage applied to the photoelectric conversion layer 209.
• the positive hole 303 generated by photoelectric effect is moved by an electric field applied to the photoelectric conversion layer 209 and reaches the interface between the photoelectric conversion layer 209 and the insulating layer 208.
• this time does not become shorter than the life time of the positive hole 303 decided by the film quality of the photoelectric conversion layer 209, the positive hole 303 cannot reach the interface between the photoelectric conversion layer 209 and the insulating layer 208 and it disappears. Therefore, it is impossible to take out the positive hole 303 as an electrical signal.
• the refresh mode shown in Fig. 2C is realized and it is possible to newly accumulate positive holes 303 in the photoelectric conversion mode by the removed number of positive holes 303. Therefore, by setting the sensor bias to be supplied at the time of refresh operation to a lower value, it is possible that a sensor does not easily become a saturation state even if much light is irradiated. Moreover, it is possible to keep the voltage applied to the photoelectric conversion layer 209 constant by this refresh operation before light irradiation. Therefore, in the state in which the refresh operation is effective, sensitivities are not changed, However, immediately after the refresh mode is changed to the photoelectric conversion mode, current due to electrons implanted into the MIS-type photoelectric conversion element 217 in the refresh mode flows.
• Fig. 3 is a circuit diagram showing the configuration of one pixel of the two-dimensional sensor (sensor unit) in an X-ray imaging apparatus. As described above, one pixel includes the TFT 216 and MIS-type photoelectric conversion element 217.
• the gate line 220 is connected to the gate of the TFT 216 and the signal line 219 is connected to the source electrode 206 of the TFT 216.
• the sensor bias line 218 for applying a voltage required to perform photoelectric conversion and refresh is connected to the MIS-type photoelectric conversion element 217.
• the signal line 219, gate line 220, and sensor bias line 218 are shared by a plurality of pixels constituting a two-dimensional sensor.
• the gate line 220 is connected to a vertical driving circuit 105 and a voltage for selectively turning on/off the TFT 216 is supplied from the vertical driving circuit 105.
• the sensor bias line 218 is connected to a sensor power supply 402.
• the sensor power supply 402 includes a power supply Vs for photoelectric conversion and a power supply Vref necessary for the refresh of a sensor, in which power supply outputs can be optionally changed by a control signal VSC.
• the signal line 219 connects the source electrode 206 of the TFT 216 and the input of a signal amplifying circuit constituted by using a current-integrating-type amplifier 401.
• Fig. 4 is a timing chart showing changes of voltages of points A and B in the pixel shown in Fig. 3.
• Fig. 3 shows a voltage (Va-Vb) applied to the photoelectric conversion layer 209 together with potential Va of the point A and potential Vb of the point B when repeating irradiation with X ray and refresh from immediately after turning-on of a power supply.
• the TFT 216 is driven synchronously with refresh or read. Moreover, when a power supply is turned on, it is assumed that the TFT 216 is regularly turned on/off. To change the operation halt state to the imaging possible state, supply of voltages is started from the sensor power supply 402, power supply Vcom, and power supply Vss. In this case, the potential Va becomes equal to the voltage Vs.
• the potential Vb is shown by the following Numerical Formula 1 when assuming the capacity of the photoelectric conversion layer 209 as Ci, the capacity of the insulating layer 208 as C SlN , and the reference power supply of the amplifier 401 as Vr. (Numerical Formula 1)
• Vb — x (Vs - Vr)+ Vr
• the potential Vb rises due to accumulation of generated electric charges because when X ray is irradiated, the photoelectric effect occurs by the light of a phosphor ' for emitting light by receiving X ray. Moreover, in the refresh mode (state in which the voltage Vref is supplied to the sensor bias line 218), because the voltage Va is changed from Vs, to
• refresh shows an effect. That is, the effect of refresh is first shown by making the state of the MIS-type photoelectric conversion element 217 approach to the saturation state.
• the voltage (Va-Vb) applied to the photoelectric conversion layer 209 is slowly lowered because electric charges generated by photoelectric effect are accumulated. Therefore, in the period a, the sensitivity of the MIS-type photoelectric conversion element 217 is slowly lowered whenever imaging is performed. Then, when imaging a plurality of sheets in the period a, a problem occurs that the contrast of an image is slowly lowered due to a change of sensitivities whenever imaging is performed or a problem occurs that sensitivities are changed between a sensor having much dose of light and a sensor having less dose of light and a problem occurs that uncomfortable feeling appears in an image in the last imaging.
• a method for continuously applying a voltage to a pixel to keep the period ⁇ is considered,
• deterioration of the characteristic is accelerated. Therefore, this is not preferable in view of the reliability of an apparatus.
