In a conventional X-ray image sensing apparatus, an X-ray beam is projected from an X-ray source via an object to be analyzed such as a patient under medical treatment. Normally, after the beam is transmitted through the object to be analyzed, an image intensifier converts X-ray radiation into a visible image, and a video camera generates an analog video signal from the visible image and displays it on a monitor. Since the analog video signal is generated, an image process for automatic luminance adjustment and image emphasis is done in the analog domain.
A high-resolution solid-state X-ray detector has already been proposed, and comprises a two-dimensional array using 3000 to 4000 detection elements represented by photodiodes or the like. Each element generates an electrical signal corresponding to the pixel luminance of an X-ray image projected onto the detector. Signals from the respective detection elements are individually read and converted into digital signals, which then undergo an image process, storage, and display.
Upon obtaining a medical X-ray image using a large-screen X-ray detector, the X-ray detector requires a certain time until actual X-ray exposure, and is driven in a preparation state to shorten that time.
In the preparation state of the detector, in order to avoid the elements in the detector from being held in a saturation state due to gradually accumulated dark currents, a dedicated read drive process is repeated at given intervals. This repetitive drive process will be referred to as “idling drive” hereinafter. Since the duration of this idling drive period is not defined in practical use, if the idling drive period is long, it shortens the service life of the apparatus, and promotes aging of various characteristics associated with detection.
The present invention has been made in consideration of the above problems, and has as its object to provide an image sensing apparatus and image sensing method, which can implement highly reliable image sensing by suppressing shortening of the service life of the apparatus and aging of various characteristics associated with detection even when the idling drive period, which is difficult to be defined in practical use, is long.
In medical radiography as that for the purpose of medical diagnosis, (spot) photographing uses X-ray photography as a combination of an intensifying screen and X-ray photo film.
When radiation such as X-rays or the like that have been transmitted through an object to be examined become incident on the intensifying screen, a phosphor contained in the intensifying screen absorbs this X-ray energy and emits fluorescence. This fluorescence exposes the X-ray photo film to form a radiation image on it. By developing and fixing the film, an X-ray image can-be visualized.
Recently, various schemes for digitally capturing a radiation image have been developed. In one scheme, using an X-ray image detector which comprises a photoelectric conversion element which has sensitivity to X-rays, converts detected X-rays into electrical signals corresponding to their intensity levels, and outputs the electrical signals, or a combination of a phosphor which absorbs X-ray energy and emits fluorescence with intensity corresponding to the absorbed energy, and a photoelectric conversion element which has sensitivity to visible light and outputs an electrical signal corresponding to its intensity, an X-ray image is converted into an electrical signal, and the electrical signal is A/D-converted to capture a digital image.
FIG. 21 is a schematic block diagram showing an example of an X-ray photographing system.
Referring to FIG. 21, reference numeral 5001 denotes an X-ray generation device; 5002, a host computer; 5003, a phosphor; 5004, a flat-panel detector as a two-dimensional array of a large number of photoelectric conversion elements each of which comprises a photodetector and switching element; 5005, a flat-panel detector controller for controlling the flat-panel detector 5004; and 5006, an object. The X-ray photographing apparatus comprises the phosphor 5003, flat-panel detector 5004, and flat-panel detector controller 5005.
The X-ray generation device 5001 has an X-ray radiation switch (not shown). When the X-ray radiation switch is pressed, a signal indicating that an X-ray generation request is generated is sent to the host computer 5002. The host computer 5002 informs the flat-panel detector controller 5005 of generation of the X-ray radiation request. Upon receiving the X-ray radiation request, the flat-panel detector controller 5005 initializes the flat-panel detector 5004. Upon completion of initialization of the flat-panel detector 5004, the flat-panel detector controller 5005 sends an X-ray radiation permission signal to the host computer 5002. Upon receiving the X-ray radiation permission signal, the host computer 5002 sends a signal indicating that X-ray radiation is permitted to the X-ray generation device 5001. Then, the X-ray generation device 5001 radiates X-rays. The radiated X-rays are transmitted through the object 5006 and are converted by the phosphor 5003 into light proportional to the incoming X-ray dose. This light is converted into an electrical signal by the flat-panel detector 5004. The flat-panel detector controller 5005 reads this electrical signal, and transfers an X-ray digital image to the host computer 5002 at the same time. The transferred X-ray digital image undergoes an image process by the host computer 5002, and the taken X-ray digital image is displayed on a display device (not shown).
FIG. 22 shows an equivalent circuit of one photoelectric conversion element. In the following description, an amorphous silicon sensor will be exemplified as the photoelectric conversion element. However, the photoelectric conversion element need not be particularly limited and, for example, elements such as other solid-state image sensing elements (charge-coupled element and the like), a photomultiplier, or the like may be used.
Referring to FIG. 22, one photoelectric conversion element 5020 comprises a photodetector 5021 and a switching element 5022 for controlling charge accumulation and read. In general, the element 20 is formed on a glass substrate using amorphous silicon (α-Si).
A capacitor 5021C in the photodiode 5021C can simply be a photodiode having a parasitic capacitance, or be a photodetector which includes a parallel circuit of a photodiode 5021D and additional capacitor 5021C to improve the dynamic range of the photodiode 5021D and detector. When X-rays hit the photodetector 5021, the photodiode 5021D generates a charge corresponding to the X-ray dose, and the generated charge is accumulated on the capacitor 5021C.
An anode A of the diode 5021D is connected to a refresh control circuit 5023. The refresh control circuit 5023 normally outputs a bias voltage Vs, but can temporarily output a refresh voltage Vr to initialize the capacitor 5021C.
A cathode K of the diode 5021D is connected to the controllable switching element 5022 used to read the charge accumulated on the capacitor 5021C. In this example, the switching element 5022 is a thin-film transistor connected between the cathode K of the diode 5021D and a charge read amplifier 5025.
A gate G of the switching element 5022 is connected to a gate control circuit 5024, which outputs a gate signal Vg to read the charge accumulated on the capacitor 5021C. The read charge is amplified by the amplifier 5025, and is A/D-converted by an A/D converter 5027 via a sample/hold circuit 5026, thereby converting the charge accumulated on the capacitor 5021C into digital data.
An initialization process of one photoelectric conversion element 5020 will be described below using FIG. 23.
Referring to FIG. 23, a refresh signal indicates the output signal from the refresh control circuit 5023, a gate signal indicates the output signal from the gate control circuit 5024 and a dark current indicates a current that flows the capacitor 5021C. Normally, the voltage of the refresh signal is equal to the bias voltage Vs, that of the gate signal is 0 V, and nearly no dark current flows.
In this state, the refresh voltage Vr is output at time T1 to initialize the capacitor 5021C. When the refresh signal has reached the refresh voltage Vr, a minus dark current flows, and the charge accumulated on the capacitor 5021C is swept out. A time period (a time period from time T1 to time T2) in which the refresh signal is equal to the refresh voltage Vr is determined in advance so as to sufficiently reduce the charge accumulated on the capacitor 5021C.
At time T2, the voltage of the refresh signal is changed to the bias voltage Vs. An operation in which the refresh control circuit 5023 temporarily outputs the refresh signal (=voltage Vr) to initialize the capacitor 5021C will be referred to as “refresh” hereinafter. Immediately after the bias voltage Vs is switched, a large plus dark current is generated, and is accumulated as a charge on the capacitor 5021C. As is known, noise to be superposed on a taken X-ray image is proportional to the square root of the charge accumulated due to the dark current.
After the refresh process, the gate control circuit 5024 temporarily outputs the gate signal Vg at time T3. As a result, the charge accumulated on the capacitor 5021C is swept out.
Time T3 is determined in advance to sufficiently reduce the dark current. In the following description, an operation in which the gate control circuit 5024 temporarily outputs the gate signal Vg to sweep out the charge accumulated on the capacitor 5021C due to the dark current will be referred to as “idle read”. When the charge accumulated on the capacitor 5021C is sufficiently swept out, the gate signal is set at 0 V at time T5. A time period (a time period from time T3 to time T5) in which the gate signal is equal to the gate voltage Vg is determined in advance so as to sufficiently reduce the charge accumulated on the capacitor 5021C.
However, even after idle read, since a slight dark current is still flowing, the capacitor 5021C gradually accumulates a charge. For this reason, the initialization process of the photoelectric conversion element 5020, which includes the refresh process and idle read process, is repeated periodically. Also, for the same reason as above, the initialization process of the photoelectric conversion element 5020 is done immediately before X-ray photographing.
FIG. 24 is a block diagram showing an example of the flat-panel detector 5004 and flat-panel detector controller 500-5.