• a conventional digital X-ray imaging apparatus restrains characteristic deterioration of the two- dimensional sensor 103 by stopping voltage supply to the sensor bias source 218, power supply Vcom, and power supply Vss when imaging is not performed and supplying no voltage to the two-dimensional sensor 103.
• Fig. 5 is a circuit diagram showing the configuration of one pixel of a two- dimensional sensor (sensor unit) in a radiation imaging apparatus (X-ray imaging apparatus) of the first embodiment of the present invention.
• a component provided with the same symbol as that in Fig. 1, 3 or the like is a device or circuit having the same function and its description is omitted.
• This embodiment is provided with a light source 601, power supply 603 for making the light source 601 emit light, and switch 605 as means for bringing the MIS-type photoelectric conversion element 217 into a saturation state before performing X-ray imaging.
• the light source 601 it is possible to use a light source capable of discharging the light having a wavelength which can be detected by the photoelectric conversion layer 209 serving as the conversion layer of an MIS-type conversion element at optional timing.
• a light source capable of discharging the light having a wavelength which can be detected by the photoelectric conversion layer 209 serving as the conversion layer of an MIS-type conversion element at optional timing.
• an MIS-type photoelectric conversion element using amorphous silicon it is possible to use a device in which a plurality of LEDs or cold-cathode tubes are arranged, a device in which a light guide plate and a LED or cold-cathode tube are combined, or EL device.
• the light having a wavelength which can be detected by the MIS-type photoelectric conversion element may includes also a radiation such as infrared rays and ultraviolet rays or the like other than the visible light.
• a control circuit 604 for controlling the light source 601 and an X-ray source 119 is provided. That is, the control circuit 604 can control light emission/no light emission of the light source 601 or control exposure of X ray from the X- ray source 119. For example, light irradiation by the light source 601 is controlled by the control circuit 604 so that the light is irradiated only for predetermined necessary time. In this case, it is preferable that the control circuit 604 sets the control circuit 604 so that it cannot expose the X- ray source 119 and X ray is not erroneously irradiated. Then, operations of one pixel constituted as described are described by referring to Fig. 6. Fig.
• FIG. 6 is a timing chart showing changes of voltages of -points C and D in the pixel shown in Fig. 5.
• Fig. 6 shows a voltage (Vc-Vd) applied to the photoelectric conversion layer 209 together with the potential Vc of the point C and the potential Vd of the point D when repeating X-ray irradiation and refresh from immediately after turning-on of a power supply.
• the pause state in which no voltage is supplied to a two-dimensional sensor is changed to the imaging state for supplying a voltage to the Mistype photoelectric conversion element 217 and TFT 216.
• the potential of the sensor bias line 218 becomes Vs and the voltage applied to the MIS-type photoelectric conversion element 217 is shown by Numerical Formula 1.
• a display for communicating that X ray can be exposed to a worker it is preferable to change a display for communicating that X ray can be exposed to a worker. Furthermore, when operations of a two-dimensional sensor are different before start of X-ray imaging and after start of X-ray imaging, it is allowed to change driving of the sensor in accordance with a signal of the control circuit 604.
• control circuit 604 is provided for a control computer 108 or program/control board 110 of a conventional X-ray imaging apparatus. Moreover, it is allowed to realize the function of the control circuit 604 by combining operations of the control computer 108 and program/control board 110. Then, a two-dimensional sensor (sensor unit) having 9 above pixels (3 x 3 pixels) and its peripheral circuits are described below.
• Fig. 7 is a circuit diagram showing the configuration of a two- dimensional sensor in which nine pixels shown in Fig. 5 are arranged like a matrix and its peripheral circuits.
• Fig. 8 is an illustration showing the configuration of an X-ray imaging apparatus using the two-dimensional sensor shown in Fig. 7.
• the two-dimensional sensor (sensor unit) 801 shown in Fig. 7 is constituted when the MIS-type photoelectric conversion element 217 (S11-S33) and thin-film transistor (TFT) 216 (T11-T33) are arranged like a matrix of 3 x 3.
• the MIS-type photoelectric conversion element 217 (S11-S33) converts light emitted from a phosphor into an electrical signal.
• the thin-film transistor 216 (T11-T33) outputs electric charges accumulated in the MIS-type photoelectric conversion element 217 at ' optional timing.
• a phosphor 101 is provided on the MIS-type photoelectric conversion element 217 as shown in Fig. 8.
• the phosphor 101 mainly contains Gd 2 O 2 S, Gd 2 O 3 and/or CsI: Tl.
• the two-dimensional sensor 801 has amplifiers
• AMPl to AMP3 respectively provided with a capacity Cf for accumulating electric charges output from the TFT 216 and connects with a signal amplifying circuit 802 for amplifying a signal.