Referring to FIG. 24, reference numeral 5007 denotes a CPU for reading an X-ray digital image from a flat-panel detector 5004, and a refresh control circuit 5008, row address select circuit 5009, and column address select circuit 5010 are connected to the CPU 5007. The CPU 5007 can control these circuits. The CPU 5007 is connected to the host computer 5002 (not shown in FIG. 24), and can transfer an X-ray digital image read from the flat-panel detector 5004 to the host computer 5002.
The flat-panel detector 5004 comprises a two-dimensional array of a large number of photoelectric conversion elements 5020 shown in FIG. 22. However, in FIG. 24, photoelectric conversion elements 5020 are two-dimensionally arranged in a 2×2 (row×column) matrix for the sake of simplicity.
As described above, the photoelectric conversion element 5020 for one pixel comprises the photodiode 5021 and switching TFT 5022. Photodetectors 5021(1,1) to 5021(2,2) correspond to the aforementioned photodetector 5021, and K and A respectively represent the cathode and anode sides of the photodetector 5021. The TFTs 5022(1,1) to 5022(2,2) correspond to the switching TFT 5022, and S, G, and D respectively represent the source, gate, and drain electrodes of the TFT.
The gate electrodes G of the TFTs 5022 of each row are connected to the row address select circuit 5009, which comprises the aforementioned gate control circuit 5024 and switches SWr1 and SWr2.
The drain electrodes D of the TFTs 5022 of each column are connected to the column address select circuit 5010, which comprises the amplifier 5025, sample/hold circuit 5026, and switches SWc1 and SWc2.
The anode sides of all the photodetectors 5021 are connected to the refresh control circuit 5008, which normally outputs a bias voltage Vs, and also outputs a refresh voltage Vr as a refresh signal. The refresh control circuit 5008 is the same as the refresh control circuit 5023 shown in FIG. 22.
The initialization process of the plurality of photoelectric conversion elements 5020 shown in FIG. 25 in the arrangement shown in FIG. 24 will be explained below.
Referring to FIG. 25, a refresh signal indicates the output signal from the refresh control circuit 5008, a gate signal indicates the output signal from the gate control circuit 5024, and SWr1 and SWr2 indicate the switches SWr1 and SWr2 in the row address select circuit 5009. Normally, the voltage of the refresh signal is equal to the bias voltage Vs, that of the gate signal is 0 V, and the switches SWr1 and SWr2 are OF-F. Therefore, the anode sides A of all the photodetectors 5021(1,1) to 5021(2,2) are set at the bias voltage Vs, and the gate electrodes G of all the TFTs 5022(1,1) to 5022(2,2) are set at 0 V.
In this state, the refresh voltage Vr is output at time T1 to initialize all the photodetectors 5021. When the refresh signal has reached the refresh voltage Vr, a minus dark current flows, and the charge accumulated on the capacitor 5021C of each photodetector 5021 is swept out. At time T2, the voltage of the refresh signal is set at the bias voltage Vs. Immediately after refresh signal is switched to the bias voltage Vs, a large plus dark current is generated, and is accumulated on the capacitor 5021C of each photodetector 5021 as a charge. Hence, after the refresh process, the gate control circuit 5024 temporarily outputs a gate signal Vg at time T3 to turn on the switch SWr1. As a result, the voltages at the gate electrodes G of the TFTs 5022(1,1) and 5022(1,2) of the first row change to Vg, and the charges accumulated on the capacitors 5021C in the photodetectors 5021 of the first row are swept out. At time T4, the switch SWr1 is turned off, and the switch SWr2 is turned on. The voltages at the gate electrodes G of the TFTs 5022(1,1) and 5022(1,2) of the first row change to 0 V, and those at the gate electrodes G of the TFTs 5022(2,1) and 5022(2,2) of the second row change to Vg. Hence, the charges accumulated on the capacitors 5021C in the photodetectors 5021 of the second row are swept out.
When the gate signal is set at 0 V and the switch SWr2 is turned off at time T5, the gate electrodes G of all the TFTs 5022(1,1) to 5022(2,2) change to 0 V, thus ending the initialization process. The initialization process is executed during the period from time T1 to time T5.
FIG. 26 shows the relationship between the initialization process of the photoelectric conversion elements 5020 and X-ray photographing. As shown in FIG. 26, when no X-ray photographing is made, the refresh and idle read processes (initialization process) are periodically repeated at intervals TI. FIG. 26 shows a case wherein the X-ray radiation switch of the X-ray generation device 5001 is pressed at time T1 (a timing other than those of the refresh and idle read processes), and the X-ray radiation request reaches the flat-panel detector controller 5005 via the host computer 5002. Upon generation of the X-ray radiation request, the host computer 5002 that has received this request changes an X-ray radiation request signal to Low.
When the X-ray radiation request signal has changed to Low, the refresh and idle read processes are executed again. Upon completion of these processes, the flat-panel detector controller 5005 outputs an X-ray radiation permission signal at time T5. That is, the controller 5005 changes the X-ray radiation permission signal to Low. When the X-ray radiation permission signal has changed to Low, X-ray radiation is permitted. Note that the time period from when the X-ray radiation request signal has changed to Low until the X-ray radiation permission signal changes to Low is called an exposure delay time period, and is indicated by TD1 in FIG. 26.
When the X-ray radiation permission signal has reached the X-ray generation device 5001 via the host computer 5002, the X-ray generation device 5001 radiates X-rays, as shown in FIG. 26. When the X-ray generation device 5001 radiates X-rays, the radiated X-rays are transmitted through the object 5006, and are converted into light proportional to the incoming X-ray dose by the phosphor 5003, and charges corresponding to the light are accumulated on the capacitors 5021C.
Upon completion of X-ray radiation, the host computer 5002 changes the X-ray radiation permission signal to High at time T6, and outputs it to the X-ray generation device 5001. When the X-ray radiation permission signal has changed to High, the X-ray radiation request signal goes High.
Also, upon completion of X-ray radiation, the gate signal is set at Vg and the switch SWr1 is turned on at time T6. As a result, the voltages at the gate electrodes G of the TFTs 5022(1,1) and 5022(1,2) of the first row shown in FIG. 24 change to Vg, the charges accumulated on the capacitors 5021C in the photodetectors 5021 of the first row are read, and the read signals are held via the amplifier 5025 and sample/hold circuit 5026. When the switch SWc1 is turned on at time T6, the held signal of the photodetector 5021(1,1) is converted into a digital value by the A/D converter 5027, and that value is transferred to the host computer 5002. Also, when the switch SWc1 is turned off and the switch SWc2 is turned on at time T7, the held signal of the photodetector 5021(1,2) is converted into a digital value by the A/D converter 5027, and that value is transferred to the host computer 5002.
When the switch SWr1 is turned off and the switch SWr2 is turned on at time T8, the voltages at the gate electrodes G of the TFTs 5022(1,1) and 5022(1,2) of the second row shown in FIG. 24 change to Vg, the charges accumulated on the capacitors 5021C in the photodetectors 5021 of the second row are read, and the read signals are held via the amplifier 5025 and sample/hold circuit 5026. When the switch SWc1 is turned on at time T8, the held signal of the photodetector 5021(2,1) is converted into a digital value by the A/D converter 5027, and that value is transferred to the host computer 5002. Also, when the switch SWc1 is turned off and the switch SWc2 is turned on at time T9, the held signal of the photodetector 5021(2,2) is converted into a digital value by the A/D converter 5027, and that value is transferred to the host computer 5002.
After all the charges accumulated on the flat-panel detector 5004 are transferred to the host computer 5002, the gate signal is set at 0 V, and the switches SWr1, SWr2, SWc1, and SWc2 are turned off at time T10. In the following description, an operation in which all charges accumulated on the flat-panel detector 5004 are read by setting the gate signal at Vg, and temporarily turning on the switches SWr1, SWr2, SWc1, and SWc2 will be referred to as “main read”.
In the aforementioned prior art, as shown in FIG. 26, when the X-ray radiation switch is pressed at a timing other than the initialization process, and the X-ray radiation request signal goes Low, the refresh and idle read processes are immediately executed.
However, when the X-ray radiation switch is pressed during the initialization process, and the X-ray radiation request signal goes Low, the current initialization process is interrupted, and is redone from the beginning. Upon generation of an X-ray radiation request, if the initialization process is interrupted and the refresh process is executed immediately, a dark current flows. As a result, many charges are accumulated on the capacitors 5021C, and some charges remain even after the idle read process. Since it is known that noise to be superposed on a taken X-ray image is proportional to the square root of the charge accumulated due to the dark current, if X-rays are radiated in this state, noise on the taken X-ray image is emphasized.
The present invention has been made in consideration of the aforementioned problems, and has as its object to prevent noise in an taken X-ray image from increasing even when an X-ray radiation request is received during initialization of a detector.
It is preferable to shorten the time period from when the X-ray radiation request is received until X-ray photographing is permitted. Therefore, it is another object of the present invention to shorten the exposure delay time period upon receiving an X-ray radiation request during initialization of the detector.