• Signal lines Sigl to Sig3 are set between the signal amplifying circuit 802 and the two-dimensional sensor 801.
• the signal lines Sigl to Sig3 are connected to the drain electrode of the TFT 216.
• the signal amplifying circuit 802 connects with an Amp reference power supply 807 serving as the reference power supply of the amplifiers AMPl to AMP3.
• the signal amplifying circuit 802 connects with a sample-and- hold circuit 803 for holding output voltage of the signal amplifying circuit 802 for an optional period and multiplexer 804 for serially outputting signals held by the sample-and-hold circuit 803.
• the sample- and-hold circuit 803 holds an electrical signal output from the signal amplifying circuit 802 until the circuit 802 is selected by the multiplexer 804.
• the signal amplifying circuit 802 connects with a buffer amplifier 805 for outputting the output of the multiplexer 804 at a low impedance and A/D converter 806 for converting an analog signal into a digital signal.
• the signal amplifying circuit 802, sample-and-hold circuit 803, multiplexer 804, and buffer amplifier 805 are included in a signal processing circuit 809.
• the two-dimensional sensor 801 connects with a power supply Vs necessary for photoelectric conversion and sensor power supply 402 provided with the power supply Vref for setting the MIS-type photoelectric conversion element 217 to a refresh mode.
• the sensor bias line 218 is connected between the N+ amorphous silicon layer of the Mistype photoelectric conversion element 217 and the sensor power supply 402.
• the sensor power supply 402 and Amp reference power supply 807 are included in a low-noise power supply 827.
• a vertical driving circuit 105 for driving gate lines VgI to Vg3 connected to the gate electrode of the TFT 216 of the two-dimensional sensor 801 is provided.
• step S1802 when an operator starts imaging by performing a necessary procedure (step S1802), a predetermined voltage is applied to the two- dimensional sensor 801, the voltage Vs is applied to the MIS-type photoelectric conversion element 217 and the voltage Vss is applied to the gate lines VgI to Vg3. Moreover, a voltage necessary for the Amp reference power supply and driving is also applied to the signal amplifying circuit 802 and an operation state is started (step S1803) .
• step S1804 After starting the imaging state, light is irradiated from the light source 601 iww the control by control circuit 604 (control computer 808) (step S1804).
• the output of the two-dimensional sensor 801 is sent to the control computer 808 after the output is converted into digital data and necessary processing is applied.
• the control circuit 604 determines whether to realize a saturation state or stop irradiation of light in accordance with the data (step S1805) . However, while light is irradiated, as shown in Fig. 6, refresh operation, photoelectric conversion operation, and read are repeated.
• the time for irradiating light can be programmed in the control computer 808. Then, it is allowed to set the irradiation time when the two- dimensional sensor 801 is fabricated or at the time of setting after fabrication. Moreover, it is allowed to monitor the output fluctuation from the MIS-type photoelectric conversion element 217 at the time of light irradiation and determine light irradiation stop time.
• control computer 808 determines stopping of irradiation of light, it stops irradiation in step S1806.
• the two-dimensional sensor 801 also starts the operation for imaging by an operator when X-ray irradiation is started (step S1807) .
• the operation for imaging a static image is described below.
• the MIS-type photoelectric conversion element 217 is first refreshed by using the sensor power supply 402.
• the MIS-type photoelectric conversion element 217 is refreshed by using the sensor power supply 402 in order to secure a large dynamic range necessary for a static image.
• the control signal VSC is first set to low (Lo) to supply the voltage Vref optimum for refresh to the sensor bias line 218.
• the refresh mode is not realized.
• the voltage Vcom is further supplied to the gate lines VgI to Vg3 by the vertical driving circuit 105 to turn on the TFT 216 and equalize the potential of the sensor bottom electrode layer 207 with potentials of the signal lines Sigl to Sig3.
• the control signal RC of the amplifiers AMPl to AMP3 is set to high (Hi) to set potentials of the signal lines Sigl to Slg3 to the reference potential Vr.
• the TFT 216 for each pixel is turned on for each line However, it is also allowed to simultaneously turn on all TFTs 216.
• the control signal SH of the sample-and-hold circuit 803 is set to high (Hi) to transfer outputs of the amplifiers Ampl to Amp3 connected to sample-and-hold capacities SHl to SH3 to sample-and-hold capacities SHl to SH3.
• the control signal SH is kept high (Hi) until voltages output from the amplifiers Ampl to Amp3 are sufficiently transferred to sample-and-hold capacities and after transfer is completed, the voltages are set to low ( Lo ) .

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